JP2014082118A - Negative electrode material for lithium ion secondary battery, and negative electrode using the same, and secondary battery - Google Patents
Negative electrode material for lithium ion secondary battery, and negative electrode using the same, and secondary battery Download PDFInfo
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Abstract
Description
本発明は、リチウムイオン二次電池の負極に用いられる材料、並びにそれを用いた負極及び二次電池に関する。 The present invention relates to a material used for a negative electrode of a lithium ion secondary battery, and a negative electrode and a secondary battery using the material.
リチウムイオン二次電池などの二次電池は、小型で大容量であるため、携帯電話やノートパソコンといった幅広い分野で用いられている。リチウムイオン二次電池の性能は、二次電池を構成する正極、負極および電解質の材料に左右される。なかでも電極に含まれる活物質材料の研究開発が活発に行われている。現在、一般的に用いられている負極活物質として、黒鉛などの炭素系材料がある。黒鉛などを負極活物質とする炭素負極は、インターカレーション反応を有することから、サイクル特性は良いものの、高容量化が困難とされている。そこで負極活物質材料として、炭素よりも高容量な珪素や珪素酸化物などの珪素系材料が検討されている。 Secondary batteries such as lithium ion secondary batteries are small and have a large capacity, and are therefore used in a wide range of fields such as mobile phones and notebook computers. The performance of the lithium ion secondary battery depends on the materials of the positive electrode, the negative electrode, and the electrolyte constituting the secondary battery. In particular, research and development of active material materials contained in electrodes are being actively conducted. Currently, there is a carbon-based material such as graphite as a commonly used negative electrode active material. A carbon negative electrode using graphite or the like as a negative electrode active material has an intercalation reaction, and thus has high cycle characteristics but is difficult to increase in capacity. Therefore, silicon-based materials such as silicon and silicon oxide having a higher capacity than carbon have been studied as negative electrode active material.
珪素系材料は、リチウムと合金化することで、１０００ｍＡｈ／ｇ以上の高容量をもつ。しかし、珪素や酸化珪素のような珪素系材料を負極活物質として用いると、充放電サイクルにより負極活物質が膨張および収縮することで体積変化することが知られている。負極活物質が膨張したり収縮したりすることで、負極活物質を集電体に保持する役割を果たす結着剤に負荷がかかり、負極活物質と集電体との密着性が低下したり、電極内の導電パスが破壊されて容量が著しく低下したり、膨張と収縮の繰り返しにより負極活物質に歪が生じて微細化して電極から脱離したり、といった問題がある。こういった種々の問題点があるため、サイクル特性に乏しいという問題がある。 A silicon-based material has a high capacity of 1000 mAh / g or more by being alloyed with lithium. However, it is known that when a silicon-based material such as silicon or silicon oxide is used as the negative electrode active material, the negative electrode active material expands and contracts due to a charge / discharge cycle, thereby changing the volume. When the negative electrode active material expands or contracts, a load is applied to the binder that holds the negative electrode active material on the current collector, and the adhesion between the negative electrode active material and the current collector decreases. There is a problem that the conductive path in the electrode is broken and the capacity is remarkably reduced, or the negative electrode active material is distorted due to repeated expansion and contraction to be refined and detached from the electrode. Because of these various problems, there is a problem that the cycle characteristics are poor.
そこで、珪素系材料として、酸化珪素（ＳｉＯｘ：ｘは０．５≦ｘ≦１．５程度）の使用が検討されている。ＳｉＯｘは、熱処理されると、ＳｉとＳｉＯ２とに分解することが知られている。これは不均化反応といい、ＳｉとＯとの比が概ね１：１の均質な固体の一酸化珪素ＳｉＯであれば、固体の内部反応によりＳｉ相とＳｉＯ２相の二相に分離する。分離して得られるＳｉ相は非常に微細である。また、Ｓｉ相を覆うＳｉＯ２相が電解液の分解を抑制する働きをもつ。したがって、体積変化の問題は依然として残るものの、Ｓｉ相とＳｉＯ２相とに分解したＳｉＯｘからなる負極活物質を用いた二次電池は、サイクル特性に優れる。 Therefore, the use of silicon oxide (SiO x : x is about 0.5 ≦ x ≦ 1.5) is being studied as a silicon-based material. It is known that SiO x decomposes into Si and SiO 2 when heat-treated. This is called a disproportionation reaction, and if it is a homogeneous solid silicon monoxide SiO having a ratio of Si to O of approximately 1: 1, it is separated into two phases of Si phase and SiO 2 phase by solid internal reaction. . The Si phase obtained by separation is very fine. Further, the SiO 2 phase covering the Si phase has a function of suppressing decomposition of the electrolytic solution. Therefore, although the problem of volume change still remains, the secondary battery using the negative electrode active material composed of SiO x decomposed into the Si phase and the SiO 2 phase has excellent cycle characteristics.
ところで、負極活物質の形状を変えて、負極活物質層における負極活物質のかさ密度を増加させることで、電池容量を増加させることが考えられている。例えば、特許文献１、２には、球状の黒鉛と、針状やフレーク状の黒鉛とを混合した負極活物質が示されている。大きな球状黒鉛の間に、小さい針状などの黒鉛を入り込ませることで、かさ密度を上げて、電池容量を増やすことができる。 By the way, it is considered to increase the battery capacity by changing the shape of the negative electrode active material to increase the bulk density of the negative electrode active material in the negative electrode active material layer. For example, Patent Documents 1 and 2 disclose a negative electrode active material in which spherical graphite and needle-like or flake-like graphite are mixed. By inserting small needle-shaped graphite or the like between large spherical graphites, the bulk density can be increased and the battery capacity can be increased.
一方で、ＳｉＯとは異なる化合物を負極活物質層に含めることで、電池特性を向上させることが提案されている。たとえば、特許文献３には、ＳｉＯと酸化鉄とを混合させた負極活物質層が提案されている。特許文献４には、活物質として、ＦｅＯＯＨの脱水反応から生成したＦｅ２Ｏ３を用いることがよいと示されている。また、特許文献５〜８には、負極活物質層に、ＳｉＯとＬｉＦｅＰＯ４とを混合させることが提案されている。 On the other hand, it has been proposed to improve battery characteristics by including a compound different from SiO in the negative electrode active material layer. For example, Patent Document 3 proposes a negative electrode active material layer in which SiO and iron oxide are mixed. Patent Document 4 shows that Fe 2 O 3 generated from the dehydration reaction of FeOOH is preferably used as the active material. Patent Documents 5 to 8 propose mixing SiO and LiFePO 4 in the negative electrode active material layer.
しかしながら、上記従来の特許文献のいずれも、充放電サイクル特性が不十分であり、更なる改良が望まれている。 However, all of the above-mentioned conventional patent documents have insufficient charge / discharge cycle characteristics, and further improvements are desired.
本発明はかかる事情に鑑みてなされたものであり、充放電サイクル特性に優れたリチウムイオン二次電池用負極材料、並びにそれを用いた負極及び二次電池を提供することを課題とする。 This invention is made | formed in view of this situation, and makes it a subject to provide the negative electrode material for lithium ion secondary batteries excellent in charging / discharging cycling characteristics, and the negative electrode and secondary battery using the same.
本発明のリチウムイオン二次電池用負極材料は、珪素酸化物からなる珪素酸化物粒子と、鉄酸化物からなる棒状の鉄酸化物粒子と、Ｌｉ（リチウム）、Ｍｇ（マグネシウム）、Ｐ（リン）及びＯ（酸素）からなる化合物よりなる化合物粒子と、の混合物を含むことを特徴とする。 The negative electrode material for a lithium ion secondary battery of the present invention comprises silicon oxide particles made of silicon oxide, rod-like iron oxide particles made of iron oxide, Li (lithium), Mg (magnesium), P (phosphorus). ) And O (oxygen) compound particles, and a mixture thereof.
本発明の負極は、上記のリチウムイオン二次電池用負極材料を有することを特徴とする。 The negative electrode of the present invention is characterized by having the above negative electrode material for a lithium ion secondary battery.
本発明のリチウムイオン二次電池は、上記の負極を有することを特徴とする。 The lithium ion secondary battery of this invention has said negative electrode, It is characterized by the above-mentioned.
本発明のリチウムイオン二次電池用負極材料は、珪素酸化物粒子と、棒状の鉄酸化物粒子と、Ｌｉ、Ｍｇ、Ｐ及びＯを含む化合物粒子とからなる混合物よりなるため、充放電サイクル特性に優れる。 Since the negative electrode material for a lithium ion secondary battery of the present invention comprises a mixture of silicon oxide particles, rod-like iron oxide particles, and compound particles containing Li, Mg, P and O, the charge / discharge cycle characteristics Excellent.
本発明の実施形態に係るリチウムイオン二次電池用負極材料、並びにそれを用いた負極及び二次電池について詳細に説明する。 A negative electrode material for a lithium ion secondary battery according to an embodiment of the present invention, and a negative electrode and a secondary battery using the same will be described in detail.
（リチウムイオン二次電池用負極材料）
リチウムイオン二次電池用負極材料は、珪素酸化物からなる珪素酸化物粒子と、鉄酸化物からなる棒状の鉄酸化物粒子と、Ｌｉ（リチウム）、Ｍｇ（マグネシウム）、Ｐ（リン）及びＯ（酸素）からなる化合物よりなる化合物粒子と、の混合物を含む。このため、充放電サイクル特性に優れる。その理由は、定かではないが、以下のように考えられる。
(Anode material for lithium ion secondary battery)
A negative electrode material for a lithium ion secondary battery includes silicon oxide particles made of silicon oxide, rod-like iron oxide particles made of iron oxide, Li (lithium), Mg (magnesium), P (phosphorus), and O And a mixture of compound particles made of a compound made of (oxygen). For this reason, it is excellent in charge / discharge cycle characteristics. The reason is not clear, but is considered as follows.
珪素酸化物粒子は、珪素酸化物からなる。珪素酸化物は負極活物質であり、Ｌｉイオンの吸蔵・放出により体積変化を伴う。 The silicon oxide particles are made of silicon oxide. Silicon oxide is a negative electrode active material and is accompanied by a volume change due to insertion and extraction of Li ions.
珪素酸化物粒子の表面に、棒状の鉄酸化物粒子と化合物粒子とが付着している。鉄酸化物粒子は、その粒子形状が棒状であるため、同じ体積の球状粒子に比べて、粒子の長さが長い。このため、珪素酸化物粒子の間に入ることで、同じ体積の球状粒子に比べて、珪素酸化物粒子間を所定間隔に保持する傾向にある。珪素酸化物粒子の体積変化を許容するスペースが確保される。ゆえに、珪素酸化物粒子が体積変化しても、珪素酸化物粒子間の間隙がさほど変化せず、負極材料全体の体積変化を抑制することができる。 Rod-shaped iron oxide particles and compound particles are attached to the surface of the silicon oxide particles. Since the iron oxide particles have a rod shape, the length of the particles is longer than that of spherical particles having the same volume. For this reason, by entering between the silicon oxide particles, the silicon oxide particles tend to be held at a predetermined interval as compared with the spherical particles having the same volume. A space allowing the volume change of the silicon oxide particles is secured. Therefore, even if the volume of the silicon oxide particles changes, the gap between the silicon oxide particles does not change so much, and the volume change of the whole negative electrode material can be suppressed.
また、鉄酸化物粒子は、その粒子形状が棒状である。棒状の粒子と球状の粒子とを同じ体積で比較した場合、棒状の粒子は、中央部が偏平で厚みが小さい。さらに、棒状の粒子は、集電体の表面に対して平行に配置する傾向にある。そのため、棒状の鉄酸化物粒子を採用することで、電極の厚み方向への体積変化が緩和される。また、珪素酸化物粒子は、接触する棒状の鉄酸化物粒子の表面で移動しやすいため、これらの粉末は、充放電中に生じる珪素酸化物粒子の体積変化に伴い再配置され、体積変化は緩和される。再配置後には粉末が密な状態になるため、体積変化の緩和のみならず、導電性の向上も期待できる。 Further, the iron oxide particles have a rod shape. When rod-shaped particles and spherical particles are compared in the same volume, the rod-shaped particles have a flat central portion and a small thickness. Furthermore, the rod-shaped particles tend to be arranged in parallel to the surface of the current collector. Therefore, by adopting rod-shaped iron oxide particles, volume change in the thickness direction of the electrode is alleviated. In addition, since the silicon oxide particles easily move on the surface of the rod-shaped iron oxide particles that come into contact, these powders are rearranged along with the volume change of the silicon oxide particles generated during charge and discharge, and the volume change is Alleviated. Since the powder is in a dense state after rearrangement, not only relaxation of the volume change but also improvement of conductivity can be expected.
また、鉄酸化物粒子は、それ自体が活物質として機能する。このため、珪素酸化物以外でも、電池反応の場が確保され、電気特性が向上する。 Further, the iron oxide particles themselves function as an active material. For this reason, even if it is other than silicon oxide, the field of a battery reaction is ensured and an electrical property improves.
鉄酸化物は、電池反応の反応速度が遅いと言われていた。しかし、本発明の二次電池用負極活物質では、棒状であることで、珪素酸化物に匹敵する反応速度が得られるものと推測される。 Iron oxide was said to have a slow reaction rate of the battery reaction. However, in the negative electrode active material for secondary batteries of the present invention, it is presumed that a reaction rate comparable to silicon oxide can be obtained due to the rod shape.
また、電池反応により、鉄酸化物粒子は、導電性を有する。このため、珪素酸化物粒子間に介在することで、Ｌｉイオン伝導パス及び電子伝導パスとして機能する。 Further, the iron oxide particles have conductivity due to the battery reaction. For this reason, by interposing between silicon oxide particles, it functions as a Li ion conduction path and an electron conduction path.
更に、鉄酸化物粒子ではＬｉイオンの吸放出を伴う電池反応が起こる。このため、鉄酸化物粒子の表面には、電解液が分解して被膜が形成される。鉄酸化物粒子は棒状を呈しているため、珪素酸化物粒子間を架橋することができる。このため、珪素酸化物粒子表面の被膜に亀裂が生じることを防止する。珪素酸化物が亀裂を通じて電解液と直接接触することを抑制し、電解液の劣化を抑えることができ、充放電サイクル特性を向上させることができる。 Furthermore, a battery reaction accompanied by absorption and release of Li ions occurs in the iron oxide particles. For this reason, the electrolytic solution is decomposed to form a film on the surface of the iron oxide particles. Since the iron oxide particles have a rod shape, the silicon oxide particles can be cross-linked. For this reason, it prevents that a crack arises in the film on the surface of silicon oxide particles. It can suppress that a silicon oxide contacts a electrolyte solution directly through a crack, can suppress degradation of electrolyte solution, and can improve charging / discharging cycling characteristics.
また、化合物粒子は、Ｌｉ（リチウム）、Ｍｇ（マグネシウム）、Ｐ（リン）及びＯ（酸素）からなる化合物よりなる。化合物粒子は、電極反応の不活性部分であるが、Ｌｉイオン伝導性を有する。化合物粒子は、珪素酸化物粒子の隙間に分散した状態で存在する。このため、負極材料内でのＬｉイオン伝導性が向上する。 The compound particles are made of a compound composed of Li (lithium), Mg (magnesium), P (phosphorus), and O (oxygen). The compound particles are an inactive part of the electrode reaction, but have Li ion conductivity. The compound particles are present in a dispersed state in the gaps between the silicon oxide particles. For this reason, Li ion conductivity in a negative electrode material improves.
化合物粒子は、珪素酸化物粒子の間に介在することで、珪素酸化物粒子の体積変化を吸収することができる。 By interposing the compound particles between the silicon oxide particles, the volume change of the silicon oxide particles can be absorbed.
化合物粒子は、リチウムと反応することなく安定に存在するとともに、電解液の分解によって生じるフッ酸をトラップする機能をもつ。このため、フッ酸と珪素酸化物との反応を防止し、過剰な固体電解質（SEI:Solid Electrolyte Interphase）被膜の形成反応が抑えられる。珪素酸化物粒子表面に安定した被膜の形成が可能となる。 The compound particles exist stably without reacting with lithium and have a function of trapping hydrofluoric acid generated by decomposition of the electrolytic solution. For this reason, the reaction between hydrofluoric acid and silicon oxide is prevented, and the formation reaction of an excessive solid electrolyte interphase (SEI) film is suppressed. A stable coating can be formed on the surface of the silicon oxide particles.
珪素酸化物粒子間に鉄酸化物粒子及び化合物粒子を介在させることにより、上記の鉄酸化物粒子と化合物粒子との効果は、互いに相殺されることなく発揮される。ゆえに、珪素酸化物粒子と鉄酸化物粒子と化合物粒子との混合物からなる負極材料は、珪素酸化物粒子単独、珪素酸化物粒子及び鉄酸化物粒子、又は珪素酸化物粒子及び化合物粒子に比べて、充放電サイクル特性が高くなると考えられる。 By interposing the iron oxide particles and the compound particles between the silicon oxide particles, the effects of the iron oxide particles and the compound particles are exhibited without canceling each other. Therefore, the negative electrode material composed of a mixture of silicon oxide particles, iron oxide particles and compound particles is compared with silicon oxide particles alone, silicon oxide particles and iron oxide particles, or silicon oxide particles and compound particles. It is considered that the charge / discharge cycle characteristics are improved.
鉄酸化物粒子を構成する鉄酸化物は、具体的には、リチウムの吸蔵および放出が可能な酸化第二鉄（Ｆｅ２Ｏ３）からなるのが好ましい。酸化第二鉄には、α相、β相、γ相、といった異なる結晶構造が存在するが、他の結晶構造よりも一般的で、入手しやすく安価であることから、α−Ｆｅ２Ｏ３からなる鉄酸化物粒子（α−Ｆｅ２Ｏ３粒子）を用いるのが好ましい。なお、言うまでもなく、構造の異なる酸化第二鉄を二種以上含む鉄酸化物粉末を使用することも可能である。 Specifically, the iron oxide constituting the iron oxide particles is preferably made of ferric oxide (Fe 2 O 3 ) capable of occluding and releasing lithium. Although ferric oxide has different crystal structures such as α phase, β phase, and γ phase, it is more common than other crystal structures, and is easily available and inexpensive. Therefore, α-Fe 2 O 3 It is preferable to use iron oxide particles (α-Fe 2 O 3 particles) made of Needless to say, it is also possible to use iron oxide powder containing two or more types of ferric oxide having different structures.
鉄酸化物粒子は、棒状である。棒状を具体的に規定するのであれば、平面視した粒子の外接長方形の長さと幅の比で規定されるアスペクト比（つまり、平均長さ／平均径）で２以上、３以上さらには４以上であるのが好ましい。アスペクト比の上限に特に規定はないが、１０以下、８．５以下さらには５以下が好ましい。具体的には、粒子の長い方向の平均長さが０．４〜０．７μｍ、粒子の短い方向の平均径が０．０８５〜０．１７μｍであるとよい。なお、本明細書において、粒子の寸法の測定は、各種顕微鏡を用いて観察して得られる顕微鏡写真からの実測値である。平均値は、複数の実測値を平均して算出する。 The iron oxide particles are rod-shaped. If the rod shape is specifically defined, the aspect ratio (that is, the average length / average diameter) defined by the ratio of the length and width of the circumscribed rectangle of the particle in plan view is 2 or more, 3 or more, or 4 or more Is preferred. Although the upper limit of the aspect ratio is not particularly specified, it is preferably 10 or less, 8.5 or less, and more preferably 5 or less. Specifically, the average length in the long direction of the particles is preferably 0.4 to 0.7 μm, and the average diameter in the short direction of the particles is preferably 0.085 to 0.17 μm. In addition, in this specification, the measurement of the dimension of particle | grains is an actual measurement value from the microscope picture obtained by observing using various microscopes. The average value is calculated by averaging a plurality of actually measured values.
また、鉄酸化物粒子は、表面に複数の細孔を備えるとよい。このような細孔は、粒子の表面で開口し、粒子の表面に対して略垂直に開口していると推測される。鉄酸化物粒子が複数の細孔を備えていることは、たとえば、比表面積を測定することにより確認できる。比表面積に特に限定はないが、３０ｍ２／ｇ以上であれば、負極活物質としての使用に適切な寸法である棒状の鉄酸化物粒子に細孔が存在すると考えて差し支えない。好ましい鉄酸化物粒子の比表面積は、８０ｍ２／ｇ以上さらには１００ｍ２／ｇ以上である。比表面積の値が大きいほど、反応面積が大きくなり、電池反応の効率はさらに向上する。一方、電解液との過剰な反応を抑制するために、比表面積を１０００ｍ２／ｇ以下さらには６００ｍ２／ｇ以下とするとよい。また、細孔容積にも特に限定はないが、０．０８ｃｍ３／ｇ以上さらには０．１０ｃｍ３／ｇ以上が好ましい。活物質充填率（活物質層における鉄系酸化物の密度）を上げる観点から、鉄酸化物粒子の細孔容積は、１．０ｃｍ３／ｇ以下さらには０．５ｃｍ３／ｇ以下が好ましい。 The iron oxide particles may have a plurality of pores on the surface. Such pores are presumed to open at the surface of the particle and open substantially perpendicular to the surface of the particle. It can be confirmed that the iron oxide particles have a plurality of pores, for example, by measuring the specific surface area. The specific surface area is not particularly limited, but if it is 30 m 2 / g or more, it may be considered that pores are present in rod-shaped iron oxide particles having a size suitable for use as a negative electrode active material. The specific surface area of the iron oxide particles is preferably 80 m 2 / g or more, more preferably 100 m 2 / g or more. The larger the specific surface area value, the larger the reaction area, further improving the efficiency of the battery reaction. On the other hand, in order to suppress an excessive reaction with the electrolytic solution, the specific surface area is preferably 1000 m 2 / g or less, more preferably 600 m 2 / g or less. Although there is no particular limitation on the pore volume, 0.08 cm 3 / g or more and more preferably 0.10 cm 3 / g or more. From the viewpoint of increasing the active material filling rate (the density of the iron-based oxide in the active material layer), the pore volume of the iron oxide particles is preferably 1.0 cm 3 / g or less, more preferably 0.5 cm 3 / g or less.
なお、本明細書において上記の比表面積および細孔容積は、鉄酸化物粉末をＢＥＴ法により測定した値を採用する。 In the present specification, values obtained by measuring the iron oxide powder by the BET method are adopted as the specific surface area and pore volume.
次に、複数の細孔を備える鉄酸化物粒子の製造方法の一例を説明する。ただし、上記の鉄酸化物粒子が得られるのであれば、この方法に限定されるものではない。また、市販品を用いることも可能である。 Next, an example of a method for producing iron oxide particles having a plurality of pores will be described. However, the method is not limited to this method as long as the iron oxide particles can be obtained. Commercial products can also be used.
たとえば、Ｆｅ２Ｏ３は、ＦｅＯＯＨ（オキシ水酸化鉄）を熱処理することで製造可能である。α−Ｆｅ２Ｏ３を得たい場合にはα−ＦｅＯＯＨ、γ−Ｆｅ２Ｏ３を得たい場合にはγ−ＦｅＯＯＨ、というように前駆体を準備すればよい。このとき、ＦｅＯＯＨの外形は熱処理前後で変化しないため、棒状のＦｅＯＯＨ粉末を準備することで、棒状のＦｅ２Ｏ３粉末が得られる。棒状のＦｅＯＯＨは市販されているが、塩化鉄などの水溶液をエージングして得られる沈殿物として合成することも容易である。熱処理は、１５０〜５００℃さらには２５０〜４００℃で１時間以上さらには１．５〜１０時間が好ましく、２〜５時間程度であってもよい。熱処理することでＦｅＯＯＨは熱分解による脱水反応が生じるが、表面からの脱水の結果として細孔が形成される。上記の温度範囲で熱処理を行うことにより、棒状のＦｅＯＯＨ粒子から、複数の細孔を備える棒状のＦｅ２Ｏ３粒子が容易に得られる。熱処理温度が高いほど、脱水反応が十分に進行して細孔が形成されやすく比表面積が大きくなる。しかし、熱処理温度が高すぎると、かえって比表面積が低下する傾向にある。これは、細孔閉塞が生じるためである。また、熱処理雰囲気に特に限定はないため、酸素含有雰囲気、たとえば大気中で行えばよい。 For example, Fe 2 O 3 can be produced by heat-treating FeOOH (iron oxyhydroxide). In order to obtain α-Fe 2 O 3 , a precursor may be prepared such as α-FeOOH, and in the case of obtaining γ-Fe 2 O 3 , γ-FeOOH. At this time, since the outer shape of FeOOH does not change before and after the heat treatment, a rod-like Fe 2 O 3 powder can be obtained by preparing a rod-like FeOOH powder. Although rod-like FeOOH is commercially available, it can be easily synthesized as a precipitate obtained by aging an aqueous solution such as iron chloride. The heat treatment is preferably 150 to 500 ° C., more preferably 250 to 400 ° C., for 1 hour or more, further preferably 1.5 to 10 hours, and may be about 2 to 5 hours. Although heat treatment causes FeOOH to undergo a dehydration reaction due to thermal decomposition, pores are formed as a result of dehydration from the surface. By performing heat treatment in the above temperature range, rod-like Fe 2 O 3 particles having a plurality of pores can be easily obtained from rod-like FeOOH particles. The higher the heat treatment temperature, the more the dehydration reaction proceeds and pores are more easily formed, resulting in a larger specific surface area. However, if the heat treatment temperature is too high, the specific surface area tends to decrease. This is because pore blockage occurs. Further, since there is no particular limitation on the heat treatment atmosphere, the treatment may be performed in an oxygen-containing atmosphere, for example, air.
化合物粒子は、Ｌｉ、Ｍｇ、Ｐ及びＯからなる化合物よりなる。この化合物粒子は、例えば、ＬｉＭｇＰＯ４で表されるオリビン型リン酸マグネシウムリチウムとすることができる。この化合物粒子の平均粒径は、珪素酸化物粒子の平均粒径より小さいことが望ましい。化合物粒子の粒径が珪素酸化物粒子の粒径より大きくなると化合物粒子の作用効果が低下するとともに、リチウムイオン二次電池の容量が低下するため実用的でない。この意味において化合物粒子の粒径は小さいほど好ましく、５μｍ以下とするのが好ましい。 この化合物粒子は、例えば実施例で示すように、メカニカルミリング(ＭＭ)処理によって製造することができる。すなわち、仕込み組成比がＬｉＭｇＰＯ４となるように、出発原料としての酸化リチウム(Ｌｉ２Ｏ)を２５モル％、酸化マグネシウム(ＭｇＯ)を５０モル％、酸化リン(Ｐ２Ｏ５)を２５モル％となるように秤量し、遊星型ボールミル装置を用いてメカニカルミリングすることにより製造できる。このとき、仕込み組成比によっては酸化マグネシウム(ＭｇＯ)が未反応で残存する場合があるが、化合物粒子中に酸化マグネシウムが含まれていても本発明の効果が損なわれることはない。 The compound particles are made of a compound composed of Li, Mg, P and O. The compound particles can be, for example, olivine-type lithium magnesium phosphate represented by LiMgPO 4 . The average particle diameter of the compound particles is preferably smaller than the average particle diameter of the silicon oxide particles. When the particle size of the compound particles is larger than the particle size of the silicon oxide particles, the function and effect of the compound particles are lowered and the capacity of the lithium ion secondary battery is lowered, which is not practical. In this sense, the particle diameter of the compound particles is preferably as small as possible, and is preferably 5 μm or less. The compound particles can be produced by, for example, mechanical milling (MM) treatment as shown in the examples. That is, 25 mol% of lithium oxide (Li 2 O) as a starting material, 50 mol% of magnesium oxide (MgO), and 25 mol of phosphorus oxide (P 2 O 5 ) so that the charged composition ratio is LiMgPO 4. % And can be manufactured by mechanical milling using a planetary ball mill apparatus. At this time, magnesium oxide (MgO) may remain unreacted depending on the charged composition ratio, but even if magnesium oxide is contained in the compound particles, the effects of the present invention are not impaired.
珪素酸化物粒子は、従来から負極活物質として用いられている珪素酸化物粉末を使用すればよい。以下に、本発明のリチウムイオン二次電池用負極活物質に最適な珪素酸化物粒子（粉末）の構成を説明する。 The silicon oxide particles may be silicon oxide powder that has been conventionally used as a negative electrode active material. Below, the structure of the silicon oxide particle (powder) optimal for the negative electrode active material for lithium ion secondary batteries of this invention is demonstrated.
珪素酸化物粒子は、ＳｉＯ２相とＳｉ相とを含むとよい。それぞれの相の効果は、既に述べた通りである。珪素酸化物粒子は、ＳｉＯｎ（０．３≦ｎ≦１．６）で表される酸化珪素からなるとよい。ｎが０．３未満であると、Ｓｉ相の占める比率が高くなるため充放電時の体積変化が大きくなりすぎてサイクル特性が低下する。またｎが１．６を超えると、Ｓｉ相の比率が低下してエネルギー密度が低下するようになる。さらに好ましいｎの範囲は、０．５≦ｎ≦１．５、０．７≦ｎ≦１．２である。 The silicon oxide particles may include a SiO 2 phase and a Si phase. The effect of each phase is as already described. The silicon oxide particles may be made of silicon oxide represented by SiO n (0.3 ≦ n ≦ 1.6). If n is less than 0.3, the proportion of the Si phase increases, so that the volume change during charge / discharge becomes too large, and the cycle characteristics deteriorate. On the other hand, when n exceeds 1.6, the ratio of the Si phase decreases and the energy density decreases. Further preferable ranges of n are 0.5 ≦ n ≦ 1.5 and 0.7 ≦ n ≦ 1.2.
一般に、酸素を断った状態であれば８００℃以上で、ほぼすべてのＳｉＯが不均化して二相に分離すると言われている。具体的には、非結晶性のＳｉＯ粉末を含む原料酸化珪素粉末に対して、真空中または不活性ガス中などの不活性雰囲気中で８００〜１２００℃、１〜５時間の熱処理を行うことで、非結晶性のＳｉＯ２相および結晶性のＳｉ相の二相を含むＳｉＯ粒子からなる粉末が得られる。 In general, when oxygen is turned off, it is said that almost all SiO is disproportionated and separated into two phases at 800 ° C. or higher. Specifically, the raw material silicon oxide powder containing non-crystalline SiO powder is subjected to heat treatment at 800 to 1200 ° C. for 1 to 5 hours in an inert atmosphere such as in a vacuum or an inert gas. A powder composed of SiO particles containing two phases of an amorphous SiO 2 phase and a crystalline Si phase is obtained.
非結晶性のＳｉＯ粉末を含む酸化珪素粉末をミリングすることでも、ＳｉＯが不均化して二相に分離する。ミリングの機械的エネルギーの一部が、粒子の固相界面における化学的な原子拡散に寄与し、ＳｉＯ２相とＳｉ相などを生成すると考えられる。酸化珪素粉末を、真空中、アルゴンガス中などの不活性ガス雰囲気下で、Ｖ型混合機、ボールミル、アトライタ、ジェットミル、振動ミル、高エネルギーボールミル等を使用してミリングするとよい。ミリング後にさらに熱処理を施すことで、珪素酸化物の不均化をさらに促進させてもよい。 By milling silicon oxide powder containing amorphous SiO powder, SiO is disproportionated and separated into two phases. A part of the mechanical energy of milling is considered to contribute to chemical atomic diffusion at the solid phase interface of the particles, and to generate SiO 2 phase and Si phase. The silicon oxide powder may be milled using a V-type mixer, a ball mill, an attritor, a jet mill, a vibration mill, a high energy ball mill or the like in an inert gas atmosphere such as vacuum or argon gas. Further heat treatment may be performed after milling to further promote disproportionation of the silicon oxide.
珪素酸化物粉末は、略球状の粒子からなるのが好ましい。リチウムイオン二次電池の充放電特性の観点からは、珪素酸化物粉末の平均粒径が小さいほど好ましい。しかし、平均粒径が小さすぎると、負極の形成時に凝集して粗大な粒子となるため、リチウムイオン二次電池の充放電特性が低下する場合がある。そのため、珪素酸化物粉末の平均粒径は、５〜２０μｍの範囲にあるとよい。 The silicon oxide powder is preferably composed of substantially spherical particles. From the viewpoint of charge / discharge characteristics of the lithium ion secondary battery, the smaller the average particle size of the silicon oxide powder, the better. However, if the average particle size is too small, the particles are agglomerated and formed into coarse particles when the negative electrode is formed, and the charge / discharge characteristics of the lithium ion secondary battery may deteriorate. Therefore, the average particle diameter of the silicon oxide powder is preferably in the range of 5 to 20 μm.
また、珪素酸化物粒子は、表面に炭素材料からなる被覆層を備えるとよい。炭素材料からなる被覆層は、珪素酸化物粒子に導電性を付与するだけでなく、珪素酸化物粒子と電解液の成分が分解されて発生するフッ酸などとの反応を防止することができ、リチウムイオン二次電池の電池特性が向上する。被覆層の炭素材料としては、天然黒鉛、人造黒鉛、コークス、メソフェーズ炭素、気相成長炭素繊維、ピッチ系炭素繊維、ＰＡＮ系炭素繊維などを用いることができる。また被覆層を形成するには、珪素酸化物と炭素材料前駆体とを混合して焼成するとよい。炭素材料前駆体としては、糖類、グリコール類、ポリピロール等のポリマー、アセチレンブラックなど、焼成により炭素材料に転化しうる有機化合物が使用可能である。その他、メカノフュージョンなどの機械的表面融合処理法、ＣＶＤなどの蒸着法を用いても、被覆層を形成することができる。 The silicon oxide particles may have a coating layer made of a carbon material on the surface. The coating layer made of a carbon material not only imparts conductivity to the silicon oxide particles, but can also prevent reaction between the silicon oxide particles and hydrofluoric acid generated by decomposition of the components of the electrolytic solution, The battery characteristics of the lithium ion secondary battery are improved. As the carbon material for the coating layer, natural graphite, artificial graphite, coke, mesophase carbon, vapor-grown carbon fiber, pitch-based carbon fiber, PAN-based carbon fiber, or the like can be used. In order to form the coating layer, silicon oxide and a carbon material precursor are mixed and fired. As the carbon material precursor, an organic compound that can be converted into a carbon material by firing, such as saccharides, glycols, polymers such as polypyrrole, and acetylene black, can be used. In addition, the coating layer can be formed by using a mechanical surface fusion treatment method such as mechano-fusion or a vapor deposition method such as CVD.
珪素酸化物粒子と被覆層の合計を１００質量％としたときに、被覆層の質量比は１〜５０質量％とすることができる。被覆層が１質量％未満では導電性向上の効果が得られず、５０質量％を超えると珪素酸化物の割合が相対的に減少して負極容量が低下してしまう。被覆層の質量比は５〜３０質量％の範囲が好ましく、５〜２０質量％の範囲がさらに望ましい。 When the total of the silicon oxide particles and the coating layer is 100% by mass, the mass ratio of the coating layer can be 1 to 50% by mass. If the coating layer is less than 1% by mass, the effect of improving the conductivity cannot be obtained. If the coating layer exceeds 50% by mass, the proportion of silicon oxide is relatively decreased and the negative electrode capacity is decreased. The mass ratio of the coating layer is preferably in the range of 5 to 30% by mass, and more preferably in the range of 5 to 20% by mass.
珪素酸化物粒子は平均粒径が１μｍ〜１０μｍの範囲にあることが望ましい。平均粒径が１０μｍより大きいとリチウムイオン二次電池の充放電特性が低下し、平均粒径が１μｍより小さいと樹脂の被覆時に凝集して粗大な粒子となるため同様にリチウムイオン二次電池の充放電特性が低下する場合がある。 The silicon oxide particles preferably have an average particle size in the range of 1 μm to 10 μm. When the average particle size is larger than 10 μm, the charge / discharge characteristics of the lithium ion secondary battery are deteriorated. When the average particle size is smaller than 1 μm, the particles are aggregated to become coarse particles when coated with the resin. Charge / discharge characteristics may deteriorate.
本発明のリチウムイオン二次電池用負極材料は、上記の珪素酸化物粒子と鉄酸化物粒子と化合物粒子との混合物を含む。この混合物全体を１００質量％としたときに、鉄酸化物粒子の質量比は１質量％以上２０質量％以下、化合物粒子を１質量％以上１５質量％以下とすることが好ましい。この場合には、サイクル特性及び電池容量が高くなる。 The negative electrode material for a lithium ion secondary battery of the present invention contains a mixture of the above silicon oxide particles, iron oxide particles and compound particles. When the total mixture is 100% by mass, the mass ratio of the iron oxide particles is preferably 1% by mass to 20% by mass, and the compound particles are preferably 1% by mass to 15% by mass. In this case, cycle characteristics and battery capacity are increased.
鉄酸化物粒子の配合量の増加に伴い、サイクル特性の安定度は向上するが、容量は低下する傾向にある。化合物粒子の配合量の増加に伴い、サイクル特性が向上するが，過剰な配合量となると却ってサイクル特性が低下する傾向にある。そのため、珪素酸化物粒子と鉄酸化物粒子との混合比率は、リチウムイオン二次電池の要求特性により適宜決定すればよい。 As the blending amount of iron oxide particles increases, the stability of cycle characteristics improves, but the capacity tends to decrease. As the compounding amount of the compound particles is increased, the cycle characteristics are improved. However, when the compounding amount is excessive, the cycle characteristics tend to be lowered. Therefore, the mixing ratio between the silicon oxide particles and the iron oxide particles may be appropriately determined according to the required characteristics of the lithium ion secondary battery.
たとえば、サイクル特性を向上させたいのであれば、混合物全体を１００質量％としたとき、鉄酸化物粒子を５質量％以上２０質量％以下、化合物粒子を５質量％以上１５質量％以下含むとよい。電池容量を高くするためには、混合物全体を１００質量％としたとき、鉄酸化物粒子を１質量％以上１０質量％以下、化合物粒子を１質量％以上１０質量％以下とすることが好ましい。 For example, if it is desired to improve the cycle characteristics, when the total mixture is 100% by mass, it is preferable to contain 5% by mass to 20% by mass of iron oxide particles and 5% by mass to 15% by mass of compound particles. . In order to increase the battery capacity, when the total mixture is 100% by mass, the iron oxide particles are preferably 1% by mass to 10% by mass and the compound particles are preferably 1% by mass to 10% by mass.
また、珪素酸化物粒子と鉄酸化物粒子と化合物粒子との混合物全体を１００質量％としたときに、珪素酸化物粒子は８０質量％以上９０質量％以下であることがよい。この場合には、二次電池の容量が高くなる。 Further, when the total mixture of silicon oxide particles, iron oxide particles and compound particles is 100% by mass, the silicon oxide particles are preferably 80% by mass or more and 90% by mass or less. In this case, the capacity of the secondary battery is increased.
また、珪素酸化物粒子及び鉄酸化物粒子は、活物質としての機能をもつ。鉄酸化物粒子と珪素酸化物粒子とを合わせた活物質を１００質量％としたとき、鉄酸化物粒子は３質量％以上１５質量％以下であり、珪素酸化物粒子は８５質量％以上９７質量％以下であることが好ましい。この場合には、二次電池の容量を高くすることができる。 Moreover, the silicon oxide particles and the iron oxide particles have a function as an active material. When the active material combining the iron oxide particles and the silicon oxide particles is 100% by mass, the iron oxide particles are 3% by mass to 15% by mass, and the silicon oxide particles are 85% by mass to 97% by mass. % Or less is preferable. In this case, the capacity of the secondary battery can be increased.
珪素酸化物粒子と鉄酸化物粒子と化合物粒子との混合物を１００質量％としたときに、珪素酸化物及び鉄酸化物を合わせた活物質の質量比は、８５質量％以上９５質量％以下であることが好ましい。この場合には、二次電池の容量を高くすることができる。 When the mixture of silicon oxide particles, iron oxide particles and compound particles is 100% by mass, the mass ratio of the active material in which silicon oxide and iron oxide are combined is 85% by mass or more and 95% by mass or less. Preferably there is. In this case, the capacity of the secondary battery can be increased.
本発明のリチウムイオン二次電池用の負極材料は、珪素酸化物粒子と鉄酸化物粒子と化合物粒子との混合物を必須として含む。負極材料は、該混合物単独で構成されていてもよいが、更に他の材料を含んでいても良い。他の材料は、例えば、他の負極活物質、導電助材、バインダー樹脂などが挙げられる。他の負極活物質は、たとえば、炭素系負極活物質が挙げられる。 The negative electrode material for a lithium ion secondary battery of the present invention contains a mixture of silicon oxide particles, iron oxide particles and compound particles as an essential component. The negative electrode material may be composed of the mixture alone, but may further contain other materials. Examples of other materials include other negative electrode active materials, conductive additives, binder resins, and the like. Examples of other negative electrode active materials include carbon-based negative electrode active materials.
導電助剤は、電極の導電性を高めるために添加される。導電助剤として、炭素質微粒子であるカーボンブラック、黒鉛、アセチレンブラック（ＡＢ）、ケッチェンブラック（ＫＢ）、気相法炭素繊維（ＶａｐｏｒＧｒｏｗｎＣａｒｂｏｎＦｉｂｅｒ：ＶＧＣＦ）等を単独でまたは二種以上組み合わせて添加することができる。導電助剤の使用量については、特に限定的ではないが、たとえば、活物質１００質量部に対して、２０〜１００質量部程度とすることができる。導電助剤の量が２０質量部未満では効率のよい導電パスを形成できず、１００質量部を超えると電極の成形性が悪化するとともにエネルギー密度が低くなる。なお、炭素材料からなる被覆層をもつ珪素酸化物粒子を用いる場合には、導電助剤の添加量を低減することができ、あるいは添加しないでもよい。 The conductive assistant is added to increase the conductivity of the electrode. Carbon black, graphite, acetylene black (AB), ketjen black (KB), vapor grown carbon fiber (Vapor Carbon Carbon Fiber: VGCF), etc., which are carbonaceous fine particles, are added alone or in combination as a conductive aid. be able to. The amount of the conductive auxiliary agent used is not particularly limited, but can be, for example, about 20 to 100 parts by mass with respect to 100 parts by mass of the active material. If the amount of the conductive auxiliary is less than 20 parts by mass, an efficient conductive path cannot be formed, and if it exceeds 100 parts by mass, the moldability of the electrode deteriorates and the energy density decreases. When silicon oxide particles having a coating layer made of a carbon material are used, the addition amount of the conductive auxiliary agent can be reduced or not added.
バインダー樹脂は、活物質および導電助剤を集電体に結着するための結着剤として用いられる。バインダー樹脂はなるべく少ない量で活物質等を結着させることが求められ、その量は、負極活物質、導電助材およびバインダー樹脂を合計で１００質量％としたときに、０．５〜５０質量％が望ましい。バインダー樹脂量が０．５質量％未満では電極の成形性が低下し、５０質量％を超えると電極のエネルギー密度が低くなる。なお、バインダー樹脂としては、ポリフッ化ビニリデン（ＰＶＤＦ）、ポリテトラフルオロエチレン（ＰＴＦＥ）等のフッ素系ポリマー、スチレンブタジエンゴム（ＳＢＲ）等のゴム、ポリイミド等のイミド系ポリマー、ポリアミドイミド、アルコキシルシリル基含有樹脂、ポリアクリル酸、ポリメタクリル酸、ポリイタコン酸などが例示される。またアクリル酸と、メタクリル酸、イタコン酸、フマル酸、マレイン酸などの酸モノマーとの共重合物を用いることもできる。中でもポリアクリル酸など、カルボキシル基を含有する樹脂が特に望ましく、カルボキシル基の含有量が多い樹脂ほど好ましい。 The binder resin is used as a binder for binding the active material and the conductive additive to the current collector. The binder resin is required to bind the active material and the like in as little amount as possible, and the amount is 0.5 to 50 mass when the negative electrode active material, the conductive additive and the binder resin are 100 mass% in total. % Is desirable. When the amount of the binder resin is less than 0.5% by mass, the moldability of the electrode is lowered, and when it exceeds 50% by mass, the energy density of the electrode is lowered. In addition, as binder resin, fluoropolymers such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), rubbers such as styrene butadiene rubber (SBR), imide polymers such as polyimide, polyamideimide, alkoxylsilyl groups Examples of the resin include polyacrylic acid, polymethacrylic acid, and polyitaconic acid. A copolymer of acrylic acid and an acid monomer such as methacrylic acid, itaconic acid, fumaric acid or maleic acid can also be used. Among them, a resin containing a carboxyl group such as polyacrylic acid is particularly desirable, and a resin having a higher carboxyl group content is more preferable.
＜リチウムイオン二次電池用負極＞
本発明のリチウムイオン二次電池の負極は、上記の負極材料を有する。負極は、例えば、集電体と、集電体表面に形成され上記の負極材料からなる負極活物質層とを有する。負極活物質層は、負極材料を、必要に応じ適量の有機溶剤を加えて混合しスラリーにしたものを、ロールコート法、ディップコート法、ドクターブレード法、スプレーコート法、カーテンコート法などの方法で集電体上に塗布し、バインダー樹脂を硬化させることによって作製することができる。
<Anode for lithium ion secondary battery>
The negative electrode of the lithium ion secondary battery of this invention has said negative electrode material. The negative electrode includes, for example, a current collector and a negative electrode active material layer formed on the current collector surface and made of the above negative electrode material. The negative electrode active material layer is prepared by mixing a negative electrode material with an appropriate amount of an organic solvent as necessary and mixing it into a slurry, such as a roll coating method, a dip coating method, a doctor blade method, a spray coating method, or a curtain coating method. It can be produced by coating on a current collector and curing the binder resin.
集電体は、金属製のメッシュ、箔または板などの形状を採用することができるが、目的に応じた形状であれば特に限定されない。集電体として、たとえば銅箔やアルミニウム箔を好適に用いることができる。 The current collector can adopt a shape such as a metal mesh, foil, or plate, but is not particularly limited as long as it has a shape according to the purpose. For example, a copper foil or an aluminum foil can be suitably used as the current collector.
本発明のリチウムイオン二次電池における負極には、リチウムがプリドーピングされていることが望ましい。負極にリチウムをドープするには、たとえば、対極に金属リチウムを用いて半電池を組み、電気化学的にリチウムをドープする電極化成法などを利用することができる。リチウムのドープ量に特に限定はなく、理論容量以上にプリドープされてもよい。 The negative electrode in the lithium ion secondary battery of the present invention is desirably pre-doped with lithium. In order to dope lithium into the negative electrode, for example, an electrode formation method in which a half cell is assembled using metallic lithium as a counter electrode and electrochemically doped with lithium can be used. There is no particular limitation on the doping amount of lithium, and it may be pre-doped beyond the theoretical capacity.
なお、リチウムをドープすることにより、あるいは本発明のリチウムイオン二次電池の初回充電後には、負極活物質に含まれるＳｉＯ２相にＬｉｘＳｉｙＯｚ（０≦ｘ≦４、０．３≦ｙ≦１．６、２≦ｚ≦４）で表される酸化物系化合物が含まれているとよい。ＬｉｘＳｉｙＯｚとしては、たとえばｘ＝０，ｙ＝１，ｚ＝２のＳｉＯ２、ｘ＝２，ｙ＝１，ｚ＝３のＬｉ２ＳｉＯ３、ｘ＝４，ｙ＝１，ｚ＝４のＬｉ４ＳｉＯ４などが例示される。たとえばｘ＝４，ｙ＝１，ｚ＝４のＬｉ４ＳｉＯ４は下記の反応により生成し、クーロン効率は約７７％と計算される。 In addition, by doping lithium or after the initial charge of the lithium ion secondary battery of the present invention, Li x Si y O z (0 ≦ x ≦ 4, 0.3 is added to the SiO 2 phase contained in the negative electrode active material. An oxide compound represented by ≦ y ≦ 1.6 and 2 ≦ z ≦ 4) is preferably included. Examples of Li x Si y O z include SiO 2 with x = 0, y = 1, and z = 2, Li 2 SiO 3 with x = 2, y = 1, and z = 3 , x = 4, y = 1, and so on. Examples thereof include Li 4 SiO 4 with z = 4. For example, Li 4 SiO 4 with x = 4, y = 1 and z = 4 is produced by the following reaction, and the Coulomb efficiency is calculated to be about 77%.
２ＳｉＯ＋８．６Ｌｉ＋＋８．６ｅ−→１．５Ｌｉ４．４Ｓｉ＋１／２Ｌｉ４ＳｉＯ４
また上記反応が途中で停止した場合には、下記の反応のようにｘ＝２，ｙ＝１，Ｚ＝３のＬｉ２ＳｉＯ３とｘ＝４，ｙ＝１，ｚ＝４のＬｉ４ＳｉＯ４の両者が生成し、この場合のクーロン効率も約７７％と計算される。
2SiO + 8.6Li + + 8.6e − → 1.5Li 4.4 Si + 1 / 2Li 4 SiO 4
Further, when the reaction is stopped halfway, x = 2, y = 1 , Z = 3 of Li 2 SiO 3 and x = 4, y = 1, z = 4 in Li 4 SiO as the following reaction 4 is generated, and the Coulomb efficiency in this case is also calculated to be about 77%.
２ＳｉＯ＋７．３５Ｌｉ＋＋７．３５ｅ−→１．４２Ｌｉ４Ｓｉ＋１／３Ｌｉ２ＳｉＯ３＋１／４Ｌｉ４ＳｉＯ４
上記反応によって生成するＬｉ４ＳｉＯ４は、充放電時の電極反応に関与しない不活性な物質であり、充放電時の活物質の体積変化を緩和する働きをする。したがってＳｉＯ２相にＬｉｘＳｉｙＯｚで表される酸化物系化合物が含まれる場合には、本発明のリチウムイオン二次電池はサイクル特性がさらに向上する。
2SiO + 7.35Li + + 7.35e − → 1.42Li 4 Si + 1 / 3Li 2 SiO 3 + 1 / 4Li 4 SiO 4
Li 4 SiO 4 produced by the above reaction is an inert substance which does not participate in the electrode reaction during charging and discharging and relieve a volume change of the active material during charging and discharging. Therefore, when the oxide compound represented by Li x Si y O z is contained in the SiO 2 phase, the cycle characteristics of the lithium ion secondary battery of the present invention are further improved.
さらに、本発明のリチウムイオン二次電池用負極は、鉄酸化物粒子をコンバージョン領域まで充放電させることで、上記のクーロン効率を７７％以上に向上させることができる。本発明者等は、低い電流密度で充放電を行った充放電試験において容量が増加し、クーロン効率であれば約９３％となることを、鋭意研究により突き止めた。この理由は明確ではないが、コンバージョン領域において生成された０価のＦｅが、珪素酸化物粒子の電池反応に対して触媒の役割を果たすと推測される。したがって、本発明のリチウムイオン二次電池用負極を用いたリチウムイオン二次電池は、鉄酸化物粒子のコンバージョン領域まで、具体的に規定するのであれば、終止電圧をリチウム基準電位で０．００５Ｖさらには０Ｖにして充放電を行うとよい。 Furthermore, the negative electrode for lithium ion secondary batteries of this invention can improve said Coulomb efficiency to 77% or more by charging / discharging iron oxide particle to a conversion area | region. The inventors of the present invention have earnestly found out that the capacity increases in a charge / discharge test in which charging / discharging is performed at a low current density and the coulomb efficiency is about 93%. The reason for this is not clear, but it is presumed that zero-valent Fe generated in the conversion region plays a role of a catalyst for the cell reaction of silicon oxide particles. Therefore, in the lithium ion secondary battery using the negative electrode for the lithium ion secondary battery of the present invention, the end voltage is 0.005 V at the lithium reference potential if specifically defined up to the conversion region of the iron oxide particles. Furthermore, charging / discharging is preferably performed at 0V.
＜リチウムイオン二次電池＞
上記した負極を用いる本発明のリチウムイオン二次電池は、特に限定されない公知の正極、電解質、セパレータを用いることができる。正極は、リチウムイオン二次電池で使用可能なものであればよい。正極は、集電体と、集電体上に結着された正極活物質層とを有する。正極活物質層は、正極材料からなる。正極材料は、正極活物質と、バインダーとを含み、さらには導電助剤を含んでもよい。正極活物質、導電助材およびバインダーは、特に限定はなく、リチウムイオン二次電池で使用可能なものであればよい。
<Lithium ion secondary battery>
The positive electrode, electrolyte, and separator which are not specifically limited can be used for the lithium ion secondary battery of this invention using the above-mentioned negative electrode. The positive electrode may be anything that can be used in a lithium ion secondary battery. The positive electrode has a current collector and a positive electrode active material layer bound on the current collector. The positive electrode active material layer is made of a positive electrode material. The positive electrode material includes a positive electrode active material and a binder, and may further include a conductive additive. The positive electrode active material, the conductive additive, and the binder are not particularly limited as long as they can be used in the lithium ion secondary battery.
正極活物質としては、ＬｉＣｏＯ２、ＬｉＮｉｐＣｏｑＭｎｒＯ２（０＜ｐ＜１、０＋ｐ＜ｑ＜１−ｐ、０＋（ｐ＋ｑ）＜ｒ＜１−（ｐ＋ｑ））、Ｌｉ２ＭｎＯ２、Ｌｉ２ＭｎＯ３、ＬｉＮｉｓＭｎｔＯ２（０＜ｓ＜１、０＋ｓ＜ｔ＜１−ｓ）、ＬｉＦｅＰＯ４、Ｌｉ２ＦｅＳＯ４を基本組成とするリチウム含有金属酸化物あるいはそれぞれを１種または２種以上含む固溶体材料などが挙げられる。望ましくは、ＬｉＣｏＯ２、ＬｉＮｉ１／３Ｃｏ１／３Ｍｎ１／３Ｏ２、Ｌｉ２ＭｎＯ２、Ｓなどが挙げられる。Ｓを含む正極活物質としては、硫黄単体（Ｓ）、ポリアクリロニトリルなどの有機化合物に硫黄を導入した硫黄変性化合物などを用いることもできる。ただし、これらの材料は、電解質イオンとなるリチウムを含まないため、負極活物質または正極活物質に予めリチウムをドープ（プレドープ）する必要がある。 As the positive electrode active material, LiCoO 2 , LiNi p Co q Mn r O 2 (0 <p <1, 0 + p <q <1-p, 0+ (p + q) <r <1- (p + q)), Li 2 MnO 2 , Li 2 MnO 3 , LiNi s Mn t O 2 (0 <s <1, 0 + s <t <1-s), LiFePO 4 , Li 2 FeSO 4 based lithium-containing metal oxide or one of each Or the solid solution material containing 2 or more types is mentioned. Desirably, such LiCoO 2, LiNi 1/3 Co 1/3 Mn 1/3 O 2, Li 2 MnO 2, S and the like. As the positive electrode active material containing S, a sulfur-modified compound obtained by introducing sulfur into an organic compound such as sulfur alone (S) or polyacrylonitrile can also be used. However, since these materials do not contain lithium as an electrolyte ion, it is necessary to dope lithium (pre-dope) in advance to the negative electrode active material or the positive electrode active material.
集電体は、アルミニウム、ニッケル、ステンレス鋼など、リチウムイオン二次電池の正極に一般的に使用されるものであればよい。導電助剤は上記の負極で記載したものと同様のものが使用できる。 The current collector is not particularly limited as long as it is generally used for the positive electrode of a lithium ion secondary battery, such as aluminum, nickel, and stainless steel. As the conductive auxiliary agent, the same ones as described in the above negative electrode can be used.
電解質は、有機溶媒に電解質であるリチウム金属塩を溶解させた電解液を用いるとよい。電解液は、特に限定されない。有機溶媒として、非プロトン性有機溶媒、たとえばプロピレンカーボネート（ＰＣ）、エチレンカーボネート（ＥＣ）、ジメチルカーボネート（ＤＭＣ）、ジエチルカーボネート（ＤＥＣ）、エチルメチルカーボネート（ＥＭＣ）、フルオロエチレンカーボネート（ＦＥＣ）等から選ばれる一種以上を用いることができる。また、溶解させる電解質としては、ＬｉＰＦ６、ＬｉＢＦ４、ＬｉＡｓＦ６、ＬｉＩ、ＮａＰＦ６、ＮａＢＦ４、ＮａＡｓＦ６、ＬｉＢＯＢ、等の有機溶媒に可溶なリチウム金属塩を用いることができる。 As the electrolyte, an electrolytic solution in which a lithium metal salt that is an electrolyte is dissolved in an organic solvent may be used. The electrolytic solution is not particularly limited. As the organic solvent, from aprotic organic solvents such as propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), fluoroethylene carbonate (FEC), etc. One or more selected can be used. As the electrolytes dissolved, it can be used LiPF 6, LiBF 4, LiAsF 6 , LiI, NaPF 6, NaBF 4, NaAsF 6, LiBOB, soluble lithium metal salt in an organic solvent and the like.
たとえば、エチレンカーボネート、ジメチルカーボネート、プロピレンカーボネート、ジメチルカーボネートなどの有機溶媒にＬｉＣｌＯ４、ＬｉＰＦ６、ＬｉＢＦ４、ＬｉＣＦ３ＳＯ３等のリチウム金属塩を０．５〜１．７モル／Ｌ程度の濃度で溶解させた溶液を使用することができる。 For example, a concentration of about 0.5 to 1.7 mol / L of a lithium metal salt such as LiClO 4 , LiPF 6 , LiBF 4 , or LiCF 3 SO 3 in an organic solvent such as ethylene carbonate, dimethyl carbonate, propylene carbonate, or dimethyl carbonate. A solution dissolved in can be used.
セパレータは、リチウムイオン二次電池に使用されることができるものであれば特に限定されない。セパレータは、正極と負極とを分離し電解液を保持するものであり、ポリエチレン、ポリプロピレン等の薄い微多孔膜を用いることができる。 A separator will not be specifically limited if it can be used for a lithium ion secondary battery. The separator separates the positive electrode and the negative electrode and holds the electrolytic solution, and a thin microporous film such as polyethylene or polypropylene can be used.
本発明のリチウムイオン二次電池は、形状に特に限定はなく、円筒型、積層型、コイン型等、種々の形状を採用することができる。いずれの形状を採る場合であっても、正極および負極にセパレータを挟装させ電極体とし、正極集電体および負極集電体から外部に通ずる正極端子および負極端子までの間を、集電用リード等を用いて接続した後、この電極体を電解液とともに電池ケースに密閉して電池となる。 The lithium ion secondary battery of the present invention is not particularly limited in shape, and various shapes such as a cylindrical shape, a stacked shape, and a coin shape can be adopted. Regardless of the shape, a separator is sandwiched between the positive electrode and the negative electrode to form an electrode body, and the space between the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal is used for current collection. After connecting using a lead or the like, the electrode body is sealed in a battery case together with an electrolytic solution to form a battery.
以上説明した本発明のリチウムマンガン系複合酸化物を活物質として用いた二次電池は、携帯電話、パソコン等の通信機器、情報関連機器の分野の他、自動車の分野においても好適に利用できる。たとえば、この二次電池を車両に搭載すれば、電気自動車用の電源として使用できる。 The secondary battery using the lithium manganese based composite oxide of the present invention described above as an active material can be suitably used in the field of automobiles as well as the field of communication devices such as mobile phones and personal computers, information-related devices. For example, if this secondary battery is mounted on a vehicle, it can be used as a power source for an electric vehicle.
＜鉄酸化物粉末の製造＞
平均長さが０．６５μｍ、平均径が０．１５μｍの棒状粒子からなるα−ＦｅＯＯＨ粉末を前駆体として用いて、α−Ｆｅ２Ｏ３粉末を製造した。熱処理は、所定の温度で大気中１０時間行った。熱処理温度は、２５０℃、３００℃、３３０℃、３６０℃、４００℃、４５０℃、又は５００℃とした。
<Manufacture of iron oxide powder>
Α-Fe 2 O 3 powder was produced using α-FeOOH powder composed of rod-shaped particles having an average length of 0.65 μm and an average diameter of 0.15 μm as a precursor. The heat treatment was performed at a predetermined temperature in the atmosphere for 10 hours. The heat processing temperature was 250 degreeC, 300 degreeC, 330 degreeC, 360 degreeC, 400 degreeC, 450 degreeC, or 500 degreeC.
熱処理前後のα−ＦｅＯＯＨ粉末について、ＣｕＫα線を用いたＸ線回折（ＸＲＤ）測定を行った。また、走査電子顕微鏡（ＳＥＭ）により、それらの形状を観察した。結果を図１および図２に示した。なお、図１に示したＸＲＤパターンは、熱処理前のα−ＦｅＯＯＨ粉末、熱処理温度２５０℃、３００℃、３３０℃、３６０℃、４００℃、４５０℃、及び５００℃で熱処理した粉末のデータである。図１には、ＦｅＯＯＨおよびα−Ｆｅ２Ｏ３の粉末回折ファイル（ＰＤＦ）の回折データを併記した。図２に示したＳＥＭ像は、熱処理前のα―ＦｅＯＯＨ粉末と熱処理温度３６０℃で１０時間熱処理した粉末の結果である。 The α-FeOOH powder before and after the heat treatment was subjected to X-ray diffraction (XRD) measurement using CuKα rays. Moreover, those shapes were observed with a scanning electron microscope (SEM). The results are shown in FIG. 1 and FIG. The XRD pattern shown in FIG. 1 is data of α-FeOOH powder before heat treatment, powder heat-treated at heat treatment temperatures of 250 ° C., 300 ° C., 330 ° C., 360 ° C., 400 ° C., 450 ° C., and 500 ° C. . FIG. 1 also shows diffraction data of powder diffraction files (PDF) of FeOOH and α-Fe 2 O 3 . The SEM image shown in FIG. 2 is the result of the α-FeOOH powder before heat treatment and the powder heat treated at 360 ° C. for 10 hours.
図１に示すように、熱処理により、α−ＦｅＯＯＨ粉末からα−Ｆｅ２Ｏ３粉末が生成したことがわかった。熱処理温度が２５０℃以上であるとき、α−Ｆｅ２Ｏ３粉末が生成したことがわかった。また、図２より、熱処理前後で外形には変化が見られないことがわかった。したがって、熱処理後の棒状粒子の平均長さおよび平均径を測定しても、平均長さは０．６５μｍ、平均径は０．１５μｍ（アスペクト比：４．３）であった。なお、平均長さおよび平均径は、ＳＥＭ像より複数の棒状粒子の長さおよび径を実測し、平均した値とした。 As shown in FIG. 1, it was found that α-Fe 2 O 3 powder was produced from α-FeOOH powder by heat treatment. It was found that α-Fe 2 O 3 powder was produced when the heat treatment temperature was 250 ° C. or higher. Moreover, it was found from FIG. 2 that there was no change in the outer shape before and after the heat treatment. Therefore, even when the average length and average diameter of the rod-like particles after heat treatment were measured, the average length was 0.65 μm and the average diameter was 0.15 μm (aspect ratio: 4.3). The average length and average diameter were values obtained by actually measuring the lengths and diameters of a plurality of rod-like particles from the SEM image.
α−ＦｅＯＯＨ粉末に熱処理を行うと、以下の脱水反応がおこり、α−Ｆｅ２Ｏ３粉末が生成するとともに、生成したＨ２Ｏの蒸発によりα−Ｆｅ２Ｏ３粒子に細孔が形成されると考えられる。
２α−ＦｅＯＯＨ（ｎ） → α−Ｆｅ２Ｏ３（ｎ） ＋ Ｈ２Ｏ↑
When heat treatment is performed on the α-FeOOH powder, the following dehydration reaction occurs, and α-Fe 2 O 3 powder is generated, and pores are formed in the α-Fe 2 O 3 particles by evaporation of the generated H 2 O. It is thought.
2α-FeOOH (n) → α-Fe 2 O 3 (n) + H 2 O ↑
＜比表面積および細孔容積の測定＞
低温低湿物理吸着によるＢＥＴ法（吸着質：窒素）を用い、α−ＦｅＯＯＨ粒子および種々の温度で熱処理して得られたα−Ｆｅ２Ｏ３粒子の比表面積および細孔容積を測定した。結果を図３に示した。熱処理温度は、２７０℃、３６０℃、５００℃及び７５０℃である。図３において、●で示す値は比表面積、□で示す値は細孔容積である。なお、図３において熱処理温度が「０℃」の位置には、未処理のα−ＦｅＯＯＨ粒子の測定結果を参考として示した。
<Measurement of specific surface area and pore volume>
Using the BET method (adsorbate: nitrogen) by low-temperature low-humidity physical adsorption, the specific surface area and pore volume of α-FeOOH particles and α-Fe 2 O 3 particles obtained by heat treatment at various temperatures were measured. The results are shown in FIG. The heat processing temperature is 270 degreeC, 360 degreeC, 500 degreeC, and 750 degreeC. In FIG. 3, the value indicated by ● is the specific surface area, and the value indicated by □ is the pore volume. In FIG. 3, the measurement result of untreated α-FeOOH particles is shown as a reference at the position where the heat treatment temperature is “0 ° C.”.
図３より、α−ＦｅＯＯＨ粒子を熱処理することで、熱処理前よりも比表面積および細孔容積が高くなったことから、熱処理後のα−Ｆｅ２Ｏ３粒子には粒子表面からの脱水によって形成された複数の細孔が存在することがわかった。特に、２７０〜３６０℃で熱処理されて得られたα−Ｆｅ２Ｏ３粒子の比表面積および細孔容積は、８０ｍ２／ｇ以上かつ０．１ｃｍ３／ｇ以上で非常に高かった。ただし、熱処理温度が５００℃以上では細孔閉塞が生じ、α−Ｆｅ２Ｏ３粒子の比表面積および細孔容積は、熱処理前のα−ＦｅＯＯＨ粒子の比表面積および細孔容積と同等かそれよりも低かった。適切な熱処理条件を選択することで、高比表面積かつ高細孔容積のα―Ｆｅ２Ｏ３粉末が得られることがわかった。 Than 3, by heat treatment of alpha-FeOOH particles, formed from the specific surface area and pore volume than before the heat treatment is increased, the dehydration of alpha-Fe 2 O 3 in the particle particle surface after heat treatment It was found that there were multiple pores. In particular, the specific surface area and pore volume of α-Fe 2 O 3 particles obtained by heat treatment at 270 to 360 ° C. were very high at 80 m 2 / g or more and 0.1 cm 3 / g or more. However, when the heat treatment temperature is 500 ° C. or higher, pore clogging occurs, and the specific surface area and pore volume of α-Fe 2 O 3 particles are equal to or more than the specific surface area and pore volume of α-FeOOH particles before heat treatment. Was also low. It was found that α-Fe 2 O 3 powder having a high specific surface area and a high pore volume can be obtained by selecting appropriate heat treatment conditions.
＜化合物粉末の製造＞
出発原料としての酸化リチウム（Ｌｉ２Ｏ）を２５モル％、酸化マグネシウム（ＭｇＯ）を５０モル％、酸化リン（Ｐ２Ｏ５）を２５モル％となるように秤量し、遊星型ボールミル装置を用いて、室温、回転数４５０ｒｐｍ、の条件で２０時間のメカニカルミリング処理を施した。仕込み組成比は、ＬｉＭｇＰＯ４となる比率である。得られた粉末のＸ線回折パターンを図４に示す。図４から、得られた粉末はＬｉＭｇＰＯ４カードデータに帰属されることから、オリビン型構造をもつＬｉＭｇＰＯ４が生成していることが明らかである。また得られた粉末のＳＥＭ像を図５に示す。図５から、ＬｉＭｇＰＯ４の粒径は約３μｍ以下となっている。
<Production of compound powder>
Lithium oxide (Li 2 O) as a starting material was weighed to 25 mol%, magnesium oxide (MgO) to 50 mol%, and phosphorus oxide (P 2 O 5 ) to 25 mol%. The mechanical milling process was performed for 20 hours under the conditions of room temperature and 450 rpm. The charged composition ratio is a ratio to be LiMgPO 4 . The X-ray diffraction pattern of the obtained powder is shown in FIG. From FIG. 4, since the obtained powder is attributed to LiMgPO 4 card data, it is clear that LiMgPO 4 having an olivine structure is generated. Moreover, the SEM image of the obtained powder is shown in FIG. From FIG. 5, the particle size of LiMgPO 4 is about 3 μm or less.
＜珪素酸化物粉末の製造＞
珪素酸化物粉末として、市販のＳｉＯｎ粉末（シグマ・アルドリッチ・ジャパン社製、平均粒径５μｍ）の粒子表面を炭素被覆した粉末を使用した。炭素被覆は、ＳｉＯｎ粉末をグルコース水溶液に添加し均一に混合した後、乾燥し、９００℃で２時間熱処理して行った。なお、ＳｉＯｎは、この熱処理によって、固体の内部反応によりＳｉ相とＳｉＯ２相の二相に分離する。分離して得られるＳｉ相は非常に微細である。
<Manufacture of silicon oxide powder>
As the silicon oxide powder, a commercially available SiO n powder (manufactured by Sigma-Aldrich Japan, average particle size 5 μm) having a carbon-coated powder surface was used. Carbon coating was performed by adding SiO n powder to a glucose aqueous solution and mixing uniformly, then drying and heat-treating at 900 ° C. for 2 hours. The SiO n is separated into two phases of Si phase and SiO 2 phase by solid internal reaction by this heat treatment. The Si phase obtained by separation is very fine.
図６に、熱処理により得られたＳｉＯ−Ｃ粉末のＳＥＭ写真を示した。図７には、ＳｉＯ−Ｃ粉末の模式図を示した。図６からわかるように、ＳｉＯ−Ｃ粒子の表面は、炭素からなる被覆層で被覆されていた。ＳｉＯ−Ｃ粒子の平均粒径は、８μｍ程度であった。ＳｉＯ−Ｃ粒子の内部は、熱処理により、Ｓｉ相とＳｉＯ２相の二相に分離していた。Ｓｉ相とＳｉＯ２相の割合は、モル比で、５０：５０であった。Ｓｉ相は、Ｌｉイオンの吸蔵・放出を伴う電池反応の場であり、体積変化が大きい。ＳｉＯ２相は電池反応に関与しない領域であり、体積変化は小さい。粒子内部をＳｉ相とＳｉＯ２相の海島構造にすることで、Ｓｉ単独相からなる粒子に比べて、体積変化の程度を約１／２程度に抑えることができる。 In FIG. 6, the SEM photograph of the SiO-C powder obtained by heat processing was shown. In FIG. 7, the schematic diagram of SiO-C powder was shown. As can be seen from FIG. 6, the surface of the SiO—C particles was covered with a coating layer made of carbon. The average particle diameter of the SiO—C particles was about 8 μm. The inside of the SiO—C particles was separated into two phases of Si phase and SiO 2 phase by heat treatment. The ratio of Si phase to SiO 2 phase was 50:50 in molar ratio. The Si phase is a battery reaction field that accompanies insertion and extraction of Li ions, and has a large volume change. The SiO 2 phase is a region that does not participate in the battery reaction, and its volume change is small. By making the inside of the particle a sea-island structure of Si phase and SiO 2 phase, the degree of volume change can be suppressed to about ½ compared to particles composed of a single Si phase.
＜リチウムイオン二次電池用負極の作製＞
鉄酸化物粉末としての上記のα−Ｆｅ２Ｏ３粉末、及び化合物粉末としてのＬｉＭｇＰＯ４粉末を用いて、負極を作製した。負極は、鉄酸化物粉末と化合物粉末の配合比を変えて、５種類作製した。５種類の負極は、負極１〜５とした。各負極の製法について説明する。
<Preparation of negative electrode for lithium ion secondary battery>
A negative electrode was produced using the α-Fe 2 O 3 powder as the iron oxide powder and the LiMgPO 4 powder as the compound powder. Five types of negative electrodes were produced by changing the mixing ratio of the iron oxide powder and the compound powder. The five types of negative electrodes were negative electrodes 1-5. The manufacturing method of each negative electrode will be described.
（負極１）
珪素酸化物粉末と鉄酸化物粉末と化合物粉末とを混合して混合粉末を得た。混合粉末の組成は、混合粉末を１００質量％としたとき、珪素酸化物粉末を８５．５質量％、鉄酸化物粉末を９．５質量％、化合物粉末を５質量％とした。珪素酸化物粉末と鉄酸化物粉末は負極活物質であるため、負極活物質と化合物粉末との組成比は、負極活物質：化合物粉末＝９５：５（質量％）となる。負極活物質を１００質量％としたときには、珪素酸化物粉末と鉄酸化物粉末との組成比は、珪素酸化物粉末：鉄酸化物粉末＝９０：１０となる。
(Negative electrode 1)
Silicon oxide powder, iron oxide powder and compound powder were mixed to obtain a mixed powder. The composition of the mixed powder was 85.5% by mass of silicon oxide powder, 9.5% by mass of iron oxide powder, and 5% by mass of compound powder when the mixed powder was 100% by mass. Since the silicon oxide powder and the iron oxide powder are negative electrode active materials, the composition ratio of the negative electrode active material and the compound powder is negative electrode active material: compound powder = 95: 5 (mass%). When the negative electrode active material is 100% by mass, the composition ratio between the silicon oxide powder and the iron oxide powder is silicon oxide powder: iron oxide powder = 90: 10.
この混合粉末８５質量部と、バインダー（固形分）１５質量部を混合してスラリー状の負極材料を調製した。バインダーには、ポリイミドの前駆体をＮ−メチル−２−ポロリドン（ＮＭＰ）に溶解したポリアミック酸溶液で、熱処理後の固形成分が１８％となる溶液を用いた。この負極材料を、厚さ１０μｍの電解銅箔（集電体）の表面にドクターブレードを用いて塗布し、銅箔上に負極活物質層を形成した。その後、ロールプレス機により、集電体と負極活物質層を強固に密着接合させた。これを真空乾燥し、活物質層の厚さが３０μｍ程度の負極を形成した。得られた負極は、負極１とした。 85 parts by mass of this mixed powder and 15 parts by mass of a binder (solid content) were mixed to prepare a slurry-like negative electrode material. The binder used was a polyamic acid solution in which a polyimide precursor was dissolved in N-methyl-2-poloridone (NMP), and the solid component after heat treatment was 18%. This negative electrode material was applied to the surface of an electrolytic copper foil (current collector) having a thickness of 10 μm using a doctor blade to form a negative electrode active material layer on the copper foil. Thereafter, the current collector and the negative electrode active material layer were firmly and closely joined by a roll press. This was vacuum dried to form a negative electrode having an active material layer thickness of about 30 μm. The obtained negative electrode was designated as negative electrode 1.
（負極２）
負極２では、混合粉末の組成は、混合粉末を１００質量％としたとき、珪素酸化物粉末を８１質量％、鉄酸化物粉末を９質量％、化合物粉末を１０質量％とした。珪素酸化物粉末と鉄酸化物粉末は負極活物質であるため、負極活物質と化合物粉末との組成比は、負極活物質：化合物粉末＝９０：１０（質量％）となる。負極活物質を１００質量％としたときには、珪素酸化物粉末と鉄酸化物粉末との組成比は、珪素酸化物粉末：鉄酸化物粉末＝９０：１０となる。その他は、負極１と同様にして、負極２を製造した。
(Negative electrode 2)
In the negative electrode 2, when the mixed powder was 100% by mass, the composition of the mixed powder was 81% by mass of silicon oxide powder, 9% by mass of iron oxide powder, and 10% by mass of compound powder. Since the silicon oxide powder and the iron oxide powder are negative electrode active materials, the composition ratio of the negative electrode active material and the compound powder is negative electrode active material: compound powder = 90: 10 (mass%). When the negative electrode active material is 100% by mass, the composition ratio between the silicon oxide powder and the iron oxide powder is silicon oxide powder: iron oxide powder = 90: 10. Otherwise, the negative electrode 2 was produced in the same manner as the negative electrode 1.
（負極３）
負極３では、混合粉末の組成は、混合粉末を１００質量％としたとき、珪素酸化物粉末を９０質量％、鉄酸化物粉末を１０質量％とした。化合物粉末は含まれていない。その他は、負極１と同様にして、負極３を製造した。
(Negative electrode 3)
In the negative electrode 3, the composition of the mixed powder was 90% by mass of silicon oxide powder and 10% by mass of iron oxide powder when the mixed powder was 100% by mass. Compound powder is not included. Otherwise, the negative electrode 3 was produced in the same manner as the negative electrode 1.
（負極４）
負極４では、混合粉末の組成は、混合粉末を１００質量％としたとき、珪素酸化物粉末を９５質量％、化合物粉末を５質量％とした。鉄酸化物粉末は含まれていない。その他は、負極１と同様にして、負極４を製造した。
(Negative electrode 4)
In the negative electrode 4, the composition of the mixed powder was 95% by mass of the silicon oxide powder and 5% by mass of the compound powder when the mixed powder was 100% by mass. Iron oxide powder is not included. Otherwise, the negative electrode 4 was produced in the same manner as the negative electrode 1.
（負極５）
負極５では、鉄酸化物粉末及び化合物粉末は含んでおらず、珪素酸化物粉末を用いて作製された。
(Negative electrode 5)
The negative electrode 5 did not contain iron oxide powder and compound powder, and was produced using silicon oxide powder.
＜電池の作製＞
上記の手順で作製した５種類の負極１〜５を評価極として用い、５種類の電池（半電池）を作製し、充放電サイクル試験に供した。
<Production of battery>
Five types of batteries (half-cells) were prepared using the five types of negative electrodes 1 to 5 prepared in the above procedure as evaluation electrodes, and subjected to a charge / discharge cycle test.
図８に示すように、評価極１１をφ１５．０ｍｍ、対極１２をφ１５．５ｍｍに裁断し、セパレータ１３（ポリエチレン製多孔質フィルム、厚さ２５μｍ）を両者の間に挟装して電極体電池とした。対極１２は、金属リチウム箔（厚さ５００μｍ）とした。電極体電池をコイン型の電池ケース１５（宝泉株式会社製ＣＲ２０３２コインセル）に収容した。また、電池ケース１５には、エチレンカーボネート（ＥＣ）とエチルメチルカーボネート（ＥＭＣ）とを体積比でＥＣ：ＥＭＣ＝３：７で混合した混合溶媒にＬｉＰＦ６を１ｍｏｌ／ｄｍ３の濃度で溶解した非水電解質１４を注入した。電池ケース１５と対極１２との間にはスプリング１６を設けた。電池ケース１５は２割体である。２割体の間にガスケット１７を介設することで、電池ケース１５内を密封した。これにより、評価用の半電池を作製した。 As shown in FIG. 8, the evaluation electrode 11 is cut to φ15.0 mm, the counter electrode 12 is cut to φ15.5 mm, and a separator 13 (polyethylene porous film, thickness 25 μm) is sandwiched between the electrodes. It was. The counter electrode 12 was a metal lithium foil (thickness: 500 μm). The electrode body battery was accommodated in a coin-type battery case 15 (CR2032 coin cell manufactured by Hosen Co., Ltd.). Further, in the battery case 15, LiPF 6 was dissolved at a concentration of 1 mol / dm 3 in a mixed solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of EC: EMC = 3: 7. Nonaqueous electrolyte 14 was injected. A spring 16 was provided between the battery case 15 and the counter electrode 12. The battery case 15 is a halved body. The battery case 15 was sealed by interposing the gasket 17 between the halves. Thereby, a half-cell for evaluation was produced.
この評価用の半電池では、評価極１１として各負極１〜５を正極側で用い、対極１２として金属リチウム箔を負極側で用いている。実際に各負極１〜５を負極側に用いたリチウムイオン二次電池でも、同じ充放電サイクル数での各負極間での相対容量は評価用半電池と同じ傾向を示すと推定される。 In this half cell for evaluation, each negative electrode 1 to 5 is used as the evaluation electrode 11 on the positive electrode side, and a metal lithium foil is used as the counter electrode 12 on the negative electrode side. Even in lithium ion secondary batteries that actually use the negative electrodes 1 to 5 on the negative electrode side, the relative capacity between the negative electrodes at the same number of charge / discharge cycles is estimated to show the same tendency as the evaluation half-cell.
＜充放電サイクル試験＞
作製した各半電池に対し、室温下で充放電サイクル試験を行った。１サイクル目は充放電電流密度０．２ｍＡ／ｃｍ２にて、２サイクル目以降は充放電電流密度０．５ｍＡ／ｃｍ２にて定電流充放電試験を行った。電位範囲は、リチウム基準電位で０〜３．０Ｖとした。６０サイクル目までの放電容量の推移を図９に示した。各負極の第１回目放電容量、第２回目放電容量及び、６０回目放電容量を表１に示した。
<Charge / discharge cycle test>
A charge / discharge cycle test was performed at room temperature on each half-cell produced. A constant current charge / discharge test was performed at a charge / discharge current density of 0.2 mA / cm 2 at the first cycle and at a charge / discharge current density of 0.5 mA / cm 2 after the second cycle. The potential range was 0 to 3.0 V at the lithium reference potential. The transition of the discharge capacity up to the 60th cycle is shown in FIG. Table 1 shows the first discharge capacity, the second discharge capacity, and the 60th discharge capacity of each negative electrode.
表１，図９より、負極１，２を用いた電池は、負極３〜５を用いた電池よりも、６０サイクルでの放電容量維持率がよかった。また、初回放電容量については、負極１〜３を用いた電池が、負極４、５よりも大きかった。２回目放電容量については、負極１がよく、次に負極２，３がよく、負極４、５は小さかった。 From Table 1 and FIG. 9, the battery using the negative electrodes 1 and 2 had a better discharge capacity maintenance rate at 60 cycles than the battery using the negative electrodes 3 to 5. Moreover, about the first time discharge capacity, the battery using the negative electrodes 1-3 was larger than the negative electrodes 4 and 5. FIG. Regarding the second discharge capacity, negative electrode 1 was good, followed by negative electrodes 2 and 3, and negative electrodes 4 and 5 were small.
このことから、各負極材料を負極側に用いたリチウムイオン二次電池でも、珪素酸化物粉末と棒状の鉄酸化物粉末と化合物粉末との３種類の混合物が混合された負極材料を用いることにより、珪素酸化物粉末と棒状鉄酸化物粉末からなる負極材料、珪素酸化物粉末と化合物粉末からなる負極材料、珪素酸化物粉末単独の負極材料を用いた場合に比べて、サイクル特性及び放電容量がよくなることがわかった。特に、負極１，２では、初回放電容量は同程度であったが、混合物を１００質量％としたときの化合物粉末の配合比が５質量％の場合（負極１）は、１０質量％であるとき（負極２）よりも、不可逆容量が小さく、２回目放電容量が大きくなった。 From this, even in the lithium ion secondary battery using each negative electrode material on the negative electrode side, by using a negative electrode material in which three kinds of mixtures of silicon oxide powder, rod-shaped iron oxide powder and compound powder are mixed. Compared with the case of using a negative electrode material made of silicon oxide powder and rod-shaped iron oxide powder, a negative electrode material made of silicon oxide powder and compound powder, and a negative electrode material made of silicon oxide powder alone, the cycle characteristics and discharge capacity are I found out that it would improve. In particular, in the negative electrodes 1 and 2, the initial discharge capacities were about the same, but when the compounding ratio of the compound powder was 5% by mass when the mixture was 100% by mass (negative electrode 1), it was 10% by mass. The irreversible capacity was smaller than the time (negative electrode 2), and the second discharge capacity was larger.
また、珪素酸化物粉末に、棒状の鉄酸化物粉末及び化合物粉末を混合することで、サイクル数の上昇に伴う放電容量の低下が抑制されることがわかった。混合物全体を１００質量％としたとき、鉄酸化物粉末を１質量％以上２０質量％以下、化合物粉末を１質量％以上１５質量％以下とすることがよいとわかった。また、前記鉄酸化物粉末を５質量％以上２０質量％以下、前記化合物粉末を５質量％以上１５質量％以下含むことで、サイクル特性が向上することがわかった。また、電池容量を高くするためには、鉄酸化物粉末を１質量％以上１０質量％以下、化合物粉末を１質量％以上１０質量％以下とすることすることがよいとわかった。 Moreover, it turned out that the fall of the discharge capacity accompanying the raise of a cycle number is suppressed by mixing rod-shaped iron oxide powder and compound powder with silicon oxide powder. It was found that when the total mixture was 100% by mass, the iron oxide powder was preferably 1% by mass to 20% by mass and the compound powder was 1% by mass to 15% by mass. Moreover, it turned out that cycling characteristics improve by containing the said iron oxide powder 5 mass% or more and 20 mass% or less, and containing the said compound powder 5 mass% or more and 15 mass% or less. Moreover, in order to make battery capacity high, it turned out that it is good to make iron oxide powder into 1 mass% or more and 10 mass% or less, and make compound powder into 1 mass% or more and 10 mass% or less.
＜参考例１：α−Ｆｅ２Ｏ３粉末の配合量及び形状の比較＞
珪素酸化物粉末と、棒状の鉄酸化物粉末または球状の鉄酸化物粉末とを用いて、各種負極を作製した。珪素酸化物粉末は、炭素で被覆されたＳｉＯ粉末（ＳｉＯ−Ｃ粉末）を用いた。ＳｉＯ−Ｃ粉末は、上記の＜珪素酸化物粒子の製造＞に示された方法で作製されたものである。鉄酸化物粉末は、棒状のものと球状のものを用いた。棒状の鉄酸化物粉末には、上記のα−ＦｅＯＯＨ粉末を３６０℃で１０時間熱処理して得たα−Ｆｅ２Ｏ３粉末（粒子の平均長さ：０．６５μｍ、粒子の平均径：０．１５μｍ（アスペクト比：４．３））を用いた。球状の鉄酸化物粉末には、市販のα−Ｆｅ２Ｏ３粉末（粒子の平均粒径：０．７μｍ（アスペクト比は略１））を用いた。棒状の鉄酸化物粉末のＢＥＴ比表面積は８７．７ｍ２／ｇであり、球状の鉄酸化物粉末のＢＥＴ比表面積は１１．６ｍ２／ｇであった。
<Reference Example 1: Comparison of blending amount and shape of α-Fe 2 O 3 powder>
Various negative electrodes were prepared using silicon oxide powder and rod-like iron oxide powder or spherical iron oxide powder. As the silicon oxide powder, SiO powder coated with carbon (SiO-C powder) was used. The SiO-C powder is produced by the method shown in <Manufacture of silicon oxide particles> above. As the iron oxide powder, rod-shaped and spherical ones were used. For the rod-like iron oxide powder, α-Fe 2 O 3 powder obtained by heat-treating the above α-FeOOH powder at 360 ° C. for 10 hours (average particle length: 0.65 μm, average particle diameter: 0) .15 μm (aspect ratio: 4.3)). Commercially available α-Fe 2 O 3 powder (average particle diameter of particles: 0.7 μm (aspect ratio is about 1)) was used as the spherical iron oxide powder. The BET specific surface area of the rod-shaped iron oxide powder was 87.7 m 2 / g, and the BET specific surface area of the spherical iron oxide powder was 11.6 m 2 / g.
ＳｉＯ−Ｃ粉末と棒状又は球状のα−Ｆｅ２Ｏ３粉末との合計質量を１００質量％としたとき、ＳｉＯ−Ｃ粉末の質量比をｘ質量％、α−Ｆｅ２Ｏ３粉末の質量比を（１００−ｘ）質量％であらわされる。棒状のα−Ｆｅ２Ｏ３粉末を用いた場合にｘ＝０、８０、９０、１００のとき、及び球状のα−Ｆｅ２Ｏ３粉末を用いた場合にｘ＝９０のとき、負極を以下の方法により作製した。 When the total mass of the SiO-C powder and the rod-like or spherical α-Fe 2 O 3 powder is 100 mass%, the mass ratio of the SiO-C powder is x mass%, and the mass ratio of the α-Fe 2 O 3 powder. Is represented by (100-x) mass%. When x = 0, 80, 90, 100 when a rod-shaped α-Fe 2 O 3 powder is used, and when x = 90 when a spherical α-Fe 2 O 3 powder is used, the negative electrode is It was produced by the method.
ＳｉＯ−Ｃ粉末と棒状又は球状のα−Ｆｅ２Ｏ３粉末との合計量が８５質量部、アセチレンブラック（ＡＢ）５質量部およびバインダー１０質量部となるようにそれぞれ混合してスラリー状の負極材料を調製した。バインダーには、ポリイミドの前駆体をＮ−メチル−２−ポロリドン（ＮＭＰ）に溶解したポリアミック酸溶液で、熱処理後の固形成分が３０％となる溶液を用いた。このスラリー状の負極材料を、厚さ１０μｍの電解銅箔（集電体）の表面にドクターブレードを用いて塗布し、銅箔上に負極活物質層を形成した。その後、ロールプレス機により、集電体と負極活物質層を強固に密着接合させた。これを真空乾燥し、活物質層の厚さが３０μｍ程度の負極を形成した。各負極を評価極として、上記と同様に半電池を作製し、同条件で充放電サイクル試験を行った。充放電のサイクル数は１００回とした。 A slurry-like negative electrode in which the total amount of SiO-C powder and rod-like or spherical α-Fe 2 O 3 powder is 85 parts by mass, acetylene black (AB) 5 parts by mass and binder 10 parts by mass, respectively. The material was prepared. The binder used was a polyamic acid solution in which a polyimide precursor was dissolved in N-methyl-2-poloridone (NMP), and the solid component after heat treatment was 30%. This slurry-like negative electrode material was applied to the surface of an electrolytic copper foil (current collector) having a thickness of 10 μm using a doctor blade to form a negative electrode active material layer on the copper foil. Thereafter, the current collector and the negative electrode active material layer were firmly and closely joined by a roll press. This was vacuum dried to form a negative electrode having an active material layer thickness of about 30 μm. Using each negative electrode as an evaluation electrode, a half cell was prepared in the same manner as described above, and a charge / discharge cycle test was performed under the same conditions. The number of charge / discharge cycles was 100.
充放電サイクル試験前後の負極活物質層をＳＥＭにより観察した。その結果を、図１０〜図１４に示した。図１０は、充放電前の負極活物質層のＳＥＭ像であり、左上段はｘ＝１００、右上段はｘ＝９０、左下段はｘ＝８０、右下段はｘ＝０の場合の負極活物質層のＳＥＭ像である。図１０で示された負極活物質層に含まれるα−Ｆｅ２Ｏ３粒子はすべて棒状粒子である。 The negative electrode active material layers before and after the charge / discharge cycle test were observed by SEM. The results are shown in FIGS. FIG. 10 is an SEM image of the negative electrode active material layer before charging and discharging. The upper left portion is x = 100, the upper right portion is x = 90, the lower left portion is x = 80, and the lower right portion is x = 0. It is a SEM image of a material layer. All the α-Fe 2 O 3 particles contained in the negative electrode active material layer shown in FIG. 10 are rod-like particles.
図１０に示すように、充電前では、ＳｉＯ−Ｃ粉末からなる負極活物質層（ｘ＝１００）では、ＳｉＯ−Ｃ粒子の表面に、炭素粒子が付着していた。ＳｉＯ−Ｃ粉末及び棒状α−Ｆｅ２Ｏ３粉末からなる負極活物質層（ｘ＝９０、８０）では、ＳｉＯ−Ｃ粒子の粒子間に、棒状のα−Ｆｅ２Ｏ３粒子が介在していた。棒状α−Ｆｅ２Ｏ３粉末からなる負極活物質層（ｘ＝０）では、棒状のα−Ｆｅ２Ｏ３粒子のみが観察された。 As shown in FIG. 10, before charging, in the negative electrode active material layer (x = 100) made of SiO—C powder, carbon particles were adhered to the surface of the SiO—C particles. In the negative electrode active material layer (x = 90, 80) made of SiO-C powder and rod-shaped α-Fe 2 O 3 powder, rod-shaped α-Fe 2 O 3 particles are interposed between the SiO-C particle particles. It was. In the negative electrode active material layer (x = 0) made of rod-shaped α-Fe 2 O 3 powder, only rod-shaped α-Fe 2 O 3 particles were observed.
図１１は、１００サイクル放電後の負極活物質層のＳＥＭ像であり、左上段は球状のα−Ｆｅ２Ｏ３粉末及びＳｉＯ−Ｃ粉末を含み且つｘ＝９０である負極活物質層のＳＥＭ像であり、右上段は棒状のα−Ｆｅ２Ｏ３粉末及びＳｉＯ−Ｃ粉末を含み且つｘ＝９０である負極活物質層のＳＥＭ像であり、左下段はＳｉＯ−Ｃ粉末を含みα−Ｆｅ２Ｏ３粉末を含まない負極活物質層のＳＥＭ像であり（ｘ＝１００）、右下段は棒状のα−Ｆｅ２Ｏ３粉末及びＳｉＯ−Ｃ粉末を含みかつｘ＝８０である負極活物質層のＳＥＭ像である。図１１では、球状のα−Ｆｅ２Ｏ３粉末を用いた場合は（ｓ）と表記し、棒状のα−Ｆｅ２Ｏ３粉末を用いた場合は（ｎ）と表記した。なお、図１０，図１１のＳＥＭ像は、集電体の表面に対して垂直方向に負極活物質層を観察したものである。 FIG. 11 is an SEM image of the negative electrode active material layer after 100 cycles of discharge, and the upper left stage includes an SEM of the negative electrode active material layer containing spherical α-Fe 2 O 3 powder and SiO—C powder and x = 90. The upper right part is an SEM image of the negative electrode active material layer containing rod-like α-Fe 2 O 3 powder and SiO—C powder and x = 90, and the lower left part contains SiO—C powder and α− Fe 2 O 3 is an SEM image of the negative electrode active material layer powder does not contain (x = 100), the negative electrode active lower right is and x = 80 include α-Fe 2 O 3 powder and SiO-C powder of the rod-shaped It is a SEM image of a material layer. In FIG. 11, when spherical α-Fe 2 O 3 powder is used, it is expressed as (s), and when rod-shaped α-Fe 2 O 3 powder is used, it is expressed as (n). 10 and 11 are obtained by observing the negative electrode active material layer in a direction perpendicular to the surface of the current collector.
図１１に示すように、１００サイクル放電後には、棒状のＦｅ２Ｏ３粉末とＳｉＯ−Ｃ粉末とを９０：１０（質量％）の配合比で含む負極活物質層（ｘ＝９０）は、ＳｉＯ−Ｃ粉末を含む場合（ｘ＝１００）に比べて、ＳｉＯ−Ｃ粒子表面が若干滑らかであった。これは、ＳｉＯ−Ｃ粒子表面に、棒状のＦｅ２Ｏ３粒子が溶けて被膜を形成したためである。この被膜には、電解液の酸化還元反応により生成したＳＥＩ成分とＦｅ２Ｏ３とが含まれていると予想される。ｘ＝９０のＳＥＭ像では、ＳｉＯ−Ｃ粒子表面の被膜の中に、わずかに棒状のＦｅ２Ｏ３粒子がみられる。また、被膜は、滑らかにみえ、まるでゲル状になっているかのようにもみえる。 As shown in FIG. 11, after 100 cycles of discharge, the negative electrode active material layer (x = 90) containing rod-like Fe 2 O 3 powder and SiO—C powder at a blending ratio of 90:10 (mass%) The surface of the SiO—C particles was slightly smoother than when the SiO—C powder was included (x = 100). This is because rod-like Fe 2 O 3 particles were dissolved on the surface of the SiO—C particles to form a film. This coating is expected to contain SEI components and Fe 2 O 3 generated by the oxidation-reduction reaction of the electrolytic solution. In the SEM image of x = 90, slightly rod-like Fe 2 O 3 particles are seen in the coating on the surface of the SiO—C particles. Also, the coating looks smooth and looks like a gel.
図１１に示すように、１００サイクル放電後には、棒状のＦｅ２Ｏ３粉末の配合量を更に増やした場合（ｘ＝８０）には、ＳｉＯ−Ｃ粒子表面に被膜が形成されており、しかも、ＳｉＯ−Ｃ粒子表面に凸状部が形成された。この凸状部は、ＳｉＯ−Ｃ粒子表面に形成された被膜の中に、棒状のＦｅ２Ｏ３粒子がその形状を留めて残っている部分である。 As shown in FIG. 11, after 100 cycles of discharge, when the blending amount of the rod-like Fe 2 O 3 powder is further increased (x = 80), a film is formed on the surface of the SiO—C particles, and A convex portion was formed on the surface of the SiO-C particles. This convex portion is a portion where rod-like Fe 2 O 3 particles remain in the shape of the coating formed on the surface of the SiO—C particles.
そして、Ｆｅ２Ｏ３粒子が球状である場合（Ｘ＝９０（ｓ））には、球状のＦｅ２Ｏ３粒子は、その形状を留めており、ＳｉＯ−Ｃ粒子表面に付着していた。 When the Fe 2 O 3 particles were spherical (X = 90 (s)), the spherical Fe 2 O 3 particles retained their shape and adhered to the SiO—C particle surface.
図１２の上段は、充放電前の負極活物質層表面のＳＥＭ像であり、下段は１００サイクル放電後の負極活物質層表面のＳＥＭ像である。上段及び下段の左図は負極活物質層がＳｉＯ−Ｃ粉末は含むがα−Ｆｅ２Ｏ３粉末を含まない場合（ｘ＝１００）、中図はＳｉＯ−Ｃ粉末及び棒状α−Ｆｅ２Ｏ３粉末を含む場合（ｘ＝９０（ｎ））、右図はＳｉＯ−Ｃ粉末及び球状α−Ｆｅ２Ｏ３粉末を含む場合（ｘ＝９０（ｓ））を示す。図１２のＳＥＭ像は、集電体の表面に対して平行方向に負極活物質層の表面を観察した。 The upper part of FIG. 12 is an SEM image of the surface of the negative electrode active material layer before charge / discharge, and the lower part is an SEM image of the surface of the negative electrode active material layer after 100 cycles of discharge. The upper and lower left figures show the case where the negative electrode active material layer contains SiO-C powder but no α-Fe 2 O 3 powder (x = 100). The middle figure shows SiO-C powder and rod-like α-Fe 2 O. When 3 powders are included (x = 90 (n)), the right figure shows a case where SiO—C powder and spherical α-Fe 2 O 3 powder are included (x = 90 (s)). In the SEM image of FIG. 12, the surface of the negative electrode active material layer was observed in a direction parallel to the surface of the current collector.
図１２の上段に示すように、充放電前では、ＳｉＯ−Ｃ粉末のみのとき（ｘ＝１００）、ＳｉＯ−Ｃ粒子が多数見られた。ＳｉＯ−Ｃ粉末及び棒状α−Ｆｅ２Ｏ３粉末を含む場合（ｘ＝９０（ｎ））、ＳｉＯ−Ｃ粒子の間に棒状のα−Ｆｅ２Ｏ３粒子が分散していた。ＳｉＯ−Ｃ粉末及び球状α−Ｆｅ２Ｏ３粉末を含む場合（ｘ＝９０（ｓ））、比較的大きなＳｉＯ−Ｃ粒子の間に、比較的小さな球状のα−Ｆｅ２Ｏ３粒子が認められた。 As shown in the upper part of FIG. 12, a large number of SiO—C particles were observed before the charge / discharge when only the SiO—C powder was used (x = 100). When SiO-C powder and rod-like α-Fe 2 O 3 powder were included (x = 90 (n)), rod-like α-Fe 2 O 3 particles were dispersed between the SiO-C particles. When SiO-C powder and spherical α-Fe 2 O 3 powder are included (x = 90 (s)), relatively small spherical α-Fe 2 O 3 particles are observed between relatively large SiO-C particles. It was.
図１２の下段に示すように、１００サイクル放電後には、ｘ＝１００及びｘ＝９０（ｓ）では、負極活物質層表面に亀裂が発生した。これに対して、ｘ＝９０（ｎ）では、負極活物質表面に亀裂は発生していなかった。このことは、以下のように推測される。棒状のα−Ｆｅ２Ｏ３粒子が、ＳｉＯ−Ｃ粒子間に分散されることで、隣り合うＳｉＯ−Ｃ粒子間を架橋する。これにより、充放電に伴う体積変化でＳｉＯ−Ｃ粒子の表面の被膜に亀裂が発生することを防止したためであると考えられる。また、充放電サイクルにより、棒状のα−Ｆｅ２Ｏ３粒子がＳｉＯ−Ｃ粒子表面にゲル状の被膜を形成し、ＳｉＯ−Ｃ粒子の体積変化を吸収するため、負極活物質層に亀裂が発生することを抑制しているとも考えられる。 As shown in the lower part of FIG. 12, after 100 cycles of discharge, cracks occurred on the surface of the negative electrode active material layer at x = 100 and x = 90 (s). On the other hand, when x = 90 (n), no crack was generated on the surface of the negative electrode active material. This is presumed as follows. The rod-like α-Fe 2 O 3 particles are dispersed between the SiO—C particles, thereby cross-linking adjacent SiO—C particles. This is considered to be because cracks were prevented from occurring in the coating film on the surface of the SiO—C particles due to the volume change accompanying charging / discharging. Further, due to the charge / discharge cycle, the rod-like α-Fe 2 O 3 particles form a gel-like film on the surface of the SiO—C particles and absorb the volume change of the SiO—C particles, so that the negative electrode active material layer is cracked. It is also considered that the occurrence is suppressed.
図１３及び図１４は、棒状又は球状のα−Ｆｅ２Ｏ３粉末を用いかつＳｉＯ−Ｃ粉末は含まない充放電前の負極活物質層のＳＥＭ像である（ｘ＝０）。図１３は、α−Ｆｅ２Ｏ３粒子が棒状である場合、図１４はα−Ｆｅ２Ｏ３粒子が球状である場合を示す。図１３に見られる粒状体（一例を矢印で示す）はアセチレンブラックであった。ＳＥＭ観察は、集電体の表面に対して垂直方向に負極活物質層の表面を観察した。図１３では、集電体の表面に対して平行な棒状粒子が多数観察された。 FIG. 13 and FIG. 14 are SEM images of the negative electrode active material layer before charge / discharge using rod-like or spherical α-Fe 2 O 3 powder and not containing SiO—C powder (x = 0). FIG. 13 shows a case where α-Fe 2 O 3 particles are rod-shaped, and FIG. 14 shows a case where α-Fe 2 O 3 particles are spherical. The granular material (an example is shown by an arrow) seen in FIG. 13 was acetylene black. In SEM observation, the surface of the negative electrode active material layer was observed in a direction perpendicular to the surface of the current collector. In FIG. 13, many rod-like particles parallel to the surface of the current collector were observed.
上記の手順で作製した２種類の電極を評価極として用い、前述の手順に従い２種類の半電池を作製した。作製したそれぞれの半電池に対し、室温下で充放電試験を行った。１サイクル目は充放電電流密度０．２ｍＡ／ｃｍ２にて、２サイクル目以降は充放電電流密度０．５ｍＡ／ｃｍ２にて定電流充放電試験を行った。電位範囲は、リチウム基準電位で０．００５〜３．０Ｖとした。 Using the two types of electrodes prepared by the above procedure as evaluation electrodes, two types of half-cells were prepared according to the above-described procedure. A charge / discharge test was performed at room temperature on each half-cell produced. A constant current charge / discharge test was performed at a charge / discharge current density of 0.2 mA / cm 2 at the first cycle and at a charge / discharge current density of 0.5 mA / cm 2 after the second cycle. The potential range was 0.005 to 3.0 V at the lithium reference potential.
１サイクル目から１８サイクル目（球状のα−Ｆｅ２Ｏ３粉末を使用した場合には２０サイクル目）までの放電容量の推移を図１５に示した。棒状のα−Ｆｅ２Ｏ３粉末を使用した電池は、１８サイクル目まで初期放電容量の８５％以上を維持した。また、棒状のα−Ｆｅ２Ｏ３粉末を使用した電池の放電容量は、２サイクル目以降であっても７００ｍＡｈ／ｇで高かった。一方、球状のα−Ｆｅ２Ｏ３粉末を使用した電池は、初期放電容量が低く、２サイクル目で初期放電容量の半分程度まで低下し、２０サイクル目にはさらに半分程度の容量に低下した。 FIG. 15 shows the transition of discharge capacity from the first cycle to the 18th cycle (20th cycle when spherical α-Fe 2 O 3 powder is used). The battery using the rod-shaped α-Fe 2 O 3 powder maintained 85% or more of the initial discharge capacity until the 18th cycle. Further, the discharge capacity of the battery using the rod-like α-Fe 2 O 3 powder was as high as 700 mAh / g even after the second cycle. On the other hand, the battery using the spherical α-Fe 2 O 3 powder has a low initial discharge capacity, which is reduced to about half of the initial discharge capacity at the second cycle, and further reduced to about half of the capacity at the 20th cycle. .
上記の評価に用いた２種類の電池において、粒子形状以外に両者に差は無い。そのため、棒状のα−Ｆｅ２Ｏ３粉末を使用した二次電池の高い容量および優れたサイクル特性は、粒子形状に起因することがわかった。そして、棒状のα−Ｆｅ２Ｏ３粉末を珪素酸化物粉末とともに用いることで、図９に示したように、珪素酸化物粉末単独では不十分であったサイクル特性および／または初期容量を向上させることができることがわかった。 In the two types of batteries used for the above evaluation, there is no difference between them except for the particle shape. Therefore, it was found that the high capacity and excellent cycle characteristics of the secondary battery using the rod-shaped α-Fe 2 O 3 powder are caused by the particle shape. Then, by using the rod-like α-Fe 2 O 3 powder together with the silicon oxide powder, as shown in FIG. 9, the cycle characteristics and / or the initial capacity, which was insufficient with the silicon oxide powder alone, are improved. I found out that I could do it.
一方、球状のα−Ｆｅ２Ｏ３粉末が珪素酸化物粉末とともに負極材料に含まれると、充放電時に珪素酸化物粒子間の接触が悪くなるという理由により、珪素酸化物粉末単独を負極材料に含む場合に比べて、容量およびサイクル特性が悪化することが予測される。 On the other hand, when spherical α-Fe 2 O 3 powder is contained in the negative electrode material together with the silicon oxide powder, the silicon oxide powder alone is used as the negative electrode material because of the poor contact between the silicon oxide particles during charge and discharge. Compared with the case where it contains, it is estimated that a capacity | capacitance and cycling characteristics deteriorate.
図１６の上段は、ＳｉＯ−Ｃ粉末と棒状のα−Ｆｅ２Ｏ３粉末とからなる負極活物質層を有する負極の断面模式図であり、図１６の下段は、ＳｉＯ−Ｃ粉末と球状のＦｅ２Ｏ３粉末とからなる負極活物質層を有する負極の断面模式図である。 The upper part of FIG. 16 is a schematic cross-sectional view of a negative electrode having a negative electrode active material layer made of SiO—C powder and rod-like α-Fe 2 O 3 powder, and the lower part of FIG. it is a schematic cross-sectional view of a negative electrode having a negative electrode active material layer consisting of Fe 2 O 3 powder.
図１６の上段に示すように、ＳｉＯ−Ｃ粉末と棒状のＦｅ２Ｏ３粉末とからなる負極活物質層では、ＳｉＯ−Ｃ粒子１の間に、比較的小さな棒状のＦｅ２Ｏ３粒子５が分散している。棒状のＦｅ２Ｏ３粒子５は、ＳｉＯ−Ｃ粒子１間を架橋する役目をもつ。ＳｉＯ−Ｃ粒子１は、図７で説明したように、ＳｉＯからなるコア部２と、コア部２を被覆するＣ（炭素）からなる被覆層３とをもつ。コア部２は、Ｓｉ相２１と、ＳｉＯ２相２２とで、海島構造を形成している。 As shown in the upper part of FIG. 16, in the negative electrode active material layer composed of SiO—C powder and rod-like Fe 2 O 3 powder, relatively small rod-like Fe 2 O 3 particles 5 are interposed between the SiO—C particles 1. Are dispersed. The rod-like Fe 2 O 3 particles 5 serve to crosslink between the SiO—C particles 1. As described with reference to FIG. 7, the SiO—C particle 1 has a core portion 2 made of SiO and a coating layer 3 made of C (carbon) covering the core portion 2. The core part 2 forms a sea-island structure with the Si phase 21 and the SiO 2 phase 22.
一般に、ＳｉＯ−Ｃ粒子１のコア部２は、Ｌｉイオンを吸蔵放出する活物質の役目を担うＳｉ相２１を有している。このため、ＳｉＯ−Ｃ粒子１が電解液に接触すると、粒子表面にＳＥＩ被膜が形成される。Ｌｉ吸蔵放出に伴いＳｉＯ−Ｃ粒子１が体積変化すると、表面に形成された被膜に亀裂が発生しやすい。被膜に亀裂が発生すると、その亀裂を通じて電解液がＳｉＯ−Ｃ粒子１に直接接触し、更に新たな被膜を形成する。被膜が形成される度に電解液は劣化していくため、この現象はサイクル特性を低下させる。 In general, the core portion 2 of the SiO—C particle 1 has a Si phase 21 that serves as an active material that absorbs and releases Li ions. For this reason, when the SiO—C particles 1 come into contact with the electrolytic solution, an SEI film is formed on the particle surface. When the SiO—C particles 1 change in volume along with Li storage / release, cracks are likely to occur in the coating formed on the surface. When a crack occurs in the coating, the electrolytic solution directly contacts the SiO-C particles 1 through the crack and further forms a new coating. This phenomenon deteriorates cycle characteristics because the electrolyte solution deteriorates every time a film is formed.
しかし、図１６の上段に示すように、ＳｉＯ−Ｃ粒子１の間に棒状のＦｅ２Ｏ３粒子５を分散させると、棒状のＦｅ２Ｏ３粒子５がＳｉＯ−Ｃ粒子１間を架橋し、被膜の亀裂発生を抑制する。また、棒状のＦｅ２Ｏ３粒子５の一部がゲル状の被膜となってＳｉＯ−Ｃ粉末表面を被覆する。この被膜は、体積変化に追従し易く、被膜に亀裂が発生しにくい。このため、電解液の劣化を抑制でき、充放電が繰り返されても高い容量を維持することができると考えられる。 However, as shown in the upper part of FIG. 16, when the rod-like Fe 2 O 3 particles 5 are dispersed between the SiO—C particles 1, the rod-like Fe 2 O 3 particles 5 cross-link between the SiO—C particles 1. Suppresses the generation of cracks in the coating. A part of the rod-shaped Fe 2 O 3 particles 5 covers the SiO-C powder surface is a gel coating. This coating is easy to follow the volume change and is difficult to crack. For this reason, it is thought that deterioration of electrolyte solution can be suppressed and a high capacity | capacitance can be maintained even if charging / discharging is repeated.
一方、図１６の下段に示すように、球状のＦｅ２Ｏ３粒子９は、その形状が球状であり、棒状粒子に比べて、ＳｉＯ−Ｃ粒子１間を架橋しにくい形状である。ゆえに、ＳｉＯ−Ｃ粒子１表面で、被膜の亀裂、生成が繰り返され、電解液の劣化や不可逆容量の増大によりサイクル特性が低下したものと考えられる。 On the other hand, as shown in the lower part of FIG. 16, the spherical Fe 2 O 3 particles 9 have a spherical shape, and are less likely to crosslink between the SiO—C particles 1 than the rod-shaped particles. Therefore, it is considered that cracking and generation of the coating were repeated on the surface of the SiO—C particles 1, and the cycle characteristics were lowered due to deterioration of the electrolytic solution and increase in irreversible capacity.
＜参考例２：棒状のα−Ｆｅ２Ｏ３粉末とＳｉＯ−Ｃ粉末の配合量の検討＞
珪素酸化物粉末と棒状の鉄酸化物粉末とを用いて、各種負極を作製した。珪素酸化物粉末は、炭素で被覆されたＳｉＯ粉末（ＳｉＯ−Ｃ粉末）を用いた。ＳｉＯ−Ｃ粉末は、上記の＜珪素酸化物粉末の製造＞に示された方法で作製された。珪素酸化物粉末と鉄酸化物粉末との配合割合を、質量比で、珪素酸化物粉末：鉄酸化物粉末＝１００：０、９０：１０、８０：２０、７０：３０、６０：４０、５０：５０、２０：８０、１０：９０、０：１００とし、負極活物質の異なる９種類の負極を作製した。負極には、化合物粉末は含まれていない。各負極のその他の構成は、上記の負極と同様である。各負極を用いて上記と同様の構成の半電池を作製した。半電池の１サイクル目の充放電容量および放電容量を図１７に示した。また、７０サイクル目までの放電容量の推移を図１８に示した。
<Reference Example 2: Examination of blending amount of rod-shaped α-Fe 2 O 3 powder and SiO—C powder>
Various negative electrodes were prepared using silicon oxide powder and rod-shaped iron oxide powder. As the silicon oxide powder, SiO powder coated with carbon (SiO-C powder) was used. The SiO-C powder was produced by the method shown in <Production of silicon oxide powder> above. The mixing ratio of the silicon oxide powder and the iron oxide powder in terms of mass ratio, silicon oxide powder: iron oxide powder = 100: 0, 90:10, 80:20, 70:30, 60:40, 50 : 50, 20:80, 10:90, and 0: 100, nine types of negative electrodes having different negative electrode active materials were produced. The negative electrode contains no compound powder. The other structure of each negative electrode is the same as that of said negative electrode. A half battery having the same configuration as described above was produced using each negative electrode. The charge / discharge capacity and discharge capacity in the first cycle of the half-cell are shown in FIG. Moreover, the transition of the discharge capacity up to the 70th cycle is shown in FIG.
図１７より、珪素酸化物粉末の配合割合が増加するにしたがって、初期容量は増加する傾向にあった。特に、珪素酸化物粉末の配合割合が８０質量％の時に容量は最大となった。珪素酸化物粉末の配合割合を７５〜９０質量％（鉄酸化物粉末の添加量であれば１０〜２５質量％）とすることで、鉄酸化物粉末を含まない場合よりも初期容量を増大させることができることがわかった。負極材料に、珪素酸化物粉末と鉄酸化物粉末に加えて、ＬｉＭｇＰＯ４からなる化合物粉末を加えた場合にも、サイクル特性に優れた二次電池を作製できると考えられる。 From FIG. 17, the initial capacity tended to increase as the blending ratio of the silicon oxide powder increased. In particular, the capacity became maximum when the blending ratio of the silicon oxide powder was 80% by mass. By setting the mixing ratio of the silicon oxide powder to 75 to 90% by mass (10 to 25% by mass if the addition amount of the iron oxide powder), the initial capacity is increased as compared with the case where the iron oxide powder is not included. I found out that I could do it. It is considered that a secondary battery having excellent cycle characteristics can also be produced when a compound powder made of LiMgPO 4 is added to the negative electrode material in addition to the silicon oxide powder and the iron oxide powder.
図１８より、鉄酸化物粉末の使用により、サイクル数増加に伴い放電容量の低下が抑制されることがわかった。鉄酸化物粉末の添加量を４０質量％以上とすることで、放電容量のサイクル推移が非常に安定した。一方、鉄酸化物粉末の添加量が１０質量％である二次電池であれば、負極活物質が珪素酸化物粉末１００質量％の二次電池と比較して、初期容量およびサイクル特性ともに優れることがわかった。すなわち、鉄酸化物粉末の添加量を５〜１５質量％（珪素酸化物粉末の配合割合であれば８５〜９５質量％）とすることで、初期容量およびサイクル特性を高いレベルで両立するリチウムイオン二次電池が得られることがわかった。負極活物質層にＬｉＭｇＰＯ４からなる化合物粉末を添加混合した場合にも、サイクル特性に優れた二次電池が得られると予想される。 From FIG. 18, it was found that the use of iron oxide powder suppresses the decrease in discharge capacity as the number of cycles increases. By making the addition amount of the iron oxide powder 40% by mass or more, the cycle transition of the discharge capacity was very stable. On the other hand, in the case of a secondary battery in which the amount of iron oxide powder added is 10% by mass, both the initial capacity and cycle characteristics are superior compared to a secondary battery in which the negative electrode active material is 100% by mass of silicon oxide powder. I understood. That is, lithium ion that makes the initial capacity and cycle characteristics compatible at a high level by adjusting the addition amount of the iron oxide powder to 5 to 15 mass% (if the mixing ratio of the silicon oxide powder is 85 to 95 mass%). It was found that a secondary battery can be obtained. Even when a compound powder made of LiMgPO 4 is added to and mixed with the negative electrode active material layer, a secondary battery excellent in cycle characteristics is expected to be obtained.
＜参考例３：ＬｉＭｇＰＯ４からなる化合物粉末の配合量の検討＞
ＳｉＯ−Ｃ粉末とＬｉＭｇＰＯ４からなるＬｉＭｇＰＯ４粉末とからなる負極材料を調製し、集電体表面に負極材料からなる負極活物質層を形成して負極を得た。負極材料には、Ｆｅ２Ｏ３粉末は含まれていない。ＳｉＯ−Ｃ粉末とＬｉＭｇＰＯ４粉末との合計質量を１００質量％としたときの、ＳｉＯ−Ｃ粉末の配合量をｙとする。ｙが９５である場合を負極６，ｙ＝９０の場合を負極７，ｙ＝８０の場合を負極８，ｙ＝１００の場合を負極９とした。その他は、上記の負極と同様である。
<Reference Example 3: Examination of compounding amount of compound powder made of LiMgPO 4 >
Preparing a negative electrode material comprising a LiMgPO 4 powder of SiO-C powder and LiMgPO 4, to obtain a negative electrode to form the anode active material layer made of negative electrode material to the current collector surface. The negative electrode material does not contain Fe 2 O 3 powder. The blending amount of the SiO—C powder is defined as y when the total mass of the SiO—C powder and the LiMgPO 4 powder is 100% by mass. A negative electrode 6 was used when y was 95, a negative electrode 7 when y = 90, a negative electrode 8 when y = 80, and a negative electrode 9 when y = 100. Others are the same as that of said negative electrode.
図１９は、負極６，７のＳＥＭ像である。図１９に示すように、粒径５〜２０μｍ程度のＳｉＯ−Ｃ粒子の表面に粒径１μｍ程度のＬｉＭｇＰＯ４粒子が付着していることがわかる。 FIG. 19 is an SEM image of the negative electrodes 6 and 7. As shown in FIG. 19, it can be seen that LiMgPO 4 particles having a particle size of about 1 μm are adhered to the surface of SiO—C particles having a particle size of about 5 to 20 μm.
各負極６〜９を用いて半電池を作製した。各半電池について上記と同様の充放電サイクル試験を行い、各サイクル毎の充放電容量を測定した。測定結果を図２０に示した。 A half battery was prepared using each of the negative electrodes 6 to 9. Each half-cell was subjected to the same charge / discharge cycle test as described above, and the charge / discharge capacity for each cycle was measured. The measurement results are shown in FIG.
図２０に示すように、負極８のようにＬｉＭｇＰＯ４粉末の含有量が多くなるとサイクル特性が低下した。このことから、混合粉末中のＬｉＭｇＰＯ４粉末の含有量は５〜１０質量％の範囲が特に好ましいこともわかる。 As shown in FIG. 20, when the content of LiMgPO 4 powder increased as in the negative electrode 8, the cycle characteristics deteriorated. From this, it can be seen that the content of LiMgPO 4 powder in the mixed powder is particularly preferably in the range of 5 to 10% by mass.
本参考例では、負極材料がＳｉＯ−Ｃ粉末とＬｉＭｇＰＯ４粉末とからなる。負極材料に更にα−Ｆｅ２Ｏ３粉末が含まれている場合にも、同様の結果を得られると予想される。 In this reference example, the negative electrode material is composed of SiO—C powder and LiMgPO 4 powder. Similar results are expected to be obtained when the negative electrode material further contains α-Fe 2 O 3 powder.
１：ＳｉＯ−Ｃ粒子、２：コア部、２１：Ｓｉ相、２２：ＳｉＯ２相、３：被覆層、５：棒状のＦｅ２Ｏ３粒子、９：球状のＦｅ２Ｏ３粒子。 1: SiO—C particles, 2: core part, 21: Si phase, 22: SiO 2 phase, 3: coating layer, 5: rod-like Fe 2 O 3 particles, 9: spherical Fe 2 O 3 particles.
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