JP2007053084A - Method of manufacturing negative electrode for lithium secondary battery - Google Patents
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Abstract
Description
本発明は、リチウム二次電池に使用される負極の製造方法に関する。 The present invention relates to a method for producing a negative electrode used for a lithium secondary battery.
リチウムイオンの吸蔵放出により充電・放電を行うリチウム二次電池は、高容量、高電圧、高エネルギー密度といった特徴を兼ね備えていることから、ＯＡ機器、なかでも携帯電話、パソコン等の携帯情報機器の電源として非常に多く使用されている。このリチウム二次電池では、充電時に正極から負極へリチウムイオンが移行し、負極に吸蔵されたリチウムイオンが放電時に正極へ移行する。 Lithium secondary batteries that charge and discharge by occluding and releasing lithium ions have characteristics such as high capacity, high voltage, and high energy density. Therefore, they are used in portable information devices such as OA devices, especially mobile phones and personal computers. Very often used as a power source. In this lithium secondary battery, lithium ions migrate from the positive electrode to the negative electrode during charging, and lithium ions occluded in the negative electrode migrate to the positive electrode during discharging.
リチウム二次電池の負極を構成する負極活物質としては、炭素粉末が多用されている。これは後で詳しく述べるが、炭素負極の容量、初期効率及びサイクル特性といった各種特性の総合的な評価が高いためである。そして、この炭素粉末は、結着剤溶液と混合されてスラリー化され、そのスラリーを集電板の表面に塗布し乾燥後、加圧する粉末混練塗布乾燥法により負極シートとされる。ちなみに、正極を構成する正極活物質としては、リチウムを含有する遷移金属の酸化物、主にＬｉＣｏＯ2 などが使用されている。 Carbon powder is frequently used as a negative electrode active material constituting a negative electrode of a lithium secondary battery. As will be described in detail later, this is because the overall evaluation of various characteristics such as the capacity, initial efficiency, and cycle characteristics of the carbon negative electrode is high. The carbon powder is mixed with a binder solution to form a slurry, and the slurry is applied to the surface of a current collector plate, dried, and then pressed to form a negative electrode sheet by a powder kneading application drying method. Incidentally, as the positive electrode active material constituting the positive electrode, an oxide of a transition metal containing lithium, mainly LiCoO 2 is used.
現在多用されている炭素負極の問題点の一つは、他の負極に比べて理論容量が小さい点である。理論容量が小さいにもかかわらず、炭素負極が多用されているのは、初期効率、サイクル寿命といった容量以外の特性が高く、諸特性のバランスが良いためである。 One of the problems of the carbon negative electrode that is widely used now is that the theoretical capacity is smaller than that of other negative electrodes. Although the theoretical capacity is small, the carbon negative electrode is frequently used because the characteristics other than the capacity such as the initial efficiency and the cycle life are high and various characteristics are well balanced.
携帯情報機器用電源として多用されるリチウム二次電池に関しては、更なる容量増大が求められており、この観点から炭素粉末より容量が大きい負極活物質の開発が進められている。このような負極活物質の一つがＳｉＯであり、ＳｉＯの理論容量は炭素の数倍に達する。このＳｉＯ負極は、炭素負極と同様、ＳｉＯの微粉末を結着剤溶液と混合してスラリー化し、そのスラリーを集電板の表面に塗布積層し乾燥後、加圧する粉末混練塗布乾燥法により作製されている（特許文献１、２）。 With respect to lithium secondary batteries that are frequently used as power sources for portable information devices, further increase in capacity has been demanded, and from this viewpoint, development of a negative electrode active material having a capacity larger than that of carbon powder is in progress. One of such negative electrode active materials is SiO, and the theoretical capacity of SiO reaches several times that of carbon. This SiO negative electrode, like the carbon negative electrode, is made by mixing a fine powder of SiO with a binder solution to form a slurry, coating and laminating the slurry on the surface of the current collector plate, drying, and then pressurizing the powder. (Patent Documents 1 and 2).
ところが、このように理論容量が大きいＳｉＯ負極も未だ実用化には至っていない。その最大の理由はＳｉＯ負極の初期効率が極端に低いからである。 However, the SiO negative electrode having such a large theoretical capacity has not yet been put into practical use. The biggest reason is that the initial efficiency of the SiO negative electrode is extremely low.
初期効率とは、初期充電容量に対する初期放電容量の比率であり、重要な電池設計因子の一つである。これが低いということは、初期充電で負極に注入されたリチウムイオンが初期放電時に十分に放出されないということであり、この初期効率が低いと如何に理論容量が大きくとも、実用負極容量が大きくならず、実用化は困難である。 The initial efficiency is a ratio of the initial discharge capacity to the initial charge capacity, and is one of important battery design factors. The fact that this is low means that lithium ions injected into the negative electrode during the initial charge are not sufficiently released during the initial discharge, and if this initial efficiency is low, the practical negative electrode capacity does not increase no matter how large the theoretical capacity is. The practical application is difficult.
加えて、これまでのＳｉＯ負極はサイクル特性も十分でない。サイクル特性とは、充放電を繰り返したときの放電容量の、１サイクル目の放電容量に対する維持特性である。これが低いと、充放電の繰り返しに伴って放電容量が低下し、実用負極容量が減少するため、実用化は困難である。つまり、二次電池用負極に求められる性能は、理論容量が大きく、同時に初期効率及びサイクル特性が高くなければならないということである。これまでのＳｉＯ負極は、後者の２特性が良くないために実用化に至っていないのである。 In addition, conventional SiO negative electrodes do not have sufficient cycle characteristics. The cycle characteristics are maintenance characteristics with respect to the discharge capacity at the first cycle of the discharge capacity when charging / discharging is repeated. If this is low, the discharge capacity decreases with repeated charge and discharge, and the practical negative electrode capacity decreases, so that practical application is difficult. That is, the performance required for the secondary battery negative electrode is that the theoretical capacity must be large, and at the same time, the initial efficiency and cycle characteristics must be high. Conventional SiO negative electrodes have not been put into practical use because the latter two characteristics are not good.
更に、リチウム二次電池は更なる小型化が要求されているが、粉末混練塗布乾燥法によって作製されるＳｉＯ負極では、ＳｉＯ層が低密度の多孔質体となるため、小型化が難しいという問題もある。 Furthermore, lithium secondary batteries are required to be further miniaturized, but the SiO negative electrode produced by the powder kneading application drying method has a problem that it is difficult to miniaturize because the SiO layer becomes a low-density porous body. There is also.
ちなみに、前記の特許文献１、２では、ＳｉＯ負極のサイクル特性を向上させるためにＳｉＯ層形成前のＳｉＯ粉末に非酸化性雰囲気で熱処理が実施されている。 Incidentally, in the above Patent Documents 1 and 2, heat treatment is performed in a non-oxidizing atmosphere on the SiO powder before forming the SiO layer in order to improve the cycle characteristics of the SiO negative electrode.
このような状況下で本発明者らは、集電体の表面に真空蒸着によりＳｉＯの緻密層（薄膜）を形成することを企画した。その結果、このような薄膜型ＳｉＯ負極では、粉末混練塗布乾燥法で形成される従来の粉末型ＳｉＯ負極と比べて単位体積当たりの容量が増加するだけでなく、そのＳｉＯ層で問題になっていた初期効率の低さが飛躍的に改善され、合わせてサイクル特性も向上することが判明した。また、真空蒸着のなかではイオンプレーティング法によって形成された薄膜が特に高性能であることが判明した。 Under such circumstances, the present inventors planned to form a dense layer (thin film) of SiO on the surface of the current collector by vacuum deposition. As a result, in such a thin film type SiO negative electrode, not only the capacity per unit volume is increased as compared with the conventional powder type SiO negative electrode formed by the powder kneading application drying method, but also the SiO layer becomes a problem. It was also found that the initial efficiency was drastically improved and the cycle characteristics were also improved. Further, it has been found that a thin film formed by an ion plating method has a particularly high performance in vacuum deposition.
このような知見を基礎として、本発明者は先に、集電体の表面にＳｉＯ、厳密にはＳｉＯｘ（０．５≦ｘ＜１．２）の緻密な薄膜を形成した薄膜型ＳｉＯ負極を特許出願し（特許文献３）、同時にその薄膜型ＳｉＯ負極の更なる改良に取り組んだ。その結果、以下の事実が判明した。 Based on such knowledge, the present inventor first formed a thin film type SiO negative electrode in which a dense thin film of SiO, strictly speaking, SiOx (0.5 ≦ x <1.2) was formed on the surface of the current collector. A patent application was filed (Patent Document 3), and at the same time, further improvement of the thin film type SiO negative electrode was tackled. As a result, the following facts were found.
ＳｉＯ薄膜の理論容量が大きいとはいえ、その膜厚が大きいほど好ましいことは言うまでもない。なぜなら、膜厚の増大に比例して単位面積当たりの負極容量が増すからである。ところが、ＳｉＯ薄膜の膜厚が５μｍ以上、とりわけ１０μｍ以上になると、サイクル特性が悪化するようになる。これは充放電の繰り返しに伴ってＳｉＯ薄膜の剥離が進行するためと考えられる。そこでＳｉＯ薄膜が形成される集電体の表面を粗くしてみた。その結果、サイクル特性は改善された。しかし、その一方で、ＳｉＯ負極に特徴的な初期効率が顕著に低下した。このため、ＳｉＯの膜厚を大きくしても、実用負極容量は期待するほどには増加しないのである。 Although the theoretical capacity of the SiO thin film is large, it goes without saying that the larger the film thickness, the better. This is because the negative electrode capacity per unit area increases in proportion to the increase in film thickness. However, when the thickness of the SiO thin film is 5 μm or more, especially 10 μm or more, the cycle characteristics are deteriorated. This is presumably because the peeling of the SiO thin film proceeds with repeated charge and discharge. Therefore, the surface of the current collector on which the SiO thin film was formed was roughened. As a result, the cycle characteristics were improved. However, on the other hand, the initial efficiency characteristic of the SiO negative electrode has been significantly reduced. For this reason, even if the film thickness of SiO is increased, the practical negative electrode capacity does not increase as expected.
なお、ＳｉＯ薄膜の形成では、ＳｉＯ粉末焼結体が真空蒸着ターゲット、スパッタリングターゲットとして多用される。スパッタリングターゲットに関しては、反応性スパッタリングにおける方法上の制約の解消を目的として、ボロン等をドープされたＳｉ粉末をＳｉＯ粉末に混合し、抵抗率を下げたＳｉ−ＳｉＯ混合粉末焼結体が、特許文献４により提案されている。 In forming the SiO thin film, a SiO powder sintered body is frequently used as a vacuum deposition target and a sputtering target. Regarding sputtering targets, Si-SiO mixed powder sintered body in which Si powder doped with boron or the like is mixed with SiO powder and the resistivity is lowered for the purpose of eliminating the limitations on the method in reactive sputtering is patented. Proposed by reference 4.
本発明の目的は、薄膜型ＳｉＯ負極で問題となる膜厚を比較的大きくし且つ集電体を粗面化してサイクル特性の低下を阻止した場合の初期効率の低下を改善でき、もって実用負極容量の大幅増大を可能にするリチウム二次電池用負極の製造方法を提供することにある。 The object of the present invention is to improve the deterioration of the initial efficiency when the thin film type SiO negative electrode has a relatively large film thickness, and the current collector is roughened to prevent the deterioration of the cycle characteristics. An object of the present invention is to provide a method for producing a negative electrode for a lithium secondary battery that enables a significant increase in capacity.
上記目的を達成するために、本発明者は、多方面からのアプローチによって初期効率を大きく向上させることが重要と考え、第１に熱処理に着目した。熱処理自体は、粉末型ＳｉＯ負極では既に公知である（特許文献１、２）。粉末型ＳｉＯ負極での熱処理では、前述したとおり、サイクル特性の向上を目的としてＳｉＯ層形成前の粉末原料に熱処理が行われる。しかし、薄膜型ＳｉＯ負極の場合は、成膜材料であるＳｉＯの析出体やその析出体から製造した焼結体に熱処理を行っても効果がない。 In order to achieve the above object, the present inventor considered that it is important to greatly improve the initial efficiency by various approaches, and focused first on heat treatment. The heat treatment itself is already known for powder-type SiO negative electrodes (Patent Documents 1 and 2). In the heat treatment in the powder-type SiO negative electrode, as described above, the heat treatment is performed on the powder raw material before the formation of the SiO layer for the purpose of improving the cycle characteristics. However, in the case of a thin-film type SiO negative electrode, there is no effect even if heat treatment is performed on a precipitate of SiO, which is a film forming material, or a sintered body manufactured from the precipitate.
このような状況下で、本発明者らは薄膜を形成した後の段階で、熱処理を実施することを試みた。その結果、この成膜後の熱処理により薄膜型ＳｉＯ負極における初期効率が向上することが判明した。 Under such circumstances, the present inventors tried to perform heat treatment at a stage after the thin film was formed. As a result, it was found that the initial efficiency in the thin-film SiO negative electrode is improved by the heat treatment after the film formation.
第２に、本発明者は成膜用ターゲットにも注目した。すなわち、リチウム二次電池用負極における酸素の存在は、酸素がリチウムイオンと結合し初期効率を低下させる原因になることから好ましくない。この観点から、本発明者は、薄膜型ＳｉＯ負極中の酸素量を低減する方法として、成膜用ターゲット中の酸素量を減少させることを企画し、実験検討を繰り返した。その結果、従来はスパッタリングの方法面から抵抗率の低さが注目されていたＳｉ−ＳｉＯ混合粉末焼結体が、真空蒸着による薄膜型ＳｉＯ負極における初期効率の向上に有効であることが判明した。 Secondly, the present inventor also paid attention to a film formation target. That is, the presence of oxygen in the negative electrode for a lithium secondary battery is not preferable because oxygen is combined with lithium ions and causes a reduction in initial efficiency. From this point of view, the present inventor planned to reduce the amount of oxygen in the film-forming target as a method for reducing the amount of oxygen in the thin-film SiO negative electrode, and repeated experimental studies. As a result, it has been found that the Si-SiO mixed powder sintered body, which has been attracting attention from the viewpoint of the sputtering method in the past, is effective in improving the initial efficiency in the thin film type SiO negative electrode by vacuum deposition. .
そして、Ｓｉ−ＳｉＯ混合粉末焼結体の使用と前述した成膜後の熱処理との組合せによれば、膜厚増大時に集電体を粗面化してサイクル特性の低下を阻止した場合の顕著な初期効率の低下が、極めて効果的に改善されることが明らかとっなった。 And, according to the combination of the use of the Si—SiO mixed powder sintered body and the heat treatment after film formation described above, the current collector is roughened when the film thickness is increased to prevent deterioration of cycle characteristics. It has been found that the reduction in initial efficiency is very effectively improved.
本発明のリチウム二次電池用負極の製造方法は、かかる知見を基礎としており、Ｓｉ粉末を５〜３０ｗｔ％含むＳｉ−ＳｉＯ混合粉末焼結ターゲットを用いて集電体の表面にＳｉＯｘ膜を形成した後に、非酸化性雰囲気中で６５０〜８５０℃の熱処理を行うものである。 The manufacturing method of a negative electrode for a lithium secondary battery of the present invention is based on such knowledge, and an SiOx film is formed on the surface of a current collector using a Si—SiO mixed powder sintered target containing 5 to 30 wt% of Si powder. Then, heat treatment at 650 to 850 ° C. is performed in a non-oxidizing atmosphere.
集電体の表面にＳｉＯｘ膜を形成するための成膜方法としては、真空蒸着、なかでもイオンプレーティング法が、ＳｉＯｘ膜中の酸素量を低減でき初期効率を向上させ得る点から好ましい。ＳｉＯｘ薄膜の厚みは、単位面積あたりの負極容量を増大させる点から５μｍ以上が好ましい。 As a film forming method for forming the SiOx film on the surface of the current collector, vacuum deposition, in particular, an ion plating method is preferable because the amount of oxygen in the SiOx film can be reduced and the initial efficiency can be improved. The thickness of the SiOx thin film is preferably 5 μm or more from the viewpoint of increasing the negative electrode capacity per unit area.
集電体の表面粗度は、ＳｉＯｘ薄膜の剥離、これによるサイクル特性の低下を阻止するために粗い方がよく、Ｒｚで５．０以上が好ましい。集電体の表面を粗くした場合に初期効率が低下するが、この初期効率の低下が、成膜用ターゲットとしてのＳｉ−ＳｉＯ混合粉末焼結体の使用と成膜後の熱処理との組合せで効果的に抑制されることは前述したとおりである。その詳しい理由は後で説明する。 The surface roughness of the current collector is preferably rough in order to prevent peeling of the SiOx thin film and the resulting deterioration in cycle characteristics, and Rz is preferably 5.0 or more. When the surface of the current collector is roughened, the initial efficiency is reduced. This decrease in initial efficiency is due to the combination of the use of a Si—SiO mixed powder sintered body as a film formation target and heat treatment after film formation. As described above, it is effectively suppressed. The detailed reason will be explained later.
本発明の方法で製造されるリチウム二次電池用負極は、薄膜型ＳｉＯ負極、厳密にはＳｉＯｘ負極（酸化珪素負極）である。以下の説明では、特にことわりのない限り、ＳｉＯ負極はＳｉＯｘ負極（酸化珪素負極）を意味する。薄膜型ＳｉＯ負極は粉末型ＳｉＯ負極に比して初期効率が高いが、その理由の一つは次のように考えられる。 The negative electrode for a lithium secondary battery produced by the method of the present invention is a thin film type SiO negative electrode, strictly speaking, a SiOx negative electrode (silicon oxide negative electrode). In the following description, the SiO negative electrode means a SiOx negative electrode (silicon oxide negative electrode) unless otherwise specified. The thin film type SiO negative electrode has higher initial efficiency than the powder type SiO negative electrode, and one of the reasons is considered as follows.
ＳｉＯ粉末は例えば次のようにして製造される。まずＳｉ粉末とＳｉＯ2 粉末の混合物を真空中で加熱することにより、ＳｉＯガスを発生させ、これを低温の析出部で析出させてＳｉＯ析出体を得る。この製法で得られるＳｉＯ析出体のＳｉに対するＯのモル比はほぼ１となる。このＳｉＯ析出体を粉砕してＳｉＯ粉末を得るのであるが、粉末にすると表面積が増大するために、粉砕時及び粉末の使用時などに大気中の酸素により酸化され、ＳｉＯ成形体のＳｉに対するＯのモル比は１を超えてしまう。加えて、ＳｉＯ粉末を粉末混練塗布乾燥法で積層する際にもＳｉＯ粉末の表面積の大きさ故に酸化が進んでしまう。こうしてＳｉＯの粉末混練塗布乾燥層ではＳｉに対するＯのモル比が高くなる。そして、粉末混練塗布乾燥層のＳｉＯ粉末のＳｉに対するＯのモル比が高いと、初期充電時に吸蔵されたリチウムイオンが放電時に放出されにくくなり、初期効率が低下することになる。 For example, the SiO powder is produced as follows. First, a mixture of Si powder and SiO 2 powder is heated in vacuum to generate SiO gas, which is precipitated at a low temperature precipitation portion to obtain a SiO precipitate. The molar ratio of O to Si in the SiO precipitate obtained by this manufacturing method is approximately 1. This SiO precipitate is pulverized to obtain SiO powder. However, since the surface area is increased when powdered, it is oxidized by oxygen in the atmosphere at the time of pulverization and use of the powder, and O in the SiO molded body with respect to Si. The molar ratio of exceeds 1. In addition, when the SiO powder is laminated by the powder kneading application drying method, oxidation proceeds due to the large surface area of the SiO powder. Thus, the molar ratio of O to Si is increased in the SiO 2 powder kneaded and dried layer. If the molar ratio of O to Si in the SiO powder of the powder kneaded coating / drying layer is high, lithium ions occluded during initial charging are difficult to be released during discharging, and initial efficiency decreases.
これに対して、真空蒸着法では、成膜を真空中で行うために酸素モル比の増加が抑制され、結果、初期効率の低下が抑えられる。加えて、真空蒸着法で形成される薄膜は緻密である。一方、粉末混練塗布乾燥層は粉末が押し固められただけの粉末集合体に過ぎず、ＳｉＯの充填率が低い。初期充電容量は負極活物質層の単位体積あたりの充電量であるために、緻密な薄膜の方が初期充電容量が高くなり、２サイクル目以降も充電容量が高くなる。 On the other hand, in the vacuum evaporation method, since the film formation is performed in a vacuum, an increase in the oxygen molar ratio is suppressed, and as a result, a decrease in initial efficiency is suppressed. In addition, the thin film formed by the vacuum deposition method is dense. On the other hand, the powder kneading application drying layer is only a powder aggregate in which the powder is pressed and compacted, and the filling rate of SiO is low. Since the initial charge capacity is the charge amount per unit volume of the negative electrode active material layer, the dense thin film has a higher initial charge capacity, and the charge capacity is higher after the second cycle.
真空蒸着法のなかでもイオンプレーティング法によって形成された薄膜が特に高性能になる理由については、Ｓｉに対するＯのモル比が１：１の酸化珪素膜を形成しようとする場合でも、その酸化珪素膜中の酸素が低下する傾向が見られることが影響していると考えられる。即ち、酸化珪素膜中の酸素はリチウムイオンとの結合性が強いために出来るだけ少ない方が望ましいところ、イオンプレーティング法を用いることにより、酸化珪素膜のＳｉに対するＯのモル比が最大で０．５程度まで低下するのである。ちなみにイオンプレーティング法で酸素モル比が低下する理由は現状では不明である。 The reason why the thin film formed by the ion plating method has a particularly high performance among the vacuum deposition methods is that even if it is intended to form a silicon oxide film having a molar ratio of O to Si of 1: 1, the silicon oxide This is considered to be due to the fact that the oxygen in the film tends to decrease. That is, it is desirable that the oxygen in the silicon oxide film is as small as possible because of its strong bonding with lithium ions. By using the ion plating method, the maximum molar ratio of O to Si in the silicon oxide film is 0. It decreases to about .5. Incidentally, the reason why the oxygen molar ratio is lowered by the ion plating method is unknown at present.
また、粉末型ＳｉＯ負極に比して薄膜型ＳｉＯ負極のサイクル特性が優れる理由は次のように考えられる。粉末型ＳｉＯ負極の場合、電気伝導性を確保するための導電助材等が粉末中に混合される。このため、リチウムイオンの吸蔵・放出時の体積膨張及び収縮により、活物質層であるＳｉＯ粉末層で微粉化が進み、且つ粉体の分離が進む。このため、集電性がしばしば悪化する。これに対し、薄膜型ＳｉＯ負極の場合は、活物質層であるＳｉＯ薄膜の集電体に対する密着性が本質的に良好であり、しかも、ＳｉＯ薄膜での活物質間の密着性も良好となる。これが薄膜型ＳｉＯ負極でサイクル特性が優れる理由と考えられる。 Moreover, the reason why the cycle characteristics of the thin film type SiO negative electrode are superior to the powder type SiO negative electrode is considered as follows. In the case of a powder-type SiO negative electrode, a conductive aid for ensuring electric conductivity is mixed in the powder. For this reason, due to volume expansion and contraction at the time of occlusion / release of lithium ions, pulverization proceeds in the SiO powder layer as the active material layer, and separation of the powder proceeds. For this reason, current collection often deteriorates. On the other hand, in the case of the thin film type SiO negative electrode, the adhesion of the SiO thin film as the active material layer to the current collector is essentially good, and the adhesion between the active materials in the SiO thin film is also good. . This is considered to be the reason why the cycle characteristics are excellent in the thin film type SiO negative electrode.
薄膜型ＳｉＯ負極で膜厚を大きくした場合にサイクル特性が悪化する理由は、膜厚の増大にしたがって集電体からの剥離が進むためと考えられる。実際、集電体の表面を粗くすると、サイクル特性の悪化が回避される。これは、集電体の表面の粗面化により、その表面に対するＳｉＯ薄膜の密着性が向上したためと考えられる。 The reason why the cycle characteristics deteriorate when the film thickness is increased in the thin film type SiO negative electrode is considered to be that the peeling from the current collector proceeds as the film thickness increases. In fact, when the surface of the current collector is roughened, deterioration of the cycle characteristics is avoided. This is presumably because the adhesion of the SiO thin film to the surface was improved by roughening the surface of the current collector.
一方、集電体の表面を粗くすることにより初期効率が低下するが、その理由は次のように考えられる。リチウム二次電池の初期効率の低下原因、特にＳｉＯ負極における低下原因としては、前述したＳｉＯ中の酸素量の増加がある。ＳｉＯ中の酸素はリチウムイオンとの結合性が強く、これを固定してしまうために、出来るだけ少ない方が良いのである。すなわち、ＳｉＯのＯ−Ｌｉ結合である。今一つの原因としては、ＳｉＯ薄膜の表面での不動態膜の生成量の増大がある。詳しく説明すると、これは電解液とＬｉが反応して負極活物質の表面上に膜を形成する現象でありＬｉロスの一因となるのである。 On the other hand, the initial efficiency is lowered by roughening the surface of the current collector. The reason is considered as follows. The cause of the decrease in the initial efficiency of the lithium secondary battery, particularly the decrease in the SiO negative electrode, is the aforementioned increase in the amount of oxygen in SiO. Since oxygen in SiO has a strong binding property to lithium ions and fixes it, it is better to have as little as possible. That is, the O—Li bond of SiO. Another cause is an increase in the amount of passivated film formed on the surface of the SiO thin film. More specifically, this is a phenomenon in which the electrolytic solution and Li react to form a film on the surface of the negative electrode active material, which contributes to Li loss.
集電体の表面を粗くすることによる初期効率の低下は主に後者が理由であると考えられる。これは集電体の表面を粗くする以上、避けることができない問題である。そして、薄膜型ＳｉＯ負極に熱処理を行うと初期効率が改善されるが、その理由は、後者の理由が避け得ないものである以上、前者の理由と考えられる。すなわち、熱処理によりＳｉＯ薄膜中のＳｉＯの一部がＳｉとＳｉＯ2 とに分解され、ＯがＳｉＯ2 の形で固定されることによりＯ−Ｌｉ結合が緩和され、充放電に寄与するＬｉ量が増加するものと考えられる。実際、本発明者らが行った各種の実験結果はこれを裏付けている。 The decrease in the initial efficiency due to the rough surface of the current collector is mainly due to the latter. This is a problem that cannot be avoided as long as the surface of the current collector is roughened. When the heat treatment is performed on the thin-film SiO negative electrode, the initial efficiency is improved. The reason is considered to be the former reason because the latter reason is unavoidable. That is, part of SiO in the SiO thin film is decomposed into Si and SiO 2 by the heat treatment, and O is fixed in the form of SiO 2 , so that the O—Li bond is relaxed, and the amount of Li contributing to charge and discharge is reduced. It is thought to increase. In fact, the results of various experiments conducted by the present inventors support this.
そして、薄膜型ＳｉＯ負極における熱処理は成膜後に行うことが重要である。すなわち、粉末型ＳｉＯ負極では、ＳｉＯ層形成前の粉末段階或いはそれより前の段階で熱処理が行われる。これと同じように成膜材料に熱処理を行うと、成膜の段階で性状が変化するために、期待する熱処理効果が得られない。これが薄膜型ＳｉＯ負極の製造工程で行う熱処理と粉末型ＳｉＯ負極の製造工程で行う熱処理とが決定的に相違する点である。ちなみに、粉末型ＳｉＯ負極の製造工程では、ＳｉＯ層の形成に加熱が伴わないため、粉末材料に行った熱処理の効果がそのままＳｉＯ層に引き継がれる。 And it is important to perform the heat treatment in the thin film type SiO negative electrode after the film formation. That is, in the powder-type SiO negative electrode, the heat treatment is performed in the powder stage before the SiO layer formation or in the stage before that. When heat treatment is performed on the film-forming material in the same manner, the expected heat treatment effect cannot be obtained because the properties change at the stage of film formation. This is a critical difference between the heat treatment performed in the thin film type SiO negative electrode manufacturing process and the heat treatment performed in the powder type SiO negative electrode manufacturing process. Incidentally, in the manufacturing process of the powder-type SiO negative electrode, since the heating is not accompanied with the formation of the SiO layer, the effect of the heat treatment performed on the powder material is directly inherited by the SiO layer.
これに加えて、薄膜型ＳｉＯ負極では、ＳｉＯ層が薄膜とはいえその膜厚は数μｍから最大でも数十μｍである。一方、粉末型ＳｉＯ負極では、粉末材料の一次粒径が数十μｍ或いはそれ以上である。このため、薄膜形成後の熱処理の方がその効果がバルク内部まで進みやすく、熱処理効果が大きい。 In addition to this, in the thin film type SiO negative electrode, although the SiO layer is a thin film, the film thickness is several μm to several tens of μm at the maximum. On the other hand, in the powder-type SiO negative electrode, the primary particle size of the powder material is several tens of μm or more. For this reason, the effect of the heat treatment after the thin film formation is more likely to proceed to the inside of the bulk, and the heat treatment effect is greater.
しかも、粉末材料の場合は、活物質の性状が初期効率やサイクル特性の全てを決めるのではなく、他の多くの要因があり、熱処理の効果が直接的に発現しにくい。その点、薄膜は組成が単純で熱処理の効果が直接的かつ効果的に発現する傾向が強い。更に、粉末の場合は表面積が大きく、熱処理の後にその効果を相殺する酸化が進みやすいが、薄膜の場合はこの問題が生じにくいという利点もある。 Moreover, in the case of a powder material, the properties of the active material do not determine all of the initial efficiency and cycle characteristics, but there are many other factors, and the effect of the heat treatment is difficult to directly express. In that respect, the thin film has a simple composition and has a strong tendency to directly and effectively express the effect of the heat treatment. Further, in the case of powder, the surface area is large, and oxidation that offsets the effect is likely to proceed after heat treatment. However, in the case of a thin film, there is also an advantage that this problem hardly occurs.
これらの点において、薄膜型ＳｉＯ負極における成膜後の熱処理は、粉末型ＳｉＯ負極における粉末材料に対する熱処理と異なり、より効果的である。なお、薄膜型ＳｉＯ負極における成膜後の熱処理では、膜厚が１μｍ程度と薄い場合でも、熱処理の効果はある程度は得られる。但し、膜厚が小さい場合は、もともとサイクル特性や初期効率の悪化が小さいために熱処理の効果は小さい。また、負極となるＳｉＯｘの膜厚が小さすぎるため、負極体積あたりの充電容量（例えば１００サイクル目の充電容量）は大きいが、電池体積あたりの充電容量は小さくなる。つまり電池の小型化が難しくなる。このためＳｉＯの膜厚は５μｍ以上が望ましいのである。 In these respects, the heat treatment after film formation in the thin film type SiO negative electrode is more effective, unlike the heat treatment for the powder material in the powder type SiO negative electrode. In the heat treatment after film formation in the thin film type SiO negative electrode, even if the film thickness is as thin as about 1 μm, the effect of the heat treatment can be obtained to some extent. However, when the film thickness is small, the effect of the heat treatment is small because the deterioration of the cycle characteristics and the initial efficiency is originally small. Moreover, since the film thickness of SiOx used as a negative electrode is too small, the charge capacity per negative electrode volume (for example, the charge capacity at the 100th cycle) is large, but the charge capacity per battery volume is small. That is, it is difficult to reduce the size of the battery. For this reason, the film thickness of SiO is desirably 5 μm or more.
熱処理の温度は６５０〜８５０℃とする。この温度が６５０℃未満の場合は、Ｏ−Ｌｉ結合の緩和による初期効率の向上が不十分である。具体的には、１サイクル目の初期充電容量は大きいが、初期放電容量は小さく、結果として初期効率の向上が不十分である。８５０℃を超える場合は充電容量が低下するが、これはＳｉＯ薄膜中に生成するＳｉ粒子が高温の熱処理によって過度に成長することが原因と考えられる。 The temperature of heat processing shall be 650-850 degreeC. When this temperature is less than 650 ° C., the initial efficiency is not sufficiently improved by relaxing the O—Li bond. Specifically, the initial charge capacity in the first cycle is large, but the initial discharge capacity is small, and as a result, the improvement of the initial efficiency is insufficient. When the temperature exceeds 850 ° C., the charge capacity decreases, but this is considered to be caused by excessive growth of Si particles generated in the SiO thin film by high-temperature heat treatment.
熱処理雰囲気は非酸化性であり、具体的には真空度が１０-2ｔｏｒｒ以下の減圧雰囲気又は不活性ガス雰囲気である。 The heat treatment atmosphere is non-oxidizing, and specifically, a reduced pressure atmosphere or an inert gas atmosphere with a vacuum degree of 10 −2 torr or less.
また熱処理時間は０．５〜５時間が好ましく、１〜３時間が特に好ましい。熱処理時間が短すぎる場合は熱処理効果、すなわちＯ−Ｌｉ結合の緩和による初期効率の向上効果が不十分になるおそれがある。逆に熱処理時間を極端に長くしても熱処理効果が向上し続けるわけではなく、経済性の悪化が問題になる。 The heat treatment time is preferably 0.5 to 5 hours, particularly preferably 1 to 3 hours. When the heat treatment time is too short, the heat treatment effect, that is, the effect of improving the initial efficiency due to relaxation of the O—Li bond may be insufficient. On the other hand, even if the heat treatment time is extremely long, the heat treatment effect does not continue to improve, and deterioration in economic efficiency becomes a problem.
ＳｉＯ薄膜の厚みは、単位面積当たりの負極容量を増大させる点から５μｍ以上が好ましく、１０μｍ以上が特に好ましい。膜厚の上限については５０μｍ以下が好ましく、３０μｍ以下が特に好ましい。なぜなら、膜厚が大きすぎると、膜状の電池を積層構造にするために巻いたり曲げたりする際に、ＳｉＯ薄膜がひび割れしたり剥離したりする可能性が高まるからである。 The thickness of the SiO thin film is preferably 5 μm or more, particularly preferably 10 μm or more, from the viewpoint of increasing the negative electrode capacity per unit area. The upper limit of the film thickness is preferably 50 μm or less, particularly preferably 30 μm or less. This is because if the film thickness is too large, the possibility of the SiO thin film cracking or peeling when the film-shaped battery is rolled or bent to form a laminated structure increases.
集電体の表面粗度は、ＳｉＯ薄膜の剥離、これによるサイクル特性の低下を阻止するために粗い方がよく、最大高さ粗さＲｚで５．０以上が好ましい。この粗度の上限については、電池特性の点からは特にないが、電池全体の体積を小さくするためには集電体として使用されるＣｕなどの薄板も薄くする必要がある。そのためＲｚが大きすぎると、集電体の機械的強度が低下するおそれがある。この観点からＲｚ≦１０が望ましい。 The surface roughness of the current collector is preferably rough in order to prevent the peeling of the SiO thin film and the resulting deterioration in cycle characteristics, and the maximum height roughness Rz is preferably 5.0 or more. The upper limit of the roughness is not particularly limited from the viewpoint of battery characteristics, but in order to reduce the volume of the entire battery, it is necessary to reduce the thickness of a thin plate such as Cu used as a current collector. Therefore, if Rz is too large, the mechanical strength of the current collector may be reduced. From this viewpoint, Rz ≦ 10 is desirable.
集電体の表面にＳｉＯ薄膜を形成する方法としては真空蒸着が好ましく、イオンプレーティング法が特に好ましい。その理由は前述したとおりである。なおスパッタリングは、膜の特性としては真空蒸着法と同等であるが、成膜速度が遅く、生産性の面で問題が残る。 As a method of forming the SiO thin film on the surface of the current collector, vacuum deposition is preferable, and an ion plating method is particularly preferable. The reason is as described above. Sputtering has the same film characteristics as the vacuum vapor deposition method, but the film forming speed is slow and there remains a problem in terms of productivity.
成膜後の熱処理と並んで重要な点は、成膜用ターゲットとして、Ｓｉ粉末を５〜３０ｗｔ％含むＳｉ−ＳｉＯ混合粉末焼結ターゲットを使用する点である。すなわち、粉末焼結ターゲットを製造する際に、Ｓｉ粉末を５〜３０ｗｔ％含み、残りが実質的にＳｉＯ粉末である混合粉末から粉末焼結により製造したターゲットを使用して、集電体の表面にＳｉＯ薄膜、厳密にはＳｉＯｘ薄膜を形成するのである。そうすることにより、ＳｉＯ膜中のＳｉに対するＯのモル比が更に下がり、最大で０．５、更にはこれ以下まで低下することにより、初期効率が向上する。なお「実質的にＳｉＯ粉末」とは、Ｓｉ及びＳｉＯ以外の粉末、例えばＳｉＯ2 粉末などが微量（好ましくは後述する膜中のＯモル比が１以上とならない範囲内で）混じってもよいということである。 An important point along with the heat treatment after film formation is that a Si—SiO mixed powder sintered target containing 5 to 30 wt% of Si powder is used as a film formation target. That is, when producing a powder sintered target, the surface of the current collector is obtained by using a target produced by powder sintering from a mixed powder containing 5 to 30 wt% of Si powder and the remainder being substantially SiO powder. In addition, a SiO thin film, strictly speaking, a SiO x thin film is formed. By doing so, the molar ratio of O to Si in the SiO film further decreases, and the initial efficiency is improved by decreasing to a maximum of 0.5 and further below this. Note that “substantially SiO powder” means that a small amount of powder other than Si and SiO, such as SiO 2 powder (preferably within a range where the O molar ratio in the film described later does not exceed 1) may be mixed. That is.
ＳｉＯ−Ｓｉ混合粉末焼結ターゲットにおけるＳｉ粉末量が５ｗｔ％未満の場合は、ＳｉＯ膜中のＳｉに対するＯのモル比を低下させる効果が不十分であり、初期効率が十分に向上しない。このＳｉ粉末量が３０ｗｔ％を超えると、初期効率は向上するが、サイクル特性が低下する。サイクル特性が低下するのは、Ｓｉ成分が多くなり過ぎると、充放電時における体積の膨張、収縮が大きくなり、充放電を繰り返すことによる破壊が問題になるからである。粉末粒径についてはＳｉ粉末、ＳｉＯ粉末ともに４５μｍ以下が好ましく、２０μｍ以下が特に好ましい。なぜなら、粉末粒径が大きすぎると、焼結前の原料の均一性が低下するからである。 When the amount of Si powder in the SiO—Si mixed powder sintered target is less than 5 wt%, the effect of reducing the molar ratio of O to Si in the SiO film is insufficient, and the initial efficiency is not sufficiently improved. When the amount of Si powder exceeds 30 wt%, the initial efficiency is improved, but the cycle characteristics are deteriorated. The reason why the cycle characteristics are deteriorated is that when the Si component is excessively large, the expansion and contraction of the volume at the time of charging / discharging become large, and destruction due to repeated charging / discharging becomes a problem. The powder particle diameter is preferably 45 μm or less, particularly preferably 20 μm or less for both the Si powder and the SiO powder. This is because if the powder particle size is too large, the uniformity of the raw material before sintering is reduced.
抵抗率を下げるための、ＳｉにおけるＢドープなどは必ずしも必要でないが、成膜方法の面から低抵抗率が必要な場合に、これを行うことを妨げるものではない。 B doping or the like in Si for lowering the resistivity is not always necessary, but this does not prevent this from being performed when a low resistivity is necessary from the viewpoint of the film formation method.
薄膜を構成するＳｉＯのＳｉに対するＯのモル比は、初期効率を向上させる点からは小さいほどよく、熱処理後の段階で１未満が好ましく、０．５以下も可能である。すなわちＳｉＯｘで表して（ｘ＜１）が好ましく、（ｘ≦０．５）も可能である。ただし、このモル比が極端に小さい場合は、リチウムイオン吸蔵時の体積膨脹が顕著になり、負極活物質層が破壊するおそれがある。 The molar ratio of SiO to Si constituting the thin film is preferably as small as possible from the viewpoint of improving the initial efficiency, and is preferably less than 1 at the stage after the heat treatment, and may be 0.5 or less. That is, (x <1) is preferable expressed by SiOx, and (x ≦ 0.5) is also possible. However, when this molar ratio is extremely small, volume expansion at the time of occlusion of lithium ions becomes significant, and the negative electrode active material layer may be destroyed.
集電体としては金属薄板が好適である。その金属としてはＣｕ、Ｎｉなどを用いることができる。板厚は１〜５０μｍが好ましい。これが薄すぎると製造が難しくなり、機械的強度の低下も問題になる。一方、厚すぎる場合は負極の小型化が阻害される。 A thin metal plate is suitable as the current collector. As the metal, Cu, Ni or the like can be used. The plate thickness is preferably 1 to 50 μm. If it is too thin, it is difficult to produce, and a decrease in mechanical strength becomes a problem. On the other hand, when it is too thick, the negative electrode is prevented from being downsized.
正極は、集電体の表面に正極活物質層を形成した構造である。正極活物質としては、ＬｉＣｏＯ2 、ＬｉＮｉＯ2 、ＬｉＭｎ2 Ｏ4 などのリチウムを含有する遷移金属の酸化物が主に使用される。正極の作製法としては、酸化物の微粉末を結着剤溶液と混合してスラリー化し、そのスラリーを集電板の表面に塗布し乾燥後、加圧する粉末混練塗布乾燥法が一般的であるが、負極と同様の成膜により形成することもできる。 The positive electrode has a structure in which a positive electrode active material layer is formed on the surface of a current collector. As the positive electrode active material, an oxide of a transition metal containing lithium such as LiCoO 2 , LiNiO 2 , LiMn 2 O 4 is mainly used. As a method for producing the positive electrode, a powder kneading application drying method is generally used in which fine oxide powder is mixed with a binder solution to form a slurry, and the slurry is applied to the surface of the current collector plate, dried, and then pressed. However, it can also be formed by film formation similar to the negative electrode.
電解液としては、例えばエチレンカーボネートを含有する非水電解質などを使用することができる。 As the electrolytic solution, for example, a non-aqueous electrolyte containing ethylene carbonate can be used.
本発明のリチウム二次電池用負極の製造方法は、Ｓｉ粉末を５〜３０ｗｔ％含むＳｉＯ−Ｓｉ混合粉末焼結ターゲットを用いて集電体の表面にＳｉＯｘ膜を形成した後に、非酸化性雰囲気中で６５０〜８５０℃の熱処理を行うことにより、ＳｉＯを負極に用いたリチウム二次電池に特徴的な初期充電容量の大きさを阻害することなく、その欠点である初期効率の低さを大幅に改善できる。また、薄膜型ＳｉＯｘ負極で問題となる膜厚を比較的大きくし且つ集電体を粗面化してサイクル特性の低下を阻止した場合の初期効率の低下を改善できる。これにより、初期容量、初期効率及びサイクル特性を高い次元で並立させることができ、実用負極容量の大幅増大を可能にする。 In the method for producing a negative electrode for a lithium secondary battery according to the present invention, a SiOx film is formed on the surface of a current collector using a SiO—Si mixed powder sintered target containing 5 to 30 wt% of Si powder, and then a non-oxidizing atmosphere. By performing heat treatment at 650 to 850 ° C., the initial charge capacity, which is characteristic of a lithium secondary battery using SiO as a negative electrode, is not hindered, and the low initial efficiency, which is a drawback thereof, is greatly reduced. Can be improved. In addition, it is possible to improve the decrease in the initial efficiency when the film thickness that is a problem in the thin film type SiOx negative electrode is relatively large and the current collector is roughened to prevent the deterioration of the cycle characteristics. Thereby, the initial capacity, the initial efficiency, and the cycle characteristics can be arranged side by side at a high level, and the practical negative electrode capacity can be greatly increased.
以下に本発明の実施形態を図面に基づいて説明する。図１は本発明の方法で製造した負極の使用例を示すリチウム二次電池の縦断面図である。 Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a longitudinal sectional view of a lithium secondary battery showing an example of use of a negative electrode produced by the method of the present invention.
図１に示すリチウム二次電池は所謂ボタン電池であり、正極面を形成する円形の偏平なケース１０を備えている。ケース１０は金属からなり、その内部には、円盤状の正極２０及び負極３０が下から順に重ねられて収容されている。正極２０は、円形の金属薄板からなる集電体２１と、その表面に形成された正極活物質層２２とからなる。同様に、負極３０は円形の金属薄板（例えば銅箔）からなる集電体３１と、その表面に形成された負極活物質層３２とからなる。そして両極は、それぞれの活物質層を対向させ、対向面間にセパレータ４０を挟んだ状態で積層されて、ケース１０内に収容されている。
The lithium secondary battery shown in FIG. 1 is a so-called button battery, and includes a circular
ケース１０内には又、正極２０及び負極３０と共に電解液が収容されている。そして、シール部材５０を介してケース１０の開口部をカバー６０で密閉することにより、収容物がケース１０内に封入されている。カバー６０は負極面を形成する部材を兼ねており、負極３０の集電体３１に接触している。正極面を形成する部材を兼ねるケース１０は、正極２０の集電体２１と接触している。
The
このリチウム二次電池において注目すべき点は、負極３０における負極活物質層３２が、集電体３１上にＳｉ−ＳｉＯの混合粉末焼結ターゲットを用いて、真空蒸着、好ましくはイオンプレーティングにより形成され、且つ非酸化性雰囲気中で熱処理を受けたＳｉＯｘの緻密な薄膜からなる点である。この薄膜を構成するＳｉＯｘのＳｉに対するＯのモル比は１未満と小さく、０．５以下も可能である。また薄膜の厚みは５μｍ以上と大きく、集電体３１の表面粗度はＲｚで５．０以上と粗い。
What should be noted in this lithium secondary battery is that the negative electrode
一方、正極２０における正極活物質層２２は、従来どおり、ＬｉＣｏＯ2 などのリチウムを含有する遷移金属の酸化物の粉末を、結着剤溶液と混合してスラリー化し、そのスラリーを集電板２１の表面に塗布し乾燥後、加圧する粉末混練塗布乾燥法により形成されている。
On the other hand, the positive electrode
このリチウム二次電池は、その負極３０の構造及び製造方法に関連して以下の特徴を有する。
This lithium secondary battery has the following characteristics in relation to the structure of the
第１に、負極活物質層３２がＳｉＯｘからなるため、炭素粉末層と比べて理論容量が格段に大きい。第２に、そのＳｉＯｘが真空蒸着等にて形成された薄膜であり、Ｓｉに対するＯのモル比が低い上に緻密であるため、初期充電容量を低減させずに初期効率を高くできる。第３に、薄膜の単位体積当たりの容量が大きく、且つ５μｍ以上と厚みが大きいため、負極容量が大である。第４に、集電体３１の表面粗度がＲｚで５．０以上と粗いため、薄膜の厚みが大であるにもかかわらず、集電体３１の表面に対する密着性が良好であり、薄膜の厚みを大きくしたときに問題となるサイクル特性の低下が阻止される。第５に、Ｓｉ−ＳｉＯ混合粉末焼結ターゲットの使用、及び薄膜に対する熱処理により、集電体３１の表面を粗くしたことによる初期効率の低下が効果的に回避される。
First, since the negative electrode
かくして、このリチウム二次電池では、初期容量、初期効率及びサイクル特性が高い次元で並立し、実用負極容量の大幅増大が実現される。 Thus, in this lithium secondary battery, the initial capacity, the initial efficiency, and the cycle characteristics are arranged side by side in a high dimension, and the practical negative electrode capacity is greatly increased.
次に、このリチウム二次電池における負極の構造及び製造方法が電池性能に及ぼす影響を調査した結果を示す。 Next, the results of investigating the influence of the structure and manufacturing method of the negative electrode on the battery performance in this lithium secondary battery are shown.
前記負極を製造するにあたり、先ず集電体として電解銅箔（厚み：１０μｍ、表面粗度：Ｒｚ＝７．０）を用意した。また、成膜用ターゲットとして、Ｓｉ粉末量を種々変更したＳｉ−ＳｉＯ混合粉末焼結ターゲットを作製すると共に、比較のためにＳｉＯ粉末焼結ターゲットを作製した。これらの作製に使用したＳｉ粉末は粒径が４５μｍ以下の破砕粉、ＳｉＯ粉末も粒径が４５μｍ以下の破砕粉であり、焼結条件は真空雰囲気中での１３７５℃加熱とした。作製されたターゲットは直径２０ｍｍ、厚み１０ｍｍの円盤である。 In manufacturing the negative electrode, first, an electrolytic copper foil (thickness: 10 μm, surface roughness: Rz = 7.0) was prepared as a current collector. Moreover, while preparing the Si-SiO mixed-powder sintered target which changed various amounts of Si powder as a film-forming target, the SiO powder sintered target was produced for the comparison. The Si powder used in these productions was crushed powder having a particle size of 45 μm or less, and the SiO powder was also crushed powder having a particle size of 45 μm or less. The sintering condition was heating at 1375 ° C. in a vacuum atmosphere. The produced target is a disk having a diameter of 20 mm and a thickness of 10 mm.
各種ターゲットが作製されると、用意した集電体の表面に、各種ターゲットを使用してイオンプレーティング法により５μｍの厚みに成膜を行った。イオンプレーティングにおける加熱源としてはＥＢガンを用い、成膜雰囲気は真空雰囲気中〔１０-3Ｐａ（１０-5ｔｏｒｒ）〕とした。そして成膜後に真空雰囲気中（１×１０-3Ｔｏｒｒ）で熱処理を実施した。熱処理温度は７５０℃、熱処理時間は２時間とした。なお、本明細書における膜厚は、平坦面に成膜を行ったと仮定したときの付着重量から算出した値である。 When various targets were produced, a film was formed on the surface of the prepared current collector to a thickness of 5 μm by ion plating using various targets. An EB gun was used as a heating source in ion plating, and the film formation atmosphere was a vacuum atmosphere [10 −3 Pa (10 −5 torr)]. And it was carried to a heat treatment in a vacuum atmosphere after the film formation (1 × 10 -3 Torr). The heat treatment temperature was 750 ° C. and the heat treatment time was 2 hours. In addition, the film thickness in this specification is a value calculated from the adhesion weight when it is assumed that the film is formed on a flat surface.
こうして製造された薄膜型ＳｉＯｘ負極を正極と組み合わせ、電解液と共にケース内に封入して前記リチウム二次電池（サイズ直径１５ｍｍ、厚さ３ｍｍ）を完成させた。正極にはＬｉＣｏＯ2 の微粉末を用い、電解液にはエチレンカーボネートを含有する非水電解質を用いた。 The thin film type SiOx negative electrode manufactured in this way was combined with the positive electrode and sealed in a case together with the electrolytic solution to complete the lithium secondary battery (size diameter 15 mm, thickness 3 mm). A fine powder of LiCoO 2 was used for the positive electrode, and a nonaqueous electrolyte containing ethylene carbonate was used for the electrolyte.
完成した二次電池の性能として初期効率、サイクル特性及び急速充放電特性を調査した。比較のために、成膜後の熱処理を省略した薄膜型ＳｉＯｘ負極を用いた二次電池についても同じ調査を行った。調査結果を表１に示す。 The initial efficiency, cycle characteristics, and rapid charge / discharge characteristics were investigated as the performance of the completed secondary battery. For comparison, the same investigation was performed on a secondary battery using a thin film type SiOx negative electrode in which the heat treatment after film formation was omitted. The survey results are shown in Table 1.
表１中のサイクル特性は１回目の放電量に対する１０回目の放電量の比率（容量維持率）により評価した。また評価値Ａは（初期効率）×（サイクル特性）2 により算出し、総合評価はＡ＜０．６の場合を不可、０．６≦Ａ＜０．７５の場合を可、０．７５≦Ａの場合を良とした。評価値Ａの算出でサイクル特性を２乗した理由は、サイクル特性の方が電池性能としての重要度が高いために、初期効率よりもサイクル特性に重きをおいた評価とすることにある。 The cycle characteristics in Table 1 were evaluated by the ratio of the 10th discharge amount (capacity maintenance ratio) to the first discharge amount. The evaluation value A is calculated by (initial efficiency) × (cycle characteristics) 2 , and comprehensive evaluation is not possible when A <0.6, 0.6 ≦ A <0.75 is acceptable, 0.75 ≦ Case A was considered good. The reason why the cycle characteristic is squared in the calculation of the evaluation value A is that, since the cycle characteristic is more important as battery performance, the evaluation emphasizes the cycle characteristic more than the initial efficiency.
またリチウム二次電池は、近年、小容量の携帯情報機器用電源だけでなく、ハイブリッド自動車用電池、エレベーター用電源などの動力用電源としても期待されている。動力用電源の場合、前述した負極容量や初期効率、サイクル特性等の諸特性に加えて、高速充放電特性に優れることが求められる。すなわち、リチウム二次電池では、充放電時の電流密度が小さいと充放電容量は変化しないが、充放電時の電流密度が大きくなるにしたがって、リチウムイオンの拡散抵抗等の影響を受け、充放電容量が減少する傾向がある。高速充放電を行ったときの充放電容量の維持性能を高速充放電特性といい、大電流密度で充放電を行ったときの充放電容量の低下率が小さい場合が、高速充放電特性が良好とされる。動力用電源の場合、充放電時の電流密度が必然的に大きくなるので、この特性の良好なことが求められる。 In recent years, lithium secondary batteries are expected not only as a power source for portable information devices with a small capacity, but also as power sources for power sources such as batteries for hybrid vehicles and power sources for elevators. In the case of a power supply for power, in addition to the above-described various characteristics such as the negative electrode capacity, initial efficiency, and cycle characteristics, it is required to be excellent in high-speed charge / discharge characteristics. That is, in a lithium secondary battery, the charge / discharge capacity does not change if the current density during charge / discharge is small, but as the current density during charge / discharge increases, it is affected by the diffusion resistance of lithium ions, etc. There is a tendency for capacity to decrease. The charge / discharge capacity maintenance performance when performing high-speed charge / discharge is called high-speed charge / discharge characteristics. It is said. In the case of a power source for motive power, the current density at the time of charging / discharging is inevitably increased, so that this characteristic is required to be good.
薄膜型ＳｉＯ負極では、バインダー等のリチウムイオンの拡散を阻害する混合物がなく、リチウムイオンの拡散性が良好なために、本質的に急速充放電特性が良好である。この急速充放電特性は前述したとおり膜厚が薄いほど良好となるが、単位面積当たりの負極容量を確保する観点から、極端に膜厚を薄くすることはできない。したがって、薄膜化によらずに急速充放電特性を高めることが重要となるが、薄膜型ＳｉＯ負極はその緻密さ故に、本質的に急速充放電特性に優れるのである。 The thin-film type SiO negative electrode has essentially no rapid charge / discharge characteristics because there is no mixture such as a binder that inhibits lithium ion diffusion and lithium ion diffusibility is good. As described above, the rapid charge / discharge characteristics are better as the film thickness is thinner. However, from the viewpoint of securing the negative electrode capacity per unit area, the film thickness cannot be extremely reduced. Therefore, it is important to enhance the rapid charge / discharge characteristics regardless of the thin film, but the thin film type SiO negative electrode is essentially excellent in rapid charge / discharge characteristics because of its denseness.
そこで、この急速放電特性を評価項目に加え、これに対する成膜後の熱処理の影響度等を調査した。評価は次の方法により実施した。定電流（０．５ｍＡ／ｃｍ2 ）で５ｍＶまで充電し、以後、定電圧（５ｍＶ）でトータル１５時間の充電を行った。その後、０．１ｍＡ／ｃｍ2 の電流密度での緩慢放電及び７．０ｍＡ／ｃｍ2 の電流密度での急速放電を行い、各放電での放電容量を調査した。放電でのカットオフ電圧は１．５Ｖとした。各サンプルにつきこの充放電を１０回繰り返し、放電容量の平均値を算出した。放電電流密度（放電速度）が大きくなるほど放電容量は低下する。０．１ｍＡ／ｃｍ2 で放電を行ったときの放電容量（ｍＡｈｒ／ｇ）と７．０ｍＡ／ｃｍ2 で放電を行ったときの放電容量（ｍＡｈｒ／ｇ）との比、すなわち前者に対する後者の比率を急速放電特性として表１に示した。 Therefore, the rapid discharge characteristics were added to the evaluation items, and the degree of influence of the heat treatment after the film formation was investigated. Evaluation was carried out by the following method. The battery was charged to 5 mV at a constant current (0.5 mA / cm 2 ), and thereafter charged at a constant voltage (5 mV) for a total of 15 hours. Thereafter, slow discharge and rapid discharge at a current density of 7.0 mA / cm 2 at a current density of 0.1 mA / cm 2, was investigated discharge capacity at each discharge. The cut-off voltage during discharge was 1.5V. This charging / discharging was repeated 10 times for each sample, and the average value of the discharge capacity was calculated. The discharge capacity decreases as the discharge current density (discharge rate) increases. 0.1 mA / cm 2 in the discharge capacity when discharge was performed (mAhr / g) and the discharge capacity when the discharging is performed 7.0mA / cm 2 (mAhr / g ) and the ratio of, that of the latter relative to the former The ratios are shown in Table 1 as rapid discharge characteristics.
表１から分かるように、絶対的な負極容量を確保するために、薄膜型ＳｉＯｘ負極におけるＳｉＯｘ膜厚を５μｍと厚くしたが、集電体の表面粗度をＲｚ＝７と粗くしたため、基本的に良好なサイクル特性が確保された。ただし、初期効率は顕著に低下した。すなわち、成膜用ターゲットとしてＳｉＯ粉末焼結ターゲットを使用し、成膜後に熱処理を行わなかった場合の初期効率は低い。 As can be seen from Table 1, in order to ensure the absolute negative electrode capacity, the SiOx film thickness in the thin film type SiOx negative electrode was increased to 5 μm, but the surface roughness of the current collector was increased to Rz = 7. In addition, good cycle characteristics were secured. However, the initial efficiency was significantly reduced. That is, the initial efficiency is low when a SiO powder sintered target is used as a film formation target and no heat treatment is performed after film formation.
しかしながら、Ｓｉ−ＳｉＯ混合粉末焼結ターゲットの使用と成膜後の熱処理との組合せにより、この初期効率は低下は飛躍的に改善された。この熱処理は急速放電特性の向上にも有効である。 However, the combination of the use of the Si—SiO mixed powder sintered target and the heat treatment after film formation has dramatically improved the initial efficiency. This heat treatment is also effective for improving rapid discharge characteristics.
１０ ケース
２０ 正極
２１ 集電体
２２ 正極活物質層
３０ 負極
３１ 集電体
３２ 負極活物質層
４０ セパレータ
５０ シール部材
６０ カバー
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| JP2008010419A (en) * | 2006-06-02 | 2008-01-17 | Matsushita Electric Ind Co Ltd | Manufacturing method of electrode for electrochemical element, and electrochemical element including it |
| JP2009024255A (en) * | 2007-06-20 | 2009-02-05 | Dainippon Printing Co Ltd | Raw material powder for evaporation source material for ion plating, evaporation source material for ion plating, method for producing the evaporation source material, and gas barrier sheet and method for producing the gas barrier sheet |
| WO2010146759A1 (en) * | 2009-06-19 | 2010-12-23 | 株式会社大阪チタニウムテクノロジーズ | Silicon oxide and negative-electrode material for a lithium-ion secondary battery |
| WO2011048756A1 (en) * | 2009-10-22 | 2011-04-28 | 株式会社大阪チタニウムテクノロジーズ | Negative electrode active material for lithium ion secondary battery |
| WO2013054476A1 (en) * | 2011-10-14 | 2013-04-18 | 信越化学工業株式会社 | Silicon oxide for negative electrode material of nonaqueous electroltye secondary cell, method for producing same, lithium ion secondary cell, and electrochemical capacitor |
| KR20200014737A (en) * | 2017-03-27 | 2020-02-11 | 시 추 | Method, apparatus and system for producing silicon-containing products using silicon sludge, a by-product produced by cutting silicon with diamond wire |
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| JP2008010419A (en) * | 2006-06-02 | 2008-01-17 | Matsushita Electric Ind Co Ltd | Manufacturing method of electrode for electrochemical element, and electrochemical element including it |
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| WO2013054476A1 (en) * | 2011-10-14 | 2013-04-18 | 信越化学工業株式会社 | Silicon oxide for negative electrode material of nonaqueous electroltye secondary cell, method for producing same, lithium ion secondary cell, and electrochemical capacitor |
| JP2013089364A (en) * | 2011-10-14 | 2013-05-13 | Shin Etsu Chem Co Ltd | Silicon oxide for negative electrode material of nonaqueous electrolyte secondary battery, production method therefor, lithium ion secondary battery and electrochemical capacitor |
| KR20200014737A (en) * | 2017-03-27 | 2020-02-11 | 시 추 | Method, apparatus and system for producing silicon-containing products using silicon sludge, a by-product produced by cutting silicon with diamond wire |
| JP2020515505A (en) * | 2017-03-27 | 2020-05-28 | シュウ, キシーCHU, Xi | Method, device and system for manufacturing silicon-containing product by using silicon sludge produced as a by-product during diamond wire cutting of silicon material |
| KR102593874B1 (en) | 2017-03-27 | 2023-10-24 | 시 추 | Method, device and system for producing silicon-containing products using silicon sludge, a by-product produced by cutting silicon with a diamond wire |
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