JP6772435B2 - Negative electrode active material for lithium ion secondary batteries and its manufacturing method - Google Patents
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本発明は、リチウムイオン2次電池用の負極活物質およびその製造方法に関するものである。 The present invention relates to a negative electrode active material for a lithium ion secondary battery and a method for producing the same.
スマートフォン、タブレット型端末などモバイル機器の高性能化や、EV、PHEVなどリチウムイオン2次電池を搭載した車両の普及に伴い、リチウムイオン2次電池の高容量化の要求が高まっている。現在、リチウムイオン2次電池の負極材には主に黒鉛が用いられているが、さらなる高容量化のため、理論容量が高く、リチウムイオンを吸蔵・放出可能なシリコン粒子を用いた負極材の開発が活発化している。 With the increasing performance of mobile devices such as smartphones and tablet terminals and the widespread use of vehicles equipped with lithium-ion secondary batteries such as EVs and PHEVs, there is an increasing demand for higher capacity lithium-ion secondary batteries. Currently, graphite is mainly used as the negative electrode material for lithium-ion secondary batteries, but in order to further increase the capacity, the negative electrode material using silicon particles that have a high theoretical capacity and can occlude and release lithium ions. Development is active.
一方、これらのシリコン粒子は、充電によってリチウムを吸蔵した際に、著しく体積膨張し、これらを用いた負極も構造が破壊されて導電性が切断される。従って、シリコン粒子を用いた負極はサイクル経過によって容量が著しく低下することが課題となっている。 On the other hand, when lithium is occluded by charging, these silicon particles remarkably expand in volume, and the structure of the negative electrode using these is also destroyed to cut the conductivity. Therefore, there is a problem that the capacity of the negative electrode using silicon particles is remarkably reduced as the cycle progresses.
この課題に対し、これらのシリコン粒子を微粉砕し、黒鉛などで複合化する手法が提案されている。このような複合粒子は、シリコン微粒子がリチウムと合金化し、膨張しても、一粒子あたりの膨張が小さく、黒鉛によって導電性が確保されるため、これらの材料を単独で負極材として用いるよりもサイクル特性が著しく向上することが知られている。例えば、特許文献1には、比表面積30m2/g以上の膨張黒鉛または薄片状黒鉛と、リチウムイオンと化合可能な電池活物質とを混合して混合物を得る混合工程と、該混合物に球形化処理を施し、黒鉛およびリチウムイオンと化合可能な電池活物質を含有する略球状のリチウム二次電池用複合負極活物質を製造する球形化工程とを有する、リチウム二次電池用複合負極活物質の製造方法が開示され、前記リチウムイオンと化合可能な負極活物質について、Si、Sn、Al、Sb、Inから選ばれる少なくとも1種の元素を含有し、平均粒子径は1μm以下が好ましいと記載されている。
To solve this problem, a method has been proposed in which these silicon particles are finely pulverized and composited with graphite or the like. In such composite particles, even if the silicon fine particles are alloyed with lithium and expanded, the expansion per particle is small and the conductivity is ensured by graphite, so that these materials are not used alone as the negative electrode material. It is known that the cycle characteristics are significantly improved. For example,
このようなシリコン微粒子を用いることにより、充電時のリチウム挿入による一粒子あたりの膨張が少なくなり、使用開始時にはサイクル特性が良好であるが、充電時、リチウムを吸蔵したシリコン微粒子が膨張とともに軟化し、粒子同士の互着が発生する。その結果、微粒子が粗大化し、粒子あたりの膨張が大きくなり、複合粒子の構造が破壊され、サイクル特性が悪化する問題があった。 By using such silicon fine particles, the expansion per particle due to lithium insertion during charging is reduced, and the cycle characteristics are good at the start of use, but the silicon fine particles that occlude lithium soften with expansion during charging. , Particles are attached to each other. As a result, there is a problem that the fine particles become coarse, the expansion per particle becomes large, the structure of the composite particle is destroyed, and the cycle characteristics deteriorate.
本発明は、シリコン系微粒子と残部が非晶質の炭素及び/又は微結晶の炭素、及び黒鉛とを含んで複合化したリチウムイオン2次電池用負極活物質に関するものであり、放電容量が大きく、サイクル寿命が長いリチウムイオン2次電池を与える負極活物質およびその製造方法を提供することにある。 The present invention relates to a negative electrode active material for a lithium ion secondary battery, which is a composite containing silicon-based fine particles, carbon having an amorphous residue and / or fine crystal carbon, and graphite, and has a large discharge capacity. The present invention is to provide a negative electrode active material that provides a lithium ion secondary battery having a long cycle life, and a method for producing the same.
本発明者らは先の課題を解決すべく鋭意検討を重ねた結果、平均粒径(D50)が0.5μm以下であり表面に1種以上の金属がドープされ、かつXRDで測定してシリコン単相のピークパターンを持つシリコン系微粒子を10〜50重量%と、残部が非晶質の炭素及び/又は微結晶の炭素、及び黒鉛であるリチウムイオン2次電池用負極活物質を用いると、充電時のシリコン系微粒子同士の互着を抑制でき、サイクル特性が良好なリチウムイオン2次電池が得られることを見出し、本発明を完成するに至った。 As a result of diligent studies to solve the above problems, the present inventors have an average particle size (D50) of 0.5 μm or less, the surface is doped with one or more metals, and silicon is measured by XRD. Using 10 to 50% by weight of silicon-based fine particles having a single-phase peak pattern, amorphous carbon and / or microcrystalline carbon, and graphite as the balance, a negative electrode active material for a lithium ion secondary battery is used. We have found that a lithium ion secondary battery capable of suppressing mutual adhesion between silicon-based fine particles during charging and having good cycle characteristics can be obtained, and have completed the present invention.
以下、本発明のリチウムイオン2次電池用負極活物質について詳細に説明する。 Hereinafter, the negative electrode active material for a lithium ion secondary battery of the present invention will be described in detail.
本発明でいうシリコン系微粒子は、1種以上の金属が表面にドープされ、かつXRDで測定してシリコン単相のピークパタ−ンを持ち、負極活物質中に含まれるリチウムと化合可能な金属(Si、Sn、Al等)やこれらを含む合金や金属酸化物とは異なり、充電時の微粒子同士の互着を抑制でき、サイクル特性を向上させる効果が大きい。ドープに用いる金属としてはAg、Al、Bi、Cd、Co、Cr、Cu、Ga、Ge、In、Li、Mg、Mn、Ni、Pb、Sb、Sn、Ti、Ta、V、W、Y、Znから選ばれた1種以上の組み合わせが望ましい。 The silicon-based fine particles referred to in the present invention have one or more metals doped on the surface, have a single-phase silicon peak pattern as measured by XRD, and are a metal that can be combined with lithium contained in the negative electrode active material ( Unlike Si, Sn, Al, etc.) and alloys and metal oxides containing these, it is possible to suppress mutual adhesion between fine particles during charging, and the effect of improving cycle characteristics is great. The metals used for doping include Ag, Al, Bi, Cd, Co, Cr, Cu, Ga, Ge, In, Li, Mg, Mn, Ni, Pb, Sb, Sn, Ti, Ta, V, W, Y, One or more combinations selected from Zn are desirable.
本発明のリチウムイオン2次電池用負極活物質において、シリコン系微粒子の平均粒径は0.5μm以下であり、0.3μm以下0.1μm以上が好ましい。0.5μmより大きいと、シリコン系微粒子と炭素前駆体との間に膨張応力による割れが発生し、導電パスが断絶されサイクル劣化が激しくなりやすい。また0.1μmより小さいと充放電の初回クーロン効率が低下してしまうことがある。なお、D50はレーザー回折法または動的光散乱法で測定した体積平均の粒子径である。 In the negative electrode active material for a lithium ion secondary battery of the present invention, the average particle size of the silicon-based fine particles is 0.5 μm or less, preferably 0.3 μm or less and 0.1 μm or more. If it is larger than 0.5 μm, cracks due to expansion stress occur between the silicon-based fine particles and the carbon precursor, the conductive path is cut off, and cycle deterioration tends to be severe. If it is smaller than 0.1 μm, the initial Coulomb efficiency of charge / discharge may decrease. D50 is a volume average particle diameter measured by a laser diffraction method or a dynamic light scattering method.
シリコン系微粒子の含有量は10〜50重量%であり、15〜40重量%が好ましい。シリコン系微粒子の含有量が10重量%未満の場合、従来の黒鉛に比べて十分に大きい容量が得られず、50重量%より大きい場合、サイクル劣化が激しくなりやすい。 The content of the silicon-based fine particles is 10 to 50% by weight, preferably 15 to 40% by weight. When the content of the silicon-based fine particles is less than 10% by weight, a sufficiently large capacity cannot be obtained as compared with the conventional graphite, and when it is larger than 50% by weight, cycle deterioration tends to be severe.
本発明でいう非晶質の炭素及び/又は微結晶の炭素は、2000℃を超える熱処理で黒鉛化する易黒鉛化炭素(ソフトカーボン)と、黒鉛化しにくい難黒鉛化炭素(ハードカーボン)がある。 The amorphous carbon and / or microcrystalline carbon referred to in the present invention includes easily graphitized carbon (soft carbon) that is graphitized by heat treatment exceeding 2000 ° C. and non-graphitized carbon (hard carbon) that is difficult to graphitize. ..
本発明でいう黒鉛には、グラフェン層がc軸に平行な結晶、または超音波等により層間剥離させたグラフェン等を用いることができる。グラフェン層がc軸に平行な結晶には、鉱石を精製した天然黒鉛、石油や石炭のピッチを黒鉛化した人造黒鉛や、これらを酸処理、酸化処理した後、熱処理することにより膨張させ黒鉛層間の一部が剥離してアコーディオン状となった膨張黒鉛もしくは膨張黒鉛の粉砕物を圧縮させたものが望ましい。形状としては鱗片状、小判状もしくは球状、円柱状もしくはファイバー状が望ましい。または黒鉛の粒子サイズは、リチウムイオン2次電池用負極活物質の粒子サイズより小さければ特に限定されない。黒鉛との複合化により本発明のリチウムイオン2次電池用負極活物質の導電性および強度が高まり、充放電のレート特性およびサイクル特性が向上する。黒鉛粒子のX線回折で測定される(002)面の面間隔d002は0.338nm以下であることが好ましく、これは高度に黒鉛化が進んだ黒鉛を意味している。d002がこの値を超える場合、黒鉛による導電性向上効果が小さくなる。 As the graphite referred to in the present invention, crystals having a graphene layer parallel to the c-axis, graphene delaminated by ultrasonic waves or the like can be used. Crystals in which the graphene layer is parallel to the c-axis include natural graphite obtained by refining ore, artificial graphite obtained by graphitizing the pitch of petroleum or coal, or graphite layers that are expanded by heat treatment after acid treatment and oxidation treatment. It is desirable that expanded graphite or a crushed product of expanded graphite, which has been partially peeled off to form an accordion, is compressed. The shape is preferably scaly, oval or spherical, columnar or fibrous. Alternatively, the particle size of graphite is not particularly limited as long as it is smaller than the particle size of the negative electrode active material for a lithium ion secondary battery. The composite with graphite enhances the conductivity and strength of the negative electrode active material for the lithium ion secondary battery of the present invention, and improves the charge / discharge rate characteristics and cycle characteristics. The interplanar spacing d002 of the (002) plane measured by X-ray diffraction of the graphite particles is preferably 0.338 nm or less, which means highly graphitized graphite. When d002 exceeds this value, the effect of improving conductivity by graphite becomes small.
本発明のリチウムイオン2次電池用負極活物質は、形状が丸みを帯びた平均粒径(D50)が1〜40μmの複合粒子であることが好ましく、特に好ましくは2〜30μmである。D50が1μm未満の場合、嵩高くなって高密度の電極が作製しにくくなり、40μmを超える場合、塗布した電極の凹凸が激しくなって均一な電極が作製しにくくなる。また、前記シリコン系微粒子の平均粒径が該負極活物質の平均粒径の1/5以下であり、非晶質の炭素及び/又は微結晶の炭素が、該負極活物質の表面を覆っていることが好ましい。 The negative electrode active material for a lithium ion secondary battery of the present invention is preferably a composite particle having a rounded shape and an average particle size (D50) of 1 to 40 μm, and particularly preferably 2 to 30 μm. If D50 is less than 1 μm, it becomes bulky and it becomes difficult to produce a high-density electrode, and if it exceeds 40 μm, the unevenness of the applied electrode becomes severe and it becomes difficult to produce a uniform electrode. Further, the average particle size of the silicon-based fine particles is 1/5 or less of the average particle size of the negative electrode active material, and amorphous carbon and / or microcrystalline carbon covers the surface of the negative electrode active material. It is preferable to have.
形状が丸みを帯びた複合粒子とは、粉砕等により生成した粒子の角が取れているもの、球状もしくは回転楕円体形状、円板もしくは小判形状で厚みを有して角が丸いもの、またはそれらが変形したもので角が丸いものなどである。形状が丸みを帯びることにより複合粒子の嵩密度が高まり、負極にした時の充填密度が高まる。また、炭素前駆体がリチウムイオン2次電池用負極活物質の粒子表面を覆っていることにより、充放電の過程で電解液に溶媒和したリチウムイオンが炭素前駆体の表面で溶媒から離れて、リチウムイオンのみがシリコン系微粒子および/または黒鉛と反応するため、溶媒の分解生成物が生成しにくくなり、充放電の効率が高まる。 Composite particles with a rounded shape are those with rounded corners, spherical or spheroidal, disc or oval, thick and rounded corners, or those. Is a deformed one with rounded corners. The rounded shape increases the bulk density of the composite particles and increases the packing density when used as a negative electrode. Further, since the carbon precursor covers the particle surface of the negative electrode active material for the lithium ion secondary battery, the lithium ions solvated in the electrolytic solution during the charging / discharging process are separated from the solvent on the surface of the carbon precursor. Since only lithium ions react with silicon-based fine particles and / or graphite, decomposition products of the solvent are less likely to be produced, and charging / discharging efficiency is increased.
本発明のリチウムイオン2次電池用負極活物質においては、前記シリコン系微粒子が非晶質の炭素及び/又は微結晶の炭素と共に0.2μm以下の厚みの黒鉛薄層の間に挟まった構造であり、その構造が積層および/または網目状に広がっており、該黒鉛薄層がリチウムイオン2次電池用負極活物質粒子の表面付近で湾曲して覆っていることが好ましい。 The negative electrode active material for a lithium ion secondary battery of the present invention has a structure in which the silicon-based fine particles are sandwiched between amorphous carbon and / or fine crystalline carbon and a thin graphite layer having a thickness of 0.2 μm or less. It is preferable that the structure is laminated and / or spreads in a mesh pattern, and the graphite thin layer is curved and covered near the surface of the negative electrode active material particles for a lithium ion secondary battery.
本発明でいう黒鉛薄層とは、先に述べた黒鉛を酸処理、酸化処理した後、熱処理することにより膨張させて黒鉛層間の一部が剥離してアコーディオン状となった膨張黒鉛もしくは膨張黒鉛の粉砕物、超音波等により層間剥離させたグラフェン等が圧縮力を受けることで生成した、グラフェン1層(厚み0.0003μm)〜数百層(厚み〜0.2μm)からなるものである。黒鉛薄層の厚みは薄い方が、黒鉛薄層間に挟まれたシリコン系微粒子と、炭素前駆体の層が薄くなって、シリコン系微粒子への電子の伝達が良くなり、厚みが0.2μmを超えると黒鉛薄層の電子伝達効果が薄まる。黒鉛薄層を断面で見て線状の場合、その長さはリチウムイオン2次電池用負極活物質の粒子サイズの半分以上あることが電子伝達に好ましく、リチウムイオン2次電池用負極活物質の粒子サイズと同等程度であることがさらに好ましい。黒鉛薄層が網目状の場合、黒鉛薄層の網がリチウムイオン2次電池用負極活物質の粒子サイズの半分以上に渡って繋がっていることが電子伝達に好ましく、リチウムイオン2次電池用負極活物質の粒子サイズと同等程度であることがさらに好ましい。 The graphite thin layer referred to in the present invention is expanded graphite or expanded graphite which is expanded by heat treatment after acid treatment and oxidation treatment of the above-mentioned graphite, and a part of the graphite layers is peeled off to form an accordion. It is composed of one layer of graphene (thickness 0.0003 μm) to several hundred layers (thickness ~ 0.2 μm) produced by receiving a compressive force on the pulverized product of graphite, graphene delaminated by ultrasonic waves, or the like. The thinner the graphite thin layer, the thinner the silicon-based fine particles sandwiched between the graphite thin layers and the carbon precursor layer, and the better the transfer of electrons to the silicon-based fine particles, resulting in a thickness of 0.2 μm. If it exceeds, the electron transfer effect of the graphite thin layer diminishes. When the graphite thin layer is linear when viewed in cross section, it is preferable for electron transfer that the length is at least half the particle size of the negative electrode active material for a lithium ion secondary battery, and that of the negative electrode active material for a lithium ion secondary battery. It is more preferable that the size is about the same as the particle size. When the graphite thin layer is mesh-like, it is preferable for electron transfer that the graphite thin layer network is connected over half or more of the particle size of the negative electrode active material for the lithium ion secondary battery, and the negative electrode for the lithium ion secondary battery. It is more preferable that the size is about the same as the particle size of the active material.
本発明においては、黒鉛薄層がリチウムイオン2次電池用負極活物質の粒子表面付近で湾曲して覆うことが好ましい。そのような形状にすることで、黒鉛薄層端面から電解液が侵入して、シリコン系微粒子や黒鉛薄層端面と電解液が直接接して、充放電時に反応物が形成され、効率が下がるというリスクが低減する。 In the present invention, it is preferable that the graphite thin layer is curved and covered near the particle surface of the negative electrode active material for a lithium ion secondary battery. By making such a shape, the electrolytic solution invades from the graphite thin layer end face, and the silicon-based fine particles and the graphite thin layer end face are in direct contact with the electrolytic solution, and a reactant is formed during charging and discharging, which reduces efficiency. Risk is reduced.
本発明のリチウムイオン2次電池用負極活物質は、BET法による比表面積が0.5〜120m2/gであることが好ましく、0.5〜50m2/gがさらに好ましい。 The negative electrode active material for a lithium ion secondary battery of the present invention preferably has a specific surface area of 0.5 to 120 m 2 / g by the BET method, and more preferably 0.5 to 50 m 2 / g.
次に、本発明のリチウムイオン2次電池用負極活物質の製造方法について説明する。 Next, a method for producing the negative electrode active material for a lithium ion secondary battery of the present invention will be described.
本発明のリチウムイオン2次電池用極活物質の製造方法は、シリコン系微粒子、炭素質物、さらに残部の黒鉛を混合する工程と、圧密化する工程と、粉砕および球形化処理して複合粒子を形成する工程と、該複合粒子を不活性雰囲気中で焼成する工程を含むものである。 The method for producing a polar active material for a lithium ion secondary battery of the present invention includes a step of mixing silicon-based fine particles, a carbonaceous substance, and the remaining graphite, a step of compacting, and a step of pulverizing and spheroidizing the composite particles. It includes a step of forming and a step of firing the composite particles in an inert atmosphere.
シリコン系微粒子の製造は、まずシリコン原料(インゴット、ウエハ、粉末などの状態)を粉砕機で粉砕し、場合によっては分級機を用いて、平均粒径(D50)が0.3μm以下のシリコン微粒子に調整する。このシリコン微粒子をアルコールスラリーとし、スラリー中に金属塩を溶解または金属酸化物を分散させ乾燥する等の方法によってシリコン微粒子表面に金属塩または金属酸化物を凝着させ、さらに窒素、アルゴン等の不活性ガス中で水素や一酸化炭素等の混合ガス中で加熱する等の方法で還元し製造する。 In the production of silicon-based fine particles, first, silicon raw materials (states such as ingots, wafers, and powders) are crushed with a crusher, and in some cases, a classifier is used to make silicon fine particles having an average particle size (D50) of 0.3 μm or less. Adjust to. The silicon fine particles are used as an alcohol slurry, and the metal salt or metal oxide is adhered to the surface of the silicon fine particles by a method such as dissolving a metal salt in the slurry or dispersing the metal oxide and drying it, and further, nitrogen, argon, etc. are not added. It is produced by reducing it by heating it in a mixed gas such as hydrogen or carbon monoxide in an active gas.
使用する金属、金属塩、金属酸化物はAg、Al、Bi、Cd、Co、Cr、Cu、Ga、Ge、In、Li、Mg、Mn、Ni、Pb、Sb、Sn、Ti、Ta、V、W、Y、Znから選ばれた1種以上の金属の単体、塩、酸化物であることが好ましく、Al、Cu、Mg、Ni、Pb、Znから選ばれた1種以上の単体、塩、酸化物であることがさらに好ましい。 The metals, metal salts and metal oxides used are Ag, Al, Bi, Cd, Co, Cr, Cu, Ga, Ge, In, Li, Mg, Mn, Ni, Pb, Sb, Sn, Ti, Ta and V. , W, Y, Zn, preferably one or more simple substances, salts, oxides, and one or more simple substances, salts selected from Al, Cu, Mg, Ni, Pb, Zn. , Oxides are more preferred.
炭素質物としては、炭素を主体とする高分子で、不活性ガス雰囲気中での熱処理により非晶質の炭素及び/又は微結晶の炭素になるものであれば特に限定はなく、石油系ピッチ、石炭系ピッチ、合成ピッチ、タール類、セルロース、スクロース、ポリ塩化ビニル、ポリビニルアルコール、フェノール樹脂、フラン樹脂、フルフリルアルコール、ポリスチレン、エポキシ樹脂、ポリアクリロニトリル、メラミン樹脂、アクリル樹脂、ポリアミドイミド樹脂、ポリアミド樹脂、ポリイミド樹脂等が使用できる。 The carbonaceous material is not particularly limited as long as it is a polymer mainly composed of carbon and becomes amorphous carbon and / or microcrystalline carbon by heat treatment in an inert gas atmosphere. Coal-based pitch, synthetic pitch, tars, cellulose, sculose, polyvinyl chloride, polyvinyl alcohol, phenol resin, furan resin, furfuryl alcohol, polystyrene, epoxy resin, polyacrylonitrile, melamine resin, acrylic resin, polyamideimide resin, polyamide Resin, polystyrene resin, etc. can be used.
残部の黒鉛には、天然黒鉛、石油や石炭のピッチを黒鉛化した人造黒鉛等、またはそれらを酸処理、酸化処理した後、熱処理することにより膨張させて黒鉛層間の一部が剥離してアコーディオン状となった膨張黒鉛もしくは膨張黒鉛の粉砕物、または超音波等により層間剥離させたグラフェン等を用いることができる。残部の黒鉛は予め混合工程で使用可能な大きさに整えて使用し、混合前の粒子サイズとしては天然黒鉛や人造黒鉛では1〜100μm、膨張黒鉛もしくは膨張黒鉛の粉砕物、グラフェンでは5μm〜5mm程度である。 The remaining graphite is natural graphite, artificial graphite obtained by graphitizing the pitch of petroleum or coal, or they are acid-treated and oxidized, and then expanded by heat treatment to peel off a part of the graphite layers and accordion. Expanded graphite in the form, a crushed product of expanded graphite, graphene delaminated by ultrasonic waves or the like can be used. The remaining graphite is adjusted to a size that can be used in the mixing process in advance, and the particle size before mixing is 1 to 100 μm for natural graphite or artificial graphite, crushed expanded graphite or expanded graphite, and 5 μm to 5 mm for graphene. Degree.
シリコン系微粒子、炭素質物、残部の黒鉛の混合は、溶媒にシリコン系微粒子、炭素質物、さらに残部の黒鉛を投入し、分散、混合し、次いで溶媒を除去することで行うことができる。用いる溶媒は、炭素質物を溶解できるものであれば特に制限なく使用できる。例えば、炭素質物としてピッチ、タール類を用いる場合には、キノリン、ピリジン、トルエン、ベンゼン、テトラヒドロフラン、クレオソート油等が使用でき、ポリ塩化ビニルを用いる場合には、テトラヒドロフラン、シクロヘキサノン、ニトロベンゼン等が使用でき、フェノール樹脂、フラン樹脂を用いる場合には、エタノール、メタノール等が使用できる。 The mixing of the silicon-based fine particles, the carbonaceous material, and the remaining graphite can be performed by adding the silicon-based fine particles, the carbonaceous material, and the remaining graphite to the solvent, dispersing and mixing them, and then removing the solvent. The solvent used is not particularly limited as long as it can dissolve carbonaceous substances. For example, when pitch and tars are used as carbonaceous substances, quinoline, pyridine, toluene, benzene, tetrahydrofuran, creosote oil and the like can be used, and when polyvinyl chloride is used, tetrahydrofuran, cyclohexanone, nitrobenzene and the like can be used. When a phenol resin or a furan resin is used, ethanol, methanol or the like can be used.
混合装置としては、撹拌槽とホモミキサーを組合わせたもの、ニーダー、ナウターミキサー、レーディゲミキサー、ヘンシェルミキサ、ハイスピードミキサー等を用いることができる。これらで溶媒中の混合を行った後、溶媒の除去はこれらの装置の内、加熱乾燥機能が付帯されているもので続けて行うか、別途、振動乾燥機、パドルドライヤー、薄膜蒸発機等の乾燥機を用いて行う。溶媒除去は混合物中の溶媒残存率が5重量%以下になるまで行うことが好ましい。 As the mixing device, a combination of a stirring tank and a homomixer, a kneader, a Nauter mixer, a Ladyge mixer, a Henschel mixer, a high speed mixer and the like can be used. After mixing in the solvent with these, the solvent can be removed continuously with one of these devices equipped with a heat drying function, or separately by using a vibration dryer, paddle dryer, thin film evaporator, etc. This is done using a dryer. It is preferable to remove the solvent until the residual ratio of the solvent in the mixture becomes 5% by weight or less.
シリコン系微粒子、炭素質物、残部の黒鉛との混合物の圧密化は、ロールプレス、ローラーコンパクタ等の圧縮機によって軽装嵩密度400g/L以上になるまで圧縮し、解砕機で0.1〜5mmのフレーク状に解砕する事が好ましい。 Consolidation of the mixture with silicon-based fine particles, carbonaceous material, and the rest of graphite is compressed by a compressor such as a roll press or roller compactor until the light bulk density is 400 g / L or more, and then 0.1 to 5 mm by a crusher. It is preferable to crush it into flakes.
圧密化物の粉砕方法は、ジェットミル、ハンマーミル、ピンミル、ディスクミル等の乾式の粉砕装置を用い粉砕する。また、粉砕後に粒度分布を整えるため、風力分級、ふるい分け等の乾式分級が用いられる。粉砕機と分級機が一体となっているタイプでは、一度に粉砕、分級が行うことが可能である。粉砕・分級後の目標の平均粒径(D50)1〜40μmである。 As a method for crushing the consolidated product, a dry crusher such as a jet mill, a hammer mill, a pin mill, or a disc mill is used for crushing. Further, in order to adjust the particle size distribution after pulverization, dry classification such as wind power classification and sieving is used. In the type in which the crusher and the classifier are integrated, it is possible to crush and classify at the same time. The target average particle size (D50) after pulverization / classification is 1 to 40 μm.
粉砕物の球形化方法は、専用の球形化装置を通す方法と、上述の処理時間を調節することで球形化する方法がある。専用の球形化装置としては、ホソカワミクロン社のファカルティ(登録商標)、ノビルタ(登録商標)、メカノフュージョン(登録商標)、日本コークス工業社のCOMPOSI、奈良機械製作所社のハイブリダイゼーションシステム、アーステクニカ社のクリプトロンオーブ、クリプトロンエディ等が挙げられる。 As a method for spheroidizing the pulverized material, there are a method of passing through a dedicated spheroidizing device and a method of spheroidizing by adjusting the above-mentioned processing time. Dedicated spherical devices include Hosokawa Micron's Faculti (registered trademark), Nobilta (registered trademark), Mechanofusion (registered trademark), Nippon Coke Industries' COMPOSI, Nara Kikai Seisakusho's hybridization system, and EarthTechnica Examples include Cryptron Orb and Cryptron Eddie.
粉砕して得られた複合粒子は、アルゴンガスや窒素ガス気流中、もしくは真空など不活性雰囲気中で焼成する。焼成温度は600〜1000℃とすることが好ましい。焼成温度が600℃未満であると、炭素質物の焼結不足により、不可逆容量が大きくなり、またサイクル特性が悪くなる為、電池の特性が低下する傾向にある。 The composite particles obtained by pulverization are fired in an argon gas or nitrogen gas stream, or in an inert atmosphere such as vacuum. The firing temperature is preferably 600 to 1000 ° C. If the firing temperature is less than 600 ° C., the irreversible capacity becomes large due to insufficient sintering of the carbonaceous material, and the cycle characteristics deteriorate, so that the characteristics of the battery tend to deteriorate.
本発明によれば、シリコン系微粒子の平均粒径(D50)0.5μm以下への微粒化による粒子当たりの膨張体積の低減と表面への金属ドープによるシリコン系微粒子同士の互着の抑制、およびシリコン系微粒子と非晶質の炭素及び/又は微結晶の炭素または非晶質の炭素及び/又は微結晶の炭素と黒鉛との複合化による導電パスの確保によりサイクル特性の優れたリチウムイオン2次電池用負極活物質を得ることができる。 According to the present invention, the expansion volume per particle is reduced by atomizing the average particle size (D50) of the silicon-based fine particles to 0.5 μm or less, and the mutual adhesion between the silicon-based fine particles is suppressed by metal doping on the surface. Lithium ion secondary with excellent cycle characteristics by securing a conductive path by combining silicon-based fine particles with amorphous carbon and / or microcrystalline carbon or amorphous carbon and / or microcrystalline carbon and graphite. A negative electrode active material for a battery can be obtained.
以下、実施例および比較例により本発明を具体的に説明する。 Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.
実施例1
「表面に金属がドープされたシリコン系微粒子の作製」
純度3.5Nの金属シリコンをエタノールに20重量%混合し、ジルコニア製メディアのビーズミルを用いて湿式粉砕し、レーザー回折式粒度分布計での平均粒子径(D50)が0.16μmのシリコン微粒子スラリーを得た。このシリコン微粒子スラリーに硝酸銅エタノール溶液をシリコンに対するCuの添加量が4重量%となる様に添加し、120℃で乾燥しエタノールを除去した後、水素/窒素混合雰囲気中、600℃で1時間焼成処理し、シリコン系微粒子を得た。この微粒子をXRD測定した所、結晶性シリコンのみの回折パターンが得られた。
Example 1
"Preparation of silicon-based fine particles whose surface is doped with metal"
20% by weight of metallic silicon with a purity of 3.5N is mixed with ethanol, wet pulverized using a zirconia media bead mill, and a silicon fine particle slurry having an average particle size (D50) of 0.16 μm on a laser diffraction type particle size distribution meter. Got A copper nitrate ethanol solution was added to this silicon fine particle slurry so that the amount of Cu added to silicon was 4% by weight, dried at 120 ° C. to remove ethanol, and then in a hydrogen / nitrogen mixed atmosphere for 1 hour at 600 ° C. It was calcined to obtain silicon-based fine particles. When these fine particles were XRD-measured, a diffraction pattern of only crystalline silicon was obtained.
「リチウムイオン2次電池用負極活物質の作製」
粒子径約0.5mm((200)面方向の幅)、厚み約0.02mmの天然黒鉛を、硝酸ナトリウム1重量%、過マンガン酸カリウム7重量%を添加した濃硫酸に24時間浸漬し、その後、水洗して乾燥し、酸処理黒鉛を得た。この酸処理黒鉛を窒素ガスを流通させた850℃のムライト管に通し、膨張させたものを捕集した。膨張黒鉛の(200)面方向の幅は約0.5mmで元の黒鉛の値を保っていたが、厚みは約4mmと約200倍に膨張し、外観はコイル状であり、SEM観察で黒鉛層が剥離し、アコーディオン状であることが確認された。
"Preparation of negative electrode active material for lithium ion secondary batteries"
Natural graphite having a particle diameter of about 0.5 mm (width in the (200) plane direction) and a thickness of about 0.02 mm was immersed in concentrated sulfuric acid containing 1% by weight of sodium nitrate and 7% by weight of potassium permanganate for 24 hours. Then, it was washed with water and dried to obtain acid-treated graphite. This acid-treated graphite was passed through a mullite tube at 850 ° C. through which nitrogen gas was passed, and the expanded graphite was collected. The width of expanded graphite in the (200) plane direction was about 0.5 mm, which was the same as the original graphite value, but the thickness was about 4 mm, which expanded about 200 times, and the appearance was coiled. Graphite was observed by SEM. The layer was peeled off and it was confirmed that it was accordion-like.
シリコン系微粒子をスラリー濃度20%でエタノールスラリーとしたものを60g、上記膨張黒鉛を24g、炭素質物としてレゾール型のフェノール樹脂を10g、エタノール1Lを撹拌容器に入れて、ホモミキサーで1時間混合撹拌した。その後、混合液からロータリーエバポレーターで60℃、減圧下で溶媒を除去し約40gの混合乾燥物(軽装嵩密度80g/L)を得た。 60 g of silicon-based fine particles as an ethanol slurry at a slurry concentration of 20%, 24 g of the expanded graphite, 10 g of a resole-type phenol resin as a carbonaceous substance, and 1 L of ethanol are placed in a stirring container and mixed and stirred with a homomixer for 1 hour. did. Then, the solvent was removed from the mixed solution with a rotary evaporator at 60 ° C. under reduced pressure to obtain about 40 g of a mixed dried product (light bulk density 80 g / L).
この混合乾燥物を3本ロールミルで、軽装嵩密度440g/Lに圧密化した。 This mixed dried product was compacted to a light bulk density of 440 g / L with a 3-roll mill.
次に、この圧密化物をニューパワーミルで粉砕・球形化し、軽装嵩密度650g/Lの球形化粉末を得た。得られた粉末を、管状炉で窒素ガス雰囲気中、900℃で1時間焼成した。その後、目開き45μmの篩を通し、平均粒子径(D50)が20μm、軽装嵩密度810g/Lの本発明のリチウムイオン2次電池用負極活物質を得た。 Next, this compacted product was pulverized and sphericalized with a new power mill to obtain a spherical powder having a light bulk density of 650 g / L. The obtained powder was calcined in a tube furnace in a nitrogen gas atmosphere at 900 ° C. for 1 hour. Then, it was passed through a sieve having an opening of 45 μm to obtain a negative electrode active material for a lithium ion secondary battery of the present invention having an average particle diameter (D50) of 20 μm and a light bulk density of 810 g / L.
図1に、本発明のリチウムイオン2次電池用負極活物質の粒子をArイオンビームで切断した断面のFE−SEMによる2次電子像を示す。粒子の内部はシリコン系微粒子が炭素前駆体、0.2μm以下の厚みの黒鉛薄層(11)の間(12)に挟まった構造が網目状に広がり、積層していた。非晶質の炭素及び/又は微結晶の炭素はシリコン系微粒子に密着して覆っており、該負極活物質の粒子表面付近では、黒鉛薄層(11)が湾曲して粒子を覆っていた。さらに、該負極活物質のBET法による比表面積は18m2/gであった。また、XRD測定で黒鉛の(002)面の面間隔d002は0.336nmであり、非晶質の炭素及び/又は微結晶の炭素、非晶質炭素に由来する非常にブロードな回折線も観察された。本発明のリチウムイオン2次電池用負極活物質を用いたリチウムイオン2次電池を以下のようにして作製した。 FIG. 1 shows a secondary electron image of a cross section of the negative electrode active material for a lithium ion secondary battery of the present invention cut with an Ar ion beam by FE-SEM. Inside the particles, a structure in which silicon-based fine particles were sandwiched between carbon precursors and graphite thin layers (11) having a thickness of 0.2 μm or less (12) spread like a mesh and were laminated. Amorphous carbon and / or microcrystalline carbon adhered to and covered the silicon-based fine particles, and the graphite thin layer (11) was curved to cover the particles in the vicinity of the particle surface of the negative electrode active material. Further, the specific surface area of the negative electrode active material by the BET method was 18 m 2 / g. In addition, the XRD measurement showed that the interplanetary spacing d002 of the (002) plane of graphite was 0.336 nm, and very broad diffraction lines derived from amorphous carbon and / or microcrystalline carbon and amorphous carbon were also observed. Was done. A lithium ion secondary battery using the negative electrode active material for the lithium ion secondary battery of the present invention was produced as follows.
「リチウムイオン二次電池用負極の作製」
得られたリチウムイオン2次電池用負極活物質を95.5重量%(固形分全量中の含有量。以下同じ。)に対して、導電助剤としてアセチレンブラック0.5重量%と、バインダとしてカルボキシメチルセルロース(CMC)1.5重量%とスチレンブタジエンゴム(SBR)2.5重量%、水とを混合して負極合剤含有スラリーを調製した。
"Manufacturing negative electrodes for lithium-ion secondary batteries"
The obtained negative electrode active material for a lithium ion secondary battery was 95.5% by weight (content in the total solid content; the same applies hereinafter), 0.5% by weight of acetylene black as a conductive auxiliary agent, and 0.5% by weight as a binder. A slurry containing a negative electrode mixture was prepared by mixing 1.5% by weight of carboxymethyl cellulose (CMC), 2.5% by weight of styrene butadiene rubber (SBR), and water.
得られたスラリーを、固形分塗布量3mg/cm2の厚みで銅箔に塗布し、大気下、110℃で0.5時間乾燥した。乾燥後、14mmφの円形に打ち抜き、圧力0.6t/cm2で一軸プレスし、さらに真空下、110℃で3時間熱処理して、リチウムイオン2次電池用負極を得た。 The obtained slurry was applied to a copper foil with a solid content coating amount of 3 mg / cm 2 and dried in the air at 110 ° C. for 0.5 hours. After drying, it was punched into a circle of 14 mmφ, uniaxially pressed at a pressure of 0.6 t / cm 2 , and further heat-treated at 110 ° C. for 3 hours under vacuum to obtain a negative electrode for a lithium ion secondary battery.
「評価用セルの作製」
評価用セルは、グローブボックス中でスクリューセルに上記負極、ポリプロピレン製セパレータ、ガラスフィルター、対極である金属リチウムおよびその基材のステンレス箔を、各々、電解液にディップした後、この順に積層し作製した。電解液はエチレンカーボネートとジエチルカーボネートを体積比1対1で混合し、LiPF6を1.2mol/Lの濃度になるように添加、さらにフルオロエチレンカーボネートを2体積%添加したものを使用した。
"Preparation of evaluation cell"
The evaluation cell is produced by dipping the negative electrode, the polypropylene separator, the glass filter, the counter electrode, metallic lithium, and the stainless foil of the base material into the screw cell in the glove box, and then laminating them in this order. did. As the electrolytic solution, ethylene carbonate and diethyl carbonate were mixed in a volume ratio of 1: 1, LiPF6 was added to a concentration of 1.2 mol / L, and fluoroethylene carbonate was further added in an amount of 2% by volume.
「評価条件」
評価用セルは25℃の恒温室にて、サイクル試験した。充電は、2mAの定電流で0.01Vまで充電後、0.01Vの定電圧で電流値が0.2mAになるまで行った。また放電は、2mAの定電流で1.5Vの電圧値まで行った。サイクル特性は、前記充放電条件にて50回充放電試験した後の放電容量を初回の放電容量と比較し、その容量維持率として評価した。
"Evaluation conditions"
The evaluation cell was cycle tested in a constant temperature room at 25 ° C. Charging was carried out at a constant current of 2 mA to 0.01 V, and then at a constant voltage of 0.01 V until the current value reached 0.2 mA. Further, the discharge was performed up to a voltage value of 1.5 V with a constant current of 2 mA. The cycle characteristics were evaluated as the capacity retention rate by comparing the discharge capacity after the charge / discharge test 50 times under the charge / discharge conditions with the initial discharge capacity.
セル評価の結果は初回放電容量615mAh/g、50回目放電容量567mAh/gであり、容量維持率が91%と高く良好であった。 The results of the cell evaluation were an initial discharge capacity of 615 mAh / g and a 50th discharge capacity of 567 mAh / g, and the capacity retention rate was as high as 91%, which was good.
比較例1
シリコン微粒子の表面に金属をドープしなかった点以外は実施例1と同様の方法でリチウムイオン2次電池用負極活物質、負極、評価用セルの順に作成し、セル評価した。
Comparative Example 1
A negative electrode active material for a lithium ion secondary battery, a negative electrode, and an evaluation cell were prepared in this order by the same method as in Example 1 except that the surface of the silicon fine particles was not doped with metal, and the cells were evaluated.
セル評価の結果は初回放電容量850mAh/g、50回目放電容量574mAh/gであり、容量維持率が67%と実施例1より劣化が速い傾向であった。 The results of the cell evaluation were an initial discharge capacity of 850 mAh / g and a 50th discharge capacity of 574 mAh / g, and the capacity retention rate was 67%, which tended to deteriorate faster than in Example 1.
参考例1
純度3.5Nの金属シリコンをエタノールに20重量%混合し、ジルコニア製メディアのビーズミルで、レーザー回折式粒度分布計で平均粒子径(D50)0.21μmのシリコン微粒子スラリーを得た。このシリコン微粒子スラリーに硝酸銅エタノール溶液をシリコンに対するCuの添加量が0.5重量%となる様に添加し、120℃で乾燥しエタノールを除去した後、水素/窒素混合雰囲気中、600℃で1時間焼成し、シリコン系微粒子を得た。この微粒子をXRD測定した所、結晶性シリコンのみの回折パターンが得られた。
Reference example 1
20% by weight of metallic silicon having a purity of 3.5N was mixed with ethanol, and a silicon fine particle slurry having an average particle size (D50) of 0.21 μm was obtained with a laser diffraction type particle size distribution meter using a zirconia media bead mill. A copper nitrate ethanol solution was added to this silicon fine particle slurry so that the amount of Cu added to silicon was 0.5% by weight, dried at 120 ° C. to remove ethanol, and then at 600 ° C. in a hydrogen / nitrogen mixed atmosphere. It was calcined for 1 hour to obtain silicon-based fine particles. When these fine particles were XRD-measured, a diffraction pattern of only crystalline silicon was obtained.
得られたシリコン系微粒子を非晶質の炭素及び/又は微結晶の炭素、黒鉛とは複合化せずリチウムイオン2次電池用負極活物質として、70重量%(固形分全量中の含有量。以下同じ。)に対して、導電助剤としてカーボンナノチューブ10重量%と、バインダーとしてポリイミド系バインダー20重量%を混合して負極合剤含有スラリーを調製した。 70% by weight (content in total solid content) of the obtained silicon-based fine particles as a negative electrode active material for a lithium ion secondary battery without being combined with amorphous carbon and / or microcrystalline carbon and graphite. The same applies hereinafter), 10% by weight of carbon nanotubes as a conductive auxiliary agent and 20% by weight of a polyimide binder as a binder were mixed to prepare a slurry containing a negative electrode mixture.
得られたスラリーを、固形分塗布量が3mg/cm2の厚みで銅箔に塗布し、大気下110℃で0.5時間乾燥した。乾燥後、14mmφの円形に打ち抜き、圧力0.6t/cm2の条件で一軸プレスし、さらに真空下、110℃で3時間熱処理して、リチウムイオン2次電池用負極を得た。評価用のセル作製方法と評価条件は実施例1と同様に行った。 The obtained slurry was applied to a copper foil with a solid content coating amount of 3 mg / cm 2 , and dried at 110 ° C. in the atmosphere for 0.5 hours. After drying, it was punched into a circle of 14 mmφ, uniaxially pressed under the condition of a pressure of 0.6 t / cm 2 , and further heat-treated at 110 ° C. for 3 hours under vacuum to obtain a negative electrode for a lithium ion secondary battery. The cell preparation method and evaluation conditions for evaluation were the same as in Example 1.
セル評価の結果は初回放電容量2673mAh/g、50回目放電容量568mAh/gであり、容量維持率が49%と複合粒子化を行った実施例1、比較例1より劣化が大きい傾向であった。 The results of the cell evaluation were an initial discharge capacity of 2673 mAh / g and a 50th discharge capacity of 568 mAh / g, and the capacity retention rate was 49%, which tended to be larger than that of Example 1 and Comparative Example 1 in which composite particles were formed. ..
充放電後のシリコン系微粒子の形状をSEM観察した所、粒子同士の互着の抑制効果が確認された(図3)。 When the shape of the silicon-based fine particles after charging and discharging was observed by SEM, the effect of suppressing mutual adhesion between the particles was confirmed (Fig. 3).
比較例2
参考例1で得られたシリコン微粒子を金属ドープせず、その後は参考例1と同様に負極、評価用セルを作製し、セル評価した。
Comparative Example 2
The silicon fine particles obtained in Reference Example 1 were not metal-doped, and then a negative electrode and an evaluation cell were prepared in the same manner as in Reference Example 1 to evaluate the cells.
セル評価の結果は初回放電容量3248mAh/g、50回目放電容量1521mAh/gであり、50サイクル維持率が47%と比較例2を若干下回った。充放電後のシリコン系微粒子の形状をSEM観察した所、粒子同士の互着が確認された(図3)。 The results of the cell evaluation were an initial discharge capacity of 3248 mAh / g and a 50th discharge capacity of 1521 mAh / g, and the 50-cycle maintenance rate was 47%, which was slightly lower than that of Comparative Example 2. When the shape of the silicon-based fine particles after charging and discharging was observed by SEM, mutual adhesion between the particles was confirmed (Fig. 3).
実施例、参考例と比較例の電池評価結果を表1に示す。 Table 1 shows the battery evaluation results of Examples, Reference Examples and Comparative Examples.
11 負極活物質内部の黒鉛薄層
12 負極活物質内部のSi微粒子
11 Graphite thin layer inside the negative electrode active material 12 Si fine particles inside the negative electrode active material
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