JP2004235057A - Negative electrode for lithium secondary battery, and the battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce the aggregation of active material particles inside a binder and improve a cycle life property in a negative electrode for a lithium secondary battery having an active material layer including the active material particles containing silicon and/or silicon alloys and the binder. <P>SOLUTION: The negative electrode for the lithium secondary battery is characterized that the active material layer is formed on the surface of a current collector made of inductive metal foil by using slurry obtained by uniformly mixing the active material particles, the binder and a dispersant in a liquid solution, and the negative electrode is obtained by firing the active material layer under a non-oxidizing atmosphere, while disposing the active material layer on the surface of the current collector. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【００５２】
（実験３）
実験１において、分散剤を用いなかったこと以外は実験１と同様にして、電池Ｂ１を作製した。
【００５３】
［充放電サイクル特性の評価］
上記の電池Ａ１〜Ａ３及びＢ１について、上記の充放電試験条件で充放電サイクル特性を評価した。すなわち、各電池を、２５℃において、電流値３．５ｍＡ／ｃｍ^２で４．２Ｖまで充電した後、電流値３．５ｍＡ／ｃｍ^２で２．７５Ｖまで放電し、これを１サイクルの充放電とした。１サイクル目の放電容量に対する１００サイクル目の放電容量を測定し、サイクル維持率とした。結果を表２に示す。なお、各電池のサイクル維持率は、電池Ａ１のサイクル維持率を１００とした指数である。
【００５４】
【表２】

【００５５】
表２から明らかなように、分散剤ａ１〜ａ３を用いた電池Ａ１〜Ａ３は、分散剤を用いない電池Ｂ１に比べ、サイクル維持率が高いことがわかる。
これは、スラリー中に分散剤を混合させることにより、スラリー中に活物質粒子が均一に分散し、活物質とバインダーが均一に混合し分散したために、充放電反応でのリチウムの吸蔵・放出に伴う活物質粒子の体積の膨張・収縮時にも強度を保つことができ、活物質粒子の脱離が抑制され、集電性を向上させることができたためと考えられる。
【００５６】
（実験４）
実験１において、活物質材料として純度９９．９％、平均粒径２．７μｍであるケイ素粉末を用い、分散剤としてＮ，Ｎ−ジメチルステアリルアミン（分散剤ａ４）を用いたこと以外は実験１と同様にして、電池Ａ４を作製した。
【００５７】
（実験５）
実験４において、分散剤を用いなかったこと以外は実験４と同様にして電池Ｂ２を作製した。
【００５８】
（実験６）
実験５において、負極合剤スラリーの攪拌回転数を３５００回転／分にしたこと以外は実験５と同様にして電池Ｂ３を作製した。
【００５９】
［充放電サイクル特性の評価］
上記の電池Ａ４、Ｂ２及びＢ３について、充放電サイクル特性を評価した。各電池を、２５℃において、電流値３．５ｍＡ／ｃｍ^２で４．２Ｖまで充電した後、電流値３．５ｍＡ／ｃｍ^２で２．７５Ｖまで放電し、これを１サイクルの充放電とした。１サイクル目の放電容量に対する１００サイクル目の放電容量を測定し、サイクル維持率とした。結果を表３に示す。なお、各電池のサイクル維持率は、電池Ａ１のサイクル維持率を１００とした指数である。
【００６０】
【表３】

【００６１】
表３から、分散剤ａ４を用いた電池Ａ４は、分散剤を用いない電池Ｂ２及び電池Ｂ３に比べ、サイクル維持率が高いことが明らかである。
分散剤を用いなかった場合には、スラリー作成時の攪拌回転数の高い電池Ｂ３の方が、スラリー作成時の攪拌回転数の低い電池Ｂ２よりもサイクル維持率が高い。これにより、機械的な攪拌工程が電池のサイクル特性に影響することがわかる。
【００６２】
しかしながら、分散剤を用いた電池Ａ４が、分散剤を用いずに攪拌回転数を高くした電池Ｂ３よりもサイクル維持率が高い。分散剤を用いた電池Ａ４は、分散剤を用いずに攪拌回転数を高くした電池Ｂ３よりも、スラリー中において、活物質粒子とバインダーとが均一に分散されていると考えられる。これにより、電池Ｂ３を作製した上記の攪拌条件だけでは、活物質粒子のバインダーへの分散が不十分であることがわかる。
【００６３】
従って、分散剤を用いると機械的な拡散工程を省略できるだけでなく、焼結後もより均一に活物質粒子が分散した電極を作製することが可能であり、それにより、充放電反応でのリチウムの吸蔵・放出に伴う活物質粒子の体積の膨張・収縮時にも強度を保つことができ、活物質粒子の脱離が抑制され、集電性を向上させることができると考えられる。
【００６４】
【発明の効果】
本発明によれば、バインダー中での活物質粒子の凝集を少なくすることができ、サイクル寿命特性を向上させることができる。
【図面の簡単な説明】
【図１】本発明に従う実施例において作製したリチウム二次電池を示す模式図。
【符号の説明】
１…正極
２…負極
３…セパレータ
４…電解液
５…外装体
６…正極タブ
７…負極タブ[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a negative electrode for a lithium secondary battery, a method for producing the same, and a lithium secondary battery using the same.
[0002]
[Prior art]
In recent years, as one of new high-power and high-energy-density secondary batteries, a lithium secondary battery that uses a non-aqueous electrolyte and moves lithium ions between a positive electrode and a negative electrode to charge and discharge has been used. I have.
[0003]
As such a negative electrode for a lithium secondary battery, a negative electrode in which an active material containing silicon is attached to a current collector having an average roughness of 0.03 μm or more with a binder has been proposed (Patent Document 1).
[0004]
However, since the active material containing silicon expands and contracts in volume when inserting and extracting lithium, it is known that the active material becomes finer with charge and discharge, or the active material is detached from the current collector. ing. For this reason, there has been a problem that the current collecting property in the electrode is reduced and the charge / discharge cycle characteristics are deteriorated.
[0005]
The present applicant solves such a problem by disposing an active material layer containing active material particles containing silicon and / or a silicon alloy and a binder on the surface of a current collector as a negative electrode for a lithium secondary battery. A negative electrode for a lithium secondary battery obtained by sintering in a non-oxidizing atmosphere has been proposed (Patent Document 2).
[0006]
As the binder used for the negative electrode, a binder such as polyimide which remains after heat treatment for sintering is preferably used.
[0007]
[Patent Document 1]
JP-A-2000-12088
[Patent Document 2]
WO 02/21616 pamphlet
[0008]
[Problems to be solved by the invention]
The present inventors have found that, in the above-described negative electrode using a binder such as polyimide, active material particles are not uniformly dispersed in the binder, and thus sufficient current collecting properties are not obtained.
[0009]
An object of the present invention is to provide a negative electrode for a lithium secondary battery capable of reducing the aggregation of active material particles in a binder and improving the cycle life, a method for producing the same, and a lithium secondary battery using the same. It is in.
[0010]
[Means for Solving the Problems]
The negative electrode for a lithium secondary battery according to the present invention comprises an active material layer containing active material particles containing silicon and / or a silicon alloy, a binder and a dispersant disposed on a surface of a current collector made of a conductive metal foil. It is characterized by being obtained by sintering in an oxidizing atmosphere.
[0011]
A negative electrode for a lithium secondary battery according to a limited aspect of the present invention is a negative electrode for a lithium secondary battery, comprising: an active material layer containing active material particles containing silicon and / or a silicon alloy; A negative electrode for a lithium secondary battery that is obtained by sintering in a non-oxidizing atmosphere and disposed thereon, wherein the binder is a binder remaining after heat treatment for sintering.
[0012]
The active material layer according to the present invention contains active material particles, a binder, and a dispersant. By containing the dispersant, the active material particles are uniformly dispersed in the binder, and the aggregation of the active material particles can be reduced. When the active material particles aggregate, no binder exists around the aggregated active material particles, and the active material particles expand and contract due to the expansion and contraction of the volume of the active material particles due to the absorption and release of lithium in the charge / discharge reaction. Desorption becomes easy, and the current collecting property in the electrode is reduced. Therefore, by reducing the aggregation of the active material particles according to the present invention, the probability that the binder exists around the active material particles can be increased, and the desorption of the active material particles in the charge / discharge reaction can be prevented. For this reason, it is possible to suppress a decrease in the current collecting property in the electrode.
[0013]
In the present invention, whether or not the dispersant remains in the active material layer after the heat treatment for sintering is not particularly limited. When the decomposition temperature of the dispersant is lower than the heat treatment temperature for sintering, it is considered that the dispersant is decomposed and does not remain after the heat treatment. However, the dispersion of the active material particles in the binder before the heat treatment is considered to be maintained even after the heat treatment, so that the active material particles can be prevented from detaching from the current collector, and the current collecting property can be improved. Can be higher.
[0014]
Further, when the dispersant is decomposed by the heat treatment, voids may be formed in the active material layer due to the decomposition of the dispersant, but the electrolyte in the voids in such an active material layer is easily contained. It is considered that this may improve the charge / discharge cycle characteristics.
[0015]
Further, when the dispersant remains without being decomposed after the heat treatment for sintering, the active material particles before the heat treatment are uniformly dispersed in the binder even after the heat treatment. From the current collector is prevented, and the current collecting property can be further improved.
[0016]
The method for producing a negative electrode for a lithium secondary battery of the present invention is a method capable of producing the negative electrode for a lithium secondary battery of the present invention, wherein active material particles containing silicon and / or a silicon alloy, a binder, A step of preparing a slurry by uniformly mixing the dispersant with the solution, a step of applying the slurry on a surface of a current collector made of a conductive metal foil to form an active material layer, Baking in a non-oxidizing atmosphere in a state where is disposed on the surface of the current collector.
[0017]
In the production method of the present invention, the active material particles can be uniformly dispersed in the slurry by mixing the dispersant in the slurry. Therefore, in the active material layer, the active material particles can be uniformly dispersed in the binder, and the aggregation of the active material particles can be reduced. For this reason, as described above, even in the active material layer after the heat treatment, the active material particles are uniformly dispersed in the binder, and the active material particles are prevented from being detached from the current collector by the charge / discharge reaction. And the current collection in the electrode can be improved.
[0018]
The dispersant used in the present invention is not particularly limited as long as the active material particles can be uniformly dispersed in the binder, but a surfactant is generally used.
[0019]
As the surfactant, an oil-soluble surfactant is particularly preferably used. The oil-soluble surfactant is one that can be easily dispersed or dissolved in an organic solvent.
In the present invention, when an organic solvent is used as a slurry solvent, oil-soluble dispersants such as nonionic surfactants and aliphatic amines are preferably used. Examples of the nonionic surfactant include glycerin fatty acid ester, sorbitan fatty acid ester, propylene fatty acid ester, polyethylene fatty acid ester, alkanolamide, and alkylpolyglucoside. Examples of the aliphatic amine include an alkylamine and an alkyldiamine. Among these, a dimethylalkyl tertiary amine having an alkyl chain having 10 to 30 carbon atoms, an alkyl diamine having one or more alkyl chains having 10 to 30 carbon atoms, and the like are preferably used. Since these have an appropriate alkyl chain length, they are easily dispersed in an organic solvent and a binder.
[0020]
In the present invention, the content of the dispersant is appropriately adjusted depending on the type of the dispersant and the type of the binder and the type of the organic solvent to be used. It is preferable that the amount be not less than part by weight. If the amount of the dispersant is smaller than this, the dispersion of the active material particles in the binder may be insufficient. Further, the amount of the dispersant is preferably 5% by weight or less based on 100 parts by weight of the total of the binder and the organic solvent. This is because even if the dispersant is present in a large amount, no further improvement in the dispersibility of the active material particles in the binder is observed.
[0021]
In the present invention, the amount of the dispersant present in the binder is preferably in the range of 1 to 50 parts by weight based on 100 parts by weight of the binder.
The active material particles in the present invention preferably have a particle size distribution in which 60% by volume or more exists within a particle size of 10 μm or less. In the case of a wide particle distribution outside such a range, between active material particles having greatly different particle sizes, there will be a large difference in the absolute amount of expansion and contraction of the volume accompanying occlusion and release of lithium, Distortion occurs in the active material layer, the binder is destroyed, and the current collecting property in the electrode is easily reduced. Further, the average particle diameter is preferably 10 μm or less, more preferably 3 μm or less. When the active material particles are silicon powder, the purity is preferably 99.9% or more.
[0022]
In the present invention, it is preferable that the surface roughness Ra of the surface of the current collector on which the active material layer is disposed is 0.2 μm or more. By using a current collector having such a surface roughness Ra, the contact area between the active material layer and the current collector can be increased, and the adhesion between the active material layer and the current collector can be improved. Can be. For this reason, the current collection in the electrode can be further improved. When the active material layers are provided on both surfaces of the current collector, the surface roughness of both surfaces of the current collector is preferably 0.2 μm or more.
[0023]
The surface roughness Ra is defined in Japanese Industrial Standard (JIS B 0601-1994). The surface roughness Ra can be measured by, for example, a surface roughness meter.
In the present invention, a current collector having a surface roughened may be used. Examples of such a roughening treatment include a plating method, a vapor deposition method, an etching method, and a polishing method. The plating method and the vapor phase growth method are methods of forming a layer having irregularities on the surface of a metal foil current collector to roughen the surface. Examples of the plating method include an electrolytic plating method and an electroless plating method. Examples of the vapor phase growth method include a sputtering method, a CVD method, and a vapor deposition method. Examples of the etching method include physical etching and chemical etching. Examples of the polishing method include sandpaper polishing and blasting.
[0024]
The current collector used in the present invention is not particularly limited as long as it is made of a conductive metal foil. Specific examples of the current collector include a metal foil of a metal such as copper, nickel, iron, titanium, and cobalt, or an alloy foil made of a combination thereof. In particular, those containing a metal element that easily diffuses into the active material particles are preferably used. Such materials include metal foils containing a copper element, particularly copper foils and copper alloy foils. Since the copper element is easily diffused into silicon by heat treatment, improvement in the adhesion between the current collector and the active material layer can be expected by performing sintering on the negative electrode.
[0025]
Further, a metal foil provided with a layer containing a copper element on the surface of the current collector may be used as the current collector. That is, a metal foil formed of a metal element other than copper and having a copper or copper alloy layer formed on the surface can be used. Examples of such a metal foil include those in which a plating film of copper or a copper alloy is formed on a metal foil by an electrolytic plating method. Specifically, a copper foil or a copper alloy plating film is formed on the surface of a nickel foil.
[0026]
In the present invention, the thickness of the current collector is not particularly limited, but is preferably in the range of 10 to 100 μm.
In the present invention, the upper limit of the surface roughness Ra of the current collector is not particularly limited, but the thickness of the current collector is preferably in the range of 10 to 100 μm. The upper limit of Ra is preferably substantially 10 μm or less.
[0027]
In the present invention, it is preferable that the thickness X of the active material layer has a relationship of 5Y ≧ X and 250Ra ≧ X with the thickness Y of the current collector and its surface roughness Ra. By satisfying such a relationship, deformation of the current collector such as wrinkles can be suppressed, and detachment of the active material layer from the current collector can be suppressed.
[0028]
The thickness X of the active material layer is not particularly limited, but is preferably 1000 μm or less, and more preferably 10 to 100 μm.
The negative electrode for a lithium secondary battery of the present invention is obtained by disposing an active material layer on the surface of a current collector and then sintering the same in a non-oxidizing atmosphere. Sintering in a non-oxidizing atmosphere is performed, for example, under a nitrogen atmosphere or an inert gas atmosphere such as argon. It may be performed in a reducing atmosphere such as a hydrogen atmosphere. The heat treatment temperature at the time of sintering is preferably a temperature equal to or lower than the melting points of the current collector and the active material particles. For example, when a copper foil is used as the current collector, the melting point is preferably 1083 ° C. or less. Generally, the heat treatment temperature for sintering is preferably in the range of 200 to 500C, more preferably in the range of 300 to 450C. As a sintering method, a spark plasma sintering method or a hot press method may be used.
[0029]
In the present invention, after forming the active material layer on the current collector, it is preferable to roll this active material layer together with the current collector before sintering. By such rolling, the packing density in the active material layer can be increased, and the adhesion between the particles of the active material and the adhesion between the particles of the active material and the current collector can be improved. Discharge cycle characteristics can be obtained.
[0030]
It is preferable that the binder used in the present invention remains without being completely decomposed even after the heat treatment for sintering. Since the binder remains without decomposition even after the heat treatment, the adhesion between the active material particles and the current collector and between the active material particles is improved by sintering. Properties can be further enhanced. In addition, when a conductive metal foil having a surface roughness Ra of 0.2 μm or more is used as a current collector, the binder enters the irregularities on the surface of the current collector, so that an anchor effect is generated between the binder and the current collector. Expression, and the adhesion is further improved. For this reason, detachment of the active material layer from the current collector due to expansion and contraction of the volume of the active material during insertion and extraction of lithium can be suppressed, and favorable charge / discharge cycle characteristics can be obtained.
[0031]
As the binder in the present invention, polyimide is preferably used. Examples of the polyimide include a thermoplastic polyimide and a thermosetting polyimide. Note that polyimide can also be obtained by heat-treating polyamic acid.
[0032]
The polyimide obtained by the heat treatment of the polyamic acid is obtained by dehydrating and condensing the polyamic acid by the heat treatment to form a polyimide. The imidization ratio of the polyimide is preferably 80% or more. The imidation ratio is a mol% of the generated polyimide with respect to the polyimide precursor (polyamic acid). Those having an imidization ratio of 80% or more can be obtained, for example, by heat-treating a polyamic acid N-methyl-2-pyrrolidone (NMP) solution at a temperature of 100 to 400 ° C. for 1 hour or more. For example, when heat treatment is performed at 350 ° C., the imidization rate becomes 80% in about 1 hour and 100% in about 3 hours.
[0033]
In the present invention, it is preferable that the binder remains without being completely decomposed even after the heat treatment for sintering. Therefore, when polyimide is used as the binder, the binder is sintered at 600 ° C. or less at which the polyimide is not completely decomposed. Preferably, a treatment is performed.
[0034]
In the present invention, the amount of the binder in the active material layer is preferably 5% by weight or more of the total weight of the active material layer. The volume occupied by the binder is preferably 5% or more of the total volume of the active material layer. If the amount of the binder in the active material layer is too small, the adhesiveness in the electrode by the binder may be insufficient. In addition, when the amount of the binder in the active material layer is too large, the resistance in the electrode increases, so that initial charging may be difficult. Therefore, the amount of the binder in the active material layer is preferably 50% by weight or less of the total weight, and the volume occupied by the binder is preferably 50% or less of the total volume of the active material layer.
[0035]
In the present invention, a conductive powder can be mixed in the active material layer. By adding the conductive powder, a conductive network is formed by the conductive powder around the active material particles, so that the current collection in the electrode can be further improved. As the conductive powder, those having the same material as the conductive metal foil can be preferably used. Specifically, it is an alloy or a mixture of metals such as copper, nickel, iron, titanium, and cobalt, or a combination thereof. In particular, copper powder is preferably used as the metal powder. Further, conductive carbon powder can also be preferably used.
[0036]
The amount of the conductive powder to be added is preferably 50% by weight or less of the total weight with the active material. If the amount of the conductive powder is too large, the mixing ratio of the active material becomes relatively small, so that the charge / discharge capacity of the electrode becomes small.
[0037]
Examples of the active material particles used in the present invention include silicon and / or silicon alloy particles. Examples of the silicon alloy include a solid solution of silicon and one or more other elements, an intermetallic compound of silicon and one or more other elements, and a eutectic alloy of silicon with one or more other elements. Can be Examples of the method for producing the alloy include an arc melting method, a liquid quenching method, a mechanical alloying method, a sputtering method, a chemical vapor deposition method, and a firing method. In particular, examples of the liquid quenching method include a single-roll quenching method, a twin-roll quenching method, and various atomizing methods such as a gas atomizing method, a water atomizing method, and a disk atomizing method.
[0038]
Further, as the active material particles used in the present invention, particles obtained by coating the surface of silicon and / or silicon alloy particles with a metal or the like may be used. Examples of the coating method include electroless plating, electrolytic plating, chemical reduction, vapor deposition, sputtering, and chemical vapor deposition. The metal covering the particle surface is preferably the same metal as the conductive metal foil used as the current collector. By coating the same metal as the conductive metal foil, the bonding property with the current collector at the time of sintering is greatly improved, and further excellent charge / discharge cycle characteristics can be obtained.
[0039]
The active material particles used in the present invention may include particles made of a material alloyable with lithium. Materials for alloying lithium include germanium, tin, lead, zinc, magnesium, sodium, aluminum, gallium, indium, and alloys thereof.
[0040]
The lithium secondary battery of the present invention is characterized by comprising the above-described negative electrode of the present invention, a positive electrode containing a positive electrode material, and a non-aqueous electrolyte.
Solvent of the non-aqueous electrolyte used in the lithium secondary battery of the present invention is not particularly limited, and ethylene carbonate, propylene carbonate, butylene carbonate, cyclic carbonate such as vinylene carbonate, and dimethyl carbonate, methyl ethyl carbonate, diethyl A mixed solvent with a chain carbonate such as carbonate is exemplified. Further, a mixed solvent of the above-mentioned cyclic carbonate and an ether-based solvent such as 1,2-dimethoxyethane and 1,2-diethoxyethane is also exemplified. As a solute of the non-aqueous electrolyte, LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , LiC (C ₂ F ₅ SO ₂ ) ₃ , LiAsF ₆ , LiClO ₄ , Li ₂ B ₁₀ Cl ₁₀ , Li ₂ B ₁₂ Cl ₁₂ And mixtures thereof. In particular, LiXF _y (Where X is P, As, Sb, B, Bi, Al, Ga, or In; when X is P, As, or Sb, y is 6; and X is Bi, Al, Ga, or In. , Y is 4), and lithium perfluoroalkylsulfonic acid imide LiN (C _m F _{2m + 1} SO ₂ ) (C _n F _{2n + 1} SO ₂ (Wherein m and n are each independently an integer from 1 to 4) or lithium perfluoroalkylsulfonic acid methide LiN (C _p F _{2p + 1} SO ₂ ) (C _q F _{2q + 1} SO ₂ ) (C _r F _{2r + 1} SO ₂ (Wherein p, q and r are each independently an integer of 1 to 4). Among them, LiPF ₆ And LiN (C ₂ F ₅ SO ₂ ) ₂ A solute mixed with is particularly preferably used. Further, as the electrolyte, a gel polymer electrolyte in which a polymer electrolyte such as polyethylene oxide or polyacrylonitrile is impregnated with an electrolyte, LiI, Li ₃ An inorganic solid electrolyte such as N is exemplified. The electrolyte of the lithium secondary battery of the present invention is not limited, as long as the lithium compound as a solute that develops ionic conductivity and the solvent that dissolves and retains the same are not decomposed at the time of charging, discharging, or storing the battery. Can be used.
[0041]
As the positive electrode material of the lithium secondary battery of the present invention, LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiMnO ₂ , LiCo _0.5 Ni _0.5 O ₂ , LiNi _0.7 Co _0.2 Mn _0.1 O ₂ Lithium-containing transition metal oxides such as MnO ₂ Examples of such metal oxides do not contain lithium. In addition, any other substance capable of electrochemically inserting and removing lithium can be used without limitation.
[0042]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited to the following examples at all, it can be implemented by appropriately changing the scope without changing the gist Things.
[0043]
(Experiment 1)
[Preparation of negative electrode]
80.4 parts by weight of silicon powder having a purity of 99.9% as an active material and an average particle diameter of 1.0 μm were mixed with 17.9 parts by weight of a polyimide as a binder and sorbitan monooleate as a dispersant (dispersant). a1) 1.8 parts by weight were mixed with an N-methylpyrrolidone solution 8.5 times the weight of the binder to prepare a negative electrode mixture slurry. Mixing of the negative electrode mixture slurry was performed for 5 minutes using a stirrer that stirred at a speed of 1400 rotations per minute (1400 rotations / minute).
[0044]
This negative electrode mixture slurry was applied to one surface (rough surface) of an electrolytic copper foil (thickness: 35 μm) having a surface roughness Ra of 0.5 μm as a current collector, dried, and then rolled. The obtained product was cut out into a disc shape having a diameter of 20 mm, and this was heat-treated at 400 ° C. for 30 hours in an argon atmosphere, and sintered to obtain a negative electrode. The thickness of the sintered body (including the current collector) was 45 μm.
[0045]
[Preparation of positive electrode]
As a starting material, Li ₂ Co ₃ And CoCO ₃ Are weighed so that the atomic ratio of Li: Co becomes 1: 1 and mixed in a mortar, and this is pressed with a mold having a diameter of 17 mm, pressed and molded. Calcined for 24 hours, LiCoO ₂ Was obtained. This was ground in a mortar and adjusted to an average particle size of 20 μm.
[0046]
[Preparation of electrolyte solution]
As an electrolytic solution, LiPF was mixed in an equal volume mixed solvent of ethylene carbonate and diethyl carbonate. ₆ Was dissolved at 1 mol / liter.
[0047]
[Production of Battery]
Using the above positive electrode, negative electrode, and electrolytic solution, a lithium secondary battery A1 was produced.
FIG. 1 shows a configuration diagram of the battery. It comprises a positive electrode 1, a negative electrode 2, a separator 3, an electrolytic solution 4, an aluminum laminate casing 5, a positive electrode tab 6, a negative electrode tab 7, and the like.
[0048]
The positive electrode 1 and the negative electrode 2 face each other with the separator 3 interposed therebetween. These are connected to the positive electrode tab 6 and the negative electrode tab 7, and have a structure capable of charging and discharging as a secondary battery.
[0049]
[Charge / Discharge Test Conditions of Examples]
Charging: constant current charging
Constant current charging: current value 3.5 mA / cm ² , Charging end voltage 4.2V
Discharge: constant current discharge
Constant current discharge: current value 3.5 mA / cm ² , Charge end voltage 2.75V
[0050]
(Experiment 2)
In Experiment 1, except that N, N-dimethyllaurylamine (dispersant a2) and N-cocoalkyl-1,3-diaminopropane (dispersant a3) were used as dispersants instead of dispersant a1. In the same manner as in Experiment 1, batteries A2 and A3 were produced.
Table 1 shows the contents of the dispersants a1 to a3.
[0051]
[Table 1]

[0052]
(Experiment 3)
In Experiment 1, a battery B1 was produced in the same manner as in Experiment 1, except that no dispersant was used.
[0053]
[Evaluation of charge / discharge cycle characteristics]
For the batteries A1 to A3 and B1, the charge / discharge cycle characteristics were evaluated under the above charge / discharge test conditions. That is, at 25 ° C., a current value of 3.5 mA / cm ² After charging to 4.2 V with a current value of 3.5 mA / cm ² To discharge to 2.75 V, which was defined as one cycle of charge and discharge. The discharge capacity at the 100th cycle with respect to the discharge capacity at the first cycle was measured, and was taken as the cycle retention rate. Table 2 shows the results. The cycle maintenance rate of each battery is an index with the cycle maintenance rate of the battery A1 as 100.
[0054]
[Table 2]

[0055]
As is clear from Table 2, the batteries A1 to A3 using the dispersants a1 to a3 have a higher cycle maintenance ratio than the battery B1 using no dispersant.
This is because the active material particles are uniformly dispersed in the slurry by mixing the dispersant in the slurry, and the active material and the binder are uniformly mixed and dispersed. It is considered that the strength could be maintained even when the volume of the active material particles expanded and contracted, desorption of the active material particles was suppressed, and the current collecting property could be improved.
[0056]
(Experiment 4)
Experiment 1 except that silicon powder having a purity of 99.9% and an average particle size of 2.7 μm was used as the active material and N, N-dimethylstearylamine (dispersant a4) was used as the dispersant. In the same manner as in the above, a battery A4 was produced.
[0057]
(Experiment 5)
In Experiment 4, a battery B2 was produced in the same manner as in Experiment 4, except that no dispersant was used.
[0058]
(Experiment 6)
In Experiment 5, a battery B3 was produced in the same manner as in Experiment 5, except that the stirring rotation speed of the negative electrode mixture slurry was set to 3500 rpm.
[0059]
[Evaluation of charge / discharge cycle characteristics]
The charge / discharge cycle characteristics of the batteries A4, B2, and B3 were evaluated. Each battery was subjected to a current value of 3.5 mA / cm at 25 ° C. ² After charging to 4.2 V with a current value of 3.5 mA / cm ² To discharge to 2.75 V, which was defined as one cycle of charge and discharge. The discharge capacity at the 100th cycle with respect to the discharge capacity at the first cycle was measured and defined as a cycle retention rate. Table 3 shows the results. The cycle maintenance rate of each battery is an index with the cycle maintenance rate of the battery A1 as 100.
[0060]
[Table 3]

[0061]
From Table 3, it is clear that the battery A4 using the dispersant a4 has a higher cycle maintenance ratio than the batteries B2 and B3 without using the dispersant.
When the dispersant was not used, the cycle maintenance rate of the battery B3 having the higher stirring rotation speed at the time of preparing the slurry was higher than the battery B2 having the lower stirring rotation speed at the time of preparing the slurry. This indicates that the mechanical stirring step affects the cycle characteristics of the battery.
[0062]
However, the battery A4 using the dispersant has a higher cycle maintenance ratio than the battery B3 in which the stirring rotation speed is increased without using the dispersant. It is considered that the active material particles and the binder are more uniformly dispersed in the slurry in the battery A4 using the dispersant than in the battery B3 in which the stirring rotation speed was increased without using the dispersant. This indicates that the dispersion of the active material particles in the binder is insufficient only under the above-described stirring conditions for producing the battery B3.
[0063]
Therefore, when a dispersant is used, not only the mechanical diffusion step can be omitted, but also an electrode in which the active material particles are dispersed more uniformly after sintering can be produced. It is considered that the strength can be maintained even when the volume of the active material particles expands and contracts due to occlusion and release of the active material, the desorption of the active material particles is suppressed, and the current collecting property can be improved.
[0064]
【The invention's effect】
According to the present invention, the aggregation of the active material particles in the binder can be reduced, and the cycle life characteristics can be improved.
[Brief description of the drawings]
FIG. 1 is a schematic view showing a lithium secondary battery manufactured in an example according to the present invention.
[Explanation of symbols]
1: Positive electrode
2 ... negative electrode
3 ... Separator
4: Electrolyte
5 ... exterior body
6 ... Positive electrode tab
7 ... Negative electrode tab

Claims (15)

An active material layer containing active material particles containing silicon and / or a silicon alloy, a binder and a dispersant is disposed on a surface of a current collector made of a conductive metal foil and sintered under a non-oxidizing atmosphere. A negative electrode for a lithium secondary battery, comprising: An active material layer containing active material particles containing silicon and / or a silicon alloy, a binder and a dispersant is disposed on a surface of a current collector made of a conductive metal foil and sintered under a non-oxidizing atmosphere. A negative electrode for a lithium secondary battery,
The negative electrode for a lithium secondary battery, wherein the binder is a binder remaining after the heat treatment for sintering. The negative electrode for a lithium secondary battery according to claim 1, wherein the binder is a polyimide. The negative electrode for a lithium secondary battery according to any one of claims 1 to 3, wherein the dispersant is an oil-soluble surfactant. The negative electrode for a lithium secondary battery according to any one of claims 1 to 4, wherein the dispersant is a sorbitan fatty acid ester or an aliphatic amine. 6. The method according to claim 1, wherein the dispersant is sorbitan monooleate, N, N-dimethyllaurylamine, N, N-dimethylstearylamine or N-cocoalkyl-1,3-diaminopropane. 4. The negative electrode for a lithium secondary battery according to claim 1. The lithium secondary according to any one of claims 1 to 6, wherein the current collector is a copper foil or a copper alloy foil, or a metal foil provided with a copper layer or a copper alloy layer on a surface. Negative electrode for battery. The said current collector is an electrolytic copper foil or an electrolytic copper alloy foil, or a metal foil provided with an electrolytic copper layer or an electrolytic copper alloy layer on the surface, The claim 1 characterized by the above-mentioned. Negative electrode for lithium secondary batteries. Preparing a slurry by uniformly mixing active material particles containing silicon and / or a silicon alloy, a binder, and a dispersant in a solution;
A step of applying the slurry on the surface of a current collector made of a conductive metal foil to form an active material layer,
Baking the active material layer in a non-oxidizing atmosphere with the active material layer disposed on the surface of the current collector. The method for producing a negative electrode for a lithium secondary battery according to claim 9, wherein the binder is a binder remaining after the heat treatment for sintering. The method for producing a negative electrode for a lithium secondary battery according to claim 9, wherein the binder is polyimide. The method for producing a negative electrode for a lithium secondary battery according to any one of claims 9 to 11, wherein the dispersant is an oil-soluble surfactant. The method for producing a negative electrode for a lithium secondary battery according to any one of claims 9 to 12, wherein the dispersant is a sorbitan fatty acid ester or an aliphatic amine. 14. The method according to claim 9, wherein the dispersant is sorbitan monooleate, N, N-dimethyllaurylamine, N, N-dimethylstearylamine or N-cocoalkyl-1,3-diaminopropane. The method for producing a negative electrode for a lithium secondary battery according to claim 1. A negative electrode according to any one of claims 1 to 8 or a negative electrode manufactured by the method according to any one of claims 9 to 14, a positive electrode including a positive electrode material, and a nonaqueous electrolyte. A lithium secondary battery characterized by the above-mentioned.

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