JP5361233B2 - Lithium secondary battery and manufacturing method thereof - Google Patents

Description

  The present invention relates to a lithium secondary battery and a method for manufacturing the same.

  Recently, in order to increase the energy density of lithium secondary batteries, an alloy material of elements such as Al, Sn, and Si that occludes lithium by an alloying reaction with lithium instead of a graphite material that is currently in practical use. However, it is being considered as a candidate for a new negative electrode active material.

  However, in an electrode using a material that is alloyed with lithium as an active material, the volume change of the active material during insertion and extraction of lithium is large, so that the active material is easily pulverized and detached from the current collector. There is a problem in that the current collecting property is lowered and the charge / discharge cycle characteristics are deteriorated.

  Therefore, in order to give high current collection in the electrode, a negative electrode obtained by sintering and arranging a mixture layer containing an active material composed of a material containing silicon and a binder in a non-oxidizing atmosphere, It has been found that it exhibits good charge / discharge cycle characteristics (Patent Document 1).

  Furthermore, it has been found that the cycle characteristics are further improved by controlling the particle size of the silicon active material particles (Patent Documents 2 to 5).

However, there is a demand for further improving the charge / discharge cycle characteristics.
Japanese Patent Laid-Open No. 2002-260637 Japanese Patent Laid-Open No. 2004-22433 JP 2006-278124 A JP 2007-73334 A JP 2007-234336 A

  An object of the present invention is to provide a lithium secondary battery that suppresses a decrease in current collecting performance in a negative electrode with the progress of a charge / discharge cycle, has a high energy density, and is excellent in charge / discharge cycle characteristics, and a method for manufacturing the same. is there.

The present invention relates to a lithium secondary battery comprising a negative electrode in which a negative electrode active material layer containing negative electrode active material particles and a negative electrode binder is formed on the surface of a negative electrode current collector, a positive electrode, and a nonaqueous electrolyte. Particles A containing silicon and / or a silicon alloy having a particle size distribution having a diameter (D ₅₀ ) of 3 μm or more and 6 μm or less and a particle diameter of 2 μm or more and 7 μm or less, and a median diameter (D ₅₀ ) is 9 μm or more and 15 μm or less and has a particle size distribution having a particle size distribution of 60% by volume or more in a particle size range of 7 μm or more and 17 μm or less, and a weight ratio (particle A: Particle B) Mixed particles mixed in a range of 10:90 to 25:75 are used as negative electrode active material particles, and a polyimide resin having the following structure is used as a negative electrode binder. It is characterized in that.

  According to the present invention, mixed particles obtained by mixing particles A containing silicon and / or silicon alloy and particles B containing silicon and / or silicon alloy in the above weight ratio range are used as negative electrode active material particles, By using a polyimide resin having a negative electrode binder as a negative electrode binder, it is possible to suppress a decrease in current collecting property in the negative electrode with the progress of charge / discharge cycle, to have a high energy density, and to be a lithium secondary battery excellent in charge / discharge cycle characteristics. it can.

  The reason why such an effect of the present invention can be obtained is as follows.

  According to the present invention, by mixing a relatively small active material particle in a small proportion with respect to a relatively large active material particle, in the negative electrode mixture layer, in the gap between the relatively large active material particles, A structure having a high filling property in which relatively small active material particles enter can be obtained. For this reason, the contact ratio between active materials increases, and the electronic conductivity in a negative mix layer increases.

  Furthermore, by controlling the mixing ratio of the relatively small active material particles to the relatively large active material particles within the range of the above weight ratio, an appropriate amount of small active material particles exists in the gaps between the large active material particles. It becomes the structure. When the ratio of the particles A is less than 10 in the above weight ratio, the amount of small active material particles (particle A) is too small, so that the contact ratio between the active materials cannot be increased, and the current collecting performance is not improved. On the other hand, when the weight ratio of the particles A is more than 25, the cycle characteristics are deteriorated because there are too many small active material particles (particles A) that are likely to be deteriorated (expanded or altered) with the passage of cycles. Small active material particles have a larger surface area per mass than large active material particles, so there is a large contact area with the electrolytic solution, and side reactions with the electrolytic solution on the active material surface easily occur. Deterioration is likely to occur.

  In addition, by using the polyimide resin having the above structure as the negative electrode binder, even when the volume change of the negative electrode active material particles occurs during charging and discharging, the arrangement between the active material particles is maintained, and high current collection is maintained. Is done. This is because the polyimide resin has high adhesion to silicon and is excellent in mechanical strength. When the adhesiveness and mechanical strength of the binder with the silicon negative electrode active material particles are low, the structure of the negative electrode mixture layer is destroyed when the volume of the silicon active material particles changes with charge / discharge. The effect of mixing small active material particles cannot be sufficiently obtained.

  The negative electrode active material particles of the present invention preferably have a crystallite size of 100 nm or less. In this case, there are many crystallites in the particle because of the small crystallite size with respect to the particle diameter. Accordingly, since many crystallite faces appear on the particle surface, fine irregularities exist on the surface. For this reason, since the anchor effect of a binder and negative electrode active material particle is acquired more, it becomes possible to obtain higher adhesiveness. Therefore, even after charge / discharge, the stability of the negative electrode mixture layer structure having a high filling property in which the small active material particles enter the gaps between the large active material particles is further improved, and higher current collecting properties can be obtained.

  Further, this anchor effect can be further promoted by performing a heat treatment at a temperature exceeding 300 ° C. which is the glass transition temperature of the polyimide resin as the binder. Due to the heat treatment at the glass transition temperature or higher, the polyimide resin becomes a plastic region, so that the polyimide enters the irregularities on the surface of the active material particles, and the heat fusion effect of the polyimide is exhibited. For this reason, greater adhesion can be obtained.

  The upper limit temperature for the heat treatment of the negative electrode is preferably 450 ° C. When the temperature exceeds 450 ° C., the polyimide resin, which is the binder of the present invention, frequently undergoes thermal decomposition, and the strength of the binder is greatly reduced. Thus, adhesion may be reduced, and charge / discharge characteristics may be reduced.

The negative electrode active material particles of the present invention have a median diameter (D ₅₀ ) of 9 μm or more and 14 μm or less and a particle size of 2 μm or more and 7 μm or less by mixing the above particles A and B in the above weight ratio. It is preferable that the negative electrode active material particles have a particle size distribution that exists in a range of volume% to 20 volume% and a particle diameter in the range of 7 μm to 17 μm in a range of 61 volume% to 86 volume%. As a result, the effect of improving the filling property by controlling the particle size of the active material and the effect of improving the adhesion property by using the polyimide resin as a binder are more effectively expressed, and the volume of the negative electrode active material accompanying charge / discharge Even when the change occurs, high current collecting property in the negative electrode mixture layer is maintained, and excellent charge / discharge cycle characteristics can be obtained.

  The effect of improving the electron conductivity by controlling the particle size of the negative electrode active material of the present invention can be obtained in the same way in the graphite active material system, but the dependence on the binder performance as in the present invention is very low. . This is because the graphite material has a small volume change during insertion and extraction of lithium compared to the silicon material, and the material itself has a high electronic conductivity. This is because the contact between the active material particles at the time of occlusion / release of lithium can be maintained without using a binder having excellent adhesion.

  Hereinafter, the negative electrode, the positive electrode, and the nonaqueous electrolyte in the present invention will be further described.

(Negative electrode active material)
The negative electrode active material of the present invention is a particle containing silicon and / or a silicon alloy. Examples of the silicon alloy include a solid solution of silicon and one or more other elements, and silicon and one or more other elements. Examples thereof include intermetallic compounds, eutectic alloys of silicon and one or more other elements.

  Moreover, as the negative electrode active material particles of the present invention, particles obtained by coating the surfaces of particles containing silicon and / or silicon alloys with a metal or the like may be used. Examples of the coating method include an electroless plating method, an electrolytic plating method, a chemical reduction method, a vapor deposition method, a sputtering method, and a chemical vapor deposition method.

  Further, as the negative electrode active material particles of the present invention, particles of silicon alone can also be preferably used.

Silicon and / or silicon alloy particles having a crystallite size of 100 nm or less can be produced by a thermal decomposition method or a thermal reduction method. The thermal decomposition method is a method for precipitating silicon produced by thermally decomposing a material containing a silane compound such as trichlorosilane (SiHCl ₃ ), monosilane (SiH ₄ ), disilane (Si ₂ H ₆ ) or the like. . In the thermal reduction method, silicon produced by thermally decomposing a material containing a silane compound such as trichlorosilane (SiHCl ₃ ), monosilane (SiH ₄ ), disilane (Si ₂ H ₆ ), etc. in a reducing atmosphere is precipitated. It is a method to make it.

In order to produce silicon particles having a smaller crystallite size by the thermal decomposition method or the thermal reduction method, it is preferable that the temperature at which the silane compound is thermally decomposed is as low as possible. The lower the temperature, the more likely that particles with a small crystallite size will be produced. When trichlorosilane (SiHCl ₃ ) is used as a raw material for the thermal decomposition method and the thermal reduction method, the minimum temperature required for thermal decomposition capable of appropriately depositing silicon is about 900 to 1000 ° C., but monosilane ( When SiH ₄ ) is used, it is about 600 to 800 ° C., and silicon can be deposited at a lower temperature. Therefore, monosilane (SiH ₄ ) is preferably used as a raw material for the production of silicon particles having a small crystallite size suitable for the present invention.

The silicon particles and silicon alloy particles of the present invention are preferably prepared by pulverizing and classifying a silicon lump prepared by a thermal decomposition method or a thermal reduction method. For example, by changing the degree of pulverization and classification, silicon particles and silicon alloy particles having different median diameters (D ₅₀ ) and particle size distributions can be obtained, and particles A and B of the present invention can be obtained.

(Negative electrode binder)
The negative electrode binder used in the present invention is a polyimide resin having the above chemical structure. The weight average molecular weight is preferably in the range of 30000-200000. As described above, after forming the negative electrode mixture layer, it is preferable to perform heat treatment at a temperature exceeding the glass transition temperature (300 ° C.) of the polyimide resin as the negative electrode binder. Thereby, a polyimide resin enters into the unevenness | corrugation of the surface of active material particle, and still greater adhesiveness can be obtained. As described above, the heat treatment temperature is preferably 450 ° C. or lower so that the thermal decomposition of the negative electrode binder does not occur.

  The amount of the negative electrode binder in the lithium secondary battery of the present invention is preferably 5% by weight or more of the total weight of the negative electrode mixture layer, and the volume occupied by the binder is preferably 5% or more of the total volume of the negative electrode active material layer. Here, the total volume of the negative electrode active material layer is the sum of the volumes of materials such as the active material and the binder contained in the active material layer. If there are voids in the active material layer, It does not include the volume occupied by voids.

(Negative electrode current collector)
As the negative electrode current collector of the present invention, a conductive metal foil is preferably used, and the surface roughness Ra of the surface on which the negative electrode active material layer is disposed is preferably 0.2 μm or more and 10 μm or less. By using a conductive metal foil having such a surface roughness Ra as the negative electrode current collector, the negative electrode binder enters the surface irregularities of the current collector, and an anchor effect is exhibited between the binder and the current collector, which is high. Since adhesiveness is obtained, even if the volume change of the silicon active material particles caused by the occlusion and release of lithium occurs, the peeling of the active material layer from the current collector is suppressed. When the negative electrode active material layers are disposed on both sides of the current collector, the surface roughness Ra is preferably 0.2 μm or more on both sides of the current collector.

  The surface roughness Ra and the average interval S between the local peaks are preferably 100Ra ≧ S. The surface roughness Ra and the average interval S between the local peaks are defined in Japanese Industrial Standard (JIS B 0601-1994), and can be measured, for example, with a surface roughness meter.

  In order to set the surface roughness Ra of the conductive metal foil to 0.2 μm or more, a method of subjecting the conductive metal foil to a roughening treatment is preferable. Examples of such roughening treatment include a plating method, a vapor phase growth method, an etching method, and a polishing method.

  Further, as described above, the negative electrode current collector in the present invention has a low softening property by heat treatment, that is, a high heat resistance when the negative electrode heat treatment for improving adhesion by thermal fusion of the negative electrode binder is performed. preferable. When the mechanical strength is reduced due to softening, the current collector is deformed in accordance with the volume change of the silicon active material during charge / discharge, so that the charge / discharge cycle characteristics are deteriorated.

  Examples of such a conductive metal foil having high mechanical strength and heat resistance include alloy foils, and in particular, elements such as copper, nickel, iron, titanium, cobalt, manganese, tin, and silicon, or combinations thereof. Alloy foil is preferred.

(Negative electrode conductive agent)
In the negative electrode of the present invention, conductive powder may be mixed as a conductive agent in the active material layer. By mixing the conductive powder, a conductive network of the conductive powder is formed around the active material particles, so that the current collecting property in the electrode can be further improved. Examples of the conductive powder include conductive carbon material powder such as graphite powder, metal such as copper, nickel, iron, titanium, cobalt, manganese, or alloy powder made of a combination thereof.

  The average particle size of the conductive powder is preferably 1 μm or more and 10 μm or less.

(Negative electrode fabrication method)
The negative electrode of the present invention can be prepared by applying a slurry in which negative electrode active material particles are uniformly mixed and dispersed in a negative electrode binder solution onto the surface of the negative electrode current collector and disposing a negative electrode mixture layer. preferable. By producing in this way, when the negative electrode binder is uniformly present in the negative electrode active material layer, the binding effect by the negative electrode binder is effectively expressed, and high adhesion is obtained in the negative electrode.

  In addition, as described above, when a thermoplastic polyimide resin is used as a negative electrode binder, the negative electrode of the present invention is formed by combining a negative electrode mixture layer with a negative electrode collection layer for further improvement of binding property by thermal fusion of the binder. It is preferable to produce it by performing a heat treatment at a temperature not lower than the glass transition temperature of the negative electrode binder and not higher than the thermal decomposition starting temperature in a state of being disposed on the electric body. Further, in the case of performing this heat treatment, when a foil containing a copper element is used as the negative electrode current collector, the heat treatment is performed at 200 ° C. or higher so that the copper element in the negative electrode current collector is in the negative electrode active material layer. By diffusing into the silicon negative electrode active material, a sintering effect can be obtained, and higher adhesion can be obtained. The atmosphere for performing the heat treatment is preferably a vacuum, a nitrogen atmosphere, an inert gas atmosphere such as argon, or a reducing atmosphere such as a hydrogen atmosphere.

(Positive electrode active material)
The positive electrode active material of the present invention has a chemical formula Li _a Ni _x Mn _y Co _z O ₂ (0 ≦ a ≦ 1.1, x + y + z = 1, and 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z. A lithium transition metal composite oxide having a layered structure represented by ≦ 1) is preferably used. As such, LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiMnO ₂ , Li Ni _0.5 Co _0.5 O ₂ , Li Ni _0.7 Co _0.3 O ₂ , Li Ni _0.8 Co _0.2 O ₂ , LiNi _0.33 Co _0.33 Mn _0.34 O _{2 and the} like are exemplified, and in particular, Li Ni _0.8 Co _0.2 O ₂ and LiCoO ₂ are preferably used. Can do. Further, at least one element selected from the group consisting of titanium, magnesium, zirconium, and aluminum may be added to the lithium transition metal composite oxide.

  Moreover, it is preferable that the average particle diameter (average particle diameter of a secondary particle) of lithium transition metal complex oxide is 20 micrometers or less. When the average particle diameter exceeds 20 μm, the movement distance of lithium ions in the lithium transition metal composite oxide particles is increased, so that the charge / discharge characteristics are deteriorated.

  As the positive electrode conductive agent, various known conductive agents can be used. For example, a conductive carbon material can be preferably used, and in particular, acetylene black or ketjen black can be preferably used.

  As the positive electrode binder, various known binders can be used without limitation as long as they do not dissolve in the solvent of the nonaqueous electrolyte in the present invention. For example, a fluorine resin such as polyvinylidene fluoride, a polyimide resin, Acrylonitrile and the like can be preferably used.

  As the positive electrode current collector, a conductive metal foil is preferably used. As such a thing, it can be used without a restriction | limiting, if it exists in the electric potential added to a positive electrode at the time of charging / discharging, without melt | dissolving in a non-aqueous electrolyte, and it can use without limitation especially aluminum foil.

(Nonaqueous electrolyte)
The solvent of the non-aqueous electrolyte in the lithium secondary battery of the present invention is not particularly limited, but cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate Chain carbonates such as fluorinated carbonates such as fluoroethylene carbonate, esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, Ethers such as 1,2-diethoxyethane, tetrahydrofuran, 1,2-dioxane, 1,3-dioxane, 1,4-dioxane and 2-methyltetrahydrofuran; Le such and, an amide such as dimethyl formamide can be used, these can be used singly or in combination. In particular, a mixed solvent of a cyclic carbonate and a chain carbonate can be preferably used.

In addition, the solute of the nonaqueous electrolyte in the present invention is not particularly limited, but is a chemical formula LiXF _y such as LiPF ₆ , LiBF ₄ , LiAsF ₆ (where X is P, As, Sb, B, Bi). , Al, Ga, or In, and when X is P, As, or Sb, y is 6, and when X is B, Bi, Al, Ga, or In, y is 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 , lithium compounds such as LiC (C ₂ F ₅ SO ₂ ) ₃ , LiClO ₄ , Li ₂ B ₁₀ Cl ₁₀ , Li ₂ B ₁₂ Cl ₁₂ can be used. Among these, LiPF ₆ can be particularly preferably used.

Furthermore, examples of the non-aqueous electrolyte in the present invention include gel polymer electrolytes in which a polymer electrolyte such as polyethylene oxide and polyacrylonitrile is impregnated with an electrolyte, and inorganic solid electrolytes such as LiI and Li ₃ N.

The nonaqueous electrolyte of the present invention preferably contains CO ₂ and / or fluoroethylene carbonate. Carbonic acid ester (fluoroethylene carbonate or the like) containing CO ₂ or F element has an effect of causing a smooth reaction with lithium on the surface of the silicon active material during charge / discharge. Thereby, the reaction uniformity is improved and the expansion of the silicon active material is suppressed, so that excellent charge / discharge cycle characteristics can be obtained.

  The non-aqueous electrolyte in the present invention can be used without limitation as long as the lithium compound as a solute that develops lithium ion conductivity and the solvent that dissolves and retains the lithium compound do not decompose during charge / discharge or storage of the battery.

  ADVANTAGE OF THE INVENTION According to this invention, it can be set as the lithium secondary battery which suppressed the fall of the current collection property in the negative electrode accompanying progress of charging / discharging cycles, has a high energy density, and was excellent in charging / discharging cycling characteristics.

  Moreover, according to the manufacturing method of the present invention, the lithium secondary battery of the present invention can be efficiently manufactured.

  Hereinafter, the present invention will be described in more detail based on examples. However, the present invention is not limited to the following examples, and can be implemented with appropriate modifications within a range not changing the gist thereof. Is.

<Experiment 1>
(Example A1)
[Production of silicon negative electrode active material]
A polycrystalline silicon lump was prepared by a thermal reduction method. Specifically, a silicon core installed in a metal reactor (reduction furnace) is heated by heating to 800 ° C., and purified high-purity monosilane (SiH ₄ ) gas vapor, purified hydrogen, By flowing a mixed gas, polycrystalline silicon was deposited on the surface of the silicon core, thereby producing a thick rod-shaped polycrystalline silicon lump.

Next, this polycrystalline silicon lump was pulverized and classified to prepare four types of polycrystalline silicon particles a1, a2, a3 and a4 having a purity of 99% and different particle sizes. The particle size distribution of the produced polycrystalline silicon particles a1, a2, a3 and a4 is shown in FIG. In addition, Table 1 shows the median diameter (D ₅₀ ) of each polycrystalline silicon particle. The median diameter was measured by a laser diffraction method.

  In these polycrystalline silicon particles, the crystallite size was 32 nm. The crystallite size was calculated by the Scherrer equation using the half width of the (111) peak of silicon in the powder X-ray diffraction pattern.

[Preparation of negative electrode mixture slurry]
The above polycrystalline silicon particles a1 and a3 were mixed with NMP (N-methyl-2-pyrrolidone) as a dispersion medium at a weight ratio of 10:90. These mixed particles, graphite powder having an average particle size of 3.5 μm as a negative electrode conductive agent, and thermoplastic polyimide resin A having a molecular structure represented by the above (Chemical Formula 1) as a negative electrode binder (glass transition temperature 300 ° C., weight Precursor varnish (solvent: NMP, concentration: 47 wt% in terms of amount of polyimide resin after heat treatment (after polymerization and imidization)), negative electrode active material powder, negative electrode conductive agent powder, and imidization It mixed so that mass ratio with subsequent polyimide resin might be set to 100: 3: 8.6, and it was set as the negative mix slurry.

  The precursor varnish of polyimide resin A is composed of m-phenylenediamine represented by the following (Chemical Formula 2) and 3,3 ′, 4,4′-benzophenone tetracarboxylic acid diethyl ester represented by the following (Chemical Formula 3). Can be produced. In addition, 3,3 ′, 4,4′-benzophenone tetracarboxylic acid diethyl ester is 3,3 ′, 4,4′-benzophenone tetracarboxylic acid shown in the following (Chemical Formula 4) in the presence of anhydride 2NMP. It can be prepared by reacting 2 equivalents of ethanol.

(Production of negative electrode)
The negative electrode mixture slurry was roughened on both sides of an 18 μm thick copper alloy foil (C7025 alloy foil, composition: Cu 96.2 wt%, Ni 3 wt%, Si 0.65 wt%, Mg 0.15 wt%). The surface of the negative electrode current collector was applied in air at 25 ° C., dried in air at 120 ° C., and then rolled in air at 25 ° C. The roughening of the copper alloy foil is performed by forming an electrolytic copper layer on the surface by a plating method, the surface roughness Ra (JIS B0601-1994) is 0.25 μm, and the average peak interval S (JIS B0601-1994). ) Was roughened to 0.85 μm.

After the rolling treatment, the current collector provided with the negative electrode mixture layer was cut out into a rectangle having a length of 380 mm and a width of 52 mm, and then heat-treated in an argon atmosphere at 400 ° C. for 10 hours to form a negative electrode active material on the surface of the negative electrode current collector. A negative electrode on which a material layer was formed was produced. The amount of the negative electrode mixture layer on the negative electrode current collector was 5.6 mg / cm ² , and the thickness of the negative electrode current collector having the mixture layer formed on both surfaces was 56 μm.

  A nickel plate as a negative electrode current collecting tab was connected to the end of the negative electrode to complete the negative electrode.

[Preparation of non-aqueous electrolyte]
After dissolving 1 mol / liter of lithium hexafluorophosphate (LiPF ₆ ) in a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a volume ratio of 3: 7, To this solution, 0.4 wt% carbon dioxide gas and 10 wt% fluoroethylene carbonate were added to prepare a non-aqueous electrolyte.

[Preparation of lithium transition metal composite oxide]
Li ₂ CO ₃ and CoCO ₃ were mixed in a mortar so that the molar ratio of Li and Co was 1: 1, then heat-treated in an air atmosphere at 800 ° C. for 24 hours, and then pulverized. A powder of lithium cobalt composite oxide represented by LiCoO ₂ having an average particle diameter of 11 μm was obtained. The obtained positive electrode active material powder had a BET specific surface area of 0.37 m ² / g.

[Production of positive electrode]
NMP as a dispersion medium, the above positive electrode active material powder, carbon powder with an average particle diameter of 30 nm as a positive electrode conductive agent, and polyvinylidene fluoride as a positive electrode binder, the weight ratio of the active material, the conductive agent and the binder Was added so as to be 95: 2.5: 2.5, and then kneaded to obtain a positive electrode mixture slurry.

This positive electrode mixture slurry was applied to the surface of an aluminum foil having a thickness of 15 μm, a length of 402 mm, and a width of 50 mm as a positive electrode current collector, with a coating portion having a length of 340 mm, a width of 50 mm, and a back surface of 270 mm and a width of 50 mm. After being coated, dried, and rolled. The amount of the active material layer of the current collector and the thickness of the positive electrode were 45 mg / cm ² and 143 μm, respectively, where the active material was formed on both sides.

  In addition, the aluminum plate was connected as a positive electrode current collection tab to the part which has not apply | coated the positive mix layer in the edge part of a positive electrode.

(Production of electrode body)
Using one sheet of the positive electrode, one sheet of the negative electrode, a polyethylene microporous membrane separator having a thickness of 20 μm, a length of 450 mm, and a width of 54.5 mm, the positive electrode and the negative electrode are opposed to each other with the separator interposed therebetween. The positive electrode tab and the negative electrode tab were wound on the outermost periphery in a spiral shape with a core having a diameter of 18 mm. After winding, the winding core was pulled out to produce a spiral electrode body. The spiral electrode body was crushed to obtain a flat electrode body.

  FIG. 3 is a perspective view showing the produced flat electrode body. A positive electrode current collecting tab 1 and a negative electrode current collecting tab 2 are drawn out from the electrode body 3.

[Production of battery]
The above flat electrode body and the above electrolytic solution were inserted and injected into an aluminum laminate outer package in an atmosphere of CO ₂ at 25 ° C. and 1 atm to produce a flat battery of the present invention A1.

  FIG. 4 is a plan view (a) and a cross-sectional view (b) showing the produced flat battery of the present invention A1. Sectional drawing (b) is sectional drawing which follows the AA line of top view (a).

  As shown in FIG. 4, the battery A1 of the present invention is a part of the heat seal closing part 5 in which the flat electrode body 3 is inserted into the exterior body 4 made of aluminum laminate, and the electrolytic solution is then placed in the exterior body 4. It is formed by sealing by heat welding.

(Examples A2 to 12 and Comparative Examples B1 to B12)
As shown in Table 2, in the same manner as in the present invention battery A1, except that the mixed particles obtained by mixing the silicon particles A and the silicon particles B in the predetermined mixing ratio shown in Table 2 are used as the negative electrode active material. A negative electrode slurry was prepared, and the present invention batteries A2 to A12 and comparative batteries B1 to B12 were prepared using the negative electrode slurry in the same manner as the present invention battery A1.

[Evaluation of charge / discharge cycle characteristics]
About said this invention battery A1-A12 and comparative battery B1-B12, the following charging / discharging cycling characteristics were evaluated on the following charging / discharging cycling conditions. The cycle number was defined as the cycle life when the capacity retention ratio (the value obtained by dividing the discharge capacity at the nth cycle by the discharge capacity at the first cycle) reached 80%.

(Charge / discharge cycle conditions)
-Charging condition in the first cycle After performing constant current charging for 4 hours at a current of 45 mA, constant current charging is performed until the battery voltage reaches 4.2 V at a current of 180 mA, and the current value is further increased at a voltage of 4.2 V. The constant voltage charge was performed until it became 45 mA.

-First cycle discharge condition Constant current charging was performed at a current of 180 mA until the battery voltage reached 2.75V.

-Charging conditions after the 2nd cycle Constant current charging was performed until the battery voltage reached 4.2 V at a current of 900 mA, and further constant voltage charging was performed until the current value reached 45 mA at a voltage of 4.2 V.

-Discharge conditions after the second cycle Constant current discharge was performed at a current of 900 mA until the battery voltage reached 2.75V.

  Table 2 shows the cycle life of the batteries A1 to 12 of the present invention and the comparative batteries B1 to B12. The cycle life of the present invention batteries A1 to A12 and the comparative batteries B1 to B12 is an index with the cycle life of the present invention battery A1 as 100.

  As shown in Table 2, in accordance with the present invention, mixed particles obtained by mixing silicon particles A and silicon particles B so that the weight ratio (particle A: particle B) is 10:90 to 25:75 are negative electrode active material particles. Inventive batteries A1 to A12 using the binder made of the polyimide resin of the present invention show excellent cycle characteristics as compared with comparative batteries B1 to B12. This seems to be based on the following effects.

  That is, in the present invention batteries A1 to A12, two kinds of large and small active material particles whose particle size distribution is specified are mixed so that the ratio of small active material particles is small with respect to large active material particles, and silicon Binders with high adhesive strength and excellent mechanical strength are used. As a result, even when the volume change of the silicon active material particles occurs due to charge / discharge, in the negative electrode mixture layer, the small active material particles have a high filling property that enters the gaps between the large active material particles, that is, between the particles. This is presumably because the contact ratio is high and a structure with high current collection is maintained.

  5-8 is a figure which shows the particle size distribution of the mixed particle | grains which mixed the silicon particle A and the silicon particle B in the predetermined ratio.

  5 to 8, 100/0, 95/5, 90/10, 80/20, 75/25, and 70/30 are respectively the mixing ratios of the respective weight percentages of silicon particles B / silicon particles A. Show.

  FIG. 5 shows the particle size distribution of mixed particles of silicon particles B having a median diameter of 9.6 μm and silicon particles A having a median diameter of 3.5 μm. FIG. 6 shows a particle size distribution of mixed particles of silicon particles B having a median diameter of 9.6 μm and silicon particles A having a median diameter of 5.7 μm. FIG. 7 shows silicon particles B having a median diameter of 14.1 μm, 8 shows the particle size distribution of mixed particles of silicon particles A having a diameter of 3.5 μm, and FIG. 8 shows the particle size distribution of mixed particles of silicon particles B having a median diameter of 14.1 μm and silicon particles A having a median diameter of 5.7 μm. Yes.

  As apparent from FIGS. 5 to 8, in the particle size distribution of the mixed particles mixed according to the present invention, a peak based on the silicon particles B and a peak or shoulder based on the silicon particles A are observed.

(Comparative Examples B13 to B36)
Comparative batteries B13 to B36 were produced in the same manner as in Examples A1 to A12 and Comparative Examples B1 to B12 except that the polyimide resin B having the molecular structure represented by the following (Chemical Formula 5) was used as the negative electrode binder. .

  Polyimide resin B has a glass transition temperature of 195 ° C. and a weight average molecular weight of 50,000. For the formation of the polyimide resin B, a varnish of the polyimide resin B (solvent; NMP, concentration 18% by weight) was used. Moreover, the negative electrode active material powder, the negative electrode conductive agent powder, and the polyimide resin B were mixed so that the weight ratio was 100: 3: 8.6 to prepare a negative electrode mixture slurry.

  For comparative batteries B13 to B36, the charge / discharge cycle characteristics were evaluated in the same manner as described above, and the evaluation results are shown in Table 3.

  As shown in Table 3, even if the mixed particles obtained by mixing the silicon particles A and the silicon particles B in the range of the weight ratio of 10:90 to 25 to 75 which are the scope of the present invention are used as the negative electrode active material, When polyimide resin other than the polyimide resin of this invention is used, it turns out that the improvement of a charge / discharge cycle characteristic is not recognized.

  This is because the binder having the structure shown in (Chemical Formula 5) has lower adhesion to silicon than the binder having the structure shown in (Chemical Formula 1). When this occurs, it is considered that the negative electrode mixture layer structure is destroyed, and the effect of improving the current collecting property by controlling the particle size of the negative electrode active material cannot be obtained.

<Experiment 2>
[Production of silicon negative electrode active material]
By pulverizing and classifying the metal silicon lump, metal silicon particles b1, b2, b3 and b4 having a purity of 98% (Al; 500 ppm by weight, Fe: 200 ppm by weight) and having different particle sizes were produced. The particle size distribution of the metal silicon particles b1, b2, b3 and b4 is shown in FIG. Table 4 shows the median diameter (D ₅₀ ).

  These metal silicon particles are all in a single crystal state in the above particle size. Therefore, the crystallite size is the same size as the particle diameter.

(Examples C1 to C12 and Comparative Examples D1 to D12)
As shown in Table 5, the metal silicon particles b1, b2, b3, and b4 described above were used as silicon particles A or silicon particles B, and particles mixed at a predetermined mixing ratio shown in Table 5 were used as negative electrode active materials. Inventive batteries C1 to C12 and comparative batteries D1 to D12 were produced in the same manner as the inventive battery or comparative battery in Experiment 1, except that the above were used.

[Evaluation of charge / discharge cycle characteristics]
About this invention battery C1-C12 and comparative battery D1-D12, it carried out similarly to the experiment 1, evaluated the charging / discharging cycle characteristic, and calculated | required cycle life. The evaluation results are shown in Table 5. The cycle life is an index with the cycle life of the present invention battery A1 as 100.

  As is apparent from the results shown in Table 5, according to the present invention, mixed particles obtained by mixing silicon particles A and silicon particles B in a weight ratio of 90:10 to 25:75 are used as the negative electrode active material, and The batteries C1 to C12 of the present invention using the polyimide resin A having the structure shown in FIG. 5) as a negative electrode binder exhibit excellent cycle characteristics as compared with the comparative batteries D1 to D12.

  However, when the present invention batteries C1 to C12 are compared with the present invention batteries A1 to A12 using the polycrystalline silicon particles having a small crystallite size of 32 nm as the negative electrode active material, the polycrystalline silicon particles having a crystallite size of 32 nm. It can be seen that the batteries A1 to A12 of the present invention using the remarkably improved charge / discharge cycle characteristics. This is because the polycrystalline silicon particles having a crystallite size as small as 32 nm have many irregularities on the surface, so that the anchor effect with the binder can be obtained more greatly, whereas the crystallite size is almost the same as the particle size. It is considered that the metal silicon particles have less irregularities on the surface, so the anchor effect is small, and the stability of the negative electrode mixture layer structure as in the case of the polycrystalline silicon active material cannot be obtained.

  9 to 12 are diagrams showing particle size distributions of mixed particles obtained by mixing single crystal metal silicon particles b, b2, b3, and b4 at a predetermined ratio.

  5 to 8, 100/0, 95/5, 90/10, 80/20, 75/25, and 70/30 are respectively a particle B having a large particle diameter and a particle A having a small particle diameter. The mixing ratio is shown.

  FIG. 9 shows a particle size distribution of mixed particles obtained by mixing particles b3 having a median diameter of 10.3 μm and particles b1 having a median diameter of 3.0 μm. FIG. 10 shows particles b3 having a median diameter of 10.3 μm, FIG. 11 shows the particle size distribution of the mixed particles with the particle b2 having a median diameter of 5.9 μm, and FIG. 11 shows the particle size distribution of the mixed particles of the particle b4 having a median diameter of 15.3 μm and the particle b1 having a median diameter of 3.0 μm. FIG. 12 shows the particle size distribution of the mixed particles of the particle b4 having a median diameter of 15.3 μm and the particle b2 having a median diameter of 5.9 μm.

    As is clear from FIGS. 9 to 12, in the mixed particles according to the present invention, a peak based on the particle B having a relatively large particle size and a peak or shoulder based on the particle A having a relatively small particle size are recognized. .

(Comparative Examples D13 to D36)
As shown in Table 6, as silicon particles A and silicon particles B, metal silicon particles b1, b2, b3 and b4 were used, and as a negative electrode binder, as in Comparative Examples B13 to B36, polyimide resin B was used. Comparative batteries D13 to D36 were produced in the same manner as described above, and the charge / discharge cycle characteristics were evaluated. The evaluation results are shown in Table 6.

  As shown in Table 6, even when single crystal metal silicon particles are used as the silicon particles, when the polyimide resin B which is outside the scope of the present invention is used as the negative electrode binder, the particle size control of the negative electrode active material is performed. It can be seen that the effect of improving the filling property by cannot be obtained. This is because, as described above, the polyimide resin B has low adhesion to silicon, and therefore, when the volume change of the silicon active material particles occurs due to charge / discharge, the negative electrode mixture layer structure is destroyed. It seems to be.

The figure which shows the particle size distribution of the polycrystalline silicon particle produced in the Example according to this invention. The figure which shows the particle size distribution of the single crystal metal silicon particle produced in the Example according to this invention. The perspective view which shows the electrode body produced in the Example according to this invention. The top view (a) and sectional drawing (b) which show the lithium secondary battery produced in the Example according to this invention. The figure which shows the particle size distribution of the mixed particle in the Example according to this invention, and a comparative example. The figure which shows the particle size distribution of the mixed particle in the Example according to this invention, and a comparative example. The figure which shows the particle size distribution of the mixed particle in the Example according to this invention, and a comparative example. The figure which shows the particle size distribution of the mixed particle in the Example according to this invention, and a comparative example. The figure which shows the particle size distribution of the mixed particle in the Example according to this invention, and a comparative example. The figure which shows the particle size distribution of the mixed particle in the Example according to this invention, and a comparative example. The figure which shows the particle size distribution of the mixed particle in the Example according to this invention, and a comparative example. The figure which shows the particle size distribution of the mixed particle in the Example according to this invention, and a comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Positive electrode current collection tab 2 ... Negative electrode current collection tab 3 ... Electrode body 4 ... Exterior body 5 ... Heat seal closing part of exterior body

Claims (4)

A lithium secondary battery comprising a negative electrode in which a negative electrode active material layer containing negative electrode active material particles and a negative electrode binder is formed on the surface of a negative electrode current collector, a positive electrode, and a non-aqueous electrolyte,
A particle A containing silicon and / or a silicon alloy having a particle size distribution having a median diameter (D ₅₀ ) of 3 μm or more and 6 μm or less and a particle size of 2 μm or more and 7 μm or less, and a median diameter ( D ₅₀ ) having a particle size distribution having a particle size distribution of not less than 9 μm and not more than 15 μm and having a particle size of not less than 7 μm and not more than 17 μm and containing 60% by volume of silicon and / or a silicon alloy and a weight ratio (particle A : Particle B) Using mixed particles mixed in the range of 10:90 to 25:75 as the negative electrode active material particles,
A lithium secondary battery using a polyimide resin having the following structure as the negative electrode binder.

  The lithium secondary battery according to claim 1, wherein a crystallite size of the negative electrode active material particles is 100 nm or less. The median diameter (D ₅₀ ) of the negative electrode active material particles is 9 μm or more and 14 μm or less, 9% by volume to 20% by volume in the range of 2 μm to 7 μm, and 61% by volume in the range of 7 μm to 17 μm. 3. The lithium secondary battery according to claim 1, wherein the negative electrode active material particles have a particle size distribution of 86% by volume or less. A method for producing the lithium secondary battery according to any one of claims 1 to 3,
Mixing the particles A containing silicon and / or a silicon alloy with the particles B containing silicon and / or a silicon alloy at the weight ratio to prepare the mixed particles;
Using the mixed particles as the negative electrode active material and producing the negative electrode using the polyimide resin as the negative electrode binder;
And a step of producing a lithium secondary battery using the negative electrode, the positive electrode, and the non-aqueous electrolyte.

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