JP2013110105A - Negative electrode for lithium ion secondary battery, and lithium ion secondary battery including the negative electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the cycle characteristics of a lithium ion secondary battery that contains a negative electrode active material composed of carbonaceous particles and Li-absorbing particles.SOLUTION: The ratio (D/D) of the median diameter D(D) of carbonaceous particles to the median diameter D(D) of Li-absorbing particles is higher than 1 and lower than or equal to 2, and the ratio (D/t) of the median diameter D(D) of the carbonaceous particles to the thickness (t) of a negative electrode active material layer is higher than or equal to 1/4 and lower than or equal to 5/6 so that the negative electrode active material layer contains many small pores. The pores reduce volume changes in charges and discharges to prevent stress concentration, and thus cracks and peeling in the negative electrode active material layer are prevented.

Description

  The present invention relates to a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery using the negative electrode.

  A lithium ion secondary battery is a secondary battery having a high charge / discharge capacity and capable of high output. Currently, it is mainly used as a power source for portable electronic devices, and further expected as a power source for electric vehicles that are expected to be widely used in the future. A lithium ion secondary battery has an active material capable of inserting and extracting lithium (Li) in a positive electrode and a negative electrode, respectively. And it operates by moving Li ions in the electrolyte provided between the two electrodes.

  In lithium ion secondary batteries, lithium-containing metal composite oxides such as lithium cobalt composite oxide are mainly used as the active material for the positive electrode, and carbon materials having a multilayer structure are mainly used as the active material for the negative electrode. Yes.

  The performance of the lithium ion secondary battery depends on the materials of the positive electrode, the negative electrode, and the electrolyte constituting the secondary battery. In particular, research and development of active material that forms an active material is being actively conducted. Currently, there is a carbon material such as graphite as a negative electrode active material generally used. A carbon negative electrode using graphite or the like as a negative electrode active material has an intercalation reaction, and thus has high cycle characteristics but is difficult to increase in capacity. Thus, silicon or silicon oxide having a higher capacity than carbon has been studied as a negative electrode active material.

  By using silicon as the negative electrode active material, a battery having a higher capacity than that using a carbon material can be obtained. However, silicon has a large volume change due to insertion and extraction of Li during charge and discharge. Therefore, there is a problem that silicon is pulverized and falls off or peels from the current collector, and the charge / discharge cycle life of the battery is short. Therefore, by using silicon oxide as the negative electrode active material, volume change associated with insertion and extraction of Li during charge and discharge can be suppressed more than silicon.

Further, the use of silicon oxide (SiO _x : x is about 0.5 ≦ x ≦ 1.5) as a negative electrode active material has been studied. It is known that SiO _x decomposes into Si and SiO ₂ when heat-treated. This is called a disproportionation reaction, and if it is a homogeneous solid silicon monoxide SiO with a ratio of Si to O of approximately 1: 1, it is separated into two phases of Si phase and SiO ₂ phase by the internal reaction of the solid. . The Si phase obtained by separation is very fine. Further, the SiO ₂ phase covering the Si phase has a function of suppressing decomposition of the electrolytic solution. Therefore, the secondary battery using the negative electrode active material made of SiO _x decomposed into Si and SiO ₂ has excellent cycle characteristics.

However, since SiO _x has low conductivity, graphite or an amorphous carbon material is mixed as a conductive material, and the negative electrode is made conductive by bringing the conductive material powder and the SiO _x powder into contact with each other at points or surfaces. For example, Japanese Patent Application Laid-Open No. 11-31518 discloses that a lithium ion secondary battery using a negative electrode material made of a mixture of lithium silicate powder and natural graphite powder has improved cycle characteristics. However, in a lithium ion secondary battery using a negative electrode material made of a mixture of SiO _x and graphite powder, cracks develop due to a difference in volume expansion due to repeated expansion and contraction of SiO _x with charge and discharge. There has been a problem that peeling occurs between the electric body and the negative electrode active material layer.

In view of this, Japanese Patent Application Laid-Open No. 2004-362789 discloses that in a negative electrode material containing Li storage particles and graphite particles, a (002) plane distance d (002) by an X-ray diffraction method is 0.3354 nm or more and 0.338 nm or less, and Raman. It has been proposed to use graphite particles in which the area ratio of G peak to D peak by spectroscopic analysis is G / D ≧ 9. It is described that Si or SiO is used as the Li storage particles, and that the cycle characteristics of the secondary battery are improved by using Si or SiO together with such graphite particles.

  Japanese Patent Laid-Open No. 2003-303588 discloses that composite particles are formed from a material containing an element that can be alloyed with Li, such as Si, and a conductive material, such as graphite, by a spray drying method or the like. Has proposed a negative electrode material having voids inside and having a void volume occupation ratio of the composite particles within a predetermined range. As described above, by using the composite particles having the void volume occupation ratio in the optimum range as the negative electrode material, there is a gap for absorbing the expansion in the composite particles, and deterioration of the electrode characteristics can be prevented. Moreover, since there are not too many gaps, a conductive network is sufficiently constructed, and a decrease in charge / discharge capacity of the nonaqueous secondary battery can be prevented.

JP 11-31518 A JP 2004-362789 A JP 2003-303588 A

  The present invention has been made in view of the above-described circumstances, and the main object of the present invention is to reduce the volume change during charge and discharge in a negative electrode material including Li storage particles and carbon-based particles. It is to prevent peeling at the interface between the negative electrode active material layer and the cycle characteristics of a lithium ion secondary battery using the negative electrode.

The negative electrode for a lithium ion secondary battery of the present invention that solves the above problems is a negative electrode for a lithium ion secondary battery comprising a current collector and a negative electrode active material layer formed on the current collector, the ratio of the negative electrode active material layer and the carbon-based particles, anda storage capable Li occluding particles lithium ions, _D 50 of _D 50 _{(D 1)} and Li occlusion particles of the carbon-based particles _{(D 2)} ( D ₁ / D ₂ ) exceeds 1 and is 2 or less, and the ratio (D ₁ / t) between D ₅₀ (D ₁ ) of the carbon-based particles and the thickness (t) of the negative electrode active material layer is 1/4 It is above and below 5/6. D ₅₀ refers to a particle diameter corresponding to an integrated volume distribution value of 50% in particle size distribution measurement by laser diffraction. That is, the D _50, it refers to the median diameter measured by volume.

  The feature of the lithium ion secondary battery of the present invention that solves the above problems is that the negative electrode of the present invention is used.

  The negative electrode active material layer made of a mixture of carbon-based particles and Li storage particles necessarily includes pores between the particles. Although the pores absorb the stress during expansion / contraction, the larger pores have a larger number of smaller pores than the smaller ones, and the stress absorption is superior. Further, it is considered that as the shape of the pore is closer to a true sphere, stress concentration can be avoided and cracks can be prevented.

However, the typical SiO _x D ₅₀ as a Li-occlusion particle is about 6.5 μm as a standard product, whereas the typical graphite particle size as a carbon-based particle is in the range of 10 μm to 20 μm. Therefore, in a negative electrode material made of a mixture of SiO _x and artificial graphite, it is difficult to have a large particle size difference and have many small pores.

So the negative electrode for a lithium ion secondary battery of the present invention, _D 50 _{(D 1)} and the ratio between _D 50 of Li occlusion particle _{(D 2) (D 1 /} D 2) is greater than 1 and 2 of the carbon-based particles since the less, so small pores is contained a number in the D ₁ and D ₂ and is close to the negative electrode active material layer.

On the other hand, the thickness of the negative electrode active material layer is desirably as thin as possible in order to reduce the electrical resistance value. However, when the thickness of the negative electrode active material layer is reduced, the D ₅₀ (D ₁ ) of the carbon-based particles and the negative electrode active material The ratio (D ₁ / t) to the layer thickness (t) also increases, and the dispersibility of the pores decreases.

Therefore, in the present invention, the ratio (D ₁ / t) between D ₅₀ (D ₁ ) of the carbon-based particles and the thickness (t) of the negative electrode active material layer is set to 1/4 or more and 5/6 or less. T is sufficiently larger than ₁ , and the dispersibility of the pores is good.

  Therefore, the lithium ion secondary battery of the present invention using the negative electrode of the present invention can relieve the stress caused by the volume change during charge and discharge by the small and highly dispersible pores contained in the negative electrode active material layer. Since peeling can be prevented, cycle characteristics are improved.

It is a SEM image of the cross section of the negative electrode which concerns on Example 2 of this invention. It is a SEM image of the cross section of the negative electrode which concerns on the comparative example 1 of this invention. It is a graph which shows the relationship between cycle number and discharge IR drop. It is a graph which shows the relationship between cycle number and discharge IR drop.

The negative electrode for a lithium ion secondary battery of the present invention includes carbon-based particles and Li storage particles. Examples of the carbon-based particles include natural graphite, artificial graphite, coke, mesophase carbon, vapor-grown carbon fiber, pitch-based carbon fiber, PAN-based carbon fiber, etc., and have excellent buffer performance and D ₅₀ of 1 μm to Graphite in the range of 15 μm is preferred. The D _{50 of the} carbon-based particles is particularly preferably 1 μm to 10 μm when the following SiO _x is used as the Li storage particles.

As the Li storage particles, silicon, tin, germanium, lead, indium, silicon oxide, tin oxide, or the like can be used, and silicon oxide represented by SiO _x (0.3 ≦ x ≦ 1.6). It is desirable to use SiO-based particles made of The SiO-based particles are composed of SiO _x decomposed into fine Si and SiO ₂ covering Si by a disproportionation reaction. When x is less than the lower limit, the Si ratio increases, so that the volume change during charge / discharge becomes too large, and the cycle characteristics deteriorate. When x exceeds the upper limit value, the Si ratio is lowered and the energy density is lowered. A range of 0.5 ≦ x ≦ 1.5 is preferable, and a range of 0.7 ≦ x ≦ 1.2 is more desirable.

In general, when oxygen is turned off, it is said that almost all SiO is disproportionated and separated into two phases at 800 ° C. or higher. Specifically, the raw material silicon oxide powder containing amorphous SiO powder is subjected to heat treatment at 800 ° C. to 1200 ° C. for 1 hour to 5 hours in an inert atmosphere such as in a vacuum or an inert gas. Thus, a powder composed of SiO _x particles including two phases of an amorphous SiO ₂ phase and a crystalline Si phase is obtained.

  The Li storage particles are preferably composed of SiO-based particles and a coating layer made of a carbon material and covering the surface of the SiO-based particles. By having the coating layer, the reaction between the SiO-based particles and hydrofluoric acid can be further prevented, and the cycle characteristics of the lithium ion secondary battery are improved. As the carbon material for the coating layer, natural graphite, artificial graphite, coke, mesophase carbon, vapor-grown carbon fiber, pitch-based carbon fiber, PAN-based carbon fiber, or the like can be used. In order to form the coating layer, silicon oxide and a carbon material precursor are mixed and fired. As the carbon material precursor, an organic compound that can be converted into a carbon material by firing, such as an organic compound such as a polymer such as sugars, glycols, or polypyrrole, or acetylene black can be used. In addition, the coating layer can be formed using a mechanical surface fusion treatment method such as mechanofusion described in Patent Document 1 or a vapor deposition method such as CVD.

The amount of the coating layer formed can be 1% by mass to 50% by mass with respect to the total of the SiO-based particles and the coating layer. If the coating layer is less than 1% by mass, the effect of improving the electrical conductivity cannot be obtained. If the coating layer exceeds 50% by mass, the proportion of SiO _x is relatively decreased and the negative electrode capacity is decreased. The formation amount of the coating layer is preferably in the range of 5% by mass to 30% by mass, and more preferably in the range of 5% by mass to 20% by mass. In addition, when providing the coating layer which consists of carbon materials on the surface of SiO type particle | grains, the mass of a coating layer is included in the mass of SiO type particle | grains. The carbon material forming the coating layer is distinguished from carbon-based particles.

Li occlusion particles is _{desirably D 50} is in the range of 1 m to 10 m. When D ₅₀ is larger than 10 μm, the charge / discharge characteristics of the lithium ion secondary battery are deteriorated. When D ₅₀ is smaller than 1 μm, the particles are aggregated into coarse particles, so that the charge / discharge characteristics of the lithium ion secondary battery are similarly decreased. There is a case.

  The mixing ratio of the carbon-based particles and the Li storage particles is preferably in the range of carbon-based particles: Li storage particles = 55: 27 to 45:37 by mass ratio. When the mass ratio of carbon-based particles: Li occluded particles = 55: 27 is larger than the mass ratio, the capacity decreases, which is not preferable. Carbon-based particles: Li occluded particles = 45: 37. If becomes small, the cycle characteristics deteriorate, which is not preferable. When the total mass of the mixture of carbon-based particles and Li storage particles, the conductive additive, and the binder resin is 100% by mass, the carbon-based particles are mixed in the range of 40% by mass to 65% by mass. Preferably it is. When the carbon-based particles are less than 40% by mass, it is difficult to improve the cycle characteristics of the lithium ion secondary battery. Even if the carbon-based particles are mixed in excess of 65% by mass, the reason is unknown, but the cycle characteristics of the lithium ion secondary battery are deteriorated as compared with the case where the carbon-based particles are 65% by mass or less. Furthermore, the mixing amount of the carbon-based particles is more optimal in the range of 45% by mass to 65% by mass.

The ratio of _D 50 _{(D 2)} of the _D 50 _{(D 1)} and Li occlusion particles of the carbon-based particles _(D 1 / _{D 2)} shall be greater than 1 and 2 below. When this ratio exceeds 2, the particle size difference becomes large, and it becomes difficult for the negative electrode active material layer to have many small pores.

  The pores preferably have a shape close to a true sphere, and the ratio (a / b) between the short diameter (a) and the long diameter (b) of the pores is preferably close to 1. By doing so, stress concentration can be prevented, and cracking and peeling can be prevented. The total volume of pores in the negative electrode active material layer is desirably smaller than the total volume of Li storage particles. When the total volume of the pores is larger than the total volume of the Li storage particles, the capacity per volume of the electrode is lowered and the capacity maintenance rate is lowered.

  The negative electrode of the lithium ion secondary battery of the present invention has a current collector and a negative electrode active material layer bound on the current collector. The negative electrode active material layer is prepared by adding a mixture of carbon-based particles and Li occlusion particles, a conductive additive, a binder resin, and an appropriate amount of an organic solvent, and mixing them into a slurry. It can be produced by coating on a current collector by a method such as a coating method, a doctor blade method, a spray coating method, or a curtain coating method, and pressing to cure the binder resin. The thickness (t) of this negative electrode active material layer can be set to 10 μm to 20 μm as in the conventional case.

The ratio (D ₁ / t) between D ₅₀ (D ₁ ) of the carbon-based particles and the thickness (t) of the negative electrode active material layer is set to ¼ or more and 5/6 or less. If this ratio (D ₁ / t) is less than ¼, the electrical resistance of the negative electrode active material layer increases, and the charge / discharge efficiency of the lithium ion secondary battery decreases. Cracks and peeling easily occur. This ratio (D ₁ / t) is particularly preferably 1/2 or more and 2/3 or less.

In addition, the ratio (D ₂ / t) between D ₅₀ (D ₂ ) of the Li storage particles and the thickness (t) of the negative electrode active material layer is such that D ₅₀ (D ₁ ) of the carbon-based particles and the Li storage particles described above. the ratio of _D 50 _{(D 2)} of _(D 1 / D _2), and the ratio of _D 50 _{(D 1)} and the anode active material layer thickness of the carbon-based particles _{(t) (D 1 / t} ) From the above relationship, it should be 1/8 or more and 2/3 or less.

  A current collector is a chemically inert electronic high conductor that keeps current flowing through an electrode during discharging or charging. The current collector can adopt a shape such as a foil or a plate, but is not particularly limited as long as it has a shape according to the purpose. As the current collector, for example, a copper foil or an aluminum foil can be suitably used.

  The conductive assistant is added to increase the conductivity of the electrode. The carbon-based particles described above also function as a conductive additive, but carbon black, acetylene black (AB), ketjen black (KB), vapor grown carbon fiber (VGCF), etc., which are carbonaceous fine particles, are used. It can add individually or in combination of 2 or more types. There is no particular limitation on the amount of conductive aid used. When Li storage particles having a coating layer made of a carbon material are used, the amount of conductive auxiliary agent added can be reduced or eliminated.

  The binder resin is used as a binder for binding the active material and the conductive additive to the current collector. The binder resin is required to bind the active material or the like in as little amount as possible, and the amount is 0.5 mass of the total of the mixture of carbon-based particles and Li storage particles, the conductive auxiliary agent, and the binder resin. % To 50% by mass is desirable. When the amount of the binder resin is less than 0.5% by mass, the moldability of the electrode is deteriorated. In addition, as binder resins, fluorinated polymers such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), rubbers such as styrene butadiene rubber (SBR), imides such as polyimide, polyamideimide, and polyamideimide silica hybrid Examples thereof include polymers, alkoxysilyl group-containing resins, polyacrylic acid, polymethacrylic acid, and polyitaconic acid. A copolymer of acrylic acid and an acid monomer such as methacrylic acid, itaconic acid, fumaric acid or maleic acid can also be used. Among them, a high binding binder having excellent binding properties is preferable, and at least one selected from a polyamideimide resin, a polyamideimide silica hybrid, and polyacrylic acid is particularly desirable.

  The negative electrode in the lithium ion secondary battery of the present invention is desirably pre-doped with lithium. In order to dope lithium into the negative electrode, for example, an electrode formation method in which a half battery is assembled using metallic lithium as the counter electrode and electrochemically doped with lithium can be used. The amount of lithium doped is not particularly limited.

By doping lithium or after the initial charging of the lithium ion secondary battery of the second embodiment of the present invention, the SiO ₂ phase of the SiO-based particles of the negative electrode active material is expressed as Li _x Si _y O _z Oxide compounds are included. As Li _x Si _y O _z , for example, x = 0, y = 1, z = 2 SiO ₂ , x = 2, y = 1, z = 3 Li ₂ SiO ₃ , x = 4, y = 1, Examples thereof include Li ₄ SiO _{4 with} z = 4. For example, Li ₄ SiO _{4 with} x = 4, y = 1 and z = ₄ is produced by the following reaction, and the Coulomb efficiency is calculated to be about 77%.
2SiO + 8.6Li + + 8.6e− → 1.5Li _4.4 Si + 1 / 2Li ₄ SiO ₄

Further, when the reaction is stopped halfway, x = 2, y = 1 , z = 3 of _Li 2 SiO ₃ and x = 4, y = 1, z = 4 in _Li 4 SiO as the following reaction ₄ is generated, and the Coulomb efficiency in this case is also calculated to be about 77%.
2SiO + 7.35Li + + 7.35e− → 1.42Li ₄ Si + 1 / 3Li ₂ SiO ₃ + 1 / 4Li ₄ SiO ₄

Li ₄ SiO ₄ produced by the above reaction is an inert substance which does not participate in the electrode reaction during charging and discharging and relieve a volume change of the active material during charging and discharging. Therefore, when the oxide compound represented by Li _x Si _y O _z is contained in the SiO ₂ phase of the SiO-based particles, the cycle characteristics of the lithium ion secondary battery of the present invention are further improved.

  The positive electrode, electrolyte solution, and separator which are not specifically limited can be used for the lithium ion secondary battery of this invention using the above-mentioned negative electrode. The positive electrode may be anything that can be used in a lithium ion secondary battery. The positive electrode has a current collector and a positive electrode active material layer bound on the current collector. The positive electrode active material layer includes a positive electrode active material and a binder, and may further include a conductive additive. The positive electrode active material, the conductive additive, and the binder are not particularly limited as long as they can be used in the lithium ion secondary battery.

Examples of the positive electrode active material include lithium metal, LiCoO ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , Li ₂ MnO ₃ , and sulfur. The current collector is not particularly limited as long as it is generally used for the positive electrode of a lithium ion secondary battery, such as aluminum, nickel, and stainless steel. As the conductive auxiliary agent, the same ones as described in the above negative electrode can be used.

The electrolytic solution is obtained by dissolving a lithium metal salt as an electrolyte in an organic solvent. The electrolytic solution is not particularly limited. As the organic solvent, from aprotic organic solvents such as fluoroethylene carbonate (FEC), propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), etc. One or more selected can be used. As the electrolyte to be dissolved, a lithium metal salt that is soluble in an organic solvent such as LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiI, LiClO ₄ , LiCF ₃ SO ₃ can be used.

For example, a lithium metal salt such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ or the like in an organic solvent such as ethylene carbonate, dimethyl carbonate, propylene carbonate, dimethyl carbonate is about 0.5 mol / l to 1.7 mol / l. A solution dissolved in a concentration can be used.

  A separator will not be specifically limited if it can be used for a lithium ion secondary battery. The separator separates the positive electrode and the negative electrode and holds the electrolytic solution, and a thin microporous film such as polyethylene or polypropylene can be used.

  The lithium ion secondary battery of the present invention is not particularly limited in shape, and various shapes such as a cylindrical shape, a stacked shape, and a coin shape can be adopted. Regardless of the shape, a separator is sandwiched between the positive electrode and the negative electrode to form an electrode body, and the space between the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal is used for current collection. After connecting using a lead or the like, the electrode body is sealed in a battery case together with an electrolyte to form a battery.

  Hereinafter, the present invention will be described in more detail with reference to examples.

<Preparation of negative electrode for lithium ion secondary battery>
The SiO powder was heat-treated at 900 ° C. for 2 hours to prepare a SiO _x powder having a D ₅₀ of 6.5 μm. With this heat treatment, if the silicon monoxide SiO is a homogeneous solid having a ratio of Si to O of approximately 1: 1, it is separated into two phases of Si phase and SiO ₂ phase by solid internal reaction. The Si phase obtained by separation is very fine.

A slurry was prepared by mixing 32 parts by mass of the obtained SiO _x powder, 50 parts by mass of graphite powder having a D ₅₀ of 9.2 μm, 8 parts by mass of carbon black, and 10 parts by mass of a binder solution. As the binder solution, a polyamideimide resin dissolved in N-methyl-2-pyrrolidone (NMP) was used. This slurry was applied to the surface of an electrolytic copper foil (current collector) having a thickness of about 20 μm to 30 μm by using a doctor blade to form a negative electrode active material layer on the copper foil. Thereafter, the current collector and the negative electrode active material layer were firmly and closely joined by a roll press. This was vacuum dried to form a negative electrode having a negative electrode active material layer thickness of 15 μm.

In this negative electrode, the ratio of _D 50 _{(D 2)} of _D 50 _{(D 1)} and SiO _x particles of the graphite particles _(D 1 / D ₂₎ is 1.42, _D 50 of the graphite particles _(D 1) And the thickness (t) of the negative electrode active material layer (D ₁ / t) is 0.61.

<Preparation of positive electrode>
L333 (Li [Mn _1/3 Ni _1/3 Co _1/3 ] O ₂ ) as a positive electrode active material, acetylene black (AB) as a conductive auxiliary agent, and polyvinylidene fluoride (PVDF) as a binder resin Were mixed to prepare a slurry-like positive electrode mixture. The composition ratio of each component (solid content) in the slurry was L333: AB: PVDF = 88: 6: 6 (mass ratio). This slurry was applied to a current collector, and a positive electrode mixture layer was laminated on the current collector. Specifically, this slurry was applied to the surface of an aluminum foil (current collector) having a thickness of 20 μm using a doctor blade.

  Then, it dried at 80 degreeC for 20 minute (s), the organic solvent was volatilized and removed from the positive mix. After drying, the electrode density was adjusted with a roll press. This was heat-cured at 200 ° C. for 2 hours in a vacuum drying furnace to obtain a positive electrode in which a positive electrode mixture layer having a thickness of about 50 μm was laminated on the upper layer of the current collector.

<Production of lithium ion secondary battery>
The positive electrode was cut into 30 mm × 25 mm and the negative electrode was cut into 31 mm × 26 mm, and accommodated with a laminate film. A rectangular sheet (40 mm × 40 mm square, thickness 30 μm) made of polypropylene resin as a separator was sandwiched between the positive electrode and the negative electrode to form an electrode plate group. The electrode plate group was covered with a set of two laminated films, and the three sides were sealed, and then the above electrolyte was poured into the bag-like laminated film. Thereafter, the remaining one side was sealed to obtain a laminate cell in which the four sides were hermetically sealed and the electrode plate group and the electrolyte were sealed. For the electrolyte, a mixed solution of FEC (fluoroethylene carbonate), EC (ethylene carbonate), MEC (methyl ethyl carbonate), DMC (dimethyl carbonate) = 0.4: 2.6: 3: 4 (volume ratio) is LiPF. ₆ was dissolved at a concentration of 1 mol / L. The positive electrode and the negative electrode were provided with a tab that could be electrically connected to the outside, and a part of this tab extended to the outside of the laminate cell. A lithium ion secondary battery in the form of a laminate cell (bipolar pouch cell) was obtained through the above steps.

A negative electrode was formed in the same manner as in Example 1 except that graphite powder having D ₅₀ of 12.5 μm was used instead of graphite powder having D ₅₀ of 9.2 μm. In this negative electrode, the ratio of the _D 50 of the graphite particles _{(D 1)} and _D 50 of the SiO _x particulate _{(D 2) (D 1 /} D 2) is 1.92, the graphite particles _D 50 _{(D 1} ) And the thickness (t) of the negative electrode active material layer (D ₁ / t) is 0.83.

  Using this negative electrode, a lithium ion secondary battery was produced in the same manner as in Example 1.

Similar SiO _x powder 42 parts Example _{1, D 50} and the graphite powder 40 parts by weight of 9.2 .mu.m, and carbon black 3 parts by weight, were mixed with a binder solution 15 parts by weight to prepare a slurry. As the binder solution, a polyamideimide resin dissolved in N-methyl-2-pyrrolidone (NMP) was used.

A negative electrode was formed in the same manner as in Example 1 except that this slurry was used. In this negative electrode, the ratio of the _D 50 of the graphite particles _{(D 1)} and _D 50 of the SiO _x particulate _{(D 2) (D 1 /} D 2) is 1.92, the graphite particles _D 50 _{(D 1} ) And the thickness (t) of the negative electrode active material layer (D ₁ / t) is 0.61. Using this negative electrode, a lithium ion secondary battery was produced in the same manner as in Example 1.

(Comparative Example 1)
A negative electrode was formed in the same manner as in Example 1 except that graphite powder having D ₅₀ of 20.0 μm was used instead of graphite powder having D ₅₀ of 9.2 μm. In this negative electrode, the ratio of the _D 50 of the graphite particles _{(D 1)} and _D 50 of the SiO _x particulate _{(D 2) (D 1 /} D 2) is 3.08, the graphite particles _D 50 _{(D 1} ) And the thickness (t) of the negative electrode active material layer (D ₁ / t) is 1.33. Using this negative electrode, a lithium ion secondary battery was produced in the same manner as in Example 1.

(Comparative Example 2)
A negative electrode was formed in the same manner as in Example 3 except that graphite powder having D ₅₀ of 20.0 μm was used instead of graphite powder having D ₅₀ of 9.2 μm. In this negative electrode, the ratio of the _D 50 of the graphite particles _{(D 1)} and _D 50 of the SiO _x particulate _{(D 2) (D 1 /} D 2) is 3.08, the graphite particles _D 50 _{(D 1} ) And the thickness (t) of the negative electrode active material layer (D ₁ / t) is 0.61. Using this negative electrode, a lithium ion secondary battery was produced in the same manner as in Example 3.

<Test>
The cross section of the negative electrode formed in Example 2 and Comparative Example 1 was observed with SEM. The SEM images are shown in FIGS. It can be seen that many smaller pores are formed in Example 2 than in Comparative Example 1. The lithium ion secondary batteries of Examples 1 and 2 and Comparative Example 1 were subjected to a constant current charge / discharge test in the first cycle at a charge / discharge current density of 0.2 mAcm ⁻² , and the charge and discharge current densities in the second and subsequent cycles. Performed at 0.5 mA cm ⁻² . The potential range was 0 V to 3.0 V at the lithium reference potential, and the test was performed at room temperature. After the first cycle, an oxide-based compound containing Li ₄ SiO ₄ and represented by Li _x Si _y O _z is generated in the SiO ₂ phase of SiO _x that is the active material in the negative electrode. In each cycle during the charge / discharge cycle test, the resistance value (discharge IR drop) of the negative electrode 10 seconds after the start of discharge was measured, and the results up to 400 cycles are shown in FIG.

<Evaluation>
From FIG. 3, since the negative electrode of Example 1 and Example 2 has a lower discharge IR drop than that of Comparative Example 1, the ratio (D ₁ / D ₂ ) is more than 1 and not more than 2 to make it conductive. It can be seen that an excellent negative electrode can be formed. Moreover, since the resistance of Example 2 is lower than that of Example 1, it is suggested that there is an optimum range for the particle size of graphite.

  The lithium ion secondary batteries of Example 3 and Comparative Example 2 were subjected to the same constant current charge / discharge test as described above, and the measurement results of the negative electrode resistance value (discharge IR drop) up to 500 cycles are shown in FIG. In Comparative Example 2, it can be seen that the resistance value after exceeding 400 cycles is significantly higher than that in Example 3, and greatly increases and decreases from a certain point close to 500 cycles. This indicates that in the negative electrode of Comparative Example 2, cracks occurred in the negative electrode active material layer, or the negative electrode active material layer peeled off from the current collector, resulting in a significant decrease in conductivity.

Claims (7)

A negative electrode for a lithium ion secondary battery comprising a current collector and a negative electrode active material layer formed on the current collector,
The negative electrode active material layer includes carbon-based particles and Li-occlusion particles capable of occluding lithium ions. The carbon-based particles D ₅₀ (D ₁ ) and the Li-occlusion particles D ₅₀ (D ₂ ) Ratio (D ₁ / D ₂ ) of more than 1 and 2 or less, and the ratio of D ₅₀ (D ₁ ) of the carbon-based particles to the thickness (t) of the negative electrode active material layer (D ₁ / A negative electrode for a lithium ion secondary battery, wherein t) is ¼ or more and 5/6 or less.   The negative electrode for a lithium ion secondary battery according to claim 1, wherein the Li storage particles are SiO-based particles. The carbonaceous particles a negative electrode for a lithium ion secondary battery according to claim 1 or claim 2 which is a graphite D ₅₀ is in the range of 1 to 15 m. The SiO-based particles composed of a SiO ₂ phase and the Si phase, the said SiO ₂ phase according to _Li x _Si y _O claims oxide-based compound represented by _z are in claim 2 or claim 3 Negative electrode for lithium ion secondary battery.   The negative electrode for a lithium ion secondary battery according to claim 1, wherein the negative electrode active material layer contains a highly binding binder.   The negative electrode for a lithium ion secondary battery according to claim 5, wherein the highly binding binder is at least one selected from a polyamideimide resin, a polyamideimide silica hybrid, and polyacrylic acid.   A lithium ion secondary battery using the negative electrode according to claim 1.