Negative electrode for lithium ion battery and lithium ion battery having the negative electrode

Abstract

To provide a negative electrode for a lithium ion battery, capable of effectively suppressing a temperature increasing generated by exposing a negative electrode alloy in an internal part of the lithium ion battery in a charged state, to the air, and a lithium ion battery having the negative electrode.SOLUTION: A negative electrode for a lithium ion battery contains a negative electrode alloy containing a negative electrode active material particle and an electrolyte. The negative electrode active material particle includes: an active material particle; and an adhesive material that is adhered to at least one part of a front surface of the active material particle. A viscosity coefficient under 20°C±1°C of the adhesive material is 90.5 mPa*s or more.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present disclosure relates to a negative electrode for a lithium ion battery and a lithium ion battery including the negative electrode.

Background Art

[0002] When a charged lithium ion battery is damaged and the internal negative electrode composite material is exposed to the atmosphere, it is known that moisture and oxygen in the atmosphere (hereinafter referred to as "moisture etc.") react with the negative electrode composite material, and the reaction heat causes a temperature rise. It is considered that such a temperature rise can be suppressed by covering the active material particles with a coating material formed of a material having a slow diffusion rate of moisture etc. Although it is not a coating material for such a purpose, a lithium ion battery including negative electrode active material particles covered with a coating material is described in Patent Document 1.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, Patent Document 1 does not assume suppressing the temperature rise caused by the internal negative electrode composite material being exposed to the atmosphere due to damage etc. of a charged lithium ion battery. Therefore, the lithium ion battery described in Patent Document 1 does not necessarily effectively suppress such a temperature rise.

[0005] In view of the above circumstances, at least one embodiment of the present disclosure provides a negative electrode for a lithium ion battery that can effectively suppress the temperature rise caused by the internal negative electrode composite material of a charged lithium ion battery being exposed to the atmosphere, and a lithium ion battery including the negative electrode.

Means for Solving the Problem

[0006] To achieve the above object, the negative electrode for a lithium ion battery according to the present disclosure includes a negative electrode composite material containing negative electrode active material particles and an electrolytic solution. The negative electrode active material particles have active material particles and an adhesive attached to at least a part of the surface of the active material particles, and the viscosity of the adhesive at 20°C ± 1°C is 90.5 mPa·s or more.

Advantages of the Invention

[0007] According to the negative electrode for a lithium ion battery of the present disclosure, since it includes a negative electrode composite material containing negative electrode active material particles with an appropriate adhesive attached to at least a part of the surface of the active material particles, it is possible to effectively suppress the temperature rise caused by exposing the negative electrode composite material inside the charged lithium ion battery to the atmosphere.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

Modes for Carrying Out the Invention

[0009] Hereinafter, the negative electrode for a lithium ion battery according to an embodiment of the present disclosure will be described with reference to the drawings. The embodiments described below show one aspect of the present disclosure and do not limit this disclosure, and can be arbitrarily changed within the scope of the technical idea of the present disclosure.

[0010] As shown in FIG. 1, a lithium-ion battery 1 according to an embodiment of the present disclosure includes a battery cell 2 and an exterior body 3 that seals the battery cell 2 therein. FIG. 1 depicts the configuration of the lithium-ion battery 1 in which one battery cell 2 is housed in the exterior body 3. However, the lithium-ion battery 1 may have a configuration in which two or more battery cells 2 are housed in the exterior body 3.

[0011] The battery cell 2 includes a positive electrode 4, a negative electrode 5, and a separator 6 provided between the positive electrode 4 and the negative electrode 5. The positive electrode 4 includes a positive electrode composite material 7 and a positive electrode current collector 8. The negative electrode 5 includes a negative electrode composite material 9 and a negative electrode current collector 10. The battery cell 2 may include an annular frame member 11, although it is not an essential configuration. In a configuration where the battery cell 2 includes the frame member 11, the separator 6 can be fixed and supported by embedding the peripheral edge of the separator 6 in the frame member 11. Also, the positive electrode current collector 8 and the negative electrode current collector 10 can be brought into surface contact with the frame member 11 on both sides of the separator 6 to fix the positive electrode 4 and the negative electrode 5 to the frame member 11.

[0012] As the positive electrode current collector 8 and the negative electrode current collector 10, current collectors used in known lithium-ion batteries can be used. For example, known metal current collectors or resin current collectors composed of a conductive material and a synthetic resin can be used.

[0013] The positive electrode composite material 7 can be a wet powder containing positive electrode active material particles and a known electrolyte solution containing an electrolyte and a non-aqueous solvent. Examples of the positive electrode active material particles include composite oxides of lithium and transition metals (e.g., LiCoO2, LiFeMnO4, LiNi 1 / 3 Mn 1 / 3 Co 1 / 3It is possible to use, for example, oxygen (O2, etc.), lithium-containing transition metal phosphates (such as LiFePO4, etc.), transition metal oxides (such as MnO2, etc.), transition metal sulfides (such as MoS2, etc.), conductive polymers (such as polyaniline, etc.), etc., and it is also possible to use a mixture of two or more kinds of these particles. From the viewpoint of battery characteristics, the volume average particle size of the positive electrode active material particles is preferably from 0.01 to 100 μm, more preferably from 0.1 to 35 μm, and even more preferably from 0.5 to 20 μm.

[0014] The negative electrode composite material 9 can be a wet powder containing negative electrode active material particles and a known electrolytic solution containing an electrolyte and a non-aqueous solvent. The negative electrode active material particles have active material particles and an adhesive material adhering to at least a part of the surface of the active material particles. As a form in which the adhesive material adheres to at least a part of the surface of the active material particles, as shown in FIG. 2, the negative electrode active material particles 20 may be constituted by the adhesive material 22 covering the surface 21a of the active material particles 21 (in FIG. 2, the entire surface 21a of the active material particles 21 is covered by the adhesive material 22, but only a part of the surface 21a may be covered by the adhesive material 22), or as shown in FIG. 3, the negative electrode active material particles 20 may be constituted by the adhesive material 22 adhering to the surface 23a of the wet powder 23 obtained by mixing the active material particles 21 and the electrolytic solution (in FIG. 3, the adhesive material 22 adheres to the entire surface 23a of the wet powder 23, but the adhesive material 22 may adhere only to a part of the surface 23a). Even in the case of FIG. 3, the adhesive material 22 covers a part of the surface 21a of the active material particles 21 contained in the wet powder 23. Regarding the configuration shown in FIG. 2, as a non-limiting example of the above-described configuration in which the adhesive material adheres to at least a part of the surface of the active material particles, the negative electrode active material particles may have a configuration in which fine particles of a first resin (such as an acrylic resin or an acrylate-based resin, etc.) that undergoes changes such as expansion, dissolution, and chemical reaction by the electrolytic solution adhere to at least a part of the surface of the active material particles, or both a first resin and a second resin in the form of fine particles (such as polytetrafluoroethylene (PTFE) or polyvinylidene fluoride, etc.) that do not undergo changes such as expansion, dissolution, and chemical reaction by the electrolytic solution adhere to at least a part of the surface of the active material particles.

[0015] As the active material particles 21, carbon-based materials (e.g., graphite, etc.), silicon-based materials (e.g., silicon oxide (SiOx), etc.), conductive polymers (e.g., polyacetylene, etc.), metals (e.g., titanium, etc.), metal oxides (e.g., titanium oxide), metal alloys (e.g., lithium-tin alloy), etc. can be used. From the viewpoint of battery characteristics, the volume average particle size of the negative electrode active material particles is preferably 0.01 to 100 μm, more preferably 0.1 to 30 μm, and even more preferably 2 to 25 μm.

[0016] As the adherent 22 attached to at least a part of the surface 21a of the active material particles 21, it is necessary that the viscosity at 20°C ± 1°C is 90.5 mPa·s or more, and preferably 90.5 mPa·s to 5701 mPa·s. However, this viscosity is measured using an E-type viscometer TV-25 commercially available from Toki Sangyo Co., Ltd. The details of this measurement method will be described in the examples described later. In order for the adherent 22 to have such a viscosity, the adherent 22 can be made from a mixture of the above-described first resin and the electrolytic solution. Such a first resin has the property of low water permeability, and its water permeability is preferably such that the water absorption rate measured by the method described in the examples described later is 0.110 mass% / min or less, and more preferably the water absorption rate is 0.010 mass% / min to 0.110 mass% / min. Further, the adherent 22 can also have a viscosity within the above-described range even if it is produced from a mixture of the above-described first resin and second resin and the electrolytic solution.

[0017] The negative electrode active material particles 20 having the configuration shown in FIG. 2 can be produced, for example, by mixing the materials of the active material particles 21 and the adherent 22. Further, the negative electrode active material particles 20 having the configuration shown in FIG. 3 can be produced, for example, by supplying the material of the adherent 22 onto the surface 23a of the wet powder 23 obtained by mixing the active material particles 21 and the electrolytic solution.

[0018] Details will become clear in each of the embodiments described later. However, when the adhesion material 22 is manufactured using the above-described first resin particles, the following operational effects can be obtained. When the lithium-ion battery 1 is damaged while the lithium-ion battery 1 is in a charged state, if the negative electrode composite material 9 is exposed to the atmosphere, since the lithium ions in the active material particles 21 have high reducibility, the temperature of the negative electrode composite material 9 may increase due to the heat of reaction generated by the reaction between the lithium ions and moisture in the atmosphere, etc., and there is a risk of an undesirable or unsafe event occurring. On the other hand, when the negative electrode composite material 9 containing the negative electrode active material particles 20 with the adhesion material 22 adhered to at least a part of the surface 21a of the active material particles 21 is exposed to the atmosphere, it becomes difficult for moisture in the atmosphere, etc. to reach the active material particles 21, so the temperature rise of the negative electrode composite material 9 can be effectively suppressed.

[0019] The principle by which the adhesion material 22 makes it difficult for moisture in the atmosphere, etc. to reach the active material particles 21 is that if the viscosity of the adhesion material 22 is 90.5 mPa·s, the diffusion of moisture in the adhesion material 22 becomes slow due to the high viscosity, and also, if the water absorption rate of the first resin constituting the adhesion material 22 is 0.110 mass% / min or less, the diffusion of moisture in the adhesion material 22 becomes slow because the penetration of moisture into the adhesion material 22 is slow.

[0020] As will be verified in each of the embodiments described later, in order to obtain the above-described operational effects, it is necessary for the adhesion material 22 to have the following characteristics. In order to describe such characteristics, a second lithium-ion battery having a configuration different from that of the lithium-ion battery 1 having the above-described configuration is required. The second lithium-ion battery includes a second negative electrode composite material having the active material particles 21 of the lithium-ion battery 1 as negative electrode active material particles. That is, the second negative electrode active material particles are the active material particles 21 that do not have the adhesion material 22.

[0021] When the negative electrode composite material 9 taken out from the fully charged lithium-ion battery 1 is exposed to the atmosphere, the temperature of the negative electrode composite material 9 rises due to the reaction heat generated by the reaction between the negative electrode composite material 9 and moisture in the atmosphere. Let the peak temperature when this temperature reaches the maximum be T1 (°C). Further, a second negative electrode composite material taken out from the second fully charged lithium-ion battery, which contains the second negative electrode active material particles having the same mass as the mass of the active material particles 21 in the negative electrode composite material 9 used when measuring the temperature change of the negative electrode composite material 9 in the lithium-ion battery 1. When the second negative electrode composite material is exposed to the atmosphere, the temperature of the second negative electrode composite material rises due to the reaction heat generated by the reaction between the second negative electrode composite material and moisture in the atmosphere. Let the peak temperature when this temperature reaches the maximum be T2 (°C). It is necessary that the difference between both peak temperatures satisfies the following conditions. T2 - T1 ≥ 4

Example

[0022] <Measurement of water absorption rate of the first resin used as a raw material for the adhering material> The acrylic resin particles of Samples 1 and 2 and the acrylate resin particles of Sample 3 shown in Table 1 below were pelletized by the following method, and the water absorption rate was measured by the following method using each pellet.

[0023]

Table 1

[0024] 1 g of each of the particles of Samples 1 to 3 was put into a mortar-shaped mold with a 16 mm diameter hole, a pestle-shaped mold was set in the mortar-shaped mold, and a pressure of 448 N / mm 2 was applied with a hand press to prepare pellets of Samples 1 to 3. The radius of each pellet was 8 mm. Using the pellets of Samples 1 to 3 thus prepared, the water absorption rate of each was measured by the method described below.

[0025] Each pellet was placed in an atmosphere at a temperature of 27°C ± 2°C and a relative humidity of 42% ± 3%, and 0.09 g of electrolyte was added onto each pellet. The electrolyte used was prepared by dissolving lithium bis(fluorosulfonyl)imide at a ratio of 2 mol / L in a mixed solvent of ethylene carbonate and propylene carbonate with a volume ratio of 1:1. The mass of each pellet after 1 minute from the addition of the electrolyte, which is the mass after water absorption m1, was measured. Assuming the mass before water absorption, which is the mass of each pellet before the addition of the electrolyte, as m0, the water absorption rate AR was calculated according to the following formula (1). The results are shown in Table 2 below. AR = 100 × (m1 - m0) ÷ m0 ··· (1)

[0026]

Table 2

[0027] <Method for Preparing Negative Electrode Active Material Particles of Lithium-Ion Battery of Example 1> The negative electrode active material particles of the lithium-ion battery of Example 1 corresponding to the negative electrode active material particles 20 of the lithium-ion battery 1 of the present disclosure were prepared by the method described below. 150 parts by mass of N,N-dimethylformamide (DMF) was charged into a four-necked flask equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen gas introduction tube, and the temperature was raised to 75°C. Next, a monomer composition containing 91 parts by mass of acrylic acid, 9 parts by mass of methyl methacrylate, and 50 parts by mass of DMF, and an initiator solution prepared by dissolving 0.3 parts by mass of 2,2'-azobis(2,4-dimethylvaleronitrile) and 0.8 parts by mass of 2,2'-azobis(2-methylbutyronitrile) in 30 parts by mass of DMF were continuously dropped into the four-necked flask over 2 hours with stirring while blowing nitrogen into the flask, and radical polymerization was carried out. After the dropping was completed, the reaction was continued at 75°C for 3 hours. Then, the temperature was raised to 80°C and the reaction was continued for 3 hours to obtain a copolymer solution with a resin concentration of 30%. The obtained copolymer solution was transferred to a Teflon (registered trademark) vat and dried under reduced pressure at 150°C and 0.01 MPa for 3 hours to distill off DMF and obtain a copolymer. After this copolymer was coarsely pulverized with a hammer, it was further pulverized in a mortar to obtain a powdery polymer compound for the adherent.

[0028] 1 part by mass of the above-mentioned polymer compound was dissolved in 3 parts by mass of DMF to obtain a polymer compound solution. 79.01 parts by mass of hard carbon powder (volume average particle diameter 25 μm, true density 1.65 g / ml, JFE Chemical Corporation) was placed in a high-speed mixer FS25 (Earth Technica Co., Ltd.), and while stirring at room temperature and 720 rpm, 21.80 parts (5.45 parts in terms of solid content) of the polymer compound solution was added dropwise over 2 minutes, and stirring was continued for another 5 minutes. Next, while stirring, 5.45 parts by mass of graphite (UP) (flake graphite, volume average particle diameter 4.5 μm, true density 2.20 g / ml), which is a conductive aid, was added in portions over 2 minutes, and stirring was continued for 30 minutes.

[0029] Furthermore, while stirring, 24.20 parts (6.05 parts in terms of solid content) of the polymer compound solution was added dropwise over 2 minutes, and stirring was continued for another 5 minutes. Next, while stirring, 4.04 parts by mass of acetylene black (AB) (Denka Black, Denka Co., Ltd.), which is a conductive aid, was added in portions over 2 minutes, and stirring was continued for 30 minutes. Then, while maintaining stirring, the pressure was reduced to 0.01 MPa, and then while maintaining stirring and the degree of pressure reduction, the temperature was raised to 140 °C, and stirring, the degree of pressure reduction, and the temperature were maintained for 8 hours to distill off volatile components. The obtained powder was classified with a sieve having an opening of 200 μm to obtain Particle 1. The true density of Particle 1 was 1.63 g / ml.

[0030] 97.85 parts by mass of Particle 1 and 1.25 parts by mass of graphite (UP) (flake graphite, aspect ratio 2.2) as the first conductive filler were mixed at 2000 rpm for 5 minutes using a planetary stirring type mixing and kneading apparatus (Avatoki Rentaro, Shinchi Co., Ltd.) to obtain Powder 1. 99.10 parts by mass of Powder 1 and 0.90 part of carbon nanofiber (CNF) (aspect ratio 30) (Donacarb Mild S-243, Osaka Gas Chemical Co., Ltd.) as the second conductive filler were mixed at 2000 rpm for 5 minutes using a planetary stirring type mixing and kneading apparatus (Avatoki Rentaro, Shinchi Co., Ltd.) to obtain the negative electrode active material particles of the lithium ion battery of Example 1.

[0031] <Method for Preparing Negative Electrode Active Material Particles of Lithium-Ion Battery of Comparative Example 1> 2.01 g of graphite (UP) (flake graphite, aspect ratio 2.2) as the first conductive filler and 0.17 g of carbon nanofiber (CNF) (aspect ratio 30) (Donacarb Mild S-243, Osaka Gas Chemical Co., Ltd.) as the second conductive filler were mixed at 1500 rpm for 10 seconds using a planetary stirring type mixing and kneading apparatus (Avatore Renta-ro, Shinchi Co., Ltd.) to obtain Powder 2. 2.18 g of Powder 2, 9.3 g of hard carbon powder, and 0.38 g of acetylene black (AB) (Denka Black, volume average particle diameter 35 nm, true density 2.20 g / ml, Denka Co., Ltd.) as a conductive assistant were mixed at 1500 rpm for 10 seconds using a planetary stirring type mixing and kneading apparatus (Avatore Renta-ro, Shinchi Co., Ltd.) to obtain negative electrode active material particles of the lithium-ion battery of Comparative Example 1.

[0032] <Method for Fabricating Lithium-Ion Batteries of Example 1 and Comparative Example 1> 3 g of each of the sample of Example 1 and the sample of Comparative Example 1 prepared by the above method and 0.3 g of the electrolyte used in the above measurement of water absorption rate were mixed at 2000 rpm for 60 seconds using a planetary stirring type mixing and kneading apparatus (Avatore Renta-ro, Shinchi Co., Ltd.), further mixed with a spatula, and then mixed again at 2000 rpm for 60 seconds to obtain negative electrode composite materials of the lithium-ion batteries of Example 1 and Comparative Example 1, respectively.

[0033] The negative electrode composite materials (negative electrodes) of the lithium-ion batteries of Example 1 and Comparative Example 1 were combined with the positive electrode through a separator (Celgard (registered trademark) 3501, Teijin) to fabricate laminate cells. These laminate cells were sealed with an outer package to obtain the lithium-ion batteries of Example 1 and Comparative Example 1, respectively.

[0034] <Measurement of Temperature Change of Negative Electrode Composite Materials of Lithium-Ion Batteries of Example 1 and Comparative Example 1> After fully charging each of the lithium-ion batteries of Example 1 and Comparative Example 1 to a fully charged state (SOC 96% - 97%), the outer casing of each was broken, and the negative electrode composite material (about 100 mg) inside each outer casing was taken out and exposed to the atmosphere. After taking out the negative electrode composite material, the infrared thermography camera (FLIR ONE (registered trademark), Teledyne FLIR LLC) was used to measure the change in the surface temperature of the negative electrode composite material over time. The results are shown in FIG. 4.

[0035] When the negative electrode composite material was taken out from inside the outer casing and exposed to the atmosphere, the temperature of the negative electrode composite material increased due to the heat of reaction generated by the reaction between the negative electrode composite material and moisture in the atmosphere. The peak temperature of each negative electrode composite material was 91.6°C in Example 1 and 95.6°C in Comparative Example 1. From these results, it was confirmed that in the lithium-ion battery of Example 1 corresponding to the lithium-ion battery of the present disclosure, the temperature rise caused by exposing the negative electrode composite material inside the charged lithium-ion battery to the atmosphere can be effectively suppressed.

[0036] Assuming that the peak temperatures of Example 1 and Comparative Example 1 are T1 and T2 respectively, since T2 - T1 = 4, it can be said that if the condition of T2 - T1 ≧ 4 is satisfied, the above-described operational effects can be obtained.

[0037] Sample 1 and Sample 2 for which the water absorption rate was measured and the polymer compound of Example 1 are common in that when mixed with the electrolyte, changes such as swelling, dissolution, and chemical reaction occur due to the electrolyte, increasing the viscosity of the electrolyte. It is considered that one of the factors for suppressing the above-described temperature rise is that the diffusion rate of moisture obtained from the atmosphere decreases as the viscosity of the electrolyte increases (the diffusion coefficient is inversely proportional to the viscosity). Therefore, it is considered that the acrylic resins of Sample 1 and Sample 2 can be raw materials for the adhesive that satisfy the above-described condition of T2 - T1 ≧ 4, similar to Example 1.

[0038] <Simulation assuming an actual lithium-ion battery> The experimental results of Example 1 and Comparative Example 1 are small-scale experimental results obtained using a sample of about 100 mg. Therefore, it is not known whether the same results can be obtained with the actual size of a lithium-ion battery. Thus, a simulation assuming the actual size of a lithium-ion battery was conducted.

[0039] The simulation was performed using COMSOL Multiphysics (registered trademark), which is simulation software. In this simulation, two conditions were assumed: one with hard carbon as the active material particles of the negative electrode active material and containing 340 g of active material particles as the actual size, and the other with 0.13 g of active material particles as the coin size. The reaction that causes the temperature to rise when the active material particles containing lithium ions are exposed to the atmosphere was set as the following reaction between water and lithium ions. 2Li + 2H2O → 2LiOH + H2

[0040] However, for the calculation of the temperature rise, the heat of reaction of the above reaction was not used as it is. Instead, so that the coin-size conditions are consistent with the respective experimental results of Example 1 and Comparative Example 1 described above, the heat of reaction for the condition where an adhesive is attached to the surface of the active material particles and the condition where no adhesive is attached to the surface of the active material particles were set to 2.3×10 6 W / m 3 and 2.5×10 6 W / m 3 respectively. For each of the coin size and the actual size, the temperature change due to the heat generation by the heat of reaction and the balance of heat dissipation to the atmosphere when these heats of reaction were applied for 50 seconds was calculated. The results are shown in Table 3 below.

[0041]

Table 3

[0042] From the results shown in Table 3, it can be judged that although the absolute value of the peak temperature is different between the actual machine size and the coin size, there is almost no difference in the peak temperature difference between the two. Therefore, it can be concluded that the results of the small-scale (coin-sized) experiment can be scaled up to the actual machine size, and thus it can be said that the present invention is applicable to the actual machine size as well.

[0043] <Measurement of Viscosity of Raw Materials of Adhesive>[ The raw materials I to V of the adhesive were prepared by the following method, and their viscosities were measured. Raw material I was prepared by adding 0.35 g of acrylic resin particles MP-2801 to 3.15 g of the electrolyte used in the above-mentioned measurement of water absorption rate, mixing with a spatula, and then leaving it in a dry environment with a dew point of -30°C or lower for one day or more. Raw material II was prepared in the same manner as the preparation method of raw material I, except that acrylic resin particles FS-107 were used instead of acrylic resin particles MP-2801. Raw material III was prepared in the same manner as the preparation method of raw material I, except that acrylate resin particles HV505E (Sumitomo Seika Co., Ltd.) were used instead of acrylic resin particles MP-2801. Raw material IV was prepared in the same manner as the preparation method of raw material I, except that 0.35 g of acrylic resin particles MP-2801 and 0.35 g of PTFE (F104, Daikin Industries, Ltd.) were added to the electrolyte. Raw material V was prepared in the same manner as the preparation method of raw material I, except that 0.35 g of PTFE (F104) was added to the electrolyte.

[0044] 1.1 mL of each of raw materials I to V and the above-mentioned electrolyte (hereinafter referred to as "measurement sample") was placed in an E-type viscometer TV-25 (cone rotor specification: 1°34’×R24). After measuring the surface temperature of the measurement sample with an infrared thermography camera (FLIR ONE (registered trademark)), torque was applied to the measurement sample by a cone rotor rotating at a rotational speed of 10 rpm, and the viscosity 30 seconds after the start of torque application was measured. The viscosity measurement results of each measurement sample are shown in Table 4 below.

[0045]

Table 4

[0046] In the negative electrode active material particles without an adherent on the surface of the active material particles, the surface of the active material particles is covered with an electrolytic solution. When the negative electrode active material particles without an adherent on the surface of the active material particles are exposed to the atmosphere, a rapid temperature rise is observed as in Comparative Example 1 described above. This is presumably because, since the viscosity of the electrolytic solution covering the surface of the active material particles is low, the rate at which moisture and the like in the atmosphere diffuse the electrolytic solution becomes high, so that the moisture and the like quickly reach the active material particles. Therefore, it is expected that even when the negative electrode active material particles having an adherent formed of Raw Material V having the same viscosity as the electrolytic solution are exposed to the atmosphere, a rapid temperature rise will be observed. On the other hand, when the negative electrode active material particles having an adherent formed of Raw Materials I to IV having a viscosity sufficiently higher than that of the electrolytic solution are exposed to the atmosphere, the rate of diffusion of each adherent becomes slower than that of the electrolytic solution, so that the rate of temperature rise is expected to become slower.

Explanation of Signs

[0047] 1 Lithium ion battery, 3 Outer package, 4 Positive electrode, 5 Negative electrode, 9 Negative electrode composite material, 20 Negative electrode active material particles, 21 Active material particles, 22 Adherent.

Claims

1. comprising a negative electrode composite material containing negative electrode active material particles and an electrolytic solution, wherein the negative electrode active material particles have active material particles and an adhesive material adhering to at least a part of the surface of the active material particles, and the viscosity of the adhesive material at 20°C ± 1°C is 90.5 mPa·s or more, a negative electrode for a lithium ion battery.

2. The negative electrode for a lithium ion battery according to Claim 1, wherein the viscosity of the adhesive material at 20°C ± 1°C is 5701 mPa·s or less.

3. The negative electrode for a lithium ion battery according to Claim 1 or 2, wherein the adhesive material contains a particulate first resin that undergoes changes such as expansion, dissolution, and chemical reaction by the electrolytic solution.

4. The negative electrode for a lithium ion battery according to Claim 3, wherein the adhesive material further contains a particulate second resin that does not undergo changes such as expansion, dissolution, and chemical reaction by the electrolytic solution.

5. The first resin has a water absorption rate AR of 0.110 mass% / min or less, wherein the water absorption rate AR 448 N / mm is applied to 1 g of the first resin ２ to form the first resin into pellets with a radius of 8 mm. Then, 0.09 g of an electrolytic solution in which lithium bis(fluorosulfonyl)imide is dissolved at a ratio of 2 mol / L in a mixed solvent of ethylene carbonate and propylene carbonate with a volume ratio of 1:1 is added onto the pellets in an atmosphere at a temperature of 27°C ± 2°C and a relative humidity of 42% ± 3%. The mass of the pellets after 1 minute has elapsed, which is the mass m after water absorption １ is measured. Let the mass before water absorption, which is the mass of the pellets before the addition of the electrolytic solution, be m ０ Then, according to the following formula AR = 100×(m １ - m ０ )÷m ０ is calculated by, a negative electrode for a lithium ion battery according to Claim 3.

6. The negative electrode for a lithium ion battery according to Claim 5, wherein the first resin has a water absorption rate AR of 0.010 mass% / min or more.

7. A lithium ion battery comprising the negative electrode according to Claim 1 or 2.

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