JP2023157080A - Negative electrode and nonaqueous electrolyte secondary battery - Google Patents

Abstract

To provide a negative electrode containing a silicon-containing substance and suppressed in cubical expansion, and a nonaqueous electrolyte secondary battery including the same.SOLUTION: Provided is a negative electrode for a nonaqueous electrolyte secondary battery, the negative electrode including a current collector, a first active material layer and a second active material layer in the order. The first active material layer includes a first binder and a graphite material, and the second active material layer includes a second binder and a silicon-containing substance. The first binder and the second binder contain resin components different from each other. A BET specific surface area of the graphite material is 1.6 m2/g or less. The resin component in the second binder includes a silicon swelling suppressing resin.SELECTED DRAWING: Figure 1

Description

The present disclosure relates to a negative electrode and a non-aqueous electrolyte secondary battery.

Patent Document 1 (Japanese Unexamined Patent Publication No. 2015-179575) proposes the use of a silicon-containing material as an active material in a negative electrode for the purpose of increasing the capacity of a non-aqueous electrolyte secondary battery.

Japanese Patent Application Publication No. 2015-179575

Silicon-containing substances tend to undergo volumetric expansion, and the thickness of the electrode plate tends to increase due to repeated charging and discharging. In order to externally suppress the reaction force (the force that pushes the battery case apart) caused by the increase in the thickness of the electrode plate, mechanical parts of a size corresponding to the reaction force are required. However, from the viewpoint of the space in which the battery is mounted, it is desirable for the mechanical components to be space-saving, and a reduction in reaction force is required.

An object of the present disclosure is to provide a negative electrode containing a silicon-containing material in which volumetric expansion is suppressed, and a non-aqueous electrolyte secondary battery including the negative electrode.

The present disclosure provides the following negative electrode and nonaqueous electrolyte secondary battery.
[1] A negative electrode for a nonaqueous electrolyte secondary battery, comprising a current collector, a first active material layer, and a second active material layer in this order, the first active material layer comprising a first binder and graphite. the second active material layer includes a second binder and a silicon-containing material, the first binder and the second binder contain different resin components, and the BET ratio of the graphite material is The negative electrode has a surface area of 1.6 m ² /g or less, and the resin component in the second binder includes a silicon swelling-inhibiting resin.
[2] The negative electrode according to [1], wherein the silicon swelling-inhibiting resin is polyacrylamide.
[3] The negative electrode according to [1] or [2], wherein the active material in the second active material layer is only the silicon-containing material.
[4] The negative electrode according to any one of [1] to [3], wherein the active material in the first active material layer is only the graphite material, or the graphite material and a silicon-containing substance.
[5] The mass ratio of the total silicon-containing material to the total active material contained in the first active material layer and the second active material layer is 40% or less, according to any one of [1] to [4]. negative electrode.
[6] The negative electrode according to any one of [1] to [5], wherein the first binder does not contain a silicon swelling-inhibiting resin.
[7] The negative electrode according to any one of [1] to [6], wherein the second binder contains only a silicon swelling-inhibiting resin.
[8] A non-aqueous electrolyte secondary battery comprising the negative electrode according to any one of [1] to [7], a positive electrode, and a non-aqueous electrolyte.

According to the present disclosure, it is possible to provide a negative electrode containing a silicon-containing substance whose volumetric expansion is suppressed, and a nonaqueous electrolyte secondary battery including the negative electrode.

FIG. 1 is a schematic diagram showing an example of the configuration of the negative electrode in this embodiment. FIG. 2 is a schematic flowchart showing a method for manufacturing a negative electrode. FIG. 3 is a schematic diagram showing an example of the configuration of a battery in this embodiment. FIG. 4 is a schematic diagram showing an example of the configuration of the electrode body in this embodiment.

Embodiments of the present invention will be described below with reference to the drawings, but the present disclosure is not limited to the following embodiments. In all the drawings below, each component is shown adjusted to an appropriate scale to make it easier to understand, and the scale of each component shown in the drawings does not necessarily match the actual scale of the component.

The negative electrode of the present disclosure will be described with reference to the drawings. A negative electrode 100 shown in FIG. 1 is a negative electrode for a nonaqueous electrolyte secondary battery. The negative electrode 100 includes a current collector 10 and a negative electrode active material layer 20. In the negative electrode active material layer 20, a first active material layer 30 and a second active material layer 40 are laminated in order from the current collector 10 side. In the negative electrode of the present disclosure, the negative electrode active material layer may be provided only on one side of the current collector, or may be provided on both sides. Although not shown, the negative electrode 100 includes another layer interposed between the current collector 10 and the first active material layer 30 and/or between the first active material layer 30 and the second active material layer 40. For example, a coating layer may be provided on the current collector 10, and an intermediate layer containing a mixture of components may be provided between the first active material layer 30 and the second active material layer 40.

Current collector 10 is a conductive sheet. The current collector 10 may include, for example, aluminum (Al) foil, copper (Cu) foil, or the like. The current collector 10 may have a thickness of, for example, 5 μm to 50 μm. For example, a coating layer may be formed on the surface of the current collector 10. The coating layer may contain, for example, a conductive carbon material. The coating layer may have a smaller thickness than, for example, the negative electrode active material layer 20.

The first active material layer 30 includes a first binder (not shown) and a graphite material 31. The first binder can bind the active materials together. The first binder can bind the active material and the current collector. The first binder and the second binder described below contain different resin components. When the first binder and the second binder described below contain mutually different resin components, the volumetric expansion of the negative electrode tends to be easily suppressed.

The first binder is, for example, polyvinylidene fluoride (PVdF), poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), carboxymethyl cellulose ( CMC), polyethylene oxide (PEO), and styrene butadiene rubber (SBR). The first binder preferably does not contain the silicon swelling-inhibiting resin described below. The amount of the first binder may be, for example, 0.1 parts by mass to 10 parts by mass, based on 100 parts by mass of the active material in the first active material layer 30.

The first active material layer 30 includes graphite material 31 as an active material. The graphite material 31 may contain at least one selected from the group consisting of graphite, soft carbon (easily graphitizable carbon), and hard carbon (non-graphitizable carbon). The BET specific surface of the graphite material 31 is 1.6 m ² /g or less, preferably 1.3 m ² /g or less, and more preferably 1.1 m ² /g or less. By using a graphite material with a BET specific surface of 1.6 m ² /g or less in the first active material layer 30, the volumetric expansion of the first active material layer 30 tends to be easily suppressed. The BET specific surface area is measured by the BET multipoint method.

The first active material layer 30 may further include a silicon-containing material as an active material. The silicon-containing material may be, for example, substantially made of Si metal (simple element of Si). The silicon-containing material may include, for example, a Si-based alloy. The silicon-containing material may include, for example, at least one selected from the group consisting of SiCu alloy, SiNi alloy, SiAl alloy, and SiZn alloy. The silicon-containing material may include, for example, a Si compound. The silicon-containing material may include, for example, silicon oxide. The silicon-containing material may include, for example, SiOx (0.5≦x≦1.5). The silicon-containing material may include, for example, a composite material of Si and other materials. The silicon-containing material may include, for example, a Si/C composite. The Si/C composite material can be formed, for example, by supporting Si metal, Si oxide, etc. on a carbon material (graphite, amorphous carbon, etc.). The silicon-containing material may include, for example, at least one selected from the group consisting of Si metal, Si-based alloy, Si oxide, and Si/C composite material.

The first active material layer 30 preferably contains only the graphite material 31 or contains the graphite material 31 and a silicon-containing substance as an active material. The first active material layer 30 may contain, for example, 50 parts by mass to 100 parts by mass of graphite material 31, preferably 60 parts by mass to 100 parts by mass, based on 100 parts by mass of the active material in the first active material layer 30. % of graphite material 31, more preferably 80 parts by mass to 100 parts by mass of graphite material 31. When the first active material layer 30 includes a graphite material 31 and a silicon-containing substance, the mass ratio of the graphite material and the silicon-containing substance may be, for example, from 99:1 to 1:99, preferably from 99:1 to 70:30. and more preferably from 99:1 to 90:10. The first active material layer 30 may further include at least one selected from the group consisting of Sn, SnO, Sn-based alloy, and Li ₄ Ti ₅ O ₁₂ as other active materials.

The second active material layer 40 includes a second binder (not shown) and a silicon-containing material 41. The second binder can bind the active materials together. The second binder can bind the active material and the second active material layer 40. The resin component in the second binder includes silicon swelling inhibiting resin. When the second active material layer 40 contains the silicon expansion-suppressing resin, the expansion of the negative electrode tends to be easily suppressed.

Examples of the silicon swelling-inhibiting resin include polyacrylamide (PAAm). As the silicon swelling inhibiting resin, a resin having a higher number of hydrogen bonds than the resin component of the first binder can be preferably used from the viewpoint of swelling inhibiting properties. It is more preferable that the first binder contains CMC and SBR and the second binder contains PAAm from the viewpoint of suppressing swelling. The resin component in the second binder can further contain a resin other than the silicon swelling-inhibiting resin. As the resin other than the silicon swelling-inhibiting resin, the above-mentioned example of the first binder is applied. The second binder preferably contains only a silicon swelling-inhibiting resin as a resin component. The amount of the second binder may be, for example, 0.1 parts by mass to 15 parts by mass based on 100 parts by mass of the active material.

The second active material layer 40 includes a silicon-containing material 41 as an active material. For the explanation of the silicon-containing material, the explanation for the first active material layer 30 described above applies. The second active material layer 40 can include the above-described graphite material in the first active material layer 30 and other active materials as active materials other than the silicon-containing material 41. The second active material layer 40 preferably contains only the silicon-containing material 41 as an active material.

The second active material layer 40 may contain, for example, from 1 part by mass to 100 parts by mass of silicon-containing material 41, preferably from 50 parts by mass to 100 parts by mass, based on 100 parts by mass of the active material in the second active material layer 40. Parts by weight of silicon-containing material 41, more preferably 80 parts by weight to 100 parts by weight silicon-containing material 41. The mass ratio of all silicon-containing materials to all active materials contained in the first active material layer and the second active material layer is 40% or less, preferably 30% or less.

The first active material layer 30 and the second active material layer 40 may include active material in the form of active material particles. The active material particles contain an active material. The active material particles may further contain components other than the active material. Active material particles can have any size. The average particle diameter of the active material particles may be, for example, 1 μm to 50 μm, 1 μm to 35 μm, or 1 μm to 25 μm. In this specification, the average particle diameter of active material particles represents a particle diameter at which the cumulative particle volume from the small particle size side is 50% of the total particle volume in a volume-based particle size distribution. In this specification, the average particle diameter of active material particles may also be expressed as D50. The average particle size can be measured by laser diffraction/scattering method.

The method for manufacturing the negative electrode 100 can include slurry preparation (A1), coating (B1), drying (C1), and compression (D1). FIG. 2 is a schematic flowchart showing a method for manufacturing the negative electrode 100. In the present disclosure, first the first active material layer 30 is formed, and then the second active material layer 40 can be formed on the first active material layer 30. Preparing the slurry (A1) may include mixing the active material, the binder, and the organic solvent. The organic solvent includes, for example, at least one selected from the group consisting of N-methyl-2-pyrrolidone (NMP), tetrahydrofuran (THF), dimethylformamide (DMF), methyl ethyl ketone (MEK), and dimethyl sulfoxide (DMSO). It's okay to stay. The amount of organic solvent used is arbitrary. That is, the slurry can have any solids concentration (mass fraction of solids). The slurry may have a solids concentration of, for example, 40% to 80%. For mixing, any stirring device, mixing device, or dispersing device can be used.

Coating (B1) can include forming a coating film by coating the surface of the base material with the slurry. In this embodiment, the slurry can be applied to the surface of the base material using any coating device. For example, a slot die coater, a roll coater, etc. may be used. The coating device may be capable of multilayer coating.

Drying (C1) can include heating and drying the coating film. In this embodiment, any drying device may be used as long as the coating can be heated. For example, the coating film may be heated using a hot air dryer or the like. The organic solvent can be evaporated by heating the coating film. This allows the organic solvent to be substantially removed.

Compression (D1) may include compressing the dried coating film to form an active material layer. In this embodiment, any compression device may be used. For example, a rolling mill or the like may be used. The dried coating film is compressed, an active material layer is formed, and the negative electrode 100 is completed. The negative electrode 100 can be cut into a predetermined planar size depending on the specifications of the battery. The negative electrode 100 may be cut to have a band-like planar shape, for example. The negative electrode 100 may be cut to have a rectangular planar shape, for example.

<Nonaqueous electrolyte secondary battery>
FIG. 3 is a schematic diagram showing an example of a battery in this embodiment. The battery is a non-aqueous electrolyte secondary battery. The battery 200 shown in FIG. 3 includes an exterior body 90. The exterior body 90 houses the electrode body 50 and an electrolyte (not shown). The electrode body 50 is connected to a positive terminal 91 through a positive current collecting member 81 . The electrode body 50 is connected to a negative electrode terminal 92 through a negative electrode current collecting member 82 . FIG. 4 is a schematic diagram showing an example of an electrode body in this embodiment. The electrode body 50 is of a wound type. The electrode body 50 includes a positive electrode 20, a separator 70, and a negative electrode 100. That is, battery 200 includes negative electrode 100. The positive electrode 60 includes a positive electrode active material layer 62 and a positive electrode current collector 61. The negative electrode 100 includes a negative electrode active material layer 20 and a current collector (negative electrode current collector) 10.

Hereinafter, the present disclosure will be explained in further detail with reference to Examples. "%" and "parts" in the examples are mass % and parts by mass unless otherwise specified.

<Example 1>
100 parts by mass of graphite (C) as a negative electrode active material, 2.5 parts by mass of styrene-butadiene rubber (SBR) as a binder, and 2.5 parts by mass of carboxymethyl cellulose (CMC) as a thickener were added to ion-exchanged water. and mixed to prepare a first negative electrode composite slurry.
100 parts by mass of silicon oxide (SiO _x ) as a negative electrode active material and 5 parts by mass of polyacrylamide (PAAm) as a binder were mixed in ion exchange water to prepare a second negative electrode composite slurry.

The first negative electrode composite slurry was applied to both sides of a negative electrode current collector made of copper foil, dried, and the coating film was pressed and rolled using a rolling roller. Next, a second negative electrode composite slurry was applied onto the dried coating film of the first negative electrode composite slurry, dried and rolled in the same manner as above. Thereby, the negative electrode active material layer including the first active material layer formed by the first negative electrode composite material slurry and the second active material layer formed by the second negative electrode composite material slurry is placed on the negative electrode current collector. A supported negative electrode sheet of Example 1 was obtained.

LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (NCM) as a positive electrode active material, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were combined into NCM:AB :PVdF=97:2:1 mass ratio in N-methylpyrrolidone (NMP) to prepare a positive electrode composite slurry. This positive electrode composite slurry was applied onto aluminum foil. Thereafter, it was dried and roll-pressed to a predetermined thickness to produce a positive electrode sheet.

A porous polyolefin sheet having a three-layer structure of PP/PE/PE was prepared as a separator. A positive electrode sheet and a negative electrode sheet were overlapped with a separator interposed therebetween and wound to obtain a wound body. This wound body was pressed to produce a flat wound electrode body.

An electrode terminal was attached to the electrode body, inserted into a case made of an aluminum laminate film, and after welding, a non-aqueous electrolyte was poured. The nonaqueous electrolyte was prepared by dissolving LiPF ₆ at a concentration of 1.0 mol/L in a mixed solvent containing ethylene carbonate (EC) and dimethyl carbonate (DEC) at a volume ratio of 3:7. Using. Thereafter, the laminate case was sealed to obtain a lithium ion secondary battery for evaluation.

<Example 2 to Example 4 and Comparative Example 1 to Comparative Example 3>
A negative electrode sheet was produced in the same manner as in Example 1 except that the materials and Si ratios shown in Table 1 were used, and a lithium ion secondary battery for evaluation was produced using it.

<Activation of lithium ion secondary battery for evaluation>
The lithium ion secondary battery for evaluation was placed in an environment of 25°C. Activation (initial charging) is performed using a constant current-constant voltage method, and after each evaluation lithium ion secondary battery is charged at a constant current of 1/3C to 4.2V, the current value is 1/50C. Constant voltage charging was performed until the battery was fully charged. Thereafter, each evaluation lithium ion secondary battery was discharged at a constant current of 1/3C to 3.0V.

<Swelling rate>
After discharging the cell to 3.0 V, several points (about 3 points) in the flat part of the cell were measured using a Mitutoyo micrometer (spindle diameter Φ6.3), and the average value was taken as the cell thickness. Changes in cell thickness are measured at regular cycle intervals, and the increase rate (%) from the initial stage is defined as the expansion rate. The increase rate (%) from the initial stage was evaluated as the swelling rate. The thickness was measured after initial activation (after discharge), and the measured thickness was taken as the initial thickness. Thereafter, a cycle test was conducted, and the final thickness was measured after 100 cycles to determine the rate of increase. If the swelling rate is less than 105%, mark "◎"; if it is 105% or more and less than 110%, mark "○"; if it is 110% or more and less than 120%, mark "△"; if it is 120% or more and less than 130%, mark "x". ”, and if it was 130% or more, it was evaluated as “XX”.

<Measurement of capacity retention rate>
Each activated lithium ion secondary battery for evaluation was placed in an environment of 25°C. One cycle of charging and discharging was repeated for 100 cycles, each cycle consisting of constant current charging to 4.2V at a current value of 0.5C and constant current discharging to 3.0V at a current value of 0.5C. The discharge capacity at the 1st cycle and the 100th cycle was measured, and the ratio of the discharge capacity at the 100th cycle to the discharge capacity at the 1st cycle was calculated as a capacity retention rate (%). At this time, if the capacity retention rate is 95% or more, it is evaluated as "◎", if it is 85% or more and less than 95%, it is evaluated as "○", and if it is less than 85%, it is evaluated as "△". The results are shown in Table 1. .

<Battery resistance measurement>
Each activated lithium ion secondary battery for evaluation was adjusted to an SOC of 50%. It was left standing for 1 hour in an environment of 25°C. Then, constant current discharge was performed for 10 seconds at a current value of 3C. The voltage drop amount ΔV at this time was obtained, and the battery resistance was calculated by dividing ΔV by the current value (3C). When the resistance of the lithium ion secondary battery for evaluation in Example 1 is taken as 100%, if it is less than 90%, mark it as "◎", if it is 90% or more and less than 120%, mark it as "○", if it is 120% or more, mark it as "◎". It was evaluated as "x" and the results are shown in Table 1.

表中、Ｓｉ含有率は負極活物質層全体のケイ素含有物質の質量比率を示す。

In the table, the Si content indicates the mass ratio of the silicon-containing material in the entire negative electrode active material layer.

In Examples 1 to 4, negative electrodes satisfying all of the expansion ratio, capacity retention ratio, and battery resistance were manufactured. In Comparative Example 1, the battery resistance tended to decrease. In Comparative Example 2, the swelling rate was significantly reduced by the cycle test. In Comparative Example 3, the effect of suppressing swelling in the entire electrode tended to be small.

1, 2 positive electrode active material, 10 current collector, 20 negative electrode active material layer, 30 first active material layer, 31 graphite material, 40 second active material layer, 41 silicon-containing material, 50 electrode body, 60 positive electrode, 61 positive electrode Current collector, 62 Positive electrode active material layer, 70 Separator, 81 Positive electrode current collecting member, 82 Negative electrode current collecting member, 90 Exterior body, 91 Positive electrode terminal, 92 Negative electrode terminal, 100 Negative electrode, 200 Battery.

Claims (8)

A negative electrode for a non-aqueous electrolyte secondary battery,
comprising a current collector, a first active material layer and a second active material layer in this order,
The first active material layer includes a first binder and a graphite material,
The second active material layer includes a second binder and a silicon-containing material,
The first binder and the second binder contain different resin components,
The BET specific surface of the graphite material is 1.6 m ² /g or less,
In the negative electrode, the resin component in the second binder includes a silicon swelling-inhibiting resin. The negative electrode according to claim 1, wherein the silicon swelling-inhibiting resin is polyacrylamide. The negative electrode according to claim 1 or 2, wherein the active material in the second active material layer is only the silicon-containing material. The negative electrode according to claim 1 or 2, wherein the active material in the first active material layer is only the graphite material, or the graphite material and a silicon-containing substance. The negative electrode according to claim 1 or 2, wherein a mass ratio of all silicon-containing materials to all active materials contained in the first active material layer and the second active material layer is 40% or less. The negative electrode according to claim 1 or 2, wherein the first binder does not contain a silicon swelling-inhibiting resin. The negative electrode according to claim 1 or 2, wherein the second binder contains only a silicon swelling-inhibiting resin. A non-aqueous electrolyte secondary battery comprising the negative electrode according to claim 1 or 2, a positive electrode, and a non-aqueous electrolyte.

Images