Negative electrode and lithium ion secondary battery

Abstract

To suppress increase of an initial resistance.SOLUTION: A negative electrode includes a negative electrode collector and a negative electrode active material layer. The negative electrode active material layer includes a first active material, a second active material, and a gel electrolyte. The gel electrolyte includes a polymer and a conductive material. The negative electrode active material layer includes the gel electrolyte around the first active material. The concentration of the gel electrolyte around the first active material is higher than that of the gel electrolyte in the regions other than region around the first active material of the negative electrode active material layer.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a negative electrode and a lithium-ion secondary battery.

Background Art

[0002] Japanese Patent Application Laid-Open No. 2021-48106 (Patent Document 1) discloses an active material for a secondary battery that has sufficient capacity and good cycle characteristics by coating silicon, which is a negative electrode active material, with polyethylene oxide, which is a polar polymer, to form a negative electrode composite material containing a conductive material.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Active materials containing silicon have large expansion and contraction during charge and discharge. Therefore, when using an active material containing silicon as the negative electrode active material, the electrolyte may flow with the expansion and contraction, resulting in a bias of the electrolyte within the electrode, which may lead to an increase in resistance.

[0005] In Patent Document 1, by coating silicon with polyethylene oxide, the increase in resistance associated with repeated charge and discharge is suppressed to some extent. On the other hand, since the negative electrode active material is coated with a polar polymer, the ionic conductivity is reduced. Therefore, there is room for improvement regarding the increase in initial resistance.

[0006] An object of the present disclosure is to suppress an increase in initial resistance.

Means for Solving the Problems

[0007] [1] including a negative electrode current collector and a negative electrode active material layer, The negative electrode active material layer contains a first active material, a second active material, and a gel electrolyte. The gel electrolyte contains a polymer and a conductive material. The negative electrode active material layer contains the gel electrolyte around the first active material. A negative electrode in which the concentration of the gel electrolyte around the first active material is higher than the concentration of the gel electrolyte in a region other than around the first active material in the negative electrode active material layer.

[0008] By disposing a gel electrolyte with low fluidity around the first active material having a large expansion and contraction during charge and discharge, the deviation of the electrolyte due to the expansion and contraction is suppressed, and as a result, an increase in resistance due to repeated charge and discharge is suppressed. In addition, since the use of the gel electrolyte is limited to around the first active material, suppression of an increase in initial resistance is also expected.

[0009] The negative electrode according to [1], wherein the polymer is a poly(vinylidene fluoride - hexafluoropropylene) copolymer.

[0010] The negative electrode according to [1] or [2], wherein the content of the first active material in the negative electrode active material layer is 5% by mass or more and 20% by mass or less.

[0011] A lithium - ion secondary battery including the negative electrode according to any one of [1] to [3].

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0013] Hereinafter, embodiments of the present disclosure (hereinafter may be abbreviated as "the present embodiments"), and examples of the present disclosure (hereinafter may be abbreviated as "the present examples") will be described. However, the present embodiments and the present examples do not limit the technical scope of the present disclosure.

[0014] <Negative electrode> FIG. 1 is a schematic diagram showing an example of the negative electrode of the present embodiment. The negative electrode 20 includes a negative electrode current collector 21 and a negative electrode active material layer 22. The negative electrode current collector 21 may include, for example, a copper (Cu) foil, a nickel (Ni) foil, or the like.

[0015] 《Negative electrode active material layer》 The negative electrode active material layer 22 includes a first active material 1, a second active material 2, and a gel electrolyte 3. The gel electrolyte 3 includes a polymer and a conductive material. The negative electrode active material layer 22 includes the gel electrolyte 3 around the first active material 1. The concentration of the gel electrolyte 3 around the first active material 1 is higher than the concentration of the gel electrolyte 3 in the negative electrode active material layer 22.

[0016] The first active material 1 includes a single active material containing an element selected from the group consisting of silicon (Si), aluminum (Al), and tin (Sn), or particles such as an oxide. From the viewpoint of capacity, the first active material 1 preferably contains Si, and examples thereof include Si single substance, silicon oxide, silicon carbide composite, and the like.

[0017] The content rate of the first active material 1 in the negative electrode active material layer 22 is 5% by mass or more and 20% by mass or less.

[0018] The average particle diameter (D50) of the first active material 1 is, for example, 3 to 20 μm, preferably 5 to 15 μm. The average particle diameter D50 in this specification is the particle diameter at which the cumulative frequency from the smaller particle diameter in the volume-based particle size distribution reaches 50%. The volume-based particle size distribution can be measured by a laser diffraction particle size distribution measuring device.

[0019] The second active material 2 contains or is one or more particles selected from the group consisting of carbon (C) such as graphite, hard carbon, soft carbon, and amorphous-coated graphite. The second active material 2 preferably contains graphite and more preferably consists of graphite. The graphite may be natural graphite or artificial graphite.

[0020] The D50 of the second active material 2 is, for example, 5 to 15 μm, preferably 8 to 10 μm. The D50 of the second active material 2 is preferably smaller than the D50 of the first active material 1.

[0021] The content of the second active material 2 in the negative electrode active material layer 22 is 75% by mass or more and 95% by mass or less.

[0022] The gel electrolyte 3 contains a polymer and a conductive material. The polymer functions as a gelling agent. The polymer may form a polymer matrix. The polymer is not particularly limited as long as it functions as a gelling agent. For example, it may be polyethylene carbonate (PEC), polyethylene oxide (PEO), polyvinylidene fluoride (PVdF), poly(vinylidene fluoride - hexafluoropropylene) copolymer (PVdF - HEP), polyacrylonitrile (PAN), PVdF - PAN, polyethylene glycol (PEG), and derivatives thereof. The polymer preferably contains PVdF - HEP and more preferably consists of PVdF - HEP.

[0023] The content of the polymer in the negative electrode active material layer 22 is 1% by mass or more and 5% by mass or less.

[0024] The conductive material forms a conductive path. Examples of the conductive material include carbon black (CB) (acetylene black (AB), ketjen black (KB)), carbon nanotube (CNT), vapor - grown carbon fiber (VGCF), etc.

[0025] The content of the conductive material in the negative electrode active material layer 22 is 1% by mass or more and 5% by mass or less.

[0026] The gel electrolyte 3 contains an electrolytic solution. The electrolytic solution contains a solvent and a Li salt. The solvent is aprotic. The solvent may contain any component. The solvent may contain, for example, at least one selected from the group consisting of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC).

[0027] The Li salt is a supporting electrolyte. The Li salt is dissolved in the solvent. The Li salt may contain, for example, at least one selected from the group consisting of LiPF6, LiTFSI, and LiBF4. The Li salt may have, for example, a molar concentration of 0.5 mol / L or more and 2.0 mol / L or less.

[0028] The electrolytic solution may further contain any additive. The electrolytic solution may contain, for example, an additive in an amount of 0.01% by mass or more and 5% by mass or less. The additive may contain, for example, at least one selected from the group consisting of vinylene carbonate (VC) and vinyl ethylene carbonate (VEC).

[0029] The negative electrode active material layer 22 contains the gel electrolyte 3 around the first active material 1. The concentration of the gel electrolyte 3 around the first active material 1 is higher than the concentration of the gel electrolyte 3 in a region other than around the first active material 1 in the negative electrode active material layer 22. By disposing the gel electrolyte 3 with low fluidity around the first active material 1 with large expansion and contraction accompanying charge and discharge, the bias of the electrolytic solution accompanying the expansion and contraction is suppressed, and as a result, an increase in resistance accompanying repeated charge and discharge is suppressed. In addition, since the use of the gel electrolyte 3 is limited to around the first active material 1, suppression of an increase in initial resistance is also expected.

[0030] Here, the "area around the first active material 1" refers to the area within a circle with a radius of a certain distance from the center of gravity of the first active material 1. Typically, it refers to the area within a circle with a radius of twice the average particle diameter of the first active material 1 from the center of gravity of the first active material 1. For example, the area around the first active material 1 with an average particle diameter of 10 μm refers to the area within a circle with a radius of 20 μm from the center of gravity of the first active material 1.

[0031] The negative electrode active material layer 22 may further contain a binder, a thickener, etc.

[0032] The binder may include, for example, PVdF, polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR), etc. The thickener may include, for example, carboxymethyl cellulose (CMC), methyl cellulose (MC), etc.

[0033] The content rates of the binder and the thickener contained in the negative electrode active material layer 22 are, for example, 0.1 mass% or more and 5 mass% or less.

[0034] The loading amount of the active material in the negative electrode active material layer 22 is preferably 20 mg / cm 2 or more. When the loading amount of the active material is 20 mg / cm 2 or more, an improvement in cycle characteristics is expected. Note that the "active material" includes both the first active material 1 and the second active material 2.

[0035] <Method for manufacturing negative electrode> The method for manufacturing the negative electrode of the present embodiment includes at least (a) a precursor particle formation step, (b) a negative electrode active material precursor layer formation step, and (c) a negative electrode active material layer formation step.

[0036] 《(a) Precursor particle formation step》 In the precursor particle formation step, the first active material, the second active material, a polymer, and a conductive material are mixed with a solvent and dried to form precursor particles. The precursor particles have a polymer and a conductive material around the first active material, and further have a second active material around them.

[0037] For example, a mixture is prepared by dispersing a first active material, a polymer, and a conductive material in a solvent at a predetermined ratio and mixing and stirring them. By dispersing a second active material in the mixture at a predetermined ratio and mixing and stirring, a first liquid paint is prepared. The solvent is not particularly limited as long as it can disperse the first active material, the second active material, the polymer, and the conductive material. Examples thereof include N-methyl-2-pyrrolidone (NMP). Thereafter, precursor particles are obtained by instantaneously drying the droplets by a method such as spray drying. In this step, it is preferable that the addition amount of the second active material is less than the addition amount of the first active material. This makes it easier to form precursor particles. The addition amount of the second active material may be 2 times or more, 3 times or more, or 4 times or more the addition amount of the first active material. Also, in this step, it is preferable that the D50 of the second active material is smaller than the D50 of the first active material. This makes it easier to form precursor particles.

[0038] <<(b) Anode active material precursor layer formation step>> In the anode active material precursor layer formation step, an anode active material precursor layer is formed by applying a second liquid paint obtained by dispersing the precursor particles obtained in the precursor particle formation step and the second active material in a solvent to the anode current collector.

[0039] For example, a second liquid paint is prepared by dispersing the precursor particles and the second active material in a solvent (for example, water). In addition to the precursor particles and the second active material, a conductive material, a binder, a thickener, etc. may be mixed. The second liquid paint is applied to the surface of the anode current collector. For coating, for example, a doctor blade or a die coater is used. By drying the second liquid paint, an anode active material precursor layer is formed. After drying, the anode active material precursor layer may be compressed.

[0040] <<(c) Anode active material layer formation step>> In the anode active material layer formation step, an anode active material layer is formed by bringing the anode active material precursor layer obtained in the anode active material precursor layer formation step into contact with an electrolytic solution.

[0041] For example, by immersing the negative electrode active material precursor layer in an electrolytic solution, the electrolytic solution penetrates into the precursor particles. Due to the penetration of the electrolytic solution, the polymer swells. The negative electrode active material precursor layer may be immersed in the electrolytic solution, for example, in a temperature environment of 40 to 60°C. The negative electrode active material precursor layer may be immersed in the electrolytic solution for, for example, 12 to 24 hours.

[0042] Note that this step may also be one step in the fabrication of a lithium-ion secondary battery. That is, after the completion of the negative electrode active material precursor layer formation step, the lithium-ion secondary battery may be assembled and the electrolytic solution may be injected to form this step.

[0043] <Lithium-ion secondary battery> FIG. 2 is a schematic diagram showing an example of the lithium-ion secondary battery (hereinafter, also simply referred to as "battery") of the present embodiment. The battery 100 may include an exterior body (not shown). The exterior body may house the power generation element 50 and an electrolytic solution (not shown). The exterior body may have any form. The exterior body may be, for example, a metal case, or may be a pouch made of a metal foil laminate film or the like. The exterior body may contain, for example, Al or the like.

[0044] The battery 100 includes a power generation element 50. The power generation element 50 may also be referred to as an electrode body or an electrode group. The power generation element 50 includes a positive electrode 10, a separator 30, and a negative electrode 20. The power generation element 50 has an arbitrary structure. For example, the power generation element 50 may be of a wound type. The positive electrode 10, the separator 30, and the negative electrode 20 may all be strip-shaped sheets. The power generation element 50 may be formed, for example, by laminating the positive electrode 10, the separator 30 (the first sheet), the negative electrode 20, and the separator 30 (the second sheet) in this order. After winding, the power generation element 50 may be formed into a flat shape.

[0045] <Positive electrode> The positive electrode 10 may include a positive electrode current collector 11 and a positive electrode active material layer 12. The positive electrode current collector 11 may include, for example, aluminum (Al) foil or the like. The positive electrode active material layer 12 contains a positive electrode active material. The positive electrode active material layer 12 may further include, for example, a conductive material, a binder, and the like.

[0046] The positive electrode active material may be, for example, in a particulate form. The positive electrode active material may have, for example, a D50 of 1 to 30 μm. The positive electrode active material may include, for example, at least one selected from the group consisting of LiCoO2, LiNiO2, LiMnO2, Li(NiCoMn)O2, and Li(NiCoAl)O2. For example, in "Li(NiCoMn)O2", "(NiCoMn)" indicates that the total of the composition ratios within the parentheses is 1. As long as the total is 1, the individual component amounts are arbitrary.

[0047] The conductive material may include, for example, AB or the like. The binder may include, for example, PVdF or the like. The conductive material and the binder may be, for example, 0.1 mass% or more and 10 mass% or less with respect to the positive electrode active material layer 12.

[0048] 《Separator》 The separator 30 is porous. The separator 30 can permeate the electrolyte. The separator 30 separates the positive electrode 10 and the negative electrode 20. The separator 30 is electrically insulating. The separator 30 may include, for example, polyolefin resins such as polyethylene (PE) and polypropylene (PP). The separator 30 may have, for example, a single-layer structure or a multilayer structure. The separator 30 may consist essentially of a PE layer, or may be formed by laminating a PP layer, a PE layer, and a PP layer in this order.

[0049] 《Electrolyte》 The electrolyte contains a solvent and a Li salt. The solvent is aprotic. The aprotic solvent and the Li salt are as described above.

[0050] The electrolyte may further contain an optional additive. For example, the electrolyte may contain an additive in an amount of 0.01% to 5% by mass. The additive may contain, for example, at least one selected from the group consisting of vinylene carbonate (VC) and vinyl ethylene carbonate (VEC).

Example

[0051] 《No.1》 As the material of the negative electrode, Si (D50: 15 μm) (5% by mass) as the first active material, artificial graphite (D50: 6 μm) (86.5% by mass) as the second active material, SBR (4% by mass) as the binder, CMC (3% by mass) as the thickener, and AB (1.5% by mass) as the conductive material were prepared. These materials were mixed in distilled water and kneaded using a kneader to obtain a negative electrode paste.

[0052] The obtained negative electrode paste was applied to the surface of a Cu foil, which is a negative electrode current collector, using a doctor blade. After application, the negative electrode paste was dried at 100 °C for 15 minutes and compressed by a roll press to obtain a negative electrode of No.1. The loading of the active material in the negative electrode active material layer is 20 mg / cm 2 and the density of the negative electrode active material layer is 1.2 to 1.4 g / cm 3 The same shall apply to the negative electrodes of No.2 and 3 described later.

[0053] As the material of the positive electrode, LiNi 0.6 Co 0.2 Mn 0.2 O2 (95% by mass), AB (2.5% by mass) as the conductive material, and PVdF (2.5% by mass) as the binder were prepared. These materials were mixed in NMP and kneaded using a kneader to obtain a positive electrode paste.

[0054] The obtained positive electrode paste was applied to the surface of an Al foil, which is a positive electrode current collector, using a doctor blade. After application, the positive electrode paste was dried at 80 °C for 15 minutes and compressed by a roll press to obtain a positive electrode. The loading of the active material in the positive electrode active material layer is 38 mg / cm2 and the density of the positive electrode active material layer was 3.2 g / cm 3 was obtained.

[0055] PE was prepared as the separator. A power generation element was formed by alternately laminating the positive electrode and the negative electrode via the separator. The power generation element included 4 positive electrodes and 5 negative electrodes.

[0056] As the exterior body, a pouch made of a laminate film was prepared. The power generation element was housed in the exterior body. As the electrolytic solution, a mixture obtained by dissolving a supporting salt (LiPF6) in a mixed solvent containing EC, DMC, and EMC at a concentration of 1.2 mol / L was prepared. The electrolytic solution was injected into the exterior body. After the injection of the electrolytic solution, the exterior body was sealed under a vacuum of -80 kPa. Thus, Battery No.1 was fabricated.

[0057] 《No.2》 As the material of the negative electrode, artificial graphite (86.5% by mass), Si (5% by mass), AB (1.5% by mass), PVdF (2% by mass) as the binder, and PVdF-HEP (5% by mass) as the polymer were prepared. Materials other than PVdF-HEP were mixed in distilled water and kneaded using a kneader, and then PVdF-HEP was added and further kneaded to obtain a negative electrode paste. Thereafter, the negative electrode of No.2 was obtained in the same manner as No.1.

[0058] The same positive electrode, separator, exterior body, and electrolytic solution as No.1 were prepared. A power generation element was formed in the same manner as No.1. The power generation element was housed in the exterior body, and the electrolytic solution was injected. After the injection of the electrolytic solution, it was held at 40 °C for 12 hours. Thereafter, the exterior body was sealed under a vacuum of -80 kPa. Thus, Battery No.2 was fabricated. In Battery No.2, the entire negative electrode active material layer is immersed in a gelled electrolytic solution (gel electrolyte).

[0059] 《No.3》 As a negative electrode material, artificial graphite (15% by mass), Si (60% by mass), PVdF-HEP (20% by mass), and AB (5% by mass) were prepared. Si, PVdF-HEP, and AB were mixed in NMP and kneaded using a kneader. Then, artificial graphite was further added and kneaded to obtain a first paste. The obtained first paste was instantaneously dried using a spray dryer to obtain precursor particles.

[0060] Artificial graphite, SBR, CMC, and AB were prepared. These materials and the obtained precursor particles were mixed in distilled water and kneaded using a kneader to obtain a second paste. The content ratio (by mass) of each material in the second paste was artificial graphite:Si:AB:PVdF-HEP:SBR:CMC = 86.5:5:1.5:1.7:2.65:2.65. Thereafter, the negative electrode of No. 3 was obtained in the same manner as No. 1.

[0061] The same positive electrode, separator, exterior body, and electrolyte as No. 1 were prepared. A power generation element was formed in the same manner as No. 1. The power generation element was housed in the exterior body and the electrolyte was injected. After the injection of the electrolyte, it was held at 40°C for 12 hours. Thereafter, the exterior body was sealed under a vacuum of -80 kPa. Thus, the battery of No. 3 was fabricated. In the battery of No. 3, a part of the negative electrode active material layer is immersed in a gelled electrolyte (gel electrolyte). That is, Si is surrounded by a gel electrolyte containing PVdF-HEP, and the concentration of the gel electrolyte in the vicinity of Si (in the range within a circle with a radius of twice the average particle diameter (30 μm) of Si from the center of gravity of Si) is higher than the concentration of the gel electrolyte in the region outside the vicinity of Si.

[0062] <Evaluation> <<Battery Resistance>> The batteries of each No. were placed in a thermostat at 40°C. The batteries were charged to 4.2 V at 0.5C. With a 10-minute break in between, the batteries were discharged to 3.0 V at 0.5C. Note that "C" is a symbol indicating the time rate. A current of 1C flows through the rated capacity of the battery in 1 hour.

[0063] Next, a battery was sandwiched between two metal plates and constrained so that a load of 0.5 MPa was applied. This was left standing in a thermostat at 25°C, and the battery was charged up to 4.1 V. Then, the battery was discharged for 10 seconds at 0.5C, 1C, and 1.5C respectively. The initial DCIR (Direct Current Internal Resistance) of each battery No. was determined from the slope of the voltage change amount and the applied current. The results are shown in Figure 3. The values in Figure 3 are shown as relative values when the value of the initial DCIR of No.1 is taken as 1.

[0064] 《Rate of increase in resistance》 Taking the above charging, resting, and discharging cycle as one cycle, in each battery No., charge and discharge were carried out 200 cycles. By determining the DCIR of the battery after each cycle, the rate of increase in resistance of each battery No. was determined. The results are shown in Figure 4. The values in Figure 4 are shown as relative values when the value of the initial DCIR of each battery No. is taken as 1.

[0065] <Results> As shown in Figure 3, it can be seen that in No.3, compared with No.2, the increase in the initial resistance is suppressed. Also, as shown in Figure 4, it can be seen that in No.3, compared with No.1, the rate of increase in resistance is reduced. From the above, it is considered that in No.3, the increase in the initial resistance is suppressed, and also the increase in the rate of increase in resistance after the cycle test is suppressed.

[0066] This embodiment and these examples are illustrative in all respects. This embodiment and these examples are not restrictive. The technical scope of the present disclosure includes all modifications within the meaning and scope equivalent to the description of the claims. For example, it has also been initially planned that any configurations are extracted from this embodiment and these examples and combined arbitrarily.

Explanation of reference numerals

[0067] 1 First active material, 2 Second active material, 3 Gel electrolyte, 10 Positive electrode, 11 Positive electrode current collector, 12 Positive electrode active material layer, 20 Negative electrode, 21 Negative electrode current collector, 22 Negative electrode active material layer, 30 Separator, 50 Power generation element, 100 Lithium ion secondary battery.

Claims

1. comprising a negative electrode current collector and a negative electrode active material layer, wherein the negative electrode active material layer contains a first active material, a second active material, and a gel electrolyte, wherein the gel electrolyte contains a polymer and a conductive material, wherein the negative electrode active material layer contains the gel electrolyte around the first active material, a negative electrode, wherein the concentration of the gel electrolyte around the first active material is higher than the concentration of the gel electrolyte in a region other than around the first active material in the negative electrode active material layer.

2. The negative electrode according to claim 1, wherein the polymer is a poly(vinylidene fluoride - hexafluoropropylene) copolymer.

3. The negative electrode according to claim 1, wherein the content of the first active material in the negative electrode active material layer is 5% by mass or more and 20% by mass or less.

4. A lithium ion secondary battery comprising the negative electrode according to any one of claims 1 to 3.

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