Lithium-ion battery

Abstract

To reduce the deterioration of storage properties associated with the use of carboxylic acid esters.SOLUTION: A lithium ion battery includes a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a positive electrode mixture. The positive electrode mixture includes a positive electrode active material and Li3PO4. The electrolyte includes a lithium salt and a solvent. The solvent includes a carboxylic acid ester in a volume fraction of 20% or more.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] This disclosure relates to lithium-ion batteries. [Background technology]

[0002] Japanese Patent Publication No. 2019-050155 (Patent Document 1) discloses an electrolyte that may contain esters. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2019-050155 [Overview of the project] [Problems that the invention aims to solve]

[0004] Generally, the electrolyte of lithium-ion batteries (hereinafter abbreviated as "batteries") contains carbonate as a solvent. Carboxylic acid esters (CAEs) can have lower viscosity than carbonates. By substituting a portion of the carbonate with CAE, improvements in input / output characteristics can be expected. However, the use of CAE tends to reduce storage characteristics. Therefore, it has been difficult to increase the volume fraction of CAE in the solvent.

[0005] The purpose of this disclosure is to mitigate the deterioration of storage properties associated with the use of carboxylic acid esters. [Means for solving the problem]

[0006] 1. A lithium-ion battery in one aspect of the present disclosure comprises the following components: The lithium-ion battery includes a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a positive electrode composite material. The positive electrode composite material includes a positive electrode active material and Li3PO4. The electrolyte includes a lithium salt and a solvent. The solvent contains 20% or more by volume of a carboxylic acid ester.

[0007] Generally, the storage properties tend to decrease as the volume fraction of CAE in the electrolyte increases. CAE tends to have a lower oxidative decomposition potential compared to carbonate. It is thought that the decrease in storage properties is accelerated by the increased decomposition of CAE at the positive electrode.

[0008] According to the new findings in this disclosure, the addition of Li3PO4 to the cathode composite material and the volume fraction of CAE in the electrolyte being 20% ​​or more are expected to mitigate the deterioration of storage properties associated with an increase in CAE. A certain amount of easily oxidatively decomposed CAE is present, which can promote the reaction between CAE and Li3PO4 on the surface of the cathode active material. This reaction is thought to rapidly form a stable protective film. After the formation of the protective film, the oxidative decomposition of CAE is expected to be reduced. In other words, the deterioration of storage properties is expected to be mitigated.

[0009] 2. The lithium-ion battery described in "1" above may have, for example, the following configuration: When fully charged, the positive electrode is 4.5V (vs.Li / Li + It is configured to have the following potentials.

[0010] Conventionally, protective coatings using Li3PO4 were 4.5V (vs. Li / Li + It has been confirmed that it is formed at high potentials exceeding 4.5V (vs.Li / Li). According to the new findings in this disclosure, when the volume fraction of CAE is 20% or more, 4.5V (vs.Li / Li + A stable protective film can be formed at positive electrode potentials below the specified range. This is expected to further reduce the deterioration of storage characteristics.

[0011] 3. The lithium-ion battery described in "1" or "2" above may have, for example, the following configuration: The carboxylic acid ester is at least one selected from the group consisting of methyl propionate and methyl acetate.

[0012] 4. The lithium-ion battery according to any one of the above "1" to "3" may have, for example, the following configuration. The mass fraction of Li3PO4 with respect to the total of the positive electrode active material and Li3PO4 is 1 to 10%. The solvent contains a carboxylic acid ester at a volume fraction of 2 20 to 70%.

[0013] 5. The lithium-ion battery in one aspect of the present disclosure may have the following configuration. The lithium-ion battery includes a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a positive electrode composite material. The positive electrode composite material includes a positive electrode active material and Li3PO4. The mass fraction of Li3PO4 with respect to the total of the positive electrode active material and Li3PO4 is 1 to 10%. The electrolyte contains a lithium salt and a solvent. The solvent contains a carboxylic acid ester at a volume fraction of 20 to 70%. When fully charged, the positive electrode is configured to have a potential of 4.5 V (vs. Li / Li + ) or less.

[0014] Hereinafter, embodiments of the present disclosure (hereinafter may be abbreviated as "the present embodiments") will be described. However, the present embodiments do not limit the technical scope of the present disclosure. The present embodiments are illustrative in all respects. The present embodiments are non-limiting. The technical scope of the present disclosure includes all modifications within the meaning and scope equivalent to the description of the claims. For example, any configurations are extracted from the present embodiments, and their arbitrary combinations are also contemplated from the beginning.

Brief Description of the Drawings

[0015] [Figure 1] It is a conceptual diagram showing an example of the lithium-ion battery in the present embodiment. [Figure 2] It is Table 1 showing experimental results. [Figure 3] It is a graph showing the relationship between the volume fraction of CAE and the remaining capacity. [Figure 4] It is a graph showing the relationship between the volume fraction of CAE and the gas generation amount.

Modes for Carrying Out the Invention

[0016] <Main Terms> The stoichiometric compositional formula shows a representative example of a compound. The compound may have a non-stoichiometric composition. For example, "Li3PO4" is not limited to compounds with a molar ratio of "Li / P / O = 3 / 1 / 4". Unless otherwise specified, "Li3PO4" refers to a compound containing Li, P, and O in any molar ratio. For example, the compound may be doped with trace elements. Some of the Li, P, and O may be substituted with other elements.

[0017] "Carboxylic acid esters (CAEs)" refer to compounds derived from the condensation of a carboxylic acid and an alcohol. Carboxylic acid esters are distinguished from carbonate esters.

[0018] "Fully charged" refers to a state where the reactive active material returns to its pre-discharge state. Fully charged can also be expressed as "fully charged," "100% charge level," or "100% SOC (state of charge)."

[0019] "V(vs.Li / Li)" + ) indicates the potential with lithium (Li) metal as the reference (zero).

[0020] Numerical ranges such as "m to n%" include upper and lower limits unless otherwise specified. That is, "m to n%" indicates a numerical range of "m% or more and n% or less". "m% or more and n% or less" includes "greater than m% and less than n%". "Greater than or equal to" and "less than or equal to" are represented by the equals sign inequality "≦". "Greater than" and "less than" are represented by the equals sign inequality "<". A number arbitrarily selected from within the numerical range may be used as a new upper or lower limit. For example, a new numerical range may be set by arbitrarily combining a number within the numerical range with a number listed in another part of this specification, in a table, in a figure, etc.

[0021] <Lithium-ion battery> Figure 1 is a conceptual diagram showing an example of a lithium-ion battery in this embodiment. The battery 100 includes a power generation element 50 and an outer casing 90. The outer casing 90 is sealed. The outer casing 90 may be, for example, a metal case, a pouch made of metal foil laminate film, etc. The outer casing 90 houses the power generation element 50 and an electrolyte (not shown).

[0022] <Electrolyte> The electrolyte is a liquid electrolyte. The electrolyte contains a Li salt and a solvent. The Li salt is dissolved in the solvent. The concentration of the Li salt may be, for example, 0.5 to 2 mol / L or 1 to 1.5 mol / L. The Li salt may be at least one selected from the group consisting of, for example, LiPF6, LiBF4, LiClO4, LiN(SO2F)2 "LiFSI", and LiN(SO2CF3)2 "LiTFSI".

[0023] The solvent contains 20% or more CAE by volume fraction. The solvent may, for example, consist of 20% or more CAE by volume fraction and the remainder being carbonate. The volume fraction of CAE may be, for example, 40% or more, 50% or more, 60% or more, 70% or more, 80% or more, or 90% or more. The volume fraction of CAE may be, for example, 100% or less, 90% or less, 80% or less, 70% or less, 60% or less, 50% or less, or 40% or less.

[0024] CAE may be at least one selected from the group consisting of, for example, methyl formate (MF), methyl acetate (MA), methyl propionate (MP), and diethyl malonate (DEM). CAE may include, for example, at least one selected from the group consisting of MP and MA.

[0025] The carbonate may include, for example, at least one selected from the group consisting of cyclic carbonates and linear carbonates. The cyclic carbonate may include, for example, at least one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and monofluoroethylene carbonate (FEC). The linear carbonate may include, for example, at least one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC).

[0026] The solvent may satisfy, for example, the following relationship: Va + Vb + Vc = 100% 20%≦Va≦40%, 0%≦Vb≦50%, 20%≦Vc≦70% Va: Volume fraction of cyclic carbonate Vb: Volume fraction of linear carbonates Vc: Volume fraction of CAE

[0027] The electrolyte may further contain optional additives in addition to the Li salt and solvent. The amount of additive (mass fraction of the total electrolyte) may be, for example, 0.01 to 5%, or 0.1 to 1%. The additives may include, for example, vinylene carbonate (VC), LiB(C2O4)2 "LiBOB," etc.

[0028] <Power generation elements> The power generation element 50 may also be referred to as, for example, an "electrode body". The power generation element 50 includes a positive electrode 10 and a negative electrode 20. The power generation element 50 may further include a separator 30. The power generation element 50 can have any form. The power generation element 50 may be, for example, a wound type or a laminated type.

[0029] <Positive electrode> The positive electrode 10 may be, for example, sheet-shaped. The positive electrode 10 may include, for example, a positive electrode current collector and a positive electrode composite material layer. The positive electrode current collector supports the positive electrode composite material layer. The positive electrode current collector may include, for example, aluminum (Al) foil or the like. The positive electrode composite material layer may be composed of a positive electrode composite material. The positive electrode composite material includes a positive electrode active material and Li3PO4. The mass fraction of Li3PO4 with respect to the total of the positive electrode active material and Li3PO4 may be, for example, 1 to 10%. The mass fraction of Li3PO4 may be any of, for example, 2% or more, 3% or more, 4% or more, 5% or more, 6% or more, 7% or more, 8% or more, or 9% or more. The mass fraction of Li3PO4 may be any of, for example, 9% or less, 8% or less, 7% or less, 6% or less, 5% or less, 4% or less, 3% or less, or 2% or less.

[0030] The positive electrode active material may be, for example, a particle group (powder). The D50 of the positive electrode active material may be, for example, either 1 to 30 μm or 5 to 15 μm. "D50" indicates the particle diameter at which the integration becomes 50% in the volume-based particle size distribution (cumulative distribution). The particle size distribution can be measured by the laser diffraction method. The positive electrode active material may contain any component. The positive electrode active material may include, for example, at least one selected from the group consisting of LiCoO2, LiNiO2, LiMnO2, LiMn2O4, Li(NiCoMn)O2, Li(NiCoAl)O2, and LiFePO4. 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. Li(NiCoMn)O2 is, for example, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, etc. may be included. Hereinafter, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 may be abbreviated as "NCM".

[0031] When fully charged, the positive terminal of battery 10 is 4.5V (vs. Li / Li + It may be configured to have a potential of ) or less. The positive electrode potential when fully charged is, for example, 4.4V (vs.Li / Li + ) or less, or 4.3V (vs. Li / Li + ) Any of the following is acceptable. The positive electrode potential when fully charged is, for example, 4.2V (vs.Li / Li + ) or higher, or 4.3V (vs. Li / Li + ) Any of the above is acceptable.

[0032] The positive electrode composite may further contain, in addition to the positive electrode active material and Li3PO4, a conductive material and a binder, for example. The conductive material may include at least one selected from the group consisting of, for example, carbon black (CB), vapor-grown carbon fibers, carbon nanotubes, and graphene flakes. The amount of conductive material may be, for example, 0.1 to 10 parts by mass per 100 parts by mass of positive electrode active material.

[0033] The binder may include, for example, at least one selected from the group consisting of polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), vinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP), and polyacrylic acid (PAA). The amount of binder may be, for example, 0.1 to 10 parts by mass per 100 parts by mass of positive electrode active material.

[0034] <Negative electrode> The negative electrode 20 may be, for example, in the form of a sheet. The negative electrode 20 may include, for example, a negative electrode current collector and a negative electrode composite layer. The negative electrode current collector supports the negative electrode composite layer. The negative electrode current collector may include, for example, copper (Cu) foil. The negative electrode composite layer may consist of a negative electrode composite. The negative electrode composite contains a negative electrode active material. The negative electrode active material may be, for example, a group of particles. The D50 of the negative electrode active material may be, for example, 1 to 30 μm or 5 to 15 μm. The negative electrode active material may contain any components. The negative electrode active material may include, for example, at least one selected from the group consisting of natural graphite, artificial graphite, soft carbon, hard carbon, silicon (Si), silicon oxide (SiO), Si-based alloys, Si-C composite materials, tin, tin oxide, and lithium titanate. The "Si-C composite material" may include, for example, composite particles. The composite particles may include, for example, porous carbon particles. Si particles may be supported within the pores of the porous carbon particles.

[0035] The negative electrode composite may further contain, for example, a binder in addition to the negative electrode active material. The binder may include, for example, at least one selected from the group consisting of styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), polyimide (PI), and polyamide-imide (PAI). The amount of binder may be, for example, 0.1 to 10 parts by mass per 100 parts by mass of negative electrode active material.

[0036] <Separator> The separator 30 is positioned between the positive electrode 10 and the negative electrode 20. The separator 30 separates the positive electrode 10 from the negative electrode 20. The separator 30 may include, for example, a porous film. The separator 30 may have, for example, a Gahl value of 200 to 400 seconds / 100 mL. The "Gahle value" can be measured by the Gahl test method. The separator 30 may have, for example, a multilayer structure. The multilayer structure may be formed, for example, by laminating a polypropylene (PP) layer, a polyethylene (PE) layer, and a PP layer in that order. [Examples]

[0037] <Preparation of test cells> Figure 2 is Table 1, which shows the experimental results. Cells (lithium-ion batteries) No. 1 to No. 27 were fabricated according to the following procedure.

[0038] Fabrication of the positive electrode The following materials were prepared. Cathode active material: NCM Conductive material: CB Binder: PVdF Dispersion medium: N-methyl-2-pyrrolidone (NMP) Positive electrode current collector: Al foil

[0039] A slurry was prepared by mixing NCM, CB, Li3PO4, PVdF, and NMP. The solid content ratio was "NCM / CB / PVdF = 87 / 10 / 3 (mass ratio)". The amount of Li3PO4 (mass fraction) was 5% of the total amount of NCM and Li3PO4. A positive electrode composite layer was formed by applying the slurry to the surface of the positive electrode current collector. The positive electrode was fabricated by compressing the positive electrode composite layer.

[0040] Fabrication of the negative electrode The following materials were prepared. Negative electrode active material: Natural graphite (D50 = 20 μm) Binder: SBR, CMC Dispersion medium: water Negative electrode current collector: Cu foil

[0041] A slurry was prepared by mixing graphite, CMC, SBR, and water. The solid content ratio was "graphite / CMC / SBR = 98 / 1 / 1 (mass ratio)". The slurry was applied to the surface of the negative electrode current collector to form a negative electrode composite layer. The basis weight of the negative electrode composite layer was adjusted so that the ratio of the negative electrode capacity to the positive electrode capacity was 1.1. The negative electrode was fabricated by compressing the negative electrode composite layer.

[0042] assembly The following materials were prepared. Separator: Three-layer structure (PP layer / PE layer / PP layer), Gaal value = 300 seconds / 100 mL Outer packaging: Pouch made of aluminum laminate film Electrolyte: LiPF6 (concentration = 1 ml / L), solvent composition (see Figure 2)

[0043] The positive electrode, separator, and negative electrode were stacked to form a power generation element. The power generation element was housed in an outer casing. Electrolyte was injected into the outer casing. After the electrolyte was injected, the outer casing was sealed to create a cell.

[0044] Activation treatment The following constant-current charging and constant-current discharging cycles were repeated three times in a constant-temperature bath (set temperature: 25°C). "C" is a symbol representing the rate of current over time. A current of 1C will fill the cell to its rated capacity in one hour. Constant current charging: Current = 0.1C, Cut-off voltage = 4.3V Constant current discharge: Current = 0.3C, Cut-off voltage = 3V

[0045] Capacity measurement The initial capacity (initial discharge capacity) was measured by the following constant current-constant voltage charging and constant current discharge methods. Constant current-constant voltage charging: Current = 0.1C, Upper voltage limit = 4.3C, Cut-off current = 0.02C Constant current discharge: Current = 0.2C, Cut-off voltage = 3V

[0046] <Preservation Test> The initial volume of the cell was measured using the Archimedes method. The cell voltage was adjusted to 4.3V by constant current-constant voltage charging (current = 0.1C, upper limit voltage = 4.3V) in a constant temperature bath (set temperature: 25°C). After voltage adjustment, the cell was left undisturbed for 100 days in a constant temperature bath (set temperature: 60°C). After 100 days, the remaining capacity was measured by discharging the cell by constant current discharge (current = 0.2C, cut-off voltage = 3V) in a constant temperature bath (set temperature: 25°C). In Figure 2 (Table 1), the value in the "Remaining Capacity" column is the percentage of the remaining capacity relative to the initial capacity. A larger remaining capacity is considered to indicate a reduced deterioration in storage characteristics.

[0047] After discharge, the storage volume of the cell was measured using the Archimedes method. The amount of gas generated was measured by subtracting the initial volume from the storage volume. A smaller amount of gas generated suggests a reduced degradation of storage characteristics.

[0048] <Result> In Figure 2 (Table 1), cells that satisfy the following conditions (A) and (B) tend to show less degradation in storage properties. In the table, the "No." of cells that satisfy the following conditions (A) and (B) is marked with an asterisk (*), for example, "No.*16". (A) The volume fraction of CAE (MP, MA) in the electrolyte (solvent) is 20% or more. (B) The positive electrode composite material contains a positive electrode active material and Li3PO4.

[0049] Figure 3 is a graph showing the relationship between the volume fraction of CAE and the remaining capacity. When Li3PO4 is not added to the cathode composite material, the remaining capacity tends to decrease monotonically as the volume fraction of CAE increases. Figure 4 is a graph showing the relationship between the volume fraction of CAE and the amount of gas generated. When Li3PO4 is not added to the cathode composite material, the amount of gas generated tends to increase monotonically as the volume fraction of CAE increases.

[0050] In Figures 3 and 4, when Li3PO4 is added to the cathode composite material, a change in the deterioration behavior of storage properties is observed at a volume fraction of 20%. Specifically, at a volume fraction of 20%, the deterioration behavior of storage properties reverses temporarily and then tends to become more gradual. It is considered that the deterioration of storage properties is reduced at volume fractions above 20%.

[0051] In Figure 2 (Table 1), the value in the "Positive Electrode Potential" column indicates the positive electrode potential when fully charged. From the results for No. 1, 12, No. 25, No. 26, and No. 27, when the solvent consists of carbonate (EC / EMC = 30 / 70), the positive electrode potential is 4.5V (vs. Li / Li + Beyond the range indicated by the above, the effect of adding Li3PO4 tends to become more pronounced.

[0052] From the results of No. 7, No. *18, No. *23, and No. *24, when the solvent contains CAE(MP), the positive electrode potential is 4.5V (vs. Li / Li + Within the range below ) the effect of adding Li3PO4 tends to be significant. [Explanation of symbols]

[0053] 10 Positive electrode, 20 Negative electrode, 30 Separator, 50 Power generation element, 90 Outer casing, 100 Battery (Lithium-ion battery).

Claims

1. It includes a positive electrode, a negative electrode, and an electrolyte, The aforementioned positive electrode includes a positive electrode composite material. The aforementioned positive electrode composite material consists of a positive electrode active material and Li 3 PO 4 Includes, The electrolyte comprises a lithium salt and a solvent, The solvent contains 40% or more of a carboxylic acid ester by volume fraction. Lithium-ion battery.

2. When fully charged, the positive terminal is 4.5V (vs. Li / Li + ) configured to have the following potentials, The lithium-ion battery according to claim 1.

3. The carboxylic acid ester is at least one selected from the group consisting of methylpropionate and methyl acetate. A lithium-ion battery according to claim 1 or claim 2.

4. The positive electrode active material and Li 3 PO 4 Li for the sum 3 PO 4 The mass fraction is between 1 and 10%, The solvent contains 40 to 70% of the carboxylic acid ester by volume fraction. A lithium-ion battery according to claim 1 or claim 2.

5. It includes a positive electrode, a negative electrode, and an electrolyte, The aforementioned positive electrode includes a positive electrode composite material. The positive electrode composite material includes a positive electrode active material and Li 3 PO 4 and contains The positive electrode active material and Li 3 PO 4 Li for the sum 3 PO 4 The mass fraction is between 1 and 10%, The electrolyte comprises a lithium salt and a solvent. The solvent contains 40 to 70% by volume of carboxylic acid ester, and When fully charged, the positive terminal is 4.5V (vs. Li / Li + ) configured to have the following potentials, Lithium-ion battery.

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