Lithium ion battery

Abstract

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 contains a positive electrode active material and Li3PO4. The electrolyte contains a lithium salt and a solvent. The solvent contains 20% or more by volume of a carboxylic acid ester.

Description

Technical Field

[0001] The present disclosure relates to lithium-ion batteries. Background Art

[0002] Japanese Patent Application Publication No. 2019-050155 discloses an electrolyte solution that may contain esters. Summary of the invention

[0003] Generally, the electrolyte of a lithium-ion battery (hereinafter may be referred to as a "battery") contains carbonate as a solvent. Carboxylic acid ester (CAE) may have a lower viscosity than carbonate. By replacing a part of the carbonate with CAE, for example, it can be expected that the input-output characteristics can be improved. However, due to the use of CAE, there is a tendency for the storage characteristics to decrease. Therefore, in the past, it was difficult to increase the volume fraction of CAE in the solvent.

[0004] An object of the present disclosure is to reduce the reduction in storage characteristics associated with the use of carboxylic acid esters.

[0005] 1. A lithium ion battery according to one aspect of the present disclosure has the following structure. The lithium ion battery comprises a positive electrode, a negative electrode, and an electrolyte. The positive electrode comprises a positive electrode composite material. The positive electrode composite material comprises a positive electrode active material and Li3PO4. The electrolyte comprises a lithium salt and a solvent. The solvent comprises 20% or more of a carboxylic acid ester by volume fraction.

[0006] Generally, the more the volume fraction of CAE in the electrolyte increases, the lower the storage characteristics tend to be. CAE tends to have a lower oxidative decomposition potential than carbonates. It is believed that the increase in the amount of CAE decomposed in the positive electrode promotes the reduction of storage characteristics.

[0007] According to the new insights disclosed in the present invention, by adding Li3PO4 to the positive electrode composite material and the volume fraction of CAE in the electrolyte being 20% ​​or more, it can be expected to reduce the reduction in storage characteristics associated with the increase in CAE. By making the CAE that is easily oxidatively decomposed above a specific amount, the reaction between CAE and Li3PO4 can be promoted on the surface of the positive electrode active material. It is believed that due to this reaction, a stable protective film is quickly formed. After the protective film is formed, it can be expected to reduce the oxidative decomposition of CAE. That is, it can be expected to reduce the reduction in storage characteristics.

[0008] 2. The lithium ion battery described in the above “1” may have the following structure, for example. The lithium ion battery is configured such that when fully charged, the positive electrode has a voltage of 4.5 V (vs. Li / Li + ) below the potential.

[0009] In the past, it was confirmed that the protective film based on Li3PO4 can be used at over 4.5V (vs. Li / Li+ ) is formed at a high potential. According to the new findings of the present disclosure, by making the volume fraction of CAE 20% or more, at 4.5V (vs. Li / Li + ) or less, a stable protective film can be formed. Thus, it can be expected that the reduction in storage characteristics can be further reduced.

[0010] 3. The lithium ion battery described in the above “1” or “2” may have the following configuration, for example: The carboxylate ester is at least one selected from the group consisting of methyl propionate and methyl acetate.

[0011] 4. The lithium ion battery described in any one of "1" to "3" above may have the following configuration, for example: The mass fraction of Li3PO4 relative to the total of the positive electrode active material and Li3PO4 is 1% to 10%. The solvent contains 20% to 70% by volume of a carboxylate ester.

[0012] 5. A lithium-ion battery according to one aspect of the present disclosure may have the following structure. The lithium-ion battery comprises a positive electrode, a negative electrode, and an electrolyte. The positive electrode comprises a positive electrode composite material. The positive electrode composite material comprises a positive electrode active material and Li3PO4. The mass fraction of Li3PO4 relative to the total of the positive electrode active material and Li3PO4 is 1% to 10%. The electrolyte comprises a lithium salt and a solvent. The solvent comprises 20% to 70% of a carboxylic acid ester by volume. The lithium-ion battery is configured such that, when fully charged, the positive electrode has a voltage of 4.5 V (vs. Li / Li + ) below the potential.

[0013] The following describes an embodiment of the present disclosure (hereinafter may be abbreviated as "this embodiment"). However, this embodiment does not limit the technical scope of the present disclosure. This embodiment is illustrative in all aspects. This embodiment is non-restrictive. The technical scope of the present disclosure includes all changes within the meaning and scope equivalent to the claims. For example, it is intended from the outset to extract any configuration from this embodiment and combine them arbitrarily. The above and other objects, features, aspects and advantages of the present disclosure will become more apparent from the following detailed description of the present disclosure when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is a conceptual diagram showing an example of a lithium ion battery in this embodiment. Figure 2 Table 1 shows the experimental results. Figure 3 It is a graph showing the relationship between the volume fraction of CAE and the residual capacity. Figure 4 It is a graph showing the relationship between the volume fraction of CAE and the amount of gas generated. DETAILED DESCRIPTION

[0015] -Terms and phrases- The stoichiometric composition formula represents a representative example of a compound. A compound may have a non-stoichiometric composition. For example, "Li3PO4" is not limited to a compound having a molar ratio (molar ratio) of "Li / P / O=3 / 1 / 4". Unless otherwise specified, "Li3PO4" represents a compound containing Li, P and O in an arbitrary molar ratio. For example, the compound may also be doped with trace elements. A portion of Li, P and O may also be replaced by other elements.

[0016] "Carboxylic acid ester (CAE)" refers to a compound derived by condensation of a carboxylic acid and an alcohol. Carboxylic acid esters are distinguished from carbonates.

[0017] "Fully charged" means charging until the active substances that can react return to the state before discharge. Fully charged may also be referred to as "full charge", "100% charge level" or "100% SOC (state of charge)".

[0018] “V(vs.Li / Li + )” represents the potential with lithium (Li) metal as the reference (zero).

[0019] Unless otherwise specified, numerical ranges such as “m% to n%” include upper and lower limits. That is, “m% to n%” means a numerical range of “above m% and below n%”. “Above m% and below n%” includes “more than m% and less than n%”. “Above” and “below” are represented by an inequality sign “≤” with an equal sign. “Exceeding” and “less than” are represented by an inequality sign “<” without an equal sign. A numerical value arbitrarily selected from the numerical range can be used as a new upper limit or lower limit. For example, a new numerical range can be set by arbitrarily combining the numerical values ​​in the numerical range with the numerical values ​​recorded in other parts, tables, figures, etc. in this specification.

[0020] -Lithium-ion battery- Figure 1 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 can be, for example, a metal shell, a bag made of a metal foil laminate film, etc. The outer casing 90 contains the power generation element 50 and an electrolyte (not shown).

[0021] -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, any one of 0.5 mol / L to 2 mol / L and 1 mol / L to 1.5 mol / L. The Li salt may be, for example, at least one selected from the group consisting of LiPF6, LiBF4, LiClO4, LiN(SO2F)2 "LiFSI" and LiN(SO2CF3)2 "LiTFSI".

[0022] The solvent contains more than 20% CAE by volume fraction. The solvent may be composed of, for example, more than 20% CAE by volume fraction and the remainder of carbonate. The volume fraction of CAE may be, for example, any one of more than 40%, more than 50%, more than 60%, more than 70%, more than 80%, and more than 90%. The volume fraction of CAE may be, for example, any one of less than 100%, less than 90%, less than 80%, less than 70%, less than 60%, less than 50%, and less than 40%.

[0023] CAE may be, for example, at least one selected from the group consisting of methyl formate (MF), methyl acetate (MA), methyl propionate (MP), and ethyl malonate (DEM). CAE may include, for example, at least one selected from the group consisting of MP and MA.

[0024] Carbonate, for example, can include at least one selected from the group consisting of cyclic carbonate and chain carbonate. Cyclic carbonate, for example, can include at least one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) and monofluoroethylene carbonate (FEC). Chain carbonate, for example, can include at least one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC).

[0025] The solvent may satisfy the following relationship, for example. Va+Vb+Vc=100% 20%≤Va≤40%, 0%≤Vb≤50%, 20%≤Vc≤70% Va: Volume fraction of cyclic carbonate Vb: Volume fraction of chain carbonate Vc: Volume fraction of CAE

[0026] The electrolyte may also contain any additives in addition to the Li salt and the solvent. The amount of addition (mass fraction relative to the total electrolyte) may be, for example, any one of 0.01% to 5% and 0.1% to 1%. The additives may include, for example, vinylene carbonate (VC), LiB(C2O4)2 "LiBOB", etc.

[0027] -Power generation elements- The power generation element 50 may be referred to as an "electrode body" or the like. 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 may have any form. For example, the power generation element 50 may be a winding type or a stacked type.

[0028] -positive electrode- The positive electrode 10 may be in sheet form, for example. 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, etc. 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 relative 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, for example, any one of 2% or more, 3% or more, 4% or more, 5% or more, 6% or more, 7% or more, 8% or more, and 9% or more. The mass fraction of Li3PO4 may be, for example, any one of 9% or less, 8% or less, 7% or less, 6% or less, 5% or less, 4% or less, 3% or less, and 2% or less.

[0029] 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, any one of 1 μm to 30 μm and 5 μm to 15 μm. "D50" represents the particle size that accumulates to 50% in the volume-based particle size distribution (cumulative distribution). The particle size distribution can be measured by laser diffraction. The positive electrode active material may contain any component. The positive electrode active material may contain, 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, "(NiCoMn)" in "Li(NiCoMn)O2" indicates that the total of the composition ratios in the brackets is 1. As long as the total is 1, the amount of each component is arbitrary. Li(NiCoMn)O2 may contain, 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. Below, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 may be abbreviated as "NCM".

[0030] The battery 100 may be configured such that when fully charged, the positive electrode 10 has a voltage of 4.5 V (vs. Li / Li + ) or less. The positive electrode potential when fully charged can be, for example, 4.4 V (vs. Li / Li + ) or less, and 4.3V(vs.Li / Li + ) or less. The positive electrode potential when fully charged can be, for example, 4.2 V (vs. Li / Li + ) and above, and 4.3V(vs.Li / Li + ) Any of the above.

[0031] The positive electrode composite material may also include, in addition to the positive electrode active material and Li3PO4, a conductive material and a binder, etc. The conductive material may include, for example, at least one selected from the group consisting of carbon black (CB), vapor-grown carbon fiber, carbon nanotubes, and graphene sheets. The amount of the conductive material may be, for example, 0.1 to 10 parts by mass relative to 100 parts by mass of the positive electrode active material.

[0032] 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 the binder may be, for example, 0.1 to 10 parts by mass relative to 100 parts by mass of the positive electrode active material.

[0033] -negative electrode- The negative electrode 20 may be, for example, in sheet form. The negative electrode 20 may include, for example, a negative electrode current collector and a negative electrode composite material layer. The negative electrode current collector supports the negative electrode composite material layer. The negative electrode current collector may include, for example, copper (Cu) foil, etc. The negative electrode composite material layer may be composed of a negative electrode composite material. The negative electrode composite material includes a negative electrode active material. The negative electrode active material may be, for example, a particle group. The D50 of the negative electrode active material may be, for example, any one of 1 μm to 30 μm and 5 μm to 15 μm. The negative electrode active material may include any component. 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 alloy, Si-C composite material, tin, tin oxide, and lithium titanate. "Si-C composite material" may include, for example, composite particles. Composite particles may include, for example, porous carbon particles. Si particles may be loaded in the pores of the porous carbon particles.

[0034] The negative electrode composite material may also include, 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), carboxymethyl cellulose (CMC), polyimide (PI) and polyamide-imide (PAI). The amount of the binder may be, for example, 0.1 to 10 parts by mass relative to 100 parts by mass of the negative electrode active material.

[0035] -Spacers- The separator 30 is disposed 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 or the like. The separator 30 may have, for example, a Gurley value of 200 sec / 100 mL to 400 sec / 100 mL. The “Gurley value” may be measured using the Gurley test method. The separator 30 may have, for example, a multilayer structure. The multilayer structure may be formed by, for example, stacking a polypropylene (PP) layer, a polyethylene (PE) layer, and a PP layer in sequence. [Example]

[0036] -Production of test unit- Figure 2 The experimental results are shown in Table 1. Cells (lithium ion batteries) No. 1 to No. 27 were produced by the following procedure.

[0037] Preparation of positive electrode Prepare the following materials. Positive electrode active material: NCM Conductive material: CB Adhesive: PVdF Dispersion medium: N-methyl-2-pyrrolidone (NMP) Positive electrode collector: Al foil

[0038] The slurry is prepared by mixing NCM, CB, Li3PO4, PVdF and NMP. The solid content ratio is "NCM / CB / PVdF=87 / 10 / 3 (mass ratio)". The amount (mass fraction) of Li3PO4 is 5% relative to the total of NCM and Li3PO4. The slurry is applied to the surface of the positive electrode collector to form a positive electrode composite material layer. The positive electrode is produced by compressing the positive electrode composite material layer.

[0039] Negative electrode preparation Prepare the following materials. Negative electrode active material: natural graphite (D50 = 20 μm) Binder: SBR, CMC Dispersion medium: water Negative electrode collector: Cu foil

[0040] The slurry is prepared by mixing graphite, CMC, SBR and water. The solid content ratio is "graphite / CMC / SBR=98 / 1 / 1 (mass ratio)". The slurry is applied to the surface of the negative electrode collector to form a negative electrode composite material layer. The unit area mass of the negative electrode composite material layer is adjusted so that the ratio of the negative electrode capacity to the positive electrode capacity is 1.1. The negative electrode is produced by compressing the negative electrode composite material layer.

[0041] Assembly Prepare the following materials. Spacer: Three-layer structure (PP layer / PE layer / PP layer), Gurley value = 300 seconds / 100mL Outer body: Bag made of Al laminated film Electrolyte: LiPF6 (concentration = 1 mol / L), solvent composition (refer to Figure 2 )

[0042] The positive electrode, separator and negative electrode are stacked to form a power generation element. The power generation element is housed in an outer casing. An electrolyte is injected into the outer casing. After the electrolyte is injected, the outer casing is sealed to produce a single body.

[0043] Activation treatment In a thermostatic chamber (set temperature: 25°C), the following constant current charge and discharge cycles were repeated three times. "C" is a symbol indicating the time rate of current. A current of 1C consumes the rated capacity of a cell in 1 hour. Constant current charging: current = 0.1C, cut-off voltage = 4.3V Constant current discharge: current = 0.3C, cut-off voltage = 3V

[0044] Capacity determination The initial capacity (initial discharge capacity) was measured by constant current-constant voltage charging and constant current discharging as described below. Constant current-constant voltage charging: current = 0.1C, upper limit voltage = 4.3, cut-off current = 0.02C Constant current discharge: current = 0.2C, cut-off voltage = 3V

[0045] -Save Test- The initial volume of the monomer was measured using the Archimedean method. The voltage of the monomer was adjusted to 4.3 V in a thermostatic chamber (set temperature: 25°C) by constant current-constant voltage charging (current = 0.1C, upper limit voltage = 4.3V). After the voltage adjustment, the monomer was left to stand for 100 days in a thermostatic chamber (set temperature: 60°C). After 100 days, the monomer was discharged by constant current discharge (current = 0.2C, cutoff voltage = 3V) in a thermostatic chamber (set temperature: 25°C), and the residual capacity was measured. Figure 2The value of the item "remaining capacity" in (Table 1) is the percentage of the remaining capacity relative to the initial capacity. It is considered that the larger the remaining capacity is, the less deterioration of the storage characteristics will be.

[0046] After discharge, the volume of the monomer after storage was measured by the Archimedean method. The amount of gas generated was measured by subtracting the initial volume from the volume after storage. It is believed that the smaller the amount of gas generated, the less degradation of storage characteristics.

[0047] -result- Figure 2 In (Table 1), monomers satisfying the following conditions (A) and (B) tend to have less deterioration in storage characteristics. In the table, "No." for monomers satisfying the following conditions (A) and (B) is marked with "*" such as "No.*16". (A) The volume fraction of CAE (MP, MA) in the electrolyte solution (solvent) is 20% or more. (B) The positive electrode composite material includes a positive electrode active material and Li3PO4.

[0048] Figure 3 Graph showing the relationship between the volume fraction of CAE and the residual capacity. When Li3PO4 is not added to the positive electrode composite material, a tendency of a monotonically decreasing residual capacity is observed as the volume fraction of CAE increases. Figure 4 Graph showing the relationship between the volume fraction of CAE and the amount of gas generated. When Li3PO4 is not added to the positive electrode composite material, a tendency of monotonically increasing the amount of gas generated is observed as the volume fraction of CAE increases.

[0049] Figure 3 and Figure 4 In the case where Li3PO4 is added to the positive electrode composite material, a change in the behavior of the decrease in storage characteristics is observed at a volume fraction of 20%. That is, at a volume fraction of 20%, a tendency is observed that the behavior of the decrease in storage characteristics is temporarily reversed and then becomes gentle. It is believed that the decrease in storage characteristics is alleviated at a volume fraction of 20% or more.

[0050] Figure 2 In (Table 1), the value of the item "Positive Electrode Potential" indicates the positive electrode potential when fully charged. According to the results of No. 1, No. 12, No. 25, No. 26, and No. 27, when the solvent is composed of carbonate (EC / EMC = 30 / 70), it was observed that the positive electrode potential exceeded 4.5 V (vs. Li / Li + ) in the range of , the effect of adding Li3PO4 tends to become significant.

[0051] According to the results of No. 7, No. *18, No. *23, and No. *24, when the solvent contained CAE (MP), it was observed that the positive electrode potential was 4.5 V (vs. Li / Li + ) below the range, the effect of adding Li3PO4 tends to become significant.

Claims

1. A lithium-ion battery, It includes a positive electrode, a negative electrode, and an electrolyte. The positive electrode comprises a positive electrode composite material, The positive electrode composite material comprises a positive electrode active material and Li3PO4, The electrolyte comprises a lithium salt and a solvent, and The solvent contains 20% or more of the carboxylic acid ester by volume fraction.

2. The lithium ion battery according to claim 1, wherein when fully charged, the positive electrode has a voltage of 4.5 V (vs. Li / Li + ) below the potential.

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

4. The lithium ion battery according to claim 1 or 2, wherein: The mass fraction of Li3PO4 relative to the total of the positive electrode active material and Li3PO4 is 1% to 10%, and The solvent contains 20 to 70% by volume of the carboxylic acid ester.

5. A lithium ion battery, It includes a positive electrode, a negative electrode, and an electrolyte. The positive electrode comprises a positive electrode composite material, The positive electrode composite material comprises a positive electrode active material and Li3PO4, The mass fraction of Li3PO4 relative to the total of the positive electrode active material and Li3PO4 is 1% to 10%, The electrolyte comprises a lithium salt and a solvent. The solvent contains 20% to 70% by volume of a carboxylic acid ester, and The lithium ion battery is configured such that when fully charged, the positive electrode has a voltage of 4.5 V (vs. Li / Li + ) below the potential.

Images