Li-ion battery

Abstract

To provide a Li-ion battery capable of mitigating the increase in internal pressure.SOLUTION: The Li-ion battery includes a positive electrode, a negative electrode, and an electrolyte. The positive electrode contains a positive electrode mixture. The positive electrode mixture contains a positive electrode active material and activated Lithium. The mass fraction of activated lithium in the positive electrode is 0.25% or more of the total of the positive electrode active material and the activated lithium. The electrolyte contains a solvent and Lithium salt. The solvent contains 40% or more carboxylic acid esters by volume.SELECTED DRAWING: Figure 5

Description

[Technical field]

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

[0002] JP2015-528640A discloses an electrolyte solution containing 10 to 90% by weight of an ester-based solvent based on the total weight of the non-aqueous solvent. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Special Publication No. 2015-528640 Summary of the Invention [Problem to be solved by the invention]

[0004] Generally, lithium ion batteries (hereinafter may be abbreviated as "batteries") are sealed systems. The decomposition reaction of the electrolyte may be accompanied by gas generation. Gas generation within the battery may cause a sustained increase in internal pressure.

[0005] An objective of the present disclosure is to mitigate the rise in internal pressure. [Means for solving the problem]

[0006] 1. A lithium ion battery according to one aspect of the present disclosure has the following configuration. The 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 active lithium. The mass fraction of the active lithium relative to the total of the positive electrode active material and the active lithium is 0.25% or more. The electrolyte includes a solvent and a lithium salt. The solvent includes a carboxylate ester in a volume fraction of 40% or more.

[0007] The gas generated by the decomposition of the electrolyte may contain various components. In particular, the composition ratio of carbon dioxide tends to be high. Carbon dioxide may contain, for example, carbon monoxide (CO), carbon dioxide (CO2), etc. In the battery, the carbon dioxide may dissolve in the solvent (electrolyte). The carbon dioxide dissolved in the electrolyte may be released from the electrolyte, for example, in response to temperature changes. It is considered that the internal pressure in the battery continues to increase while the carbon dioxide is repeatedly dissolved and released. When the solvent contains 40% or more of a carboxylate ester, a part of the carbon dioxide dissolved in the electrolyte may change into a solid compound by reacting with, for example, lithium (Li) at least in one of the positive electrode and the negative electrode. That is, a part of the carbon dioxide may be fixed to the electrode. It is considered that the fixed carbon dioxide may be removed from the dissolution-release cycle.

[0008] Furthermore, the positive electrode contains chemically active Li (hereinafter also referred to as "active Li"). The presence of a specific amount or more of active Li in the positive electrode can promote fixation of carbon dioxide gas in the positive electrode. It is expected that the synergistic effect of these actions will mitigate the increase in internal pressure.

[0009] 2. The lithium ion battery described in "1" above may include, for example, the following configuration. The negative electrode includes a negative electrode mixture. The negative electrode mixture includes a negative electrode active material and amorphous carbon. The amorphous carbon covers at least a part of the surface of the negative electrode active material. The mass fraction of the amorphous carbon with respect to the total of the negative electrode active material and the amorphous carbon is 0.5 to 1%.

[0010] The presence of a specific amount of amorphous carbon on the surface of the negative electrode active material can promote fixation of carbon dioxide gas in the negative electrode, which is expected to further mitigate the increase in internal pressure.

[0011] 3. The lithium ion battery according to the above "1" or "2" may include, for example, the following configuration: The mass fraction of active lithium relative to the positive electrode active material is 0.25 to 0.5%. The solvent contains 40 to 70% by volume of a carboxylic acid ester.

[0012] 4. The lithium ion battery according to any one of the above items "1" to "3" may include, for example, the following configuration: The carboxylic acid ester is at least one selected from the group consisting of methyl propionate and methyl acetate.

[0013] 5. A lithium ion battery according to an 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 mixture. The positive electrode mixture includes a positive electrode active material and active lithium. The mass fraction of the active lithium relative to the total of the positive electrode active material and the active lithium is 0.25 to 0.5%. The negative electrode includes a negative electrode mixture. The negative electrode mixture includes a negative electrode active material and amorphous carbon. The amorphous carbon covers at least a portion of the surface of the negative electrode active material. The mass fraction of the amorphous carbon relative to the total of the negative electrode active material and the amorphous carbon is 0.5 to 1%. The electrolyte includes a solvent and a lithium salt. The solvent includes a carboxylic acid ester in a volume fraction of 40 to 70%. The carboxylic acid ester is at least one selected from the group consisting of methyl propionate and methyl acetate.

[0014] Hereinafter, an embodiment of the present disclosure (hereinafter may be abbreviated as "the present embodiment") and an example of the present disclosure (hereinafter may be abbreviated as "the present embodiment") will be described. However, the present embodiment and the present embodiment do not limit the technical scope of the present disclosure. The present embodiment and the present embodiment are illustrative in all respects. The present embodiment and the present embodiment are non-restrictive. The technical scope of the present disclosure encompasses all modifications within the meaning and scope equivalent to the description of the claims. For example, it is also intended from the beginning that any configuration may be extracted from the present embodiment and arbitrarily combined. [Brief description of the drawings]

[0015] [Figure 1] FIG. 1 is a conceptual diagram showing an example of a lithium ion battery according to the present embodiment. [Diagram 2] FIG. 2 is Table 1 showing the experimental results. [Diagram 3]FIG. 3 is Table 2 showing the experimental results. [Figure 4] FIG. 4 is Table 3 showing the experimental results. [Diagram 5] FIG. 5 is a graph showing the relationship between the volume fraction of CAE, the amount of active Li, and the amount of cell shrinkage. [Figure 6] FIG. 6 is a graph showing the relationship between the volume fraction of CAE, the amount of amorphous coating, and the amount of cell shrinkage. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] <Key terms> "Amount of active Li" refers to the mass fraction of active Li relative to the total of the positive electrode active material and active Li. The amount of active Li is measured using the following procedure. First, the amount of substance (unit: mol) of lithium hydroxide (LiOH) attached to the positive electrode active material is measured. The positive electrode active material is subjected to a water washing treatment. After washing, the amount of substance of LiOH attached to the positive electrode active material is measured again. The amount of substance of LiOH before washing is subtracted from the amount of substance of LiOH after washing to determine the increase in the amount of LiOH. The increase in the amount of LiOH (amount of substance) is converted to mass to determine the mass of active Li. The amount of active Li (mass fraction) is determined by dividing the mass of active Li by the total mass of the positive electrode active material and active Li.

[0017] The "amorphous coating amount" refers to the mass fraction of amorphous carbon relative to the total of the negative electrode active material and amorphous carbon. The coating process with amorphous carbon can be carried out by the following procedure. A carbon raw material (e.g., pitch, etc.) is mixed with the negative electrode active material to prepare a mixture. The mixture can be heat-treated to convert the carbon raw material into amorphous carbon. The mass fraction of the carbon raw material relative to the entire mixture during the coating process is considered to be the "amorphous coating amount."

[0018] "Carboxylic acid ester (CAE)" refers to a compound derived from the condensation of a carboxylic acid with an alcohol. CAEs are to be distinguished from carbonates.

[0019] 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 "more than m% and less than n%". "More than" and "less than" are represented by an inequality sign with an equal sign "≦". "More than" and "less than" are represented by an inequality sign without an equal sign "<". A numerical value arbitrarily selected from within the numerical range may be set as a new upper or lower limit. For example, a new numerical range may be set by arbitrarily combining a numerical value within the numerical range with a numerical value described in another part of this specification, in a table, in a figure, or the like.

[0020] <Lithium-ion battery> 1 is a conceptual diagram showing an example of a lithium ion battery in this embodiment. The battery 100 includes a power generating element 50 and an exterior body 90. The exterior body 90 is sealed. The exterior body 90 may be, for example, a metal case or a pouch made of a metal foil laminate film. The exterior body 90 houses the power generating element 50 and an electrolyte (not shown).

[0021] <Electrolyte> The electrolyte is a liquid electrolyte. The electrolyte includes a solvent and a Li salt. 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, 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 40% or more CAE by volume. The solvent may contain, for example, 40 to 70% CAE by volume. The solvent may be, for example, 40 to 70% CAE by volume, with the remainder being carbonate. The volume fraction of the CAE may be, for example, 50% or more, or 60% or more. The CAE may have a lower viscosity than the carbonate. By increasing the volume fraction of the CAE, for example, improvement of rate characteristics and the like is expected. The volume fraction of the CAE may be, for example, 60% or less, or 50% or less.

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

[0024] The carbonate may include at least one selected from the group consisting of cyclic carbonates and chain carbonates. The cyclic carbonate may include at least one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and monofluoroethylene carbonate (FEC). The chain carbonate may 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, for example, satisfy the following relationship: Va+Vb+Vc=100% 20%≦Va≦40%, 0%≦Vb≦30%, 40%≦Vc≦70% Va: Volume fraction of cyclic carbonate Vb: Volume fraction of chain carbonate Vc: Volume fraction of CAE

[0026] The electrolyte may further contain an arbitrary additive in addition to the solvent and the Li salt. The amount of additive (mass fraction relative to the entire electrolyte) may be, for example, 0.01 to 5%, or 0.1 to 1%. The additive may include, for example, vinylene carbonate (VC), LiB(C2O4)2 "LiBOB", etc.

[0027] <Power generation elements> The power generating element 50 may also be referred to as, for example, an "electrode body." The power generating element 50 includes a positive electrode 10 and a negative electrode 20. The power generating element 50 may further include a separator 30. The power generating element 50 may have any form. The power generating element 50 may be, for example, either a wound type or a laminated type.

[0028] <Positive electrode> The positive electrode 10 may be, for example, in the form of a sheet. The positive electrode 10 may include, for example, a positive electrode current collector and a positive electrode composite layer. The positive electrode current collector supports the positive electrode composite layer. The positive electrode current collector may include, for example, aluminum (Al) foil or the like. The positive electrode composite layer may be made of a positive electrode composite. The positive electrode composite includes a positive electrode active material and active Li. The amount of active Li is 0.25% or more. The amount of active Li may be, for example, any of 0.5% or more, 0.75% or more, or 1% or more. The amount of active Li may be, for example, any of 2% or less, 1% or less, 0.75% or less, or 0.5% 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, 1 to 30 μm or 5 to 15 μm. "D50" indicates a particle diameter at which the cumulative distribution is 50% in a volume-based particle size distribution (cumulative distribution). The particle size distribution can be measured by a laser diffraction method. 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 sum of the composition ratios in parentheses is 1. As long as the sum is 1, the amount of each component is 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 It may contain O2, etc. 1 / 3 Co 1 / 3 Mn 1 / 3 O2 may be abbreviated as "NCM."

[0030] The positive electrode mixture may further contain, for example, a conductive material and a binder in addition to the positive electrode active material and active Li. The conductive material may contain, for example, at least one selected from the group consisting of carbon black (CB), vapor-grown carbon fiber, carbon nanotubes, and graphene flakes. The amount of the conductive material may be, for example, 0.1 to 10 parts by mass with respect to 100 parts by mass of the positive electrode active material.

[0031] The binder may include 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 with respect to 100 parts by mass of the positive electrode active material.

[0032] <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 or the like. The negative electrode composite layer may be made of a negative electrode composite. The negative electrode composite 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, 1 to 30 μm or 5 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, silicon (Si), silicon oxide (SiO), Si-based alloy, Si-C composite material, 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, and Si particles may be supported within the pores of the porous carbon particles.

[0033] The negative electrode mixture may further contain amorphous carbon in addition to the negative electrode active material. Amorphous carbon has lower crystallinity than graphite (crystalline carbon). The R value of amorphous carbon may be, for example, 0.5 or more, 1.0 or more, or 1.5 or more. The R value is calculated by the following formula. It is considered that the larger the R value, the lower the crystallinity.

[0034] R value = I 1360 / I 1580 I 1360 : In the Raman spectrum of the object, 1360 cm -1 Intensity of nearby peak (D band) I 1580: In the Raman spectrum of the object, 1580 cm -1 Intensity of nearby peaks (G band)

[0035] The amorphous carbon may cover at least a part of the surface of the negative electrode active material. The amorphous carbon may form a coating layer. The thickness of the coating layer may be, for example, 5 to 10 nm. The amount of the amorphous coating may be, for example, 0.25% or more, 0.5% or more, or 1.0% or more. The amount of the amorphous coating may be, for example, 1.5% or less, 1% or less, or 0.5% or less.

[0036] The negative electrode mixture may further contain, for example, a binder in addition to the negative electrode active material and the amorphous carbon. The binder may contain, 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 with respect to 100 parts by mass of the negative electrode active material.

[0037] <separator> 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 to 400 seconds / 100 mL. The "Gurley value" can be measured by a Gurley test method. The separator 30 may have, for example, a multi-layer structure. The multi-layer structure may be formed by, for example, laminating a polypropylene (PP) layer, a polyethylene (PE) layer, and a PP layer in this order. EXAMPLES

[0038] <Preparation of test cell> FIG. 2 is Table 1 showing the experimental results. FIG. 3 is Table 2 showing the experimental results. FIG. 4 is Table 3 showing the experimental results. Cells (lithium ion batteries) No. 1 to No. 58 were produced by the following procedure.

[0039] Preparation of the positive electrode The following materials were prepared: Positive electrode active material: NCM (active Li amount: see Figures 2 to 4) Conductive material: CB Binder: PVdF Dispersion medium: N-methyl-2-pyrrolidone (NMP) Positive electrode current collector: Aluminum foil

[0040] NCM, CB, PVdF, and NMP were mixed to prepare a slurry. The solid content ratio was "NCM / CB / PVdF = 87 / 10 / 3 (mass ratio)". The slurry was applied to the surface of the positive electrode current collector to form a positive electrode composite layer. The positive electrode composite layer was compressed to produce a positive electrode.

[0041] Preparation of negative electrode The following materials were prepared: Negative electrode active material: artificial graphite (D50 = 15 μm, amorphous coating amount: see Figures 2 to 4) Binder: SBR, CMC Dispersion medium: water Negative electrode current collector: Cu foil

[0042] 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 composite layer was compressed to produce a negative electrode.

[0043] ·assembly The following materials were prepared: Separator: Three-layer structure (PP layer / PE layer / PP layer), Gurley value = 300 seconds / 100 mL Exterior: Pouch made of aluminum laminated film Electrolyte: LiPF6 (concentration = 1 mol / L), solvent composition (see Figures 2 to 4)

[0044] A power generating element was formed by stacking the positive electrode, the separator, and the negative electrode. The power generating element was housed in an exterior body. An electrolyte solution was injected into the exterior body. After the electrolyte solution was injected, the exterior body was sealed to prepare a cell.

[0045] Activation treatment The following cycle of constant current charging and discharging was repeated three times in a thermostatic chamber (set temperature: 25°C). "C" is the symbol that represents the time rate of current. A current of 1C will drain the rated capacity of the cell in one hour. Constant current charging: Current = 0.1C, Cutoff voltage = 4.3V Constant current discharge: Current = 0.3C, Cut voltage = 3V

[0046] Capacity measurement The initial capacity (initial discharge capacity) was measured by the following constant current-constant voltage charging and constant current discharging. Constant current-constant voltage charging: current = 0.1C, upper limit voltage = 4.3, cut current = 0.02C Constant current discharge: Current = 0.2C, Cut voltage = 3V

[0047] <Preservation test> The initial volume of the cell was measured by 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 thermostatic bath (set temperature: 25°C). After adjusting the voltage, the cell was left to stand in a thermostatic bath (set temperature: 60°C) for 100 days. After 100 days, the cell was discharged by constant current discharging (current = 0.2C, cut voltage = 3V) in a thermostatic bath (set temperature: 25°C). After discharging, the post-storage volume of the cell was measured by the Archimedes method. The post-storage volume was divided by the initial volume to obtain the "volume change (dimensionless quantity)". It is considered that the smaller the value shown in the "volume change" item in Figures 2 to 4, the more mitigated the increase in internal pressure is.

[0048] <Result> In the tables of Figures 2 to 4, cells that satisfy the following conditions (A) and (B) tend to have small volume changes. In the tables, the "No." of cells that satisfy the following conditions (A) and (B) is marked with an "*", such as "No.*21". (A) The volume fraction of CAE (MP, MA) in the electrolyte is 40% or more. (B) The amount of active Li in the positive electrode is 0.25% or more.

[0049] In the table, cells that satisfy the following condition (C) in addition to the above conditions (A) and (B) tend to have even smaller volume changes. (C) The amount of amorphous coating on the negative electrode is 0.5 to 1%.

[0050] FIG. 5 is a graph showing the relationship between the volume fraction of CAE, the amount of active Li, and the amount of cell shrinkage. The amount of cell shrinkage is measured in the following procedure. After the storage test is completed, the cell is left to stand for 7 days in a thermostatic chamber (set temperature: 25°C). After 7 days, the volume of the cell after standing is measured by the Archimedes method. The amount of cell shrinkage (unit: cm) is calculated by subtracting the volume after storage from the volume after standing. 3 ) is obtained. The amount of cell shrinkage is usually a negative value. It is believed that the larger the absolute value of the amount of cell shrinkage, the more easily the carbon dioxide gas generated during the storage test is fixed to the electrode. When the volume fraction of CAE reaches 40% or more, the amount of cell shrinkage (absolute value) increases sharply. There is a tendency for the amount of cell shrinkage (absolute value) to increase further as the amount of active Li increases.

[0051] Figure 6 is a graph showing the relationship between the volume fraction of CAE, the amount of amorphous coating, and the amount of cell shrinkage. When the volume fraction of CAE is 40% or more, the amount of cell shrinkage (absolute value) increases rapidly. By providing an amorphous coating to the negative electrode active material, the amount of cell shrinkage (absolute value) tends to increase further. [Explanation of symbols]

[0052] 10 Positive electrode, 20 Negative electrode, 30 Separator, 50 Power generating element, 90 Exterior body, 100 Battery (lithium ion battery).

Claims

1. A positive electrode, a negative electrode, and an electrolyte solution, The positive electrode includes a positive electrode mixture, The positive electrode mixture includes a positive electrode active material and active lithium, a mass fraction of the active lithium relative to the total mass of the positive electrode active material and the active lithium is 0.25% or more; The electrolyte solution includes a solvent and a lithium salt, and The solvent contains a carboxylic acid ester in a volume fraction of 40% or more. Lithium-ion battery.

2. The negative electrode includes a negative electrode mixture, The negative electrode mixture includes a negative electrode active material and amorphous carbon, The amorphous carbon covers at least a part of the surface of the negative electrode active material, and The mass fraction of the amorphous carbon relative to the total mass of the negative electrode active material and the amorphous carbon is 0.5 to 1%.

2. The lithium ion battery of claim 1.

3. The mass fraction of the active lithium relative to the positive electrode active material is 0.25 to 0.5%, and The solvent contains 40 to 70% by volume of the carboxylic acid ester.

2. The lithium ion battery of claim 1.

4. The carboxylic acid ester is at least one selected from the group consisting of methyl propionate and methyl acetate. The lithium ion battery according to any one of claims 1 to 3.

5. A positive electrode, a negative electrode, and an electrolyte solution, The positive electrode includes a positive electrode mixture, The positive electrode mixture includes a positive electrode active material and active lithium, a mass fraction of the active lithium relative to the total of the positive electrode active material and the active lithium is 0.25 to 0.5%; The negative electrode includes a negative electrode mixture, The negative electrode mixture includes a negative electrode active material and amorphous carbon, the amorphous carbon covers at least a portion of a surface of the negative electrode active material, a mass fraction of the amorphous carbon relative to the total mass of the negative electrode active material and the amorphous carbon is 0.5 to 1%; The electrolyte solution includes a solvent and a lithium salt, The solvent comprises 40 to 70% by volume of a carboxylic acid ester; and The carboxylic acid ester is at least one selected from the group consisting of methyl propionate and methyl acetate. Lithium-ion battery.

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