JP7756695B2 - Lithium-ion battery and manufacturing method of lithium-ion battery - Google Patents

Description

The present invention relates to a lithium-ion battery and a method for manufacturing a lithium-ion battery.

Vehicles such as EVs (Electric Vehicles) and HEVs (Hybrid Electric Vehicles) are equipped with storage batteries that supply power to motors and other devices. Storage batteries typically contain multiple secondary batteries.

Lithium-ion batteries (LIBs) are widely used as secondary batteries in EVs and HEVs. Because they are lightweight and offer high energy density, lithium-ion batteries are preferred as high-output power sources for vehicles.

In order to improve the output characteristics of such vehicle-mounted lithium-ion batteries, it is desirable to reduce both the initial resistance value and the resistance value after deterioration due to charge/discharge cycles.

In addition, because lithium-ion batteries for vehicle use require a high level of safety, it is necessary to suppress heat generation from the positive electrode during discharge.

In order to improve safety in lithium-ion batteries due to heat generation, for example, lithium-ion battery electrolytes with enhanced flame retardancy have been disclosed, which are made by selecting materials used as solvents for the electrolyte and mixing them within specific ranges (see, for example, Patent Documents 1 to 4).

Japanese Patent Application Laid-Open No. 2005-340223 Japanese Patent Application Laid-Open No. 2001-93572 Japanese Patent Application Laid-Open No. 2021-82516 Japanese Patent Application Publication No. 10-12272

However, while the electrolyte solutions for lithium-ion batteries disclosed in Patent Documents 1 to 4 improve the flame retardancy of lithium-ion batteries, they do not sufficiently improve the charge-discharge cycle characteristics.

The present application aims to solve the above problems by improving the charge/discharge cycle characteristics of lithium-ion batteries, which will ultimately contribute to greater energy efficiency.

In light of the above-mentioned background, the present inventors have discovered a new finding that by using a mixture of specific materials in specific ratios as a solvent for a lithium ion battery and optimizing the conditions for initial charging, at least one of at least a portion of the particle surface of the positive electrode active material contained in the positive electrode active material layer and at least a portion of the particle surface of the negative electrode active material contained in the negative electrode active material layer is coated with a coating containing fluorine, and the coating contains S ₂ F ₂ NO ₄ , thereby improving the charge-discharge cycle characteristics of the lithium ion battery.

[1] A positive electrode having a positive electrode current collector and a positive electrode active material layer;
a negative electrode having a negative electrode current collector and a negative electrode active material layer, the negative electrode facing the positive electrode;
an electrolyte layer located between the positive electrode and the negative electrode and containing an electrolyte solution;
A lithium ion battery having:
the electrolytic solution contains at least a solvent containing 1,2-dimethoxyethane and a fluorinated ether, and a lithium imide compound dissolved in the solvent;
at least one of at least a portion of the particle surface of the positive electrode active material contained in the positive electrode active material layer and at least a portion of the particle surface of the negative electrode active material contained in the negative electrode active material layer is coated with a coating containing fluorine,
A _lithium - _ion battery, wherein the coating comprises _S2F2NO4 .

In the lithium ion battery of the present invention, at least one of at least a portion of the particle surface of the positive electrode active material contained in the positive electrode active material layer and at least a portion of the particle surface of the negative electrode active material contained in the negative _electrode active material layer is coated with a coating containing fluorine, and the coating contains _S2F2NO4 , so that the _{lithium ion} battery has excellent capacity retention rate during charge and discharge cycles.

[2] The lithium ion battery according to [1], wherein the count number of the S ₂ F ₂ NO ₄ in the coating that covers at least a part of the particle surface of the positive electrode active material by time-of-flight secondary ion mass spectrometry is 5000 or more.

The lithium ion battery of the present invention has a count number of S ₂ F ₂ NO ₄ of 5000 or more in the coating that covers at least a portion of the particle surface of the positive electrode active material, as determined by time-of-flight secondary ion mass spectrometry, and therefore has excellent capacity retention rate during charge-discharge cycles.

[3] The lithium ion battery according to claim 1, wherein the count number of the S ₂ F ₂ NO ₄ in the coating that covers at least a part of the particle surface of the negative electrode active material by time-of-flight secondary ion mass spectrometry is 3000 or more.

The lithium ion battery of the present invention has a count number of 3000 or more of the S ₂ F ₂ NO ₄ in the coating that covers at least a portion of the particle surface of the negative electrode active material, as determined by time-of-flight secondary ion mass spectrometry, and therefore has excellent capacity retention rate during charge-discharge cycles.

[4] An activation step of activating the lithium ion battery in an environment of 60°C or higher at a charge rate of 0.3C or less in the first cycle,
The lithium ion battery includes a positive electrode having a positive electrode current collector and a positive electrode active material layer, and an electrolyte layer having a negative electrode current collector and a negative electrode active material layer, the electrolyte layer being positioned between the positive electrode and the negative electrode and containing an electrolyte solution;
the electrolytic solution contains at least a solvent containing 1,2-dimethoxyethane and a fluorinated ether, and a lithium imide compound dissolved in the solvent;
a fluorine-containing coating on at least a portion of the particle surface of the positive electrode active material contained in the positive electrode active material layer, or a fluorine-containing coating on at least a portion of the particle surface of the negative electrode active material contained in the negative electrode active material layer.

The lithium-ion battery of the present invention includes an activation process in which the lithium-ion battery is placed in an environment of 60°C or higher and the charge rate for the first cycle is set to 0.3 C or less, making it possible to produce a lithium-ion battery with excellent capacity retention during charge-discharge cycles.

The present invention makes it possible to improve the charge/discharge cycle characteristics of lithium-ion batteries.

1 is a schematic cross-sectional view showing an example of a layer structure of a lithium ion battery according to one embodiment of the present invention. FIG. 10 is a diagram showing a dQ/dV curve during charging in Verification Example 1. FIG. 10 is a diagram showing a dQ/dV curve during charging in Verification Example 2. FIG. 10 is a diagram showing the relationship between the temperature and the reaction amount of a lithium ion battery during initial charging in Verification Example 3. FIG. 10 is a diagram showing the measurement results of TOF-SIMS for the positive electrode in Verification Example 4. FIG. 10 is a diagram showing the results of TOF-SIMS measurement of the negative electrode in Verification Example 4. FIG. 10 is a graph showing the relationship between the content of S ₂ F ₂ NO ₄ in the coatings that coat the particle surfaces of the positive electrode active material and the negative electrode active material and the temperature of the lithium ion battery during initial charging in Verification Example 4.

Hereinafter, a lithium ion battery according to one embodiment of the present invention will be described with reference to the drawings.
The embodiments shown below are specifically described to provide a better understanding of the gist of the invention, and unless otherwise specified, do not limit the present invention. Furthermore, the drawings used in the following description may show essential parts enlarged for the sake of convenience in order to make the features of the present invention easier to understand, and the dimensional proportions of each component may not necessarily be the same as those in reality.

[Lithium-ion battery]
An example of the configuration of a lithium ion battery according to one embodiment of the present invention will be described.
FIG. 1 is a schematic cross-sectional view showing an example of the layer structure of a lithium ion battery according to one embodiment of the present invention.

The lithium-ion battery (LIB) 10 is composed of a stack of a positive electrode 13 having a positive electrode current collector 11 and a positive electrode active material layer 12 located on one side of the positive electrode current collector 11, a negative electrode 16 having a negative electrode current collector 14 and a negative electrode active material layer 15 located on one side of the negative electrode current collector 14 and facing the positive electrode 13, and an electrolyte layer 17 located between the positive electrode 13 and the negative electrode 16.

The positive electrode active material layer 12 is a layer containing a positive electrode mixture. The positive electrode mixture contains a positive electrode active material, lithium carbonate, a conductive additive, and a binder.

The positive electrode active material can be an electrode active material that can reversibly absorb and release ions, desorb and insert ions (intercalation), or dope and dedope ions with their counter anions (e.g., PF ₆ ⁻ ).

Specific examples of the positive electrode active material include lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganese oxide (LiMnO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), composite metal oxides (ternary compounds) represented by the general formula: LiNi _x Co _y Mn _z M _a O ₂ (x + y + z + a = 1, 0≦x<1, 0≦y<1, 0≦z<1, 0≦a<1, M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr), lithium vanadium compounds (LiV ₂ O ₅ ), and olivine-type LiMPO ₄ (wherein M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, _{and Zr} , or VO), composite metal oxides such as lithium _titanate ( _Li4Ti5O12 ) and _{LiNixCoyAlzO2} ( _0.9 <x+y+z<1.1), polyacetylene, _polyaniline , polypyrrole, polythiophene, and polyacene.
In this embodiment, a ternary compound containing Ni, Co, and Mn is used as the positive electrode active material contained in the positive electrode mixture.

Known binders can be used as the binder contained in the positive electrode mixture of the positive electrode active material layer 12. Examples include fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), and polyvinyl fluoride (PVF).

Examples of the conductive additive contained in the positive electrode mixture of the positive electrode active material layer 12 include carbon powders such as carbon blacks, carbon nanotubes, carbon materials, metal fine powders such as copper, nickel, stainless steel, and iron, mixtures of carbon materials and metal fine powders, and conductive oxides such as ITO.
The positive electrode mixture constituting the positive electrode active material layer 12 uses Ketjen black, which is a type of carbon black that has particularly excellent conductivity.
In addition, when sufficient conductivity can be ensured by the positive electrode mixture alone, the positive electrode mixture does not need to contain a conductive additive.

The negative electrode active material layer 15 contains a negative electrode active material and a binder as a negative electrode mixture, and optionally contains a conductive additive. Known negative electrode active materials can be used as the negative electrode active material. Examples of the negative electrode active material include metallic lithium, graphite (natural graphite, artificial graphite) capable of absorbing and releasing lithium ions, carbon materials such as carbon nanotubes, non-graphitizable carbon, easily graphitizable carbon, and low-temperature fired carbon, metals capable of combining with lithium such as aluminum, silicon, and tin, amorphous compounds mainly composed of oxides such as SiO _x (0<x<2) and tin dioxide, and particles containing lithium titanate (Li ₄ Ti ₅ O ₁₂ ).

The conductive additive and binder contained in the negative electrode mixture can be the same as those used in the positive electrode active material layer 12. In addition to the binders listed for the positive electrode active material layer 12, other binders that can be used in the negative electrode mixture include, for example, carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyimide (PI), polyamideimide (PAI), polyacrylic acid (PAA), etc.

When the lithium-ion battery 10 is charged, the potential of the negative electrode 16, which includes the negative electrode active material layer 15, changes as lithium ions enter between the layers of the carbon material, which is an example of a negative electrode active material.

The electrolyte layer (separator) 17 is located between the positive electrode 13 and the negative electrode 16, allows lithium ions to pass through, and separates the positive electrode 13 and the negative electrode 16. The electrolyte layer (separator) 17 is made of, for example, a porous membrane made of a resin material or a nonwoven fabric. The electrolyte layer (separator) 17 contains an electrolytic solution.

The electrolyte used in the lithium-ion battery 10 of this embodiment is a solvent mixture of 1,2-dimethoxyethane and a fluorinated ether, with a lithium imide compound dissolved in this solvent.

Examples of fluorinated ethers (fluorine-containing chain ethers) used as solvents for electrolyte solutions include compounds having a structure in which some or all of the hydrogen atoms in 1,2-ethoxyethane (DEE) or ethoxymethoxyethane (EME) are replaced with fluorine atoms.

When the number of carbon atoms in a fluorinated ether is small, it tends to have a low boiling point, which can lead to evaporation during high-temperature operation of the battery. On the other hand, if the number of carbon atoms is too large, the viscosity of the chain ether increases, which can reduce the conductivity of the electrolyte. Therefore, it is preferable for the number of carbon atoms to be between 4 and 10.

Specific examples of fluorine-containing chain ethers include 2,2,3,3,3-pentafluoropropyl 1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, 1H,1H,2'H,3H-decafluorodipropyl ether, 1,1,2,3,3,3-hexafluoropropyl-2,2-difluoroethyl ether, isopropyl 1,1,2,2-tetrafluoroethyl ether, propyl 1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, and 1H,1H,5H-perfluoropentyl-1, 1,2,2-tetrafluoroethyl ether, 1H,1H,2'H-perfluorodipropyl ether, 1H-perfluorobutyl-1H-perfluoroethyl ether, methyl perfluoropentyl ether, methyl perfluorohexyl ether, methyl 1,1,3,3,3-pentafluoro-2-(trifluoromethyl)propyl ether, 1,1,2,3,3,3-hexafluoropropyl 2,2,2-trifluoroethyl ether, ethyl nonafluorobutyl ether, ethyl 1,1,2,3,3,3-hexafluoropropyl ether, 1,1,2,3,3,3-hexafluoropropyl 1H,1H-heptafluorobutyl ether, biphenyls, bis(2,2,3,3-tetrafluoropropyl) ether, bis(2,2,3,3,3-pentafluoropropyl) ether, 1H,1H,5H-octafluoropentyl 1,1,2,2-tetrafluoroethyl ether, 1H,1H,2'H-perfluorodipropyl ether, heptafluoropropyl 1,2,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 2,2,3,3,3-pentafluoropropyl-1,1,2,2-tetrafluoroethyl ether, ethyl nonafluorobutyl ether, methyl nonafluorobutyl ether, 2,2,3,4,4, Examples include 4-hexafluorobutyl-difluoromethyl ether, 1,1-difluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1,1-difluoroethyl-1H,1H-heptafluorobutyl ether, bis(1H,1H-heptafluorobutyl)ether, nonafluorobutyl methyl ether, 2,2-difluoroethyl-1,1,2,2-tetrafluoroethyl ether, bis(2,2-difluoroethyl)ether, bis(1,1,2-trifluoroethyl)ether, 1,1,2-trifluoroethyl-2,2,2-trifluoroethyl ether, and bis(1,1,2,2-tetrafluoroethyl)ether.

Of the above-mentioned fluorinated ethers (fluorine-containing chain ethers), in this embodiment, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) is used from the standpoints of voltage resistance, boiling point, etc.

The fluorinated ether may be contained in a proportion of 50% by mass or more and 90% by mass or less, more preferably 75% by mass or more and 90% by mass or less, based on the mass of the entire solvent. By containing the fluorinated ether in a proportion of 75% by mass or more and 90% by mass or less, based on the mass of the entire solvent, corrosion of aluminum by the electrolyte can be prevented when aluminum is used as the positive electrode current collector 11. This makes it possible to further improve the charge/discharge cycle of the lithium-ion battery 10.
The concentration of the fluorinated ether in the electrolyte may be in the range of 1 mol/L or more and 5 mol/L or less.

1,2-dimethoxyethane (DME) used as a solvent for the electrolyte is a water-soluble liquid with a relatively high boiling point (85° C.) as an organic solvent.
The 1,2-dimethoxyethane may be contained in an amount of 10% by mass or more and 30% by mass or less, more preferably 15% by mass or more and 25% by mass or less, based on the total mass of the electrolyte solution. The concentration of 1,2-dimethoxyethane in the electrolyte solution may be in the range of 3 mol/L or more and 7 mol/L or less.

The 1,2-dimethoxyethane contained in the electrolyte can also be replaced with ethylene carbonate.
Ethylene carbonate is a glassy solid (room temperature) with a melting point of 34°C to 37°C, and is an ester of ethylene glycol with carbonic acid. Ethylene carbonate is a polar solvent, and adding it to the solvent of an electrolyte can increase the dielectric constant.

Ethylene carbonate may be contained in a concentration range of 2 mol/L or less relative to 1,2-dimethoxyethane.
Alternatively, ethylene carbonate may be contained in a concentration range of less than 8 mass % relative to the total mass of the solvent.
By setting the concentration of ethylene carbonate to a range of less than 8 mass % with respect to the entire solvent, it is possible to prevent corrosion of aluminum by the electrolyte when, for example, aluminum is used as the positive electrode current collector 11. This makes it possible to further improve the charge/discharge cycle of the lithium ion battery 10.

Lithium imide compounds are electrolytes dissolved in the solvent of the electrolyte solution and are generally produced by the reaction of lithium amide with lithium hydride. Examples of lithium imide compounds used in this embodiment include lithium bis-fluorosulfonylimide (LiFSI) and lithium bis-trifluoromethanesulfonylimide (LiTFSI). Adding these lithium bisimides to the non-aqueous electrolyte solution as lithium salts improves low-temperature output characteristics and suppresses decomposition of the positive electrode surface, which can occur during high-temperature cycle operation, thereby preventing oxidation of the electrolyte solution.

Such lithium imide compounds may be contained in the electrolyte at a concentration ranging from 1 mol/L to 3 mol/L inclusive.

When the electrolyte solution of the present embodiment having the above-described configuration is used in the lithium-ion battery 10 of the present embodiment, at least one of at least a portion of the particle surface of the positive electrode active material contained in the positive electrode active material layer 12 and at least a portion of the particle surface of the negative electrode active material contained in the negative electrode active material layer 15 is coated with a coating containing fluorine.
For example, a fluorine-containing organic coating is formed on the particle surface of the positive electrode active material by the fluorinated ether in the electrolyte. Thus, when at least a portion of the particle surface of the positive electrode active material is coated with a fluorine-containing coating, reversible capacity loss can be reduced. This leads to good discharge characteristics during charge-discharge cycles. Furthermore, by improving the surface stability of the positive electrode active material, oxygen release at high temperatures can be delayed, and the heat generation onset temperature can be raised, improving high-temperature resistance and safety.
Furthermore, a coating of a fluorine-containing organic material is formed on the particle surface of the negative electrode active material by the fluorinated ether in the electrolyte. When at least a portion of the particle surface of the negative electrode active material is coated with a coating containing fluorine, the reversible capacity loss can be reduced.

In the lithium-ion battery 10 of this embodiment, the coating that covers at least a portion of the particle surface of the positive electrode active material and the coating that covers at least a portion of the particle surface _of the negative electrode active material contain _{S2F2NO4 .} Because the coating contains _S2F2NO4 , the lithium _- ion battery 10 has _an excellent capacity retention rate during charge-discharge cycles.

The S ₂ F ₂ NO ₄ contained in the coating can be analyzed by time-of-flight secondary ion mass spectrometry (TOF-SIMS).

It is preferable that the count number of S ₂ F ₂ NO ₄ in the coating that covers at least a portion of the particle surface of the positive electrode active material by TOF-SIMS is not less than 5000. When the count number of S ₂ F ₂ NO ₄ contained in the coating is not less than the lower limit, the lithium ion battery 10 has a better capacity retention rate during charge and discharge cycles.

It is preferable that the count number of S ₂ F ₂ NO ₄ in the coating that covers at least a portion of the particle surface of the negative electrode active material by TOF-SIMS is not less than 3000. When the count number of S ₂ F ₂ NO ₄ contained in the coating is not less than the lower limit, the lithium ion battery 10 has a better capacity retention rate during charge and discharge cycles.

The number of S ₂ F ₂ NO ₄ counted in the coating by TOF-SIMS can be converted into the mass % of S ₂ F ₂ NO ₄ in the total amount (total mass) of any substance contained in the coating. For example, in the lithium-ion battery 10 of this embodiment, the main anion molecules detected by TOF-SIMS from the coating covering at least a portion of the particle surface of the positive electrode active material or the coating covering at least a portion of the particle surface of the negative electrode active material are six: F, PO ₂ , SO ₃ , CN, Li ₂ F ₃ , and S ₂ F ₂ NO ₄ . Since the molecular weights of F, _PO2 , _SO3 , CN, _Li2F3 , and _S2F2NO4 are 18.99, 62.97, _{80.06 ,} 26.02, _70.88 , _and 180.13, respectively, the mass % _{of S2F2NO4} contained in the six anion molecules can be obtained by calculating the ratio of the count number of _S2F2NO4 multiplied by the molecular weight _{of S2F2NO4} to the sum of the count numbers _multiplied by the molecular weight of _{S2F2NO4 .}

According to the above conversion method, the content of _S2F2NO4 relative to the total mass (100 mass%) of the six anion molecules in the _coating that covers at least a portion of the particle surface of the positive _{electrode active material is preferably 65 mass%, more preferably 68 mass% or more. When the content of S2F2NO4 relative to} the total mass _of the coating is equal to or greater than the lower limit, the cycle characteristics are improved.

Furthermore, by the above conversion method, the content of _S2F2NO4 relative to the total mass (100 mass%) of the coating that covers at least a portion of the particle surface of _the negative electrode active material is preferably 73 mass% and more preferably 74 _mass % or more. When the content of _S2F2NO4 relative to the total mass _{of the} coating is equal to or greater than the lower limit, the cycle characteristics are improved.

According to the lithium ion battery 10 of this embodiment, at least one of at least a portion of the particle surface of the positive electrode active material contained in the positive electrode active material layer 12 and at least a portion of the particle surface of the negative electrode active material contained in the negative _electrode active material layer ₁₅ is coated with a coating containing fluorine, and the coating contains _S2F2NO4 , so that a lithium ion battery 10 with excellent capacity retention rate during charge and discharge cycles can be realized.
_S2F2NO4 is _a specific product produced by decomposition _of a _{lithium imide} compound, which is an electrolyte dissolved in a solvent of an electrolytic solution, and among the anion molecules detected from the coating covering at least a portion of the particle surface of the positive electrode active material of the _lithium ion battery 10 of this embodiment or the coating covering at least a portion of the particle surface of the negative electrode active material, S2F2NO4 has a molecular formula close to that of a lithium imide compound.
From this, it is presumed that the fluorine or nitrogen contained in S ₂ F ₂ NO ₄ acts effectively to improve the capacity retention rate during charge-discharge cycles.

[Method of manufacturing lithium-ion batteries]
A method for manufacturing a lithium ion battery according to one embodiment of the present invention includes an activation step of activating a lithium ion battery in an environment of 60° C. or higher at a charge rate of 0.3 C or less in the first cycle.
The charge rate in the first cycle is preferably equal to or less than 0.05 C. The charge voltage in the first cycle is not particularly limited and may be any value.
The discharge rate in the first cycle is not particularly limited and may be any value. The discharge voltage in the first cycle is also not particularly limited and may be any value.

The lithium-ion battery in the manufacturing method for a lithium-ion battery of this embodiment comprises a positive electrode having a positive electrode current collector and a positive electrode active material layer, a negative electrode having a negative electrode current collector and a negative electrode active material layer and facing the positive electrode, and an electrolyte layer located between the positive electrode and the negative electrode and containing an electrolyte solution, the electrolyte solution containing at least a solvent containing 1,2-dimethoxyethane and a fluorinated ether, and a lithium imide compound dissolved in the solvent, and at least a portion of the particle surface of the positive electrode active material contained in the positive electrode active material layer or at least a portion of the particle surface of the negative electrode active material contained in the negative electrode active material layer is coated with a coating containing fluorine.

The method for manufacturing a lithium ion battery according to this embodiment may include a manufacturing step of a lithium ion battery before the activation step.
The lithium ion battery before the activation step is obtained by stacking the above-mentioned positive electrode and the above-mentioned negative electrode with an electrolyte layer interposed therebetween.

According to the method for producing a lithium ion battery of this embodiment, S 2 F 2 NO 4 is produced in at least one of the coating covering at least a portion of the particle surface of the positive electrode active material and the coating covering at least a portion of the _particle surface of the negative electrode active material by _{the activation step ,} thereby obtaining the lithium ion battery of the above embodiment.

The above describes embodiments of the present invention, but these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be embodied in a variety of other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as set forth in the claims.

The present invention will be explained in more detail below using examples, but the present invention is not limited to the following examples.

The effects of the present invention were verified.
[Electrolyte layer]
Electrolyte solutions (samples) having the following compositions were prepared. The concentration of each sample is shown relative to the total electrolyte.
Electrolyte: LiFSI (1.7 mol/L), solvent: EC/DME/TTE (0.9 mol/L/2.2 mol/L/3.8 mol/L)
LiFSI: lithium bis-fluorosulfonylimide EC: ethylene carbonate DME: 1,2-dimethoxyethane TTE: 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether The electrolyte solution of each of these samples was impregnated into a sheet-shaped porous substrate to obtain an electrolyte layer.

[Positive electrode]
A kneaded mixture of LiCoNiMnO ₆ (NMC), lithium carbonate, Ketjen black (KB), and polyvinylidene fluoride (PVDF) was used as the positive electrode mixture. This kneaded mixture was applied to an aluminum thin film using a blade coater and dried to obtain a positive electrode.

[Negative electrode]
The negative electrode was made of graphite and silicon oxide.

The positive electrode, negative electrode, and electrolyte layer described above were stacked to create the lithium-ion battery used for verification.

<Verification Example 1>
Using the above-described lithium-ion battery, the capacity of the lithium-ion battery after initial charging was measured. The temperatures of the lithium-ion battery during initial charging were 25°C, 45°C, 60°C, and 65°C. The temperature of the lithium-ion battery during initial charging was the temperature measured by a thermocouple from the temperature of the lithium-ion battery cell. The charge rate during initial charging was 0.05C and 4.15V. The dQ/dV curve during charging is shown in Figure 2. It was found that as the temperature of the lithium-ion battery during charging increased, the area of the peak observed at 2.5V-2.7V, which indicates the amount of reaction resulting from the formation of a coating covering at least a portion of the particle surface of the positive electrode active material and a coating covering at least a portion of the particle surface of the negative electrode active material, increased.

<Verification Example 2>
The lithium-ion battery described above was repeatedly charged and discharged 50 times, and the capacity retention rate (%) at the 50th charge cycle relative to the initial charge was measured. Table 1 and Figure 3 show the area of the peak observed at 2.5 V-2.7 V on the dQ/dV curve, which indicates the reaction amount resulting from the formation of a coating covering at least a portion of the particle surface of the positive electrode active material and a coating covering at least a portion of the particle surface of the negative electrode active material during the initial charge.

The results shown in Table 1 and Figure 3 indicate that if the cell temperature is 60°C or higher, a sufficient amount of film is formed, resulting in a high capacity retention rate.

FIG. 4 shows the relationship between the temperature and the reaction amount of the lithium ion battery during the initial charge.
From the results shown in FIG. 4, it was found that as the temperature of the lithium-ion battery during charging increases, the amount of reaction resulting from the formation of a coating that covers at least a portion of the particle surface of the positive electrode active material and a coating that covers at least a portion of the particle surface of the negative electrode active material increases.

<Verification Example 3>
An initial charge was performed using the above-described lithium ion battery. The temperatures of the lithium ion battery during the initial charge were 25°C, 45°C, and 65°C. The temperature of the lithium ion battery during the initial charge was the temperature measured by a thermocouple from the cell temperature of the lithium ion battery. The charge rate during the initial charge was 0.05 C and 4.15 V. TOF-SIMS was used to measure the components formed in the coating covering at least a portion of the particle surface of the positive electrode active material and the coating covering at least a portion of the particle surface of the negative electrode active material by charging. The conditions for measuring the anion components formed in the coating by TOF-SIMS were as follows:
(Primary ion source)
Ion species: Bi ³⁺⁺ (bismuth ions with three clusters and two charges)
Acceleration voltage: 30 kV
Current value: approximately 0.2 pA
Measurement area: Approximately 100 μm square (sputter ion source)
Ion species: Ar cluster ions Acceleration voltage: 2.5 kV
Measurement area: Approximately 400 μm square Cycles (number of sputtering): 100

The results of TOF-SIMS anion component measurements are shown in Tables 2, 3, 5, and 6. Tables 2 and 5 show the TOF-SIMS measurement results for the positive electrode. Table 3 and 6 show the TOF-SIMS measurement results for the negative electrode.

From the results shown in Tables 2 and 3, it was confirmed that F, PO ₂ , SO ₃ , CN, Li ₂ F ₃ , and S ₂ F ₂ NO ₄ were produced in the coatings covering the particle surfaces of the positive electrode active material and negative electrode active material during the initial charge.
5 is a diagram showing the relationship between the reaction amount and the count number of S ₂ F ₂ NO ₄ in the coating covering the particle surface of the positive electrode active material. In FIG. 5, a linear equation is shown that approximately represents the distribution of the three measurement results. From this equation, the count number of S ₂ F ₂ NO ₄ when the temperature of the lithium ion battery during the initial charge was set to 60°C was predicted. As a result, as shown in Table 2, when the temperature of the lithium ion battery during the initial charge was set to 60°C, the count number of S ₂ F ₂ NO ₄ in the coating covering the particle surface of the positive electrode active material was 5473.2.
6 is a diagram showing the relationship between the reaction amount and the count number of S ₂ F ₂ NO ₄ in the coating coating the particle surface of the negative electrode active material. In FIG. 6, a linear equation is shown that approximately represents the distribution of the three measurement results. From this equation, the count number of S ₂ F ₂ NO ₄ was predicted when the temperature of the lithium ion battery during the initial charge was set to 60°C. As a result, as shown in Table 3, when the temperature of the lithium ion battery during the initial charge was set to 60°C, the count number of S ₂ F ₂ NO ₄ in the coating coating the particle surface of the negative electrode active material was 3125.6.

The number of S ₂ F ₂ NO ₄ counted in the coating by TOF-SIMS can be converted into the mass % of S ₂ F ₂ NO ₄ in the total amount (total mass) of any substance contained in the coating. For example, in the lithium-ion battery 10 of this embodiment, the main anion molecules detected by TOF-SIMS from the coating covering at least a portion of the particle surface of the positive electrode active material or the coating covering at least a portion of the particle surface of the negative electrode active material are six: F, PO ₂ , SO ₃ , CN, Li ₂ F ₃ , and S ₂ F ₂ NO ₄ . Since the molecular weights of F, _PO2 , _SO3 , CN, _Li2F3 , and _S2F2NO4 are 18.99, 62.97, _{80.06 ,} 26.02, _70.88 , _and 180.13, respectively, the mass % _{of S2F2NO4} contained in the six anion molecules can be obtained by calculating the ratio of the count number of _S2F2NO4 multiplied by the molecular weight _{of S2F2NO4} to the sum of the count numbers _multiplied by the molecular weight of _{S2F2NO4 .}

According to the above conversion method, the content of _S2F2NO4 relative to the total mass (100 mass%) of the six anion molecules in the _coating that covers at least a portion of the particle surface of the positive _{electrode active material is preferably 65 mass%, more preferably 68 mass% or more. When the content of S2F2NO4 relative to} the total mass _of the coating is equal to or greater than the lower limit, the cycle characteristics are improved.

Furthermore, by the above conversion method, the content of _S2F2NO4 relative to the total mass (100 mass%) of the coating that covers at least a portion of the particle surface of _the negative electrode active material is preferably 73 mass% and more preferably 74 _mass % or more. When the content of _S2F2NO4 relative to the total mass _{of the} coating is equal to or greater than the lower limit, the cycle characteristics are improved.

The results shown in Tables 2 and 3 indicate that when the temperature of the lithium-ion battery during initial charging was 25° C., the S ₂ F ₂ NO ₄ content in the coating film coating the particle surfaces of the positive electrode active material was 57.1% by mass, and the S ₂ F ₂ NO ₄ content in the coating film coating the particle surfaces of the negative electrode active material was 69.6% by mass. Furthermore, the results shown in Tables 2 and 3 indicate that when the temperature of the lithium-ion battery during initial charging was 45° C., the S ₂ F ₂ NO ₄ content in the coating film coating the particle surfaces of the positive electrode active material was 59.1% by mass, and the S ₂ F 2 NO ₄ content in the coating film coating the particle surfaces of the negative _electrode active material was 71.1% by mass. Furthermore, from the results shown in Tables 2 and 3, when the temperature of the lithium ion battery during initial charge was 60° C., the content of S ₂ F ₂ NO ₄ in the coating coating the particle surfaces of the positive electrode active material was 66.0 mass %, and the content of S ₂ F ₂ NO ₄ in the coating coating the particle surfaces of the negative electrode active material was 73.3 mass %. Furthermore, from the results shown in Tables 2 and 3, when the temperature of the lithium ion battery during initial charge was 65° C., the content of S ₂ F ₂ NO ₄ in the coating coating the particle surfaces of the positive electrode active material was 68.2 mass %, and the content of S 2 F 2 NO ₄ in the coating coating the particle surfaces of the _{negative electrode} active material was 74.0 mass %.

7 is a graph showing the relationship between the temperature of the lithium-ion battery during the initial charge and the change in capacity retention rate with respect to the number of cycles. It was found that the capacity retention rate curve shown in FIG _. 7, in which the temperature of the lithium-ion battery during the initial charge was 60°C or higher, corresponds to the curve in which the _S2F2NO4 content in the coating is the highest, as shown in Tables 2 and ₃ .

The lithium-ion battery of the present invention can improve the capacity retention rate during charge/discharge cycles. When used as a secondary battery in vehicles such as EVs and HEVs, such a lithium-ion battery can achieve long-distance driving on a single charge and improve energy efficiency. Therefore, it has industrial applicability.

10 Lithium ion battery 11 Positive electrode current collector 12 Positive electrode active material layer 13 Positive electrode 14 Negative electrode current collector 15 Negative electrode active material layer 16 Negative electrode 17 Electrolyte layer

Claims (2)

a positive electrode having a positive electrode current collector and a positive electrode active material layer;
a negative electrode having a negative electrode current collector and a negative electrode active material layer, the negative electrode facing the positive electrode;
an electrolyte layer located between the positive electrode and the negative electrode and containing an electrolyte solution;
A lithium ion battery having:
the electrolytic solution contains at least a solvent containing 1,2-dimethoxyethane and a fluorinated ether, and a lithium imide compound dissolved in the solvent;
at least one of at least a portion of the particle surface of the positive electrode active material contained in the positive electrode active material layer and at least a portion of the particle surface of the negative electrode active material contained in the negative electrode active material layer is coated with a coating containing fluorine,
A lithium ion battery in which the content of S2F2NO4 in the coating covering at least a portion of the particle surface of the positive electrode active material is 68 mass% or more relative to the total mass (100 mass%) of the anions of F , _PO2 , _SO3 , _{CN ,} Li2F3 _, and _S2F2NO4 , as _detected by measurement using time-of-flight secondary _{ion mass spectrometry} . an activation step of activating the lithium ion battery in an environment of 60°C or higher at a charge rate of 0.3C or less in the first cycle;
The lithium ion battery includes a positive electrode having a positive electrode current collector and a positive electrode active material layer, a negative electrode having a negative electrode current collector and a negative electrode active material layer and facing the positive electrode, and an electrolyte layer located between the positive electrode and the negative electrode and containing an electrolyte solution,
the electrolytic solution contains at least a solvent containing 1,2-dimethoxyethane and a fluorinated ether, and a lithium imide compound dissolved in the solvent;
a fluorine-containing coating on at least a portion of the particle surface of the positive electrode active material contained in the positive electrode active material layer, or a fluorine-containing coating on at least a portion of the particle surface of the negative electrode active material contained in the negative electrode active material layer.