阅读说明：本技术 锂二次电池 (Lithium secondary battery ) 是由 坂本纯一 蚊野聪 于 2019-05-27 设计创作，主要内容包括：本公开提供一种循环特性优异的锂二次电池。锂二次电池(10)具备正极(11)、负极(12)以及具有锂离子传导性的非水电解质。负极(12)中,通过充电而形成锂金属,并且锂金属通过放电而向非水电解质中溶出。非水电解质含有溶剂和锂盐。锂盐包含第1锂盐和第2锂盐。第1锂盐是由锂离子和酸根型络合物阴离子构成的盐。非水电解质中含有的第1锂盐和第2锂盐的合计浓度为3.0mol/L以上。(Disclosed is a lithium secondary battery having excellent cycle characteristics. A lithium secondary battery (10) is provided with a positive electrode (11), a negative electrode (12), and a nonaqueous electrolyte having lithium ion conductivity. In the negative electrode (12), lithium metal is formed by charging, and the lithium metal is eluted into the nonaqueous electrolyte by discharging. The nonaqueous electrolyte contains a solvent and a lithium salt. The lithium salt includes a 1 st lithium salt and a 2 nd lithium salt. The 1 st lithium salt is a salt composed of a lithium ion and an acid group type complex anion. The total concentration of the 1 st lithium salt and the 2 nd lithium salt contained in the nonaqueous electrolyte is 3.0mol/L or more.)

1. A lithium secondary battery comprising a positive electrode, a negative electrode and a non-aqueous electrolyte having lithium ion conductivity,

Lithium metal is deposited on the surface of the negative electrode during charging of the lithium secondary battery,

Wherein the lithium metal is eluted from the surface of the negative electrode into the nonaqueous electrolyte during discharge of the lithium secondary battery,

The non-aqueous electrolyte contains a solvent and a lithium salt,

The lithium salt includes a 1 st lithium salt and a 2 nd lithium salt,

The 2 nd lithium salt is different from the 1 st lithium salt,

The 1 st lithium salt is composed of a lithium ion and an acid group type complex anion, and

the total concentration of the 1 st lithium salt and the 2 nd lithium salt contained in the nonaqueous electrolyte is 3.0mol/L or more.

2. The lithium secondary battery according to claim 1,

The solvent contains ether as a main solvent.

3. The lithium secondary battery according to claim 2,

The volume ratio of the ether to the solvent is 60% or more.

4. The lithium secondary battery according to claim 2 or 3,

The ether is represented by the formula R¹-O-R³-O-R²It is shown that,

wherein the content of the first and second substances,

R¹An alkyl group having 1 to 4 carbon atoms,

R²An alkyl group having 1 to 4 carbon atoms,

R³Is an alkylene group having 2 to 4 carbon atoms.

5. the lithium secondary battery according to any one of claims 1 to 4,

The concentration of the 1 st lithium salt contained in the nonaqueous electrolyte is 0.01mol/L to 2 mol/L.

6. the lithium secondary battery according to any one of claims 1 to 5,

The acid radical type complex anion contains a boron atom.

7. The lithium secondary battery according to claim 6,

The acid-group-type complex anion is at least one selected from the group consisting of a bis (oxalato) borate anion and a difluoro (oxalato) borate anion.

8. The lithium secondary battery according to any one of claims 1 to 7,

The 2 nd lithium salt is composed of lithium ions and anions of imide.

9. The lithium secondary battery according to claim 8,

The concentration of the 2 nd lithium salt contained in the nonaqueous electrolyte is 1.5mol/L or more.

10. The lithium secondary battery according to claim 2,

the concentration of the 2 nd lithium salt contained in the nonaqueous electrolyte is 1.5mol/L or more.

11. The lithium secondary battery according to claim 8 or 9,

The anion of the imide is represented by the formula N (SO)₂C_mF_2m+1)(SO₂C_nF_2n+1)^-Wherein m and n are each independently an integer of 0 or more.

12. The lithium secondary battery according to any one of claims 1 to 11,

The non-aqueous electrolyte further includes at least one selected from among ethylene carbonate, vinylene carbonate, fluoroethylene carbonate and vinyl ethylene carbonate.

13. The lithium secondary battery according to any one of claims 1 to 12,

The positive electrode has a positive electrode active material having a crystal structure belonging to the space group R-3 m.

Technical Field

The present disclosure relates to a lithium secondary battery using lithium metal as a negative electrode active material. More particularly, the present disclosure relates to an improvement of a nonaqueous electrolyte in a lithium secondary battery.

background

Lithium metal is deposited on the negative electrode during charging of the lithium secondary battery. Lithium metal is dissolved in the non-aqueous electrolyte upon discharge.

Lithium metal has a high reducing power, and therefore side reactions easily occur between lithium metal deposited on the negative electrode and the nonaqueous electrolyte during charging. This side reaction may degrade the cycle characteristics of the lithium secondary battery. The dendrite of lithium metal precipitated on the negative electrode further deteriorates the cycle characteristics.

Patent document 1 discloses that the ten-point average roughness Rz of the lithium metal deposition surface of the negative electrode current collector is 10 μm or less in order to suppress dendrite deposition of lithium metal.

on the other hand, a nonaqueous electrolyte for a lithium secondary battery generally includes a solvent and a lithium salt dissolved in the solvent. Patent document 1 discloses that LiPF is dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate (volume ratio 1:1) at a concentration of 1mol/L₆The non-aqueous electrolyte of (1). Patent document 2 discloses an electrolytic solution containing lithium bis (fluorosulfonyl) imide at a concentration of 0.7 to 4mol/L, a cyclic carbonate at a volume ratio of more than 0% and 15% or less, and a chain carbonate at a volume ratio of 85% or more and 99% or less.

Patent document 3 discloses a composition containing LiPF₆And the 1 st lithium salt, the 2 nd lithium salt, and a tertiary carboxylic acid ester. Examples of the 2 nd lithium salt include at least two selected from a lithium salt having an oxalic acid skeleton, a lithium salt having a phosphoric acid skeleton, and a lithium salt having an S ═ O group.

Prior art documents

Patent document 1: japanese patent laid-open No. 2001 and 243957

Patent document 2: japanese laid-open patent publication No. 2015-79636

patent document 3: international publication No. 2016/017809 handbook

Disclosure of Invention

Problems to be solved by the invention

Disclosed is a lithium secondary battery having excellent cycle characteristics.

Means for solving the problems

The disclosed lithium secondary battery comprises a positive electrode, a negative electrode, and a nonaqueous electrolyte having lithium ion conductivity,

Lithium metal is deposited on the surface of the negative electrode during charging of the lithium secondary battery,

Wherein the lithium metal is eluted from the surface of the negative electrode into the nonaqueous electrolyte during discharge of the lithium secondary battery,

The non-aqueous electrolyte contains a solvent and a lithium salt,

The lithium salt includes a 1 st lithium salt and a 2 nd lithium salt,

The 2 nd lithium salt is different from the 1 st lithium salt,

the 1 st lithium salt is composed of lithium ions and acid group type complex anions,

The total concentration of the 1 st lithium salt and the 2 nd lithium salt contained in the nonaqueous electrolyte is 3.0mol/L or more.

ADVANTAGEOUS EFFECTS OF INVENTION

excellent cycle characteristics can be obtained in a lithium secondary battery.

Drawings

Fig. 1A is a longitudinal sectional view of a lithium secondary battery according to an embodiment.

Fig. 1B is an enlarged cross-sectional view taken along line 1B in fig. 1A in a fully discharged state of the lithium secondary battery.

Detailed Description

Hereinafter, a lithium secondary battery according to an embodiment will be described with reference to the drawings. The lithium secondary battery of the present embodiment includes a positive electrode, a negative electrode, and a nonaqueous electrolyte having lithium ion conductivity.

(non-aqueous electrolyte)

The nonaqueous electrolyte has lithium ion conductivity. The nonaqueous electrolyte contains a lithium salt and a solvent. The lithium salt is composed of lithium ions and anions. The lithium salt is dissolved in a solvent. Generally, the lithium salt is dissociated in the non-aqueous electrolyte, and exists in the form of lithium ions and anions. The nonaqueous electrolyte is usually in a liquid state.

The liquid nonaqueous electrolyte is prepared by dissolving a lithium salt in a solvent. The lithium salt is dissolved in the solvent to generate lithium ions and anions, but the non-aqueous electrolyte may contain a lithium salt that is not dissociated.

The nonaqueous electrolyte may contain not only a liquid nonaqueous electrolyte but also a matrix polymer as long as the diffusion of lithium ions on the surface of the negative electrode is not hindered. Examples of matrix polymers are polymeric materials that absorb solvents to increase viscosity. Examples of the polymer material are a fluororesin, an acrylic resin, or a polyether resin.

in the lithium secondary battery of the present embodiment, the lithium salt contained in the nonaqueous electrolyte includes a 1 st lithium salt and a 2 nd lithium salt. The 1 st lithium salt is composed of a lithium ion and an acid group type complex anion. The 2 nd lithium salt is a lithium salt other than the 1 st lithium salt. That is, the 2 nd lithium salt has a different composition from the 1 st lithium salt.

The total concentration of the 1 st lithium salt and the 2 nd lithium salt is 3.0mol/L or more. Hereinafter, the total of the concentrations of the 1 st lithium salt and the 2 nd lithium salt is referred to as "total concentration". Since the total concentration is 3.0mol/L or more, a large amount of lithium ions can be supplied to the surface of the negative electrode. As a result, high diffusibility of lithium ions can be ensured. By supplying a large amount of lithium ions to the surface of the negative electrode, the 1 st lithium salt can effectively function, and a solid electrolyte interphase coating (hereinafter referred to as "SEI coating") having a good film quality can be formed more uniformly. The SEI film allows charge and discharge reactions to proceed more uniformly. As a result, a lithium secondary battery having high cycle characteristics is provided.

(solvent)

As the solvent, a nonaqueous solvent is generally used. Examples of the nonaqueous solvent are ethers, esters, nitriles, amides, or halogen substitutes thereof. Two or more nonaqueous solvents may be used for the nonaqueous electrolyte. The halogen substituent has a chemical structure in which at least one hydrogen atom contained in an ether, an ester, a nitrile, or an amide is substituted with a halogen atom. The halogen atom means a fluorine atom, a chlorine atom, a bromine atom and an iodine atom.

The solvent is preferably an ether. The oxygen atoms contained in the ether skeleton strongly interact with lithium ions. Thus, the desolvation energy of the ether with respect to lithium ions increases. In the case where desolvation energy of ether is large, lithium ions are captured by ether molecules. As a result, lithium ions are difficult to be reduced to lithium metal on the surface of the negative electrode as a whole. However, in the present embodiment, since the total concentration of the 1 st lithium salt and the 2 nd lithium salt is a high value of 3.0mol/L or more, a large amount of lithium ions can be supplied to the surface of the negative electrode. Therefore, the diffusion limitation of lithium ions on the surface of the negative electrode is alleviated, and the transport characteristics of lithium ions are improved. Thus, even in the case of using a solvent containing an ether, lithium ions are hardly solvated by the ether molecules. As a result, charge and discharge are more uniformly performed.

The lowest unoccupied orbital (hereinafter referred to as "LUMO") of the ether exists at a high energy level. Therefore, the ether is not easily reductively decomposed even when it is brought into contact with lithium metal having a strong reducing power. Thus, even when an ether-containing solvent is used, the effect of the SEI film formation using the 1 st lithium salt can be sufficiently exhibited. From such a viewpoint, when the solvent containing ether is used, the effect of improving the cycle characteristics is further improved. This effect is remarkably exhibited when a solvent containing ether as a main solvent is used. The solubility of the 1 st lithium salt and the 2 nd lithium salt is improved if a solvent containing ether as a main solvent is used.

The ether is represented by the following chemical formula (1).

(in the formula, R³Is an alkylene group, m is an integer of 0 or more)

R¹May be a hydrocarbon group or an organic group containing a hetero atom. R²May be a hydrocarbon group or an organic group containing a hetero atom. Examples of the hydrocarbon group include an aliphatic hydrocarbon group, an alicyclic hydrocarbon group, and an aromatic hydrocarbon group such as an alkyl group. Examples of the hetero atom contained in the organic group include an oxygen atom, a sulfur atom, a nitrogen atom, or a silicon atom. The organic group may be an aliphatic organic group containing a heteroatom, or may be a heterocyclic group containing a heteroatom as a ring-constituting atom.

R³It may be a hydrocarbon group or an alkylene group. The number of carbon atoms of the alkylene group is, for example, 1 or more and 4 or less, or 2 or more and 3 or less.

From the viewpoint of high ion conductivity, the value of m may be an integer of 0 or more and 6 or less, or an integer of 0 or more and 3 or less.

From the viewpoint of high fluidity of the nonaqueous electrolyte, for example, dialkoxyalkanes may be used. The dialkoxyalkane has R in the formula (1)¹And R²Is a hydrocarbyl group, and m is equal to 1. In dialkoxyalkanes, R¹～R³At least one of (a) and (b) may have the above-mentioned organic group. From the viewpoint of high fluidity, R¹～R³any of which may not have an organic group. R¹And R²The number of carbon atoms of the alkyl group of (a) is, for example, 1 or more and 6 or less, or 1 or more and 4 or less, or 1 or more and 2 or less, respectively. Di C_1-4Alkoxy radical C_2-4Alkane (i.e., R)¹Is an alkyl group having 1 to 4 carbon atoms, R²Is an alkyl group having 1 to 4 carbon atoms, R³An alkylene group having 2 or more and 4 or less carbon atoms, and m is 1) has high fluidity. Thus, even if two C_1-4Alkoxy radical C_2-4the concentration of the lithium salt contained in the alkane solvent is increased, and the nonaqueous electrolyte also has high ion conductivity.

Examples of the ether include cyclic ethers and linear ethers. Two or more ethers may be used in combination. The ether has high resistance to reduction, and therefore the ether is difficult to decompose even in the case of being exposed to a low potential environment on the surface of the negative electrode. Therefore, if ether is used as the nonaqueous electrolyte, the effect of suppressing the side reaction generated between the nonaqueous electrolyte and the lithium metal can be enhanced.

Examples of cyclic ethers are 1, 3-dioxolane, 4-methyl-1, 3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1, 2-butylene oxide, 1, 3-bisAlkane, 1, 4-diAlkane, 1,3, 5-trisAlkane, furan, 2-methylfuran, 1, 8-cineole or crown ether.

Examples of the chain ether are diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, amyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1, 2-dimethoxyethane, 1, 2-diethoxyethane, 1, 2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1-dimethoxymethane, 1-diethoxyethane, triethylene glycol dimethyl ether or tetraethylene glycol dimethyl ether.

Examples of esters are carbonates or carboxylates.

Examples of cyclic carbonates are ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate, vinyl ethylene carbonate or vinylene carbonate.

Examples of the chain carbonate are dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate or methyl isopropyl carbonate.

Examples of cyclic carboxylic acid esters are gamma-butyrolactone or gamma-valerolactone.

Examples of the chain carboxylic acid ester are methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, or methyl fluoropropionate.

From the viewpoint of easily forming an SEI film having a more uniform film quality on the surface of the negative electrode, at least one selected from the group consisting of ethylene carbonate, vinylene carbonate, fluoroethylene carbonate and vinyl ethylene carbonate may be used.

Examples of nitriles are acetonitrile, propionitrile or benzonitrile.

Examples of amides are dimethylformamide or dimethylacetamide.

The solvent of the nonaqueous electrolyte may contain ether as a main solvent. The phrase "ether is the main solvent" means that the volume ratio of the ether to the whole solvent is 50% or more. The volume ratio is more than 60%, more than 70%, more than 80% or more than 90%. The solvent may consist of ether alone.

in the present embodiment, the volume ratio of the solvent to the entire solvent is a volume ratio at 25 ℃.

(lithium salt)

(lithium salt 1)

The anion of the 1 st lithium salt is an anion of an acid group type complex. Examples of the anion of the acid group-type complex include at least one selected from the group consisting of a boron atom and a phosphorus atom. The salt composed of anion of acid radical complex and lithium ion is represented by the chemical formula LiBX₄Or LiPX₆And (4) showing. Wherein X is a ligand having a high electronegativity. Examples of X include a fluorine atom and an oxygen-containing ligand. X may be a monodentate ligand. X may also be a polydentate ligand. Examples of the oxygen-containing ligand include those represented by the formula^-OOC-COO^-Oxalic acid ligand of the formula. Examples of the anion of the acid group-type complex having an oxalic acid ligand include those represented by the formula [ B (OOC-COO)₂]^-The bis oxalato borate anion shown.

Examples of the anion of the 1 st lithium salt include those represented by the formula B (C)₂O₄)₂ ^-Bis (oxalato) borate anion represented by the formula BF₂(C₂O₄)^-The difluoro oxalic acid boronic acid anion shown. From the viewpoint of forming a thin and dense SEI film, an anion containing a fluorine atom may be used. When an anion containing a fluorine atom is used, an SEI film containing a fluorine atom is formed.

Examples of the anion of the acid group-type complex are represented by the following chemical formulae (I) to (IV).

The 1 st lithium salt may include an acid-type complex ion. The lithium salt 1 may also contain two or more acid group-type complex ions. By using the 1 st lithium salt, the anion interacts with lithium,This makes it easy to precipitate lithium metal in a relatively large and uniform particle form. From the viewpoint of enhancing the effect of uniformly precipitating lithium metal in a relatively large particle form, a compound selected from the group consisting of bisoxalatoborate anion and BF can be used₂(C₂O₄)^-At least one of the above.

The concentration of the 1 st lithium salt in the nonaqueous electrolyte may be 0.01mol/L or more, 0.05mol/L or more, 0.3mol/L or more, or 0.5mol/L or more. When the concentration of the 1 st lithium salt is 0.01mol/L or more, a more uniform SEI film is easily formed. The concentration of the 1 st lithium salt in the nonaqueous electrolyte may be 2mol/L or less, 1.5mol/L or less, or 1.0mol/L or less from the viewpoint of high solubility of the 2 nd lithium salt.

(lithium salt of 2 nd)

The composition of the 2 nd lithium salt is different from that of the 1 st lithium salt. As the 2 nd lithium salt, a known lithium salt used for a nonaqueous electrolyte of a lithium secondary battery can be used. Examples of anions of the 2 nd lithium salt are BF₄ ^-、ClO₄ ^-、PF₆ ^-、AsF₆ ^-、SbF₆ ^-、AlCl₄ ^-、SCN^-、CF₃SO₃ ^-、CF₃CO₂ ^-Or the anion of an imide (i.e., of the formula R)₁CON^-COR₂An anion of wherein R₁And R₂Each independently an organic group). The nonaqueous electrolyte may contain one kind of imide anion, or may contain two or more kinds.

examples of anions of imides are N (SO)₂C_mF_2m+1)(SO₂C_nF_2n+1)^-(m and n are each independently an integer of 0 or more). The values of m and n may be 0 or more and 3 or less, respectively. The values of m and n may each independently be 0, 1 or 2. Examples of anions of imides are N (SO)₂CF₃)₂ ^-、N(SO₂C₂F₅)₂ ^-or N (SO)₂F)₂ ^-. Hereinafter, N (SO)₂F)₂ ^-Referred to as FSI^-. Will be formed by FSI^-And lithium ions are called LiFSI.

The 2 nd lithium salt may be composed of one lithium salt. Alternatively, the 2 nd lithium salt may be a mixture of two or more lithium salts.

(total of concentrations of the 1 st lithium salt and the 2 nd lithium salt)

The total concentration of the 1 st lithium salt and the 2 nd lithium salt contained in the nonaqueous electrolyte (i.e., "total concentration") is 3.0mol/L or more. The total concentration may be 3.5mol/L or more. When the total concentration is 3.0mol/L or more, the 1 st lithium salt easily acts and a more uniform and dense SEI film is easily formed. Thus, the effect of the cycle characteristic is further improved. From the viewpoint of an appropriate viscosity of the nonaqueous electrolyte, the total concentration of the 1 st lithium salt and the 2 nd lithium salt contained in the nonaqueous electrolyte may be 5mol/L or less. The total concentration may be 4.5mol/L or less.

The concentration of the 2 nd lithium salt in the nonaqueous electrolyte may be 1.5mol/L or more, 2mol/L or more, 2.5mol/L or more, or 3.0mol/L or more. When the concentration of the 2 nd lithium salt is 1.5mol/L or more, a large amount of lithium ions are supplied to the surface of the negative electrode. As a result, the diffusibility of lithium ions is easily improved, and lithium metal is more uniformly deposited on the negative electrode during charging. In addition, the SEI film derived from the 1 st lithium salt is more uniformly deposited. Thus, the effect of forming the coating of the 1 st lithium salt can be sufficiently exhibited. The concentration of the 2 nd lithium salt contained in the nonaqueous electrolyte may be 4.5mol/L or less. The concentration may be 4.0mol/L or less.

When a salt composed of lithium ions and anions of imide is used as the 2 nd lithium salt, gelation of the nonaqueous electrolytic solution is easily suppressed even if the total concentration of the 1 st lithium salt and the 2 nd lithium salt is a high value of 3.0mol/L or more, and therefore, the lithium ion conductivity can be easily improved. A salt composed of a lithium ion and an anion of an imide is easily dissolved in a solvent containing ether as a main solvent. Therefore, even when the nonaqueous electrolyte contains the salt at a high concentration, gelation of the nonaqueous electrolyte can be suppressed, and therefore, the ion conductivity can be improved. In a non-aqueous electrolyte of a lithium secondary battery containing ether as a main solvent, the effect of the 1 st lithium salt on the cycle characteristics in the case where the concentration of a salt composed of lithium ions and anions of imide is 1.5mol/L or more is completely different from the effect in the case where the concentration is less than 1.5 mol/L. Therefore, the concentration of the salt composed of the lithium ion and the anion of the imide may be 1.5mol/L or more, 2mol/L or more, 2.5mol/L or more, or 3.0mol/L or more.

In the case of using a salt composed of lithium ions and anions other than imide-based anions as the 2 nd lithium salt, the concentration of the 2 nd lithium salt is, for example, 0.5mol/L or less, 0.1mol/L or less, or 0.01mol/L or less. The non-aqueous electrolyte may include only a salt composed of lithium ions and anions of imide as the 2 nd lithium salt. In this case, the effects of the 1 st lithium salt and the 2 nd lithium salt can be further exhibited, and the cycle characteristics can be further improved. Such an effect can be exerted particularly effectively when a solvent containing ether as a main solvent is used.

The concentration of each lithium salt in the nonaqueous electrolyte is the sum of the concentration of lithium ions derived from the dissociated lithium salt (i.e., ionized lithium atoms) and the concentration of undissociated lithium salt (i.e., non-ionized lithium atoms).

(others)

The nonaqueous electrolyte may contain an additive. Examples of additives are vinylene carbonate, fluoroethylene carbonate or vinyl ethylene carbonate. An additive may be used. Two or more additives may also be used.

the additive decomposes at a potential lower than that at which the 1 st lithium salt decomposes to form a thin film on the negative electrode. A thin film derived from the additive is formed on the negative electrode, so that the charge and discharge reactions proceed more uniformly. If a film derived from the additive is formed on the SEI film derived from the 1 st lithium salt, the uniformity of the SEI film can be further improved. As a result, the charge and discharge reactions proceed more uniformly. This suppresses the formation of dendrites, and thus suppresses the volume change of the negative electrode associated with charge and discharge. As a result, the lithium secondary battery of the present embodiment has a high discharge capacity, and the reduction in cycle characteristics of the lithium secondary battery of the present embodiment is further suppressed.

(insight underlying the present disclosure)

In a lithium secondary battery, ions contained in a nonaqueous electrolyte receive electrons at a negative electrode during charging, and lithium metal is deposited on the negative electrode. The precipitated lithium metal is dissolved in the nonaqueous electrolyte at the time of discharge. The precipitation and dissolution of the lithium metal are performed to perform charge and discharge. Lithium metal has extremely high reducing power and is liable to cause side reactions with nonaqueous electrolytes. In a lithium secondary battery, lithium metal is almost always present in the negative electrode except in a fully discharged state. Therefore, in the lithium secondary battery, the chance of contact between the lithium metal and the nonaqueous electrolyte increases, and a side reaction between the two tends to become significant.

In lithium secondary batteries, lithium metal is easily precipitated in the form of dendrites. If lithium metal precipitates in a dendritic form, the specific surface area of lithium metal increases. Thereby, the side reaction of the lithium metal with the electrolyte further increases. As a result, the discharge capacity is remarkably reduced, and the cycle characteristics are likely to be greatly reduced.

The reason why lithium metal precipitates in a dendritic form is, first, that the diffusion of lithium ions in the negative electrode is likely to become uneven. The electrodeposition reaction of lithium metal in the negative electrode, which occurs during charging, is divided into two processes, i.e., a diffusion process of lithium ions into the surface of the negative electrode and an electron transfer process on the surface of the negative electrode. When diffusion is limited in a state where the diffusion process is very slow compared to the electron transfer process, lithium ions used for the electrodeposition reaction may be locally depleted. In this case, the electrodeposition reaction occurs preferentially in the portion where lithium ions exist, and lithium metal tends to be easily precipitated in a dendritic form.

Further, the second is that the thickness of the SEI film formed on the negative electrode tends to be uneven during charging. The SEI film is formed by decomposition and/or reaction of components contained in the electrolyte. In the negative electrode of a lithium secondary battery, lithium metal is precipitated during charging, and an SEI film is formed. Therefore, the thickness of the SEI film tends to be uneven. In addition, the SEI film may cause crystal defects.

If the SEI film has crystal defects, the electrical resistance at that site decreases. Therefore, during charging, lithium ions enter the inside from the low-resistance part and reach the negative electrode current collector or the negative electrode active material, and lithium metal is deposited. If lithium metal is unevenly precipitated in the negative electrode, the SEI film is locally stressed by the precipitated lithium metal. The stress preferentially breaks the fragile portion of the SEI film. Lithium metal precipitates so as to be squeezed out from the portion where the SEI film is broken, and a dendritic lithium metal is formed.

From the viewpoint of making the diffusion of lithium ions on the surface of the negative electrode more uniform, it is considered to increase the concentration of the lithium salt in the nonaqueous electrolyte so that a large amount of lithium ions can be supplied to the surface of the negative electrode. However, if the concentration of the lithium salt is increased, the viscosity becomes too high, and the ionic conductivity is lowered. In addition, if a nonaqueous electrolyte containing a high concentration of lithium salt is used, the film quality of the SEI film tends to be uneven, and the SEI film tends to be brittle. Therefore, stress is applied by local precipitation of lithium metal, and the SEI film tends to be more uneven along with breakage of the SEI film. This makes the deposition of lithium metal more uneven.

On the other hand, in conventional lithium secondary batteries, LiPF is used as in patent document 1₆and the like, are used for the non-aqueous electrolyte, and mainly use carbonate as a solvent. As shown in patent document 2, carbonate is commonly used as a solvent for a nonaqueous electrolyte in a lithium ion battery. In patent document 2, an imide lithium salt is used for the nonaqueous electrolyte. In patent document 3, two or more lithium salts having an oxalic acid skeleton and a phosphoric acid skeleton are mixed with LiPF at predetermined concentrations in order to suppress an increase in impedance after high-temperature storage₆And the like in combination.

However, as a result of earnest studies, the present inventors have found that the action of the lithium salt varies depending on the concentration of the lithium salt in the nonaqueous electrolyte, and the cycle characteristics change greatly. The following more specifically describes the present invention.

In the case of using the 1 st lithium salt of an acid group-type complex anion and lithium ions, even if the 2 nd lithium salt different from the 1 st lithium salt is combined, the cycle characteristics are degraded when the total concentration of the 1 st lithium salt and the 2 nd lithium salt is less than 3.0 mol/L. However, if the total concentration of the 1 st lithium salt and the 2 nd lithium salt is 3.0mol/L or more, the cycle characteristics in the case of using the 1 st lithium salt and the 2 nd lithium salt in combination are greatly improved as compared with the case of having the total concentration of less than 3.0 mol/L. In this manner, in the nonaqueous electrolyte of a lithium secondary battery, the cycle characteristics may be greatly different depending on the total concentration of the 1 st lithium salt and the 2 nd lithium salt.

The detailed cause of the reduction in the cycle characteristics in the case of using a nonaqueous electrolyte containing the 1 st lithium salt and the 2 nd lithium salt and having a total concentration of these lithium salts of less than 3.0mol/L is not determined, but is presumed to be due to the following reasons.

In a lithium secondary battery, the chance of contact between lithium metal and a nonaqueous electrolyte increases, and side reactions tend to become more pronounced. If the concentration of the lithium salt is low, it is difficult to rapidly supply a sufficient amount of lithium ions to the surface of the negative electrode. In such a state, deposition of lithium metal is likely to occur locally on the surface of the negative electrode, and the contact area between the lithium metal and the nonaqueous electrolyte is likely to increase. Although the 1 st lithium salt easily generates a good SEI film, when the deposition state of lithium metal on the surface of the negative electrode is not uniform, reductive decomposition of the 1 st lithium salt easily occurs, and the SEI film derived from the 1 st lithium salt is also easily non-uniform. If an uneven SEI film is formed, the deposition of lithium metal becomes more uneven. Thereby, the cycle characteristics are greatly reduced.

In addition, since the lithium secondary battery is charged and discharged by precipitation and dissolution of lithium metal in the negative electrode as described above, the volume change accompanying expansion and contraction of the negative electrode during charging and discharging is particularly remarkable. If the negative electrode expands greatly during charging, the electrode may crack and/or break due to stress caused by the expansion. Such damage of the electrode deteriorates cycle characteristics.

The present inventors have conceived of a lithium secondary battery according to the present disclosure based on the above-described problems and differences in cycle characteristics.

A lithium secondary battery according to one aspect of the present disclosure includes a positive electrode, a negative electrode, and a nonaqueous electrolyte having lithium ion conductivity. Lithium metal is deposited on the negative electrode by charging, and the lithium metal is eluted into the nonaqueous electrolyte by discharging. The nonaqueous electrolyte contains a solvent and a lithium salt. The lithium salt includes a 1 st lithium salt and a 2 nd lithium salt. The lithium salt 1 is a salt of a lithium ion and an acid group type complex anion. The total concentration of the 1 st lithium salt and the 2 nd lithium salt in the nonaqueous electrolyte is 3.0mol/L or more.

According to the above aspect, the 1 st lithium salt and the 2 nd lithium salt are used in combination in the lithium secondary battery, and the total concentration of these lithium salts is set to 3.0mol/L or more. Thereby greatly improving cycle characteristics. The detailed reason for the improvement of the cycle characteristics is not determined, but is considered to be probably due to the following reasons.

By increasing the total concentration of the 1 st lithium salt and the 2 nd lithium salt, a large amount of lithium ions are supplied to the surface of the negative electrode. Therefore, the limitation of diffusion of lithium ions on the surface of the negative electrode can be reduced, and the transport characteristics of lithium ions can be improved. This can reduce local precipitation of lithium metal. In addition, by supplying a large amount of lithium salt to the surface of the negative electrode, the effect of the 2 nd lithium salt can be sufficiently exhibited, and a dense and thin SEI film can be more uniformly deposited on the surface of the negative electrode. Therefore, the starting points of precipitation of lithium metal can be made more uniform, and precipitation of lithium metal in a dendritic form can be suppressed. Thereby suppressing the contact area of the lithium metal with the nonaqueous electrolyte from becoming excessively large, and suppressing the reductive decomposition of the 2 nd lithium salt. As a result, the charge and discharge reactions proceed more uniformly. Further, by making the charge-discharge reaction more uniform and suppressing the generation of dendritic lithium metal, the volume change due to the expansion and contraction of the electrode can be suppressed. It is considered that the cycle characteristics are improved for these reasons.

(Structure of lithium Secondary Battery)

A lithium secondary battery includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. A separator is generally disposed between the positive electrode and the negative electrode. Hereinafter, the structure of the lithium secondary battery will be described with reference to the drawings.

Fig. 1A is a longitudinal sectional view of a lithium secondary battery according to an embodiment. Fig. 1B is an enlarged cross-sectional view taken along line 1B in fig. 1A in a fully discharged state of the lithium secondary battery.

The lithium secondary battery 10 is a cylindrical battery including a cylindrical battery case, a wound electrode group 14 housed in the battery case, and a nonaqueous electrolyte (not shown). The battery case includes a case body 15 and a sealing member 16 for sealing an opening of the case body 15, and the case body 15 is a bottomed cylindrical metal container. A gasket 27 is disposed between the case main body 15 and the sealing body 16, thereby ensuring the sealing property of the battery case. In the case body 15, insulating plates 17 and 18 are disposed on one side or both side surfaces of the wound electrode group 14.

The case body 15 has, for example, a stepped portion 21 formed by pressing a side wall portion of the case body 15 from the outside. The step portion 21 may be formed in a ring shape along the outer circumferential direction of the case main body 15 at the side wall of the case main body 15. In this case, sealing body 16 is supported by the opening side surface of step portion 21.

Sealing body 16 includes filter 22, lower valve element 23, insulating member 24, upper valve element 25, and cap 26. In the sealing body 16, these members are stacked in this order. Sealing body 16 is attached to the opening of case body 15 such that lid 26 is positioned outside case body 15 and filter 22 is positioned inside case body 15. Each of the members constituting the sealing body 16 has, for example, a disk shape or a ring shape. The members other than the insulating member 24 are electrically connected to each other.

The electrode group 14 has a positive electrode 11, a negative electrode 12, and a separator 13. The positive electrode 11, the negative electrode 12, and the separator 13 are all in the form of a belt. The positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween, so that the width direction of the strip-shaped positive electrode 11 and the negative electrode 12 is parallel to the winding axis. In a cross section perpendicular to the winding axis of the electrode group 14, the positive electrodes 11 and the negative electrodes 12 are alternately stacked in the radial direction of the electrode group 14 with the separator 13 interposed therebetween.

The positive electrode 11 is electrically connected to a cap 26 serving also as a positive electrode terminal via a positive electrode lead 19. One end of positive electrode lead 19 is electrically connected to positive electrode 11 in a strip shape. The positive electrode lead 19 extending from the positive electrode 11 extends to the filter 22 through a through hole (not shown) formed in the insulating plate 17. The other end of the positive electrode lead 19 is welded to the electrode group 14 side surface of the filter 22.

The negative electrode 12 is electrically connected to the case main body 15 serving also as a negative electrode terminal via a negative electrode lead 20. One end of the negative electrode lead 20 is electrically connected to the strip-shaped negative electrode 12. The other end of the negative electrode lead 20 is welded to the inner bottom surface of the case main body 15.

As shown in fig. 1B, the positive electrode 11 includes a positive electrode current collector 30 and positive electrode mixture layers 31 disposed on both surfaces of the positive electrode current collector 30. The negative electrode 12 includes a negative electrode current collector 32. Lithium metal is deposited on the negative electrode 12 of the lithium secondary battery 10 by charging. On the other hand, the precipitated lithium metal is dissolved in the nonaqueous electrolyte by discharge.

(Positive electrode 11)

The positive electrode 11 includes, for example, a positive electrode current collector 30 and a positive electrode mixture layer 31 formed on the positive electrode current collector 30. The positive electrode mixture layer 31 may be formed on both surfaces of the positive electrode current collector 30. The positive electrode mixture layer 31 may be formed on one surface of the positive electrode current collector 30.

The positive electrode mixture layer 31 contains a positive electrode active material as an essential component. The positive electrode mixture layer 31 may contain a conductive material, a binder, and an additive. A conductive carbon material may be disposed between the positive electrode current collector 30 and the positive electrode mixture layer 31.

The positive electrode 11 can be obtained, for example, by applying a slurry containing the constituent components of the positive electrode mixture layer 31 and a dispersion medium to the surface of the positive electrode current collector 30, drying the coating film, and then rolling the coating film. Examples of the dispersion medium are water, an organic medium or a mixture thereof. A conductive carbon material may be applied to the surface of the positive electrode current collector 30.

The positive electrode active material occludes and releases lithium ions. Examples of the positive electrode active material are lithium-containing transition metal oxides, transition metal fluorides, polyanions, fluorinated polyanions, or transition metal sulfides. The positive electrode active material is preferably a lithium-containing transition metal oxide from the viewpoints of high average discharge voltage and low cost.

Examples of the transition metal element contained in the lithium-containing transition metal oxide include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, and W. The lithium-containing transition metal oxide may include one transition metal element. The lithium-containing transition metal oxide may also contain two or more transition metal elements. The lithium-containing transition metal oxide may include typical metal elements. Examples of typical metal elements are Mg, Al, Ca, Zn, Ga, Ge, Sn, Sb, Pb or Bi. The lithium-containing transition metal oxide may include one typical metal element. The lithium-containing transition metal oxide may also contain two or more typical metal elements.

There is no limitation on the crystal structure of the positive electrode active material. An example of the crystal structure of the positive electrode active material is a crystal structure belonging to space group R-3 m. The positive electrode active material having a crystal structure belonging to the space group R-3m is less likely to deteriorate even in a nonaqueous electrolyte. This is because the expansion and contraction of the crystal lattice accompanying charge and discharge of the crystal structure belonging to space group R-3m is relatively small. As a result, excellent cycle characteristics are easily obtained.

The lithium secondary battery of the present embodiment can be assembled in a discharged state. The positive electrode active material having a crystal structure belonging to the space group R-3m may be an oxide containing at least one selected from Ni, Co, Mn, and Al. In the positive electrode active material formed of such an oxide, the ratio of the number of atoms of Ni to the total number of atoms of Ni, Co, Mn, and Al may be 0.5 or more. For example, when the positive electrode active material contains Ni, Co, and Al, the ratio of the number of atoms of Ni may be 0.5 or more or 0.8 or more. When the positive electrode active material contains Ni, Co, and Mn, the ratio of the number of atoms in the Ni may be 0.5 or more.

An example of the conductive material is a carbon material. Examples of carbon materials are carbon black, carbon nanotubes or graphite. Examples of carbon black are acetylene black or ketjen black. The positive electrode mixture layer 31 may contain a conductive material. The positive electrode mixture layer 31 may contain two or more conductive materials. At least one selected from these carbon materials can be used as the conductive carbon material present between the positive electrode current collector 30 and the positive electrode mixture layer 31.

Examples of the binder are fluororesin, polyacrylonitrile, polyimide resin, acrylic resin, polyolefin resin, or rubbery polymer. Examples of the fluororesin are polytetrafluoroethylene or polyvinylidene fluoride. The positive electrode mixture layer 31 may contain a binder. The positive electrode mixture layer 31 may contain two or more kinds of binders.

Examples of the material of the positive electrode current collector 30 include metals such as Al, Ti, Fe, Al alloy, Ti alloy, and Fe alloy, and alloys thereof. The Fe alloy may be stainless steel called SUS. Examples of the shape of the positive electrode collector 30 include a foil and a film. The positive electrode current collector 30 may be porous. For example, a metal mesh may be used as the positive electrode collector 30.

(cathode 12)

Lithium metal is deposited on the negative electrode 12 of the lithium secondary battery 10 by charging. More specifically, lithium ions contained in the nonaqueous electrolyte receive electrons in the negative electrode 12 by charging, and are deposited as lithium metal in the negative electrode 12. The lithium metal deposited on the negative electrode 12 is discharged to become lithium ions, and the lithium ions are dissolved in the nonaqueous electrolyte. The lithium ions contained in the nonaqueous electrolyte are at least one type of lithium ions selected from the group consisting of lithium ions derived from a lithium salt added to the nonaqueous electrolyte and lithium ions supplied from the positive electrode active material by charging.

The negative electrode 12 includes a negative electrode current collector 32. The negative electrode collector 32 is generally made of a conductive sheet. The conductive sheet may be composed of lithium metal or lithium alloy. The conductive sheet may be made of a conductive material other than lithium metal and lithium alloy. The conductive material may be a metal material such as a metal or an alloy. The metal material may be a material that does not react with lithium. Such a material may be a material that does not react with lithium metal and lithium ions, and more specifically, may be a material that does not form an alloy and an intermetallic compound with lithium. Examples of such a metal material include copper, nickel, iron, and alloys containing these metal elements. The alloy may be copper alloy, SUS, or the like. The metal material may be copper and/or a copper alloy from the viewpoints of high conductivity, high capacity, and high charge-discharge efficiency. The conductive sheet may contain one kind of these conductive materials, or may contain two or more kinds.

As the conductive sheet, a foil, a film, or the like can be used. The conductive sheet may be porous. The conductive sheet may be a metal foil or may be a metal foil containing copper from the viewpoint of high conductivity. Such metal foil may be a copper foil or a copper alloy foil. The content of copper contained in the metal foil may be 50 mass% or more or 80 mass% or more. As the metal foil, in particular, a copper foil containing substantially only copper as a metal element can be used.

From the viewpoint of high volumetric energy density, the negative electrode 12 may include only the negative electrode current collector 32 in a fully discharged state of the lithium secondary battery. In this case, the negative electrode current collector 32 may be made of a material that does not react with lithium. In the fully discharged state, the negative electrode may include a negative electrode current collector and a negative electrode active material layer disposed on a surface of the negative electrode current collector, from the viewpoint of high charge/discharge efficiency. When the battery is assembled, only the negative electrode current collector 32 may be used as the negative electrode 12, or a negative electrode including a negative electrode active material layer and a negative electrode current collector may be used.

Examples of the negative electrode active material contained in the negative electrode active material layer are (i) metallic lithium, (ii) a lithium alloy, or (iii) a material capable of reversibly occluding and releasing lithium ions. An example of a lithium alloy is a lithium-aluminum alloy. Examples of the material capable of reversibly occluding and releasing lithium ions are carbon materials or alloy materials. Examples of the carbon material include a graphite material, soft carbon, hard carbon, or amorphous carbon. Examples of the alloy material include a material containing silicon or tin. Examples of the material containing silicon are elemental silicon, a silicon alloy, or a silicon compound. Examples of the material containing tin are a tin simple substance, a tin alloy, or a tin compound. Examples of the silicon compound are silicon oxide or silicon nitride. Examples of tin compounds are tin oxides or tin nitrides.

The anode active material layer can be formed by depositing an anode active material on the surface of an anode current collector by a vapor phase method such as electrodeposition or vapor phase deposition. The negative electrode active material layer may be formed by applying a negative electrode mixture containing a negative electrode active material and a binder to the surface of a negative electrode current collector. The negative electrode mixture may contain at least one of a conductive agent, a thickener, and an additive as necessary.

The thickness of the negative electrode active material layer is not limited. The thickness of the negative electrode active material layer is, for example, 30 μm or more and 300 μm or less in a fully discharged state of the lithium secondary battery. The thickness of the negative electrode current collector 32 is, for example, 5 μm or more and 20 μm or less.

In the present specification, the fully discharged State of the lithium secondary battery refers to a State of Charge (hereinafter referred to as "SOC" or "State of Charge", where C denotes the rated capacity of the battery) in which the lithium secondary battery is discharged to 0.05 × C or less, assuming that the rated capacity of the battery is C. For example, the fully discharged state of the lithium secondary battery refers to a state in which the lithium secondary battery is discharged to a lower limit voltage at a constant current of 0.05C. An example of the lower limit voltage is 2.5V.

the anode 12 may further include a protective layer. The protective layer may be formed on the surface of the negative electrode collector 32. In the case where the anode 12 has an anode active material layer, a protective layer may be formed on the surface of the anode active material layer. The protective layer has the effect of making the surface reaction of the electrode more uniform. The protective layer facilitates more uniform deposition of lithium metal on the negative electrode. The protective layer is made of at least one selected from among organic substances and inorganic substances, for example. As a material of the protective layer, a material that does not impair lithium ion conductivity is selected. Examples of the organic material include polymers having lithium ion conductivity. Examples of such polymers are polyethylene oxide or polymethyl methacrylate. Examples of inorganic substances are ceramics or solid electrolytes. Examples of ceramics are SiO₂、Al₂O₃Or MgO.

There is no limitation on the material of the solid electrolyte constituting the protective layer. Examples of the material of the solid electrolyte constituting the protective layer include a sulfide-based solid electrolyte, a phosphate-based solid electrolyte, a perovskite-based solid electrolyte, and a garnet-based solid electrolyte. From the viewpoint of low cost and easy availability, the solid electrolyte is preferably a sulfide-based solid electrolyte or a phosphoric acid-based solid electrolyte.

The sulfide-based solid electrolyte contains a sulfur component and has lithium ion conductivity. The sulfide-based solid electrolyte may contain, for example, S, Li and element 3. Examples of the element 3 include at least one selected from the group consisting of P, Ge, B, Si, I, Al, Ga and As. An example of the material of the sulfide-based solid electrolyte is Li₂S-P₂S₅、70Li₂S-30P₂S₅、80Li₂S-20P₂S₅、Li₂S-SiS₂Or LiGe_0.25P_0.75S₄。

The phosphoric acid-based solid electrolyte contains a phosphoric acid component and has lithium ion conductivity. Examples of the material of the phosphoric acid-based solid electrolyte include Li_1+XAl_XTi_2-X(PO₄)₃(wherein 0 < x < 2, for example, is Li)_1.5Al_0.5Ti_1.5(PO₄)₃) Or Li_{1+ X}Al_XGe_2-X(PO₄)₃. The value of X may be 1 or less.

(diaphragm 13)

As the separator 13, a porous sheet having ion permeability and insulation properties is used. Examples of the porous sheet include a microporous film, woven fabric, and nonwoven fabric. The material of the separator is not limited. Examples of the material of the separator include a polymer material. Examples of the polymer material include olefin resin, polyamide resin, and cellulose. Examples of the olefin resin are (i) polyethylene, (ii) polypropylene, or (iii) an olefin-based copolymer containing at least one of ethylene and propylene as a monomer unit. The separator 13 may contain an additive. Examples of additives are inorganic fillers.

The separator 13 may be a laminated body. Examples of the laminate include (i) a laminate of a polyethylene microporous membrane and a polypropylene microporous membrane, or (ii) a laminate of a nonwoven fabric comprising cellulose fibers and a nonwoven fabric comprising thermoplastic resin fibers. Another example of the laminate is a laminate in which a coating film made of a polyamide resin is laminated on the surface of a microporous membrane, woven fabric, or nonwoven fabric. Such a diaphragm 13 has high durability, and therefore, even if pressure is applied to the diaphragm 13 in a state where the diaphragm 13 is in contact with the plurality of convex portions, damage to the diaphragm 13 can be suppressed. The separator 13 may have a layer containing an inorganic filler on a surface facing the positive electrode 11 and a surface facing the negative electrode 12 from the viewpoint of at least one of heat resistance and strength.

(others)

In fig. 1A, the lithium secondary battery is a cylindrical lithium secondary battery including a cylindrical battery case. However, the lithium secondary battery according to the present disclosure is not limited to the lithium secondary battery shown in fig. 1A. The lithium secondary battery according to the present disclosure may be, for example, a prismatic battery including a prismatic battery case. The lithium secondary battery according to the present disclosure may be a laminated battery including a resin exterior such as an aluminum laminate sheet. The electrode assembly need not be wound. The electrode group may be, for example, a laminated electrode group in which a plurality of positive electrode layers and a plurality of negative electrode layers are alternately laminated with separators interposed therebetween.

In a lithium secondary battery including a wound electrode group, the electrode may be cracked or broken due to stress caused by expansion of the negative electrode during charging. The thickness of the electrode of the lithium secondary battery having the laminated electrode group is greatly increased by the influence of the large expansion of the negative electrode accompanying the charging. However, in the lithium secondary battery according to the present disclosure, since the nonaqueous electrolyte is used as described above, the negative electrode can be inhibited from swelling. Therefore, when either one of the wound electrode group and the laminated electrode group is used, the battery characteristics including the cycle characteristics and the degradation of the battery characteristics due to the swelling of the negative electrode can be suppressed.