JP2020191157A - Lithium secondary battery - Google Patents

Abstract

To provide a lithium secondary battery having excellent cycle characteristics.SOLUTION: A lithium secondary battery (10) includes a positive electrode (20), a negative electrode (30) and electrolyte (50). The positive electrode (20) has a positive electrode collector (21), and a positive electrode active material layer (22) supported by the positive electrode collector (21). The electrolyte (50) contains a solvent, and a negative electrode mediator dissolved into the solvent, and is in contact with the positive electrode (20) and the negative electrode (30). The negative electrode (30) has a negative electrode collector (31), and a negative electrode active material(32) in contact with the negative electrode collector (31). The negative electrode mediator sends and receives electrons between the negative electrode active material (32) and the negative electrode collector (31).SELECTED DRAWING: Figure 1

Description

The present disclosure relates to a lithium secondary battery.

A lithium secondary battery is well known as a typical example of a non-aqueous electrolyte secondary battery. The characteristics of a lithium secondary battery vary depending on the type of active material. For example, when a lithium metal is used as the negative electrode active material, a lithium secondary battery having a high energy density can be obtained. However, if lithium metal is used as the negative electrode active material, the lithium metal may precipitate in a dendrite shape when the lithium secondary battery is charged, causing an internal short circuit.

On the other hand, when graphite is used as the negative electrode active material, it is possible to prevent the lithium metal from precipitating in the form of dendrites during charging. The reaction in which lithium ions are inserted between graphite layers and the reaction in which lithium ions are desorbed from the graphite layers are topotropic reactions and are excellent in reversibility. Because of these advantages, lithium secondary batteries using graphite as the negative electrode active material have been put into practical use.

Japanese Patent No. 4898737 Japanese Patent No. 3733065

The theoretical volume density of lithium metal is 3884 mAh / g. On the other hand, the theoretical volume density of graphite is 372 mAh / g, which is about 1/10 of the theoretical volume density of lithium metal. The capacity density of graphite in an actual lithium secondary battery is also approaching the theoretical capacity density, and it is difficult to further increase the capacity of a lithium secondary battery using graphite as a negative electrode active material.

As a negative electrode active material that replaces graphite, a material that forms an alloy with lithium is drawing attention. Aluminum, silicon, tin and the like are known as materials for forming an alloy with lithium. The theoretical volume densities of these materials are significantly higher than the theoretical volume densities of graphite.

One of the problems of lithium secondary batteries using these materials is cycle characteristics.

The present disclosure provides a lithium secondary battery having excellent cycle characteristics.

This disclosure is
A positive electrode having a positive electrode current collector and a positive electrode active material layer supported by the positive electrode current collector,
With the negative electrode
An electrolytic solution containing a solvent and a negative electrode mediator dissolved in the solvent and in contact with the positive electrode and the negative electrode.
With
The negative electrode has a negative electrode current collector and a negative electrode active material in contact with the negative electrode current collector.
The negative electrode mediator provides a lithium secondary battery that transfers electrons between the negative electrode active material and the negative electrode current collector.

According to the present disclosure, it is possible to provide a lithium secondary battery having excellent cycle characteristics.

FIG. 1 is a schematic cross-sectional view of a lithium secondary battery according to an embodiment of the present disclosure. FIG. 2A is a schematic view showing the state of the electrolytic solution when the negative electrode active material is separated from the negative electrode current collector. FIG. 2B is a graph showing the relationship between the distance from the negative electrode current collector and the concentration of the complex (Md · Li). FIG. 3 is a graph showing the relationship between the ratio of the volume of triglime to the volume of the solvent of the electrolytic solution and the potential value of the potential measurement cell in the potential measurement cells of Samples 1 to 9. FIG. 4 is a graph showing the results of the charge / discharge test of the lithium secondary batteries of the samples 10, 11 and 13. FIG. 5 is a graph showing the results of a charge / discharge test of the lithium secondary battery of sample 14.

(Findings underlying this disclosure)
When materials such as aluminum, silicon, and tin are used as the negative electrode active material in lithium secondary batteries, these materials electrochemically form an alloy with lithium during charging. In the present specification, these materials forming an alloy with lithium are also referred to as "alloying materials".

The negative electrode using the alloying material expands when lithium is occluded and contracts when lithium is released. When expansion and contraction are repeated, the alloying material is atomized and separated from the negative electrode current collector. Even if the alloying material stays in the negative electrode, electrical contact between the alloying material and the negative electrode current collector cannot be obtained, and charging / discharging becomes difficult. That is, the capacity drops sharply as charging and discharging are repeated. This phenomenon is the main cause that hinders the improvement of the cycle characteristics of lithium secondary batteries using alloyed materials.

The present inventors have diligently studied a technique for overcoming the above-mentioned problems caused by the charging / discharging mechanism of the alloying material. As a result, the lithium secondary battery of the present disclosure shown below has been completed.

(Summary of one aspect relating to this disclosure)
The lithium secondary battery according to the first aspect of the present disclosure is
A positive electrode having a positive electrode current collector and a positive electrode active material layer supported by the positive electrode current collector,
With the negative electrode
An electrolytic solution containing a solvent and a negative electrode mediator dissolved in the solvent and in contact with the positive electrode and the negative electrode.
With
The negative electrode has a negative electrode current collector and a negative electrode active material in contact with the negative electrode current collector.
The negative electrode mediator transfers electrons between the negative electrode active material and the negative electrode current collector.

According to the first aspect, even if the negative electrode active material is pulverized or peeled from the negative electrode current collector, the lithium secondary battery can be reliably charged and discharged via the negative electrode mediator. For example, it is possible to provide a lithium secondary battery having a high energy density and excellent cycle characteristics while using a high-capacity alloying material as a negative electrode active material.

In the second aspect of the present disclosure, for example, in the lithium secondary battery according to the first aspect, the positive electrode active material layer may occlude or release lithium ions, and the negative electrode active material may be alloyed with lithium. .. According to the second aspect, a lithium secondary battery having a high energy density can be obtained.

In the third aspect of the present disclosure, for example, in the lithium secondary battery according to the first or second aspect, the negative electrode active material is composed of a group consisting of Al, Zn, Si, Sn, Ge, Cd, Pb, Bi and Sb. It may contain at least one selected. According to the second aspect, a lithium secondary battery having a high energy density can be obtained.

In the fourth aspect of the present disclosure, for example, in the lithium secondary battery according to any one of the first to third aspects, the negative electrode mediator may contain a condensed aromatic compound. According to the fourth aspect, it is possible to provide a lithium secondary battery having excellent cycle characteristics.

In the fifth aspect of the present disclosure, for example, in the lithium secondary battery according to any one of the first to fourth aspects, the negative electrode mediator is phenanthrene, biphenyl, o-terphenyl, triphenylene, acenaphthene, acenaphthylene, fluoranthene. , Benzyl and at least one selected from the group consisting of naphthalene. According to the fifth aspect, it is possible to provide a lithium secondary battery having excellent cycle characteristics.

In the sixth aspect of the present disclosure, for example, in the lithium secondary battery according to any one of the first to fifth aspects, the concentration of the negative electrode mediator in the electrolytic solution is 0.00625 mol / L or more and 0.0125 mol / L. It may be as follows. According to the sixth aspect, a sufficient discharge capacity of the lithium secondary battery can be secured.

In the seventh aspect of the present disclosure, for example, in the lithium secondary battery according to any one of the first to sixth aspects, the electrolytic solution may contain ether. According to the seventh aspect, it is possible to provide a lithium secondary battery having excellent cycle characteristics.

In the eighth aspect of the present disclosure, for example, in the lithium secondary battery according to any one of the first to sixth aspects, the electrolytic solution is at least one selected from the group consisting of cyclic ether, grime and sulfolane. It may be included. According to the eighth aspect, it is possible to provide a lithium secondary battery having excellent cycle characteristics.

In the ninth aspect of the present disclosure, for example, in the lithium secondary battery according to the eighth aspect, the cyclic ether is 2-methyltetrahydrofuran, tetrahydrofuran, 2,5-dimethyltetrahydrofuran, 1,3-dioxolane and 4-methyl-. It may contain at least one selected from the group consisting of 1,3-dioxolane. According to the ninth aspect, it is possible to provide a lithium secondary battery having excellent cycle characteristics.

In the tenth aspect of the present disclosure, for example, in the lithium secondary battery according to the eighth or ninth aspect, the grime is at least one selected from the group consisting of monogrime, jigglime, triglime, tetraglime, and polyethylene glycol dimethyl ether. May include. According to the tenth aspect, it is possible to provide a lithium secondary battery having excellent cycle characteristics.

In the eleventh aspect of the present disclosure, for example, in the lithium secondary battery according to any one of the eighth to tenth aspects, the sulfolane may contain 3-methylsulfolane. According to the eleventh aspect, a lithium secondary battery having excellent cycle characteristics can be provided.

In the twelfth aspect of the present disclosure, for example, the lithium secondary battery according to any one of the first to eleventh aspects may further include a separator arranged between the positive electrode and the negative electrode. According to the twelfth aspect, the safety of the lithium secondary battery can be sufficiently ensured.

In the thirteenth aspect of the present disclosure, for example, in the lithium secondary battery according to the twelfth aspect, the separator may be made of a material that allows the passage of lithium ions and the negative electrode mediator. According to the thirteenth aspect, since the material of the separator is widely selected, the degree of freedom in designing the lithium secondary battery is increased.

Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The present disclosure is not limited to the following embodiments.

(Embodiment)
FIG. 1 shows a cross section of a lithium secondary battery according to an embodiment of the present disclosure. The lithium secondary battery 10 includes a positive electrode 20, a negative electrode 30, and an electrolytic solution 50. The electrolytic solution 50 contains a solvent and a negative electrode mediator, and is in contact with the positive electrode 20 and the negative electrode 30. The negative electrode mediator is dissolved in the solvent of the electrolytic solution 50. The negative electrode 30 has a negative electrode current collector 31 and a negative electrode active material 32.

In the present embodiment, the negative electrode active material 32 is in contact with the negative electrode current collector 31. Specifically, a part of the negative electrode active material 32 is physically and electrically in contact with the negative electrode current collector 31. When a part of the negative electrode active material 32 is physically and electrically in contact with the negative electrode current collector 31, two reactions, a reaction that proceeds through the negative electrode mediator and a reaction that proceeds without the negative electrode mediator, occur at the same time. proceed. When the negative electrode active material 32 is pulverized or separated from the negative electrode current collector 31, the redox reaction of the negative electrode active material 32 in the negative electrode 30 proceeds via the negative electrode mediator. The negative electrode mediator transfers and receives electrons between the negative electrode active material 32 and the negative electrode current collector 31.

According to the present embodiment, even if the negative electrode active material 32 is pulverized or separated from the negative electrode current collector 31, the lithium secondary battery 10 can be reliably charged and discharged via the negative electrode mediator. For example, it is possible to provide a lithium secondary battery 10 having a high energy density and excellent cycle characteristics while using a high-capacity alloying material as the negative electrode active material 32.

The lithium secondary battery 10 further includes a container 60. The container 60 is hermetically sealed. The container 60 is made of a material having insulating properties and corrosion resistance. The positive electrode 20, the negative electrode 30, and the electrolytic solution 50 are arranged inside the container 60.

The positive electrode 20 has a positive electrode current collector 21 and a positive electrode active material layer 22. The positive electrode active material layer 22 is supported by the positive electrode current collector 21. Specifically, the positive electrode active material layer 22 is arranged on the positive electrode current collector 21. The positive electrode current collector 21 and the positive electrode active material layer 22 are in electrical contact with each other.

The positive electrode current collector 21 is made of a material having electron conductivity such as stainless steel, copper, nickel, and carbon. The shape of the positive electrode current collector 21 is not particularly limited, and is, for example, a plate shape.

The positive electrode active material layer 22 is a layer containing a positive electrode active material. The positive electrode active material can be a material having the property of reversibly occluding and releasing lithium ions. Examples of the positive electrode active material include transition metal oxides, fluorides, polyanions, fluorinated polyanions, transition metal sulfides, and phosphorus oxides having an olivine structure. Examples of the transition metal oxide include LiCoO ₂ , LiNiO ₂ , Li ₂ Mn ₂ O _{4, and the} like. Examples of the phosphor oxide include LiFePO ₄ , LiNiPO ₄ , and LiCoPO ₄ . The positive electrode active material layer 22 may contain a plurality of types of positive electrode active materials.

The positive electrode active material layer 22 may contain additives such as a conductive agent, an ionic conduction auxiliary agent, and a binder, if necessary.

The negative electrode 30 has a negative electrode current collector 31 and a negative electrode active material 32. Both the negative electrode current collector 31 and the negative electrode active material 32 are immersed in the electrolytic solution 50. In the present embodiment, the negative electrode active material 32 is in contact with the negative electrode current collector 31. The electrolytic solution 50 surrounding the negative electrode 30 contains a negative electrode mediator. The transfer of electrons between the negative electrode active material 32 and the negative electrode current collector 31 may be performed directly or may be performed via the negative electrode mediator. The charge / discharge reaction can proceed regardless of whether or not the direct contact between the negative electrode current collector 31 and the negative electrode active material 32 is maintained. This improves the degree of freedom in designing the shapes and dimensions of the negative electrode current collector 31 and the negative electrode active material 32.

The negative electrode current collector 31 has a surface that acts as a reaction field for the negative electrode mediator. As the negative electrode current collector 31, a material that is stable with respect to the electrolytic solution 50 can be used. Further, as the negative electrode current collector 31, a material that is stable against an electrochemical reaction that is an electrode reaction can be used. For example, as the negative electrode current collector 31, a material having electron conductivity such as metal or carbon can be used. Examples of the metal include stainless steel, iron, copper, nickel and the like.

The negative electrode current collector 31 may have a structure in which the surface area thereof is increased. Examples of the structure having an increased surface area include a mesh, a non-woven fabric, a surface roughened plate, and a sintered porous body. When the negative electrode current collector 31 has these structures, the oxidation reaction or reduction reaction of the negative electrode mediator tends to proceed.

The negative electrode active material 32 may have a property of occluding and releasing lithium.

When the lithium secondary battery 10 is charged, the negative electrode mediator is reduced on the surface of the negative electrode current collector 31. The reduced negative electrode mediator is oxidized by the negative electrode active material 32. The negative electrode active material 32 occludes lithium.

When the lithium secondary battery 10 is discharged, the negative electrode active material 32 that stores lithium reduces the negative electrode mediator and releases lithium. The reduced negative electrode mediator is oxidized on the surface of the negative electrode current collector 31.

The negative electrode active material 32 contains, for example, a material that forms an alloy with lithium during charging. The negative electrode active material 32 includes, for example, at least one selected from the group consisting of Al, Zn, Si, Sn, Ge, Cd, Pb, Bi and Sb. By using these alloying materials, a lithium secondary battery 10 having a high energy density can be obtained.

The negative electrode active material 32 may contain Al. In this case, a LiAl alloy is produced when the lithium secondary battery 10 is charged. The composition of the LiAl alloy may be at least one selected from the group consisting of LiAl, Li ₂ Al ₃ and Li ₄ Al ₅ .

The negative electrode active material 32 may contain Zn. In this case, a LiZn alloy is produced when the lithium secondary battery 10 is charged. The composition of the LiZn alloy may be at least one selected from the group consisting of Li ₂ Zn ₃ , Li Zn ₂ , Li ₂ Zn ₅ , Li Zn ₄ and Li Zn.

The negative electrode active material 32 may contain Si. In this case, a LiSi alloy is produced when the lithium secondary battery 10 is charged. The composition of the LiSi alloy may be at least one selected from the group consisting of Li ₂₂ Si ₅ , Li ₁₃ Si ₄ , Li ₇ Si ₃ and Li ₁₂ Si ₇ .

The negative electrode active material 32 may contain Sn. In this case, a LiSn alloy is produced when the lithium secondary battery 10 is charged. The composition of the LiSn alloy may be at least one selected from the group consisting of Li ₂₂ Sn ₅ , Li ₇ Sn ₂ , Li ₁₃ Sn ₅ , Li ₇ Sn ₃ , Li ₅ Sn ₂ , Li Sn and Li ₂ Sn ₅ .

The negative electrode active material 32 may contain Ge. In this case, a LiGe alloy is produced when the lithium secondary battery 10 is charged. The composition of the LiGe alloy may be at least one selected from the group consisting of Li ₅ Ge ₂₂ and Li ₃ Ge.

The negative electrode active material 32 may contain Cd. In this case, a LiCd alloy is produced when the lithium secondary battery 10 is charged. The composition of the LiCd alloy may be at least one selected from the group consisting of LiCd ₃ and Li ₃ Cd.

The negative electrode active material 32 may contain Pb. In this case, a LiPb alloy is produced when the lithium secondary battery 10 is charged. The composition of the LiPb alloy may be at least one selected from the group consisting of LiPb, Li ₃ Pb, Li ₅ Pb ₂ , Li ₄ Pb and Li ₁₀ Pb ₃ .

The negative electrode active material 32 may contain Bi. In this case, a LiBi alloy is produced when the lithium secondary battery 10 is charged. The composition of LiBi alloys can be at least one selected from the group consisting of LiBi and Li ₃ Bi.

The negative electrode active material 32 may contain Sb. In this case, a LiSb alloy is produced when the lithium secondary battery 10 is charged. The composition of the LiSb alloy may be at least one selected from the group consisting of Li ₂ Sb and Li ₃ Sb.

The negative electrode active material 32 may contain at least one selected from the group consisting of Al, Zn, Si, Sn, Ge, Cd, Pb, Bi and Sb. The negative electrode active material 32 may contain a carbon material such as graphite.

The negative electrode active material 32 may be a carbon material such as graphite. The compound generated at the negative electrode 30 when the lithium secondary battery 10 is charged may be an alloy such as a LiAl alloy or a graphite intercalation compound such as C6Li.

The shape of the negative electrode active material 32 is not particularly limited. The negative electrode active material 32 may be in the form of powder or pellets. The negative electrode active material 32 may be hardened by a binder. Examples of the binder include resins such as polyvinylidene fluoride, polypropylene, polyethylene, and polyimide.

The negative electrode active material 32 may be a material insoluble in the electrolytic solution 50.

The lithium secondary battery 10 may include a separator 40. The separator 40 is arranged between the positive electrode 20 and the negative electrode 30 to prevent direct contact between the positive electrode 20 and the negative electrode 30. According to the separator 40, the safety of the lithium secondary battery 10 can be sufficiently ensured.

The separator 40 has lithium ion conductivity. The material of the separator 40 is not particularly limited as long as the passage of lithium ions is allowed. The separator 40 may be made of a material that allows the passage of the negative electrode mediator. In this case, since the material of the separator 40 can be selected widely, the degree of freedom in designing the lithium secondary battery 10 is increased.

As the material of the separator 40, a material that does not dissolve in the electrolytic solution 50 can be used. Examples of the material of the separator 40 include a solid electrolyte, a lithium cation exchange resin, and a porous material. The separator 40 may cover only a part of the positive electrode 20, or may cover the entire positive electrode 20. The separator 40 may have the shape of a film. When the separator 40 is a porous film, the porous film includes a woven fabric, a non-woven fabric, a porous film made of a polyolefin resin, a porous film made of glass paper obtained by weaving glass fibers into the non-woven fabric, and the like. Can be mentioned. These porous membranes are impregnated with the electrolytic solution 50 and exhibit lithium ion conductivity.

Even if the separator 40 is omitted, the lithium secondary battery 10 can be charged and discharged. If the reaction rate between the negative electrode mediator and the negative electrode active material 32 exceeds the reaction rate between the negative electrode mediator and the positive electrode 20, the lithium secondary battery 10 is charged. The reaction rate between the negative electrode mediator and the positive electrode 20 decreases as the concentration of the negative electrode mediator in the electrolytic solution 50 decreases.

The electrolytic solution 50 contains a solvent and a negative electrode mediator. The electrolytic solution 50 fills the inside of the container 60. The solvent can be a non-aqueous solvent.

In the lithium secondary battery 10 of the present embodiment, the electrolytic solution 50 may contain at least one selected from the group consisting of cyclic ether, grime and sulfolane. The electrolytic solution 50 may contain ether. Examples of the ether include cyclic ether and glycol ether. The glycol ether may be a grime represented by the composition formula CH ₃ (OCH ₂ CH ₂ ) _n OCH ₃ . In the above composition formula, n is an integer of 1 or more. The electrolytic solution 50 may contain a mixture of cyclic ether and grime or cyclic ether as a solvent. By using these solvents, a lithium secondary battery 10 having excellent cycle characteristics can be provided.

According to the above configuration, an electrolytic solution containing a negative electrode mediator can be realized as the electrolytic solution 50. That is, since the solution containing the negative electrode mediator is an ether solution having no electron conductivity, the ether solution itself can have properties as an electrolytic solution.

As the cyclic ether, tetrahydrofuran (THF), 2-methyltetrahydrofuran (2MeTHF), 2,5-dimethyltetrahydrofuran, 1,3-dioxolane (1,3DO), 4-methyl-1,3-dioxolane (4Me1,3DO) And so on. One kind or a mixture of two or more kinds selected from these can be used.

Examples of the glyme include monoglyme (1,2-dimethoxyethane), diglyme (diethylene glycol dimethyl ether), triglime (triethylene glycol dimethyl ether), tetraglyme (tetraethylene glycol dimethyl ether), pentaethylene glycol dimethyl ether, polyethylene glycol dimethyl ether and the like. The grime may be a mixture of tetraglime and pentaethylene glycol dimethyl ether.

Examples of sulfolane include 3-methylsulfolane.

By using the above solvent, a lithium secondary battery 10 having excellent cycle characteristics can be provided.

The electrolyte solution 50 may contain an electrolyte salt. As the electrolyte _{salt, LiPF 6, LiBF 4, LiSbF} 6, LiAsF 6, LiSO 3 CF 3, LiN (SO 2 CF 3) 2, LiN (SO 2 C 2 F 5) 2, LiN (SO 2 CF 3) ( Examples thereof include lithium salts such as SO ₂ C ₄ F ₉ ), LiC (SO ₂ CF ₃ ) ₃ , LiClO ₄ , and lithium bisoxalate borate. Lithium may be dissolved in the electrolytic solution 50.

In the lithium secondary battery 10 of the present embodiment, the negative electrode mediator may be present on both the surface of the positive electrode 20 and the surface of the negative electrode 30. The surface of the negative electrode 30 includes a surface of the negative electrode current collector 31 and a surface of the negative electrode active material 32. Since a dedicated electrolytic solution is not required for the positive electrode 20 and it is not necessary to block the electrolytic solution 50 from coming and going between the positive electrode 20 and the negative electrode 30, the structure of the lithium secondary battery 10 can be simplified. Even when the separator 40 is used to prevent contact between the positive electrode 20 and the negative electrode 30, there are few restrictions imposed on the separator 40.

When charging the lithium secondary battery 10 of the present embodiment, the negative electrode mediator can be reduced on the surface of the negative electrode current collector 31.

When the lithium secondary battery 10 of the present embodiment is discharged, the negative electrode mediator can be oxidized on the surface of the negative electrode current collector 31.

When the negative electrode mediator is reduced on the negative electrode current collector 31 during charging, the negative electrode mediator is oxidized on the negative electrode current collector 31 during discharging. Since the negative electrode mediator is involved in the discharge, the capacity corresponding to the oxidation of the negative electrode mediator is added to the discharge capacity. That is, the discharge capacity can be increased. When the negative electrode mediator is oxidized on the negative electrode current collector 31 during discharge, the negative electrode mediator is reduced on the negative electrode current collector 31 during charging. Since the negative electrode mediator is involved in charging, the capacity corresponding to the reduction of the negative electrode mediator is added to the charging capacity. That is, the charging capacity can be increased.

For example, when the electrolytic solution 50 comes into contact with the negative electrode current collector 31, the negative electrode mediator is oxidized or reduced by the negative electrode current collector 31.

For example, when the electrolytic solution 50 comes into contact with the negative electrode active material 32, a reduction reaction of the negative electrode mediator by the negative electrode active material 32 occurs, or an oxidation reaction of the negative electrode mediator by the negative electrode active material 32 occurs. The electrolytic solution 50 may contain only the negative electrode mediator as a compound that is oxidized and reduced by the negative electrode active material 32. According to such a configuration, it is possible to provide a lithium secondary battery 10 having excellent cycle characteristics.

The negative electrode mediator can be a material that gives the electrolytic solution 50 an equilibrium potential equal to or lower than the upper limit potential at which a compound of lithium and the negative electrode active material 32 is formed by dissolving it in the solvent of the electrolytic solution 50 together with lithium. The negative electrode mediator may contain only one or more compounds that give the electrolytic solution 50 an equilibrium potential equal to or lower than the upper limit potential at which a compound of lithium and the negative electrode active material 32 is formed. The negative electrode mediator may contain only one compound that gives the electrolytic solution 50 an equilibrium potential equal to or lower than the upper limit potential at which a compound of lithium and the negative electrode active material 32 is formed. The negative electrode mediator may be a condensed aromatic compound. According to these configurations, it is possible to provide a lithium secondary battery 10 having excellent cycle characteristics.

The electrolytic solution 50 does not contain a compound that is given to the electrolytic solution 50 by dissolving an equilibrium potential higher than the upper limit potential at which a compound of lithium and the negative electrode active material 32 is formed in the solvent of the electrolytic solution 50 together with lithium. Good. According to such a configuration, a lithium secondary battery 10 having a high battery voltage can be realized.

The electrolytic solution 50 contains only the compound given to the electrolytic solution 50 by dissolving the equilibrium potential equal to or lower than the upper limit potential at which the compound of lithium and the negative electrode active material 32 is formed together with lithium in the solvent of the electrolytic solution 50 as the negative electrode mediator. You may.

The electrolytic solution 50 in which the condensed aromatic compound is dissolved has a property of releasing solvated electrons of lithium and dissolving lithium as a cation. In other words, the condensed aromatic compound has a property of receiving electrons released when lithium is dissolved in the solvent of the electrolytic solution 50 as solvated electrons and dissolving in the solvent of the electrolytic solution 50.

The solution containing the condensed aromatic compound has the ability to dissolve lithium. The solution containing the condensed aromatic compound may be, for example, an ether solution. Lithium easily separates electrons and becomes a cation. Therefore, electrons are passed to the condensed aromatic compound in the solution to become a cation and dissolve in the solution. At this time, the condensed aromatic compound that received the electron is solvated with the electron. By solvating with electrons, the condensed aromatic compound behaves as an anion. Therefore, the solution itself containing the condensed aromatic compound has ionic conductivity. Here, in the solution containing the condensed aromatic compound, Li cations and electrons are present in equivalent amounts. Therefore, the solution itself containing the condensed aromatic compound can be provided with a strongly reducing property, in other words, a potentially low property.

For example, if an electrode chemically inactive with respect to lithium is immersed in the solvent of the electrolytic solution 50 in which the condensed aromatic compound is dissolved and the potential of the electrode with respect to lithium metal is measured, a considerably low potential is observed. .. The observed potential is determined by the degree of solvation between the condensed aromatic compound and the electrons, i.e., the type of condensed aromatic compound.

By using an active material having a relatively low equilibrium potential (vs. Li ^/ Li ⁺ ) as the negative electrode active material 32, a compound having a relatively low equilibrium potential (vs. Li ^/ Li ⁺ ) can be used as the negative electrode mediator. .. The active material having a relatively low equilibrium potential as the negative electrode active material 32 is, for example, aluminum. A compound having a relatively low equilibrium potential as a negative electrode mediator is, for example, a condensed aromatic compound. As a result, the negative electrode 30 of the lithium secondary battery 10 having a lower potential can be realized. Therefore, a lithium secondary battery 10 having a high battery voltage can be realized.

When the negative electrode active material 32 contains aluminum, the aluminum contained in the negative electrode active material 32 reacts with lithium and is reduced to a LiAl alloy when the lithium secondary battery 10 is charged. Therefore, if a condensed aromatic compound exhibiting a potential equal to or lower than the upper limit potential at which a LiAl alloy is formed is used as the negative electrode mediator, the mediator type negative electrode 30 can be constructed. The upper limit potential at which the LiAl alloy is formed is, for example, 0.18 Vvs. Li / Li ⁺ . The negative electrode mediator is 0.18 Vvs. By dissolving it in the solvent of the electrolytic solution 50 together with lithium, for example. It is a compound that gives an equilibrium potential of Li / Li ⁺ or less to the electrolytic solution 50.

When the negative electrode active material 32 contains tin, the tin contained in the negative electrode active material 32 reacts with lithium and is reduced to a LiSn alloy when the lithium secondary battery 10 is charged. Therefore, if a condensed aromatic compound having a potential equal to or lower than the upper limit potential at which a LiSn alloy is formed is used as the negative electrode mediator, the mediator type negative electrode 30 can be constructed. The upper limit potential at which the LiSn alloy is formed is, for example, 0.25 Vvs. Li / Li ⁺ . The negative electrode mediator is dissolved in the solvent of the electrolytic solution 50 together with lithium, for example, to obtain 0.25 Vvs. It is a compound that gives an equilibrium potential of Li / Li ⁺ or less to the electrolytic solution 50.

When the negative electrode active material 32 contains graphite, the graphite contained in the negative electrode active material 32 reacts with lithium and is reduced to C ₆ Li when the lithium secondary battery 10 is charged. Therefore, if a condensed aromatic compound having a potential equal to or lower than the upper limit potential at which C ₆ Li is formed is used as the negative electrode mediator, the mediator type negative electrode 30 can be constructed. The upper limit potential at which C ₆ Li is formed is, for example, 0.15 Vvs. Li / Li ⁺ . The negative electrode mediator is dissolved in the solvent of the electrolytic solution 50 together with lithium, for example, to obtain 0.15 Vvs. It is a compound that gives an equilibrium potential of Li / Li ⁺ or less to the electrolytic solution 50.

The above description of aluminum, tin and graphite also applies to Zn, Si, Ge, Cd, Pb, Bi and Sb. The negative electrode mediator can be appropriately selected depending on the upper limit potential at which each of LiZn, LiSi, LiGe, LiCd, LiPb, LiBi and LiSb is formed.

Condensed aromatic compounds showing a low potential include phenanthrene, biphenyl, o-terphenyl, triphenylene, acenaphthene, acenaphthene, fluorantene, 2,2'-bipyridyl, trans-stilben, 2,4'-bipyridyl, 2,3. Included are'-bipyridyl, cis-stilben, propiophenone, butyrophenone, valerophenone, ethylenediamine, benzyl, tetraphenylcyclopentadienone, naphthalene and the like.

Condensed aromatic compounds that exhibit a sufficiently low potential include, for example, phenanthrene, biphenyl, o-terphenyl, triphenylene, acenaphthene, acenaphthylene, fluoranthene, benzyl and naphthalene. That is, the negative electrode mediator may contain at least one selected from the group consisting of phenanthrene, biphenyl, o-terphenyl, triphenylene, acenaphthene, acenaphthylene, fluoranthene, benzyl and naphthalene. When these compounds are used as a negative electrode mediator, a lithium secondary battery 10 having excellent cycle characteristics can be provided.

The concentration of the negative electrode mediator in the electrolytic solution 50 is, for example, 0.00625 mol / L or more and 0.05 mol / L or less. When the concentration of the negative electrode mediator is appropriately adjusted, the reaction rate between the charged negative electrode mediator and the positive electrode 20 can be reduced. As a result, the discharge capacity of the lithium secondary battery can be secured. The upper limit of the concentration of the negative electrode mediator in the electrolytic solution 50 may be 0.025 mol / L or 0.0125 mol / L.

When lithium is dissolved in an ether solution of a condensed aromatic compound, the potential of the ether solution differs slightly depending on the type of solvent. Cyclic ether has a low boiling point and therefore easily volatilizes. Therefore, cyclic ether and grime having a relatively high boiling point may be mixed and used. When cyclic ether is used as the solvent of the ether solution, the potential of the ether solution tends to be further lowered when THF or 2MeTHF is used as the cyclic ether. When grime is used as the solvent of the ether solution, the potential of the ether solution is lowered most when grime is used as the grime. Therefore, as the solvent of the ether solution, a mixture of THF or 2MeTHF and triglime may be used. The higher the ratio of grime in the solvent of the ether solution, the higher the potential of the ether solution tends to be. Therefore, the ratio of the volume of the cyclic ether to the volume of grime in the solvent of the electrolytic solution 50 may be 10: 0 to 7: 3.

When charging is performed via the negative electrode mediator, the negative electrode mediator solvated on the negative electrode current collector 31 is reduced to form a complex containing solvated electrons and Li cations. When this composite comes into contact with the negative electrode active material 32, the negative electrode active material 32 receives Li cations and solvated electrons, and a compound of lithium and the negative electrode active material 32 is formed. After the complex releases Li cations and solvated electrons, the solvated negative electrode mediator is reduced again on the negative electrode current collector 31. By this circulation, the negative electrode active material 32 is reduced to a compound of lithium and the negative electrode active material 32, and the solvated negative electrode mediator in the electrolytic solution 50 is reduced. As a result, charging of the lithium secondary battery 10 is completed.

When charging is performed via the negative electrode mediator, first, the reductant of the solvated negative electrode mediator in the electrolytic solution 50 releases solvated electrons and Li cations on the negative electrode current collector 31. The electrons move to the positive electrode 20 through an external circuit. The Li cation moves to the positive electrode 20 through the separator 40. As the discharge progresses, the concentration of the negative electrode mediator that has released solvated electrons and Li cations increases in the electrolytic solution 50. As a result, the potential of the electrolytic solution 50 rises. When the potential of the electrolytic solution 50 exceeds the equilibrium potential of the compound of lithium and the negative electrode active material 32, Li cations and solvated electrons are supplied to the negative electrode mediator from the compound of lithium and the negative electrode active material 32, and the Li cations are contained again. A complex is formed. The complex containing the Li cation sustains the discharge of the lithium secondary battery 10. When Li cations and solvated electrons are emitted from the compound of lithium and the negative electrode active material 32, and the complex containing Li cations in the electrolytic solution 50 releases Li cations and solvated electrons, the lithium secondary battery 10 The discharge ends.

In the present embodiment, the lithium secondary battery 10 can be charged and discharged by the negative electrode mediator. Therefore, the electrolytic solution 50 does not require a compound that is given to the electrolytic solution 50 by dissolving an equilibrium potential higher than the upper limit potential at which the compound of lithium and the negative electrode active material 32 is formed in the solvent of the electrolytic solution 50 together with lithium. Since only the difference between the reduction potential and the oxidation potential in the negative electrode mediator affects the charge / discharge voltage difference, the charge / discharge voltage difference is basically small. Therefore, in the lithium secondary battery 10 of the present embodiment, it is possible to suppress a decrease in power efficiency during charging and discharging. As a result, the lithium secondary battery 10 has both high energy density and high reliability.

However, the electrolytic solution 50 may contain a plurality of types of negative electrode mediators. The potential of the solution in which lithium is dissolved changes depending on the type of the negative electrode mediator, the concentration of the negative electrode mediator, and the composition of the solvent. For example, when a lithium metal is dissolved in 2MeTHF containing biphenyl or phenanthrene as a negative mediator at a concentration of 0.1 mol / L, the resulting solution exhibits an equilibrium potential of approximately 0 V (vs. Li ^/ Li ⁺ ). On the other hand, when trans-stilbene is used as the negative electrode mediator, the solution exhibits an equilibrium potential of 0.3 V (vs. Li ^/ Li ⁺ ). Using cis-stilbene as the negative electrode mediator, the solution exhibits an equilibrium potential of 0.43 V (vs. Li / Li ⁺ ). That is, when trans-stilbene or cis-stilbene is used as the negative electrode mediator, the solution exhibits a noble equilibrium potential. Therefore, a mediator showing an equilibrium potential lower than the upper limit potential at which the Li alloy is formed (for example, biphenyl or phenanthrene) is used as a charging mediator, and a mediator showing an equilibrium potential higher than the upper limit potential at which the Li alloy is formed is used. (For example, trans-stilbene or cis-stilbene) can be used as the discharge mediator. Since the discharge potential is governed by the potential of the discharge mediator, if the difference between the potential of the discharge mediator and the upper limit potential at which the Li alloy is formed is large, the potential loss tends to be large. In order to suppress the potential loss, the difference between the potential of the discharge mediator and the upper limit potential at which the Li alloy is formed may be 0.2 V or less.

The charge / discharge process of the lithium secondary battery 10 of the present embodiment will be described. The following specific example shows the charge / discharge when the negative electrode active material 32 is separated from the negative electrode current collector 31. When the negative electrode active material 32 is in electrical contact with the negative electrode current collector 31, electrons can be directly transferred between the negative electrode active material 32 and the negative electrode current collector 31.

In this operation example, the negative electrode current collector 31 is made of stainless steel. The electrolytic solution 50 is an ether solution in which the negative electrode mediator is dissolved. The negative electrode mediator is one kind of condensed aromatic compound (hereinafter referred to as Md). The negative electrode active material 32 is aluminum. The positive electrode 20 includes a positive electrode current collector 21 made of stainless steel and a positive electrode active material layer 22 containing lithium iron phosphate (LiFePO ₄ ) as a positive electrode active material.

[Description of charging process]
First, the charging reaction will be described.

Charging is performed by applying a voltage between the positive electrode current collector 21 and the negative electrode current collector 31.

(Reaction on the positive electrode side)
By applying a voltage, an oxidation reaction of the positive electrode active material occurs at the positive electrode 20. That is, lithium ions are released from the positive electrode active material. As a result, electrons are emitted from the positive electrode 20 to the outside of the lithium secondary battery 10.

For example, in this operation example, the following reaction occurs.
^{_{LiFePO 4 → FePO 4 + Li +}} + e -

A part of the generated lithium ions (Li ⁺ ) moves toward the negative electrode 30 through the separator 40.

(Reaction on the negative electrode side)
By applying a voltage, electrons are supplied to the negative electrode current collector 31 from the outside of the lithium secondary battery 10. As a result, the reduction reaction of the negative electrode mediator occurs on the negative electrode current collector 31.

For example, in this operation example, the following reaction occurs.
^{Md + Li + + e - →} Md · Li

The reduced negative electrode mediator is oxidized by the negative electrode active material 32. That is, the negative electrode active material 32 is reduced by the negative electrode mediator. As a result, the negative electrode active material 32 reacts with lithium to become LiAl.

For example, in this operation example, the following reaction occurs.
Al + Md ・ Li → LiAl + Md

As described above, the negative electrode mediator has not changed in the total reaction.

On the other hand, the negative electrode active material 32 located at a location away from the negative electrode current collector 31 is in a charged state.

The above charging reaction can proceed until either the negative electrode active material 32 is in the charged state or the positive electrode active material is in the charged state.

[Description of discharge process]
Next, the discharge reaction from full charge will be described.

When fully charged, the negative electrode active material 32 and the positive electrode active material are in a charged state.

In the discharge reaction, electric power is taken out from between the positive electrode current collector 21 and the negative electrode current collector 31.

(Reaction on the positive electrode side)
By discharging the lithium secondary battery 10, electrons are supplied to the positive electrode 20 from the outside of the lithium secondary battery 10. As a result, in the positive electrode 20, a reduction reaction of the positive electrode active material occurs.

For example, in this operation example, the following reaction occurs.
_{^{FePO 4 + Li + + e -}} → LiFePO 4

A part of the lithium ion (Li ⁺ ) moves toward the positive electrode 20 through the separator 40.

(Reaction on the negative electrode side)
Due to the discharge of the lithium secondary battery 10, an oxidation reaction of the negative electrode mediator occurs on the negative electrode current collector 31. As a result, electrons are emitted from the negative electrode current collector 31 to the outside of the lithium secondary battery 10.

For example, in this operation example, the following reaction occurs.
^{Md · Li → Md + Li +} + e -

The oxidized negative electrode mediator is reduced by the negative electrode active material 32. That is, the negative electrode active material 32 is oxidized by the negative electrode mediator. As a result, the negative electrode active material 32 releases lithium.

For example, in this operation example, the following reaction occurs.
LiAl + Md → Al + Md ・ Li

As described above, the negative electrode mediator has not changed in the total reaction.

On the other hand, the negative electrode active material 32 located at a location away from the negative electrode current collector 31 is in a discharged state.

The above discharge reaction can proceed until either the negative electrode active material 32 is in the discharged state or the positive electrode active material is in the discharged state.

FIG. 2A shows the state of the electrolytic solution 50 when the negative electrode active material 32 is separated from the negative electrode current collector 31. In the lithium secondary battery 10 of the present embodiment, the electrolytic solution 50 containing the negative electrode mediator is in contact with both the positive electrode 20 and the negative electrode 30. The negative electrode mediator receives electrons on the negative electrode current collector 31. A composite of the negative electrode mediator and the Li cation is formed on the negative electrode current collector 31. When the composite of the negative electrode mediator and the Li cation comes into contact with the negative electrode active material 32, the Li cation and electrons are donated from the composite to the negative electrode active material 32.

Since the complex of the negative electrode mediator and the Li cation diffuses in the electrolytic solution 50, the complex also exists on the surface of the positive electrode 20. As shown in FIG. 2B, the concentration of the complex (Md · Li) decreases with distance from the negative electrode current collector. When the concentration of the composite (Md · Li) on the surface of the positive electrode 20 is sufficiently low, the lithium secondary battery 10 of the present embodiment can be repeatedly charged and discharged.

The present disclosure will be specifically described based on examples. However, the present disclosure is not limited to the following examples.

<Experiment 1>
A copper foil having a size of 2 cm × 2 cm was wrapped with a polypropylene microporous separator. The separator was then wrapped in lithium metal foil. Next, tabs were attached to each of the copper foil and the lithium metal foil. Then, the copper foil, the separator and the lithium metal foil were put into the outer case. Next, a 2MeTHF solution containing a condensed aromatic compound at the concentration (mol / L) shown in Table 1 was injected into the case, and the case was sealed. As a result, a cell for potential measurement was obtained.

The potential (V vs. Li ^/ Li ⁺ ) was measured using a lithium metal standard using a potential measurement cell. The results are shown in Table 1. The potentials shown in Table 1 are values 100 hours after the potential measurement cell was prepared.

In the potential measurement cell, a lithium metal solution is formed by dissolving a part of the lithium metal in the solution. The potentials shown in Table 1 are the potentials of the lithium metal solution. Although 2MeTHF was used as the ether in this measurement, other ethers can be used in the same manner.

When the potential shown in Table 1 is equal to or lower than the upper limit potential at which a compound of lithium and the negative electrode active material 32 is formed, the negative electrode active material 32 has an ability to alloy with lithium. A condensed aromatic compound exhibiting such a potential can be used as a negative electrode mediator.

<Experiment 2>
A copper foil having a size of 2 cm × 2 cm was wrapped with a polypropylene microporous separator. The separator was then wrapped in lithium metal foil. Next, tabs were attached to each of the copper foil and the lithium metal foil. Then, the copper foil, the separator and the lithium metal foil were put into the outer case. Next, an ether solution containing biphenyl at a concentration of 0.1 mol / L was injected into the case, and the case was sealed. The solvent of the ether solution contained 2 MeTHF as the cyclic ether and further contained triglime as the grime. The ratio of the volume of triglime to the volume of solvent in the ether solution was 0.2. In other words, the ratio of the volume of 2MeTHF to the volume of triglime was 8: 2. 1 mol / L LiPF ₆ was dissolved as a supporting salt in the ether solution. From the above, the potential measurement cell of Sample 1 was prepared. FIG. 3 shows the potential (V vs. Li ^/ Li ⁺ ) measured on the basis of lithium metal using this potential measurement cell. The value of the potential immediately after preparing the potential measurement cell is about 0.24 Vvs. It was Li / Li ⁺ . The value of the potential 100 hours after the cell for measuring the potential was prepared was about 0.15 Vvs. It was Li / Li ⁺ .

Next, potential measurement cells of Sample 2 to Sample 9 were prepared by the same method as above, except that the ratio of the volume of triglime to the volume of the solvent of the ether solution was changed. In the potential measurement cells of Samples 2 to 9, the ratio of the volume of triglime to the volume of the solvent of the ether solution was 0.3, 0.4, 0.5, 0.6, 0.7, 0, respectively. It was 8, 0.9 and 1.0. For these potential measurement cells, the potential value immediately after the potential measurement cell was produced and the potential value 100 hours after the potential measurement cell was produced were measured. FIG. 3 shows the relationship between the ratio of the volume of triglime to the volume of the solvent of the ether solution and the potential value of the potential measurement cell. In FIG. 3, the broken line indicates the potential value immediately after the potential measurement cell is produced. The solid line shows the potential value 100 hours after the potential measurement cell was produced.

Next, aluminum was added to each of the ether solutions of the potential measurement cells of Samples 1 to 9. The surface of the aluminum was observed 100 hours after the aluminum was charged. As a result, the ratio of the volume of 2MeTHF to the volume of triglime is 8: 2, and the ratio of the aluminum charged into the potential measurement cell of Sample 1 and the volume of 2MeTHF to the volume of triglime is 7: 3. It was confirmed that the aluminum charged into the potential measurement cell of Sample 2 was transformed into a powdery LiAl alloy. It was confirmed that the aluminum charged into the potential measurement cell of Sample 3 in which the ratio of the volume of 2MeTHF to the volume of triglime was 6: 4 had a rough surface only, and a part of the aluminum was changed to a LiAl alloy. It was. This aluminum maintained its shape when placed in an ether solution. No change was confirmed from the aluminum charged into the potential measurement cell of Sample 4 to Sample 9 in which the ratio of the volume of 2MeTHF to the volume of triglime was 5: 5 to 0:10, and a LiAl alloy was formed. There wasn't. From the above results, the potential of the ether solution was 0.18 Vvs. If it is Li / Li ⁺ or less, it can be seen that a LiAl alloy is formed. Such an ether solution can be used as a solvent for an electrolytic solution of a lithium secondary battery.

<Experiment 3>
(Sample 10)
LiFePO ₄ equivalent to 7.8 mAh as the positive electrode active material, Zn foil equivalent to 2.049 mAh as the negative electrode active material, surfaced copper foil as the negative electrode current collector, lithium salt as the electrolytic solution, and a 2MeTH F solution containing biphenyl as the negative electrode mediator. Got ready. Zn foil was overlaid on copper foil as a negative electrode current collector and spot welded at 9 points. Using these materials, a lithium secondary battery of sample 11 was prepared. The concentration of biphenyl in the electrolytic solution was 0.00625 mol / L. The positive electrode was wrapped with a separator made of a microporous membrane. LiPF ₆ was used as the lithium salt. The concentration of LiPF _{6 in} the electrolytic solution was 1 mol / L.

(Sample 11)
A lithium secondary battery of sample 11 was prepared under the same conditions as sample 10 except that the concentration of biphenyl in the electrolytic solution was changed to 0.0125 mol / L.

(Sample 12)
The lithium secondary battery of sample 12 was prepared in the same manner as in sample 10 except that the electrolytic solution did not contain a negative electrode mediator and that an aluminum foil also used as a negative electrode active material was used as the negative electrode current collector. Was produced.

(Sample 13)
The lithium secondary battery of sample 13 was prepared in the same manner as in sample 10 except that the electrolytic solution did not contain a negative electrode mediator and that a Zn foil that was also used as a negative electrode active material was used as the negative electrode current collector. Was produced.

(Sample 14)
A lithium secondary battery of sample 14 was prepared in the same manner as in sample 10 except that the Zn foil was not welded to the negative electrode current collector, but was wrapped in a separator and placed on the negative electrode current collector.

(Charge / discharge test)
The charge / discharge test of the obtained lithium secondary battery was performed. The charge / discharge test was performed under the conditions that the charge / discharge current was 1 mA, the charging time was 2 hours, and the final voltage at the time of discharge was 2 V. The results are shown in FIGS. 4 and 5. FIG. 4 shows the results of charge / discharge tests of the lithium secondary batteries of samples 10, 11 and 13. FIG. 5 shows the results of a charge / discharge test of the lithium secondary battery of sample 14.

The vertical axis of FIGS. 4 and 5 shows the charge / discharge efficiency (unit:%). The charge / discharge efficiency is the ratio (%) of the discharge capacity of each cycle to the initial charge capacity (2 mAh).

As shown in FIG. 4, the charging / discharging efficiency of the lithium secondary batteries of Samples 10 and 11 did not substantially decrease even after 30 times of charging / discharging. The charge / discharge efficiency of the lithium secondary battery of sample 10 was superior to the charge / discharge efficiency of the lithium secondary battery of sample 11. That is, as the concentration of the negative electrode mediator decreased, the charge / discharge efficiency improved. The charging capacity was 2 mAh, which was much larger than the capacity of biphenyl. This means that Zn forms a LiZn alloy and is involved in charging and discharging. It is considered that Zn changed to LiZn alloy and the LiZn alloy was pulverized, but discharge was possible by the action of the negative electrode mediator.

The reason why the charge / discharge efficiency at the initial stage of the charge / discharge cycle is low is that the charged negative electrode mediator comes into contact with the positive electrode and chemically short-circuits. When multiple charge / discharge cycles are performed, Zn is atomized and its surface area is increased, so that the reaction rate between the charged negative electrode mediator and Zn is increased, and the chemical short circuit between the charged negative electrode mediator and the positive electrode is caused. Exceed speed. As a result, the charge / discharge efficiency gradually increases.

Since the chemical short circuit between the charged negative electrode mediator and the positive electrode is not zero, the charge / discharge efficiency did not reach 100% even after repeating the charge / discharge cycle.

In a lithium secondary battery, lithium moves back and forth between the positive and negative electrodes. If lithium is inactivated in the process, the charge capacity is reduced and cannot be restored. In a conventional lithium secondary battery using an alloying material, the loss of electrical contact between the alloying material and the negative electrode current collector can be said to be one type of inactivation.

In the lithium secondary battery of the present embodiment, even if a chemical short circuit occurs, lithium returns to the positive electrode without being inactivated, so that the charge capacity of the lithium secondary battery is restored. Therefore, charging / discharging can be repeated even if the charging / discharging efficiency does not reach 100%.

The results shown in FIG. 4 are considered to be less dependent on the type of negative electrode mediator. This is because the reaction rate depends on the concentration of the negative electrode mediator in the electrolyte.

On the other hand, the discharge capacity of the lithium secondary battery of sample 13 was drastically reduced by performing charging and discharging 30 times. Since the lithium secondary battery of sample 13 did not contain the negative electrode mediator, it is considered that it became difficult to transfer electrons between the pulverized Zn and the negative electrode current collector.

The lithium secondary battery of sample 12 was rechargeable, but could not be discharged. It is considered that as a result of aluminum as the negative electrode active material being pulverized by charging, it became difficult for the negative electrode current collector to collect electrons during discharge.

FIG. 5 shows the charge / discharge cycle characteristics of the lithium secondary battery of sample 14. In sample 14, Zn, which is a negative electrode active material, was previously arranged away from the negative electrode current collector. Comparing the result of sample 10 (FIG. 4) and the result of sample 14 (FIG. 5), the lithium secondary battery of sample 10 showed slightly higher charge / discharge efficiency.

The lithium secondary battery of the present disclosure can be suitably used as, for example, a power storage device or a power storage system.

10 Lithium secondary battery 20 Positive electrode 21 Positive electrode current collector 22 Positive electrode active material layer 30 Negative electrode 31 Negative electrode current collector 32 Negative electrode active material 40 Separator 50 Electrolyte 60 Container

Claims (13)

A positive electrode having a positive electrode current collector and a positive electrode active material layer supported by the positive electrode current collector,
With the negative electrode
An electrolytic solution containing a solvent and a negative electrode mediator dissolved in the solvent and in contact with the positive electrode and the negative electrode.
With
The negative electrode has a negative electrode current collector and a negative electrode active material in contact with the negative electrode current collector.
The negative electrode mediator is a lithium secondary battery that transfers electrons between the negative electrode active material and the negative electrode current collector. The positive electrode active material layer occludes or releases lithium ions,
The lithium secondary battery according to claim 1, wherein the negative electrode active material is alloyed with lithium. The lithium secondary battery according to claim 1 or 2, wherein the negative electrode active material comprises at least one selected from the group consisting of Al, Zn, Si, Sn, Ge, Cd, Pb, Bi and Sb. The lithium secondary battery according to any one of claims 1 to 3, wherein the negative electrode mediator contains a condensed aromatic compound. The negative electrode mediator according to any one of claims 1 to 4, wherein the negative electrode mediator comprises at least one selected from the group consisting of phenanthrene, biphenyl, o-terphenyl, triphenylene, acenaphthene, acenaphthylene, fluoranthene, benzyl and naphthalene. Lithium secondary battery. The lithium secondary battery according to any one of claims 1 to 5, wherein the concentration of the negative electrode mediator in the electrolytic solution is 0.00625 mol / L or more and 0.0125 mol / L or less. The lithium secondary battery according to any one of claims 1 to 6, wherein the electrolytic solution contains ether. The lithium secondary battery according to any one of claims 1 to 6, wherein the electrolytic solution contains at least one selected from the group consisting of cyclic ether, grime and sulfolane. 8. The cyclic ether comprises at least one selected from the group consisting of 2-methyltetrahydrofuran, tetrahydrofuran, 2,5-dimethyltetrahydrofuran, 1,3-dioxolane and 4-methyl-1,3-dioxolane. The lithium secondary battery described. The lithium secondary battery according to claim 8 or 9, wherein the grime comprises at least one selected from the group consisting of monogrime, jiggrime, triglime, tetraglime, and polyethylene glycol dimethyl ether. The lithium secondary battery according to any one of claims 8 to 10, wherein the sulfolane contains 3-methylsulfolane. The lithium secondary battery according to any one of claims 1 to 11, further comprising a separator arranged between the positive electrode and the negative electrode. The lithium secondary battery according to claim 12, wherein the separator is made of a material that allows the passage of lithium ions and the negative electrode mediator.

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