JP7322684B2 - lithium ion secondary battery - Google Patents

Description

The present invention relates to lithium ion secondary batteries.

Lithium ion secondary batteries are also widely used as power sources for mobile devices such as mobile phones and laptop computers, and hybrid cars. With the development of these fields, lithium ion secondary batteries are required to have higher performance.

One of the performances is to increase the capacity of the lithium-ion secondary battery. The theoretical capacity of graphite, which is a typical negative electrode active material, is about 360 mAh/g. In recent years, attention has been focused on alloy-based negative electrode active materials such as silicon and silicon oxide as materials that can replace graphite. The theoretical capacity of silicon is said to be about 3600 mAh/g, which is expected to be nearly ten times that of graphite.

However, even if an alloy-based negative electrode active material such as silicon or silicon oxide is used, it is difficult for a lithium ion secondary battery to obtain a sufficient capacity close to the theoretical value. One of the reasons for this is that a lithium-containing layer is formed on the surface of the negative electrode active material layer during the initial charge. Formation of the lithium-containing layer is an irreversible reaction, and lithium trapped in the lithium-containing layer cannot contribute to subsequent charging and discharging.

In recent years, a technique has been proposed in which lithium is pre-doped to form the lithium-containing layer. For example, Patent Literature 1 discloses a technique of pre-doping by including lithium particles in a negative electrode active material layer. Further, for example, Patent Document 2 describes that a positive electrode material having a large initial charge capacity and an irreversible capacity is used as a pre-dope material. Non-Patent Document 1 also describes that the use of Li ₂ Co ₃ as a pre-dopant increases the initial charge capacity and the irreversible capacity.

JP 2008-98151 A JP 2016-12620 A

Lijuan Fan et al., Nano Research 2016, 9(12):3903-3913.

The technique of using a pre-dopant for alloy-based negative electrodes has not yet been fully established. When a pre-doping material is used in a lithium ion secondary battery using an alloy negative electrode, sufficient initial efficiency cannot be obtained.

The present disclosure has been made in view of the above problems, and an object thereof is to provide a lithium-ion secondary battery capable of improving the initial efficiency of the alloy-based negative electrode.

In order to solve the above problems, the following means are provided.

(1) A lithium ion secondary battery according to a first aspect has a positive electrode, a negative electrode, a separator sandwiched between the positive electrode and the negative electrode, and an electrolyte solution, and the positive electrode is pre-doped material, the pre-doping material includes a composite of a lithium compound that decomposes at a voltage of 4.5 V (vs Li/Li ⁺ ) or higher and a carbon material, and the negative electrode includes silicon or silicon oxide, The electrolytic solution contains fluoroethylene carbonate and vinylene carbonate, and the ratio X of the fluoroethylene carbonate in the electrolytic solution and the ratio Y of the vinylene carbonate in the electrolytic solution are 0.001≦Y/X≦0. .01.

(2) In the lithium ion secondary battery according to the above aspect, the lithium compound may be lithium carbonate.

The lithium-ion secondary battery according to the aspect described above has improved initial efficiency.

1 is a schematic diagram of a lithium ion secondary battery according to a first embodiment; FIG.

Hereinafter, embodiments will be described in detail with appropriate reference to the drawings. In the drawings used in the following description, characteristic portions may be enlarged for convenience in order to make the characteristics easier to understand, and the dimensional ratios and the like of each component may differ from the actual. The materials, dimensions, and the like exemplified in the following description are examples, and the present invention is not limited to them, and can be implemented with appropriate modifications without changing the gist of the invention.

"Lithium-ion secondary battery"
FIG. 1 is a schematic diagram of a lithium ion secondary battery according to a first embodiment. A lithium-ion secondary battery 100 shown in FIG. 1 includes a power generation element 40, an exterior body 50, and an electrolytic solution (not shown). The exterior body 50 covers the periphery of the power generation element 40 . The power generation element 40 is connected to the outside by a pair of connected terminals 60 and 62 . The electrolytic solution is contained in the exterior body 50 and impregnates the power generation element 40 .

(power generation element)
The power generation element 40 includes a positive electrode 20 , a negative electrode 30 and a separator 10 .

<Separator>
Separator 10 is sandwiched between positive electrode 20 and negative electrode 30 . The separator 10 separates the positive electrode 20 and the negative electrode 30 and prevents short circuit between the positive electrode 20 and the negative electrode 30 . Lithium ions can pass through the separator 10 .

The separator 10 has, for example, an electrically insulating porous structure. The separator 10 is, for example, a monolayer of a film made of polyolefin such as polyethylene or polypropylene, a stretched film of a laminate or a mixture of the above resins, or selected from the group consisting of cellulose, polyester, polyacrylonitrile, polyamide, polyethylene and polypropylene. A fibrous nonwoven fabric made of at least one constituent material can be mentioned.

Separator 10 may be, for example, a solid electrolyte. Solid electrolytes are polymer solid electrolytes, oxide-based solid electrolytes, and sulfide-based solid electrolytes, for example. The polymer solid electrolyte is, for example, a polyethylene oxide polymer dissolved in an alkali metal salt. Examples of oxide-based solid electrolytes include Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ (Nasicon type), Li _1.07 Al _0.69 Ti _1.46 (PO ₄ ) ₃ (glass _{ceramics )} , _{Li0.34La0.51TiO2.94} ( _{perovskite type ) , Li7La3Zr2O12} (garnet type), _{Li2.9PO3.3N0.46} ( _amorphous , LIPON) , 50Li ₄ SiO ₄ .50Li ₂ BO ₃ (glass), and 90Li ₃ BO ₃ .10Li ₂ SO ₄ (glass ceramics). Sulfide-based solid electrolytes include, for example, Li _3.25 Ge _0.25 P _0.75 S ₄ (crystal), Li ₁₀ GeP ₂ S ₁₂ (crystal, LGPS), Li ₆ PS ₅ Cl (crystal, aldirodite type) , Li _9.54 Si _1.74 P _1.44 S _11.7 Cl _0.3 (crystal), Li _3.25 P _0.95 S ₄ (glass ceramics), Li ₇ P ₃ S ₁₁ (glass ceramics) , 70Li ₂ S.30P ₂ S ₅ (glass), 30Li ₂ S.26B _{2 S} 3.44LiI (glass), 50Li ₂ S.17P ₂ S _{5.33LiBH 4} (glass), 63Li ₂ S.36SiS _{2.Li 3PO4 (} glass), _{57Li2S.38SiS2.5Li4SiO4} ( _{glass ) .}

<Positive electrode>
The positive electrode 20 has a positive electrode current collector 22 and a positive electrode active material layer 24 . The positive electrode active material layer 24 is formed on at least one surface of the positive electrode current collector 22 . The positive electrode active material layer 24 may be formed on both sides of the positive electrode current collector 22 .

[Positive collector]
The positive electrode current collector 22 is, for example, a conductive plate. The positive electrode current collector 22 is, for example, a metal thin plate made of aluminum, copper, nickel, titanium, stainless steel, or the like.

[Positive electrode active material layer]
The positive electrode active material layer 24 has, for example, a positive electrode active material, a conductive aid, a pre-dopant, and a binder.

The positive electrode active material causes reversible absorption and release of lithium ions, desorption and insertion (intercalation) of lithium ions, or doping and dedoping of lithium ions and counter anions.

Examples of positive electrode active materials include lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate (LiMnO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), and general formula: LiNi _x Co _y Mn _z M _a2 (x+y+z+a=1, 0≤x<1, 0≤y<1, 0≤z<1, 0≤a<1, M is selected from Al, Mg, Nb, Ti, Cu, Zn, Cr one or more elements), lithium vanadium compound (LiV ₂ O ₅ ), olivine-type LiMPO ₄ (where M is Co, Ni, Mn, Fe, Mg, Nb, Ti, _one or more _elements selected from Al _and Zr _, or VO), lithium titanate ( _Li4Ti5O12 ) _{, LiNixCoyAlzO2} (0.9<x+y+z<1.1), etc. Examples include composite metal oxides, polyacetylene, polyaniline, polypyrrole, polythiophene, and polyacene. Moreover, what mixed these may be sufficient as a positive electrode active material.

The conductive aids are scattered in the positive electrode active material layer 24 . The conductive aid enhances the conductivity between the positive electrode active materials in the positive electrode active material layer 24 . Examples of conductive aids include carbon powders such as carbon blacks, carbon nanotubes, carbon materials, metal fine powders such as copper, nickel, stainless steel and iron, mixtures of carbon materials and metal fine powders, and conductive oxides such as ITO. . A carbon material such as carbon black is preferable as the conductive aid. If the active material alone can ensure sufficient conductivity, the positive electrode active material layer 24 does not need to contain a conductive aid.

The pre-dopant decomposes during charging to produce lithium ions. The pre-dopant supplements lithium ions that are trapped in the negative electrode and cannot be used reversibly. The pre-dopant is scattered in the positive electrode active material layer 24 .

The pre-dopant is a composite in which a lithium compound and a carbon material are combined before the initial charge/discharge. A lithium compound decomposes at a voltage of 4.5 V (vs Li/Li ⁺ ) or more.

vsLi/Li ⁺ indicates that the equilibrium potential of Li is used as a reference. Also, "decompose" is not limited to the complete completion of decomposition, and may be at least partially decomposed. Also, "composite" means a state of being physically attached and integrated. For example, a state in which a lithium compound and a carbon material are entangled with each other and cannot be separated is an example of a composite state. Examples of lithium compounds that decompose at a voltage of 4.5 V (vs Li/Li ⁺ ) or higher are Li ₂ Co ₃ , Li ₂ O, CH ₃ CO ₂ Li, LiOOCCOOLi, and LiI.

The pre-dopant is in two states after the initial charge/discharge. The first state is a state in which only the carbon material remains after the lithium compound is decomposed. The first state is a state in which the lithium compound is removed from the pre-dopant, leaving only the skeleton of the carbon material. For example, it is a state in which voids are included in a skeleton made of a carbon material. The skeleton made of the carbon material has, for example, a cage shape, an egg shell shape, or the like. The second state is a composite state in which at least part of the lithium compound remains without being decomposed, and the lithium compound and the carbon material are composited. When all of the lithium compound decomposes, the second state is not reached, but some of the lithium compound remains, so the complex also remains. That is, the lithium compound is detected even from the lithium ion secondary battery after charging and discharging.

The binder binds together the positive electrode active materials 25 in the positive electrode active material layer 24 . A known binder can be used. The binder is, for example, fluororesin. Fluororesins include, for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF), and the like.

In addition to the above, binders include, for example, vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based fluororubber), vinylidene fluoride-pentafluoropropylene fluororubber (VDF-PFP fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene fluororubber (VDF-PFP-TFE fluororubber), vinylidene fluoride Vinylidene fluoride-based fluorine such as Ride-perfluoromethyl vinyl ether-tetrafluoroethylene-based fluororubber (VDF-PFMVE-TFE-based fluororubber), vinylidene fluoride-chlorotrifluoroethylene-based fluororubber (VDF-CTFE-based fluororubber) Rubber may be used.

<Negative Electrode>
The negative electrode 30 has, for example, a negative electrode current collector 32 and a negative electrode active material layer 34 . The negative electrode active material layer 34 is formed on at least one surface of the negative electrode current collector 32 .

[Negative electrode current collector]
The negative electrode current collector 32 is, for example, a conductive plate. The negative electrode current collector 32 can be the same as the positive electrode current collector 22 .

[Negative electrode active material layer]
The negative electrode active material layer 34 contains a negative electrode active material. Moreover, a conductive aid and a binder may be included as necessary.

A negative electrode active material is a material that causes an alloying reaction with lithium. The negative electrode active material contains silicon or silicon oxide. Silicon or silicon oxide absorbs lithium ions by being alloyed with lithium. This reversible reaction also releases lithium ions. The negative electrode 30 is sometimes called a Si negative electrode.

As the conductive aid and binder, the same materials as those used for the positive electrode 20 can be used. The binder in the negative electrode 30 may be, for example, cellulose, styrene/butadiene rubber, ethylene/propylene rubber, polyimide resin, polyamideimide resin, acrylic resin, etc., in addition to those listed for the positive electrode 20 . The cellulose may be, for example, carboxymethylcellulose (CMC).

(Terminal)
Terminals 60 and 62 are connected to positive electrode 20 and negative electrode 30, respectively. A terminal 60 connected to the positive electrode 20 is a positive terminal, and a terminal 62 connected to the negative electrode 30 is a negative terminal. Terminals 60 and 62 are responsible for electrical connection with the outside. Terminals 60, 62 are made of a conductive material such as aluminum, nickel, or copper. The connection method may be welding or screwing. Terminals 60, 62 are preferably protected with insulating tape to prevent short circuits.

(Exterior body)
The exterior body 50 seals the power generation element 40 and the electrolytic solution therein. The exterior body 50 prevents the leakage of the electrolytic solution to the outside and the intrusion of moisture into the inside of the lithium ion secondary battery 100 from the outside.

The exterior body 50 has a metal foil 52 and a resin layer 54 laminated on each surface of the metal foil 52, as shown in FIG. 1, for example. The exterior body 50 is a metal laminate film in which a metal foil 52 is coated from both sides with polymer films (resin layers 54).

For example, aluminum foil can be used as the metal foil 52 . A polymer film such as polypropylene can be used for the resin layer 54 . The material forming the resin layer 54 may be different between the inner side and the outer side. For example, a polymer with a high melting point such as polyethylene terephthalate (PET) or polyamide (PA) is used as the outer material, and polyethylene (PE) or polypropylene (PP) is used as the inner polymer film material. be able to.

(Electrolyte)
The electrolytic solution is enclosed in the exterior body 50 and impregnates the power generating element 40 . The electrolytic solution is a non-aqueous electrolytic solution, and has, for example, a non-aqueous solvent and an electrolyte. The electrolyte is dissolved in a non-aqueous solvent.

Non-aqueous solvents include, for example, cyclic carbonates and chain carbonates. Cyclic carbonates solvate electrolytes. Cyclic carbonates are, for example, ethylene carbonate, propylene carbonate and butylene carbonate. Chain carbonates reduce the viscosity of cyclic carbonates. Chain carbonates are, for example, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate. Non-aqueous solvents may also include methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, and the like. .

The volume ratio of the cyclic carbonate and the chain carbonate in the non-aqueous solvent is, for example, 1:9 to 1:1.

The electrolyte is, for _example , _{LiPF6 , LiClO4} , _LiBF4 , _LiCF3SO3 , _{LiCF3CF2SO3 ,} LiC( _{CF3SO2 ) 3 ,} LiN( _CF3SO2 ) ₂ , _LiN ( _CF3CF2 SO ₂ ) ₂ , LiN(CF ₃ SO ₂ )(C ₄ F ₉ SO ₂ ), LiN(CF ₃ CF ₂ CO) ₂ , LiBOB and the like.

The electrolyte contains fluoroethylene carbonate and vinylene carbonate. Although the details will be described later, fluoroethylene carbonate and vinylene carbonate inhibit decomposition of the electrolytic solution on the surface of the pre-doping material.

The ratio of fluoroethylene carbonate in the electrolytic solution and the ratio of vinylene carbonate in the electrolytic solution satisfy 0.001≦Y/X≦0.01. Here, X is the proportion of fluoroethylene carbonate in the electrolyte and Y is the proportion of vinylene carbonate in the electrolyte.

The proportion X of fluoroethylene carbonate in the electrolytic solution is, for example, 20 wt % or less, and may be 5 wt % or more and 20 wt % or less. The proportion Y of vinylene carbonate in the electrolytic solution is, for example, 0.05 wt % or less, and may be 0.005 wt % or more and 0.5 wt % or less.

"Manufacturing method of lithium ion secondary battery"
First, a pre-dope material is produced. A pre-dope material is obtained by compounding a lithium compound and a carbon material. For example, when a lithium compound and a carbon material are mixed while applying mechanical energy, the lithium compound and the carbon material are combined. Mechanical energy is, for example, impact, compression, shear, shear stress, friction, and the like. In the compounding step, the lithium compound and the carbon material may be mixed while being pulverized by mechanical energy. Mixing using a ball mill and mechanochemical treatment are examples of compounding treatment. The mechanochemical treatment can be performed using, for example, Nobilta manufactured by Hosokawa Micron Corporation.

Next, the positive electrode 20 is produced. The positive electrode 20 is prepared by mixing a positive electrode active material, a pre-dopant, a conductive aid, a binder, and a solvent to prepare a pasty positive electrode slurry. The mixing method of these components constituting the positive electrode slurry is not particularly limited, and the mixing order is also not particularly limited. Next, the positive electrode slurry is applied to the positive electrode current collector 22 . The coating method is not particularly limited. For example, a slit die coating method and a doctor blade method can be used.

Subsequently, the solvent in the positive electrode slurry applied on the positive electrode current collector 22 is removed. A removal method is not particularly limited. For example, the positive electrode current collector 22 coated with the positive electrode slurry is dried in an atmosphere of 80.degree. C. to 150.degree. Then, the positive electrode 20 is obtained by pressing the obtained coating film to increase the density of the positive electrode active material layer 24 . As a means of pressing, for example, a roll press machine, a hydrostatic press machine, or the like can be used.

Next, the negative electrode 30 is produced. The negative electrode 30 can be produced in the same manner as the positive electrode 20 . The negative electrode 30 is produced by mixing a negative electrode active material, a binder and a solvent to prepare a pasty negative electrode slurry. The negative electrode 30 is obtained by applying the negative electrode slurry to the negative electrode current collector 32 and drying it.

Next, the positive electrode 20 and the negative electrode 30 are laminated so that the separator 10 is positioned between them to produce the power generation element 40 . When the power generating element 40 is a wound body, the positive electrode 20, the negative electrode 30, and the separator 10 are wound around one end side of the separator.

Finally, the power generation element 40 is enclosed in the exterior body 50 . The electrolytic solution is injected into the exterior body 50 . After injecting the electrolytic solution, the power generation element 40 is impregnated with the electrolytic solution by depressurizing, heating, or the like. The lithium ion secondary battery 100 is obtained by applying heat or the like to seal the exterior body 50 .

Then, the produced lithium ion secondary battery 100 is aged. Aging is a step of precharging the manufactured lithium ion secondary battery, and can remove defective products. Aging is performed, for example, at a voltage of 4.5 V (vs Li/Li ⁺ ) or higher.

During aging, some of the lithium ions are trapped in the negative electrode. Lithium ions trapped in the negative electrode cannot be used in subsequent reactions. Lithium ions trapped in the negative electrode cause irreversible capacity of the lithium ion secondary battery 100 . Irreversible capacity is the difference between discharge capacity and charge capacity. Lithium ions corresponding to the irreversible capacity are trapped in the negative electrode, thereby stabilizing the subsequent charge-discharge reaction. On the other hand, when lithium ions are trapped in the negative electrode, the amount of lithium ions that contribute to charging and discharging decreases. The reduced amount of lithium ions is replenished by decomposition of the lithium compound contained in the pre-dopant.

The initial efficiency of the lithium ion secondary battery 100 according to the first embodiment is improved. The initial efficiency is the discharge efficiency of the lithium-ion secondary battery 100 with respect to the initial charge. The reason for this will be explained below.

As described above, sufficient decomposition of the lithium compound contained in the pre-dopant ensures the amount of lithium ions that contribute to charging and discharging. On the other hand, if the decomposition of the lithium compound contained in the pre-dopant is inhibited, a sufficient amount of lithium ions that contribute to charging and discharging cannot be secured, resulting in deterioration of initial characteristics.

For example, a lithium ion secondary battery in which the positive electrode contains a pre-dopant and the electrolytic solution does not contain fluoroethylene carbonate and vinylene carbonate cannot obtain sufficient initial characteristics. This example is hereinafter referred to as a comparative example. The inventors considered that the reason for this is that a thick film is formed on the surface of the pre-dope material due to the decomposition of the electrolytic solution. When the lithium ion secondary battery according to the comparative example is charged at a high voltage (for example, a voltage of 4.5 V (vs Li/Li ⁺ ) or higher), the electrolyte is decomposed on the surface of the pre-dope material. The decomposed electrolyte forms a thick film on the surface of the pre-dopant. A thick coating inhibits the decomposition reaction of lithium compounds. When the decomposition of the lithium compound is inhibited, a sufficient amount of lithium ions that contribute to charging and discharging cannot be secured, resulting in deterioration of initial characteristics.

In contrast, the lithium ion secondary battery according to the first embodiment contains fluoroethylene carbonate and vinylene carbonate in the electrolyte. Fluoroethylene carbonate and vinylene carbonate inhibit the decomposition reaction of the electrolytic solution.

When fluoroethylene carbonate is added to the electrolytic solution, a film composed of LiF and an organic lithium compound is formed on the surface of the pre-dopant. This coating inhibits the decomposition of the electrolyte on the surface of the pre-dopant. This coating is also thin and contains lithium, unlike the thick coating that results from decomposition of the electrolyte. Therefore, the decomposition reaction of the lithium compound is not significantly hindered, and the movement of lithium ions generated by the decomposition of the lithium compound is not significantly hindered.

Also, when vinylene carbonate is added to the electrolytic solution, the ratio of the organolithium compound contained in the film increases. As the proportion of the organolithium compound increases, the coverage of the coating increases and the thickness of the coating decreases. When the coverage of the film increases, the decomposition reaction of the electrolytic solution on the surface of the pre-dopant is further suppressed. As a result, the decomposition reaction of the pre-dopant proceeds, the pre-doping of the negative electrode proceeds sufficiently, and the initial efficiency of the lithium ion secondary battery 100 improves.

As described above, the embodiments of the present invention have been described in detail with reference to the drawings. , substitutions, and other modifications are possible.

"Example 1"
(Preparation of positive electrode)
First, a pre-dope material was produced. 80 parts by mass of Li ₂ Co ₃ and 20 parts by mass of carbon black were prepared and mixed in a ball mill for 1 hour. A pre-dopant in which Li ₂ Co ₃ and carbon black were combined was obtained by mixing with a ball mill.

Next, 60 parts by mass of the positive electrode active material, 20 parts by mass of the pre-dopant, 10 parts by mass of the conductive aid, and 10 parts by mass of the binder were mixed to prepare a positive electrode mixture. Lithium cobaltate (LiCoO ₂ ) was used as the positive electrode active material, carbon black was used as the conductive agent, and polyvinylidene fluoride (PVDF) was used as the binder. This positive electrode mixture was dispersed in N-methyl-2-pyrrolidone to prepare a positive electrode slurry. Then, the slurry was applied to one surface of an aluminum foil having a thickness of 20 μm so that the amount of the positive electrode active material applied was 9.0 mg/cm ² . After the coating, it was dried at 100° C. to remove the solvent and form a positive electrode active material layer.

(Preparation of negative electrode)
A negative electrode mixture was prepared by mixing a negative electrode active material, a conductive material, and a binder. Silicon was used as the negative electrode active material, carbon black was used as the conductive material, and carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) were used as the binder. The mass ratio of the negative electrode active material, the conductive material, and the binder was 90:5:5. This negative electrode mixture was dispersed in distilled water to prepare a negative electrode slurry. Then, the negative electrode slurry was applied to one surface of a copper foil having a thickness of 10 μm. After coating, the coating was dried at 100° C. to remove the solvent and form a negative electrode active material layer.

(Production of cells)
The produced negative electrode and positive electrode were punched into a predetermined shape, and laminated alternately via a polypropylene separator having a thickness of 25 μm to produce a laminate by laminating 9 negative electrodes and 8 positive electrodes.

An opening was formed by inserting the laminate into an outer package made of an aluminum laminate film and heat-sealing the laminate except for one peripheral portion. A non-aqueous electrolyte was injected into the exterior body. The non-aqueous electrolyte was obtained by dissolving 1.0 mol/L of lithium hexafluorophosphate (LiPF ₆ ) in a solvent in which equal amounts of ethylene carbonate (EC) and dimethyl carbonate (DEC) were mixed. Further, 5 wt % of fluoroethylene carbonate and 0.01 wt % of vinylene carbonate were added to the non-aqueous electrolyte. The ratio Y/X of fluoroethylene carbonate and vinylene carbonate in the electrolytic solution was 0.002. Then, the remaining one portion was heat-sealed while the pressure was reduced by a vacuum sealer, and a lithium ion secondary battery according to Example 1 was produced.

(Evaluation of initial efficiency)
The initial efficiency of the fabricated lithium-ion battery was evaluated. Using a charge/discharge test device (manufactured by Hokuto Denko Co., Ltd.), the battery was charged in a constant temperature bath (manufactured by Espec Co., Ltd.) at 25 ° C. by constant current charging at a current density of 80 μA / cm ² until the battery voltage reached 4.6 V. After that, the battery was discharged at a constant current density of 80 μA/cm ² until the battery voltage reached 2.8V. The initial efficiency was calculated according to "initial efficiency=initial discharge capacity/initial charge capacity×100", and the obtained values are shown in Table 1.

"Examples 2 and 3"
Examples 2 and 3 differ from Example 1 in that the amount of vinylene carbonate added to the electrolytic solution was changed.
In Example 2, the amount of vinylene carbonate added to the electrolytic solution was 0.005 wt%. The ratio Y/X of fluoroethylene carbonate and vinylene carbonate in the electrolytic solution of Example 2 was 0.001.
In Example 3, the amount of vinylene carbonate added to the electrolytic solution was 0.05 wt %. The ratio Y/X of fluoroethylene carbonate and vinylene carbonate in the electrolytic solution of Example 3 was 0.01. Examples 2 and 3 were also evaluated for initial efficiency in the same manner as in Example 1.

"Examples 4-6"
Examples 4 to 6 differ from Example 1 in that the ratio Y/X of the ratio of fluoroethylene carbonate to vinylene carbonate in the electrolytic solution was kept constant, and the amount of fluoroethylene carbonate and vinylene carbonate to be added was changed.
In Example 4, the amount of fluoroethylene carbonate added to the electrolytic solution was 3 wt %, and the amount of vinylene carbonate was 0.006 wt %.
In Example 5, the amount of fluoroethylene carbonate added to the electrolytic solution was 10 wt %, and the amount of vinylene carbonate added was 0.02 wt %.
In Example 6, the amount of fluoroethylene carbonate added to the electrolytic solution was 20 wt %, and the amount of vinylene carbonate added was 0.04 wt %.
Examples 4 to 6 were also evaluated for initial efficiency in the same manner as in Example 1.

"Example 7"
Example 7 differs from Example 1 in that the type of lithium compound contained in the pre-dopant was changed.
In Example 7, Li ₂ O was used as the lithium compound contained in the pre-dopant.
In Example 7, the initial efficiency was evaluated in the same manner as in Example 1.

"Examples 8-10"
Examples 8 to 10 differ from Example 1 in that the positive electrode active material is a mixed system of lithium cobaltate (LiCoO ₂ ) and other positive electrode active material. The total mass of the positive electrode active material in the positive electrode active material layer was kept at 60 parts by mass, and the breakdown of the positive electrode active material was 40 parts by mass of lithium cobaltate (LiCoO ₂ ) and 20 parts by mass of other positive electrode active material. .
Example 8 used LiNi _0.80 Co _0.10 Mn _0.10 O ₂ (NCM) as another positive electrode active material.
Example 9 used LiNi _0.80 Co _0.15 Al _0.05 O ₂ (NCA) as another positive electrode active material.
Example 10 used LiFePO ₄ (LFP) as another positive electrode active material.
Examples 8 to 10 were also evaluated for initial efficiency in the same manner as in Example 1.

"Comparative Example 1"
Comparative Example 1 differs from Example 1 in that fluoroethylene carbonate and vinylene carbonate were not added to the electrolytic solution. Comparative Example 1 was also evaluated for initial efficiency in the same manner as in Example 1.

"Comparative Example 2"
Comparative Example 2 differs from Example 1 in that only fluoroethylene carbonate was added to the electrolytic solution and vinylene carbonate was not added. In Comparative Example 2, the amount of fluoroethylene carbonate added to the electrolytic solution was 5 wt %. Comparative Example 2 was also evaluated for initial efficiency in the same manner as in Example 1.

The results of Examples 1 to 10 and Comparative Examples 1 and 2 are summarized in Table 1 below.

10 Separator 20 Positive electrode 22 Positive electrode current collector 24 Positive electrode active material layer 30 Negative electrode 32 Negative electrode current collector 34 Negative electrode active material layer 40 Power generating element 50 Exterior 52 Metal foil 54 Resin layers 60, 62 Terminal 100 Lithium ion secondary battery

Claims (2)

a positive electrode, a negative electrode, a separator sandwiched between the positive electrode and the negative electrode, and an electrolytic solution;
The positive electrode contains a pre-dopant,
The pre-dopant includes a composite of a lithium compound that decomposes at a voltage of 4.5 V (vs Li/Li ⁺ ) or higher and a carbon material,
the negative electrode comprises silicon or silicon oxide;
The electrolytic solution contains fluoroethylene carbonate and vinylene carbonate,
The proportion X of the fluoroethylene carbonate in the electrolytic solution is 5 wt% or more and 20 wt% or less,
The ratio Y of the vinylene carbonate in the electrolytic solution is 0.006 wt% or more and 0.2 wt% or less,
A lithium ion secondary battery, wherein a ratio X of the fluoroethylene carbonate in the electrolytic solution and a ratio Y of the vinylene carbonate in the electrolytic solution satisfy 0.001≦Y/X≦0.01. 2. The lithium ion secondary battery according to claim 1, wherein said lithium compound is lithium carbonate.

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