JP2016085837A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery having a high initial discharge capacity and a satisfactory discharge rate characteristic even in the case of a positive electrode including, as a positive electrode active material, LiMPO(where 0.8≤x≤1.1; M represents at least one of Co, Ni, Mn, Fe, V and VO).SOLUTION: A lithium ion secondary battery of the present invention comprises: a positive electrode; a negative electrode; a separator; and a nonaqueous electrolytic solution having a nonaqueous solvent and an electrolyte. The positive electrode includes, as a positive electrode active material, LiMPO(where 0.8≤x≤1.1; M represents at least one of Co, Ni, Mn, Fe, V and VO). The nonaqueous electrolytic solution includes, as the electrolyte, 0.05-0.2 mol/L of LiBF, and 0.9-1.5 mol/L of LiPF.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a lithium ion secondary battery.

  Lithium ion secondary batteries are lighter and have higher capacity than nickel cadmium batteries and nickel metal hydride batteries, and thus are widely applied as power sources for portable electronic devices. In addition, it has begun to be used as a power source mounted for hybrid vehicles and electric vehicles. In recent years, the miniaturization and high functionality of portable electronic devices have progressed, and in order to improve the mileage of electric vehicles and the like, further increase in capacity of lithium ion secondary batteries serving as these power sources is expected. ing.

At the same time, a lithium ion battery having higher safety than ever has been demanded, and a lithium metal composite oxide represented by LiMPO ₄ (M is a transition metal, etc.) having high thermal stability is used as a positive electrode active material. It is increasingly used as a substance. A representative example is LiFePO ₄ , and LiMnPO ₄ that operates at a higher voltage has been actively developed. (For example, Patent Documents 1 and 2 and Non-Patent Document 1)

One of the problems common to the LiMPO ₄ is that the resistance is high and it is difficult to exhibit the inherent capacity. For this, the surface of the positive electrode active material is covered with a carbon material conductive auxiliary agent to impart conductivity. (For example, Patent Documents 3 and 4) In many cases, it has been confirmed that the initial discharge capacity is increased by this method.

JP-A-9-134725 US Pat. No. 5,910,382 Canadian Patent No. 2270771 Japanese translation of PCT publication No. 2004-509058

A. K. Padhi, K .; S. Nanjundaswami, and J.M. B. Goodenough, Journal of electrochemical society, vol. 144, 1188 (1997)

However, even if the surface of the positive electrode active material is covered with a conductive additive, the capacity of LiMPO ₄ is still insufficient. Furthermore, the movement of lithium ions is hindered by being covered with a conductive additive, and the discharge capacity tends to decrease as the current value during discharge increases.

The present invention has been made in view of the above problems, and Li _x MPO ₄ (where 0.8 ≦ x ≦ 1.1, where M is Co, Ni, Mn, Fe, Even in the case of including at least one of V and VO, an object is to provide a lithium ion secondary battery having a large initial discharge capacity and good discharge rate characteristics.

In order to solve the above problems, a lithium ion secondary battery according to the present invention includes a positive electrode, a negative electrode, a separator, and a nonaqueous electrolytic solution having a nonaqueous solvent and an electrolyte, and the positive electrode has a positive electrode active material. Li _x MPO ₄ (where 0.8 ≦ x ≦ 1.1, and M represents at least one of Co, Ni, Mn, Fe, V, and VO), and the non-aqueous electrolyte solution Is characterized by containing LiBF ₄ having a content of 0.05 to 0.2 mol / L and LiPF ₆ having a content of 0.9 to 1.5 mol / L as an electrolyte.

According to such a configuration, LiBF ₄ works selectively near the surface of the positive electrode active material, and the diffusion coefficient of lithium ions is greatly increased. Therefore, it is speculated that the initial discharge capacity can be sufficiently exhibited even when the positive electrode active material contains Li _x MPO ₄ (.

If the concentration of LiPF ₆ contained in the non-aqueous electrolyte is 0.9 mol / L or more, the ion conductivity is high, and a sufficient initial discharge capacity can be obtained even if the positive electrode active material is Li _x MPO _4. Can do. On the other hand, when the concentration of LiPF ₆ is 1.5 mol / L or less, the viscosity of the non-aqueous electrolyte does not become too high, and good discharge rate characteristics can be obtained. When the concentration of LiBF ₄ contained in the non-aqueous electrolyte is 0.05 mol / L or more, the diffusion coefficient of lithium ions in the non-aqueous electrolyte greatly increases, and the positive electrode active material is Li _x MPO _4. In addition, sufficient initial discharge capacity and discharge rate characteristics can be obtained. On the other hand, when the concentration of LiBF ₄ is 0.2 mol / L or less, good initial discharge capacity and discharge rate characteristics can be obtained without a decrease in the ionic conductivity of the non-aqueous electrolyte.

The positive electrode active material is preferably LiFePO ₄ . In addition to the material properties of LiFePO _4, the proper initial discharge capacity can be exhibited by including appropriate amounts of LiPF ₆ and LiBF ₄ in the non-aqueous electrolyte.

It is also preferable that the positive electrode active material is β-LiVOPO ₄ . Since β-LiVOPO ₄ has a different crystal structure from other Li _x MPO ₄ and is more stable, the effect of using a non-aqueous electrolyte containing appropriate amounts of LiPF ₆ and LiBF ₄ will be more prominent.

Since a large discharge capacity can be obtained even when the cell volume of the lithium ion secondary battery is the same, the positive electrode preferably further contains a layered lithium compound as a positive electrode active material. When a layered lithium compound such as LiCoO ₂ or Li (Ni, Mn, Co) O ₂ is included in addition to Li _x MPO ₄ , lithium ions are selectively released from the layered lithium compound due to the difference in conductivity, and Li The initial discharge capacity of _x MPO ₄ is not sufficiently exhibited. However, by including appropriate amounts of LiPF ₆ and LiBF ₄ in the non-aqueous electrolyte, lithium ions can be sufficiently extracted from Li _x MPO ₄ and a good initial discharge capacity can be exhibited.

The non-aqueous electrolyte preferably further contains 1,3,2-dioxathiolane-2,2-dioxide represented by the formula (1). Thereby, the impedance of the lithium ion secondary battery can be lowered, and better discharge rate characteristics can be obtained.

According to the present invention, Li _x MPO ₄ (provided that 0.8 ≦ x ≦ 1.1 and M is at least one of Co, Ni, Mn, Fe, V, and VO) 2), it is possible to provide a lithium ion secondary battery having a large initial discharge capacity and good discharge rate characteristics.

It is a schematic cross section of a lithium ion secondary battery.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. However, the non-aqueous electrolyte for lithium ion secondary batteries and the lithium ion secondary battery according to the present invention are not limited to the following embodiments. In addition, the dimensional ratio of drawing is not restricted to the ratio of illustration.

(Lithium ion secondary battery)
Next, the electrode and the lithium ion secondary battery according to this embodiment will be briefly described with reference to FIG.

  The lithium ion secondary battery 100 mainly includes a power storage element 30, a case 50 that houses the power storage element 30 in a sealed state, and a pair of leads 60 and 62 connected to the power storage element 30.

  The power storage element 30 is configured such that a pair of electrodes 10 and 20 are disposed to face each other with the separator 18 interposed therebetween. The positive electrode 10 is a product in which a positive electrode active material layer 14 is provided on a positive electrode current collector 12. The negative electrode 20 is a product in which a negative electrode active material layer 24 is provided on a negative electrode current collector 22. The positive electrode active material layer 14 and the negative electrode active material layer 24 are in contact with both sides of the separator 18. An electrolyte solution is contained inside the positive electrode active material layer 14, the negative electrode active material layer 24, and the separator 18. Leads 60 and 62 are connected to the end portions of the positive electrode current collector 12 and the negative electrode current collector 22, respectively, and the end portions of the leads 60 and 62 extend to the outside of the case 50.

(Negative electrode)
The negative electrode 20 includes a negative electrode active material layer 24 on both sides of a negative electrode current collector 22. Furthermore, the negative electrode active material layer 24 is formed by applying a paint containing a negative electrode active material, a conductive additive, and a binder to the negative electrode current collector 22.

The negative electrode active material contains at least one selected from carbon materials such as natural graphite, artificial graphite (non-graphitizable carbon, graphitizable carbon, low temperature calcined carbon, etc.), MCF (mesocarbon fiber), and the like. Among these, artificial graphite is preferable because it exhibits a favorable negative electrode capacity and cycle characteristics, and from the viewpoint of improving electrode density, it is more preferable to use artificial graphite mixed with natural graphite.
In addition, for example, a metal capable of forming a compound with lithium such as Al, Si and Sn, an amorphous compound mainly composed of an oxide such as SiO ₂ and SnO ₂ , lithium titanate (Li ₄ Ti ₅ O _A known negative electrode active material such as ₁₂ ) may be mixed with a carbon material.

  The conductive auxiliary agent is not particularly limited, and a known conductive auxiliary agent can be used. Examples thereof include carbon materials such as pyrolytic carbon such as carbon black, cokes, glassy carbons, organic polymer compound fired materials, carbon fibers, and activated carbon. Moreover, you may add negative electrode active material materials, such as non-graphitizable carbon, graphitizable carbon, and graphite, changing a shape.

  As carbon black, acetylene black, ketjen black and the like are particularly preferable, and ketjen black is particularly preferable. By including an electron-conductive porous body, pores can be formed at the interface between the particles of the negative electrode active material and the binder, and the penetration of the non-aqueous electrolyte into the negative electrode active material layer 24 is facilitated by the pores. Therefore, it is preferable.

  The binder is not particularly limited as long as it can bind the negative electrode active material particles and the conductive auxiliary particles. For example, fluororesins such as polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and the like can be given. In addition, the binder contributes not only to the binding of the negative electrode active material particles and the conductive additive particles but also to the negative electrode current collector 22.

  As the negative electrode current collector 22, various known metal foils used for current collectors for lithium ion secondary batteries can be used. Specifically, it is preferable to use a copper foil.

(Positive electrode)
The positive electrode 10 includes a positive electrode active material layer 14 on both surfaces of a positive electrode current collector 12. Furthermore, the positive electrode active material layer 14 is preferably formed by applying a paint containing a positive electrode active material, a conductive additive, and a binder to the positive electrode current collector 12.

The positive electrode active material includes insertion and extraction of lithium ions, desorption and insertion of lithium ions (intercalation), or doping and dedoping of lithium ions and a counter anion (for example, ClO ₄ ⁻ ) of the lithium ions. Li _x MPO ₄ capable of proceeding reversibly (where 0.8 ≦ x ≦ 1.1, and M represents at least one of Co, Ni, Mn, Fe, V, and VO) including. The surface of these positive electrode active materials is preferably covered with a carbon material conductive additive.

Specifically, the positive electrode active material is preferably LiFePO ₄ . LiFePO ₄ has a small calorific value and contributes to improving the safety of the lithium ion secondary battery.

As the positive electrode active material, β-LiVOPO ₄ is also preferable. Like LiFePO ₄ , it contributes to safety improvement and can be operated at a high voltage of 3.7 V average voltage.

It is preferable that the positive electrode active material further contains a layered lithium compound. Examples of the layered lithium compound include lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), general formula: LiNi _x Co _y Mn _z O ₂ (x + y + z = 1) and Li _a composite metal oxide represented by _a Ni _x Co _y Al _1-xy O ₂ (0.98 <a <1.2, 0 <x, y <1).

  Furthermore, the same material as that used in the negative electrode 20 can be used as each constituent element (conductive auxiliary agent and binder) other than the positive electrode active material. Therefore, the binder contained in the positive electrode 10 contributes not only to the binding between the particles of the positive electrode active material and the particles of the conductive auxiliary agent, but also to the binding to the positive electrode current collector 12. Yes.

  As the positive electrode current collector 12, various known metal foils used in current collectors for lithium ion secondary batteries can be used. Specifically, it is preferable to use an aluminum foil.

(Separator)
As long as the separator 18 is formed of an insulating porous material, the material, the manufacturing method, and the like are not particularly limited, and a known separator used in the lithium ion secondary battery 100 can be used. For example, as the insulating porous material, a known polyolefin resin, specifically, a crystalline homopolymer or copolymer obtained by polymerizing polyethylene, polypropylene, 1-butene, 4-methyl-1-pentene, 1-hexene, or the like. A polymer is mentioned. These homopolymers or copolymers can be used alone or in combination of two or more. Further, it may be a single layer or a multilayer.

(Nonaqueous electrolyte)
The non-aqueous electrolyte has a non-aqueous solvent and an electrolyte, and includes 0.05 to 0.2 mol / L LiBF ₄ and 0.9 to 1.5 mol / L LiPF _{6 as} the electrolyte.

Although the mechanism of the effect expression by including LiBF ₄ and LiPF ₆ simultaneously is not clear, the present inventors consider as follows.

In the case where Li _x MPO _{4 is included} as the positive electrode active material (provided that 0.8 ≦ x ≦ 1.1, and M represents at least one of Co, Ni, Mn, Fe, V, and VO), Li By covering the periphery of _x MPO ₄ with a conductive auxiliary agent, the diffusion of lithium ions in the vicinity of the electrode is prevented, the lithium ion conductivity of the non-aqueous electrolyte is lowered, and the discharge capacity of the lithium ion secondary battery, etc. The characteristic of was deteriorated. Therefore, by simultaneously including LiBF ₄ and LiPF ₆ as the electrolyte, lithium ions can be easily diffused in the vicinity of the electrodes while maintaining a state in which lithium ions easily migrate between the positive electrode and the negative electrode. As a result, it is estimated that the lithium ion conductivity of the non-aqueous electrolyte is improved and the discharge capacity and discharge rate characteristics of the lithium ion secondary battery are improved.

LiBF ₄ is contained in the non-aqueous electrolyte at 0.05 to 0.2 mol / L. When the concentration of LiBF ₄ contained in the non-aqueous electrolyte is 0.05 mol / L or more, the diffusion coefficient of lithium ions in the non-aqueous electrolyte near the surface of the positive electrode active material greatly increases, and the positive electrode active material Even if Li _x MPO ₄ is used, sufficient initial discharge capacity and discharge rate characteristics can be obtained. On the other hand, if the concentration of LiBF ₄ is 0.2 mol / L or less, lithium ions are likely to migrate between the positive electrode and the negative electrode, and the lithium ion conductivity of the non-aqueous electrolyte is maintained high. Good initial discharge capacity and discharge rate characteristics can be obtained.

Since a large initial discharge capacity can be obtained, LiBF ₄ is preferably contained in the nonaqueous electrolytic solution in an amount of 0.08 to 0.17 mol / L. Furthermore, since the discharge rate characteristics are also greatly improved, it is more preferable that the content is 0.1 to 0.15 mol / L.

LiPF ₆ is contained in the non-aqueous electrolyte at 0.9 to 1.5 mol / L. If the concentration of LiPF ₆ contained in the non-aqueous electrolyte is 0.9 mol / L or more, the ion conductivity is high, and a sufficient initial discharge capacity can be obtained even if the positive electrode active material is Li _x MPO _4. Can do. On the other hand, when the concentration of LiPF ₆ is 1.5 mol / L or less, the viscosity of the non-aqueous electrolyte does not become too high, and the mobility of lithium ions can be sufficiently secured. Can be obtained.

Since a large initial discharge capacity can be obtained, LiPF ₆ is preferably contained in the non-aqueous electrolyte at 1.0 to 1.3 mol / L. Furthermore, since the discharge rate characteristics are also greatly improved, it is more preferable that 1.0 to 1.2 mol / L is contained.

Other electrolytes include LiClO ₄ , LiPOF ₂ , LiAsF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CF ₂ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ CF ₂ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiN (CF ₃ CF ₂ CO) _{2, and the} like may be used, but an appropriate amount thereof may be mixed.

  The non-aqueous solvent preferably contains a cyclic carbonate and a low viscosity solvent. The cyclic carbonate refers to an organic solvent having a dielectric constant of 20 or more so as to promote dissociation of the lithium salt that is an electrolyte, and the low viscosity solvent is an organic solvent having a viscosity of 1.0 cP or less so as to improve the mobility of lithium ions. .

  As the cyclic carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, and the like can be used. Among them, ethylene carbonate is preferably included. You may mix and use ethylene carbonate with propylene carbonate and butylene carbonate.

  In addition, as a low viscosity solvent, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, 1,2-dimethoxyethane, 1,2-diethoxyethane, etc. may be used. it can. In particular, it is preferable to contain diethyl carbonate from the viewpoint of improving high-temperature storage characteristics. Diethyl carbonate may be used by mixing with other low viscosity solvents.

  The ratio of the cyclic carbonate and the low viscosity solvent in the non-aqueous solvent is preferably 1: 9 to 1: 1 by volume.

The non-aqueous electrolyte preferably further contains 1,3,2-dioxathiolane-2,2-dioxide represented by the formula (1). By adding 1,3,2-dioxathiolane-2,2-dioxide, the interface resistance between the positive electrode and the non-aqueous electrolyte is lowered, and in particular, the discharge rate characteristics are improved.

  Since 1,3,2-dioxathiolane-2,2-dioxide improves the discharge rate characteristics without reducing the initial discharge capacity, it is preferably contained in the non-aqueous electrolyte at 1 to 6% by weight.

  In addition, a carbonate compound having an unsaturated bond such as vinylene carbonate and a sulfur-containing compound such as 1,3-propane sultone may be further added.

(Exterior body)
The case 50 may be made of any material as long as it can seal the power generation element 30 and the electrolyte solution therein.

  The leads 60 and 62 are made of a conductive material such as aluminum or nickel.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated in more detail, this invention is not limited to these Examples at all.

  The non-aqueous electrolyte for lithium ion secondary batteries and lithium ion secondary batteries of Examples 1 to 59 and Comparative Examples 1 to 10 were prepared and evaluated by the following procedure.

Example 1
First, a negative electrode was produced. In the production of the negative electrode, artificial graphite (90% by mass) as a negative electrode active material, carbon black (2% by mass) as a conductive auxiliary agent, and polyvinylidene fluoride (PVDF) (8% by mass) as a binder are mixed. Was dispersed in N-methyl-2-pyrrolidone to obtain a slurry. The obtained slurry was applied to an electrolytic copper foil as a current collector by a doctor blade method and dried at 110 ° C. After drying, rolling was performed to obtain a negative electrode.

Next, a positive electrode was produced. Also in the production of the positive electrode, LiFePO ₄ (90% by mass) as the positive electrode active material, carbon black (6% by mass) as the conductive auxiliary, and PVDF (4% by mass) as the binder are mixed and dispersed in NMP. A slurry was obtained. The obtained slurry was applied to an aluminum foil as a current collector, dried, rolled, and a positive electrode was obtained.

Next, a non-aqueous electrolyte was prepared. LiBF ₄ was added at a ratio of 0.05 mol / L and LiPF ₆ was added at a ratio of 1.0 mol / L to a solution in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 3: 7 to obtain a nonaqueous electrolytic solution.

  A laminated body (element body) was obtained by laminating a separator made of polyethylene between the obtained negative electrode and positive electrode. The obtained laminate is placed in an aluminum laminate pack, a non-aqueous electrolyte is injected into the aluminum laminate pack, and then vacuum-sealed to produce a lithium ion secondary battery (length: 60 mm, width: 85 mm, thickness: 3 mm). did.

(Examples 2-56 and Comparative Examples 1-10)
Table shows the type of positive electrode active material, the combination of solvents in the non-aqueous electrolyte, LiBF ₄ and LiPF ₆ contents, and the amount of 1,3,2-dioxathiolane-2,2-dioxide added to the non-aqueous electrolyte Except having changed as shown to 1-3, it carried out similarly to Example 1, and produced the lithium ion secondary battery of Examples 2-56 and Comparative Examples 1-10. In Tables 1 to 3, “LFP” of the positive electrode active material is LiFePO ₄ , “β-LVP” is β-LiVOPO ₄ , and “NCM” is LiNi _1/3 Co _1/3 Mn _1/3 O _2. “NCA” represents LiNi _0.8 Co _0.15 Al _0.05 O ₂ . When the positive electrode active material is a mixture of a layered lithium compound such as “NCM / LFP” and LiMPO ₄ , the weight ratio is 1/1. In the nonaqueous solvent, “EC” represents ethylene carbonate, “DEC” represents diethyl carbonate, “EMC” represents ethyl methyl carbonate, and “PC” represents propylene carbonate. “DTD content” represents the content of 1,3,2-dioxathiolane-2,2-dioxide in the non-aqueous electrolyte.

(Initial discharge capacity evaluation test)
After the production of the lithium ion secondary battery, the first charge was performed in an environment set at 25 ° C. in a thermostatic bath, and the first discharge was performed immediately after that. The charging was performed at a constant current and constant voltage up to 4.2 V at 30 mA, and the discharging was performed at a constant current of 2.5 mA at 30 mA. The measured value of the discharge capacity (mAh) in the first discharge is shown as “initial discharge capacity (mAh)” in Tables 1 to 3.

(Discharge rate evaluation test)
After the discharge capacity evaluation test, constant current and constant voltage charging was performed up to 4.2 V at 30 mA in an environment set to 25 ° C. in a thermostatic bath. The battery was fully charged and allowed to stand for 10 minutes, and then a constant current discharge was performed up to 2.5 V at 300 mA. A value obtained by dividing the discharge capacity measured here by the initial discharge capacity and multiplying by 100 is shown as “2C discharge capacity (%)” in Tables 1 to 3.

(Examples 1 to 7 and Comparative Examples 1 and 2)
First, Table 1 shows the evaluation results when the concentration contained in the nonaqueous electrolyte solution of LiPF ₆ is fixed at 1.0 mol / L and the concentration contained in the nonaqueous electrolyte solution of LiBF ₄ is changed. . The positive electrode active material was LiFePO ₄ , the negative electrode active material was artificial graphite, and the components other than the content of LiBF ₄ were fixed and compared in the configuration shown in Table 1.

In the initial discharge capacity, the comparative example is less than 140 mAh, whereas the lithium ion secondary battery of the example in which the content of LiBF ₄ is 0.05 to 0.2 mol / L is 140 mAh or more. It was confirmed that it had an initial discharge capacity. In particular, when the content of LiBF ₄ was 0.08 to 0.17 mol / L, the initial discharge capacity was 150 mAh or more, indicating a good value.

In the 2C discharge capacity, the comparative example is less than 75%, whereas the lithium ion secondary battery of the example in which the content of LiBF ₄ is 0.05 to 0.2 mol / L is 75% or more. Was confirmed to have a sufficient 2C discharge capacity. In particular, when the content of LiBF ₄ was 0.1 to 0.15 mol / L, the 2C discharge capacity was 85% or more, indicating a good value.

(Examples 8 to 13 and Comparative Examples 3 and 4)
Next, Table 2 shows the evaluation results when the concentration contained in the nonaqueous electrolyte solution of LiBF ₄ was fixed at 0.1 mol / L and the concentration contained in the nonaqueous electrolyte solution of LiPF ₆ was changed. Show. The positive electrode active material was LiFePO ₄ , the negative electrode active material was artificial graphite, and the components other than the LiPF ₆ content were fixed and compared in the configuration shown in Table 2.

In the initial discharge capacity, the comparative example is less than 140 mAh, whereas the lithium ion secondary battery of the example in which the content of LiPF ₆ is 0.9 to 1.5 mol / L is 140 mAh or more, both of which are sufficient. It was confirmed that it had an initial discharge capacity. In particular, when the content of LiPF ₆ was 1.0 to 1.3 mol / L, the initial discharge capacity was 150 mAh or more, indicating a good value.

In the 2C discharge capacity, the comparative example is less than 75%, whereas the lithium ion secondary battery of the example in which the content of LiPF ₆ is 0.9 to 1.5 mol / L is 75% or more. Was confirmed to have a sufficient 2C discharge capacity. In particular, when the content of LiPF ₆ was 1.0 to 1.2 mol / L, the 2C discharge capacity was 85% or more, indicating a good value.

(Examples 14 to 56 and Comparative Examples 5 to 10)
Table 3 shows the types of positive electrode active materials, combinations of nonaqueous solvents, 1,3,2-dioxathiolane-2,2-dioxide content, and LiBF ₄ and LiPF ₆ contents. Went.

In the initial discharge capacity, the comparative example is less than 140 mAh, whereas the content of LiBF ₄ is 0.05 to 0.2 mol / L and the content of LiPF ₆ is 0.9 to 1.5 mol / L. The lithium ion secondary batteries of the examples were 140 mAh or more, and it was confirmed that all had sufficient initial discharge capacity. In particular, when the content of LiBF ₄ is 0.08 to 0.17 mol / L and the content of LiPF ₆ is 1.0 to 1.3 mol / L, the initial discharge capacity is 150 mAh or more, which is favorable. The value is shown.

In particular, when the positive electrode active material is β-LiVOPO ₄ , it shows a larger initial discharge capacity than LiFePO _4, and the positive electrode active material is LiNi _1/3 Co _1/3 Mn _1/3 O ₂ or LiNi. _In the case of a mixture of a layered lithium compound of _0.8 Co _0.15 Al _0.05 O ₂ and LiFePO ₄ or β-LiVOPO ₄ , a larger initial discharge capacity was exhibited.

In the 2C discharge capacity, the comparative example is less than 75%, whereas the content of LiBF ₄ is 0.05 to 0.2 mol / L and the content of LiPF ₆ is 0.9 to 1.5 mol. The lithium ion secondary battery of the example of / L was 75% or more, and it was confirmed that all had sufficient 2C discharge capacity. In particular, when the content of LiBF ₄ is 0.1 to 0.15 mol / L and the content of LiPF ₆ is 1.0 to 1.2 mol / L, the 2C discharge capacity is 85% or more, which is good Showed a good value.

  In particular, when 1,3,2-dioxathiolane-2,2-dioxide was included, a tendency for the 2C discharge capacity to increase was confirmed.

(Examples 57 to 59)
Next, in order to confirm the characteristics when silicon oxide was included in the negative electrode, the negative electrode was produced. Silicon oxide (18% by weight) and artificial graphite (65% by weight) as the negative electrode active material, carbon black (5% by weight) as the conductive additive, and polyimide (12% by weight) as the binder are mixed, and the solvent N -Disperse in methyl-2-pyrrolidone to obtain a slurry. The obtained slurry was applied to an electrolytic copper foil as a current collector by a doctor blade method and dried at 110 ° C. Furthermore, in order to harden a polyimide, it heated to about 320 degreeC in the vacuum, rolled after cooling, and obtained the negative electrode.

A combination of the solvent as shown in Table 4 and a nonaqueous electrolyte solution containing LiBF ₄ and LiPF ₆ were added to the laminate using the negative electrode thus obtained and the LiFePO ₄ positive electrode. Examples 57 to 59 A lithium ion secondary battery was prepared and each measurement was performed.

  In Examples 57 to 59, the initial discharge capacity was 140 mAh or more, and the 2C discharge capacity was 75% or more, and it was confirmed that all had sufficient initial discharge capacity and 2C discharge capacity. That is, even when silicon oxide was included in the negative electrode, the same tendency as when only graphite was included was obtained.

  DESCRIPTION OF SYMBOLS 10 ... Positive electrode, 20 ... Negative electrode, 12 ... Positive electrode collector, 14 ... Positive electrode active material layer, 18 ... Separator, 22 ... Negative electrode collector, 24 ... Negative electrode Active material layer, 30 ... electricity storage element, 50 ... case, 60, 62 ... lead, 100 ... lithium ion secondary battery

Claims (5)

A positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte solution having a nonaqueous solvent and an electrolyte,
The positive electrode contains Li _x MPO ₄ (provided that 0.8 ≦ x ≦ 1.1, and M represents at least one of Co, Ni, Mn, Fe, V, and VO) as a positive electrode active material. ,
The non-aqueous electrolyte contains LiBF ₄ having a content of 0.05 to 0.2 mol / L and LiPF ₆ having a content of 0.9 to 1.5 mol / L as electrolytes. Secondary battery. The lithium ion secondary battery according to claim 1, wherein the positive electrode active material is LiFePO ₄ . The lithium ion secondary battery according to claim 1, wherein the positive electrode active material is β-LiVOPO ₄ .   The lithium ion secondary battery according to any one of claims 1 to 3, wherein the positive electrode further contains a layered lithium compound as a positive electrode active material. The lithium according to claim 1, further comprising 1,3,2-dioxathiolane-2,2-dioxide represented by the formula (1) as the non-aqueous electrolyte. Ion secondary battery.

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