JP4901089B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte and a non-aqueous electrolyte secondary battery using the same, and in particular, non-aqueous electrolysis using ethylene carbonate, propylene carbonate, and γ-butyrolactone simultaneously as a solvent for the non-aqueous electrolyte. The present invention relates to a liquid and a high-capacity nonaqueous electrolyte secondary battery using the same.

  In recent years, mobile information terminals such as mobile phones, notebook personal computers, and PDAs have been rapidly reduced in size and weight, and batteries as drive power sources are required to have higher capacities. Non-aqueous electrolyte secondary batteries that charge and discharge by moving lithium ions between positive and negative electrodes have high energy density and high capacity, so they are widely used as a driving power source for such mobile information terminals. Has been. In recent years, the use of such features has led to the development of not only mobile applications such as mobile phones, but also medium to large battery applications ranging from electric tools, electric vehicles, and hybrid vehicles.

Here, as the non-aqueous electrolyte used in the non-aqueous electrolyte secondary battery, a lithium salt such as LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ is usually dissolved in an aprotic solvent. Examples of the aprotic solvent include carbonates such as ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate, and esters such as γ-butyrolactone and methyl acetate. Ethers such as diethoxyethane are used, and in particular, cyclic carbonate compounds having a high dielectric constant such as ethylene carbonate and propylene carbonate are widely used.

  Since the above-mentioned cyclic carbonate compound having a high dielectric constant has a low freezing point, generally, a low-boiling solvent such as a chain carbonate is mixed in an amount of 50% by volume or more. Problems such as a decrease in dielectric constant and a decrease in flash point occur in the non-aqueous electrolyte. Therefore, especially in recent years when high energy density and large size of the battery are required, it is indispensable to improve the charge / discharge characteristics and reliability of the battery. Cannot meet the demands.

Therefore, in consideration of the above problems, a nonaqueous electrolyte secondary battery using LiBF ₄ as a solute and a nonaqueous electrolytic solution mainly composed of a mixed solvent composed of ethylene carbonate, propylene carbonate, γ-butyrolactone, and vinylene carbonate. Has been proposed (see Patent Document 1 below).

JP 2003-203675 A

  Here, recently, in order to increase the energy density of a battery, a carbon material such as graphite is used as a negative electrode active material to improve the initial efficiency. However, when a nonaqueous electrolytic solution mainly composed of a mixed solvent composed of ethylene carbonate, propylene carbonate, γ-butyrolactone, and vinylene carbonate as described above is used, and a carbon material is used as the negative electrode active material, propylene carbonate or Since γ-butyrolactone is contained, a decomposition reaction of these solvents occurs on the negative electrode during initial charging, and sufficient charge / discharge characteristics cannot be obtained. For this reason, it has a problem that it cannot cope with diversifying uses such as high energy density and large size.

  The present invention has been made in view of the above-described problems, and provides a non-aqueous electrolyte with high energy density and high reliability and a non-aqueous electrolyte secondary battery using the same.

In order to achieve the above object, the invention according to claim 1 of the present invention includes a positive electrode using a lithium-containing transition metal oxide as a positive electrode active material, a negative electrode using a carbon material as a negative electrode active material, and a solute and a solvent. In a non-aqueous electrolyte secondary battery having a non-aqueous electrolyte consisting of
The solute contains a lithium salt having an oxalato complex as an anion and another type of lithium salt, the solvent is mainly composed of ethylene carbonate, propylene carbonate, and γ-butyrolactone, and the lithium having the oxalato complex as an anion. The salt has the chemical formula Li [M (C ₂ O ₄ ) _x R _y ] (wherein M is boron or phosphorus, R is a group selected from a halogen, an alkyl group, and a halogen-substituted alkyl group, and x Is a positive integer, y is 0 or a positive integer), and a lithium salt having the oxalato complex as an anion is contained in an amount of 0.01 to 0.5 mol / liter with respect to the solvent. It is characterized by that.

  As described above, when a lithium salt having an oxalate complex as an anion is contained in a solute of a non-aqueous electrolyte mainly composed of ethylene carbonate, propylene carbonate, and γ-butyrolactone, a non-aqueous electrolyte using the non-aqueous electrolyte is used. The charge / discharge characteristics of the water electrolyte secondary battery can be improved. This is because the lithium salt containing an oxalato complex as an anion is contained in the electrolyte, thereby suppressing the decomposition of propylene carbonate and γ-butyrolactone on the surface of the negative electrode provided with the carbon material, and being stable and permeable to lithium ions. This is thought to be due to the formation of an excellent film. Specifically, it is as follows.

The lithium salt having the oxalato complex as an anion is a lithium salt having an anion in which C ₂ O ₄ ²⁻ is coordinated to a central atom, and the reduction potential of the lithium salt is coordinated with the C ₂ O ₄ ^2−. Therefore, it is considered to be higher than LiPF ₆ , LiBF _{4 and the} like, and further higher than propylene carbonate and γ-butyrolactone. For this reason, with the above configuration, lithium salt having an oxalato complex as an anion is reduced before propylene carbonate or γ-butyrolactone reacts with the negative electrode and decomposes, and is stable on the negative electrode surface and has excellent lithium ion permeability. It is considered that a thick film is formed. As a result, reduction of propylene carbonate and γ-butyrolactone during initial charging can be suppressed, and the charge / discharge efficiency of the non-aqueous electrolyte secondary battery using this non-aqueous electrolyte is improved, resulting in high energy density. Is achieved.

  The reason why ethylene carbonate, propylene carbonate, and γ-butyrolactone are used as the nonaqueous solvent of the nonaqueous electrolyte secondary battery in the present invention is as follows. First, propylene carbonate and ethylene carbonate are used because propylene carbonate and ethylene carbonate have a high dielectric constant and high boiling points of 242 ° C. and 238 ° C., respectively. This is because an electrolyte secondary battery can be provided. In addition, γ-butyrolactone is used as the nonaqueous solvent of the nonaqueous electrolyte secondary battery in the present invention because γ-butyrolactone has a low viscosity and a boiling point as high as 204 ° C. This is because the reliability of the non-aqueous electrolyte secondary battery can be increased without lowering the ionic conductivity and the most suitable solvent as the third butterfly mixed with ethylene carbonate and propylene carbonate.

As described above, a lithium salt having an oxalato complex as an anion is a lithium salt having an anion in which C ₂ O ₄ ²⁻ is coordinated to a central atom, and the chemical formula Li [M (C ₂ O ₄ ) _x R _y However, the present invention is not limited to this.

The invention according to claim 1 is characterized in that the element M in the lithium salt having the oxalato complex as an anion is boron or phosphorus.
The reason for this regulation is that when the element M is boron or phosphorus, a stable film is formed particularly on the negative electrode, so that the charge / discharge characteristics of the nonaqueous electrolyte secondary battery of the present invention are further improved. .

Examples of the element M in the lithium salt having an oxalato complex as an anion are boron or phosphorus, such as lithium-bis (oxalato) borate (Li [B (C ₂ O ₄ ) ₂ ]), lithium-difluoro (oxalato). Borate (Li [B (C ₂ O ₄ ) F ₂ ]), lithium-difluorobis (oxalato) phosphate (Li [P (C ₂ O ₄ ) ₂ F ₂ ]), lithium-tetrafluoro (oxalato) phosphate (Li [P (C ₂ O ₄ ) F ₄ ]) and the like are exemplified, but not limited thereto.

According to a second aspect of the present invention, in the first aspect of the invention, the lithium salt having the oxalato complex as an anion is lithium-bis (oxalato) borate (Li [B (C ₂ O ₄ ) ₂ ]). And
In this way, if lithium-bis (oxalato) borate is used as the lithium salt with an oxalato complex as an anion, it becomes a particularly stable complex in the electrolyte and forms a stable film on the negative electrode in the electrolyte. This is because the charge / discharge characteristics are improved.

The invention according to claim 1 is characterized in that the solute in the non-aqueous electrolyte contains a lithium salt having an oxalato complex as an anion and another type of lithium salt.
If other types of lithium salt are contained in addition to the lithium salt having an oxalato complex as an anion as in the above configuration, it is possible to suppress a decrease in the charge capacity of the present nonaqueous electrolyte secondary battery.

As other types of lithium salts, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (C _l F _{2l + 1} SO ₂ ) (C _m F _{2m + 1} SO ₂ ) (l, m are an integer of 1 or _{more), LiC (C P F 2P} + 1 SO 2) (C q F 2q + 1 SO 2) (C r F 2r + 1 SO 2) (p, q, r is an integer of 1 or more) such as Is exemplified. These solutes may be used alone or in combination of two or more. Moreover, it is preferable that content of these lithium salts is 0.5-1.5 mol / l with respect to a solvent.

The invention according to claim 1 is characterized in that the lithium salt having the oxalato complex as an anion is contained in an amount of 0.01 to 0.5 mol / liter with respect to the solvent.
In this way, the effect of the invention may not be sufficiently obtained when the content of the lithium salt having an oxalato complex as an anion is less than 0.01 mol / liter, whereas 0.5 mol / liter If it exceeds 1, the film formed on the negative electrode surface becomes thick, the reaction resistance increases, and the discharge characteristics deteriorate.

A third aspect of the invention is characterized in that, in the first aspect of the invention, the lithium salt having the oxalato complex as an anion is contained in an amount of 0.1 to 0.4 mol / liter with respect to the solvent.
Thus regulation, operation and effect of claim 1 can be more exerted.

  ADVANTAGE OF THE INVENTION According to this invention, there exists an outstanding effect that the high energy density and the improvement of reliability of a nonaqueous electrolyte and a nonaqueous electrolyte secondary battery using the same can be aimed at.

  Hereinafter, the present invention will be described in more detail. However, the present invention is not limited to the following best modes, and can be appropriately modified and implemented without departing from the scope of the present invention.

(Production of working electrode)
97.5% by mass of artificial graphite powder, 1.5% by mass of styrene butadiene rubber (SBR), and 1% by mass of carboxymethyl cellulose are mixed to form a negative electrode mixture, and the negative electrode mixture is dispersed in water to form a negative electrode slurry. After the production, this negative electrode slurry was applied to a copper foil, and further dried and rolled to produce a working electrode serving as a negative electrode.

[Production of counter electrode]
As a counter electrode, a lithium rolled plate was used.

(Preparation of non-aqueous electrolyte)
Ethylene carbonate (EC), γ-butyrolactone (γ-BL), and propylene carbonate (PC) were mixed at a volume ratio of 20:20:60 to prepare a mixed solvent. Lithium fluorophosphate (LiPF ₆ ) was dissolved at a rate of 0.9 mol / liter, and lithium-bis (oxalato) borate (Li [B (C ₂ O ₄ ) ₂ ]) was dissolved at a rate of 0.1 mol / liter. . 4 parts by mass of vinylene carbonate (VC) and 4 parts by mass of trioctyl phosphate (TOP) were added to 100 parts by mass of this non-aqueous electrolyte to prepare a non-aqueous electrolyte.

[Production of evaluation battery]
As shown in FIG. 1, the working electrode 1 and the counter electrode 2 are immersed in the electrolytic solution 4 filled in a glass three-electrode beaker cell 5 and a reference electrode 3 made of lithium metal. A three-pole evaluation battery was prepared using
The evaluation battery is configured to evaluate the charge / discharge characteristics of the negative electrode and the electrolytic solution according to the present invention. Therefore, when current flows in the direction of electrochemically discharging the working electrode, the working electrode is charged with lithium. Ions are occluded and charged. Further, when a current passes in a direction in which the working electrode is electrochemically charged, lithium ions are released from the working electrode and discharged. Further, this evaluation battery is configured with a large excess of metallic lithium in terms of electric capacity, and the characteristics of this evaluation battery can be evaluated as indicating the characteristics of the negative electrode of the present invention and the electrolytic solution. .

Example 1
As Example 1, the battery shown in the best mode for carrying out the invention was used.
The battery for evaluation thus produced is hereinafter referred to as the present invention battery A1.

(Example 2)
As the solute of the non-aqueous electrolyte, 0.8 mol / liter of lithium hexafluorophosphate (LiPF ₆ ) and 0.2 mol of lithium-bis (oxalato) borate (Li [B (C ₂ O ₄ ) ₂ ]) A battery for evaluation was produced in the same manner as in Example 1 except that dissolution was performed at a ratio of 1 / liter.
The evaluation battery thus produced is hereinafter referred to as the present invention battery A2.

Example 3
As a solute of the non-aqueous electrolyte, 0.7 mol / liter of lithium hexafluorophosphate (LiPF ₆ ) and 0.3 mol of lithium-bis (oxalato) borate (Li [B (C ₂ O ₄ ) ₂ ]) A battery for evaluation was produced in the same manner as in Example 1 except that dissolution was performed at a ratio of 1 / liter.
The evaluation battery thus produced is hereinafter referred to as the present invention battery A3.

Example 4
As the solute of the non-aqueous electrolyte, 0.6 mol / liter of lithium hexafluorophosphate (LiPF ₆ ) and 0.4 mol of lithium-bis (oxalato) borate (Li [B (C ₂ O ₄ ) ₂ ]) An evaluation cell was prepared in the same manner as in Example 1 except that the sample was dissolved at a ratio of 1 / liter.
The evaluation battery thus produced is hereinafter referred to as the present invention battery A4.

(Example 5)
As a solute of the non-aqueous electrolyte, 0.5 mol / liter of lithium hexafluorophosphate (LiPF ₆ ) and 0.5 mol of lithium-bis (oxalato) borate (Li [B (C ₂ O ₄ ) ₂ ]) A battery for evaluation was produced in the same manner as in Example 1 except that dissolution was performed at a ratio of 1 / liter.
The evaluation battery thus produced is hereinafter referred to as the present invention battery A5.

(Comparative Example 1)
A battery for evaluation was produced in the same manner as in Example 1 except that only 1.0 mol / liter of lithium hexafluorophosphate (LiPF ₆ ) was dissolved as the solute of the nonaqueous electrolytic solution.
The evaluation battery thus produced is hereinafter referred to as comparative battery X1.

(Comparative Example 2)
A mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) (volume ratio EC: PC = 20: 80) is used as a solvent for the nonaqueous electrolytic solution, and lithium hexafluorophosphate (LiPF ₆ ) is used as a solute. Was dissolved in the same manner as in Example 1 except that only 1.0 mol / liter was dissolved.
The evaluation battery thus produced is hereinafter referred to as comparative battery X2.

(Comparative Example 3)
A mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) (volume ratio EC: PC = 20: 80) is used as a solvent for the nonaqueous electrolytic solution, and lithium hexafluorophosphate (LiPF ₆ ) is used as a solute. In the same manner as Example 1 except that 0.7 mol / liter and lithium-bis (oxalato) borate (Li [B (C ₂ O ₄ ) ₂ ]) were dissolved at a rate of 0.3 mol / liter. A battery for evaluation was prepared.
The evaluation battery thus produced is hereinafter referred to as comparative battery X3.

(Experiment)
Since the charge / discharge characteristics (initial charge capacity, initial discharge capacity, and initial charge / discharge efficiency) of the negative electrodes in the present invention batteries A1 to A5 and comparative batteries X1 to X3 were measured, the results are shown in Table 1. Specific charging / discharging conditions are as follows.

(Charging / discharging conditions)
Charge and discharge were performed in the order of the following (1) to (4) [In (1) to (3), lithium ions were occluded in the working electrode, and in (4), lithium ions were released from the working electrode] .
(1) Discharge to a discharge end potential of 0 V (vsLi / Li ⁺ ) at a current of 0.5 mA / cm ² (2) Discharge to a discharge end potential of 0 V (vsLi / Li ⁺ ) at a current of 0.25 mA / cm ² (3) at 0.1 mA / cm ² current, discharged up to discharge cutoff potential ^{0V (vsLi / Li +) (} 4) at 0.1 mA / cm ² of current, the charging end potential 1.0 V (vs Li / Li ⁺⁾ Charge up to

As is apparent from Table 1, the batteries A1 to A5 of the present invention have higher charge / discharge efficiency than the comparative battery X1, and in particular, lithium-bis (oxalato) borate (Li [B (C ₂ It is recognized that the charge / discharge efficiency increases as the content of O ₄ ) ₂ ]) increases. This is because the use of a lithium salt having an oxalato complex as an anion as a solute of a non-aqueous electrolysis solution containing ethylene carbonate, propylene carbonate and γ-butyrolactone results in the formation of a high-quality lithium ion permeable film on the negative electrode surface. In particular, it is considered that this effect is exhibited more as the content of lithium-bis (oxalato) borate increases. Specifically, it is as shown below.

That is, in the batteries A1 to A5 of the present invention, lithium-bis (oxalato) borate (Li [B (C ₂ O ₄ ) ₂ ]) having a reduction potential of about 1.6 V to 1.7 V (vsLi / Li ⁺ )). Is used as a solute. Therefore, since lithium-bis (oxalato) borate is reduced before propylene carbonate and γ-butyrolactone having a reduction potential lower than that of the solute is reduced, a high-quality lithium ion permeable coating is formed on the negative electrode surface. It is formed. As a result, since it can suppress that propylene carbonate and (gamma) -butyrolactone are reduce | restored, it is thought that it became the above experimental result.

  On the other hand, it is recognized that the batteries A1 to A5 of the present invention have a smaller charge capacity than the comparative battery X1, and the charge capacity decreases as the content of lithium-bis (oxalato) borate increases. It is recognized that

From the above, from the viewpoint of maximizing the discharge capacity, the lithium-bis (oxalato) borate content is 0.3 mol / liter (lithium-bis (oxalato) in the solute as in the present invention battery A3. It is understood that it is most desirable that the ratio of borate is about 30%).

In addition, the battery X2 for comparative evaluation which does not contain lithium-bis (oxalato) borate (Li [B (C ₂ O ₄ ) ₂ ]) and γ-butyrolactone cannot be charged / discharged. Further, in the comparative evaluation battery X3 not containing γ-butyrolactone, similarly to the battery A3 of the present invention, the lithium-bis (oxalato) borate contained 0.3 mol / liter with respect to the nonaqueous electrolytic solution. Therefore, charging / discharging was impossible. This shows that the non-aqueous electrolyte containing lithium-bis (oxalato) borate according to the present invention exhibits its effect by containing γ-butyrolactone in a solvent composed of ethylene carbonate and propylene carbonate.

  From the above, in a non-aqueous electrolyte secondary battery having a positive electrode made of a lithium-containing transition metal oxide, a negative electrode containing a carbon material, and a non-aqueous electrolyte solution made of a solute and a solvent, the solute has an oxalato complex as an anion. If a lithium salt [for example, the above lithium-bis (oxalato) borate] is contained and the solvent is mainly composed of ethylene carbonate, propylene carbonate, or γ-butyrolactone, it has high energy density and excellent reliability. In addition, a non-aqueous electrolyte secondary battery can be provided.

[Other matters]
(1) In the above examples, an evaluation battery was prepared to evaluate the electrolytic solution. However, the structure of the battery is not limited to such a structure of the evaluation battery, and is cylindrical, rectangular, flat or flat. The present invention can be applied to non-aqueous electrolyte secondary batteries having various shapes.
The non-aqueous electrolyte secondary battery includes a positive electrode active material, a negative electrode active material, a non-aqueous electrolyte, a separator, a battery case, and a battery component such as a current collector that holds the active material and also collects current It is comprised.

(2) The negative electrode active material is not limited to the above graphite, and other carbon materials such as graphite and coke can be used.
(3) The positive electrode active material is not limited to the above lithium metal, but includes lithium such as lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), and lithium manganese oxide (LiMn ₂ O ₄ ). Transition metal oxides can be used. When such a positive electrode material is used, these materials are kneaded with a conductive agent such as acetylene black and carbon black, and a binder such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF). Thus, a positive electrode can be produced by using a mixture.

(4) As the nonaqueous solvent of the nonaqueous electrolyte, three kinds of mixed solvents of ethylene carbonate, propylene carbonate and γ-butyrolactone were used. Cyclic carbonate having a C═C unsaturated bond in such a mixed solvent. More preferably, at least one ester compound is contained. In this case, a cyclic carbonate having a C═C unsaturated bond is particularly preferred. Examples of such cyclic carbonates include vinylene carbonate, 4,5-dimethylvinylene carbonate, 4,5-diethylvinylene carbonate, 4 , 5-dipropyl vinylene carbonate, 4-ethyl-5-methyl vinylene carbonate, 4-ethyl-5-propyl vinylene carbonate, 4-methyl-5-propyl vinylene carbonate, 4-ethyl-5-methyl vinylene carbonate, vinyl ethylene Examples include carbonate and divinylethylene carbonate. Among these, vinylene carbonate and vinyl ethylene carbonate are more preferable because they form a good film on the negative electrode and can suppress the decomposition of propylene carbonate and γ-butyrolactone.

  In addition, the ratio of the cyclic ester compound which has a C = C unsaturated compound in a non-aqueous electrolyte is 1-15 mass parts with respect to electrolyte solution, More preferably, it is 2-10 mass parts. This is because if the content is too small, the effect of improving the charge / discharge characteristics may not be sufficiently obtained, whereas if the content is too large, the coating formed on the negative electrode surface becomes thick and the reaction resistance of the negative electrode increases. This is because the charge / discharge characteristics may be deteriorated.

  Moreover, in order to lower the viscosity of the nonaqueous electrolytic solution, a low boiling point solvent may be mixed. In this case, the proportion of the low boiling point solvent is preferably 30% by volume or less, and particularly preferably 15% by volume or less. Examples of the low boiling point solvent include chain carbonates such as dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate.

  Furthermore, the volume ratio of propylene carbonate to the total volume of the solvent is desirably 10% by volume or more and 70% by volume or less. This is because if the volume ratio of propylene carbonate exceeds 70% by volume, the solvent tends to decompose on the negative electrode surface and the charge capacity decreases, whereas if the volume ratio of propylene carbonate is less than 10% by volume, non-aqueous electrolysis This is because the dielectric constant of the liquid is lowered. In consideration of this, the volume ratio of propylene carbonate to the total volume of the solvent is particularly preferably 20% by volume or more and 60% by volume or less. Although the ratio of ethylene carbonate is not regulated, when the volume ratio of ethylene carbonate increases, the viscosity of the non-aqueous electrolyte increases, and the decrease in liquid content and the decrease in ionic conductivity increase. The volume ratio of ethylene carbonate to the total volume of the solvent is preferably 30% by volume or less.

(5) In the present invention, in order to improve the wettability to the separator, it is preferable to add a surfactant such as trioctyl phosphate or ester to the non-aqueous electrolyte. In this case, the addition amount of the surfactant is preferably about 0.5 to 5 parts by mass with respect to 100 parts by mass of the non-aqueous electrolyte.

(6) In the manufacturing process of the nonaqueous electrolyte secondary battery according to the present invention, it is preferable that the first charge after the electrolyte injection is performed at a current value of 10 hour rate (0.1 It) or less. This is because when the initial charging current is larger than 10 hours, the formation of a film by the cyclic carbonate compound having a C═C unsaturated bond is not uniformly performed, and the decomposition of propylene carbonate and γ-butyrolactone is promoted, and the discharge characteristics. This is because of a decrease. In addition, regarding this initial charging, it is preferable to perform a capacity of 10% or more of the battery capacity at a current value of 10 hours or less, and thereafter, charging can be performed at a current density greater than 10 hours.

  The present invention can be applied not only to a driving power source of a mobile information terminal such as a mobile phone, a notebook computer, and a PDA, but also to a large battery such as an in-vehicle power source of an electric vehicle or a hybrid vehicle.

It is explanatory drawing which shows the structure of the battery for evaluation.

Explanation of symbols

1 Working Electrode 2 Counter Electrode 3 Reference Electrode 4 Electrolyte 5 Tripolar Beaker Cell

Claims (3)

In a non-aqueous electrolyte secondary battery having a positive electrode using a lithium-containing transition metal oxide as a positive electrode active material, a negative electrode using a carbon material as a negative electrode active material, and a non-aqueous electrolyte composed of a solute and a solvent,
The solute contains a lithium salt having an oxalato complex as an anion and another type of lithium salt, the solvent is mainly composed of ethylene carbonate, propylene carbonate, and γ-butyrolactone, and the lithium having the oxalato complex as an anion. The salt has the chemical formula Li [M (C ₂ O ₄ ) _x R _y ] (wherein M is boron or phosphorus, R is a group selected from a halogen, an alkyl group, and a halogen-substituted alkyl group, and x Is a positive integer, y is 0 or a positive integer), and a lithium salt having the oxalato complex as an anion is contained in an amount of 0.01 to 0.5 mol / liter with respect to the solvent. A non-aqueous electrolyte secondary battery . The nonaqueous electrolyte secondary battery according to claim 1, wherein the lithium salt having the oxalato complex as an anion is lithium-bis (oxalato) borate (Li [B (C ₂ O ₄ ) ₂ ]). The nonaqueous electrolyte secondary battery according to claim 1, wherein a lithium salt having the oxalato complex as an anion is contained in an amount of 0.1 to 0.4 mol / liter with respect to the solvent.