JP2014146517A - Nonaqueous electrolyte for nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte for a nonaqueous electrolyte secondary battery having excellent high temperature durability and rate characteristics.SOLUTION: Disclosed is a nonaqueous electrolyte used for a nonaqueous secondary battery. The nonaqueous electrolyte contains a fluorine-containing phosphorous acid diester. The fluorine-containing phosphorous acid diester is represented by general formula (1) and Rf1 and Rf2 are independently a C1-C3 alkyl group, and at least one of Rf1 and Rf2 contains fluorine.

Description

  The present invention relates to a nonaqueous electrolyte for a nonaqueous electrolyte secondary battery and a nonaqueous electrolyte secondary battery using the same.

  In a non-aqueous electrolyte secondary battery, it is known that a non-aqueous electrolyte that is a liquid non-aqueous electrolyte is reduced and decomposed at the interface with the negative electrode. The vaporized component generated by the reductive decomposition of the non-aqueous electrolyte and the coating on the negative electrode surface cause a decrease in battery performance. Therefore, by adding an additive to the non-aqueous electrolyte, a film accompanying the reductive decomposition of the additive is formed on the negative electrode surface before the non-aqueous electrolyte is reductively decomposed, thereby reducing and decomposing the non-aqueous electrolyte. Suppression is being studied. Since the film derived from the additive formed on the negative electrode surface by this reductive decomposition is a film excellent in ion permeability, the electrochemical characteristics are improved by the efficient use of the negative electrode active material. For example, Patent Document 1 discloses that a non-aqueous electrolyte contains phosphite diester as an additive, which describes that the charge / discharge efficiency of a lithium negative electrode or a lithium alloy negative electrode is improved. ing.

Japanese Patent Laid-Open No. 5-190205

  However, the additive described in Patent Document 1 is not effective for high temperature durability and rate characteristics.

  The objective of this invention is providing the nonaqueous electrolyte for nonaqueous electrolyte secondary batteries excellent in high temperature durability and a rate characteristic, and a nonaqueous electrolyte secondary battery using the same.

  The nonaqueous electrolyte for a nonaqueous electrolyte secondary battery according to the present invention contains a fluorine-containing phosphite diester, the fluorine-containing phosphite diester is represented by the general formula (1), and Rf1 and Rf2 are independent of each other. An alkyl group having 1 to 3 carbon atoms, and at least one of Rf1 and Rf2 contains fluorine.

  The non-aqueous electrolyte secondary battery according to the present invention includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The non-aqueous electrolyte contains a fluorine-containing phosphite diester, and the fluorine-containing phosphite diester is In the formula (1), Rf1 and Rf2 are each independently an alkyl group having 1 to 3 carbon atoms, and at least one of Rf1 and Rf2 contains fluorine.

  The nonaqueous electrolyte for a nonaqueous electrolyte secondary battery according to the present invention and the nonaqueous electrolyte secondary battery using the same are excellent in high temperature durability and rate characteristics.

It is a figure which shows the relationship between the addition amount of an additive, and a discharge capacity maintenance factor about Examples 1-4 and the comparative example 1. FIG. It is a figure which shows the relationship between the addition amount of an additive, and a discharge capacity maintenance factor about Examples 5-8 and the comparative example 2. FIG.

  Hereinafter, embodiments according to the present invention will be described in detail. The nonaqueous electrolyte secondary battery according to the embodiment of the present invention has a configuration in which, for example, an electrode body in which a positive electrode and a negative electrode are stacked via a separator, and a nonaqueous electrolyte are housed in an exterior body. Below, each structural member of a nonaqueous electrolyte secondary battery is explained in full detail.

[Positive electrode]
The positive electrode includes, for example, a positive electrode current collector such as a metal foil and a positive electrode active material layer formed on the positive electrode current collector. As the positive electrode current collector, a metal foil that is stable in the potential range of the positive electrode or a film in which a metal that is stable in the potential range of the positive electrode is arranged on the surface layer is used. As the metal stable in the potential range of the positive electrode, it is preferable to use aluminum (Al). The positive electrode active material layer includes, for example, a conductive agent, a binder and the like in addition to the positive electrode active material, and these are mixed with an appropriate solvent, applied onto the positive electrode current collector, dried and rolled. Layer.

  The positive electrode active material has a particle shape, and a transition metal oxide containing an alkali metal element or a transition metal oxide in which a part of the transition metal element contained in the transition metal oxide is substituted with a different element is used. Can do. Examples of the alkali metal element include lithium (Li) and sodium (Na). Among these alkali metal elements, lithium is preferably used. The transition metal element includes at least one selected from the group consisting of scandium (Sc), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), yttrium (Y), and the like. Various transition metal elements can be used. Among these transition metal elements, it is preferable to use Mn, Co, Ni or the like. As the different element, at least one different element selected from the group consisting of magnesium (Mg), aluminum (Al), lead (Pb), antimony (Sb), boron (B) and the like can be used. Of these different elements, Mg, Al, etc. are preferably used.

Specific examples of such a positive electrode active material include LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiMnO ₂ , LiNi _1-y Co _y O ₂ as lithium-containing transition metal oxides using lithium as an alkali metal element. (0 <y <1), LiNi 1-yz Co y Mn z O 2 (0 <y + z <1), LiFePO 4 , and the like. The positive electrode active material 24 may be used alone or in combination of two or more.

  The conductive agent is conductive powder or particles, and is used to increase the electronic conductivity of the positive electrode active material layer. As the conductive agent, a conductive carbon material, metal powder, organic material, or the like is used. Specifically, acetylene black, ketjen black, and graphite are used as the carbon material, aluminum is used as the metal powder, and a phenylene derivative is used as the organic material. These conductive agents may be used alone or in combination of two or more.

  The binder is a polymer having a particle shape or a network structure, maintains a good contact state between the particle shape positive electrode active material and the powder or the particle shape conductive agent, and is a positive electrode with respect to the surface of the positive electrode current collector. Used to enhance the binding properties of active materials and the like. As the binder, a fluorine polymer, a rubber polymer, or the like can be used. Specifically, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), or modified products thereof as the fluorine-based polymer, ethylene-propylene-isoprene copolymer, ethylene-propylene- Examples thereof include butadiene copolymers. The binder may be used in combination with a thickener such as carboxymethyl cellulose (CMC) or polyethylene oxide (PEO).

[Negative electrode]
The negative electrode includes, for example, a negative electrode current collector such as a metal foil and a negative electrode active material layer formed on the negative electrode current collector. As the negative electrode current collector, a metal foil that does not form an alloy with lithium in the potential range of the negative electrode or a film in which a metal that does not form an alloy with lithium in the potential range of the negative electrode is disposed on the surface layer is used. As a metal that does not form an alloy with lithium in the potential range of the negative electrode, it is preferable to use copper that is easy to process at low cost and has good electron conductivity. The negative electrode active material layer includes, for example, a negative electrode active material, a binder, and the like, mixed with water or an appropriate solvent, applied onto the negative electrode current collector, and then dried and rolled. It is.

  The negative electrode active material can be used without particular limitation as long as it is a material that can occlude and release alkali metal ions. As such a negative electrode active material, for example, carbon, silicon in which a carbon material, a metal, an alloy, a metal oxide, a metal nitride, and an alkali metal are occluded in advance can be used. Examples of the carbon material include natural graphite, artificial graphite, and pitch-based carbon fiber. Specific examples of the metal or alloy include lithium (Li), silicon (Si), tin (Sn), germanium (Ge), indium (In), gallium (Ga), lithium alloy, silicon alloy, tin alloy, and the like. It is done. A negative electrode active material may be used individually by 1 type, and may be used in combination of 2 or more type.

  As the binder, as in the case of the positive electrode, a fluorine-based polymer, a rubber-based polymer, or the like can be used. Is preferably used. The binder may be used in combination with a thickener such as carboxymethylcellulose (CMC).

  As the negative electrode current collector, a metal foil that does not form an alloy with lithium in the potential range of the negative electrode or a film in which a metal that does not form an alloy with lithium in the potential range of the negative electrode is disposed on the surface layer is used. As a metal that does not form an alloy with lithium in the potential range of the negative electrode, it is preferable to use copper that is easy to process at low cost and has good electron conductivity.

[Non-aqueous electrolyte]
The non-aqueous electrolyte includes a non-aqueous solvent, an electrolyte salt that dissolves in the non-aqueous solvent, and an additive.

  As the non-aqueous solvent, cyclic carbonic acid ester, cyclic carboxylic acid ester, cyclic ether, chain carbonic acid ester, chain carboxylic acid ester, chain ether, nitriles, amides and the like can be used. More specifically, ethylene carbonate (EC) or propylene carbonate (PC) as a cyclic carbonate, γ-butyrolactone (γ-GBL) or the like as a cyclic carboxylic acid ester, ethyl methyl carbonate (EMC) as a chain carbonate, Dimethyl carbonate (DMC) or the like can be used. Among them, it is preferable to use a mixture of ethylene carbonate (EC) as a cyclic carbonate which is a high dielectric constant solvent and ethyl methyl carbonate (EMC) as a chain carbonate which is a low viscosity solvent. Moreover, the halogen substituted body which substituted the hydrogen atom of the said non-aqueous solvent with halogen atoms, such as a fluorine atom, can be used. As the halogen-substituted product, for example, 4-fluoroethylene carbonate (FEC) is preferable.

As the electrolyte salt, an alkali metal salt can be used, and for example, a lithium salt is more preferable. As the lithium salt, LiPF ₆ , LiBF ₄ , LiClO ₄ or the like generally used as a supporting salt in a conventional nonaqueous electrolyte secondary battery can be used. These lithium salts may be used alone or in combination of two or more.

  The fluorine-containing phosphite diester as an additive is reduced and decomposed at the negative electrode potential at the time of initial charge, and forms an ion-permeable film on the negative electrode surface, thereby suppressing the reaction between the non-aqueous electrolyte and the negative electrode active material. It is thought that it has the function to do. Here, the negative electrode surface is an interface between the non-aqueous electrolyte and the negative electrode active material that contributes to the reaction, that is, the surface of the negative electrode active material layer and the surface of the negative electrode active material. The formed coating derived from the fluorine-containing phosphite diester improves high temperature durability and rate characteristics. The fluorine-containing phosphite diester is represented by the following formula (1), Rf1 and Rf2 are each independently an alkyl group having 1 to 3 carbon atoms, and at least one of Rf1 and Rf2 contains fluorine ( That is, it is preferably a compound in which at least one hydrogen atom is substituted with a fluorine atom. Furthermore, it is more preferable that each of Rf1 and Rf2 is a compound containing fluorine. Examples of Rf1 and Rf2 include trifluoromethyl group, pentafluoroethyl group, octafluoropropyl group, 2,2,2-trifluoroethyl group, 3,3,3-trifluoropropyl group, 2,2,3 , 3,3-pentafluoropropyl group and the like. Furthermore, a fluorine-containing phosphite diester represented by the following formula (2) is more preferable.

  An additive may be used individually by 1 type and may be used in combination of 2 or more type. The ratio of the additive to the nonaqueous electrolyte is preferably an amount that can sufficiently form a film derived from the additive, and is preferably 0.01% by mass or more and 10% by mass or less with respect to the total amount of the nonaqueous electrolyte. Furthermore, it is more preferable that it is 0.01 mass% or more and 1 mass% or less.

[Separator]
As the separator, a porous film having ion permeability and insulating properties disposed between the positive electrode and the negative electrode is used. Examples of the porous film include a microporous thin film, a woven fabric, and a non-woven fabric. As a material used for the separator, polyolefin is preferable, and more specifically, polyethylene, polypropylene, and the like are preferable.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated in detail more concretely, this invention is not limited to a following example. Below, the nonaqueous electrolyte secondary battery used for Examples 1-8 and Comparative Examples 1-2 was produced. A specific method for producing the nonaqueous electrolyte secondary battery is as follows.

<Example 1>
[Production of positive electrode]
As the positive electrode active material, a lithium-containing transition metal oxide represented by the composition formula LiNi _0.33 Co _0.33 Mn _0.33 O ₂ was used. The positive electrode was produced as follows. First, the positive electrode active material represented by LiNi _0.33 Co _0.33 Mn _0.33 O ₂ is 92% by mass, acetylene black as the conductive agent 26 is 5% by mass, and the polyvinylidene fluoride powder as the binder 28 is 3% by mass. This was mixed with N-methyl-2-pyrrolidone (NMP) solution to prepare a slurry. The slurry was applied to both surfaces of a 15 μm thick aluminum positive electrode current collector by a doctor blade method to form a positive electrode active material layer. Then, it compressed using the compression roller and produced the positive electrode.

[Production of negative electrode]
As the negative electrode active material, three types of natural graphite, artificial graphite, and artificial graphite whose surface was coated with amorphous carbon were prepared and used in various blends. The negative electrode was produced as follows. First, the negative electrode active material was mixed to 98% by mass, the styrene-butadiene copolymer (SBR) as the binder was 1% by mass, and the sodium carboxymethylcellulose as the thickener was 1% by mass. A slurry was prepared by mixing with water, and the slurry was applied to both surfaces of a copper negative electrode collector having a thickness of 10 μm by a doctor blade method to form a negative electrode active material layer. Then, it compressed to the predetermined density using the compression roller, and produced the negative electrode.

[Production of non-aqueous electrolyte]
A liquid nonaqueous electrolyte is prepared by dissolving 1.0 mol / L of LiPF ₆ as an electrolyte salt in a nonaqueous solvent in which 4-fluoroethylene carbonate (FEC) and ethylmethyl carbonate (EMC) are mixed at a volume ratio of 1: 3. It was set as the non-aqueous electrolyte which is. Next, 0.5% by mass of the fluorine-containing phosphite diester represented by the above formula (2) was added as an additive to this non-aqueous electrolyte, and this was used for battery preparation as a non-aqueous electrolyte. .

[Production of battery]
The positive electrode obtained above was cut to a size of 30 mm × 40 mm, and the negative electrode was cut to a size of 32 mm × 42 mm, and a lead terminal was attached to each of the positive electrode and the negative electrode. Next, the positive electrode and the negative electrode were opposed to each other through a separator to obtain an electrode body. Next, the electrode body and the non-aqueous electrolyte were put in a battery outer body made of an aluminum laminate, and the battery outer body was sealed to prepare a battery. The rated capacity of the battery was 50 mAh. The produced battery was subjected to a charge / discharge test to stabilize it. In the charge / discharge test, the battery was further charged with a constant current of 0.5 It (25 mA) until the voltage reached 4.4 V, and then charged with a constant voltage of 4.4 V until the current reached 2.5 mA. Left for 20 minutes. Next, discharging was performed at a constant current of 0.5 It (25 mA) until the voltage reached 3.0V. This charge / discharge test was performed for 3 cycles.

[Rate characteristics evaluation]
The rate characteristics of the battery after stabilization were evaluated. As an evaluation method, after charging until a voltage of 4.4 V is reached with a constant current of 0.5 It (25 mA), the battery is further charged until a current of 2.5 mA is reached with a constant voltage of 4.4 V, Then it was left for 20 minutes. Next, discharging was performed at a constant current of 5 It (250 mA) until the voltage reached 3.0 V, and the discharge capacity Q5 at 5 It was measured. Thereafter, the sample was left for 20 minutes, discharged at a constant current of 0.05 It (2.5 mA) until the voltage reached 3.0 V, and the discharge capacity Q0.05 was measured. Based on the discharge capacity Q5 and the discharge capacity Q0.05 obtained above, the discharge capacity retention rate (%) calculated by the following formula was evaluated as a rate characteristic.
Discharge capacity maintenance rate (%) = Q5 / (Q5 + Q0.05) × 100

[High temperature durability test]
The battery after stabilization was subjected to a 60 ° C. trickle charge storage test for the purpose of grasping the high temperature durability. Here, trickle charging means continuous charging. As a test method, after charging to a voltage of 4.4 V at a constant current of 0.5 It (25 mA), the battery was further charged to a current of 2.5 mA at a constant voltage of 4.4 V, Then it was left for 20 minutes. Next, discharging was performed at a constant current of 0.5 It (25 mA) until the voltage reached 3.0 V, and the discharge capacity Q0 was measured. Next, it was charged at a constant voltage of 4.4 V for 96 hours in a constant temperature bath at 60 ° C. Next, after the battery was cooled to room temperature, it was discharged at a constant current of 0.5 It (25 mA) at room temperature until the voltage reached 3.0 V, and the discharge capacity Q1 was measured. The capacity retention rate (%) after trickle charge calculated by the following formula based on the discharge capacity Q0 and discharge capacity Q1 obtained above was evaluated as high temperature durability.
Capacity maintenance rate after trickle charge (%) = Q1 / Q0 × 100

<Comparative Example 1>
In Example 1, rate characteristics and high-temperature durability tests were conducted in the same manner as in Example 1 except that the fluorine-containing phosphite diester represented by the above formula (2) as an additive was not added.

<Comparative Example 2>
In Example 1, the fluorine-containing phosphite diester represented by the above formula (2) was changed to diethyl phosphite containing no fluorine represented by the following formula (3) as an additive, and a non-aqueous electrolyte was obtained. On the other hand, the rate characteristics were evaluated and the high-temperature durability test was conducted in the same manner as in Example 1 except that 0.5% by mass was added.

<Comparative Example 3>
In Example 1, the fluorine-containing phosphite diester represented by the above formula (2) was changed to dimethyl phosphite containing no fluorine represented by the following formula (4) as an additive, and a non-aqueous electrolyte solution was obtained. On the other hand, the rate characteristics were evaluated and the high-temperature durability test was conducted in the same manner as in Example 1 except that 0.5% by mass was added.

  Table 1 shows a summary of the discharge capacity maintenance rates for Example 1 and Comparative Examples 1 to 3 and the capacity maintenance rates after trickle charge.

  From Table 1, in Example 1 in which the fluorine-containing phosphite diester of the present invention was contained in the non-aqueous electrolyte, the discharge capacity retention rate was higher than that in Comparative Example 1 without addition. Moreover, the same effect was acquired also in the comparative examples 2 and 3 using the conventional phosphorous acid diester which does not contain a fluorine. That is, the rate characteristics were improved by using the fluorine-containing phosphite diester of the present invention. This is presumably because the fluorine-containing phosphite diester is reductively decomposed at a potential higher than the potential at which the nonaqueous electrolytic solution undergoes reductive decomposition, thereby forming a film having excellent lithium ion permeability on the negative electrode surface.

  Further, in Example 1 in which the fluorine-containing phosphite diester of the present invention was contained in the non-aqueous electrolyte, the capacity retention rate after charge of trickle was not reduced, and was comparable to that in Comparative Example 1 without addition. It was. On the other hand, in Comparative Examples 2 and 3 in which the conventional phosphorous acid diester containing no fluorine was added, a decrease in capacity retention rate after trickle charge was observed. This is because if the additive does not contain fluorine, the film formed by reductive decomposition of the additive dissolves in the non-aqueous electrolyte during storage for 96 hours at an ambient temperature of 60 ° C. in the trickle charge storage test. This is probably because a side reaction with the non-aqueous electrolyte or the negative electrode occurs.

  Thus, by using the fluorine-containing phosphite diester, the rate characteristics can be improved without reducing the high-temperature durability.

  Next, for the purpose of grasping the amount of fluorine-containing phosphite diester that is effective as an additive, the amount of addition was changed and the same rate characteristics as in Example 1 were evaluated.

<Example 2>
In Example 1, rate characteristics were the same as in Example 1 except that the addition amount of the fluorine-containing phosphite diester represented by the above formula (2) was changed to 0.1% by mass with respect to the nonaqueous electrolyte. Was evaluated.

<Example 3>
In Example 1, rate characteristics were the same as in Example 1 except that the addition amount of the fluorine-containing phosphite diester represented by the above formula (2) was changed to 0.2% by mass with respect to the nonaqueous electrolyte. Was evaluated.

<Example 4>
In Example 1, the rate characteristics were evaluated in the same manner as in Example 1 except that the addition amount of the fluorine-containing phosphite diester represented by the above formula (2) was changed to 1% by mass with respect to the nonaqueous electrolyte. Went.

  Table 2 shows a summary of the amount of fluorine-containing phosphite diester added and the discharge capacity maintenance rate for Examples 1 to 4 and Comparative Example 1. Moreover, FIG. 1 is a figure which shows the relationship between the addition amount and discharge capacity maintenance factor about Examples 1-4 and the comparative example 1. FIG.

  From Table 2 and FIG. 1, in Examples 1-4 which made the non-aqueous electrolyte contain the fluorine-containing phosphite diester of this invention, compared with the additive-free Comparative Example 1, all have a high discharge capacity maintenance factor. For this reason, the addition amount of the fluorine-containing phosphite diester effective for obtaining a high discharge capacity retention rate is preferably 0.1% by mass or more and 1% by mass or less with respect to the total amount of the nonaqueous electrolyte.

  Here, as shown in FIG. 1, the discharge capacity retention rate is such that a gentle parabola is drawn when the addition amount of the fluorine-containing phosphite diester is 0.5% by mass, and the addition amount is 0.1%. The effect is considered to be exhibited even at 1% by mass or less. From this, it is considered that the addition amount of the fluorinated phosphite diester is preferably within the above range with respect to the total amount of the non-aqueous electrolyte, but the addition amount of the fluorinated phosphite diester and the discharge capacity maintenance shown in FIG. From the relationship with the rate, it is presumed that the effect can be obtained if the fluorine-containing phosphite diester is added. Since the fluorine-containing phosphite diester is expensive, considering the cost, the addition amount of the fluorine-containing phosphite diester is preferably added, and it should be more than the amount sufficient to form a film. Since it is considered, it is more preferable that it is 0.01 mass% or more and 1 mass% or less with respect to the total amount of nonaqueous electrolyte.

  In the above, it was confirmed that when the fluorine-containing phosphite diester was added to a non-aqueous electrolyte using 4-fluoroethylene carbonate (FEC) containing fluorine as a non-aqueous solvent, it was excellent in rate characteristics and high-temperature durability. However, the rate characteristics were evaluated by changing the nonaqueous solvent to ethylene carbonate (EC) for the purpose of grasping the effect when the nonaqueous solvent was changed to a nonaqueous solvent containing no fluorine.

<Example 5>
The rate characteristics were evaluated in the same manner as in Example 2 except that 4-fluoroethylene carbonate (FEC) as the nonaqueous solvent in Example 2 was changed to ethylene carbonate (EC).

<Example 6>
The rate characteristics were evaluated in the same manner as in Example 3 except that 4-fluoroethylene carbonate (FEC) as the nonaqueous solvent in Example 3 was changed to ethylene carbonate (EC).

<Example 7>
The rate characteristics were evaluated in the same manner as in Example 1 except that 4-fluoroethylene carbonate (FEC) as the nonaqueous solvent in Example 1 was changed to ethylene carbonate (EC).

<Example 8>
The rate characteristics were evaluated in the same manner as in Example 4 except that 4-fluoroethylene carbonate (FEC) as the nonaqueous solvent in Example 4 was changed to ethylene carbonate (EC).

<Comparative Example 2>
The rate characteristics were evaluated in the same manner as in Comparative Example 1 except that 4-fluoroethylene carbonate (FEC) as the nonaqueous solvent in Comparative Example 1 was changed to ethylene carbonate (EC).

  Table 3 summarizes the contents of the nonaqueous solvent, the amount added, and the discharge capacity retention rate for Examples 5 to 8 and Comparative Example 2. Moreover, FIG. 2 is a figure which shows the relationship between the addition amount and discharge capacity maintenance factor about Examples 5-8 and the comparative example 2. FIG.

  The contents of Table 3 and FIG. 2 are as follows. For Examples 5 to 8 and Comparative Example 2, 4-fluoroethylene carbonate (FEC) as a nonaqueous solvent in Examples 1 to 4 and Comparative Example 1 of Table 2 was changed to ethylene carbonate ( It is the result of changing to EC) and performing the same evaluation.

  About each of Examples 5-8 and Comparative Example 2, compared with Examples 1-4 and Comparative Example 1, it became a result that a discharge capacity maintenance factor fell. However, Examples 5-8 all had a high discharge capacity retention rate compared to Comparative Example 2, and the rate characteristics were improved even when ethylene carbonate (EC) was used. That is, even when ethylene carbonate (EC) is used, rate characteristics are improved by incorporating the fluorine-containing phosphite diester into the non-aqueous electrolyte. This is because the fluorine-containing phosphite diester undergoes reductive decomposition at a potential higher than the potential at which a non-aqueous electrolyte containing ethylene carbonate (EC) as a non-aqueous solvent is reduced, and forms a film with excellent ion permeability on the negative electrode surface. It is thought to do. Thus, the fluorine-containing phosphite diester exhibits an effect as an additive not only in 4-fluoroethylene carbonate (FEC) but also in ethylene carbonate (EC).

  Moreover, the addition amount of the fluorine-containing phosphite diester effective for obtaining a high discharge capacity retention rate is preferably 0.1% by mass or more and 1% by mass or less with respect to the total amount of the nonaqueous electrolyte.

  Here, as shown in FIG. 2, the discharge capacity retention rate is the best when the addition amount of the fluorine-containing phosphite diester is 0.2% by mass, and decreases before and after that. However, although a decrease in the discharge capacity retention rate is observed before and after that, both results are better than the case of no addition, and it is considered that the effect is exhibited even when the addition amount is 0.1% by mass or less from the relationship. From this, it is considered that the addition amount of the fluorine-containing phosphite diester is preferably within the above range with respect to the total amount of the nonaqueous electrolyte, but the addition amount of the fluorine-containing phosphite diester and the discharge capacity maintenance shown in FIG. From the relationship with the rate, it is presumed that the effect can be obtained if the fluorine-containing phosphite diester is added. Since the fluorine-containing phosphite diester is expensive, considering the cost, the addition amount of the fluorine-containing phosphite diester is preferably added, and it should be more than the amount sufficient to form a film. Since it is considered, it is more preferable that it is 0.01 mass% or more and 1 mass% or less with respect to the total amount of nonaqueous electrolyte.

  Thus, even in a non-aqueous solvent containing no fluorine, it is possible to improve rate characteristics by adding a fluorine-containing phosphite diester.

  Here, according to Paragraph 0032 of the same document of Japanese Patent Application Laid-Open No. 2011-49157, there is a description that battery performance decreases when fluorine-containing phosphite as a fluorine-containing phosphite is contained, and the above idea is easy. is not. However, according to the inventors' idea, the fluorine-containing phosphite diester as an additive suppresses the reaction between the negative electrode and the non-aqueous electrolyte by forming a film having excellent ion permeability on the negative electrode surface, and the high temperature It was confirmed to improve durability and rate characteristics.

  Thus, the nonaqueous electrolyte for a nonaqueous electrolyte secondary battery containing a fluorine-containing phosphite diester, and the nonaqueous electrolyte secondary battery having a nonaqueous electrolyte for a nonaqueous electrolyte secondary battery have high-temperature durability, Excellent rate characteristics.

Claims (4)

A non-aqueous electrolyte used in a non-aqueous electrolyte secondary battery,
The non-aqueous electrolyte contains a fluorine-containing phosphite diester,
The fluorine-containing phosphite diester is represented by the general formula (1), Rf1 and Rf2 are each independently an alkyl group having 1 to 3 carbon atoms, and at least one of Rf1 and Rf2 contains fluorine. A nonaqueous electrolyte for a nonaqueous electrolyte secondary battery.

In the nonaqueous electrolyte for nonaqueous electrolyte secondary batteries according to claim 1,
The non-aqueous electrolyte for a non-aqueous electrolyte secondary battery, wherein the fluorine-containing phosphorous acid diester is a compound represented by the chemical formula (2).

The nonaqueous electrolyte for a nonaqueous electrolyte secondary battery according to claim 1 or 2,
The non-aqueous electrolyte for a non-aqueous electrolyte secondary battery, wherein the fluorine-containing phosphite diester contains 0.01% by mass or more and 1% by mass or less with respect to the total amount of the non-aqueous electrolyte. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte,
The non-aqueous electrolyte contains a fluorine-containing phosphite diester,
The fluorine-containing phosphite diester is represented by the general formula (1), Rf1 and Rf2 are each independently an alkyl group having 1 to 3 carbon atoms, and at least one of Rf1 and Rf2 contains fluorine. A non-aqueous electrolyte secondary battery.

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