JP6344100B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

The present invention relates to a non-aqueous electrolyte secondary battery.

  As an additive for preventing overcharge of a non-aqueous electrolyte secondary battery typified by a lithium secondary battery, a technique is known in which a disulfide compound having an aromatic ring such as diphenyl disulfide is added to an electrolyte solution.

Patent Document 1 discloses that the best battery performance during overcharge is obtained by applying a non-aqueous electrolyte containing 1 to 10% by weight of diphenyl disulfide to a lithium secondary battery including a positive electrode using LiCoO ₂ as a positive electrode active material. It is shown that the ultimate temperature could be lowered.

Patent Document 2 discloses that a lithium secondary battery including a positive electrode having LiCoO ₂ or LiNi _0.8 Co _0.2 O ₂ as a positive electrode active material, as an electrolyte additive, 2 to 3% by weight of cyclohexylbenzene, A lithium secondary battery is described that is used in combination with 0.2-0.3 wt% bis (4-methoxyphenyl) disulfide, bis (4-ethoxyphenyl) disulfide or bis (4-chlorophenyl) disulfide. Yes. Paragraph 0017 of Patent Document 2 states that “by including the cyclohexylbenzene together with the disulfide derivative, the cycle characteristics can be improved over a long period of time even under severe conditions. It is presumed that the disulfide derivative is preferentially oxidatively decomposed before the nonaqueous solvent electrolyte in the battery at the minute overvoltage part where the potential of the material surface is excessively increased, thereby preventing the oxidative decomposition of the solvent. In this way, it is considered that the cyclohexylbenzene having an overcharge-preventing action is normally used because it has the effect of suppressing the decomposition of the electrolytic solution and cyclohexylbenzene without impairing the normal reaction of the battery. Occasionally there is no oxidative degradation, so an overcharge test is performed after 300 cycles. Also, cyclohexylbenzene is considered to have the effect of sufficiently ensuring safety with decomposition. "Is described as.

Patent Document 3 contains, as Example 2, a positive electrode using LiCoO ₂ as a positive electrode active material, a negative electrode in which a layer of natural graphite is formed on a copper foil, and 0.1% by weight of di-n-butyl disulfide. It describes that a non-aqueous electrolyte was injected to prepare a lithium secondary battery, and charge and discharge were performed 50 cycles at a charge end voltage of 4.2 V and a discharge end voltage of 2.7 V. In addition, if the content of the disulfide derivative is excessively large, battery performance may be deteriorated, and sufficient battery performance that is expected to be excessively small cannot be obtained. In the range of 0.001 to 2% by weight, particularly 0.01 to 0.5% by weight, the cycle characteristics are improved. ”(Paragraph 0009).

Japanese Patent No. 4974404 Japanese Patent No. 4423785 Japanese Patent No. 4042082

  In the manufacturing process of a non-aqueous electrolyte secondary battery, if the non-aqueous electrolyte is injected and left for a long time before the first charge, there is a problem that the battery performance deteriorates. Therefore, it is necessary to perform process management so that the time until the first charge is performed after the nonaqueous electrolyte is injected is not long.

  The configuration and operational effects of the present invention will be described with the technical idea. However, the action mechanism includes estimation, and the correctness does not limit the present invention. It should be noted that the present invention can be implemented in various other forms without departing from the spirit or main features thereof. For this reason, the following embodiments or experimental examples are merely examples in all respects and should not be interpreted in a limited manner. Further, all modifications and changes belonging to the equivalent scope of the claims are within the scope of the present invention.

The present invention is a nonaqueous electrolyte secondary battery comprising a positive electrode, a negative electrode comprising a negative electrode current collector containing copper, and a nonaqueous electrolyte, wherein the nonaqueous electrolyte is represented by the general formula R ¹ —S—. A non-aqueous electrolyte secondary battery represented by S—R ² , wherein each of R ¹ and R ² independently contains a disulfide compound that is an alkyl group having 18 or less carbon atoms.

  ADVANTAGE OF THE INVENTION According to this invention, even if it is a case where it is a case where it is left for a long time until the first charge after injecting a nonaqueous electrolyte, the nonaqueous electrolyte battery by which the fall of battery performance was suppressed can be provided.

The figure which shows the CV measurement result of the nonaqueous electrolyte which concerns on an Example The figure which shows the CV measurement result of the nonaqueous electrolyte which concerns on a comparative example The figure which shows the CV measurement result of the nonaqueous electrolyte which concerns on an Example External perspective view showing one embodiment of a nonaqueous electrolyte secondary battery Schematic showing a power storage device configured by assembling a plurality of nonaqueous electrolyte secondary batteries

About the action mechanism in which the said effect is show | played, this inventor estimates as follows. It is conceivable that a current collector containing copper (Cu) is used in the negative electrode as a cause of deterioration in battery performance when the nonaqueous electrolyte is injected and left for a long time until the first charge. . In a non-aqueous electrolyte, copper is stable at a base potential near 0 V (vs. Li ^/ Li ⁺ ), but has a property of dissolving at a noble potential of 3 V (vs. Li ^/ Li ⁺ ) or higher. On the other hand, many of the negative electrode active materials used for the negative electrode of the non-aqueous electrolyte secondary battery have a natural potential of 3 V (vs. Li ^/ Li ⁺ ) or more in the non-aqueous electrolyte in a state where lithium ions are not occluded. . Therefore, when a non-aqueous electrolyte is injected in a battery manufacturing process in which such a negative electrode active material is used, the negative electrode potential naturally becomes 3 V (vs. Li ^/ Li ⁺ ) or more at that time. Further, the voltage between the terminals is not 0 V at the time of injection, but a phenomenon in which the voltage between the terminals becomes 0 V is often observed when left for a while after the injection. Although the behavior of the negative electrode potential at this time is not detailed, there is a possibility that the negative electrode potential is further increased from the time of injection. Therefore, there is a possibility that copper is eluted from the negative electrode current collector when it is left for a long time in the injected state. Since the negative electrode of the nonaqueous electrolyte battery has a negative electrode mixture layer containing a negative electrode active material on the negative electrode current collector, when copper is eluted from the negative electrode current collector, the negative electrode current collector, the negative electrode mixture layer, It is considered that the battery performance deteriorates because a gap is formed between the two and the contact is deteriorated.

In the nonaqueous electrolyte battery according to the present invention, the nonaqueous electrolyte is represented by the general formula R ¹ —S—S—R ² , and R ¹ and R ² are each independently an alkyl group having 18 or less carbon atoms. Contains certain disulfide compounds. When R ¹ and R ² are phenyl groups, the effects of the present invention are not achieved. The alkyl group having 18 or less carbon atoms may be a branched alkyl group, but is preferably a linear alkyl group.

  According to the present invention, it is presumed that when the non-aqueous electrolyte contains a specific disulfide compound, the disulfide compound covers the surface of the negative electrode current collector and elution of copper is suppressed.

A larger carbon number of the alkyl group is preferable because the disulfide compound can effectively cover the negative electrode current collector. In this respect, the alkyl group preferably has 4 or more carbon atoms, more preferably 5 or more, and still more preferably 6 or more. However, when the carbon number of the alkyl group is too large, the solubility in the non-aqueous electrolyte decreases. For this reason, since a disulfide compound cannot fully cover a negative electrode collector as a result, it is unpreferable. In this respect, the alkyl group preferably has 17 or less carbon atoms, and more preferably 16 or less carbon atoms. The number of carbon atoms of R ¹ and R ² may be different, towards the number of carbon atoms in R ¹ and R ² are the same it is a negative electrode current collector surface covered densely more uniform, copper Since the elution suppression effect increases, it is preferable.

  The amount of the disulfide compound contained in the nonaqueous electrolyte is preferably a certain amount or more, but is preferably not too much. In this respect, the amount of the disulfide compound contained in the nonaqueous electrolyte is preferably 0.3% by mass or more, more preferably 0.5% by mass or more, and further preferably 0.6% by mass or more. Moreover, 4 mass% or less is preferable and 3 mass% or less is more preferable.

  The negative electrode current collector provided in the non-aqueous electrolyte secondary battery according to the present invention is not limited in terms of copper content as long as it contains copper. For example, the content ratio of copper may be 5% by mass. Ni-Cu alloys, Ni-Sn-Cu alloys, Cu-Al alloys, and the like can be used as long as they do not adversely affect battery performance. From the viewpoint of flexibility of the current collector or cost, a copper foil containing 70% by mass or more of copper is preferable. 90 mass% or more is more preferable, 99 mass% or more is further more preferable, and 99.9 mass% or more is the most preferable. The copper foil may be a rolled copper foil or an electrolytic copper foil.

The positive electrode active material that can be used for the positive electrode included in the nonaqueous electrolyte battery according to the present invention is not limited at all, and various oxides, sulfides, and the like can be mentioned. For example, a composite oxide represented by Li _x MO _y (M represents at least one transition metal) (Li _x CoO ₂ , Li _x NiO ₂ , Li _x Mn ₂ O ₄ , Li _x MnO ₃ , Li _x Ni _{y Co (1-y) O} 2, Li x Ni y Mn z Co (1-y-z) O 2, Li x Ni y Mn (2-y) O 4 , _{etc.), Li w Me x (XO} y) _z (Me represents at least one transition metal, X represents P, Si, B, V, for example) (LiFePO ₄ , LiMnPO ₄ , LiNiPO ₄ , LiCoPO ₄ , Li ₃ V ₂ (PO) ₄ ) ₃ , Li ₂ MnSiO ₄ , Li ₂ CoPO ₄ F, etc.). The elements or polyanions in these compounds may be partially substituted with other elements or anion species. Examples of the positive electrode active material further include conductive polymer compounds such as disulfide, polypyrrole, polyaniline, polyparastyrene, polyacetylene, and polyacene-based materials, and pseudographite-structured carbonaceous materials. In a positive electrode active material, 1 type of these compounds may be used independently, and 2 or more types may be mixed and used for it.

The negative electrode material used for the negative electrode of the non-aqueous electrolyte secondary battery according to the present invention is not limited, and any negative electrode material that can deposit or occlude lithium ions may be selected. For example, alloy-based materials such as Si, Sb, and Sn-based lithium metal; including lithium metal such as lithium-silicon, lithium-aluminum, lithium-lead, lithium-tin, lithium-aluminum-tin, lithium-gallium, and wood alloy Examples include lithium alloys such as alloys; lithium composite oxides such as lithium-titanium; silicon oxides; alloys capable of inserting and extracting lithium; carbon materials such as graphite, hard carbon, low-temperature fired carbon, and amorphous carbon. These have a working potential of less than 1 V (vs. Li ^/ Li ⁺ ) and are preferable.

  It is desirable that the positive electrode active material powder and the negative electrode material powder have an average particle size of 100 μm or less. In particular, the positive electrode active material powder is desirably 10 μm or less for the purpose of improving the high output characteristics of the non-aqueous electrolyte battery. In order to obtain the powder in a predetermined shape, a pulverizer or a classifier is used. For example, a mortar, a ball mill, a sand mill, a vibrating ball mill, a planetary ball mill, a jet mill, a counter jet mill, a swirling air flow type jet mill or a sieve is used. At the time of pulverization, wet pulverization in the presence of water or an organic solvent such as hexane may be used. There is no particular limitation on the classification method, and a sieve, an air classifier, or the like is used as needed for both dry and wet methods.

  The positive electrode active material and the negative electrode material, which are the main components of the positive electrode and the negative electrode, have been described in detail above. In addition to the main components, the positive electrode and the negative electrode include a conductive agent, a binder, a thickener, and a filler. Etc. may be contained as other constituents.

  The conductive agent is not limited as long as it is an electron conductive material that does not adversely affect battery performance. Usually, natural graphite such as scaly graphite, scaly graphite, and earth graphite; artificial graphite; carbon black; acetylene black; Carbon black; Carbon fiber; Metal powder such as copper, nickel, aluminum, silver, and gold; Metal fiber; Conductive material such as conductive ceramic material may be included as one kind or a mixture thereof.

  Among these, as the conductive agent, acetylene black is desirable from the viewpoints of electron conductivity and coatability. The addition amount of the conductive agent is preferably 0.1% by weight to 50% by weight, and particularly preferably 0.5% by weight to 30% by weight with respect to the total weight of the positive electrode or the negative electrode. In particular, it is desirable to use acetylene black by pulverizing it into ultrafine particles of 0.1 to 0.5 μm because the required amount can be reduced. These mixing methods are physical mixing, and the ideal is uniform mixing. Therefore, powder mixers such as V-type mixers, S-type mixers, crackers, ball mills, and planetary ball mills can be mixed dry or wet.

  Examples of the binder include thermoplastic resins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyethylene, and polypropylene; ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, and styrene butadiene. Polymers having rubber elasticity such as rubber (SBR) and fluororubber can be used as one kind or a mixture of two or more kinds. The addition amount of the binder is preferably 1 to 50% by weight, particularly preferably 2 to 30% by weight, based on the total weight of the positive electrode or the negative electrode.

  As the filler, any material that does not adversely affect the battery performance may be used. Usually, olefin polymers such as polypropylene and polyethylene, amorphous silica, alumina, zeolite, glass, carbon and the like are used. The addition amount of the filler is preferably 30% by weight or less with respect to the total weight of the positive electrode or the negative electrode.

  The positive electrode and the negative electrode are prepared by mixing the main constituents (positive electrode active material in the positive electrode, negative electrode material in the negative electrode) and other materials into a mixture and mixing with an organic solvent such as N-methylpyrrolidone or toluene or water. Then, the obtained mixed liquid is applied or pressure-bonded onto a current collector described in detail below, and is heat-treated at a temperature of about 50 ° C. to 250 ° C. for about 2 hours, so that it is suitably produced. . About the application method, for example, it is desirable to apply to any thickness and any shape using means such as roller coating such as applicator roll, screen coating, doctor blade method, spin coating, bar coater, etc. It is not limited.

  As the separator, it is preferable to use a porous film or a non-woven fabric exhibiting excellent high rate discharge performance alone or in combination. Examples of the material constituting the separator for a non-aqueous electrolyte battery include polyolefin resins typified by polyethylene and polypropylene; polyester resins typified by polyethylene terephthalate and polybutylene terephthalate; polyvinylidene fluoride; vinylidene fluoride-hexa Fluoropropylene copolymer; vinylidene fluoride-perfluorovinyl ether copolymer; vinylidene fluoride-tetrafluoroethylene copolymer; vinylidene fluoride-trifluoroethylene copolymer; vinylidene fluoride-fluoroethylene copolymer; Vinylidene fluoride-hexafluoroacetone copolymer; vinylidene fluoride-ethylene copolymer; vinylidene fluoride-propylene copolymer; vinylidene fluoride-trifluoropropylene copolymer; vinylidide fluoride - tetrafluoroethylene - hexafluoropropylene copolymer; a vinylidene fluoride - ethylene - can be mentioned tetrafluoroethylene copolymer.

  The porosity of the separator is preferably 98% by volume or less from the viewpoint of strength. Further, the porosity is preferably 20% by volume or more from the viewpoint of charge / discharge characteristics.

  The separator may be a polymer gel composed of a polymer such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinyl pyrrolidone, polyvinylidene fluoride, and an electrolyte. Use of the non-aqueous electrolyte in the gel state as described above is preferable in that it has an effect of preventing leakage.

  Furthermore, it is desirable that the separator be used in combination with the above-described porous film, non-woven fabric, or the like and a polymer gel because the liquid retention of the electrolyte is improved. That is, by forming a film in which the surface of the polyethylene microporous membrane and the microporous wall are coated with a solvophilic polymer having a thickness of several μm or less, and holding the electrolyte in the micropores of the film, Gels.

  Examples of the solvophilic polymer include polyvinylidene fluoride, an acrylate monomer having an ethylene oxide group or an ester group, an epoxy monomer, a polymer having a monomer having an isocyanate group, and the like crosslinked. The monomer can be subjected to a crosslinking reaction using a radical initiator in combination with heating or ultraviolet rays (UV), or using an actinic ray such as an electron beam (EB).

  The configuration of the nonaqueous electrolyte secondary battery according to the present invention is not particularly limited, and examples thereof include a cylindrical battery, a square battery (rectangular battery), a flat battery, and the like. FIG. 4 is a schematic view of a rectangular nonaqueous electrolyte secondary battery 1 which is an embodiment of the nonaqueous electrolyte secondary battery according to the present invention. In the figure, the inside of the container is seen through. In the nonaqueous electrolyte secondary battery 1 shown in FIG. 4, the electrode group 2 is housed in a battery container 3. The electrode group 2 is formed by winding a positive electrode including a positive electrode active material and a negative electrode including a negative electrode active material via a separator. The positive electrode is electrically connected to the positive electrode terminal 4 via the positive electrode lead 4 ′, and the negative electrode is electrically connected to the negative electrode terminal 5 via the negative electrode lead 5 ′.

  The present invention can also be realized as a power storage device including a plurality of the above non-aqueous electrolyte secondary batteries. One embodiment of a power storage device is shown in FIG. In FIG. 5, the power storage device 30 includes a plurality of power storage units 20. Each power storage unit 20 includes a plurality of nonaqueous electrolyte secondary batteries 1. The power storage device 30 can be mounted as a power source for vehicles such as an electric vehicle (EV), a hybrid vehicle (HEV), and a plug-in hybrid vehicle (PHEV). FIG. 12 is a conceptual diagram of the automobile 100 on which the power storage device 30 is mounted. Here, the automobile 100 includes a power storage device 30 and a vehicle body 40 that houses the power storage device 30.

  By the way, the non-aqueous electrolyte secondary battery has a problem that its performance is likely to deteriorate when it is overdischarged. For example, in a non-aqueous electrolyte secondary battery used as a battery for a vehicle such as a hybrid vehicle or a plug-in hybrid vehicle, a large current instantaneously flows when the engine is started. In particular, when the battery is in a low SOC state and the overdischarge protection circuit fails, there is a possibility that the battery is overdischarged when the engine is started. A non-aqueous electrolyte battery usually employs a design in which the capacity balance between the positive electrode and the negative electrode is limited by the negative electrode. Accordingly, the potential of the negative electrode increases in the overdischarge state.

  ADVANTAGE OF THE INVENTION According to this invention, since the elution of copper from a negative electrode electrical power collector can be suppressed when the electric potential of a negative electrode raises, the nonaqueous electrolyte secondary battery by which the performance fall at the time of an overdischarge was suppressed can be provided.

This effect is remarkably exhibited in a battery having a positive electrode potential of 3.8 V (vs. Li ^/ Li ⁺ ) or less when discharged until the terminal voltage becomes 0 V. The reason is as follows. Even in the non-aqueous electrolyte containing the disulfide compound according to the present invention, elution of copper is observed at a potential exceeding 3.8 V (vs. Li ^/ Li ⁺ ). However, if the non-aqueous electrolyte secondary battery has a positive electrode potential of 3.8 V (vs. Li ^/ Li ⁺ ) or less when discharged until the voltage between terminals becomes 0 V, the voltage between terminals should be reduced during overdischarge. Even if the negative electrode potential is increased to 0 V and the negative electrode potential is increased, the negative electrode potential is not nobler than the positive electrode potential of 3.8 V (vs. Li ^/ Li ⁺ ) at that time. The effect of suppressing the elution of copper from the current collector is reliably exhibited. Therefore, it is possible to reliably provide a nonaqueous electrolyte secondary battery in which the performance degradation when overdischarged is suppressed.

The type of positive electrode active material used for a battery having a positive electrode potential of 3.8 V (vs. Li ^/ Li ⁺ ) or less when discharged until the terminal voltage becomes 0 V is not limited as long as it is designed as such. . Lithium iron phosphate (LiFePO ₄ ) is exemplified as a preferred positive electrode active material because it exhibits a flat operating potential in the vicinity of 3.3 V (vs. Li ^/ Li ⁺ ).

(Preparation of positive electrode plate)
A material in which carbon was coated on the surface of lithium iron phosphate particles represented by the composition formula LiFePO ₄ was used as a positive electrode active material. The positive electrode active material, acetylene black as a conductive agent, and polyvinylidene fluoride (PVdF) as a binder are contained in a mass ratio of 88: 5: 7, and N-methyl-2-pyrrolidone (NMP) is used as a solvent. A positive electrode paste was prepared. The positive electrode paste was applied to both sides of a 20 μm-thick strip-shaped aluminum foil as a positive electrode current collector, dried, and then pressed. In this way, a positive electrode plate was produced.

(Preparation of negative electrode plate)
A negative electrode paste containing graphitizable carbon and polyvinylidene fluoride (PVdF) as a binder in a mass ratio of 95: 5 and N-methyl-2-pyrrolidone (NMP) as a solvent was prepared. The negative electrode paste was applied to both sides of a strip-shaped copper foil having a thickness of 15 μm, which was a negative electrode current collector, dried, and then pressed. In this way, a negative electrode plate was produced.

(Assembling of non-aqueous electrolyte battery)
A power generating element formed by flatly winding the positive electrode plate and the negative electrode plate through a separator made of a polypropylene microporous film having a thickness of 25 μm was housed in a rectangular battery case, and a nonaqueous electrolyte was injected. The design capacity is 480 mAh. The kind of nonaqueous electrolyte used and the injection method are as follows.

Ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 1: 1: 1, containing 1% by weight vinylene carbonate and 1.0 mol / l LiPF _6. The non-aqueous electrolyte is adjusted. This is designated as non-aqueous electrolyte A.

Example 1
A non-aqueous electrolyte in which 0.5% by mass of di-n-butyl disulfide (C ₄ H ₉ —S—S—C ₄ H ₉ ) was further added to and dissolved in the non-aqueous electrolyte A was prepared. This non-aqueous electrolyte was injected to assemble a non-aqueous electrolyte battery.

(Example 2-1)
A non-aqueous electrolyte in which 0.5% by mass of dihexyl disulfide (C ₆ H ₁₃ —S—S—C ₆ H ₁₃ ) was further added to and dissolved in the non-aqueous electrolyte A was prepared. This non-aqueous electrolyte was injected to assemble a non-aqueous electrolyte battery.

(Example 2-2)
To the non-aqueous electrolyte A, 1% by mass of dihexyl disulfide (C ₆ H ₁₃ —S—S—C ₆ H ₁₃ ) was further added to prepare a dissolved non-aqueous electrolyte. This non-aqueous electrolyte was injected to assemble a non-aqueous electrolyte battery.

(Example 2-3)
When 3% by mass of dihexyl disulfide (C ₆ H ₁₃ —S—S—C ₆ H ₁₃ ) was further added to the non-aqueous electrolyte A, an undissolved residue was generated. Therefore, 3% by mass of dihexyl disulfide was introduced into the rectangular battery case, and then the nonaqueous electrolyte A was injected. In this way, a nonaqueous electrolyte battery was assembled.

(Example 2-4)
4% by mass of dihexyl disulfide (C ₆ H ₁₃ —S—S—C ₆ H ₁₃ ) was charged into the rectangular battery case, and then the non-aqueous electrolyte A was injected. In this way, a nonaqueous electrolyte battery was assembled.

(Example 3)
A non-aqueous electrolyte in which 0.5% by mass of diundecyl disulfide (C ₁₁ H ₂₃ —S—S—C ₁₁ H ₂₃ ) was further added to the non-aqueous electrolyte 1 and dissolved therein was prepared. This non-aqueous electrolyte was injected to assemble a non-aqueous electrolyte battery.

Example 4
When 0.3% by mass of dihexadecyl disulfide (C ₁₆ H ₃₃ —S—S—C ₁₆ H ₃₃ ) was further added to the non-aqueous electrolyte A, an undissolved residue was generated. Therefore, 0.3% by mass of dihexadecyl disulfide with respect to the injection amount was introduced into the rectangular battery case, and then the nonaqueous electrolyte A was injected. In this way, a nonaqueous electrolyte battery was assembled.

(Example 5)
When 0.3% by mass of dioctadecyl disulfide (C ₁₈ H ₃₇ —S—S—C ₁₈ H ₃₇ ) was further added to the non-aqueous electrolyte A, most of it remained undissolved. By visual observation, it was estimated that about 0.1% by mass was dissolved. Therefore, 0.3% by mass of dioctadecyl disulfide with respect to the injection amount was introduced into the rectangular battery case, and then the nonaqueous electrolyte A was injected. In this way, a nonaqueous electrolyte battery was assembled.

(Comparative Example 1)
The nonaqueous electrolyte A was injected to assemble a nonaqueous electrolyte battery.

(Comparative Example 2)
A non-aqueous electrolyte in which 0.5% by mass of diphenyl disulfide was further added to and dissolved in the non-aqueous electrolyte A was prepared. This non-aqueous electrolyte was injected to assemble a non-aqueous electrolyte battery.

  The non-aqueous electrolyte batteries according to the comparative example and the example assembled as described above were subjected to an acceleration test in order to simulate a situation in which they were left for a long period of time from the injection to the first charge. It was left in a constant temperature bath at 7 ° C. for 7 days. Next, a total of 7 cycles of charge / discharge tests were performed at 25 ° C. under the following conditions. All charge / discharge tests were constant current charging with an upper limit voltage of 3.55V and constant current discharge with a lower limit voltage of 2.0V. However, in the first cycle, the charging current and the discharging current are both 0.1 CmA, in the first to fourth cycles, both the charging current and the discharging current are 0.2 CmA, and in the fifth to seventh cycles, the charging current and the discharging current are set. Both were 1 CmA. The discharge capacity at the seventh cycle was recorded as “1C discharge capacity (mAh)”.

(Reference example)
A non-aqueous electrolyte battery was assembled in the same manner as in Comparative Example 1, and was not allowed to stand at 45 ° C. for 7 days.

The results are shown in Table 1. In addition, Example 1, Example 2-4, and Example 5 are reference examples.

As can be seen from the results in Table 1, as R ¹ and R ² of the disulfide compound represented by the general formula R ¹ —S—S—R ² , C _n H _{2n + 1} (n is 18 or less) which is a linear alkyl group The nonaqueous electrolyte battery according to the example using the nonaqueous electrolyte containing) was left standing for a long time as compared with the nonaqueous electrolyte battery according to Comparative Example 1 using the nonaqueous electrolyte containing no disulfide compound. It was found that the capacity drop was suppressed.

  In addition, it was recognized that the larger the carbon number of the alkyl group, the better the effect of suppressing the decrease in capacity due to standing for a long time. However, the nonaqueous electrolyte battery according to Example 5 using dioctadecyl disulfide, which is a disulfide compound having 18 carbon atoms, is different from Example 4 using dihexadecyl disulfide, which is a disulfide compound having 16 carbon atoms. The effect of suppressing the decrease in capacity was smaller than that of the nonaqueous electrolyte battery. This is probably because the amount of the disulfide compound contained in the nonaqueous electrolyte according to Example 5 was as small as about 0.1% by mass.

  On the other hand, the non-aqueous electrolyte battery according to Comparative Example 2 using a disulfide compound having an aromatic ring was left for a longer period than the non-aqueous electrolyte battery according to Comparative Example 1 using a non-aqueous electrolyte containing no disulfide compound. On the contrary, it deteriorated in terms of capacity reduction.

In the above examples, the present inventor can suppress the deterioration in battery performance when left for a long period of time after the first injection and the elution of Cu contained in the negative electrode current collector at a noble potential. I guessed it was because it was suppressed. Therefore, in order to verify this, a cell having metal Cu as a working electrode and metal Li as a counter electrode and a reference electrode is assembled, an electrolytic solution is injected, and a scanning range is 3.6 V (vs. Li / Li ⁺ ) to Cyclic voltammetry (CV) measurement was performed with 3.0 V (vs. Li / Li ⁺ ), a scanning speed of 10 mV / sec, and a scanning frequency of 2 cycles. (A) Example 2-1, (b) Example 3, (c) Example 4, (d) Example 5 and (e) Nonaqueous electrolytes prepared according to Comparative Example 1 as the electrolyte Using. Since the profiles of (a) to (d) were almost the same, the profile of (c) is representatively shown in FIG. 1, the profile of (e) is shown in FIG. 2, and the second cycle of 3.6V ( The current value at the time of reaching vs. Li / Li + is shown in Table 2. Note that Example 5 is a reference example.

As can be seen from FIG. 1-2 and Table 2, the value of the oxidation current at a potential of 3.6 V (vs. Li ^/ Li ⁺ ) is about 8 mA in (a) using a non-aqueous electrolyte containing no disulfide compound. On the other hand, in (b)-(d) using the non-aqueous electrolyte containing a disulfide compound, it was 0.01-0.05 mA. Therefore, by using a non-aqueous electrolyte containing a disulfide compound represented by the general formula R ¹ —S—S—R ² , wherein R ¹ and R ² are alkyl groups having 4 to 18 carbon atoms, 3 It was found that the elution reaction of Cu at potentials up to .6 V (vs. Li / Li ⁺ ) was suppressed to 1/100 or less.

Next, after the test, for the cells according to (a), (b), (c), and (d), the scanning range is 4.3 V (vs. Li ^/ Li ⁺ ) to 3.0 V (vs. Li). / Li ⁺ ) and CV measurement at the third cycle was performed. Since the results were the same for all the cells, the profile of (d) is representatively shown in FIG.

As can be seen from FIG. 3, no reaction current was observed at a potential of 3.8 V (vs. Li ^/ Li ⁺ ) or less, but it exceeded 3.8 V (vs. Li ^/ Li ⁺ ) and reached 4.3 V (vs. Li ^/ Li ⁺ ). oxidation current between Li / Li ⁺⁾ until the observed, 4.3V (vs.Li/Li ⁺⁾ current value upon reaching reached about 20mA. From this, it is considered that Cu elution occurs at a potential exceeding 3.8 V (vs. Li ^/ Li ⁺ ).

Claims (1)

A nonaqueous electrolyte secondary battery comprising a positive electrode, a negative electrode including a negative electrode current collector containing copper, and a nonaqueous electrolyte, wherein the nonaqueous electrolyte is represented by the general formula R ¹ —S—S—R ^2. Wherein R ¹ and R ² each independently contains 3% by mass or less of a disulfide compound which is an alkyl group having 6 to 16 carbon atoms , and the positive electrode contains lithium iron phosphate . A non-aqueous electrolyte secondary battery.