JP2017168373A - 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, which makes possible to form a nonaqueous electrolyte secondary battery arranged so that the reduction in capacity-keeping rate owing to charge and discharge cycles can be suppressed.SOLUTION: A nonaqueous electrolyte with a magnesium pyrophosphate added thereto is used. The nonaqueous electrolyte contains a pyrophosphoric acid anion and a magnesium cation.SELECTED DRAWING: None

Description

  The present invention relates to a nonaqueous electrolyte for a nonaqueous electrolyte secondary battery.

  Nonaqueous electrolyte secondary batteries represented by lithium secondary batteries have been used as power sources for mobile devices such as notebook computers and mobile phones. In recent years, non-aqueous electrolyte secondary batteries are also used as power sources for automobiles such as electric vehicles (EV), hybrid vehicles (HEV), plug-in hybrid vehicles (PHEV) and the like.

The positive electrode active material constituting the nonaqueous electrolyte secondary battery is a lithium-containing transition metal oxide, the negative electrode active material is a carbon material typified by graphite, and the nonaqueous electrolyte is a cyclic carbonate such as ethylene carbonate and diethyl. A solution in which an electrolyte such as lithium hexafluorophosphate (LiPF ₆ ) is dissolved in a nonaqueous solvent containing a chain carbonate such as carbonate as a main constituent is widely known.

Patent Document 1 describes a nonaqueous electrolyte secondary battery (Table 51) using Mg ₂ P ₂ O ₇ as a negative electrode active material.

  Patent Document 2 describes a nonaqueous electrolyte battery (Example 12) using an electrolytic solution to which potassium pyrophosphate is added.

Japanese Patent Laid-Open No. 10-233208 JP 2001-357874 A

  The non-aqueous electrolyte secondary battery has a problem of a decrease in capacity maintenance rate due to a charge / discharge cycle.

  One aspect of the present invention is a nonaqueous electrolyte for a nonaqueous electrolyte secondary battery containing magnesium pyrophosphate.

  According to one aspect of the present invention, it is possible to provide a nonaqueous electrolyte for a nonaqueous electrolyte secondary battery that can be a nonaqueous electrolyte secondary battery in which a decrease in capacity retention rate due to charge / discharge cycles is suppressed.

1 is an external perspective view showing a nonaqueous electrolyte secondary battery according to an embodiment of the present invention. 1 is a schematic diagram showing a power storage device configured by assembling a plurality of nonaqueous electrolyte secondary batteries according to an embodiment of the present invention.

  One aspect of the present invention is a nonaqueous electrolyte for a nonaqueous electrolyte secondary battery containing magnesium pyrophosphate.

  According to one aspect of the present invention, it is possible to provide a nonaqueous electrolyte for a nonaqueous electrolyte secondary battery that can be a nonaqueous electrolyte secondary battery in which a decrease in capacity retention rate due to charge / discharge cycles is suppressed.

  Another aspect of the present invention is a nonaqueous electrolyte secondary battery including the nonaqueous electrolyte.

  According to another aspect of the present invention, it is possible to provide a nonaqueous electrolyte secondary battery in which a decrease in capacity maintenance rate due to charge / discharge cycles is suppressed.

  Another aspect of the present invention is a method for producing the nonaqueous electrolyte secondary battery.

  According to another aspect of the present invention, it is possible to provide a method for manufacturing a nonaqueous electrolyte secondary battery in which a decrease in capacity retention rate due to charge / discharge cycles is suppressed.

  A configuration and operational effects according to one aspect of the present invention will be described with a 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.

It does not limit about the method of manufacturing the nonaqueous electrolyte for nonaqueous electrolyte secondary batteries which concerns on this embodiment. For example, a step of adding and stirring, for example, about 0.01 to 2% by mass of magnesium pyrophosphate to a nonaqueous solvent or a solution in which an electrolyte salt is dissolved in a nonaqueous solvent can be employed. Since magnesium pyrophosphate has low solubility in a non-aqueous solvent, undissolved magnesium pyrophosphate remains in the non-aqueous solvent or the solution. The remaining magnesium pyrophosphate may be removed by filtration or the like, or may be dispersed or settled in the nonaqueous electrolyte as it is. The non-aqueous electrolyte obtained by this method contains a saturated dissolution amount of magnesium pyrophosphate, and the amount thereof is very small. In this way, a non-aqueous electrolyte to which magnesium pyrophosphate is added can be produced. A non-aqueous electrolyte to which magnesium pyrophosphate is added contains pyrophosphate anion (P ₂ O ₇ ⁴⁻ ) and magnesium cation (Mg ²⁺ ).

Further, instead of adding magnesium pyrophosphate, a compound capable of dissociating pyrophosphate anion (P ₂ O ₇ ⁴⁻ ) and a compound capable of dissociating magnesium cation (Mg ²⁺ ) may be added. A non-aqueous electrolyte containing P ₂ O ₇ ⁴⁻ ) and a magnesium cation (Mg ²⁺ ) can be produced. The non-aqueous electrolyte thus produced also corresponds to the “non-aqueous electrolyte for non-aqueous electrolyte secondary battery containing magnesium pyrophosphate” referred to in the present specification, and belongs to the technical scope of the present invention. . Here, by not using a compound that can dissociate calcium cation or potassium cation as a compound that can dissociate pyrophosphate anion (P ₂ O ₇ ⁴⁻ ) or a compound that can dissociate magnesium cation (Mg ²⁺ ), Since the possibility that the effect of the invention is suppressed can be reduced, it is preferable.

The nonaqueous electrolyte for a nonaqueous electrolyte secondary battery according to this embodiment is particularly excellent when applied to a nonaqueous electrolyte secondary battery having a positive electrode potential of 4.4 V (vs. Li ^/ Li ⁺ ) or more during charging. The effect is demonstrated.

  The non-aqueous solvent contained in the non-aqueous electrolyte for a non-aqueous electrolyte secondary battery according to this embodiment is not limited, and non-aqueous solvents that are generally proposed for use in lithium batteries and the like can be used. For example, cyclic carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate, fluoroethylene carbonate, vinylene carbonate; cyclic esters such as γ-butyrolactone and γ-valerolactone; dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate Chain carbonates such as methyl formate, methyl acetate and methyl butyrate; tetrahydrofuran or derivatives thereof; 1,3-dioxane, 1,4-dioxane, 1,2-dimethoxyethane, 1,4- Ethers such as dibutoxyethane and methyldiglyme; nitriles such as acetonitrile and benzonitrile; dioxolane or derivatives thereof; ethylene sulfide, sulfolane, sultone or the like Examples thereof include, but are not limited to, derivatives alone or a mixture of two or more thereof.

  As will be described later, when a fluorine-substituted cyclic carbonate such as fluoroethylene carbonate is used as the cyclic carbonate, for example, a nonaqueous electrolyte secondary battery excellent in charge / discharge cycle performance can be provided due to a synergistic effect with magnesium pyrophosphate. Therefore, it is preferable.

  When the non-aqueous solvent contains a cyclic carbonate such as ethylene carbonate and a chain carbonate such as ethyl methyl carbonate and diethyl carbonate, the volume ratio of the cyclic carbonate in the total volume of the cyclic carbonate and the chain carbonate is 5 volume% or more is preferable and 10 volume% or more is more preferable. Moreover, 50 volume% or less is preferable and 30 volume% or less is more preferable.

Examples of the electrolyte salt used for the non-aqueous electrolyte include LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiPF ₆ , LiSCN, LiBr, LiI, Li ₂ SO ₄ , Li ₂ B ₁₀ Cl ₁₀ , NaClO ₄ , NaI, NaSCN, Inorganic ion salts containing one of lithium (Li), sodium (Na) or potassium (K) such as NaBr, KClO ₄ , KSCN, LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , LiC (C ₂ F ₅ SO ₂ ) ₃ , (CH ₃ ) ₄ NBF ₄ , _{(CH 3) 4 NBr, (} C 2 H 5) 4 NClO 4, (C 2 H 5) 4 NI, (C 3 H 7) 4 NBr, (n-C _{H 9) 4, NClO 4,} (n-C 4 H 9) 4 NI, (C 2 H 5) 4 N-maleate, (C 2 H 5) 4 N-benzoate, (C 2 H 5) 4 N- Examples thereof include organic ionic salts such as phthalate, lithium stearyl sulfonate, lithium octyl sulfonate, and lithium dodecylbenzene sulfonate, and these ionic compounds can be used alone or in admixture of two or more.

There is no restriction | limiting in particular as a positive electrode active material used for the positive electrode of the nonaqueous electrolyte secondary battery which concerns on this embodiment, A various material can be used suitably. Among them, those capable of electrochemically inserting and removing lithium ions and having a positive electrode potential of 4.4 V (vs. Li ^/ Li ⁺ ) or more are preferable, and are generally positive electrode active materials for non-aqueous electrolyte secondary batteries. Lithium transition metal composite oxide, polyanion compound, etc. used in the above can be used. Examples of the lithium transition metal composite oxide include lithium nickel manganese composite oxide and lithium nickel cobalt manganese composite oxide. The lithium-nickel-manganese composite oxide, for _{example, Li x Ni y Mn 2-} y O 4-δ (0 <x having a spinel structure such as _{LiNi 0.5 Mn 1.5 O 4 <1.1,0} . 45 <transition _{y <0.55,0 ≦ δ <0.4)} , the LiMeO ₂ (Me having alpha-NaFeO ₂ type crystal structure such as LiNi _{1/2 Mn} 1/2 _{O 2} containing Ni, and Mn Metal) and Li _{1 + α} Me _1-α O ₂ (0 <α) having an α-NaFeO ₂ type crystal structure such as Li _1.11 Ni _0.29 Mn _0.60 O ₂ (Li / Me = 1.25). , Me is a transition metal containing Ni and Mn), and as the lithium nickel cobalt manganese composite oxide, for example, the same α-NaFeO ₂ type crystal structure such as LiNi _1/3 Co _1/3 Mn _1/3 O ₂ is used. LiMeO _{2 with} (Me is a transition metal containing Ni, Co and Mn), Li _1.09 Co _0.11 Ni _0.18 Mn _0.62 O ₂ (Li / Me = 1.2), Li _1.11 Co _{0. 11} Ni _0.18 Mn _0.60 O ₂ (Li / Me = 1.25) and other Li _{1 + α} Me _1-α O ₂ having an α-NaFeO ₂ type crystal structure (0 <α, Me is Ni, Co and Transition metals including Mn) can be used. As the polyanion compound, for example, LiNiPO ₄ , LiCoPO ₄ , Li ₂ CoPO ₄ F, Li ₂ MnSiO _4, or the like can be used.

The negative electrode active material used for the negative electrode constituting the non-aqueous electrolyte secondary battery according to this embodiment is one that can electrochemically insert and desorb lithium ions, and has a positive electrode potential of 4.4 V (vs. Li). / Li ⁺ ) that can be used at a high voltage in combination with a positive electrode that is more than or equal to, a carbonaceous material generally used as a negative electrode active material of a non-aqueous electrolyte secondary battery, a metal oxide such as tin oxide or silicon oxide, Metal composite oxides, lithium alloys such as lithium alone and lithium aluminum alloys, metals that can form alloys with lithium such as Sn and Si, and the like can be used. Examples of the carbonaceous material include natural graphite, artificial graphite, coke, non-graphitizable carbon, low-temperature calcinable graphitizable carbon, fullerene, carbon nanotube, carbon black, activated carbon and the like. These may be used individually by 1 type, or may use 2 or more types together by arbitrary combinations and a ratio. Of these, carbonaceous materials are preferable from the viewpoint of safety, and graphite is particularly preferable.

  The positive electrode active material powder and the negative electrode active material powder preferably 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 secondary 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.

  As described above, the positive electrode active material and the negative electrode active material which are main components of the positive electrode and the negative electrode have been described in detail. In addition to the main component, the positive electrode and the negative electrode include a conductive agent, a binder, a thickener, A filler etc. may be contained as another structural component.

  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, flake graphite, and lump 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.

  The filler may be any material as long as it does not adversely affect battery performance. 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 based on the total weight of the positive electrode or the negative electrode.

  The positive electrode and the negative electrode are prepared by mixing the main constituent components (positive electrode active material in the positive electrode, negative electrode active material in the negative electrode) and other materials into a mixture, an organic solvent such as N-methylpyrrolidone and toluene, After mixing with water, the obtained mixed solution is applied or pressure-bonded onto a current collector described in detail below, and heat-treated at a temperature of about 50 ° C. to 250 ° C. for about 2 hours. 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 secondary battery include polyolefin resins typified by polyethylene, polypropylene, etc .; polyester resins typified by polyethylene terephthalate, polybutylene terephthalate, etc .; polyvinylidene fluoride; -Hexafluoropropylene 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; Den - 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, and polyvinylidene fluoride and a non-aqueous 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, when the separator is used in combination with the above-described porous film, nonwoven fabric or the like and a polymer gel, it is desirable because the liquid retention of the electrolyte is improved. For example, by forming a microporous film made of a solvophilic polymer having a thickness of several μm or less on the surface of the polyethylene porous membrane and the microporous wall, and holding the nonaqueous electrolyte in the micropores of the film, The solvophilic polymer 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 by irradiation with an electron beam (EB) or heating or ultraviolet (UV) irradiation with a radical initiator added.

  FIG. 1 shows a schematic diagram of a rectangular nonaqueous electrolyte secondary battery 1 which is a nonaqueous electrolyte secondary battery according to an embodiment of the present invention. In the figure, the inside of the container is seen through. In the nonaqueous electrolyte secondary battery 1 shown in FIG. 1, an 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 configuration of the non-aqueous electrolyte secondary battery is not particularly limited, and examples thereof include a cylindrical battery having a positive electrode, a negative electrode, and a roll separator, a square battery, and a flat battery. One embodiment of the present invention can also be realized as a power storage device including a plurality of the above nonaqueous electrolyte secondary batteries. One embodiment of a power storage device is shown in FIG. In FIG. 2, 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).

<Initial performance test>
In this specification, the initial performance test of the nonaqueous electrolyte secondary battery is performed under the following conditions. The nonaqueous electrolyte secondary battery is first subjected to an initial charge / discharge process of two cycles at 25 ° C. All voltage control is performed on the voltage between the positive and negative terminals. The charging is a constant current / constant voltage charging with a current of 0.2 CmA and a voltage of 4.35 V for 8 hours, and the discharging is a constant current discharging with a current of 0.2 CmA and a final voltage of 2.75 V. After charging and discharging, a pause time of 10 minutes is set. In the present specification, graphite is used for the negative electrode active material in the nonaqueous electrolyte secondary batteries according to the examples. In this case, it is known that the value of the positive electrode potential at the end of charging is approximately 0.05 V larger than the value of the battery voltage. Therefore, the positive electrode potential at the end of charging is about 4.4 V (vs. Li / Li ⁺ ). The discharge capacity at the second cycle is defined as “initial discharge capacity (mAh)”. After charging again under the same conditions as described above, an “initial internal resistance (mΩ)” is measured by applying an alternating current (AC) of 1 kHz using an impedance meter. Further, the battery thickness is measured with a caliper and is set as “initial battery thickness (mm)”.

<Charge / discharge cycle test>
In this specification, the charge / discharge cycle test of the nonaqueous electrolyte secondary battery is performed under the following conditions. The nonaqueous electrolyte secondary battery is charged and discharged at 45 ° C. after the initial performance test. All voltage control is performed on the voltage between the positive and negative terminals. The charging is a constant current / constant voltage charging with a current of 1.0 CmA and a voltage of 4.35 V for 3 hours, and the discharging is a constant current discharging with a current of 1.0 CmA and a final voltage of 2.75 V. After charging and discharging, a pause time of 10 minutes is set.

  After 75 cycles of charge / discharge, 1 cycle charge / discharge is performed at 25 ° C. under the same conditions as in the initial performance test, and the “75th cycle discharge capacity (mAh)” is obtained. Next, after charging under the same conditions as described above, an “75th cycle internal resistance (mΩ)” is measured by applying an alternating current (AC) of 1 kHz using an impedance meter. Further, the battery thickness is measured with a caliper and is defined as “75th cycle battery thickness (mm)”. The percentage of the “75th cycle discharge capacity (mAh)” with respect to the “initial discharge capacity (mAh)” is defined as “discharge capacity maintenance ratio (%)”. The percentage of the “75th cycle internal resistance (Ω)” relative to the “initial internal resistance (Ω)” is defined as “internal resistance increase rate (%)”. The percentage of “75th cycle battery thickness (mm)” with respect to “initial battery thickness (mm)” is defined as “battery thickness increase rate (%)”.

<Cycle life test>
In this specification, the cycle life test of the nonaqueous electrolyte secondary battery is performed under the following conditions. Charging / discharging is continued under the above charging / discharging cycle test conditions, and the number of cycles charged / discharged until the discharge capacity (mAh) decreases to 60% of the “initial discharge capacity (mAh)” is measured. "Life".

(Comparative Example 1)
An electrolyte solution in which LiPF ₆ was dissolved at a concentration of 1.0 mol / l in a mixed solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 3: 7 was used. Use water electrolyte.

Example 1
1.0% by mass of magnesium pyrophosphate (Mg ₂ P ₂ O ₇ , manufactured by Wako Pure Chemical Industries, Ltd.) was added to the non-aqueous electrolyte according to Comparative Example 1, and the mixture was sufficiently stirred and allowed to stand. Were collected. Therefore, in this non-aqueous electrolyte, magnesium pyrophosphate is saturated, and the amount of dissolved magnesium pyrophosphate is less than 1.0% by mass. In this way, a nonaqueous electrolyte to which magnesium pyrophosphate was added was produced. This is the nonaqueous electrolyte according to Example 1.

(Comparative Example 2)
1.0% by mass of calcium pyrophosphate (Ca ₂ P ₂ O ₇ , manufactured by Strem Chemicals) is added to the nonaqueous electrolyte according to Comparative Example 1, and the mixture is sufficiently stirred and allowed to stand, and the supernatant is collected. did. This is the nonaqueous electrolyte according to Comparative Example 2.

(Comparative Example 3)
1.0% by mass of potassium pyrophosphate (K ₄ P ₂ O ₇ , manufactured by Wako Pure Chemical Industries, Ltd.) was added to the non-aqueous electrolyte according to Comparative Example 1, and the mixture was sufficiently stirred and allowed to stand. Were collected. This is the nonaqueous electrolyte according to Comparative Example 3.

(Preparation of non-aqueous electrolyte secondary battery)
Using each of these non-aqueous electrolytes, non-aqueous electrolyte secondary batteries were produced by the following procedure. In the following examples, a lithium transition metal composite oxide having an α-NaFeO ₂ type crystal structure and represented by LiNi _1/3 Mn _1/3 Co _1/3 O ₂ was used as a positive electrode active material.

  A positive electrode paste containing the positive electrode active material, acetylene black (AB) and polyvinylidene fluoride (PVdF) in a mass ratio of 94: 3: 3 (in terms of solid content) and N-methylpyrrolidone (NMP) as a solvent. After making and apply | coating to both surfaces of a 20-micrometer-thick strip | belt-shaped aluminum foil electrical power collector, NMP was volatilized. The positive electrode was pressure-molded with a roller press to form a positive electrode active material layer, and then vacuum-dried at 100 ° C. for 14 hours to volatilize water in the electrode plate. In this way, a positive electrode plate was produced.

  Graphite, styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) were mixed at a mass ratio of 97: 2: 1 (in terms of solid content) to prepare a negative electrode paste using water as a solvent, and having a thickness of 10 μm. After coating on both sides of the strip-shaped copper foil current collector, water was volatilized. The negative electrode was pressure-molded with a roller press to form a negative electrode active material layer, and then vacuum-dried at 100 ° C. for 12 hours to volatilize moisture in the electrode plate. In this way, a negative electrode plate was produced.

  A positive electrode plate and a negative electrode plate are laminated through a separator made of a polyethylene porous membrane, wound in a flat shape to produce a power generation element, housed in an aluminum square battery case, and attached with positive and negative electrode terminals . The container was sealed after injecting a nonaqueous electrolyte into the container. In this manner, a nonaqueous electrolyte secondary battery having a design capacity of 800 mAh (1 CmA = 800 mAh) according to Example 1 and Comparative Examples 1 to 4 was produced.

  According to the above procedure, an initial performance test and a charge / discharge cycle test were performed, and “discharge capacity maintenance rate (%)”, “internal resistance increase rate (%)”, and “battery thickness increase rate (%)” were obtained. The results are shown in Table 1.

  From Table 1, the non-aqueous electrolyte secondary battery of Example 1 using a non-aqueous electrolyte containing magnesium pyrophosphate as an additive is the non-aqueous electrolyte of Comparative Example 1 containing no additive or other pyrroline. It turned out that the fall of the capacity | capacitance maintenance factor by a charging / discharging cycle is suppressed compared with the nonaqueous electrolyte secondary battery using the nonaqueous electrolyte of the comparative examples 2 and 3 using an acid compound. Moreover, it turned out that the increase in internal resistance by a charging / discharging cycle is suppressed. Moreover, it turned out that the increase in the battery thickness by a charging / discharging cycle is suppressed.

(Comparative Example 4)
According to Comparative Example 4, an electrolytic solution in which LiPF ₆ was dissolved at a concentration of 1.0 mol / l in a mixed solvent in which fluoroethylene carbonate (FEC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 1: 9 was used. A non-aqueous electrolyte is used.

(Example 2)
1.0% by mass of magnesium pyrophosphate (Mg ₂ P ₂ O ₇ , manufactured by Wako Pure Chemical Industries, Ltd.) was added to the nonaqueous electrolyte according to Comparative Example 4 and the mixture was sufficiently stirred and allowed to stand, and the supernatant Were collected. This is designated as the nonaqueous electrolyte according to Example 2.

  Using the nonaqueous electrolytes of Examples 1 and 2 and Comparative Examples 1 and 4, using the same material as the nonaqueous electrolyte secondary battery of Example 1 described above, the nonaqueous electrolyte secondary battery was manufactured in the same procedure. Produced. However, the design capacity was 850 mAh (1 CmA = 850 mAh).

  According to the above procedure, an initial performance test and a charge / discharge cycle test were performed to obtain a “cycle life”. The results are shown in Table 2.

  From Table 2, the non-aqueous electrolyte for a non-aqueous electrolyte secondary battery according to this embodiment has excellent cycle life due to the effects of the present invention even if the type of non-aqueous solvent contained is different. I understand that. In particular, a nonaqueous electrolyte secondary battery using the nonaqueous electrolyte of Example 2 containing magnesium pyrophosphate as an additive and fluoroethylene carbonate (FEC), which is a fluorine-substituted cyclic carbonate, as a cyclic carbonate, Compared with the non-aqueous electrolyte secondary battery using the non-aqueous electrolyte of Comparative Example 4 that does not contain magnesium, the cycle life is significantly superior due to the synergistic effect of fluoroethylene carbonate (FEC) and magnesium pyrophosphate. Recognize.

In recent years, high energy density is also required for non-aqueous electrolyte secondary batteries used as power sources for automobiles. For this reason, there has been a demand for a non-aqueous electrolyte secondary battery that can exhibit excellent performance even when a method of use for charging at a higher voltage than before is employed. The nonaqueous electrolyte for a nonaqueous electrolyte secondary battery according to this embodiment is particularly excellent when applied to a nonaqueous electrolyte secondary battery having a positive electrode potential of 4.4 V (vs. Li ^/ Li ⁺ ) or more during charging. Therefore, the industrial applicability is high.

(Explanation of symbols)
DESCRIPTION OF SYMBOLS 1 Nonaqueous electrolyte secondary battery 2 Electrode group 3 Battery container 4 Positive electrode terminal 4 'Positive electrode lead 5 Negative electrode terminal 5' Negative electrode lead 20 Power storage unit 30 Power storage device

Claims (4)

  A non-aqueous electrolyte for a non-aqueous electrolyte secondary battery, containing magnesium pyrophosphate.   The nonaqueous electrolyte for a nonaqueous electrolyte secondary battery according to claim 1, comprising a fluorine-substituted cyclic carbonate.   A nonaqueous electrolyte secondary battery using the nonaqueous electrolyte for a nonaqueous electrolyte secondary battery according to claim 1. A nonaqueous electrolyte secondary battery is produced using the nonaqueous electrolyte for a nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the nonaqueous electrolyte secondary battery is produced.