JP2017021949A - Nonaqueous lithium battery and application method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous lithium battery that is enhanced in thermal stability.SOLUTION: A nonaqueous lithium battery includes a positive electrode containing lithium nickel composite oxide represented by the general formula: LiNiMO(in the formula, M represents at least one selected from the group consisting of Co, Al, Mg, Fe and Ti, x satisfies 0.1≤x≤0.5), a negative electrode, and nonaqueous electrolytic solution which is interposed between the positive electrode and the negative electrode and contains LiPFand LiFSI, LiFSI being contained in the range from 10 or more mol% to 90 or less mol% with respect to the total amount of LiPFand LiFSI. According to a method of using the present invention, the nonaqueous lithium battery described above is charged and discharged in a range where the potential based on lithium of positive electrode active material does not exceed 4.2 V.SELECTED DRAWING: None

Description

  The present invention relates to a non-aqueous lithium battery and a method for using the same.

In recent years, it has been proposed to use LiFSI (lithium bis (fluorosulfonyl) imide) in this type of battery. For example, Patent Document 1 describes a battery using a nonaqueous electrolytic solution in which LiPF ₆ and LiFSI are dissolved in a mixed solvent of ethylene carbonate and ethyl methyl carbonate. In this battery, lithium cobaltate is used as the positive electrode and graphite is used as the negative electrode.

JP2015-62154A

  However, in the battery of Patent Document 1, in the heating test at about 150 ° C., the internal pressure of the battery may rise to 20 atmospheres or more, and thus the thermal stability may be low. For this reason, it has been desired to increase the thermal stability.

  The present invention has been made to solve such a problem, and has as its main object to provide a battery capable of further improving the thermal stability.

In order to achieve the above-described object, the present inventors have intensively studied. And in a battery comprising a positive electrode such as LiNi _0.8 Co _0.15 Al _0.05 O ₂ and a nonaqueous electrolytic solution containing LiPF ₆ and LiFSI and having 10 mol% or more and 90 mol% or less of LiFSI out of the total of both, The present inventors have found that the thermal stability is higher and have completed the present invention.

That is, the non-aqueous lithium battery of the present invention is
General formula LiNi _1-x M _x O ₂ (wherein M is one or more selected from the group consisting of Co, Al, Mg, Fe, Ti, and x satisfies 0.1 ≦ x ≦ 0.5) A positive electrode containing a lithium nickel composite oxide represented by:
A negative electrode,
A nonaqueous electrolyte solution that is interposed between the positive electrode and the negative electrode, includes LiPF ₆ and LiFSI, and 10 mol% or more and 90 mol% or less of the total of LiPF ₆ and LiFSI is LiFSI;
It is equipped with.

The method of use of the present invention is as follows:
A method of using the non-aqueous lithium battery described above,
Charging / discharging is performed in a range where the lithium-based potential of the positive electrode active material does not exceed 4.2V.

  In this non-aqueous lithium battery, the thermal stability can be further improved. The reason why such an effect is obtained is estimated as follows, for example. That is, on the surface of the nickel-containing oxide positive electrode active material, the nickel element exposed on the surface of the active material and LiFSI in the electrolytic solution have a strong interaction, thereby acting as a protective film that enhances thermal stability. This is presumed to suppress heat generation due to the reaction between the substance and the electrolyte.

1 is a schematic diagram showing an example of a non-aqueous lithium battery 10.

  The non-aqueous lithium battery of the present invention includes a positive electrode including a positive electrode active material capable of occluding and releasing lithium, a negative electrode including a negative electrode active material capable of occluding and releasing lithium, and a lithium ion interposed between the positive electrode and the negative electrode. A non-aqueous electrolyte that conducts heat.

The positive electrode contains a lithium nickel composite oxide represented by the general formula LiNi _1-x M _x O ₂ as a positive electrode active material. In the formula, M is one or more selected from the group consisting of Co, Al, Mg, Fe, and Ti, and x satisfies 0.1 ≦ x ≦ 0.5. The lithium nickel composite oxide preferably includes at least Co as M, more preferably includes Co and Al, and further preferably includes Co, Al, and Mg. In such a thing, thermal stability can be improved more. In the lithium nickel composite oxide, the value of x is not particularly limited as long as it is in the range of 0.1 or more and 0.5 or less, but is preferably 0.15 or more, more preferably 0.2 or more, and 45 or less is preferable and 0.4 or less is more preferable. If x is 0.1 or more, the lithium nickel composite oxide is unlikely to deteriorate and the durability, which is the basic performance of the battery, is considered high. If x is 0.5 or less, it is considered that the nickel element is sufficiently exposed on the surface of the active material, and the thermal stability of the battery can be improved. The positive electrode may contain at least one of LiNi _0.8 Co _0.15 Al _0.05 O ₂ and LiNi _0.75 Co _0.15 Al _0.05 Mg _0.05 O ₂ as a lithium nickel composite oxide. The lithium nickel composite oxide may be a layered oxide. In the compounds represented by the general formula or chemical formula, some of the constituent elements shown in the general formula or chemical formula may be missing, excessive, or substituted with other elements (the same applies hereinafter). ).

  For the positive electrode, for example, the above positive electrode active material, a conductive material and a binder are mixed, and an appropriate solvent is added to form a paste-like positive electrode mixture, which is applied to the surface of the current collector and dried. The electrode may be compressed to increase the electrode density. The conductive material is not particularly limited as long as it is an electron conductive material that does not adversely affect the battery performance of the positive electrode. For example, graphite such as natural graphite (scale-like graphite, scale-like graphite) or artificial graphite, acetylene black, carbon black, What mixed 1 type (s) or 2 or more types, such as ketjen black, carbon whisker, needle coke, carbon fiber, metal (copper, nickel, aluminum, silver, gold, etc.) can be used. Among these, as the conductive material, carbon black and acetylene black are preferable from the viewpoints of electron conductivity and coatability.

  The binder serves to bind the active material particles and the conductive material particles. For example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), a fluorine-containing resin such as fluororubber, polypropylene, Thermoplastic resins such as polyethylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM rubber, natural butyl rubber (NBR) and the like can be used alone or as a mixture of two or more. In addition, an aqueous dispersion of cellulose or styrene butadiene rubber (SBR), which is an aqueous binder, can also be used.

  Examples of the solvent for dispersing the positive electrode active material, the conductive material and the binder include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethylenetriamine, N, Organic solvents such as N-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran can be used. Moreover, a dispersing agent, a thickener, etc. may be added to water, and an active material may be slurried with latex, such as SBR. As the thickener, for example, polysaccharides such as carboxymethyl cellulose and methyl cellulose can be used alone or as a mixture of two or more.

  Examples of the application method include roller coating such as applicator roll, screen coating, doctor blade method, spin coating, bar coater, and the like, and any of these can be used to obtain an arbitrary thickness and shape.

  Current collectors include aluminum, titanium, stainless steel, nickel, iron, calcined carbon, conductive polymer, conductive glass, and aluminum, copper, etc. for the purpose of improving adhesion, conductivity, and oxidation resistance. A surface treated with carbon, nickel, titanium, silver or the like can be used. For these, the surface can be oxidized. Examples of the shape of the current collector include foil, film, sheet, net, punched or expanded, lath, porous, foam, and formed fiber group. The thickness of the current collector is, for example, 1 to 500 μm.

  The negative electrode may be formed by closely adhering the negative electrode active material and the current collector. For example, the negative electrode active material, a conductive material, and a binder are mixed, and an appropriate solvent is added to form a paste-like negative electrode material. The product may be applied to the surface of the current collector and dried, and may be compressed to increase the electrode density as necessary. Examples of the negative electrode active material include an inorganic compound such as lithium metal, a lithium alloy, and a tin compound, a carbonaceous material capable of inserting and extracting lithium ions, a composite oxide containing a plurality of elements, and a conductive polymer. Examples of the carbonaceous material include cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, and carbon fibers. Of these, graphites such as artificial graphite and natural graphite have an operating potential close to that of metallic lithium, can be charged and discharged at a high operating voltage, and suppresses self-discharge when a lithium salt is used as a supporting salt. In addition, the irreversible capacity during charging can be reduced, which is preferable. Examples of the composite oxide include lithium titanium composite oxide and lithium vanadium composite oxide. Among these, as the negative electrode active material, a carbonaceous material is preferable from the viewpoint of safety, and graphite is more preferable. In addition, as the conductive material, binder, solvent, and the like used for the negative electrode, those exemplified for the positive electrode can be used. The negative electrode current collector includes copper, nickel, stainless steel, titanium, aluminum, calcined carbon, conductive polymer, conductive glass, Al-Cd alloy, etc., as well as improved adhesion, conductivity and reduction resistance. For the purpose, for example, a copper surface treated with carbon, nickel, titanium, silver or the like can be used. For these, the surface can be oxidized. The shape of the current collector can be the same as that of the positive electrode.

The non-aqueous electrolyte contains a lithium salt and a non-aqueous solvent. The non-aqueous electrolyte contains a mixture of LiPF ₆ and LiFSI (lithium bis (fluorosulfonyl) imide) as a lithium salt, and 10 mol% or more and 90 mol% or less of the total of LiPF ₆ and LiFSI is LiFSI. If 10 mol% or more is LiFSI, the effect of the thermal stability improvement by LiFSI is acquired. Moreover, if 90 mol% or less is LiFSI, the heat_generation | fever starting temperature of a positive electrode and a negative electrode can be raised, and thermal stability can be improved. LiFSI is preferably at least 20 mole% of the total of LiPF ₆ and LiFSI, more preferably at least 30 mol%, more preferably at least 70 mol%. This is because the higher the LiFSI ratio, the higher the thermal stability. LiFSI may be 85% or less of the total of LiPF ₆ and LiFSI, or may be 80% or less. This lithium salt preferably has a concentration in the non-aqueous electrolyte of 0.1 mol / L or more and 5 mol / L or less, and more preferably 0.5 mol / L or more and 2 mol / L or less. When the concentration for dissolving the lithium salt is 0.1 mol / L or more, a sufficient current density can be obtained, and when the concentration is 5 mol / L or less, the electrolytic solution can be made more stable.

  Examples of the non-aqueous solvent include carbonates, esters, ethers, nitriles, furans, sulfolanes and dioxolanes, and these can be used alone or in combination. Specifically, as carbonates, cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate, butylene carbonate, chloroethylene carbonate, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl Chain carbonates such as carbonate (DEC), ethyl-n-butyl carbonate, methyl-t-butyl carbonate, di-i-propyl carbonate, t-butyl-i-propyl carbonate, γ-butyllactone, γ-valerolactone Cyclic esters such as methyl formate, chain acetates such as methyl acetate, ethyl acetate, methyl butyrate, ethers such as dimethoxyethane, ethoxymethoxyethane, diethoxyethane, acetonitrile, Nitriles such as benzonitrile, furans such as tetrahydrofuran and methyltetrahydrofuran, sulfolanes such as sulfolane and tetramethylsulfolane, and dioxolanes such as 1,3-dioxolane and methyldioxolane. Among these, it is preferable that a non-aqueous solvent contains EC. Further, the non-aqueous solvent is preferably a combination of a cyclic carbonate and a chain carbonate, for example, a combination of EC that is a cyclic carbonate and a chain carbonate is more preferable, and EC that is a cyclic carbonate and a chain carbonate. A combination with at least one selected from the group consisting of DMC, EMC and DEC is more preferred. According to the combination of cyclic carbonates and chain carbonates, not only the cycle characteristics representing the battery characteristics by repeated charge and discharge are excellent, but also the viscosity of the electrolyte, the electric capacity of the obtained battery, the battery output, etc. It can be balanced. The cyclic carbonates are considered to have a relatively high relative dielectric constant and increase the dielectric constant of the electrolytic solution, and the chain carbonates are considered to suppress the viscosity of the electrolytic solution.

The nonaqueous electrolytic solution may contain a supporting salt other than the lithium salt described above. Examples of the supporting salt include LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiSbF ₆ , LiSiF ₆ , LiAlF ₄ , LiSCN, LiClO ₄ , Examples include LiCl, LiF, LiBr, LiI, LiAlCl _{4 and the} like. When such a supporting salt is included, the concentration of the lithium salt and the supporting salt in the nonaqueous electrolytic solution is preferably 0.1 mol / L or more and 5 mol / L or less, and 0.5 mol / L or more and 2 mol / L or less. More preferably, it is L or less. In addition, the non-aqueous electrolyte may be a phosphor to which a flame retardant such as phosphorus or halogen is added. The nonaqueous electrolytic solution may be a gel electrolyte in which the nonaqueous electrolytic solution is contained in a saccharide such as a polymer such as polyvinylidene fluoride, polyethylene glycol, or polyacrylonitrile, an amino acid derivative, or a sorbitol derivative.

  The nonaqueous lithium battery of the present invention may include a separator between the positive electrode and the negative electrode. The separator is not particularly limited as long as it has a composition that can withstand the range of use of the non-aqueous lithium battery. For example, a polymer nonwoven fabric such as a polypropylene nonwoven fabric or a polyphenylene sulfide nonwoven fabric, or a thin fine olefin resin such as polyethylene or polypropylene is used. A porous membrane is mentioned. These may be used alone or in combination.

The shape of the nonaqueous lithium battery of the present invention is not particularly limited, and examples thereof include a coin type, a button type, a sheet type, a laminated type, a cylindrical type, a flat type, and a square type. Moreover, you may apply to the large sized thing etc. which are used for an electric vehicle etc. FIG. 1 is a schematic diagram showing an example of a non-aqueous lithium battery 10 of the present invention. This non-aqueous lithium battery 10 includes a positive electrode sheet 13 in which a positive electrode mixture 12 is formed on a current collector 11, a negative electrode sheet 18 in which a negative electrode mixture 17 is formed on the surface of the current collector 14, and the positive electrode sheet 13 and the negative electrode sheet. 18 and a non-aqueous electrolyte solution 20 that fills the space between the positive electrode sheet 13 and the negative electrode sheet 18. In this nonaqueous lithium battery 10, the separator 19 is sandwiched between the positive electrode sheet 13 and the negative electrode sheet 18, and these are wound and inserted into the cylindrical case 22, and the positive electrode terminal 24 and the negative electrode sheet 18 connected to the positive electrode sheet 13. And a negative electrode terminal 26 connected to each other. Here, the positive electrode mixture 12 has a general formula LiNi _1-x M _x O ₂ (wherein M is one or more selected from the group consisting of Co, Al, Mg, Fe, Ti, and x is 0.1 ≦ x ≦ 0.5) is included. Further, the nonaqueous electrolyte solution 20 may include LiPF ₆ and LiFSI, LiPF ₆ and 90 mol% 10 mol% or more of total LiFSI following is LiFSI. As the battery case (the cylindrical case 22 or the like), for example, nickel-plated iron, nickel, titanium, or the like can be used.

  In the nonaqueous lithium battery detailed above, the thermal stability can be further improved. Further, the battery performance (for example, initial resistance) is equal to or higher than that in the case where LiFSI is not used.

The usage method of the present invention is a usage method of the non-aqueous lithium battery described above, in which charging / discharging is performed in a range where the lithium-based potential of the positive electrode active material does not exceed 4.2 V (vs. Li ⁺ / Li). It is. In Patent Document 1 described above, the coating on the electrode surface is optimized by charging at least the initial charge until the battery voltage becomes 4.3 V or higher. However, in the present invention, the battery voltage exceeds 4.3 V. Thermal stability can be further improved even when charging / discharging is performed in a range that does not exist (for example, a range in which the lithium-based potential of the positive electrode active material does not exceed 4.2 V). The usage method of the present invention may perform charging and discharging in a range where the lithium-based potential of the positive electrode active material does not exceed 4.15V. The non-aqueous lithium battery of the present invention described above may be charged / discharged in a potential range other than such a potential range.

  It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be implemented in various modes as long as it belongs to the technical scope of the present invention.

  Below, the example which produced the non-aqueous lithium battery of this invention concretely is demonstrated as an experiment example. Experimental examples 1 to 6 correspond to examples, and experimental examples 7 to 12 correspond to comparative examples.

[Experiment 1]
(Electrolyte adjustment)
A mixture having a LiPF ₆ : LiFSI molar ratio of 80:20 was used as a lithium salt, and the mixture was dissolved in a non-aqueous solvent so that the total amount of the lithium salt was 1 mol / L to prepare an electrolytic solution. As the non-aqueous solvent, a mixed solvent in which EC and EMC were mixed so as to have a volume ratio of 3: 7 was used.

(Test battery preparation)
LiNi _0.8 Co _0.15 Al _0.05 O ₂ was used as the positive electrode active material. 85% by mass of the positive electrode active material, 10% by mass of carbon black as a conductive material, and 5% by mass of PVdF as a binder were mixed, and an appropriate amount of NMP was added and dispersed as a dispersing material to obtain a slurry-like positive electrode mixture. The slurry-like positive electrode mixture was applied to both sides of an aluminum foil current collector having a thickness of 15 μm, dried, and then densified with a roll press to obtain a positive electrode sheet. The amount of positive electrode active material deposited was about 7.0 mg / cm ² per side.

Amorphous coated graphite was used as the negative electrode active material. 95% by mass of the negative electrode active material and 5% by mass of PVdF as a binder were mixed, and an appropriate amount of NMP was added and dispersed as a dispersion material to obtain a slurry-like negative electrode mixture. The slurry-like negative electrode mixture was applied to both surfaces of a 10 μm thick copper foil current collector and dried, and then densified with a roll press to obtain a negative electrode sheet. In addition, the adhesion amount of the negative electrode active material was about 5.0 mg / cm ² per side.

  A positive electrode current collector tab lead and a negative electrode current collector tab lead were welded to the sheet-like positive electrode and negative electrode, respectively. A roll electrode body was fabricated by sandwiching a polyethylene microporous membrane separator having a thickness of 20 μm between the positive electrode and the negative electrode, and winding the positive electrode, the negative electrode, and the separator.

  Subsequently, this roll electrode body was inserted into an 18650 type cylindrical battery case comprising an outer can and a cap. The cylindrical battery case is made of nickel-plated iron and is provided with a rupture (rupture disk) on the bottom of the can. When the battery internal pressure rises and exceeds 20 atm, the valve is opened. The positive electrode current collecting lead was connected to the positive electrode current collecting tab arranged on the cap side of the battery case by welding, and the negative electrode current collecting lead was connected to the negative electrode current collecting tab arranged on the bottom of the outer can by welding.

  Next, the battery case was impregnated with the electrolytic solution prepared as described above, and then the cap was subjected to caulking to seal the battery case, thereby producing a cylindrical non-aqueous lithium secondary battery.

(conditioning)
Using the cylindrical non-aqueous lithium secondary battery produced by the above method, an initial charge / discharge test was performed at an environmental temperature of 20 ° C. In the first cycle, the constant current method is used to charge up to an upper limit voltage of 4.1 V with a constant current of 0.2 mA / cm ² (equivalent to 0.2 C), and the lower limit voltage is set at a current density of 0.2 mA / cm ² with the constant current method. The battery was discharged to 3.0V. The second cycle was a constant current-constant voltage method. After charging at a constant current of 0.2 mA / cm ² during charging, after reaching 4.1 V, constant voltage charging (4.1 V) was performed for 1 hour. After discharging at a constant current of 0.1 mA / cm ² (equivalent to 0.1 C), the voltage reached 3.0 V and then a constant voltage discharge (3.0 V) to prepare a test battery.

(Heating test)
In the heating test, first, a conditioned test battery was charged by the constant current constant voltage method at an environmental temperature of 20 ° C. The current density was 0.2 mA / cm ² and constant voltage charging (4.1 V) was performed for 1 hour to prepare a battery with SOC 100% (battery voltage 4.1 V). The battery in a state of SOC 100% was heated from room temperature to a predetermined temperature at 5 ° C./min in an environmental tester and held at the predetermined temperature for 60 minutes. The result of the heating test was judged as “no event” or “valve open”. “No event” means that nothing happens to the battery, and “open” means that the internal pressure of the battery rises and the rupture portion provided at the bottom of the battery can open. The temperature of the heating test (predetermined temperature) was measured at three points of 150 ° C, 170 ° C and 190 ° C. Table 1 shows the results of the heating test. In the table, “◯” indicates “no event”, and “×” indicates “open”.

[Experimental Examples 2 and 3]
A battery of Experimental Example 2 was fabricated in the same manner as in Experimental Example 1 except that a mixture of LiPF ₆ : LiFSI having a molar ratio of 50:50 was used as the lithium salt of the electrolytic solution, and a heating test was performed. A battery of Experimental Example 3 was produced in the same manner as in Experimental Example 1 except that a mixture having a LiPF ₆ : LiFSI molar ratio of 20:80 was used as the lithium salt of the electrolytic solution, and a heating test was performed.

[Experimental Examples 4 to 6]
A battery of Experimental Example 4 was produced in the same manner as in Experimental Example 1 except that LiNi _0.75 Co _0.15 Al _0.05 Mg _0.05 O ₂ was used as the positive electrode active material, and a heating test was performed. Experimental Example 5 similar to Experimental Example 1 except that LiNi _0.75 Co _0.15 Al _0.05 Mg _0.05 O ₂ was used as the positive electrode active material, and a mixture of LiPF ₆ : LiFSI having a molar ratio of 50:50 was used as the lithium salt of the electrolytic solution. A battery was prepared and subjected to a heating test. Experimental Example 6 as in Experimental Example 1 except that LiNi _0.75 Co _0.15 Al _0.05 Mg _0.05 O ₂ was used as the positive electrode active material and a mixture of LiPF ₆ : LiFSI having a molar ratio of 20:80 was used as the lithium salt of the electrolytic solution. A battery was prepared and subjected to a heating test.

[Experimental Examples 7 and 8]
A battery of Experimental Example 7 was produced in the same manner as Experimental Example 1 except that a LiPF ₆ : LiFSI molar ratio of 100: 0 was used as the lithium salt of the electrolytic solution, and a heating test was performed. A battery of Experimental Example 8 was produced in the same manner as Experimental Example 1 except that a LiPF ₆ : LiFSI molar ratio of 0: 100 was used as the lithium salt of the electrolytic solution, and a heating test was performed.

[Experimental Examples 9 and 10]
Experimental Example 9 as in Experimental Example 1 except that LiNi _0.75 Co _0.15 Al _0.05 Mg _0.05 O ₂ was used as the positive electrode active material, and the lithium salt of the electrolytic solution was a LiPF ₆ : LiFSI molar ratio of 100: 0. A battery was prepared and subjected to a heating test. Experimental Example 10 as in Experimental Example 1 except that LiNi _0.75 Co _0.15 Al _0.05 Mg _0.05 O ₂ was used as the positive electrode active material, and the LiPF ₆ : LiFSI molar ratio was 0: 100 as the lithium salt of the electrolytic solution. A battery was prepared and subjected to a heating test.

[Experimental Examples 11 and 12]
A battery of Experimental Example 11 was fabricated in the same manner as in Experimental Example 1 except that LiCoO ₂ was used as the positive electrode active material and a LiPF ₆ : LiFSI molar ratio of 100: 0 was used as the lithium salt of the electrolytic solution. Went. A battery of Experimental Example 12 was fabricated in the same manner as in Experimental Example 1 except that LiCoO ₂ was used as the positive electrode active material and a mixture of LiPF ₆ : LiFSI having a molar ratio of 20:80 was used as the lithium salt of the electrolytic solution. Went.

[Experimental result]
In Experimental Example 7 in which LiNi _0.8 Co _0.15 Al _0.05 O ₂ was used as the positive electrode active material and LiPF ₆ was 100% as the lithium salt of the electrolytic solution, the valve was opened by a 150 ° C. heating test. In Experimental Examples 1 to 3 using LiFSI added as a lithium salt of the electrolytic solution, when the amount of LIFSI added increased, the valve opening temperature increased, that is, the thermal stability as a battery improved. However, in Experimental Example 8 in which the lithium salt of the electrolytic solution was 100% LiFSI, the thermal stability was lowered and the valve was opened at 150 ° C. Such a tendency was similarly confirmed in Experimental Examples 4 to 6, 9, and 10 using LiNi _0.75 Co _0.15 Al _0.05 Mg _0.05 O ₂ as the positive electrode active material.

Separately, the differential scanning calorimetry (DSC measurement) of the positive electrode and the electrolyte in the charged state and the DSC measurement of the negative electrode and the electrolyte in the charged state were performed. Rose. From this, it was found that heat generation was suppressed by the presence of LiFSI. On the other hand, when LiFSI was set to 100%, the heat generation start temperature of the negative electrode decreased rapidly and the heat generation rate also increased. This is because, for example, the SEI film formed on the negative electrode when an electrolyte containing almost no LiPF ₆ is used is more thermally stable than the SEI film formed on the negative electrode when LiPF ₆ is included. It was guessed that it was low. In addition, when LiFSI was set to 100%, the heat generation start temperature of the positive electrode also decreased rapidly (specifically, it decreased to almost the same as when LiPF ₆ was 100%). The reason for this was presumed that, for example, the interaction between the charged positive electrode and LiFSI was very strong. For these reasons, it was considered that in Examples 8 and 10 in which LiFSI was 100% as the lithium salt of the electrolytic solution, the valve was opened at 150 ° C.

On the other hand, in Experimental Examples 11 and 12 using LiCoO ₂ as the positive electrode active material, no increase in the valve opening temperature was confirmed even when LiFSI was added.

When LiNi _0.8 Co _0.15 Al _0.05 O ₂ or LiNi _0.75 Co _0.15 Al _0.05 Mg _0.05 O ₂ was used as the positive electrode active material, the valve opening temperature increased due to the addition of LiFSI, whereas LiCoO ₂ was used as the positive electrode active material. In this case, the reason why the valve opening temperature did not increase is assumed as follows. For example, similar to the difference in catalytic ability of nickel and cobalt as methane reforming catalysts (M. Hatano and K. Otsuka, Inorg. Chim. Acta., 146, 243 (1988)), It is considered that the difference in the catalytic action between the nickel element and the cobalt element on the active material surface affects the heat generation of the positive electrode. That is, it is considered that nickel element and cobalt element exposed on the surface of the active material catalyze an exothermic reaction at the positive electrode and advance the reaction. Here, since the active material surface is in a highly oxidized state, the FSI anion is adsorbed. And probably nickel element on the surface of active material has strong interaction with FSI anion, and the reaction with electrolyte has suppressed the thermal stability, whereas cobalt element has interaction with FSI anion. Is not so strong, and it can be assumed that the thermal stability could not be improved.

From the above, LiNi _1-x M _x O ₂ (M is one or more selected from the group consisting of Co, Al, Mg, Fe, Ti, 0.1 ≦ x ≦ 0.5) positive electrode, LiPF ₆ and LiFSI It has been clarified that the heat test resistance can be further improved by using an electrolytic solution containing the above mixture.

  The present invention is not limited to the experimental examples described above, and it goes without saying that the present invention can be implemented in various modes as long as it belongs to the technical scope of the present invention.

  The present invention can be used in the field of the battery industry.

  DESCRIPTION OF SYMBOLS 10 Nonaqueous lithium battery, 11 Current collector, 12 Positive electrode mixture, 13 Positive electrode sheet, 14 Current collector, 17 Negative electrode mixture, 18 Negative electrode sheet, 19 Separator, 20 Nonaqueous electrolyte, 22 Cylindrical case, 24 Positive electrode terminal , 26 Negative terminal.

Claims (5)

General formula LiNi _1-x M _x O ₂ (wherein M is one or more selected from the group consisting of Co, Al, Mg, Fe, Ti, and x satisfies 0.1 ≦ x ≦ 0.5) A positive electrode containing a lithium nickel composite oxide represented by:
A negative electrode,
A nonaqueous electrolyte solution that is interposed between the positive electrode and the negative electrode, includes LiPF ₆ and LiFSI, and 10 mol% or more and 90 mol% or less of the total of LiPF ₆ and LiFSI is LiFSI;
A non-aqueous lithium battery.   The non-aqueous lithium battery according to claim 1, wherein the positive electrode includes a lithium nickel composite oxide having Co as M.   The positive electrode includes at least one of a lithium nickel composite oxide in which the M is Co and Al, and a lithium nickel composite oxide in which the M is Co, Al, and Mg. The nonaqueous lithium battery as described. 4. The non-aqueous lithium battery according to claim 1, wherein the non-aqueous electrolyte is LiFSI in a total of 20 mol% to 80 mol% of the total of the LiPF ₆ and the LiFSI. It is a usage method of the non-aqueous lithium battery of any one of Claims 1-4,
Charging / discharging is performed in a range where the lithium-based potential of the positive electrode active material does not exceed 4.2V
how to use.

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