JP2013145732A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery excellent in cycle characteristic at high temperature.SOLUTION: A lithium secondary battery comprises a positive electrode containing a positive electrode active material capable of occluding and discharging lithium ion; a negative electrode containing a negative electrode active material capable of occluding and discharging lithium ion; and a nonaqueous electrolyte. The nonaqueous electrolyte contains an electrolyte (1); a compound represented by a general formula (2) (XSO)(X'SO)NLi(where X, X' are a fluorine atom, 1-6C alkyl group, or 1-6C fluoroalkyl group, and at least one of X and X' is a fluorine atom), and a solvent. Provided that an existence ratio of all atoms by the XPS measurement in the range of less than or equal to 10 nm from a surface of the positive electrode or the negative electrode is 100%, an existence ratio of S atoms is more than or equal to 0.5%.

Description

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

  In recent years, since lithium secondary batteries have high energy density, they have been used as power sources for mobile communication devices, portable information terminals, etc., and the market has grown rapidly with the spread of terminals, ensuring safety. Various studies have been conducted for the purpose of improving cycle characteristics, energy density, high-temperature storage characteristics, and the like.

  Among the lithium secondary batteries, in the field of lithium ion batteries using a non-aqueous electrolyte, a positive electrode is obtained by adding a cyclic carbonate having an unsaturated bond such as vinylene carbonate or vinyl ethylene carbonate to the non-aqueous electrolyte. / It is known to improve cycle characteristics by forming a film on the negative electrode and suppressing decomposition of the electrolyte contained in the non-aqueous electrolyte (Patent Documents 1 and 2). However, when a cyclic carbonate having such an unsaturated bond forms a film on the negative electrode, the internal resistance of the battery (of all the resistances, intentionally added to the circuit as a resistor or visible wiring in the circuit) There was a problem of causing a voltage drop at the initial stage of discharge (IR drop), and there was room for improvement.

JP 2004-165151 A JP-A-11-185806

  The present invention has been made paying attention to the above-described circumstances, and an object thereof is to provide a lithium secondary battery excellent in cycle characteristics at high temperatures.

The present inventors have found that by using a specific compound in the non-aqueous electrolyte, an appropriate film can be formed on the positive electrode / negative electrode, and the lithium secondary battery that achieves the above problems can be provided. The present invention has been completed.
That is, the lithium secondary battery of the present invention is
A positive electrode containing a positive electrode active material capable of occluding and releasing lithium ions, a negative electrode containing a negative electrode active material capable of occluding and releasing lithium ions, and a lithium secondary battery comprising a non-aqueous electrolyte,
The non-aqueous electrolyte has the general formula (1); (XSO ₂ ) (X′SO ₂ ) N ^— Li ⁺ (X and X ′ are a fluorine atom, an alkyl group having 1 to 6 carbon atoms, or 1 to 1 carbon atoms. A fluoroalkyl group of 6, wherein at least one of X and X ′ is a fluorine atom), and a solvent,
The abundance ratio of S atoms is 0.5% or more when the abundance ratio of all atoms on the surface of the positive electrode or the negative electrode is 100% by XPS measurement.

  The lithium secondary battery of the present invention achieves improved cycle characteristics at high temperatures. The lithium secondary battery of the present invention is considered to exhibit high performance with little deterioration even in fields used at high temperatures, such as for automobiles (EV, PHEV, HEV) and stationary power supply batteries.

The XPS S2p spectrum of Experimental Example 1 is shown. The S2p spectrum of XPS of Experimental example 2 is shown.

The lithium secondary battery of the present invention that has solved the above problems is
A positive electrode containing a positive electrode active material capable of occluding and releasing lithium ions, a negative electrode containing a negative electrode active material capable of occluding and releasing lithium ions, a lithium secondary battery comprising a non-aqueous electrolyte, The electrolytic solution is represented by the general formula (1); (XSO ₂ ) (X′SO ₂ ) N ^— Li ⁺ (X and X ′ are fluorine atoms, alkyl groups having 1 to 6 carbon atoms, or fluoro having 1 to 6 carbon atoms) An alkyl group, and at least one of X and X ′ is a fluorine atom.), And a solvent,
The lithium secondary battery is characterized in that the abundance ratio of S atoms on the surface of the positive electrode or negative electrode is 0.5% or more when the abundance ratio of all atoms by XPS measurement is 100%, or
A positive electrode containing a positive electrode active material capable of occluding and releasing lithium ions, a negative electrode containing a negative electrode active material capable of occluding and releasing lithium ions, and a lithium secondary battery comprising a non-aqueous electrolyte,
The non-aqueous electrolyte has the general formula (1); (XSO ₂ ) (X′SO ₂ ) N ^— Li ⁺ (X and X ′ are a fluorine atom, an alkyl group having 1 to 6 carbon atoms, or 1 to 1 carbon atoms. A fluoroalkyl group of 6, wherein at least one of X and X ′ is a fluorine atom), and a solvent,
The lithium secondary battery is characterized in that the normalized value of the S atom-containing fragment of the positive secondary ion on the surface of the positive electrode or the negative electrode measured by TOF-SIMS is 2.0 × 10 ⁻⁴ or more. Have

  The present inventors have repeatedly studied to develop a higher performance lithium secondary battery. By using a compound represented by the general formula (1) in the non-aqueous electrolyte, the surface of the electrode is obtained. The present inventors have found that a coating film can be formed on the surface, and that it is possible to maintain cycle characteristics at high temperatures, thereby completing the present invention. The present invention will be described below.

<S concentration on electrode surface>
In the lithium secondary battery of the present invention, the abundance ratio of S atoms is 0.5% or more, assuming that the abundance ratio of all atoms on the surface of the positive electrode or negative electrode by XPS measurement is 100%. The presence of this S atom is due to the formation of a film of a compound represented by the general formula (1) described later on the surface of the positive electrode or the negative electrode. Preferably it is 0.7% or more, More preferably, it is 1.0% or more. If the abundance ratio of S atoms is 0.5% or less when the abundance ratio of all atoms by XPS measurement is 100%, the film formed by the compound represented by the general formula (1) is thin and sufficient effect is obtained. Can not demonstrate.
Further, among the atoms belonging to S2p among S atoms, when the area of atoms having a peak top at 165 eV to 174 eV is S1, and the area of atoms having a peak top at 160 eV to 165 eV is S2, all the existing S When the number of atoms is 100%, the presence ratio of S2 is more preferably 5% or more. More preferably, it is 7% or more, More preferably, it is 10% or more. If the content ratio of S2 is 5% or less, the film formed by the compound represented by the general formula (1) is thin, and a sufficient effect cannot be exhibited. The abundance ratio of S atoms on the surface of the positive electrode or the negative electrode can be measured, for example, as follows. In order to suppress changes in the chemical state of the surface until measurement, it is preferable to disassemble the cell, clean the electrodes, sample, and transport the sample to the apparatus in an inert atmosphere such as argon.

  The measurement of the abundance ratio of S atoms by XPS measurement can be performed as follows, for example. Al Kα ray (1486.6 eV) was used as the excitation X-ray. The X-ray diameter is 200 μm, and the photoelectron escape angle is 45 °. As a correction of Binding Energy, the peak of a peak having a peak top at 283 eV to 285 eV in the C1s peak (assigned to C—C and CHx components) was set to 284.6 eV. 9 points smoothing and peak splitting.

In the lithium secondary battery of the present invention, the total normalized value of fragments containing S atoms of positive secondary ions measured by TOF-SIMS in the range of the surface of the positive electrode or negative electrode of 5 nm or less is 2.0 × 10 ⁻⁴ or more. It is. The normalized value is a ratio to the total value of peak intensities (counts) of all ionic species (fragments) that can be assigned by TOF-SIMS. Examples of the fragment containing S atom include CH ₂ S ⁺ and C ₂ H ₅ S ⁺ among all detected ionic species, and the total value of all ionic species containing S is within the above range. That's fine. This fragment containing S atoms is due to the fact that a film made of a compound represented by the following general formula (1) is formed on the surface of the positive electrode or the negative electrode. The fragment containing S atoms on the surface of the positive electrode or the negative electrode can be measured as follows.

By irradiating Bi ₃ ⁺⁺ as primary ions, secondary ions emitted from the sample surface detect positive secondary ions and negative secondary ions.

<Lithium secondary battery>
The lithium secondary battery according to the present invention has a structure in which a positive electrode and a negative electrode are housed in a case via a separator impregnated with the electrolytic solution according to the present invention. The shape of the lithium secondary battery according to the present invention is arbitrary, and may be any of a cylindrical shape, a square shape, a laminate shape, a coin shape, a large size, and the like. Moreover, it is good also as a battery module comprised by connecting each battery in series as a high voltage power supply which requires several 10V-several 100V for mounting in an electric vehicle, a hybrid electric vehicle, etc.

<Positive electrode>
The positive electrode is a sheet in which a positive electrode active material composition containing a positive electrode active material, a conductive additive, a binder, a dispersion solvent and the like is supported on a positive electrode current collector, and is usually in a sheet form.

  The method for producing the positive electrode is, for example, a method in which the positive electrode active material composition is applied to the positive electrode current collector by the doctor blade method or the like and then dried; the positive electrode active material composition is kneaded, molded and dried. A method of joining the sheet to the positive electrode current collector via a conductive adhesive, pressing and drying; forming a positive electrode active material composition to which a liquid lubricant has been added on the positive electrode current collector; The method of removing and then extending | stretching to a uniaxial or multiaxial direction etc. is mentioned.

  The material of the positive electrode current collector is not particularly limited, and for example, a conductive metal such as aluminum, an aluminum alloy, SUS, or titanium can be used. Among these, aluminum is preferable from the viewpoint of being easily processed into a thin film and being inexpensive.

  Examples of the conductive assistant include acetylene black, carbon black, graphite, metal powder material, single-phase carbon nanotube, multi-walled carbon nanotube, and vapor grown carbon fiber.

  As binders, fluorine resins such as polyvinylidene fluoride and polytetrafluoroethylene; synthetic rubbers such as styrene-butadiene rubber and nitrile butadiene rubber; polyamide resins such as polyamideimide; polyolefin resins such as polyethylene and polypropylene A poly (meth) acrylic resin; a polyacrylic acid; a cellulose resin such as carboxymethylcellulose; These binders may be used alone or in combination of two or more. These binders may be dissolved in a solvent or dispersed in a solvent.

  The blending amount of the conductive auxiliary agent or the binder can be appropriately adjusted in consideration of the intended use of the battery (emphasis on output, importance on energy, etc.), ion conductivity, and the like.

  Examples of the solvent used in the positive electrode active material composition when producing the positive electrode include N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, tetrahydrofuran, acetone, ethanol, ethyl acetate, and water.

<Positive electrode active material>
In the lithium secondary battery of the present invention, the positive electrode active material only needs to be able to occlude and release lithium ions, and a positive electrode active material used in a conventionally known lithium secondary battery can be used.

Specifically, lithium cobalt acid, lithium nickel acid, lithium manganese _{acid, LiNi 1-x-y Co} x Mn y O 2 or _{LiNi 1-x-y Co x} Al y O 2 (0 ≦ x ≦ 1,0 ≦ y ≦ 1) transition metal oxides such as ternary oxides, compounds having an olivine structure such as LiAPO ₄ (A = Fe, Mn, Ni, Co), and solid solutions incorporating a plurality of transition metals A material (a solid solution of an electrochemically inactive layered Li ₂ MnO ₃ and an electrochemically active layered LiMO (transition metals such as M = Co and Ni)) can be used. These may be used alone or in combination.

<Negative electrode>
The negative electrode is a sheet in which a negative electrode active material composition containing a negative electrode active material, a dispersion solvent, a binder, and a conductive auxiliary agent as necessary is supported on a negative electrode current collector, and is usually in the form of a sheet.

  As a material of the negative electrode current collector, a conductive metal such as copper, iron, nickel, silver, and stainless steel SUS (SUS stainless steel) can be used. However, copper is preferable from the viewpoint of easy processing into a thin film.

  As a negative electrode active material, the negative electrode active material used with a conventionally well-known lithium secondary battery can be used, What is necessary is just what can occlude / release lithium ion. Specifically, graphite materials such as artificial graphite and natural graphite, mesophase fired bodies made from coal / petroleum pitch, carbon materials such as non-graphitizable carbon, silicon Si-based negative electrode materials such as Si, Si alloy, and SiO Sn-based negative electrode materials such as Sn alloy, and lithium alloys such as lithium metal and lithium-aluminum alloy can be used.

In addition, the manufacturing method of a negative electrode can employ | adopt the method similar to the manufacturing method of a positive electrode. As the conductive auxiliary agent, binder, and material dispersion solvent that can be used for the negative electrode, the same ones as those used for the positive electrode can be used.
<Separator>
The separator is disposed so as to separate the positive and negative electrodes. The separator is not particularly limited, and a conventionally known separator can be used. For example, a porous sheet (for example, a polyolefin-based microporous separator or a cellulose-based separator) made of a polymer that absorbs and retains a nonaqueous electrolytic solution, a nonwoven fabric separator, a porous metal body, and the like can be given. Among these, a polyolefin microporous separator having a property of being chemically stable with respect to an organic solvent is preferable.

  Examples of the material for the porous sheet include polyethylene, polypropylene, and a laminate having a three-layer structure of polypropylene / polyethylene / polypropylene.

  Examples of the material of the nonwoven fabric separator include cotton, rayon, acetate, nylon, polyester, polypropylene, polyethylene, polyimide, aramid, glass, and the like, depending on the mechanical strength required for the nonaqueous electrolyte layer or the like. Used by mixing.

<Non-aqueous electrolyte>
[Compound represented by general formula (1)]
The non-aqueous electrolyte of the present invention is characterized by containing a compound (1) represented by the general formula (1); (XSO ₂ ) (X′SO ₂ ) N ^— Li ⁺ . It has been found that by containing the compound (1) represented by the general formula (1) in the electrolyte, a film can be formed on the electrode, the increase in internal resistance can be suppressed, and the discharge voltage can be maintained at a high value. It was.

  In the general formula (1), X and X ′ represent a fluorine atom, an alkyl group having 1 to 6 carbon atoms or a fluoroalkyl group having 1 to 6 carbon atoms, and at least one of X and X ′ is a fluorine atom. . The alkyl group having 1 to 6 carbon atoms is preferably a linear alkyl group, and examples thereof include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, a pentyl group, and a hexyl group. Examples of the fluoroalkyl group having 1 to 6 carbon atoms include those in which some or all of the hydrogen atoms of the alkyl group are substituted with fluorine atoms, such as a fluoromethyl group, a difluoromethyl group, and a trifluoromethyl group. , Fluoroethyl group, difluoroethyl group, trifluoroethyl group, pentafluoroethyl group and the like. Among the alkyl groups or fluoroalkyl groups, a fluorine atom, a trifluoromethyl group, and a pentafluoroethyl group are preferable. Preferred compounds (1) include lithium bis (fluorosulfonyl) imide, lithium (fluorosulfonyl) (trifluoromethylsulfonyl) imide, lithium (fluorosulfonyl) (methylsulfonyl) imide, lithium (fluorosulfonyl) (pentafluoroethylsulfonyl) ) Imide, lithium (fluorosulfonyl) (ethylsulfonyl) imide, and more preferably lithium bis (fluorosulfonyl) imide, lithium (fluorosulfonyl) (trifluoromethylsulfonyl) imide, lithium (fluorosulfonyl) (pentafluoroethyl) Sulfonyl) imide, more preferably lithium bis (fluorosulfonyl) imide.

  The concentration of the compound (1) in the nonaqueous electrolytic solution of the present invention is preferably in the range of 0.01 mol / L to 2.0 mol / L, more preferably 0.05 mol / L to 1.0 mol / L. More preferably, it is 0.1 mol / L to 0.5 mol / L. When the concentration of the compound (1) is 0.01 mol / L or less, the effect on the film on the electrode is not sufficient, and the desired battery performance may not be obtained. When the concentration of the compound (1) is 2.0 mol / L or more, the viscosity of the non-aqueous electrolyte is increased, the conductivity is lowered, and the battery performance may not be sufficiently exhibited.

  In the nonaqueous electrolytic solution of the lithium secondary battery of the present invention, the compound (1) may be a main electrolyte or another electrolyte may be a main electrolyte, but another electrolyte (an electrolyte (2) described later) may be used. The main electrolyte is preferred.

  As the compound (1), a commercially available product may be used, or a compound synthesized by a conventionally known method may be used.

[Electrolyte (2)]
In the lithium secondary battery of the present invention, it is preferable to contain other electrolyte (electrolyte (2)) in addition to the compound (1) in the nonaqueous electrolytic solution. The electrolyte (2) is not particularly limited, and any conventionally known electrolyte used as an electrolyte in the electrolyte solution of various power storage devices can be used. The electrolyte (2) is preferably one that has a large dissociation constant in the electrolytic solution and generates an anion that is difficult to solvate with the solvent described later. Specifically, one or more compounds selected from the group consisting of LiPF ₆ , LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiSbF ₆ , LiAlO ₄ , LiAlCl ₄ , LiCl, and LiI can be used as the electrolyte (2 ). Among these, LiPF ₆ and LiBF ₄ are preferable, and LiPF ₆ is more preferable. The electrolyte (2) may be used alone or in combination of two or more.

  Moreover, it is preferable that the density | concentration of the electrolyte (2) in a non-aqueous electrolyte is 0.1 mol / L or more and below a saturation concentration. More preferably, it is 0.1 mol / L to 2.5 mol / L, more preferably 0.3 mol / L to 2 mol / L, still more preferably 0.4 mol / L to 1.5 mol / L, and most preferably Is 0.5 mol / L to 1.2 mol / L. If the concentration of the electrolyte (2) is too high, the viscosity of the non-aqueous electrolyte is increased, the conductivity is lowered, and the battery performance may not be sufficiently exhibited.

  The concentration of the compound (1) and the electrolyte (2) in the non-aqueous electrolyte is such that the sum of the concentrations of the compound (1) and the electrolyte (2) is 0.8 mol / L to 2.5 mol / L. Is more preferable, 1.0 mol / L to 2.0 mol / L, more preferably 1.2 mol / L to 1.5 mol / L.

  The proportion of the electrolyte (2) in the nonaqueous electrolytic solution is preferably 30 mol% or more in the total amount of the electrolyte contained in the nonaqueous electrolytic solution of the present invention. More preferably, it is 50 mol% or more. The upper limit may be 99 mol% or less.

  The compounding amount of the compound (1) in the nonaqueous electrolytic solution of the present invention is such that the compound (2) is 50 mol% or less and 0.1 mol% when the total of the electrolyte (1) and the compound (2) is 100 mol%. The above is preferable. More preferably, they are 0.5 mol% or more and 30 mol% or less, More preferably, they are 1 mol% or more and 20 mol% or less. When the compounding amount of the compound (2) is too small, the effect on the film on the electrode is not sufficient, and the desired battery performance may not be obtained.

[solvent]
In the non-aqueous electrolyte of the present invention, various non-aqueous solvents conventionally used in non-aqueous electrolytes can be used as the solvent for dissolving the electrolytes. For example, cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and chloroethylene carbonate; chain carbonates such as dimethyl carbonate, ethylmethyl carbonate, and diethyl carbonate; tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1 Ethers such as 1,2-dimethoxyethane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane; γ-butyrolactone, γ-valerolactone, α-methyl-γ-butyrolactone, etc. Lactones of the above; chain carboxylic acid esters such as methyl propionate and methyl butyrate can be used. Among these non-aqueous solvents, carbonate solvents such as cyclic carbonates and chain carbonates are preferably used because they are difficult to decompose upon application of voltage and are stable. In addition, the said non-aqueous solvent may be used independently and may mix and use 2 or more types.

[Additive]
The nonaqueous electrolytic solution of the lithium secondary battery of the present invention may contain additives other than the compound (1), the electrolyte (2), and the solvent in order to improve cycle characteristics and safety. As additives, cyclic carbonates having unsaturated bonds such as vinylene carbonate (VC), vinyl ethylene carbonate (VEC), methyl vinylene carbonate (MVC), ethyl vinylene carbonate (EVC); fluoroethylene carbonate, trifluoropropylene carbonate, Carbonate compounds such as phenylethylene carbonate and erythritan carbonate; succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride, cyclohexanedicarboxylic anhydride, cyclopentanetetra Carboxylic anhydrides such as carboxylic dianhydride and phenyl succinic anhydride; ethylene sulfite, 1,3-propane sultone, 1,4-butane sultone, methane sulfone Sulfur-containing compounds such as methyl, busulfan, sulfolane, sulfolene, dimethyl sulfone, tetramethylthiuram monosulfide; 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone, 3-methyl-2-oxazolidinone, 1,3- Nitrogen-containing compounds such as dimethyl-2-imidazolidinone and N-methylsuccinimide; phosphates such as monofluorophosphate and difluorophosphate; hydrocarbon compounds such as heptane, octane and cycloheptane It is done. When these additives are used in the non-aqueous electrolyte, the concentration is preferably 0.1% by weight to 5% by weight.

  It is preferable that the density | concentration of the cyclic carbonate which has the said unsaturated bond in the non-aqueous electrolyte of this invention is 0.01 mass%-5 mass%. More preferably, it is 0.05 mass%-4 mass%. More preferably, it is 0.1 mass%-3 mass%.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

[Preparation of electrolyte]
Electrolyte adjustment example 1
Electrolyte (1) lithium hexafluorophosphate as (LiPF _6, manufactured by Kishida Chemical Co., LBG grade) 1.52 g (10 mmol), lithium bis (fluorosulfonyl) as the compound (2) imide (LiFSI) 0.38 g (2 mmol ) Was measured in a 10 mL volumetric flask, and the volume ratio of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (both manufactured by Kishida Chemical Co., Ltd., LBG grade) was 3/7 (EC / EMC). An electrolyte was prepared by measuring the volume with a solvent, and this was designated as electrolyte 1.

Electrolyte adjustment example 2
An electrolyte solution was prepared in the same manner as in the electrolyte solution adjustment example 1 except that 1.82 g (12 mmol) of lithium hexafluorophosphate (LiPF ₆ , LBG grade, manufactured by Kishida Chemical Co., Ltd.) was used as the electrolyte (1). Was used as electrolytic solution 2.

[Production of laminate cell and charge / discharge before test]
One commercially available positive electrode sheet (active material: lithium cobaltate, 3 mAh / cm ² ) having an area of the positive electrode active material layer of 12 cm ² (3 × 4 cm) and an area of the negative electrode active material layer of 13.44 cm ² (3. 2 × 4.2 cm) of a commercially available negative electrode sheet (active material: graphite, 3.2 mAh / cm ² ) was laminated so as to face each other, and one polyolefin separator was sandwiched therebetween. The positive and negative electrode sheets were sandwiched between two aluminum laminate films, and the aluminum laminate film was filled with electrolytes 1 and 2 and sealed in a vacuum state.

  Using a charge / discharge test apparatus (ACD-01, manufactured by Asuka Electronics Co., Ltd.), charge / discharge was performed once at 30 ° C. at a charge / discharge rate of 0.2 C (constant current mode) and 3.5 V to 4.2 V. Thereafter, the laminate cell was opened and then sealed again in a vacuum state. Under the same conditions, charging / discharging was repeated 5 times to complete a laminated lithium battery.

  For each of the batteries obtained, after the high-temperature storage test and for the battery stored at 25 ° C. without performing the high-temperature storage test, cell disassembly, electrode cleaning, sampling, and sample transport to the apparatus are performed in an argon atmosphere. XPS and TOF-SIMS were measured. The electrolytic solution and experimental conditions are shown in Table 1.

[High temperature storage test]
The obtained laminated lithium battery was charged / discharged at 30 ° C. at a charge / discharge rate of 0.2 C (constant current mode). The discharge capacity at this time is defined as the initial discharge capacity (A). The laminated lithium battery was charged to 4.2 V at 30 ° C. at a charge / discharge rate of 0.2 C (constant current mode), and stored at 80 ° C. for 1 week. Thereafter, discharging was performed at 30 ° C. and 0.2 C. Let the discharge capacity at this time be the discharge capacity (B) after high-temperature storage. The capacity retention rate before and after high temperature storage was calculated by the following formula. Table 2 summarizes the results.
Capacity retention rate = discharge capacity after high temperature storage (B) / initial discharge capacity (A) × 100

[XPS measurement conditions]
Apparatus: Quantera SXM (manufactured by PHI)
Excitation X-ray: Monochromatic Al Kα ray (1486.6 eV)
X-ray diameter: 200 μm
Photoelectron escape angle: 45 ° (inclination of detector with respect to sample surface)
Measurement elements: Lithium, carbon, oxygen, fluorine, nitrogen, phosphorus, cobalt Data processing conditions Smoothing: 9 points smoothing
Horizontal axis correction: The peak of a peak having a peak top at 283 eV to 285 eV (attributed to C—C, CHx component) in the C1s peak was set to 284.6 eV.

[TOF-SIMS measurement conditions]
Apparatus: TOF. SIMS5 (made by ION-TOF)
Primary ion: Bi3 ⁺⁺
Secondary ion polarity: Positive [XPS measurement result]
Table 3 shows the analysis results of the negative electrode surface of each battery. In the table, the abundance ratio indicates the abundance ratio of S1 and S2 with respect to the total S atoms present, and the abundance ratio of S atoms indicates the abundance ratio of S atoms calculated with the total atoms obtained by measurement as 100%. Moreover, the XPS spectrum which analyzed the measurement result by data processing conditions was shown in FIG. 1, FIG.

  In Experimental Examples 3 and 4 in which LiFSI was not included in the electrolytic solution, no peak derived from S was observed. In contrast, in Experimental Examples 1 and 2, a peak derived from S (sulfur) could be observed. This S-derived peak is considered to be derived from sulfur contained in LiFSI. From this result, it was found that LiFSI contained in the electrolytic solution formed a film on the negative electrode surface.

[TOF-SIMS measurement results]
Table 4 shows the measurement results of the positive secondary ions of TOF-SIMS of the negative electrode of each battery, and Table 5 shows the measurement results of the positive electrode. In addition, the value in a table | surface represents the numerical value normalized with respect to the total secondary ion intensity which can be assigned. Since CH ₂ S ⁺ and C ₂ H ₅ S ⁺ were detected as S-containing fragments, the sum of the two detected values was taken as the S-containing fragment.

From the results of Tables 4 and 5, it can be seen that S-containing fragments are observed more strongly in Experimental Examples 1 and 2 than in Experimental Examples 3 and 4. In Experimental Examples 1 and 2 containing LiFSI in the electrolytic solution, sulfur atoms contained in LiFSI were observed, and it was found that a film by LiFSI was formed on the electrode. In addition, the difference in the strength of the S-derived fragment due to the presence or absence of LiFSI appeared significantly on the positive electrode side than the negative electrode, and the difference was further increased in the battery after being stored at high temperature in a fully charged state. . It was clarified that LiFSI forms a film on the electrode by charging and discharging in the battery. Moreover, from the results in Table 2, it was found that the battery using the electrolytic solution 1 had a higher capacity retention rate after high-temperature storage than when the electrolytic solution 2 was used. From this, it can be seen that when LiFSI is contained in the electrolytic solution, a film is formed on the surfaces of the positive electrode and the negative electrode, and the battery performance is improved.

Claims (6)

A positive electrode containing a positive electrode active material capable of occluding and releasing lithium ions, a negative electrode containing a negative electrode active material capable of occluding and releasing lithium ions, and a lithium secondary battery comprising a non-aqueous electrolyte,
The non-aqueous electrolyte has the general formula (1); (XSO ₂ ) (X′SO ₂ ) N ^— Li ⁺ (X and X ′ are a fluorine atom, an alkyl group having 1 to 6 carbon atoms, or 1 to 1 carbon atoms. A fluoroalkyl group of 6, wherein at least one of X and X ′ is a fluorine atom), and a solvent,
A lithium secondary battery, wherein an abundance ratio of S atoms is 0.5% or more when an abundance ratio of all atoms by XPS measurement is 100% on a surface of a positive electrode or a negative electrode. A positive electrode containing a positive electrode active material capable of occluding and releasing lithium ions, a negative electrode containing a negative electrode active material capable of occluding and releasing lithium ions, and a lithium secondary battery comprising a non-aqueous electrolyte,
The non-aqueous electrolyte has the general formula (1); (XSO ₂ ) (X′SO ₂ ) N ^— Li ⁺ (X and X ′ are a fluorine atom, an alkyl group having 1 to 6 carbon atoms, or 1 to 1 carbon atoms. A fluoroalkyl group of 6, wherein at least one of X and X ′ is a fluorine atom), and a solvent,
A lithium secondary battery, wherein a total of normalized values of fragments containing S atoms of positive secondary ions measured by TOF-SIMS is 2.0 × 10 ⁻⁴ or more on a positive electrode or negative electrode surface.   The lithium secondary battery according to claim 1 or 2, further comprising an electrolyte (2) in the non-aqueous electrolyte. The lithium secondary battery according to claim 1, wherein the electrolyte (2) is LiPF ₆ .   The lithium secondary battery according to any one of claims 1 to 6, wherein the compound represented by the general formula (1) is lithium bis (fluorosulfonyl) imide.   The lithium secondary battery according to claim 1, wherein the solvent contains a carbonate-based solvent.

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