JP2006004822A - Nonaqueous electrolyte battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte battery containing a high capacity oxide of Si in a negative electrode and having high safety. <P>SOLUTION: In a nonaqueous electrolyte secondary battery equipped with a negative active material comprising a core and the surface layer having different composition each other, the core has an oxide containing Si and the surface layer has a compound containing Li and at least one element selected from N, P, and S, and the negative active material layer does not have exothermic peak at 20°C or less in DSC (differential scanning calorimetry). <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

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

  In recent years, non-aqueous electrolyte batteries such as lithium ion batteries have been widely used as power sources for mobile electronic devices such as mobile and digital cameras. As these devices have higher performance, their power sources are required to have higher capacity and higher output, and higher performance of nonaqueous electrolyte batteries is required. A carbon material such as graphite is used for the negative electrode active material of current lithium ion batteries, and lithium cobalt oxide or the like is used for the positive electrode active material. Along with the improvement in performance of these active materials, search for new active materials is also widely performed.

New negative electrode active materials include metals such as Si, Sn, Al, Pb, and Zn that are alloyed with Li, and oxides thereof (for example, N. Li, CR Martin, and B. Scrosati, Electrochemical and Solid-State Letters, 3 , 316 (2000)).

  Among them, since the oxide of Si exhibits a large capacity of 1000 mAh / g or more, it is particularly attracting attention as a negative electrode active material for next-generation lithium ion batteries (Patent Document 1, Patent Document 2, etc.).

  However, since non-aqueous electrolyte batteries such as lithium ion batteries use a flammable organic solvent in the electrolyte, there is a possibility that a safety failure may occur due to misuse of the battery. Since the capacity of the Si oxide is larger than that of graphite, the possibility increases.

JP-A-6-325765 Special table 2000-515672

  As described above, since Si oxides have a large capacity of 1000 mAh / g or more, they are promising as negative electrode active materials for next-generation lithium ion batteries. However, since non-aqueous electrolyte batteries such as lithium ion batteries use a flammable organic solvent in the electrolyte, there is a possibility that safety may be deteriorated due to misuse of the batteries. It is an important practical issue to prevent defects from occurring even if misused, that is, to improve safety.

  The present invention has been completed based on the above circumstances, and an object of the present invention is to provide a non-aqueous electrolyte battery that has a high-capacity Si oxide and is highly safe.

  As means for achieving the above object, the invention of claim 1 includes a positive electrode including a positive electrode active material capable of occluding and releasing lithium ions, a core and a surface layer capable of occluding and releasing lithium ions and having different compositions from each other. And a nonaqueous electrolyte secondary battery comprising a nonaqueous electrolyte, wherein the core of the negative electrode active material includes an oxide containing Si, and the surface layer includes B, N, P And a compound containing at least one element selected from S and Li element, and having no exothermic peak at 200 ° C. or lower when the DSC (differential scanning calorimetry) of the negative electrode active material layer is measured. Features.

  According to a second aspect of the present invention, in the nonaqueous electrolyte secondary battery according to the first aspect, the substance contains at least one element selected from C and F.

  In the nonaqueous electrolyte battery of the present invention, in the invention of claim 1, a negative electrode active material having a core and a surface layer having different compositions is provided in the negative electrode, and the core of the negative electrode active material is an oxide containing Si. The surface layer includes at least one compound selected from B, N, P, and S and a compound containing Li, and the DSC (differential scanning calorimetry) of the negative electrode active material layer is measured. In this case, since there is no exothermic peak at 200 ° C. or less, it has a high capacity and good safety.

  In the invention of claim 2, it is particularly preferred that the compound contains at least one element selected from C or F, which has a high capacity and good safety.

  Therefore, the nonaqueous electrolyte battery having such a configuration according to the present invention achieves high energy density while maintaining safety.

  The negative electrode of the nonaqueous electrolyte battery of the present invention includes a negative electrode active material including a core and a surface layer having different compositions, the core includes an oxide containing Si, and the surface layer includes B, N, P, It comprises at least one compound selected from the group consisting of S and a compound containing Li and has no exothermic peak at 200 ° C. or lower in DSC (differential scanning calorimetry) of the negative electrode active material layer. To do. Further, the negative electrode active material is not necessarily formed of only two layers of the core and the surface layer, and may be three or more layers.

Here, the surface layer includes one kind of element selected from B, N, P, and S and Li, and may exist as a compound of Li such as various salts, organic or inorganic compounds. preferable. Specifically, examples of the B source for producing the compound include LiBF ₄ , LiBC ₄ O ₈ , boron trifluoride (BF ₃ ), H ₃ BO _3, etc., and the N source includes LiN (SO ₂ CF _{3) 2,} phosphazene _{(N 3 P 3 F 6)} , tetramethyl ammonium - hexafluorophosphate _{((CH 3) 4 NPF 6} ), ammonia gas, nitrogen gas, N- (methoxymethyl) -N- methyl - 2- (such as triphenylphosphonium Ani isopropylidene) acetamide. Examples of the P source, LiPF6, phosphazenes _{(N 3} P _{3 F} 6), tetramethyl ammonium - hexafluorophosphate _((CH 3) 4 _NPF 6), Phosphorus trifluoride (PF ₃ ), phosphorus pentafluoride (PF ₅ ), phosphoric acid (H ₃ PO ₄ ), N- (methoxy) -N-methyl-2- (triphes) Nylphosphoanilidene) acetamide and the like, and examples of the S source include LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , hydrogen sulfide (H ₂ S), and the like.

  More preferably, the compound contains at least one element selected from C and / or F.

As a C source for producing the compound, other electrolytes such as ethane (C ₂ H ₄ ) or an additive used for improving the utilization rate of the negative electrode containing C, specifically, LiC ₄ BO ₈ , LiSCN , LiCF ₃ CO ₂ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ CF ₂ CF ₃ ) ₂ , LiN (COCF ₃ ) ₂ and LiN (COCF ₂ CF ₃ ) _2, etc. And a mixture thereof, tetramethylammonium-hexafluorophosphoric acid ((CH ₃ ) ₄ NPF ₆ ), N- (methoxy) -N-methyl-2- (triphenylphosphoanilidene) acetamide and the like. Further, it may be used for a non-aqueous electrolyte and may contain C. Specifically, ethylene carbonate (EC), propylene carbonate, diethyl carbonate (DEC), diethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, sulfolane, dimethyl sulfoxide, acetonitrile, dimethylformamide, dimethylacetamide, 1,2-dimethoxyethane, Examples thereof include solvents such as 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dioxolane, methyl acetate, methyl acetate, and mixed solvents thereof.

Examples of the F source include boron trifluoride (BF ₃ ), phosphorus trifluoride (PF ₃ ), phosphorus pentafluoride (PF ₅ ), etc., and F as an additive used for improving the utilization rate of the electrolyte or the negative electrode. In particular, LiPF ₆ , LiBF ₄ , LiCF ₃ CO ₂ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ CF ₂ CF ₃ ) ₂ , LiN (COCF ₃ ) ₂ And LiN (COCF ₂ CF ₃ ) ₂ , hydrogen fluoride (HF), fluorine-containing ester solvents, tetraethylammonium fluoride hydrogen fluoride complex (TEAFHF), or derivatives thereof, compounds such as phosphazenes, and mixtures thereof Is mentioned.

  In addition, the compound can be provided on the surface of the negative electrode active material by various methods. Specific methods include an electrochemical method, a CVD method, a dipping method, and a sputtering method, but other methods may be used.

  By including these elements on the surface of the oxide, it is presumed that the reaction between the oxide and the electrolytic solution is suppressed even when a large current such as a short circuit is passed.

  By measuring the DSC (differential scanning calorimetry) of the negative electrode active material layer, the safety of the battery can be known. The measurement conditions are described below.

  Disassemble the charged battery in a dry room and take out the negative electrode. The active material layer of the negative electrode is scraped off from the current collector, and the active material layer and the electrolyte used for the battery are weighed at a mass ratio of 1: 1 and sealed in a differential scanning calorimeter container. The container is immediately set on a differential scanning calorimeter (manufactured by SII, DSC220C), and DSC measurement is performed in the range from room temperature to 200 ° C at a temperature rising rate of 10 ° C / min. From the result, the heat generation rate (unit: W / g · ° C.) of the oxide is obtained. Here, when the exothermic rate calculated from the DSC measurement result is 0.1 W / g · ° C. or less without exothermic peak, when the active material layer is measured by DSC, the exothermic peak is below 200 ° C. It is characterized by not having.

A battery provided with such a negative electrode active material in the negative electrode does not generate heat even when it is heated to a high temperature due to misuse, thus improving safety. The DSC of the negative electrode active material layer can be measured by mixing with an organic solvent containing a lithium salt that is a non-aqueous electrolyte (see, for example, GS News Technical Report, 55 , 21 (1996)). Moreover, it is preferable to measure DSC in the state which electrically supplied 700 mAh / g or more of electricity with the negative electrode active material. This is because, when charged, the negative electrode increases its reducibility and becomes more dangerous. This is because safety can be confirmed by measuring in a more dangerous state.

The oxide containing Si is preferably one whose composition formula is SiO _x . In particular, those in which the atomic ratio x of O to Si is 0 <x <2 are more preferable because the discharge capacity is large and the charge / discharge cycle performance is good.

Further, it is preferably a substance containing both phases of Si and SiO _x (1 <x ≦ 2). In the X-ray diffraction measurement using CuKα ray of the substance, the Si (111) plane and Si (220) It is preferable that at least one of the half widths of the diffraction peaks of the surface is less than 3 °.

  The oxides include typical nonmetallic elements such as B, C, N, P, F, Br, and I, and typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, and Ge. , Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and other transition metal elements may be included.

  Moreover, in order to improve electronic conductivity, the said oxide and electronic conductive materials, such as a carbon material, can be mixed, or it can compound, and what coat | covered the surface with the electronic conductive material can be used.

  The oxide may be used alone, or may be mixed with at least one of those capable of occluding and releasing lithium ions or metallic lithium.

Those capable of occluding and releasing lithium ions, a carbon material, _{oxide, Li 3-P M P N} ( provided that, M is a transition metal, 0 ≦ P ≦ 0.8) nitride and a lithium alloy, such as Is exemplified. Carbon materials include coke, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fibers, pyrolytic vapor-grown carbon fibers and other graphitizable carbon, phenol resin fired bodies, polyacrylonitrile-based carbon fibers, pseudo-isotropic Carbon, non-graphitizable carbon such as a furfuryl alcohol resin fired body, natural graphite, artificial graphite, graphitized MCMB, graphitized mesophase pitch-based carbon fiber, graphite whisker and other graphite materials, and a mixture thereof be able to. As the lithium alloy, an alloy of lithium and aluminum, zinc, bismuth, cadmium, antimony, lead, tin, gallium, germanium, or indium can be used. As the oxide, the oxide of the lithium alloy can be used.

  The negative electrode used for the nonaqueous electrolyte battery of the present invention includes a mixture layer containing a negative electrode active material and a current collector. The mixture layer may contain a conductive agent in addition to the active material. The negative electrode can be produced by applying a slurry obtained by mixing a negative electrode active material and a binder in a solvent or solution to a current collector and then drying the slurry.

  For the current collector of the negative electrode, iron, copper, stainless steel, nickel, or the like can be used. Examples of the shape include a sheet, a foam, a sintered porous body, and an expanded lattice. Furthermore, you may use the electrical power collector which opened the hole with arbitrary shapes.

  Various carbon materials can be used for the conductive agent used in the negative electrode. Examples of the carbon material include graphite such as natural graphite and artificial graphite, carbon black such as acetylene black, and amorphous carbon such as needle coke.

  Examples of the binder for the negative electrode include polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene copolymer (P (VdF / HFP)), polytetrafluoroethylene (PTFE), fluorinated polyvinylidene fluoride, and ethylene. -Propylene-diene terpolymer (EPDM), styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), fluoro rubber, polyvinyl acetate, polymethyl methacrylate, polyethylene, nitrocellulose, or derivatives thereof. These can be used alone or in combination.

  The solvent or solution for mixing the negative electrode active material and the binder dissolves or disperses the binder, and a non-aqueous solvent or an aqueous solution can be used. Non-aqueous solvents include N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, NN-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran and the like. On the other hand, as the aqueous solution, water or a solution obtained by adding a dispersant, a thickener or the like to this can be used. When a latex such as SBR is added to the aqueous solution, a slurry can be made.

  The positive electrode of the nonaqueous electrolyte battery of the present invention includes a mixture layer containing a positive electrode active material and a current collector. The positive electrode can be manufactured by applying a slurry obtained by mixing an active material, a conductive agent, and a binder in a solvent or a solution to a current collector and then drying the slurry.

The positive electrode active material of the nonaqueous electrolyte battery of the present invention includes a transition metal oxide containing at least one element selected from Ti, V, Cr, Mn, Fe, Co, Ni, and Cu, or this transition A composite oxide of a metal oxide and lithium can be used. These transition metal oxides or composite oxides may contain typical nonmetallic elements and typical metal elements. In particular, the metal oxide composition formula is _{Li y Ni z Mn 2-z} O 4 (0 ≦ y ≦ 1.1,0.45 ≦ z ≦ 0.6) is a preferred active material. This is because the average discharge potential of the positive electrode to which Li _y Ni _z Mn _2−z O ₄ is applied is about 4.7 V (vs. Li / Li ⁺ ), and that of LiCoO ₂ , which is a conventional representative positive electrode active material, is about 3 This is because the voltage is 0.9 V (vs. Li / Li ⁺ ), and the former is more noble. That is, a battery in which the former positive electrode and the negative electrode including the negative electrode active material of the present invention are combined has a higher energy density because the terminal voltage is higher than that using the latter.

  Iron, copper, aluminum, stainless steel, or nickel can be used for the current collector of the positive electrode. Examples of the shape include a sheet, a foam, a sintered porous body, and an expanded lattice. Furthermore, you may use the electrical power collector which opened the hole with arbitrary shapes.

  Various carbon materials can be used for the conductive agent of the positive electrode. Examples of the carbon material include graphite such as natural graphite and artificial graphite, carbon black such as acetylene black, and amorphous carbon such as needle coke.

  Examples of the positive electrode binder include polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene copolymer (P (VdF / HFP)), polytetrafluoroethylene (PTFE), fluorinated polyvinylidene fluoride, and ethylene. -Propylene-diene terpolymer (EPDM), styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), fluoro rubber, polyvinyl acetate, polymethyl methacrylate, polyethylene, nitrocellulose, or derivatives thereof. These can be used alone or in combination.

  The nonaqueous electrolyte battery of the present invention is composed of these negative electrode and positive electrode, a nonaqueous electrolyte and a separator. As the non-aqueous electrolyte, a non-aqueous electrolyte, a polymer solid electrolyte, a gel electrolyte, or an inorganic solid electrolyte can be used. The electrolyte may have pores.

The non-aqueous electrolyte is composed of a non-aqueous solvent and a solute. Non-aqueous solvents are ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, sulfolane, dimethyl sulfoxide, acetonitrile, dimethylformamide, dimethylacetamide, 1,2-dimethoxyethane, 1,2-di Examples thereof include solvents such as ethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dioxolane, methyl acetate, methyl acetate, and mixed solvents thereof. The solutes include LiPF ₆ , LiBF ₄ , LiC ₄ BO ₈ , LiSCN, LiCF ₃ CO ₂ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ CF ₂ CF ₃ ) ₂ , LiN. Examples include salts such as (COCF ₃ ) ₂ and LiN (COCF ₂ CF ₃ ) ₂ and mixtures thereof.

In addition, for the purpose of improving the utilization factor of the negative electrode, in the above solvent, ethylene sulfide (ES), hydrogen fluoride (HF), triazole-based cyclic compound, fluorine-containing ester solvent, hydrogen fluoride complex of tetraethylammonium fluoride ( TEAFHF), or derivatives thereof, phosphazenes, amide group-containing compounds, imino group-containing compounds, nitrogen-containing compounds, or gases such as CO ₂ , NO ₂ , CO, SO ₂ may be added as additives.

  As the polymer solid electrolyte, a polymer obtained by adding the above solute to a polymer such as polyethylene oxide, polypropylene oxide, polyethylene imide, or a mixture thereof can be used.

  As the gel electrolyte, a polymer obtained by adding the above solvent and solute to the above polymer can be used.

The inorganic solid electrolyte can be crystalline or amorphous. The former includes LiI, Li ₃ N, Li _{1 + x} M _x Ti _2-x (PO ₄ ) ₃ (M = Al, Sc, Y, La), Li _0.5-3x R _{0.5 + x} TiO ₃ (R = La, Pr, Nd, Sm), or LISICON represented by Li _4-x Ge _1-x P _x S ₄ can be used, and the latter includes LiI-Li ₂ O—B ₂ O ₅ system, Li ₂ Oxide glass such as O—SiO ₂ system or sulfide such as LiI—Li ₂ S—B ₂ S ₃ system, LiI—Li ₂ S—SiS ₂ system, Li ₂ S—SiS ₂ —Li ₃ PO ₄ system Glass can be used. Moreover, these mixtures can be used.

  A microporous polymer membrane can be used for the separator of the nonaqueous electrolyte battery of the present invention, and the material thereof is nylon, cellulose acetate, nitrocellulose, polysulfone, polyacrylonitrile, polyvinylidene fluoride, polypropylene, polyethylene, Examples include polyolefins such as polybutene. Of these, polyolefin microporous membranes are particularly preferred. Alternatively, a microporous film in which polyethylene and polypropylene are laminated may be used.

  Below, the nonaqueous electrolyte battery of the present invention will be described in more detail based on examples. However, the present invention is not limited to the following examples.

[Example 1]
The SiO particles were heat-treated at 870 ° C. for 6 hours in an argon atmosphere, then immersed in a 0.5 mol / dm ³ hydrofluoric acid solution for 1 hour, and then washed and dried. This X-ray diffraction pattern of SiO was broad, and the crystal structure was found to be amorphous. A negative electrode was produced using the SiO particles as an active material. A paste was prepared by dispersing 80% by mass of SiO particles, 5% by mass of acetylene black, and 15% by mass of PVdF in N-methyl-2-pyrrolidone (NMP). This paste was applied on a copper foil having a thickness of 15 μm and then dried at 150 ° C. to evaporate NMP. This operation was performed on both sides of the copper foil. This was roll-pressed, then cut into a width of 2.5 cm and a length of 40 cm, and a negative electrode 1 having a mixture layer on both sides of the current collector was produced. The mass of the negative electrode layer applied on both sides of the copper foil was 0.010 g / cm ² . When the discharge capacity of SiO is 1000 mAh / g, the capacity of this negative electrode is about 520 mAh.

Next, in order to form a substance containing P, C, and F elements and Li element on the surface of SiO particles using an electrochemical method, the negative electrode 1 is used as a working electrode, and a Li foil is used as a counter electrode. A battery was produced. As the non-aqueous electrolyte, a solution obtained by dissolving 1 mol / dm ³ of LiPF ₆ in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 1: 1 was used. At 25 ° C., 40 mA was applied from the working electrode to the counter electrode until the battery voltage reached 0.05 V, and then charged at that value for 5 hours. Next, 40 mA was applied from the counter electrode to the working electrode until the battery voltage was 1.5 V to discharge, and then the negative electrode 1 was taken out from the test cell. The electrolyte remaining in the negative electrode 1 was evaporated by vacuum drying at room temperature. Thus, a part of electrolyte was decomposed | disassembled and the substance containing C, P, F element, and Li element was formed in the surface of the active material. This was designated as negative electrode 2.

A paste was prepared by dispersing 90% by mass of LiCoO ₂ , 5% by mass of acetylene black, and 5% by mass of PVdF in NMP. This paste was applied onto an aluminum foil having a thickness of 20 μm and then dried at 150 ° C. to evaporate NMP. This operation was performed on both sides of the aluminum foil. After this was roll-pressed, it was cut into a width of 2.5 cm and a length of 40 cm to produce the positive electrode 1. The mass of the positive electrode layer applied on both sides of the aluminum foil was 0.045 g / cm ² . When the discharge capacity of LiCoO ₂ is 150 mAh / g, the capacity of this positive electrode is about 730 mAh.

The negative electrode 2 and the positive electrode 1 prepared in this manner were wound while being overlapped with a polyethylene porous separator having a thickness of 20 μm and a porosity of 40% interposed therebetween. This was inserted into a battery case mainly composed of iron. Finally, by injecting a non-aqueous electrolyte into the battery, a rectangular battery of the present invention (A1) having a height of 48 mm, a width of 30 mm, a thickness of 4.2 mm and a rated capacity of 500 mAh was obtained. . As this nonaqueous electrolytic solution, a solution obtained by dissolving 1 mol / dm ³ of LiPF ₆ in a mixed solvent of EC and DEC in a volume ratio of 1: 1 was used.

[Example 2]
When a material containing B, C and F elements and Li element is formed on the surface of the negative electrode active material using an electrochemical technique, 1 mol / dm ^{3 in} a mixed solvent of EC and DEC in a volume ratio of 1: 1. A battery (A2) of the present invention was produced in the same manner as in Example 1 except that a non-aqueous electrolyte solution in which LiBF ₄ was dissolved was used.

[Example 3]
When a material containing S, C, F elements and Li element is formed on the surface of the negative electrode active material using an electrochemical technique, 1 mol / dm ^{3 in} a mixed solvent of EC and DEC in a volume ratio of 1: 1. A battery (A3) of the present invention was produced in the same manner as in Example 1 except that a nonaqueous electrolyte solution in which LiCF ₃ SO ₃ was dissolved was used.

[Example 4]
When a material containing N, S, C and F elements and Li element is formed on the surface of the negative electrode active material using an electrochemical technique, 1 mol / l in a mixed solvent of EC and DEC in a volume ratio of 1: 1. A battery (A4) of the present invention was produced in the same manner as in Example 1 except that a nonaqueous electrolytic solution in which LiN (SO ₂ CF ₃ ) ₂ of dm ³ was dissolved was used.

[Example 5]
When a material containing B, N, P, C, and F elements and Li element is formed on the surface of the negative electrode active material using an electrochemical method, a mixture solvent with a volume ratio of EC and DEC of 1: 1 is used. The present invention was carried out in the same manner as in Example 1 except that a nonaqueous electrolytic solution in which 1 mol / dm ³ of LiBF ₄ was dissolved and 10% by mass of phosphazene (N ₃ P ₃ F ₆ ) was added was used. A battery (A5) was produced.

[Example 6]
When a material having B and C elements and Li element is formed on the surface of the negative electrode active material using an electrochemical method, 0.5 mol / dm ³ of LiBC ₄ O ₈ is added to 1,2-dimethoxyethane. A battery of the present invention (A6) was produced in the same manner as in Example 1 except that the dissolved nonaqueous electrolytic solution was used.

[Example 7]
When a material containing N, P, C and F elements and Li element is formed on the surface of the negative electrode active material using an electrochemical method, 1 mol / l in a 1: 1 mixed solvent of EC and DEC is used. In the same manner as in Example 1 except that a non-aqueous electrolyte solution in which 10% by mass of phosphazene (N ₃ P ₃ F ₆ ) was added was dissolved in LiPF ₆ of dm ³ A7) was prepared.

[Example 8]
When a material containing N, P, C and F elements and Li element is formed on the surface of the negative electrode active material using an electrochemical method, 1 mol / l in a 1: 1 mixed solvent of EC and DEC is used. Example 1 except that a nonaqueous electrolyte solution in which 10% by mass of tetramethylammonium-hexafluorophosphoric acid ((CH ₃ ) ₄ NPF ₆ ) was further dissolved and dm ³ of LiPF ₆ was dissolved was used. Similarly, the battery (A8) of the present invention was produced.

[Example 9]
The negative electrode 1 produced in Example 1 and the Li metal foil were inserted into a chamber in a DC plasma CVD apparatus. After heating to 400 ° C., the degree of vacuum in the chamber was set to 1 × 10 ⁻⁶ Torr. Next, a mixed gas of PF ₃ , O _2, and C ₂ H ₄ was supplied into the chamber. The internal pressure of the chamber was kept at 0.7 Torr. Next, DC power of 400 W was applied to the negative electrode 1 to produce a negative electrode 3 in which a material containing P, C and F elements and a Li element was formed on the surface of the active material.

  A battery (A9) of the present invention was produced in the same manner as in Example 1 except that the negative electrode 3 prepared using the CVD method was used.

[Example 10]
When forming a material containing P, C, and F elements and Li element on the surface of the negative electrode active material using the CVD method, a mixed gas of PF ₅ , O _2, and C ₂ H ₄ was supplied into the chamber. Except for this, a battery of the present invention (A10) was produced in the same manner as in Example 9.

[Example 11]
When forming a material containing B, C, and F elements and Li element on the surface of the negative electrode active material using the CVD method, a mixed gas of BF ₃ , O _2, and C ₂ H ₄ was supplied into the chamber. Except for this, a battery of the present invention (A11) was produced in the same manner as in Example 9.

[Example 12]
When a material containing N and C elements and Li element is formed on the surface of the negative electrode active material by using the CVD method, a mixed gas of NH ₃ , O ₂ and C ₂ H ₄ is supplied into the chamber. Otherwise, the battery of the present invention (A12) was produced in the same manner as in Example 9.

[Example 13]
The battery of the present invention was the same as in Example 9 except that H ₂ S was supplied into the chamber when a material containing S element and Li element was formed on the surface of the negative electrode active material using the CVD method. (A13) was produced.

[Example 14]
When forming a material containing N element and Li element on the surface of the negative electrode active material using the CVD method, the battery of the present invention (as in Example 9) except that N ₂ was supplied into the chamber. A14) was prepared.

[Example 15]
When forming a material containing B, N, C, and F elements and Li element on the surface of the negative electrode active material using the CVD method, a mixed gas of BF ₃ , NH ₃ , O _2, and C ₂ H ₄ is used. A battery (A15) of the present invention was produced in the same manner as in Example 9, except that it was supplied into the chamber.

[Example 16]
When a material containing B, P, C, and F elements and Li element is formed on the surface of the negative electrode active material using a CVD method, a mixed gas of BF ₃ , PF ₃ , O _2, and C ₂ H ₄ is used. A battery of the present invention (A16) was produced in the same manner as in Example 9 except that the material was supplied into the chamber.

[Example 17]
A mixed gas of BF ₃ , H ₂ S, O _2, and C ₂ H ₄ when a material containing B, S, C, and F elements and a Li element is formed on the surface of the negative electrode active material using a CVD method. A battery of the present invention (A17) was produced in the same manner as in Example 9, except that was supplied into the chamber.

[Example 18]
A mixed gas of PF ₃ , H ₂ S, O _2, and C ₂ H ₄ when a material containing P, S, C, and F elements and Li element is formed on the surface of the negative electrode active material using the CVD method. A battery of the present invention (A18) was produced in the same manner as in Example 9, except that was supplied into the chamber.

[Example 19]
When a material containing B, P, S, C, and F elements and Li element is formed on the surface of the negative electrode active material using the CVD method, BF ₃ , PF ₃ , H ₂ S, O _2, and C ₂ H _A battery of the present invention (A19) was produced in the same manner as in Example 9 except that the mixed gas with No. ₄ was supplied into the chamber.

[Example 20]
When a material containing B, N, S, C, and F elements and Li element is formed on the surface of the negative electrode active material using the CVD method, BF ₃ , NH ₃ , H ₂ S, O _2, and C ₂ H _A battery of the present invention (A20) was produced in the same manner as in Example 9 except that the gas mixture with No. ₄ was supplied into the chamber.

[Example 21]
When a material containing N, P, S, C, and F elements and Li element is formed on the surface of the negative electrode active material using the CVD method, NH ₃ , PF ₃ , H ₂ S, O _2, and C ₂ H _A battery of the present invention (A21) was produced in the same manner as in Example 9 except that the gas mixture with No. ₄ was supplied into the chamber.

[Example 22]
When a material containing B, N, P, S, C and F elements and a Li element is formed on the surface of the negative electrode active material using the CVD method, BF ₃ , NH ₃ , PF ₃ , H ₂ S and O _A battery (A22) of the present invention was produced in the same manner as in Example 9, except that a mixed gas of ₂ and C ₂ H ₄ was supplied into the chamber.

[Example 23]
In order to form a substance containing P element and Li element on the surface of the negative electrode active material by using the dipping method, the negative electrode 1 produced in Example 1 was placed in an NMP solution in which LiOH, H ₃ PO ₄ and PVdF were dissolved. After soaking, it was dried at 80 ° C. to evaporate NMP. This was designated as negative electrode 6.

A battery (A23) of the present invention was produced in the same manner as in Example 1, except that the negative electrode 6 thus prepared was used. As this nonaqueous electrolytic solution, a solution obtained by dissolving 1 mol / dm ³ of LiPF ₆ in a mixed solvent of EC and DEC in a volume ratio of 1: 1 was used.

[Example 24]
In order to form a substance comprising B element and Li element on the surface of the negative electrode active material by using the dipping method, the same procedure as in Example 23 was performed except that the substance was immersed in an NMP solution in which LiOH, H ₃ BO ₃ and PVdF were dissolved. Thus, a battery (A24) of the present invention was produced.

[Example 25]
LiOH and N- (methoxy) -N-methyl-2- (triphenylphosphoanilidene) are used to form a material comprising N, P, C elements and Li elements on the surface of the negative electrode active material using an immersion method. ) A battery of the present invention (A25) was produced in the same manner as in Example 23 except that it was immersed in an NMP solution in which acetamide and PVdF were dissolved.

[Comparative Example 1]
When a material containing Cl, C element, and Li element is formed on the surface of the negative electrode active material using an electrochemical method, 1 mol / dm ³ of LiClO in a mixed solvent of EC and DEC in a volume ratio of 1: 1. _A comparative battery (B1) was produced in the same manner as in Example 1 except that the nonaqueous electrolytic solution in which ₄ was dissolved was used.

[Comparative Example 2]
When a material containing As and C elements and Li element is formed on the surface of the negative electrode active material using an electrochemical method, 1 mol / dm ³ of LiAsF is mixed in a mixed solvent of EC and DEC in a volume ratio of 1: 1. _A comparative battery (B2) was produced in the same manner as in Example 1 except that the nonaqueous electrolytic solution in which ₆ was dissolved was used.

[Comparative Example 3]
When a material containing Sb, C element and Li element is formed on the surface of the negative electrode active material using an electrochemical method, 1 mol / dm ³ of LiSbF is mixed in a mixed solvent of EC and DEC in a volume ratio of 1: 1. _A comparative battery (B3) was produced in the same manner as in Example 1 except that the nonaqueous electrolytic solution in which ₆ was dissolved was used.

[Comparative Example 4]
The same as Example 9 except that when a material containing Cl element and Li element was formed on the surface of the negative electrode active material using the CVD method, a mixed gas of Cl ₂ and O ₂ was supplied into the chamber. Thus, a comparative battery (B4) was produced.

[Comparative Example 5]
When forming the substance containing Cl element and Li element on the surface of the negative electrode active material using the dipping method, the same procedure as in Example 23 was used except that an NMP solution in which KClO ₄ and PVdF were dissolved was used. A comparative battery (B5) was produced.

[Comparative Example 6]
When forming the substance containing C element and Li element on the surface of the negative electrode active material using the dipping method, the same procedure as in Example 23 was used except that an NMP solution in which polyacrylic acid and LiOH were dissolved was used. Thus, a comparative battery (B6) was produced.

  The batteries of these examples and comparative examples were charged and discharged at 25 ° C. under the following conditions. Charging was performed at a current of 0.1 CmA (50 mA) up to 4.0 V, and then the voltage was constant voltage charging for 5 hours. The discharge was up to 3.0 V with the same current. When the average discharge voltage is 3.4 V, the volume energy density is about 290 Wh / l.

  The following tests were performed using these batteries.

[1. Safety test]
The following nail penetration test was performed to evaluate the safety of the battery. The battery after the second cycle charge was pierced at 25 ° C. with a 1 mm diameter nail at a speed of 3 cm / sec. The safety was judged as bad if the battery temperature was 130 ° C. or higher or the safety valve was activated, and the others were good.

[2. XPS measurement]
The battery after the second cycle charge was disassembled in the glove box, and the negative electrode was taken out. The negative electrode was vacuum dried to evaporate the organic solvent, and then cut into a size of 5 mm × 5 mm. This was set on a support in the glove box. Air is shut off using a transfer vessel, and this support is set on an XPS measuring device (manufactured by Shimadzu / KRATOS, high performance photoelectron spectrometer AXIS-HS), and then XPS measurement is performed in a high vacuum state. The compounds contained in were estimated. It confirmed that it corresponded to each compound from the binding energy which the peak of Li, B, N, P, S, C, F, Cl, O, As, and Sb shows. For example, in the case of _Li 3 PO _4, it was confirmed Li, P, O, the binding energy corresponding to _Li 3 PO ₄ in the elements.

[3. DSC measurement]
The battery after the second cycle charge was disassembled in a dry room, and the negative electrode was taken out. The active material layer of the negative electrode was scraped off from the copper foil, and 3 mg of this active material layer was weighed. To this, 3 mg of an electrolytic solution in which 1 mol / dm ³ of LiPF ₆ was dissolved in a mixed solvent of EC and DEC in a volume ratio of 1: 1 was added and immediately set in a differential scanning calorimeter (SII, DSC220C). DSC measurement was performed in the range from room temperature to 200 ° C. at a rate of temperature increase of 10 ° C./min. From the result, the heat generation rate (unit: W / g · ° C.) of the negative electrode active material layer was determined. As an example, FIG. 1 shows DSC curves of negative electrode active material layers of Example Battery A1 and Comparative Example Battery B1.

Tables 1 to 5 show the determination results of the safety of each battery, the compounds on the negative electrode surface by XPS measurement, and the maximum value of the heat generation rate of each oxide by DSC measurement.

<result>
The batteries A1 to A25 of the example showed good safety, but the batteries B1 to B6 of the comparative example were defective. This result shows the correlation with the elements contained in the material on the negative electrode surface analyzed by XPS. In other words, in Examples Batteries A1 to A25 having good safety, the material present on the surface of the negative electrode active material contains at least one element selected from B, N, P, and S and Li element. It turned out to be included. On the other hand, it was found that Comparative Examples B1 to B6, which had poor safety, did not contain these elements.

  In addition, in DSC measurement of the negative electrode active material layer of each battery, no exothermic peak was observed in the A1 to A25 batteries of the present invention, and the maximum heat generation rate was 0.03 to 0.1 W / g · ° C. . On the other hand, in the negative electrode active material layers of Comparative Examples B1 to B7, an exothermic peak was observed, and the maximum exothermic rate was 0.5 W / g · ° C. or higher. That is, the battery of this example provided with the negative electrode active material layer having no exothermic peak in DSC is consistent with the test result that the safety was good.

  From the above results, a negative electrode active material having a core and a surface layer having different compositions from each other is provided in the negative electrode, the core of the negative electrode active material is provided with an oxide containing Si, and the surface layer is formed of B, N, P , Comprising at least one element selected from S and a compound containing Li, and good when no exothermic peak is observed at 200 ° C. or lower when DSC (differential scanning calorimetry) of the negative electrode active material layer is measured It was found to show a safe.

  This is presumably because the compound formed on the surface of the negative electrode active material used in the battery of the present invention suppressed the reaction with the electrolytic solution and improved safety. Further, when the data of the batteries of Examples A1 and A23, Examples A2 and A24, and Examples A3 and A13 were compared, those containing the elements C and F had a smaller maximum heat generation rate. It has been found that the compound formed on the surface of the negative electrode active material containing C and F elements is preferable for improving the safety of the battery.

  In addition, although SiO whose value of the atomic ratio x of O with respect to Si was x = 1 was used for the negative electrode active material of an Example, it is not limited to this. Those in the range of 0 <x <2 are preferable active materials. It was found that the substance provided on the surface of the oxide by the above method is not related to the element ratio x of O to Si.

  Moreover, the substance formed on the surface of the oxide containing Si is not limited to those listed in Table 1, but includes a compound containing at least one element selected from B, N, P, and S and Li. It may be sufficient and other elements may be included. As the method for forming the substance, an electrochemical method, a direct-current plasma CVD method, and an immersion method are shown, but other methods can also be used. In the electrochemical method, the salt type, concentration, and solvent type and ratio of the electrolytic solution are not limited to these, and are effective. In addition, additives such as phosphazenes and tetramethylammonium hexafluorophosphoric acid in Examples 5, 7 and 8 may be added. These contain N and P as elements, but are not limited to the compounds of the compositions of the examples, and derivatives thereof can also be used.

  Moreover, you may add another additive as needed. In CVD, not only DC plasma but also high frequency plasma, light, laser and heat methods are effective. The type and composition of the gas to be supplied can be changed as appropriate.

  In the dipping method, the solvent for dipping the negative electrode is not limited to NMP in the examples, but dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, NN-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran The drying temperature can be changed to an appropriate value as long as the solvent can be evaporated.

The electrolyte solution of the nonaqueous electrolyte battery of the example was prepared by dissolving 1 mol / dm ³ of LiPF ₆ in a mixed solvent of EC and DEC in a volume ratio of 1: 1. The type and mixing ratio are not limited to these, and an appropriate one can be selected. Moreover, you may add various additives as needed.

The figure which showed the DSC measurement result of the negative electrode active material layer taken out from this invention battery A1 and comparative example battery B1 of charge state.

Claims (2)

In a non-aqueous electrolyte secondary battery in which a negative electrode active material having a core and a surface layer having different compositions is provided in a negative electrode, the core includes an oxide containing Si, and the surface layer includes B, N, P, S Non-water characterized by comprising at least one compound containing one element selected from the group consisting of Li and Li and having no exothermic peak at 200 ° C. or lower in DSC (differential scanning calorimetry) of the negative electrode active material layer Electrolyte secondary battery 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the surface layer contains C or (and) F.

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