JP6011607B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

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

  Some non-aqueous electrolyte secondary batteries discharge and charge when the active material absorbs and releases lithium ions. Such non-aqueous electrolyte secondary batteries are small and have a large capacity, and are therefore used in a wide range of fields such as mobile phones and notebook computers. In recent years, it has been studied that lithium ion secondary batteries are also used as driving sources for vehicles.

  The non-aqueous electrolyte secondary battery is composed of a positive electrode, a negative electrode, and an electrolytic solution. The positive electrode is coated with a positive electrode active material composed of a metal composite oxide of lithium and a transition metal, such as a lithium / manganese composite oxide, a lithium / cobalt composite oxide, or a lithium / nickel composite oxide, and a positive electrode active material. Current collector.

The negative electrode is formed by covering a current collector with a negative electrode active material capable of inserting and extracting lithium ions. In recent years, the use of silicon oxide (SiOx: about 0.5 ≦ x ≦ 1.5) has been studied as a negative electrode active material capable of inserting and extracting lithium ions. It is known that silicon oxide SiOx decomposes into Si and SiO ₂ when heat-treated. This is called a disproportionation reaction, and if it is a homogeneous solid silicon monoxide SiO in which the ratio of Si to O is approximately 1: 1, it is separated into two phases of Si phase and SiO ₂ phase by solid internal reaction. To do. The Si phase obtained by separation is very fine and is covered with the SiO ₂ phase. The Si phase contains silicon alone capable of inserting and extracting Li ions, and the volume expands and contracts due to the expansion and contraction of Li ions. The SiO ₂ phase suppresses the decomposition reaction of the electrolytic solution by absorbing the expansion / contraction of the Si phase and improves the charge / discharge cycle characteristics of the battery.

  When the non-aqueous electrolyte secondary battery is charged and discharged, lithium ions are inserted and desorbed between the positive electrode active material and the negative electrode active material through the electrolytic solution. At that time, the electrolytic solution is partially reduced and decomposed, and the decomposition product covers the surface of the negative electrode active material to form a film. This film is a film in which lithium ions can easily pass but electrons cannot easily pass through, and is called a solid electrolyte interphase (SEI). The coating covers the surface of the negative electrode active material, thereby preventing direct contact between the electrolytic solution and the negative electrode active material, thereby suppressing degradation of the electrolytic solution.

  In recent years, in order to improve battery characteristics, components in the electrolytic solution have been studied. For example, JP 2006-294518 A, JP 2009-087934 A, International Publication No. 2009-028567, JP 2007-504628 A, and JP 2006-172811, as a solvent for an electrolytic solution, It is disclosed that the charge / discharge cycle characteristics of a battery are improved by using fluoroethylene carbonate (FEC).

JP 2006-294518 A JP 2009-087934 A International Publication No. 2009-028567 Special table 2007-504628 JP 2006-172811 A

  However, it is necessary to further improve the charge / discharge cycle characteristics of the battery. Therefore, the inventor of the present application diligently searched to improve the electrolytic solution.

  This invention is made | formed in view of this situation, and makes it a subject to provide the nonaqueous electrolyte secondary battery excellent in charging / discharging cycling characteristics.

The non-aqueous electrolyte secondary battery of the present invention comprises a negative electrode having a negative electrode active material capable of occluding and releasing lithium ions, a positive electrode having a positive electrode active material capable of occluding and releasing lithium ions, and an electrolyte dissolved in a solvent. A non-aqueous electrolyte secondary battery having an electrolyte solution, wherein the solvent of the electrolyte solution contains fluorine-based ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate.

  According to the non-aqueous electrolyte secondary battery of the present invention, since the electrolytic solution contains fluorine-based ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate, the battery is excellent in charge / discharge cycle characteristics.

It is a diagram which shows the result of the charging / discharging cycle test of samples 1-3. It is a diagram which shows the result of the charging / discharging cycle test of the samples 1 and 8. FIG.

  A nonaqueous electrolyte secondary battery according to an embodiment of the present invention will be described in detail.

  The electrolyte used for the non-aqueous electrolyte secondary battery contains fluorine-based ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate. Fluorine-based ethylene carbonate is a kind of cyclic carbonate, and by containing fluorine, the reduction potential is low and is easily reduced. For this reason, fluorine-based ethylene carbonate is easily reduced and decomposed in preference to other components in the electrolytic solution. The decomposition component of fluorine-based ethylene carbonate forms a film on the surface of the negative electrode active material. Since fluorine-based ethylene carbonate is easily reduced and decomposed, a thin film is formed on the entire surface of the negative electrode active material. For this reason, most of the active sites of the negative electrode active material are covered with the film in the initial use of the battery, and direct contact with the electrolytic solution is suppressed. Therefore, the production | generation of the further coating film is suppressed and the thickening of a film is suppressed. Therefore, an increase in the resistance of the negative electrode is suppressed, and a decrease in the discharge capacity of the battery when charging / discharging is repeated is suppressed. That is, the charge / discharge cycle characteristics of the battery are improved.

  Furthermore, the solvent of the electrolytic solution contains dimethyl carbonate (hereinafter referred to as DMC) and ethyl methyl carbonate (hereinafter referred to as EMC) in addition to fluorine-based ethylene carbonate. DMC and EMC are types of chain carbonates. In this case, although the reason is not clear, the decrease in the discharge capacity of the battery accompanying use is remarkably suppressed.

In the fluorine-based ethylene carbonate, fluorine is preferably bonded directly to carbon forming the cyclic skeleton of the cyclic carbonate. Examples of the fluorinated ethylene carbonate include fluorinated ethylene carbonate, difluorinated ethylene carbonate, trifluorinated ethylene carbonate, trifluoropropylene carbonate, 4-methyl-5-fluoro-1,3-dioxolan-2-one, and the like. . Examples of the fluorinated ethylene carbonate include 4-fluoro-1,3-dioxolan-2-one (fluoroethylene carbonate, FEC ) . The difluoride ethylene carbonate include D FEC (difluoroethylene carbonate). DFEC includes 4,5-difluoro-1,3-dioxolan-2-one. Examples of the trifluoropropylene carbonate include ethylene trifluoromethylene carbonate (4-trifluoromethyl-1,3-dioxolan-2-one) . In view of acid resistance, it is particularly preferable to use FEC.

  The volume ratio of fluorine-based ethylene carbonate when the solvent of the electrolytic solution is 100% by volume is preferably 1% by volume to 40% by volume, and more preferably 15% by volume to 30% by volume. When the volume ratio of the fluorine-based ethylene carbonate is less than 1% by volume, the discharge capacity of the battery may decrease as the battery is used. When the volume ratio of the fluorine-based ethylene carbonate exceeds 40% by volume, the viscosity of the electrolytic solution is increased, the lithium ion flowability is lowered, and the discharge capacity of the battery may be lowered.

When the total volume of EMC and DMC in the electrolytic solution is 100% by volume, the volume ratio of EMC is preferably 14% by volume or more and 86% by volume or less. The conductivity of EMC is 1.1 mS · cm ⁻¹ , and the conductivity of DMC is 2.0 mS · cm ⁻¹ . The conductivity of DMC is higher than that of EMC, but it is volatile and difficult to handle. If the DMC is too low, the conductivity of the electrolyte may be reduced. If the EMC is too low, the DMC may be relatively increased, and the volume of the electrolyte may be reduced due to volatilization of the DMC, making it difficult to handle. There is.

  When the combined volume of EMC and DMC in the electrolytic solution is 100% by volume, the FEC volume ratio is preferably 10% by volume to 60% by volume. When this volume ratio is too small, a film is only formed on a part of the surface of the negative electrode active material in the initial stage of battery use. For this reason, every time charging / discharging of the battery is repeated, the negative electrode active material and the electrolytic solution come into contact with each other to gradually form a film, and a relatively thick film tends to be formed. For this reason, the electrical resistance of a negative electrode active material becomes high by repetition of charging / discharging, and there exists a possibility that the discharge capacity of a battery may fall. On the other hand, when the volume ratio is excessive, the viscosity of the electrolytic solution becomes high, the fluidity of lithium ions is lowered, and the discharge capacity of the battery may be lowered.

  The solvent of the electrolytic solution is preferably a non-aqueous organic solvent. The organic solvent contained in the electrolytic solution is preferably an aprotic organic solvent, and requires fluorine-based ethylene carbonate, DMC, and EMC. Fluorine-based ethylene carbonate is a kind of cyclic carbonate, and DMC and EMC are kinds of chain carbonate. Since cyclic carbonate has a high dielectric constant and chain carbonate has low viscosity, the inclusion of both cyclic carbonate and chain carbonate in the electrolyte does not hinder the movement of Li ions in the electrolyte, and the discharge capacity of the battery Can be improved.

  When the entire solvent of the electrolytic solution is 100% by volume, the cyclic carbonate is preferably 30 to 50% by volume, and the chain carbonate is preferably 50 to 70% by volume. This volume ratio is a ratio when the cyclic carbonate contains fluorine-based carbonate and the chain carbonate contains DMC and EMC. Cyclic carbonate has a high dielectric constant and a high viscosity while increasing the conductivity of the electrolyte. If the viscosity is high, the movement of Li ions is hindered, resulting in poor conductivity. A chain carbonate has a low dielectric constant, but a low viscosity. By blending them in a well-balanced range within the above blending ratio, the conductivity of the solvent can be increased to some extent and the viscosity can be decreased, a solvent having good conductivity can be adjusted, and the discharge capacity of the battery can be improved. .

The cyclic carbonate used for the electrolytic solution may contain the following cyclic carbonate. For example, at least one selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate, γ-butyrolactone, vinylene carbonate, 2-methyl-gamma butyrolactone, acetyl-gamma butyrolactone, and gamma valerolactone You can leave.

  The chain carbonate used in the electrolytic solution contains DMC and EMC as essential components and may contain other chain carbonates. For example, one or more selected from diethyl carbonate (DEC), dibutyl carbonate, dipropyl carbonate, propionic acid alkyl ester, malonic acid dialkyl ester, and acetic acid alkyl ester may be included.

  Further, the solvent of the electrolytic solution may contain ethers. Examples of ethers contained in the solvent of the electrolytic solution include tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane, and the like. Can be used.

  The electrolytic solution does not contain ethylene carbonate (EC), and preferably contains fluorine-based ethylene carbonate. The content of the fluorine-based ethylene carbonate is preferably 15% by volume or more and 30% by volume or less when the entire solvent of the electrolytic solution is 100% by volume. Further, the electrolytic solution preferably contains FEC without EC. In this case, the content of FEC is preferably 15% by volume or more and 30% by volume or less when the entire solvent of the electrolytic solution is 100% by volume. In this case, the charge / discharge cycle characteristics are further improved.

  Moreover, the solvent of electrolyte solution contains EC in addition to FEC, EMC, and DMC, and the whole solvent should consist only of FEC, EMC, DMC, and EC. Also in this case, the cycle characteristics are good.

  The electrolyte contained in the electrolytic solution is preferably a fluoride salt, and is preferably an alkali metal fluoride salt that is soluble in an organic solvent. As an alkali metal fluoride, for example, LiPF₆, LiBF₄, LiAsF₆, NaPF₆, NaBF₄, And NaAsF₆It is good to use at least 1 sort (s) chosen from the group of these. As the fluoride salt, LiClO₄, LiN (SO₂CF₃)₂, LiN (SO₂C₂F₅)₂, LiC (SO₂CF₃)₃, LiPF ₄(CF₃)₂, LiPF₃(C₂F₅)₃, LiPF₃(CF₃)₃, LiPF₃(Iso-C₃F₇)₃, LiPF₅(Iso-C₃F₇), LiSbF₆, LiCF₃SO ₃, LiC₄F₉SO₃, LiCF₃CO₂, Li₂C₂F₄(SO₃)₂, LiN (CF₃SO₂)₂And LiC_nF_{2n + 1}SO₃At least one selected from the group consisting of (n ≧ 2) may be used. Of these, LiPF provides good charge / discharge characteristics₆And LiC₄F₉SO₃It is particularly preferable to use Note that the non-aqueous electrolyte in the non-aqueous electrolyte secondary battery of the present invention may contain an electrolyte salt other than the fluorine-containing electrolyte salt. For example, LiClO₄LiI or the like can be used alone or in admixture of two or more with the fluorine-containing electrolyte salt.

The concentration of the electrolyte in the nonaqueous electrolytic solution is not particularly limited, but is preferably about 0.3 to 1.7 mol / dm ³ , particularly about 0.4 to 1.5 mol / dm ³ . The electrolyte concentration here refers to the concentration of all electrolytes including the fluorine-containing electrolyte salt.

  Moreover, in order to improve the safety | security and storage characteristic of a battery, you may make an non-aqueous electrolyte contain an aromatic compound. As the aromatic compound, benzenes having an alkyl group such as cyclohexylbenzene or t-butylbenzene, biphenyl, or fluorobenzenes are preferably used.

  This aromatic compound is an additive that may be added to the electrolytic solution. The aromatic compound may be at least one selected from the group consisting of polycyclic hydrocarbon compounds and derivatives thereof. The aromatic compound is at least one selected from a polycyclic hydrocarbon compound in which C (carbon) of a cyclic hydrocarbon group is directly bonded to C contained in a benzene ring and a derivative thereof. The cyclic hydrocarbon group is a cyclic hydrocarbon group having 5 or more carbon atoms, and may be an alicyclic hydrocarbon group, an aromatic hydrocarbon group, or both of them. . Further, it may be monocyclic or polycyclic. Examples of such a cyclic hydrocarbon group include a 6-membered ring such as a phenyl group (benzene ring) and a cyclohexyl group, a 5-membered ring such as a cyclopentyl group, and a condensed ring such as a naphthyl group. It is done. Specific examples of the additive having such a cyclic hydrocarbon group include biphenyl, cyclohexylbenzene, dicyclohexylbenzene, diphenylcyclohexane, naphthylbenzene and the like.

  According to the lithium ion secondary battery of the present invention, these aromatic compounds can generate hydrogen gas at the time of overcharging and actuate the current interrupting means at an early stage to prevent further progress of overcharging. Moreover, decomposition | disassembly of electrolyte solution can be suppressed by adding these aromatic compounds to electrolyte solution. Although the reason is not clear, it is considered that the polycyclic hydrocarbon compound is oxidized at the positive electrode during the activation process or during storage, and forms a polymer film in which the cyclic hydrocarbon groups are linked in a turtle shell shape on the positive electrode. And it is thought that this polymerized film suppresses decomposition of the electrolytic solution in the positive electrode. And it is thought that malfunctions, such as insulation film formation resulting from decomposition | disassembly of electrolyte solution, are suppressed, and the charge / discharge capacity fall after storage is suppressed.

  The negative electrode has a negative electrode active material capable of inserting and extracting lithium ions. The negative electrode is preferably composed of a current collector provided with at least the surface of the negative electrode active material. As the current collector, nickel, stainless steel, or the like can be used, and the current collector may have any shape such as a foil, a plate, and a mesh.

  The negative electrode active material capable of inserting and extracting lithium ions may be composed of an element compound capable of being alloyed with lithium and / or an element compound having an element capable of being alloyed with lithium. Elements capable of alloying with lithium are Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Ti, Ag, Zn, Cd, Al, Ga, In, Si, Ge. , Sn, Pb, Sb, and Bi may be at least one selected from the group. Among these, silicon (Si) or tin (Sn) is preferable. The elemental compound having an element capable of alloying with lithium is preferably a silicon compound or a tin compound. The silicon compound is preferably SiOx (0.5 ≦ x ≦ 1.5). Examples of the tin compound include a tin alloy (Cu—Sn alloy, Co—Sn alloy, etc.), a tin alloy (Cu—Sn alloy, Co—Sn alloy, etc.), and the like.

  Moreover, as a negative electrode active material which can occlude / release lithium ions, for example, natural graphite, artificial graphite, an organic compound heating body such as a phenol resin, and a powdery body of a carbon material such as coke can be used.

  Further, as the negative electrode active material capable of inserting and extracting lithium ions, metallic lithium or a lithium alloy may be used. Thus, a battery using metallic lithium or a lithium alloy as a negative electrode active material is called a lithium secondary battery. A battery using an element other than metallic lithium or a lithium alloy as a negative electrode active material is called a lithium ion secondary battery.

  The negative electrode active material may contain Si (silicon). This is because the negative electrode active material containing Si has a large discharge capacity. The negative electrode active material containing Si has a large volume change due to insertion and extraction of Li ions during charge and discharge. Since the film formed on the surface of the negative electrode active material has a relatively small thickness, it flexibly follows the volume change of the negative electrode active material. For this reason, even if it repeats charging / discharging, a film is hard to destroy and it is excellent in charging / discharging cycling characteristics.

  The negative electrode active material containing Si may be capable of occluding and releasing lithium ions and may be made of silicon or / and a silicon compound. The negative electrode active material may have SiOx (0.5 ≦ x ≦ 1.5). Silicon has a large theoretical discharge capacity. On the other hand, since the volume change of silicon at the time of charging / discharging is large, the volume change can be reduced by using SiOx.

The negative electrode active material preferably has a Si phase and a SiO ₂ phase. The Si phase is composed of simple silicon, and is a phase that can occlude and release Li ions, and expands and contracts as Li ions are occluded and released. The SiO ₂ phase is made of SiO ₂ and absorbs expansion and contraction of the Si phase. By Si phase is covered by SiO ₂ phase, it may form a negative electrode active material composed of a Si phase and SiO ₂ phase. Furthermore, it is preferable that a plurality of miniaturized Si phases are covered with a SiO ₂ phase and integrated to form one particle, that is, a negative electrode active material. In this case, the volume change of the whole negative electrode active material can be suppressed effectively.

The mass ratio of the SiO ₂ phase to the Si phase in the negative electrode active material is preferably 1 to 3. When the mass ratio is less than 1, the negative electrode active material is greatly expanded and contracted, and cracks may occur in the negative electrode active material layer composed of the negative electrode active material. On the other hand, when the mass ratio exceeds 3, the amount of insertion / extraction of Li ions in the negative electrode active material is small, and the electric capacity may be lowered.

The negative electrode active material may be composed only of the Si phase and the SiO ₂ phase. Further, the negative electrode active material is mainly composed of a Si phase and a SiO ₂ phase, but in addition, a known active material may be included as a component of the negative electrode active material particles. Specifically, Me _x At least one of Si _y O _z (Me is Li, Ca, etc., x, y, and z are integers) may be mixed.

As a raw material for the negative electrode active material, a raw material powder containing silicon monoxide may be used. In this case, silicon monoxide in the raw material powder is disproportionated into two phases of SiO ₂ phase and Si phase. In disproportionation of silicon monoxide, silicon monoxide (SiOn: n is 0.5 ≦ n ≦ 1.5), which is a homogeneous solid having an atomic ratio of Si to O of approximately 1: 1, is a reaction inside the solid. To separate into two phases of SiO ₂ phase and Si phase. The silicon oxide powder obtained by disproportionation includes a SiO ₂ phase and a Si phase.

  The disproportionation of silicon monoxide in the raw material powder proceeds by applying energy to the raw material powder. As an example, a method of heating or milling the raw material powder can be mentioned.

When the raw material powder is heated, it is generally said that almost all silicon monoxide is disproportionated and separated into two phases at 800 ° C. or higher if oxygen is removed. Specifically, the raw material powder containing the amorphous silicon monoxide powder is subjected to heat treatment at 800 to 1200 ° C. for 1 to 5 hours in an inert atmosphere such as vacuum or in an inert gas. A silicon oxide powder containing two phases of an amorphous SiO ₂ phase and a crystalline Si phase is obtained.

  When milling the raw material powder, part of the mechanical energy of the milling contributes to chemical atomic diffusion at the solid phase interface of the raw material powder, and generates an oxide phase, a silicon phase, and the like. In milling, the raw material powder may be mixed using a V-type mixer, a ball mill, an attritor, a jet mill, a vibration mill, a high energy ball mill or the like in an inert gas atmosphere such as vacuum or argon gas. Further heat treatment may be performed after milling to further promote disproportionation of silicon monoxide.

  The negative electrode active material constitutes a negative electrode material that covers at least the surface of the current collector. In general, the negative electrode is configured by pressing the negative electrode material as a negative electrode active material layer onto a current collector. As the current collector, for example, a metal mesh or metal foil such as copper or copper alloy may be used. In addition to the negative electrode active material, the negative electrode material may contain a binder, a conductive additive, and the like.

  The binder is not particularly limited, and a known one may be used. For example, a resin that does not decompose even at a high potential, such as a fluorine-containing resin such as polytetrafluoroethylene or polyvinylidene fluoride, can be used. The blending ratio of the binder is preferably a mass ratio of negative electrode active material: binder = 1: 0.05 to 1: 0.5. This is because when the amount of the binder is too small, the moldability of the electrode is lowered, and when the amount of the binder is too large, the energy density of the electrode is lowered.

  As the conductive auxiliary agent, a material generally used for electrodes of non-aqueous electrolyte secondary batteries may be used. For example, it is preferable to use conductive carbon materials such as carbon black (carbonaceous fine particles) such as acetylene black and ketjen black, and carbon fibers. Besides conductive carbon materials, known conductive materials such as conductive organic compounds are also used. An auxiliary agent may be used. One of these may be used alone or in combination of two or more. The blending ratio of the conductive assistant is, in mass ratio, negative electrode active material: conductive assistant = 1: 0.01 to 1: 0.5. This is because if the amount of the conductive auxiliary is too small, an efficient conductive path cannot be formed, and if the amount of the conductive auxiliary is too large, the moldability of the electrode is deteriorated and the energy density of the electrode is lowered.

  The positive electrode used in the non-aqueous electrolyte secondary battery of the present invention is preferably composed of a current collector and a positive electrode material that has a positive electrode active material and covers the surface of the current collector. The positive electrode material includes a positive electrode active material capable of inserting and extracting lithium ions, and preferably further includes a binder and / or a conductive aid. The conductive auxiliary agent and the binder are not particularly limited, and any conductive auxiliary agent and binder can be used as long as they can be used in the nonaqueous electrolyte secondary battery. As described above, the binder and the conductive auxiliary used in the positive electrode can be the same as the binder and the conductive auxiliary used in the negative electrode.

As the positive electrode active material, for example, a metal composite oxide of lithium and a transition metal such as a lithium / manganese composite oxide, a lithium / cobalt composite oxide, or a lithium / nickel composite oxide is used. Specific examples include LiCoO ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , Li ₂ MnO ₃ , and S. As the positive electrode active material, an active material that does not contain lithium, for example, sulfur alone or a sulfur-modified compound can be used. However, when neither positive electrode nor negative electrode contains lithium, it is necessary to pre-dope lithium.

  The current collector for the positive electrode is not particularly limited as long as it is generally used for the positive electrode of a lithium ion secondary battery, such as aluminum, nickel, and stainless steel, and may have various shapes such as a mesh and a metal foil.

  A separator is used as needed. The separator separates the positive electrode and the negative electrode and holds the non-aqueous electrolyte, and a thin microporous film such as polyethylene or polypropylene can be used.

It is preferable that a separator coated with ceramic is provided between the positive electrode and the negative electrode. A ceramic film is preferably formed on one side or both sides of the separator. The ceramic film is preferably made of at least one of alumina (Al ₂ O ₃ ) and magnesia (MgO). Charge / discharge cycle characteristics are improved by using a separator on which a ceramic film is formed. The reason is considered that the ceramic film captures hydrogen fluoride and suppresses the excessive formation of the SEI film on the surface of the negative electrode active material.

  A separator is sandwiched between the positive electrode and the negative electrode as necessary to form an electrode body. Lithium ion secondary battery in which a non-aqueous electrolyte is impregnated in the electrode body after connecting between the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal leading to the outside using a current collecting lead or the like It is good to do.

  The shape of the lithium ion secondary battery is not particularly limited, and various shapes such as a cylindrical shape, a stacked shape, a coin shape, and a laminated shape can be adopted.

  The non-aqueous electrolyte secondary battery may be mounted on a vehicle. By driving the driving motor with the non-aqueous electrolyte secondary battery, it can be used for a long time with a large capacity and a large output. The vehicle may be a vehicle that uses electric energy generated by a non-aqueous electrolyte secondary battery for all or a part of its power source. For example, the vehicle may be an electric vehicle or a hybrid vehicle. When a non-aqueous electrolyte secondary battery is mounted on a vehicle, a plurality of non-aqueous electrolyte secondary batteries may be connected in series to form an assembled battery. Examples of non-aqueous electrolyte secondary batteries include various home electric appliances, office equipment, and industrial equipment driven by batteries, such as personal computers and portable communication devices, in addition to vehicles.

  Fourteen types of samples 1 to 14 were prepared as follows, and a charge / discharge cycle evaluation test was performed. Samples 1, 2, 8, and 10 to 14 are examples of the present invention, and samples 3 to 7 and 9 are reference examples of the present invention.

(Sample 1)
First, commercially available SiO powder was put in a ball mill and milled for 20 hours at 450 rpm in an Ar atmosphere. Thereafter, heat treatment was performed for 2 hours at 900 ° C. in an inert gas atmosphere. Thereby, SiO powder was disproportionated and the negative electrode active material was obtained. When this negative electrode active material was subjected to X-ray diffraction (XRD) measurement using CuKα, a specific peak derived from simple silicon and silicon dioxide was confirmed. From this, it was found that elemental silicon and silicon dioxide were generated in the negative electrode active material.

  The prepared negative electrode active material, natural graphite powder and ketjen black as a conductive additive, and polyamideimide as a binder were mixed, and a solvent was added to obtain a slurry mixture. The solvent was N-methyl-2-pyrrolidone (NMP). The mass ratio of the negative electrode active material, natural graphite, ketjen black, and polyamideimide was, as a percentage, negative electrode active material particles / natural graphite particles / acetylene black / polyamideimide = 32/50/8/10. .

  Next, the slurry mixture was formed into a film on one side of a copper foil as a current collector using a doctor blade, pressed at a predetermined pressure, heated at 200 ° C. for 2 hours, and allowed to cool. Thereby, the negative electrode formed by fixing the negative electrode active material layer on the current collector surface was formed.

Next, a lithium / nickel composite oxide LiNi _1/3 Co _1/3 Mn _1/3 O ₂ as a positive electrode active material, acetylene black, and polyvinylidene fluoride (PVDF) as a binder are mixed to form a slurry. It became. This slurry was applied to one side of an aluminum foil as a current collector, pressed and fired. The mass ratio of the lithium / nickel composite oxide, acetylene black and polyvinylidene fluoride was lithium / nickel composite oxide / acetylene black / polyvinylidene fluoride = 88/6/6. This obtained the positive electrode formed by fixing a positive electrode active material layer on the surface of a collector.

A polypropylene porous membrane as a separator was sandwiched between the positive electrode and the negative electrode. The electrode body which consists of this positive electrode, a separator, and a negative electrode was laminated | stacked. The periphery of the two aluminum films was sealed by heat-welding except for a part to make a bag shape. The laminated electrode body was put in a bag-like aluminum film, and an electrolytic solution was further put. The electrolytic solution is obtained by dissolving LiPF ₆ as an electrolyte in an organic solvent. The organic solvent was prepared by mixing fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of FEC / EMC / DMC = 30/30/40. The concentration of LiPF ₆ in the electrolytic solution was 1 mol / L (M).

  Then, the opening part of the aluminum film was completely airtightly sealed while evacuating. At this time, the tips of the positive electrode side and negative electrode side current collectors were projected from the edge portions of the film to be connectable to external terminals to obtain a lithium ion secondary battery. The lithium ion secondary battery was subjected to a conditioning treatment in which initial charge / discharge was repeated three times.

(Sample 2)
In the lithium ion secondary battery of Sample 2, fluoroethylene carbonate (FEC), ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were used as the organic solvent of the electrolytic solution. The compounding ratio of the organic solvent was FEC / EC / EMC / DMC = 4/26/30/40 by volume%. Others are the same as those of Sample 1.

(Sample 3)
In the lithium ion secondary battery of Sample 3, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were used as the organic solvent for the electrolytic solution. The compounding ratio of the organic solvent was EC / EMC / DMC = 30/30/40 by volume%. Others are the same as those of Sample 1.

<Battery charge / discharge cycle test (1)>
The charge / discharge cycle test was performed on the lithium ion secondary batteries of Samples 1 to 3. The cycle test was performed at 25 ° C., the charging condition was 1 C, 4.2 V CC (constant current) charging, and the discharging condition was 1 C, 2.5 V CC (constant current) discharging. The first charge / discharge after the conditioning treatment was regarded as the first cycle, and the same charge / discharge was repeated until the 600th cycle. In FIG. 1, the result of the charging / discharging cycle test of the samples 1-3 was shown. The horizontal axis of FIG. 1 shows the number of charge / discharge cycles in the cycle test, and the vertical axis shows the capacity maintenance rate of the discharge current for each charge / discharge cycle. The capacity maintenance rate of the discharge current is indicated by the ratio (%) of the capacity of the discharge current of each battery in each cycle when the capacity of the discharge current of each battery before the charge / discharge cycle test is 100%. .

  As shown in FIG. 1, the cycle characteristics of Samples 1 and 2 were much better than Sample 3. Samples 1 and 2 showed substantially the same discharge capacity maintenance rate up to the 500th cycle, but the discharge capacity maintenance rate of sample 1 was higher than that of sample 2 near the 600th cycle.

  Moreover, the mass increase rate of the negative electrode active material layer by the charge / discharge cycle test of the batteries of Samples 1 to 3 was measured. The mass increase rate is the ratio of the mass of the negative electrode active material layer of each battery at the 600th cycle of the charge / discharge cycle test, where the mass of the negative electrode active material layer of each battery before the charge / discharge cycle test is 1. did. The mass increase rate is shown in Table 1.

  As shown in Table 1, Samples 1 and 2 containing FEC in the electrolyte had a lower mass increase rate of the negative electrode active material layer than Sample 3 containing no FEC. In samples 1 and 2, sample 1 having a higher FEC concentration had a lower mass increase rate of the negative electrode active material layer.

  From the above results, in Samples 1 and 2, in the initial stage of the cycle test, FEC in the electrolytic solution decomposes prior to the other electrolytic solution components, and the decomposed components are thin over almost the entire surface of the negative electrode active material. A coating was formed, and the increase in the thickness of the coating was suppressed by suppressing the increase in the thickness of the coating beyond that. In addition, it is considered that the thin coating prevented direct contact between the negative electrode active material and the electrolytic solution, and the decomposition of the electrolytic solution other than FEC was suppressed. In Samples 1 and 2, since the coating covering the surface of the negative electrode active material is thin, it flexibly follows the surface of the negative electrode active material that expands and contracts due to charge and discharge, and cracks are less likely to occur. It is considered that this also improved the charge / discharge cycle characteristics.

  Since Sample 1 has a higher FEC concentration in the electrolytic solution than Sample 2, it is considered that the increase in the weight of the negative electrode active material is suppressed, the decomposition of the electrolytic solution is suppressed, and the cycle characteristics are improved.

  Next, in order to examine components (DMC, EMC) other than FEC of the electrolytic solution, a lithium ion secondary battery including an electrolytic solution in which components other than FEC were changed was produced, and a charge / discharge cycle test was performed.

(Sample 4)
In the lithium ion secondary battery of Sample 4, ethylene carbonate (EC) and diethyl carbonate (DEC) were used as organic solvents for the electrolytic solution. The compounding ratio of the organic solvent was EC / DEC = 30/70 by volume%. Others are the same as those of Sample 1.

(Sample 5)
In the lithium ion secondary battery of Sample 5, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were used as the organic solvent for the electrolytic solution. The compounding ratio of the organic solvent was EC / EMC = 30/70 by volume%. Others are the same as those of Sample 1.

(Sample 6)
In the lithium ion secondary battery of Sample 6, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were used as the organic solvent for the electrolytic solution. The compounding ratio of the organic solvent was EC / EMC / DMC = 30/30/40 by volume%. Others are the same as those of Sample 1.

(Sample 7)
In the lithium ion secondary battery of Sample 7, ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) were used as the organic solvent for the electrolytic solution. The compounding ratio of the organic solvent was EC / DEC / DMC = 30/30/40 in volume%. Others are the same as those of Sample 1.

<Battery charge / discharge cycle test (2)>
About the lithium ion secondary battery of samples 4-7, the cycle test of charging / discharging was done. The cycle test was performed in the same manner as the above <Battery charge / discharge cycle test (1)> except that the cycle test was performed at 55 ° C. The charge / discharge cycle test was performed 500 times, and the capacity retention rate of the discharge current at that time is shown in Table 2.

  As shown in Table 2, the capacity retention rate of Sample 6 was the highest among Samples 4-7. In the <Battery charge / discharge cycle test (2)>, the samples 4 to 7 are tested at 55 [deg.] C., but the cycle test is performed at 25 [deg.] C. as shown in <Battery charge / discharge cycle test (1)>. It is thought that the result of the same cycle characteristic is shown.

  Moreover, the structure of the solvent of the electrolyte solution of the sample 3 is the same as that of the sample 1 except that EC is used instead of FEC. FEC is a compound in which hydrogen of EC is substituted with fluorine, and its chemical structure is very similar to EC. The electrolytic solution of Sample 1 includes FEC instead of EC, and is the same as Sample 6 in that it includes DMC and EMC. Sample 1 is considered to exhibit the same cycle characteristics as Sample 6.

Samples 4-7 all contain EC that is very similar in structure to FEC. Furthermore, sample 6 contains EMC and DMC, while sample 5 contains EMC and does not contain DMC. Sample 7 contains DEC and DMC, but does not contain EMC. In such a solvent configuration, the sample 6 has the best cycle characteristics because it contains EMC and DMC in addition to EC. It is considered that the same cycle characteristics as those of Samples 4 to 7 can be obtained for a battery in which FEC having a structure similar to EC is added to the electrolyte. Therefore, it can be said that the charge / discharge cycle characteristics are remarkably improved by including FEC, EMC, and DMC in the solvent of the electrolytic solution. FEC is a cyclic carbonate, and EMC and DMC are chain carbonates. Cyclic carbonates are highly conductive but highly viscous. While chain carbonate has low conductivity, it has low viscosity. By including both cyclic carbonate and chain carbonate, the electrolytic solution can have high conductivity and low viscosity. The electric conductivity of EMC is 1.1 mS · ^{cm -1,} the conductivity of the DMC is 2.0 mS · ^{cm -1,} the conductivity of the DEC is 0.6 mS · ^{cm -1.} Although chain carbonates generally have a low conductivity, EMC and DMC have a relatively high conductivity among chain carbonates. For this reason, it is considered that the conductivity of the electrolytic solution can be further enhanced by including EMC and DMC in the electrolytic solution among the chain carbonates, and the cycle characteristics are improved.

(Sample 8)
The lithium ion secondary battery of Sample 8 is the same as Sample 1 except that an alumina film is formed on the separator. An alumina film is formed on both sides of the separator. The thickness of the alumina film is 25.5 μm. The porosity of the separator is 53%.

  The alumina coating method is as follows. To the polyparaphenylene terephthalamide (PPTA) solution, an N-methyl-2-pyrrolidone solution (NMP) of polyimide that has been imidized is added, and finally an isotropic phase having a PPTA concentration of 1.3% by weight is added. Prepared in solution and stirred for 60 minutes. Alumina fine particles (manufactured by Nippon Aerosil Co., Ltd .; Alumina C, average particle size 0.013 μm) were mixed in the above solution and stirred for 240 minutes. The slurry solution in which the alumina fine particles were sufficiently dispersed was filtered and then defoamed under reduced pressure to obtain a coating slurry solution. The slurry solution for coating is applied on both sides of the separator to form an alumina film, and after depositing the heat-resistant nitrogen-containing aromatic polymer on the alumina film, the polar organic solvent is removed from the alumina film, and the alumina film Dried.

<Battery charge / discharge cycle test (3)>
A charge / discharge cycle test was performed on the lithium ion secondary batteries of Samples 1 and 8. The cycle test was performed at 25 ° C., and was performed under the same conditions as <Battery charge / discharge cycle test (1)>. The results of the charge / discharge cycle test of the lithium ion secondary batteries of Samples 1 and 8 are shown in FIG. As shown in FIG. 2, Sample 8 in which an alumina film was formed on both surfaces of the separator had better charge / discharge cycle characteristics than Sample 1 using a separator in which no alumina film was formed.

(Sample 9)
Similar to Sample 1, a disproportionation treatment was performed on the SiO powder to obtain a negative electrode active material. This negative electrode active material, natural graphite powder as a conductive additive, ketjen black, and polyamideimide as a binder were mixed, and a solvent was added to obtain a slurry mixture. The solvent was N-methyl-2-pyrrolidone (NMP). The mass ratio of the negative electrode active material, natural graphite, ketjen black, and polyamideimide was, as a percentage, negative electrode active material particles / natural graphite particles / acetylene black / polyamideimide = 32/50/8/10. .

  Next, the slurry mixture was formed into a film on one side of a copper foil as a current collector using a doctor blade, pressed at a predetermined pressure, heated at 200 ° C. for 2 hours, and allowed to cool. Thereby, the negative electrode formed by fixing the negative electrode active material layer on the current collector surface was formed.

Next, a lithium / nickel composite oxide LiNi _1/3 Co _1/3 Mn _1/3 O ₂ as a positive electrode active material, acetylene black, and polyvinylidene fluoride (PVDF) as a binder are mixed to form a slurry. This slurry was applied to one side of an aluminum foil as a current collector, pressed and fired. The mass ratio of the lithium / nickel composite oxide, acetylene black and polyvinylidene fluoride was lithium / nickel composite oxide / acetylene black / polyvinylidene fluoride = 88/6/6. This obtained the positive electrode formed by fixing a positive electrode active material layer on the surface of a collector.

A polypropylene porous membrane as a separator was sandwiched between the positive electrode and the negative electrode. The electrode body which consists of this positive electrode, a separator, and a negative electrode was laminated | stacked. Similarly to the sample 1, the electrode body and the electrolytic solution were sealed in a bag-shaped aluminum film to obtain a lithium ion secondary battery. The electrolytic solution is obtained by dissolving LiPF ₆ as an electrolyte in an organic solvent. The organic solvent was prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) at a volume ratio of EC / EMC / DMC = 30/30/40. The concentration of LiPF ₆ in the electrolytic solution was 1 mol / L (M). The lithium ion secondary battery was subjected to a conditioning treatment in which initial charge / discharge was repeated three times.

(Sample 10)
In this sample 10, fluoroethylene carbonate (FEC), ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were used as the organic solvent of the electrolytic solution. The compounding ratio of the organic solvent was FEC / EC / EMC / DMC = 4/26/30/40 by volume%. Others are the same as those of the sample 9.

(Sample 11)
This sample 11 is different from the sample 10 in that LiNi _0.5 Co _0.2 Mn _0.3 O ₂ is used as the positive electrode active material. Others are the same as those of the sample 10.

(Sample 12)
In this sample 12, fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were used as the organic solvent of the electrolytic solution. The compounding ratio of the organic solvent was FEC / EMC / DMC = 30/30/40 by volume%. Others are the same as those of the sample 9.

<Battery charge / discharge cycle test (4)>
About the lithium ion secondary battery of samples 9-12, the cycle test of charging / discharging was done. The cycle test was performed at 25 ° C., the charging condition was 1 C, 4.2 V CC (constant current) charging, and the discharging condition was 1 C, 2.5 V CC (constant current) discharging. The first charge / discharge after the conditioning treatment was regarded as the first cycle, and the same charge / discharge was repeated until the 500th cycle. The ratio (%) of the capacity of the discharge current of each battery at the time of 500 cycles when the capacity of the discharge current of each battery before the charge / discharge cycle test was taken as 100% was determined. This ratio is shown in Table 3 as the capacity retention rate at 500 cycles.

  As shown in Table 3, the capacity retention rate of Sample 9 that did not include FEC in the solvent of the electrolytic solution was zero at the time of 500 cycles. In contrast, Samples 10 to 12 containing FEC in the solvent had a high capacity retention rate. The discharge capacity of sample 11 containing 30% by volume of FEC was slightly higher than that of sample 10 containing 4% by volume of FEC in the solvent. From this, it was found that when the solvent of the electrolytic solution was 100% by volume, the cycle characteristics were particularly improved when FEC contained 1% by volume to 40% by volume. Further, when FEC was added to the electrolytic solution, even when the type of the positive electrode active material was changed as in Samples 10 and 11, the discharge capacity retention rate was almost the same.

(Sample 13)
This sample 13 is different from the sample 10 in that the electrolyte solution further contains cyclohexylbenzene (CHB) and biphenyl (BP) as additives. When the entire electrolytic solution was 100% by mass, the CHB content was 2% by mass, and the BP content was 2% by mass. Others are the same as those of the sample 10.

(Sample 14)
This sample 14 is different from the sample 13 in that LiNi _0.5 Co _0.2 Mn _0.3 O ₂ is used as the positive electrode active material. Others are the same as those of the sample 13.

<Battery charge / discharge cycle test (5)>
For the lithium ion secondary batteries of Samples 10, 13, and 14, a charge / discharge cycle test was performed. In this charge / discharge cycle test, the capacity retention rate at 500 cycles was measured in the same manner as in <Battery charge / discharge cycle test (4)> except that the temperature during the test was 60 ° C. The results are shown in Table 4.

As shown in Table 4, even when CHB and BP were added to the electrolytic solution, high cycle characteristics could be realized by including FEC. CHB and BP are compounds that can generate gas during overcharge. Even in a high temperature state where the battery characteristics are likely to deteriorate, high cycle characteristics were obtained. For this reason, it is considered that a substance that can generate gas during overcharge regardless of temperature does not adversely affect the battery. By promoting gas generation at the time of overcharging, it is possible to more reliably implement an overcharging countermeasure such as operating the current interrupting means at an early stage and preventing further progress of overcharging.

Claims (6)

Non-aqueous electrolyte secondary having a negative electrode having a negative electrode active material capable of occluding and releasing lithium ions, a positive electrode having a positive electrode active material capable of occluding and releasing lithium ions, and an electrolyte obtained by dissolving an electrolyte in a solvent A battery,
The solvent of the electrolytic solution includes fluorine-based ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate,
The fluorine-based ethylene carbonate is composed of one or more selected from the group consisting of trifluoropropylene carbonate, in which fluorine is directly bonded to carbon forming the cyclic skeleton of the cyclic carbonate,
The solvent of the electrolytic solution does not contain ethylene carbonate,
The electrolytic solution includes an aromatic compound,
The negative active material includes a silicon oxide powder containing crystalline Si phase and non-crystalline SiO ₂ phase,
The negative electrode further includes a polyamideimide,
The positive electrode includes the positive electrode active material containing a metal composite oxide of lithium and a transition metal. A non-aqueous electrolyte secondary battery (excluding a positive electrode containing lithium phosphate).   2. The non-aqueous electrolyte secondary battery according to claim 1, wherein a volume ratio of the fluorine-based ethylene carbonate when the solvent of the electrolytic solution is 100 volume% is 1 volume% or more and 40 volume% or less.   The non-aqueous electrolyte secondary battery according to claim 2, wherein a volume ratio of the fluorine-based ethylene carbonate when the solvent of the electrolytic solution is 100% by volume is 15% by volume or more and 30% by volume or less.   2. The volume ratio of the ethyl methyl carbonate when the combined volume of the ethyl methyl carbonate and the dimethyl carbonate in the electrolytic solution is 100% by volume is 14% by volume or more and 86% by volume or less. The nonaqueous electrolyte secondary battery according to any one of to 3.   The volume ratio of the fluorine-based ethylene carbonate is 10% by volume or more and 60% by volume or less when the combined volume of the ethyl methyl carbonate and the dimethyl carbonate in the electrolytic solution is 100% by volume. The nonaqueous electrolyte secondary battery according to any one of 1 to 4.   The nonaqueous electrolyte secondary battery according to any one of claims 1 to 5, further comprising a separator coated with ceramic between the positive electrode and the negative electrode.

Images