Patent Publication Number: US-2023145387-A1

Title: Non-aqueous electrolyte secondary battery

Description:
TECHNICAL FIELD 
     The present disclosure relates to a nonaqueous electrolyte secondary battery. 
     BACKGROUND ART 
     Nonaqueous electrolyte secondary batteries represented by lithium ion secondary batteries have high energy density and high output, and expected to be promising as a power supply of mobile devices such as smart-phones, a power source of a vehicle such as an electric vehicle, and a storage device of natural energy such as sunlight. As the positive electrode active material of a nonaqueous electrolyte secondary battery, for example, a composite oxide containing lithium and a transition metal is used. 
     Patent Literature 1 has proposed forming a cover layer including a phosphorus compound on a surface of a composite oxide including lithium and manganese. The composite oxide is a positive electrode active material for a nonaqueous electrolyte secondary battery. For the above-described phosphorus compound, at least one selected from the group consisting of Li 3 PO 4 , Li 4 P 2 O 7 , and LiPO 3  (hereinafter, referred to as Li 3 PO 4  and the like) is used. The above-described cover layer contains, along with the phosphorus compound, an oxide or a fluoride including at least one element selected from the group consisting of Mg, Al, and Cu. 
     Patent Literature 2 has proposed attaching an organic phosphoric acid compound to particle surfaces of a spinel structured composite oxide including lithium, manganese, and nickel. The composite oxide is a positive electrode active material for a nonaqueous electrolyte secondary battery. The above-described organic phosphoric acid compound is phosphoric acid triester represented by PO(OR) 3  (R is an organic group such as alkyl group, aryl group, and the like). 
     CITATION LIST 
     Patent Literature 
     PLT1: Japanese Laid-Open Patent Publication No. 2011-187193 
     PLT2: WO2016/084966 
     SUMMARY OF INVENTION 
     The Li 3 PO 4  and the like described in Patent Literature 1 may coagulate during coverage by a liquid phase method, and may be distributed in an island form. The island form distribution is due to, for example, differences in the densities of the raw material and the final product during the covering process (heating and drying step) when the final product has a higher density than that of the raw material (by sintering the product densely), and gas generation involved with reaction of the raw material. For example, when using (NH 4 ) 2 HPO 4  and Li 2 CO 3  for raw materials to produce Li 3 PO 4  having a higher density than that of (NH 4 ) 2 HPO 4 , the density difference between (NH 4 ) 2 HPO 4  and Li 3 PO 4  is large, and NH 3  and CO 2  gasses may be generated during reaction, which may easily cause Li 3 PO 4  to coagulate. Furthermore, the organic phosphoric acid compound described in Patent Literature 2 easily seeps out into the nonaqueous electrolyte. 
     Distribution of Li 3 PO 4  and the like in an island form and seeping of the organic phosphoric acid compound into the nonaqueous electrolyte may cause insufficient coverage of the composite oxide, and may cause reduction in cycle characteristics along with decomposition due to contacts between the nonaqueous electrolyte and composite oxide. 
     Also, for a purpose of improvement in negative electrode performance or the like, a lithium salt with an oxalate complex as an anion such as LiBF 2 (C 2 O 4 ) may be added to the nonaqueous electrolyte. However, when the charging voltage is increased to 4.1 V or more, in the positive electrode side with a high potential, the oxalate complex makes contact with the composite oxide, which may easily cause side reactions. The side reaction deteriorates the composite oxide, and impairs cycle characteristics. 
     In view of the above, an aspect of the present disclosure relates to a nonaqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a nonaqueous electrolyte, wherein the positive electrode includes a composite oxide containing lithium and a transition metal, and an additive covering at least a portion of a surface of the composite oxide; the additive includes a cyclic inorganic phosphoric acid compound, the nonaqueous electrolyte includes lithium ion and an anion, and the anion includes an anion of an oxalate complex. 
     Effects of Invention 
     The present disclosure allows for improvement in cycle characteristics of nonaqueous electrolyte secondary batteries. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a partially cutaway oblique perspective view of a nonaqueous electrolyte secondary battery in an embodiment of the present disclosure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     [Nonaqueous Electrolyte Secondary Battery] 
     The positive electrode for a nonaqueous electrolyte secondary battery of an embodiment of the present disclosure includes a positive electrode, a negative electrode, and a nonaqueous electrolyte, wherein the positive electrode includes a composite oxide (positive electrode active material) containing lithium and a transition metal, and an additive covering at least a portion of a surface of the composite oxide; the additive includes a cyclic inorganic phosphoric acid compound (hereinafter, also referred to as compound A), and the nonaqueous electrolyte includes lithium ion and an anion, and the anion includes an anion of an oxalate complex. 
     With the nonaqueous electrolyte including the anion of the oxalate complex, when the negative electrode includes the negative electrode active material, an excellent film (SEI: Solid Electrolyte Interphase) derived from the oxalate complex is formed on the negative electrode active material surface. Also, when lithium metal is deposited on the negative electrode surface at the time of charging, deposition of the lithium metal in a dendritic state is suppressed. Due to the interaction between the anion of the oxalate complex and lithium, the lithium metal tends to be deposited in a fine particulate form uniformly. Therefore, local deposition of lithium metal can be easily suppressed. 
     On the other hand, when the charging voltage is increased to 4.1 V or more, in the positive electrode side with a high potential, the oxalate complex makes contact with the composite oxide, which may cause side reactions, and the side reaction may deteriorate the composite oxide and impair cycle characteristics. 
     To cope with these, in the present disclosure, the composite oxide surface is covered with the additive including the compound A, which suppresses contact between the oxalate complex and the composite oxide, and suppresses composite oxide deterioration and cycle characteristics impairment involved with the deterioration due to the contact. When the additive includes the compound A, the composite oxide surface is covered stably and sufficiently with the additive. The compound A has excellent oxidation resistance, and is stably present in the high potential positive electrode. Suppressing the composite oxide deterioration in the high potential positive electrode allows for easily achieving a battery with a high output. 
     The compound A does not easily coagulate in the process of coverage by a liquid phase method. With the compound A, in the coverage by the liquid phase, a raw material with a small density difference from the final product, and which does not easily generate gas during reaction can be used. Furthermore, the compound A may have a cyclic structure including a plurality of P atoms, and a plurality of oxygen atoms each bonded to the plurality of P atoms may become O −  by anion formation. In this case, the anions of the compound A easily interact with P of other inorganic phosphoric acid compound molecules surrounding them, and easily bond. Thus, when the additive includes the compound A, the composite oxide surface can be covered widely with the additive as a layer. 
     The compound A easily forms an anion and bonds with the transition metal of the composite oxide, and does not easily seep out into the nonaqueous electrolyte. Thus, when the additive includes the compound A, the composite oxide surface can be stably covered with the additive. The compound A has excellent lithium ion conductivity, and the lithium ions transfer smoothly between the composite oxide and the nonaqueous electrolyte through the cover layer including the compound A. 
     In view of its molecular structure, the cyclic inorganic phosphoric acid compound (e.g., cyclic polyphosphoric acid) does not easily become dense, has a small density, and does not easily coagulate, compared with chain inorganic phosphoric acid compounds (e.g., chain polyphosphoric acid). When using a raw material with a smaller density than that of a cyclic inorganic phosphoric acid compound and the chain inorganic phosphoric acid compound, the cyclic inorganic phosphoric acid compound has a smaller density difference with that of the raw material, and does not easily coagulate compared with the chain inorganic phosphoric acid compound. 
     The additive may include at least a compound A, and an inorganic phosphoric acid compound other than the compound A. The inorganic phosphoric acid compound other than the compound A may include Li 3 PO 4 , Li 4 P 2 O 7 , and LiPO 3 , and the like, or a chain polyphosphoric acid such as tetra polyphosphoric acid. A content of phosphorus (P) derived from the compound A relative to a total of the composite oxide and the additive is, for example, 0.01 mass % or more, 0.01 mass % or more and 0.5 mass % or less, or 0.1 mass % or more and 0.5 mass % or less. 
     The additive does not substantially include an organic phosphoric acid compound that easily seeps out into the nonaqueous electrolyte. For example, when using an aqueous solution including H 3 PO 4  and LiOH for a raw material solution, the additive does not include an organic phosphoric acid compound. Even when the organic phosphoric acid compound is included, the amount of phosphorus derived from the organic phosphoric acid compound attached to 100 parts by mass of the composite oxide is, for example, 0.001 parts by mass or less. Thus, insufficient coverage by the additive of the composite oxide by seeping of the organic phosphoric acid compound into the nonaqueous electrolyte can be avoided. The amount of the organic phosphoric acid compound, which is included in the additive in the battery, seeping into the nonaqueous electrolyte can be estimated by determining the organic phosphoric acid compound content in the nonaqueous electrolyte, when the nonaqueous electrolyte does not contain the organic phosphoric acid compound at the time of nonaqueous electrolyte preparation (before injection of nonaqueous electrolyte to battery). The organic phosphoric acid compound content in the nonaqueous electrolyte is determined by gas chromatograph mass spectrometry (GC/MS) and the like. 
     The compound A preferably includes at least one selected from the group consisting of a cyclic polyphosphoric acid and a salt thereof. The salt of the cyclic polyphosphoric acid includes, for example, an alkali metal salt such as lithium salt. The anion of cyclic polyphosphoric acid has a plurality of O −  bonded to P, and easily bonds with the transition metal in the composite oxide. The cyclic polyphosphoric acid may have a composition represented by, for example, a general formula: (HPO 3 ) n . The “n” is, for example, 3 or more and 6 or less. In particular, cyclic polyphosphoric acid preferably includes hexametaphosphoric acid (H 6 P 6 O 18 ) corresponding to the case of n=6. 
     The compound A goes through anion formation, and easily interact and bond with the transition metal in the composite oxide, Li +  and H +  in the nonaqueous electrolyte, and P in the surrounding inorganic phosphoric acid compound, and hereinafter these components are referred to as a “transition metal in composite oxide and the like”. By bonding of the anion of the compound A with the P of the surrounding inorganic phosphoric acid compound, the composite oxide surface is easily covered widely with the additive in a layer form. By bonding of the anion of the compound A with the transition metal in the composite oxide, the composite oxide surface is stably covered with the additive. By easily bonding the anion of compound A with Li +  in the nonaqueous electrolyte, lithium ion transferring between the composite oxide and the nonaqueous electrolyte can be smoothly performed. 
     The anion of hexametaphosphoric acid has a structure represented by a formula (I) below. O −  bonded to P in the formula (I) may bond with the transition metal in the composite oxide and the like. It has many O −  to be bonded with P, and easily forms many bonds with the transition metal in the composite oxide and the like. 
     
       
         
         
             
             
         
       
     
     The component (compound A) of the additive covering the composite oxide surface can be determined, for example, by the method below. 
     The battery is disassembled and the positive electrode is removed. The positive electrode is washed with a nonaqueous solvent; the nonaqueous electrolyte adhering to the positive electrode is removed; and the nonaqueous solvent is removed by drying. The positive electrode mixture layer is taken from the positive electrode, suitably ground, and dispersed in water. The positive electrode mixture dispersion liquid is filtrated to obtain a filtrate as a sample solution. Also, the positive electrode material (composite oxide particle with its surface covered with an additive) may be dispersed in water, and then the positive electrode material dispersion liquid is filtrated to obtain a filtrate as a sample solution. The components included in the above-described sample solution are analyzed by X-ray diffraction (XRD). When the additive covering the composite oxide surface includes the compound A, the compound A is dissolved in the sample solution (water), and the peak attributed to the compound A can be seen in the XRD pattern. The above-described sample solution may be subjected to nuclear magnetic resonance (NMR) spectroscopy analysis. 
     The cover material of the composite oxide surface may also be analyzed by an XRD pattern obtained by XRD and an electron beam diffraction pattern obtained by transmission electron microscope (TEM) on the positive electrode material. 
     In the positive electrode, the phosphorus (P) content (P amount derived from the additive) relative to a total of the composite oxide and the additive may be 0.1 mass % or more and 0.75 mass % or less, or 0.2 mass % or more and 0.55 mass % or less. When the P content relative to a total of the composite oxide and the additive is 0.1 mass % or more, the composite oxide is sufficiently covered with the additive, which easily improve cycle characteristics. When the P content relative to a total of the composite oxide and the additive is 0.75 mass % or less, the composite oxide is sufficiently secured in the positive electrode, which easily allows for a high capacity battery. 
     The P content (mass ratio relative to total of composite oxide and additive) in the positive electrode can be determined by the method below. 
     The battery is disassembled and the positive electrode is removed. The positive electrode is washed with a nonaqueous solvent; the nonaqueous electrolyte adhering to the positive electrode is removed; and the nonaqueous solvent is removed by drying. The positive electrode mixture is taken out from the positive electrode, and a mass W1 of the positive electrode mixture is measured. The positive electrode mixture is made into a solution with a predetermined acid, and subjected to filtration to separate residues of the carbon material (acetylene black) and resin material (polyvinylidene fluoride) to obtain a sample solution. A mass W2 of the dried residue is measured. (W1−W2) is determined as a total mass of the composite oxide and additive. Using the obtained sample solution, a mass W3 of P in the sample solution is determined by inductively coupled plasma (ICP) emission spectrometry. Using the obtained (W1−W2) and W3, W3/(W1−W2)×100 is calculated to be regarded as the above-described P content. 
     Also, after determining a mass WA of the positive electrode material (composite oxide particle surface with covered with an additive), the positive electrode material may be made into a solution with a predetermined acid to obtain a sample solution, a mass WB of P in the sample solution is determined by ICP emission spectroscopy, and WB/WA×100 may be determined as the above-described P content. 
     The positive electrode may include a positive electrode material including composite oxide particles, and an additive including the compound A, which covers the composite oxide particle surface. In this case, the positive electrode may include a positive electrode current collector, a positive electrode mixture layer supported on the positive electrode current collector, and the positive electrode mixture layer may include the above-described positive electrode material. 
     The distribution state of the P in the positive electrode material can be checked by performing element analysis (element mapping) on cross sections of the positive electrode mixture layer or positive electrode material using an electron beam probe micro analyzer (EPMA) or energy dispersive X-ray (EDX) analyzer. 
     The positive electrode material production method includes, for example, a first step, in which a raw material solution is attached to the composite oxide particle surface, and a second step, in which the composite oxide particles with their surface having the raw material solution attached thereto are heated and dried. 
     The composite oxide is synthesized by using coprecipitation or the like, and for example, obtained by mixing a lithium compound with a compound containing a metal Me (transition metal) other than lithium obtained by coprecipitation or the like, and baking the obtained mixture under predetermined conditions. The composite oxide is usually forming secondary particles, in which a plurality of primary particles are agglomerated. The composite oxide particles have an average particle size (D50) of, for example, 3 μm or more and 25 μm or less. The average particle size (D50) of the composite oxide particles means a particle size (volume average particle size) at a volume integrated value of 50% in the volume-based particle size distribution measured by the laser diffraction scattering method. The first step may also serve as a step for washing the synthesized composite oxide particles. This is advantageous in terms of improved productivity. 
     In the first step, for example, composite oxide particles are added to the raw material solution, and the mixture is stirred to disperse the composite oxide particles in the raw material solution. The raw material solution is, for example, an aqueous solution including H 3 PO 4  and LiOH, and is produced by adding a suitable amount of an aqueous LiOH solution to an aqueous H 3 PO 4  solution. When the raw material solution includes an acid component such as H 3 PO 4  and the like, by adding an alkaline component such as LiOH, the acid component is partly neutralized to reduce the effects from the acid component to the composite oxide. In view of easily preparing the raw material solution, the amount of the aqueous LiOH solution to be added is preferably adjusted so that the raw material solution has a pH in a range of less than 8. The amount of the composite oxide particles to be added is, for example, 500 g or more and 2000 g or less per 1 L of the raw material solution. 
     When the raw material composition in the raw material solution (aqueous solution of H 3 PO 4  and LiOH) is represented by Li z H (3-z) PO 4 , z may be 1.0 or more and 1.8 or less, or 1.2 or more and 1.8 or less. In this case, the pH of the raw material solution is easily adjusted to be in a range of about 6 or more and less than 8. The effects from the acid component to the composite oxide can be avoided, and the composite oxide can sufficiently work as the positive electrode active material. The raw material solution can be easily prepared, and the compound A can be efficiently obtained. 
     When the alkaline component (LiOH and the like) used for the synthesis remained in the composite oxide, the raw material solution attached to the composite oxide surface increases the z value to some extent from the effects of the alkaline component. For example, when the raw material solution having a raw material composition with the z value of 1 is attached to the composite oxide with the remained alkaline component, the z value is increased to more than 1. The z value changes based on the mole ratio of LiOH to H 3 PO 4 . For example, when the aqueous LiOH solution is not added, z=0. 
     The second step (heating step) works as a step in which the dispersion medium attached to the composite oxide particle surface is removed by heating and drying, and also as a step in which the raw material attached to the composite oxide particle surface is allowed to react to form the compound A. The heating temperature is, for example, 180° C. or more and 450° C. or less. This allows for efficient drying of the composite oxide particle surface and production of the compound A at the surface. The compound A produced during heating in the second step can be bonded to the metal Me (transition metal and the like) of the composite oxide particle as an anion. 
     In the second step, for example, water in the raw material solution (aqueous solution including H 3 PO 4  and LiOH) attached to the composite oxide particle surface decreases by heating, and Li 3 PO 4  and LiH 2 PO 4  are produced to deposit. Furthermore, Li 3 PO 4  and LiH 2 PO 4  react to produce hexametaphosphoric acid. Water produced at the time of reaction also evaporates by heating. At this time, cyclic polyphosphoric acid other than hexametaphosphoric acid such as tetra metaphosphoric acid, and chain polyphosphoric acid such as tetra polyphosphoric acid may be produced in a small amount. Unreacted components such as Li 3 PO 4  may remain in a small amount. Hexametaphosphoric acid has a smaller density than Li 3 PO 4  and LiH 2 PO 4 , and therefore does not easily coagulate. 
     The positive electrode active material includes a composite oxide containing lithium and a metal Me other than lithium. The metal Me includes at least a transition metal. The transition metal may include at least one element selected from the group consisting of nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), copper (Cu), chromium (Cr), titanium (Ti), niobium (Nb), zirconium (Zr), vanadium (V), tantalum (Ta), and molybdenum (Mo). 
     The metal Me may include a metal other than a transition metal. The metal other than the transition metal may include at least one selected from the group consisting of aluminum (Al), magnesium (Mg), calcium (Ca), strontium (Sr), zinc (Zn), and silicon (Si). In addition to the metal, the composite oxide may further include boron (B) or the like. 
     In view of achieving a high capacity, the transition metal preferably includes at least Ni. The metal Me may include Ni and at least one selected from the group consisting of Co, Mn, Al, Ti, and Fe. In view of increasing the capacity and output, among others, the metal Me preferably contains Ni and at least one selected from the group consisting of Co, Mn, and Al, and more preferably contains Ni, Co, and Mn and/or Al. When the metal Me contains Co, the phase transition of the composite oxide containing Li and Ni is suppressed during charge and discharge, the stability of the crystal structure is improved, and the cycle characteristics are easily improved. When the metal Me contains Mn and/or Al, the thermal stability is improved. 
     In view of easily increasing the capacity, in the composite oxide, the atomic ratio of Ni to the metal Me:Ni/Me is preferably 0.3 or more and less than 1, more preferably 0.5 or more and less than 1, and still more preferably 0.75 or more and less than 1. 
     In view of improving cycle characteristics and obtaining higher output, the positive electrode active material may include a composite oxide having a layered rock salt type crystal structure and containing Ni and/or Co, or a composite oxide having a spinel type crystal structure and containing Mn. In view of a higher capacity, in particular, preferred is a composite oxide having a layered rock salt type crystal structure and containing Ni, and having an atomic ratio of Ni relative to metal Me:Ni/Me of 0.3 or more (hereinafter, the composite oxide is also referred to as a nickel-based composite oxide). 
     The additive including the compound A covering the composite oxide surface is excellent in lithium ion conductivity, and allows the composite oxide to smoothly absorb and release lithium ions. Also, by including the alkaline component in the raw material solution, upon coverage with the additive including the compound A, deterioration of the composite oxide by the acid component in the raw material solution is suppressed. Thus, when covering the nickel-based composite oxide surface with the additive including the compound A, the high capacity of the positive electrode including the nickel-based composite oxide can be sufficiently brought out. 
     The nickel-based composite oxide has a relatively unstable crystal structure, is prone to deterioration due to elution of Ni caused by contacts with the nonaqueous electrolyte (oxalateborate complex) in a high potential positive electrode, and easily reduces cycle characteristics. Thus, in the case of the nickel-based composite oxide, improvement effects of cycle characteristics by the coverage of the composite oxide surface with the additive including the compound A are significant. Also, the Ni-based composite oxide may exhibit alkalinity by the remaining alkaline component used for the synthesis, and deterioration of the composite oxide by the acid component in the raw material solution used for covering the composite oxide surface with the additive is easily suppressed. 
     The composite oxide may have a composition having a layered rock salt type crystal structure, and represented by a general formula (1): LiNi α M 1-α O 2  (0.3≤α&lt;1 is satisfied, and M is at least one element selected from the group consisting of Co, Mn, Al, Ti, and Fe). When a is in the above-described range, the effect of Ni and the effect of element M can be obtained in a well-balanced manner. 
     In view of obtaining improvement in cycle characteristics, a higher capacity and higher output, the composite oxide may have a composition having a layered rock salt type crystal structure and represented by a general formula (2): LiNi x Co y M 1-x-y O 2 . In the general formula (2), 0.3≤x&lt;1, 0&lt;y≤0.5, and 0&lt;1-x-y≤0.35 are satisfied, and M is at least one selected from the group consisting of Al and Mn. In this case, the effect of Ni, the effect of Co, and the effect of element M can be obtained in a well-balanced manner. When covering the surface of the composite oxide represented by the general formula (2) with the additive including the compound A, the high capacity of the composite oxide can be sufficiently brought out. In particular, M is preferably Al in the general formula (2). The value of x may be in the range of 0.5≤x&lt;1. The value of y may be in the range of 0&lt;y≤0.35. 
     In view of improvement in cycle characteristics and a higher output, the composite oxide may have a spinel structured crystal structure, and a composition represented by a general formula (3): LiMn β Ni 2-β O 4  (0.1≤β&lt;2). Also, in the general formula (3), β is 0.5 or more and less than 2. 
     Hereinafter, the configuration of the nonaqueous electrolyte secondary battery will be described more specifically. 
     (Positive Electrode) 
     The positive electrode includes, for example, a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed by applying a positive electrode slurry in which the positive electrode mixture is dispersed in a dispersion medium on a surface of the positive electrode current collector, and drying the slurry. The dried coating film may be rolled, if necessary. The positive electrode mixture layer may be formed on one surface of the positive electrode current collector, or may be formed on both surfaces thereof. The positive electrode mixture includes, as an essential component, the above-described positive electrode material. The positive electrode mixture may include, as an optional component, a binder, conductive agent, and the like. Examples of the dispersion medium include N-methyl-2-pyrrolidone (NMP). 
     Examples of the binder include resin materials such as, for example, fluororesin, polyolefin resin, polyamide resin, polyimide resin, acrylic resin, and vinyl resin. Examples of the fluororesin include polytetrafluoroethylene (PTFE), and polyvinylidene fluoride (PVDF). A kind of binder may be used singly, or two or more kinds thereof may be used in combination. 
     Examples of the conductive agent include carbon blacks such as acetylene black; conductive fibers such as carbon fibers and metal fibers; and carbon fluoride. A kind of conductive agent may be used singly, or two or more kinds thereof may be used in combination. 
     As the positive electrode current collector, for example, a metal foil can be used. As a metal composing the positive electrode current collector, aluminum (Al), titanium (Ti), alloys containing these metal elements, and stainless steel are exemplified. The thickness of the positive electrode current collector is not particularly limited, but is, for example, 3 to 50 μm. 
     (Negative Electrode) 
     The negative electrode includes at least a negative electrode current collector, and may be a type of negative electrode in which lithium metal deposits during charging and lithium metal dissolves into the nonaqueous electrolyte during discharging. For example, the negative electrode may include a negative electrode current collector or a negative electrode mixture layer supported on a negative electrode current collector surface, and the lithium metal may be deposited on the surface of the negative electrode current collector or negative electrode mixture layer. 
     As the negative electrode current collector, for example, a metal foil can be used. As a metal composing the negative electrode current collector, a metal that does not react with lithium metal is preferable, and copper (Cu), nickel (Ni), iron (Fe), and an alloy containing any of these metal elements are exemplified. The thickness of the negative electrode current collector is not particularly limited, and is, for example, 5 μm or more and 300 μm or less. 
     The negative electrode may include a negative electrode current collector and a negative electrode mixture layer supported on the surface of the negative electrode current collector. In view of increasing the capacity, the thickness of the negative electrode mixture layer may be set to be sufficiently thin so that lithium metal can be deposited on the negative electrode during charging. In this case, the design capacity Cn involved with a negative electrode active material in the negative electrode mixture layer relative to the design capacity Cp of the positive electrode satisfies Cn/Cp&lt;1 and may satisfy Cn/Cp&lt;0.8. In these cases, lithium metal is deposited on the surface of the negative electrode mixture layer during charging, and lithium metal deposited on the surface of the negative electrode mixture layer is dissolved in the nonaqueous electrolyte during discharging. 
     The negative electrode mixture layer can be formed, for example, by applying a negative electrode slurry in which the negative electrode mixture is dispersed in a dispersion medium on a surface of the negative electrode current collector and drying the slurry. The dried coating film may be rolled, if necessary. The negative electrode mixture layer may be formed on one surface of the negative electrode current collector, or on both surfaces thereof. As the dispersion medium, for example, water or NMP is used. 
     The negative electrode mixture contains a negative electrode active material as an essential component, and may contain a binder, a conductive agent, a thickener, and the like as an optional component. As the binder and the conductive agent, those exemplified for the positive electrode can be used. Examples of the binder include rubber materials such as styrene-butadiene rubber (SBR). Examples of the thickener include carboxymethylcellulose (CMC) and a modified product thereof (such as Na salts). 
     The negative electrode active material may contain a carbon material which absorbs and releases lithium ions. Examples of the carbon material absorbing and releasing lithium ions include graphite (natural graphite, artificial graphite), non-graphitizable carbon (soft carbon), and graphitizable carbon (hard carbon). Preferred among them is graphite, which is excellent in stability during charging and discharging and has a small irreversible capacity. 
     The negative electrode active material may include an alloy-type material. The alloy-type material is a material containing at least one kind of metal capable of forming an alloy with lithium, and includes, for example, silicon, tin, a silicon alloy, a tin alloy, and a silicon compound. As the alloy-type material, a composite material having a lithium ion conductive phase and silicon particles dispersed in the phase may be used. As the lithium ion conductive phase, a silicate phase, a silicon oxide phase in which 95 mass % or more is silicon dioxide, a carbon phase, and/or may be used. 
     As the negative electrode active material, the alloy-type material and the carbon material can be used in combination. In this case, the mass ratio of the carbon material to the total of the alloy-type material and the carbon material is, for example, preferably 80 mass % or more, and more preferably 90 mass % or more. 
     (Nonaqueous Electrolyte) 
     The nonaqueous electrolyte contains lithium ions and anions, and has lithium ion conductivity. The nonaqueous electrolyte may be in a liquid form. The liquid nonaqueous electrolyte contains, for example, lithium ions, anions, and a nonaqueous solvent. The liquid nonaqueous electrolyte is prepared by dissolving a lithium salt in a nonaqueous solvent. By dissolving the lithium salt in a nonaqueous solvent, lithium ions and anions are generated. 
     The nonaqueous electrolyte may be in a gel form. The gel nonaqueous electrolyte contains, for example, lithium ions, anions, and a matrix polymer, and may further contain a nonaqueous solvent. As the matrix polymer, for example, a polymer material which absorbs and gels the nonaqueous solvent is used. Examples of the polymer material include fluororesin, acrylic resin, and polyether resin. 
     The anion includes at least an anion of an oxalate complex. The anion of the oxalate complex may contain boron and/or phosphorus. Specific examples of the anion of the oxalate complex include B(C 2 O 4 ) 2   − , difluoro (oxalate)borate anion: BF 2 (C 2 O 4 ) − , PF 4 (C 2 O 4 ) − , and PF 2 (C 2 O 4 ) 2   − . The anion of the oxalate complex may be used singly, or two or more kinds thereof may be used in combination. 
     The anion of the oxalate complex and another anion may be combined. Examples of another anion include BF 4   − , ClO 4   − , PF 6   − , CF 3 SO 3   − , CF 3 CO 2   − , and anions of imides. Examples of the anions of the imides include N(SO 2 CF 3 ) 2   − , N(C m F 2m+1 SO 2 ) x  (C n F 2n+1 SO 2 )y −  (m and n are each independently 0 or an integer of 1 or more, and x and y are each independently 0, 1 or 2, and satisfy x+y=2). Another anion may be PF 6   −  and/or the anion of the imide. Another anion may be used singly, or two or more kinds thereof may be used in combination. 
     The concentration of the anion in the nonaqueous electrolyte may be 0.5 mol/L or more and 3.5 mol/L or less. Further, the concentration of the anion of the oxalate complex in the nonaqueous electrolyte may be 0.05 mol/L or more and 1 mol/L or less. 
     Examples of the nonaqueous solvent include ester, ether, nitrile, amide, or halogen-substituted products thereof. A kind of nonaqueous solvent may be used singly, or two or more kinds thereof may be used in combination. Examples of the halogen-substituted product include fluoride. 
     Examples of the ester include cyclic carbonates, chain carbonates, cyclic carboxylates, and chain carboxylates. Examples of the cyclic carbonate include ethylene carbonate (EC) and propylene carbonate (PC). Examples of the chain carbonate include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC). Examples of the cyclic carboxylate include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of the chain carboxylate include ethyl acetate, propyl acetate, and methyl propionate (PM). 
     Ether includes cyclic ether and chain ether. Examples of the cyclic ether include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, and 2-methyltetrahydrofuran. Examples of the chain ether include 1,2-dimethoxyethane, diethyl ether, ethyl vinyl ether, and 1,2-diethoxyethane. 
     The cyclic carbonate may include fluorinated cyclic carbonate such as fluoroethylene carbonate (FEC), and cyclic carbonate having a carbon-carbon unsaturated bond such as vinylene carbonate (VC), and vinyl ethylene carbonate. In this case, an excellent film is formed on the negative electrode active material surface. Also, a good film is formed on the surface of the negative electrode (negative electrode current collector or negative electrode mixture layer), and the generation of dendrite of lithium metal is suppressed. 
     (Separator) 
     Usually, it is desirable to interpose a separator between the positive electrode and the negative electrode. The separator has excellent ion permeability and suitable mechanical strength and electrically insulating properties. As the separator, there may be used, a microporous thin film, a woven fabric, or a nonwoven fabric, and the like. The separator is preferably made of, for example, polyolefin such as polypropylene and polyethylene. 
     In an example of a structure of the nonaqueous electrolyte secondary battery, an electrode group and a nonaqueous electrolyte are accommodated in an outer package, and the electrode group has a positive electrode and a negative electrode wound with a separator interposed therebetween. Alternatively, instead of the wound-type electrode group, other forms of electrode groups may be applied, such as a laminated electrode group in which the positive electrode and the negative electrode are laminated with a separator interposed therebetween. The nonaqueous electrolyte secondary battery may be any shape, for example, a cylindrical type, a rectangular type, a coin-type, a button type, or a laminate type. 
       FIG.  1    is a partially cutaway oblique perspective view of a nonaqueous electrolyte secondary battery in an embodiment of the present disclosure. 
     The battery includes a bottomed rectangular battery case  4 , and an electrode group  1  and a nonaqueous electrolyte (not shown) accommodated in the battery case  4 . The electrode group  1  has a negative electrode in the form of a long strip, a positive electrode in the form of a long strip, and a separator interposed therebetween for preventing direct contact therebetween. The electrode group  1  is formed by winding the negative electrode, the positive electrode, and the separator around a flat core and removing the core. 
     One end of a negative electrode lead  3  is attached to a negative electrode current collector of the negative electrode by welding or the like. The other end portion of the negative electrode lead  3  is electrically connected to a negative electrode terminal  6  provided in a sealing plate  5  with a resin insulating plate (not shown). The negative electrode terminal  6  is insulated from the sealing plate  5  by a gasket  7  made of a resin. To a positive electrode current collector of the positive electrode, one end of a positive lead  2  is attached by welding or the like. The other end of the positive lead  2  is connected to the rear surface of the sealing plate  5  through an insulating plate. That is, the positive electrode lead  2  is electrically connected to the battery case  4  which also serves as the positive electrode terminal. The insulating plate separates the electrode group  1  and the sealing plate  5  and separates the negative electrode lead  3  and the battery case  4 . The periphery of the sealing plate  5  is fitted to the open end of the battery case  4 , and the fitting portion is laser welded. In this manner, the opening of the battery case  4  is sealed by the sealing plate  5 . The injection hole for the nonaqueous electrolyte provided in the sealing plate  5  is plugged by a sealing plug  8 . 
     In the following, the present disclosure will be described in detail based on Examples and Comparative Examples, but the present invention is not limited to Examples below. 
     Example 1 
     [Positive Electrode Material Production] 
     Surfaces of composite oxide particle (average particle size (D50) 11.1 μm) were covered with an additive including the compound A by a liquid phase method based on the following processes: the composite oxide was a layered rock salt type, and had a composition of LiNi 0.9 Co 0.05 Al 0.05  (NCA). 
     First, an aqueous LiOH solution (concentration 1 mol/L) was added to an aqueous H 3 PO 4  solution (concentration 1 mol/L) to produce a raw material solution (aqueous solution including H 3 PO 4  and LiOH). The amount of the aqueous LiOH solution to be added to the aqueous H 3 PO 4  solution was adjusted so that the value of z was as shown in Table 1, when the raw material composition in the raw material solution is represented by Li z H (3-z) PO 4 . The aqueous LiOH solution was added in an amount so that the raw material solution had a pH of less than 8. 
     The composite oxide particles were added to the raw material solution. The composite oxide particles were added in an amount of 1250 g per 1 L of the raw material solution. The raw material solution including the composite oxide particles was stirred for 15 minutes to disperse the composite oxide particles in the raw material solution. Afterwards, the composite oxide particles in the dispersion liquid were separated by filtration, and the composite oxide particles having the raw material solution attached to their surfaces were heated at 450° C. for 3 hours to dry. A positive electrode material with the composite oxide particle surface covered with the additive was produced in this manner. The positive electrode material had a P content of 0.21 mass % as determined by the above described method. 
     The components of the additive covering the composite oxide particle surface was checked by the method described, and it was confirmed that it contained hexametaphosphoric acid (Li 6 P 6 O 18 ) as the compound A, lithium orthophosphate (Li 3 PO 4 ), and lithium pyrophosphate (Li 4 P 2 O 7 ). 
     [Positive Electrode Production] 
     To the positive electrode mixture, N-methyl-2-pyrrolidone (NMP) was added, and stirred to prepare a positive electrode slurry. As the positive electrode mixture, a mixture of the positive electrode material obtained as described above, acetylene black (AB), and polyvinylidene fluoride (PVDF) was used. In the positive electrode mixture, the mass ratio of the positive electrode material, AB, and PVDF was set to 100:2:2. 
     The positive electrode slurry was applied to the surface of aluminum foil as a positive electrode current collector, and the coating film was dried, and then rolled to form a positive electrode in which a positive electrode mixture layer (thickness 40 μm, density 3.6 g/cm 3 ) is formed on one surface of the aluminum foil. 
     [Nonaqueous Electrolyte Preparation] 
     A lithium salt was dissolved in a nonaqueous solvent to prepare a nonaqueous electrolyte. A solvent mixture including fluoroethylene carbonate (FEC) and dimethyl carbonate (DMC) at a volume ratio of 2:8 was used for the nonaqueous solvent. LiPF 6  and LiBF 2 (C 2 O 4 ) (LiFOB) were used for the lithium salt. The nonaqueous electrolyte had a LiPF 6  concentration of 1 mol/L. The nonaqueous electrolyte had a LiFOB concentration of 0.5 mol/L. 
     [Production of Cell for Evaluation] 
     A cell for positive electrode evaluation was made with the above-described positive electrode to which an Al made lead was attached, a counter electrode (Li electrode) to which a Ni made lead was attached, and the above-described nonaqueous electrolyte. Specifically, for the counter electrode, an electrolytic copper foil with lithium metal foil attached to its one surface was used. The positive electrode and the counter electrode were stacked with a polyethylene-made separator interposed therebetween so that the positive electrode mixture layer and the lithium metal foil faced each other, thereby producing an electrode group. The electrode was accommodated in a bag-type outer package formed with an Al laminate film, the nonaqueous electrolyte was injected to immerse the positive electrode mixture layer with the nonaqueous electrolyte, and then the opening of the outer package was sealed by heating. A portion of the Al made lead and a portion of the Ni made lead were exposed from the outer package to the outer side. The evaluation cell was prepared in a dry air atmosphere having a dew point of −60° C. or less. The evaluation cell was fixed under a pressure of 3.2 MPa. 
     Comparative Example 1 
     An evaluation cell B1 of Comparative Example 1 was produced in the same manner as in Example 1, except that in the positive electrode production, the composite oxide particle surface was not covered with the additive including the compound A. 
     Comparative Example 2 
     An evaluation cell B2 of Comparative Example 2 was produced in the same manner as in Example 1, except that in the nonaqueous electrolyte preparation, only LiPF 6  was used for the lithium salt, and the LiPF 6  concentration in the nonaqueous electrolyte was set to 1 mol/L. 
     Comparative Example 3 
     In the positive electrode production, the composite oxide particle surface was not covered with the additive including the compound A. In the nonaqueous electrolyte preparation, only LiPF 6  was used for the lithium salt, and the LiPF 6  concentration in the nonaqueous electrolyte was set to 1 mol/L. Except for the above, an evaluation cell B3 of Comparative Example 3 was produced in the same manner as in Example 1. 
     The evaluation cells Al and B1 to B3 produced as described above were evaluated as below. 
     [Evaluation: Charge/Discharge Cycle Test] 
     Constant current charging was performed at a current of 0.2 C until the cell voltage reached 4.3 V, and thereafter, constant voltage charging was performed at a cell voltage of 4.3 V until the electric current reached 0.05 C. Afterwards, constant current discharging was performed at a current of 0.2 C until the cell voltage reached 2.5 V. The batteries were allowed to rest for 10 minutes between the charging and discharging. The charge/discharge were conducted under an environment of 25° C. 
     With the above-described charge/discharge as 1 cycle, 40 cycles were performed. The ratio of the discharge capacity at the 40th cycle relative to the discharge capacity at the 1st cycle was determined as the capacity retention rate. 
     The evaluation results are shown in Table 1. The capacity retention rate improvement percentage of the cell A1 in Table 1 is an increase percentage of the capacity retention rate of the cell Al relative to the cell B1, and is a value obtained by (a1−b1)/b1×100, setting the capacity retention rates of the battery Al and the battery B1 as al and b1, respectively. The capacity retention rate improvement percentage of the cell B2 is the increase percentage of the capacity retention rate of the cell B2 relative to the cell B3, and is a value obtained by (b2−b3)/b3×100, setting the capacity retention rates of the cell B2 and the cell B3 as b2 and b3, respectively. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                   
                   
                   
                 Improvement 
               
               
                   
                 Coverage of 
                   
                   
                 rate of 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Coverage with Additive 
                 composite oxide 
                   
                 Capacity 
                 Capacity 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Raw 
                   
                   
                 particle surface 
                 LiFOB 
                 retention rate 
                 retention rate 
               
               
                   
                   
                 material 
                 Heating 
                 P 
                 with additive 
                 content in 
                 at 40th 
                 by coverage 
               
               
                 Cell for 
                 Composite 
                 composition 
                 temperature 
                 Content 
                 including 
                 nonaqueous 
                 Cycle 
                 with additive 
               
               
                 Evaluation 
                 oxide 
                 Li z H (3−z) PO 4   
                 (° C.) 
                 (mass %) 
                 compound A 
                 electrolyte 
                 (%) 
                 (%) 
               
               
                   
               
               
                 A1 
                 NCA 
                 z = 1.6 
                 450 
                 0.25 
                 Yes 
                 Yes 
                 93.1 
                 1.2 
               
               
                 B1 
                 NCA 
                 — 
                 — 
                 — 
                 No 
                 Yes 
                 92.0 
                 — 
               
               
                 B2 
                 NCA 
                 z = 1.6 
                 450 
                 0.25 
                 Yes 
                 No 
                 96.2 
                 0.1 
               
               
                 B3 
                 NCA 
                 — 
                 — 
                 — 
                 No 
                 No 
                 96.1 
                 — 
               
               
                   
               
            
           
         
       
     
     The improvement percentage of the capacity retention rate of the cell Al relative to the cell B1 was 1.2%, and significantly improved more than the improvement percentage of the capacity retention rate of the cell B2 relative to the cell B3 of 0.1%. When including LiFOB in the nonaqueous electrolyte, by covering the composite oxide surface with the additive including the compound A, cycle characteristics significantly improved. 
     The discharge capacity at 40th cycle was determined for the cell A1, in which the nonaqueous electrolyte including LiFOB was used, and for the cells B2 and B3, in which the nonaqueous electrolyte not containing LiFOB was used, and the above-described charging was performed. Deposition of the lithium metal at the counter electrode (Li electrode) surface after charging was checked using a scanning electron microscope (SEM). As a result, it was confirmed that in the counter electrode of the cell Al, dendritic lithium metal deposition was suppressed more compared with the counter electrode of the cells B2 and B3. 
     INDUSTRIAL APPLICABILITY 
     The nonaqueous electrolyte secondary battery according to the present disclosure is preferably used as, for example, a power supply of a mobile device such as a smart phone, a power source of a vehicle such as an electric vehicle, or a storage device of natural energy such as sunlight. 
     REFERENCE SIGNS LIST 
     
         
           1  Electrode Group 
           2  Positive Electrode Lead 
           3  Negative Electrode Lead 
           4  Battery Case 
           5  Sealing Plate 
           6  Negative Electrode Terminal 
           7  Gasket 
           8  Sealing Plug