Non-aqueous electrolyte secondary battery

Abstract

This non-aqueous electrolyte secondary battery comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode comprises: a composite oxide containing lithium and a transition metal; and an additive that covers at least a portion of the surface of the composite oxide. The additive contains an annular inorganic phosphate compound. The non-aqueous electrolyte contains lithium ions and anions, and the anions contain oxalate complex anions.

Description

Technical Field

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery.

Background Art

[0002] Non-aqueous electrolyte secondary batteries typified by lithium-ion secondary batteries have high energy density and high output, and are expected to be used as power sources for mobile devices such as smartphones, power sources for vehicles such as electric vehicles, and storage devices for natural energy such as sunlight. As the positive electrode active material of the non-aqueous electrolyte secondary battery, a composite oxide containing lithium and a transition metal is used.

[0003] In Patent Document 1, it has been proposed to form a coating layer containing a phosphorus compound on the surface of a composite oxide containing lithium and manganese, which is the positive electrode active material of a non-aqueous electrolyte secondary battery. The above phosphorus compounds include Li 3 PO 4 、Li 4 P 2 O 7 and LiPO 3 At least one selected from the group consisting of (hereinafter referred to as Li 3 PO 4 etc.).) is used. The above coating layer also contains an oxide or fluoride containing at least one element selected from the group consisting of Mg, Al, and Cu together with the phosphorus compound.

[0004] Further, in Patent Document 2, it has been proposed to attach an organic phosphoric acid compound to the particle surface of a spinel structure composite oxide containing lithium, manganese, and nickel, which is the positive electrode active material of a non-aqueous electrolyte secondary battery. The above organic phosphoric acid compound is a triester phosphate represented by PO(OR) 3 (R is an organic group such as an alkyl group or an aryl group.).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

[0006] Li described in Patent Document 1 3 PO 4 etc. may aggregate during coating by the liquid phase method and may be distributed in an island shape. This is due to the density difference (shrinkage) between the raw material and the final product when the density of the final product is greater than that of the raw material during the coating process (heating and drying process), and the influence of gas generation accompanying the reaction of the raw material. For example, when using (NH 4 ) 2 HPO 4 and Li 2 CO 3 to produce Li 4 ) 2 HPO 4 with a density greater than that of Li 3 PO 4 the density difference between (NH 4 ) 2 HPO 4 and Li 3 PO 4 is large, and gases such as NH 3 and CO 2 can be generated during the reaction, so Li 3 PO 4 tends to aggregate. Also, the organic phosphate compound described in Patent Document 2 tends to flow out into the non-aqueous electrolyte.

[0007] Li 3 PO 4 etc. being distributed in an island shape or the organic phosphate compound flowing out into the non-aqueous electrolyte may result in insufficient coating of the composite oxide, and the cycle characteristics may deteriorate due to decomposition caused by contact between the non-aqueous electrolyte and the composite oxide.

[0008] Also, for the purpose of improving the negative electrode performance, etc., LiBF 2 (C 2 O 4A lithium salt having an oxalate complex or the like as an anion may be included. However, when the charging voltage rises to 4.1 V or higher, the oxalate complex comes into contact with the composite oxide on the high-potential positive electrode side, and side reactions are likely to occur. Along with the side reactions, the composite oxide deteriorates and the cycle characteristics deteriorate.

[0009] In view of the above, one aspect of the present disclosure relates to a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, the positive electrode including a composite oxide containing lithium and a transition metal, and an additive covering at least a part of the surface of the composite oxide, the additive including a cyclic inorganic phosphate compound, the non-aqueous electrolyte including lithium ions and an anion, and the anion including an anion of an oxalate complex.

Advantages of the Invention

[0010] According to the present disclosure, the cycle characteristics of the non-aqueous electrolyte secondary battery can be improved.

Brief Description of the Drawings

[0011]

Figure 1

Embodiments for Carrying Out the Invention

[0012] [Non-aqueous Electrolyte Secondary Battery] The positive electrode for a non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode includes a composite oxide (positive electrode active material) containing lithium and a transition metal, and an additive covering at least a part of the surface of the composite oxide. The additive includes a cyclic inorganic phosphate compound (hereinafter also referred to as Compound A). The non-aqueous electrolyte includes lithium ions and an anion, and the anion includes an anion of an oxalate complex.

[0013] By including the anion of the oxalate complex in the non-aqueous electrolyte, when the negative electrode contains a negative electrode active material, a high-quality film (SEI: Solid Electrolyte Interphase) derived from the oxalate complex is formed on the surface of the negative electrode active material. Further, when lithium metal is deposited on the surface of the negative electrode during charging, the deposition of lithium metal in a dendrite shape is suppressed. Due to the interaction between the anion of the oxalate complex and lithium, lithium metal is likely to deposit uniformly in fine particle form. Therefore, it becomes easier to suppress the local deposition of lithium metal.

[0014] On the other hand, when the charging voltage rises to 4.1 V or more, a side reaction occurs due to the contact between the oxalate complex and the composite oxide on the high-potential positive electrode side, and accordingly, the composite oxide deteriorates and the cycle characteristics may deteriorate.

[0015] In contrast, in the present disclosure, by coating the surface of the composite oxide with an additive containing compound A, the contact between the oxalate complex and the composite oxide is suppressed, and the deterioration of the composite oxide caused by the contact and the resulting decrease in cycle characteristics are suppressed. When the additive contains compound A, the surface of the composite oxide is sufficiently and stably covered with the additive. Compound A has excellent oxidation resistance and exists stably in the positive electrode at a high potential. By suppressing the deterioration of the composite oxide in the positive electrode at a high potential, it becomes easier to increase the output of the battery.

[0016] Compound A is less likely to aggregate during the coating process by the liquid phase method. In the case of compound A, in the coating by the liquid phase method, the density difference from the final product is small, and a raw material that hardly generates gas during the reaction can be used. Further, compound A may have a ring structure containing a plurality of P atoms, and a plurality of oxygen atoms each bonding to the plurality of P atoms by anionization can be O - and can become. In this case, the anion of compound A is likely to interact with and bond to the P of other surrounding inorganic phosphate compounds. Therefore, when the additive contains compound A, the surface of the composite oxide can be widely and layerwise covered with the additive.

[0017] Compound A can be anionized and easily bind to the transition metal of the composite oxide, and it is difficult to flow out into the non-aqueous electrolyte. Therefore, when the additive contains Compound A, the surface of the composite oxide can be stably covered with the additive. Compound A has excellent lithium ion conductivity, and the transfer of lithium ions between the composite oxide and the non-aqueous electrolyte through the coating layer containing Compound A occurs smoothly.

[0018] Cyclic inorganic phosphate compounds (e.g., cyclic polyphosphoric acid) are less likely to be dense from the perspective of molecular structure, have a lower density, and are less likely to aggregate compared to chain-like inorganic phosphate compounds (e.g., chain-like polyphosphoric acid). When using a raw material with a lower density than cyclic and chain-like inorganic phosphate compounds, the density difference between the cyclic inorganic phosphate compound and the raw material is smaller than that of the chain-like inorganic phosphate compound, and it is less likely to aggregate.

[0019] The additive contains at least Compound A and may contain inorganic phosphate compounds other than Compound A. The inorganic phosphate compounds other than Compound A may include Li 3 PO 4 、Li 4 P 2 O 7 and LiPO 3 etc., and may include chain-like polyphosphoric acids such as tetrapolyphosphoric acid. The content of phosphorus (P) derived from Compound A is, for example, 0.01 mass% or more with respect to the total of the composite oxide and the additive, and may be 0.01 mass% or more and 0.5 mass% or less, or may be 0.1 mass% or more and 0.5 mass% or less.

[0020] The additive substantially does not contain organic phosphate compounds that are likely to flow out into the non-aqueous electrolyte. For example, in the raw material solution, H 3 PO 4When an aqueous solution containing LiOH and LiOH is used, the additive does not contain an organic phosphoric acid compound. Even when an organic phosphoric acid compound is contained, 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. Therefore, it is possible to prevent the organic phosphoric acid compound from leaking into the non-aqueous electrolyte from becoming insufficient in covering the composite oxide with the additive. The amount of the organic phosphoric acid compound contained in the additive leaking into the non-aqueous electrolyte in the battery can be estimated by determining the content of the organic phosphoric acid compound in the non-aqueous electrolyte when the non-aqueous electrolyte does not contain an organic phosphoric acid compound during preparation of the non-aqueous electrolyte (before the non-aqueous electrolyte is injected into the battery). The content of the organic phosphoric acid compound in the non-aqueous electrolyte can be determined by gas chromatography mass spectrometry (GC / MS) or the like.

[0021] Compound A preferably contains at least one selected from the group consisting of cyclic polyphosphoric acid and salts thereof. The salt of cyclic polyphosphoric acid includes, for example, an alkali metal salt such as a lithium salt. The anion of cyclic polyphosphoric acid is O bonded to P. - Cyclic polyphosphoric acid has a plurality of groups, and easily bonds with transition metals in the composite oxide. 3 ) n The compound may have a composition represented by the formula: n is, for example, 3 or more and 6 or less. Among them, the cyclic polyphosphoric acid may be hexametaphosphoric acid (H 6 P 6 O 18 ) is preferably included.

[0022] Compound A becomes anionized and reacts with the transition metal in the composite oxide and Li in the nonaqueous electrolyte. + Or H + The anion of compound A easily interacts with and bonds to P in the surrounding inorganic phosphate compounds (hereinafter referred to as transition metals in the composite oxide, etc.). When the anion of compound A bonds with P in the surrounding inorganic phosphate compounds, the surface of the composite oxide is easily covered in a layer of additives. When the anion of compound A bonds with the transition metals in the composite oxide, the surface of the composite oxide is stably covered with additives. When the anion of compound A bonds with Li in the non-aqueous electrolyte, the surface of the composite oxide is stably covered with additives. +It is easy to combine with, and the transfer of lithium ions between the composite oxide and the non-aqueous electrolyte occurs smoothly.

[0023] The anion of hexametaphosphoric acid has a structure represented by the following formula (I). The O bonded to P in formula (I) - can bond with transition metals and the like in the composite oxide. The O bonded to P - has a large number and is likely to form a large number of bonds with transition metals and the like in the composite oxide.

[0024]

Chemical formula

[0025] The component (compound A) of the additive coating the surface of the composite oxide can be determined, for example, by the following method.

[0026] Disassemble the battery and take out the positive electrode. Wash the positive electrode with a non-aqueous solvent, remove the non-aqueous electrolyte adhering to the positive electrode, and remove the non-aqueous solvent by drying. Collect the positive electrode mixture layer from the positive electrode, appropriately pulverize it, and disperse it in water. Filter the dispersion of the positive electrode mixture to obtain the filtrate as a sample solution. Also, after dispersing the positive electrode material (composite oxide particles whose surface is coated with an additive) in water, the dispersion of the positive electrode material can be filtered to obtain the filtrate as a sample solution. Analyze the components contained in the sample solution obtained above by X-ray diffraction (XRD) method. When the additive coating the surface of the composite oxide contains compound A, compound A is dissolved in the sample solution (water), and peaks based on compound A are observed in the XRD pattern. The above sample solution may be analyzed by nuclear magnetic resonance (NMR) spectroscopy.

[0027] Also, based on the XRD pattern obtained by the XRD method of the positive electrode material or the electron diffraction pattern obtained by transmission electron microscopy (TEM), the analysis of the coating material on the surface of the composite oxide may be performed.

[0028] The content of phosphorus (P) in the positive electrode (the amount of P derived from the additive) may be 0.1% by mass or more and 0.75% by mass or less, or may be 0.2% by mass or more and 0.55% by mass or less, based on the total of the composite oxide and the additive. When the content of P is 0.1% by mass or more based on the total of the composite oxide and the additive, the composite oxide is sufficiently coated with the additive, and the cycle characteristics are likely to be improved. When the content of P is 0.75% by mass or less based on the total of the composite oxide and the additive, the composite oxide is sufficiently ensured in the positive electrode, and the battery is likely to have a high capacity.

[0029] The content of P in the positive electrode (mass ratio to the total of the composite oxide and the additive) can be determined by the following method.

[0030] Disassemble the battery and take out the positive electrode. Wash the positive electrode with a non-aqueous solvent to remove the non-aqueous electrolyte adhering to the positive electrode, and remove the non-aqueous solvent by drying. Collect the positive electrode mixture from the positive electrode and measure the mass W1 of the positive electrode mixture. Dissolve the positive electrode mixture in a predetermined acid, filter off the residues of the carbon material (acetylene black) and the resin material (polyvinylidene fluoride) by filtration to obtain a sample solution. Measure the mass W2 of the dried residue. Determine (W1 - W2) as the total mass of the composite oxide and the additive. Using the obtained sample solution, determine the mass W3 of P in the sample solution by inductively coupled plasma (ICP) optical emission spectrometry. Calculate W3 / (W1 - W2)×100 using the obtained (W1 - W2) and W3, and use it as the above-mentioned content of P.

[0031] Alternatively, after measuring the mass WA of the positive electrode material (composite oxide particles whose surface is coated with an additive), dissolve the positive electrode material in a predetermined acid to obtain a sample solution, determine the mass WB of P in the sample solution by ICP optical emission spectrometry, and determine WB / WA×100 as the above-mentioned content of P.

[0032] The positive electrode may include a positive electrode material including composite oxide particles and an additive that covers the surface of the composite oxide particles and contains compound A. In this case, the positive electrode includes a positive electrode current collector and a positive electrode mixture layer supported on the positive electrode current collector, and the positive electrode mixture layer may include the above-mentioned positive electrode material.

[0033] The state of the distribution of P in the positive electrode material can be confirmed by performing elemental analysis (element mapping) on the cross-section of the positive electrode binder layer or the positive electrode material using an electron probe microanalyzer (EPMA) or an energy-dispersive X-ray (EDX) analyzer.

[0034] The method for producing the positive electrode material includes, for example, a first step of attaching a raw material solution to the surface of composite oxide particles, and a second step of heating and drying the composite oxide particles with the raw material solution attached to the surface.

[0035] The composite oxide is synthesized using a coprecipitation method or the like. For example, a lithium compound and a compound containing a metal Me (transition metal) other than lithium obtained by a coprecipitation method or the like are mixed, and the resulting mixture is fired under predetermined conditions. The composite oxide usually forms secondary particles in which a plurality of primary particles are aggregated. The average particle size (D50) of the composite oxide particles is, for example, 3 μm or more and 25 μm or less. The average particle size (D50) of the composite oxide particles means the particle size (volume average particle size) at which the volume integration value becomes 50% in the volume-based particle size distribution measured by the laser diffraction scattering method. The first step may also serve as a step of washing the synthesized composite oxide particles. In this case, it is advantageous in terms of improving productivity.

[0036] In the first step, for example, composite oxide particles are added to the raw material solution and stirred to disperse the composite oxide particles in the raw material solution. The raw material solution is, for example, an aqueous solution containing H 3 PO 4 and LiOH, and is obtained by adding an appropriate amount of an aqueous solution of LiOH to an aqueous solution of H 3 PO 4 When the raw material solution is an aqueous solution of H 3 PO 4When containing an acid component such as etc., by adding an alkali component such as LiOH, a part of the acid component is neutralized, and the influence of the acid component on the composite oxide can be reduced. From the viewpoint of facilitating the preparation of the raw material solution, the amount of the aqueous LiOH solution added is preferably adjusted within a range where the pH of the raw material solution is less than 8. The input amount of the composite oxide particles is, for example, 500 g or more and 2000 g or less per 1 L of the raw material solution.

[0037] Raw material solution (H 3 PO 4 and the aqueous solution of LiOH), when the raw material composition in it is represented by Li z H (3-z) PO 4 z may be 1.0 or more and 1.8 or less, and may also be 1.2 or more and 1.8 or less. In this case, it is easy to adjust the pH of the raw material solution within the range of about 6 or more and less than 8. The influence of the acid component on the composite oxide is avoided, and the composite oxide can sufficiently play the role as a positive electrode active material. The raw material solution is easily prepared, and Compound A can be efficiently obtained.

[0038] In addition, when an alkali component (such as LiOH) used in its synthesis remains in the composite oxide, when the raw material solution adheres to the surface of the composite oxide particles, the z value shifts in a direction of slightly increasing due to the influence of the alkali component. For example, when a raw material solution with a z value of 1 in the raw material composition adheres to the composite oxide with the alkali component remaining, the z value becomes larger than 1. The z value changes according to the molar ratio of LiOH to H 3 PO 4 . For example, when no aqueous LiOH solution is added, z = 0.

[0039] The second step (heating step) serves both as a step of removing the dispersion medium adhering to the surface of the composite oxide particles by heat drying and a step of reacting the raw materials adhering to the surface of the composite oxide particles to produce Compound A. The heating temperature is, for example, 180 °C or more and 450 °C or less. In this case, the drying of the surface of the composite oxide particles and the production of Compound A on the surface are efficiently carried out. During the heating in the second step, the produced Compound A can bind to the metal Me (transition metal, etc.) of the composite oxide particles as an anion.

[0040] In the second step, for example, by heating, the water in the raw material solution (aqueous solution containing H 3 PO 4 and LiOH) adhering to the surface of the composite oxide particles decreases, and Li 3 PO 4 and LiH 2 PO 4 are generated and precipitate. Further, Li 3 PO 4 and LiH 2 PO 4 react to form hexametaphosphoric acid. The water generated during the reaction also evaporates by heating. At this time, cyclic polyphosphoric acids other than hexametaphosphoric acid such as tetrametaphosphoric acid and chain polyphosphoric acids such as tetrapolyphosphoric acid may also be generated in small amounts. Also, unreacted components such as Li 3 PO 4 may remain in small amounts. Since hexametaphosphoric acid has a lower density than Li 3 PO 4 and LiH 2 PO 4 , it is less likely to aggregate.

[0041] The positive electrode active material contains a composite oxide containing lithium and a metal Me other than lithium. The metal Me contains at least a transition metal. The transition metal may contain 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).

[0042] The metal Me may contain a metal other than a transition metal. The metal other than the transition metal may contain at least one selected from the group consisting of aluminum (Al), magnesium (Mg), calcium (Ca), strontium (Sr), zinc (Zn), and silicon (Si). Further, the composite oxide may further contain boron (B) or the like in addition to the metal.

[0043] From the perspective of high capacity, the transition metal preferably contains at least Ni. The metal Me may contain Ni and at least one selected from the group consisting of Co, Mn, Al, Ti, and Fe. From the perspectives of high capacity and high 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, during charge and discharge, the phase transition of the composite oxide containing Li and Ni is suppressed, the stability of the crystal structure is improved, and the cycle characteristics are likely to be improved. When the metal Me contains Mn and / or Al, the thermal stability is improved.

[0044] From the perspective of being easily capable of achieving high 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.

[0045] From the perspectives of improving cycle characteristics and high output, the positive electrode active material may contain a composite oxide containing Ni and / or Co having a layered rock salt-type crystal structure, or may contain a composite oxide containing Mn having a spinel-type crystal structure. Among others, from the perspective of high capacity, a composite oxide having a layered rock salt-type crystal structure, containing Ni, and having an atomic ratio of Ni to the metal Me: Ni / Me of 0.3 or more (hereinafter also referred to as a nickel-based composite oxide) is preferred.

[0046] The additive containing compound A that covers the surface of the composite oxide is excellent in lithium ion conductivity, and the occlusion and release of lithium ions by the composite oxide are smoothly carried out. Also, in the coating of the additive containing compound A, by including an alkaline component in the raw material solution, the deterioration of the composite oxide due to the acid component in the raw material solution is suppressed. Therefore, when the surface of the nickel-based composite oxide is coated with the additive containing compound A, the high capacity of the positive electrode containing the nickel-based composite oxide can be fully utilized.

[0047] Nickel-based composite oxides have a relatively unstable crystal structure and are prone to deterioration due to elution of Ni and the like caused by contact with a non-aqueous electrolyte (oxalate borate complex) at a positive electrode with a high potential, and the cycle characteristics are likely to deteriorate. Therefore, in the case of nickel-based composite oxides, a remarkable effect of improving the cycle characteristics can be obtained by coating the surface of the composite oxide with an additive containing compound A. In addition, Ni-based composite oxides may exhibit alkalinity due to the remaining alkaline components used in their synthesis, and deterioration of the composite oxides due to the acid components in the raw material solution used during coating with the additive on the surface of the composite oxides is likely to be suppressed.

[0048] The composite oxide has a layered rock salt-type crystal structure and has a general formula (1): LiNi α M 1-α O 2 (satisfying 0.3 ≦ α < 1, and M is at least one element selected from the group consisting of Co, Mn, Al, Ti, and Fe.) and may have a composition represented thereby. When α is in the above range, the effects of Ni and the effects of element M can be obtained in a well-balanced manner.

[0049] From the viewpoints of improving cycle characteristics, increasing capacity, and increasing output, the composite oxide has a layered rock salt-type crystal structure and has a general formula (2): LiNi x Co y M 1-x-y O 2 and may have a composition represented thereby. In general formula (2), 0.3 ≦ x < 1, 0 < y ≦ 0.5, and 0 < 1 - x - y ≦ 0.35 are satisfied, and M is at least one element selected from the group consisting of Al and Mn. In this case, the effects of Ni, the effects of Co, and the effects of element M can be obtained in a well-balanced manner. When the surface of the composite oxide represented by general formula (2) is coated with an additive containing compound A, the high capacity of the composite oxide can be sufficiently extracted. Among them, M in general formula (2) is preferably Al. The x value may be in the range of 0.5 ≦ x < 1. The y value may be in the range of 0 < y ≦ 0.35.

[0050] From the viewpoints of improving cycle characteristics and increasing output power, the composite oxide has a spinel-type crystal structure and has a composition represented by the general formula (3): LiMn β Ni 2-β O 4 (0.1 ≦ β < 2). Further, in the general formula (3), β may be 0.5 or more and less than 2.

[0051] Hereinafter, the configuration of the non-aqueous electrolyte secondary battery will be described in detail.

[0052] (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 a positive electrode mixture is dispersed in a dispersion medium onto the surface of the positive electrode current collector and drying it. 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 on both surfaces. The positive electrode mixture contains the above positive electrode material as an essential component. The positive electrode mixture may contain a binder, a conductive agent, etc. as optional components. As the dispersion medium, N-methyl-2-pyrrolidone (NMP) or the like is used.

[0053] Examples of the binder include resin materials such as fluororesin, polyolefin resin, polyamide resin, polyimide resin, acrylic resin, and vinyl resin. Examples of the fluororesin include polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF). The binder may be used alone or in combination of two or more.

[0054] Examples of the conductive agent include carbon blacks such as acetylene black; conductive fibers such as carbon fibers and metal fibers; and carbon fluoride. The conductive agent may be used alone or in combination of two or more.

[0055] For the positive electrode current collector, for example, a metal foil can be used. Examples of the metal constituting the positive electrode current collector include aluminum (Al), titanium (Ti), alloys containing these metal elements, and stainless steel. The thickness of the positive electrode current collector is not particularly limited, but is, for example, 3 to 50 μm.

[0056] (Negative electrode) The negative electrode may be a type of negative electrode that includes at least a negative electrode current collector, and lithium metal is deposited on the negative electrode during charging and dissolved in the non-aqueous electrolyte during discharging. For example, the negative electrode may include a negative electrode current collector or a negative electrode active material layer supported on the surface of the negative electrode current collector, and lithium metal may be deposited on the surface of the negative electrode current collector or the negative electrode active material layer.

[0057] For the negative electrode current collector, for example, a metal foil can be used. The metal constituting the negative electrode current collector is preferably a metal that does not react with lithium metal, and examples thereof include copper (Cu), nickel (Ni), iron (Fe), and alloys containing these metal elements. 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.

[0058] The negative electrode may include a negative electrode current collector and a negative electrode active material layer supported on the surface of the negative electrode current collector. From the viewpoint of increasing the capacity, the thickness of the negative electrode active material 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 borne by the negative electrode active material in the negative electrode active material layer with respect to the design capacity Cp of the positive electrode satisfies Cn / Cp < 1, and may satisfy Cn / Cp < 0.8. In this case, lithium metal is deposited on the surface of the negative electrode active material layer during charging, and the lithium metal deposited on the surface of the negative electrode active material layer during discharging is dissolved in the non-aqueous electrolyte.

[0059] The negative electrode active material layer can be formed, for example, by applying a negative electrode slurry in which a negative electrode active material is dispersed in a dispersion medium to the surface of the negative electrode current collector and drying it. The dried coating film may be rolled if necessary. The negative electrode active material layer may be formed on one surface of the negative electrode current collector or on both surfaces. As the dispersion medium, for example, water and NMP are used.

[0060] The negative electrode mixture contains a negative electrode active material as an essential component, and may contain, as optional components, a binder, a conductive agent, a thickener, and the like. As the binder and the conductive agent, those exemplified for the positive electrode can be used. As the binder, a rubber material such as styrene-butadiene rubber (SBR) may be used. As the thickener, for example, carboxymethyl cellulose (CMC) and its modified products (such as Na salt) can be mentioned.

[0061] The negative electrode active material may contain a carbon material that occludes and releases lithium ions. Examples of the carbon material that occludes and releases lithium ions include graphite (natural graphite, artificial graphite), graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), and the like. Among them, graphite is preferable because of its excellent charge-discharge stability and low irreversible capacity.

[0062] The negative electrode active material may contain an alloy-based material. The alloy-based material is a material containing at least one metal capable of forming an alloy with lithium, and examples thereof include silicon, tin, silicon alloys, tin alloys, silicon compounds, and the like. As the alloy-based material, a composite material having a lithium ion conduction phase and silicon particles dispersed in the phase may be used. As the lithium ion conduction phase, a silicate phase, a silicon oxide phase in which 95% by mass or more is silicon dioxide, a carbon phase, or the like may be used.

[0063] The negative electrode active material may be used in combination with an alloy-based material and a carbon material. In this case, the mass ratio of the carbon material in the total of the alloy-based material and the carbon material is preferably, for example, 80% by mass or more, and more preferably 90% by mass or more.

[0064] (Non-aqueous electrolyte) The non-aqueous electrolyte contains lithium ions and anions and has lithium ion conductivity. The non-aqueous electrolyte may be liquid. The liquid non-aqueous electrolyte contains, for example, lithium ions, anions, and a non-aqueous solvent. The liquid non-aqueous electrolyte is prepared by dissolving a lithium salt in a non-aqueous solvent. When the lithium salt is dissolved in the non-aqueous solvent, lithium ions and anions are generated.

[0065] The non-aqueous electrolyte may be in a gel form. The gel-like non-aqueous electrolyte contains, for example, lithium ions, anions, and a matrix polymer, and may further contain a non-aqueous solvent. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. Examples of the polymer material include fluororesin, acrylic resin, polyether resin, and the like.

[0066] The anion contains at least an anion of an oxalate complex. The anion of the oxalate complex may contain boron and / or phosphorus. Examples of the anion of the oxalate complex include B(C 2 O 4 ) 2 - , difluorooxalate borate anion: BF 2 (C 2 O 4 ) - , PF 4 (C 2 O 4 ) - , PF 2 (C 2 O 4 ) 2 - and the like. The anion of the oxalate complex may be used alone or in combination of two or more.

[0067] The anion of the oxalate complex may be combined with other anions. Examples of other anions include, for example, BF 4 - , ClO 4 - , PF 6 - , CF 3 SO 3 - , CF 3 CO 2 - , anions of imides, and the like. Examples of the anion of 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 an integer of 0 or 1 or more, x and y are each independently 0, 1, or 2, and x + y = 2 is satisfied.) etc. may be mentioned. Other anions may be PF 6 - and / or an anion of imides. Other anions may be used alone or in combination of two or more kinds.

[0068] The concentration of the anion in the non-aqueous electrolyte may be 0.5 mol / L or more and 3.5 mol / L or less. Also, the concentration of the anion of the oxalate complex in the non-aqueous electrolyte may be 0.05 mol / L or more and 1 mol / L or less.

[0069] Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, or halogenated derivatives thereof. The non-aqueous solvent may be used alone or in combination of two or more kinds. Examples of the halogenated derivative include fluorides etc.

[0070] Examples of the ester include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, chain carboxylic acid esters, etc. Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), etc. Examples of the chain carbonate include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), etc. Examples of the cyclic carboxylic acid ester include γ-butyrolactone (GBL), γ-valerolactone (GVL), etc. Examples of the chain carboxylic acid ester include ethyl acetate, propyl acetate, methyl propionate (PM), etc.

[0071] Examples of the ether include cyclic ethers and chain ethers. Examples of the cyclic ether include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, etc. Examples of the chain ether include 1,2-dimethoxyethane, diethyl ether, ethyl vinyl ether, 1,2-diethoxyethane, etc.

[0072] In addition, the cyclic carbonate may include a fluorinated cyclic carbonate such as fluoroethylene carbonate (FEC), and may also include a cyclic carbonate having a carbon-carbon unsaturated bond such as vinylene carbonate (VC) and vinyl ethylene carbonate. In this case, a high-quality film is formed on the surface of the negative electrode active material. Also, a high-quality film is formed on the surface of the negative electrode (negative electrode current collector or negative electrode binder layer), and the generation of lithium metal dendrites is suppressed.

[0073] (Separator) Generally, it is desirable to interpose a separator between the positive electrode and the negative electrode. The separator has a high ion permeability and appropriate mechanical strength and insulation. As the separator, a microporous thin film, a woven fabric, a non-woven fabric, etc. can be used. As the material of the separator, polyolefins such as polypropylene and polyethylene are preferable.

[0074] As an example of the structure of the non-aqueous electrolyte secondary battery, there is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and a non-aqueous electrolyte are housed in an exterior body. Alternatively, instead of the wound electrode group, other forms of electrode groups such as a laminated electrode group formed by laminating a positive electrode and a negative electrode with a separator interposed therebetween may be applied. The non-aqueous electrolyte secondary battery may be in any form such as a cylindrical shape, a square shape, a coin shape, a button shape, a laminate shape, etc.

[0075] FIG. 1 is a schematic perspective view of a part of a non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure, with a cutout.

[0076] The battery includes a bottomed angular battery case 4, an electrode group 1 accommodated in the battery case 4, and a non-aqueous electrolyte (not shown). The electrode group 1 has a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed therebetween and preventing direct contact. The electrode group 1 is formed by winding the negative electrode, the positive electrode, and the separator around a flat-shaped winding core and extracting the winding core.

[0077] One end of a negative electrode lead 3 is attached to the negative electrode current collector of the negative electrode by welding or the like. The other end of the negative electrode lead 3 is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a resin insulating plate (not shown). The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. One end of a positive electrode lead 2 is attached to the positive electrode current collector of the positive electrode by welding or the like. The other end of the positive electrode lead 2 is connected to the back surface of the sealing plate 5 via an insulating plate. That is, the positive electrode lead 2 is electrically connected to the battery case 4 that also serves as a positive electrode terminal. The insulating plate isolates the electrode group 1 from the sealing plate 5 and isolates the negative electrode lead 3 from the battery case 4. The peripheral edge of the sealing plate 5 is fitted to the opening end of the battery case 4, and the fitting portion is laser welded. In this way, the opening of the battery case 4 is sealed by the sealing plate 5. The injection hole for the non-aqueous electrolyte provided in the sealing plate 5 is blocked by a seal 8.

[0078] Hereinafter, the present disclosure will be specifically described based on examples and comparative examples, but the present invention is not limited to the following examples.

[0079] 《Example 1》 [Fabrication of Positive Electrode Material] By the following procedure, the surface of composite oxide particles (average particle size (D50) 11.1 μm) having a layered rock salt type composition of LiNi 0.9 Co 0.05 Al 0.05 (NCA) was coated with an additive containing Compound A.

[0080] First, H 3 PO 4To an aqueous solution (concentration 1 mol / L), an aqueous LiOH solution (concentration 1 mol / L) was added to obtain a raw material solution (an aqueous solution containing H 3 PO 4 and LiOH). When the raw material composition in the raw material solution is represented by Li z H (3-z) PO 4 , the amount of the aqueous LiOH solution added to the H 3 PO 4 aqueous solution was adjusted so that z would be the value shown in Table 1. The amount of the aqueous LiOH solution added was such that the pH of the raw material solution would be less than 8.

[0081] Composite oxide particles were added to the raw material solution. At this time, the amount of the composite oxide particles added was 1250 g per 1 L of the raw material solution. The raw material solution containing the composite oxide particles was stirred for 15 minutes to disperse the composite oxide particles in the raw material solution. Then, the composite oxide particles in the dispersion were filtered off, and the composite oxide particles with the raw material solution adhering to the surface were heated at 450 °C for 3 hours and dried. In this way, a positive electrode material in which the surface of the composite oxide particles was coated with an additive was obtained. The P content in the positive electrode material determined by the method described above was 0.21% by mass.

[0082] When the components of the additive coating the surface of the composite oxide particles were examined by the method described above, it was confirmed that it contained hexametaphosphoric acid (Li 6 P 6 O 18 ), which is compound A, lithium orthophosphate (Li 3 PO 4 ), and lithium pyrophosphate (Li 4 P 2 O 7 ).

[0083] [Fabrication of Positive Electrode] N-methyl-2-pyrrolidone (NMP) was added to the positive electrode mixture and stirred to prepare a positive electrode slurry. As the positive electrode mixture, a mixture of the positive electrode material obtained 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 100:2:2.

[0084] The positive electrode slurry was applied to the surface of the aluminum foil serving as the positive electrode current collector, and after drying the coating film, it was rolled to form a positive electrode mixture layer (thickness 40 μm, density 3.6 g / cm 3 ) on one side of the aluminum foil.

[0085] [Preparation of non-aqueous electrolyte] A non-aqueous electrolyte was prepared by dissolving a lithium salt in a non-aqueous solvent. As the non-aqueous solvent, a mixed solvent containing fluoroethylene carbonate (FEC) and dimethyl carbonate (DMC) in a volume ratio of 2:8 was used. As the lithium salt, LiPF 6 and LiBF 2 (C 2 O 4 )(LiFOB) was used. The concentration of LiPF 6 in the non-aqueous electrolyte was 1 mol / L. The concentration of LiFOB in the non-aqueous electrolyte was 0.5 mol / L.

[0086] [Preparation of evaluation cell] An evaluation cell for the positive electrode was constructed using the above positive electrode with an Al lead attached, a counter electrode (Li electrode) with a Ni lead attached, and the above non-aqueous electrolyte. Specifically, as the counter electrode, a lithium metal foil was attached to one side of an electrolytic copper foil. The positive electrode and the counter electrode were laminated via a polyethylene separator so that the positive electrode mixture layer and the lithium metal foil faced each other, and an electrode group was obtained. The electrodes were housed in a bag-shaped exterior body formed of an Al laminate film, the non-aqueous electrolyte was injected, the positive electrode mixture layer was made to contain the non-aqueous electrolyte, and then the opening of the exterior body was sealed by heat. A part of the Al lead and the Ni lead was exposed to the outside from the exterior body. The preparation of the evaluation cell was carried out in a dry air atmosphere with a dew point of -60°C or lower. The evaluation cell was fixed under pressure at 3.2 MPa.

[0087] 《Comparative Example 1》 In the preparation of the positive electrode, an evaluation cell B1 of Comparative Example 1 was prepared in the same manner as in Example 1, except that the surface of the composite oxide particles was not coated with an additive containing Compound A.

[0088] 《Comparative Example 2》 In the preparation of the non-aqueous electrolyte, only LiPF 6 was used, and the evaluation cell B2 of Comparative Example 2 was prepared in the same manner as in Example 1, except that the concentration of LiPF 6 in the non-aqueous electrolyte was 1 mol / L.

[0089] 《Comparative Example 3》 In the preparation of the positive electrode, the surface of the composite oxide particles was not coated with the additive containing Compound A. In the preparation of the non-aqueous electrolyte, only LiPF 6 was used, and the concentration of LiPF 6 in the non-aqueous electrolyte was 1 mol / L. The evaluation cell B3 of Comparative Example 3 was prepared in the same manner as in Example 1, except for the above.

[0090] The following evaluations were performed on the evaluation cells A1 and B1 to B3 obtained above.

[0091] [Evaluation: Charge-discharge cycle test] Constant current charging was performed at a current of 0.2C until the cell voltage reached 4.3V, and then constant voltage charging was performed at a cell voltage of 4.3V until the current reached 0.05C. Thereafter, constant current discharging was performed at a current of 0.2C until the cell voltage reached 2.5V. The rest time between charging and discharging was 10 minutes. The charge and discharge were performed in an environment at 25°C.

[0092] Taking the above charge and discharge as one cycle, 40 cycles were performed. The ratio of the discharge capacity at the 40th cycle to the discharge capacity at the first cycle was determined as the capacity retention rate.

[0093] The evaluation results are shown in Table 1. The improvement rate of the capacity retention rate of cell A1 with respect to cell B1 is the increase rate of the capacity retention rate of cell A1 with respect to cell B1. When the capacity retention rates of battery A1 and battery B1 are a1 and b1, respectively, it is a value obtained by (a1 - b1) / b1 × 100. Also, the improvement rate of the capacity retention rate of cell B2 is the increase rate of the capacity retention rate of cell B2 with respect to cell B3. When the capacity retention rates of cell B2 and cell B3 are b2 and b3, respectively, it is a value obtained by (b2 - b3) / b3 × 100.

[0094]

Table 1

[0095] The improvement rate of the capacity retention rate of cell A1 with respect to cell B1 was 1.2%, which was significantly increased compared to 0.1%, the improvement rate of the capacity retention rate of cell B2 with respect to cell B3. When LiFOB was included in the non-aqueous electrolyte, the cycle characteristics were significantly improved by covering the surface of the composite oxide with an additive containing compound A.

[0096] For cell A1 using a non-aqueous electrolyte containing LiFOB and cells B2 and B3 using a non-aqueous electrolyte not containing LiFOB, after obtaining the discharge capacity at the 40th cycle, respectively, the same charging as above was further performed. The deposition state of lithium metal on the surface of the counter electrode (Li electrode) after charging was examined using a scanning electron microscope (SEM). As a result, it was confirmed that the deposition of dendritic lithium metal was suppressed at the counter electrode of cell A1 compared to the counter electrodes of cells B2 and B3.

Industrial Applicability

[0097] The non-aqueous electrolyte secondary battery according to the present disclosure is suitably used, for example, as a power source for mobile devices such as smartphones, a power source for vehicles such as electric vehicles, and a storage device for natural energy such as sunlight.

Explanation of Signs

[0098] 1 Electrode group 2 Positive electrode lead 3 Negative electrode lead 4 Battery case 5 Sealing plate 6 Negative electrode terminal 7 Gasket 8 Bolt

Claims

1. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode includes a composite oxide containing lithium and a transition metal, and an additive covering at least a part of the surface of the composite oxide, the additive includes a cyclic inorganic phosphate compound, the non-aqueous electrolyte includes lithium ions and anions, and the anion includes an anion of an oxalate complex.

2. The non-aqueous electrolyte secondary battery according to Claim 1, wherein the anion of the oxalate complex includes a difluorooxalate borate anion.

3. The non-aqueous electrolyte secondary battery according to Claim 1 or 2, wherein the phosphate compound includes at least one selected from the group consisting of cyclic polyphosphoric acid and its salts.

4. The non-aqueous electrolyte secondary battery according to Claim 3, wherein the cyclic polyphosphoric acid includes hexametaphosphoric acid.

5. The non-aqueous electrolyte secondary battery according to any one of Claims 1 to 4, wherein the phosphorus content in the positive electrode is 0.1% by mass or more and 0.75% by mass or less based on the total of the composite oxide and the additive.

6. the composite oxide has a layered rock salt-type crystal structure, and General formula: LiNi ｘ Co ｙ M １－ｘ－ｙ O ２ The non-aqueous electrolyte secondary battery according to any one of claims 1 to 5, having a composition represented by the formula, wherein in the general formula, 0.3 ≦ x < 1, 0 < y ≦ 0.5 and 0 < 1 - x - y ≦ 0.35 are satisfied, and M is at least one selected from the group consisting of Al and Mn.

7. the negative electrode includes at least a negative electrode current collector, and lithium metal is deposited on the negative electrode during charging and dissolved in the non-aqueous electrolyte during discharging. The non-aqueous electrolyte secondary battery according to any one of Claims 1 to 6.

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