Nonaqueous electrolyte secondary battery and method of manufacturing the same

Abstract

This nonaqueous electrolyte secondary battery comprises a positive electrode, a negative electrode, and a nonaqueous electrolyte. The positive electrode contains positive electrode active material particles. The positive electrode active material particles each contain a composite oxide including lithium and a transition metal, and a coating material for coating at least a portion of the surface of the composite oxide. The coating material contains a metal oxide and a phosphorus compound for coating at least a portion of the surface of the metal oxide.

Description

[Technical Field]

[0001] The present invention relates to a non-aqueous electrolyte secondary battery and a method for manufacturing the same. [Background technology]

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

[0003] Incidentally, Patent Document 1 proposes "a positive electrode active material for a lithium ion secondary battery comprising particles (III) having, on the surface of a lithium-containing composite oxide containing lithium and a transition metal element, a coating layer containing a metal oxide (I) containing at least one metal element selected from the group consisting of Groups 3 and 13 of the periodic table and lanthanoids, and a compound (II) containing Li and P, wherein the atomic ratio of the P to the metal element (P / metal element) contained within a surface layer of 5 nm of the particles (III) is 0.03 to 0.45."

[0004] Furthermore, Patent Document 1 proposes "a method for producing a positive electrode active material for a lithium ion secondary battery, the method comprising: a first contacting step of contacting a powder of a lithium-containing composite oxide containing lithium and a transition metal element with a first aqueous solution containing a cation having at least one metal element selected from the group consisting of Groups 3 and 13 of the periodic table and lanthanoids; a second contacting step of contacting the powder of the lithium-containing composite oxide with a second aqueous solution containing an anion having P but not containing a cation having the metal element; and a heating step of heating the treated powder of the lithium-containing composite oxide obtained after the first and second contacting steps to 250 to 700°C, wherein |(number of moles of the anion contained in the second aqueous solution × valence of the anion)| / (number of moles of the cations contained in the first aqueous solution × valence of the cation) is less than 1 in the entire aqueous solution obtained by combining the first aqueous solution and the second aqueous solution." [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2013 / 047877 Brochure Summary of the Invention [Problem to be solved by the invention]

[0006] The coating layer in Patent Document 1 is formed by contacting a composite oxide with cations and anions using a liquid phase method, followed by heating to 250 to 700° C. In such a method, the cations and anions are complexly combined, making it difficult to control the structure of the coating layer, which may in turn result in a deterioration in cycle characteristics. [Means for solving the problem]

[0007] In view of the above, one aspect of the present invention relates to a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the positive electrode includes positive electrode active material particles, the positive electrode active material particles including a composite oxide containing lithium and a transition metal, and a coating material covering at least a portion of a surface of the composite oxide, and the coating material includes a metal oxide and a phosphorus compound covering at least a portion of the surface of the metal oxide.

[0008] Another aspect of the present invention relates to a method for manufacturing a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, the method including: (1A) forming a positive electrode mixture layer containing a composite oxide containing lithium and a transition metal on a surface of a positive electrode current collector; (1B) attaching a metal oxide to at least a portion of the surface of the composite oxide in the positive electrode mixture layer by atomic layer deposition or the like to obtain a positive electrode intermediate; and (2) bringing a phosphate ester compound into contact with the surface of the metal oxide to form a coating material containing the metal oxide and a phosphorus compound that covers at least a portion of the surface of the metal oxide, thereby obtaining a positive electrode.

[0009] Yet another aspect of the present invention relates to a method for manufacturing a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, the method including: (1a) a step of attaching a metal oxide to at least a portion of a surface of a composite oxide containing lithium and a transition metal by a sputtering method or the like; (1b) a step of forming a positive electrode mixture layer containing the composite oxide to which the metal oxide has been attached on a surface of a positive electrode current collector, thereby obtaining a positive electrode intermediate; and (2) a step of bringing a phosphate ester compound into contact with the surface of the metal oxide, thereby forming a coating material containing the metal oxide and a phosphorus compound that covers at least a portion of the surface of the metal oxide, thereby obtaining a positive electrode. [Effects of the Invention]

[0010] According to the present invention, the cycle characteristics of a non-aqueous electrolyte secondary battery can be improved. The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a partially cutaway schematic perspective view of a nonaqueous electrolyte secondary battery according to one embodiment of the present invention. [Figure 2] 1 is a cross-sectional TEM photograph of a first region of a positive electrode active material particle, where region (a) shows the surface of the composite oxide, region (b) shows the metal oxide of the coating material, and region (c) shows the phosphorus coating of the coating material. [Figure 3] FIG. 10 is a diagram showing the amount of phosphorus MO detected by EDX analysis of the first region. [Figure 4] 1 is a cross-sectional TEM photograph of a second region of a positive electrode active material particle, where region (a) shows the surface of the composite oxide and region (b) shows the region where the binder is present. [Figure 5] FIG. 10 shows the amount of phosphorus MS detected in EDX analysis of the second region. DETAILED DESCRIPTION OF THE INVENTION

[0012] A non-aqueous electrolyte secondary battery according to one embodiment of the present invention includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode includes positive electrode active material particles. The positive electrode active material particles include a composite oxide containing lithium and a transition metal (hereinafter also referred to as "composite oxide A") and a coating material that covers at least a portion of the surface of the composite oxide.

[0013] The coating material contains a metal oxide (hereinafter also referred to as "metal oxide B") and a phosphorus compound that covers at least a portion of the surface of metal oxide B. Hereinafter, the phosphorus compound that covers at least a portion of the surface of metal oxide B will also be referred to as "phosphorus coat." The structure of the phosphorus coat is not particularly limited, and the presence of the phosphorus coat can be confirmed by detecting the P element.

[0014] Complex oxide A is the part (positive electrode active material) that generates capacity through charging and discharging of lithium ions. On the other hand, coating material or metal oxide B is an additive to the positive electrode active material and does not need to generate capacity itself. In other words, complex oxide A is different from metal oxide B.

[0015] The coating material has a laminated structure. Metal oxide B is attached to the surface of composite oxide A. A phosphorus coat is attached to the surface of metal oxide B. The phosphorus coat is derived from, for example, a phosphorus compound contained in the non-aqueous electrolyte. The phosphorus compound contained in the non-aqueous electrolyte may be a phosphate ester compound. When the phosphorus compound attaches to the surface of metal oxide B (or reacts with the surface of metal oxide B), at least a portion of the surface of metal oxide B is covered with a phosphorus coat. The phosphorus coat may be a portion of the phosphorus compound contained in the non-aqueous electrolyte, or may be a portion of a residue of the phosphorus compound contained in the non-aqueous electrolyte.

[0016] The phosphorus compound contained in the non-aqueous electrolyte may be a phosphate ester compound having at least one alkenyl group in one molecule. Hereinafter, the phosphate ester compound having at least one alkenyl group in one molecule will also be referred to as compound A. Compound A has a high affinity with the surface of metal oxide B.

[0017] Both the metal oxide B and the phosphorus coat have the function of protecting the surface of the composite oxide A and limiting side reactions (hereinafter referred to as the "protection function"), and the function of suppressing metal elution (hereinafter referred to as the "elution suppression function") from the composite oxide A. Both the protection function and the elution suppression function contribute to improving the cycle characteristics.

[0018] It is believed that the phosphorus coat adheres to the surface of metal oxide B and protects metal oxide B, allowing the metal oxide B to continuously exhibit the above-mentioned functions during charge-discharge cycles. On the other hand, since the phosphorus coat can be stably present on the surface of composite oxide B via metal oxide B, the above-mentioned functions of the phosphorus coat are also sufficiently and continuously exhibited, thereby improving cycle characteristics.

[0019] When metal oxide B and phosphorus coat are used in combination, the amount of metal oxide B required to increase the coverage of the surface of composite oxide A with the coating material is smaller than when metal oxide B alone is used as the coating material. The thickness of metal oxide B attached to the surface of composite oxide A can be sufficiently thin. The coating material can be composed of a small amount of metal oxide B and a small amount of phosphorus coat. The increase in internal resistance due to such a coating material is slight.

[0020] The surface of the composite oxide A may have a first region covered with island-shaped metal oxide B and a second region other than the first region. The second region does not need to be covered with metal oxide B, but may contain a smaller amount of metal oxide B than in the first region. In the second region, lithium ions enter and exit the composite oxide A more easily than in the first region. The second region contributes to further reducing the internal resistance.

[0021] When metal oxide B is detected in the second region, the amount MA of metal contained in metal oxide B detected in the first region and the amount MB of metal contained in metal oxide B detected in the second region satisfy the relationship MA > MB, and may also satisfy the relationship MA > 2MB or MA > 3MB. The quantitative relationship between MA and MB is determined by the abundance ratio of metal atoms contained in metal oxide B.

[0022] The first regions may be scattered on the surface of the composite oxide A. That is, the surface of the composite oxide A may be covered with scattered island-like metal oxide B. By scattering the island-like metal oxide B evenly or evenly on the surface of the composite oxide A, the protective function and elution-suppressing function of the coating material are more likely to be exerted over the entire surface of the composite oxide A. Similarly, the effect of reducing internal resistance by the second regions is more likely to be exerted over the entire surface of the composite oxide A.

[0023] Since the compound A preferentially or selectively covers the surface of the metal oxide B to form a phosphorus coat, the second region is less likely to be covered with the phosphorus coat. The second region does not necessarily have to be covered with the phosphorus coat.

[0024] When the second region is covered with a phosphorus coating, the amount of phosphorus detected on the surface of the metal oxide B (i.e., the first region) M O and the amount of phosphorus detected in the second region M S may satisfy the relationship M O > M S. M O > 2 M S may be satisfied, or even M O > 3 M S. The quantitative relationship between M O and M S is determined by the abundance ratio of phosphorus atoms contained in the phosphorus coating.

[0025] The distinction between the first and second regions, the distribution of metal oxide B on the surface of composite oxide A, the quantitative relationship between MA and MB (the ratio of MA to MB), the distribution of phosphorus, and the quantitative relationship between MO and MS (the ratio of MO to MS) can be confirmed or quantitatively measured by, for example, performing elemental analysis or elemental mapping of a cross section of a positive electrode or positive electrode active material particle using an electron probe micro analyzer (EPMA), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), or the like.

[0026] In elemental analysis of the outermost surface of the positive electrode mixture, the atomic ratio of the metal (MY) derived from metal oxide B to the metal (MX) derived from composite oxide A, i.e., MY / MX, is preferably, for example, 2 or less. In this case, it can be assumed that a thin layer of metal oxide B (e.g., with a thickness of 1 nm or more and 3 nm or less) is distributed in an island pattern on the surface of composite oxide A.

[0027] (Compound A) Compound A is a phosphate ester (organophosphate) having at least one alkenyl group in one molecule. Compound A may be mixed into the non-aqueous electrolyte used in the battery. By producing a battery using a non-aqueous electrolyte containing compound A, a phosphorus coat can be easily formed on the surface of metal oxide B, which covers the surface of composite oxide A.

[0028] The carbon-carbon double bond of the alkenyl group is preferably close to the tip of the alkenyl group. Such compound A is considered to have a high interaction with metal oxide B. The number of carbon atoms in the alkenyl group may be, for example, 2 or more and 5 or less. In this case, compound A is easily dissolved in the non-aqueous electrolyte and easily adheres to the surface of metal oxide B. The viscosity of the non-aqueous electrolyte containing compound A can also be appropriately reduced. From the same viewpoint, the alkenyl group is preferably linear. When compound A has multiple alkenyl groups, the multiple alkenyl groups may be the same or different.

[0029] Specifically, the alkenyl group may be at least one selected from the group consisting of a vinyl group, a 1-propenyl group, a 2-propenyl group (allyl group), an isopropenyl group, a 1-butenyl group, a 2-butenyl group, and a 3-butenyl group. Of these, the alkenyl group is preferably at least one of an allyl group and a 3-butenyl group, and more preferably an allyl group.

[0030] Compound A has, for example, a structure represented by the following formula (I):

[0031] [ka]

[0032] In formula (I), R 1 , R 2 and R 3 At least one of R is an alkenyl group. 1 , R 2 and R 3 It is preferable that all of the alkenyl groups are alkenyl groups. When compound A represented by formula (I) has a plurality of alkenyl groups, the plurality of alkenyl groups may be the same or different. Some of the hydrogen atoms contained in the alkenyl groups may be substituted with other atoms or groups (e.g., halogen atoms such as chlorine atoms). The number of carbon atoms in the alkenyl groups is, for example, 2 or more and 5 or less. The alkenyl group may be linear or branched. The alkenyl group is preferably a group such as CH2=CH-(CH2)n n may be 0 or more and 3 or less, and n=1 is more preferred.

[0033] In formula (I), R 1 , R 2 and R 3 One or two of the groups may be a hydrocarbon group other than an alkenyl group. The hydrocarbon group other than an alkenyl group may be an alkyl group or the like. Some of the hydrogen atoms contained in the hydrocarbon group other than an alkenyl group (such as an alkyl group) may be substituted with other atoms or groups (for example, halogen atoms such as chlorine atoms). When compound A represented by formula (I) has two hydrocarbon groups other than an alkenyl group, the hydrocarbon groups other than an alkenyl group may be the same or different. The number of carbon atoms in the alkyl group is, for example, 2 or more and 5 or less. The alkyl group may be linear or branched. The alkyl group may be a methyl group, an ethyl group, a propyl group, or the like.

[0034] Compound A includes, for example, at least one selected from the group consisting of a phosphoric acid monoester, a phosphoric acid diester, and a phosphoric acid triester. Among these, a phosphoric acid triester is preferred. The phosphoric acid triester preferably accounts for 50% by mass or more of compound A, and may account for 70% by mass or more, or even 90% by mass or more.

[0035] The phosphate triester preferably contains triallyl phosphate. Triallyl phosphate is easily soluble in the non-aqueous electrolyte, allowing the preparation of a non-aqueous electrolyte with lower viscosity. Triallyl phosphate preferably accounts for 50% by mass or more of the phosphate triester, and may be 70% by mass or more, or even 90% by mass or more.

[0036] The content of compound A in the non-aqueous electrolyte may be 2% by mass or less, 0.25% by mass or more and 2% by mass or less, 0.25% by mass or more and 1.5% by mass or less, or 0.25% by mass or more and 1.3% by mass or less. For example, the content of compound A may be within the above range when the non-aqueous electrolyte is prepared (before being poured into the battery). In this case, the cycle characteristics are likely to be improved.

[0037] When the content of compound A during preparation of the non-aqueous electrolyte is 2% by mass or less, the content of compound A in the non-aqueous electrolyte in the initial battery (e.g., after injection of the non-aqueous electrolyte or after several charge / discharge cycles) may be, for example, 1% by mass or less, 100 ppm or less, or even a trace amount close to the detection limit. If the presence of compound A can be confirmed in the non-aqueous electrolyte in the battery, it is estimated that compound A is attached to the surface of metal oxide B to some extent. The content of compound A in the non-aqueous electrolyte can be determined by gas chromatography mass spectrometry (GC / MS) or the like.

[0038] (Metal oxide B) The metal contained in metal oxide B may be, for example, at least one selected from the group consisting of aluminum, silicon, titanium, magnesium, zirconium, niobium, germanium, calcium, and strontium. Metal oxide B may be, for example, at least one selected from the group consisting of aluminum oxide (e.g., Al2O3 or silica alumina), silicon oxide (e.g., SiO2), titanium oxide (e.g., TiO2), magnesium oxide (e.g., MgO), zirconium oxide (e.g., ZrO2), niobium oxide, germanium oxide, calcium oxide, and strontium oxide.

[0039] Among these, it is preferable that the metal oxide B contains at least one selected from the group consisting of aluminum oxide, silicon oxide, and silica alumina, which are inexpensive and have excellent chemical and thermal stability.

[0040] The thickness of the metal oxide B attached to the surface of the composite oxide A is, for example, in the range of 1 nm or more and 5 nm or less. When the thickness of the metal oxide B is 5 nm or less, lithium ions tend to move smoothly between the composite oxide A and the non-aqueous electrolyte, and the internal resistance can be maintained low. The thickness of the metal oxide B may be 1 nm or more and 3 nm or less.

[0041] (Composite oxide A) The composite oxide A is a portion that exhibits capacity as a positive electrode active material. The composite oxide A contains lithium and a metal other than lithium. The metal other than lithium includes at least a transition metal, and may include a metal other than the transition metal. The transition metal may include, for example, at least one selected from the group consisting of Ni, Co, Mn, Fe, Cu, Cr, Ti, Nb, Zr, V, Ta, and Mo. The metal other than the transition metal may include, for example, at least one selected from the group consisting of Al, Mg, Ca, Sr, Zn, and Si.

[0042] The composite oxide A can be obtained, for example, by mixing a lithium compound with a compound containing a metal other than lithium obtained by a coprecipitation method or the like, and firing the mixture under predetermined conditions.

[0043] The composite oxide A is, for example, a secondary particle formed by agglomeration of a plurality of primary particles. The particle size of the primary particles is generally 0.05 μm or more and 1 μm or less. The average particle size of the secondary particles is, for example, 3 μm or more and 30 μm or less, and may be 5 μm or more and 25 μm or less.

[0044] In this specification, the average particle size refers to the particle size (volume average particle size) at which the volume integrated value is 50% in the particle size distribution measured by a laser diffraction scattering method. Such an average particle size is sometimes referred to as D50.

[0045] From the viewpoint of increasing capacity, it is preferable that the transition metal contains at least Ni. Ni is cheaper than Co and is advantageous for increasing capacity. The atomic ratio of Ni to all metals other than lithium may be, for example, 0.3 or more and less than 1, 0.5 or more and less than 1, or 0.75 or more and less than 1.

[0046] The composite oxide A may have a layered rock salt structure. The composite oxide A may contain Ni, Co, and at least one of Al and Mn. In this case, the atomic ratio of the metal derived from the metal oxide B to the Ni derived from the composite oxide A at the outermost surface of the positive electrode mixture may be 2 or less. For example, when the surface of the composite oxide A is coated with a coating material, Al2O3, the atomic ratio of Al derived from the Al2O3 to Ni derived from the composite oxide A at the outermost surface of the positive electrode mixture, Al / Ni, may be 2 or less. In this case, it can be assumed that thin layers of the metal oxide B are distributed in an island pattern on the surface of the composite oxide A.

[0047] The contents of the elements constituting the composite oxide A can be measured by an inductively coupled plasma atomic emission spectroscopy (ICP-AES), EPMA, EDX, or the like.

[0048] (Method of coating the surface of composite oxide A with a coating material) A method for coating the surface of composite oxide A with a coating material includes, for example, a first step of coating the surface of composite oxide A with metal oxide B, and a second step of coating the surface of metal oxide B with a phosphorus coat. The first step may be a step of forming island-like first regions scattered on the surface of composite oxide A. In this case, the surface of composite oxide A has second regions that are not coated with metal oxide B. The second step may be a step of attaching compound A or a residue thereof to the surface of metal oxide B.

[0049] (1st step) The surface of composite oxide A can be coated with metal oxide B by a liquid-phase method or a gas-phase method. Examples of liquid-phase methods include a wet method, a spray coating method, and a dip coating method. Examples of gas-phase methods include a chemical vapor deposition (CVD) method, an atomic layer deposition (ALD) method, and a sputtering method.

[0050] (First method) The first step includes, for example, a step (1A) of forming a positive electrode mixture layer containing a composite oxide A on the surface of a positive electrode current collector, and a step (1B) of attaching a metal oxide B to at least a portion of the surface of the composite oxide A in the positive electrode mixture layer to obtain a positive electrode intermediate.

[0051] In step (1A), for example, a positive electrode slurry in which a positive electrode mixture is dispersed in a dispersion medium is applied to the surface of a positive electrode current collector, the coating is dried, and if necessary, rolled to form a positive electrode mixture layer. The positive electrode mixture layer may be formed on one surface or both surfaces of the positive electrode current collector. The positive electrode mixture contains at least composite oxide A and may further contain a binder, a conductive agent, etc. As the dispersion medium, N-methyl-2-pyrrolidone (NMP) or the like is used.

[0052] As the binder, a resin material is used, and examples thereof include fluororesin, polyolefin resin, polyamide resin, polyimide resin, acrylic resin, vinyl resin, etc. Examples of fluororesin include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.

[0053] Examples of the conductive agent include carbon black, carbon fiber, metal fiber, and carbon fluoride.

[0054] The positive electrode current collector may be, for example, a metal foil. Examples of metals constituting the positive electrode current collector include aluminum, titanium, 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.

[0055] In step (1B), metal oxide B can be attached to the surface of composite oxide A by a wet method, spray coating, dip coating, chemical vapor deposition (CVD), atomic layer deposition (ALD), sputtering, or the like. For example, metal oxide can be attached to the surface of composite oxide by ALD. While gas-phase methods such as CVD are generally performed at temperatures of 400 to 900°C, ALD can be performed at temperatures of 100 to 400°C. In other words, ALD is advantageous in that it can suppress thermal damage to the electrode.

[0056] The ALD method is a film-forming method in which a precursor gas containing a metal (such as Al) and an oxidant are alternately supplied to a reaction chamber in which the target is placed, forming a metal oxide on the surface of the target. The ALD method has a self-limiting effect, allowing metal oxide to be deposited on the surface of the target in atomic layers.

[0057] In the ALD method, by controlling the film formation conditions, it is possible to form island-shaped metal oxide B dotted on the surface of composite oxide A. For example, the deposition amount of metal oxide B can be controlled by the number of cycles, where one cycle is "supply of source gas → exhaust (purging) of source gas → supply of oxidant → exhaust (purging) of oxidant." By limiting the deposition amount of metal oxide B (in other words, by making metal oxide B thinner), island-shaped metal oxide B is formed so as to be dotted on the surface of composite oxide A.

[0058] Examples of oxidizing agents used in the ALD method include water, oxygen, ozone, etc. The oxidizing agent may be supplied to the reaction chamber as plasma using the oxidizing agent as a raw material.

[0059] The precursor can be any of various organometallic compounds conventionally used in ALD. For example, precursors containing Al include trimethylaluminum ((CH3)3Al) and triethylaluminum ((C2H5)3Al).

[0060] (Second method) The first step may include, for example, a step (1a) of attaching a metal oxide B to at least a portion of the surface of a composite oxide A, and a step (1b) of forming a positive electrode mixture layer containing the composite oxide A to which the metal oxide B has been attached on the surface of a positive electrode current collector to obtain a positive electrode intermediate.

[0061] In step (1a), metal oxide B can be attached to the surface of complex oxide A by a wet method, spray coating, dip coating, chemical vapor deposition (CVD), sputtering, or the like. Step (1a) includes, for example, step (1a-1) of attaching a raw material solution to the surface of complex oxide A, and step (1a-2) of heating and drying the complex oxide A with the raw material solution attached to its surface. Step (1a-1) is, for example, a step of adding complex oxide A to the raw material solution and dispersing it by stirring. Step (1a-2) serves both as a step of removing the dispersion medium attached to the surface of complex oxide A by heating and drying, and a step of reacting the raw materials attached to the surface of complex oxide A to produce metal oxide B.

[0062] The raw material solution may be, for example, an aqueous solution containing raw materials. The raw material may be a compound that can generate metal oxide B through a decomposition reaction caused by heating. Examples of such compounds include metal salts of organic acids such as citric acid, maleic acid, and lactic acid, and organometallic complexes. By limiting the amount of raw material contained in the raw material solution, island-like metal oxide B is formed so as to be scattered on the surface of composite oxide A.

[0063] In step (1b), similarly to step (1A), a positive electrode slurry is prepared, the positive electrode slurry is applied to the surface of a positive electrode current collector, the coating is dried, and if necessary, is rolled to form a positive electrode mixture layer.

[0064] (2nd process) The second step is a step of bringing compound A into contact with the surface of metal oxide B to form a coating material containing metal oxide B and a phosphorus coating that covers at least a portion of the surface of metal oxide B, thereby obtaining a positive electrode. The second step includes, for example, step (2A) of preparing a non-aqueous electrolyte containing compound A, and step (2B) of bringing the non-aqueous electrolyte containing compound A into contact with composite oxide A whose surface is coated with metal oxide B. In step (2B), compound A, which has a high affinity for metal oxide B, preferentially or selectively adheres to or reacts with the surface of metal oxide B.

[0065] In step (2B), for example, an electrode group may be formed including the positive electrode intermediate obtained in step (1B) or step (1b), a negative electrode, and a separator disposed between the positive electrode intermediate and the negative electrode, and the electrode group may be brought into contact with a non-aqueous electrolyte. For example, a battery may be assembled by housing the electrode group in a battery case, injecting a non-aqueous electrolyte into the battery case containing the electrode group, and closing the opening of the battery case with a sealing plate.

[0066] As described above, when a phosphorus coating of compound A is formed after the positive electrode mixture layer is formed, contact points between the composite oxides A are easily formed, and a conductive network between the composite oxides A is easily secured.

[0067] The configuration of the nonaqueous electrolyte secondary battery will be described in further detail below. (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 contains positive electrode active material particles as an essential component. The positive electrode active material particles include a composite oxide A and a coating material attached to the surface of the composite oxide A.

[0068] (Negative electrode) The negative electrode has at least a negative electrode current collector, and may have a negative electrode mixture layer supported on the surface of the negative electrode current collector. That is, the nonaqueous electrolyte secondary battery may be a lithium secondary battery (lithium metal secondary battery) in which lithium metal is deposited at the negative electrode during charging and dissolved during discharging, or a lithium ion secondary battery in which lithium ions are absorbed and released during charging and discharging.

[0069] 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, to the surface of the negative electrode current collector, drying the coating, and optionally rolling it. The negative electrode mixture layer may be formed on one surface or both surfaces of the negative electrode current collector. Examples of the dispersion medium include water and NMP.

[0070] The negative electrode mixture contains a negative electrode active material as an essential component, and may contain optional components such as a binder, a conductive agent, and a thickener. The binder and conductive agent may be the same as those exemplified for the positive electrode. The binder may be a rubber material such as styrene-butadiene copolymer rubber (SBR). Examples of thickeners include carboxymethyl cellulose (CMC) and its modified form (such as the Na salt).

[0071] The negative electrode active material may contain a carbon material that absorbs and releases lithium ions. Examples of carbon materials that absorb and release lithium ions include graphite (natural graphite and artificial graphite), easily graphitizable carbon (soft carbon), and hardly graphitizable carbon (hard carbon). Among these, graphite is preferred because of its excellent charge / discharge stability and low irreversible capacity.

[0072] The negative electrode active material may include an alloy material. An alloy material is a material containing at least one metal capable of forming an alloy with lithium. Examples of alloy materials include silicon, tin, silicon alloys, tin alloys, and silicon compounds. As the silicon compound, a composite material having a lithium ion conductive phase and a silicon phase dispersed in the phase may be used. As the lithium ion conductive phase, a silicate phase such as a lithium silicate phase, a silicon oxide phase containing 95% or more by mass of silicon dioxide, a carbon phase, or the like may be used.

[0073] The negative electrode active material may be a combination of an alloy material and a carbon material. The proportion of the carbon material in the total of the alloy material and the carbon material is, for example, preferably 80 mass % or more, and more preferably 90 mass % or more.

[0074] The shape and thickness of the negative electrode current collector can be selected from the shape and range corresponding to those of the positive electrode current collector. Examples of metals constituting the negative electrode current collector include copper (Cu), nickel (Ni), iron (Fe), and alloys containing these metal elements.

[0075] (non-aqueous electrolyte) The non-aqueous electrolyte contains a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The non-aqueous electrolyte may contain Compound A. The concentration of the lithium salt in the non-aqueous electrolyte is preferably, for example, 0.5 mol / L or more and 2 mol / L or less. By controlling the lithium salt concentration within the above range, a non-aqueous electrolyte having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0076] Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC). Examples of cyclic carbonates include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), and cyclic carbonates having a carbon-carbon unsaturated bond such as vinylene carbonate (VC) and vinylethylene carbonate. Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL), γ-valerolactone (GVL), and examples of chain carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. The non-aqueous solvent may be used alone or in combination of two or more.

[0077] Known lithium salts can be used as the lithium salt. Preferred lithium salts include, for example, LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, and LiB 10 Cl 10 Examples of the lithium salt include lithium bis(fluorosulfonyl)imide (LiN(FSO2)2), lithium bistrifluoromethanesulfonylimide (LiN(CF3SO2)2), lithium trifluoromethanesulfonyl nonafluorobutanesulfonylimide (LiN(CF3SO2)(CF9SO2)), and lithium bispentafluoroethanesulfonylimide (LiN(CF5SO2)2). One type of lithium salt may be used alone, or two or more types may be used in combination.

[0078] (separator) Generally, a separator is interposed between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator can be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. The separator is preferably made of polyolefin such as polypropylene or polyethylene.

[0079] An example of the structure of a nonaqueous electrolyte secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween is housed in an outer casing together with a nonaqueous electrolyte. However, this is not limited thereto, and other forms of electrode groups may also be applied. For example, a laminated electrode group in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween may also be used. The shape of the nonaqueous electrolyte secondary battery is also not limited, and may be, for example, a cylindrical type, a prismatic type, a coin type, a button type, a laminate type, or the like.

[0080] Hereinafter, the structure of a prismatic nonaqueous electrolyte secondary battery will be described as an example of the nonaqueous electrolyte secondary battery according to the present invention with reference to FIG.

[0081] The battery includes a bottomed, rectangular battery case 4, and an electrode group 1 and a nonaqueous electrolyte (not shown) housed within the battery case 4. The electrode group 1 includes a long, strip-shaped negative electrode, a long, strip-shaped positive electrode, and a separator interposed therebetween. The negative electrode current collector is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode current collector is electrically connected to the back surface of the sealing plate 5 via a positive electrode lead 2. In other words, the positive electrode is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The periphery of the sealing plate 5 fits into the open edge of the battery case 4, and the fitting is laser-welded. The sealing plate 5 has an injection hole for the nonaqueous electrolyte, which is closed with a seal plug 8 after injection.

[0082] The present invention will be specifically described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0083] Examples 1 to 4 [Preparation of positive electrode intermediate] NMP was added to the positive electrode mixture and stirred to prepare a positive electrode slurry. The positive electrode mixture was a mixture of positive electrode active material particles, acetylene black (AB), and polyvinylidene fluoride (PVDF). The positive electrode active material was layered rock salt type LiNi 0.35 Co 0.35 Mn 0.30 Composite oxide particles (average particle size (D50) 4 μm) having a composition of (NCM) were used. In the positive electrode mixture, the mass ratio of the positive electrode active material, AB, and PVDF was 100:2:2.

[0084] The positive electrode slurry was applied to the surface of the aluminum foil, the coating was dried, and then rolled to form a positive electrode mixture layer. The positive electrode mixture layer was formed on both sides of the aluminum foil. Furthermore, island-like Al2O3 was deposited on the surface of the composite oxide particles in the positive electrode mixture layer using the ALD method (temperature: 120°C, precursor: trimethylaluminum, oxidant: HO, pressure: several Torr, 10 cycles) so that they were dotted. In this way, a positive electrode intermediate was obtained. The atomic ratio Al / Ni at the outermost surface of the composite oxide particles in the positive electrode intermediate, determined by the method described above, was 2 or less.

[0085] [Preparation of negative electrode] Water was added to the negative electrode mixture and stirred to prepare a negative electrode slurry. The negative electrode mixture was a mixture of artificial graphite (average particle size 20 μm), styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC). The mass ratio of the artificial graphite, SBR, and CMC in the negative electrode mixture was 100:1:1. The negative electrode slurry was applied to the surface of copper foil, the coating was dried, and then rolled to produce a negative electrode with a negative electrode mixture layer formed on both sides of the copper foil.

[0086] [Preparation of non-aqueous electrolyte] A non-aqueous electrolyte was prepared by dissolving LiPF6 in a mixed solvent of fluoroethylene carbonate (FEC) and dimethyl carbonate (DMC) (volume ratio 2:8) and further adding triallyl phosphate (TP). The concentration of LiPF6 in the non-aqueous electrolyte was 1 mol / L. The content of TP in the non-aqueous electrolyte was as shown in Table 1.

[0087] [Fabrication of non-aqueous electrolyte secondary battery] An Al positive electrode lead was attached to the positive electrode intermediate obtained above. A Ni negative electrode lead was attached to the negative electrode obtained above. The positive electrode intermediate and the negative electrode were spirally wound in an inert gas atmosphere with a two-layer polyethylene thin film (separator) interposed between them to produce a wound electrode assembly. The electrode assembly was housed in a bag-shaped exterior body formed of a laminate sheet with an Al layer, and the nonaqueous electrolyte was injected. The exterior body was then sealed to produce a nonaqueous electrolyte secondary battery. When the electrode assembly was housed in the exterior body, portions of the positive electrode lead and the negative electrode lead were exposed to the outside from the exterior body. Within the battery, the nonaqueous electrolyte solution permeated into the positive electrode mixture layer, and TP adhered to the surface of the Al2O3 scattered in island-like patterns on the surfaces of the composite oxide particles, forming a coating material. In Table 1, the batteries of Examples 1 to 4 are designated A1 to A4, respectively.

[0088] Comparative Example 1 In the preparation of the positive electrode intermediate, the surfaces of the composite oxide particles in the positive electrode mixture layer were not coated with Al2O3. In the preparation of the non-aqueous electrolyte, TP was not added to the non-aqueous electrolyte. Other than the above, Battery B1 was prepared in the same manner as Battery A1 of Example 1.

[0089] Comparative Example 2 Battery B2 was produced in the same manner as Battery A1 in Example 1, except that TP was not added to the non-aqueous electrolyte in the preparation of the non-aqueous electrolyte.

[0090] Comparative Example 3 Battery B3 was produced in the same manner as Battery A2 in Example 2, except that in the production of the positive electrode intermediate, the surfaces of the composite oxide particles in the positive electrode mixture layer were not coated with Al2O3.

[0091] The batteries A1 to A4 of Examples 1 to 4 and the batteries B1 to B3 of Comparative Examples 1 to 3 were evaluated as follows.

[0092] Battery A4 was disassembled, the positive electrode removed and cleaned, and a cross-sectional photograph of the positive electrode active material particle was taken. Figure 2 is a cross-sectional TEM photograph of the first region of the positive electrode active material particle. Region (a) shows the surface of the composite oxide, region (b) shows the metal oxide coating material, and region (c) shows the phosphorus coating material. Figure 3 shows the amount of phosphorus (MO) detected by EDX analysis of the first region. Figure 4 is a cross-sectional TEM photograph of the second region of the positive electrode active material particle. Region (a) shows the surface of the composite oxide, and region (b) shows the region where PDVF is present. Figure 5 shows the amount of phosphorus (MS) detected by EDX analysis of the second region.

[0093] The cross-sectional photographs confirmed that the surface of composite oxide A (positive electrode active material) was uniformly dotted with island-shaped first regions (Fig. 2) coated with metal oxide B (Al2O3), and that second regions (Fig. 4), in which metal oxide B (Al2O3) was almost completely absent, were also present throughout the surface of composite oxide A.

[0094] As shown in Figure 3, it was confirmed that a large amount of P was present on the surface of the island-shaped metal oxide B (region (c)). On the other hand, as shown in Figure 5, it was confirmed that there was relatively little P in the second region. When normalized by the Al count number, the ratio of the P count number in region (c) in Figures 2 and 3 to that in region (b) in Figures 4 and 5 was 3.47:1.

[0095] [Evaluation 1: Capacity retention rate at 151 cycles] (1) First charge / discharge The battery was charged at a constant current of 0.2 C until the voltage reached 4.5 V, then charged at a constant voltage of 4.5 V until the current reached 0.05 C. It was then discharged at a constant current of 0.2 C until the voltage reached 2.5 V. The rest time between charges and discharges was 60 minutes. Charging and discharging were performed at 25°C.

[0096] (2)Second charge / discharge The battery was charged at a constant current of 0.3 C until the voltage reached 4.5 V, and then discharged at a constant current of 0.5 C until the voltage reached 2.5 V. The rest time between charge and discharge was 10 minutes. The charge and discharge were carried out at 25°C.

[0097] (3) Measurement of capacity retention at 151 cycles Six sets of the process, each consisting of one cycle of the first charge / discharge described in (1) above followed by 24 cycles of the second charge / discharge described in (2) above, were performed. That is, the first charge / discharge described in (1) above was performed at the 1st, 26th, 51st, 76th, 101st, 126th, and 151st cycles. The second charge / discharge described in (2) above was performed at the other cycles. The ratio of the discharge capacity of the first charge / discharge at the 151st cycle to the discharge capacity of the first charge / discharge at the 1st cycle was calculated as the capacity retention rate at the 151st cycle.

[0098] [Evaluation 2: Internal resistance at 101st cycle] After the first charge / discharge in the 101st cycle, battery A3 and batteries B1 to B3 were charged to 50% of their full charge capacity. Then, constant-current discharge was performed at a current I of 0.3 C for 30 seconds, and the voltage drop ΔV from the start of discharge to 30 seconds after the start of discharge was measured, and ΔV / I was calculated as the internal resistance.

[0099] [Evaluation 3: Metal elution after 151 cycles] After Evaluation 1, the battery was disassembled, and the two-layer polyethylene thin film (separator) and the negative electrode were removed. The negative electrode side of the two-layer polyethylene thin film was peeled off, and the total amount of metal derived from the positive electrode active material contained therein and metal derived from the positive electrode active material present in the negative electrode was measured as the amount of metal eluted from the positive electrode, and the mass ratio (ppm) of this to the positive electrode active material contained in the positive electrode was calculated. The evaluation results are shown in Table 1.

[0100] [Table 1]

[0101] Batteries A1 to A4 exhibited higher capacity retention rates than batteries B1 to B3. In battery B1, a coating material was not formed on the surface of composite oxide A, which resulted in contact between the nonaqueous electrolyte and the composite oxide particles, causing side reactions and metal elution, resulting in a decrease in capacity retention rate. In battery B2, the surface of composite oxide A was coated with Al2O3, but a phosphorus coating was not formed, resulting in a decrease in capacity retention rate. In battery B3, the surface of composite oxide A was coated with TP, but the surface of composite oxide A was not coated with Al2O3, resulting in an increase in internal resistance (resistance of the positive electrode) and a decrease in capacity retention rate. In battery A3, although the TP content was higher than in battery B3, the surface of composite oxide A was coated with a coating material, resulting in a decrease in internal resistance (resistance of the positive electrode) compared to battery B3. [Industrial Applicability]

[0102] The nonaqueous electrolyte secondary battery according to the present invention is suitable for use as a power source for mobile devices such as smartphones, a power source for vehicles such as electric cars, and a storage device for natural energy such as sunlight. While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention. [Explanation of symbols]

[0103] 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

Claims

1. A positive electrode, a negative electrode, and a non-aqueous electrolyte, the positive electrode has a positive electrode mixture containing positive electrode active material particles, the positive electrode active material particles include a composite oxide containing lithium and a transition metal, and a coating material that covers at least a portion of a surface of the composite oxide; the coating material includes a metal oxide and a phosphorus compound that covers at least a portion of a surface of the metal oxide; the composite oxide has a layered rock salt structure and contains Ni, Co, and at least one of Al and Mn; a surface of the composite oxide having a first region covered with the metal oxide in an island shape and a second region other than the first region;

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the first regions are scattered on the surface of the composite oxide.

3. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein an amount of phosphorus detected on the surface of said metal oxide, MO, and an amount of phosphorus detected in said second region, MS, satisfy the relationship MO>MS.

4. The nonaqueous electrolyte secondary battery according to claim 3 , wherein MO>3MS is satisfied.

5. 5. The nonaqueous electrolyte secondary battery according to claim 1, wherein the metal oxide contains at least one element selected from the group consisting of aluminum, silicon, titanium, magnesium, zirconium, niobium, germanium, calcium, and strontium.

6. 6. The nonaqueous electrolyte secondary battery according to claim 1, wherein the atomic ratio of the metal derived from the metal oxide to the Ni derived from the composite oxide on the outermost surface of the positive electrode mixture is 2 or less.

7. the non-aqueous electrolyte contains a phosphate ester compound, 7. The nonaqueous electrolyte secondary battery according to claim 1, wherein the phosphate ester compound has at least one alkenyl group in one molecule.

8. 8. The non-aqueous electrolyte secondary battery according to claim 7, wherein the content of the phosphate ester compound in the non-aqueous electrolyte is 2% by mass or less.

9. 9. The nonaqueous electrolyte secondary battery according to claim 7, wherein the alkenyl group includes at least one selected from the group consisting of a vinyl group, a 1-propenyl group, a 2-propenyl group, an isopropenyl group, a 1-butenyl group, a 2-butenyl group, and a 3-butenyl group.

10. 10. The non-aqueous electrolyte secondary battery according to claim 7, wherein the phosphate ester compound contains triallyl phosphate.

11. A positive electrode, a negative electrode, and a non-aqueous electrolyte, the positive electrode has a positive electrode mixture containing positive electrode active material particles, the positive electrode active material particles include a composite oxide containing lithium and a transition metal, and a coating material that covers at least a portion of a surface of the composite oxide; the coating material includes a metal oxide and a phosphorus compound that covers at least a portion of a surface of the metal oxide; the non-aqueous electrolyte contains a phosphate ester compound, The phosphate ester compound has at least one alkenyl group in one molecule, a surface of the composite oxide having a first region covered with the metal oxide in an island shape and a second region other than the first region;

12. A method for manufacturing a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, comprising: (1A) forming a positive electrode mixture layer including a composite oxide containing lithium and a transition metal on a surface of a positive electrode current collector; (1B) a step of attaching a metal oxide to at least a portion of the surface of the composite oxide in the positive electrode mixture layer to obtain a positive electrode intermediate; (2) A method for producing a nonaqueous electrolyte secondary battery, comprising: a step of bringing a phosphate ester compound into contact with a surface of the metal oxide to form a coating material containing the metal oxide and a phosphorus compound that covers at least a portion of the surface of the metal oxide, thereby obtaining a positive electrode.

13. 13. The method for producing a nonaqueous electrolyte secondary battery according to claim 12, wherein in the step of obtaining the positive electrode intermediate, the metal oxide is attached to the surface of the composite oxide by at least one method selected from the group consisting of a wet method, a spray coating method, a dip coating method, a chemical vapor deposition (CVD) method, an atomic layer deposition (ALD) method, and a sputtering method.

14. A method for manufacturing a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, comprising: (1a) a step of attaching a metal oxide to at least a portion of the surface of a composite oxide containing lithium and a transition metal; (1b) forming a positive electrode mixture layer containing the composite oxide to which the metal oxide is attached on the surface of a positive electrode current collector to obtain a positive electrode intermediate; (2) A method for producing a nonaqueous electrolyte secondary battery, comprising: a step of bringing a phosphate ester compound into contact with a surface of the metal oxide to form a coating material containing the metal oxide and a phosphorus compound that covers at least a portion of the surface of the metal oxide, thereby obtaining a positive electrode.

15. 15. The method for producing a nonaqueous electrolyte secondary battery according to claim 14, wherein the step of adhering the metal oxide is carried out by at least one method selected from the group consisting of a wet method, a spray coating method, a dip coating method, a chemical vapor deposition (CVD) method, and a sputtering method.

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