Coated active material, electrode, battery, and method for producing coated active material

Abstract

A coated active material 130 disclosed herein comprises an active material 110 and a covering layer 120 that covers at least a part of the surface of the active material 110, wherein: the covering layer 120 has a first layer 111 containing a first solid electrolyte and a second layer 112 containing a second solid electrolyte; the first layer 111 is positioned between the second layer 112 and the active material 110; the first solid electrolyte contains Ti and F; the second solid electrolyte contains Ti, F, and O; and the proportion of Ti-O bonds in a group of bonds to Ti contained in the second solid electrolyte is higher than the proportion of Ti-O bonds in a group of bonds to Ti contained in the first solid electrolyte.

Description

Coated active material, electrode, battery, and method for producing coated active material

[0001] The present disclosure relates to a coated active material, an electrode, a battery, and a method for producing the coated active material.

[0002] It is known that coating an active material with a solid electrolyte improves the durability of a battery. For example, the fluoride solid electrolyte disclosed in Patent Document 1 exhibits high ionic conductivity and provides good output characteristics to the battery.

[0003] International Publication No. 2021 / 186809

[0004] The solid electrolyte described in Patent Document 1 exhibits reactivity to oxygen, and therefore, if a solvent with a high oxygen content is used in the electrolyte solution, there is a possibility that the solid electrolyte may react with the electrolyte solution during charging and discharging of the battery. If the solid electrolyte coating the active material reacts with the electrolyte solution, the resistance of the battery increases and the output characteristics deteriorate. Therefore, there is a demand for a material that is less likely to deteriorate during charging and discharging of the battery.

[0005] The present disclosure provides a coated active material comprising: an active material; and a coating layer coating at least a portion of a surface of the active material, wherein the coating layer has a first layer containing a first solid electrolyte and a second layer containing a second solid electrolyte, the first layer being located between the second layer and the active material, the first solid electrolyte containing Ti and F, and the second solid electrolyte containing Ti, F, and O, wherein a ratio of Ti—O bonds in bonding groups for Ti contained in the second solid electrolyte is higher than a ratio of Ti—O bonds in bonding groups for Ti contained in the first solid electrolyte.

[0006] The coated active material of the present disclosure is less likely to be altered during charging and discharging of the battery, and can improve the output characteristics of the battery.

[0007] Fig. 1 is a cross-sectional view showing a schematic configuration of a coated active material according to a first embodiment of the present disclosure. Fig. 2 is a process diagram showing a method for producing a coated active material. Fig. 3 is a schematic cross-sectional view of a lithium secondary battery according to a second embodiment of the present disclosure. Fig. 4 is an XPS spectrum of the outermost surface of the coated active materials of Example 1 and Comparative Example 1. Fig. 5 is an XPS spectrum of LTAF used in producing the coated active material immediately after synthesis and after exposure to the atmosphere for 7 days.

[0008] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The present disclosure is not limited to the following embodiments.

[0009] (Embodiment 1) Fig. 1 is a cross-sectional view showing a schematic configuration of a coated active material according to embodiment 1 of the present disclosure. The coated active material 130 includes an active material 110 and a coating layer 120. The active material 110 is, for example, particulate in shape. The coating layer 120 coats at least a portion of the surface of the active material 110. The coating layer 120 includes a first layer 111 and a second layer 112. The first layer 111 is located between the second layer 112 and the active material 110. The first layer 111 is a layer containing a first solid electrolyte. The second layer 112 is a layer containing a second solid electrolyte.

[0010] The first solid electrolyte is a solid electrolyte containing Li, Ti, and F. The second solid electrolyte is a solid electrolyte containing Li, Ti, F, and O. The ratio of Ti—O bonds in the bonding groups for Ti contained in the second solid electrolyte is higher than the ratio of Ti—O bonds in the bonding groups for Ti contained in the first solid electrolyte.

[0011] The first solid electrolyte and the second solid electrolyte are fluoride solid electrolytes and have excellent oxidation resistance due to the high electronegativity of fluorine. When the active material 110 is coated with the coating layer 120, direct contact between the active material 110 and the electrolyte solution is prevented. In particular, according to this embodiment, the second solid electrolyte contained in the second layer 112 contains more Ti—O bonds than the first solid electrolyte contained in the first layer 111. That is, the ratio of Ti atoms bonded to oxygen atoms is relatively high in the surface portion of the coating layer 120. The presence of many Ti—O bonds suppresses the activity of the second solid electrolyte with respect to the electrolyte solution. Therefore, even when the electrolyte solution contains a solvent with a high oxygen content, the coating layer 120 is less likely to deteriorate during battery charge and discharge. As a result, the output characteristics of a battery using the coated active material 130 are improved. In particular, the retention rate of discharge capacity at high rates is improved.

[0012] The bonding groups to Ti refer to the bonding groups of anions to Ti. Titanium (Ti), fluorine (F), and oxygen (O) typically form the compounds TiF4, TiOF2, and TiO2. Therefore, the bonding groups to Ti are represented by Ti-F bonds, Ti-O-F bonds, and Ti-O bonds. By adjusting the ratio of Ti-O bonds in these bonding groups, a coated active material 130 having a coating layer 120 that is resistant to deterioration can be obtained.

[0013] The respective ratios of Ti—F bonds, Ti—O—F bonds, and Ti—O bonds can be calculated by the following method. First, XPS (X-ray Photoelectron Spectroscopy) measurement is performed on the coated active material 130. The Ti2p spectrum observed in the binding energy range of 448 eV to 486 eV is separated into peaks for TiF4, TiOF2, and TiO2. The TiF4 peak is near 462.0 eV. The TiOF2 peak is near 460.2 eV. The TiO2 peak is near 458.3 eV. The ratio of the area of ​​the peak attributed to TiF4 to the total area of ​​the separated peaks is calculated as the Ti—F bond ratio (unit: %). The ratio of the area of ​​the peak attributed to TiOF2 to the total area of ​​the separated peaks is calculated as the ratio of the Ti-O-F bond (unit: %). The ratio of the area of ​​the peak attributed to TiO2 to the total area of ​​the separated peaks is calculated as the ratio of the Ti-O bond (unit: %).

[0014] By performing XPS measurement on coated active material 130 without any special processing, the ratio of each bond in the second solid electrolyte can be determined. By performing XPS measurement after removing a portion of coating layer 120 by sputtering, the ratio of each bond in the first solid electrolyte can be determined.

[0015] In the second solid electrolyte contained in the second layer 112, the proportion of Ti—O bonds is, for example, greater than 5%. In other words, the sum of the proportion of Ti—F bonds and the proportion of Ti—O—F bonds is less than 95%. By making the proportion of Ti—O bonds in the second solid electrolyte greater than 5%, a coated active material 130 having a coating layer 120 that is resistant to deterioration can be obtained.

[0016] The proportion of Ti—O bonds in the second solid electrolyte may be 40% or more, 50% or more, or 80% or more. In other words, the sum of the proportion of Ti—F bonds and the proportion of Ti—O—F bonds may be 60% or less, 50% or less, or 20% or less. By sufficiently increasing the proportion of Ti—O bonds in the second solid electrolyte, the effect of suppressing deterioration of coating layer 120 is enhanced.

[0017] The upper limit of the ratio of Ti—O bonds in the second solid electrolyte is not particularly limited, and is, for example, 95%.

[0018] In the first solid electrolyte contained in the first layer 111, the sum of the ratio of Ti—F bonds and the ratio of Ti—O—F bonds is, for example, 63% or more. In other words, the ratio of Ti—O bonds is 37% or less. In the first solid electrolyte, a large sum of the ratio of Ti—F bonds and the ratio of Ti—O—F bonds means that the ratio of Ti atoms bonded to oxygen atoms is low. With this configuration, the coating layer 120 tends to exhibit high ionic conductivity.

[0019] The sum of the ratio of Ti—F bonds and the ratio of Ti—O—F bonds in the first solid electrolyte may be 85% or more. In other words, the ratio of Ti—O bonds may be 15% or less. By increasing the sum of the ratio of Ti—F bonds and the ratio of Ti—O—F bonds, coating layer 120 tends to exhibit high ionic conductivity.

[0020] The upper limit of the total ratio of Ti—F bonds and Ti—O—F bonds in the first solid electrolyte is not particularly limited, and is, for example, 98%.

[0021] In the present embodiment, the second layer 112 is a layer that includes the outermost surface of the coated active material 130. In other words, at least a portion of the outermost surface of the coated active material 130 is formed by the second layer 112. With this configuration, it is possible to effectively prevent the coating layer 120 from reacting with the electrolyte and being altered during charging and discharging of the battery.

[0022] In this embodiment, the first layer 111 is a layer that exists in a region that is 1 nm or more away from the outermost surface of the coated active material 130. This configuration effectively prevents the coating layer 120 from reacting with the electrolyte and being altered during charging and discharging of the battery. The thickness of the second layer 112 can be 1 nm or more, but the thickness of the second layer 112 may be less than 1 nm.

[0023] The above-described relationship that holds between the first solid electrolyte contained in the first layer 111 and the second solid electrolyte contained in the second layer 112 can also hold between the outermost surface of the coated active material 130 and an intermediate position in the thickness direction of the coating layer 120. That is, the ratio of Ti—O bonds in the bonding groups for Ti contained in the outermost surface of the coated active material 130 is higher than the ratio of Ti—O bonds in the bonding groups for Ti contained in the coating layer 120 at an intermediate position between the outermost surface of the coated active material 130 and the surface of the active material 110. The ratio of Ti—O bonds in the bonding groups for Ti contained in the outermost surface of the coated active material 130 is equal to the ratio of Ti—O bonds in the bonding groups for Ti contained in the second solid electrolyte. The ratio of Ti—O bonds to the bonding groups for Ti contained in coating layer 120 at an intermediate position between the outermost surface of coated active material 130 and the surface of active material 110 is equal to the ratio of Ti—O bonds to the bonding groups for Ti contained in the first solid electrolyte.

[0024] The coating layer 120 may have a surface-most portion and a portion that forms the interface between the coating layer and the active material, with the intermediate position in the thickness direction as a reference. The ratio of Ti—O bonds in the bonding groups for Ti contained in the surface-most portion may be higher than the ratio of Ti—O bonds in the bonding groups for Ti contained in the portion that forms the interface. With this configuration, the coating layer 120 is less likely to be altered during charging and discharging of the battery.

[0025] In the structure shown in FIG. 1, the portion on the outermost surface side may correspond to the second layer 112 , and the portion constituting the interface between the coating layer 120 and the active material 110 may correspond to the first layer 111 .

[0026] The proportion of Ti—O bonds at the outermost surface of the coated active material 130 is, for example, greater than 5%. In other words, the sum of the proportion of Ti—F bonds and the proportion of Ti—O—F bonds is less than 95%. By making the proportion of Ti—O bonds at the outermost surface greater than 5%, a coated active material 130 having a coating layer 120 that is resistant to deterioration can be obtained.

[0027] The ratio of Ti—O bonds at the outermost surface of the coated active material 130 may be 40% or more, 50% or more, or 80% or more. In other words, the sum of the ratio of Ti—F bonds and the ratio of Ti—O—F bonds may be 60% or less, 50% or less, or 20% or less. By sufficiently increasing the ratio of Ti—O bonds at the outermost surface, the effect of suppressing alteration of the coating layer 120 is enhanced.

[0028] The upper limit of the ratio of Ti—O bonds on the outermost surface is not particularly limited, and is, for example, 95%.

[0029] At the intermediate position of the coating layer 120, the sum of the ratio of Ti—F bonds and the ratio of Ti—O—F bonds is, for example, 63% or more. In other words, the ratio of Ti—O bonds is 37% or less. A large sum of the ratio of Ti—F bonds and the ratio of Ti—O—F bonds at the intermediate position means that the ratio of Ti atoms bonded to oxygen atoms is low. With this configuration, the coating layer 120 tends to exhibit high ionic conductivity.

[0030] At the intermediate position of the coating layer 120, the sum of the ratio of Ti—F bonds and the ratio of Ti—O—F bonds may be 85% or more. In other words, the ratio of Ti—O bonds may be 15% or less. By increasing the sum of the ratio of Ti—F bonds and the ratio of Ti—O—F bonds, the coating layer 120 tends to exhibit high ionic conductivity.

[0031] The upper limit of the total of the ratio of Ti—F bonds and the ratio of Ti—O—F bonds at the intermediate position of the coating layer 120 is not particularly limited, and is, for example, 98%.

[0032] The first solid electrolyte and the second solid electrolyte will now be described in detail.

[0033] The first solid electrolyte and the second solid electrolyte each have lithium ion conductivity. The coated active material 130 is suitable for lithium ion secondary batteries.

[0034] The first solid electrolyte may consist essentially of Li, Ti, and F. "The first solid electrolyte consists essentially of Li, Ti, and F" means that the molar ratio (i.e., molar fraction) of the total amount of substance of Li, Ti, and F to the total amount of substance of all elements constituting the first solid electrolyte is 90% or more. In one example, the molar ratio may be 95% or more. The first solid electrolyte may consist only of Li, Ti, and F.

[0035] The first solid electrolyte may further contain at least one element selected from the group consisting of Ca, Mg, Al, Y, and Zr. By containing these elements, the ionic conductivity of the first solid electrolyte can be improved. From the viewpoints of cost and ionic conductivity, Al is preferred.

[0036] The first solid electrolyte may consist essentially of Li, Ti, M, and F. M is at least one selected from the group consisting of Ca, Mg, Al, Y, and Zr. "The first solid electrolyte consists essentially of Li, Ti, M, and F" means that the molar ratio (i.e., molar fraction) of the total amount of substance of Li, Ti, M, and F to the total amount of substance of all elements constituting the first solid electrolyte is 90% or more. In one example, the molar ratio may be 95% or more. The first solid electrolyte may consist only of Li, Ti, M, and F.

[0037] However, the first solid electrolyte may contain unavoidably mixed elements, such as hydrogen, oxygen, and nitrogen, which may be contained in the raw material powder of the first solid electrolyte or may be present in the atmosphere used for producing and storing the first solid electrolyte.

[0038] To further increase the ionic conductivity of the first solid electrolyte, the ratio of the amount of substance of Li to the total amount of substance of Ti and M may be 1.7 or more and 4.2 or less.

[0039] The first solid electrolyte has, for example, a composition represented by the following formula (1): Formula (1) satisfies 0≦x<1 and 0<b≦2. When the first solid electrolyte has a composition represented by the following formula (1), the first solid electrolyte exhibits good ionic conductivity.

[0040] Li 6-(4-x)b (Ti 1-x M x ) b F6...Formula (1)

[0041] In order to increase the ionic conductivity of the first solid electrolyte, formula (1) may satisfy 0.1≦x≦0.9.

[0042] In order to increase the ionic conductivity of the first solid electrolyte, formula (1) may satisfy 0.8≦b≦1.2.

[0043] The first solid electrolyte may have a crystalline phase or may contain an amorphous phase.

[0044] In order to increase ionic conductivity, the first solid electrolyte may contain anions other than F. The anions other than F are at least one selected from the group consisting of Cl, Br, I, and Se.

[0045] The second solid electrolyte may consist essentially of Li, Ti, F, and O. "The second solid electrolyte consists essentially of Li, Ti, F, and O" means that the molar ratio (i.e., molar fraction) of the total amount of substance of Li, Ti, F, and O to the total amount of substance of all elements constituting the second solid electrolyte is 90% or more. In one example, the molar ratio may be 95% or more. The second solid electrolyte may consist only of Li, Ti, F, and O.

[0046] The second solid electrolyte may further contain at least one element selected from the group consisting of Ca, Mg, Al, Y, and Zr. By containing these elements, the ionic conductivity of the second solid electrolyte can be improved. From the viewpoints of cost and ionic conductivity, Al is preferred.

[0047] The second solid electrolyte may consist essentially of Li, Ti, M, F, and O. M is at least one selected from the group consisting of Ca, Mg, Al, Y, and Zr. "The second solid electrolyte consists essentially of Li, Ti, M, F, and O" means that the molar ratio (i.e., molar fraction) of the total amount of substance of Li, Ti, M, F, and O to the total amount of substance of all elements constituting the second solid electrolyte is 90% or more. In one example, the molar ratio may be 95% or more. The second solid electrolyte may consist only of Li, Ti, M, F, and O.

[0048] However, the second solid electrolyte may contain elements that are inevitably mixed in. Examples of such elements include hydrogen and nitrogen.

[0049] The second solid electrolyte is obtained by coating the active material 110 with the first solid electrolyte and then oxidizing a portion of the first solid electrolyte. In this case, the composition of the second solid electrolyte, excluding oxygen, is identical to the composition of the first solid electrolyte. According to this method, the process of coating the active material 110 with the solid electrolyte is completed in one step.

[0050] However, the composition of the second solid electrolyte when oxygen is ignored may be different from the composition of the first solid electrolyte when oxygen is ignored. The constituent elements of the second solid electrolyte other than oxygen may be different from the constituent elements of the first solid electrolyte other than oxygen. For example, by coating the active material 110 with a first solid electrolyte having a low Ti—O bond ratio and then coating the active material 110 with a second solid electrolyte having a high Ti—O bond ratio, a first layer 111 containing the first solid electrolyte and a second layer 112 containing the second solid electrolyte can be formed in this order on the active material 110.

[0051] The first solid electrolyte and the second solid electrolyte preferably do not contain sulfur. A sulfur-free solid electrolyte does not generate hydrogen sulfide even when exposed to the atmosphere, and therefore is highly safe.

[0052] The coating layer 120 may cover only a portion of the surface of the particles of the active material 110. In this case, the particles of the active material 110 come into direct contact with each other via the portions not covered by the coating layer 120, thereby ensuring electronic conductivity between the particles of the active material 110. However, the coating layer 120 may also cover the surfaces of the particles of the active material 110 uniformly.

[0053] The first layer 111 of the coating layer 120 may contain the first solid electrolyte as a main component, or may contain only the first solid electrolyte. "Main component" refers to the component that is contained in the largest amount by mass. "Containing only the first solid electrolyte" means that, with the exception of inevitable impurities, no materials other than the first solid electrolyte are intentionally added.

[0054] The second layer 112 of the coating layer 120 may contain the second solid electrolyte as a main component, or may contain only the second solid electrolyte. "Main component" refers to the component that is contained in the largest amount by mass. "Containing only the second solid electrolyte" means that, with the exception of inevitable impurities, no materials other than the second solid electrolyte are intentionally added.

[0055] The thickness of the coating layer 120 is preferably 1 nm or more and 500 nm or less, more preferably 1 nm or more and 100 nm or less, and even more preferably 1 nm or more and 20 nm or less. When the thickness of the coating layer 120 is appropriately adjusted, contact between the active material 110 and the electrolyte solution can be sufficiently suppressed. The thickness of the coating layer 120 can be determined by slicing the coated active material 130 using a method such as ion milling and observing the cross section of the coated active material 130 with a transmission electron microscope. The average value of thicknesses measured at any multiple positions (e.g., five points) can be considered to be the thickness of the coating layer 120. Observation with a transmission electron microscope allows the position of the interface between the coating layer 120 and the active material 110 in the coated active material 130 to be identified.

[0056] The active material 110 includes a material that has the property of absorbing and releasing metal ions such as lithium ions.

[0057] In this embodiment, the active material 110 may be an oxide. The active material 110 may be an oxide that does not contain Ti, except in cases where Ti is unavoidably mixed in.

[0058] When the active material 110 is a positive electrode active material, the active material 110 may be a lithium-containing transition metal oxide, a lithium-containing transition metal phosphate, a transition metal fluoride, a polyanion material, a fluorinated polyanion material, a transition metal sulfide, a transition metal oxysulfide, a transition metal oxynitride, or the like. In particular, when a lithium-containing transition metal oxide or a lithium-containing transition metal phosphate is used as the active material 110, the manufacturing cost of the battery can be reduced and the average discharge voltage can be increased. Examples of lithium-containing transition metal oxides include lithium cobalt oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, and lithium nickel manganese oxide. Examples of lithium-containing transition metal phosphates include lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, and lithium nickel phosphate.

[0059] The active material 110 may be a negative electrode active material, such as lithium titanate, graphite, silicon, a silicon compound, or a NiBi alloy.

[0060] There are no particular limitations on the shape of the particles of the active material 110. The shape of the particles of the active material 110 may be spherical, oval, scaly, or fibrous.

[0061] 2 is a process chart showing a method for producing the coated active material 130. According to the production method of this embodiment, the coated active material 130 can be produced efficiently.

[0062] In step S1, the active material 110 is coated with a solid electrolyte containing Li, Ti, and F. The solid electrolyte containing Li, Ti, and F is a first solid electrolyte.

[0063] A solid electrolyte containing Li, Ti, and F can be synthesized, for example, by the following method. First, raw material powders are mixed so as to obtain a solid electrolyte having a desired composition. The raw material powders are, for example, fluorides of the respective elements. If the desired composition is Li, Ti, and F, 2.7 Ti 0.3 Al 0.7In the case of F, LiF, TiF, and AlF are mixed in a molar ratio of 2.7:0.3:0.7. The raw material powders are then reacted with each other using a mixing device such as a planetary ball mill. That is, the raw material powders are reacted with each other by mechanochemical milling. This results in a solid electrolyte having the desired composition.

[0064] The process of coating the active material 110 with the solid electrolyte is, for example, as follows: A powder of the active material 110 and a powder of the solid electrolyte are mixed in an appropriate ratio to obtain a mixture. The mixture is milled to impart mechanical energy to the mixture. As a result, the active material 110 is coated with the solid electrolyte. A mixing device such as a ball mill can be used for the milling process. To suppress oxidation of the material, the milling process may be performed in a dry and inert atmosphere.

[0065] In step S2, an oxygen-containing liquid is applied to the active material 110 coated with the solid electrolyte. The oxygen-containing liquid is used to oxidize a portion of the solid electrolyte layer and increase the Ti—O bonds. When heat treatment is performed with the oxygen-containing liquid applied, the portion of the solid electrolyte layer can be efficiently oxidized.

[0066] The oxygen-containing liquid includes, for example, water. Water forms a bond with Ti and can efficiently oxidize the solid electrolyte through heat treatment. The oxygen-containing liquid may be ethanol, N-methylpyrrolidone, or the like. The oxygen-containing liquid may include multiple components.

[0067] In step S3, the active material 110 coated with the solid electrolyte is heat-treated so as to remove the liquid containing oxygen, thereby obtaining the coated active material 130 of this embodiment.

[0068] The ambient temperature in the heat treatment is, for example, 180° C. or higher. At such a temperature, a portion of the solid electrolyte layer can be efficiently oxidized. The upper limit of the ambient temperature in the heat treatment is not particularly limited, and is, for example, 300° C.

[0069] 3 is a schematic cross-sectional view of a lithium secondary battery according to a second embodiment of the present disclosure. The lithium secondary battery 100 includes a positive electrode 53, a negative electrode 56, an electrolyte layer 57, an exterior casing 58, and a non-aqueous electrolyte solution 59. The positive electrode 53 includes a positive electrode current collector 51 and a positive electrode active material layer 52. The positive electrode active material layer 52 is disposed on the positive electrode current collector 51. The negative electrode 56 includes a negative electrode current collector 54 and a negative electrode active material layer 55. The negative electrode active material layer 55 is disposed on the negative electrode current collector 54. An electrolyte layer 57 is disposed between the positive electrode 53 and the negative electrode 56. The electrolyte layer 57 is a separator. The positive electrode 53, the negative electrode 56, the electrolyte layer 57, and the non-aqueous electrolyte solution 59 are housed in an exterior casing 58.

[0070] The positive electrode 53 or the negative electrode 56 contains the coated active material 130 of Embodiment 1. Specifically, the positive electrode active material layer 52 or the negative electrode active material layer 55 contains the coated active material 130 of Embodiment 1. The coated active material 130 has excellent durability against the nonaqueous electrolyte solution 59, and therefore improves the output characteristics of the lithium secondary battery 100.

[0071] The coated active material 130 is typically included in the positive electrode 53 .

[0072] The positive electrode current collector 51 is a foil made of a metal material such as aluminum, stainless steel, titanium, or an alloy thereof. The negative electrode current collector 54 is a foil made of a metal material such as stainless steel, nickel, copper, or an alloy thereof.

[0073] The positive electrode active material layer 52 and the negative electrode active material layer 55 may contain a conductive additive, an ion conductor, a binder, and the like.

[0074] The non-aqueous electrolyte 59 is impregnated into the positive electrode 53, the negative electrode 56, and the electrolyte layer 57. The non-aqueous electrolyte 59 may fill the internal space of the exterior casing 58.

[0075] The non-aqueous electrolyte 59 contains a non-aqueous solvent and a lithium salt.

[0076] Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, esters, cyclic ethers, chain ethers, nitriles, amides, and lactones. One solvent selected from these may be used, or two or more may be used in combination. Examples of solvents with a high oxygen content include lactones. Specific examples of lactones include γ-butyrolactone. When lactones are contained in the electrolyte solution, the coated active material 130 of the present disclosure can exhibit greater effectiveness. "Lactones" refers to compounds having a lactone ring. A lactone ring refers to a heterocycle containing an ester group within the ring.

[0077] Examples of lithium salts include lithium hexafluorophosphate (LiPF), lithium tetrafluoroborate (LiBF), lithium perchlorate (LiClO), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bisperfluoroethylsulfonylimide (LiN(SO2C2F5)2), LiAsF6, LiCF3SO3, and lithium difluoro(oxalato)borate. One selected from these electrolyte salts may be used, or two or more may be used in combination.

[0078] The electrolyte layer 57 has lithium ion conductivity. There are no particular limitations on the material of the electrolyte layer 57 as long as it allows the passage of lithium ions. The material of the electrolyte layer 57 can be at least one selected from the group consisting of solid electrolytes, gel electrolytes, ion exchange resin membranes such as lithium cation exchange resins, semipermeable membranes, and porous membranes. If the electrolyte layer 57 is made of these materials, the safety of the lithium secondary battery 100 can be sufficiently ensured. Examples of solid electrolytes include sulfide solid electrolytes such as Li2S-P2S5, Li7La3Zr2O 12Examples of the electrolyte layer 57 include oxide solid electrolytes such as LLZ. Examples of the gel electrolyte include gel electrolytes containing fluororesins such as PVdF. Examples of the ion exchange resin membrane include cation exchange membranes and anion exchange membranes. Examples of the porous membrane include porous membranes made of polyolefin resin and porous membranes made of glass paper obtained by weaving glass fibers into nonwoven fabric. The electrolyte layer 57 has a thickness of, for example, 0.001 μm or more and 500 μm or less.

[0079] There is no particular limitation on the shape of the lithium secondary battery 100. The lithium secondary battery 100 may have various shapes such as a coin type, a cylindrical type, a square type, a sheet type, a button type, a flat type, and a laminated type.

[0080] The lithium secondary battery 100 may be a solid-state battery that does not use a non-aqueous electrolyte. Even in a solid-state battery, the coated active material 130 may be altered by other materials such as a solid electrolyte. Suppressing the alteration of the coated active material 130 improves the output characteristics of the lithium secondary battery.

[0081] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0082] (Technology 1) A coated active material comprising: an active material; and a coating layer that coats at least a portion of a surface of the active material, wherein the coating layer has a first layer containing a first solid electrolyte and a second layer containing a second solid electrolyte, the first layer being located between the second layer and the active material, the first solid electrolyte containing Li, Ti, and F, and the second solid electrolyte containing Li, Ti, F, and O, wherein a ratio of Ti—O bonds in bonding groups for Ti contained in the second solid electrolyte is higher than a ratio of Ti—O bonds in bonding groups for Ti contained in the first solid electrolyte.

[0083] The coated active material of the present disclosure is less likely to be altered during charging and discharging of the battery, and can improve the output characteristics of the battery.

[0084] (Technology 2) The coated active material according to Technology 1, wherein the ratio of the Ti—O bonds in the second solid electrolyte is greater than 5%. By increasing the ratio of the Ti—O bonds in the second solid electrolyte to greater than 5%, a coated active material having a coating layer that is resistant to deterioration can be obtained.

[0085] (Technology 3) The coated active material according to Technology 1 or 2, wherein in the first solid electrolyte, the sum of the ratio of Ti—F bonds and the ratio of Ti—O—F bonds is 63% or more. With this configuration, the coating layer tends to have high ionic conductivity.

[0086] (Technology 4) The coated active material according to any one of Technologies 1 to 3, wherein the first solid electrolyte and the second solid electrolyte further contain at least one element selected from the group consisting of Ca, Mg, Al, Y, and Zr. By containing these elements, the ionic conductivity of the first solid electrolyte can be improved.

[0087] (Technology 5) The coated active material according to any one of Technologies 1 to 4, wherein the second layer is a layer including the outermost surface of the coated active material. With this configuration, it is possible to effectively prevent the coating layer from reacting with the electrolyte and being altered during charging and discharging of the battery.

[0088] (Technology 6) The coated active material according to any one of Technologies 1 to 5, wherein the first layer is a layer present in a region at a distance of 1 nm or more from the outermost surface of the coated active material. With this configuration, it is possible to effectively prevent the coating layer from reacting with the electrolyte and being altered during charging and discharging of the battery.

[0089] (Technology 7) The coated active material according to any one of Technologies 1 to 6, wherein the coating layer has a thickness of 1 nm to 500 nm. When the thickness of the coating layer is appropriately adjusted, contact between the active material and the electrolyte solution can be sufficiently suppressed.

[0090] (Technology 8) A coated active material comprising: an active material; and a coating layer that coats at least a portion of the surface of the active material, wherein a ratio of Ti—O bonds in the bonding groups for Ti contained in the outermost surface of the coated active material is higher than a ratio of Ti—O bonds in the bonding groups for Ti contained in the coating layer at an intermediate position between the outermost surface and the surface of the active material.

[0091] The coated active material of the present disclosure is less likely to be altered during charging and discharging of the battery, and can improve the output characteristics of the battery.

[0092] (Technology 9) The coating layer has, with respect to the intermediate position as a reference, the outermost surface portion and a portion that constitutes the interface between the coating layer and the active material, and the ratio of Ti—O bonds in the bonding groups for Ti contained in the outermost surface portion is higher than the ratio of Ti—O bonds in the bonding groups for Ti contained in the portion that constitutes the interface, in the coated active material according to Technology 8. With this configuration, the coating layer is less likely to deteriorate during charge and discharge of the battery.

[0093] (Technology 10) The coated active material according to Technology 8 or 9, wherein the proportion of the Ti—O bonds on the outermost surface is greater than 5%. By making the proportion of the Ti—O bonds on the outermost surface greater than 5%, a coated active material having a coating layer that is resistant to deterioration can be obtained.

[0094] (Technology 11) The coated active material according to any one of Technologies 8 to 10, wherein the sum of the ratio of Ti—F bonds and the ratio of Ti—O—F bonds at the intermediate position is 63% or more. With this configuration, the coating layer tends to have high ionic conductivity.

[0095] (Technology 12) The coated active material according to any one of Technologies 8 to 11, wherein the coating layer further contains at least one element selected from the group consisting of Ca, Mg, Al, Y, and Zr. By containing these elements, the ionic conductivity of the coating layer can be improved.

[0096] (Technology 13) An electrode comprising the coated active material according to any one of Technologies 1 to 12. The coated active material has excellent durability against an electrolyte solution, and therefore improves the output characteristics of a battery.

[0097] (Technology 14) A battery comprising the electrode according to Technology 13. The coated active material has excellent durability against the electrolyte, and therefore improves the output characteristics of the battery.

[0098] (Technology 15) The battery according to Technology 14, further comprising an electrolyte solution containing lactones. When the electrolyte solution contains lactones, the coated active material of the present disclosure can exhibit a higher effect.

[0099] (Technology 16) A method for producing a coated active material, comprising: coating an active material with a solid electrolyte containing Li, Ti, and F; attaching a liquid containing oxygen to the active material coated with the solid electrolyte; and heat-treating the active material coated with the solid electrolyte so as to remove the liquid.

[0100] According to the manufacturing method of the present disclosure, the coated active material can be manufactured efficiently.

[0101] (Technology 17) The method for producing a coated active material according to Technology 16, wherein the liquid contains water. Water forms bonds with Ti and F, and can efficiently oxidize the solid electrolyte through heat treatment.

[0102] (Technology 18) The method for producing a coated active material according to Technology 16 or 17, wherein the ambient temperature in the heat treatment is 180° C. or higher. At such a temperature, a part of the solid electrolyte layer can be efficiently oxidized.

[0103] Hereinafter, the present disclosure will be described in detail using examples and comparative examples. However, the present disclosure is not limited to the following examples.

[0104] Example 1 [Preparation of Fluoride Solid Electrolyte] In an argon atmosphere with a dew point of −60°C or lower, raw material powders LiF, TiF, and AlF were weighed in a molar ratio of LiF:TiF:AlF = 2.7:0.3:0.7. These were placed in a 45 cc planetary ball mill pod along with 25 g of 0.5 mm diameter balls. γ-Butyrolactone (GBL) was added dropwise to the pod to achieve a solids content of 50% by mass. The solids content was calculated by {(mass of raw materials) / (mass of raw materials + mass of solvent)} × 100. The mixture was milled for 12 hours at 500 rpm using a planetary ball mill. After milling, the balls were separated to obtain a slurry. The slurry was dried using a mantle heater under nitrogen flow at 200°C for 1 hour. The obtained solid was pulverized in a mortar to obtain a powder of a fluoride solid electrolyte. 2.7 Ti 0.3 Al 0.7 F6 (hereinafter referred to as "LTAF").

[0105] [Preparation of Coated Active Material] Powder of LiCoO2 (hereinafter referred to as "LCO") was prepared as the positive electrode active material. Next, a coating layer made of LTAF was formed on the surface of the LCO. The coating layer was formed by a compressive shear treatment using a particle composite device (NOB-MINI, manufactured by Hosokawa Micron Corporation). Specifically, LCO and LTAF were weighed to have a volume ratio of 100:3, and treated under the following conditions: blade clearance: 2 mm, rotation speed: 6000 rpm, and treatment time: 30 min. This resulted in an LCO coated with LTAF. The thickness of the coating layer was measured using the method described above. The thickness of the coating layer was 25 nm.

[0106] The LTAF-coated LCO was mixed with water and kneaded for 1 hour. The mass ratio of the LTAF-coated LCO to water was 10:1. The LTAF-coated LCO was then heat-treated at an ambient temperature of 200°C for 24 hours. This gave the coated active material of Example 1.

[0107] Comparative Example 1 A coated active material of Comparative Example 1 was produced in the same manner as in Example 1, except that the step of kneading with water and heat treating was omitted.

[0108] [XPS Measurement of Coated Active Material] XPS measurement of the coated active materials of Example 1 and Comparative Example 1 was carried out under the following conditions. For the XPS measurement, a Quantera SXM (manufactured by ULVAC-PHI, Inc.) was used. The measurement conditions were as follows.

[0109] X-ray source: Al monochromator (25W, 15kV) Electron / ion neutralization gun: ON Photoelectron take-off angle: 45 degrees

[0110] Ti was selected as the measurement element, and the binding energy scan range was set to 450 eV to 480 eV (Ti 2p orbital). Peak separation of the XPS spectrum in the range of 455 eV to 486 eV was performed, and the area of ​​each peak was calculated. Specifically, the spectrum was separated into peaks attributable to Ti-F bonds, Ti-O-F bonds, and Ti-O bonds, and the area of ​​each peak (integrated peak area) was calculated. The ratio (unit: %) of the area of ​​the peak attributable to Ti-O bonds to the total area of ​​the peaks was calculated. The ratio (unit: %) of the area of ​​the peak attributable to Ti-O-F bonds to the total area of ​​the peaks was calculated. The ratio (unit: %) of the area of ​​the peak attributable to Ti-F bonds to the total area of ​​the peaks was calculated. In analyzing the XPS spectrum, peak fitting was performed using a Gauss-Lorentz function.

[0111] FIG. 4 shows XPS spectra of the outermost surfaces of the coated active materials of Example 1 and Comparative Example 1.

[0112] After XPS measurement of the outermost surface of the coated active material of Example 1 and Comparative Example 1, the surface layer of the coated active material was removed by argon sputtering. Then, XPS measurement was performed again. By repeating argon sputtering and XPS measurement, XPS measurement was performed at depths of 1 nm, 3 nm, 5 nm, 10 nm, 15 nm, 20 nm, and 25 nm from the outermost surface. The results are shown in Table 1.

[0113]

[0114] As shown in Table 1, the proportion of Ti—O bonds on the outermost surface of the coated active material of Example 1 was as high as 80%, while the proportion of Ti—O bonds on the outermost surface of the coated active material of Comparative Example 1 was as low as 5%. It is presumed that the Ti—O bonds on the outermost surface of the coated active material of Comparative Example 1 are due to natural oxidation.

[0115] Because the active material was an oxide, Ti—O bonds were observed at depths of 1 nm to 25 nm in both Example 1 and Comparative Example 1. In other words, it is presumed that Ti—O bonds were formed when the LTAF particles were strongly pressed against the surfaces of the LCO particles during the preparation of the coated active material. The ratio of Ti—O bonds at the outermost surface of the coated active material of Comparative Example 1 was lower than the ratio of Ti—O bonds at depths of 1 nm, 3 nm, and 5 nm. In contrast, the ratio of Ti—O bonds at the outermost surface of the coated active material of Example 1 was significantly higher than the ratio of Ti—O bonds at depths of 1 nm, 3 nm, and 5 nm. The ratio of Ti—O bonds at the outermost surface of the coated active material of Example 1 was more than two times higher than the ratio of Ti—O bonds at a depth of 1 nm. Therefore, in Example 1, the interface between the first layer and the second layer is located in a region less than 1 nm away from the outermost surface of the coated active material, and the thickness of the second layer in Example 1 is less than 1 nm. The ratios of Ti—O bonds at depths of 1 nm, 3 nm, and 5 nm in Example 1 were greater than the ratios of Ti—O bonds at depths of 1 nm, 3 nm, and 5 nm in Comparative Example 1. Note that the depth of 25 nm is the result of observing the vicinity of the interface between the coating layer and the active material.

[0116] [Fabrication of Lithium Secondary Battery] The coated active material of Example 1, acetylene black as a conductive additive, polytetrafluoroethylene as a binder, and a solvent were mixed to obtain a positive electrode mixture paste. The ratio of the coated active material, acetylene black, and polytetrafluoroethylene, by mass, was coated active material:acetylene black:polytetrafluoroethylene=90:5:5. The positive electrode mixture paste was dried to obtain a positive electrode mixture.

[0117] 100 mg of the positive electrode mixture was subjected to 10 kN / cm 2The mixture was pressed at a pressure of 1000 kJ / cm2 to obtain a pellet (diameter: 10 mm). The pellet was dried at 200° C. to obtain the positive electrode of Example 1.

[0118] The positive electrode of Example 1, a polypropylene nonwoven fabric, and a negative electrode were placed in this order inside a coin case. 90 mg of electrolyte was poured into the coin case. The coin case was sealed with a lid to obtain a lithium secondary battery of Example 1. The electrolyte was prepared by dissolving lithium fluoroborate (LiBF4) in γ-butyrolactone. The concentration of lithium fluoroborate in the electrolyte was 1.0 mol / L. Metallic Li was used for the negative electrode.

[0119] A lithium secondary battery of Comparative Example 1 was fabricated in the same manner as in Example 1, except that the coated active material of Comparative Example 1 was used instead of the coated active material of Example 1.

[0120] [Charge / Discharge Test] A charge / discharge test was carried out on the lithium secondary batteries of Example 1 and Comparative Example 1 according to the following procedure. At an ambient temperature of 25°C, constant current charging was carried out at a current value of 0.02 C until the voltage reached 4.5 V. After a rest period of 1 hour, constant current discharging was carried out at a current value of 2 C until the voltage reached 3.0 V, and the initial discharge capacity was measured. The 2C discharge capacities of the lithium secondary batteries of Example 1 and Comparative Example 1 were as shown in Table 1. The 2C discharge capacity shown in Table 1 is the discharge capacity per unit mass of coated active material.

[0121]

[0122] As shown in Table 2, the discharge capacity of the lithium secondary battery of Example 1 at a 2C rate was significantly greater than the discharge capacity of the lithium secondary battery of Comparative Example 1 at a 2C rate.

[0123] Reference Example XPS measurement was carried out on LTAF used in the preparation of the coated active material immediately after synthesis and after exposure to the atmosphere for 7 days. The results are shown in FIG.

[0124] As can be seen from the two spectra shown in Figure 5, the change in the LTAF spectrum before and after 7 days of air exposure was minimal. In other words, the change caused by oxidation treatment using water and heat, as in Example 1, is significantly different from the change caused by air exposure. Even if the coated active material of Comparative Example 1 is exposed to air for a long period of time, it is predicted that the ratio of Ti-O bonds will not change significantly from the value (5%) shown in Table 1.

[0125] The techniques of the present disclosure are useful for coating active materials used in batteries.

Claims

1. A coated active material comprising: an active material; and a coating layer coating at least a portion of a surface of the active material, wherein the coating layer has a first layer containing a first solid electrolyte and a second layer containing a second solid electrolyte, the first layer being located between the second layer and the active material, the first solid electrolyte containing Li, Ti, and F, and the second solid electrolyte containing Li, Ti, F, and O, a ratio of Ti—O bonds in bonding groups for Ti contained in the second solid electrolyte is higher than a ratio of Ti—O bonds in bonding groups for Ti contained in the first solid electrolyte.

2. The coated active material according to claim 1, wherein the ratio of the Ti--O bonds in the second solid electrolyte is greater than 5%.

3. The coated active material according to claim 1, wherein in said first solid electrolyte, the sum of the ratio of Ti-F bonds and the ratio of Ti-O-F bonds is 63% or more.

4. The coated active material according to claim 1, wherein the first solid electrolyte and the second solid electrolyte further contain at least one element selected from the group consisting of Ca, Mg, Al, Y and Zr.

5. The coated active material according to claim 1, wherein the second layer is a layer including the outermost surface of the coated active material.

6. The coated active material according to claim 1, wherein the first layer is a layer present in an area at a distance of 1 nm or more from the outermost surface of the coated active material.

7. The coated active material according to claim 1, wherein the coating layer has a thickness of 1 nm or more and 500 nm or less.

8. A coated active material comprising: an active material; and a coating layer covering at least a portion of a surface of the active material, wherein a ratio of Ti—O bonds in a group of bonds for Ti contained in an outermost surface of the coated active material is higher than a ratio of Ti—O bonds in a group of bonds for Ti contained in the coating layer at an intermediate position between the outermost surface and the surface of the active material.

9. The coated active material according to claim 8, wherein the coating layer has, with respect to the intermediate position, a portion on the outermost surface side and a portion that constitutes an interface between the coating layer and the active material, and a ratio of Ti—O bonds in the bonds for Ti contained in the portion on the outermost surface side is higher than a ratio of Ti—O bonds in the bonds for Ti contained in the portion that constitutes the interface.

10. The coated active material according to claim 8, wherein the ratio of the Ti--O bonds on the outermost surface is greater than 5%.

11. The coated active material according to claim 8, wherein the sum of the ratio of Ti--F bonds and the ratio of Ti--O--F bonds at the intermediate position is 63% or more.

12. The coated active material according to claim 8, wherein the coating layer further contains at least one element selected from the group consisting of Ca, Mg, Al, Y and Zr.

13. An electrode comprising the coated active material according to any one of claims 1 to 12.

14. A battery comprising the electrode according to claim 13.

15. The battery of claim 14, further comprising an electrolyte solution containing a lactone.

16. A method for producing a coated active material, comprising: coating an active material with a solid electrolyte containing Li, Ti, and F; applying a liquid containing oxygen to the active material coated with the solid electrolyte; and heat-treating the active material coated with the solid electrolyte so as to remove the liquid.

17. The method for producing a coated active material according to claim 16, wherein the liquid includes water.

18. The method for producing a coated active material according to claim 16, wherein the ambient temperature in the heat treatment is 180° C. or higher.

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