Solid electrolyte, coated active material, electrode, battery, and method for producing solid electrolyte

Abstract

A solid electrolyte 10 disclosed herein contains Li, Ti, F, and O, and the proportion of Ti—O bonds in a group of bonds to Ti is 6% or more. The solid electrolyte 10 may further contain at least one element selected from the group consisting of Be, Mg, Ca, Sr, Ba, Sc, Y, Al, Ga, In, Zr, and Sn. The solid electrolyte 10 can be manufactured by adhering a liquid containing oxygen to a temporary solid electrolyte containing Li, Ti and F, and heat-treating the temporary solid electrolyte such that the liquid is removed.

Description

Solid electrolyte, coated active material, electrode, battery, and method for producing solid electrolyte

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

[0002] As described in Patent Document 1, fluoride solid electrolytes have been attracting attention as solid electrolytes with excellent oxidation resistance.

[0003] International Publication No. 2021 / 186809

[0004] Fluoride solid electrolytes have the problem of low ionic conductivity. There is a need to further improve the output characteristics of batteries by utilizing fluoride solid electrolytes with high ionic conductivity.

[0005] The present disclosure provides a solid electrolyte comprising Li, Ti, F, and O, wherein the ratio of Ti—O bonds to the bonding groups for Ti is 6% or more.

[0006] The solid electrolyte of the present disclosure can improve the output characteristics of a battery.

[0007] FIG. 1 is a diagram showing a solid electrolyte according to a first embodiment of the present disclosure. FIG. 2 is a process diagram showing a method for producing a solid electrolyte. FIG. 3 is a cross-sectional view showing a schematic configuration of a coated active material according to a second embodiment of the present disclosure. FIG. 4 is a schematic cross-sectional view of a lithium secondary battery according to a third embodiment of the present disclosure. FIG. 5 is a graph showing the 2C discharge capacity of the lithium secondary batteries of Examples 1 to 3 and Comparative Example 1. FIG. 6 is an XPS spectrum of LTAF 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] 1 is a diagram showing a solid electrolyte 10 according to a first embodiment of the present disclosure. The solid electrolyte 10 contains Li, Ti, F, and O. The solid electrolyte 10 is a fluoride solid electrolyte and has excellent oxidation resistance due to the high electronegativity of fluorine.

[0010] The ratio of Ti—O bonds to the bonding groups relative to Ti contained in the solid electrolyte 10 is 6% or more. In other words, the sum of the ratio of Ti—F bonds and the ratio of Ti—O—F bonds is 94% or less. The solid electrolyte 10 of this embodiment is obtained by incorporating oxygen into a solid electrolyte containing Li, Ti, and F. The solid electrolyte 10 of this embodiment can improve the output characteristics of a battery compared to a solid electrolyte with a low ratio of Ti—O bonds, i.e., a solid electrolyte that does not contain oxygen in an amount exceeding that of natural oxidation. In particular, the discharge capacity at high rates is improved. This is thought to be because the incorporation of oxygen improves the lithium ion conductivity of the solid electrolyte containing Li, Ti, and F.

[0011] The bonding groups to Ti refer to the bonding groups of anions to Ti. Titanium (Ti), fluorine (F), and oxygen (O) typically form compounds such as 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 solid electrolyte 10 suitable for improving the output characteristics of a battery can be obtained.

[0012] 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 of the solid electrolyte 10 is performed. The Ti2p spectrum observed in the binding energy range of 448 eV to 486 eV is separated into peaks of TiF, TiOF, and TiO. The TiF peak is near 462.0 eV. The TiOF peak is near 460.2 eV. The TiO peak is near 458.3 eV. The ratio of the area of ​​the peak attributed to TiF to the total area of ​​the separated peaks is calculated as the ratio of 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: %).

[0013] The ratio of Ti—O bonds may be 30% or more. In other words, the sum of the ratio of Ti—F bonds and the ratio of Ti—O—F bonds may be 70% or less. With this configuration, the effect of improving the output characteristics of the battery can be more sufficiently obtained.

[0014] The upper limit of the ratio of Ti—O bonds is not particularly limited. The upper limit of the ratio of Ti—O bonds may be 80% or 40%. In other words, the lower limit of the sum of the ratio of Ti—F bonds and the ratio of Ti—O—F bonds may be 20% or 60%. With this configuration, oxygen can be incorporated into the solid electrolyte 10 while maintaining the skeletal structure (crystalline structure) of the solid electrolyte 10. By maintaining the skeletal structure of the solid electrolyte 10, the solid electrolyte 10 exhibits high ionic conductivity.

[0015] The ratio of Ti—O bonds may be 6% or more and 80% or less, 6% or more and 40% or less, 30% or more and 80% or less, or 30% or more and 40% or less. The sum of the ratio of Ti—F bonds and the ratio of Ti—O—F bonds may be 20% or more and 94% or less, 60% or more and 94% or less, 20% or more and 70% or less, or 60% or more and 70% or less.

[0016] The solid electrolyte 10 has lithium ion conductivity and is suitable for lithium ion secondary batteries.

[0017] The solid electrolyte 10 may consist essentially of Li, Ti, F, and O. "The solid electrolyte 10 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 solid electrolyte 10 is 90% or more. In one example, the molar ratio may be 95% or more. The solid electrolyte 10 may consist only of Li, Ti, F, and O.

[0018] The solid electrolyte 10 may further contain at least one element selected from the group consisting of Be, Mg, Ca, Sr, Ba, Sc, Y, Al, Ga, In, Zr, and Sn. The inclusion of these elements can improve the ionic conductivity of the solid electrolyte 10. From the viewpoints of cost and ionic conductivity, at least one element selected from the group consisting of Ca, Mg, Al, Y, and Zr is desirable, and Al is more desirable.

[0019] The solid electrolyte 10 may consist essentially of Li, Ti, M, F, and O. M is at least one selected from the group consisting of Be, Mg, Ca, Sr, Ba, Sc, Y, Al, Ga, In, Zr, and Sn. "The solid electrolyte 10 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 solid electrolyte 10 is 90% or more. In one example, the molar ratio may be 95% or more. The solid electrolyte 10 may consist only of Li, Ti, M, F, and O.

[0020] However, the solid electrolyte 10 may contain elements that are inevitably mixed in. Examples of such elements include hydrogen and nitrogen. Such elements may be contained in the raw material powder of the solid electrolyte 10 or may be present in the atmosphere during the production and storage of the solid electrolyte 10.

[0021] To further increase the ionic conductivity of the solid electrolyte 10, 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.

[0022] The solid electrolyte 10 has a composition represented by, for example, the following formula (1): Formula (1) satisfies 0≦x<1, 0<b≦2, and 0<y≦2.8. When the solid electrolyte 10 has a composition represented by the following formula (1), the solid electrolyte 10 exhibits good ionic conductivity.

[0023] Li 6-(4-x)b (Ti 1-x M x ) b F 6-2y O y ...Formula (1)

[0024] To increase the ionic conductivity of the solid electrolyte 10, formula (1) may satisfy 0.1≦x≦0.9.

[0025] In order to increase the ionic conductivity of the solid electrolyte 10, the formula (1) may satisfy 0.8≦b≦1.2.

[0026] In order to increase the ionic conductivity of the solid electrolyte 10, the formula (1) may satisfy 0.1≦y≦2.5.

[0027] The solid electrolyte 10 may have a crystalline phase or may contain an amorphous phase.

[0028] To increase ionic conductivity, the solid electrolyte 10 may contain anions other than F and O. The anions other than F and O are at least one selected from the group consisting of Cl, Br, I, and Se.

[0029] It is desirable that the solid electrolyte 10 does not contain sulfur. A sulfur-free solid electrolyte does not generate hydrogen sulfide even when exposed to the atmosphere, and therefore is highly safe.

[0030] The shape of the solid electrolyte 10 is not particularly limited. The solid electrolyte 10 may have a particle shape. Examples of the particle shape include an acicular shape, a spherical shape, and an oval spherical shape. The solid electrolyte 10 may have a pellet, a plate, or a thin film shape.

[0031] When the solid electrolyte 10 is particulate, the particles of the solid electrolyte 10 may have a median diameter of 0.01 μm or more and 100 μm or less. This allows the solid electrolyte 10 to exhibit higher ionic conductivity. Furthermore, when the solid electrolyte 10 is mixed with other materials such as active materials, the solid electrolyte 10 and the other materials are well dispersed. The "median diameter" refers to the particle size at which the cumulative volume in the volume-based particle size distribution is 50%. The volume-based particle size distribution is measured, for example, using a laser diffraction particle size distribution analyzer.

[0032] 2 is a process diagram showing a method for manufacturing the solid electrolyte 10. According to the manufacturing method of this embodiment, the solid electrolyte 10 can be manufactured efficiently.

[0033] In step 1, a temporary solid electrolyte containing Li, Ti, and F is synthesized. The temporary 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 temporary solid electrolyte having a target composition. The raw material powders are, for example, fluorides of each element. If the target composition is Li, 2.7 Ti0.3 Al 0.7 In 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 temporary solid electrolyte having the desired composition.

[0034] In step S2, a liquid containing oxygen is applied to the temporary solid electrolyte. The liquid containing oxygen is used to oxidize the temporary solid electrolyte and increase the Ti—O bonds. When heat treatment is performed with the liquid containing oxygen applied, the temporary solid electrolyte can be efficiently oxidized.

[0035] 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.

[0036] In step S3, the temporary solid electrolyte is heat-treated to remove the liquid containing oxygen, thereby obtaining the solid electrolyte 10 of this embodiment.

[0037] The ambient temperature in the heat treatment is, for example, 180° C. or higher. At such a temperature, the solid electrolyte 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.

[0038] (Embodiment 2) FIG. 3 is a cross-sectional view of a coated active material according to Embodiment 2 of the present disclosure. The coated active material 130 includes an active material 110 and a coating layer 120. The coating layer 120 covers at least a portion of the surface of a particle of the active material 110. The active material 110 may be a secondary particle formed by aggregation of a plurality of primary particles. The coating layer 120 may cover at least a portion of the surface of the secondary particle of the active material 110. The coating layer 120 includes the solid electrolyte 10 of Embodiment 1. When the active material 110 is coated with the solid electrolyte 10, direct contact between the active material 110 and other materials, such as an electrolyte solution, is prevented. In particular, according to this embodiment, the solid electrolyte 10 contained in the coating layer 120 contains a large number of Ti—O bonds. The large number of Ti—O bonds suppresses the activity of the solid electrolyte 10 in relation 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 charging and discharging of the battery. As a result, the output characteristics of a battery using the coated active material 130 are improved.

[0039] 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.

[0040] The coating layer 120 may contain the solid electrolyte 10 as a main component, or may contain only the solid electrolyte 10. "Main component" means the component that is contained in the largest amount by mass. "Containing only the solid electrolyte 10" means that, with the exception of inevitable impurities, no materials other than the solid electrolyte 10 are intentionally added.

[0041] 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.

[0042] The active material 110 includes a material that has the property of absorbing and releasing metal ions such as lithium ions.

[0043] 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.

[0044] 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.

[0045] The active material 110 may be a negative electrode active material, such as lithium titanate, graphite, silicon, a silicon compound, or a NiBi alloy.

[0046] 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.

[0047] The coated active material 130 can be manufactured by the following method.

[0048] A powder of the active material 110 and a powder of the solid electrolyte 10 are mixed in an appropriate ratio to obtain a mixture. The mixture is then milled to impart mechanical energy to the mixture. A mixing device such as a ball mill can be used for the milling process. The milling process may be performed in a dry and inert atmosphere to prevent oxidation of the materials.

[0049] The coated active material 130 may be manufactured by a dry particle compounding method. The treatment by the dry particle compounding method includes applying at least one mechanical energy selected from the group consisting of impact, compression, and shear to the active material 110 and the solid electrolyte 10. The active material 110 and the solid electrolyte 10 are mixed in an appropriate ratio.

[0050] The apparatus used in producing the coated active material 130 is not particularly limited, and may be an apparatus capable of applying mechanical energy of impact, compression, and shear to the mixture of the active material 110 and the solid electrolyte 10. Examples of apparatus capable of applying mechanical energy include processing apparatuses (particle composite apparatuses) such as a ball mill, "Mechanofusion" (manufactured by Hosokawa Micron Corporation), "Nobilta" (manufactured by Hosokawa Micron Corporation), and "Balance Gran" (manufactured by Freund Turbo Corporation). The coated active material 130 may be produced by a liquid phase method such as a sol-gel method.

[0051] A second coating layer may be present between the active material 110 and the coating layer 120. The second coating layer may be formed of an oxide solid electrolyte such as LISICON or lithium niobate.

[0052] It is not essential that the solid electrolyte 10 be used as a coating material for the active material 110. The solid electrolyte 10 may be used in the electrolyte layer of a solid-state battery.

[0053] (Embodiment 3) Figure 4 is a schematic cross-sectional view of a lithium secondary battery according to embodiment 3 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 provided 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 provided 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.

[0054] The positive electrode 53 or the negative electrode 56 includes the solid electrolyte 10 of Embodiment 1. Specifically, the positive electrode active material layer 52 or the negative electrode active material layer 55 includes the coated active material 130 of Embodiment 2. The solid electrolyte 10 and the coated active material 130 have excellent durability against the nonaqueous electrolyte solution 59, and therefore improve the output characteristics of the lithium secondary battery 100.

[0055] The coated active material 130 is typically included in the positive electrode 53 .

[0056] 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.

[0057] 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.

[0058] 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.

[0059] The non-aqueous electrolyte 59 contains a non-aqueous solvent and a lithium salt.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0066] (Technology 1) A solid electrolyte containing Li, Ti, F, and O, in which the ratio of Ti—O bonds to the bonding groups for Ti is 6% or more.

[0067] The solid electrolyte of the present disclosure can improve the output characteristics of a battery.

[0068] (Technology 2) The solid electrolyte according to Technology 1, further comprising at least one element selected from the group consisting of Be, Mg, Ca, Sr, Ba, Sc, Y, Al, Ga, In, Zr, and Sn. By including these elements, the ionic conductivity of the solid electrolyte can be improved.

[0069] (Technology 3) A coated active material comprising: an active material; and a coating layer covering at least a part of the surface of particles of the active material, wherein the coating layer contains the solid electrolyte according to Technology 1 or 2.

[0070] The coated active material of the present disclosure can improve the output characteristics of a battery.

[0071] (Technology 4) An electrode including the solid electrolyte according to Technology 1 or 2. According to the present disclosure, an electrode having excellent output characteristics can be provided.

[0072] (Technology 5) A battery including the solid electrolyte according to Technology 1 or 2. According to the present disclosure, a battery having excellent output characteristics can be provided.

[0073] (Technology 6) A method for producing a solid electrolyte containing Li, Ti, F, and O, comprising: attaching a liquid containing oxygen to a temporary solid electrolyte containing Li, Ti, and F; and heat-treating the temporary solid electrolyte so as to remove the liquid.

[0074] According to the manufacturing method of the present disclosure, the solid electrolyte of the present disclosure can be efficiently manufactured.

[0075] (Technology 7) The method for producing a solid electrolyte according to Technology 6, wherein the liquid contains water. Water forms a bond with Ti and can efficiently oxidize the solid electrolyte through heat treatment.

[0076] (Technology 8) The method for producing a solid electrolyte according to Technology 6, wherein the ambient temperature in the heat treatment is 180° C. or higher. At such a temperature, the solid electrolyte can be efficiently oxidized.

[0077] 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.

[0078] [Preparation of Temporary 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 a 0.5 mm diameter ball (25 g). γ-butyrolactone (GBL) as an organic solvent 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 using a planetary ball mill at 500 rpm for 12 hours. 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 temporary solid electrolyte. 2.7 Ti 0.3 Al 0.7 F6 (hereinafter referred to as "LTAF").

[0079] Example 1 LTAF powder was placed in a crucible, sprayed with water in an amount equal to the mass of LTAF, and left in the air for 1 hour. Thereafter, the crucible was heat-treated at an ambient temperature of 200° C. for 24 hours. Thus, a solid electrolyte of Example 1 was obtained.

[0080] Example 2 A solid electrolyte of Example 2 was prepared in the same manner as in Example 1, except that the amount of water was increased to five times the mass of LTAF.

[0081] Example 3 A solid electrolyte of Example 3 was prepared in the same manner as in Example 1, except that the amount of water was increased to 17 times the mass of LTAF.

[0082] Comparative Example 1 The prepared LTAF was used as the solid electrolyte of Comparative Example 1. No treatment with water or heat was carried out.

[0083] [XPS Measurement] XPS measurement of the solid electrolytes of Examples 1 to 3 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:

[0084] X-ray source: Al monochromator (25W, 15kV) Electron / ion neutralization gun: ON Photoelectron take-off angle: 45 degrees

[0085] 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 was performed in the range of 455 eV to 486 eV, 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. Peak fitting was performed using the Gauss-Lorentz function in the XPS spectrum analysis. The results are shown in Table 1.

[0086]

[0087] As shown in Table 1, the proportion of Ti—O bonds in the solid electrolyte of Example 1 was 6%, which exceeded the proportion of Ti—O bonds in the solid electrolyte of Comparative Example 1. It is presumed that the Ti—O bonds in the solid electrolyte of Comparative Example 1 are due to natural oxidation. The proportion of Ti—O bonds in the solid electrolyte of Example 2 was 36%. The proportion of Ti—O bonds in the solid electrolyte of Example 3 was 45%. In other words, the proportion of Ti—O bonds increased as the amount of water increased.

[0088] [Preparation of Coated Active Material] Powder of LiCoO2 (hereinafter referred to as "LCO") was prepared as a positive electrode active material. Next, a coating layer made of the solid electrolyte of Example 1 was formed on the surface of the LCO. The coating layer was formed by a compressive shear treatment using a particle compositer (NOB-MINI, manufactured by Hosokawa Micron Corporation). Specifically, LCO and the solid electrolyte of Example 1 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 the production of the coated active material of Example 1. The thickness of the coating layer was measured using the method described above. The thickness of the coating layer was 25 nm.

[0089] The coated active materials of Examples 2, 3 and 1 were prepared in the same manner as in Example 1, except that the solid electrolyte of Example 2, Example 3 or Comparative Example 1 was used instead of the solid active material of Example 1.

[0090] [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.

[0091] 100 mg of the positive electrode mixture was subjected to 10 kN / cm 2 The 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.

[0092] 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.

[0093] Lithium secondary batteries of Examples 2, 3 and 1 were fabricated in the same manner as in Example 1, except that the coated active material of Example 1 was replaced with the coated active material of Example 2, Example 3 or Comparative Example 1.

[0094] Charge / Discharge Test The charge / discharge tests of the lithium secondary batteries of Examples 1 to 3 and Comparative Example 1 were carried out 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 4.05 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 results are shown in Figure 5.

[0095] 5 is a graph showing the 2C discharge capacity of the lithium secondary batteries of Examples 1 to 3 and Comparative Example 1. The 2C discharge capacity on the vertical axis represents the discharge capacity per unit mass of coated active material (unit: mAh / g). As shown in FIG. 5, the 2C discharge capacity of the lithium secondary batteries of Examples 1, 2, and 3 exceeded the 2C discharge capacity of the lithium secondary battery of Comparative Example 1. By using a solid electrolyte with a high ratio of Ti—O bonds in the lithium secondary battery, the output characteristics of the lithium secondary battery, particularly the discharge capacity at high rates, were improved.

[0096] (Reference Example) XPS measurement was carried out on LTAF immediately after synthesis and after exposure to the atmosphere for 7 days. The results are shown in FIG.

[0097] As can be seen from the two spectra shown in Figure 6, the change in the LTAF spectrum before and after 7 days of air exposure was minimal. In other words, the changes caused by oxidation treatment using water and heat, as in the Examples, are significantly different from the changes caused by air exposure. Even if the solid electrolyte 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.

[0098] The technology of the present disclosure is useful for lithium secondary batteries.

Claims

1. A solid electrolyte containing Li, Ti, F, and O, in which the ratio of Ti--O bonds to the bonding groups for Ti is 6% or more.

2. The solid electrolyte according to claim 1, further comprising at least one element selected from the group consisting of Be, Mg, Ca, Sr, Ba, Sc, Y, Al, Ga, In, Zr, and Sn.

3. A coated active material comprising: an active material; and a coating layer covering at least a portion of a surface of a particle of the active material, the coating layer including the solid electrolyte according to claim 1 or 2.

4. An electrode comprising the solid electrolyte according to claim 1 or 2.

5. A battery comprising the solid electrolyte according to claim 1 or 2.

6. A method for producing a solid electrolyte containing Li, Ti, F, and O, comprising: attaching a liquid containing oxygen to a temporary solid electrolyte containing Li, Ti, and F; and heat-treating the temporary solid electrolyte so as to remove the liquid.

7. The method for producing a solid electrolyte according to claim 6, wherein the liquid includes water.

8. The method for producing a solid electrolyte according to claim 6, wherein the ambient temperature in the heat treatment is 180° C. or higher.

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