Coated active material, positive electrode, and secondary battery

Abstract

A coated active material (10) according to the present disclosure comprises: a positive electrode active material; and an apatite-containing particle layer (13) that contains apatite-containing particles (12) and covers at least a part of the surfaces of particles (11) of the positive electrode active material. A positive electrode according to the present disclosure comprises: a positive electrode current collector; and a positive electrode active material layer supported by the positive electrode current collector. The positive electrode active material layer includes said coated active material (10) according to the present disclosure.

Description

Coated active material, positive electrode, and secondary battery

[0001] The present disclosure relates to a coated active material, a positive electrode, and a secondary battery.

[0002] It is known that conventional secondary batteries are subject to side reactions, such as reduction and leaching of transition metals contained in the active material, which in turn cause decomposition of the electrolyte. These side reactions accelerate the deterioration of the secondary battery and result in disadvantages such as reduced cycle characteristics.

[0003] Patent Document 1 discloses a positive electrode that can increase the capacity retention rate during charge-discharge cycles, the positive electrode containing first compound particles that contain a lithium transition metal composite oxide as a positive electrode active material, and second compound particles that contain Ca and P.

[0004] Japanese Patent Application Laid-Open No. 2018-137147

[0005] The present disclosure provides a coated active material that can effectively suppress side reactions in a positive electrode and improve the cycle characteristics of a secondary battery.

[0006] The coated active material of the present disclosure includes a positive electrode active material, and an apatite-containing particle layer that includes apatite-containing particles and that coats at least a portion of the surface of particles of the positive electrode active material.

[0007] According to the present disclosure, it is possible to provide a coated active material that can effectively suppress side reactions in the positive electrode and improve the cycle characteristics of a secondary battery.

[0008] Fig. 1 is a cross-sectional view showing a schematic configuration of a coated active material according to embodiment 1. Fig. 2 is a cross-sectional view showing a schematic configuration of a positive electrode according to embodiment 2. Fig. 3 is a longitudinal cross-sectional view showing a schematic configuration of a secondary battery according to embodiment 3.

[0009] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The present disclosure is not limited to the following embodiments.

[0010] 1 is a cross-sectional view showing a schematic configuration of a coated active material according to Embodiment 1. The coated active material 10 includes a positive electrode active material and an apatite-containing particle layer 13 containing apatite-containing particles 12 that coats at least a portion of the surface of a particle 11 of the positive electrode active material. In other words, the coated active material 10 according to Embodiment 1 is a composite particle in which the positive electrode active material particle 11 is a mother particle and the apatite-containing particles 12 are child particles, and at least a portion of the surface of the mother particle is coated by the child particles.

[0011] In a battery, when a positive electrode active material comes into contact with an electrolyte, side reactions such as metal elution from the positive electrode active material and gas generation due to decomposition of the electrolyte occur, resulting in battery degradation. In the coated active material 10 according to the first embodiment, at least a portion of the surface of the positive electrode active material particles 11 is coated with an apatite-containing particle layer 13. This configuration reduces the contact area between the positive electrode active material and the electrolyte, thereby suppressing side reactions such as metal elution from the positive electrode active material. Therefore, the coated active material 10 according to the first embodiment can effectively suppress side reactions at the positive electrode even when charge and discharge are repeated, thereby improving the cycle characteristics of the secondary battery.

[0012] Typically, if a coating made of a material other than the positive electrode active material is formed on the surface of the positive electrode active material to prevent contact between the positive electrode active material and the electrolyte, the coating acts as a resistance factor, resulting in a decrease in the output characteristics (i.e., discharge capacity) of the positive electrode. On the other hand, the coated active material 10 according to embodiment 1 is less likely to exhibit a decrease in output characteristics, even though the positive electrode active material is coated with the apatite-containing particle layer 13. For example, by adjusting the ratio of the apatite-containing particle layer 13 to the entire coated active material 10 or adjusting the size of the apatite-containing particles 12, the presence of the apatite-containing particle layer 13 can actually improve the output characteristics of the positive electrode. This effect is believed to be unique to the apatite-containing particle layer 13. The presence of the apatite-containing particles 12 on the surface of the positive electrode active material particles 11 is believed to improve the Li ion conductivity rate. That is, the apatite-containing particles 12 present on the surfaces of the positive electrode active material particles 11 help to speed up the movement of Li ions, so that even if the apatite-containing particles 12 are attached to the surfaces of the positive electrode active material, the output characteristics are unlikely to decrease, and it is thought that the output characteristics may actually be improved. Due to this special Li ion conductivity improving effect of the apatite-containing particles 12, the coated active material 10 according to the first embodiment can also achieve the effect of improving the output characteristics in addition to improving the cycle characteristics.

[0013] In this specification, apatite refers to a compound containing calcium (Ca), a phosphate group (PO), and at least one selected from the group consisting of an OH group, F, and Cl, and refers to calcium phosphate apatite.

[0014] The apatite contained in the apatite-containing particles 12 may contain an OH group. That is, the apatite-containing particles 12 may contain hydroxyapatite. With this configuration, the coated active material 10 according to the first embodiment can more effectively suppress side reactions in the positive electrode, thereby further improving the cycle characteristics of the secondary battery. The apatite-containing particles 12 may be made of, for example, hydroxyapatite.

[0015] The apatite-containing particles 12 may contain apatite having a composition represented by the following formula (1): x (PO4) y (A) z ... (1) Here, in formula (1), A is at least one selected from the group consisting of OH, F, and Cl, and 0<x<10, 0<y<6, and 0≦z<2 are satisfied.

[0016] Since the apatite-containing particles 12 contain apatite having a composition represented by the above formula (1), the coated active material 10 according to the first embodiment can more effectively suppress side reactions at the positive electrode, and can further improve the cycle characteristics of the secondary battery.

[0017] In order to more reliably improve the cycle characteristics, A in the above formula (1) may be OH. That is, the apatite-containing particles 12 may contain Ca x (PO4) y (OH) z The hydroxyapatite may contain hydroxyapatite having a composition represented by the following formula:

[0018] In the above formula (1), x may satisfy 2<x<8, 4<x<6, 4.5<x<5.5, or x=5.

[0019] In the above formula (1), y may satisfy 1<y<5, 2<y<4, 2.5<y<3.5, or y=3.

[0020] In the above formula (1), z may satisfy 0<z<2, 0.5<z<1.5, or z=1.

[0021] The apatite-containing particles 12 may contain at least one selected from the group consisting of Ca(PO)(OH), Ca(PO)F, and Ca(PO)Cl as the apatite having the composition represented by the above formula (1), or may contain Ca(PO)(OH).

[0022] The crystalline structure of the apatite contained in the apatite-containing particles 12 can be identified by, for example, particle observation using a transmission electron microscope (TEM) and electron beam diffraction. The composition of the apatite contained in the apatite-containing particles 12 can be identified by, for example, energy dispersive X-ray spectroscopy using a transmission electron microscope (TEM-EDX) or ICP optical emission spectroscopy.

[0023] The apatite contained in the apatite-containing particles has, for example, a hexagonal crystal structure. Apatite having such a crystal structure is, for example, Ca(PO)(OH) or Ca(PO)F. With this configuration, the coated active material 10 according to the first embodiment can more effectively suppress side reactions in the positive electrode, thereby further improving the cycle characteristics of the secondary battery.

[0024] The particle diameter of the apatite-containing particles 12 is, for example, 1 μm or less, and may be 500 nm or less. Apatite-containing particles 12 having such a particle diameter are likely to coat the surfaces of the positive electrode active material particles 11. Therefore, the contact area between the positive electrode active material and the electrolyte is likely to be further reduced. This more effectively suppresses side reactions at the positive electrode, resulting in better cycle characteristics. Furthermore, apatite-containing particles 12 having such a particle diameter are more likely to exhibit the above-mentioned Li ion conductivity improvement effect, thereby further improving the output characteristics of the secondary battery. The lower limit of the particle diameter of the apatite-containing particles 12 is not particularly limited, but is, for example, 5 nm.

[0025] In the coated active material 10 according to the first embodiment, the particle diameter of the apatite-containing particles 12 can be measured, for example, using a scanning electron microscope (SEM) image obtained by observing the coated active material 10 with an SEM, in which the cross section of the apatite-containing particles 12 is exposed. The particle diameter of the apatite-containing particles 12 is the maximum diameter of the apatite-containing particles 12.

[0026] The positive electrode active material may be a material capable of absorbing and releasing lithium ions. Examples of the positive electrode active material include lithium-containing transition metal oxides, lithium-containing transition metal phosphates, transition metal fluorides, polyanionic materials, fluorinated polyanionic materials, transition metal sulfides, transition metal oxysulfides, and transition metal oxynitrides. In particular, using a lithium-containing transition metal oxide or a lithium-containing transition metal phosphate as the positive electrode active material can reduce the manufacturing cost of the battery and increase the average discharge voltage. 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.

[0027] The positive electrode active material may contain lithium nickel oxide having a layered rock salt crystal structure. The proportion of Ni among metal elements other than Li contained in the lithium nickel oxide may be 50 atomic % or more. The lithium nickel oxide may contain other transition metals. Lithium nickel oxide is useful for achieving a high operating voltage. The configuration of the coated active material according to embodiment 1 can suppress the elution of metal ions from the positive electrode active material. Because nickel is easily eluted from the positive electrode active material, the technology of the present disclosure is particularly expected to be effective in suppressing elution.

[0028] The lithium nickel oxide may be represented by the following composition formula (2): Element M1 is at least one selected from the group consisting of V, Co, and Mn. Element M2 is at least one selected from the group consisting of Mg, Al, Ca, Ti, Cu, Zn, and Nb. Composition formula (2) satisfies 0.9≦α≦1.10, −0.05≦β≦0.05, 0.5≦x1<1, 0≦x2≦0.5, and 0<1−x1−x2≦0.5.

[0029] Li α Ni x1 M1 x2 M2 (1-x1-x2) O 2+β ... (2)

[0030] There are no particular limitations on the shape of the positive electrode active material particles 11. The shape of the particles 11 is, for example, needle-like, scale-like, spherical, or oval-spherical.

[0031] In the coated active material 10 according to the first embodiment, the particle diameter of the positive electrode active material particles 11 is larger than the particle diameter of the apatite-containing particles 12. The ratio of the particle diameter of the positive electrode active material particles 11 to the particle diameter of the apatite-containing particles 12 (particle diameter of the positive electrode active material particles 11 / particle diameter of the apatite-containing particles 12) is, for example, 2 or more and 40,000 or less, preferably 5 or more and 1,500 or less, and more preferably 10 or more and 150 or less. In the coated active material 10 according to the first embodiment, the particle diameter of the positive electrode active material particles 11 can be measured, for example, using an SEM image obtained by SEM observation of the coated active material 10 in which the cross section of the positive electrode active material particles 11 is exposed. The particle diameter of the positive electrode active material particles 11 is the maximum diameter of the positive electrode active material particles 11.

[0032] In the coated active material 10 according to the first embodiment, the proportion of the apatite-containing particles 12 is determined taking into account the surface area of ​​the positive electrode active material particles 12 and the particle diameter of the apatite-containing particles 12. For example, when apatite-containing particles 12 having a particle diameter of 20 nm to 300 nm are coated on base particles having a particle diameter of 1.0 μm to 5.0 μm, the ratio of the apatite-containing particles to the base particles is, for example, 0.1% by mass or more and 15% by mass or less, preferably 0.3% by mass or more and 12% by mass or less, and more preferably 0.3% by mass or more and 6.0% by mass or less. When the proportion of the apatite-containing particles 12 is 0.1% by mass or more, the surface area of ​​the positive electrode active material particles 11 coated with the apatite-containing particles 12 increases, thereby further reducing the contact area between the positive electrode active material and the electrolyte. This more effectively suppresses side reactions at the positive electrode, resulting in better cycle characteristics. By setting the proportion of the apatite-containing particles 12 to 15% by mass or less, it is possible to improve the output characteristics of the secondary battery by suppressing an increase in the resistance of the apatite-containing particle layer 13. To further improve the output characteristics of the secondary battery, it is desirable that the proportion of the apatite-containing particles 12 be 6.0% by mass or less.

[0033] The proportion of the apatite-containing particles 12 in the coated active material 10 according to the first embodiment can be measured using, for example, TEM-EDX.

[0034] The average particle diameter of the coated active material 10 according to the first embodiment is, for example, 1 μm or more and 20 μm or less, preferably 1 μm or more and 10 μm or less, and more preferably 1 μm or more and 6 μm or less. The average particle diameter of the coated active material 10 refers to the particle diameter (median diameter) when 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.

[0035] The coated active material 10 according to the first embodiment can be produced by the following method.

[0036] The apatite-containing particle layer 13 is obtained by attaching apatite-containing particles 12 to the surfaces of particles 11 of the positive electrode active material.

[0037] For example, apatite-containing particles 12 are attached to the surfaces of positive electrode active material particles 11 by a mechanical composite method. Specifically, apatite-containing particles 12 are attached to the surfaces of positive electrode active material particles 11 by, for example, a mechanochemical method, to form an apatite-containing particle layer 13. The positive electrode active material and the apatite-containing particles are mixed in an appropriate ratio to obtain a mixture, and mechanical energy is applied to the mixture. Devices that can apply mechanical energy include ball mills, processing devices (particle composite devices) such as "Mechanofusion" (manufactured by Hosokawa Micron), "Nobilta" (manufactured by Hosokawa Micron), and "Balance Gran" (manufactured by Freund Turbo), and a "Hybridization System" (manufactured by Nara Machinery Manufacturing Co., Ltd.).

[0038] 2 is a cross-sectional view showing a schematic configuration of a positive electrode 20 according to embodiment 2. The positive electrode 20 includes a positive electrode current collector 21 and a positive electrode active material layer 22 supported on the positive electrode current collector 21. The positive electrode active material layer 22 includes the coated active material according to embodiment 1.

[0039] The positive electrode active material layer 22 may contain only the coated active material according to the first embodiment as the positive electrode active material, or may further contain a positive electrode active material different from the coated active material according to the first embodiment.

[0040] The positive electrode active material layer 22 may contain other materials such as a conductive additive, an ion conductor, and a binder, as needed.

[0041] The conductive additive and the ion conductor are used to reduce the resistance of the positive electrode 20. Examples of the conductive additive include carbon materials and conductive polymer compounds. Examples of the carbon material include carbon black, graphite, acetylene black, carbon nanotubes, carbon nanofibers, graphene, fullerene, and graphite oxide. Examples of the conductive polymer compound include polyaniline, polypyrrole, and polythiophene. At least one selected from these conductive additives can be used.

[0042] The binder is used to improve the binding properties of the materials that make up the positive electrode 20. Examples of binders include polymer materials such as polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, polytetrafluoroethylene, carboxymethyl cellulose, polyacrylic acid, styrene-butadiene copolymer rubber, polypropylene, polyethylene, and polyimide. At least one selected from these binders can be used.

[0043] The positive electrode current collector 21 may be a sheet or film made of a metal material such as stainless steel, nickel, copper, or an alloy thereof. The sheet or film may be porous or non-porous. Examples of the sheet or film include metal foil and metal mesh. A carbon material such as carbon may be applied to the surface of the positive electrode current collector 21 as a conductive auxiliary material.

[0044] The positive electrode 20 according to the second embodiment can be produced, for example, by mixing the material constituting the positive electrode active material layer 22 with a solvent to prepare a positive electrode slurry, applying the positive electrode slurry onto the positive electrode current collector 21 to form a coating film, and then drying the coating film.

[0045] (Embodiment 3) A secondary battery according to embodiment 3 includes the positive electrode according to embodiment 2, a negative electrode, and an electrolyte. By using the positive electrode according to embodiment 2, the cycle characteristics of the secondary battery can be improved.

[0046] 3 is a longitudinal cross-sectional view schematically illustrating a secondary battery 100 according to embodiment 3. The secondary battery 100 is a cylindrical battery including a cylindrical battery case, a wound electrode group 34, and an electrolyte (not shown). The electrode group 34 is housed in the battery case and is in contact with the electrolyte.

[0047] The battery case is composed of a case body 35, which is a cylindrical metal container with a bottom, and a sealing body 36 that seals the opening of the case body 35. A gasket 47 is disposed between the case body 35 and the sealing body 36. The gasket 47 ensures that the battery case is airtight. Within the case body 35, insulating plates 37 and 38 are disposed on both ends of the electrode group 34 in the direction of the winding axis of the electrode group 34.

[0048] The case body 35 has, for example, a step portion 41. The step portion 41 can be formed by partially pressing the side wall of the case body 35 from the outside. The step portion 41 may be formed in an annular shape on the side wall of the case body 35 along the circumferential direction of an imaginary circle defined by the case body 35. In this case, the sealing body 36 is supported by, for example, the surface of the step portion 41 on the opening side.

[0049] The sealing body 36 includes a filter 42, a lower valve body 43, an insulating member 44, an upper valve body 45, and a cap 46. These components are stacked in this order in the sealing body 36. The sealing body 36 is attached to the opening of the case body 35 so that the cap 46 is located outside the case body 35 and the filter 42 is located inside the case body 35.

[0050] Each of the above-mentioned members constituting the sealing body 36 has, for example, a disk or ring shape. The above-mentioned members, except for the insulating member 44, are electrically connected to one another.

[0051] The electrode group 34 has a positive electrode 31, a separator 32, and a negative electrode 33. The positive electrode 31, the separator 32, and the negative electrode 33 are all strip-shaped. The width direction of the strip-shaped positive electrode 31 and the negative electrode 33 is, for example, parallel to the winding axis of the electrode group 34. The separator 32 is disposed between the positive electrode 31 and the negative electrode 33. The positive electrode 31 and the negative electrode 33 are spirally wound with the separator 32 interposed between these electrodes.

[0052] When observing the cross section of the secondary battery 100 in a direction perpendicular to the winding axis of the electrode group 34, the positive electrodes 31 and negative electrodes 33 are stacked alternately in the radial direction of the imaginary circle defined by the case body 35, with a separator 32 interposed between these electrodes.

[0053] The positive electrode 31 is electrically connected to a cap 46, which also serves as a positive electrode terminal, via a positive electrode lead 39. One end of the positive electrode lead 39 is connected, for example, to near the center of the positive electrode 31 in the longitudinal direction of the positive electrode 31. The positive electrode lead 39 extends from the positive electrode 31 to the filter 42 through a through-hole formed in the insulating plate 37. The other end of the positive electrode lead 39 is welded, for example, to the surface of the filter 42 on the electrode group 34 side.

[0054] The negative electrode 33 is electrically connected to the case body 35, which also serves as a negative electrode terminal, via a negative electrode lead 40. One end of the negative electrode lead 40 is connected to, for example, an end of the negative electrode 33 in the longitudinal direction of the negative electrode 33. The other end of the negative electrode lead 40 is welded to, for example, the inner bottom surface of the case body 35.

[0055] Each component of the secondary battery 100 will be specifically described below.

[0056] The positive electrode 31 is the positive electrode according to the second embodiment.

[0057] The negative electrode 33 includes the negative electrode active material according to this embodiment. The negative electrode 33 includes, for example, a negative electrode current collector and a negative electrode mixture layer carried on the surface of the negative electrode current collector.

[0058] The negative electrode current collector may be a sheet or film made of a metal material such as stainless steel, nickel, copper, or an alloy thereof. The sheet or film may be porous or non-porous. Examples of the sheet or film include metal foil and metal mesh. A carbon material such as carbon may be applied to the surface of the negative electrode current collector as a conductive auxiliary material.

[0059] The negative electrode mixture layer contains the negative electrode active material according to this embodiment. The negative electrode mixture layer may contain other materials such as a conductive additive, an ion conductor, and a binder, as needed. The conductive additive, ion conductor, and binder materials described above for the positive electrode mixture layer can also be used for the negative electrode mixture layer.

[0060] The electrolyte may contain a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the electrolyte may be, for example, 0.5 mol / L or more and 2 mol / L or less. By controlling the lithium salt concentration within the above range, an electrolyte having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0061] The non-aqueous solvent may be a cyclic carbonate, a chain carbonate, a cyclic ether, a chain ether, a nitrile, an amide, etc. One selected from these solvents may be used, or two or more may be used in combination.

[0062] Examples of lithium salts that can be used 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.

[0063] It is usually desirable to interpose a separator between the positive electrode and the negative electrode. The separator 32 has high ion permeability and adequate mechanical strength and insulating properties. The separator 32 can be made of a microporous thin film, a woven fabric, a nonwoven fabric, or the like. The separator 32 can be made of a polymer, for example. The polymer can be a polyolefin such as polypropylene or polyethylene.

[0064] In the secondary battery of the present disclosure, the electrolyte may be impregnated into a polymer provided as a separator, for example, that is, the secondary battery of the present disclosure may have a structure in which the electrolyte and the polymer are used in combination.

[0065] The secondary battery of the present disclosure may further include a solid electrolyte as an electrolyte. That is, the secondary battery of the present disclosure may have a hybrid structure in which an electrolytic solution and a solid electrolyte are used in combination. Examples of solid electrolyte materials include halide solid electrolytes, sulfide solid electrolytes, oxide solid electrolytes, and organic polymer solid electrolytes. In the present disclosure, a "halide solid electrolyte" refers to a solid electrolyte containing a halogen element as the main anion component. A "sulfide solid electrolyte" refers to a solid electrolyte containing sulfur as the main anion component. An "oxide solid electrolyte" refers to a solid electrolyte containing oxygen as the main anion component. The "main anion component" refers to the anion with the largest substance amount among all the anions constituting the solid electrolyte. The coating material in the secondary battery of the present disclosure may include a halide solid electrolyte.

[0066] In the present disclosure, as an example of the structure of a secondary battery according to embodiment 3, the configuration example shown in FIG. 3 is described, i.e., a secondary battery in which a stacked electrode group formed by stacking positive and negative electrodes with a separator interposed therebetween and an electrolyte solution are housed in an exterior body. However, the secondary battery according to the present disclosure is not limited to this configuration example. The secondary battery according to the present disclosure may have any shape, such as a cylindrical shape, a prismatic shape, a coin shape, a button shape, or a laminate shape. Furthermore, instead of a stacked electrode group in the secondary battery according to the present disclosure, an electrode group of another shape, such as an electrode group formed by winding positive and negative electrodes with a separator interposed therebetween, may be used.

[0067] The application of the coated active material and positive electrode of the present disclosure is not limited to the secondary battery 100. In addition to lithium secondary batteries, the coated active material and positive electrode of the present disclosure can be applied to various secondary batteries such as sodium secondary batteries and magnesium secondary batteries.

[0068] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0069] (Technology 1) A coated active material comprising: a positive electrode active material; and an apatite-containing particle layer containing apatite-containing particles, the apatite-containing particle layer covering at least a portion of the surface of particles of the positive electrode active material.

[0070] With this configuration, the coated active material according to Technology 1 can effectively suppress side reactions in the positive electrode and improve the cycle characteristics of the secondary battery.

[0071] (Technology 2) The coated active material according to Technology 1, wherein the apatite-containing particles contain apatite having a composition represented by the following formula (1): Ca x (PO4) y (A) z ...(1) In formula (1), A is at least one selected from the group consisting of OH, F, and Cl, and 0<x<10, 0<y<6, and 0≦z<2 are satisfied.

[0072] With this configuration, the coated active material according to Technology 2 can more effectively suppress side reactions in the positive electrode, and can further improve the cycle characteristics of the secondary battery.

[0073] (Technology 3) The coated active material according to Technology 2, wherein A is OH, and 0<z<2 is satisfied.

[0074] With this configuration, the coated active material according to Technology 3 can more effectively suppress side reactions in the positive electrode, and can further improve the cycle characteristics of the secondary battery.

[0075] (Technology 4) The coated active material according to Technology 2, wherein the apatite-containing particles contain at least one selected from the group consisting of Ca5(PO4)3(OH), Ca5(PO4)3F, and Ca5(PO4)3Cl as the apatite having a composition represented by formula (1).

[0076] With this configuration, the coated active material according to Technology 4 can more effectively suppress side reactions in the positive electrode, and can further improve the cycle characteristics of the secondary battery.

[0077] (Technology 5) The coated active material according to any one of Technologies 1 to 4, wherein the apatite contained in the apatite-containing particles has a hexagonal crystal structure.

[0078] With this configuration, the coated active material according to Technology 5 can more effectively suppress side reactions in the positive electrode, and can further improve the cycle characteristics of the secondary battery.

[0079] (Technology 6) The coated active material according to any one of Technologies 1 to 5, wherein the apatite-containing particles have a particle diameter of 1 μm or less.

[0080] With this configuration, the coated active material according to Technology 6 can more effectively suppress side reactions in the positive electrode, and can further improve the cycle characteristics of the secondary battery.

[0081] (Technology 7) The coated active material according to any one of Technologies 1 to 6, wherein the proportion of the apatite-containing particles in the coated active material is 0.1 mass % or more and 15 mass % or less.

[0082] With this configuration, the coated active material according to Technology 7 can more effectively suppress side reactions in the positive electrode, and can further improve the cycle characteristics of the secondary battery as well as the output characteristics.

[0083] (Technology 8) The coated active material according to any one of Technologies 1 to 7, wherein a ratio of a particle diameter of the particles of the positive electrode active material to a particle diameter of the apatite-containing particles (the particle diameter of the particles of the positive electrode active material / the particle diameter of the apatite-containing particles) is 2 or more and 40,000 or less.

[0084] With this configuration, the coated active material according to Technology 8 can more effectively suppress side reactions in the positive electrode, and can further improve the cycle characteristics of the secondary battery.

[0085] (Technology 9) A positive electrode comprising: a positive electrode current collector; and a positive electrode active material layer supported on the positive electrode current collector, wherein the positive electrode active material layer contains the coated active material according to any one of Technologies 1 to 8.

[0086] With this configuration, the positive electrode according to Technology 9 can effectively suppress side reactions and further improve the cycle characteristics of the secondary battery.

[0087] (Technology 10) A secondary battery comprising: the positive electrode according to Technology 9; a negative electrode; and an electrolyte.

[0088] With this configuration, the secondary battery according to the tenth technique can improve the cycle characteristics.

[0089] The present disclosure will be described in more detail below using examples. The following examples are merely examples of embodiments, and are not intended to limit the scope of the present disclosure.

[0090] (Example 1) (Preparation of coated active material) LiNi 0.88 Co 0.085 Mn 0.035A powder of a positive electrode active material having a composition of O and a powder of hydroxyapatite having a composition of Ca(PO)(OH) were prepared. The average particle size of the positive electrode active material particles was 4.5 μm. The average particle size of the hydroxyapatite particles was 200 nm. These average particle sizes were median diameters determined by a laser diffraction scattering method.

[0091] The positive electrode active material and hydroxyapatite were weighed to a mass ratio of 88.4:11.6. These materials were placed in a dry particle compositing device "Hybridization System" (manufactured by Nara Machinery Manufacturing Co., Ltd.) and mechanically composited at a rotation speed of 9600 / min, a peripheral speed of 60 m / s, and a processing time of 3 minutes. In this way, a coated active material was obtained in which the surfaces of the positive electrode active material particles were coated with an apatite-containing particle layer.

[0092] [Preparation of Evaluation Cell] The coated active material prepared as described above, acetylene black (AB), and polyvinylidene fluoride (PVDF) were mixed with N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode slurry. The mass ratio of these materials in the positive electrode active material layer was coated active material:AB:PVDF = 96:2:2.

[0093] The positive electrode slurry was applied to the surface of an aluminum foil (1.45 cm x 1.45 cm), the coating was dried, and then rolled to form a positive electrode active material layer. In this way, a positive electrode was obtained.

[0094] The nonaqueous electrolyte was prepared as follows. First, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of EC:EMC:DMC = 20:5:75 to prepare a nonaqueous solvent. LiPF was dissolved in the resulting nonaqueous solvent to a concentration of 1 mol / L to obtain a solution. This solution was used as the nonaqueous electrolyte.

[0095] A laminate half-cell was fabricated using a positive electrode, a Li metal foil (2 cm × 2 cm, 200 μm thick) as a counter electrode, a separator, and a non-aqueous electrolyte. The separator was a PP / PE / PP trilayer separator.

[0096] In this manner, the evaluation cell of Example 1 was obtained.

[0097] Example 2 In the preparation of the coated active material, the mass ratio of the positive electrode active material to the hydroxyapatite was changed to positive electrode active material:hydroxyapatite = 95.0:5.0. Except for this, the coated active material and the evaluation cell were prepared in the same manner as in Example 1.

[0098] Comparative Example In the preparation of the positive electrode, the positive electrode active material used in Example 1 was used without being coated with an apatite-containing particle layer. 0.88 Co 0.085 Mn 0.035 The evaluation cell was fabricated in the same manner as in Example 1 except for the positive electrode active material having a composition of 0, an average particle diameter of 4.5 μm, and a surface that was not coated with an apatite-containing particle layer.

[0099] [Battery Evaluation] (Power Output Test) The evaluation cells of Examples 1, 2, and the Comparative Example were charged at a constant current of 0.2 C at an ambient temperature of 25°C until the voltage reached 4.5 V, and then charged at a constant voltage of 4.5 V. Charging was terminated at a current of 0.02 C. After a 20-minute pause, constant current discharge was performed at a current of 0.1 C until the voltage reached 2.5 V, followed by a 20-minute pause. Next, charging was performed again under the above charging conditions to determine the initial charge capacity. After a 20-minute pause, constant current discharge was performed at a current of 0.2 C until the voltage reached 2.5 V, and the initial discharge capacity was determined. The initial charge and discharge efficiencies were measured using the obtained initial charge and discharge capacities. The initial discharge capacities of the evaluation cells of Examples 1, 2, and the Comparative Example were then determined. The output characteristics were evaluated based on the initial charge and discharge efficiencies calculated from the initial charge and discharge capacities.

[0100] (Trickle Test) After the output characteristics were evaluated by the above method, the test cells of Example 1, Example 2, and Comparative Example were subjected to a trickle test according to the following procedure. Constant current charging was performed at a current value of 0.2 C until the voltage reached 4.6 V, and then trickle charging was performed at an ambient temperature of 55°C, maintaining the voltage at 4.6 V for 72 hours. The integrated charge capacity from 48 to 72 hours of trickle charging was calculated from the trickle charge curve. This integrated charge capacity correlates with the amount of side reactions in the battery, and it can be determined that the larger the integrated charge capacity, the greater the amount of side reactions.

[0101] (Cycle Test) After the output characteristics were evaluated by the above method, the evaluation cells of Example 1, Example 2, and Comparative Example were changed to an ambient temperature of 55°C. These cells were charged at a constant current of 0.2 C until the voltage reached 4.5 V, followed by a 20-minute pause. Subsequently, they were discharged at a constant current of 0.2 C until the voltage reached 2.5 V, followed by a 20-minute pause. This cycle of charge and discharge was repeated 45 times to determine the discharge capacity, charge / discharge efficiency, and capacity retention. From these results, the cycle characteristics of the battery were evaluated using the ratio of the discharge capacity at the 42nd cycle to the discharge capacity at the 3rd cycle (discharge capacity retention) and the charge / discharge efficiency at the 42nd cycle.

[0102] The evaluation results of the above power test, trickle test, and cycle test are shown in Table 1.

[0103]

[0104] (Discussion) From the results of the cycle test, the secondary batteries of Examples 1 and 2, which used coated active materials in which a positive electrode active material was coated with an apatite-containing particle layer, had significantly improved capacity retention rates (discharge capacity at the 42nd cycle / discharge capacity at the 3rd cycle) compared to the secondary battery of the comparative example, which used a positive electrode active material not coated with an apatite-containing particle layer. Furthermore, the secondary batteries of Examples 1 and 2 had charge-discharge efficiencies of over 99% at the 42nd cycle, which was improved compared to the secondary battery of the comparative example. Thus, the coated active material in which a positive electrode active material was coated with an apatite-containing particle layer was able to improve the cycle characteristics of the secondary battery. Furthermore, from the results of the trickle test, the secondary batteries of Examples 1 and 2 had smaller amounts of side reactions than the secondary battery of the comparative example, confirming that the configuration in which the positive electrode active material was coated with an apatite-containing particle layer effectively suppressed side reactions at the positive electrode.

[0105] As described above, the results of the cycle test and trickle test confirmed that a coated active material in which a positive electrode active material is coated with an apatite-containing particle layer can effectively suppress side reactions in the positive electrode and improve cycle characteristics.

[0106] Furthermore, the results of the output test showed that the secondary batteries of Examples 1 and 2 had output characteristics almost equal to or better than those of the secondary battery of the comparative example, even though the positive electrode active material was coated with the apatite-containing particle layer 13. As can be seen from the results of the output test for the secondary battery of Example 2, by adjusting the ratio of the apatite-containing particle layer to the entire coated active material to, for example, 6.0 mass % or less, the provision of the apatite-containing particle layer actually improved the output characteristics. This is presumably due to the special Li-ion conductivity-enhancing effect of the apatite-containing particles 12, as described above.

[0107] The technology of the present disclosure is useful for, for example, lithium ion secondary batteries.

Claims

1. A coated active material comprising: a positive electrode active material; and an apatite-containing particle layer containing apatite-containing particles, the apatite-containing particle layer covering at least a portion of the surface of particles of the positive electrode active material.

2. The coated active material according to claim 1, wherein the apatite-containing particles contain apatite having a composition represented by the following formula (1): Ca x (PO4) y (A) z In the formula (1), A is at least one selected from the group consisting of OH, F, and Cl, and 0<x<10, 0<y<6, and 0≦z<2 are satisfied.

3. The coated active material according to claim 2, wherein A is OH and 0<z<2 is satisfied.

4. The coated active material according to claim 2, wherein the apatite-containing particles contain at least one selected from the group consisting of Ca5(PO4)3(OH), Ca5(PO4)3F, and Ca5(PO4)3Cl as the apatite having a composition represented by formula (1).

5. The coated active material according to claim 1, wherein the apatite contained in the apatite-containing particles has a hexagonal crystal structure.

6. The coated active material according to claim 1, wherein the apatite-containing particles have a particle size of 1 μm or less.

7. The coated active material according to claim 1, wherein the proportion of the apatite-containing particles in the coated active material is 0.1 mass % or more and 15 mass % or less.

8. The coated active material according to claim 1, wherein a ratio of a particle diameter of the particles of the positive electrode active material to a particle diameter of the apatite-containing particles (the particle diameter of the particles of the positive electrode active material / the particle diameter of the apatite-containing particles) is 2 or more and 40,000 or less.

9. A positive electrode comprising: a positive electrode current collector; and a positive electrode active material layer supported on the positive electrode current collector, wherein the positive electrode active material layer contains the coated active material according to any one of claims 1 to 8.

10. A secondary battery comprising: the positive electrode according to claim 9; a negative electrode; and an electrolyte.

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