Positive electrode for lithium secondary batteries and lithium secondary battery using same

Abstract

The present invention provides a means that is capable of suppressing an increase in the internal resistance of a battery in a lithium secondary battery in which a positive electrode active material layer contains a sulfide solid electrolyte and a positive electrode active material that includes a lithium transition metal composite oxide. The present invention provides a positive electrode for lithium secondary batteries, the positive electrode being provided with a positive electrode active material layer which contains a sulfide solid electrolyte and a positive electrode active material that includes a lithium transition metal composite oxide. With respect to the positive electrode, the surfaces of particles of the lithium transition metal composite oxide are covered with a lithium ion conductive oxide wherein the energy of electrochemical reaction with the sulfide solid electrolyte is more than -0.574 [eV / atom] at the lithium reference potential (4.25 [V]) and the energy of electrochemical reaction with the lithium transition metal composite oxide is 0 [eV / atom] or more at the lithium reference potential (4.25 [V]).

Description

Positive electrode for lithium secondary battery and lithium secondary battery using the same

[0001] The present invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery using the same.

[0002] In recent years, research and development into all-solid-state lithium secondary batteries using oxide- or sulfide-based solid electrolytes has been actively pursued. Solid electrolytes are materials primarily composed of ionic conductors capable of ion conduction in a solid state. Therefore, all-solid-state lithium secondary batteries, in principle, do not encounter the various problems associated with flammable organic electrolytes that occur in conventional liquid-based lithium secondary batteries. In addition, the use of high-potential, high-capacity positive electrode materials and high-capacity negative electrode materials generally leads to significant improvements in the battery's power density and energy density.

[0003] Conventionally, in all-solid-state lithium secondary batteries, a cathode active material layer is prepared by molding a mixture containing a cathode active material and a solid electrolyte by a method such as powder compaction. When a cathode active material layer is prepared by such a method, there is a problem that the interfacial resistance between the cathode active material and the solid electrolyte increases, which in turn increases the internal resistance of the battery. Aiming to address this problem, for example, Japanese Patent Application Laid-Open No. 2009-266728 discloses a technology in which a resistance-layer-formation-suppressing coating layer is provided on the surface of the cathode active material, and the coating layer is prevented from containing fine particles of the cathode active material. According to Japanese Patent Application Laid-Open No. 2009-266728, this configuration makes it possible to reduce the resistance between the cathode active material and the solid electrolyte material, thereby obtaining an all-solid-state lithium secondary battery in which a decrease in output is suppressed. It should be noted that Japanese Patent Application Laid-Open No. 2009-266728 discloses that the coating layer is made of lithium niobate (LiNbO ) as a constituent material. ３ ) and lithium titanate (Li ２ TiO ３ ) is disclosed. Also, lithium zirconate (Li ２ ZrO ３ A technique for coating the surface of a positive electrode active material with a fluorine-containing compound is also known.

[0004] According to the investigations of the present inventors, it has been found that even when a secondary battery is fabricated using the technology described in JP 2009-266728 A, the internal resistance of the battery may not be sufficiently reduced. Specifically, it has been found that in a lithium secondary battery in which the positive electrode active material layer contains a positive electrode active material including a lithium transition metal composite oxide and a sulfide solid electrolyte and which is intended for use at a high cell voltage of 4.2 V or more, an increase in the internal resistance of the battery may still not be sufficiently prevented.

[0005] Therefore, an object of the present invention is to provide a means for suppressing an increase in the internal resistance of a lithium secondary battery in which the positive electrode active material layer contains a positive electrode active material including a lithium transition metal composite oxide and a sulfide solid electrolyte.

[0006] The present inventors have conducted extensive research to solve the above problems. ３ ) and lithium zirconate (Li ２ ZrO ３ The authors used computational science techniques to explore the reason why an increase in the internal resistance of a battery cannot be sufficiently suppressed when a coating layer is formed by coating the surface of a positive electrode active material with lithium niobate (LiNbO ３ ) was found to have insufficient stability (electrochemical stability) when exposed to high potentials. Also, unexpectedly, lithium zirconate (Li ２ ZrO ３ ) also proved to have insufficient stability (electrochemical stability) when exposed to a high potential. Based on these results, the inventors further explored conditions under which the electrochemical stability of both the lithium transition metal composite oxide and the sulfide solid electrolyte is excellent even when exposed to a high potential, and found that the above-mentioned problems can be solved by coating the surface of a positive electrode active material containing a lithium transition metal composite oxide with a material that satisfies these conditions, thereby completing the present invention.

[0007] One aspect of the present invention relates to a positive electrode for a lithium secondary battery, comprising a positive electrode active material layer containing a positive electrode active material including a lithium transition metal composite oxide and a sulfide solid electrolyte, wherein the surfaces of particles of the lithium transition metal composite oxide are coated with a lithium ion conductive oxide, and the electrochemical reaction energy of the lithium ion conductive oxide with the sulfide solid electrolyte is greater than −0.574 eV / atom at a lithium reference potential (4.25 V), and the electrochemical reaction energy of the lithium ion conductive oxide with the lithium transition metal composite oxide is greater than or equal to 0 eV / atom at a lithium reference potential (4.25 V).

[0008] Fig. 1 is a perspective view showing the appearance of a flat laminated all-solid-state lithium ion secondary battery, which is one embodiment of a secondary battery including a positive electrode for a lithium secondary battery according to the present invention. Fig. 2 is a cross-sectional view taken along line 2-2 shown in Fig. 1.

[0009] One embodiment of the present invention is a positive electrode for a lithium secondary battery including a positive electrode active material layer containing a positive electrode active material including a lithium transition metal composite oxide and a sulfide solid electrolyte, wherein surfaces of particles of the lithium transition metal composite oxide are coated with a lithium ion conductive oxide, and the electrochemical reaction energy of the lithium ion conductive oxide with the sulfide solid electrolyte is greater than −0.574 eV / atom at a lithium reference potential (4.25 V), and the electrochemical reaction energy of the lithium ion conductive oxide with the lithium transition metal composite oxide is equal to or greater than 0 eV / atom at a lithium reference potential (4.25 V).

[0010] Another aspect of the present invention is a cathode material for use in a lithium secondary battery cathode comprising a cathode active material layer containing a cathode active material and a sulfide solid electrolyte, the cathode material comprising a lithium transition metal composite oxide, the surfaces of particles of the lithium transition metal composite oxide being coated with a lithium ion conductive oxide, the electrochemical reaction energy of the lithium ion conductive oxide with the sulfide solid electrolyte being greater than −0.574 eV / atom at a lithium reference potential (4.25 V), and the electrochemical reaction energy of the lithium ion conductive oxide with the lithium transition metal composite oxide being 0 eV / atom or greater at a lithium reference potential (4.25 V). According to the present invention including these aspects, an increase in the internal resistance of a lithium secondary battery can be suppressed in which the cathode active material layer includes a cathode active material containing a lithium transition metal composite oxide and a sulfide solid electrolyte.

[0011] Hereinafter, embodiments of the positive electrode for a lithium secondary battery according to the present embodiment will be described with reference to the drawings. However, the technical scope of the present invention should be defined based on the description of the claims and is not limited to the following embodiments. Note that the dimensional proportions in the drawings are exaggerated for the convenience of explanation and may differ from the actual proportions.

[0012] FIG. 1 is a perspective view showing the appearance of a flat-layered all-solid-state lithium ion secondary battery, which is one embodiment of a secondary battery including a positive electrode for a lithium secondary battery according to the present invention. FIG. 2 is a cross-sectional view taken along line 2-2 shown in FIG. 1. By adopting a layered structure, the battery can be made compact and have a high capacity. In this specification, the flat-layered non-bipolar lithium ion secondary battery shown in FIGS. 1 and 2 (hereinafter also simply referred to as a "layered battery") will be described in detail as an example.

[0013] 1, the stacked battery 10a has a flat, rectangular shape, with a negative electrode current collector 25 and a positive electrode current collector 27 extending from both sides for extracting power. The power generating element 21 is wrapped in the battery exterior material (laminate film 29) of the stacked battery 10a, and the periphery is heat-sealed, with the negative electrode current collector 25 and the positive electrode current collector 27 extending to the outside.

[0014] 2, the stacked battery 10a of this embodiment has a structure in which a flat, generally rectangular power generating element 21, where charge and discharge reactions actually proceed, is sealed inside a laminate film 29, which is a battery exterior material. Here, the power generating element 21 has a configuration in which a positive electrode, a solid electrolyte layer 17, and a negative electrode are stacked. The positive electrode has a structure in which positive electrode active material layers 15 containing a positive electrode active material are disposed on both sides of a positive electrode current collector 11". The negative electrode has a structure in which negative electrode active material layers 13 containing a negative electrode active material are disposed on both sides of a negative electrode current collector 11'. Specifically, the positive electrode, solid electrolyte layer, and negative electrode are stacked in this order such that one positive electrode active material layer 15 faces an adjacent negative electrode active material layer 13 with a solid electrolyte layer 17 interposed therebetween. As a result, the adjacent positive electrode, solid electrolyte layer, and negative electrode constitute one unit cell layer 19. Therefore, it can be said that the stacked battery 10a shown in FIG. 1 has a structure in which a plurality of unit cell layers 19 are stacked and electrically connected in parallel.

[0015] Hereinafter, the main components of a secondary battery to which the positive electrode for a lithium secondary battery according to this embodiment is applied will be described.

[0016] [Current Collector] The current collector has a function of mediating the transfer of electrons from the electrode active material layer. There are no particular limitations on the material constituting the current collector. For example, metals and conductive resins can be used as the material constituting the current collector.

[0017] The current collector may have a single layer structure made of a single material, or may have a laminate structure made of an appropriate combination of layers made of these materials. From the viewpoint of reducing the weight of the current collector, it is preferable that the current collector include at least a conductive resin layer made of a resin having electrical conductivity. Furthermore, from the viewpoint of blocking the movement of lithium ions between the cell layers, a metal layer may be provided on a part of the current collector.

[0018] [Positive Electrode Active Material Layer] In the positive electrode for a lithium secondary battery according to this embodiment, the positive electrode active material layer contains a positive electrode active material. The positive electrode active material contained in the positive electrode active material layer essentially contains a lithium transition metal composite oxide. The lithium transition metal composite oxide is a compound belonging to the space group R3m and has a layered structure (layered rock salt structure) in which lithium atomic layers and transition metal atomic layers are alternately stacked. Therefore, the use of such a positive electrode active material can improve the battery capacity of a lithium ion secondary battery.

[0019] Examples of lithium transition metal composite oxides include LiCoO ２ , LiNiO ２ , LiMnO ２ , Li(Ni-Mn-Co)O ２ , Li(Ni-Co-Al)O ２ and those in which a part of these transition metals has been substituted with other elements. In some cases, two or more positive electrode active materials may be used in combination. More preferably, a composite oxide containing lithium and nickel is used, and even more preferably, Li(Ni—Mn—Co)O ２ and those in which part of these transition metals is replaced by other elements (hereinafter also referred to simply as "NMC composite oxides") or Li(Ni-Co-Al)O ２ Also, oxides in which part of these transition metals has been replaced by other elements (hereinafter simply referred to as "NCA composite oxides") are used, and NMC composite oxides are particularly preferred.

[0020] As described above, the NMC composite oxide and the NCA composite oxide also include composite oxides in which a portion of the transition metal element is replaced by another metal element. In this case, the other element may be Al, Sn, Nb, Ti, Zr, Mg, W, P, V, Ca, Sr, Cr, Fe, B, Ga, In, Si, Mo, Y, V, Cu, Ag, or Zn, preferably Al, Sn, Nb, Ti, Zr, Mg, W, P, V, Ca, Sr, or Cr, more preferably Al, Sn, Nb, Ti, Zr, Mg, P, or Cr, and even more preferably Al, Sn, Nb, Ti, Zr, or Mg from the viewpoint of improving cycle characteristics. However, the other metal element that can replace the transition metal element of the NCA composite oxide is other than Al.

[0021] The lithium transition metal composite oxide as the positive electrode active material has a high theoretical discharge capacity, and is therefore preferably a lithium transition metal composite oxide represented by the general formula (1): Li ｘ Ni ａ M ｂ N ｃ O ２ (wherein x, a, b, c, and d satisfy 0.8≦x≦1.1, a+b+c=1, 0.33≦a≦0.95, 0.05≦b≦0.67, and 0≦c≦0.10; M is one or more elements selected from the group consisting of Mn and Co, and N is one or more elements selected from the group consisting of Al, Sn, Nb, Ti, Zr, and Mg). Here, x represents the atomic ratio of Li, a represents the atomic ratio of Ni, b represents the atomic ratio of M, and c represents the atomic ratio of N. The composition of each element can be measured, for example, by inductively coupled plasma (ICP) emission spectrometry.

[0022] From the viewpoint of realizing a higher discharge capacity, it is preferable that in general formula (1), 0.80≦a≦0.95 (i.e., high-nickel composite oxide). Furthermore, the effect of configuring the positive electrode for a lithium secondary battery according to this embodiment is particularly pronounced when a high-nickel composite oxide is used as the positive electrode active material. Therefore, from this viewpoint as well, it is preferable that the positive electrode active material contains a high-nickel composite oxide.

[0023] Here, the content of the lithium transition metal composite oxide in the total amount (100% by mass) of the positive electrode active material is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, still more preferably 90% by mass or more, particularly preferably 95% by mass or more, and most preferably 100% by mass.

[0024] The positive electrode for a lithium secondary battery according to this embodiment is characterized in that the surfaces of the particles of the lithium transition metal composite oxide contained in the positive electrode active material layer are coated with a predetermined lithium ion conductive oxide (hereinafter also referred to as the "oxide of the present invention").

[0025] The oxide of the present invention has the following characteristics (1) and (2): (1) the electrochemical reaction energy with the sulfide solid electrolyte contained in the positive electrode active material layer is greater than −0.574 eV / atom at a lithium reference potential (4.25 V); (2) the electrochemical reaction energy with the lithium transition metal composite oxide contained in the positive electrode active material layer is 0 eV / atom or greater at a lithium reference potential (4.25 V).

[0026] According to the positive electrode for a lithium secondary battery of this embodiment, the surface of the particles of the positive electrode active material (lithium transition metal composite oxide) is coated with the lithium ion conductive oxide having the above-mentioned characteristics, thereby suppressing an increase in the internal resistance of the battery. Here, the above-mentioned characteristic (1) indicates that the oxide of the present invention is electrochemically stable with respect to a sulfide solid electrolyte. Furthermore, the above-mentioned characteristic (2) indicates that the oxide of the present invention is electrochemically stable with respect to a lithium transition metal composite oxide. Here, "electrochemically stable" means that the oxide of the present invention is unlikely to decompose under relatively high potential conditions (lithium reference potential (4.25 [V])), assuming a situation in which lithium ions are able to conduct inside the lithium ion conductive oxide. Therefore, by coating the lithium transition metal composite oxide, which is the positive electrode active material, with the oxide of the present invention having the above-mentioned characteristics (1) and (2), the coating layer is unlikely to decompose even under relatively high potential conditions at the end of charging. As a result, it is presumed that a high-resistance layer is unlikely to form on the surface of the particles of the positive electrode active material (lithium transition metal composite oxide), thereby suppressing an increase in the internal resistance of the battery.

[0027] The process by which the above characteristics (1) and (2) were derived as conditions that the oxide of the present invention must satisfy will be described below.

[0028] First, as described above, lithium niobate (LiNbO ３ ) and lithium zirconate (Li ２ ZrO ３ In order to investigate the cause of this, the inventors have investigated the use of lithium niobate (LiNbO ３ ) or lithium zirconate (Li ２ ZrO ３We attempted to calculate the electrochemical reaction energy at the interface between the cathode active material layer and the solid electrolyte layer when the coating layer is made of lithium-based transition metal composite oxide (Li-MTC). To calculate the electrochemical reaction energy, we used the "pymatgen.analysis.interface_reactions module" (see, for example, https: / / github.com / materialsproject / pymatgen / blob / master / pymatgen / analysis / interface_reactions.py), an analysis package in pymatgen, to simulate a system consisting of a cathode active material layer (containing a lithium transition metal composite oxide), a coating layer (containing a lithium ion conductive oxide), and a solid electrolyte layer (containing a sulfide solid electrolyte). The "Materials Project" mentioned below is a database of first-principles calculation results for materials (https: / / next-gen.materialsproject.org / ).

[0029] Specifically, first, lithium niobate (LiNbO ３ ) or lithium zirconate (Li ２ ZrO ３ The electrochemical reaction energy at the interface between lithium niobate (LiNbO ３ ) or lithium zirconate (Li ２ ZrO ３ The electrochemical reaction energy at the interface between the lithium-transition metal composite oxide and the cation exchanger was calculated.

[0030] Here, the following parameters were used as input parameters when calculating the electrochemical reaction energy by the above simulation.

[0031] (i) The type of each compound present on both sides of the interface; (ii) The energy value of the compound (i) above; (iii) The energy values ​​of all compounds that can theoretically be constructed from the constituent elements of the compound (i) above; (iv) The applied potential (set to 4.25 [V]); (v) The type of ion carrier in the electrochemical reaction when the potential is applied (set to lithium (Li)).

[0032] Regarding the above (i), the lithium transition metal composite oxide used in deriving feature (1) is a high nickel NMC composite oxide "LiNi ０．８ Mn ０．１ C ０．１ O ２ On the other hand, as the sulfide solid electrolyte for deriving feature (2), an argyrodite-type sulfide solid electrolyte "Li ６ P.S. ５ The coating layer was made of lithium niobate (LiNbO ３ ) or lithium zirconate (Li ２ ZrO ３ ) was set.

[0033] Next, for (ii) above, the "energy value of the compound (i) above" was the predicted formation energy value published in the Materials Project. Furthermore, for (iii) above, "all compounds that can theoretically be constructed from the constituent elements of the compound (i) above" were all compounds published in the Materials Project that could theoretically be constructed, taking into account the types and composition ratios of compounds present on both sides of the assumed interface (which were varied when calculating the electrochemical reaction energy). The energy values ​​of these compounds were, as above, the predicted formation energy values ​​published in the Materials Project.

[0034] Next, for the simulation using pymatgen, lithium niobate (LiNbO ３ ) or lithium zirconate (Li ２ ZrO ３ ) and sulfide solid electrolyte (Li ６ P.S. ５This section explains how to calculate the electrochemical reaction energy at the interface with Cl as an example. pymatgen is a Python library that allows easy handling of various physical property data such as crystal structure and band structure. pymatgen incorporates various classes, one of which is an interface called the MPRester class, which makes it possible to download data via the Materials Project API.

[0035] In calculating the electrochemical reaction energy, first, the input parameters (i), (iv), and (v) are input (as described above, the sulfide solid electrolyte in (i) is Li ６ P.S. ５ Cl). Then, the data (ii) and (iii) above are retrieved from the Materials Project database by an interface calculation using a Python program. Then, when the composition ratio of each compound present on both sides of the interface is varied in the range of 0:100 to 100:0, the reaction with the smallest mutual reaction energy is identified as the reaction most likely to proceed. As a result of these interface calculations, lithium zirconate (Li ２ ZrO ３ ) for sulfide solid electrolyte (Li ６ P.S. ５ The electrochemical reaction energy with lithium niobate (LiNbO ３ ) for sulfide solid electrolyte (Li ６ P.S. ５ The electrochemical reaction energy with HCl was −0.294 eV / atom at a lithium reference potential (4.25 V).

[0036] In addition, lithium niobate (LiNbO ３ ) or lithium zirconate (Li ２ ZrO ３ ) and the lithium transition metal composite oxide.６ P.S. ５ Cl) into a lithium transition metal composite oxide (LiNi ０．８ Mn ０．１ C ０．１ O ２ The same procedure as above was used except that the lithium zirconate (Li ２ ZrO ３ ) for lithium transition metal composite oxide (LiNi ０．８ Mn ０．１ C ０．１ O ２ The electrochemical reaction energy with lithium niobate (LiNbO ３ ) for lithium transition metal composite oxide (LiNi ０．８ Mn ０．１ C ０．１ O ２ The electrochemical reaction energy with lithium niobate (LiNbO ３ It was speculated that the reason why the coating layer made of lithium zirconate (Li) has a poor effect of suppressing the increase in internal resistance is that the electrochemical reaction energy with the lithium transition metal composite oxide is too small under high potential conditions. ２ ZrO ３ It was speculated that the reason why the coating layer made of the sulfide) is less effective in suppressing an increase in internal resistance is that the electrochemical reaction energy with the sulfide solid electrolyte is too small under high potential conditions.

[0037] Based on the above speculation, the inventors have derived the following condition that the constituent material of the coating layer must satisfy: that the electrochemical reaction energy with the sulfide solid electrolyte under high potential conditions is greater than a predetermined value (the above-mentioned feature (1)); and that the electrochemical reaction energy with the lithium transition metal composite oxide under high potential conditions is equal to or greater than a predetermined value (the above-mentioned feature (2)). These have led to the completion of the present invention.

[0038] Next, the inventors carried out high-throughput interface calculations to specifically search for a constituent material (lithium ion conductive oxide) of the coating layer that satisfies the above characteristics (1) and (2). Specifically, the inventors carried out simulations using the same method as above, using approximately 6,800 materials selected as lithium ion conductive oxides for forming the coating layer as a result of sequentially carrying out the following screenings (A) to (E).

[0039] (A) Compounds registered in the Materials Project (approximately 145,000 types); (B) Lithium-containing compounds (approximately 20,000 types); (C) E listed in the Materials Project ｈｕｌｌ (Energy above hull) value is E ｈｕｌｌ (D) The band gap value published in the Materials Project is greater than 0.5 eV (approximately 10,000 types); (E) The number of constituent elements is four or less (approximately 6,800 types).

[0040] Among the above screening criteria, (B) is based on the fact that if the coating layer does not contain lithium ions, the interfacial resistance between the positive electrode active material (lithium transition metal composite oxide) and the sulfide solid electrolyte would increase significantly. (C) imposes the condition that the lithium ion conductive oxide must have high thermodynamic stability, because if the thermodynamic stability of the lithium ion conductive oxide itself is low, it will self-decompose, thereby defeating the purpose of providing the coating layer. (D) imposes the condition that the lithium ion conductive oxide must have high electronic insulation, because if the lithium ion conductive oxide has low electronic insulation, it will promote the decomposition reaction of the sulfide solid electrolyte in contact with the coating layer when a potential is applied. (E) imposes the condition that the number of constituent elements must be relatively small, because the greater the number of constituent elements, the more difficult the synthesis becomes, leading to increased costs.

[0041] As a result of the high-throughput interface calculations described above, among approximately 6,800 kinds of lithium ion conductive oxides, sulfide solid electrolytes (Li ６P.S. ５ The electrochemical reaction energy with the lithium transition metal composite oxide (LiNiCl) is greater than -0.574 [eV / atom] at a lithium reference potential (4.25 [V]) (Feature (1)), and ０．８ Mn ０．１ C ０．１ O ２ The following compounds were selected as compounds that satisfy the condition (characteristic (2)) that the electrochemical reaction energy with lithium ion conductive oxide (NMC811) is 0 eV / atom or more at a lithium reference potential (4.25 V). By forming a coating layer using these lithium ion conductive oxides as constituent materials, it is possible to prevent an increase in the internal resistance of the battery under high potential conditions, which was insufficient with lithium niobate and lithium zirconate.

[0042] Li ４ CrFe ３ O ８ , NaLiTm ２ F ８ , LiYbAlF ６ , LiCaAlF ６ , SrLiAlF ６ , LiMgAlF ６ , LiLuF ４ , LiY ９ (Si ３ O １３ ) ２ , Li ２ Mg ２ (SO ４ ) ３ , LiYF ４ , Na ３ Li ３ Al ２ F １２ , K. ２ LiAlF ６ , CsLiSO ４ , RbLiSO ４ , KLiSO ４ , Li ２ Ca ４ Si ４ O １３ , LiCa ２ Nb ３ O １０ , LiScSiO ４ , Sr ２L&L ３ O １０ 、Sr ２ Li ２ Nb ４ O １３ 、LiTmSiOO ４ 、L&YSiOO ４ 、 ４ Ni ７ (OF ７ ) ２ 、Sr ３ Li ４ Nb ６ O ２０ 、 ２ CaTa ２ O ７ 、SrLi ２ Ta ２ O ７ 、 ２ Ca ２ Si ２ O ７ 、L-& ２ OF ３ 、L-& ５ O ３ F ５ 、SrLi ２ Nb ２ O ７ 、 ２ Hf ２ O ５ 、 ２ Zr ２ O ５ 、 ４ Ni ３ Sb ５ O １６ 、 ４ Ni ３ (OF ３ ) ２ 、L-L-OO ４ 、 ４ Ni ３ Se ５ O １６ 、 ４ Zr ３ O ８ 、L)#O ２ 、L-1O ３ 、L---CO ４ 、L---2O ４ 、L-& ２ O ２ F、L& ２Caｳiｯ ４ 、i ２ 59 ４ 、iInｯ ２ 、LiAlｮiｯ ３ 、, ８ Li ９ Cr ４ 9 １６ 、i ３ ii ４ 9 ４ 0. ３ 、ｳrLi ２ 3iｯ ４ 、LiNd(Moｯ ４ ) ) ２ 、LiNiOF ４ 、i ５ 6 ２ ii ５ 9 １２ 、i ２ iiｳnｯ ４ 、i ４ ii ３ 9 ３ 0. ４ 、i ２ iiｳnｯ ４ 、i １１ b ４ (e) ３ 9 ８ ) ) ３ 、i ２ 8iｯP ２ 、i ３ bbbiｯ ２ ) ) ４ 、i ３ ii ２ 9 ２ 0. ３ 、Baii ２ 3iｯ ４ 、i ５ 3m ３ (3bｯ) ６ ) ) ２ 、LRhｯ ２ 、bbiｭoｯ ４ 、i ３ ii ４ bｯ ８ 、i ２ fｯ ３ 、i ５ 6 ２ ii ３ 9 １０ 、i ２ 3.9３ , Li ３ Ni ２ SbO ６ , Li ４ Ni ３ WO ８ , Li ２ Ni ２ SbO ６ , Li ２ Ni ３ OW ３ .

[0043] In the above study, the electrochemical stability of the lithium ion conductive oxide (i.e., the resistance of the oxide to decomposition when it is assumed that lithium ions are able to conduct inside the lithium ion conductive oxide under conditions of a relatively high potential (lithium reference potential (4.25 [V])))) was used as an index. Separately, the present inventors also evaluated the stability (hereinafter also referred to as "chemical stability") of the lithium ion conductive oxide under conditions in which lithium ions are not able to conduct inside the lithium ion conductive oxide (under conditions in which the total amount of lithium ions in the system is constant). As a result, it was found that, among the compounds listed above, nine types of lithium ion conductive oxides shown in Table 1 below exhibited better electrochemical stability than lithium niobate and lithium zirconate when used in sulfide solid electrolytes (Li ６ P.S. ５ It also has excellent stability when in contact with lithium transition metal composite oxides (LiNi ０．８ Mn ０．１ C ０．１ O ２ It was also found that the stability when in contact with lithium niobate (LiNbO ３ ) and lithium zirconate (Li ２ ZrO ３ ) are also shown.

[0044]

[0045] From the results shown in Table 1 above, it can be said that in a preferred embodiment of the present invention, the lithium ion conductive oxide constituting the coating layer preferably contains, in addition to the Li element and the O element, the nine types of compounds listed in Table 1, and further contains: Hf element; Si element and an element other than Li, O, and Si; or Cr element and Fe element. Furthermore, from the viewpoint of higher stability (higher electrochemical reaction energy) with respect to the positive electrode active material and the sulfide solid electrolyte, the lithium ion conductive oxide is preferably Li ２ HfO ３ , Li ２ Hf ２ O ５ , Li ２ CaSiO ４ , Li ２ Ca ２ Si ２ O ７ , LiYSiO ４ , LiTmSiO ４ , Li ４ CrFe ３ O ８ and compounds in which a part of these constituent elements is substituted with other elements, and ２ Ca ２ Si ２ O ７ , LiYSiO ４ , LiTmSiO ４ , Li ４ CrFe ３ O ８ and compounds in which a part of these constituent elements is substituted with other elements, and ４ , LiTmSiO ４ , Li ４ CrFe ３ O ８ and compounds in which a part of these constituent elements is substituted with other elements, and ４ CrFe ３ O ８It is particularly preferred that the compound contains one or more compounds selected from the group consisting of compounds in which some of the constituent elements are substituted with other elements.

[0046] Furthermore, in another preferred embodiment, from the viewpoint of further increasing the lithium ion conductivity of the coating layer made of a lithium ion conductive oxide and thereby reducing the internal resistance of the battery, it is preferable that the crystalline structure of the lithium ion conductive oxide has a lithium ion conduction path along at least one linear direction. Whether or not a certain lithium ion conductive oxide satisfies this requirement can be determined by referring to the crystalline structure of the oxide published in a database such as Materials Project, and using as an index whether or not lithium elements are arranged along at least one direction (dimension).

[0047] As described above, in the positive electrode active material used in the positive electrode for a lithium secondary battery according to this embodiment, the surface of the lithium transition metal composite oxide particle is coated with a predetermined lithium ion conductive oxide. Here, the coverage is not particularly limited, but is preferably 50% or more. Here, "coverage" refers to the percentage of the surface area of ​​the lithium transition metal composite oxide particle that is covered with the lithium ion conductive oxide. From the viewpoint of more effectively achieving the effects of the present invention, the coverage is preferably 60% or more, more preferably 70% or more, even more preferably 80% or more, and particularly preferably 90% or more. There is also no particular upper limit to the coverage, but it is usually 99% or less, preferably 95% or less. The "coverage" value can be calculated from an image of the positive electrode active material observed using a scanning electron microscope (SEM).

[0048] The thickness of the coating layer made of a lithium ion conductive oxide that coats the surface of the lithium transition metal composite oxide particles is not particularly limited, but is preferably 2 to 20 nm, more preferably 2 to 15 nm, even more preferably 2 to 10 nm, and particularly preferably 3 to 8 nm. When the thickness of the coating layer made of a lithium ion conductive oxide is within this range, the relative volume of the coating layer with respect to the volume of the lithium transition metal composite oxide is very small, and therefore, the decrease in the volumetric energy density of the secondary battery due to the arrangement of the coating layer can be minimized.

[0049] There are no particular limitations on the method for coating the surface of lithium transition metal composite oxide particles with a lithium ion conductive oxide to obtain the positive electrode active material according to this embodiment. For dry methods, a milling device such as a planetary ball mill can be used as a conventionally known method for adhering (attaching) a large number of other particles to the surface of the particles. In some cases, a mechanofusion system from Hosokawa Micron Corporation may also be used. Furthermore, when forming a coating layer by a wet method, in addition to the method described in the above-mentioned document (JP 2009-266728 A), it is possible to carry out the method by referring to known documents (Xiaowen Xhan et al., Electrochimia Acta 300 (2019) 36-44, Seitaro Ito et al., Journal of Power Sources 248 (2014) 943-950), etc.

[0050] The positive electrode active material layer further includes a sulfide solid electrolyte. When the positive electrode active material layer includes a sulfide solid electrolyte, the ionic conductivity of the positive electrode active material layer can be improved.

[0051] Examples of sulfide solid electrolytes include LiI-Li ２ S-SiS ２ , LiI-Li ２ S-P ２ O ５ , LiI-Li ３ P.O. ４ -P ２ S ５ , Li ２ S-P２ S ５ , LiI-Li ３ P.S. ４ , LiI-LiBr-Li ３ P.S. ４ , Li ３ P.S. ４ , Li ２ S-P ２ S ５ - LiI, Li ２ S-P ２ S ５ -Li ２ O, Li ２ S-P ２ S ５ -Li ２ O-LiI, Li ２ S-SiS ２ , Li ２ S-SiS ２ - LiI, Li ２ S-SiS ２ - LiBr, Li ２ S-SiS ２ -LiCl, Li ２ S-SiS ２ -B ２ S ３ - LiI, Li ２ S-SiS ２ -P ２ S ５ - LiI, Li ２ S-B ２ S ３ , Li ２ S-P ２ S ５ -Z ｍ S ｎ (where m and n are positive numbers, and Z is Ge, Zn, or Ga), Li ２ S-GeS ２ , Li ２ S-SiS ２ -Li ３ P.O. ４ , Li ２ S-SiS ２ -Li ｘ MO ｙ (where x and y are positive numbers, and M is any of P, Si, Ge, B, Al, Ga, and In). ２ S-P２ S ５ " is written by Li ２ S and P ２ S ５ The same applies to other descriptions.

[0052] The sulfide solid electrolyte is, for example, Li ３ P.S. ４ It may have a Li framework. ４ P ２ S ７ It may have a Li framework. ４ P ２ S ６ It may have a Li skeleton. ３ P.S. ４ Examples of sulfide solid electrolytes having a skeleton include LiI-Li ３ P.S. ４ , LiI-LiBr-Li ３ P.S. ４ , Li ３ P.S. ４ In addition, Li ４ P ２ S ７ Examples of the sulfide solid electrolyte having a skeleton include a Li-P-S solid electrolyte called LPS (for example, Li ７ P ３ S １１ ) can be mentioned. In addition, examples of sulfide solid electrolytes include Li （４－ｘ） Ge （１－ｘ） P ｘ S ４ (x satisfies 0<x<1), or the like. Among them, the sulfide solid electrolyte is preferably a sulfide solid electrolyte containing a P element, and the sulfide solid electrolyte is preferably a sulfide solid electrolyte containing Li ２ S-P ２ S ５ It is more preferable that the sulfide solid electrolyte contains a halogen (F, Cl, Br, I). As the sulfide solid electrolyte containing a halogen, an argyrodite-type solid electrolyte (Li ６ P.S. ５CX (wherein X is a halogen atom) is also a material that can be preferably used.

[0053] In addition, the sulfide solid electrolyte is Li ２ S-P ２ S ５ In the case of the system, Li ２ S and P ２ S ５ The ratio is the molar ratio of Li ２ S:P ２ S ５ = 50:50 to 100:0, and among these, Li ２ S:P ２ S ５ It is preferable that the ratio is 70:30 to 80:20.

[0054] The sulfide solid electrolyte may be sulfide glass, crystallized sulfide glass, or a crystalline material obtained by a solid-phase method. The sulfide glass can be obtained, for example, by mechanical milling (ball mill, etc.) a raw material composition. The crystallized sulfide glass can be obtained, for example, by heat treating the sulfide glass at a temperature equal to or higher than the crystallization temperature. The lithium ion conductivity of the sulfide solid electrolyte at room temperature (25° C.) is, for example, 1×10 －５ S / cm or more, and preferably 1×10 －４ The lithium ion conductivity of the sulfide solid electrolyte can be measured by an AC impedance method.

[0055] The positive electrode active material layer may further contain at least one of a conductive additive and a binder in addition to the above-mentioned positive electrode active material and solid electrolyte.

[0056] Examples of conductive additives include metals such as aluminum, stainless steel (SUS), silver, gold, copper, and titanium, alloys or metal oxides containing these metals, carbon fibers (specifically, vapor-grown carbon fibers (VGCF), polyacrylonitrile-based carbon fibers, pitch-based carbon fibers, rayon-based carbon fibers, activated carbon fibers, etc.), carbon nanotubes (CNT), and carbon black (specifically, acetylene black, Ketjen Black (registered trademark), furnace black, channel black, thermal lamp black, etc.). Also, particulate ceramic materials or resin materials coated with the above-mentioned metal materials by plating or the like can also be used as conductive additives. Among these conductive additives, from the viewpoint of electrical stability, it is preferable to include at least one selected from the group consisting of aluminum, stainless steel, silver, gold, copper, titanium, and carbon, more preferably at least one selected from the group consisting of aluminum, stainless steel, silver, gold, and carbon, and even more preferably at least one carbon. These conductive additives may be used alone or in combination of two or more.

[0057] The conductive additive is preferably in the form of particles or fibers. When the conductive additive is in the form of particles, the shape of the particles is not particularly limited, and may be any shape such as powder, sphere, rod, needle, plate, column, irregular shape, scale, or spindle shape.

[0058] When the conductive additive is in a particulate form, the average particle size (primary particle size) is not particularly limited, but is preferably 0.01 to 10 μm from the viewpoint of the electrical characteristics of the battery.

[0059] When the positive electrode active material layer contains a conductive additive, the content of the conductive additive in the positive electrode active material layer is not particularly limited, but is preferably 0 to 10 mass %, more preferably 2 to 8 mass %, and even more preferably 4 to 7 mass %, relative to the total mass of the positive electrode active material layer. Within such a range, a stronger electron conduction path can be formed in the positive electrode active material layer, which can effectively contribute to improving battery characteristics.

[0060] On the other hand, the binder is not particularly limited, but examples thereof include the following materials:

[0061] Thermoplastic polymers such as polybutylene terephthalate, polyethylene terephthalate, polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements), carboxymethyl cellulose, polyethylene, polypropylene, polymethylpentene, polybutene, polyether nitrile, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and hydrogenated products thereof, styrene-isoprene-styrene block copolymer and hydrogenated products thereof, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (P fluororesins such as vinylidene fluoride-hexafluoropropylene-based fluororubbers (VDF-HFP-based fluororubbers), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubbers (VDF-HFP-TFE-based fluororubbers), vinylidene fluoride-pentafluoropropylene-based fluororubbers (VDF-PFP-based fluororubbers), Examples of the fluororubber include vinylidene fluoride-based fluororubbers such as vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubbers (VDF-PFP-TFE-based fluororubbers), vinylidene fluoride-perfluoromethylvinyl ether-tetrafluoroethylene-based fluororubbers (VDF-PFMVE-TFE-based fluororubbers), and vinylidene fluoride-chlorotrifluoroethylene-based fluororubbers (VDF-CTFE-based fluororubbers), and epoxy resins. Among these, polyimide, styrene-butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are more preferred.

[0062] The thickness of the positive electrode active material layer varies depending on the intended configuration of the lithium secondary battery, but is preferably within the range of 0.1 to 1000 μm, for example.

[0063] [Negative Electrode (Negative Electrode Active Material Layer)] In the lithium secondary battery according to the above embodiment, the negative electrode active material layer 13 contains a negative electrode active material. The type of negative electrode active material is not particularly limited, but examples include carbon materials, metal oxides, and metal active materials. Furthermore, a lithium-containing metal may be used as the negative electrode active material. Such a negative electrode active material is not particularly limited as long as it is a lithium-containing active material, and examples thereof include lithium metal and lithium-containing alloys. Examples of lithium-containing alloys include alloys of Li with at least one of In, Al, Si, and Sn. In some cases, two or more negative electrode active materials may be used in combination. Of course, negative electrode active materials other than those described above may also be used. The negative electrode active material preferably contains lithium metal or a lithium-containing alloy, a silicon-based negative electrode active material, or a tin-based negative electrode active material, and more preferably contains lithium metal or a lithium-containing alloy. When lithium metal or a lithium-containing alloy is used as the negative electrode active material, the lithium secondary battery as an electrical device may be a so-called lithium deposition type in which lithium metal as the negative electrode active material is deposited on the negative electrode current collector during the charging process. Therefore, in this configuration, the thickness of the negative electrode active material layer increases as the charging process progresses, and the thickness of the negative electrode active material layer decreases as the discharging process progresses. Although the negative electrode active material layer may not be present during full discharge, in some cases, a negative electrode active material layer consisting of a certain amount of lithium metal may be present during full discharge.

[0064] The shape of the negative electrode active material may be, for example, particulate (spherical, fibrous), thin film, etc. When the negative electrode active material is particulate, the average particle diameter is, for example, preferably in the range of 1 nm to 100 μm, more preferably in the range of 10 nm to 50 μm, even more preferably in the range of 100 nm to 20 μm, and particularly preferably in the range of 1 to 20 μm.

[0065] The content of the negative electrode active material in the negative electrode active material layer is not particularly limited, but is preferably in the range of 40 to 99 mass %, and more preferably in the range of 50 to 90 mass %. The negative electrode active material layer may further contain a solid electrolyte, a conductive additive, and / or a binder, and specific and preferred forms thereof can be similarly adopted as those described in the section on the positive electrode active material layer above.

[0066] The thickness of the negative electrode active material layer varies depending on the intended configuration of the lithium secondary battery, but is preferably within the range of 0.1 to 1000 μm, for example.

[0067] [Solid Electrolyte Layer] In the lithium secondary battery according to the above-described embodiment, the solid electrolyte layer is interposed between the above-described positive electrode active material layer and negative electrode active material layer, and is a layer essentially containing a solid electrolyte. The specific form of the solid electrolyte contained in the solid electrolyte layer is not particularly limited, and the exemplary and preferred forms described in the section on the positive electrode active material layer may be similarly adopted. The solid electrolyte layer may also contain an oxide solid electrolyte. However, from the viewpoint of excellent ion conductivity and further improvement of battery performance, it is preferable that the solid electrolyte layer contains a sulfide solid electrolyte.

[0068] The solid electrolyte layer may further contain a binder in addition to the above-described solid electrolyte. The examples and preferred embodiments described in the section on the positive electrode active material layer may also be used for the binder that can be contained in the solid electrolyte layer.

[0069] The thickness of the solid electrolyte layer varies depending on the configuration of the intended lithium secondary battery, but from the viewpoint of improving the volumetric energy density of the battery, it is preferably 600 μm or less, more preferably 500 μm or less, and even more preferably 400 μm or less. On the other hand, there is no particular restriction on the lower limit of the thickness of the solid electrolyte layer, but it is preferably 10 μm or more, more preferably 50 μm or more, and even more preferably 100 μm or more.

[0070] [Positive electrode current collector plate and negative electrode current collector plate] The material constituting the current collector plate (25, 27) is not particularly limited, and known highly conductive materials conventionally used as current collector plates for secondary batteries can be used. Metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferred as constituent materials of the current collector plate. From the viewpoints of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The positive electrode current collector plate 27 and the negative electrode current collector plate 25 may be made of the same material or different materials.

[0071] [Positive Electrode Lead and Negative Electrode Lead] Although not shown in the drawings, the current collectors (11", 11') and the current collector plates (27, 25) may be electrically connected via a positive electrode lead or a negative electrode lead. As the constituent materials of the positive electrode and the negative electrode lead, materials used in known lithium ion secondary batteries can be similarly adopted. Note that the portion removed from the exterior is preferably covered with a heat-resistant, insulating heat-shrinkable tube or the like so as to prevent contact with peripheral devices or wiring, etc., causing electrical leakage and affecting products (for example, automobile parts, particularly electronic devices, etc.).

[0072] [Battery Exterior Material] As the battery exterior material, a known metal can case can be used. Alternatively, a bag-shaped case using an aluminum-containing laminate film 29 that can cover the power-generating element can be used, as shown in Figures 1 and 2 . The laminate film can be, for example, a three-layer laminate film formed by laminating PP, aluminum, and nylon in this order, but is not limited thereto. A laminate film is desirable from the viewpoint of achieving high output and excellent cooling performance, making it suitable for use in batteries for large equipment such as EVs and HEVs. Furthermore, an aluminum-containing laminate film is more preferable as the exterior material because it allows for easy adjustment of the collective pressure applied to the power-generating element from the outside.

[0073] The stacked battery according to the present embodiment has a configuration in which a plurality of unit cell layers are connected in parallel, and therefore has high capacity and excellent cycle durability, and is therefore suitable for use as a power source for driving EVs and HEVs.

[0074] Although one embodiment of a lithium secondary battery has been described above, the present invention is not limited to the configuration described in the above embodiment, and can be modified as appropriate based on the claims.

[0075] For example, the type of battery to which the lithium secondary battery according to the present invention is applied includes a bipolar battery including a bipolar electrode having a positive electrode active material layer electrically coupled to one surface of a current collector and a negative electrode active material layer electrically coupled to the opposite surface of the current collector.

[0076] Furthermore, the lithium secondary battery according to the present embodiment does not have to be an all-solid-state type. That is, the solid electrolyte layer may further contain a conventionally known liquid electrolyte (electrolytic solution). There is no particular limitation on the amount of liquid electrolyte (electrolytic solution) that can be contained in the solid electrolyte layer, but it is preferably an amount that allows the shape of the solid electrolyte layer formed by the solid electrolyte to be maintained and prevents leakage of the liquid electrolyte (electrolytic solution).

[0077] The liquid electrolyte (electrolytic solution) that can be used has a form in which a lithium salt is dissolved in an organic solvent. Examples of the organic solvent that can be used include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propionate (MP), methyl acetate (MA), methyl formate (MF), 4-methyldioxolane (4MeDOL), dioxolane (DOL), 2-methyltetrahydrofuran (2MeTHF), tetrahydrofuran (THF), dimethoxyethane (DME), propylene carbonate (PC), butylene carbonate (BC), dimethyl sulfoxide (DMSO), and γ-butyrolactone (GBL). Among these, from the viewpoint of further improving the rapid charging characteristics and output characteristics, the organic solvent is preferably a chain carbonate, more preferably at least one selected from the group consisting of diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC), and more preferably selected from ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC).

[0078] The lithium salt is Li(FSO ２ ) ２ N(lithium bis(fluorosulfonyl)imide; LiFSI), Li(C ２ F ５ SO ２ ) ２ N, LiPF ６ , LiBF ４ , LiClO ４ , LiAsF ６ , LiCF ３ SO ３ Among them, the lithium salt is preferably Li(FSO ２ ) ２ N(LiFSI).

[0079] The liquid electrolyte (electrolytic solution) may further contain additives other than the above-mentioned components. Specific examples of such compounds include ethylene carbonate, vinylene carbonate, methyl vinylene carbonate, dimethyl vinylene carbonate, phenyl vinylene carbonate, diphenyl vinylene carbonate, ethyl vinylene carbonate, diethyl vinylene carbonate, vinyl ethylene carbonate, 1,2-divinyl ethylene carbonate, 1-methyl-1-vinyl ethylene carbonate, 1-methyl-2-vinyl ethylene carbonate, 1-ethyl-1-vinyl ethylene carbonate, and 1-ethyl-2-vinyl ethylene carbonate. Examples of the additive include ethylene carbonate, vinyl vinylene carbonate, allyl ethylene carbonate, vinyloxymethyl ethylene carbonate, allyloxymethyl ethylene carbonate, acryloxymethyl ethylene carbonate, methacryloxymethyl ethylene carbonate, ethynyl ethylene carbonate, propargyl ethylene carbonate, ethynyloxymethyl ethylene carbonate, propargyloxyethylene carbonate, methylene ethylene carbonate, and 1,1-dimethyl-2-methylene ethylene carbonate. These additives may be used alone or in combination of two or more. Furthermore, when an additive is used in the electrolyte solution, the amount used can be adjusted as appropriate.

[0080] The following embodiments are also included within the scope of the present invention: a positive electrode according to claim 1 having the features of claim 2; a positive electrode according to claim 1 or 2 having the features of claim 3; a positive electrode according to claim 3 having the features of claim 4; a positive electrode according to any one of claims 1 to 4 having the features of claim 5; a positive electrode according to any one of claims 1 to 5 having the features of claim 6; a lithium secondary battery comprising a positive electrode according to any one of claims 1 to 6; and a lithium secondary battery according to claim 7 having the features of claim 8.

[0081] This application claims priority to Japanese Patent Application No. 2023-141521, filed on August 31, 2023, the contents of which are incorporated herein by reference in their entirety.

[0082] 10a: laminated battery, 11': negative electrode current collector, 11'': positive electrode current collector, 13: negative electrode active material layer, 15: positive electrode active material layer, 17: solid electrolyte layer, 19: single cell layer, 21: power generating element, 25: negative electrode current collector, 27: positive electrode current collector, 29: laminate film.

Claims

1. A positive electrode for a lithium secondary battery comprising a positive electrode active material layer containing: a positive electrode active material including a lithium transition metal composite oxide; and a sulfide solid electrolyte, wherein surfaces of particles of the lithium transition metal composite oxide are coated with a lithium ion conductive oxide, the electrochemical reaction energy of the lithium ion conductive oxide with the sulfide solid electrolyte is greater than -0.574 [eV / atom] at a lithium reference potential (4.25 [V]), and the electrochemical reaction energy of the lithium ion conductive oxide with the lithium transition metal composite oxide is equal to or greater than 0 [eV / atom] at a lithium reference potential (4.25 [V]).

2. The positive electrode for a lithium secondary battery according to claim 1, wherein the crystal structure of the lithium ion conductive oxide has at least one linear lithium ion conductive path.

3. The positive electrode for a lithium secondary battery according to claim 1, wherein the lithium ion conductive oxide further contains, in addition to Li and O, an Hf element, or further contains Si and an element other than Li, O and Si, or further contains Cr and Fe elements.

4. The lithium ion conductive oxide is Li ２ HfO ３ , Li ２ Hf ２ O ５ , Li ２ CaSiO ４ , Li ２ Ca ２ S ２ O ７ , LiYSiO ４ , LiTmSiO ４ , Li ４ CrFe ３ O ８ 4. The positive electrode for a lithium secondary battery according to claim 3, comprising one or more compounds selected from the group consisting of compounds in which a part of the constituent elements of these elements is substituted with other elements.

5. The lithium transition metal composite oxide is represented by the general formula (1): Li ｘ Ni ａ M ｂ N ｃ O ２ (wherein x, a, b, c, and d satisfy 0.8≦x≦1.1, a+b+c=1, 0.80≦a≦0.95, 0.05≦b≦0.20, and 0≦c≦0.10; M is one or more elements selected from the group consisting of Mn and Co, and N is one or more elements selected from the group consisting of Al, Sn, Nb, Ti, Zr, and Mg).

6. The sulfide solid electrolyte is Li ６ P.S. ５ 2. The positive electrode for a lithium secondary battery according to claim 1, comprising X, wherein X is a halogen atom.

7. A lithium secondary battery comprising the positive electrode for lithium secondary batteries according to any one of claims 1 to 6.

8. The lithium secondary battery according to claim 7, which is an all-solid-state lithium secondary battery.

9. A positive electrode material for use in a positive electrode for a lithium secondary battery having a positive electrode active material layer containing a positive electrode active material and a sulfide solid electrolyte, comprising a lithium transition metal composite oxide, wherein surfaces of particles of the lithium transition metal composite oxide are coated with a lithium ion conductive oxide, the lithium ion conductive oxide has an electrochemical reaction energy with the sulfide solid electrolyte that is greater than -0.574 [eV / atom] at a lithium reference potential (4.25 [V]), and the lithium ion conductive oxide has an electrochemical reaction energy with the lithium transition metal composite oxide that is equal to or greater than 0 [eV / atom] at a lithium reference potential (4.25 [V]).

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