Positive electrode for all-solid-state secondary battery, and all-solid-state secondary battery

Abstract

Provided are: an all-solid-state secondary battery having high capacity and excellent heat resistance; and a positive electrode for constituting the all-solid-state secondary battery. The all-solid-state secondary battery of the present invention relates to targets 3, 7, 11, and 12 of SDGs. This positive electrode for an all-solid-state secondary battery contains a positive electrode active material and a sulfide-based solid electrolyte, and is characterized in that at least a portion of the surface of the positive electrode active material is coated with a coating layer comprising a composition containing a compound containing a P-O bond and a compound containing a B-O bond. Furthermore, the all-solid-state secondary battery according to the present invention has at least one electrode body in which a negative electrode and a positive electrode face each other with a solid electrolyte layer interposed therebetween, and the positive electrode for all-solid-state secondary batteries according to the present invention is provided as the positive electrode.

Description

Technical Field

[0001] This invention relates to a high-capacity, heat-resistant all-solid-state secondary battery and a positive electrode for constituting the all-solid-state secondary battery. Background Technology

[0002] In recent years, with the development of portable electronic devices such as mobile phones and laptop computers, and the practical application of electric vehicles, there is a need for small, lightweight, high-capacity, and high-energy-density batteries.

[0003] Currently, in lithium secondary batteries, especially lithium-ion secondary batteries, that can meet this requirement, lithium-containing composite oxides such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2) are used as positive electrode active materials, graphite is used as negative electrode active materials, and organic electrolytes containing organic solvents and lithium salts are used as non-aqueous electrolytes.

[0004] Furthermore, with the further development of applications for lithium-ion secondary batteries, there is a growing demand for lithium-ion secondary batteries to have longer lifespans, higher capacity, and higher energy density, as well as high reliability.

[0005] However, the organic electrolytes used in lithium-ion secondary batteries contain organic solvents, which are flammable substances. Therefore, in the event of a short circuit or other abnormal situation, the organic electrolyte may generate abnormal heat. Furthermore, with the increasing energy density of lithium-ion secondary batteries and the growing trend of increasing organic solvent content in organic electrolytes in recent years, the reliability of lithium-ion secondary batteries is becoming increasingly critical.

[0006] Under the aforementioned conditions, an all-solid-state lithium secondary battery (all-solid-state secondary battery) that does not use organic solvents was also studied. The all-solid-state lithium secondary battery uses a molded solid electrolyte that does not use organic solvents to replace the conventional organic solvent-based electrolyte, eliminating concerns about abnormal heat generation of solid electrolytes and exhibiting high reliability.

[0007] Furthermore, all-solid-state rechargeable batteries not only possess high safety but also high reliability and environmental resistance, along with a long lifespan. Therefore, they are expected to contribute to social development while offering a safe and sustainable maintenance-free solution. Providing all-solid-state rechargeable batteries to society can contribute to the achievement of the 17 Sustainable Development Goals (SDGs) set by the United Nations: Goal 3 (ensuring healthy lifestyles and promoting the well-being of people of all ages), Goal 7 (ensuring access to affordable, reliable and sustainable modern energy for all), Goal 11 (achieving inclusive, safe, resilient and sustainable cities and human settlements), and Goal 12 (ensuring sustainable production and consumption patterns).

[0008] Most solid-state secondary batteries use sulfide-based solid electrolytes with excellent ion conductivity. Compositions containing Li3PO4, Li3BO3, Li2SO4, etc. have also been explored as solid electrolytes (Non-Patent Literature 1-3).

[0009] Furthermore, for the positive electrode of an all-solid-state secondary battery, a molded body containing a positive electrode compound such as a positive electrode active material, a solid electrolyte, and conductive additives is typically used, or a positive electrode with a layer (positive electrode compound layer) formed of the positive electrode compound on the current collector is formed. However, if the positive electrode active material and the solid electrolyte are in direct contact within the positive electrode, the solid electrolyte may oxidize and form a resistive layer, potentially reducing the ionic conductivity within the positive electrode. This problem is particularly prone to occur when a sulfide-based solid electrolyte is used as the solid electrolyte contained in the positive electrode compound.

[0010] On the other hand, research has also been conducted on the application of coatings containing niobium composite oxides such as LiNbO3, Al2O3, and Li3PO4 on the surface of the positive electrode active material in all-solid-state secondary batteries (Patent Documents 1-4, Non-Patent Documents 4 and 5). By using this method, direct contact between the positive electrode active material and the solid electrolyte can be prevented, and the decrease in ionic conductivity within the positive electrode caused by oxidation of the solid electrolyte can be suppressed.

[0011] Existing technical documents

[0012] Patent documents

[0013] Patent Document 1: Japanese Patent Application Publication No. 2014-154407

[0014] Patent Document 2: Japanese Patent Application Publication No. 2016-201342

[0015] Patent Document 3: International Publication No. 2021 / 221000

[0016] Patent Document 4: Japanese Patent Application Publication No. 2022-83502

[0017] Non-patent literature

[0018] Non-patent literature 1: Journal of Power Sources, 2014, Vol. 270, pp. 603-607 Non-patent literature 2: Solid State Ionics, 2017, Vol. 308, pp. 68-76

[0019] Non-patent literature 3: Bulletin of the Chemical Society of Japan, 2019, Vol. 92, pp. 1100-1106

[0020] Non-patent literature 4: Solid State Ionics, 2016, Vol. 285, pp. 79-82

[0021] Non-patent literature 5: Nano Letters, 2022, Vol. 22, pp. 7477-7483 Summary of the Invention

[0022] The problem that the invention aims to solve

[0023] However, as mentioned above, all-solid-state secondary batteries, by using a solid electrolyte instead of an organic solvent-based electrolyte, can be expected to have improved heat resistance, for example, for applications requiring high-temperature environments. However, in all-solid-state secondary batteries using a positive electrode active material containing a coating layer of LiNbO3 or Al2O3 and a sulfide-based solid electrolyte, according to the research of the present inventors, the degradation of the sulfide-based solid electrolyte within the positive electrode leads to the progression of resistive layer formation, which easily increases the resistance of the positive electrode, thereby causing a decrease in battery performance.

[0024] On the other hand, when using all-solid-state secondary batteries with positive electrode active materials containing Li3PO4 coating, although the resistance of the positive electrode is difficult to increase when placed in a high-temperature environment, due to the low lithium (Li) ion conductivity of Li3PO4, there are problems such as high initial resistance of the positive electrode (resistance immediately after manufacturing) and small discharge capacity, resulting in poor battery characteristics.

[0025] The present invention was made in view of the above circumstances, and its object is to provide a high-capacity all-solid-state secondary battery with excellent heat resistance and a positive electrode for constituting the all-solid-state secondary battery.

[0026] Solution for solving the problem

[0027] The positive electrode for all-solid-state secondary batteries of the present invention is characterized in that it contains a positive electrode active material and a sulfide-based solid electrolyte, wherein at least a portion of the surface of the positive electrode active material is coated with a coating layer, the coating layer having a composition comprising a compound containing a PO bond and a compound containing a BO bond.

[0028] Furthermore, the all-solid-state secondary battery of the present invention is characterized in that it has at least one stack in which the positive electrode and the negative electrode face each other with a solid electrolyte layer in between, and the positive electrode is the positive electrode for the all-solid-state secondary battery of the present invention.

[0029] Invention Effects

[0030] According to the present invention, a high-capacity all-solid-state secondary battery with excellent heat resistance and a positive electrode for constituting the all-solid-state secondary battery can be provided. Attached Figure Description

[0031] Figure 1 This is a cross-sectional view schematically illustrating an example of the all-solid-state secondary battery of the present invention.

[0032] Figure 2 This is a top view schematically illustrating another example of the all-solid-state secondary battery of the present invention.

[0033] Figure 3 yes Figure 2 Sectional view of line II. Detailed Implementation

[0034] Positive electrode for all-solid-state secondary batteries

[0035] The positive electrode (hereinafter sometimes simply referred to as "positive electrode") for all-solid-state secondary batteries of the present invention contains a positive electrode active material and a sulfide-based solid electrolyte. At least a portion of the surface of the positive electrode active material is coated with a coating layer, the coating layer having a composition comprising a compound containing a PO bond and a compound containing a BO bond.

[0036] The materials used as the coating layer for the positive electrode active material in all-solid-state secondary batteries, LiNbO3 and Al2O3, are anions composed solely of O. 2- The oxides formed therefore undergo oxidation at voltages above 4.1V. 2- A side reaction that extracts electrons. If such a side reaction occurs, highly reactive oxygen free radicals are generated, which react with sulfur (S), a component of sulfide-based solid electrolytes. 2- PS4 3- The reaction leads to the formation of a resistive layer at the interface between the positive electrode active material and the sulfide-based solid electrolyte, which hinders ion conduction. The formation of such a resistive layer is particularly prone to occur when all-solid-state secondary batteries are placed in high-temperature environments. Therefore, when using all-solid-state secondary batteries in high-temperature environments, by keeping the battery in a charging state, the formation of this resistive layer progresses, the internal resistance increases, and the battery characteristics deteriorate.

[0037] On the other hand, as mentioned above, in all-solid-state secondary batteries using positive electrode active materials containing a Li3PO4 coating layer that has a resistive layer that is difficult to form even at high temperatures, there is a problem of reduced initial battery characteristics (initial discharge capacity, load characteristics, etc.) due to insufficient ion conductivity of the coating layer itself.

[0038] Therefore, in this invention, when constructing a positive electrode having a positive active material and a sulfide-based solid electrolyte, at least a portion of the surface of the positive active material is coated with a coating layer, the coating layer having a composition comprising a compound containing a PO bond and a compound containing a BO bond.

[0039] Unlike coatings composed of LiNbO3 and Al2O3, coatings containing the aforementioned composition are less prone to side reactions that generate oxygen free radicals, as described above. Therefore, even when the all-solid-state secondary battery (the all-solid-state secondary battery of the present invention) having the positive electrode of the present invention is placed in a high-temperature environment, for example, around 160°C, the formation of a resistive layer at the interface between the positive electrode active material and the sulfide-based solid electrolyte can be suppressed, thereby inhibiting the increase in internal resistance.

[0040] Additionally, for example, the Li ion conductivity of Li3PO4 is 10. -8 In the S / cm range, compounds with PO bonds exhibit very low Li-ion conductivity, whereas in the aforementioned composition where both PO-containing and BO-containing compounds coexist, the Li-ion conductivity reaches as high as 10. -6 ~10 -5 The ratio is in the S / cm range. Therefore, even when the surface of the positive electrode active material is coated with the composition, the movement of Li ions between the positive electrode active materials and between the positive electrode active material and the sulfide-based solid electrolyte proceeds smoothly. As a result, the increase in the initial resistance of the positive electrode for all-solid-state secondary batteries, and consequently the initial internal resistance of the all-solid-state secondary batteries, is suppressed.

[0041] Through these effects, the positive electrode for the all-solid-state secondary battery of the present invention can form an all-solid-state secondary battery with high capacity and excellent heat resistance.

[0042] Examples of positive electrodes include positive electrodes (particles, etc.) consisting only of a molded body of a positive electrode compound containing a positive electrode active material and a sulfide-based solid electrolyte, and positive electrodes with a structure in which a layer (positive electrode compound layer) consisting of a molded body of a positive electrode compound is formed on a current collector.

[0043] As a positive electrode active material, any positive electrode active material previously known to be used in non-aqueous electrolyte secondary batteries, that is, an active material capable of absorbing and releasing Li ions, can be used, preferably satisfying any one of the following general formulas (1) to (4).

[0044] LiCo 1-x M 1 x O2(1)

[0045] In the general formula (1), M 1 The element is selected from the group consisting of Al, Mg, Ti, V, Cr, Zr, Fe, Ni, Cu, Zn, Ga, Ge, Nb, Mo, Sn, Sb and Ba, where 0 ≤ x ≤ 0.5.

[0046] LiNi 1-a-b Mn a M2 b O2(2)

[0047] In the general formula (2), M 2 To select at least one element from the group consisting of Co, Mg, Al, B, Ti, V, Cr, Fe, Cu, Zn, Zr, Mo, Sn, Ca, Sr, and W, 0 <a≤1,0≤b<1,0<a+b≤1。

[0048] LiNi 1-c M 3 c O2(3)

[0049] In the general formula (3), M 3 The element is selected from the group consisting of Al, Mg, Ti, Zr, Fe, Co, Cu, Zn, Ga, Ge, Nb, Mo, Sn, Sb and Ba, where 0 ≤ c ≤ 0.5.

[0050] LiM 4 d Mn 2-d O 4-e X e (4)

[0051] In the general formula (4), M 4 X is selected from at least one element in the group consisting of Li, B, Mg, Ca, Sr, Ba, Ti, V, Cr, Fe, Co, Ni, Cu, Al, Sn, Sb, In, Nb, Mo, W, Y, Ru and Rh, and X is selected from at least one element in the group consisting of F, Cl, Br and S, where 0 ≤ d ≤ 1.2 and 0 ≤ e ≤ 0.5.

[0052] Regardless of the type of active material, the effects of this invention can be obtained, but the higher the potential applied to the positive electrode during charging, the greater the effect.

[0053] It should be noted that, as long as it does not impair the effect of the present invention, impurities such as sodium sulfate generated during the synthesis of the positive electrode active material can also be attached to the surface of the positive electrode active material.

[0054] From the viewpoint of further reducing side reactions that cause battery capacity degradation and increasing the density of the positive electrode, the average particle size of the positive electrode active material is preferably 0.1 μm or more, more preferably 0.5 μm or more, and preferably 25 μm or less, more preferably 10 μm or less. It should be noted that the positive electrode active material can be primary particles or secondary particles formed by the aggregation of primary particles. If a positive electrode active material with an average particle size within the above-mentioned range is used, the interface between the material and the sulfide-based solid electrolyte contained in the positive electrode increases, thus further improving the load characteristics of the all-solid-state secondary battery using this positive electrode.

[0055] The average particle size of the positive electrode active material and the solid electrolyte mentioned later in this specification refers to the value of 50% of the diameter in the cumulative fraction of the volume reference when calculating the integral volume starting from the smallest particle using a particle size distribution measuring device (such as the Microtrac particle size distribution measuring device "HRA 9320" manufactured by Nikko Co., Ltd.). 50 ).

[0056] At least a portion of the surface of the positive electrode active material is coated with a coating layer having a composition comprising a compound containing a PO bond and a compound containing a BO bond.

[0057] Specific examples of compounds containing PO bonds include Li3PO4, Li4P2O7, LiTi2(PO4)3, LiLa(PO3)4, LiCs(PO3)2, and P2O5. Specific examples of compounds containing BO bonds include Li3BO3, Li2B4O7, LiBO2, Li2B2O4, B2O3, and LiBa(B3O5)3.

[0058] The composition constituting the coating layer preferably further comprises a compound containing SO bonds. In this case, the effect of reducing the initial resistance of the positive electrode and the effect of improving the heat resistance of the positive electrode (the effect of suppressing the formation of a resistive layer at the interface between the positive electrode active material and the sulfide-based solid electrolyte under high temperature conditions) are further enhanced.

[0059] Specific examples of compounds containing SO bonds include Li2SO4 and Li2S2O7.

[0060] The coating layer may contain inorganic particles along with the composition. By including inorganic particles in the coating layer, the coating rate of the coating layer on the surface of the positive electrode active material can be easily increased, further enhancing the effect brought about by the formation of the coating layer.

[0061] Specific examples of inorganic particles contained in the coating layer include Al2O3, BaTiO3, and Li4Ti5O. 12Examples of suitable coating materials include Li4GeO4, ZrO2, Li2ZrO3, LiNbO3, LiTaO3, WO3, LiWO2, MoO3, and Li2MoO4. It should be noted that when coating the positive electrode active material with oxide particles such as Al2O3, as mentioned above, the formation of a resistive layer at the interface between the positive electrode active material and the sulfide-based solid electrolyte progresses by placing the charged battery in a high-temperature environment. However, when the coating layer is composed of such oxide particles and compositions containing compounds with PO bonds and compounds containing BO bonds, although the reason is not yet certain, it has been determined that even when the charged battery is placed in a high-temperature environment, the formation of a resistive layer at the interface between the positive electrode active material and the sulfide-based solid electrolyte can be suppressed.

[0062] In the composition constituting the coating layer, the relationship between the content of phosphorus (P) from the compound containing PO bonds and the content of boron (B) from the compound containing BO bonds is preferably P ≥ B in elemental ratio. When the composition contains the compound containing PO bonds and the compound containing BO bonds in such a relationship, the Li ion conductivity of the composition is further improved, thereby further reducing the initial resistance of the positive electrode.

[0063] In the composition constituting the coating layer, from the viewpoint of improving the heat resistance of the all-solid-state secondary battery while achieving high capacity, the contents of the compound containing PO bonds and the compound containing BO bonds are preferably as follows: Regarding the contents of the compound containing PO bonds, when the total of P, B, and S elements is set to 100 mol%, P element is preferably 40 mol% or more, more preferably 45 mol% or more, more preferably 95 mol% or less, and more preferably 80 mol% or less. Furthermore, regarding the contents of the compound containing BO bonds, when the total of P, B, and S elements is set to 100 mol%, B element is preferably 5 mol% or more, more preferably 10 mol% or more, more preferably 50 mol% or less, and more preferably 40 mol% or less.

[0064] Furthermore, when the composition constituting the coating layer contains a compound containing SO bonds, from the viewpoint of better ensuring the aforementioned effects brought about by using a compound containing SO bonds, when the total amount of P, B, and S elements is set to 100 mol%, the S element is preferably 1 mol% or more, more preferably 3 mol% or more. Additionally, if the amount of the compound containing SO bonds in the composition constituting the coating layer is too high, the amount of compounds containing PO bonds and BO bonds will decrease, and the effects they bring may diminish. Therefore, regarding the content of the compound containing SO bonds in the composition constituting the coating layer, when the total amount of P, B, and S elements is set to 100 mol%, the S element is preferably 40 mol% or less, more preferably 35 mol% or less.

[0065] Regarding the content of the composition in the coating layer, from the viewpoint of better ensuring its effect, it is preferably 10% by mass or more, more preferably 20% by mass or more. It should be noted that the coating layer may consist solely of the composition, therefore the upper limit of the content of the composition in the coating layer is 100% by mass.

[0066] When the coating layer contains inorganic particles, from the viewpoint of ensuring the effects derived from its use, the content of inorganic particles in the coating layer is preferably 3% by mass or more, more preferably 5% by mass or more. It should be noted that the upper limit of the content of inorganic particles in the coating layer is preferably set within the range where the content of the composition in the coating layer satisfies the aforementioned preferred lower limit.

[0067] The compositional analysis of the coating and composition was performed according to the following steps. The cut surface of the electrode was smoothed by ion milling, followed by elemental analysis using either field emission electron beam microscopy (FE-EPMA) or Auger electron microscopy. Alternatively, the electrode cross-section could be fabricated by FIB processing of the electrode surface, and elemental analysis using scanning transmission electron microscopy (STEM) based on energy dispersive X-ray spectrophotometer (EDX) or electron energy loss spectrophotometer (EELS). During elemental analysis, to eliminate the influence of P, B, and S, which are components of the solid electrolyte, it was preferable to observe the interparticle gaps in the primary particles of the condensate of the positive electrode active material particles embedded in the coating. The elemental ratios of P, B, and S were calculated from the obtained elemental ratios.

[0068] Even with a thin coating layer between the positive electrode active material particles and the sulfide-based solid electrolyte, direct contact between the particles and the electrolyte can be prevented. This effectively suppresses the increase in internal resistance of the all-solid-state secondary battery caused by oxidation of the sulfide-based solid electrolyte. From the viewpoint of ensuring this effect even better, the average thickness of the coating layer is preferably 1 nm or more, more preferably 5 nm or more. It should be noted that even with an excessively thick coating layer, not only does the effect become saturated, but the Li-ion conductivity at the interfaces between the positive electrode active materials, at the interface between the positive electrode active material and the sulfide-based solid electrolyte, and the electronic conductivity between the positive electrode active material and the conductive additive tends to decrease. Therefore, the average thickness of the coating layer is preferably 100 nm or less, more preferably 30 nm or less.

[0069] In addition to compositional analysis of the coating and the composition, the average thickness of the coating layer can also be determined by performing various elemental mappings. Specifically, it can be evaluated by taking the arithmetic mean of the thickness of a region containing P, B, and O within the interval between a region containing a transition metal element that is a constituent element of the positive electrode active material and a region containing S, a constituent element of the solid electrolyte, and, when the solid electrolyte contains a halogen element, any one of F, Cl, Br, or I. The boundary of the region is defined as the location where the intensity of the signal attributable to the transition metal element that is a constituent element of the positive electrode active material and S, F, Cl, Br, or I, a constituent element of the solid electrolyte, is halved compared to the average intensity on the main body.

[0070] Furthermore, from the viewpoint of better ensuring the effect brought about by the coating layer, the coating rate of the coating layer on the surface of the positive electrode active material (the proportion of the area of ​​the portion covered by the coating layer in the surface area of ​​the positive electrode active material) is preferably 50% or more, more preferably 70% or more, and particularly preferably 100%.

[0071] Regarding the coating rate, for the test sample used in the thickness measurement, the average value is obtained by dividing the length of the boundary line between the coating layer of each positive electrode active material and the positive electrode active material particle by the perimeter of the cross section of the positive electrode active material particle for five or more particles.

[0072] When forming a coating layer on the positive electrode active material, the following method can be used: prepare an aqueous solution containing the constituent components of the composition (a compound containing a PO bond, a compound containing a BO bond, etc.), impregnate the positive electrode active material in the aqueous solution, or spray the aqueous solution onto the surface of the positive electrode active material so that the aqueous solution adheres to the surface of the positive electrode active material, and then dry to remove the water.

[0073] In addition, when forming a coating layer that also contains inorganic particles, the following method can be adopted: after the inorganic particles are attached to the surface of the positive electrode active material in a wet or dry manner, an aqueous solution containing the above composition is used, and the aqueous solution is attached in the same manner as above and then dried. By forming the coating layer using such a method, the coating rate of the coating layer in the surface of the positive electrode active material can be further improved.

[0074] The content of the positive electrode active material in the positive electrode mixture is preferably 40 to 90% by mass.

[0075] As the sulfide-based solid electrolyte in the positive electrode, in addition to particles such as Li2S-P2S5, Li2S-SiS2, Li2S-P2S5-GeS2, Li2S-B2S3-based glass, etc., thio-LISICON type substances [Li 10 GeP2S 12 、Li 9.54 Si 1.74 P 1.44 S 11.7 Cl 0.3 等Li 12-12g-h+i+6j- k M 5 3+g-h-i-j M 6 h M 7 i M 8 j M 9 12-k X k (where M 5 is Si, Ge or Sn, M 6 is P or V, M 7 is Al, Ga, Y or Sb, M 8 is Zn, Ca or Ba, M 9 is S, or S and O, X is F, Cl, Br or I, 0≤g<3, 0≤h+i+j≤3, 0≤k≤3), silver thiogermanate type substances [Li6PS5Cl, etc. composed of Li 7-k PS 6-k X k (where X represents one or more halogen elements, 0.2<k<2.0) substances, Li 7-m+ n PS 6-o Cl o+p (where 0.05≤m≤0.9, -3.0m+1.8≤n≤-3.0m+5.7) substances, Li 7-q PS 6-q ​r Br s (where q = r + s, 0 < q ≤ 1.8, 0.1 ≤ r / s ≤ 10.0), etc.

[0076] Among these sulfide-based solid electrolytes, a sulfide-based solid electrolyte of the argyrodite type with high Li ion conductivity and high chemical stability is particularly preferred.

[0077] In addition, the positive electrode may contain, together with the sulfide-based solid electrolyte, a solid electrolyte other than the sulfide-based solid electrolyte (such as a hydride-based solid electrolyte, a halide-based solid electrolyte, an oxide-based solid electrolyte, etc.).

[0078] Examples of the hydride-based solid electrolyte include LiBH4, a solid solution of LiBH4 and the following alkali metal compounds (for example, a solid solution with a molar ratio of LiBH4 to the alkali metal compound of 1:1 to 20:1), etc. Examples of the alkali metal compound in the solid solution include at least one selected from the group consisting of lithium halides (LiI, LiBr, LiF, LiCl, etc.), rubidium halides (RbI, RbBr, RbF, RbCl, etc.), cesium halides (CsI, CsBr, CsF, CsCl, etc.), lithium amide, rubidium amide, and cesium amide.

[0079] Examples of the halide-based solid electrolyte include monoclinic LiAlCl4, defect spinel-type or layered-structured LiInBr4, monoclinic Li 6-3m Y m X6 (where 0 < m < 2 and X = Cl or Br), etc. In addition, for example, known substances described in International Publication No. 2020 / 070958 and International Publication No. 2020 / 070955 can be used.

[0080] Examples of the oxide-based solid electrolyte include Li2O - Al2O3 - SiO2 - P2O5 - TiO2-based glass ceramics, Li2O - Al2O3 - SiO2 - P2O5 - GeO2-based glass ceramics, Li3PO4 - Li3BO3 - Li2SO4-based glass, garnet-type Li7La3Zr2O 12 、NASICON-type Li 1+O Al 1+O Ti 2-O (PO4)3, Li 1+p Al 1+p Ge 2-p (PO4)3, perovskite-type Li 3q La 2 / 3-q TiO3, inverse perovskite-type Li3HX (X = O, S, Se, Te), etc.

[0081] From the viewpoint of reducing grain boundary resistance, the average particle size of the solid electrolyte is preferably 0.1 μm or more, more preferably 0.2 μm or more. On the other hand, from the viewpoint of forming a sufficient contact interface between the positive electrode active material and the solid electrolyte, it is preferably 10 μm or less, more preferably 5 μm or less.

[0082] The content of solid electrolyte in the positive electrode mixture is preferably 5 to 60% by mass. It should be noted that when using solid electrolytes other than sulfide-based solid electrolytes, and setting the total amount of solid electrolyte in the positive electrode mixture to 100% by mass, the proportion of sulfide-based solid electrolyte is preferably 50% by mass or more.

[0083] The positive electrode may contain conductive additives. Specific examples of conductive additives include graphite (natural graphite, artificial graphite), graphene, carbon black, carbon nanofibers, carbon nanotubes, and other carbon materials. The preferred content of the conductive additive in the positive electrode mixture is 1 to 10% by mass.

[0084] In the positive electrode of the positive electrode compound, since it contains a sulfide-based solid electrolyte, the positive electrode compound can ensure good moldability even without a binder. Therefore, the positive electrode may or may not contain a binder, or it may contain fluoropolymers such as polyvinylidene fluoride (PVDF) as a binder.

[0085] The content of binder in the positive electrode mixture is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 0% by mass (i.e., no binder).

[0086] When the positive electrode has a current collector, the current collector can be made of metal foil, perforated metal, mesh, expanded metal, foamed metal, carbon sheet, etc., such as aluminum, nickel, stainless steel, etc.

[0087] For example, the positive electrode can be formed by compressing a positive electrode mixture prepared by mixing positive electrode active materials and sulfide-based solid electrolytes into a molded body using pressure molding or the like.

[0088] In the case of a positive electrode having a current collector, it can be manufactured by bonding a molded body of a positive electrode mixture formed using the method described above to the current collector by pressing or other means.

[0089] Alternatively, the positive electrode agent can be mixed with a solvent to prepare a composition containing the positive electrode agent, which is then coated onto a substrate such as a current collector and a solid electrolyte layer opposite to the positive electrode. After drying, it is pressed to form a molded body of the positive electrode agent, thereby manufacturing the positive electrode.

[0090] The solvent for the composition containing the positive electrode agent is preferably a solvent that does not easily degrade the solid electrolyte. In particular, since sulfide-based and hydride-based solid electrolytes undergo chemical reactions with trace amounts of water, nonpolar and aprotic solvents, such as hydrocarbon solvents like hexane, heptane, octane, nonane, decane, decahydronaphthalene, toluene, xylene, styrene, and tetrahydronaphthalene, are preferred. In particular, ultra-dehydrated solvents with a water content of 0.001% by mass (10 ppm) or less are more preferred. In addition, fluorinated solvents such as "Vertrel (registered trademark)" manufactured by Mitsui-DuPont Fluorochemicals, "ZEORORA (registered trademark)" manufactured by ZEON Corporation of Japan, and "Novec (registered trademark)" manufactured by Sumitomo 3M Corporation, as well as non-aqueous organic solvents such as dichloromethane, diethyl ether, and anisole, can also be used.

[0091] The thickness of the molded positive electrode compound (in the case of a current collector, the thickness of the molded positive electrode compound on one side of the current collector; the same applies below) is typically 100 μm or more, but from the viewpoint of increasing the capacity of all-solid-state secondary batteries, it is preferably 200 μm or more. Furthermore, the thickness of the molded positive electrode compound is typically 3000 μm or less.

[0092] It should be noted that, in the case of a positive electrode manufactured by forming a positive electrode mixture layer on a current collector using a composition containing a solvent and a positive electrode mixture, the thickness of the positive electrode mixture layer is preferably 10 to 1000 μm.

[0093] As described above, the positive electrode has the coating layer on the surface of the positive electrode active material, therefore, in its Raman spectrum, in the range of 900–950 cm⁻¹... ～1 The region contains two peaks: one from compounds containing PO bonds and the other from compounds containing BO bonds.

[0094] Raman spectra of the positive electrode can be obtained by observing the polished surface of the electrode using a confocal laser Raman microscope (RAMANTouch, Nanophoton) and an airtight cell (LIBcell, Nanophoton). The polished surface of the electrode can be prepared by removing the current collector from the positive electrode in a glove box under Ar gas conditions with a dew point below -60°C, or by polishing with a coarse-grained polishing film, and finally by polishing with a polishing film with a particle size of 4000 or higher. The sample is sealed with the polished surface of the obtained sample facing the quartz window of the airtight cell, and observed using an excitation laser with a wavelength of 532 nm and an objective lens with a magnification of 50x or higher (with a glass correction ring). When determining the observation area, it is more preferable to obtain Raman micrographs of the sample, investigate the position of the condensate of the positive electrode active material from the position of the peak attributed to the positive electrode active material, and perform point observation of the coating layer located in the space between these particles.

[0095] In addition, as described above, the positive electrode has the coating layer on the surface of the positive electrode active material, and therefore has peaks at 134±1 eV and 191.5±1 eV in its XPS (X-ray photoelectron spectroscopy) spectrum (peaks from compounds containing PO bonds and peaks from compounds containing BO bonds).

[0096] The XPS spectrum of the positive electrode can be obtained as follows. A polished surface of the positive electrode is prepared in the same manner as for the Raman spectroscopy determination, with the polished surface facing upwards. The battery is fixed to the sample stage using insulating double-sided tape. A conductive metal clamp is mounted on the sample stage, and the clamp's elastic force is used to press its front end against the polished surface, thereby balancing the polished surface with the Fermi level of the spectrometer. For samples where the influence of charge is significant, charge neutralization can be achieved through low-energy electron beam irradiation or low-energy ion beam irradiation. The binding energy of the obtained XPS spectrum is corrected by setting the peak position of the C1s spectrum attributable to hydrocarbons adsorbed on the sample surface to 284.8 eV. However, if the peak intensity of the carbon constituting the conductive additive is high, and even after peak separation, the position of the peak attributable to hydrocarbons adsorbed on the sample surface is uncertain, Au sputtering treatment can be performed on the polished surface of the sample, setting the Au4f peak position to 83.95 eV for binding energy correction.

[0097] All-solid-state rechargeable batteries

[0098] The all-solid-state secondary battery of the present invention is a battery having at least one stack, wherein the positive electrode and the negative electrode are opposite each other separated by a solid electrolyte layer, and the positive electrode is the positive electrode used in the all-solid-state secondary battery of the present invention. Regarding the components other than the positive electrode, various configurations used in conventionally known all-solid-state secondary batteries can be applied.

[0099] A cross-sectional view schematically illustrating an example of the all-solid-state secondary battery of the present invention is shown in the figure. Figure 1 . Figure 1 The all-solid-state battery 1 shown contains a positive electrode 10, a negative electrode 20, and a solid electrolyte layer 30 between the positive electrode 10 and the negative electrode 20, encapsulated in an outer casing formed by an outer casing 40, a sealing casing 50, and a resin gasket 60 between them. The positive electrode 10 is the positive electrode for the all-solid-state secondary battery of the present invention.

[0100] The sealing can 50 is fitted into the opening of the outer can 40 through the gasket 60. The opening end of the outer can 40 is tightened inward, so that the gasket 60 abuts against the sealing can 50, thereby sealing the opening of the outer can 40 and making the interior of the component a sealed structure.

[0101] The outer packaging can and sealing can be made of stainless steel, etc. In addition, besides polypropylene and nylon, the raw materials for the gaskets can also be heat-resistant resins with melting points exceeding 240°C, depending on the application of the battery and the required heat resistance. Examples of such heat-resistant resins include fluoropolymers (such as tetrafluoroethylene-perfluoroalkoxyethylene copolymer (PFA)), polyphenylene ether (PPE), polysulfone (PSF), polyarylate (PAR), polyethersulfone (PES), polyphenylene sulfide (PPS), and polyetheretherketone (PEEK). Furthermore, when the battery is used in applications requiring heat resistance, its sealing can also utilize hermetic glass seals.

[0102] in addition, Figure 2 and Figure 3 The figure shown schematically illustrates another example of the all-solid-state secondary battery of the present invention. Figure 2 This is a top view of an all-solid-state secondary battery. Figure 3 yes Figure 2 Sectional view of line II.

[0103] Figure 2 and Figure 3 The all-solid-state secondary battery 100 shown houses an electrode body 200, composed of a positive electrode, a solid electrolyte layer, and a negative electrode, within a laminated film outer casing 500 made of two metal laminated films. The outer casing 500 is sealed at its outer periphery by thermally fusing the upper and lower metal laminated films. It should be noted that... Figure 3 In order to avoid complicating the accompanying drawings, the layers constituting the laminated membrane outer body 500, the positive electrode constituting the electrode body, the solid electrolyte layer, and the negative electrode are not shown separately.

[0104] The positive electrode of the electrode body 200 is connected to the positive electrode external terminal 300 inside the battery 100. Additionally, although not shown, the negative electrode of the electrode body 200 is also connected to the negative electrode external terminal 400 inside the battery 100. Furthermore, the positive electrode external terminal 300 and the negative electrode external terminal 400 are extended at one end to the outside of the laminated film outer casing 500 in a manner that allows connection to external devices or the like.

[0105] (negative electrode)

[0106] The negative electrode of an all-solid-state secondary battery is, for example, a molded body containing a negative electrode compound containing negative electrode active material, a lithium sheet, or a lithium alloy sheet.

[0107] In the case where the negative electrode is a molded body containing a negative electrode active material, examples include molded bodies (particles, etc.) formed by molding the negative electrode mixture, and structures formed on the current collector consisting of a layer (negative electrode mixture layer) composed of the molded body of the negative electrode mixture.

[0108] When the negative electrode has a molded body containing a negative electrode compound, one or more mixtures of carbon-based materials capable of absorbing and releasing lithium can be used as the negative electrode active material, such as graphite, pyrolytic carbon, coke, glassy carbon, sintered organic polymer compounds, mesophase carbon microspheres (MCMB), carbon fibers, etc. Additionally, elements, compounds, and alloys containing Si, Sn, Ge, Bi, Sb, In, etc.; compounds containing lithium nitrides or lithium oxides that can be charged and discharged at low voltages close to that of lithium metal; lithium metal; and lithium / aluminum alloys can also be used as negative electrode active materials. For example, Li4Ti5O can be used. 12 TiO2, NbO 2.5-δ (0≤δ≤0.5), MoO 3-δ (0≤δ≤1),WO 3-δ (0≤δ≤1), TiNb2O7 and other metal oxides; WS2, MoS2 and other metal sulfides, one or more of these are used as negative electrode active materials.

[0109] The content of negative electrode active material in the negative electrode mixture is preferably 50-95% by mass.

[0110] The negative electrode mixture may contain a solid electrolyte. The solid electrolyte contained in the negative electrode mixture may be one or more of the sulfide-based, hydride-based, and oxide-based solid electrolytes exemplified above, which are also solid electrolytes that can be contained in the positive electrode mixture. Among the solid electrolytes exemplified above, from the perspective of high Li-ion conductivity and the function of improving the formability of the negative electrode mixture, a sulfide-based solid electrolyte is more preferred, and a sulfide-based solid electrolyte of the argentite type is even more preferred.

[0111] The content of solid electrolyte in the negative electrode mixture is preferably 4-70% by mass.

[0112] The negative electrode mixture may contain conductive additives. Specific examples include graphite (natural graphite, artificial graphite), graphene, carbon black, vapor-grown carbon fibers, carbon nanofibers, carbon nanotubes, and other carbon materials. The preferred content of the conductive additive in the negative electrode mixture is 1-10% by mass.

[0113] The negative electrode mixture may or may not contain a binder. Specific examples include substances similar to the binders previously exemplified as substances that can be included in the positive electrode mixture. It should be noted that, for example, in the case where the negative electrode mixture contains a sulfide-based solid electrolyte, if good formability can be ensured in forming the molded body of the negative electrode mixture even without the use of a binder, the negative electrode mixture may be made without a binder.

[0114] In the negative electrode binder, when a binder is required, its content is preferably 15% by mass or less, and more preferably 0.5% by mass or more. On the other hand, when good formability can be obtained even if the negative electrode binder does not contain a binder, its content is preferably 0.5% by mass or less, more preferably 0.3% by mass or less, and even more preferably 0% by mass (i.e., it does not contain a binder).

[0115] When using a current collector at the negative electrode, the current collector can be made of copper or nickel foil, perforated metal, mesh, expanded metal, foamed metal, carbon sheet, etc.

[0116] The molded body of the negative electrode compound can be formed, for example, by compressing a negative electrode compound prepared by mixing a negative electrode active material, a conductive additive and a solid electrolyte, and a binder added as needed, using pressure molding or the like.

[0117] In the case of a negative electrode having a current collector, it can be manufactured by bonding a molded body of a negative electrode mixture formed using the method described above to a current collector by pressing or other means.

[0118] Alternatively, the negative electrode agent can be mixed with a solvent to prepare a composition containing the negative electrode agent, which is then coated onto a substrate such as a current collector and a solid electrolyte layer opposite to the negative electrode, dried, and pressed to form a molded body of the negative electrode agent.

[0119] The solvent used in the composition containing the negative electrode agent is preferably selected in the same way as the solvent used in the composition containing the positive electrode agent, and is less likely to degrade the solid electrolyte. It is preferred to use the various solvents exemplified above as solvents for the composition containing the positive electrode agent, and it is particularly preferred to use a super-dehydrating solvent with a water content of 0.001% by mass (10 ppm) or less.

[0120] The thickness of the molded negative electrode compound (in the case of an electrode with a current collector, the thickness of the molded negative electrode compound on one side of the current collector; the same applies below) is typically 100 μm or more, but from the viewpoint of increasing the capacity of all-solid-state secondary batteries, it is preferably 200 μm or more. Furthermore, the thickness of the molded negative electrode compound is typically 3000 μm or less.

[0121] It should be noted that, in the case of a negative electrode manufactured by forming a negative electrode mixture layer on a current collector using a composition containing a solvent and a negative electrode mixture, the thickness of the negative electrode mixture layer is preferably 10 to 1000 μm.

[0122] (Solid electrolyte layer)

[0123] In the solid electrolyte layer of an all-solid-state secondary battery, one or more of the sulfide-based, hydride-based, and oxide-based solid electrolytes that can be included as a positive electrode additive can be used. Among the solid electrolytes exemplified above, a sulfide-based solid electrolyte is more preferred to improve battery performance, and a sulfide-based solid electrolyte of the argyrodite type is even more preferred.

[0124] Solid electrolyte layers can be formed by methods such as: compressing solid electrolytes by pressure molding; coating a solid electrolyte layer forming composition prepared by dispersing solid electrolytes in a solvent onto a substrate, a positive electrode, and a negative electrode, drying it, and then pressing it as needed, etc.

[0125] In addition, the solid electrolyte layer can also have a porous material such as resin-based nonwoven fabric as a support.

[0126] The solvent used in the composition for forming the solid electrolyte layer is preferably selected in the same way as the solvent used in the composition containing the positive electrode agent, and is not likely to degrade the solid electrolyte. It is preferable to use the various solvents exemplified above as solvents for the composition containing the positive electrode agent, and it is particularly preferable to use a super-dehydrating solvent with a water content of 0.001% by mass (10 ppm) or less.

[0127] The thickness of the solid electrolyte layer is preferably 10–500 μm.

[0128] (Electrode)

[0129] The positive and negative electrodes can be used in a battery as a stacked electrode body formed by layering solid electrolyte layers, and then the stacked electrode body can be wound into a wound electrode body.

[0130] It should be noted that, from the viewpoint of improving the mechanical strength of the electrode body, it is preferable to perform pressure molding while the positive electrode, negative electrode and solid electrolyte layer are stacked.

[0131] (Battery form)

[0132] Regarding the form factor of all-solid-state secondary batteries, besides Figure 1 The battery shown has an outer casing consisting of an outer can, a sealing can, and a gasket; this is commonly referred to as a coin-shaped battery or a button-shaped battery. Figure 2 and Figure 3 In addition to the form shown, which has an outer casing made of a resin film or a metal-resin laminate, it can also be an outer casing made of metal with a bottom cylindrical (cylindrical or square) outer casing and a sealing structure that seals its opening.

[0133] Example

[0134] The present invention will now be described in detail based on embodiments. However, the following embodiments do not limit the present invention.

[0135] (Example 1)

[0136] <The Making of the Positive Electrode>

[0137] (Preparation of positive electrode active material)

[0138] 10.62 g of Li₂CO₃ and 50.75 g of NiMn composite oxide (molar ratio: Ni / Mn = 1 / 3) were mixed with air using a vibratory mill to obtain a precursor for spinel-type cathode active material. This precursor was heated from room temperature to 1000 °C at 11 °C / min, calcined at 1000 °C for 1 hour, cooled from 1000 °C to 850 °C at 1.7 °C / min, calcined at 850 °C for 12 hours, cooled from 850 °C to 550 °C at 3.0 °C / min, calcined at 550 °C for 12 hours, cooled from 550 °C to 200 °C at 3.0 °C / min, and then naturally cooled from 200 °C to room temperature. Finally, it was crushed in a mortar to obtain spinel-type cathode active material (LiNi). 0.5 Mn 1.5 O4) particles.

[0139] (Formation of the coating layer)

[0140] A 7.5% (w / w) citric acid aqueous solution was prepared. 4.32 g of the citric acid aqueous solution, 122.4 mg of Li3PO4, 31.2 mg of Li3BO3, and 155.0 mg of Al2O3 nanoparticles were mixed and ultrasonically treated for 1 minute, then stirred until colorless and transparent. The pH of the aqueous solution was adjusted to 7 using 10% (w / w) ammonia water, and then 0.25 g of ethanol was added to prepare the coating solution.

[0141] Add 5g of the spinel-type positive electrode active material and the total amount of the coating liquid to an agate mortar, mix thoroughly, and then place the agate mortar on a heating plate at 60°C, mixing and drying simultaneously. Place the resulting powder into an alumina crucible for heat treatment. The heat treatment is set to increase the temperature from room temperature to 650°C at a rate of 2°C / min, calcine at 650°C for 5 hours, and then decrease the temperature from 650°C to 200°C at a rate of 2.5°C / min. After natural cooling to room temperature, crush the resulting powder in an agate mortar for 10 minutes, thereby forming a coating layer on the surface of the spinel-type positive electrode active material particles.

[0142] In the coating layer covering the surface of the positive electrode active material, based on the results of STEM-EELS analysis of the sample, when the total content of the composition containing Li3PO4 and Li3BO3 is set to 100 mol%, Li3PO4 is 70 mol%, Li3BO3 is 30 mol% (the relationship between the content of P from Li3PO4 and the content of B from Li3BO3 is P≥B in elemental ratio), the content of each component in the total coating layer is: the composition containing Li3PO4 and Li3BO3 is 50 wt%, and Al2O3 particles are 50 wt%. Furthermore, the average thickness of the coating layer is 17 nm, and the coating coverage of the positive electrode active material surface is 88%.

[0143] A positive electrode mixture was prepared by mixing the surface-coated positive electrode active material, vapor-grown carbon fiber (conductive additive), and Li6PS5Cl (sulfide-based solid electrolyte) as described above. The mixing ratio of the positive electrode material, conductive additive, and sulfide-based solid electrolyte was 75:3:22 by mass. 56 mg of this positive electrode mixture was added to a powder molding die with a diameter of 7.5 mm and pressed using a press at 6000 kgf / cm². 2 The positive electrode is formed by pressing and molding, and is composed of a cylindrical positive electrode mixture.

[0144] The obtained positive electrode exhibits Raman spectra at 900–950 cm⁻¹ ～-1 It has two peaks in the region, and also has peaks at 134±1 eV and 191.5±1 eV in the XPS spectrum.

[0145] <Formation of Solid Electrolyte Layer>

[0146] 8 mg of the same sulfide-based solid electrolyte used in the positive electrode was added to the positive electrode mixture molded body within the powder molding die, and then pressed using a press at 1000 kgf / cm³. 2 The pressure is used to form a solid electrolyte layer on the positive electrode mixture.

[0147] <Making the Negative Electrode>

[0148] Lithium titanate (Li4Ti5O) 12 The negative electrode active material, the same sulfide solid electrolyte used in the solid electrolyte layer, and graphene (conductive additive) are mixed in a mass ratio of 55:36:9 and thoroughly kneaded to prepare the negative electrode mixture. Next, 95 mg of the negative electrode mixture is added to the solid electrolyte layer within the powder molding die, and pressed using a press at 10000 kgf / cm². 2The pressure is used to form a negative electrode, which is composed of a negative electrode mixture, on the solid electrolyte layer, thereby producing a stacked electrode body with a positive electrode, a solid electrolyte layer and a negative electrode.

[0149] Assembly of all-solid-state secondary batteries

[0150] The flexible graphite sheet "PERMA-FOIL" (product name) manufactured by Toyo Carbon Co., Ltd. (thickness: 0.1 mm, apparent density: 1.1 g / cm³) 3 Two graphite sheets are prepared, each the same size as the stacked electrode body, and one of them is placed on the inner bottom surface of a stainless steel sealed can with an embedded annular gasket made of polypropylene. Next, the stacked electrode body is overlapped on the graphite sheet with the negative electrode on the graphite sheet side, and the other graphite sheet is placed on top. Then, the stainless steel outer can is covered, and the opening end of the outer can is sealed by folding the seam inwards. This creates a flat, all-solid-state secondary battery with a diameter of approximately 9 mm, where the graphite sheets are respectively placed between the inner bottom surface of the sealed can and the stacked electrode body, and between the inner bottom surface of the outer can and the stacked electrode body.

[0151] (Example 2)

[0152] Except for removing Al2O3 nanoparticles from the composition of the coating solution, spinel-type positive electrode active material particles with a coating layer on the surface were obtained in the same manner as in Example 1.

[0153] In the coating layer covering the surface of the positive electrode active material, based on the results of STEM-EELS analysis of the sample, when the total content of Li3PO4 and Li3BO3 in the composition is set to 100 mol%, Li3PO4 is 70 mol% and Li3BO3 is 30 mol% (the relationship between the content of P from Li3PO4 and the content of B from Li3BO3, expressed as an elemental ratio, is P≥B). Furthermore, the average thickness of the coating layer is 10 nm, and the coating coverage of the positive electrode active material surface is 75%.

[0154] Then, except that the positive electrode active material with the coating layer is used to coat the surface, a flat all-solid-state secondary battery is fabricated in the same manner as in Example 1.

[0155] (Example 3)

[0156] Al2O3 nanoparticles were removed from the composition of the coating solution. The heat treatment conditions were changed to increase the temperature from room temperature to 350°C at a rate of 1°C / min, calcine at 350°C for 5 hours, and then decrease the temperature from 350°C to 200°C at a rate of 0.8°C / min. Otherwise, the same procedure as in Example 1 was followed to obtain spinel-type positive electrode active material particles with a coating layer on the surface.

[0157] In the coating layer covering the surface of the positive electrode active material, based on the results of STEM-EELS analysis of the sample, when the total content of Li3PO4 and Li3BO3 in the composition is set to 100 mol%, Li3PO4 is 70 mol% and Li3BO3 is 30 mol% (the relationship between the content of P from Li3PO4 and the content of B from Li3BO3, expressed as an elemental ratio, is P≥B). Furthermore, the average thickness of the coating layer is 11 nm, and the coating coverage of the positive electrode active material surface is 76%.

[0158] Then, except that the positive electrode active material with the coating layer is used to coat the surface, a flat all-solid-state secondary battery is fabricated in the same manner as in Example 1.

[0159] (Example 4)

[0160] Al2O3 nanoparticles were removed from the composition of the coating solution, and the lithium salt was changed to 96.2 mg of Li3PO4, 27.0 mg of Li3BO3 and 37.1 mg of Li2SO4·H2O. Otherwise, the same procedure as in Example 1 was followed to obtain spinel-type positive electrode active material particles with a coating layer on the surface.

[0161] In the coating layer covering the surface of the positive electrode active material, based on the results of STEM-EELS analysis of the sample, when the total content of the composition containing Li3PO4, Li3BO3, and Li2SO4 is set to 100 mol%, Li3PO4 is 55 mol%, Li3BO3 is 26 mol%, and Li2SO4 is 19 mol% (the relationship between the content of P from Li3PO4 and the content of B from Li3BO3 is P≥B in elemental ratio). Furthermore, the average thickness of the coating layer is 10 nm, and the coating coverage of the positive electrode active material surface is 75%.

[0162] Then, except that the positive electrode active material with the coating layer is used to coat the surface, a flat all-solid-state secondary battery is fabricated in the same manner as in Example 1.

[0163] (Example 5)

[0164] Al2O3 nanoparticles were removed from the composition of the coating solution. The amount of citric acid aqueous solution was changed to 23.8 g, and the lithium salts were changed to 385.8 mg of Li3PO4, 229.3 mg of Li3BO3, and 215.3 mg of Li2SO4·H2O. Otherwise, the operation was the same as in Example 1 to obtain spinel-type positive electrode active material particles with a coating layer on the surface.

[0165] In the coating layer covering the surface of the positive electrode active material, based on the results of STEM-EELS analysis of the sample, when the total content of the composition containing Li3PO4, Li3BO3, and Li2SO4 is set to 100 mol%, Li3PO4 is 40 mol%, Li3BO3 is 40 mol%, and Li2SO4 is 20 mol% (the relationship between the content of P from Li3PO4 and the content of B from Li3BO3 is P≥B in elemental ratio). Furthermore, the average thickness of the coating layer is 55 nm, and the coating coverage of the positive electrode active material surface is 80%.

[0166] Then, except that the positive electrode active material with the coating layer is used to coat the surface, a flat all-solid-state secondary battery is fabricated in the same manner as in Example 1.

[0167] (Comparative Example 1)

[0168] The same LiNi used in Example 1 0.5 Mn 1.5 O4 was used as the positive electrode active material without a coating layer. Otherwise, a flat all-solid-state secondary battery was fabricated in the same manner as in Example 1.

[0169] (Comparative Example 2)

[0170] 5g of LiNi 0.5 Mn 1.5 O4 and 0.155g of Al2O3 nanoparticles were mixed in an agate mortar for 40 minutes, subjected to the same heat treatment as in Example 1, and then crushed in an agate mortar for 10 minutes to obtain the positive electrode active material.

[0171] In addition to using LiNi with Al2O3 nanoparticle-coated surface as described above, 0.5 Mn 1.5 Except for O4 as the positive electrode active material, a flat all-solid-state secondary battery was fabricated in the same manner as in Example 1.

[0172] (Comparative Example 3)

[0173] Al2O3 nanoparticles were removed from the composition of the coating solution, and the lithium salt was replaced with only 175 mg of Li3PO4. Otherwise, spinel-type positive electrode active material particles with a coating layer on the surface were obtained in the same manner as in Example 1.

[0174] In addition to using LiNi with a surface coated with Li3PO4 as described above 0.5 Mn 1.5 Except for O4 as the positive electrode active material, a flat all-solid-state secondary battery was fabricated in the same manner as in Example 1.

[0175] The following evaluations were performed on the flat all-solid-state secondary batteries of the embodiments and comparative examples.

[0176] <Initial Characteristic Evaluation>

[0177] For each flat all-solid-state secondary battery of the examples and comparative examples, constant current charging was performed at a current value of 0.07C until the voltage reached 3.5V, then constant voltage charging was performed until the current value reached 0.01C, the open-circuit terminal voltage was measured for 1 hour, and then discharged at a current value of 0.07C until the voltage reached 1.5V, and the discharge capacity (initial capacity) at this time was measured.

[0178] The voltage obtained by subtracting the voltage 1 second after the initial discharge from the open-circuit voltage measured above is used to calculate the DC resistance (DCR).

[0179] <Heat Resistance Evaluation>

[0180] For each flat all-solid-state secondary battery of the Examples and Comparative Examples (batteries different from those evaluated for initial characteristics), they were placed in a constant temperature bath at 150°C for 30 minutes immediately after fabrication, and then removed from the constant temperature bath and allowed to reach room temperature. For each subsequent battery, the DCR was measured under the same conditions as during the initial characteristic evaluation, and the resulting value was divided by the DCR during the initial characteristic evaluation and expressed as a percentage to calculate the DCR rise rate.

[0181] These results are shown in Table 1. It should be noted that in Table 1, the initial capacity is expressed as a relative value when the value of the battery of Comparative Example 3 is set to 100.

[0182] [Table 1]

[0183]

[0184] The initial characteristics of the flat all-solid-state secondary batteries in Examples 1-5, including discharge capacity and DCR (discharge rate of increase) of heat resistance, are superior to those of the batteries in Comparative Examples 1-3. Examples 1-5 use positive electrodes containing a positive electrode active material, the surface of which is coated with a coating layer having a composition containing appropriate components. This is believed to be because, in the positive electrodes of the examples, the coating layer is uniform and has high ionic conductivity, thereby reducing isolated positive electrode active material particles.

[0185] It should be noted that in Comparative Example 3, where a positive electrode active material with insufficient ionic conductivity, having a coating layer formed on its surface, was used, the expected increase in DCR during the initial performance evaluation did not occur. This is likely due to the low coating coverage of the positive electrode active material. Consequently, during the initial performance evaluation, the area without a coating layer widened, ensuring good electronic and ionic conductivity, resulting in a lower DCR. Furthermore, during the heat resistance evaluation, oxidation occurred in the wide area without a coating layer through heat treatment, forming a resistive layer, which significantly increased the DCR.

[0186] This invention can also be implemented in ways other than those described above without departing from its spirit. The embodiments disclosed in this application are examples, and the invention is not limited to these embodiments. Regarding the scope of the invention, the appended claims are preferred over the description in the foregoing specification, and all modifications within the scope of the claims are included in the claims.

[0187] Industrial utilization potential

[0188] The all-solid-state secondary battery of the present invention can be applied to the same uses as conventionally known secondary batteries, but as mentioned above, due to its excellent heat resistance, it is preferably used for applications exposed to high temperatures. The positive electrode of the all-solid-state secondary battery of the present invention can be used to construct the all-solid-state secondary battery of the present invention.

[0189] Symbol Explanation

[0190] 1. 100: All-solid-state rechargeable battery.

[0191] 10: Positive electrode,

[0192] 20: Negative electrode

[0193] 30: Solid electrolyte layer,

[0194] 40: Outer packaging can,

[0195] 50: Sealed jars

[0196] 60: Gasket,

[0197] 200: Electrode body,

[0198] 300: Positive external terminal,

[0199] 400: External negative terminal

[0200] 500: Laminated outer casing.

Claims

1. A positive electrode for an all-solid-state secondary battery, characterized in that, Contains positive electrode active material and sulfide-based solid electrolyte. At least a portion of the surface of the positive electrode active material is coated with a coating layer having a composition comprising a compound containing a PO bond and a compound containing a BO bond.

2. The positive electrode for an all-solid-state secondary battery according to claim 1, wherein, The relationship between the content of P and the content of B in the composition, expressed as an elemental ratio, is P ≥ B.

3. The positive electrode for an all-solid-state secondary battery according to claim 1, wherein, The composition also contains compounds containing SO bonds.

4. The positive electrode for an all-solid-state secondary battery according to claim 1, wherein, The average thickness of the coating layer is 1–100 nm.

5. The positive electrode for an all-solid-state secondary battery according to claim 1, wherein, The positive electrode active material satisfies any one of the following general formulas (1) to (4). LiCo 1-x M 1 x O2(1) In the general formula (1), M 1 The element to be selected is chosen from the group consisting of Al, Mg, Ti, V, Cr, Zr, Fe, Ni, Cu, Zn, Ga, Ge, Nb, Mo, Sn, Sb, and Ba, where 0 ≤ x ≤ 0.

5. LiNi 1-a-b Mn a M 2 b O2(2) In the general formula (2), M 2 To select at least one element from the group consisting of Co, Mg, Al, B, Ti, V, Cr, Fe, Cu, Zn, Zr, Mo, Sn, Ca, Sr, and W, 0 <a≤1,0≤b<1,0<a+b≤1, LiNi 1-c M 3 c O2(3) In the general formula (3), M 3 The element to be selected is chosen from the group consisting of Al, Mg, Ti, Zr, Fe, Co, Cu, Zn, Ga, Ge, Nb, Mo, Sn, Sb, and Ba, where 0 ≤ c ≤ 0.

5. Glue 4 d Mn 2-d O 4-e X e (4) In the general formula (4), M 4 X is selected from at least one element in the group consisting of Li, B, Mg, Ca, Sr, Ba, Ti, V, Cr, Fe, Co, Ni, Cu, Al, Sn, Sb, In, Nb, Mo, W, Y, Ru and Rh, and X is selected from at least one element in the group consisting of F, Cl, Br and S, where 0 ≤ d ≤ 1.2 and 0 ≤ e ≤ 0.

5.

6. An all-solid-state secondary battery, characterized in that, It has at least one stack in which the positive and negative electrodes face each other separated by a solid electrolyte layer. The positive electrode has any one of claims 1 to 5 as the positive electrode for an all-solid-state secondary battery.

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