Lithium ion batteries are secondary batteries having a structure in which, during charging, lithium dissolves out as ions from a positive electrode and migrates to the negative electrode to be stored therein; and on the contrary, during discharging, lithium ions return from the negative electrode to the positive electrode. Since lithium ion batteries have features such as high energy density and a long life cycle, lithium ion batteries are widely used as power supplies for domestic appliances such as video cameras; portable electronic devices such as laptop computers and mobile telephones; and electric tools such as power tools. Recently, lithium ion batteries are also applied to large-sized batteries that are mounted in electric vehicles (EV), hybrid electric vehicles, and the like.
A lithium ion battery of this kind is configured to include a positive electrode, a negative electrode, and an ion conducting layer interposed between these two electrodes, and as this ion conducting layer, a separator formed from a porous film of polyethylene, polypropylene or the like, which is filled with a non-aqueous liquid electrolyte, is generally used. However, since such an organic liquid electrolyte which uses a flammable organic solvent as the solvent is used as an electrolyte, improvements in view of structure and material for preventing volatilization or leakage are needed, and installation of a safety device for suppressing temperature increase at the time of a short circuit and improvements in view of structure and material for preventing a short circuit are also needed.
In contrast, an all-solid lithium ion battery formed by solidifying the whole battery using a solid electrolyte that uses lithium sulfide (Li2S) or the like as a starting material, does not use a flammable organic solvent. Therefore, simplification of safety devices can be attempted, and the battery can be made as a battery which is excellent in terms of production cost or productivity. Also, the battery has a feature that the solid electrolyte can be laminated in series in a cell, and thus voltage increase can be promoted. Furthermore, in a solid electrolyte of this kind, since nothing but Li ion moves, side reactions caused by movement of anions do not occur, and it is expected that this leads to enhancement of safety and durability.
A solid electrolyte used in such a battery is required to have high ionic conductivity as far as possible and to be stable chemically and electrochemically. For example, lithium halide, lithium nitride, lithium oxoate, and derivatives of these compounds are known as candidate materials for the solid electrolyte.
In regard to solid electrolytes of this kind, for example, Patent Document 1 discloses a sulfide-based solid electrolyte obtainable by incorporating a high temperature lithium ion conductive compound formed from lithium phosphate (Li3PO4), into a lithium ion conductive sulfide glass represented by general formula: Li2S—X (provided that X represents at least one sulfide among SiS2, GeS2, and B2S3).
Furthermore, Patent Document 2 discloses, as a material that is crystalline and exhibits a very high ionic conductivity such as an ionic conductivity at room temperature of 6.49×10−5 Scm−1, a sulfide-based solid electrolyte characterized by including a lithium ion conductive substance as a composite compound represented by general formula: Li2S—GeS2—X (provided that X represents at least one of Ga2S3 and ZnS).
Patent Document 3 discloses a lithium ion conductive sulfide ceramic having high lithium ion conductivity and a high decomposition voltage, the sulfide ceramic containing Li2S and P2S5 as main components and having a composition in which Li2S=82.5 to 92.5 and P2S5=7.5 to 17.5, as expressed in mol %, and preferably a composition (compositional formula: Li7PS6) in which Li2S/P2S5=7 as a molar ratio.
Patent Document 4 discloses a lithium ion conductive material having a silver germanium sulfide mineral type crystal structure represented by chemical formula: Li+(12-n-x)Bn+X2−(6-x)Y−x (wherein Bn+ represents at least one selected from P, As, Ge, Ga, Sb, Si, Sn, Al, In, Ti, V, Nb, and Ta; X2− represents at least one selected from S, Se, and Te; and Y− represents at least one selected from F, Cl, Br, I, CN, OCN, SCN, and N3; while 0≦x≦2).
Patent Document 5 discloses, as a solid compound that can be produced into a single layer in addition to the high fluidity of lithium ions, a lithium silver germanium sulfide mineral represented by general formula (I): Li+(12-n-x)Bn+X2−6-xY−x, and in this formula, Bn+ is selected from the group consisting of P, As, Ge, Ga, Sb, Sn, Al, In, Ti, V, Nb and Ta; X2− is selected from the group consisting of S, Se and Te; and Y− is selected from the group consisting of Cl, Br, I, F, CN, OCN, SCN, and N3, while 0≦x≦2.