A solid electrolyte material contains Li, M, and X. M is at least one selected from metallic elements, and X is at least one selected from the group consisting of Cl, Br, and I. A plurality of atoms of X form a sublattice having a closest packed structure. An average distance between two adjacent atoms of X among the plurality of atoms of X is 1.8% or more larger than a distance between two adjacent atoms of X in a rock-salt structure composed only of Li and X.

A solid electrolyte material contains Li, M, and X. M is at least one selected from metallic elements, and X is at least one selected from the group consisting of Cl, Br, and I. A plurality of atoms of X form a sublattice having a closest packed structure. An average distance between two adjacent atoms of X among the plurality of atoms of X is 1.8% or more larger than a distance between two adjacent atoms of X in a rock-salt structure composed only of Li and X.

A solid electrolyte material is made of Li, Ca, Y, Gd, X, O, and H, where X is at least one selected from the group consisting of F, Cl, Br, and I; and the molar ratio of O to the sum of Y and Gd is greater than 0 and less than 0.82.

A production method for producing a halide, the method includes a heat treatment step of heat-treating a mixed material containing (NH.sub.4).sub.aYα.sub.3+a, (NH.sub.4).sub.bSmβ.sub.3+b, Liγ, and Caδ.sub.2 in an inert gas atmosphere, wherein α, β, γ, and δ are each independently at least one selected from the group consisting of F, Cl, Br, and I, and the following three formulas: 0≤a≤3, 0≤b≤3, and 0<a+b≤6, are satisfied.

The production method of the present disclosure includes heat-treating a material mixture containing a compound containing Y, a compound containing Sm, NH.sub.4α, Liβ, and Caγ.sub.2 in an inert gas atmosphere. The compound containing Y is at least one selected from the group consisting of Y.sub.2O.sub.3 and Yδ.sub.3, and the compound containing Sm is at least one selected from the group consisting of Sm.sub.2O.sub.3 and Smε.sub.3. The material mixture contains at least one selected from the group consisting of Y.sub.2O.sub.3 and Sm.sub.2O.sub.3, and α, β, γ, δ, and ε are each independently at least one selected from the group consisting of F, Cl, Br, and I.

The production method of the present disclosure includes heat-treating a material mixture containing a compound containing Y, a compound containing Sm, NH.sub.4α, Liβ, and Caγ.sub.2 in an inert gas atmosphere. The compound containing Y is at least one selected from the group consisting of Y.sub.2O.sub.3 and Yδ.sub.3, and the compound containing Sm is at least one selected from the group consisting of Sm.sub.2O.sub.3 and Smε.sub.3. The material mixture contains at least one selected from the group consisting of Y.sub.2O.sub.3 and Sm.sub.2O.sub.3, and α, β, γ, δ, and ε are each independently at least one selected from the group consisting of F, Cl, Br, and I.

A positive-electrode material according to the present disclosure includes a positive-electrode active material and a cover layer 111 containing a first solid electrolyte and covering at least partially the surface of the positive-electrode active material. The positive-electrode active material and the cover layer constitute a covered active material; the positive-electrode active material has a pore volume V.sub.α, the covered active material has a pore volume V.sub.β, the positive-electrode active material has a specific surface area Sa, the covered active material has a specific surface area Sp, and at least one selected from the group consisting of 0.20<V.sub.β/V.sub.α<0.88 and 0.81<S.sub.β/S.sub.α<0.97 is satisfied.

A production method for producing a halide, the method includes a heat treatment step of heat-treating a mixed material containing (NH.sub.4).sub.aYα.sub.3+a, (NH.sub.4).sub.bGdβ.sub.3+b, Liγ, and Caδ.sub.2 in an inert gas atmosphere, wherein α, β, γ, and δ are each independently at least one selected from the group consisting of F, Cl, Br, and I, and the formulas: 0≤a≤3, 0≤b≤3, and 0<a+b≤6, are satisfied.

A production method for producing a halide, the method includes a heat treatment step of heat-treating a mixed material containing (NH.sub.4).sub.aYα.sub.3+a, (NH.sub.4).sub.bGdβ.sub.3+b, Liγ, and Caδ.sub.2 in an inert gas atmosphere, wherein α, β, γ, and δ are each independently at least one selected from the group consisting of F, Cl, Br, and I, and the formulas: 0≤a≤3, 0≤b≤3, and 0<a+b≤6, are satisfied.

The present invention concerns new lithium rare earth halides that may be used as solid electrolytes or in electrochemical devices. The invention also refers to wet and dry processes for the synthesis of such lithium rare earth halides and lithium rare earth halides susceptible to be obtained by these processes.