A method for producing a halide includes heat-treating a mixed material in an inert gas atmosphere, the mixed material being a mixture of M.sub.2O.sub.3, NH.sub.4X, and LiZ. The M includes at least one element selected from the group consisting of Y, a lanthanoid, and Sc. The X is at least one element selected from the group consisting of Cl, Br, I, and F. The Z is at least one element selected from the group consisting of Cl, Br, I, and F.

A method for producing a halide includes heat-treating a mixed material in an inert gas atmosphere, the mixed material being a mixture of M.sub.2O.sub.3, NH.sub.4X, and LiZ. The M includes at least one element selected from the group consisting of Y, a lanthanoid, and Sc. The X is at least one element selected from the group consisting of Cl, Br, I, and F. The Z is at least one element selected from the group consisting of Cl, Br, I, and F.

A production method for producing a halide includes a heat-treatment step of heat-treating, in an inert gas atmosphere, a mixed material in which LiBr and YBr.sub.3 are mixed. In the heat-treatment step, the mixed material is heat-treated at higher than or equal to 200° C. and lower than or equal to 650° C.

A production method for producing a halide includes a heat-treatment step of heat-treating, in an inert gas atmosphere, a mixed material in which LiX and YZ.sub.3 are mixed, where X is an element selected from the group consisting of Cl, Br, and I, and Z is an element selected from the group consisting of Cl, Br, and I. In the heat-treatment step, the mixed material is heat-treated at higher than or equal to 200° C. and lower than or equal to 650° C.

A production method for producing a halide includes heat-treating, in an inert gas atmosphere, a mixed material in which LiX, YZ.sub.3, and at least one of LiX′ or YZ′.sub.3 are mixed, where X is an element selected from the group consisting of Cl, Br, and I; Z is an element selected from the group consisting of Cl, Br, and I and different from X; X is an element selected from the group consisting of Cl, Br, and I and different from either X or Z; and Z′ is an element selected from the group consisting of Cl, Br, and I and different from either X or Z. In the heat-treatment, the mixed material is heat-treated at higher than or equal to 200° C. and lower than or equal to 650° C.

A solid electrolyte material is composed of Li, M, and X. M is at least one selected from the group consisting of metal elements other than Li and metalloids. X is at least one selected from the group consisting of F, Cl, Br, and I. FWHM/2θ.sub.p≤0.015 is satisfied, wherein FWHM represents a half width of an X-ray diffraction peak in an X-ray diffraction pattern obtained by performing X-ray diffraction measurement on the solid electrolyte material by using Cu-Kα radiation, the X-ray diffraction peak having the highest intensity within a range of diffraction angles 2θ greater than or equal to 25° and less than or equal to 35°, and 2θ.sub.p represents a diffraction angle at a center of the X-ray diffraction peak.

A solid electrolyte material is composed of Li, M, and X. M is at least one selected from the group consisting of metal elements other than Li and metalloids. X is at least one selected from the group consisting of F, Cl, Br, and I. FWHM/2θ.sub.p≤0.015 is satisfied, wherein FWHM represents a half width of an X-ray diffraction peak in an X-ray diffraction pattern obtained by performing X-ray diffraction measurement on the solid electrolyte material by using Cu-Kα radiation, the X-ray diffraction peak having the highest intensity within a range of diffraction angles 2θ greater than or equal to 25° and less than or equal to 35°, and 2θ.sub.p represents a diffraction angle at a center of the X-ray diffraction peak.

A method for producing a halide includes heat-treating a mixed material in an inert gas atmosphere, the mixed material being a mixture of (NH.sub.4).sub.aMX.sub.3+a and LiZ. The M includes at least one element selected from the group consisting of Y, a lanthanoid, and Sc. The X is at least one element selected from the group consisting of Cl, Br, I, and F. The Z is at least one element selected from the group consisting of Cl, Br, I, and F. Furthermore, 0<a≤3 is satisfied.

A method for producing a halide includes heat-treating a mixed material in an inert gas atmosphere, the mixed material being a mixture of (NH.sub.4).sub.aMX.sub.3+a and LiZ. The M includes at least one element selected from the group consisting of Y, a lanthanoid, and Sc. The X is at least one element selected from the group consisting of Cl, Br, I, and F. The Z is at least one element selected from the group consisting of Cl, Br, I, and F. Furthermore, 0<a≤3 is satisfied.

A production method for producing a halide includes heat-treating, in an inert gas atmosphere, a mixed material in which LiCl and YCl.sub.3 are mixed. In the heat-treatment, the mixed material is heat-treated at higher than or equal to 200° C. and lower than or equal to 650° C.