Solid electrolyte

A solid electrolyte glass ceramic represented by the formula (1):

• LaMbPcSdXe (1)
• wherein:
• L is an alkali metal;
• M is one or more elements selected from the group consisting of B, Al, Si, Ge, As, Se, Sn, Sb, Te, Pb and Bi;
• and
• X includes Br and optionally further includes at least one halogen element selected from the group consisting of I, Cl, and F;
• and
• a to e independently satisfy:
• 0<a≤12,
• 0≤b≤0.2,
• c=1,
• 0<d≤9, and
• 0<e≤9,
• wherein the solid electrolyte glass ceramic has one exothermic peak in a temperature range of 150° C. to 350° C. as determined by differential scanning calorimetric measurement (in a dry nitrogen atmosphere at a temperature-elevating speed of 10° C./min from 20 to 600° C.), and
• the solid electrolyte glass ceramic is obtained by subjecting a solid electrolyte glass which has two exothermic peaks that are separated from each other in a temperature range of 150° C. to 350° C. as determined by differential scanning calorimetric measurement (in a dry nitrogen atmosphere at a temperature-elevating speed of 10° C./min from 20 to 600° C.) to a heat treatment at a temperature between the two exothermic peaks.