A composite material of formula (I) is provided: (LPS)a(MPS)b  (I) wherein each of a and b is a mass % value from 1% to 99% such that a+b is 100%; (LPS) is a material selected from the group consisting of Li3PS4, Li7P3S11, Li10GeP2S11, and a material of formula (II): xLi2S.yP2S5.(100−x−y)LiX  (II) wherein X is I, Cl or Br, each of x and y is a mass % value of from 33.3% to 50% such that x+y is from 75% to 100% and the total mass % of Li2S, P2S5 and LiX is 100%; and (MPS) is a material of formula (III): mLi2S.nMS.oP2S5.(100−m−n−o)LiX  (III) wherein MS is a transition metal sulfide or a semi metal sulfide, X is I, Cl or Br, each of m, n and o is a mass % value greater than 0 such that (m+n+o) is from 75% to 100% and the total mass % of Li2S, P2S5 and LiX is 100%. Solid state batteries containing the composite material are also provided.