We report on a new ultrafast solid electrolyte of the composition Li₁₁Si₂PS₁₂, which exhibits a higher room-temperature Li ion diffusivity than the present record holder Li₁₀GeP₂S₁₂. We discuss the high-pressure synthesis and ion dynamics of tetragonal Li₁₁Si₂PS₁₂, and comparison is made with our investigations of related members of the LMePS family, i.e. electrolytes of the general formula Li_11−xMe_2−xP_1+xS₁₂ with Me = Ge, Sn : Li₁₀GeP₂S₁₂, Li₇GePS₈, Li₁₀SnP₂S₁₂. The structure and dynamics were studied with multiple complementary techniques and the macroscopic diffusion could be traced back to fast Li ion hopping in the crystalline lattice. A clear correlation between the diffusivity and the unit cell volume of the LGPS-type electrolytes was observed.