Despite remarkable recent advances in the field of solid electrolytes for lithium ion batteries, there is still considerable room for improvements with respect to ionic conductivity, cost and electrochemical stability. This study tests systematically how much Sn in the superionic conductor Li10SnP2S12 can be replaced by Si or Al, while retaining its tetragonal structure. For comparison, also the known superionic conductors Li10SnP2S12, Li10GeP2S12, and Li10SiP2S12 are synthesized and characterized with respect to their grain and grain boundary resistances. The results show that due to the negligible grain boundary resistance of the new compound Li10Si0.3Sn0.7P2S12, its total Li+ ion conductivity is only 10–20% lower than that of the expensive superionic conductor Li10GeP2S12 and about four times higher than that of Li10SiP2S12.