Abstract
Lithium superionic conductor (LISICON)-related compositions Li4 +/- xSi1-xXxO4 (X = P, Al, or Ge) are important materials that have been identified as potential solid electrolytes for all solid state batteries. Here, we show that the room temperature lithium ion conductivity can be improved by several orders of magnitude through substitution on Si sites. We apply a combined computer simulation and experimental approach to a wide range of compositions (Li4SiO4, Li3.75Si0.75P0.25O4, Li4.25Si0.75Al0.25O4, Li4Al0.33Si0.33P0.33O4, and Li4Al1/3Si1/6Ge1/6P1/3O4) which include new doped materials. Depending on the temperature, three different Li+ ion diffusion mechanisms are observed. The polyanion mixing introduced by substitution lowers the temperature at which the transition to a superionic state with high Li+ ion conductivity occurs. These insights help to rationalize the mechanism of the lithium ion conductivity enhancement and provide strategies for designing materials with promising transport properties.