Insights into the Performance Limits of the Li₇P₃S₁₁ Superionic Conductor : A Combined First-Principles and Experimental Study

The Li₇P₃S₁₁ glass-ceramic is a promising superionic conductor electrolyte (SCE) with an extremely high Li⁺ conductivity that exceeds that of even traditional organic electrolytes. In this work, we present a combined computational and experimental investigation of the material performance limitations in terms of its phase and electrochemical stability, and Li⁺ conductivity. We find that Li₇P₃S₁₁ is metastable at 0 K but becomes stable at above 630 K (∼360°C) when vibrational entropy contributions are accounted for, in agreement with differential scanning calorimetry measurements. Both scanning electron microscopy and the calculated Wulff shape show that Li₇P₃S₁₁ tends to form relatively isotropic crystals. In terms of electrochemical stability, first-principles calculations predict that, unlike the LiCoO₂ cathode, the olivine LiFePO₄ and spinel LiMn₂O₄ cathodes are likely to form stable passivation interfaces with the Li₇P₃S₁₁ SCE. This finding underscores the importance of considering multicomponent integration in developing an all-solid-state architecture. To probe the fundamental limit of its bulk Li⁺ conductivity, a comparison of conventional cold-press sintered versus spark-plasma sintering (SPS) Li₇P₃S₁₁ was done in conjunction with ab initio molecular dynamics (AIMD) simulations. Though the measured diffusion activation barriers are in excellent agreement, the AIMD-predicted room-temperature Li⁺ conductivity of 57 mS cm^-1 is much higher than the experimental values. The optimized SPS sample exhibits a room-temperature Li⁺ conductivity of 11.6 mS cm^-1, significantly higher than that of the cold-pressed sample (1.3 mS cm^-1) due to the reduction of grain boundary resistance by densification. We conclude that grain boundary conductivity is limiting the overall Li⁺ conductivity in Li₇P₃S₁₁, and further optimization of overall conductivities should be possible. Finally, we show that Li⁺ motions in this material are highly collective, and the flexing of the P₂S₇ ditetrahedra facilitates fast Li⁺ diffusion. (Graph Presented). © 2016 American Chemical Society.