Triggering dynamically disordered lithium sublattice in superionic conductors

The design of superionic conductors has been largely focused on static structural features, with the dynamic ion transport mechanism less explored. Here, we explore a paradigm that harnesses the polyanion rotations to trigger the dynamically disordered Li sublattice as well as the liquid-like cation diffusion for superior ionic conductivity in crystals. A descriptor called rotation tolerance factor was proposed as a predictive metric for identifying the potential fast-rotating anion clusters with the low mass and reduced valence charge for given structural frameworks. Guided by this factor, halides with rotational polyanions, namely Li₃Y(SH)₆, Li₃Y(NH₂)₆, Li₂Zr(NH₂)₆, and an oxide (Li_6.5La₃Zr₂O_11.5(NH₂)_0.5) have been designed with synergistic polyanion rotation and Li⁺ sublattice disorder, which lead to enhanced Li ionic conductivities at room temperature compared to the counterparts without polyanions. The experimentally synthesized NH₂^- incorporated Li₂ZrCl_5.92(NH₂)_0.08 demonstrates a four-fold higher conductivity over Li₂ZrCl₆ control, enabling all-solid-state Li-In | |LiCoO₂ and Li-In | |LiNi_0.88Co_0.09Mn_0.03O₂ cells with 96.5% and 97.4% capacity retention after 190 cycles at 140 and 200 mA g^-1, respectively. This work provides an insight that flexible anion rotations could promote the dynamically disordered lithium sublattice distribution as well as the ionic conductivity.