The glass phase in the grain boundary of NaZrSiPO, created by gallium modulation.

NaZrSiPO has been proven to be a promising electrolyte for solid-state sodium batteries. However, its poor conductivity prevents application, caused by the large ionic resistance created by the grain boundary. Herein, we propose an additional glass phase (Na-Ga-Si-P-O phase) to connect the grain boundary Ga ion introduction, resulting in enhanced sodium-ion conduction and electrochemical performance. The optimized NaZrSiPO-0.15Ga electrolyte exhibits Na conductivity of 1.65 mS cm at room temperature and a low activation energy of 0.16 eV, with 20% newly formed glass phase enclosing the grain boundary. Temperature-dependent NMR line shapes and spin-lattice relaxation were used to estimate the Na self-diffusion and Na ion hopping. The dense glass-ceramic electrolyte design strategy and the structure-dynamics-property correlation from NMR, can be extended to the optimization of other materials.