Insight into Ion Diffusion Dynamics/Mechanisms and Electronic Structure of Highly Conductive Sodium-Rich...

Solid-state electrolytes (SSEs) have attracted considerable attention as an alternative for liquid electrolytes to improve safety and durability. Sodium Super Ionic CONductor (NASICON)-type SSEs, typically Na₃Zr₂Si₂PO₁₂, have shown great promise because of their high ionic conductivity and low thermal expansivity. Doping La into the NASICON structure can further elevate the ionic conductivity by an order of magnitude to several mS/cm. However, the underlying mechanism of ionic transportation enhancement has not yet been fully disclosed. Herein, we fabricate a series of Na₃₊LaZr_2-Si₂PO₁₂ (0 ≤ x ≤ 0.5) SSEs. The electronic and local structures of constituent elements are studied via synchrotron-based X-ray absorption spectroscopy, and the ionic dynamics and Na-ion conduction mechanism are investigated by solid-state nuclear magnetic resonance spectroscopy. The results prove that La³⁺ ions exist in the form of phosphate impurities such as Na₃La(PO₄)₂ instead of occupying the Zr⁴⁺ site. As a result, the increased Si/P ratio in the NASICON phase, accompanied by an increase in the sodium ion occupancy, makes a major contribution to the enhancement of ionic conductivity. The spin-lattice relaxation time study confirms the accelerated Na⁺ motions in the altered NASICON phase. Modifications on the Si/P composition can be a promising strategy to enhance the ionic conductivity of NASICON.