A hierarchical approach to designing a Na-rich phosphide solid-state electrolyte for Na-ion batteries

Abstract

Solid-state electrolytes (SSEs) have brought significant advancements to secondary battery technology, but phosphides remain relatively unexplored compared to the extensively studied oxides, sulfides, and halides. In this study, we introduce a hierarchical approach for designing a Na-rich phosphide SSE. Starting with Na₃P, one of the most Na-rich materials, we apply the concept of dilute element compounds, incorporating a small amount of a dopant element to preserve its Na-richness. This leads us to identify Na₇TaP₄ as a promising candidate. Subsequently, through phase engineering utilizing the inorganic crystal structure database, we pinpoint a superior phase for Na₇TaP₄ that exhibits significantly enhanced Na diffusion kinetics. Lastly, employing defect engineering, we determine that partial substitution of Ta with Zr or Hf can stabilize Na₇TaP₄ in the desired phase over the experimentally synthesized phase. This hierarchical approach not only reduces the Na diffusion activation energy of the initial material Na₃P from 0.67 to 0.25 eV but also increases its band gap from 1.0 to 2.4 eV. These advancements show considerable promise for SSE applications. Furthermore, this design strategy is expected to find broad applicability in the development of SSEs for various types of secondary batteries.