A quasi-zero-strain layered Nb4P2S21 cathode for high-energy solid-state polymer Na–metal batteries

Abstract

Solid-state polymer sodium–metal batteries (SPSMBs) are considered an essential solution to the safety issues and low energy density of conventional liquid electrolyte sodium-ion batteries. The unstable cathode electrolyte interface (CEI) caused by the volume expansion of the cathode hinders the construction of high-energy and long-life SPSMBs. Herein, a quasi-zero-strain layered Nb₄P₂S₂₁ cathode and a localized high-concentration solid-state polymer electrolyte (LHC-SPE) are designed to address the interfacial issues in SPSMBs. The fabricated Na|SPE|Nb₄P₂S₂₁ SPSMBs present a high capacity (315 mA h g⁻¹ at 0.5C), high-rate performance (167 mA h g⁻¹ at 10C), and good cycling performance (294 mA h g⁻¹ at 1C after 150 cycles). Moreover, a high energy density of 500 W h kg⁻¹ and a high power density of 2.7 kW kg⁻¹ can be achieved, which are superior to the electrochemical performance of the Nb₄P₂S₂₁ cathode in a liquid electrolyte (LE). Further cyclic voltammetry (CV) and in situ Raman characterization studies reveal a dual-redox mechanism based on Nb⁴⁺/Nb³⁺ and S⁻/S²⁻, and in situ XRD manifests quasi-zero-strain sodium insertion behavior. Moreover, a NaF-rich CEI is revealed from the X-ray photoelectron spectroscopy (XPS) results of the cycled electrode. This study has significant implications for the development of SPSMBs with high energy and a long cycling life.