Na-Sn Alloy Regulated Protective Coating Layer Enables the Long Life Dendrite-Free Sodium Metal Batteries

Abstract

Sodium metal anodes are highlighted as the most potential candidates for next-generation batteries due to their high theoretical capacity (1166 mAh/g), low electrochemical potential (-2.71 V vs standard hydrogen electrode), and cheaper material cost. However, practical application of sodium metal batteries is limited due to the uneven solid electrolyte interphase layer and insignificant coulombic efficiency. Designing a protective coating layer as an artificial solid electrolyte interphase layer on a sodium metal anode is one of the main approaches to maintaining the homogenous deposition and long-term stability during the process of electrochemical plating/stripping. Herein, a protective layer of (Na-Sn) alloy and Na-Cl insulator was formed by establishing the spontaneous self-alloying reaction. Most importantly, the proposed Na-Sn metal alloy layer has high ionic conductivity, stable chemical/electrochemical interphase, and robustness to enhance the homogenous Na ion deposition and prevent the uncontrollable sodium dendrites. Based on these effects, using the Na-Sn alloy symmetric cell exhibits the stable electrochemical performance (500 h at 2 mA/cm2 & 2 mAh areal capacity). The Full cell was assembled using the Na3V2(PO4)3 cathode and Na-Sn alloy over sodium metal anode in diglyme electrolyte, showing excellent cycle stability up to 100 cycles at 1 C rate and discharge capacity of 168 mAh/g at 0.1 C rate. This work presents a new technique to produce a uniform Na ion deposition and safe dendrites-free sodium metal batteries (SMBs).