Highly Fluorinated and Boron Containing Anion Derived Stable Anode and Cathode Interfaces for Long Cycle and High Voltage Sodium Metal Batteries

Abstract

Sodium metal is a propitious anode for the development of next generation metal batteries owing to its low reduction potential (-2.71 V) and high theoretical capacity (1165 mAh g-1)1. Among various electrolytes, carbonate solvents can be useful to develop high voltage metal batteries with wide temperature range2. However, the aggressive reactivity of sodium metal in carbonate electrolytes lead to the formation of unstable solid electrolyte interface on the metal anode which renders poor long cycling performance to the battery3. In this work, we have fabricated thin and inorganic components (F and B rich) rich solid electrolyte interface (SEI) on Na metal anode and cathode electrolyte interface (CEI) on oxide-based cathode to improve long cycling stability of sodium metal battery. Here, fluorine (F) and boron (B) containing anion is compared with conventional hexafluorophosphate anion (PF6-) in binary carbonate solvent system (EC:EMC). The stable stripping/plating behaviour of both symmetric (Na/Na) and asymmetric cell (Al/Na) in case of F and B containing anion is found compared to the PF6 - anion.The excellent long-term cycling stability of full cell (Na/Na0.67Fe0.5Mn0.5O2) is further attributed to the improved electrochemical stability window and better aluminium passivation on employing F and B containing anion in the electrolyte system as compared to PF6 - anion. The dual role of F and B containing anion to form stable interfaces on both the anode and cathode side provides a pathway in the development of fluorinated electrolytes for high voltage sodium metal batteries.