Factors Affecting Electrochemical Properties of P’2-Type Mn-Based Layered Oxides for Sodium Ion Batteries

Abstract

In recent years, the rapid growth in the market of electric vehicles has resulted in the increase of prices for lithium, cobalt, and nickel resources used in lithium-ion batteries. Therefore, a great research effort has been conducted to explore new battery systems based on more abundant resources with lower costs. Recently, rechargeable sodium-ion batteries have been attracting much attention for energy storage applications.[1] In this study, Mn-based sodium layered oxide, P'2 Na 2/3 MnO 2 is revisited as a positive electrode material for sodium-ion batteries, and factors affecting its electrochemical performances is examined. The cycle stability of Na 2/3 MnO 2 is remarkably improved by increasing the lower cut-off voltage during cycling. This is because the electrochemical cycling in a higher voltage range maintains a higher average oxidation state of Mn ions in the electrode ( i.e., less local distortion induced by Jahn-Teller Mn 3+ ions). Furthermore, the use of highly concentrated electrolytes, in which the presence of free solvent molecules is eliminated, effectively suppresses the Mn dissolution, thus enabling stable cycling with >80% capacity retention for more than 1000 cycles. From these results, the possibility of Na 2/3 MnO 2 as a practical positive electrode for sodium-ion battery applications is discussed. Reference [1] T. Sato et al ., and N. Yabuuchi, Energy Material Advances , 2021 , 9857563 (2021).