Synergistic voltage and electrolyte mediation improves sodiation kinetics in µ-Sn alloy-anodes

Abstract

Alloying electrodes, such as tin (Sn), are promising candidates for sodium-ion batteries because of their high specific capacity, electronic conductivity, and low sodium insertion voltage. However, sizeable volumetric change and electrode-electrolyte interface evolution in Sn preclude prolonged performance. The electrochemical potential window, compounded by the choice of electrolyte and additive combination, plays a critical role in the interface instability, which yet remains unresolved. This study, based on a comprehensive set of electrochemical, microscopy, and spectroscopic analyses, sheds light into the interface instability and reveals that the use of fluoroethylene carbonate additives in carbonate-based electrolytes can dramatically improve the interface stability of such alloying anodes. Electrochemical and morphological analyses show that without the additive, a higher end-of-charge voltage can cause breakdown and reformation of an unstable passivating layer, leading to rapid electrochemical performance decay. A novel three-electrode-based analytics reveals that superior interphase stability with higher microstructural integrity of the Sn electrode can alleviate the detriments from the upper cut-off voltage restrictions. Addressing the hitherto unresolved role of the electrochemical potential window, this study comprehensively examines and elucidates the causality of interfacial instability and the underpinnings of electrochemical complexations in sodium-alloying anodes.