A Glass-Interphase for Highly Reversible and Stable RT-Na/S Batteries

Abstract

Owing to the high theoretical specific capacity (1165 mA h g-1), high natural abundance, and low redox potential (-2.71 V vs standard hydrogen electrode), sodium metal is considered as an ideal anode material for the next-generation high-energy batteries. Though, a high-temperature variant of the sodium-sulfur batteries has already been commercialized, its compromised safety (due to high-temperature operation, 250 – 300 ⁰C) has triggered the research interest in developing its room-temperature analogue. At room-temperature, the thermodynamic instability of Na metal with the liquid electrolytes often leads to develop a thick and chemically unstable interphase, often called the solid electrolyte interphase (SEI). Upon Na plating/stripping, the SEI grows and dissolves repeatedly, results in consuming both the Na anode and the electrolyte inventory. The Na dendrites, the primary unknown risk associated with Na metal-based batteries, are inclined to develop from the fractured sites of the SEI. The SEI exfoliation can be attributed to weak chemical adhesion between the SEI and the Na metal anode. Functional additives, engineered solvents, and the high concentration electrolytes have often been used to alter the mechanical and electrochemical properties of the SEI. It is apparent that, a stable and durable SEI is the key to creating a secure, dendrite-free, and long-lasting RT-Na/S batteries.More recently, the idea of glass-electrolytes to stabilize the metal anodes is disclosed by the Nobel laureate Prof. J. Goodenough. The glass-electrolyte is used to suppress the dendrite growth and ensured improved safety of the high-energy metal-sulfur batteries. However, the cell experiences a huge interfacial resistance due to metal anode/glass-electrolyte interface.The present work relates to the development of a glass-interphase for RT-Na/S batteries. The interphase is grown directly over the sodium metal anode through a simple and scalable approach. To growth the interphase, a novel organic molecule is used to facilitate the formation of the glass-interphase, and called as “glass-interphase forming additive (GIFA). In general, GIFAs contains the hydrolysable group, which is commonly an amine, acyloxy, alkoxy, or halogen on one side, and one Si atom. It contains organo-functional group, such as epoxide, on other side. The presence of oxide or hydroxide is essential in realizing interaction of GIFA with the sodium metal surface.