Ion–Solvent Complexes Promote Gas Evolution from Electrolytes on a Sodium Metal Anode

Abstract

Lithium and sodium metal batteries are considered as promising next-generation energy storage devices due to their ultrahigh energy densities. The high reactivity of alkali metal toward organic solvents and salts results in side reactions, which further lead to undesirable electrolyte depletion, cell failure, and evolution of flammable gas. Herein, first-principles calculations and in situ optical microscopy are used to study the mechanism of organic electrolyte decomposition and gas evolution on a sodium metal anode. Once complexed with sodium ions, solvent molecules show a reduced LUMO, which facilitates the electrolyte decomposition and gas evolution. Such a general mechanism is also applicable to lithium and other metal anodes. We uncover the critical role of ion-solvent complexation for the stability of alkali metal anodes, reveal the mechanism of electrolyte gassing, and provide a mechanistic guidance to electrolyte and lithium/sodium anode design for safe rechargeable batteries.

The Origin of the Reduced Reductive Stability of Ion–Solvent Complexes on Alkali and Alkaline Earth Metal Anodes

“…Despite as maller theoretical capacity (1160 mAh g À1 ), sodium metal anodes can be applied to large-scale energy storage owing to the much lower price of sodium comparing with lithium metal. [9][10][11] More seriously,b oth lithium and sodium prefer ad endritic growth during plating process. [7] Although having great advantages in theoretical energy density,b oth alkali and alkaline earth metal batteries face many challenging issues.S pecifically,t he intrinsic instability of conventional organic electrolytes against metal anodes seriously impedes their practical applications.…”

“…[18] Besides,the artificial protective layer is particularly designed to avoid the direct interactions between electrolytes and anodes to impede the electrolyte decomposition. [10] However,the applicability of this principle to other alkali metal anodes and even alkaline earth metal anodes should be further explored. Recently,t he concept of ionsolvent complexes has been proposed to explain the electrolyte decompositions on sodium metal anodes as the ionsolvent complex with am uch lower energy level of LUMO (the lowest unoccupied molecular orbital) than that of pure solvents can promote the electrolyte gassing.…”

Schwefel‐basierte Elektroden mit Mehrelektronenreaktionen für Raumtemperatur‐Natriumionenspeicherung

“…[2b, 4] Wissenschaftler sind daher bestrebt, diese Schwelle zu überwinden. [15] Natriumanoden haben ein relativ niedriges Redoxpotenzial (À2.714 V) und hohe theoretische gravimetrische Kapazitäten (1165 mA hg À1 ). [5] Interkalationsanoden wie ungeordneter Kohlenstoff ("harter Kohlenstoff"), [6] reduziertes Graphenoxid (RGO) [7] und Titandioxid (TiO 2 ) [8] führen zu relativ niedrigen Kapazitäten.…”

A Stable Quasi‐Solid‐State Sodium–Sulfur Battery

“…[1] Among Na battery systems,s odium-sulfur (Na-S) batteries based on ac onversion chemistry have drawn tremendous attention because of the high energy density and low cost of sulfur (S) cathodes. Meanwhile,t he in situ formed polymer electrolyte with high ionic conductivity and enhanced safety successfully stabilizes the Na anode/electrolyte interface,and simultaneously immobilizes soluble Na polysulfides.T he as-developed quasi-solid-state Na-S cells exhibit ah igh reversible capacity of 877 mA hg À1 at 0.1 Ca nd an extended cycling stability.…”

“…[1] Among Na battery systems,s odium-sulfur (Na-S) batteries based on ac onversion chemistry have drawn tremendous attention because of the high energy density and low cost of sulfur (S) cathodes. [1] Among Na battery systems,s odium-sulfur (Na-S) batteries based on ac onversion chemistry have drawn tremendous attention because of the high energy density and low cost of sulfur (S) cathodes.…”