Highly Durable and Stable Sodium Superoxide in Concentrated Electrolytes for Sodium–Oxygen Batteries

Abstract

A great interest has emerged on metal–oxygen batteries with a hope for ultra-high-energy-density batteries [1]; of these, the intrinsic high energy efficiency and reversibility make sodium–oxygen batteries as one of the most promising post-lithium-ion batteries. In contrast to the lithium peroxide formation in lithium–oxygen batteries, the formation of sodium superoxide discharge product brings out unique electrochemical properties of sodium–oxygen batteries. Intense studies have revealed that the high capacity and low polarization of sodium–oxygen batteries are attributable to the solution phase formation and decomposition of sodium superoxide, which has substantial stability and solubility in ethereal electrolytes.[2-4] However, recent studies have raised out the issue on chemical instability that sodium superoxide undergoes parasitic reactions with the ethereal electrolytes in sodium–oxygen batteries.[4-8] We have previously demonstrated the mechanism that the dissolution of sodium superoxide into the electrolytes liberates reactive superoxide anion and triggers the parasitic reactions.[4] Other products formed as a result of parasitic reactions require higher polarization to be recharged and do not involve oxygen evolution during the charge process, which seriously deteriorates cell efficiency and reversibility during storage period of batteries. These phenomena should be closely linked with the storage properties or shelf-life of batteries. The detrimental effects of storage after discharge in sodium–oxygen batteries may be amplified when it comes to be applied for practical batteries in electric vehicles (EVs) since EVs sometimes cannot be immediately recharged depending on the consumers’ utilization patterns. Therefore, proper strategies to accomplishment long living sodium superoxide have to be addressed for maintaining high efficiency and reversibility of sodium–oxygen batteries. A rational tuning of the electrolyte possesses the key to control the chemical stability of discharge products and improve storage properties of the sodium–oxygen batteries. In this study, we explore a concentrated electrolyte to prevent the dissolution and parasitic reactions of sodium superoxide in sodium–oxygen batteries. Through a couple of solution characterization techniques, unique solvation structures of concentrated electrolytes are revealed with the elimination of free solvents. Time-resolved ex situ characterizations confirm that sodium superoxide stored in the concentrated electrolyte exhibits prolonged lifetime compared to that in normal electrolytes. We finally demonstrate that highly durable sodium superoxide in the concentrated electrolyte succeeds to preserve both high energy efficiency and oxygen reversibility of sodium–oxygen batteries despite the insertion of storage period after discharge. This is the first study to successfully improve the chemical stability and lifetime of sodium superoxide by simple tuning of electrolyte concentrations in sodium–oxygen batteries. This finding is complementary to the previous understanding on the solution chemistry of sodium–oxygen batteries, highlighting the importance for understanding the role of electrolytes on the sodium–oxygen chemistry.