Reactions in the Rechargeable Lithium–O₂ Battery with Alkyl Carbonate Electrolytes

TLDR

The authors propose mechanisms for the discharge and charge reactions in a lithium–oxygen battery using an alkyl carbonate electrolyte. Discharge proceeds via electrolyte decomposition to form lithium carbonate, formate, acetate, CO₂, and H₂O, while charge oxidizes these products, producing CO₂, H₂O, and a gel on the lithium anode from the remaining carbonate groups. The identified pathways explain the observed voltage gap and show that accumulation of lithium carbonate, formate, acetate, and alkyl carbonate on the cathode, along with continuous electrolyte consumption, leads to capacity fading and cell failure.

Abstract

The nonaqueous rechargeable lithium-O(2) battery containing an alkyl carbonate electrolyte discharges by formation of C(3)H(6)(OCO(2)Li)(2), Li(2)CO(3), HCO(2)Li, CH(3)CO(2)Li, CO(2), and H(2)O at the cathode, due to electrolyte decomposition. Charging involves oxidation of C(3)H(6)(OCO(2)Li)(2), Li(2)CO(3), HCO(2)Li, CH(3)CO(2)Li accompanied by CO(2) and H(2)O evolution. Mechanisms are proposed for the reactions on discharge and charge. The different pathways for discharge and charge are consistent with the widely observed voltage gap in Li-O(2) cells. Oxidation of C(3)H(6)(OCO(2)Li)(2) involves terminal carbonate groups leaving behind the OC(3)H(6)O moiety that reacts to form a thick gel on the Li anode. Li(2)CO(3), HCO(2)Li, CH(3)CO(2)Li, and C(3)H(6)(OCO(2)Li)(2) accumulate in the cathode on cycling correlating with capacity fading and cell failure. The latter is compounded by continuous consumption of the electrolyte on each discharge.