Stability and recharging of aprotic Li-O₂ batteries

Abstract

Non-aqueous rechargeable lithium-air (O₂) batteries are receiving intense interest because of their high theoretical specific energy, which are several times greater than that of lithium-ion batteries. To achieve it, the highly reversible formation/decomposition of Li₂O₂ is required to occur in the cathode during cycling. Due to the reactivity of reduced O₂ species, the aprotic electrolyte and carbon electrode substrate would be attacked and then decomposed. The organic carbonate decomposed on discharge, forming C₃H₆(OCO₂Li)₂, Li₂CO₃, HCO₂Li, CH₃CO₂Li, CO₂ and H₂O. Part of these by-products decomposed on the subsequent charge process and the rest remained and blocked the electrode surface. Finally, the cell cycling stopped because of the depletion of electrolyte and the passivation of the electrode surface. Possible mechanisms are proposed for reactions on discharge and charge. Some other types of aprotic solvents were investigated in the same way. Ethers, amides, sulfones, dimethyl sulfoxide (DMSO), etc reveal better stability than organic carbonates. Reversible formation/decomposition was observed together with minor side-reactions. Besides electrolytes, carbon substrate of electrode also slightly decomposed. Several other substrate materials were studied. If the carbon electrodes were replaced with the nanoporous gold electrodes, less side-reaction was observed in the cells, and the cell sustained 100 cycles without severe polarisation and capacity fading. The charge performance of a Li-O₂ cell remains a challenge. Great voltage polarisation even at modest rate was observed because of the difficulty of charge transfer between solid electrode surface and solid Li₂O₂. Redox mediators were used in a Li-O₂ cell, which transported the charge between electrode surface and solid Li₂O₂, acting as an electron-hole transfer agent. The oxidation of solid Li₂O₂ was facilitated and the cell with mediator demonstrated 100 charge/discharge cycles.