Nonaqueous Electrolytes

The electrolyte in the non-aqueous (aprotic) lithium air battery has a profound influence on the reactions that occur at the anode and cathode, and hence its overall operation on discharge/charge. It must possess a wide range of attributes, exceeding the requirements of electrolytes for Lithium ion batteries by far. The most important additional issues are stability at both anode and cathode in the presence of O2. The known problems with cycling the Li metal/non-aqueous electrolyte interface are further complicated by O2. New and much less understood are the reactions at the O2 cathode/electrolyte interface where the highly reversible formation/decomposition of Li2O2 on discharge/charge is critical for the operation of the non-aqueous lithium air battery. Many aprotic electrolytes exhibit decomposition at the cathode during discharge and charge due to the presence of reactive reduced O2 species affecting potential, capacity and kinetics on discharge and charge, cyclability and calendar life. Identifying suitable electrolytes is one of the key challenges for the non-aqueous lithium air battery at the present time. Following the realisation that cyclability of such cells in the initially used organic carbonate electrolytes is due to back-to-back irreversible reactions the stability of the non-aqueous electrolytes became a major focus of research on rechargeable lithium air batteries. This realisation led to the establishment of a suite of experimental and computational methods capable of screening the stability of electrolytes. These allow for greater mechanistic understanding of the reactivity and guide the way towards designing more stable systems. A range of electrolytes based on ethers, amides, sulfones, ionic liquids and dimethyl sulfoxide have been investigated. All are more stable than the organic carbonates, but not all are equally stable. Even though it was soon realised, by a number of groups, that ethers exhibit side reactions on discharge and charge, they still remain the choice in many studies. To date dimethyl sulfoxide and dimethylacetamide were identified as the most stable electrolytes. In conjunction with the investigation of electrolyte stability the importance of electrode stability became more prominent. The stability of the electrolyte cannot be considered in isolation. Its stability depends on the synergy between electrolyte and electrode. Carbon based electrodes promote electrolyte decomposition and decompose on their own. Although great progress has been made in only a few years, future work on aprotic electrolytes for Li-O2 batteries will need to explore other electrolytes in the quest for yet lower cost, higher safety, stability and low volatility.