The Lithium and Oxygen Concentration Dependence in Li-O2 Batteries Studied with Electrochemical Quartz Crystal Microbalance and Cylindrical Microelectrodes

The Li-O2 battery is a promising chemistry suggesting very high energy densities. In the literature it is agreed upon that during the discharge process Li2O2 is the final product formed via the intermediate LiO2. However, the nature of the intermediate is not fully understood, e.g. how long is the lifetime and how does it proceed to form the final product (chemical or electrochemical reaction). This study sets out to shed light on the reaction mechanism of the Li-O2 battery by studying the effects of different concentrations of lithium and oxygen. Experimentally this is done using an electrochemical quartz crystal microbalance (EQCM) and a cylindrical microelectrode. The EQCM results show that the system is unable to return to its initial mass, i.e. there is a buildup of reaction product during cycling. Further, it is not possible to recover all the discharge electrons during recharge. The intermediate is believed to diffuse away from the electrode rendering it unavailable for recharge. This result is further strengthened by the measurements on microelectrodes where intermediate is believed to diffuse away from the electrode more easily. There is also a discrepancy when looking at the change in mass and the expected mass change from the corresponding current suggesting that both electrochemical and chemical processes occurs. In conclusion it can be said that this study provides additional knowledge to the surface confined processes in the Li-O2 battery. The EQCM provides information that otherwise would go unnoticed, i.e. solely measuring the current response will not provide detailed quantitative information of the mass response.