Effects of Electrolyte Salt Concentration on Performance of Li-Oxygen Batteries

Abstract

Developing batteries with high specific capacity and power density is essential in many applications such as electric vehicles and portable electronic devices. The Li-oxygen battery has a very high theoretical energy density of 11 kWh kg-1 and is considered as a promising battery technology. The concentration of the lithium ion in battery electrolyte is typically 1 M in both Li-ion and Li-oxygen batteries. Considering the high cost of the lithium salt and low current rates of Li-oxygen battery, this study investigated effects of salt concentration (LiPF6 in Tetraethylene glycol dimethyl ether) on battery performance through experiments and model simulations. Results showed that when the salt concentration decreased from 1 M to 0.05 M, the specific discharge capacity decreased from 557.7 mAh/g to 390.9 mAh/g, and the specific charge capacity also decreased from 512.7 mAh/g to 339.5 mAh/g at the current rate of 0.1 mA/cm2. Both the discharge and charge capacity decreased significantly to less than 31.1 mAh/g when the lithium salt concentration was lower than 0.005M. The electrolyte conductivity that can affect the ion transport in the electrolyte was also measured in the experiment. The measured conductivity decreased from 3010.0 μS/cm to 16.6 μS/cm when the concentration of electrolyte decreased from 1 M to 0.005 M, while the molar conductivity was almost constant (3.518 +/- 0.048 S.cm2/mol) in this concentration range. A one-dimensional model was also developed to illustrate the importance of mass transfer in the electrode and simulate the electrochemical performance of Li-oxygen batteries.