First-Principles Study of Low Electron Conductivity in Li peroxide for Li-air Batteries

Su-Huai Wei, Joongoo Kang, Yufeng Zhao, Chunmei Ban, and Anne C. Dillon
National Renewable Energy Laboratory

Rechargeable lithium batteries represent one of the most important developments in electrical energy storage and application. However, the energy density of state-of-the-art Li-ion batteries is still too small for some practical applications. Recently, Li-air batteries (LABs) have received new attention as a promising energy storage system beyond Li-ion batteries because their specific energy densities could be 5-10 times greater than those of the Li-ion batteries. However, although LABs offer the promise of very high energy density, its utilizations are hindered by both poor rate capability and significant polarization in cell voltage, primarily due to the formation of Li2O2 in the air cathode and poor electron conductivity in Li2O2. Here, using hybrid density functional theory, we demonstrate that the self-trapping of electrons in small polarons could be the origin of the low electron mobility in Li2O2. The low electron mobility is an intrinsic property of Li2O2 that originates from the molecular nature of the conduction band states of Li2O2 and the strong spin polarization of the O 2p state. We will discuss in detail how the low electron mobility affects the performance of LABs. Furthermore, based on our understanding of the mechanism, we will propose approaches to improve the performance of LABs at high current densities, such as selecting optimal growth direction of Li2O2 via substrate control, designing alternative carrier conduction paths for the cathode reaction, and/or introducing electron-deficient nano boron-carbon sheets to enhance the conductivity.