UC Irvine

Lithium oxygen battery has the potential to outdo the best battery system on the market due to its high theoretical specific energy of 11,400 Wh/kg (Li) which is comparable to that of gasolines. However, their practical implementation is still facing challenges, including low cyclic performance, high charging voltage, insoluble discharge product formation, and electrolyte degradation. Air cathodes, where oxygen reacts with Li ions and electrons to produce insoluble discharge oxides, are often considered as the most challenging component in nonaqueous Li-air batteries. To understand the voltage loss mechanisms in air cathodes due to insoluble discharge oxides, a mathematical model is developed, which incorporates the major thermodynamic, transport, and kinetic processes, and comprehensive analysis is conducted. Experiment is also designed and conducted for the model formulation and validation. Li battery components were fabricated, then assembled in an argon filled glove box. Electrochemical testing was conducted on the experimental Li air battery by using the PARSTAT MC Multichannel Potentiostat. An X-ray Diffraction, scanning electron microscope, and high-precision mass scale were employed to determine the composition, morphology, and spatial variation of Li oxide products. Through the model analysis, it is found that the electric passivation and oxygen transport blockage caused by the Li oxides precipitates reduce the battery voltage and energy capacity. The first stage of voltage drop is dominated by the electrode passivation and surface loss, while the latter stage of voltage drop is dominated by the oxygen blockage. In addition, several morphologies of oxides are identified in the cathode structure under various current densities, thus the proposed surface coverage model is superior to the traditional film-resistor approach. Further, it is found there exists spatial variations in discharge Li oxides in terms of mass and morphology, and experimental data show a good agreement with the theoretical analysis in term of the spatial variation of the Li oxides mass. The Da number is identified as a major parameter governing the spatial variation.