Performance of Li-Ion Batteries: Contribution of Electronic Factors to the Battery Voltage

The voltage is a key parameter for the performance of Li-ion cells. An increased average voltage results in a higher energy- and power density. Theoretically, the potential of Li-intercalation electrodes is related to the chemical potentials of both electrons and Li+-ions, and the voltage of a cell to their respective differences. As experimentally these quantities cannot be directly obtained, the exact contribution of electrons and Li+-ions to the voltage is still not known for the different materials. In this contribution, we investigate the correlation of the average cell voltage with the electronic ionization potential of different thin film cathode materials. Specifically, we relate the ionization potential obtained by photoelectron spectroscopy to the electron chemical potential at the onset of the charging plateau, and its difference to the ionization potential of lithium to the electronic contribution of the cell voltage. As experimental data both layered and olivine cathode materials are considered and we find a reasonable correlation between ionization potential and average voltage, as well as the electronic contribution as major factor for the battery voltage. The possible reasons for the deviation such as the ionic contribution and the presence of surface dipole potentials are discussed.