Generally speaking, a higher specific capacity or average battery voltage of the electrode is undoubtedly beneficial to improving the battery's energy density. The average battery voltage is determined by the enthalpy of free energy of the lithium-ion exchange reaction, which includes the intercalation and deintercalation of lithium ions into the active electrode material. For ordinary cathode materials, the electrode reaction may appear at first glance to be a redox reaction; however, this viewpoint does not consider the interaction between electrons and ions within the material. It is precisely because of these interactions that the electronic state of the transition metal ions participating in the reaction depends on the degree of lithium-ion intercalation.
Given the limitations of traditional methods in understanding the average voltage of batteries, Professor Wolfram Jaegermann of the Technical University of Darmstadt, Germany, proposes a novel method for analyzing the average electrode potential of batteries. As shown in the figure above, which illustrates the Fermi level and density of electronic states (DOS) of the electrode, it can be seen that under normal conditions, the Fermi level of the positive electrode material is located in the TM-3d derived band and shifts with changes in charge state; its displacement is related to the corresponding shift in the electron chemical potential. However, the information obtained from this figure alone is incomplete, as it does not consider the differences in lithium-ion chemical potential between lithium metal and other negative electrodes.
To more accurately reflect the electrode voltage of a lithium battery, it can be represented by the change in Gibbs free energy ΔG, or by the lithium chemical potential difference (μCLi, μALi) between the positive and negative electrodes:
Formally, the chemical potential of lithium can be divided into two parts: the electronic chemical potential Δμe and the ionic chemical potential ΔμLi+, further describing the changes in Gibbs free energy at the positive and negative electrodes, and their relationship with electron and ion exchange. It is generally believed that electrons play a dominant role; for example, by substituting transition metals in olivine materials to generate transition metal ions with higher ionization potentials, a higher average voltage can be obtained, a conclusion supported by many reports (J. Electrochem. Soc., 144, 1188 (1997); J. Phys. Chem. B, 108, 16093 (2004)). To gain a deeper understanding of the fundamental processes of electron and ion exchange in the electrodes and their related impact on battery performance, Ceder et al. developed the first first-principles calculations (Phys. Rev. B, 56(3), 1354 (1997)). However, to date, no experiment has clearly described the effect of electron-ion interactions on battery voltage.
Voltage is a key parameter determining the performance of lithium-ion batteries; the higher the average voltage, the higher the energy and power density. Theoretically, the potential of the lithium intercalation electrode is closely related to the chemical potential of electrons and ions, as well as the difference in battery voltage. However, since these quantities cannot be directly obtained experimentally, the precise contributions of electrons and lithium ions to the voltage remain unclear.
