Kinetics Study and Degradation Analysis through Raman Spectroscopy of Graphite as a Negative-Electrode Material for Potassium-Ion Batteries

The development of K-ion batteries composed of abundant elements is a promising way to address the concerns generated by the resource constraint due to the increased demand for Li-ion batteries in the future. The expectation that K-ion batteries can supplement Li-ion batteries originates from the weaker Lewis acidity of K ions than that of Li ions. However, there are limited reports on the kinetics of charge-transfer reactions. Herein, we focused on graphite, which has attracted much attention as a negative-electrode material, and we studied the activation energy of the desolvation of K+ at the graphite/electrolyte interface in which the electrolyte typically had a composition of 1 mol kg(-1) potassium bis(fluorosulfonyl)amide (KFSA) dissolved in ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 50:50. Despite the larger solvation number of K+ than that of Li+ at the same salt concentration, analyzed by Raman spectroscopy measurements, the transference number of K+ resulted in a higher value of 0.43 compared to Li+ (0.39). The temperature dependence of the charge-transfer resistance associated with K+ desolvation using highly oriented pyrolytic graphite as a model electrode, without any binder and conductive additives, was investigated. The activation energy of K+ desolvation was found to be approximately 40 kJ mol(-1), which is much lower than that of the Li+ system. Thus, we experimentally demonstrated that the weaker Lewis acidity of K ions than that of Li ions has the energetically favorable kinetics on the charge-transfer reaction. Moreover, it was found that graphite in composite electrodes undergoes disordering by repeated K+-intercalation/deintercalation. The disordering determined by the initial co-intercalation of the K+-complex followed by the formation of surface layers has a significant relation with cyclability.