Charge transfer kinetics strongly impact high-power applications of lithium-ion batteries such as fast charging. At such high current applications, the classical Butler¬¬ Volmer (BV) model may fail to predict overpotentials without deviations [1]. One of the approaches that has succeeded to explain these deviations is the coupled ion-electron transfer (CIET) theory [2]; however, its validation requires high overpotentials that are difficult to access experimentally. As a solution, the Nonlinear Frequency Response Analysis (NFRA) can be used to precisely extract the nonlinear high current-overpotential relationship [3]. Therefore, we present the first CIET-based interpretation of lithium-ion charge-transfer kinetics, using NFRA experiments [4].
For this purpose, we conducted experiments, using electrochemical impedance spectroscopy (EIS) and NFRA on Nickel-Manganese-Cobalt | Li half-cells to extract nonlinear cathode charge transfer behavior. The power of the CIET theory was examined by varying the temperature and state of charge. Extracting the charge transfer coefficients, we parametrized a frequency-domain porous electrode model (wP2D). To minimize issues of direct CIET integration and to promote imitation, we modified the BV theory to capture the high current trends by relaxing the constraint that charge transfer coefficients sum to unity. This relaxation was reported as an indication of different charge transfer kinetics [5, 6].
Our results showed that the charge transfer coefficients exhibit a temperature-independent trend across state of charge. Additionally, the exchange current density shows a distinct deviation from the BV prediction [7] and closer agreement with the CIET theory. Therefore, quantitative interpretation with CIET was conducted: The charge transfer coefficients were mapped to the reorganization energy of the coupled theory, ranging from 0.10 to 0.24 eV. The sum of the coefficients was always below 1, with an average of 0.55. Our findings advance the validity of CIET as a unified theory for capturing high current kinetics. The varying trends in the charge transfer coefficients and exchange current density do not support modeling approaches such as classical BV and BV with constant film resistance. Finally, the identified charge transfer trends promote future applications in state of charge estimation and optimized operation strategies. Our method also serves as a foundation for further studies on nonlinear charge transfer kinetics.