Understanding Coupled-Ion Electron Transfer Kinetics at Li-Ion Battery Interfaces

Enhancing the charge transfer kinetics of intercalation at the electrode-electrolyte interface is critical to further increase the power and energy performance of Li-ion batteries. Despite significant advancements in understanding Li-ion diffusion and discoveries of new electrodes and electrolytes, the molecular process of ion intercalation across electrode-electrolyte interfaces remains poorly understood. Liintercalation kinetics has been traditionally treated by the empirical Butler-Volmer kinetics, but remains poorly measured and understood. Here, by developing electrochemical characterizations, combined with a coupled-ion electron transfer (CIET) model,¹ we gain insights into the ion intercalation kinetics across the interface in Li-ion batteries. We developed experimental electrochemical methods using current-voltage responses and reaction-limited capacities to probe Li⁺ (de-)intercalation kinetics, for common intercalation electrode materials including LiCoO₂ and NMCs. A universal dependence of the intercalation rate on the lithium-ion filling fraction was revealed. Further, the temperature and electrolyte effects supported the microscopic Li⁺intercalation mechanism of CIET, which describes classical ion transfer from the electrolyte is coupled with quantum-mechanical electron transfer from the electrode.^2,3 We further quantified the three kinetic parameters that govern ion intercalation kinetics and their dependence on electrode and electrolyte materials. Finally, rate capability tests on thin, porous electrodes showed that the CIET reaction limitation governed the usable capacity at low-to-moderate (dis)charging rates. Our findings suggest that the proposed mechanism applies to a variety of intercalation materials used in energy storage, and governs the power and energy density at reaction-limited conditions. The understanding of CIET reaction limitation also helps to set usable capacity and extend lifetime by avoiding large overpotentials. The possibility of modifying the reaction-limited current with electrodes and electrolytes opens new directions for interfacial engineering.<br/><br/><br/>References:<br/>1 Y. Zhang, D. Fraggedakis, T. Gao, S. Pathak, D. Zhuang, C. Grosu, Y. Samantaray, A. R. C. Neto, S. R. Duggirala, B. Huang, Y. G. Zhu, L. Giordano, R. Tatara, H. Agarwal, R. M. Stephens, M. Z. Bazant and Y. Shao-Horn, 2024. DOI: 10.26434/chemrxiv-2024-d00cp.<br/>2 M. Z. Bazant, <i>Faraday Discuss.</i>, 2023, <b>246</b>, 60–124.<br/>3 D. Fraggedakis, M. McEldrew, R. B. Smith, Y. Krishnan, Y. Zhang, P. Bai, W. C. Chueh, Y. Shao-Horn and M. Z. Bazant, <i>Electrochimica Acta</i>, 2021, <b>367</b>, 137432.