Equations and Electrochemical Methods for Measuring the Interfacial Charge-Transfer Kinetics of Li-ion Battery Active Materials at High Current Densities

Experimental measurements and quantitative models of the interfacial charge-transfer kinetics of Li-ion battery (LIB) active materials (AM) are essential for accurate predictions of LIB rate performance, safety, and lifetime. The Butler-Volmer (BV) equation is commonly used to describe interfacial kinetics in LIBs as a function of the transfer coefficient (α) and exchange current (I0). It is tacitly assumed that α ≈ 0.5, so experimental measurements of α for LIB AMs have rarely been attempted. In this work, mathematical models are derived to fit the apparent α and I0 values from the electrochemical data at high current densities by reformulating the BV equation to describe the current dependence of charge-transfer resistance (Rct) and differential charge-transfer resistance (R’ct). Pseudo-steady-state extrapolation chronopotentiometry (S3E-CP), large-amplitude galvano EIS (LA-GEIS), and operando galvano EIS (O-GEIS) techniques are developed, and each is shown to be capable of accurately and precisely measuring the values of α and I0 while maintaining the conditions of stability, stationarity, and linearity. Symmetric coin cells are demonstrated as a simple and widely accessible tool for achieving the most accurate kinetic measurements, and preliminary results are reported for LiCoO2 symmetric cells at 50% state-of-charge. S3E-CP and LA-GEIS measurements yield apparent α values of, respectively, 0.420 and 0.431, while O-GEIS measurements show that these data are consistent with a two-step reaction with α1 = 0.5 and α2 = 3. The equations and electrochemical methods developed herein are broadly applicable for empirically measuring and modeling the interfacial charge-transfer kinetics in rechargeable batteries.