Equations and electrochemical methods for measuring the interfacial charge-transfer kinetics of Li-ion battery active materials at high current densities
Kevin Scanlan, Arumugam Manthiram
Abstract
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 ( I 0 ). 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 I 0 values from the electrochemical data at high current densities by reformulating the BV equation to describe the current dependence of charge-transfer resistance ( R ct ) 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 α and I 0 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 LiCoO 2 at 50 % state-of-charge. S3E-CP, LA-GEIS, and O-GEIS measurements yield average α values of, respectively, 0.420 ± 0.016, 0.432 ± 0.012, and 0.437 ± 0.014, and it is suggested that these data are consistent with a two-step reaction mechanism with α 1 = 0.5 and α 2 = 3. The equations and electrochemical methods developed herein are broadly applicable for empirically measuring and modeling the interfacial charge-transfer kinetics in rechargeable batteries.