Characterization of Chemical and Electrochemical Lithiation/ Delithiation of LiFePO4

Abstract

Redox kinetics for the electroactive compounds used in a lithium-ion battery are a key indicator of their power density. The kinetics and phase transformation of these compounds can be studied using potentiostatic methods, which apply constant thermodynamic driving force while the material is dispersed within a composite electrode. The current response, in some cases, is interpreted as a phase transformation of the solid electroactive material. Most studies apply electrochemical potential on the fabricated electrode, which gives convoluted data as inactive carbon, binders, and current collectors play a significant role in charge transfer. In this work, chemical redox was investigated for LiFePO4 to study the chemical lithiation and delithiation process at different temperatures. The kinetics were analyzed using a Johnson-Mehl-Avrami-Kolmogerov model and compared to electrochemical kinetics determined using coin cells. It was found that different pathways were followed for chemical and electrochemical redox. The apparent activation energy was an order of magnitude larger for chemical redox relative to electrochemical redox.