In Situ Measurement of Strain and Stress Evolution in Lithium Iron Phosphate Electrodes during Electrochemical Cycling

Abstract

Rechargeable lithium-ion batteries have been widely commercialized in portable electronics. However, adoption of lithium-ion batteries in more demanding applications such as electric vehicles and renewable energy storage has been hindered by capacity loss and poor performance. Although there has been enormous progress on the advancement of high capacity anode materials such as silicon, the current stage of the cathode performance is a limiting factor for the development of higher performance Li-ion batteries. Lithium iron phosphate, LiFePO4, (LFP) is a promising candidate material because it is inexpensive, environmentally friendly and has a high theoretical capacity (170 mAh g-1). Unfortunately, LFP suffers from mechanical degredations and surface instabilities that lead to capacity fade. In this work, we investigate in situ stress and strain generation in LFP composite electrodes during battery charging – discharging. Repeated lithiation and delithiation cause continuous stress and strain evolution in the electrode due to lithium intercalation and the interaction between electrode and electrolyte species. Electrodes are constrained on substrate for stress measurements, whereas free-standing electrodes are fabricated for strain measurements. Digital image correlation (DIC) is utilized to measure in situ strain generation, whereas beam curvature technique is employed to monitor in situ stress evolution in the electrode. Electrodes are cycled between 2.6 to 4.4 V at different scan rates in various electrolytes. Strain and stress derivatives are calculated with respect to applied potential. The derivatives provide remarkable information about the evolution of surface stress and structural changes in the electrode due to phase transformations. Electrochemical stiffness is calculated by combining stress and strain. Changes in stiffness during electrochemical cycling reveal underlying mechanisms governing stress and strain evolution in the LFP electrode. Acknowledgement This work was supported as part of the Center for Electrochemical Energy Science (CEES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.”