In Situ Chemo-Mechanical Responses of Lithium Iron Phosphate Electrodes during Electrochemical Cycling

Abstract

Rechargeable Li-ion batteries (LIBs) have been successfully commercialized for portable electronics in the last two decades. Although there has been a great interest to use LIBs in more demanding applications such as electric vehicles, adoption of LIBs has been hindered by capacity loss and poor performance. The current stage of the cathode performance is a limiting factor for the development of higher performance LIBs. Lithium intercalation induces considerable volumetric changes with associated stress generation in the electrode. Even small strain can cause particle fractures in brittle cathode materials. The formation of a fracture particles leads to the capacity fade and limits the lifetime of batteries. To investigate chemo-mechanical response of the cathodes, we previously calculated electrochemical stiffness changes cathodes by coordinating in situ stress and strain measurements1. The electrochemical stiffness response of lithium manganese oxide revealed that the underlying mechanisms governing stress and strain are intrinsically different. In this study, our objective is to evaluate chemo-mechanical responses of lithium iron phosphate (LFP) cathode. In situ stress measurements were carried out with the beam curvature method whereas digital image correlation (DIC) is utilized to measure in situ strain generation. Electrodes are constrained on the cantilever for stress measurement, whereas free-standing electrodes were fabricated for strain measurements. LFP electrodes were fabricated with a composition of 8:1:1 weight ratio of LFP, carboxymethyl cellulose binder and carbon black. Electrodes are cycled with cyclic voltammetry technique at different scan rates (between 10 and 250μV/s) in various electrolytes. Stress and strain derivatives are calculated with respect to applied potential. Derivatives present crucial information to probe potential-dependent surface stress evolutions and structural changes in the electrode. As expected, strain derivatives are well-correlated with the changes in the crystal structure during delithiation and lithiation. Interestingly, stress derivatives reveal that electrolyte solution has a role on the evolution of surface stress. Changes in stress and strain during electrochemical cycling reveal underlying mechanisms governing chemo-mechanical responses in the LFP electrode. References 1- Ö. Ö. Çapraz, K. L. Bassett, A. A. Gewirth, and N. R. Sottos, Adv. Energy Mater., 1601778–7 (2017). 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.”