Electrochemical strain evolution in iron phosphate composite cathodes during lithium and sodium ion intercalation

Sodiation-induced electrochemical strain generation in composite iron phosphate (FePO4) host material is compared with lithiation-induced strain evolution. FePO4 composite materials are prepared by an electrochemical displacement technique using pristine composite LiFePO4 as the starting material. The composite FePO4 electrodes have identical composition, binder, conductive additives and particle morphology for both Na+ and Li+ ion intercalation. We employ digital image correlation to investigate potential-dependent mechanical changes in FePO4 host material during alkali-metal ion intercalation via cyclic voltammetry and galvanostatic cycling. The FePO4 electrode experience much larger strains during the first sodiation (∼2.40%) compared to the first lithiation (∼0.60%). Strains in the subsequent cycles slowly decreased and become more reversible upon both Na+ and Li+ ion intercalation. Analysis of strain derivatives during lithiation, delithiation and sodiation exhibit a single peak that coincide with the associated phase transformation. The relative expansion in the composite electrode during Na+ ion intercalation with respect to Li+ ion intercalation is much greater than the relative expansions in electrode cell volume reported by the previous diffraction studies. We hypothesize that amorphization and slower Na+ diffusion in the electrode can lead to additional strain development compared to Li intercalation. Our results provide new insights into the mechanics of alkali metal-ion intercalation in cathodes.