The Role of Fluoroethylene Carbonate as an Electrolyte Additive on the Chemomechanical Stabilities of NaCrO2 Cathodes in Na-Ion Batteries

Sodium-ion batteries are promising energy storage devices beyond Li-ion batteries; however, their cycling stability is limited by chemomechanical instabilities in transition-metal oxide cathodes. Although electrolyte additives and other surface modification strategies have been widely used to improve electrochemical performance, the mechanisms underlying these improvements remain poorly understood. Here, sodium chromium oxide (NaCrO2) and fluoroethylene carbonate (FEC) were selected as model cathode and electrolyte additive systems, respectively. FEC improved the capacity retention of the NaCrO2 cathode from 66% to 85% after 50 cycles at C/20 rate. Operando digital image correlation (DIC), together with ex situ X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), high-resolution transmission electron microscopy (HR-TEM), and X-ray diffraction (XRD) techniques, was employed to investigate the mechanical, chemical, and structural evolution of the NaCrO2 cathode. The cathode exhibited nearly identical chemomechanical deformations and phase evolution regardless of FEC except during the first charge, where a distinct deformation response indicated the interfacial reconstruction associated with cathode-electrolyte interphase formation. XPS, FTIR, and HR-TEM measurements demonstrated the formation of a thinner, more uniform NaF-rich cathode-electrolyte interphase with improved preservation of surface Cr3+ species in the presence of FEC. Additional XPS analysis of the Na-metal anode revealed NaF-rich solid-electrolyte interphase formation in the presence of the FEC additive. The enhanced cycling stability of the NaCrO2 cathode against the Na-metal anode is primarily attributed to the cohesive impact of the improvement in chemical stability of the electrodes' interphases rather than bulk chemomechanical deformations of the cathode alone at a slower rate. The operando mechanical measurements revealed a unique mechanical signature associated with interfacial reconstruction in the presence of FEC, providing a direct mechanistic link between interphase formation and electrochemical performance.