Probing Electrochemical Strain Generation in Sodium Chromium Oxide (NaCrO2) Cathode in Na-ion Batteries during Charge/Discharge

Sodium chromium oxide, NaCrO2, exhibits promising features as a cathode electrode in Na-ion batteries, yet it encounters challenges with its capacity fading and poor cycle life. NaCrO2 undergoes multiple phase transitions during Na ion intercalation, eventually leading to chemical instabilities and mechanical deformations. This study aims to investigate the reversible and irreversible mechanical deformations in NaCrO2 using an operando digital image correlation (DIC) coupled with electrochemical techniques. Electrochemical strains were recorded during either cyclic voltammetry or galvanostatic cycling. The electrode undergoes significant irreversible mechanical deformations in the initial cycle, and irreversibility decreases in the subsequent cycles. During desodiation and sodiation, the electrode initially undergoes volume contraction followed by expansion. The strain derivative peaks align well with the peak of the capacitive derivatives, indicating the phase-transition-induced deformations in the electrode. Cumulative irreversible strains demonstrate a linear relationship with the square root of cycling time, indicating irreversibility arising from forming a cathode-electrolyte interphase layer. The outcome of this study provides valuable insights into mechanical deformations in NaCrO2 electrodes during battery cycling and guides the design of mechanically robust cathodes for Na-ion batteries.