Probing electrochemical strain generation in sodium chromium oxide (NaCrO2) cathode in Na-ion batteries during charge/discharge

Abstract

Sodium chromium oxide, NaCrO₂, exhibits promising features as a cathode 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. Here, we employed the digital image correlation (DIC) technique to probe electrochemical strain generation in the cathode during cycling via cyclic voltammetry and 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 at a lower state-of-(dis)charge followed by expansions at a higher state-of-(dis)charge. The similar progression between strain and capacitive derivatives points out the phase-transition-induced deformations in the electrode. The evolution of cumulative irreversible strains with cycling time indicates the irreversibility rising from the formation of cathode-electrolyte interphase layers. The study demonstrates valuable insights into mechanical deformations in NaCrO₂ electrodes during battery cycling, which is critical to engineer mechanically robust cathodes for Na-ion batteries.