Rate-dependent electrochemical strain generation in composite iron phosphate cathodes in Li-ion batteries

The performance of battery electrodes is significantly impacted by chemo-mechanical instabilities at faster charge/discharge rates. This study reports rate-dependent mechanical deformations in the LiFePO4 cathodes during battery cycling by synchronizing in situ digital image correlation and electrochemical techniques. The electrode undergoes larger mechanical deformations in the early cycles and irreversible strains become negligible in the subsequent cycles. Cumulative irreversible strains show a linear relationship with the square root of cycling time, and the slope of the cumulative irreversible strains is greater at faster rates. The study compares the irreversible strains in LiFePO4 for Li-ion batteries with its analogous NaFePO4 cathodes for Na-ion batteries. Rate-dependent mechanical deformations are reported as the LiFePO4 electrode undergoes larger strains per capacity at faster rates. Pulsed current charge/discharge experiments coupled with strain measurements suggest a delay in the phase transformations at faster rates. The study provides new insights into rate-dependent chemo-mechanical deformations in the LiFePO4 electrodes.