(592e) The Role of Intercalation Rate on Mechanical Behavior of Sodium Iron Phosphate Cathode for Na-Ion Batteries

The ever-growing energy demand associated with the increasing human population, environmental concerns and technological developments puts pressure on society to harvest electricity from renewable sources. Due to their intermittent nature, the adoption of renewable energy depends on further developments in energy storage technology. Although Li-ion batteries dominate the current energy-storage landscape, their application in large-scale energy storage is constrained by continuously growing lithium prices as well as limited Li resources. Sodium-ion batteries have attracted attention in the search for cost-effective batteries with a minimum sacrifice on the performance. Already-existing LIB facilities can be adopted for manufacturing NIBs without requiring expensive upgrades. Similar to Li-ion batteries, performance of sodium-ion batteries also depends on mechanical integrity of the electrodes and interfacial stability associated with solid-electrolyte interface formation. However, diffusion of Na ions is slower in the electrode matrix due to their larger ionic size, compared to the Li ions. Therefore, it is expected that Na-ion electrodes experience more mechanical instabilities at faster scan rates due to diffusion-limited ion transport. In this study, we experimentally characterize the electrochemical strain generations in composite sodium iron phosphate (NFP) and developed an analytical model to predict Na concentration gradients and misfit strain generation in the NFP particles.

Digital image correlation technique was used to measure in-situ strain evolution in the composite sodium iron phosphate cathodes. The composite cathode consists of NFP active materials with CMC binder and conductive carbon in 8:1:1 mass ratio, respectively. The electrodes were galvanostatic cycled at 1C, C/10 and C/25 rates between 2.0-4.0 V. Electrochemical strains linearly increased with the discharge capacity at all scan rates. When the electrode was cycled at C/25 rate, the electrode expanded by 0.85% at the end of the discharge and capacity was 130 mAh/g. The electrodes experience additional 0.05 and 0.15% more composite strains at C/10 and 1C rates compared to the electrode cycled at C/25 rate when the state of discharge was same in all the electrodes. An analytical model was developed based on the coupling between Na transport and mechanics. At slower rates, the concentration gradient was homogeneously distributed through the electrode particle; however sharp concentration gradients occurred near electrode surface in the electrode at faster scan rates. As a result, larger misfit strain generations were calculated for faster scan rates. In conclusion, experimentally measured composite strains and predicted particle strains demonstrates the similar behavior of rate dependent strain evolution in the NFP electrodes.