Chemo-mechanics of iron phosphate cathode in alkali metal-ion batteries

Abstract

Utilization of renewable energy sources requires the use of grid-scale stationary energy storage that requires low-cost, safe, and nontoxic systems. For these applications, “beyond” lithium-ion battery chemistries, such as sodium and potassium-ion batteries, are possible alternatives due to their abundance and lower cost. However, their larger ionic radius and chemical reactivity can cause performance degradation in the long-term because of chemo-mechanical instabilities. The main goal of the work is to elucidate the relationship between chemo-mechanics of different alkali-metal ion intercalation and chemo-physical response of electrode materials. Investigation of this phenomena carried out by utilizing in-situ strain measurement coupled with in-situ XRD, HR-TEM, and mathematical model. First stage of the investigation focused on the effect of different alkali metals on the same host structure. Initial findings indicated that iron phosphate host structure experienced larger-than-expected expansion during first lithium and sodium intercalation, which became more reversible in subsequent cycles. During potassium intercalation, in-situ XRD and HR-TEM results showed the amorphization of iron phosphate structure. By employing DIC technique, reversible deformations in the amorphous phase was tracked during electrochemical redox reaction. Comparing the effect of these alkali metal on redox chemistry and mechanical deformation showed that strain rate, instead of absolute value of the strain, is critical factor in the amorphization of crystal structure. Second stage of the research focused on the effect of cycling rate on the mechanical deformation of electrode materials. In-situ strain measurements, coupled with GITT analysis and transport-mechanics model indicated that, lower diffusivity of sodium in the cathode results in the steep concentration gradient and misfit strain generation at faster scan rates. In the case of lithium intercalation, in situ strain measurements during pulsed current charge/discharge experiments indicated that at faster scan rates, phase transformation was delayed. In the last stage, DIC system was employed to investigate mechanical deformation of LAGP solid electrolyte for all-solid-state battery applications. During this study, increase strains at the metal/solid electrolyte interphase coincided with increase in the overpotential. This result experimentally showed the relationship between overpotential generation and strain evolution between metal/solid electrolyte interphase. These findings indicate that 1) strain rate is critical to the amorphization of crystal structure and 2) chemical reactivity of different alkali metals cause difference in mechanical response of electrode materials when batteries cycled at different scan rates. Understanding the similarities and differences between alkali metals on mechanical deformation will provide new insights into the selection of battery materials for beyond lithium-ion battery applications.