Thermally activated prussian blue analogues low-spin iron for enhanced sodium ion storage performance

Abstract

Iron-based Prussian Blue Analogues (PBAs) are widely utilized in sodium-ion battery cathodes owing to their low cost and high theoretical capacity (170 mAh g⁻¹). However, the electrochemical inactivity of low-spin iron (LS-Fe) significantly limits the operating voltage and capacity, thereby hindering the energy density of the battery. This study introduces a method for thermally activating the LS-Fe capacity of Prussian Blue Analogues under a nitrogen atmosphere. The results show that the LS-Fe capacity increases progressively with temperature. However, excessively high temperatures can lead to decomposition and oxide formation, which subsequently reduces the capacity. The sample thermally activated at 200 °C (PB-200) demonstrates the highest specific capacity of 119.86 mAh g⁻¹ at 0.2C and optimal rate performance. Moreover, after 500 cycles at 5C, PB-200 retains 84.6 % of its initial capacity. In-situ Electrochemical Impedance Spectroscopy (EIS) and Galvanostatic Intermittent Titration Technique (GITT) measurements reveal a significant improvement in the diffusion process following heat treatment, attributed to phase changes during heating, resulting in lattice misalignment and crack formation. The full cell exhibits a stable capacity of 84.81 mAh g⁻¹ at 0.2C. After 100 cycles at 1C, its capacity remains at 70.7 %.