GITT Analysis of Cation-Insertion Kinetics in Prussian Blue Analogues

Abstract

Prussian blue analogues (PBAs) can accommodate a wide variety of cations and are promising electrode materials for post-lithium-ion batteries. To clarify the factors affecting cation-insertion kinetics in PBA, the insertion reactions of monovalent alkali metal, divalent alkaline-earth metal, and trivalent metal cations were investigated by galvanostatic intermittent titration technique (GITT). The relaxation profiles were analyzed by separating the overvoltage into an instantaneous component, ηinst, mainly arising from charge-transfer overvoltage and resistance polarization, and a time-dependent component, ηtime, corresponding to concentration overvoltage associated with cation diffusion. The ηinst values were within a relatively narrow range of 15–30 mV for all cations. In contrast, ηtime showed a much stronger dependence on cation species and exhibited a close correlation with the rate capability reported previously, indicating that ηtime serves as a semi-quantitative descriptor of the cation-diffusion rate governing high-rate performance. Furthermore, ηtime was correlated with ionic radius. For monovalent alkali-metal cations, ηtime showed a minimum at an ionic radius of approximately 1.5–1.6 Å, whereas for divalent and trivalent cations it generally decreased with increasing ionic radius. Comparison with the estimated vacant-site size in the PBA lattice (1.67 Å) suggests that cation diffusion is facilitated when the ionic size approaches the lattice-void size. These findings show that cation-insertion kinetics in PBA are strongly influenced by ionic size and valence, providing a structural guideline for designing high-power insertion materials.