Mechanistic study of the electrochemical extraction of K+ from KFeSO4F

Abstract

KFeSO₄F was recently reported as a cathode candidate for the removal and insertion of Li⁺, Na⁺ and K⁺. Here the mechanism for the electrochemical extraction of K⁺ from KFeSO₄F is unveiled using density functional calculations. Consistent with experimental observations, first-principles calculations predict a thermodynamically favourable phase transition when the potassium content in K_xFeSO₄F is below 50% (x ≤0.5). The mechanism that drives this phase transition is carefully investigated. It is shown that the extraction of K⁺ can be expressed as a sequential removal of K⁺ and selective oxidation of Fe²⁺ ions. Before the composition reaches K_0.5FeSO₄F the selective oxidation only produces neighbouring Fe²⁺–Fe³⁺ pairs. The crystal structure is stable in this stage. Further removal of K⁺ after the composition reaches K_0.5FeSO₄F generates Fe³⁺–Fe³⁺ pairs. It triggers the phase transition in order to minimize the strong electrostatic repulsion between highly charged Fe³⁺ ions. The voltage profile for the removal of K⁺ from KFeSO₄F is also affected by this special mechanism. These results not only provide a mechanistic explanation of the experimental observations, but also suggest the cation repulsion is a key factor that affects the structural stability and the voltage profile of the cathode material.