Diffusion Kinetics in Multi-Phase K-Ion Cathodes

Abstract

Prussian blue analogues (PBAs) are a framework material that are of great interest for use as cathode materials in Na- and K-ion batteries. Na-ion PBA batteries have been commercialized for use in stationary storage and are now in development for use in electric vehicles. The high operating potential of K-ion PBAs and their compatibility with pre-existing graphite anodes offer the exciting prospect of use in electric vehicles where high specific energy density is necessary [1].The leading cathode material is the Manganese/Iron PBA – K x Mn[Fe(CN)6] y (0<x<2, 0.67<y<1.00). Attempts to maximise the specific capacity of PBA cathodes have been made by eliminating [Fe(CN)6] vacancies (y=1) which incorporates a full x = 2 formula units of K+ ions in the structure through electronic balancing [2]. Whilst making these low-vacancy materials has been made possible by altering the synthesis, optimising the electrochemical performance has proved more challenging [3]. Accessing the full capacity with high retention on cycling is one problem, and achieving the same excellent rate capability of the previously studied high-vacancy PBAs is also non-trivial [3,4] .High-vacancy PBAs have an apparent simple cubic structure, and cycle electrochemically in a solid-solution [4]. Conversely, the low-vacancy PBAs with high K-ion content have coherent structural distortions and undergo phase transitions on cycling [3,5]. The phase transitions are believed to be responsible for poor capacity retention and rate capability [2]. There is also the theory that vacancies provide additional diffusion pathways for K+ ions [Fig. 1] [2]. For the development of this material as a cathode for K-ion batteries to be successful, it is vital that the diffusion kinetics and structural changes on electrochemical cycling are well-understood from a fundamental perspective.For the first time we report diffusion coefficients by using Potentiostatic Intermittent Titration across multiple states of charge of a PBA cathode [6]. Linking the ionic diffusion in the material with the structural phase(s) present from operando X-ray diffraction gives us a mechanistic insight into the cathode’s charge/discharge behaviour. This informs materials design for tuning the structure of the pristine material to optimise its electrochemical performance in a K-ion battery.