Filling Vacancies in a Prussian Blue Analogue Using Mechanochemical Post-Synthetic Modification

Abstract

Mechanochemical grinding of polycrystalline powders of the Prussian blue analogue (PBA) Mn[Co(CN) 6 ] 2/3 1/3 · xH 2 O and K 3 Co(CN) 6 consumes the latter and chemically modifies the former. A combination of inductively-coupled plasma and X-ray powder diffraction measurements suggests the hexacyanometallate vacancy fraction in this modified PBA is reduced by approximately one third under the specific conditions we explore. We infer the mechanochemically-driven incorporation of [Co(CN) 6 ] 3− ions onto the initially-vacant sites, coupled with intercalation of charge-balancing K + ions within the PBA framework cavities. Our results offer a new methodology for the synthesis of low vacancy PBAs, unlocking novel, high capacity PBA battery materials.Prussian blue analogues (PBAs) are an important and topical family of battery materials with a number of considerable advantages over longer-established charge storage systems. 1-3 On the one hand, they are chemically versatile, inexpensive, and easily made by aqueous precipitation from widely-available precursors. And, on the other hand, PBA electrochemistry is characterised by a combination of high specific capacity for Na + /K + insertion, rapid charge/discharge rates, and long cycle lives. 4 One key limitation of PBA chemistry as battery materials is the remarkably strong tendency for vacancy incorporation within the PBA structure. 5,6 Whilst hexacyanometallate vacancies help improve structural stability and increase bulk ionic conductivity, their presence reduces specific capacity. 4 Increasing energy density therefore requires synthetic routes that minimise vacancies. 7