LOW VACANCY Fe-SUBSTITUTED Mn-BASED PRUSSIAN BLUE ANALOGUE

A system and method implementing and manufacturing transition metal cyanide coordination compounds (TMCCC) comprising Na, Fe, Mn, C, H, N, S, and O, wherein the TMCCC have 0-14% hexacyanometallate vacancies such as for application in electrochemical cells, including sodium ion secondary batteries; further including both sodium and potassium with and without sulfate.