An effective approach to reaching the theoretical capacity of a low-cost and environmentally friendly Na<inf>4</inf>Fe<inf>3</inf>(PO<inf>4</inf>)<inf>2</inf>(P<inf>2</inf>O<inf>7</inf>) cathode for Na-ion batteries

Abstract

M4Fe3(PO4)2(P2O7) specific family has appeared as a new class of polyanionic compounds for sodium-ion batteries, capable of offering a higher operating voltage than individual phosphates and pyrophosphates. The study addresses the issue of Na4Fe3(PO4)2P2O7 (NFPP) sol-gel synthesis when both phosphates and pyrophosphates act as reactants, leading to successful production of NFPP under controlled synthesis conditions, capable of reaching the theoretical capacity. Spontaneous citric-assisted sol-gel reaction, between PO43− and P2O72− units occurring at pH of 3 (which follows NFPP stoichiometry), leads to the formation of pyrophosphate (Na2FeP2O7, NFP) with a certain amount of the mixed phase. Fe-oxalate coordination is dominant at low pH while the citric acid protonation suppresses direct Fe-citric complexation. pH adjustment to a neutral value changes the complexation and reaction pathway, allowing direct Fe(II)-citric coordination and subsequent oxidation. The exchange of Fe-oxalate with the soluble ferric ammonium citrate complex happens under neutral pH and therefore leads to the formation of NFPP as the dominant phase, liberated from NFP. Furthermore, a series of samples, developed by varying citric-to-Fe molar ratio and controlling pH, served as a platform to identify and solve problems regarding the unambiguous FTIR assignment of the polyanionic NFP/NFPP mixture. FTIR and CV methods are proposed as assisting tools for XRD to identify NFP admixture. Finally, and most importantly, NFPP phase formed under neutral pH has a higher sodiation/desodiation capacity than NFPP/NFP heterostructure, reaching a theoretical value at a rather high current of 1 A g−1, which has not been attained in the literature.