An effective approach to reaching the theoretical capacity of a low-cost and environmentally friendly Na4Fe3(PO4)2(P2O7) cathode for Na-ion batteries

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 Feoxalate 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.