On the enhanced electrochemical properties of dihydrate hydroxyl ferric phosphate: effects of intrinsic crystal water and additive graphene oxide

Abstract Crystal water of lithium-ion battery electrode active substances is electrochemically inert in reversible charge-discharge processes and may exert a positive effect on charge transfer, but these are still controversial in view of the absolutely adverse effects of adsorbed water/moisture. In this paper, hydrated, partial dehydration and anhydrous hydroxyl ferric phosphate Fe5(PO4)4(OH)3•nH2O (n = 2, 1.3 and 0) and their graphene oxide (GO) doped composites of Fe5(PO4)4(OH)3•nH2O/GO are separately prepared and applied as low-voltage lithium-ion battery (LIB) cathodes, aiming to prove a kinetic improvement mechanism of crystal water for the first time. Within 1.5-4.5 V vs. Li+/Li at 50 mA g−1, pristine Fe5(PO4)4(OH)3•nH2O and corresponding GO-doped composites deliver a reversible capacity of 128.7 (pristine n=2), 63.5 (pristine n=1.3), 46.5 (pristine n=0), 155.1 (composite n=2), 117.5 (composite n=1.3) and 98.0 mAh g−1 (composite n=0) in the 120th cycle, respectively. According to the theoretical capacity of Fe5(PO4)4(OH)3•2H2O (180 mAh g−1), it is the synergistic effect of intrinsic crystal water and additive GO that leads to the high performances of composite Fe5(PO4)4(OH)3•2H2O/GO (e.g., a reversible capacity ∼140.6 mAh g−1; 180 mA g−1; the 250th cycle) for potential application purposes.