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

•Fe5(PO4)4(OH)3•nH2O (n = 2, 1.3 or 0) spherical aggregates are separately obtained.•Two-dimensional H2O channels of lattice Fe5(PO4)4(OH)3•2H2O are stimulated.•Crystal water kinetically contributes to lithiation of hydroxyl ferric phosphates.•Crystal water content determines the performance of Fe5(PO4)4(OH)3•nH2O cathodes.•A synergistic effect (1 + 1 > 2) between intrinsic H2O and additive GO is proposed. 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. [Display omitted]