Enhancing electrochemical lithium-storage properties of hydrated iron oxalate (FeC₂O₄.2H₂O) anode material by combining with dual-states copper

The application of iron oxalate (FeC2O4) as a new high-energy anode material for lithium-ion batteries is astricted by difficulty in obtaining 100 % anhydrous material and inhibitive role of crystal water on lithium storage. Herein, we fabricated the trepang-liked hydrated iron oxalate (FeC2O4.2 H2O) anode materials by combining with dual-states copper and investigated the special function of copper. Because of the differentiated distribution between C2O42- and oxalic acid complexes ([Fe(C2O4)x]-2(x-1) and [Cu(C2O4)x]-2(x-1)), the CuC2O4.xH2O nanosheets with compound state adsorb and imbed on surface of rod-like FeC2O4.2 H2O particles. Meanwhile, copper element with ion state can effectively doped into the crystal structure of FeC2O4.2 H2O, due to the similar three-dimensional crystal structure between transition metal oxalates. In virtue of the differentiated electrochemical behavior of crystal water for iron oxalate and copper oxalate, the copper derivatives (Cu, CuO, Cu2O et al.), formed by the reaction between CuC2O4.xH2O and Li+, exhibit high electrochemical activity and electrical conductivity, which can significantly enhance the lithium storage ability of dihydrate iron oxalate. Hence, hydrated iron oxalate combined with dual-states copper, suggests superior reversible capacity (-550 mAh g-1 at 0.1 A g-1) and stable long-term cycling performance (-520 mAh g-1 at 0.5 A g-1 after 200 cycles). This paper provides a novel opportunity to weaken the inhibitive role of crystal water on lithium storage and enhance the electrochemical properties of hydrous oxalate materials.