Tunable polymorph and morphology synthesis of iron oxalate nanoparticles as anode materials for lithium ion batteries

Abstract

Iron oxalate (FeC2O4), a promising anode material for lithium ion batteries (LIBs), is typically crystallized in orthorhombic-type phase. When the synthesis temperature rises, β-FeC2O4·H2O can transform into high-crystalline α-phase. At present, the detail evolution mechanism of crystal structure for two kinds of iron oxalate polymorphs is seldom studied. Through in-depth experiments, the tunable polymorph and morphology of iron oxalate can be obtained by controlling certain conditions. The oxalic acid complex ([Fe(C2O4) x]−2(x−1)), as unstable intermediate state, can change the reaction thermodynamics and kinetics, thus promoting the nucleation and growth of crystals. Higher temperature and longer reaction time would availably reinstitute stable hydrogen bonds and enhance the disordered state between successive stack layers, then resulting in various morphologies and phase-transition process. Due to short diffusion path and stable effective channels for Li+ ions, the mixed state of iron oxalate exhibits higher reversible capacity and more excellent cycling stability. Understanding the phase evolution mechanism of iron oxalate is critical to controllable synthesis, and ultimately, enhancing the performance of future LIBs.