Morphology-controlled solvothermal synthesis of LiFePO4 as a cathode material for lithium-ion batteries

Abstract

LiFePO₄ (LFP) nanoparticles (∼50 nm in size), nanoplates (100 nm thick and 800 nm wide) and microplates (300 nm thick and 3 μm wide) have been selectively synthesized by a solvothermal method in a water–polyethylene glycol (PEG) binary solvent using H₃PO₄, LiOH•H₂O and FeSO₄•7H₂O as precursors. The morphology and size of the LFP particles were strongly dependent on synthetic parameters such as volume ratio of PEG to water, temperature, concentration, and feeding sequence. The carbon coated nanoparticles and nanoplates could deliver a discharge capacity of >155 mAh g⁻¹ at 0.1C rate (i.e. 17 mA g⁻¹ of current density); in comparison, the carbon coated microplates had a discharge capacity as low as 110 mAh g⁻¹ at 0.1C rate. The Li-ion diffusion coefficients of the carbon coated nanoparticles, nanoplates, and microplates were calculated to be 6.4 × 10⁻⁹, 4.2 × 10⁻⁹, and 2.2 × 10⁻⁹ cm² s⁻¹, respectively. When the content of conductive Super P carbon (SP) was increased to 30 wt.%, the prepared electrodes could charge–discharge at a rate as high as 20C. Over 1000 cycles at 20C, the nanoparticle electrode could maintain 89% of its initial capacity (126 mAh g⁻¹), the nanoplate electrode showed 79% capacity retention compared to an initial capacity (129 mAh g⁻¹), and the microplate electrode retained 80% of its initial capacity (63.5 mAh g⁻¹).