Ultrathin and uniform carbon-layer-coated hierarchically porous LiFePO4 microspheres and their electrochemical performance

• Surface-modified LiFePO4 particles form in supercritical ethanol.
• Subsequent calcination forms uniform and complete coverage of ultrathin carbon layer.
• Hierarchically porous LiFePO4 resulted with carbon layer of 1.5–2.0 nm. Carbon-coated LiFePO4 Exhibits 84 mAh g−1 at the fast rate of 20 C.
• The carbon layer coating enhanced charge transfer kinetics and Li ion diffusivity.

Uniform and complete coverage of ultrathin carbon layer coatings on the surface of nano-sized LiFePO4 (LFP) particles prepared using a supercritical alcohol route was investigated. In the presence of oleylamine, nanosized oleylamine-modified LFP particles were synthesized in supercritical ethanol because particle growth was inhibited. When the oleylamine-modified LFP was calcined, uniform coverage of carbon layers with thicknesses of 1.5–2.0 nm on individual LFP particles was achieved. The carbon-coated LFP (C-LFP) exhibited hierarchical porous nano/micro sphere morphology with a porosity of 45.6%. In comparison with bare LFP synthesized in the absence of oleylamine, C-LFP exhibited excellent Li ion uptake performance; C-LFP delivered a reversible discharge capacity of 158.4 mAh g−1 after 100 cycles at 0.1 C and a rate performance of 84.0 mAh g−1 at 20 C. In addition, after 700 continuous cycles at 10 C, the C-LiFePO4 electrode exhibited a reversible discharge capacity of 84.0 mAh g−1 with small capacity loss. Electrochemical impedance tests revealed that the uniform and ultrathin carbon coating improved charge transfer kinetics and lithium diffusion into the active material.

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