Facile synthesis and electrochemical performances of secondary carbon-coated LiFePO₄-C composite for Li-ion capacitors based on neutral aqueous electrolytes

In order to obtain LiFePO₄ (LFP) with the fine particle size and high electronic conductivity, the secondary carbon-coated LFP-C composite was synthesized by a facile two-step method with polyethylene glycol (PEG) as a grain growth inhibitor and sucrose as a main carbon source. For comparison, commercial LFP and LFP prepared without PEG and sucrose were also studied. Li-ion capacitors (LICs) using environmentally friendly, safe and low-cost LiNO₃ aqueous electrolyte were assembled with LFP samples as the positive electrode. Results show that the secondary carbon-coated LiFePO₄-C composite with nanometer-sized particles was synthesized at 550 °C successfully and the carbon content was about 23.4 wt%. The lamellar carbon coating, with much graphitic nature, wraps and connects LFP particles, ensuring the good electronic connection between LFP particles. The results of cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance show that the electrochemical performances of secondary carbon-coated LFP dramatically increase due to the decrease of particle size through adding PEG and the increased electronic conductivity by the introduction of carbon coating. It exhibits the C _pe of 845.2 F g⁻¹ at the scanning rate of 5 mV s⁻¹. The LIC delivers a C _ps of 59.3 F g⁻¹ and E _p of 8.2 Wh kg⁻¹ at the current density of 2 mA cm⁻². The LIC exhibits an excellent cycling performance and it maintains 98 % of its initial C _ps after 500 cycles. The secondary carbon-coated LiFePO₄-C composite is the suitable positive electrode material for LICs with neutral aqueous electrolytes.