Preparation And Properties Of Secondary Carbon-coated Lithium Iron Phosphate Cathode Materials

The secondary carbon-coated Li Fe PO₄@C composite was synthesized by a facile two-step method.First,Li₂CO₃,Fe C₂O₄·2H₂O and NH₄H₂PO₄were dissolved in the deionized water as the starting materials and polyethylene glycol(PEG)as the grain growth inhibitor to prepared the Li Fe PO₄by calcination.Then,the secondary carbon-coated Li Fe PO₄@C composite was prepared from the Li Fe PO₄.Sucrose,PVP and sodium carboxymethyl cellulose were respectively used as a main carbon source to form the secondary carbon coating on Li Fe PO₄.The obtained samples were characterized by thermogravimetry(TG),X-ray diffraction(XRD),Scanning Electronic Microscopy(SEM)and Transmission Electronic Microscopy(TEM)and Raman analysis.Li-ion capacitors(LICs)using environmentally friendly,safe and low-cost Li NO₃aqueous electrolyte were assembled with the LFP sample as the positive electrode and active carbon(AC)as the negative electrode.Results show that:(1)The Li Fe PO₄/C composite was prepared successfully by using PEG as a grain growth inhibitor.It is due to the fact that PEG has the impact on the formation of final products and it can reduce the particle size.CV results show that the sample prepared at 550°C with 1.0 g PEG exhibits the highest C_peof 725 F/g at the scanning rate of 5 m V/s.The LIC exhibits good electrochemical performances.When the current density is 2 m A/cm²,the LIC shows the C_psof 49.6 F/g;(2)The secondary carbon-coated LFP@C composite was synthesized by using sucrose as a carbon source.CV results show that the sample prepared at 550°C with 1.0 g sucrose exhibits the highest C_peof 845 F/g at the scanning rate of 5 m V/s.The LIC exhibits good electrochemical performances.When the current density is 2 m A/cm²,the LIC show the C_psof 59.3 F/g.(3)the secondary carbon-coated LFP@C composite synthesized by using sucrose as carbon source exhibits the best electrochemical performances.The composite owns the improved electrochemical performance compared with LFP due to its increased electronic conductivity through the carbon coating.(4)The LIC based on Li NO₃aqueous electrolyte exhibits the best electrochemical performances and the E_pincreases with the increase of potential,and the highest cell voltage can be achieved at1.6 V.