Surface treatment of LiFePO4 cathode material with PPy/PEG conductive layer

In this work, we studied LiFePO4 particles coated with thin films of highly conductive polypyrrole (PPy) and their electrochemical performance in cathode layers of lithium cells. Carbon-free LiFePO4 particles were synthesized by a solvothermal method. Besides this, a part of the experiments were carried out on commercial carbon-coated LiFePO4 for comparison. Polypyrrole coated LiFePO4 particles (PPy-LiFePO4) were obtained by a straightforward oxidative polymerization of dissolved pyrrole on LiFePO4 particles dispersed in water. The use of polyethylene glycol (PEG) as an additive during the polymerization was decisive to achieve high electronic conductivities in the final cathode layers. The carbon-free and carbon-coated LiFePO4 particles were prepared with PPy and with PPy/PEG coating. The obtained PPy-LiFePO4 and PPy/PEG-LiFePO4 powders were characterized by SEM, EIS, cyclic voltammetry, and galvanostatic charge/discharge measurements in lithium-ion cells with lithium metal as counter and reference electrode. Carbon-free LiFePO4 coated with PPy/PEG hybrid films exhibited very good electrode kinetics and a stable discharge capacity of 156 mAh/g at a rate of C/10. Impedance measurements showed that the PPy/PEG coating decreases the charge-transfer resistance of the corresponding LiFePO4 cathode material very effectively, which was attributed to a favorable mixed ionic and electronic conductivity of the PPy/PEG coatings.