Towards flexible secondary lithium batteries: polypyrrole-LiFePO4 thin electrodes with polymer electrolytes

Abstract

A thin flexible polypyrrole-lithium iron phosphate (PPy-LiFePO₄) based cathode has been fabricated. A slurry containing carbon black, a binder and the active material prepared by direct polymerization of pyrrole on the surface of LiFePO₄ (LFP) was spread on an Al/carbon film substrate by the doctor blade method. Transmission electron micrographs reveal that PPy nanoparticles form a web like structure over the surface of LFP particles. After doping with lithium ions the PPy network becomes conducting. When evaluated as a cathode of 180 μm thickness together with a gel polymer electrolyte and a lithium anode, the charge–discharge performance reveals that the electrochemical properties of LFP are influenced to a considerable extent by the PPy. The cells show high initial discharge capacities of 135 and 110 mA h g⁻¹ for 0.041 (C/10) and 0.21 (C/2) mA cm⁻², respectively, and high active material utilization. Furthermore the cells exhibit stable cycle properties even at 0.21 mA cm⁻² with a low capacity fade per cycle (∼0.3%).