This study highlights the effects of a biomass-derived carbon coating on the properties of flexible lithium iron phosphate polymer batteries. Pure LiFePO4 (LFP) and carbon-coated LiFePO4 (C-LFP) cathode materials are synthesized by a modified mechanical activation process. LiFePO4 was uniformly coated with biomass-derived carbon by the addition of orange peel during the synthesis. C-LFP with a particle size of approximately 90 nm, having a homogeneous particle size distribution with particles smaller than that of pure LFP (140 nm), is obtained with 6.0 wt% carbon content. The electrochemical properties are evaluated at room temperature by cyclic voltammetry and charge-discharge performance analysis using porous carbon current collector with a lithium metal anode and gel polymer electrolyte that is prepared by the phase inversion method. C-LFP exhibits a high discharge capacity of 147.3 mAh g−1 (corresponding to 87% of the theoretical capacity) at 0.5 C-rate and 139.8 mAh g−1 at 1 C-rate. Good rate capability and stable cycle performance were realized for flexible lithium polymer cells employing the biomass derived-carbon coated LiFePO4.