The poor contact between the cathode and the solid polymer electrolyte (SPE) is one of the primary causes for the low specific capacities of lithium ion batteries. In this work, a composite cathode material containing LiFePO4 nanoparticles encapsulated by PEG-based polymer electrolyte (coated LiFePO4) are prepared in order to increase the interfacial contact area between SPE and cathode materials. The LiFePO4/SPE/Li half-cell battery with coated LiFePO4 (C-LFP) and uncoated LiFePO4 (U-LFP) are assembled and their electrochemical performances are studied. The optimal LiFePO4/SPE weight ratio in the composites cathode is 60/15. When the cathode thickness is 100 pm, the specific capacity of battery with C-LFP can reach up to 138 mAhg(-1), which is 2.5 times higher than that with U-LFP under 1C charge-and-discharge rate at 60 degrees C. Moreover, the capacity retention of C-LFP is 91% after 300 cycles, while that of U-LFP is only 18%. Large cracks emerge in the cathode with U-LFP, while no damage can be found in that with C-LFP after 300 cycles at 1C. This should be ascribed to the existence of polymer buffer layers among LiFePO4 nanoparticles to alleviate the stress induced by their volume changes during the charge-and-discharge cycling process. This enhancement effect increases along with the increase of cathode thickness. When the cathode thickness increases up to 200 mu m, the specific capacity of battery with C-LFP cathode is almost 4 times higher than that with U-LFP.