In order to investigate the effects of electrode density and thickness on the room temperature electrochemical performance of lithium metal polymer batteries (LMPBs), four LiFePO₄ cathodes with different electrode density/thickness (1.6 g cm⁻³/20 μm, 1.6 g cm⁻³/40 μm, 2.0 g cm⁻³/20 μm, and 2.0 g cm⁻³/40 μm) are prepared. Several types of unit cells employing each cathode are prepared using in situ thermally cross-linked polymer electrolytes controlled under the same manufacturing condition. The unit cells employing the thinnest cathode with highest density achieve the most improved rate capability and cycle life, which seems to be attributed to the higher electrical conductivity of the cathode and shorter diffusion length of Li⁺ ions within the cathode pores. In addition, the LMPB with an optimized LiFePO₄ cathode (2.0 g cm⁻³, 20 μm) and polymer electrolyte (5 vol% of the crosslinking agent with non-volatile liquid electrolytes) delivers a high and stable charge/discharge capacity under very fast charging and discharging conditions (at a 2 C rate) at room temperature.