Hydrothermally prepared 100 nm-sized LiCoO2 with a plate morphology packed in a crucible resulted in 40 mu m-sized particles that consisted of aggregated <1 micron-sized particles after annealing at 900 degrees C for 3 h. In the condition where the optimized electrode pore volume was 20%, 4.1 g/cc of electrode density was obtained, which corresponded to 3 Wh/cc, which is the highest value among the cathode materials. Furthermore, the LiCoO2 showed excellent capacity retention of 78% after 290 cycles in a Li-ion cell under 7 C rate cycling.