Porous LiFePO4 particles have been investigated as cathode materials for lithium ion batteries by introducing supercritical carbon dioxide (scCO2). First we produced LiFePO4 through hydrothermal method. Then scCO2 was introduced to change the morphology of the particle. By controlling the experiment parameter, porous LiFePO4 was finally obtained. A solid-to-porous transformation mechanism was supposed based on the dissolution of LiFePO4. Electrochemical measurements showed that porous structure greatly improved the performance of LiFePO4.