A unique approach for improving the thermal stability of LiCoO2 cathode is presented, which is based on the treatment of LiCoO2 with ionic liquid (BDMIPF6, butyldimethylimidazolium-hexafluorophosphate), instead of traditional electrolyte modification. The electrolyte modification by the BDMIPF6 addition deteriorates the C-rate performances especially at high charge/discharge rates, with slightly improving the exothermic reaction between delithiated (charged) LiCoO2 and electrolytes. On the other hand, the BDMIPF6-modified cathode noticeably improves the thermal stability as well as little changes the C-rate performances. This is believed mainly due to the preferential location of BDMIPF6 in the LiCoO2 cathode rather than in the carbon anode, which might make it possible that the BDMIPF6 could effectively contribute to the suppression of exothermic reaction of delithiated LiCoO2 with electrolytes, with little affecting the electrolyte and the carbon anode.