Enhanced Overdischarge Stability of LiCoO₂by a Solution Deposited AlPO₄Coating

Abstract

Enhancing the stability of lithium ion cathode active materials to over-insertion of lithium can improve overdischarge tolerance in cells that have excess reversible lithium compared to the stoichiometric cathode capacity. In the present work, a solution deposited coating of AlPO4 on LiCoO2, known to stabilize overcharge, is applied to test its effect on the overdischarge (or over-insertion) tolerance of LiCoO2. Cathode testing versus lithium metal is performed with constant current charge to 140 mAh/gLiCoO2, constant current discharge to 3.0 V vs. Li/Li+ and 7 mAh/gLiCoO2 lithium over-insertion at fixed resistive load. Results show that the AlPO4 coating maintains >99% of the discharge energy (compared to 96% for the as-received LiCoO2) after 10 cycles. X-ray diffraction analysis of the relative intensity of {003} peak of the space group indicates that after repeated lithium over-insertion, the AlPO4 coating suppresses irreversible cation exchange between Li and Co ions in the octahedral layers of LiCoO2. Thus, AlPO4 surface coatings can prevent crystal structure changes detrimental to charge/discharge performance in LiCoO2 caused by over-insertion of lithium ions.