Improvements on Capacity Retention and Metal Dissolution of NCM Cathode Material By the Use of Alternative Conducting Salts

Ni_1/3Co_1/3Mn_1/3O₂ (NCM) cathode material can be charged to different potentials. When using the standard salt LiPF₆, elevated cutoff potentials lead to increased energy densities and better energy quality, but reduce the cycle life dramatically. Metal dissolution is catalyzed by acids, like HF, derived from deterioration of the salt. To exclude the influence of HF, next to LiPF₆ the less hydrolysis sensitive LiBF₄ and the fluorine-free conducting salt LiClO₄ were investigated. To determine the metal dissolution behavior of NCM, these electrodes were stored in 1 M EC/DMC (1/1, by wt.) electrolyte containing different salts for 28 days at different states of charge. The measurement of metal ion content was carried out by inductively coupled plasma/optical emission spectrometry (ICP-OES) technique. Constant current cycling experiments were performed with different charge potential limits. In contrast to LiPF₆, LiBF₄ showed suppressed metal dissolution and superior cycling performance with low capacity fading even at high cutoff-potentials (4.6 V vs. Li/Li⁺). At low potentials, metal dissolution, as well as capacity fading, is rather low for LiClO₄ but escalates at potentials above 4.4 V vs. Li/Li⁺.