In situ constructing a stable interface film on high-voltage LiCoO₂ cathode via a novel electrolyte additive

We propose a novel electrolyte additive, 5-acetylthiophene-2-carbonitrile (ATCN) with three functional groups (thiophene, nitrile and carbonyl), to in situ construct a stable cathode interface film that can significantly improve the cycling stability of LiCoO₂ cathode under high-voltage. Adding 0.2% of ATCN into a base electrolyte, the capacity retention of LiCoO₂/Li cell under 4.5 V is enhanced from 53% to 91% after 200 cycles at 1 C, and the cycle number of commercial LiCoO₂/graphite pouch cell (34 Ah) with 10% capacity loss at 0.5 C under a cut-off voltage of 4.45 V is increased from 550 to 800. Experimental characterizations and theoretical calculations reveal that ATCN is preferentially oxidized on LiCoO₂ cathode and utilizes its decomposition intermediates to convert the detrimental components, the hydrogen fluoride and water present in the electrolyte, and the lithium oxide and carbonate resulting from the electrolyte decomposition, into a unique film texture comprised of underneath compacted lithium salts and outer thiophene polymers. The as-constructed film significantly improves the cathode/electrolyte interface stability and the cycling stability of the cell. Such an effective strategy to address the interface instability has never been reported before and paves a new path to improve the energy density of commercial lithium-ion batteries via enhancing cut-off charge voltage.