We investigated an N-heterocyclic-carbene-PF5 adduct (NHC-PF5) as additive for overcharge protection of a lithium nickel cobalt oxide (NCM) cathode by linear potential sweep methods and constant current experiments. The NHC-PF5 additive was oxidized at a potential of 4.57 V vs. Li/Li+ thus protecting the NCM cathode from overcharge. No detrimental influence of the additive on the cycling behavior of NCM/graphite full cells was observed. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.006210jes] All rights reserved.