(644h) Interfacial Origins of Degradation in High Energy Li-Rich NMC Cathodes Probed By Online Electrochemical Mass Spectrometry

Layered lithium-rich Ni, Mn, Co (NMC) oxide cathodes, with theoretical capacities greater than 250 mAh/g, can provide a game-changing boost to the energy density of current Li-ion batteries. Their high capacities stem for the participation of the oxygen anions of the oxide in charge compensation during Li-extraction at voltages above 4.5 V vs. Li^+/0. As a result of this, however, they suffer from rapid capacity and voltage fade during battery cycling, concomitant with surface O₂ release, spinel formation, crack propagation, transition metal migration etc. in the electrode. We study the interfacial origins and effects of this degradation employing Online Electrochemical Mass Spectrometry (OEMS). We demonstrate effective strategies of electrode surface passivation as well as electrolyte modification, that independently cause a significant suppression of the first cycle CO₂ and O₂ evolution, leading to improved longer-term cycling performance.