Understanding the Stability of NMC811 in Lithium-Ion Batteries with Water-in-Salt Electrolytes

The high practical capacity and high average de-/lithation potential of LiNi0.8Mn0.1Co0.1O2 (NMC811) renders it one of the most prominent cathode materials for lithium-ion batteries. Here, we investigate the compatibility of NMC811 with non-flammable water-in-salt electrolytes. These highly concentrated aqueous solutions possess a much extended electrochemical stability window compared to common dilute aqueous electrolytes and can comfortably accommodate 4 V-class cathodes. We find that common degradation phenomena observed when cycling NMC811 in organic electrolytes such as surface phase transformation, transition metal dissolution, and particle cracking, also occur in water-in-salt electrolytes, but the enhanced salt concentration of a water-in-salt/ionic-liquid hybrid electrolyte effectively diminishes these effects. Furthermore, we find that self-discharge reactions of NMC811 at a high state of charge with aqueous electrolytes lead to NMC811 protonation and irreversible capacity losses. Protonation represents an additional challenge that needs to be overcome when combining NMC811 with non-flammable aqueous electrolytes.