Degradation Mechanism of Monocrystalline Ni-Rich Li[Ni _x Mn _y Co _z ]O ₂ (NMC) Active Material in Lithium Ion Batteries

Lithium ion batteries are the enabler for electric vehicles and, hereby, a sustainable and green mobility in the future. However, there are high requirements regarding electric vehicles which can be translated into great demands of life time and sustainibility on cell level. Ni-rich Li[Ni _x Mn _y Co _z ]O ₂ (NMC), where x ≥ 0.6, became the state of the art electrode material for the positive electrode to meet energy and power demands. However, further optimization is required to increase the life time and safety of those materials. An approach is the change from polycrystalline NMC to single crystals to increase the intrinsic stability by suppressing degradation phenomena like particle cracking. In this work, we show that particle cracking is still an issue for monocrystalline Ni-rich NMC811 under moderate abusive conditions. Intragranular cracking, i.e. cracking within the primary particle, was revealed as a result of structural degradation of the NMC structure accompanied with oxygen release and cross-talks which affected the SEI and, ultimately, accelerated the ageing of the single crystal NMC811 containing cell compared to its polycrystalline counterpart.