  Challenges associated with in-service mechanical degradation of Li-ion
battery cathodes has prompted a transition from polycrystalline to single
crystal cathode materials. Whilst for single crystal materials,
dislocation-assisted crack formation is assumed to be the dominating failure
mechanism throughout battery life, there is little direct information about
their mechanical behaviour, and mechanistic understanding remains elusive.
Here, we demonstrated, using in situ micromechanical testing, direct
measurement of local mechanical properties within LiNi0.8Mn0.1Co0.1O2 single
crystalline domains. We elucidated the dislocation slip systems, their critical
stresses, and how slip facilitate cracking. We then compared single crystal and
polycrystal deformation behaviour. Our findings answer two fundamental
questions critical to understanding cathode degradation: What dislocation slip
systems operate in Ni-rich cathode materials? And how does slip cause fracture?
This knowledge unlocks our ability to develop tools for lifetime prediction and
failure risk assessment, as well as in designing novel cathode materials with
increased toughness in-service.
