Origins and importance of intragranular cracking in layered lithium transition metal oxide cathodes

Li-ion batteries have a pivotal role in the transition towards electric transportation. Ni-rich layered transition metal oxide (LTMO) cathode materials promise high specific capacity and lower cost but exhibit faster degradation compared to lower Ni alternatives. Here, we employ high resolution electron microscopy and spectroscopies to investigate the nanoscale origins and impact on performance of intragranular cracking (within primary crystals) in Ni-rich LTMOs. We find that intragranular cracking is widespread in charged specimens early in cycle life, but uncommon in discharged samples even after cycling. The distribution of intragranular cracking is highly inhomogeneous. We conclude that intragranular cracking is caused by local stresses that can have several independent sources: neighbouring particle anisotropic expansion/contraction, Li- and TM-inhomogeneities at the primary and secondary particle levels and interfacing of electrochemically active and inactive phases. Our results suggest that intragranular cracks can manifest at different points of life of the cathode and can potentially lead to capacity fade and impedance rise of LTMO cathodes through plane gliding and particle detachment that lead to exposure of new surfaces to the electrolyte and loss of electrical contact.