Real-Time Observation of Chemomechanical Breakdown in a Layered Nickel-Rich Oxide Cathode Realized by In Situ Scanning Electron Microscopy

It is well-accepted that massive cracks in Ni-rich cathode secondary particles are the determining factors for long-term performance degradation; however, the corresponding crack generation and the state of dynamic propagation are still unknown. In this work, we utilize in situ scanning electron microscopy to reveal the dynamical morphological evolution of a single LiNi0.8Mn0.1Co0.1O2 secondary particle embedded in a cathode blend during electrochemical cycling. These observations show that very few cracks appear in the particle when cycled at a normal cutoff voltage of 4.1 V, but when the cutoff voltage is increased to 4.7 V as an extreme working condition, several cracks were clearly initially generated in the core region and propagated radially along the grain boundaries, finally reaching the particle's surface. Impressively, crack propagation follows a repeat "grow–stagnate–grow" phenomenon during charge–discharge cycling. Our direct in situ investigation provides a full map of crack evolution in the cathode under electrochemical cycling during early stages.