Disintegration of Meatball Electrodes for LiNi_xMn_yCo_zO₂ Cathode Materials

Mechanical degradation of Li-ion batteries caused by the repetitive swelling and shrinking of electrodes upon electrochemical cycles is now well recognized. Structural disintegration of the state-of-art cathode materials of a hierarchical structure is relatively less studied. We track the microstructural evolution of different marked regimes in LiNi_xMn_yCo_zO₂ (NMC) electrodes after lithiation cycles. Decohesion of primary particles constitutes the major mechanical degradation in the NMC materials, which results in the loss of connectivity of the conductive network and impedance increase. We find that the structural disintegration is largely dependent on the charging rate – slow charging causes more damage, and is relatively insensitive to the cyclic voltage window. We use finite element modeling to study the evolution of Li concentration and stresses in a NMC secondary particle and employ the cohesive zone model to simulate the interfacial fracture between primary particles. We reveal that microcracks accumulate and propagate during the cyclic lithiation and delithiation at a slow charging rate.