Formation Cycle Control for Enhanced Structural Stability of Ni-Rich LiNiCoMn_1-x-yO₂ Cathodes

Nickel-rich NCM cathode materials promise lithium-ion batteries with a high energy density. However, an increased Ni fraction in the cathode leads to complex phase transformations with electrode-electrolyte side reactions, which cause rapid capacity fading. Here, we show that an initial formation cycle at 0.1 C with a higher cutoff voltage (≥4.35 V) increases the stability of Ni-rich NCM (LiNi_0.88Co_0.08Mn_0.04O₂) particles during cycling at 1 C. We unveil that the formation of intragranular nanovoids is directly associated with the initial formation cycle at a lower charging cutoff voltage when oxygen vacancies are introduced at the Ni-rich NCM particle surface, due to irreversible electrolyte decomposition at the cathode-electrolyte interface. Nanovoid evolution of the Ni-rich NCM particles after 50 cycles increases the NiO-like rock salt phase; it results in intragranular cracks, which cause structural instability via heterogeneous phase distribution. This work demonstrates the importance of controlling Ni-rich NCM surface chemistry from the initial formation cycle to achieve better cycling stability.