Formation Cycle Control for Enhanced Structural Stability of Ni-Rich LiNixCoyMn1-x-yO2 Cathodes

Abstract

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.