Air Instability-Induced Mechanical Degradation Plaguing Cyclability of Polycrystalline Nickel-Rich Layered Cathode

Increasing Ni content in layered lithium transition-metal oxides can boost energy density and reduce the cost of lithium-ion batteries, but its drawbacks on cycling stability and air stability remain as critical challenges. Herein, the two issues are investigated based on a Ni-rich layered cathode, and air instability-induced cycling degradations are revealed by diverse characterization techniques. We show that both single-crystalline and polycrystalline LiNi_0.8Co_0.1Mn_0.1O₂ (PC-NCM811) suffer similar surface degradations during air storage, while additional intergranular modification is revealed for PC-NCM811. Air degradation-induced Li₂CO₃ impurity is formed not only on the particle surface but also at the intergranular space of PC-NCM811. Intergranular Li₂CO₃ shows a bonding effect, leading to increased initial mechanical strength measured by an in situ nanoindenter. However, Li₂CO₃ impurity is verified to be very detrimental to cycling stability, which not only incurs unwanted interfacial degradations but also intergranular cracks, contributing to capacity fade during cycling. This work highlights that air storage of polycrystalline layered cathode can cause severe mechanical cracking degradations during subsequent battery cycling, which offers new insights into the air degradation mechanisms. Therefore, improving the air storage stability of polycrystalline Ni-rich layered cathode materials requires greater attention to the stabilization of the intergranular sites.