Degradation Pathways of Cobalt-free LiNiO2 Cathode in Lithium Batteries

Electrode-electrolyte reactivity (EER) and particle cracking (PC) are considered two main causes of capacity fade in high-nickel layered oxide cathodes in lithium-based batteries. However, whether EER or PC is more critical remains debatable. Herein, the fundamental correlation between EER and PC is systematically investigated with LiNiO2 (LNO), the ultimate cobalt-free lithium layered oxide cathode. Specifically, EER is found more critical than secondary particle cracking (SPC) in determining the cycling stability of LNO; EER leads to primary particle cracking (PPC), but contrary to conventional wisdom, prevents SPC. Two surface degradation pathways are identified for cycled LNO under low and high EERs. A common blocking surface reconstruction layer (SRL) containing electrochemically-inactive Ni3O4 spinel and NiO rock-salt phases is formed on LNO at the charged state in an electrolyte with high EER; in contrast, an electrochemically-active SRL featuring regions of electron- and lithium-ion-conductive LiNi2O4 spinel phase is formed on LNO at the charged state in an electrolyte with low EER, even though bulk LiNi2O4 crystals are believed to be non-existent. These findings unveil the intrinsic degradation pathways of LNO cathode and are foreseen to provide new insights into the development of lithium-based batteries with minimized EER and maximized service life.