Carbon-encapsulated Li2NiO2 lithium compensator: decoding failure mechanisms and enabling high-performance pouch cells

Abstract

Li₂NiO₂ has emerged as a promising cathode pre-lithiation additive capable of substantially enhancing the energy density and cycling durability of next-generation lithium-ion batteries. However, its practical deployment is hindered by intrinsic surface structural instability under ambient conditions. Although prior studies have reported residual alkali formation on Li₂NiO₂ surfaces and proposed coating strategies, critical knowledge gaps persist regarding the temporal evolution of alkali byproducts and industrially viable modification approaches. Through multiscale in situ characterizations combining X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS), we reveal a stratified residual alkali architecture: the inner layer predominantly comprises Li₂CO₃ while the outer layer is dominated by LiOH, despite minimal bulk structural alterations. Leveraging these insights, we developed a facile carbon-coating strategy enabling scalable synthesis of hundred-gram batches. The conformal carbon layer effectively mitigates structural degradation and suppresses alkali formation, facilitating the integration of high-content pre-lithiation additives. LiFePO₄||graphite pouch cells incorporating 2.5% modified Li₂NiO₂ demonstrate enhanced specific capacity with exceptional stability—exhibiting negligible energy decay (99.58% retention) over 500 cycles at 0.5P and maintaining 81.15% energy retention under aggressive 4P/4P cycling conditions over 1000 cycles. Remarkably, pouch cells with 8% additive loading achieve zero energy density decay after 1000 cycles at 4P/4P. This work provides a practical and scalable solution for advancing high-energy–density lithium-ion battery technologies.