Delineating the Factors Impacting the Electrochemical Behavior of Single-Crystal High-Nickel Layered Oxide Cathodes

High-nickel (Ni) (≥80%) single-crystal LiNi_1-x-yMn_xCo_yO₂ (NMCs) have garnered recent interest as cathodes in lithium (Li)-ion batteries (LIBs). However, capacity fade at high voltages, particularly after the onset of the H2-H3 phase transition, hampers their viability. In this study, single-crystal LiNi_0.8Mn_xCo_0.2-xO₂ (x = 0.2, 0.1, 0) are synthesized and tested in LiPF₆ in ethyl methyl carbonate-based electrolytes, with and without monofluoroethylene carbonate and LiF₂PO₂ additives, to clarify the effects of Co/Mn ratio and surface stabilization on high-voltage cycling degradation. By imposing a kinetic barrier to the accessible H2-H3 capacity, surface reconstruction is identified as the primary driver of high-voltage capacity loss, being greater in the Co-free cathode and in the absence of fluorinated electrolyte components. This is attributed to a synergy between increased mechanical stress due to worsened bulk and interfacial H2-H3 kinetics and decreased interfacial stability due to the poor passivating capability of the electrolyte. The findings highlight the importance of limiting cathode impedance growth during high-voltage cycling, which can be achieved by tuning bulk dopants and electrolyte chemistry.