Facet-Dependent Ni Segregation in a Micron-Sized Single-Crystal Li_1.2Ni_0.2Mn_0.6O₂ Cathode

Elemental surface segregation in cathode materials is critical for determining the phase and interfacial reaction between the electrode and electrolyte, which consequently affects the electrochemical properties. Single-crystal cathodes of Li_1.2Ni_0.2Mn_0.6O₂ and Li_1.2Ni_0.2Mn_0.6O_1.95F_0.05 with octahedral morphologies of (102)- and (003)-dominated facets have been manifested to show enhanced electrochemical properties. However, the surface structural features of such single crystals have not been investigated. Herein, using scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, and electron energy loss spectroscopy, we probe the elemental surface segregation characteristics in these single-crystal cathodes. We reveal that Ni surface segregation shows dependence on the crystal facet such that it occurs on crystal facets with a mix of cations and anions but not on the facets with only cations or anions. Furthermore, facet-dependent surface reconstructions are observed, featuring a spinel-like structure at the Ni-rich facet but a rock-salt structure at the facet without Ni segregation. The commonly known Mn reduction appears at the single-crystal surfaces and is more pronounced at the facet without Ni segregation. We further reveal that fluorination leads to stabilization of surface oxygens. This study provides detailed structural and chemical information about the facet-dependent Ni surface segregation and the resulting phase formation in the rather less explored micron-sized octahedral Li_1.2Ni_0.2Mn_0.6O₂ and Li_1.2Ni_0.2Mn_0.6O_1.95F_0.05 single crystals, which is key to further exploration of the electrochemical properties of the cathodes in the form of microsized single crystals.