Collinear Jahn-Teller Ordering Induces Monoclinic Distortion in "Defect-Free" LiNiO₂

Lithium nickel oxide, LiNiO₂ (LNO), and its doped derivatives are promising battery cathode materials with high gravimetric capacity and operating voltages. They are also of interest to the field of quantum magnetism due to the presumed S = 1/2 triangular lattice and associated geometric frustration. However, the tendency for Li/Ni substitutional defects and off-stoichiometry makes fundamental studies challenging. In particular, there is still a discrepancy between the rhombohedral (R3̅m) bulk structure and the Jahn-Teller (JT) distortions of the NiO₆ octahedra inferred on the basis of local structural probes. Karger et al. (Chem. Mater. 2023, 35, 648-657) recently used Na/Li ion exchange to synthesize "defect-free" LNO by exploiting the absence of antisite disorder in NaNiO₂ (NNO). Here we characterize the short- and long-range structure of this ion-exchanged material and observe splittings of key Bragg reflections at 100 K in X-ray and neutron diffraction (XRD and NPD), indicative of a monoclinic distortion induced by a cooperative collinear JT distortion, similar to that seen in NNO. Variable temperature XRD reveals a second-order phase transition from the monoclinic (C2/m) low-temperature structure to a rhombohedral (R3̅m) structure above ∼400 K. We propose that this collinear JT ordering is also present in solid-state synthesized LNO with the domain size and extent of monoclinic distortion controlled by defect concentration. This new structural description of LNO will help advance our understanding of its electronic and magnetic properties and the series of phase transformations that this material undergoes upon electrochemical cycling in Li-ion batteries.