Kinetic Pathways Templated by Low-Temperature Intermediates during Solid-State Synthesis of Layered Oxides

Layered oxides have been the dominant cathodes in lithium-ion batteries, and among them, high-nickel (Ni) systems are attractive because of their high capacity. For practical use, synthetic control of stoichiometry and structural ordering is crucial but has been nontrivial due to the complexity inherent to synthesis reactions, which often proceed via nonequilibrium pathways. In this paper, we report here a combined in situ synchrotron X-ray diffraction and ab initio study of solid-state synthesis of layered oxides starting from acetate precursors for LiCoO<sub>2</sub> and LiNiO<sub>2</sub> and their solid solution LiNi<sub>0.8</sub>Co<sub>0.2</sub>O<sub>2</sub>. While all three systems ultimately evolve into the same thermodynamically stable layered phase ($R\\overline{3}m$), each chemistry involves distinct metastable intermediates. We explain the phase progressions using a structural template model, demonstrating that during the synthesis of LiCoO<sub>2</sub>, the formed metastable spinel polymorph (Li<sub>2</sub>Co<sub>2</sub>O<sub>4</sub>; $Fd\\overline{3}m$) is a kinetically facile lithiation product of spinel Co<sub>3</sub>O<sub>4</sub>—the low-temperature (LT) intermediate from the decomposed Co-acetate. Similarly, in the Ni-based systems, the acetate decomposition products, rocksalts (Ni,Co)O, topotactically template the kinetic pathways of forming disordered rocksalts (Li<sub>x</sub>(Ni,Co)<sub>2–x</sub>O<sub>2</sub>; $Fm\\overline{3}m$), consequently leading to off-stoichiometric Li<sub>x</sub>(Ni,Co)O<sub>2</sub> with undesired high Li/Ni mixing. These findings highlight new opportunities for engineering precursors to form LT intermediates that template the synthesis of target phases and structural properties.