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

Abstract

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. 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 2 and LiNiO 2 and their solid solution LiNi 0.8 Co 0.2 O 2 . While all three systems ultimately evolve into the same thermodynamically stable layered phase (R3̅ m), each chemistry involves distinct metastable intermediates. We explain the phase progressions using a structural template model, demonstrating that during the synthesis of LiCoO 2 , the formed metastable spinel polymorph (Li 2 Co 2 O 4 ; Fd3̅ m) is a kinetically facile lithiation product of spinel Co 3 O 4 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 x (Ni,Co) 2−x O 2 ; Fm3̅ m), consequently leading to off-stoichiometric Li x (Ni,Co)O 2 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.

The Importance of Surface Oxygen for Lithiation and Morphology Evolution during Calcination of High‐Nickel NMC Cathodes

“…The first reaction seen by both methods occurred near 100 °C, corresponding to the dehydration of the lithium salt to LiOH, which resulted in noticeably more uniform powder lines suggesting that the LiOH had a finer grain structure (Figure S3a,b, Supporting Information). Between 250–300 °C, the Ni 0.8 Mn 0.1 Co 0.1 (OH) 2 also lost water, rapidly converting to a high‐symmetry cubic phase previously described with a rock salt‐like structure [ 11–13 ] ( Figure 1 f; Figure S2e,g, Supporting Information). The peaks associated with this rock salt phase formed from the conventional precursor were initially broad and accompanied by increased small‐angle X‐ray scattering (SAXS), both indicating nano‐scale primary particles.…”

“…While both the hydroxide precursor and final LiNi y Mn z Co 1−y−z O 2 cathode share a layered structure, prior research has implicated an intermediate cubic structure resembling rock salt, [11] recently confirmed using in situ powder X-ray diffraction (XRD). [12,13] These phases share an oxygen sub-lattice, and so the mechanism has been ascribed to a topotactic transformation involving extensive cation ordering. [14,15] In addition, several competing processes occur at high temperatures, where cation mixing and particle growth have been shown to depend on calcination temperature and hold time.…”

Kinetic Control of Long‐Range Cationic Ordering in the Synthesis of Layered Ni‐Rich Oxides

“…The revealed phase transition from disordered rock-salt structure to ordered layered LNCM622O is similar to previous studies. [29][30][31] Meanwhile, the integrated intensity of all reflections is continuously increasing. The average apparent crystallite sizes, obtained from the Rietveld refinement analyses of SRD patterns, increase from 13(5) nm at 500 °C to 83(5) nm at 900 °C, and the corresponding average micro-strains in these particles reduce from 0.0074(2) and 0.0018 (2), which manifest the crystallization (crystal growth) of powder crystals upon calcination.…”

Synthesis and Processing by Design of High‐Nickel Cathode Materials

“…The electrochemical performance of high‐Ni cathodes is often compromised by their off‐stoichiometry and cationic disordering formed during synthesis, which may be tuned by manipulating the low‐temperature phases [57] . As illustrated in Figure 15, the synthesis reaction of layered metal oxides proceeded along distinctively different routes when Ni and Co acetates were employed as precursors respectively.…”