Chemical and Structural Evolution during the Synthesis of Layered Li(Ni,Co,Mn)O₂ Oxides

Abstract

The discovery of Li-containing transition-metal (TM) oxides has attracted broad interest and triggered intensive studies on these oxides as cathodes for lithium-ion batteries over decades.Unfortunately, a clear picture of how Li/TM/O ions are transported and electrons are transferred during synthesis of these compounds is still missing, especially when cubic close-packed (ccp) anion sublattices are involved, as it is the case for spinel, layered, or rock-salt systems. In the present study, a series of layered Li(Ni,Co,Mn)O 2 oxides was chosen as target materials to elucidate the underlying formation mechanism of these compounds during high-temperature lithiation reaction. The consistent experimental results demonstrate that, as lithium ions are inserted from surface to bulk, some transition metal cations located within the bulk of crystallites are able to diffuse to the near-surface region. They create cation vacancies for the inserted lithium ions, the mass transport behavior of these elements is driven by chemical potential gradient. Concurrently, oxygen anions from lithium oxides and/or ambient oxygen are adsorbed and incorporated into the ccp oxygen lattice on the surface structure, connecting the relocated transition metal cations and the incorporated lithium ions by forming ionic bonds. This process is concomitant with crystal growth, surface reorganization caused by phase transformation, occurrence and disappearance of pores.

Probing Dopant Redistribution, Phase Propagation, and Local Chemical Changes in the Synthesis of Layered Oxide Battery Cathodes

“…Our results are consistent with the in situ XRD study of the LiNiO 2 formation reported by Bianchini et al. [ 14b ] Together with our XPS and XAS results, the XRD result further confirms that the metal hydroxide phase is still present at the temperature (below 300 °C) when Mg/Ti dopants diffuse into the matrix. To rationalize the dopant diffusion in the nickel hydroxide matrix, we hypothesize that the larger NiO and NiM distances in the metal hydroxide (larger lattice volume) result in lower energy barriers for dopants to diffuse (Figure 4c).…”

Phosphoric acid and thermal treatments reveal the peculiar role of surface oxygen anions in lithium and manganese-rich layered oxides

“…0.2Ni 2+ / 0.2Ni 4+ + 0.4e À ). 46 The lower capacity of LLNMO-TH4 and LLNMO-TH5 cathodes in this region could be due to the migration of some Ni 2+ cations into the octahedral sites in the rock-salt structure and/or the tetrahedral sites in the spinel structure. The two plateaus at about 4.7 V found in the LLNMO-TH5 cathode (see Fig.…”

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

“…Previous studies have suggested that the spinel TM 3 O 4 phase obtained at high temperatures (> 600 °C) contains a higher amount of Co and Mn cations because the formation of Co 3 O 4 and Mn 2 O 3 is thermodynamically favored over a temperature range from 25 to 900 °C in air as opposed to the thermally stable NiO. [ 21,34 ] The oxidation state of TM cations for the NCM622CO after being heated at 800 °C for 12 h (namely NCM622O‐800) was analyzed by K‐edge X‐ray absorption near‐edge structure (XANES) spectroscopy using linear combination fitting (LCF) approach, see Figure S6, Supporting Information. The valence state of Ni, Co, and Mn in NCM622O‐800 can be assigned close to 2.0+, 2.4+, and 3.3+, respectively, which matches precisely with the SRD results above.…”

“…[ 20 ] Recently, our previous work suggested that the pathway of Li(Ni x Co y Mn 1− x − y )O 2 formation is closely correlated with the oxidation state, site preference, and chemical composition of transition metal (TM) cations. [ 21 ] Since these pathways cannot be easily predicted today, the kinetics of the lithiation reaction for the synthesis of layered Ni‐rich cathode materials remains to be elucidated.…”