2020/10/14 by Halyna Volkova, Kevin Pachuta, Kyle Crowley +6 · 10 citations
Materials Science · Physics and Astronomy · #Advanced Thermoelectric Materials and Devices #Analytical Chemistry (journal) #Annealing (glass) #Band gap #Chemical and Physical Properties of Materials #Conductivity #Copper-based nanomaterials and applications #Electrical resistivity and conductivity #Electron energy loss spectroscopy #Spectral line #Spectroscopy #Transmission electron microscopy #cond-mat.mes-hall #cond-mat.mtrl-sci #cond-mat.str-el
paper · pdf · doi:10.1103/physrevmaterials.5.015401
published in Physical Review Materials 5(1) (American Physical Society)
arxiv created 2020/10/14 · openalex created_date 2020/10/22 · openalex publication_date 2021/01/06 · arxiv updated 2021/01/13 · openalex updated_date 2026/08/05
Chemically exfoliated nanoscale few-layer thin LixCoO2 samples are studied as a function of annealing at various temperatures, using transmission electron microscopy and electron energy-loss spectroscopy (EELS) in various energy ranges, probing the O-K and Co-L2,3 spectra as well as low-energy interband transitions. These spectra are compared with first-principles density functional theory (DFT) calculations. A gradual disordering of the Li and Co cations in the lattice is observed starting from a slight distortion of the pure LiCoO2\phantom\rule4pt0exR3m to C2/m due to the lower Li content, followed by a P2/m phase forming at \ensuremath≈200\phantom\rule0.16em0ex^\ensuremath∘C indicative of Li-vacancy ordering, formation of a spinel-type Fd3m phase around 250\phantom\rule0.16em0ex^\ensuremath∘C, and ultimately a rocksalt-type Fm3m phase above 350\phantom\rule0.16em0ex^\ensuremath∘C. This disordering leads to a lowering of the band gap as established by low-energy EELS. The Co-L2,3 spectra indicate a change of average Co valence from an initial value of about 3.5 consistent with Li-deficiency related Co4+, down to 2.8 and 2.4 in the Fd3m and Fm3m, indicative of the increasing presence of Co2+ in the higher-temperature phases. The O-K spectra of the rocksalt phase are only reproduced by a calculation for pure CoO and not for a model with random distribution of Li and Co. This indicates that there may be a loss of Li from the rocksalt regions of the sample at these higher temperatures. The conductivity measurements indicate a gradual drop in conductivity above 200\phantom\rule0.16em0ex^\ensuremath∘C. This loss in conductivity is clearly related to the more Li-Co interdiffused phases, in which a low-spin electronic structure is no longer valid and stronger correlation effects are expected. Calculations for these phases are based on DFT+U with Hubbard-U terms with a random distribution of magnetic moment orientations, which lead to a gap even in the paramagnetic phase of CoO.