Electron microscopy and spectroscopic study of structural changes, electronic properties, and conductivity in annealed LixCoO2

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-L-2,L-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 LiCoO2R (3) over barn to C2/m due to the lower Li content, followed by a P2/m phase forming at approximate to 200 degrees C indicative of Li-vacancy ordering, formation of a spinel-type Fm (3) over barm phase around 250 degrees C, and ultimately a rocksalt-type Fm (3) over barm phase above 350 degrees C. This disordering leads to a lowering of the band gap as established by low-energy EELS. The Co-L-2,L-3 spectra indicate a change of average Co valence from an initial value of about 3.5 consistent with Lideficiency related Co4+, down to 2.8 and 2.4 in the Fm (3) over barm and Fm (3) over barm, indicative of the increasing presence of Co(2+ )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 degrees 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in conductivity is clearly related to the more Li-Co interdiffused phases, in which a low-spin electronic structure is no longer valid and stronger correlation effects are expected.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.

Automated summary

Chemically exfoliated nanoscale few-layer thin LIxCoO2 samples were studied after annealing at various temperatures, revealing a gradual disordering of Li and Co cations in the lattice leading to a decrease in band gap and conductivity loss, with an increasing presence of Co(2+) in higher-temperature phases. The O-K spectra of the rocksalt phase indicate a potential loss of Li at higher temperatures. Conductivity measurements show a gradual drop above 200 degrees C, which is attributed to the presence of more Li-Co interdiffused phases with stronger correlation effects. Calculations based on DFT + U with Hubbard-U terms indicate a gap even in the paramagnetic phase of CoO.