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

Chemically exfoliated nanoscale few-layer thin ${\mathrm{Li}}_x{\mathrm{CoO}}₂$ 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-${L}_2,3$ 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 ${\mathrm{LiCoO}}₂\phantom{\rule{4pt}{0ex}}R\overline{3}m$ to $C2/m$ due to the lower Li content, followed by a $P2/m$ phase forming at $\ensuremath{\approx}200{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ indicative of Li-vacancy ordering, formation of a spinel-type $Fd\overline{3}m$ phase around $250{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, and ultimately a rocksalt-type $Fm\overline{3}m$ phase above $350{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. This disordering leads to a lowering of the band gap as established by low-energy EELS. The Co-${L}_2,3$ spectra indicate a change of average Co valence from an initial value of about 3.5 consistent with Li-deficiency related ${\mathrm{Co}}⁴⁺$, down to 2.8 and 2.4 in the $Fd\overline{3}m$ and $Fm\overline{3}m$, indicative of the increasing presence of ${\mathrm{Co}}²⁺$ 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 $200{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. 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in conductivity is clearly related to the more Li-Co interdiffused phases, in which a low-spin electronic structure is no longer valid and stronger correlation effects are expected.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 $\text{DFT}+U$ with Hubbard-$U$ terms with a random distribution of magnetic moment orientations, which lead to a gap even in the paramagnetic phase of CoO.