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Chemically exfoliated nanoscale few-layer thin Li$_x$CoO$_2$ samples arestudied as function of annealing at various temperatures, using transmissionelectron microscopy (TEM) and Electron Energy Loss Spectroscopies (EELS),probing the O-K, Co-L$_{2,3}$ spectra along with low energy interbandtransitions. These spectra are compared with first-principles DFT calculationsof -Im$[\varepsilon^{-1}(q,\omega)]$ and O-2p Partial Densities of Statesweighted by dipole matrix elements with the core wavefunction and including theO-1s core-hole and with known trends of the L$_2$/L$_3$ peak ratio to averageCo valence. Trends in these spectra under the annealing procedures areestablished and correlated with the structural phase changes observed fromdiffraction TEM and High Resolution TEM images. The results are also correlatedwith conductivity measurements on samples subjected to the same annealingprocedures. A gradual disordering of the Li and Co cations in the lattice isobserved starting from a slight distortion of the pure LiCoO$_2$ $R\bar{3}m$ to$C2/m$ due to the lower Li content, followed by a $P2/m$ phase forming at200$^o$C indicative of Li-vacancy ordering, formation of a spinel type$Fd\bar{3}m$ phase around 250$^o$C and ultimately a rocksalt type $Fm\bar{3}m$phase above 350$^o$C. This disordering leads to a lowering of the band gap asestablished by low energy EELS. The O-K spectra of the rocksalt phase are onlyreproduced by a calculation for pure CoO and not for a model with randomdistribution of Li and Co. This indicates that there may be a loss of Li fromthe rocksalt regions of the sample at these higher temperatures. Theconductivity measurements indicate a gradual drop in conductivity above200$^o$C, which is clearly related to the more Li-Co interdiffused phases, inwhich a low-spin electronic structure is no longer valid and strongercorrelation effects are expected.

Electron microscopy and spectroscopic study of structural changes, electronic properties and conductivity in annealed Li$_x$CoO$_2$