MgO/LiCoO

This section will compare the interactions between electrolyte and the LiCoO surfaces under various conditions, pristine or nano-MgO modified, charged or uncharged, and find out the reasons for the improved electrochemical performances of MgO/LiCoO, cathode materials. 

The above electrolyte residue on nano-MgO and commercial LiCoO is in fact a mixture of EC, LiPF and their decomposition products. Figure 24 shows the CH stretching bands of the fresh electrolyte and the electrolyte residue on different surfaces. Clearly the spectrum of electrolyte residue on MgO/LiCoO is somewhat similar to that of the electrolyte, but the spectra of the electrolyte... 

Figure 26 shows the IR spectra of some ring stretching modes of EC in the electrolyte residue. As marked with asterisk (at 1020 cm ), DMC and LiPF are almost undetectable in the electrolyte residue on commercial LiCoO and nano-MgO. Compared with the spectrum of pure EC and the electrolyte, a new component is observed at approx. 1197 cm in the electrolyte residue on the three substrates. Another two new components are detected at 1210 and 1088 cm" in the residue on MgO/LiCoO. As the content of the decomposed... 

Figure 20 SEM images of pristine LiCo02 (a, upper left), LiCoO coated with 1.5 mol% of MgO (b, lower left), the surface of MgO-coated LiCoO electrode after 70 cycles (c, upper right), and an MgO-coated LiCo02 particle (d, lower right) in the crack in (c).

The IR absorption spectra of commercial LiCoOj, nano-MgO and nano-MgO coated LiCoO have been shown in Figure 22. It is seen that LiCoOj has two strong absorption peaks at 522 and 610 cm while nano-MgO has a broad hump at around 1483 cm . Another two strong bands are observed at 640 cm and 420 cm in nano-MgO. These broad peaks are characteristic of nanometer sized MgO. However, when the surface of LiCoOj particle is coated with nano-MgO, no obvious spectral variation is observed. The two peaks of pristine LiCoO are still there and their relative intensities remain unchanged. The reason for the spectral features is that the content of nano-MgO on LiCoOj is very low (about 1.5 mol% ). Therefore some surface-sensitive characterization techniques are necessary for the identification of the MgO/LiCoOj interlayer properties. The other peaks observed in Figure 22 are attributed to the instrumental error (the sharp peak at around 1400 cm" , for example) or some contamination to the KBr pellets because these weak peaks reappear in all these samples. 

Figure 26 Selected IR spectra of the C=0 stretching modes of EC (a), the electrolyte (b) and the electrolyte residue on commercial LiCoO (c), nano-MgO (d) and MgO-coated LiCoO, (e) (the peak with an asterisk overhead is from the overlapping of the DMC and LiPF, bands). Reproduced from [122] with permission of The Electrochemical Society Inc.

Table 5 New bands observed on MgO-modified LiCoO charged to various voltages.

Comparison of Table 4 with Table 5 indicates that the composition of the SEI film on commercial LiCoO is quite similar to but not the same as that on MgO/LiCoOj at high charge voltages. The SEI film on the commercial and MgO/LiCoOj cathodes are mainly composed of ROLi and ROCO Li but species... 

Figure 33 Comparison of O Is spectra of commercial (left) and MgO-coated (right) LiCoO charged to various voltages. Reproduced from [122] with permission of The Electrochemical Society Inc.

---