Figure 6. Top (left panel) and cross-sectional (right panel)
SEM images of NCM with two electrolytes: fresh NCM cathode (a-b), after
100 cycles with pristine electrolyte(c-d) and optimized electrolyte
(e-f).
In addition to the positive effects of the dual additives on the
deposited Li metal, the effect on the NCM cathodes was also explored.
Top and cross-sectional morphological analysis of the NCM cathodes after
100 cycles with two different electrolytes (Figure 6). It was shown that
NCM developed many cracks after 100 cycles with E1 electrolytes (Figure
6c). In contrast, the morphology of NCM is better preserved after 100
cycles with E2 electrolytes (Figure 6e). According to the
cross-sectional SEM images (Figure 6b, d, f), the NCM cycled with E2
electrolytes is more compact. In addition, the existence of the fluorine
on the surface of the NCM cycled with E2 electrolytes was confirmed by
EDX measurements (Figure S3). This observation indicates that
introducing dual additives of LiAsF6 and FEC facilitates
the formation of fluorine-containing composites on the surface of the
NCM cathode. These composites protect the NCM and improve its cycling
stability. Raman tests were performed on the NCM cathodes at different
cycling states, as shown in Figure 7. Bands between 300 and 750
cm−1 correspond to metal-oxygen vibrations. Bands at
1350 and 1600 cm−1 refer to the Raman shifts of the
carbon black conductive additive.[39, 40] For the
NCM cathode cycled with E1 electrolyte, the peak of at 450 and 580
cm−1 decreased after charge to 4.3V. It is likely that
a kind of structural (dis-)ordering transition (hexagonal phases H1,
H1+H2) happens during the charging process when the electrode is
partially delithiated.[41] Subsequently, the peaks
increased after full discharge. However, after 100 cycles, these peaks
become weak, indicating that NCM cathode materials’ structure
drastically degraded. In contrast, with the E1 electrolyte, the Raman
peaks of the NCM cathode are higher at 450 and 580
cm−1 (Figure 7b), indicating that more NCM materials
are active with E2 after 100 cycles. It is confirmed that the dual
additives help form more stable CEI layers on the NCM surface, improving
stability. Therefore, the dual additives synergistically improve the
stability of both positive and negative electrodes.