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.