Figure 7. Raman spectra measured fresh NCM cathodes and cycled at different cycling states with E1 (a) and E2 (b).
The effect of various cycling conditions was further explored. It has long been established that using lower Li plating current density help improve AFLBs’ lifetime by forming a compact Li metal anode.[42] For this, an asymmetric cycling protocol was designed with 0.1 C-rate for charge and 0.25 C-rate for discharge. This cycling protocol help to minimize lithium inventory loss during Li plating and stripping.[43] As shown in Figure 8a, the discharge capacity decreased. However, higher capacity retention of 60% after 100 cycles was demonstrated, with a similar average CE of 98.2% (Figure 8b). Therefore, using the asymmetric cycling protocol helps to improve the stability of the battery capacity. Then, we examine the influence of the depth of discharge on the lifetime of anode-free cells. Limited depth of discharge has previously been shown to be beneficial to the lifetime of Li metal cells.[44] Therefore, a limited cutoff voltage (3.6-4.3 V) was applied together with an asymmetric cycling protocol, which may improve the NCM cathodes’ stability. The specific capacity further decreased when a limited cutoff voltage was used (Figure 8a). However, the capacity retention and CE are similar to the cutoff voltage of 3.0-4.0V (Figure 8b).