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).