For both cells, the first discharge only provides a fraction of the charge capacity. However, for the cell cycled at 450 MPa, the discharge capacity and therefore the coulombic efficiency quickly reaches values of >38 mAh/g and > 95 % within the first 10 cycles. After around 120 cycles, a decrease in both the discharge capacity and the columbic efficiency was observed due to a disruption of the pressure cell set up as the potentiostat was changed. For the cell cycled at ambient pressure, the coulombic efficiency slowly approximates 90 % within the first 100 cycles. This indicates that the carbon side reactions mainly occur during the earlier cycles, The energy efficiency of the cell cycled at ambient pressure reaches a maximum of ~40 % after 50 cycles, then starts to decrease over the next cycles. This is also visible in the cycling curves, as the overpotential during charging continually increases from the 50 th cycle.