Figure 5. Coulombic efficiencies (CE) of ZSONT/CC//Li, CC//Li, and Cu//Li cells at (a)1 mA cm-2/ 1 mA h cm-2 and at (b) 2 mA cm-2/ 2 mA h cm-2. Galvanostatic cycling performance of Li/ZSONT/CC, Li/CC, and Li/Cu symmetric cells at (c) 4 mA cm-2/ 1 mA h cm-2 (The inset images are the corresponding enlarged curves of the 61-65th, 351-355th, and 651-655th cycles) and at (d) 4 mA cm-2/ 4 mA h cm-2 (The inset images are corresponding enlarged curves of the 21-25th, 151-155th, and 251-255th cycles)
Coulombic efficiencies (CE) were tested in ZSONT/CC//Li, CC//Li, and Cu//Li batteries with lithium foil as the counter electrodes in ether electrolyte. The CE value was estimated as the ratio of stripped Li capacity to plated capacity. Figure 5a showed the CE comparison of Li on ZSONT/CC host, CC host, and Cu foil at 1 mA cm-2/ 1 mA h cm-2. The ZSONT/CC electrodes exhibited a stable CE of ~99.2% over 400 cycles. By contrast, the CE of Cu foil fluctuated drastically with a CE below 80.4% in 50 cycles and the CE of the CC host decreased to 74.3% after 205 cycles. When the current density and capacity were respectively increased to 2 mA cm-2 and 2 mA h cm-2, the average CE of the ZSONT/CC was over 99.0% in 180 cycles, whereas that of the Cu foil was below 60.1% in 50 cycles and the CE of the CC host dropped to 43.9% after 130 cycles (Fig. 5b ). The average CE of ZSONT/CC was found to >99% in 150 cycles even at 4 mA cm-2/1 mA h cm-2, which is much better than CC host (97.2% in 116 cycles) and Cu foil (76.8% in 60 cycles) (Fig. S4 ). The lithiophilic property of the as-prepared ZSONT/CC was accessed by measuring the nucleation overpotentials, taking the Li as the anode and the hosting electrodes as the cathodes. The nucleation overpotential was estimated as ~10.6 mV for ZSONT/CC electrode, while that of Cu was 325.7 mV (Fig. S5 ). Figure S6 showed the rate performance of the ZSONT/CC, CC and Cu host at different current densities ranging from 1 to 10 mA cm-2 with a fixed capacity of 1 mA h cm-2 in symmetrical cells. It was noted that the overpotential was small and steady for ZSONT/CC-Li electrode, specifically, a small voltage hysteresis of 46.7 mV was observed at a current density of 1 mA cm-2. A relatively low overpotential of 263.6 mV was obtained, even at 10 mA cm-2. In contrast, the overpotential of CC was 333.6 mV and that of Cu was 534.2 mV at 10 mA cm-2. These results suggested superior Li ion transport kinetics using the ZSONT/CC-Li electrode. We further compared the cycling stability of the electrodes at small and large capacities. The ZSONT/CC host exhibited stable performance over 650 cycles at 4 mA cm-2/ 1 mA h cm-2 in a symmetrical cell, while CC host only had a life span of ~100 cycles and the cell with Cu foil only ran ~90 cycles (Fig. 5c ). At an even higher capacity of 4 mA h cm-2, the symmetrical battery ran stably near 320 cycles (Fig 5d ), while the cell with CC only lasted for 25 cycles and with Cu lasted for ~13 cycles.