Figure 3 (a) Cyclic Voltammetry (CV) curves of ZSONT/CC electrode in the
first three cycles. (b) XRD pattern of ZSONT/CC electrode after 100
cycles. Nyquist plots of Li/ZSONT/CC and Li/CC symmetric cells (c)
before cycle and (d) after 100 cycles.
Cyclic voltammetry (CV) analysis of the ZSONT/CC sample was performed in
the potential range of 0.01 to 1.0 V (the set potential range was also
used in the pre-lithiation process) to see the redox reaction when
applied ZSONT/CC and CC hosts. Figure 3a showed its CV plots of
the first three cycles. The first cathodic peak at 0.7 V can be assigned
to the lithiation of ZnS and ZnO that leads to the formation of
Li2S, Li2O, and Zn metal, and the small
peak at ~0.5 V can be attributed to the formation of
Li-Zn alloys, with respective reactions shown in eqs 1 and 2.[41,42]
ZnS + ZnO + 2Li+ + 4e- = 2Zn +
Li2S + Li2O (1)
Li+ + Zn + e-= LiZn (2)
A broad peak in 0.2-0.4 V can be ascribed to the generation of the
solid-electrolyte interface layer and the decomposition of the
electrolyte. [43] In the following anodic sweep,
three oxidation peaks located at 0.25, 0.54, and 0.69 V were observed,
corresponding to the multistep delithiation process of Li-Zn alloy to
metal Zn. As the potential cut-off is set at 1 V, the oxidation of
Li2S /Li2O back to ZnS /ZnO (with
oxidation potential ~1.26 V) did not occur, and hence in
the subsequent cycles, only redox peaks related to Li-Zn were present.
The CV results showed that the Li2S and
Li2O were formed in the pre-lithiation process, and the
Zn formed in the cathodic process functioned as lithiophilic sites in
the following cycles. XRD of the ZSONT/CC samples after being washed by
EC: DEC electrolyte was conducted after they were cycled for 100 runs
and lithiated (to 0.05 V). From Figure 3b , the peak located at
the 30.1°, 32.3°, 34.9°, and 36.8° can be ascribed to the 002, 011, 102,
and 111 reflections of Li2O. The peak at 74.9° is the
422 reflections of LiZn. The two peaks at 44.1° and 51.2° were
attributed to the Li2S. The left three peaks located at
33.0°, 40.4°, and 38.3° came from Li2CO3(0002 and 1012) and LiF (111). The results suggested the persistence of
Li2O and Li2S in the sample after
cycling, in addition to LiZn, Li2CO3,
and LiF, with the latter two commonly found in SEI of Li metal
electrodes. [44]
Electrochemical impedance spectroscopy tests of Li/ZSONT/CC symmetric
cells were carried out to elucidate the fast ion transport kinetics and
stable electrolyte-anode interface. Nyquist plots of Li/ZSONT/CC and
Li/CC were displayed in Figure 3 . The fitted results were
listed in Table S1 (RSEI: SEI resistance,
Rct: charge-transfer resistance, R1:
internal resistance). From Figure 3c , the Li/ZSONT/CC and Li/CC
composite anodes have similar interfacial resistance (26.92 Ω and 24.80
Ω, respectively) before cycling. After 100 cycles, the
RSEI of two electrodes dropped and the
Rct of the Li/ZSONT/CC and Li/CC decreased to 4.03 Ω and
2.20 Ω (Fig. 3d ). Such a reduced resistance is commonly
generated by the SEI self-optimizing upon cycling.[45,46] A more significant reduction in
RSEI was observed in the Li/ZSONT/CC composite anode
(from 26.92 to 8.55 Ω) than that of the Li/CC composite anode (from
24.80 to 14.48 Ω).