Figure 5. (a) Raman spectra of ν-SO42-band of ZnSO4 of different concentrations. (b) zinc ion association in MOF channel.
As a proof-of-concept demonstration, the MOF-growing zinc anode was paired with a MnO2 cathode (Figure S3, supporting information) to assemble full cells. The Zn@Zn2(bim)4//MnO2 cell demonstrated improved rate performance in higher discharge capacities than the cell with bare Zn anode as the current density increased from 0.1 to 2 A g-1 (Figure 6a). Although the discharge capacities of cells with Zn and Zn@Zn2(bim)4 were almost the same at 0.1 A g-1, the gap between the two cells gradually amplified with an increase in current density. The detailed voltage profiles at 0.5 and 1 A g-1 are shown in Figure 6b. Additionally, the Zn@Zn2(bim)4//MnO2 cell exhibited a slightly smaller discharge capacity (115.5 mAh g-1) compared to the bare zinc full cell (139.6 mAh g-1) but a much more stable cycling performance over 1200 cycles at 1 A g-1, retaining 48% of the capacity for Zn@Zn2(bim)4//MnO2vs 23% for Zn//MnO2 (Figure 6c). A similar trend is found at a smaller cathode loading (e.g., 1.2 mg cm-2) (Figure S4, supporting information). Furthermore, both cells were disassembled after cycling, and the zinc deposition morphology was viewed from both the cross-section and top views. For bare zinc, the zinc deposition was highly porous and stuck to the glass fiber separator, as shown in Figure 6d and f. In contrast, the surface of the Zn@Zn2(bim)4 after cycling was smooth, and the zinc was densely deposited underneath the MOF layer (Figure 6e and g). Therefore, compared to the bare zinc cell, the repeated stripping and depositing of zinc with Zn@Zn2(bim)4 are less likely to pierce the separator, thereby leading to an increased cycle life in symmetric cells.