School of Materials Science and Engineering, South China University of Technology,

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1 Supporting information Zn/MnO 2 Battery Chemistry With H + and Zn 2+ Co-Insertion Wei Sun, Fei Wang, Singyuk Hou, Chongyin Yang, Xiulin Fan, Zhaohui Ma, Tao Gao, Fudong Han, Renzong Hu, Min Zhu *, Chunsheng Wang * Department of Chemical and Bimolecular Engineering, University of Maryland, College Park, Maryland 20740, United States *Corresponding author Prof. C. Wang address: cswang@umd.edu School of Materials Science and Engineering, South China University of Technology, Guangzhou , China *Corresponding author Prof. M. Zhu address: memzhu@scut.edu.cn W.S. and F.W. contributed equally. S1

2 Experiment Section Synthesis of MnO electrode. The MnO electrode was synthesized by electrodeposition that carried out in a three-electrode cell with carbon fiber paper (CFP, Sigracet) as the working electrode, Zn metal foil as both counter and reference electrodes in the 2 M ZnSO M MnSO 4 aqueous solution. The three-electrode cell was first galvanostatically charged at 0.2 ma cm -2 to 1.8 V (vs Zn/Zn 2+ ) and then maintained at 1.8 V for 8 h using an Arbin battery test station (BT2000, Arbin Instruments). The as-obtained MnO electrode was dried in vacuum oven at 80 overnight for material characterization. Zn/MnO aqueous batteries design. The Zn/MnO aqueous batteries were assembled in the CR2032-type coin cell using the Zn metal plate as the anode, ZnSO 4 +MnSO 4 solution as the electrolyte, and the carbon fiber paper (CFP) as the cathode current collector. The cell was first charged to 1.8 V (vs. Zn/Zn 2+ ) and kept at 1.8 V for 8 h to electrodeposit the MnO 2 on the cathode current collector CFP. Four different concentration of MnSO 4 (0.1, 0.2, 0.3, 0.5 M), and three different charge time period (4, 8, and 12 h) were applied to optimize the electrodeposition process. Electrochemical Measurements: The cyclic voltammetry (CV) testing of zinc metal and MnO electrode was performed in a three-electrode cell with zinc metal as counter and reference electrodes, using the electrochemical working station (CHI 600E) at a scanning rate of 1 mv s -1. The coin cell was used to test the cycling and charge-discharge behavior on a battery test system (LAND CT-2001A). Galvanostatic intermittent titration technique (GITT) was obtained by a series of galvanostatic discharge pulses of 120 s at 50 ma g -1 followed by a 4 h rest. EIS were obtained by an electrochemistry work station (Gamry Interface3000) over a S2

3 frequency range from 1 MHz to 0.1 Hz, with an AC amplitude of 5 mv. All the electrochemical measurements were performed at room temperature. Materials Characterizations. The crystalline structure of the electrode was characterized by X- ray diffraction (XRD) on a D8 Advance X-ray diffraction with a Cu Kα radiation. The morphology of the sample was investigated by Scanning electron microscopy (SEM, Hitachi SU- 70) and transmission electron microscopy (TEM, JOEL 2100F). X-ray photoelectron spectroscopy (XPS) was conducted on a high sensitivity Kratos AXIS 165 X-ray photoelectron spectrometer with Mg Kα radiation. All binding energy values were referenced to the C 1S peak of carbon at ev. S3

4 Supplementary Figures Figure S1. Photograph images of carbon fiber paper (CFP) before and after electrodeposition. The black dense layer coated on the CFP was the electrodeposited MnO 2. Figure S2. XPS full survey of MnO cathode. S4

5 Figure S3. SEM images of MnO with different magnification. Figure S4. SEM and elemental mapping images of MnO electrode (scale bar: 10 µm). S5

6 Figure S5. TEM images of MnO with different magnification, showing the nanocrystalline feature of electrodeposited MnO 2. Figure S6. The electrochemical performance of MnO cathode obtained from different protocols at 50 µa cm -2 between 1.0 and 1.8 V. These cells were synthesized by galvanostatically charging at 200 µa cm -2 to 1.8 V and then maintaining at 1.8 V for different times (4, 8, and 12 h, as shown in a) in different electrolytes (2 M ZnSO 4 + X M MnSO 4 (X=0.1, 0.2, 0.3 and 0.5), as shown in b). Among the different protocols, the MnO 2 deposited in 2 M ZnSO M MnSO 4 electrolyte for 8 h exhibits the best cycling S6

7 stability. Therefore, the MnO 2 deposited by this electrodeposition protocol is selected to evaluate the electrochemical performances afterward. Figure S7. Typical profiles of voltage and current versus electrodeposition time. The cell is first galvanostatically charged at 200 µa cm -2 to 1.8V (vs. Zn/Zn 2+ ) and then maintained at 1.8 V for 8 h in 2 M ZnSO M MnSO 4 electrolyte. The deposited MnO 2 mass is obtained from weight difference between bare CFP and electrodeposited MnO electrode (0.6 mg cm -2 for this deposition protocol). And the charge/discharge current density per area is converted to current per mass of MnO 2 for convenience to compare with previous report. S7

8 Figure S8. Galvanostatic charge-discharge profiles of the Zn/a-MnO 2 cell at 100 ma g -1 between 1.0 and 1.8 V. The cathode prepared on stainless foil using the commercial available a-mno 2 nanorod (200nm, Sigma-Aldrich) as the active material, the PTFE as binder, and the super P as the conductive agent. Figure S9. Ex situ XRD patterns of the pristine and recharged MnO electrodes. S8

9 Figure S10. Cycling performance of Zn/MnO 2 batteries in 2 M ZnSO M MnSO 4 electrolyte between 1 V and 1.8 V at 0.3 C. Figure S11. SEM images of MnO electrode after 1, 2, 100, and 300 cycles at 1.3C in 2 M ZnSO M MnSO 4 electrolyte. S9

10 Figure S12. EIS plots of Zn/MnO battery after 1, 2, and 100 cycles at 1.3 C in 2 M ZnSO M MnSO 4 electrolyte. All the EIS tests were performed on the charge state. Figure S13. Cyclic voltammetric (CV) curves of Zn/MnO battery with different sweep rates. S10

11 Figure S14. Galvanostatic charge and discharge curves of the Zn/MnO cell in organic Zn based electrolyte with and without adding H 2 O. As shown by the black curve, MnO electrode demonstrated a very limited capacity (capacitor behaviour) due to the absence of H + ions. For comparison, we added 1 wt.% H 2 O into the non-aqueous electrolyte. As presented by the red curve, a much larger capacity was delivered and a long plateau around 1.2 V could be observed. From this result, we speculate that the insertion of Zn 2+ in the second step happens after the reaction between H + and MnO 2 electrode in the first step. S11

12 Figure S15. Ex situ XRD patterns of the pristine MnO electrode and the MnO electrode after discharged to 1.3V. Figure S16. Ex situ XRD patterns of the MnO cathode at discharge potential of 1.3V and 1.0V, respectively. S12

13 Figure S17. The weight loss of the MnO 2 electrode discharged to 1.3V during the thermal gravimetric analysis (TGA) test. The electrode was washed by H 2 O and dried at 60 C for 8 h to exclude the influence of surface adsorbed water, and the current collector was not used in this experiment to exclude its influence on weight change. The discharged MnO 2 electrode without drying treatment and the MnOOH powder were also tested for comparing. All the TGA experiments were conducted from room temperature up to 350 C under Argon flow. To demonstrate whether the water insert into the structure of MnO 2 with H + during the discharge process, we conducted a TGA test on the MnO 2 electrodes discharged to 1.3V to monitor the weight change and compared the results with pure MnOOH powder. If the water could insert into the structure of MnO 2 with H +, the weight loss during the TGA test should be larger than the MnOOH powder. As shown in Figure S17, the MnOOH demonstrated a weight loss of 11.5% after heating to 350 C, ascribing to the transformation of MnOOH into Mn 3 O 4 and oxygen and water release, which is consistent with previous reports. 1,2 For the MnO 2 electrode after drying at 60 C, the weight loss (8.7%) is smaller than the pure MnOOH, indicating that there is no additional water insertion in the structure of MnO 2. For the discharged MnO 2 electrode sample without drying, the higher weight loss in the low-temperature region (<80 C) mainly comes from the absorbed water on the surface. S13

14 Figure S18. SEM and elemental mapping images of discharged MnO electrode (scale bar: 20 µm). Table S1 The ph value of the different electrolytes. Electrolyte ph value ZnSO 4 + MnSO ZnSO 4 + MnSO 4 (after discharge) 5.00 MnSO Reference (1) Folch, B.; Larionova, J.; Guari, Y.; Guérin, C.; Reibel, C. Journal of Solid State Chemistry 2005, 178, (2) Bai, Z.; Sun, B.; Fan, N.; Ju, Z.; Li, M.; Xu, L.; Qian, Y. Chemistry-a European Journal 2012, 18, S14