Supporting Information Aqueous Magnesium Zinc Hybrid Battery: An Advanced High Voltage and High Energy MgMn 2 O 4 Cathode Vaiyapuri Soundharrajan a, Balaji Sambandam a, Sungjin Kim a, Vinod Mathew a, Jeonggeun Jo a, Seokhun Kim a, Jun Lee a, Saiful Islam a, Kwangho Kim b,c,yang-kook Sun d and Jaekook Kim a * a Department of Materials Science and Engineering, Chonnam National University, Gwangju 500-757, South Korea. b Global Frontier Center for Hybrid Interface Materials, Pusan National University, Busan 609-735, South Korea c School of Materials Science and Engineering, Pusan National University, Busan 609-735, South Korea d Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea. These authors contributed equally. 1
1. Experimental Section 1.1. Material Preparation First, solution A was prepared by dissolving magnesium acetate (1 mmol) and manganese acetate (2 mmol) in 80 ml of ethylene glycol (EG). Next, 20 ml of liquid paint thinner (solution B) was added to solution A and stirred well; the mixture was then transferred to an aluminum boat which was maintained at 250 C and ignited using a gas torch. This resulted in rapid precipitation followed by combustion. The as-obtained product was kept at 450 C for 2 h and used for subsequent characterizations. 1.2. Material Characterization Powder X-ray diffraction (PXRD, Cu Kα radiation, with λ = 1.5406 Å) measurements were conducted using a Shimadzu X-ray diffractometer. The surface morphology was analyzed through field-emission scanning electron microscopy (FE-SEM, S-4700 Hitachi). The lattice fringes were analyzed using field-emission transmission electron microscopy (FE-TEM, Philips Tecnai F20 at 200 kv). Surface elemental oxidation states were examined by X-ray photoelectron spectroscopy (XPS, Thermo VG Scientific Instruments, Multilab 2000), using Al Kα as the X-ray source. The spectrometer was calibrated relative to the C 1s peak binding energy of 284.6 ev. Thermogravimetric analysis (TGA) was carried out using an SDT Q600 thermobalance in air with a temperature change of 5 C min -1. The structural transformation of the MgMn 2 O 4 spinel during electrochemical Mg/Zn-ion egress/ingress was measured using in situ synchrotron XRD performed at the 1D beamline KIST-PAL (Pohang Accelerator Laboratory). The 2θ values of the synchrotron XRD patterns were recorded using the wavelength of 1.00076 Å and converted to Cu K radiation (λ = 1.5414 Å) before replotting. Kapton tapes were used to cover the apertures in the outer cases of the test cell used in this in situ synchrotron XRD analysis. The electrochemical charge/discharge measurements were carried out at room temperature using a BTS 2004H battery tester (Nagano Keiki Co. Ltd., Tokyo, Japan). The cells were cycled in the voltage range of 1.9 0.5 V vs. Zn 2+ /Zn. The 2
monochromator energy was calibrated with the K-edge position of a Mn foil at 6539 ev. The obtained spectra were then analyzed using ATHENA software. 1.3. Electrochemical characterization Pastes containing 70, 20, and 10 wt% of the active material, Ketjenblack and teflonized acetylene black (TAB), respectively, were pressed onto a stainless-steel mesh before being vacuum dried at 120 C for 12 h for use as the cathode. Active mass loading of that cathode was found to be 3 3.5 mg. Zn metal foil and 1 mol L -1 MgSO 4 + ZnSO 4 +0.1mol L -1 MnSO 4 (MZM) were used as the anode and electrolyte, respectively. While preparing 2032-type coin cells, a glass-fiber separator soaked in the electrolyte was pressed between the prepared cathode and anode under open-air conditions and aged for 24 h. Then, electrochemical discharge/charge measurements were performed using a battery tester at different current densities in the potential range of 1.9 0.5 V vs. Zn 2+/ Zn. For cyclic voltammetry (CV) scans and electrochemical impedance spectroscopy (EIS), a potentiostat model workstation (AUTOLAB PGSTAT302N) was used. Specific energy calculation is based on the specific capacity and average working potential. S1 3
Figure S1. (a) PXRD pattern for as prepared MMO sample via pyro-synthesis at 250 o C and (b) its corresponding TGA/DSC profiles. Figure S2. Cyclic voltammetry profiles at (a) ZnSO 4 /MnSO 4 and (b) MgSO 4 /MnSO 4 electrolytes under similar electrochemical condition. 4
Figure S3. (a) Cyclic voltammetry patterns for MMO-400 in MZ electrolyte. (b) EIS profiles for MMO-400 in MZM and MZ electrolyte before cycling. It is clearly evident that the redox behavior is nearly same as the MZM electrolyte in the initial cycles; however, the peak current increment in the anodic scan is limited when compared to the MZM electrolyte, indicating slower activation during the cycling process (Figure S3a). This could be due to the fact that cation insertion is further enhanced by the MnSO 4 salt in the MZM electrolyte. S2 The Nyquist plot of both curves (Figure S3b) exhibits two parts: a perfect semicircle at a high medium frequency region that corresponds to the electrode electrolyte charge transfer reaction (Rct) at the interface, and a sloping line at a low frequency corresponding to cation insertion and extraction (i.e. Warburg impedance, Zw). It is clearly seen that the charge transfer resistance for the electrode in the MZM electrolyte is higher than that for the electrode in the MZ electrolyte. The increased charge transfer resistance is ascribed to the interference of ion diffusion by the Mn 2+ ions in the MZM electrolyte during the initial cycle, which is in agreement with CV data and previous studies on spinel-based electrodes with MnSO 4 as an additive. S3 5
Figure S4. (a) Charge/discharge patterns at the 10, 15, 25 and 50th cycles for MMO-400 in MZM electrolyte at 100 ma g -1 rate. (b) Charge/discharge profiles for MMO-400 in MZ electrolyte at 100 ma g -1 rate. 6
Figure S5. (a) Multi-scan CV profile for MMO-400 in MZM electrolyte, the contribution ratio of the diffusion-limited capacities and capacitive capacities at (b) 0.1 and (c) 0.5 mv s -1. (d) Bar chart for capacity contribution ratios at different scan rates. The resultant CV curves preserve the original profile with a little broadening in the shape (Figure S5a). Thu s, the two pairs of reduction/oxidation peaks situated at 1.54/1.16 and 1.6/1.36 V move slightly to higher or lower voltages, respectively. Surface-originated capacitive and diffusion-induced processes control the total current, and consequently the specific capacity. The two processes can be elucidated from the following equation: S4 i=av b ------- (3) 7
In the above equation, i is the measured peak current, v is the scan rate (mv s -1 ), a and b are the variable parameters. In general, b varies within the range of 0.5 1.0. When b=0.5, the total capacity is contributed to diffusion-controlled phenomena and b=1 indicates the existence of a capacitive process. For MMO-400, the b-values of the three peaks, obtained by the slope of the redox peaks log i vs log v, are 0.61, 0.73 and 0.78. (Figure S6) clearly shows that the majority of the electrode reaction is diffusion-controlled. Moreover, the quantitative separation of the capacitive distribution and diffusion-induced processes, at a certain potential upon electrochemical cycling, is described below: S5 i= k 1 v + k 2 v 1/2 --------- (4) Here, (k 1 v) corresponds to the capacitive process and (k 2 v 1/2 ) represents the diffusioncontrolled process at a certain sweep rate. For example, at the sweep rates of (0.1 and 0.5 mv s -1 ), the shaded portion relates to the surface capacitive properties, assigned to 37 and 73 % of the total specific capacity at their respective scan rates (Figures S5b and S5c). The influence of capacitive growth at different scan rates was calculated and is given in Figure S5d. It is obvious, that the capacitive contribution increases with faster scan rates, growing from 0.1 to 0.8 mv s -1, indicating that the surface reaction (capacitive) is predominant at higher scan rates. 8
log(i, peak current (ma)) 0.6 0.3 0.0-0.3-0.6-1.0-0.8-0.6-0.4-0.2 0.0 log(v, scan rate) peak1; b=0.61 peak2; b=0.73 peak3; b=0.78 Figure S6. log(i)-log(v) plots for specific peak currents at different sweep rates to interpret the capacity/diffusion behaviors. 267 151 9
Figure S7. In situ XRD patterns during the second charging process within selected scanning angle (2θ) domains of (a) 7-9, (b) 15-22 and (c) 28-45. Figure S8. (a) Ex situ XRD patterns at various cycle ends of charge and discharge depths for Zn/Mg hybrid ion battery with a MgMn 2 O 4 cathode, (b) corresponding electrochemical charge/discharge curves. Figure S9. Ex-situ XPS spectra of (a) Mg 2p and (b) Zn 2p prints of the MMO cathode at different states.. 10
The corresponding Mg 2p finger print at 49.5 ev is varied with respect to charged (considerably decreased from the pristine electrode) and discharged states (located in between the initial and charged states), indicating that the magnesium ions is not fully inserted back, probably due to the co-insertion of zinc ions during discharge-process (Figure S7a). On the other hand, Zn 2p finger print is absent in the pristine sample and existence of slightly low intense Zn 2p peaks in the charged process which cannot be avoided during ex situ analysis (Figure S7b). However, for the insertion electrode (discharge) there are strong peaks at 1022.1 (2p 3/2 ) and 1045.2 (2p 1/2 ) ev further confirming presence of zinc at the end of discharged state in the MMO-400 host. (a) (b) 11
Voltage / V Figure S10. Ex-situ ESM/EDS spectra of Zn anode (a) before and (b) after 3 complete cycles of the electrochemical reaction. 2.0 1.6 1.2 MgMn 2 O 4 0.8 0.4 0.0 LiNi 1/3 Co 1/3 Mn 1/3 O 2 /Zn 40 LiFePO 4 /Zn 41 Li 3 V 2 (PO 4 ) 3 /Zn17 Na 3 V 2 (PO 4 ) 3 /Zn42 V 2 O 5 /Zn 44 NiHCF/Zn 43 Na 2 MnFe(CN) 6 /Zn 45 NaFe-PB/Zn 7 LiMn 0.8 Fe 0.2 PO 4 /Zn 9 100 200 300 400 Specific Energy (Wh kg -1 ) Figure S11. Comparison of specific-energy of some hybrid aqueous batteries with Zn metal as anode. Specific power and specific energy calculation: Specific power (W Kg -1 ): I*V/2m Where I is the applied current (A), V is the cell voltage (V), m is the total active mass at cathode (g). S5 Specific energy (Wh Kg -1 ): specific capacity * voltage (average working voltage). S1 References (S1) Zhao, J.; Li, Y.; Peng, X.; Dong, S.; Ma, J.; Cui, G.; Chen, L. High-Voltage Zn/LiMn 0. 8 Fe 0. 2 PO 4 aqueous Rechargeable Battery by Virtue of water-in-salt electrolyte. Electrochem. commun. 2016, 69, 6 10. (S2) Islam, S.; Alfaruqi, M. H.; Mathew, V.; Song, J.; Kim, S.; Kim, S.; Jo, J.; Baboo, J. P.; Tung, D. P.; Putro, D. Y.; et al. Facile Synthesis and the Exploration of Zinc Storage 12
Mechanism of β-mno 2 Nanorod with Exposed (101) Planes as a Novel Cathode Material for High Performance Eco-Friendly Zinc-Ion Battery. J. Mater. Chem. A 2017, 5, 23299 23309. (S3) Wu, X.; Xiang, Y.; Peng, Q.; Wu, X.; Li, Y.; Tang, F.; Song, R.; Liu, Z.; He, Z.; Wu, X. Green-Low-Cost Rechargeable Aqueous Zinc-Ion Batteries Using Hollow Porous Spinel ZnMn 2 O 4 as the Cathode Material. J. Mater. Chem. A 2017, 5, 17990 17997. (S4) He, P.; Yan, M.; Zhang, G.; Sun, R.; Chen, L.; An, Q.; Mai, L. Layered VS 2 Nanosheet-Based Aqueous Zn Ion Battery Cathode. Adv. Energy Mater. 2017, 7, 1601920. (S5) Sambandam, B.; Soundharrajan, V.; Kim, S.; Alfaruqi, M. H.; Jo, jeonggeun; Kim, S.; Mathew, V.; Sun, Y.-K.; Kim, J. Aqueous Rechargeable Zn-Ion Battery: An Imperishable and High-Energy Zn 2 V 2 O 7 Nanowire Cathode through Intercalation Regulation. J. Mater. Chem. A 2018, 6, 3850 3856. 13