3D Porous Copper Skeleton Supported Zinc Anode towards High

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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3D Porous Copper Skeleton Supported Zinc Anode toward High Capacity and Long Cycle Life Zinc Ion Batteries Zhuang Kang,† Changle Wu,† Liubing Dong,*,†,§ Wenbao Liu,†,∥ Jian Mou,† Jingwen Zhang,† Ziwen Chang,† Baozheng Jiang,†,‡ Guoxiu Wang,*,§ Feiyu Kang,†,∥ and Chengjun Xu*,†

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†

Shenzhen Geim Graphene Center, Graduate School at Shenzhen, and ‡Tsinghua-Berkeley Shenzhen Institute (TBSI), Tsinghua University, Shenzhen 518055, China § Centre for Clean Energy Technology, University of Technology Sydney, Ultimo NSW 2007, Australia ∥ State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: Zinc ion batteries (ZIBs) have attracted extensive attention in recent years, benefiting from their high safety, eco-friendliness, low cost, and high energy density. Although many cathode materials for ZIBs have been developed, the poor stability of zinc anodes caused by uneven deposition/stripping of zinc has inevitably limited the practical application of ZIBs. Herein, we report a highly stable 3D Zn anode prepared by electrodepositing Zn on a chemically etched porous copper skeleton. The inherent excellent electrical conductivity and open structure of the 3D porous copper skeleton ensure the uniform deposition/stripping of Zn. The 3D Zn anode exhibits reduced polarization, stable cycling performance, and almost 100% Coulombic efficiency as well as fast electrochemical kinetics during repeated Zn deposition/stripping processes for 350 h. Furthermore, full cells with a 3D Zn anode, ultrathin MnO2 nanosheet cathode, and Zn2+-containing aqueous electrolyte delivered a record-high capacity of 364 mAh g−1 at a current density of 0.1 A g−1 and good cycling stability with a retained capacity of 173 mAh g−1 after 300 charge/discharge cycles at 0.4 A g−1. This work provides a pathway for developing high-performance ZIBs. KEYWORDS: Zinc anode, Porous copper skeleton, Zinc ion battery, MnO2 nanosheet

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INTRODUCTION Lithium ion batteries (LIBs) currently dominate the rechargeable battery market due to their good cycling stability and high energy density.1 However, insufficient lithium resources, flammable organic electrolytes, and safety issues limit further large-scale application of LIBs.2 In recent years, multivalent ion batteries, such as Mg2+, Zn2+, and Al3+ batteries,3−7 have been studied as alternatives to LIBs for electrochemical energy storage. Rechargeable aqueous zinc ion batteries (ZIBs) use MnO2 with tunnel structures as cathodes and Zn metal as anodes,4 which have become one of the most promising battery systems owing to their high safety, ecofriendliness, low cost, and high energy density. In ZIB battery systems, much attention was focused on seeking for new cathode materials, including manganese oxides and vanadium oxides,8−14 whereas little investigation has been devoted to Zn metal anodes. This is because in currently reported ZIBs the Zn anodes are excessive in mass. The excessive Zn anode mass results in the under-utilization of Zn metal and a very low energy density, which limits the practical © XXXX American Chemical Society

application of ZIBs. In fact, metal anodes in battery systems such as lithium and sodium metal anodes always suffer from the serious problem of unstable deposition (further causing shape deformation/protrusion/dendrite formation), which leads to poor stability and safety risks for these metal electrodes in batteries.15,16 Therefore, many efforts have been made to improve the stability of these metal anodes, like an electrolyte additive,17 carbon-containing composite,18−25 and 3D structured anode.26−31 In ZIBs, the plating/stripping process of Zn (Zn2+) is uneven when the mass of Zn metal anode is not excessive. This will cause the Zn protrusions/ dendrites, electrode pulverization, surface passivation, or hydrogen evolution,32,33 thus giving rise to an abrupt capacity decay or failure of Zn-anode-based ZIB batteries. To develop ZIBs with high capacity and long cycle life, highperformance cathode materials should also be applied to match Received: October 28, 2018 Revised: December 11, 2018 Published: January 7, 2019 A

DOI: 10.1021/acssuschemeng.8b05568 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. (a) Schematic illustration of the procedures to prepare 3D Zn anode. SEM images of (b) porous copper skeleton, after depositing different amounts of Zn(c) 0.5 mAh cm−2, (d) 1 mAh cm−2, and (e) 2 mAh cm−2and then stripping different amounts of Zn(f) 1 mAh cm−2, (g) 1.5 mAh cm−2, and (h) 2 mAh cm−2. (i) SEM image of a 3D Zn anode (with 2 mAh cm−2 of Zn) after repeated deposition/stripping for 15 cycles. The rectangular symbols show the amount of Zn metal in each image; each solid rectangle represents 0.5 mAh cm−2 of Zn. Zn deposition/stripping states (b−i) are indicated in (j) a galvanostatic discharge/charge voltage profile at a current density of 1 mA cm−2 in 2 M ZnSO4 electrolyte.

ance, and almost 100% Coulombic efficiency (CE), as well as fast electrochemical kinetics, during repeated Zn deposition/ stripping processes for 350 h. Furthermore, the 3D Zn anode and ultrathin MnO2 nanosheet cathode were assembled into full cells with Zn2+-contained aqueous electrolyte. The ultrathin MnO2 nanosheets cathode delivers a record-high capacity of 364 mAh g−1, originating from the conversion reaction between MnO2 and MnOOH and the insertion/ extraction of Zn2+ into/from MnO2. Furthermore, the 3D Zn anode|MnO2 nanosheet cathode full cells show a good cycling stability with a retained capacity of 173 mAh g−1 after 300 charge/discharge cycles at 0.4 A g−1.

with stable Zn anodes. Previously several cathode materials were reported for ZIBs, including Mo6S8 (134 mAh g−1), Co3O4 (162 mAh g−1), and the Prussian Blue analogs (about 60 mAh g−1). However, their specific capacity is low.34−36 Although the capacity of vanadium oxides is relatively high as a Zn2+ storage host (e.g., Na0.33V2O5, 367.1 mAh g−1), vanadium oxides (e.g., V2O5, 372 mAh g−1) are toxic and therefore not environmentally benign.37−39 Manganese dioxides, especially MnO2, are abundant, low cost, and have a high zinc storage capacity. They are considered as the most promising cathode materials for ZIBs.1,4 Despite these positive features, the electrochemical performance of MnO2 cathode material is affected by its tunnel structure and micromorphology (for instance, bulk MnO2 generally has poor electrical conductivity and slow electrochemical kinetics).40,41 Meanwhile, the Zn2+ storage mechanism in MnO2 is still debatable. Therefore, synthesis of high-performance MnO2 cathode materials and elucidation of the Zn2+ storage mechanism are critically important. Herein, we developed a new strategy for preparing stable Zn anodes by electrodepositing Zn on chemically etched 3D porous copper skeletons, which are then coupled with highcapacity MnO2 nanosheet cathodes to assemble high-performance ZIB full cells. The inherent excellent electrical conductivity and open structure of the 3D porous copper skeleton ensure the uniform deposition of Zn metal. The 3D porous copper skeleton supported Zn (denoted as “3D Zn”) anode exhibits a reduced polarization, stable cycling perform-

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EXPERIMENTAL SECTION

Preparation of 3D Zn Anode. A copper foil (10 μm in thickness, GoodFellow) was washed using 5% HCl and then deionized water to remove surface impurities. The copper foil was subsequently immersed in 100 mL of 5 wt % NH3·H2O solution for 15 h, followed by washing with deionized water and drying at room temperature to obtain a 3D porous copper skeleton. A galvanostatic electrodeposition method was used to deposit Zn on the 3D porous copper skeleton to prepare 3D Zn anode in a three-electrode system, in which a saturated calomel electrode, stainless steel foil, and the porous copper skeleton served as reference electrode, counter electrode, and working electrode, respectively. Typically, 62.5 g of ZnSO4·7H2O, 62.5 g of Na2SO4, and 10 g of H3BO3 were dissolved in 500 mL of deionized water as electrolyte, and the electrodeposition was conducted under a current of 40 mA cm−2 for 9 min (6 mAh cm−2). Prepared anode was washed with deionized water and finally cut into 1 × 1 cm2 sheets. B


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Figure 2. Electrochemical behaviors of symmetric cells with 2 M ZnSO4 electrolyte. (a) Discharge/charge voltage profiles at 1 mA cm−2 of Zn plating/stripping, and the inset enlarges the voltage profiles in the range of 0.9−1.1 mAh cm−2. (b) CE values vs cycle number. (c) Cycling performance at a constant current of 0.5 mA cm−2 (the amount of Zn deposited in each cycle is 0.5 mAh cm−2). (d) Long-term cycling stability at varying current densities. Synthesis of MnO2 Nanosheets. Ultrathin MnO2 nanosheets were synthesized by a self-reacting microemulsion method. A 22.22 g portion of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) was added into 500 mL of isooctane. Then, 27 mL of 0.1 M KMnO4 aqueous solution was introduced. The mixture was stirred for 30 min, before filtration and sufficient washing using distilled water and ethanol. After drying at 80 °C for 24 h, ultrathin MnO2 nanosheets could be obtained. To prepare MnO2 cathode, MnO2 nanosheets, acetylene black, and polyvinylidene fluoride binder, with a weight ratio of 7:2:1, were homogeneously mixed in N-methylpyrrolidone. The mixture was subsequently coated on a stainless foil and dried in a vacuum oven for 12 h. The mass loading of MnO2 is about 0.9−1.1 mg cm−2. The mass ratio of 3D porous Zn anode and MnO2 nanosheet cathode is about 1.4:1. Characterization. The phase of the samples was analyzed by Xray diffraction (XRD) (Rigaku 2500) with copper Kα radiation. The diffraction patterns were recorded from 10° to 80° with a step size of 0.02°. Micromorphologies of materials and electrodes were observed using scanning electron microscopy (SEM) ( ZEISS SUPRA55) with energy dispersive X-ray spectroscopy and high-resolution transmission electron microscopy (HRTEM) (FEI Tecnai G2 F30). Assembly of Symmetric Cells. CR2032-type coin cells were assembled. Two 3D Zn electrodes or two planar Zn foil electrodes were utilized as electrodes and 2 M ZnSO4 with/without 0.5 M MnSO4 aqueous solution was used as electrolyte. Glass fiber membranes with a diameter of 19 mm served as separators. The cell was sealed in air and left for 8 h before electrochemical tests. Assembly of ZIB Full Cells. CR2032-type coin cells were assembled. 3D Zn anodes and planar Zn foil anodes (1 × 1 cm2) were separately matched with MnO2 nanosheet cathodes. The electrolyte used was 2 M ZnSO4 with/without 0.5 M MnSO4 aqueous solution. Glass fiber membranes with diameter of 19 mm were used as separator. The cells were sealed in air and left for 8 h before electrochemical tests.

Electrochemical Measurements. Deposition/stripping behaviors of the zinc on the 3D porous copper skeletons were investigated in a two-electrode system, in which 3D copper skeleton was used as a work electrode and Zn foil was used as both counter and reference electrodes. Galvanostatic discharge/charge measurements of symmetric cells and full cells were performed on a LAND battery-testing instrument. Cyclic voltammetry (CV) tests were carried out on a BioLogic VMP3 electrochemical station.

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RESULTS AND DISCUSSION The 3D porous copper skeleton was prepared from a planar copper foil via a chemical etching method, as illustrated in Figure 1a. The major chemical reaction during chemical etching can be described by eq 1 2Cu + O2 + 8NH3·H 2O → 2Cu(NH3)4 2 + + 4OH− + 6H 2O

(1)

SEM images in Figures 1b and S1 (Supporting Information, SI) show that the chemically etched copper foil forms a 3D porous structure, which is significantly different from the original planar copper foil with a dense structure. The 3D porous copper has sizes in the range of tens of nanometers to several micrometers with an increased specific surface area of 0.1 m2 g−1 [Figure S2 (SI); this value is increased by ∼400% from that of the original copper foil]. It should be noted that chemical etching does not introduce impurity phases, e.g., copper oxide, based on the XRD analysis in Figure S3 (SI). This ensures the excellent electrical conductivity of the 3D porous copper (the electrical conductivity of copper oxide is lower by several orders of magnitude than that of metallic copper). The 3D porous copper was then used as skeleton for C


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Figure 3. Schematic diagrams of Zn deposition/stripping processes on 3D Zn electrodes and planar Zn foil electrodes.

Figure 4. Electrochemical performance of full cells. (a) Schematic of 3D Zn anode|MnO2 nanosheet cathode full cell. (b) Specific capacities at current density of 0.1 A g−1 with different electrolytes. (c) CV curve at 0.5 mV s−1 in electrolyte containing 2 M ZnSO4 and 0.5 M MnSO4. (d) Cycling performance of full cells with 3D Zn anodes or planar Zn foil anodes at 0.4 A g−1 in electrolyte containing 2 M ZnSO4 and 0.5 M MnSO4.

the electrodeposition of Zn. Overall, the deposited Zn can uniformly coat on the copper skeleton to form a 3D porous

copper skeleton supported Zn sample [i.e., 3D Zn anode; Figures 1c−e and S1c (SI)]. Specifically, we studied the D


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Figure 5. (a) XRD patterns of cathodes at different charge/discharge voltages. (b) XRD pattern, (c) Mn 3s XPS spectra, and (d) HRTEM image of the fully discharged cathode. (e) Schematic of the energy storage mechanism of MnO2 nanosheet cathodes in ZIBs.

the copper skeleton gradually recovers to its original porous structure. After repeated Zn deposition/stripping cycles, the 3D Zn electrode still remains a uniform micro-morphology and intact appearance (Figure 1i), indicating that the 3D porous copper skeleton is robust and elastic enough to tolerate the changing stresses caused by the Zn deposition/stripping. Therefore, a stable 3D Zn anode was obtained by electrodepositing Zn on the chemically etched copper skeleton. The electrochemical performance of the 3D Zn electrode was evaluated in 3D Zn|3D Zn symmetric cells with 2 M ZnSO4 electrolyte. For comparison, the electrochemical behaviors of planar Zn foil were also tested under the same condition. Except for the first cycle, the charge/discharge curves of the 3D Zn electrode-based symmetric cells remain almost identical (Figure 2a), accompanied by a stable and small voltage hysteresis of only ∼40 mV. These suggest the good stability of the 3D Zn electrodes. CV curves also show consistent results, as shown in Figure S6 (SI). The CE of the cells remains at about 100% after the initial several cycles

deposition/stripping behavior of Zn on/off the 3D porous copper skeleton in an electrolyte of 2 mol L−1 (M) ZnSO4 aqueous solution. As Zn is deposited continuously, the pores inside the 3D copper skeleton are gradually filled with Zn (Figure 1b−e). Meanwhile, the obtained 3D Zn anode shows a homogeneous macro-morphology (Figure S4, SI), which confirms the relatively uniform distribution of Zn in the internal spaces of the copper skeleton. By contrast, the electrodeposition of Zn on carbon fiber cloth and some other substrates often results in a less homogeneous macromorphology. Except for the advantage of uniformity, the electrodeposition curves of Zn on different substrates show a very low deposition overpotential on the 3D porous copper skeleton (Figure S5, SI). This could be associated with the excellent electrical conductivity and open structure of the 3D copper skeleton. Importantly, the deposited Zn on the 3D porous copper skeleton is able to reversibly dissolve (Figure 1f−h) and almost completely strip from the copper skeleton when the 3D Zn anode is charged to 0.4 V (Figure 1h,j), while E


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presented in Figures 5a,b and S10 (SI), respectively. When the ZIBs are discharged from 1.9 to 1.5, 1.3, and 1.0 V, new peaks at 2θ of 8°, 21°, 26°, 32°, and 58° emerge in the XRD patterns, and their intensity gradually increases (Figure 5a). These new peaks that appeared during the discharge process indicate a phase change of the MnO2 nanosheet cathode. To further confirm the phase change, XRD patterns of MnO2 nanosheet at a fully discharged state of 1.0 V (Figure 5b) were carefully analyzed. Compared with the original state or the fully charged state (Figure 5a), there are several new diffraction peaks emerging at 29.3°, 33.0°, 38.9°, and 59.0°, which depicts the formation of ZnxMn2O4 (JCPDS #24-1133). This can be attributed to the insertion of Zn2+ into the MnO2 tunnels, accompanied by the valence decrease of positive quadrivalent manganese. EDS mapping of a MnO2 cathode in a fully discharged state also shows that both Mn and S are found with few voids in their distribution, while Zn signal is seen uniformly everywhere (Figure S10c, SI). According to X-ray photoelectron spectroscopy (XPS) for Mn 3s and O 1s spectra of the MnO2 nanosheet at the fully discharged state in Figures 5c and S13 (SI), the average valences of manganese are calculated to be 3.22−3.27.44−46 Besides, MnOOH (JCPDS #74-1049) and Zn4SO4(OH)6·4H2O (JCPDS #44-0673) phases are also detected (Figure 5b,d). This is inconsistent with the results reported by Liu’s group.47 The presence of MnOOH and Zn4SO4(OH)6·4H2O suggests that the MnO2 nanosheets also undergo another conversion reaction. MnO2 reacts with a proton (H+) from water (or a hydronium ion H3O+) to form MnOOH. Meanwhile, the subsequent OH− reacts with Zn2+, SO42−, and H2O in the aqueous electrolyte to form large flakelike Zn4SO4(OH)6·4H2O (Figure S10d, SI). Furthermore, when the MnO2 nanosheet cathode is fully charged to 1.9 V, the XRD pattern is highly consistent with the pattern of the original state (Figure 5a). Therefore, the high cathode capacity of MnO2 nanosheets originates from the conversion reaction between MnO2 and MnOOH and the insertion/extraction of Zn2+ in MnO2 tunnels. Those can be described by eqs 2−4 and illustrated by the scheme in Figure 5e.

(Figure 2b, 99.53% after 66 cycles), confirming the excellent reversibility of Zn deposition/stripping on/from the 3D porous copper skeleton. The accurate Coulombic efficiency of Zn deposition/stripping is shown in Table S1 (SI). Longterm cycling behaviors of symmetric cells with 3D Zn electrodes and planar Zn foil electrodes are presented in Figure 2c,d. At a current density of 0.5 mA cm−2, the cell with the 3D Zn electrode can be repeatedly and stably charged/ discharged for up to 350 h, while the cell with a planar Zn foil electrode exhibits larger voltage hysteresis (about 100 vs 40 mV for 3D Zn electrode constructed cell) as well as severe fluctuations in cycling curves and finally failure within 110 h. Similarly, under different current densities of 0.1−1 mA cm−2, the 3D Zn electrode-based symmetric cells maintain a stable operating status for 350 h, whereas short circuit damage to the planar Zn foil electrode-based cells occurs at about 120 h (Figure 2d). The results of raising current densities and areal capacities are shown in the Supporting Information (Figures S14−S16). SEM observations display that after long-term cycles, the 3D Zn electrodes remain intact, while the planar Zn foil electrodes become defective in appearance (Figure S7, SI). Furthermore, large Zn protrusions/dendrites and partial oxidation are observed for planar Zn foil electrode. Obviously, these suggest that the Zn deposition/stripping process is not stable on planar Zn foils. This is because the planar Zn foils simultaneously serve as current collector and active material in the cell, so their volume and electrical properties are changing during the Zn deposition/stripping on/off the foil surface, thus resulting in unstable electrochemical behavior (as graphically depicted in Figure 3). In contrast, for 3D Zn electrodes, the 3D copper skeleton always maintains high electrical conductivity and their internal pores can effectively accommodate the volume change caused by deposition/stripping of Zn. Consequently, the cycling performance of the 3D Zn electrodes is significantly improved. To produce high-performance ZIB full cells for practical application, high-capacity ultrathin MnO2 nanosheets were synthesized via a self-reacting microemulsion method and employed as cathode material to couple with the aforementioned stable 3D Zn anodes (as illustrated in Figure 4a). The ultrathin MnO2 nanosheets possess a thickness of ∼2 nm and a width of ∼6 nm (Figure S8, SI). Such nanoscale size can shorten the transmission distance for electrons and enable the electrolyte ions to insert/extract more easily in MnO2 tunnels.42,43 Figure 4b−d show the electrochemical properties of ZIB full cells. In the 2 M ZnSO4 aqueous electrolyte solution (Figure 4b), the full cells deliver a capacity of 234 mAh g−1 (for a better comparison with previous reports, the capacity is calculated on the basis of the mass of MnO2 cathode) at a current density of 0.1 A g−1. This is higher than the capacity of the cell with MnO2 cathode and planar Zn foil anode. From the CV curves in Figure S9 (SI), we can see that the potential gap between oxidation peak and reduction peak of 3D Zn anode based cells is smaller than that of planar Zn foil anode based cells (250 vs 295 mV), which indicates a better reversibility of 3D Zn anodes. It should also be pointed out that the copper skeleton inside 3D Zn anode does not participate in the electrochemical reactions. The energy storage mechanism of the ultrathin MnO2 nanosheet cathodes was also investigated. The ZIBs using MnO2 nanosheet cathodes were first charged to 1.9 V and then discharged to 1.0 V. XRD patterns and micromorphologies of MnO2 cathode at different charge and discharge voltages are

MnO2 + H+ + e− → MnOOH

(2)

MnO2 + x Zn 2 + + 2x e− → ZnxMnO2

(3)

4Zn 2 + + SO4 2 − + 6OH− + 4H 2O → Zn4SO4 (OH)6 · 4H 2O (byproduct)

(4)

Due to manganese dissolution, electrochemical performance of MnO2 itself is not satisfactory in pure ZnSO4 electrolyte. The addition of Mn2+ in ZnSO4 electrolyte is capable of significantly optimizing the electrochemical properties of MnO2 cathode in ZIBs,48 thus a mixed solution of 2 M ZnSO4 and 0.5 M MnSO4 was chosen as the electrolyte. By using the mixed electrolyte, the MnO2 cathode|3D Zn anode cells operating in the voltage range of 1.0−1.9 V achieved a record-high capacity of 364 mAh g−1 (Figure 4b,c). The full cells using 3D Zn anodes exhibited a superior cycling stability (Figure 4d), and their CE value maintained at about 100% over 300 charge/discharge cycles. By contrast, the cells using planar Zn foil anodes show rapid capacity decay with increasing charge/discharge cycles, accompanying severely fluctuating capacity and CE values. A conclusion can be easily made by comparing the above two cells where the 3D Zn F


ACS Sustainable Chemistry & Engineering anodes possess much better stability than the planar Zn foils, even in the mixed electrolyte of 2 M ZnSO4 and 0.5 M MnSO4. To confirm the good stability of the 3D Zn electrodes in the mixed ZnSO4/MnSO4 electrolyte, the electrochemical behaviors were evaluated in both a three-electrode system and symmetric 3D Zn|3D Zn cells. As exhibited in Figures S11 and S12 (SI), the electrochemical performance of the 3D Zn electrodes in the mixed electrolyte is very similar to that in the pure ZnSO4 electrolyte (Figures 1 and 2). The Zn deposition/ stripping can reversibly and stably occur on the 3D porous copper skeleton, accompanying a relatively small voltage hysteresis and almost 100% CE values during repeated Zn deposition/stripping processes. Cycle life test results shown in Figure S12c clearly display the excellent stability of the 3D Zn electrode, compared with the planar Zn foil electrode (which can work only for ∼110 h).

ACKNOWLEDGMENTS

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REFERENCES

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CONCLUSION In summary, 3D porous copper skeleton supported Zn anodes were fabricated that endow reversible deposition/stripping of Zn. The 3D Zn anodes exhibit reduced polarization, stable cycling performance, and almost 100% CE, as well as fast electrochemical kinetics, during repeated Zn deposition/ stripping processes for 350 h. Meanwhile, ultrathin MnO2 nanosheets were also synthesized through a self-reacting microemulsion method and achieved a high capacity of 364 mAh g−1 as cathode materials in ZIBs. Such a high capacity originates from the conversion reaction between MnO2 and MnOOH and the insertion/extraction of Zn2+ in MnO2. The assembled ZIB full cells based on the 3D Zn anodes and the ultrathin MnO2 nanosheet cathodes show high capacity and good cycling performance. This work provides a stable 3D Zn anode and high-capacity ultrathin MnO2 nanosheets cathode for developing high-performance ZIBs. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b05568. Electrochemical peformance of symmetric cells when the current density is raised, electrochemical peformance of cells with electrolyte containing 2 M ZnSO4 and 0.5 M MnSO4, various experimental details, and other additional data (Figures S1−S16 and Tables S1 and S2) (PDF)

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The authors appreciate the financial support from Shenzhen Technical Plan Project (No. JCYJ20160301154114273), National Key Basic Research (973) Program of China (No. 2014CB932400), International Science & Technology Cooperation Program of China (No. 2016YFE0102200), and Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01N111). We also acknowledge the financial support of the Australian Research Council through the ARC Discovery projects (DP160104340 and DP170100436).

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AUTHOR INFORMATION

Corresponding Authors

*L.D. e-mail: [email protected]. *G.W. e-mail: [email protected]. *C.X. e-mail: [email protected]. ORCID

Liubing Dong: 0000-0002-1787-8595 Guoxiu Wang: 0000-0003-4295-8578 Chengjun Xu: 0000-0002-7775-6311 Notes

The authors declare no competing financial interest. G


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