Highly Reversible Zinc-Ion Intercalation into ... - ACS Publications

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May 16, 2016 - Nanocubes and Applications for Advanced Zinc-Ion Batteries ... Mo6S8 can host Zn2+ ions reversibly in both aqueous and nonaqueous.
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Highly Reversible Zinc-Ion Intercalation into Chevrel Phase Mo6S8 Nanocubes and Applications for Advanced Zinc-Ion Batteries Yingwen Cheng,† Langli Luo,‡ Li Zhong,‡,§ Junzheng Chen,† Bin Li,† Wei Wang,† Scott X. Mao,§ Chongmin Wang,‡ Vincent L. Sprenkle,† Guosheng Li,*,† and Jun Liu*,† †

Energy Processes and Materials Division, Energy and Environment Directorate, and ‡Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99354, United States § Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States S Supporting Information *

ABSTRACT: This work describes the synthesis of Chevrel phase Mo6S8 nanocubes and its application as the anode material for rechargeable Zn-ion batteries. Mo6S8 can host Zn2+ ions reversibly in both aqueous and nonaqueous electrolytes with specific capacities around 90 mAh/g, and exhibited remarkable intercalation kinetics and cyclic stability. In addition, we assembled full cells by integrating Mo6S8 anodes with zinc-polyiodide (I−/I3−)−based catholytes, and demonstrated that such full cells were also able to deliver outstanding rate performance and cyclic stability. This first demonstration of a zinc-intercalating anode could inspire the design of advanced Zn-ion batteries.

KEYWORDS: Chevrel, Mo6S8 nanocubes, Zn battery, Zn2+ intercalation, nondendrite anode, stationary energy storage, polyiodide catholytes

A

standard electrode potential of Zn2+ ion deposition in aqueous electrolytes (E° = −0.733 V vs standard hydrogen electrode (SHE)) is more negative than that for hydrogen evolution, and this could bring challenges of electrolyte stability in the aqueous phase.16 The use of advanced electrolytes and novel electrolyte additives has been shown to be beneficial to minimize growth of dendrites and improve Coulombic efficiency to a certain extent, but substantial further improvements are still required.17,18 These challenges associated with zinc metal suggest that it is also necessary to develop alternative anode materials in order to exploit the advantages of the cathode materials discussed above, but such an approach has rarely been pursued for Zn2+ thus far. Carbon materials are conventionally used as anode materials for batteries based on monovalent cations (Li+, Na+, and K+ ions) but they fail to show good activity for housing multivalent cations.19,20 Anode materials that host cations through a chemical conversion reaction have shown some promise for divalent ions such as Mg ions recently, but it is not clear whether they are applicable with Zn2+ ions.21−24 Therefore, critical knowledge is still missing for designing anode materials suitable for hosing Zn2+ ions and applying to rechargeable device.

dvanced rechargeable batteries are required for many practical applications ranging from transportation and grid energy storage to portable electronics.1,2 In recent years, the research on post-lithium-ion battery (LIB) techniques has been vigorous because of the inherent safety and cost challenges associated with LIBs.3,4 In this regard, there is considerable interest in energy storage techniques using multivalent metal elements, such as Mg2+ and Al3+.5,6 The use of Zn2+ ions for the energy carrier is attractive because they can be used in a wide variety of electrolytes and can deliver reasonably high specific energy when paired with suitable electrode materials, particularly in aqueous electrolytes.7 In fact, they have been used extensively in several types of commercial primary batteries, such as alkaline batteries (Zn-MnO2). Furthermore, Zn2+ ions also have the advantage of being able to react with various types of cathode materials with excellent reversibility and capacities. Some typical examples of cathode materials are MnO2,8,9 Prussian blue compounds,10,11 and oxygen (for zinc− air batteries).12−14 The use of zinc metal as the anode material has many advantages, such as low cost, natural abundance, and high energy density both gravimetrically (820 mAh/g) and volumetrically (5855 mAh/cm3). Its application in practical energy storage devices, however, faces substantial challenges associated with safety and reliability, largely because of formation of Zn dendritic structures upon charging.15,16 In addition, the © 2016 American Chemical Society

Received: March 16, 2016 Accepted: May 16, 2016 Published: May 16, 2016 13673

DOI: 10.1021/acsami.6b03197 ACS Appl. Mater. Interfaces 2016, 8, 13673−13677

Letter

ACS Applied Materials & Interfaces

Figure 1. (a, b) Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) of Chevrel phase nanocubes; scale bars are 500 nm. (c) STEM-HAADF image of a single cubic Mo6S8 particle. (d) Atomic scale STEM-HAADF image of the boxed area from (c) shows large spaces between Chevrel clusters. (e) Schematic crystal structure of Mo6S8 phase and corresponding STEM image (purple balls are Mo atoms; yellow balls are S atoms, which are not shown on the HAADF image).

Figure 2. CV of Mo6S8 nanocubes for intercalating Zn2+ ions (a) in 1.0 M ZnSO4 in water and (b) in 1.0 M Zn(ClO4)2 in acetonitrile. The scan rates were 0.05 mV/s.

The Chevrel phase compounds are molybdenum chalcogenides with the general formula MxMo6T8 (M = metal; T = S, Se, or Te). They have a unique crystal structure consisting of three-dimensional arrays of Mo6T8 units that form tridirectional channels consisting of metal ions, and each unit consists of a distorted Mo6 octahedron surrounded by a cubic unit (T8) of chalcogenide atoms. Chevrel phase compounds have outstanding capability to host various cations with remarkable ionic and electronic transparent kinetics,24 and could have good potential for Zn2+ ion intercalation.23 In this study, we synthesized Mo6S8 nanocubes using a solution-based method that has been outlined in our previous work.25

The synthesis was processed through an intermediate compound of Cu2Mo6S8, and Mo6S8 was obtained through acid leaching of Cu2+ ions (see the Supporting Information for details). Control of the size and shape of the nanocubes was achieved by using defect sites on graphene sheets to manipulate the heterogeneous nucleation and growth of particles, and overall this method yielded particles with well-defined cubic shape and sizes ranging from 20 to 150 nm, with an average size of ∼100 nm, as shown in Figure 1a, b. The scanning transmission electron microscopy high-angle annular darkfield (STEM-HAADF) image in Figure 1c shows the particles were well-faceted with {100} surfaces and cornered by {010} surfaces. The atomic structure of Mo6S8 is resolved in Figure 13674

DOI: 10.1021/acsami.6b03197 ACS Appl. Mater. Interfaces 2016, 8, 13673−13677

Letter

ACS Applied Materials & Interfaces

Figure 3. (a) XRD of Mo6S8 electrode before (pristine) and after Zn2+ intercalation (discharged). (b) Rate capability of Mo6S8 for Zn2+ intercalation in1.0 M ZnSO4 aqueous electrolytes. (c) Cyclic stability in 1.0 M ZnSO4 aqueous electrolytes.

Figure 4. (a) Cyclic voltammetry of Mo6S8 electrodes in an electrolyte consisting of a mixed solution of 1.0 M ZnI2 and 0.2 M I2. The scan rate was 0.05 mV/s, and the reference and counter electrodes were two pieces of metallic zinc foil, respectively. The highlighted areas are electrochemical reactions corresponding to anode and cathode reactions in a full cell. (b) Photograph of the full cell; a cation exchange membrane (CEM) was used to separate cathode electrolyte from anode electrolyte. (c) Voltage profiles for different C-rates. (d) Cyclic stability of the cell at the rate of 600 mA/g for 350 cycles.

istics that are generally the same, and both had two sets of redox peaks (two anodic peaks at 0.68 and 0.37 V, and two cathodic peaks at 0.50 and 0.34 V). In addition, by comparing the first and second intercalation cycles, some irreversible capacities were observed for second pair peak appeared at higher voltage, which is similar to observations with Mg2+ and Al3+ cations (Figure S2).26,27 This is presumably due to cation trapping in the lattice of Mo6S8, which is an inherent property of this material that can be improved through reducing the particle size, increasing the operating temperature, or substitution of S by Se or Te.28 The electrochemical results suggest that the intercalation of Zn2+ with Mo6S8 should proceed through a multistep process that corresponds to intercalation into different sites in Mo6S8; it can be best understood as the electrochemical reactions outlined below and has a theoretical capacity of 129 mAh/g

1d, e, i.e., the motif (Chevrel cluster) is composed of 6 Mo atoms (the white dots) and 8 S atoms (not seen in HAADF images), and the large space (∼6.5 Å) between the clusters provides ideal hosting sites for Zn2+ ions. The small size and good crystallinity are both beneficial for intercalating ions as opposed to microparticles. The X-ray diffraction patterns suggest the as-synthesized Cu2Mo6S8 and the Mo6S8 powders both had good crystallinity and purity, and no impurity phases were evident (Figures S1). The electrochemical Zn2+ intercalation in Mo6S8 nanocubes was first analyzed using cyclic voltammetry (CV) at room temperature, and examined in both aqueous (1.0 M ZnSO4 in water) and nonaqueous electrolytes (1.0 M Zn(ClO4)2 in acetonitrile). Two pieces of Zn foil were used as the reference and the counter electrode. The scan rate was 0.05 mV/s and the voltage range was 0.25−1.0 V vs Zn. CV profiles (Figure 2) in these two electrolytes exhibited electrochemical character13675

DOI: 10.1021/acsami.6b03197 ACS Appl. Mater. Interfaces 2016, 8, 13673−13677

Letter

ACS Applied Materials & Interfaces Zn 2 + + Mo6S8 + 2e− → ZnMo6S8

The H-cell was then tested with charge−discharge tests at different rates. Figure 4c shows the charge−discharge profiles at different rates for the full cell. The cell had outstanding rate capacity as expected. The capacity was determined based on the weight of anode materials, and the cell capacity was ∼55 mAh/ g for all the current ranges studied. As the current increased (Figure 4c), the cell exhibited increased polarization and lower discharge voltage, and this is believed to mainly originate from the inherent limitation of the H-cell, which exhibits a long ionpermeation path length. Further improvements with optimized cell design could reduce the polarization substantially. In addition to the remarkable rate capability, the full cell also had excellent cyclic stability (Figure S5), and the capacity retention for 350 charge−discharge cycles conducted at 600 mA/g was more than 90% (Figure 4d). These results highlight the stability of the full cell and outstanding potential for applying Mo6S8 anodes. In conclusion, we demonstrate that the Chevrel phase compound Mo6S8 has remarkable activity for reversible intercalation of Zn2+ ions, and its application as an anode material for Zn-ion batteries. They were able to deliver a reversible capacity of 85 mAh/g, and exhibited outstanding rate capacities and cyclic stability. Prototype full cells integrated with a zinc polyiodide catholyte were also assembled and were able to deliver similar capacities. This success could inspire further research on Zn-ion battery technology to mitigate zinc dendrite formation in practical Zn-based energy storage applications.

Zn 2 + + ZnMo6S8 + 2e− → Zn2Mo6S8

The successful intercalation of Zn2+ into Mo6S8 was also verified with X-ray diffraction (XRD). The comparison of diffraction patterns before and after discharge shown in Figure 3a suggests formation of Zn2Mo6S8 phases. The Mo6S8 electrodes were further assembled as coin cells (Zn−Mo6S8 half-cell) with discs of Zn foil as the anodes, and were tested for rate capability and cyclic stability. Figure 3b shows typical charge−discharge profiles at different current densities in the aqueous electrolyte. The discharge curves exhibited two distinct plateaus that are consistent with the two set of redox peaks observed in the CV tests. The specific capacity of the Zn2+ ion intercalation for the first cycle was 120 mAh/g (Figure S3), which is close to the theoretical capacity of 129 mAh/g as determined from formation of Zn2Mo6S8. For the subsequent cycles, the capacity stabilized at 83 mAh/g due to the partial cation trapping discussed above. Nevertheless, the Mo6S8 compound showed outstanding rate capacity and had remarkable capacity retention as the current density was increased, particularly for the set of plateaus at low voltage. Overall, the electrodes were able to deliver typical specific capacities of 79 and 63 mAh/g at the current densities of 45 and 180 mA/g, respectively. In addition, the cyclic stability was also evaluated at a current density of 180 mA/g. The cells exhibited outstanding cyclic stability, and were able to maintain a capacity of ∼60 mAh/g after 150 charge−discharge cycles (Figure 3c). In addition to aqueous electrolytes, outstanding rate capability and cyclic stability was also observed with the organic electrolyte of 1.0 M Zn(ClO4)2 in acetonitrile (Figure S3). These results demonstrated remarkable properties of Mo6S8 for reversibly hosting Zn2+ ions. In addition to Zn−Mo6S8 half cells, we further assembled full cells by integrating Mo6S8 electrodes with cathodes based on zinc polyiodide catholytes. The zinc polyiodide solution consists of a highly soluble iodide/triiodide redox couple and can enable high-energy-density redox flow batteries that have been recently demonstrated.29 The I−/I3− couple has a standard potential of 0.536 V vs SHE (1.299 V vs Zn) and has great potential for practical application due to its high solubility and high electrochemical potential.30,31 The viability of this catholyte with the Mo6S8 anode was verified with CV tests in 0.2 M ZnI2 electrolyte and using Mo6S8 as the working electrode. The CV profile (Figure 4a) clearly shows two sets of redox peaks that correspond to electrochemical reactions: Zn2+ intercalation (highlighted in red) and I−/I3− redox reactions (highlighted in blue). On the basis of these results, the full cell was assembled as static H-cells using a cation exchange membrane (CEM) due to the unique electrochemical properties of the catholyte (a photograph is shown in Figure 4b). The catholyte used here was 1.5 M ZnI2 and 0.2 M I2 dissolved in water, and the electrolyte used for the anode side was 1.1 M ZnSO4 aqueous solution. In addition, the anode reaction involved in such full cells was the charge−discharge plateaus at ∼0.35 V that correspond to Zn2+ + ZnMo6S8 + 2e− ←→ Zn2Mo6S8, Figure 4a. It should be noted that the Chevrel phase has particularly good rate capability in this voltage range, as shown in Figure S4, and has capacity retention of more than 85% as the current density was increased by a factor of 10 (62 and 53 mAh/g at 60 and 600 mA/g, respectively).



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b03197. Detailed experimental methods and additional figures(PDF)



AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy (DOE) Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, under Award KC020105FWP12152. The electron microscope characterizations were conducted at the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle for the DOE under Contract DEAC05-76RL01830.



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DOI: 10.1021/acsami.6b03197 ACS Appl. Mater. Interfaces 2016, 8, 13673−13677

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DOI: 10.1021/acsami.6b03197 ACS Appl. Mater. Interfaces 2016, 8, 13673−13677