Performance Enhancement and Mechanistic Studies of Magnesium−Sulfur Cells with an Advanced Cathode Structure Xingwen Yu and Arumugam Manthiram* Electrochemical Energy Laboratory & Materials Science and Engineering Program, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *
ABSTRACT: Magnesium−sulfur cells based on abundant, safe Mg and S are demonstrated with a cathode containing a preactivated carbon nanofiber (CNF) electrode matrix filled with sulfur active material and a CNF-coated separator. The CNF coating on the separator serves as a polysulfide trapper and an upper current collector for facilitating high sulfur utilization and enhancing the cycle life. Mechanisms regarding the performance enhancement and the charge−discharge processes of the Mg−S cells are investigated with spectroscopic, microscopic, and electrochemical analyses.
high theoretical capacity of the sulfur cathode (1675 mAh g−1) and the high theoretical capacity of magnesium anode (2230 mAh g−1), a magnesium−sulfur (Mg−S) cell is expected to provide a theoretical energy density >1600 Wh kg−1. The Mg− S cell was first reported in 2011.36 One of the key factors for the operation of the Mg−S cells is the discovery of proper electrolytes.36−41 With a non-nucleophilic electrolyte synthesized from the reaction between hexamethyldisilazide magnesium chloride and aluminum trichloride, the electrochemical conversion of sulfur to magnesium sulfide was first successfully demonstrated by Kim et al.36 This type of non-nucleophilic electrolyte was then modified by Zhao-Karger et al. to improve the performance of the Mg−S cells.37 Recently, two relatively simple electrolytes have been reported.39,40 A “Li+-ion mediation” approach was also proposed, and it successfully improved the reversibility of the Mg−S cells.38 Progress from the above studies shed light on the development of rechargeable Mg−S batteries. However, at the current stage, the Mg−S cells still suffer from a quick capacity decline after the first cycle. In light of the above research with the non-nucleophilic Mgelectrolyte and our previous experience with the Li−S (and RT Na−S) battery systems, herein we present an approach to enhance the performance of Mg−S cells with an advanced
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agnesium is an earth-abundant element. Owing to a naturally low formula weight and being able to provide a two-electron charge transfer upon oxidation, magnesium has been considered as an attractive anode material for electrochemical energy-storage systems. Primary magnesium batteries have been practically used for military applications since the early 20th century.1−3 Secondary magnesium batteries are currently being pursued as a promising candidate for replacing Li+-ion batteries.4−6 Generally, Li-based batteries require an intercalation cathode and anode because of the safety concerns of Li metal. The magnesium cells offer a significant advantage of using a solid magnesium metal anode, providing a higher energy density than the Li-intercalation anodes. In addition, magnesium does not form metal dendrites during electrodeposition (charging), which is a significant advantage over the lithium metal anode.7−9 Since the first rechargeable magnesium cell was reported in 2000,10 much progress has been made with respect to the anodes,11−13 electrolytes,14−16 and especially the cathode materials, including the chevrel phase Mo6S8,17−19 metal sulfides,20−22 manganese oxides,23−25 molybdenum oxides,26 vanadium oxides,27,28 and polyanion-based materials.29−31 However, the specific capacity of the above insertion-type cathodes is usually low, which limits the overall energy density of the magnesium batteries. With the rapid progress of lithium−sulfur batteries,32−34 it is natural to couple the high-capacity sulfur cathode with other alkali metal or alkali earth metal anodes. Recently, an increasing number of reports on room-temperature sodium−sulfur (RT Na−S) batteries have appeared in the literature.35 Based on the © XXXX American Chemical Society
Received: June 18, 2016 Accepted: July 27, 2016
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DOI: 10.1021/acsenergylett.6b00213 ACS Energy Lett. 2016, 1, 431−437
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Letter
ACS Energy Letters
with a thin layer (∼0.2 mg cm−2) of activated carbon nanofiber, which was fabricated through a vacuum-filtration process as schematized in Figure 1b and as described in the Supporting Information. The resulting coated separator is pictured in Figure S2. During cell assembly, the coated side of the separator faced the sulfur cathode. Figure 1c shows the scanning electron microscopy (SEM) image of the CNF coating on the separator. The diameter of an individual CNF fiber is ∼100−200 nm (Figure 1c). The resulting CNF coating exhibits micrometer-scale interspaces between the single fibers. Both the CNF cathode matrix and the CNF coating on the separator were preactivated to enhance the surface area and to obtain the desired porous structure with the material. The activation process has been presented in our previous reports42,43 and is briefly described in the Supporting Information. In this study, the cells prepared with the CNFcoated separator were termed Mg∥CNF-GF∥S. The control cells prepared with the uncoated separator were termed Mg∥GF∥S. GF refers to glass fiber separator. The electrochemical performances of the Mg−S cells were characterized with a coin-cell configuration. Figure 2a compares the discharge−charge profiles of the Mg∥CNF-GF∥S and Mg∥GF∥S cells for a few cycles, from which the benefit of the CNF-coated separator can be evidently seen. At an identical cycling rate (C/50), both the cells delivered a similar first-cycle discharge capacity. (It took almost the same time-duration for the two cells to complete their first discharge.) However, after the first cycle, a significant shrinkage in the charge and discharge times was observed for the Mg∥GF∥S cell (Figure 2a top), while the Mg∥CNF-GF∥S cell maintained relatively constant charge−discharge time-durations in the subsequent cycles (Figure 2a bottom). Rate capability of the Mg∥CNFGF∥S cell is presented in Figure 2b with the voltage profiles obtained at different cycling rates (the third-cycle profiles are displayed here as representatives). At a relatively low rate (C/ 50), the discharge profile of the Mg∥CNF-GF∥S cell exhibits
cathode structure. The employed cathode comprises a preactivated carbon nanofiber (CNF) electrode matrix filled with sulfur active material. A thin layer of CNF film was deposited on the separator, acting as an upper current collector to enhance the utilization of sulfur and improve the cyclability of the cell. In addition, the charge−discharge processes of the Mg−S cells are investigated with spectroscopic and electrochemical analyses. Figure 1a shows the schematic of the cell configuration of the Mg−S cells in this study. The anode of the cell was prepared
Figure 1. (a) Schematic of a Mg−S cell with an activated-CNFcoated separator. (b) Schematic of the formation of the CNFcoated separator. (c) Scanning electron microscopy image of the CNF coating (inset is a magnified image).
with a piece of magnesium foil cut into a disc shape. The nonnucleophilic magnesium electrolyte was synthesized according to the method reported elsewhere37,41 and as described in the Experimental Section in the Supporting Information. The cathode matrix was a piece of preactivated carbon nanofiber (CNF) paper (with a diameter of 1.2 cm, an area of 1.13 cm2, and an areal weight of ∼1.0 mg cm−2) prepared as presented in our previous studies for the Li−S and Na−S battery systems42,43 and as briefly described in the Supporting Information. The cathode was prepared by spreading 45 μL of a 0.025 mg μL−1 sulfur (dispersed in a tetraglyme solvent) onto the activated CNF paper electrode, as schematized in Figure S1. The separator was a piece of glass fiber (GF) coated
Figure 2. (a) Voltage versus time profiles of Mg∥CNF-GF∥S and Mg∥GF∥S cells at C/50 rate. (b) Discharge−charge profiles of the Mg∥CNFGF∥S cells at various C-rates. (c) Cyclic voltammograms of the Mg∥CNF-GF∥S cell at a scan rate of 0.05 mV s−1. (d) Discharge capacities and (e) discharge/charge efficiencies as a function of cycle number of the Mg∥GF∥S and the Mg∥CNF-GF∥S cells at different C-rates. (f) Electrochemical impedance spectroscopy (EIS, Nyquist plot) of the Mg∥CNF-GF∥S cell after different cycles. 432
DOI: 10.1021/acsenergylett.6b00213 ACS Energy Lett. 2016, 1, 431−437
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ACS Energy Letters two voltage plateaus at ∼1.55 and 0.85 V. However, at the cycling rates of C/20 and C/10, the second voltage plateau was not very obvious possibly because of the low conductivity of the deep discharge products. Cyclic voltammetry (CV) profiles of the Mg∥CNF-GF∥S cell are presented in Figure 2c. The cell was first scanned from the open-circuit voltage (OCV ∼ 2.1 V) to 0.2 V in a negative direction (black line in Figure 2c). Then a sequence of cyclic scans were performed between 0.2 and 3.2 V (the first, second, and the fifth cycles are shown here as representatives). The cathodic scan shows a wide current wave between 1.8 and 0.6 V, which covers the voltage values (1.55 and 0.85 V) of the two plateaus in the discharge curve (Figure 2b). The anodic current increases almost linearly with the voltage between 2.0 and 2.8 V and then reaches a peak value at ∼2.8 V. The above features of the CV are consistent with the charge−discharge profiles shown in Figure 2b. Also, as seen in Figure 2c, the current responses for the initial half (black line in Figure 2c) and the first cycle are relatively lower, indicating a prolonged preconditioning of the cell during the first cycle. Upon a full cycle, the CVs of the cell become steady in the subsequent cycles. Panels d and e of Figure 2 present the discharge capacities and discharge/charge efficiencies (ratio of discharge capacity to charge capacity), respectively, as a function of cycle number for the Mg−S cells (with the uncoated or the CNF-coated separator). Consistent with the results reflected from Figure 2a, the Mg∥CNF-GF∥S cell and the Mg∥GF∥S cell yield a similar first discharge capacity. After the first cycle, the discharge capacity of the Mg∥GF∥S cell declines sharply. There is also a slow capacity fade with the Mg∥CNF-GF∥S cell, but the fade rate is much lower in contrast to the cell with the uncoated separator. Throughout the 20 cycles (after the first discharge), the specific capacity of the Mg∥CNF-GF∥S cell is significantly higher than the cell with the uncoated separator. In addition, as seen in Figure 2e, the discharge/charge efficiency of the Mg−S cell with the coated separator is relatively higher and more stable than that for the cell with the uncoated separator. It is also noticed that the cells with the coated separator show a relatively lower first discharge capacity compared to those in the subsequent few cycles. This is consistent with the CV results shown in Figure 2c in which the current loop of the first cycle is relatively smaller than those of the subsequent cycles. This phenomenon is assumed to be due to the relatively slow preconditioning of the cell as reflected from the impedance measurement results shown in Figure 2f. The as-prepared fresh cell shows relatively larger impedance. The decrease in the impedance of the cell is seen after the first cycle. Then the impedance of the cell becomes relatively stable. It should be noted that the discharge/ charge efficiency of the Mg−S cells (Figure 2e) is relatively lower than those for the Li−S and Na−S systems.34,35 Also as seen in Figure 2c, both the anodic and cathodic scans of the second and fifth cycles exhibit increased charges in comparison to that of the first scan. However, the increase in the anodic scans is relatively more obvious. These phenomena could be due to some undesired side reactions during the charge process, which are not so clear at the moment and will be investigated further in the future. The charge−discharge processes of the Li−S and the RT Na−S batteries have been relatively well understood.32−35 To mechanistically understand the electrochemical processes of the Mg−S cells, ultraviolet−visible spectroscopy (UV−vis) and Xray photoelectron spectroscopy (XPS) experiments were
performed to analyze the charge/discharge and intermediate products of the sulfur cathodes. Figure 3 presents the UV−vis analysis results of the sulfur cathodes from the Mg−S cells that have been discharged or
Figure 3. (a) Illustration of the sample preparations for ultraviolet− visible spectroscopy (UV−vis) analyses with a typical discharge− charge profile of a Mg∥CNF-GF∥S cell. The samples were collected from the S/CNF cathodes taken from the cells, which were terminated upon discharge or charge to various states as indicated with the red ovals in the figure. (b) UV−vis absorption spectra of the fresh sulfur electrode and the electrodes discharged or charged to various states in the Mg∥CNF-GF∥S cell.
charged to various status (as specified with the red ovals in Figure 3a). The liquid samples were collected by immersing the discharged or charged cathodes and the separators in a tetraglyme solvent for 48 h. According to the literature,37,44 the absorbance at a wavelength of
