Letter pubs.acs.org/NanoLett
The Electrochemistry with Lithium versus Sodium of Selenium Confined To Slit Micropores in Carbon Sen Xin,†,§ Le Yu,‡ Ya You,‡ Huai-Ping Cong,† Ya-Xia Yin,‡ Xue-Li Du,† Yu-Guo Guo,*,‡ Shu-Hong Yu,*,†,∥ Yi Cui,*,⊥,# and John B. Goodenough*,§ †
School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, People’s Republic of China Institute of Chemistry, Chinese Academy of Sciences (CAS), and Beijing National Laboratory for Molecular Sciences (BNLMS), Beijing 100190, People’s Republic of China § Materials Science and Engineering Program and Texas Materials Institute, The University of Texas at Austin, 1 University Station, C2201, Austin, Texas 78712, United States ∥ Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China ⊥ Department of Materials Science and Engineering and #Department of Chemistry, Stanford University, Stanford, California 94305, United States ‡
S Supporting Information *
ABSTRACT: Substitution of selenium for sulfur in the cathode of a rechargeable battery containing Sx molecules in microporous slits in carbon allows a better characterization of the electrochemical reactions that occur. Paired with a metallic lithium anode, the Sex chains are converted to Li2Se in a single-step reaction. With a sodium anode, a sequential chemical reaction is characterized by a continuous chain shortening of Sex upon initial discharge before completing the reduction to Na2Se; on charge, the reconstituted Sex molecules retain a smaller x value than the original Sex chain molecule. In both cases, the Se molecules remain almost completely confined to the micropore slits to give a long cycle life.
KEYWORDS: Energy storage, metal−selenium battery, microporous carbon, confined selenium chain, selenium electrochemistry
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technical limitations in material characterization it is hard to resolve some specific information on the confined S molecules because of interference from the surrounding carbon wall, which has made controversial the conclusion about the confined S existence drawn largely from the electrochemical data. Guo et al. have shown that Raman signals from Se are more easily detected than those from S even when confined in carbon micropores.26 Therefore, by loading Se molecules into the carbon micropores, one may clarify the interpretation of the electrochemical data for the cathode with S confined to slit micropores because Se and S exhibit similar atomic, molecular, and electrochemical properties.27 For example, both elements form 8-atom rings or chains with similar configuration parameter, including bond lengths and angles. Also, there has been interest in the use of Se in place of S for batteries of high energy density because Se has a theoretical volumetric capacity (3250 mA h cm−3) comparable to that of S
he Li-ion batteries that power portable devices have too low an energy density and too high a cost for application in electric family cars and stationary storage of electric energy from wind and solar energy.1−13 Batteries with an alkali-metal anode and a sulfur or air cathode promise theoretically the needed low cost and energy density.14−24 However, experimentally the Li−air cells show too low an efficiency for use, and the sulfur cathode shows a limited volumetric capacity density and poor cyclability.14,23 To make a cyclable Li−S battery with high volumetric energy density and cycle life, a principal strategy for fabricating a S cathode is confinement of the S particles in porous carbon; the carbon transports electrons to and from the S particles and confinement of the S to carbon pores suppresses escape of soluble intermediates of the cell reaction to the electrolyte.5,14,23 The size and shape of the carbon pore directly influences retention of active S on the cathode. A successful example is confining the S to extremely narrow carbon slit pores 0.5 nm in diameter in which the S molecules deform from rings to short S2−4 chains, which enables novel electrochemical reactions with Li+ and Na+ with the shuttle effect almost eliminated.22,25 However, due to © XXXX American Chemical Society
Received: May 4, 2016 Revised: June 6, 2016
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DOI: 10.1021/acs.nanolett.6b01819 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 1. (a) Schematic diagram showing the preparation of Se/(CNT@MPC), (b) TEM image of a Se/(CNT@MPC) nanocable (inset shows the EDX pattern), (c) high-resolution TEM image of the white dotted square in panel b, (d) annular dark-field TEM image and elemental mappings of (e) Se and (f) C of the composite.
(3470 mA h cm−3), and its electrochemistry is more stable than that of S.26,28−32 As a result, a Li−Se battery is expected to deliver a comparable volumetric energy density to that of a Li− S battery and a gravimetric specific energy exceeding that of a Li-ion battery because the Se cathode holds a theoretical gravimetric capacity (675 mA h g−1) 2−5 times higher than that of the traditional intercalation cathodes for Li-ion batteries, which offsets its deficiency in working voltage. However, a Se cathode has many problems that hinder its practical use: (1) bulk Se has limited reactivity with an alkali metal; (2) soluble intermediate molecules may shuttle to the anode or fail to be reconstituted to an active Se particle on the carbon, which leads to capacity fade on discharge/charge cycling; (3) the low voltage requires realization of an alkali-metal anode although testing of the cathode performance with an alkali-metal anode is safe; and (4) Se has a lower abundance than S, leading to a significantly higher cost almost 10 times as much as that of S.26,28,30 Although rechargeable alkaline batteries based on the Se cathode are expected to yield competitive energy outputs, it should be avoided to oversell the practical advantages of using Se as a cathode material. However, it is worth understanding the fundamental electrochemistry of the confined Se molecules so that one can provide insights on the rational design of an S cathode for metal-S batteries. Aiming at the above purpose, we have loaded Se as single-chain molecules into microporous slits of high surface density in a carbon coat supported on a carbon nanotube fiber; the slit morphology contains Sex linear molecules that react with Li+ + e− in a one-step reversible reaction, which provides stable cycling and shows voltage profiles resembling the previously reported Li−S2−4 cell.22 The reaction of the slit-confined Sex chains with Na+ + e− involves an initial chain decomposition into smaller Se species that are not reconstituted but remain confined; after the initial reaction, the confined Se species react reversibly with Na+ + e− to Na2Se. The data for the Se cathodes not only clarify the previously controversial S existence in the carbon slits and the interpretation of confined S electrochemistry, they also shed
light on how to reach an optimized S retention in the cathode. The data also show that the larger Se atoms are better matched in size to the slit width of the micropores, which means a stronger confinement and, therefore, a cathode with longer cycle life. This cathode strategy offers practical Li−Se and Na− Se rechargeable batteries of high energy density (Figure S1, Supporting Information) if provided a valid alkali-metal anode that can be realized. A microporous carbon (MPC) substrate was designed for the accommodation of Se (Figure 1a); it has a coaxial cable-like structure consisting of a multiwalled carbon nanotube core (Figure S2a, Supporting Information) coated by a pyrolytic microporous carbon sheath (CNT@MPC, Figure S2b, Supporting Information); it can be prepared as reported elsewhere.22 The cable has a mean diameter of ∼250 nm as confirmed by the scanning electron microscopic (SEM) and the transmission electron microscopic (TEM) images (Figure S2b,c, Supporting Information), and it is clearly observed from Figure S2c (Supporting Information) that a uniform MPC sheath of ∼100 nm in thickness is coated on all the CNTs. The amorphous nature of the MPC sheath is proven by the Raman spectra (Figure S3a, Supporting Information), which reveals a significantly increased relative intensity of D-band (denoting the disordered carbon components) and, correspondingly, a much attenuated G-band (denoting the graphitic carbons) of the CNT@MPC compared with the uncoated CNTs. The CNT@MPC consists of numerous short-range-ordered slit pores of ∼0.5 nm in width on average and several nanometers in length according to the Horvath−Kawazoe method and the “house of cards” theory (inset of Figure S2b, Supporting Information)33,34 and is provided with a large micropore volume of >0.44 cm3 g−1, which corresponds to a high Se loading of >65 wt % (calculated based on the lowest density of amorphous Se, 4.26 g cm−3).26 With slit pores of high surface density in the MPC, Se can be loaded into the carbon substrate via a simple mixing-heating route (Figure 1a) in which Se (Figure S2d, Supporting B
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Figure 2. Electrochemical properties of Li−Se batteries: (a) cycling performance, (b) GDC profiles, and (c) energy density at 0.1 C, (d) cycling performance, (e) GDC profiles, and (f) energy density at various rates (from 0.1 to 10 C), (g) extended cycling performance at 1 C. Insets in (c,g) show the variation of energy density with increasing cycle number, while the inset in (f) shows the variation of energy density with increasing rate. The blue scale bars in (a,b,d,e,g) denote the capacities based on the composite, while the black scale bars denote the capacities based on Se (the same for Figure 3).
Information) melted and penetrated into the C micropores to form the Se−C composite (Se/(CNT@MPC)); bulk Se disappeared after heating (Figure S2e, Supporting Information), as was also made evident by the disappearance of hexagonal Se peaks in the X-ray diffraction (XRD) patterns obtained during the loading process (Figure S4, Supporting Information). A TEM image (Figure 1b) shows that the Se/ (CNT@MPC) still retains its cable structure after heating with only the lattice planes of the CNTs (d = 0.34 nm, corresponding to the (002) plane, Figure 1c) being observed. However, both energy-dispersive X-ray (EDX) analysis and elemental mapping reveal a rich and uniform Se distribution in the MPC layer (inset of Figure 1b, Figure 1d−f), confirming
the amorphous nature of Se in the MPC. The Se content in Se/ (CNT@MPC) is measured to be 50.2 wt % by thermogravimetry (TG) (Figure S2f, Supporting Information). It is known that Se has four major allotropes, that is, cycloSe8 in monoclinic crystals, helical Sex chains in hexagonal crystals, a deformed chain allotrope in amorphous Se, and polymeric Se rings containing >1000 atoms in vitreous Se.27 To clarify the Se structure in the MPCs, theoretical calculations were performed on different Se allotropes (except for polymeric Se, which apparently cannot be accommodated in the micropore). Results show that only helical Sex chains (isolated from hexagonal Se) have the dimensions to fit into the C slit pore (Figure S5, Supporting Information).27,35 Given that the C
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Figure 3. Electrochemical properties of Na−Se batteries: (a) cycling performance, (b) GDC profiles, (c) energy density at 0.1 C, (d) cycling performance, (e) GDC profiles, (f) energy density at 1 C. Insets in (c,f) show the variation of energy density with increasing cycle number.
cells.14,23,28 The different voltage signature also confirms a different Se existence in the composite. The gravimetric capacity upon initial discharge is 990 (495) mA h g−1 (the value in parentheses throughout is based on Se/(CNT@MPC) and the one outside is based on Se), exceeding the theoretical gravimetric capacity of Se, 675 mA h g−1. The excessive capacity, according to a GDC test with the same current density applied onto the bare CNT@MPC, is largely contributed by the irreversible capacity loss of the carbon substrate in Cycle 1 (320 mA h g−1, see Figure S6a, Supporting Information). Therefore, the net capacity of Se is ∼670 (335) mA h g−1 in Cycle 1, which is very close to the theoretical capacity of Se, implying a complete Se reduction to Li2Se at the end of discharge. The conclusion is further supported by ex situ X-ray photoelectron spectroscopy (XPS), which demonstrates a reduction from Se0 to Se2− after discharge (Figure S7a,b, Supporting Information). The reverse charge process also brings a single plateau with little polarization. A charge capacity of 680 (340) mA h g−1 (also exceeds the theoretical capacity of
hexagonal Se is the most stable form a little below its melting point, the melted Se may keep its chain structure while penetrating into the micropore. Because of the space confinement of the pore, the penetrated Se does not crystallize upon cooling, but is stored as isolated Sex chains. This deduction is confirmed by Raman spectra collected during the Se capture process, which shows a blue shift from 233 cm−1 (hexagonal Se) to 257 cm−1 (disordered Sex chain) (Figure S3a, Supporting Information), indicating the generation of Se chains in the MPC.35−37 On the basis of the Se/(CNT@MPC) cathode, prototype Li−Se and Na−Se batteries were separately assembled to study the electrochemistry of the confined Sex chains. Figure 2b shows the galvanostatic discharge−charge (GDC) profiles of the Se cathode cycled at 0.1 C (the rate and capacity throughout is based on Se mass). During the initial discharge process, the composite shows a long plateau starting from 1.9 V (vs Li+/Li), which is significantly different from the stepped discharge behavior reported previously on Li−S and Li−Se D
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Figure 4. Electrochemistry of a Li−Sex battery: (a) energy (per electron) released in one-step lithiation of Sex, (b) sectional view and front view showing the Li+ migration in a Sex preoccupied slit pore, and electrochemistry of (c) Cycle 1 and (d) Cycle 2.
Se due to a capacity contribution of 30 mA h g−1 from carbon, see Figure S6a, Supporting Information), and the restoration of Se 0 XPS peaks confirms a completely reversible Li + deintercalation from Se (Figure 2b, S7c, Supporting Information). In subsequent cycles, the profiles still show a single plateau with a tiny voltage drop of 88% retention of the theoretical capacity), corresponding to a volumetric capacity density of 2536 (602) mA h cm−3 (see Figure 2a, the value in parentheses is based on the composite density, 2.022 g cm−3, and the one outside is based on the density of amorphous Se, 4.260 g cm−3). In the Na−Se battery, a high Coulombic efficiency of ∼100% is maintained upon cycling, and the
cathode is able to deliver a reversible gravimetric capacity of 578 (289) mA h g−1, or a capacity density of 2461 (584) mA h cm−3 after 100 cycles at 0.1 C (Figure 3a). The stable Se electrochemistry and the space confinement of MPC also helps to avoid the formation and escape of electrolyte-soluble polyselenides upon cycling. According to the ex situ SEM images and EDX spectra (Figure S16, Supporting Information), no selenium was detected on the cycled Li and Na anodes even after the batteries were cycled at a very low GDC rate of 0.05 C, where the electrochemical reactions between Se and Li/Na were much like quasi-static processes. The absorption of the liquid electrolyte into a mat of fibers with a porous MPC shell and a robust CNT core form a mixed conducting network, allowing the migration of Li+/Na+ to the active Se molecules while bringing a fast and stable transmission of e−; this cathode design gives impressively high rate capabilities of Se/(CNT@MPC). In the Li−Se battery, the cathode delivers a stable electrochemistry while gradually raising the rate from 0.1 to 10 C, as is supported by the singleplateau GDC profiles and ∼100% efficiency at each rate (Figure 2d,e). Tested at an ultrahigh rate of 10 C, a reversible capacity of 250 (125) mA h g−1 (or 1065 (253) mA h cm−3) is still maintained (Figure 2d). An extended cycling test at 1 C shows that the composite can output 954 (477) mA h g−1 (or 4064 (964) mA h cm−3) upon the initial discharge and reversibly output ∼600 (300) mA h g−1 in the following cycles (Figure 2g). After 1000 cycles, a reversible capacity of 352 (176) mA h g−1 (or 1500 (356) mA h cm−3) is retained, still higher than 50% of Se’s theoretical capacity (338 mA h g−1). In the Na−Se battery, the composite holds an almost unchanged electrochemistry at 1 C (Figure 3e) with a high discharge capacity of 882 (441) mA h g−1 (or 3757 (892) mA h cm−3) and a Coulombic efficiency of 77.2% delivered in Cycle 1 (Figure 3d). From Cycle 2, the efficiency is kept around 100%, and a reversible capacity of 442 (221) mA h g−1 (or 1883 (447) mA h cm−3) is obtained after 100 cycles (Figure 3d). With the high capacities and the fine electrochemical performances of the confined Se molecules, high-energy Li− Se and Na−Se batteries are realized. According to Figure 2c, the initial volumetric energy density of the Li−Se battery is 2308 W h L−1 (based on active Se and Li) at 0.1 C, surpassing the theoretical energy density of a Li−S battery (2200 W h L−1). Even calculated from the composite, a high energy density of 1123 W h L−1 is still delivered, superior to Li-ion batteries (theoretically 1015 W h L−1 for a LiCoO2−graphite battery). After 100 cycles, an energy density of 1869 (766) W h L−1 is retained (the value in parentheses is calculated from Se/ (CNT@MPC) and Li/Na; the one outside is calculated from Se and Li/Na). Tested at 10 C, the battery can still output a high energy density of 893 W h L−1 (Figure 2f, based on active Se and Li). The extended cycling test at 1 C yields a high energy output of 2310 (1109) W h L−1 upon the first discharge and an energy retention of >50% (1195 (418) W h L−1) upon the 1000th discharge with the Li−Se system (Figure 2g), demonstrating its potential as a high-energy battery with long cycle life. For the Na−Se battery, a volumetric energy density of 1143 (661) W h L−1 is initially delivered at 0.1 C, which stabilizes at 1050 (522) W h L−1 after 100 cycles (Figure 3c). Cycled at 1 C, the energy density still keeps 1140 (653) W h L−1 upon the first discharge and 715 (324) W h L−1 upon the 100th discharge (Figure 3f). Though lower than that delivered by the Li−Se system, the energy output is still comparable with the LiCoO2/ G
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C battery and the battery is cheaper due to the reduced cost of Na. In summary, we have demonstrated that Se is confined as single-chain molecules in carbon with microporous slits. Through ex situ characterizations made possible by Se versus S as well as theoretical calculations, we made a deep investigation into the electrochemistry of these confined Se chains that revealed a reversible one-step reaction with Li and a nonreconstituted chain-shortening reaction with Na to yield highly active small Se molecules after the initial discharge. The deep investigation into the electrochemistry of confined Se molecules has clarified the chemical processes of previously reported micropore-confined small sulfur molecules,22 In that work, the single-plateau discharge/charge profiles of sulfur confined in carbon slit pores was ascribed to the Li reactions with short-chain S2−4 molecules due to a missing plateau at 2.3−2.4 V versus Li+/Li (which is generally related to the transformation between cyclo-S8 and Li2S4). The conclusion was made largely based on the interpretation of electrochemical data, yet a lack of concrete proof made it controversial. Herein, with our comprehensive material characterizations including ex situ Raman spectra and theoretical calculations, it is confirmed that the Sex chains confined in the above carbon slits showed the same single-plateau discharge/charge profiles. Given the similarities between Se and S, the single-plateau profiles could result from either small S2−4 molecules or chainlike S molecules. Moreover, the conclusions obtained from this work may offer suggestions on the rational desigin of a S cathode for metal−S batteries. As shown in Figure S17a,b (Supporting Information), the slit size of carbon substrate for S cathodes in Li−S batteries can be further reduced by 60 pm due to the smaller helix diameter of the S chain (396 pm) compared with that of a Se chain (443 pm),27 which may not impede the Li+ migration yet will provide an improved trapping of soluble polysulfides. For the cathodes in Na−S batteries, it is better to enlarge the carbon pore size by 35 pm so that a free Na+ migration can be guaranteed, which we believe will benefit the kinetics upon sodiation/desodiation. Finally, the Li−Se and Na−Se batteries based on the novel Se electrochemistry exhibit high specific capacities, good cycling stabilities, and superior rate capabilities, which promise practical batteries with high energy densities (2308 W h L−1 for a Li−Se battery and 1143 W h L−1 for a Na−Se battery) for compact applications in electronics (Figure S1b, Supporting Information) and automobiles.2,19 The pairing with low-cost Na anode may offset the cost disadvantage of Se in Na−Se batteries used in large-scale stationary storage systems such as those for the grid.4,7
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: *E-mail: *E-mail: *E-mail:
[email protected].
[email protected].
[email protected].
[email protected].
Author Contributions
S.X., L.Y., and Y.Y. contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21403050, 51225204, 21431006), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant 21521001), the National Basic Research Program of China (Grants 2014CB931800, 2013CB931800), the Chinese Academy of Sciences (Grant KJZD-EW-M01-1), the Fundamental Research Funds for the Central Universities (Grants J2014HGBZ0126, 2014HGQC0015), the Anhui Provincial Natural Science Foundation (1608085MB32), and the Beijing National Laboratory for Molecular Sciences (Grant 20140156). The ex situ Raman characterization and electrochemistry study of the Se cathode in Austin, TX, were supported by the Lawrence Berkeley National Laboratory BMR Program (Grant 7223523). The authors also thank Professor Dr. Hongbin Yao and Professor Dr. Hengxing Ji from the University of Science and Technology of China for their helpful discussions.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b01819. Additional information on the electrochemistry of Li−Se and Na−Se batteries and electrochemical measurements, ex situ characterizations and the results, morphological and structural characterization results, and theoretical calculations and molecular simulations.(PDF) H
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