Elemental Selenium for Electrochemical Energy Storage - The Journal

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Perspective

Elemental Selenium for Electrochemical Energy Storage Yu-Guo Guo J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/jz502405h • Publication Date (Web): 26 Dec 2014 Downloaded from http://pubs.acs.org on January 1, 2015

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Elemental Selenium for Electrochemical Energy Storage

Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:

The Journal of Physical Chemistry Letters jz-2014-02405h.R1 Perspective 15-Dec-2014 Yang, Chun-Peng; Institute of Chemistry, Chinese Academy of Sciences, CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, and Beijing National Laboratory for Molecular Sciences Yin, Ya-Xia; Institute of Chemistry, Chinese Academy of Sciences, Guo, Yu-Guo; Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Science (CAS),

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Elemental Selenium for Electrochemical Energy Storage Chun-Peng Yang,ab Ya-Xia Yin,a and Yu-Guo Guo*a a

CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, and Beijing National

Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, P. R. China b

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

AUTHOR INFORMATION Corresponding Author * Tel/Fax: (+86)-10-82617069, E-mail: [email protected]


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ABSTRACT

To meet the increasing demand for electrochemical energy storage with high energy density, elemental Se is proposed as a new attractive candidate with high volumetric capacity density similar to that of S. Se is chemically and electrochemically analogous to S to a large extent but is saliently featured owing to its semi-conductivity, compatibility with carbonate-based electrolytes, and activity with Na anode. Despite only short-term studies, many advanced Se-based electrode materials have been developed for rechargeable Li batteries, Na batteries, and Li-ion batteries. In this Perspective, we review the advances in Se-based energy storage materials and the challenges of Li–Se battery in both carbonate-based and ether-based electrolytes. We also discuss the rational design strategies for future Se-based energy storage systems based on the strengths and weaknesses of Se.

TOC GRAPHICS

KEYWORDS electrochemical energy storage, rechargeable batteries, electrode materials, selenium, Se–C nanocomposite


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Efficient energy storage systems are crucial for the clean and sustainable utilization of energy given the current energy crisis. Among various energy storage devices, electrochemical batteries are considered as the best option because of their efficient energy storage and conversion.1 Over the past two decades, lithium-ion (Li-ion) batteries have gained considerable success and found diverse applications, especially in portable electronics.2-4 Sodium-ion (Na-ion) batteries have recently been recognized as an alternative technology for grid energy storage because of the low cost and abundant reserves of Na precursors.5-7 However, the energy density of Li-ion batteries is limited and can hardly satisfy the requirements of many rapid growing markets, such as electric vehicles (EVs), in which high specific energy (gravimetric energy density) and volumetric energy density are critical.8 In this scenario, rechargeable metallic batteries are appealing as post Li-ion systems for the next-generation batteries because of their astonishing energy density.9 Metallic batteries are coupled with a metallic anode (such as Li, Na, Mg, etc.) and a highenergy cathode material, primarily elements in Group 16 (VIA). Oxygen and sulfur are the most compelling cathode materials because of their amazing energy density and wide availability. However, Li–O2 batteries (not even practical Li–air batteries) suffer from poor cycle life and many other problems, far from mass commercialization.10-13 Li–S batteries have been under scrutiny for several decades and are more close to practical realization.14-23 S is inexpensive, abundant, and nontoxic, as well as exhibits high theoretical specific capacity and volumetric capacity density. Despite these considerable advantages, the use of Li–S batteries is still restricted by their inherent limitations, including the insulating nature of S and the dissolution of intermediary polysulfide species into the electrolyte. These limitations result in low utilization of S, rapid capacity decay, and “shuttle effect” (repeated migration of the polysulfides between cathode and anode during cycling), thus greatly hindering the practical applications of Li–S


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batteries. Making effort to address these issues of S cathode, researchers are also attempting to explore new electrode materials for advanced energy storage systems. Elemental selenium, another element in Group 16, is thus proposed as a novel prospective candidate, possessing many merits of S and many more beyond S. Se, [Ar] 4s23d104p4, is chemically similar to S. The major allotropes of Se include trigonal Se (t-Se, thermodynamic stable phase constructed from Se chains) and various monoclinic Se (constructed from Se8 rings).24, 25 Due to its unique photovoltaic and photoconductive properties, Se has been investigated and applied in photocells, sensors, rectifiers, xerography, etc.,26 and as the candidate to build other functional materials, such as CdSe,27 CuInSexS2-x,28 and et al. Nevertheless, elemental Se was not applied in electrochemical energy storage systems until recently. Se is proven a potential electrode material for rechargeable Li and Na batteries. The overall redox reaction of Se is Se + 2M+ + 2e− ↔ M2Se (M = Li, Na). The output voltages, theoretical capacities, and specific energies of Li–Se and Na–Se batteries are summarized in Table 1 in comparison with Li-ion and Li–S batteries. The theoretical specific capacity of Se, as well as the specific energies of Li–Se and Na–Se batteries, is much higher than the conventional Li-ion batteries but is lower than S. However, because of the high density of Se (4.82 g cm−3, ca. 2.4 times that of S), Se can provide a high volumetric capacity density that is comparable with S (Table 1). It is noteworthy that in energy storage devices used in portable devices and EVs, the volumetric energy density is considerably crucial because of the limited battery packing space.29, 30

As a potential energy storage material, Se rivals S in many aspects (such as volumetric

capacity) while exhibits some unique advantages over S cathode. Most importantly, Se displays a considerable benign electronic conductivity, approximately 20 orders of magnitude higher than S.31 The conductivity greatly benefits the electrochemical properties of Se in batteries, including


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better electrochemical activity, higher utilization rate, and faster electrochemical reaction rate. Therefore, Se is expected to be a promising candidate that can provide a high volumetric energy density similar to S while overcoming some limitations of S. Nevertheless, due to a lack of recognition of the merits of Se, elemental Se as an electrode material has long been disregarded. Table 1. Voltages, capacities, and specific energies of several rechargeable batteries. Li-ion*

Li–S

Li–Se

Na–Se

Cell voltage (V)

3.8

2.2

2.0

1.5

Specific capacity of cathode (mA h g−1)

137

1675

675

675

Capacity density of cathode (mA h cm−3)

700

3467

3254

3254

Theoretical specific 387 2567 1155 644 energy (W h kg-1)** * Data of the LiCoO2/graphite battery (electrochemical reaction: 2Li0.5CoO2 + LiC6 ↔ 2LiCoO2 + C6) as an example of Li-ion batteries.10 ** Theoretical data based on the total mass of active materials on the cathode and the anode. The seminal work of developing Se cathode was conducted by Amine and co-workers in 2012.32 In the cathode, Se is embedded within carbon nanotubes, though bulk Se is present as well. The Se–C cathode is proven rechargeable in Li battery and Na battery with carbonate-based electrolytes. According to the electrochemistry (Figure 1a) and the pair distribution function (PDF) analysis for the pristine samples and the discharge/charge product (Figure 1b), for the Li– Se battery, the initial discharge (Li insertion) process involves only one plateau at 2 V with trigonal Se directly reduced to antifluorite Li2Se. The ensuing charge (Li extraction) process occurs at 2.4 V, where Li2Se is converted to Se. The second charge plateau at 3.75 V is actually unrelated to redox reaction. For Na–Se battery (Figure 1a), the initial discharge process (Na insertion) exhibits two plateaus at 1.9 V (short) and 1.5 V (long), indicating an intermediate product; the ensuing charging (Na extraction) occurs at 2.1 V. In Li–Se and Na–Se batteries, Se


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is completely reduced to Se2−, delivering an initial capacity approaching the theoretical capacity. However, in subsequent cycles, the electrochemical reaction of Se becomes complex, exhibiting more discharge/charge potentials than the intrinsic redox potential of Se. Moreover, the capacity decreases and the polarization increases dramatically after the initial cycle (Figure 1c). This is possibly because of the bulk-sized trigonal Se in the Se–C cathode that would deteriorate the battery performance. Nevertheless, this work has paved the way for the new promising material, elemental Se, for high-energy batteries. It is also proposed that mixed Se–S systems can be a new class of battery materials with advanced comprehensive performance.

Figure 1. (a) Discharge/charge voltage profiles of Li/Se–C and Na/Se–C batteries during the first cycle; (b) PDFs for the pristine Se–C electrode at various states of discharge/charge (structural representations of trigonal Se and antifluorite Li2Se are shown); (c) Discharge/charge voltage profiles of Li/Se–C battery during selected cycles. Reprinted from Ref. 32. It is known that the organic electrolytes have a great impact on the electrochemistry of Li–O2 and Li–S batteries.33, 34 Likewise, electrochemistry of Se highly depends on the nature of the electrolytes. Cui et al. further conducted an in-depth study on the lithiation/delithiation mechanism of Se cathode in ether-based electrolytes in 2013.35 In situ synchrotron high-energy


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X-ray diffraction (XRD) and in situ Se K-edge X-ray absorption spectroscopy (XAS) are utilized to track the evolution of the crystalline phase and oxidization state of Se during cell cycling. The comprehensive study has proposed a stepwise lithiation/delithiation mechanism of Li–Se batteries in an ether-based electrolyte, viz., 1 M lithium bis(trifluoromethanesulphonyl)imide (LiTFSI) in a mixture of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME), as illustrated in Figure 2. During the discharge, Se is first reduced to lithium polyselenide Li2Sen (n ≥ 4) at the upper plateau (ca. 2.04 V) and then reduced to Li2Se2 and Li2Se at the lower plateau (ca. 1.95 V). During the charge, Li2Se is first oxidized to Li2Sen (n ≥ 4) and then to elemental Se. Because of the similar nature of Se and S, the electrochemical reaction process of Li–Se batteries (with ether-based electrolytes) is almost the same as the well-known stepwise reaction in Li–S batteries. Similarly, polyselenides are also soluble in ether (as shown in the inset in Figure 2). A minor difference is that the polyselenides may contain more than 8 atoms depending on the pristine Se structure. In addition, for different Se cathodes, the reduction voltages may be slightly different. Despite these insignificant differences, the electrochemistry, and therefore the virtues and issues, of Li–Se batteries and Li–S batteries (with ether-based electrolytes) share much in common. The soluble polyselenides favor the electrochemical activity and kinetics of the redox reaction with liquid phase involved, but lead to loss of Se in cathode and the shuttle effect, as in the case of Li–S batteries.


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Figure 2. A typical discharge/charge voltage profile of Li–Se battery with an ether-based electrolyte and the evolution of Se species during reaction. The inset shows the solubility of Se, Li2Se, and Li2Sen in DOL/DME solvent. Reprinted from Ref. 35. These two types of electrolytes are generally applied in subsequent researches of Li–Se battery batteries. The first type is carbonate-based electrolyte that is LiPF6 dissolved in carbonate mixtures, such as ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate. The second type is ether-based electrolyte that is Li salt (typically LiTFSI) in ether mixtures, such as DOL and DME. The conventional carbonate-based electrolytes have gained more attention because of their low cost and high-voltage stability. Historically, carbonate-based electrolytes have been explored for O2 and S cathodes. The electrolytes suffer decomposition in Li–O2 batteries and most Li–S batteries, because of the nucleophilic attack to the carbonyl groups.33, 34 In contrast, carbonate-based electrolytes are feasibly used in most Se-based batteries. By encapsulating Se in a proper conductive matrix, Se cathode is easy to deliver excellent electrochemical performance in carbonate-based electrolytes. A remarkable example is Se confined in ordered mesoporous carbon material CMK-3.36 Through a melting-diffusion method, Se is filled in the mesopores (3–4 nm) of CMK-3. The porous carbon can form a conducting network and maintain intimate electrical contact during cycling. On the other hand, the space


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confinement reduces the size of Se, thus shortening the diffusion path of electrons and Li ions. Used as the cathode of Li–Se battery with a carbonate-based electrolyte, the Se molecules can deliver a stable reversible capacity of ca. 650 mA h g−1 (Figure 3a), approaching the theoretical value. Different from the previous report in which Se displays unstable discharge plateaus,32 Se confined in CMK-3 display a single well-defined plateau at 1.9 V without the subsequent multiple discharge voltages or enlarged polarization. It is also found that if poly(vinylidene fluoride) (PVDF) binder (dissolved in N-methyl-2-pyrrolidone) is used to prepare Se–C cathode, bulk Se will crystallize on the electrode, resulting in inferior battery performance. Water-soluble sodium alginate (SA) binder is employed to prevent the space-confined Se from crystallizing and guarantee its outstanding electrochemical performance (Figure 3b). Insightful investigations are conducted to disclose the reaction mechanism of the confined Se. Raman measurement is helpful to probe the variety of Se, because sufficient Raman data of Se have been provided for an unambiguous frequency and symmetry assignment.24 The Raman spectra (Figure 3c) demonstrate that crystalline Se is stored in the mesopores in the form of cyclic Se8 molecules due to the space confinement. After the initial cycle, cyclic Se8 molecules convert to chain-like Sen molecules, strongly constrained and stabilized by the carbon matrix as illustrated in Figure 3d. The Se molecules are directly reduced to Li2Se without the intermediate polyselenides, and thus without shuttle effect. This result indicates that to gain superior Se cathodes for Li–Se batteries, particularly for those with carbonate-based electrolytes, it is important to get Se well confined in carbon matrix, and avoid bulk-sized Se crystals. It is noteworthy that Se is thermodynamic stable as chains, different from S that is stable as rings. The Se8 rings are electronic insulating; only Se chains with ordered arrangement can facilitate the electronic conduction.25 The chain structures with good conductivity could be


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advantageous in an electrochemical system and may be electrochemically preferred compared with ring structures. This is because each chain has two active terminal atoms and is therefore more electrochemically active than the ring structure. Furthermore, the chain-like molecules can form stronger interaction with the substrate and are stable during electrochemical cycling. Therefore, the chain-like Se molecules confined in porous carbon stably exhibit a superior capacity.

Figure 3. (a) Discharge/charge voltage profiles of Se/CMK-3 cathode with SA binder in the voltage range of 1.0–3.0 V (vs. Li+/Li) at 0.1 C; (b) Specific capacity and Coloumbic efficiency of Se/CMK-3 with SA or PVDF binder; (c) Raman spectra of Se, Se–CMK-3 mixture, Se/CMK3 composite, and Se/CMK-3 cathodes at various discharge/charge states; (d) Schematic


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presentation of the proposed lithiation/delithiation processes of Se/CMK-3. Reproduced from Ref. 36 with permission from Wiley-VCH. More Se–C composite materials have been developed as cathodes for Li–Se batteries, including Se confined in porous carbon,37-40 Se with polyacrylonitrile-derived carbon materials,41 and graphene–Se@carbon nanotubes.42 Even pure Se (with some carbon additives), such as nanoporous Se43 and nanofibrous Se,44 are explored as cathode materials. These Se-based cathode materials can be classified into two types: space-confined Se in porous carbon and bulk Se crystals with less homogeneous dispersion in conductive matrices. The two types of Se cathodes show quite different electrochemical behaviors when applied in Li–Se batteries with carbonate-based electrolytes. The former shows well-defined discharge/charge voltage plateaus and reversibly delivers high capacity, consistent with the Se/CMK-3 composite; the latter behaves in a different way. Taking nanoporous Se (NP-Se) as an example,43 which is large in size (Figure 4a) and can be taken as “bulk-size” compared with Se confined in pores usually smaller than 5 nm. NP-Se shows pronounced diffraction peaks as strong as the commercial Se (CP-Se) (Figure 4b). The bulk Se crystals used as a cathode material, show unstable discharge voltage (rather than the single discharge voltage in Figure 3a), large polarization, and fast-fading capacity (Figures 4c and 4d), in contrast to the space-confined Se. In summary, the spaceconfined Se can work easily and stably in carbonate-based electrolytes, whereas the bulk crystalline Se cannot.


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Figure 4. (a) Transmission electron microscope (TEM) image of NP-Se particles; (b) XRD patterns of NP-Se and CP-Se; (c) Discharge/charge voltage profiles of NP-Se at the first and the 10th cycles at a current rate of 100 mA g-1; (d) Cycling performances of NP-Se and CP-Se at a current rate of 100 mA g-1. Reproduced from Ref. 43 with permission from The Royal Society of Chemistry. This property of Se is distinctive from S cathode. Most S cathodes, even if confined in host materials, fail to function in Li–S batteries with carbonate-based electrolytes. For example, S/CMK-3 composite does not reversibly work with carbonate-based electrolytes whereas Se/CMK-3 does. It is believed that S anions are nucleophilic and reactive with the carbonyl groups in the carbonate-based electrolytes.34 Only in strict conditions, such as when confined in small micropores (pore size < 0.5 nm), can S work without such side reaction. In this case, the confined S are small molecules45 and the small micropores physically obstruct S from the electrolytes46 because of the steric effect. However, although Se anions are also nucleophilic, Se is more adaptive to carbonate-based electrolytes. A slew of Se–C composites show excellent battery performance in carbonate-based electrolytes, only except those bulk-sized crystalline Se cathodes. Therefore, Se cathode is more compatible with carbonate-based electrolytes than S.


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Nevertheless, the reason for the compatibility of space-confined Se with carbonate-based electrolytes and the incompatibility of crystalline Se remains ambiguous. A possible reason is that polyselenides are insoluble in carbonated solvents and absent in Li– Se batteries in carbonate-based electrolytes. Cui et al. found that Se, Li2Se, and Li2Sen (n ≥ 4) are insoluble in carbonate solvents..47 By in situ XRD and XAS analyses, they concluded that Se is directly reduced to Li2Se in carbonate-based electrolytes without intermediate Li2Sen (n ≥ 4), exhibiting a single discharge plateau The absence of polyselenides in carbonate-based system endows unique advantages of Se cathode over S: Se is more compatible with the cheap carbonate-based electrolytes, in which shuttle effect is exempted. However, the insolubility of polyselenides imposes new challenges as well. As no dissolved phase is involved in this system, redox reactions between Se and Li2Se occur in solid phase, leading to a slow Li+ diffusion therein. To work in carbonated-based batteries, Se should be homogenously dispersed and confined in conductive matrices; otherwise, repeated cycles transpire with declined capacity and enlarged polarization, as in the case of bulk sized crystalline Se cathodes. However, multi-step reactions are still observed in some crystalline Se cathode in carbonatebased electrolytes,32, 41, 43, 44 indicating the formation of polyselenide intermediates. Liu et al. disassembled and examined discharged Li-Se batteries with bulk Se cathode and Se–C composite cathode.41 Red polyselenides are found migrated to the separator in the former after the fifth discharge but are not found in the latter. Thus, it is concluded that polyselenides exist after discharging but can be constrained by the carbon host material; it would be more convincing if the red product and electrolyte after discharge were extracted and carefully analyzed. A recent impressive Se cathode that is confined in porous carbon nanospheres (PCN) further confirms this point.48 Se is confined in microporous carbon host with a homogeneous distribution (Figure 5a).


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The confined Se cathode works stably in carbonate-based electrolytes and exhibits a minor irreversible initial plateau at 2.2 V in addition to the reversible discharge plateau at approximately 2.0 V (Figure 5b). Based on the electrochemical behavior and further XPS study, it is believed that the ring-like Se8 are firstly reduced to chain-like Se82– at 2.2 V, then to Li2Se at around 2.0 V (Figure 5c). These findings are inconsistent with the conclusion of Cui et al. that Se is directly reduced to Li2Se without polyselenides.47 Maybe the one-step electrochemical reaction of Li–Se batteries with carbonate-based electrolytes still depends on the pristine structure of Se cathodes. Carbonate-based electrolytes have been proven unstable in Li–O2 and most Li–S batteries because of nucleophilic reactions.33, 34 As polyselenides are also nucleophilic, they are believed reactive with the carbonate-based electrolytes, although no experiment has confirmed this point yet.35 It is therefore interesting why most Se cathodes can function stably in carbonatebased electrolytes. A possible reason is the protection by the space confinement and passivating solid electrolyte interface (SEI) films. The SEI films are considered vital to preserve the electrode materials and prevent Se cathodes from irreversible reaction with the carbonyl groups. In Li–Se batteries in carbonate-based electrolytes, SEI films have been identified by ex situ TEM observation, X-ray photoelectron spectroscopic (XPS), and electrochemical impedance spectroscopy (EIS) analysis.35, 38, 48, 49 For the Se/PCN composite, after electrochemical reaction, SEI film containing F and O is immediately generated on the Se–C composite after the first charging process, as demonstrated by the elemental mappings in Figure 5d. The multi-elemental images of Se, O, and F clearly show the passivating SEI film that covers the S/C particle. The formation of SEI film is further disclosed through XPS, which verifies the formation of F and O on the surface. According to EIS analysis along electrochemical cycling, the resistance of the SEI film is stabilized after the initial discharge. The stable SEI film is attributed to the structure


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stability of the carbon host material, which helps to alleviate the volume change of Se during lithiation/delithiation. Once stabilized, the SEI film will no longer cause capacity loss and polarization in the ensuing cycles. The porous carbon and the SEI film plays a crucial role precluding the polyselenides (if any) from dissolution into the electrolyte and detrimental reaction with the carbonates, so Se cathodes can reversibly work in carbonate-based electrolytes. For the bulk crystalline Se cathodes, SEI may not completely cover the Se particles. Moreover, the volume change of bulk Se during cycling is not cushioned with host materials, leading to repeated formation and destruction of SEI films. The unstable SEI films finally results in capacity drop and unstable redox potential, as is often observed in the bulk Se cathodes.

Figure 5. (a) TEM image of a Se/PCN composite particle and its corresponding elemental mappings of C and Se; (b) Discharge-charge voltage profiles of Se/PCNs in the initial 3 cycles at 0.2 C; (c) Proposed (de)lithiation processes of Se/PCNs; (d) Elemental mappings and overlapped


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multi-elemental mappings of C, Se, O, and F of Se/PCNs after one cycle at the charged state (3.0 V). Reproduced from Ref. 48 with permission from Elsevier Ltd. It is now known that, compared with S, Se cathodes (specifically space-confined Se) are more compatible with cheap carbonate-based electrolytes. Se cathodes in carbonate-based electrolytes exhibit outstanding capacity and stability, only except the crude crystalline Se. However, the detailed electrochemical mechanism of carbonated-based Li–Se batteries is still not clear. Further studies are necessary to understand the electrochemical reactions comprehensively, such as the electrochemistry of Se chains, the reactivity of polyselenides in carbonate solvents, the contribution of space confinement, and so on. Multiple in-situ and/or postmortem analysis methods that helped to understand the electrochemistry of Li–O2 and Li–S batteries, such as nuclear magnetic resonance, mass spectrometry, ultraviolet–visible absorption spectroscopy, liquid chromatography, and et al., should be utilized to reveal the electrochemistry of Li–Se batteries as well. Although incompatible with carbonate-based electrolytes, crystalline Se possesses several advantages. The most common and stable variety, t-Se, is a p-type semiconductor and has the highest conductivity and density of all Se allotropes.25,

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Moreover, Se cathodes based on

crystalline Se are anticipated to have higher Se content than the carbon-confined amorphous Se. Therefore, trigonal Se is expected to have higher electrochemical activity and volumetric capacity than other varieties of Se, which are exactly the major advantages of Se cathodes. As crystalline Se is not suitable for carbonate-based electrolytes, ether-based electrolytes should be adopted. As revealed by Cui et al., the electrochemistry of Se cathodes in ether-based electrolytes resembles that of S cathodes, so the issues of Li–S batteries also challenge the Li–Se batteries (with ether-based electrolytes).35 Lithium polyselenides are generated during


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electrochemical reaction and are soluble in ether solvents, thus resulting in the loss of active materials and causing shuttle effect. Fortunately, present research results and knowledge of S cathode are instructive for Se cathode, which can greatly facilitate the research on Se-based cathode materials. Similar to S cathode materials, it is vital to design rational Se cathodes with conductive matrices to restrain the polyselenides and avoid the shuttle effect.50-52 In addition to the cathode design, innovation of other battery components is constructive. For example, optimization of electrolytes has been proven advantageous based on the previous results in Li–S batteries. Electrolyte additives such as LiNO3 widely applied in Li–S batteries may play the same effect in Li–Se batteries. It is reported that the high Li salt concentration in the electrolyte can significantly improve the capacity retention of the cathode and stabilize the Li anode.53, 54 Lee et al. studied the impact of the electrolyte modification in Li–Se battery by using Se infiltrated carbide-derived carbon (Se–CDC) composite with about 62% Se as the cathode, LiTFSI in DOL/DME as the electrolyte, and 0.2 M LiNO3 as the additive.55 With the Li salt concentration increased from 1 M to 7 M (particularly from 1 M to 5 M), Se utilization and reversible capacity are improved (Figures 6a and 6b). The cell with 5 M Li salt even exhibits a slow activation process and an initial capacity increase, thus retaining the highest capacity after 150 cycles (~100% of the initial capacity). Although the increased electrolyte molarity results in higher viscosity and lower ion mobility, the capacities of Se–CDC in the concentrated electrolytes exceed the cell with common 1 M electrolyte. The improved capacity retention and rate capability in Li–Se batteries with elevated Li salt concentration are mainly attributed to the reduced polyselenide dissolution because of the common salt effect, as in the case of Li–S batteries. Intriguingly, Se–CDC exhibits considerably better capacity retention and reduced voltage polarization in 5 M electrolyte compared with the similar S–CDC composite (Figures 6c


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and 6d). This finding is probably due to the higher electrical conductivity of Se than S and higher ionic conductivity of Li2Se than Li2S,56 indicating the inherent advantages of electrochemical properties of Se over S.

Figure 6. Discharge capacities of Se–CDC composites in electrolytes with different Li salt concentrations: (a) long-term cycling at 0.2 C and (b) cycling at different C-rates. Electrochemical performances of Se and S: (c) discharge capacity retention rate and (d) normalized discharge/charge voltage profiles of Se–CDC and S–CDS cathodes in the same electrolyte (5 M LiTFSI in DOL/DME), where the contents of Se or S in the composite is close (ca. 60%). Reproduced from Ref. 55 with permission from Wiley-VCH. In addition to optimizing the electrolytes, modification of the separator is also beneficial to resolve the shuttle effect in Li–S batteries. Carbon paper interlayers are proven effective to intercept the dissolved polysulfides by Manthiram and co-workers.57-59 Likewise, carbon paper can also be employed as the interlayer to capture and retain the polyselenides within the cathode side in Li–Se batteries, thus improving the battery performance.60


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Compared with Li–Se employing carbonate-based electrolytes, researches focusing on Li–Se batteries utilizing ether-based electrolytes are even fewer. As crystalline Se represents the major virtues of Se cathode, i.e., satisfactory conductivity and prominent volumetric capacity density, it deserves further investigation. Although crystalline Se fails to work in carbonate-based electrolyte according to current researching results, it is unclear whether bulk crystalline Se is in any way possible to work in a carbonate-based Li-Se battery. Most current Se-C cathodes applied carbonate-based Li-Se batteries have a relatively low Se content. To utilize elemental Se in energy storage systems, advanced Se cathode materials with a reasonably high Se content are crucial for a high energy density. A Li-Se battery consisting of a crystalline Se cathode with a high Se content and a carbonate-based electrolyte is essential because it can deliver a high energy density at a low cost. This may be challenging to realize because solid phase diffusion of Li+ through the crystalline Se could be rate-limited, but it is worth trying in the future research. In addition to protocols imitating S cathode, development of crystalline Se cathodes should take advantages of Se itself. Superior to S, Se can be easily prepared with various morphologies in nano size.44, 61-64 Nano-sized Se like nanoporous and nanofibrous Se would greatly shorten the diffusion path of Li+ and electrons, thus favoring the electrochemical reaction. We have now seen that, as energy storage materials, S and Se each have their merits and limits. Se possesses superior conductivity and cycling stability but inferior specific capacity and cost; S is just the opposite. It is therefore rational to design mixed chalcogenide system SexSy as the cathode materials, aiming at the optimal comprehensive performance. The mixed chalcogenide has been proven feasible in carbonate-based or ether-based electrolytes.32,

35

Because of the

analogous chemical nature of S, Se, and SexSy, the strategies toward advanced S and Se cathodes are applicable to the mix system. Luo et al. further improved the electrochemical performance of


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SeSx with carbonized polyacrylonitrile (CPAN) matrix.49 The SeSx/CPAN composite reversibly delivers an impressive capacity of ca. 780 mA h g-1 for 120 cycles and shows excellent rate performance, in a carbonate-based electrolyte (LiPF6 in EC/DEC). As the ratio of S and Se are continuously adjustable in Se–S solid solution, the mixed chalcogenide materials have a bright researching prospect. The optimal SexSy cathode is promising to deliver superior energy density at cheap price as S cathode, and meanwhile have the satisfactory electrochemical activity and cycle stability like Se cathode. In addition to the cathode application in metallic batteries, Se is also possible to serve as the anode in energy storage systems. The redox potential of Se is approximately 2.0 V, and can be used as the anode with a high lithiation potential in Li-ion batteries. The high lithiation potential can avoid the precipitation of Li metal, which is the potential safety hazard of graphite anode. Li4Ti5O12 is a typical anode material with safe lithiation potential (1.55 V), but its capacity is too low (175 mA h g−1) to meet the stringent demand for energy density.65, 66 Se provides a much higher capacity than Li4Ti5O12 and is feasible in carbonate-based electrolytes widely applied in Li-ion batteries. Thus, Se is eligible as a new anode material for Li-ion batteries with exceptional capacity and safety features. This hypothesis has been realized by Ye et al.38 An advanced Se–C electrode material is achieved using hierarchically micro-/mesoporous carbon spheres as the host material. According to Raman spectra, Se is originally confined as chain-like molecules due to the narrow pore size. As discussed above, the chain-like Se molecules are extremely electrochemically active and are stable with cycles. Li2Se is directly reduced from Se chains without any intermediate species. Because of the advantageous electrochemical activity of the Se chains and strong interaction between the Se chains and enclosing carbon, the resultant Se–C composite delivers exceptional high reversible capacity (660 mA h g−1) with long cycle lifespan.


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Full cells are assembled with this advanced Se–C composite as anode and a commercial LiNi1/3Mn1/3Co1/3O2 (NMC) cathode, as illustrated in Figure 7a. The as-designed Li-ion full cell shows an output voltage of 2.0 V (Figure 7b). During prolonged cycle, this Li-ion full cell shows steady capacity retention with long lifespan of more than 1000 cycles (Figure 7c). As a proof of concept, this Li-ion full cell successfully ascertains the possibility of Se as a novel anode material.

Figure 7. (a) Schematic representation of a full cell composed of NMC cathode, Se–C anode, and EC/DMC electrolyte; (b) Discharge/charge voltage profile of the full cell at 0.2 C; (c) Capacity retention and efficiency of the full cell at 1 C. Figures b and c reproduced from Ref. 38 by permission of The Royal Society of Chemistry Hereinabove we reviewed the electrochemical performance of elemental Se. Due to the shortterm study of Se-based electrodes, the electrochemistry of Se in different electrochemical systems is still fully understood, which may be the greatest hindrance for developing advanced Se electrode material and thus needs continuous investigation. In addition to the electrochemistry,


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for possible future applications we have to evaluate several other benchmarks of Se in energy storage applications, which are listed in Table 2 in comparison with S. The density and electronic conductivity of Se are its advantages while other characteristics raise some concerns (and some unnecessary concerns) about the application prospect of Se-based energy storage materials. Table 2. Comparison of several parameters of S and Se. Sulfur

Selenium

Density (g/cm3)67

2.07 (α-S)

4.8 (t-Se)

Electrical resistivity (µΩ cm)31

2×1023 (20 °C)

1.2 (0 °C) [sic]

Crustal abundance (mg/kg)67

350

0.05

68100000

2240

3

6.7

114

217–222

235

N/A

68

World production (2012, t)

69

Toxicity (LD50 Oral, rat, g/kg) 69

Melting point (°C)

69

Autoignition temperature (°C) 70

Price (2013, $/kg)

0.124 (mine and/or plant) 77 (refined)

(1) Safety and environmental concerns. As shown in Table 2, the toxicity of Se is comparable to S, even a little lower. Although H2Se and some Se compounds are toxic, elemental Se and metal selenide involved in Se-based batteries are practically nontoxic.67 Generally, Se-containing electrode materials ordinarily pose no threat to human health or the environment, and incur few environmental issues that cannot be resolved. Moreover, since the melt point and autoignition point of Se is higher than S, Se cathode should be safer than S cathode in emergency conditions, such as short circuit and overheating. (2) Abundance in earth. Admittedly, Se is less abundant in the crust than S. However, consumption of Se is also far less. According to the U. S. Geological Survey, the present selenium reserve bases are adequate and can satisfy the demand for Se for several decades


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without difficulty.71 The reserve of Se should not be a hindrance for electrochemical energy storage systems, before Li–air batteries become commercial realization. (3) Cost. S is indeed a low-cost resource and Se is more expensive than S. The undesirable high cost is a major drawback of Se-based electrode materials. Limited by the cost of Se, the large-scale application of Li–Se batteries could be much more impeded compared with Li–S batteries. However, the cost of the cathode can be reduced by mixing S with Se as discussed. One the other hand, the cost of Se-based cathode could be compensated by lowering the cost of other parts of the energy storage system, including the electrolyte and the anode. (a) Inexpensive electrolytes are available for Se cathodes. In contrast to S that is largely limited to ether-based electrolytes, Se is friendly to the cheap carbonate-based electrolytes. Moreover, in practical high-energy Li–S batteries, large amount of ether-based electrolytes is required to dissolve polysulfides and enable the polysulfide redox reaction. In contrast, Se cathode in carbonate-based electrolytes involves in solid-phase reaction,47 and is thus independent of the amount of electrolyte. Taking the lower price and amount of electrolyte into account, the cost of Se-based energy storage devices can be much reduced. (b) Cheap metallic anode can be chosen as the anode. Considering the ample reserves of Na and the low cost of its precursors,5 Na is a compelling choice as the anode in Se-based energy storage devices, to compensate the annoying cost and deficient reserve of Se. For S, Na–S batteries can only deliver less than half of the theoretical capacity of S even at elevated temperature (300–350 °C) because of its low electrochemical activity.72 Only some very special designed S cathodes are available for room temperature Na–S batteries with acceptable activity.73 In contrast, Se shows good activity in Na batteries and can completely discharge to Na2Se at ambient temperature, delivering high capacity.32, 37 Another attractive metallic anode is


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Mg, which is of strikingly high volumetric capacity density (3832 mA h cm−3, vs. Li 2062 mA h cm−3 of Li and 1128 mA h cm−3 of Na) and is safe, cheap, and abundant.29 Both with high volumetric capacity density, Mg and Se may be gold partners for energy storage systems, yet rechargeable Mg–Se battery has not been built to date. Using Na or Mg substituting for Li as the anode, it is hopeful to develop Se-based energy storage devices with affordable price. By cutting the total expense of the whole system, Se-based energy storage devices can be significantly alleviated from the cost burden, exhibiting wide application prospects. The elements with high energy density are mainly located in Group 16 by no accident. This is because chalcogens have high reduction potentials and are involved in two-electron reduction (per atom) process. Among the elements in Group 16 as energy storage materials, Li–O2 battery is still some way off from practical application (let alone Li–air battery),10 while Te is only limited to anode materials and is too rare and expensive.74, 75 S and Se are the only elements that are close to practical application in energy storage in the near future. Elemental Se is a promising electrode material for electrochemical energy storage systems, particularly for those with strong aspiration for volumetric energy density, such as batteries powering EVs and portable electronics. The advantages of Se include its remarkable capacity density, conductivity, stability, and compatibility with carbonate-based electrolytes and Na anodes, whereas the key hindrance is its unfavorable expensive cost. Although development of Se cathode is still in its infancy, it grows fast, which is considerably benefited from the investigation results of existing S cathode. There are approximately thirty publications investigating the Li–Se batteries since the first report in 2012 and the number is increasing sharply. Se has found applications in Li–Se batteries, Na–Se batteries, Se/S mixed cathode, and a potential safe and high-capacity anode material for Li-ion batteries. Some other fields for Se-based energy storage systems that have been missed still need


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further study. For example, given that Se is conductive and electrochemically active, it is possible to be applied in batteries for a wide temperature range and in all-solid Li–Se batteries. Further researching will build various exciting Se-based electrochemical energy storage devices for versatile applications. Development of Se-based energy storage systems in the future should avoid blind echoes of Li–S batteries. Instead, emphasis of investigations should be put on maximizing the above-mentioned strengths of Se and minimizing the cost of Se-based energy storage systems. Only in this way can we build promising Se-based materials for efficient storage of energy with economic effectiveness. AUTHOR INFORMATION Corresponding Author * Tel/Fax: (+86)-10-82617069, E-mail: [email protected] Notes The authors declare no competing financial interests. Biographies Chun-Peng Yang received his B.S. degree in Material Physics from University of Science and Technology of China in 2011. He is currently a Ph.D. candidate under the supervision of Prof. Yu-Guo Guo at Institute of Chemistry, Chinese Academy of Sciences (ICCAS). His research focuses on advanced electrode materials for rechargeable metallic batteries. Ya-Xia Yin is an associate Professor at ICCAS. She received her Ph.D. in Beijing University of Chemical Technology in 2012. Her current research focuses on nanostructured electrode materials for advanced Li-ion and Li–S batteries.


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Yu-Guo Guo is a Professor of Chemistry at ICCAS. He received his Ph.D. in Physical Chemistry from ICCAS in 2004. He worked at the Max Planck Institute for Solid State Research at Stuttgart (Germany) first as a Guest Scientist and then a Staff Scientist from 2004 to 2007. He joined ICCAS as a full professor in 2007. His research focuses on electrochemical energy storage and nanostructured energy materials. See http://spm.iccas.ac.cn/guoyuguo/ for more details. ACKNOWLEDGMENT This work was supported by the “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant No. XDA09010300), the National Natural Science Foundation of China (Grant Nos. 51225204, and U1301244), the National Basic Research Program of China (Grant Nos. 2011CB935700, and 2013AA050903), and the Chinese Academy of Sciences. REFERENCES (1)

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Quotes Se is a novel electrode material that has attractive volumetric energy density comparable with S while better electrical conductivity than S. The space-confined Se displays excellent electrochemical performance in Li–Se batteries with carbonate-based electrolytes; the crystalline Se possessing high volumetric capacity and conductivity, however, are only suitable for ether-based electrolytes. S cathode and Se cathode share many properties in common; each of them has its advantages and disadvantages. Rational combination of S and Se should promise an optimal cathode material. Despite the unfavorable high cost of Se, the price of Se-based energy storage devices can be reduced to an affordable level.


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Elemental Selenium for Electrochemical Energy Storage - The Journal

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