Advances in Lithium–Sulfur Batteries Based on Multifunctional Cathodes and Electrolytes Quan Pang†, Xiao Liang†, Chun Yuen Kwok† and Linda F

REVIEW ARTICLE PUBLISHED: 8 SEPTEMBER 2016 | ARTICLE NUMBER: 16132 | DOI: 10.1038/NENERGY.2016.132

Advances in lithium–sulfur batteries based on multifunctional cathodes and electrolytes Quan Pang†, Xiao Liang†, Chun Yuen Kwok† and Linda F. Nazar*

Amid burgeoning environmental concerns, electrochemical energy storage has rapidly gained momentum. Among the con- tenders in the ‘beyond lithium’ energy storage arena, the lithium–sulfur (Li–S) battery has emerged as particularly promising, owing to its potential to reversibly store considerable electrical energy at low cost. Whether or not Li–S energy storage will be able to fulfil this potential depends on simultaneously solving many aspects of its underlying conversion chemistry. Here, we review recent developments in tackling the dissolution of polysulfides — a fundamental problem in Li–S batteries — focusing on both experimental and computational approaches to tailor the chemical interactions between the sulfur host materials and polysulfides. We also discuss smart cathode architectures enabled by recent materials engineering, especially for high areal sulfur loading, as well as innovative electrolyte design to control the solubility of polysulfides. Key factors that allow long-life and high-loading Li–S batteries are summarized.

ithium-ion battery technology has enabled the development and shuttling of LiPSs over long-term cycling. This results in loss of portable electronic devices over recent decades. The goal of of active materials, accumulation of insulating layers on the anode increasing the share of electric vehicles on the roads, however, and capacity fading. The second problem is the large electrolyte vol- L –1 calls for energy storage devices that embrace lower cost and higher ume/sulfur ratio (typically >15:1 μl mg ) required to wet the large energy density as well as longer cycling life1,2. Li–S batteries that volume of porous carbon (usually >40 wt% in the electrode) and couple Earth-abundant and high-capacity sulfur positive electrodes to solubilize the LiPSs. This greatly reduces the volumetric energy (cathodes) coupled with lithium negative electrodes (anodes) are density of Li–S batteries, in particular for high sulfur loading cells. considered among the most promising candidates to achieve a low- In the past few years, we have witnessed Li–S batteries with greatly cost and high-energy-density system3–6. Contributing factors are the improved capacity and cycling life enabled by multifunctional very high abundance of sulfur, its relatively low mass and its ability cathodes and electrolytes. The interfacial phenomenon — strong to adopt a wide variety of oxidation states. The batteries rely on the chemical interaction between the host materials and the dissolved reversible reaction: LiPSs — is essential to suppress LiPS diffusion and thus achieve a long cell lifetime. Discharge In this Review, we show that a variety of chemical interactions + – 2 Li + 2 e + x S Li2Sx 1 < x < 8 (1) can be realized by different sulfur host materials. The surface func- Charge tionality, intrinsic polarity, electro-/nucleophilicity and/or redox potential play an important role in determining the strength of the The redox chemistry proceeds through lithium polysulfide (LiPS) interaction. Special attention is paid to spectroscopic and compu- intermediates consisting of Li2Sx chains reaching the end member, tational simulation probes. We also present materials engineering Li2S, where x = 1. Fundamental challenges facing Li–S batteries orig- efforts to build smart cathode architectures to allow high sulfur inate from the insulating properties of elemental sulfur and lithium loading. Finally, we describe newly emerged approaches involv- sulfides, the dissolution of lithium polysulfides in the electrolyte, the ing modifications of current ether-based electrolytes and explora- volume changes at the cathode on cycling and the need to passivate tion of new electrolyte systems to control polysulfide dissolution membranes at the anode to inhibit dendrite formation. Moreover, it and sulfide deposition. We note that although here we choose to is now widely realized that high sulfur loading electrodes are essen- focus on cathodes and liquid electrolytes, other topics such as chal- tial for Li–S technology in the marketplace7–9. In such cathodes, the lenges at the lithium-metal negative electrode and the prospect of aforementioned problems are amplified due to the difficulties in -cre solid-state sulfur batteries are important and merit attention in ating a continuous electron/Li+ pathway over thick cathodes and in future perspectives. protecting the anodes from LiPS corrosion and dendrite formation at high currents. Trapping lithium polysulfides Infiltrating molten sulfur into porous conductive carbon mate- LiPSs, long- or short-chain, are intrinsically polar species with the rials was the main approach in the early stages10–15, resulting in terminal sulfur bearing most of the negative charge18–21. Physically interconnected conducting networks and enhanced physical confining LiPSs in the pores of non-polar carbon materials proves entrapment of the LiPSs. Examples of hosts included meso- and effective only on short- and medium-term cycling (~a few hundred microporous carbons, carbon nanotubes and fibres, graphene, and cycles). Owing to weak intermolecular interactions, the LiPSs diffuse mixtures thereof, reviewed elsewhere3–5,16,17. There are two funda- out of the cathode and eventually migrate to the anode. Recently, mental problems with this approach. As illustrated in Fig. 1, simple sulfur host materials that exhibit strong chemical interactions with physical (spatial) entrapment is not sufficient to prevent diffusion LiPSs have been studied and appear to be an effective approach to

University of Waterloo, Department of Chemistry and the Waterloo Institute of Nanotechnology, 200 University Avenue, Waterloo, Ontario N2L 3G1, Canada. †These authors contributed equally to this work. *e-mail: [email protected]

REVIEW ARTICLE NATURE ENERGY DOI: 10.1038/NENERGY.2016.132

a b

Discharge – + Charge

LiX/ether LiX/ether

LiPSs LiPSs

Li C/S Li Shuttle mechanism C/ Li2S

Self discharge

Figure 1 | Fundamental problems with using porous carbon as a sulfur host material. Physical entrapment of sulfur and lithium polysulfides (LiPSs) in the positive electrode is not sufficient to prevent dissolution and diffusion into the electrolyte.a , On discharge, sulfur is ideally reduced to LiPSs and insoluble Li2S at the cathode. Diffusion of soluble LiPSs to the anode results in self-discharge via their chemical reduction at the lithium surface, build-up of an insulating

Li2S/Li2S2 layer and loss of active material. b, On charge, Li2S is ideally converted to sulfur at the cathode; however, redox shuttling of soluble LiPSs between the electrodes gives rise to poor coulombic efficiency and leads to precipitation of 2Li S at the exterior cathode surface. Stick-and-ball structures represent long- and short-chain LiPSs (Li2S8, Li2S4, Li2S2); LiX: electrolyte lithium salts. Black and green balls represent sulphur and lithium atoms, respectively. stabilizing the capacity (up to ~2,000 cycles). This strategy is based Porous carbons and graphenes doped with heteroatoms repre- on interfacial phenomena rather than spatial confinement22. Here, sent another class of polar carbonaceous materials (Fig. 2b)30–35. we highlight the latest developments regarding chemical interac- Introduction of electronegative N atoms into the carbon lattice, tions with LiPSs realized by different sulfur host materials, includ- first suggested in studies of mesoporous N-doped carbon30, induces ing polar–polar interactions, Lewis acid–base interactions and asymmetric charge distribution. This affects the net polarity, cre- sulfur-chain catenation. ating sites for binding LiPSs. Pair distribution analysis of X-ray scattering data for polysulfide-contacted carbon revealed strong Polar–polar interaction based on polar surfaces. The polar–polar Li-N interactions34. However, the upper threshold of N doping con- interaction is a strong chemical interaction between polar LiPSs and centrations (~14.5 at%)36 limits improvement of these materials. polar host materials that can be tuned to adsorb LiPSs (Fig. 2a). A Graphitic carbon nitride (g-C3N4) shows high LiPS adsorptivity due wide variety of such host materials have been recently developed to its ultrahigh N concentration (53%; Fig. 2b)37,38, but an ultrahigh to chemically interact with LiPSs, including modified carbona- concentration of nitrogen also lowers the electronic conductiv- ceous materials, functional polymeric materials and beyond-carbon ity38. N, S dual-doped graphene and nanoporous carbon have both materials. For these host materials, sufficient electronic conductiv- shown high LiPS adsorptivity and long-term cycling stability39,40. ity is also important to ensure high sulfur utilization, especially in X-ray photoelectron spectroscopy (XPS) studies have revealed 2– thick sulfur electrodes. Partially reduced graphene oxides (rGOs) highly synergistic chemical interaction of Li–N and Sx -doped S are the most widely studied sulfur host materials, due to their oxy- (ref. 40). gen functional groups, 2D nanosheet structure and easy composite Polymeric materials containing functional groups have been fabrication23 (Fig. 2b). The rGOs contain hydroxyl, carboxyl and used in conjunction with carbonaceous materials. Early studies 20,24 ester groups that have been shown to effectively bind S8 and showed that wrapping a thin layer of polar polyethylene glycol onto 10 Li2Sx. There has been a progressive understanding of the function of a mesoporous carbon composite was advantageous . Functional rGOs in the Li–S battery field and several groups have studied dif- groups comprising highly electronegative elements (N, O, S) have ferent perspectives. The presence of O–S and C–S bonding between since been reported to be effective anchor sites, including nitriles, sulfur and rGOs was confirmed using X-ray absorption and emis- amines, pyrrolidones, esters and thiophenes41–45. The polar–polar 24,25 2– •– + 10,41,42,44,46 sion spectroscopies . The interaction of S3 (or S3 ) with rGOs interaction of Li with these electron-rich groups is key . was studied using first-principles calculations and charge transfer Polyvinylpyrrolidone modifies tubular carbon to create a polar sur- was observed20. Although there is no universal understanding on face for LiPSs binding43 (Fig. 2c). The improved binding efficiency the exact configuration of the interactions due to the complexity of compared with non-coated surfaces is evidenced by reduced Li2S rGOs, their strong effect in suppressing the polysulfide shuttle has detachment from the inner walls after full discharge. Alternatively, led to several reports of stable cycling performance. A sulfur com- coating sulfur nanoparticles with conductive poly(3,4-ethylenedi- posite prepared by depositing nanosulfur on rGOs exhibited a low oxythiophene) can enhance both conductivity and LiPS binding capacity fading of 0.039% per cycle over 1,500 cycles in an ionic capability45 (Fig. 2c). A minimum thickness of this layer (~20 nm) liquid electrolyte26, and wrapping rGOs around sulfur or carbon– is required to sustain the LiPS binding and entrapment over pro- sulfur composites was highly effective compared with bare sulfur longed cycling. Examination of the LiPS binding strength and electrodes27–29. With recent developments in large-scale produc- cycling stability of nitrile-rich molecular sorbents has clearly dem- tion of rGO, we consider it to be a very promising material for the onstrated that introduction of 2 wt% of these molecules (imidazo- commercialization of Li–S batteries23. lium and acrylonitrile) can lead to stabilized cycling, owing to their

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Graphene oxides Doped carbons g-C3N4

Li–O/N S–S c d

PVP Li S 2 8 Li–O/S Li2S2 Li–O/S Ti–S [ ]n

Ti4O7 PEDOT Li2S4 Li2S

[ ]n TiS2

Figure 2 | Summary of the polar–polar chemical interactions between lithium polysulfides and polar sulfur hosts. a, A representative discharge–charge voltage profile of a Li–S cell, showing sequential formation of lithium polysulfides at different redox stages. The LiPSs strongly interact with polar sulfur hosts, such as graphene oxides, heteroatom-doped carbons and graphitic carbon nitride (b), functional polymers, for example polyvinylpyrrolidone (PVP) and poly(3,4-ethylenedioxythiophene) (PEDOT) (c), metal oxides and sulfides d( ). As the black arrows indicate, these types of interaction are + 2– characterized by the bonding between N, O and S polar units (or metal centres, M) and Li (or Sx ) ions where charge transfer is involved. Representative interaction sites for each case are indicated by a dotted circle. Black, red, yellow, green, blue, light grey and light blue balls represent carbon, oxygen, sulfur, lithium, nitrogen, hydrogen and titanium atoms, respectively.

interaction with LiPSs41. A clever twist on functional polymer coat- Metal–sulfur bonding. Instead of relying on polar–polar interac- ings was recently demonstrated by using a layer-by-layer growth of tions that involve Li+ and polar units, suitably tailored hosts can 2– polyelectrolytes to create an onion-skin-like ion-selective, flexible also bind LiPSs via metal–sulfur bonding. Polysulfide anions (Sx ) membrane on the surface of carbon–sulfur composites47. are soft Lewis bases owing to the sulfur lone electron pairs. Host Beyond carbonaceous materials, metal chalcogenides (oxides materials that exhibit Lewis acid characteristics are therefore able and sulfides) have been evaluated (Fig. 2d)48–53. These materi- to interact strongly with LiPSs and thus trap them within or on als possess intrinsic network polarity, where the surface metal the host surface57 (Fig. 3a). Examples of such materials are metal– 2– + or chalcogen ions synergistically interact with the Sx and Li organic frameworks (MOFs), MXene phases and sub-stoichiometric ions. Addition of mesoporous SiO2 and TiO2 additives to the metal chalcogenides. carbon–sulfur composites results in the reversible storage and MOFs represent a class of host materials with high surface release of polysulfide intermediates in and from the pores during area, tunable porosity and variable surface functionality, but low discharge48,51. Spectroscopic evidence of the chemical interaction conductivity. MIL-100(Cr) was the first to be used to encapsulate 51 between these oxides and LiPSs is clear . TiO2 and TiS2 coatings and bind the LiPSs owing to its unique pore structure and surface 58 59,60 have also been successfully used to encapsulate sulfur or Li2S par- functional groups , with MIL-101(Cr) and ZIF-8 following . It ticles, to provide both physical confinement and chemical adsorp- was realized that the abundant open coordination metal sites in

tion for LiPSs. A well-designed sulfur–TiO2 yolk–shell structure the MOFs are soft Lewis acids and thus coordinate readily to poly- exhibited low capacity fading of 0.033% per cycle50,53. Nickel nitrate sulfide anions. Benefitting from this approach, improved cycling 54 hydroxide combined with carbon materials is also promising , stability using a Ni-based MOF (Ni6(BTB)4(BP)3) as a sulfur host but its low conductivity is disadvantageous. Cui and co-workers was reported57. Computational studies revealed that the Lewis acid designed a hybrid electrode of conductive indium tin oxide pat- Ni(ii) centre prefers to coordinate with the terminal sulfur atom as terned on a non-polar carbon substrate, providing visual evidence an axial ligand (Fig. 3b). This strong interaction was experimentally of preferential adsorption and thus reduction of LiPSs on the oxide evidenced by shifts in XPS binding energies, studies which further- surface55. This illustrates a way to spatially control deposition of more correlated low LiPS binding energy with inferior cycling sta- 57 Li2S within the electrode. The same approach utilized the metallic bility . It was suggested that the strength of the Lewis acid–base 61 Magnéli phase Ti4O7 as an excellent bifunctional sulfur host in a interaction is governed by the number of acidic sites in the MOFs . full cell (Fig. 2d)22,56. The shift of the XPS Sp 2 spectrum towards a Each Cu2+, Fe3+ and H+ site in the MOF-525 frameworks offers two,

higher binding energy for the Ti4O7-contacted Li2S4 indicates elec- one and zero Lewis acid sites, respectively. This was correlated with 2– 22 tron transfer from the Sx to the metal oxide surface . Operando the cycling stability of the corresponding sulfur electrodes, further X-ray absorption near-edge spectroscopy analysis on the cycling confirming the interaction. electrode showed a greatly diminished concentration of LiPS inter- To overcome the insulating nature of MOFs, the strategy of

mediates and earlier precipitation of Li2S, owing to the intimate entrapping LiPSs via Lewis acid–base bonding has been extended to metal oxide–LiPSs interface. other classes of materials that possess high electronic conductivity.

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Lewis base Lewis acid

+ M . M Filled orbital Vacant orbital Coordination

b c

Li S H O Melt diusion Discharge Discharge Ti C N Ni

Figure 3 | Entrapping lithium polysulfides by metal-sulfur bonding.a , Schematic of the interaction between the polysulfide Lewis base and the transition metal acid centre. Coordination occurs between the filled orbital (lone electron pairs) of the polysulfide anion and the vacant orbital of the transition metal.

The schematics show that the porous Ni–MOF, Ni6(BTB)4(BP)3, binds Li2S4 through the Ni centre (green) (b) and that the Ti2C MXene nanosheets bind S8,

Li2S4 and Li2S through the surface Ti atoms (via a Ti–S bond) (c). See legend for atom colours. Panel b reproduced from ref. 57, American Chemical Society; panel c adapted from ref. 63, Wiley-VCH.

2– 2– MXenes are a family of semiconducting or metallic early-transition- thiosulfate groups (S2O3 ) on the surface. Long-chain Sx (x > 4) is 62 2– metal carbides or carbonitrides . A fully delaminated MXene catenated into the S–S bond of S2O3 species to form polythionate 2– phase, Ti2C, combines metallic conductivity with a 2D surface that complexes (O3S–Sx−2–SO3) , leaving the short-chain Li2S and Li2S2 63 69 is abundant with Lewis acid Ti sites and hydroxyl groups (Fig. 3c) . in electrical contact with the MnO2 host (Fig. 4b). These catenated A strong Lewis acid-based interaction of the lithium polysulfides S–S chains in the polythionate are electrochemically active during with the surface is demonstrated by metal–sulfur (S–Ti–C) binding the following discharge–charge processes. The LiPS intermediates at the interface. Both mitigated diffusion of LiPSs and abundant Li2S are thus anchored, creating a good interface for Li2S deposition. nucleation sites act to control Li2S deposition on discharge. Stable Li–S cells exhibit very low LiPS solubility, greatly suppressed self- cycling performance with a capacity fade rate of 0.05% per cycle discharge and long life (up to 2,000 cycles). The key conversion was achieved. Ti has also been electrochemically extracted from reaction is triggered by metal oxides, such as MnO2 and VO2, with + the MAX (Mn+1AXn, layered metal carbides and nitrides)-phase a redox potential between 2.40 and 3.05 V vs. Li/Li . Materials with Ti2SC to form carbon–sulfur (C–S) nanolaminates that exhibit C–S higher redox potentials over-oxidize LiPSs to inactive sulfate moie- bonding64. In the case of under-coordinated transition metals in ties, whereas those with lower potentials are redox inactive70. The sub-stoichiometric oxides and sulfides (Ti4O7 and Co9S8) and other same conversion reaction was observed for rGO (which is, in prac- sulfides (CoS2 and FeS2), Lewis acid–base coordination also partially tice, only partially reduced GO), possibly accounting for its excellent contributes to the strong interaction in addition to the polar–polar properties69. The study opens up the exploration of other materials interaction discussed above65–67. with targeted redox potentials. This principle was further extended to in situ formation of scalable ‘yolk–shell’ composites containing a

Surface chemistry for polysulfide grafting and catenation.Sulfur core of 75 wt% micrometre-sized sulfur, surrounded by a thin MnO2 71 readily undergoes catenation reactions where sulfur chains are shell . The bifunctional MnO2 shell provides polysulfide entrap- grafted into organic or inorganic materials68,69. A class of sulfur-rich ment via physical confinement and chemical interactions, leading compounds, lithium polysulfidophosphates (Li3PS4+n), was gener- to very stable cycling. 3– 68 ated by grafting sulfur atoms to the PS4 anions of Li3PS4 (Fig. 4a) . These promising new compounds preserve the ionic conductivity of Understanding chemical interactions –5 –6 –1 + LiPS4 (10 to 10 S cm ), favouring Li -ion diffusion in the solid Experimental techniques including XPS, infrared and Raman spec- state and reversible electrochemical reactions that involve breaking troscopies22,34,41,46 have been routinely used to probe the chemical and reforming S–S bonds. interaction between LiPSs and sulfur hosts. Computational stud- Recently, our group reported entrapment of LiPSs by a differ- ies based on first-principles calculations also provide very valu- ent mechanism that relies on chemical catenation of LiPSs by the able metrics regarding the strength of the interaction based on 69,70 thiosulfate–polythionate conversion . First reported for δ-MnO2 binding energies. Non-polar carbon materials do not exhibit spe- nanosheets, the surface Mn(iv) ions undergo redox reaction cific bonding configurations with LiPSs: very low binding ener- 2– with the electrochemically formed Sx anions to form functional gies (0.1–0.7 eV) are observed and are attributed to van der Waals

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THF +

Li3PS4 S8

Li3PS(4+n) b

S O 2– 2 3 Li P Nucleophilic attack S O + 2– S (in Li3PS4 or S2O3 )

Li2S Li2Sx Polythionate

Figure 4 | Strategy of binding polysulfides by sulfur-chain catenation via nucleophilic substitution. a, The sulfur chain from the S8 ring can be grafted into

Li3PS4 by nucleophilic attack of the P–S bonds in tetrahydrofuran (THF). b, Trapping polysulfides by thiosulfate–polythionate conversion, realized by using metal oxides with appropriate redox potentials. Panel a adapted from ref. 68, Wiley-VCH.

forces36,37,43,72. Other materials exhibit much higher values, and are can lead to the reactive dissociation of polysulfides that potentially categorized on this basis in Fig. 5. deteriorate the sulfur redox reaction19,75. Functional polymeric materials lie in the first region, exhibiting The binding energies of MOFs and some crystal faces of non-

relatively weak LiPS binding (Fig. 5a). A computational screening stoichiometric metal oxides and sulfides (Ti4O7, Co9S8) for poly- of functional groups grafted on the framework of vinyl polymers sulfides are among the highest reported, which is due to Lewis • 56,57 demonstrates their binding strength towards Li2S and LiS spe- acid–base bonding (Fig. 5d) . Metal centres with free d-orbitals 72 2– cies . Electron-rich groups with lone pairs of electrons tend to are able to coordinate to nucleophilic Sx anion clusters. It was bond with Li+, with the carbonyl (C=O) groups showing the high- reported that Ni(ii) and Co(ii) MOFs show high LiPS binding ener- est binding energy. This type of interaction can be generalized gies of 4.0–6.0 eV, and Ni–S (Co–S) bond formation for LiPSs was in the form of Li–X (X = N, O, S), with a relatively low binding observed. It was also found that under-coordinated Ti sites on the 41,73 energy of 0.5–1.3 eV (Fig. 5a) . However, it remains unclear as (1–20) surface of Ti4O7 are favourable adsorption sites for S clus- to whether this originates from the electro-/nucleophilicity of the ters56. Recently, the binding energies of polysulfides on three crystal + δ– + Li and X ions, the electrostatic forces between the X and Li ions, facets of Co9S8 with different surface Co/S ratios were examined. or a combination of both41,72. In the second class, carbon materials The (008) facet terminated with under-coordinated Co exhibits a doped with heteroatoms (N, O, S) show higher binding energies in much higher binding energy than the (002) facet, which relies on 30,34,74 65 the range of 1.3–2.6 eV (Fig. 5b) . The binding geometry of Li2S Li–S interactions . In practice these oxide and sulfide materials are 74 and Li2S4 on two different doped N sites was computed , showing able to bind LiPSs via both polar–polar Li–S(O) interaction and that Li+ ions bond directly with pyridinic nitrogen, but only indi- Lewis acid–base bonding, depending on the exposed facets. rectly with quaternary nitrogen via the neighbouring carbon atoms In this stage of first-principles calculations, the computational because of electron delocalization. N, S dual-doped carbons show parameters used have not been universal, nor have many differ- superior cycling stability over N-doped carbon. First-principle cal- ent LiPSs been explored, making systematic comparisons difficult. culations40 revealed the main reason is a synergistic interaction of However, the geometrical configurations and binding energy val- 2– Li–N and Sx -doped S. The chemical bonding of LiSH with thionic ues available to date provide a highly valuable understanding of the S was also identified39. mechanism and strength of the LiPS binding, which should inspire The third class of sulfur hosts with higher binding energy exploration of new sulfur host materials. (2.6–3.5 eV) are represented by stoichiometric metal oxides and sulfides (Fig. 5c). A study of layered materials, including oxides, Optimizing cathode architecture sulfides and chlorides of different transition metals, shed light on Techno-economic analysis suggests that cathodes with areal sul- their interaction with LiPSs19. Layered slabs terminated with O, S or fur loadings of >7 mg cm–2 are necessary for Li–S batteries to find Cl anions without dangling bonds were used as the host substrates, practical applications in transportation8. While significant efforts and LiPSs at different lithiation stages were examined. This inter- have been devoted to entrapping LiPSs within the cathode and rea- action exists mainly in the form of Li–O/S/Cl bonding and gener- sonably stable capacity retention over long-term cycling has been ally metal oxides show higher binding energy for LiPSs than metal attained, building high-sulfur-loading Li–S cells poses a great chal-

sulfides. Recently, the interaction of MoS2, MnO2 and Fe2O3 with lenge. The areal specific capacity of a Li–S cell scales linearly with long-chain polysulfides (Li2S6, Li2S8) was studied in the presence of increasing cathode thickness up to a critical value, followed by a ether solvent molecules75. In addition to the Li–O/S interaction, it decrease. The poor sulfur utilization in thick electrodes is attributed was found that for some metal exposed facets, the S–M (M = Mo, to the significant changes in the mass transport and spatial distribu- Mn, Fe) interaction contributes significantly to polysulfide adsorption of dissolved polysulfides throughout the electrode. This leads to tion. Importantly, in both reports, it was pointed out that for some limited diffusion pathways, kinetics and electrolyte uptake that may

metal oxides (for example, Fe2O3, V2O5), the strong M–O interaction not be able to support high current densities. Challenges in thick

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a b c d

MOF, non-stoichiometric Li2S2 transition metal chalcogenides Stoichiometric metal chalcogenides 3.5–7.0 eV H Heteroatom-doped 2.6–3.5 eV Lewis acid-base Li (N, S, O) 2– N-, O- or S- containing bonding (M–Sx ) O Li+–O(S)–M polymers/molecules 1.3–2.6 eV interactions C N 0.5–1.3 eV Li+–N/S/O, S–S Eb interactions S + Li –N/O/S Ti interactions Ni

Figure 5 | Binding energy metrics for chemical interactions between lithium polysulfides and sulfur hosts. Results are derived from computations. The interactions are intuitively categorized by the strength of the calculated binding energy, Eb. The Li2S2 molecule is used as the representative polysulfide. + a, Functional polymers interact in the form of Li –N/O/S interactions, with Eb ranging from 0.5 to 1.3 eV. b, Heteroatom-doped carbons interact mainly via + + Li –N/O or S–S bonding, with Eb between 1.3 and 2.6 eV. c, Stoichiometric metal chalcogenides interact mainly in the form of Li –O/S–M, with Eb ranging from 2.6 to 3.5 eV (M = metal). d, Metal organic frameworks (MOFs) and non-stoichiometric metal chalcogenides interact in the form of Lewis acid–base bonding, exhibiting the highest Eb values ranging from 3.5 to 7.0 eV. Panel d adapted from ref. 57, American Chemical Society. electrodes arise in designing an architecture that can accommodate is attributed to the excellent electron and ion conduction pathway a large amount of sulfur and the corresponding volume change, through the electrode and reasonable polysulfide confinement. effectively entrap the LiPSs, and maintain the ionic and electronic conducting pathways over cycling, while minimizing the electrolyte Cross-linking sulfur hosts. While 3D frameworks may provide supe- volume. Moreover, the overall density is also an important factor rior areal sulfur loading, ionic and electronic conductive pathways in reaching high energy density. This requires not only high areal and mechanical properties compared to their 2D counterparts, they sulfur loading, but also a low fraction of electrochemical inactive have intrinsic disadvantages of lower volumetric energy density due materials, compact packing of the sulfur components and opti- to two reasons. First, the carbon scaffold accounts for a large fraction mal porosity within the cathode. Simply increasing carbon and/or of the electrode volume. Second, the excess void space in these 3D binder content only neutralizes the advantageous energy density of porous current collectors requires a larger amount of electrolyte to wet the overall cell. Constructing free-standing 3D electrodes and cross- the electrode8,9. Therefore, modifying conventional sulfur composites linking conventional nanostructured sulfur hosts are the two main so that they can be slurry-cast on current collectors to form crack- approaches adopted to date for high-areal-loading cathodes (Fig. 6). free films, while ensuring sufficient electron transport through the composite, is an attractive alternative approach. This can be accom- Free-standing 3D framework electrodes. Constructing 3D car- plished by cross-linking the individual sulfur–carbon nanoparticles bonaceous architectures to directly impregnate sulfur has been to form large secondary structured particles34,83 (Fig. 6g). Recently, proposed (Fig. 6a). This facilitates charge transfer and provides individual Ketjen black particles were connected using conductive mechanical support to accommodate the volume change. Examples amorphous carbon derived from in situ polymerization, forming an include porous carbon cloths and graphene sponges, impregnated integrated structure. Crack-free cathodes with sulfur loadings up to with either sulfur or a polysulfide containing electrolyte (catho- 8 mg cm–2 were achieved83. Also, CNTs were used to interpenetrate lyte)20,39,76–78. A free-standing activated carbon fibre cloth was N-doped porous carbon to construct micrometre-size secondary first employed to load up to 6.5 mg cm–2 sulfur and stable perfor- spheres. Owing to additional nitrogen functional groups in the CNT mance for 80 cycles was achieved76. Notably, a catholyte cell using framework, Li–S cells with a sulfur loading of ~5 mg cm–2 exhibited a N, S dual-doped graphene sponge can deliver an areal capacity stable capacity retention over 200 cycles (Fig. 6h)34. A long-range 3D –2 39 of 5.7 mA h cm over 200 cycles (Fig. 6b) . A 3D graphene foam interconnected architecture fabricated from Co9S8 nanosheets allows (Fig. 6c) can improve the elasticity of the overall electrode, exempli- sulfur electrodes with up to a 4.5 mg cm–2 loading and 75% sulfur fied by infiltrating a conventional sulfur–carbon composite slurry content to cycle without notable capacity fading65. into the large pores of a polydimethylsiloxane-coated graphene Achieving high areal sulfur loading with a low electrolyte/sulfur foam (Fig. 6d)78. The graphene foam provides a sufficient electron ratio is vital at this stage of research. Fine tuning of the conductive pathway, while the polymer coating ensures the network flexibil- pathways, the void space volume and the elasticity of the electrodes is ity78. The resulting Li–S cell underwent 1,000 cycles with a fade rate needed. Compared with lithium-ion intercalation electrodes such as of 0.07% per cycle at a sulfur loading of 10.1 mg cm–2. lithium nickel cobalt manganese oxide, sulfur electrodes are inferior Layer-by-layer fabrication of 3D electrodes (Fig. 6e) offers a prac- with respect to volumetric energy density, as a consequence of the tical approach towards high sulfur loading. The porous scaffold can lower density of sulfur and a high fraction of carbon additives. Issues be made of carbon nanofibres or carbon nanotubes (CNTs)79–82. The associated with the lithium anode are also masked due to the shal- top and bottom carbon layers function as current collectors, and the low cycling of thin electrodes, and the resulting Li–S cell may seem central layers serve as electron connective layers. The areal sulfur to be stable for hundreds or even thousands of cycles. No doubt, the loading can be augmented by simply adjusting the number of layers anode issue will only become more problematic for thick electrodes. (Fig. 6f)80. A six-layer cathode was fabricated, corresponding to an Therefore, stabilizing the lithium anode and/or minimizing poly- areal sulfur loading of 11.4 mg cm–2, which survived over 100 cycles. sulfide cross-over is critical. We note that the reported capacity reten- A laminated nanostructure cathode was designed by cross-stacking tion profile is usually less than 200 cycles, and typically ~50 cycles. multiple aligned CNT sheets with sandwiched CMK-3/S composite This indicates that other problems, such as the polysulfide diffusion particles81. The multi-layered cathode achieves up to 20 mg cm–2 sul- and shuttle effect and cathode and anode impedance layer build-up, fur with an initial capacity of 900 mA h g–1. The high areal capacity are still prominent in these cells; factors that must be addressed84.

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a c e g

Sulfur–carbon composite Elemental sulfur Carbon scaold

b d f h

Upper layer

Middle layer

Bottom layer 5 µm 150 µm 100 µm 20 µm

Figure 6 | Various cathode architectures for high areal sulfur loading in Li–S cells. Top: Schematic models of free-standing 3D graphene sponge–sulfur (a), sulfur–carbon–graphene foam (c), a multilayer carbon paper–sulfur sandwiched structure (e) and cross-linked sulfur hosts (g). Scanning electron microscopy images of representative electrode structures for each model. b, N, S co-doped graphene sponge; d, polymer-coated 3D graphene foam; f, stacked carbon nanofibre layers;h , N-doped carbon nanotube-interpenetrated carbon spheres. The insets in b, d and f are photographic images of the corresponding free-standing 3D electrodes used in actual Li–S cells. Panels adapted from: b, ref. 39, NPG; d, ref. 78, Elsevier; f, ref. 80, Wiley-VCH; h, ref. 34, Wiley-VCH.

Role of electrolytes redox couples that undergo electrochemical reduction or oxidation To overcome the LiPS dissolution and shuttle problem, studies at the electrode and diffuse to the active material. They allow the have been devoted to the development of new electrolytes with redox of insoluble species to be electrochemically coupled to the suppressed LiPS solubility and redox mediators that facilitate electrode surface, even when the active material is not in direct elec- electrochemical processes. tronic contact. Their electrochemical potential must be matched to that of the process that they are mediating. They have been widely

Conventional electrolytes and additives. Generally, electro- exploited in Li–O2 batteries to oxidize insulating Li2O2, but their lytes for the studies discussed above were based on ethers such as roles in the sulfur cell are just emerging. 1,2-dimethoxyethane (DME) and tetra(ethylene glycol) dimethyl Sulfur redox chemistry takes place as two electrochemically ether (TEGDME). The cyclic ether 1,3-dioxolane (DOL) is used as distinct steps on discharge; a sloping region centred at ~2.3 V cor-

a co-solvent as a result of its lower viscosity and ability to form a responding to conversion of elemental sulfur to Li2S4, followed by protective polymeric film on lithium metal because of ring-opening a low-potential plateau at ~2.1 V attributed to the precipitation of 85 polymerization . Up to ~6 M Li2S8 can be dissolved (total atomic insoluble Li2S deposits via disproportionation of polysulfides, such 3,7 S concentration) in a 1 M lithium bis(trifluoromethane)sulfonimide as Li2S4, from solution . Oxidizing insulating Li2S requires a signifi- (LiTFSI)–TEGDME electrolyte86. The advantage of dissolved LiPS cant initial overpotential if the soluble polysulfides — which act as intermediates is that they provide fast kinetics for redox reactions internal redox mediators — are fully consumed on discharge and/or

compared with solid-solid reactions. However, without effective if the electron transfer to the Li2S deposits is limited by their large entrapment, the soluble sulfur species diffuse to the lithium anode dimensions. In a cell assembled in a discharged state using bulk

surface, where — without anode protection — they are chemically Li2S as the cathode, a redox mediator is required. It must have an reduced to form thick insoluble and insulating layers of Li2S2/Li2S, equilibrium potential just above that of Li2S, so it can be oxidized resulting in high surface impedance (Fig. 7a). Partial electrochemical at the electrode surface and diffuse to, and oxidize, the Li2S par- oxidation of this layer on the charge cycle leads to the ‘shuttle’ effect ticles. Aurbach and co-workers were the first to demonstrate that and low coulombic efficiency. The shuttle at around 2.4 V gives rise metallocenes are highly effective91. The metallocene oxidizes the 87 to a significant parasitic current , which is exacerbated with high- Li2S to polysulfides, in turn is reduced, whereupon it diffuses to the sulfur-loading electrodes. Diffusion of polysulfides accounts for electrode surface to be re-oxidized for another cycle.

the self-discharge of the Li–S batteries and shortened shelf life. To To control precipitation of Li2S, an important new approach has uti- 88 mitigate its impact, LiNO3 is used as an electrolyte additive and lized benzo[ghi]peryleneimide (BPI) as a mediator on the lower volt- 89 P2S5 has been proposed . A new approach utilizes lithium iodide to age plateau where reduction of Li2S4 to Li2S occurs. As the reduction induce formation of a protective coating on both the cathode (Li2S) potential of BPI is slightly less than the plateau voltage, it is reduced at and Li anode via the I• radical-initiated polymerization of the DME the electrode surface and diffuses away to reduce polysulfides in solu- electrolyte solvent. The in situ formed layer contains iodine and pre- tion, remote from the surface. Formation of large domains of insulat-

vents dissolution of polysulfides on the cathode side and reduction ing intractable films of Li2S that cover the cathode and shut down the 90 of polysulfides on the anode side . cell is thus avoided. Instead, Li2S is preferentially deposited on pre- existing sulfide nuclei, favouring localized three-dimensional sulfide 92 Redox mediators. Controlling precipitation of Li2S on the cathode deposition and doubling the capacity . host on discharge, and lowering the overpotential for its oxidation on charge are two very important aspects. Both can be facilitated Development of LiPS non-solvent electrolytes. The dissolution of by redox mediators. Redox mediators are molecules with reversible LiPSs, like any other Li salts, relies in part on the solvation of Li+

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REVIEW ARTICLE NATURE ENERGY DOI: 10.1038/NENERGY.2016.132

a b Carbons 1 M LiTFSI DME/DOL

+ – Li (ACN)2TFSI Li2S6

Solid Li2S + – Li (TEGDME)nTFSI

Li2S4 Li anode Li anode

+ – Li (DME)nTFSI

Solid sulfur species Polysulﬁde catholyte

Figure 7 | Concept of using non-solvents compared with typical ether-based electrolytes. The non-solvents are exemplified by ionic liquids: TEGDME–

LiTFSI solvates, DME–LiTFSI and (ACN)2–LiTFSI. ACN = acetonitrile. a, In a conventional ether-based electrolyte, polysulfides are partially dissolved, enabling their crossover to and reduction on the Li anode to form an insulating Li2S layer. b, In non-solvent electrolytes, polysulfides are formed but largely confined within the cathodes in the solid state; in principle, the Li anodes are free of sulfide coatings. The extremely low solubility of the polysulfide in these non-solvent systems arises from the strong complexation of solvent molecules with Li+ (and/or TFSI–), leaving little available free solvent for polysulfide solvation. ions. Electrolyte solutions with lower electron donor ability are less and mass transport resistance — limiting the rate capabilities and able to solvate Li+ ions and thus can suppress the LiPS solubility energy efficiency during cycling at room temperature. The high den- (Fig. 7b). Reported ‘non-solvent’ electrolytes include amide-based sity and cost of the salts add to their disadvantages. An alternative room-temperature ionic liquids (RTILs) and solvates of Li salts approach to developing new electrolytes is to maximize LiPS solu- (mostly LiTFSI) in conventional solvent systems. RTILs, consist- bility using high donor number solvents such as dimethylacetamide ing of coordinated cations and anions, are well known for their and dimethylsulfoxide98, or even water. Fast reaction kinetics are high electrochemical stability, non-flammability and non-volatility. expected for these electrolytes and the existence of the trisulfur Lacking any free solvent molecules, the donor ability of RTILs is radical can tune the equilibria among LiPSs, leading to catalytic oxi- solely dependent on the anionic component, which is related to the dation of Li2S (refs 99,100). However, as these electrolytes are reac- LiPS solubility93. LiPSs have much lower solubility in an ionic liquid tive with lithium, anode protection is necessary. Considering the with a low Gutmann donor number (DN) of ~10 kcal mol−1 than advanced architectures described in the last section, we believe that one with a DN of ~20 kcal mol−1 (ref. 93). Other RTILs with amide combining an electrolyte with high LiPS solubility with an effective anions also show suppressed polysulfide solubility93. LiPS adsorptive host could lead to a superior cell configuration with The intrinsic drawback with these RTILs is their poor rate capabil- minimum electrolyte volume. ity due to their low Li+ ion diffusion coefficients and+ Li transference number. The design of a non-solvent electrolyte based on organic sol- Outlook vents is more promising. Adding a high concentration of Li salts into Significant advancement in understanding the electrochemistry ether solvents can decrease the ability to solvate other lithium species, governing the aprotic Li–S battery within the last few years — in that is, LiPSs. The concept was first explored in a glyme–Li equimolar tailoring surfaces, electrode architecture and electrolytes to extend complex94. Glyme is favourable for the solvation of Li+ ions due to cycle life — has brought the prospect of its practical commercializa- its Lewis-basic oxygen atoms, especially for long chain glymes such tion ever closer. Key factors discussed in this review are summarized as TEGDME (‘G4’)94. Since both Li(G4)+ and TFSI− behave as dis- in Fig. 8. They remain critical as foci for future progress. crete ions, LiPS dissolution is mitigated, as it is in TFSI–-based RTIL Work on the mechanism of entrapment of polysulfides at the electrolytes. Glyme–Li salt equimolar complexes also show RTIL-like positive electrode using both computation and experiment indicates behaviour, such as high thermal stability, low volatility and a wide that ‘sulfiphilic’ cathode host surfaces are correlated with a long electrochemical window. Similar solvation properties were reported cycle life. These include metal oxides and sulfides, N- and S- doped for highly concentrated salt DME–DOL electrolytes (up to 7 M)95. carbons and graphene oxides, MOFs, functionalized metallic car- Most of the solvent molecules coordinate with the Li+ cations, leading bide nanosheets and new materials awaiting discovery. We under- to low LiPS solubility. These electrolytes showed promising results for stand why hydrophobic carbons lead to poor capacity retention on the Li–S system, especially in terms of coulombic efficiency. long-term cycling: non-specific deposition of lithium sulfide causes We recently reported an electrolyte for the Li–S cell based on pore clogging and cell failure. Polysulfide binding with surfaces via an (acetonitrile)2–LiTFSI complex, in which all acetonitrile mol- polar or Lewis acid–base interactions, or via catenation is vital. A ecules are bound by complexation96. Two benefits are the suppressed balance between too strong versus too weak interactions must be electrolyte reaction with lithium metal and LiPS solubility. To lower struck to enable polysulfide mobility, however. To ensure high cell the viscosity and enhance the ionic conductivity, 1,1,2,2-tetra- capacities and a long cycle life, especially at high sulfur loading, not fluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) was added as a only the surface chemistry but also the electronic conductivity of co-solvent. The HFE is too fluorinated to participate effectively in the host is paramount. Li+ solvation, and can therefore minimize LiPS solubility, as first To accommodate the large volume expansion and contraction on described for glymes97. A combination of operando X-ray absorption full electrochemical conversion between sulfur and lithium sulfide, as spectroscopy at the S K-edge and electrochemical studies demon- well as to mitigate polysulfide dissolution, successful approaches have strated that LiPSs are indeed formed in these acetonitrile-complexed been developed. Both polar, flexible functional coatings that encap- electrolytes. Their very limited dissolution and mobility in the sulate sulfur–conductive carbon composites and inorganic yolk–shell electrolyte strongly affect the speciation and polysulfide equilibria, structures have been shown to be effective for long life cycling. We leading to enhanced capacity and controlled formation of Li2S. note that much previous work was conducted with low loading cath- These polysulfide non-solvent based electrolytes exhibit rela- odes. Breakthroughs are still required to demonstrate a low-cost, scal- tively low ionic conductivity — due to their somewhat high viscosity able, high-areal-sulfur-loading, long-life cathode that sustains low

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Controlled Li2S deposition/dissolution

Surface chemistry

High rate capability Polysulﬁde binding Low polarization Conductivity Computation Controlled polysulﬁde shuttling Long cycle life High sulfur loading High energy density

Elasticity to withstand volume expansion Cathode Electrolyte Optimal polysulﬁde dissolution Continuous e–/Li+ architecture optimization High sulfur utilization pathways

Figure 8 | A summary of five aspects to be addressed to allow long-life and high-loading Li–S batteries.High conductivity of the cathodes is required for high rate capability and energy efficiency. Strong polysulfide interaction with the cathode is essential to control shuttling and the deposition and

dissolution of Li2S. This can be achieved by exploiting clever surface chemistry guided by computation. To achieve Li–S batteries with high areal loading and low electrolyte volume, it is also critical to design smart cathode architectures that are elastic and conductive, along with optimized electrolytes that enable high sulfur utilization.

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NATURE ENERGY DOI: 10.1038/NENERGY.2016.132 REVIEW ARTICLE

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