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Stabilizing lithium–sulphur cathodes using polysulphide reservoirs

Stabilizing lithium–sulphur cathodes using polysulphide reservoirs ARTICLE DOI: 10.1038/ncomms1293 Received 28 Dec 2010 | Accepted 28 mar 2011 | Published 24 may 2011 stabilizing lithium–sulphur cathodes using polysulphide reservoirs 1 1 1 1 Xiulei Ji , scott Evers , Robert Black & Linda F. nazar The possibility of achieving high-energy, long-life storage batteries has tremendous scientific and technological significance. An example is the Li– s cell, which can offer a 3–5-fold increase in energy density compared with conventional Li-ion cells, at lower cost. Despite significant advances, there are challenges to its wide-scale implementation, which include dissolution of intermediate polysulphide reaction species into the electrolyte. Here we report a new concept to mitigate the problem, which relies on the design principles of drug delivery. our strategy employs absorption of the intermediate polysulphides by a porous silica embedded within the carbon–sulphur composite that not only absorbs the polysulphides by means of weak binding, but also permits reversible desorption and release. It functions as an internal polysulphide reservoir during the reversible electrochemical process to give rise to long-term stabilization and improved coulombic efficiency. The reservoir mechanism is general and applicable to Li/s cathodes of any nature. Department of Chemistry and Department of Electrical Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1. Correspondence and requests for materials should be addressed to L.F.N. (email: [email protected]). nATuRE C ommunICATIons | 2:325 | DoI: 10.1038/ncomms1293 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. ARTICLE nATuRE C ommunICATIons | DoI: 10.1038/ncomms1293 ithium–sulphur (Li–S) batteries are one of the most promis- desorption of the polysulphide anions. Herein, as a significant ing candidates to couple renewable energy sources for green step towards solving the challenge of polysulphide dissolution, we Ltransportation and large-scale energy storage owing to their present a new approach to sulphur cathode composites that com- various desirable characteristics including competitive cost, and prise a carbon–sulphur nanocomposite together with a small frac- low environmental impact. Moreover, their high theoretical energy tion of a mesoporous silica additive. We were inspired by studies density is over 5 times larger than that of conventional Li-ion bat- using triblock copolymer-templated SBA-15 as a reversible drug teries based on intercalation electrodes . Despite these advantages, delivery system. SBA-15 is a well-developed mesoporous silica that massive implementation of Li–S batteries remains hindered by exhibits high surface area, large pore volume, bi-connected porous various challenges that mainly arise from the cathode. e Th major structure and highly hydrophilic surface properties . es Th e char - problem is rapid capacity fading, which is mainly due to dissolu- acteristics allow it to adsorb certain kinds of drugs and release 2 − tion of polysulphide anions (S )—intermediate reaction species them in a highly reproducible and predictable manner. It has been formed on charge and discharge—from the cathode into the elec- shown that both small and large molecular drugs can be entrapped 3,4 trolyte . Once the polysulphide ions diffuse out from the nano- within the mesopores by an impregnation process and liberated via structured cathode, not only will their reaction with the Li anode a diffusion-controlled mechanism . Although the Si–O groups that cause active mass loss, but also the redeposition of sulphide species cover its internal surface are too weak to engage in molecular drug from the electrolyte back onto the cathode surface generates large interaction without being first functionalized, they serve as ideal agglomerates at the end of discharge. e Th polysulphide anions also binding sites for the sulphur cathode. e Th cavities not only accom - act as an internal redox shuttle, which gives rise to low coulombic modate polysulphide anions through weak binding via the posi- efficiency: namely, a charge capacity larger than the corresponding tively charged silica surface, but also permit reversible desorption. discharge capacity . Various electrolyte systems have been employed u Th s, they function as an internal sulphide reservoir to absorb and to solve the problem, including polymers . A physical barrier release the active material for electrochemical reduction–oxidation that prevents diffusion of the polysulphide species could solve the during cycling. We employed SBA-15 with a platelet morphology, dissolution problem, but in the long term this could be compro- that not only exhibits good absorption properties but can also be mised. A fast-responding sulphur battery requires facile transport easily dispersed with carbon–sulphur particles. Other silicas with of electrolyte/Li into and out of the sulphur electrode, but eventu- suitable pore sizes, such as zeolite beta or MCM-41—which have ally some soluble polysulphide ions will diffuse over the physical also been employed for targeted drug delivery —should be equally barrier, which initiates the shuttle phenomenon. effective. Furthermore, we have developed and utilized a new large- Recently, there has been some promising progress made on new pore ( > 10 nm) porous carbon for the sulphur/carbon nanocompo- sulphur battery configurations. Li S has been employed as an ini- site to illustrate the efficacy of the SBA-15 additive in binding the tial active material in the sulphur electrode, to couple with non- polysulphide anions. Polysulphide diffusion is more of a concern in 8 9 15 metallic lithium anodes, that is, graphite , a carbon/tin alloy and highly open pore systems than in CMK-3/S , which was examined Si nanowires . es Th e sulphur batteries still suffer capacity loss, previously, and hence it necessitates an effective absorption addi - however, due to the untackled polysulphide dissolution problem. tive. An additional benefit is that this combination can oer ff better Cobalt polysulphides have been investigated as cathode materials to rate performance owing to faster kinetics as we show herein. reduce the solubility of polysulphide ions in the electrolyte . Efforts devoted to electrolytes include rational selection of solvents and Results introduction of additives such as LiNO (ref. 13), which diminishes Synthesis and characterization of the mesoporous carbon the effect of the polysulphide shuttle mechanism as a result of com - (SCM). e Th mesoporous carbon was prepared by replicating a silica plex reactions on the Li metal anode. Ordered mesoporous carbons monolith. This methodology was originally reported by Jaroniec 22,23 have been employed as nanosized reaction chamber assemblies et al. and is modified in this study. e Th monolith is formed by for the sulphur electrode . This approach overcomes the low con - drying a commercialized silica colloid (LUDOX HS-40 40wt%, ductivity of the sulphur electrode, but falls short of completely Sigma-Aldrich). e Th mesoporous carbon prepared from it by the controlling polysulphide dissolution. Our group has developed a replica technique is referred to as SCM (silica colloidal monolith), polymer-decorated mesoporous carbon/sulphur-interwoven nano- which exhibits the N absorption and desorption isotherm shown composite as a sulphur cathode, where the surface polymer chains in Figure 1a. SCM exhibits a Brunauer-Emmett-Teller (BET)- 15 2 − 1 can effectively retard the polysulphide anion dissolution . Impor- specific surface area of 1,100 m g , and a very narrow pore size tantly, the agglomeration of precipitated sulphides on the cathode distribution centered at 12.5 nm, as determined by the BJH method. 3 −1 is greatly reduced. This is important because such a build-up on the This carbon exhibits a specific pore volume of 2.3 cm g . As shown surface results in capacity fading owing to poor contact with the in a representative high resolution SEM image of a fractured surface conductive carbon surface underneath, and the barrier to Li-ion (Fig. 1b), the pores (~12 nm in diameter; in good accord with the diffusion that is created. BET data) are distributed with no strict long-range order, and are With all these considerations in mind, it is still vital to retain the inter-connected. e Th porous structure can also be observed in the polysulphide anions within the cathode layer by using additional corresponding dark-field STEM image ( Fig. 1c). methods. On-site adsorption is one promising concept. Oxide An advantage of SCM as the carbon framework for the sulphur additives, including Mg Ni O (ref. 16) and Al O (ref. 17) nano- electrode is that the particle size can be controlled by varying the 0.6 0.4 2 3 particles (~50 nm), were employed as adsorbents in the sulphur grinding force and duration on the carbon monolith. e Th SCM used cathode and they showed some ee ff ct to increase an otherwise low here for the cathode exhibits an irregular morphology and an aver- coulombic ec ffi iency; however, the ee ff ct of these additives can only age particle size of ~10 µm, as shown in the low-resolution SEM be observed at a low level of sulphur loading in the cathode, which image in Figure 2a. e Th SCM/S electrode will exhibit a higher tap is probably due to their limited adsorption surface area. Additives density than counterparts with smaller carbon particle sizes. Impor- based on porous carbons have also been described . However, elec- tantly, the micron-sized SCM/S structures still preserve all the bene- tronically conductive additives will facilitate the reduction of the ts fi of nano-dimensions due to their fine porous structure. As shown polysulphides to insoluble sulphides onto their surface during the by the high resolution SEM image in Figure 2b, which can be com- charge cycle. Once the adsorbents are covered, their function ceases. pared to empty SCM shown in Figure 1c, the surface morphology of A successful additive should not only be inert to redox reac- SCM is altered aer ft the melt-diffusion process for sulphur impreg - tivity and show strong absorption capacity, but also allow facile nation. e Th corresponding STEM image ( Fig. 2c) shows much less nATuRE C ommunICATIons | 2:325 | DoI: 10.1038/ncomms1293 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. n ATu RE Commun ICATIons | Do I: 10.1038/ncomms1293 ARTICLE a a b 1,500 0.015 1,200 0.010 0.005 0.0 0.2 0.4 0.6 0.8 1.0 P/P d c 0.000 0 10 20 30 40 50 Pore size (nm) Figure 2 | Electron micrographs of the SCM/sulphur and SBA-15/SCM/ sulphur electrode materials. (a) Low-resolution s Em of s Cm , scale bar = 20 µm; (b) high-resolution s Em and (c) dark-field s TEm images of s Cm /s (sulphur 70 wt%). (d) s Em of s Cm /s with s BA-15 additive (the white platelets highlighted by the red box), scale bar = 1 µm; inset: s BA-15 for reference (scale bar = 1 µm). (b, c) s cale bar = 300 nm. a b c Figure 3 | Schematic diagram illustrating the concept of the ‘polysulphide reservoir’ afforded by the SBA-15 platelets in the SCM/S electrode. (a) Before discharge: the black area represents carbon with pores infiltrated by sulphur and grey particles on the cube surface are s BA-15. (b) Discharge to 2 − 2.15 V: the green-coloured area denotes polysulphide ions (s , 3 ≤ n ≤8), Figure 1 | Porosity characterization of the ‘silica colloidal monolith’- which are ‘concentrated’ in the s BA-15. (c) Discharge to 1.5 V: polysulphide derived mesoporous carbon (SCM). (a) Pore size distribution for s Cm ions diffuse out of s BA-15 platelets and are further reduced into solid carbon. Inset: adsorption/desorption isotherm; (b) high-resolution sulphides (Li s /Li s ) within the s Cm carbon framework. 2 2 2 s Em image of s Cm ; (c) dark-field s TEm image of s Cm . (b, c) s cale bar = 300 nm. aggregated particles by the mixing process; they are also visible on porosity aer ft sulphur-filling than before ( Fig. 1d), which is con- the surface as shown in the SEM image in Figure 2d. eir Th charac - firmed by pore volume measurements of the SCM/S composite teristic shape makes them easy to identify, which is important for 3 −1 (0.31 cm g ). e Th particle size of the SCM/S has benefits for elec - the EDX(Energy dispersive X-ray spectroscopy) studies to verify the trode preparation as well. Extensive efforts have been devoted to sulphur reservoir concept (vide infra). A schematic that illustrates fabricating electrode materials with decreased particle sizes but it the incorporation of the SBA-15 into the SCM/S is illustrated in has been shown that the superior performance of nanoparticles can Figure 3a, along with the concept of the function and benefit of the come at the expense of necessity of binder overuse, lowered tap den- polysulphide reservoirs (Fig. 3b,c) that will be discussed in detail sity and potential safety concerns . e Th large particle size of SCM/S later. e Th electrical conductivity of the electrode materials both with −1 means that the amount of the polymer binder necessary to prepare and without the SBA-15 additive was equivalent, ~6 S cm showing electrodes is reduced to 5 wt% (vide infra) compared to the typical that the silica has no effect owing to its low overall concentration. content of 20–28 wt% for electrode materials comprising nanoparti- cles . u Th s the composite exhibits the advantage of bulk-sized elec - Electrochemistry. Electrochemical measurements of SCM/S elec- trode materials but with internal nanostructure. trodes were carried out to investigate the influence of the SBA-15 incorporation. Figure 4a shows the galvanostatic discharge/charge −1 −1 Incorporation of the polysulphide ‘reservoirs’ into the SCM. profiles recorded at a current rate of C/5 (334 mA g or 0.4 mA cm ). −1 To homogeneously incorporate the silica SBA-15 (10 wt%) within e Th initial discharge capacity of the cell with SBA-15 is 960 mA h g , the SCM/S (90 wt%), the solids were well dispersed and mixed by where the mass (g) refers to the active sulphur component, follow- −1 sonication (see Methods). e Th platelets are incorporated within the ing convention. This is greater than the capacity of 920 mA h g n ATu RE Commun ICATIons | 2:325 | Do I: 10.1038/ncomms1293 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. 3 –1 –1 dV/dr (cm g angstrom ) Volume adsorbed 3 –1 (cm g ) ARTICLE nATuRE C ommunICATIons | DoI: 10.1038/ncomms1293 3.5 e Th theoretical sorption capacity is dependent on whether sur - face adsorption or pore absorption (or a combination of the two) governs the process. We based our estimate on the assumption of 3.0 complete dissolution of all of the sulphur in the cathode as polysul- phide, which we know does not occur but serves as a boundary 2.5 point. For the former case—adsorption—a lower limit of about 20% adsorption is predicted, based on the surface area of the SBA-15 2.0 2 −1 26 2 − (850 m g ) and that of the intermediate polysulphide S species − 19 2 2 − (2.2×10 m /molecule S ; see Methods). In the latter case— 1.5 absorption—nearly 100% of the polysulphide can be absorbed into the volume in the upper limit of complete pore filling (based on the 2 − estimated volume of the intermediate polysulphide S species of 0 200 400 600 800 1,000 − 29 3 2 − –1 6.6×10 m /molecule S , and a typical pore volume of SBA-15 of Specific capacity (mA h g s) 6 −1 1.2 cc g (ref. 26). A reasonable estimate of the sorption capacity lies somewhere between these two extremes. Of course, it is not neces- 1,000 sary for SBA-15 to hold all the polysulphide ions at any stage owing to dynamic equilibrium. er Th efore, we believe that the fraction of SBA-15 employed in this study is not only valid to demonstrate the concept but adequate to effectively prevent polysulphide ions from diffusing away from the cathode side. To confirm the role of the SBA-15 additive in stabilizing the cycling and its mechanism, we carried out analytical measurements on the electrodes at various stages of cycling. EDX was used to inves- tigate whether electrochemically generated polysulphide anions are absorbed by SBA-15 platelets and desorbed when necessary, that is, 0 10 20 30 40 near the end of discharge. We employed tetraethylene glycol dime- Cycle number thyl ether as the electrolyte solvent (containing 1 M LiPF ) in the cells for this EDX study and for the analysis of the sulphur concentration Figure 4 | Electrochemical data for SCM/S and SBA-15-SCM/S in the electrolyte. Elemental analysis could not be performed using demonstrating the positive effect of silica incorporation. Data were the electrolyte based on ethyl methyl sulfone, as it contains sulphur. obtained on galvanostatic cycling at a C/5 rate (corresponding to a current − 1 − 2 e Th concentration of LiPF should be a constant value within SBA- density of 334 mA g or 0.4 mA cm ). (a) Comparison of the discharge– 6 15 particles in the sulphur electrode throughout cycling. e Th phos - charge profiles of the first cycle of s Cm/s (black solid line) and s Cm/s phorus signal acts as an internal reference via determination of the with the sBA-15 additive (red dotted line). ( b) Comparison of the cycling S/P ratio. To determine the absorption capacity of SBA-15 additive stability of s Cm/s (blue) and s Cm/s with sBA-15 additive showing for polysulphide anions, the electrode material was extracted (in an the capacity stabilization in the latter case. Discharge capacity: empty Ar filled glovebox) from a cell which was discharged to 2.15 V in its symbols; charge capacity: solid symbols. −1 −2 40th cycle at a current rate of C/5 (334 mA g or 0.4 mA cm ). At this potential, elemental sulphur is completely converted to soluble 2 − + polysulphide species, that is, S ·2Li . e Th cathode was investigated exhibited by the cell without SBA-15. Both cells exhibit some irre- by SEM and EDX. As shown in Figure 5a, EDX signals collected versible capacity in the first cycle, but it is less with the SBA-15 from an SBA-15 particle (marked by the red square) show a very additive. It is also evident that cell polarization with SBA-15 is only high sulphur/phosphorus (S/P) atomic ratio of 3.4 averaged from slightly greater than in the case without it. Overall, the presence of 20 spots. er Th efore, one can expect that the polysulphide anion con - SBA-15 in the sulphur electrode greatly improves the overall elec- centration in the electrolyte will be much lower in the presence of trochemical performance. As Figure 4b (blue curve) shows, with- SBA-15 in the cathode layer, as conceptualized in Figure 3b. This out SBA-15, the cell suffers both capacity fading and an increasing will greatly hinder the redox shuttle in the electrolyte and, in turn, divergence between the charge and discharge capacity as a result of prevent active mass loss on both electrodes. the polysulphide shuttle mechanism. It is likely that the large pore To determine whether the absorbed polysulphide can be size of SCM carbon permits significantly more polysulphide dis - desorbed on demand, as schematically illustrated in Figure 3c, elec- solution than CMK-3, for example . With the addition of SBA-15, trode material was obtained from another cell which was discharged as Figure 4b (black curve) illustrates, although the cell experiences to 1.5 V at the end of the 40th discharge. A much lower average S/P some initial capacity fading (~30%), from the tenth cycle onward, ratio of 0.2 in the SBA-15 was measured (30 spots), as shown in this is almost completely curtailed. A discharge capacity well above Figure 5b. By comparing the S/P ratio at 2.15 V and 1.5 V, it is esti- −1 650 mA·h g is steadily maintained aer ft 40 cycles. Both cells exhibit mated that ~94% of the sulphur mass in the SBA-15 particles was slightly uc fl tuating cycling behaviour, which is not yet well under - desorbed and participated in electrochemical reactions even during stood. We speculate that the release rate of the polysulphide ions the 40th cycle. Most importantly, we did not observe glassy sulphide from the silica may not be constant, and sudden expulsion may give agglomeration phase on either electrode surface. Due to the fact rise to higher capacity. Further investigation is needed to study this that the SBA-15 polysulphide nano-reservoirs reside on the surface phenomenon. Importantly, the coulombic efficiency is maintained of SCM/S particles in addition to being contained within the bulk, at above 95% for 30 cycles, which indicates the effective suppression polysulphide ions can easily diffuse back within the pores of SCM of a polysulphide shuttle mechanism in the presence of the SBA-15 instead of being reduced on the surface to form agglomerates. e Th additive. reversible absorption and desorption of polysulphide anions is also facilitated by the insulating properties of the silica. If the absorb- Discussion ent is electronically conductive, we believe that sulphide agglom- Back-of-the-envelope calculations suggest, in fact, that a large frac- eration will rapidly occur on the surface of the absorbent. We also tion of polysulphide can be accommodated in the SBA-15 particles. measured the sulphur electrolyte concentration in the cells with nATuRE C ommunICATIons | 2:325 | DoI: 10.1038/ncomms1293 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. –1 Specific capacity (mA h g s) Voltage (V) versus Li*/Li n ATu RE Commun ICATIons | Do I: 10.1038/ncomms1293 ARTICLE a a 0.6 0.5 0.4 0.3 0.2 SiK S K 0.1 P K 0.0 2.00 2.50 0 5 10 15 20 25 30 Cycle number b 600 SiK P K S K 1.60 2.40 Figure 5 | SEM and EDX results of the SBA-15-SCM/S electrode at 0 10 20 30 40 different cell voltages. The EDX results are collected from the area Cycle number marked by the red boxes shown at the left bottom corner of the images. (a) Discharge to 2.15 V at the 40th cycle reveals an s /P ratio of 3.4, Figure 6 | Effect of SBA-15 incorporation on sulphur dissolution in indicating that a high concentration of residual polysulphide is absorbed in electrochemical cells and high-rate cycling behaviour of cathodes. the s BA-5 particles. s cale bar=200 nm; (b) Discharge to 1.5 V at the 40th (a) Percentage of sulphur dissolution into the electrolyte from the s Cm /s cycle reveals an s /P ratio of 0.2, demonstrating that 94% of the sulphur is cathode (open symbols); from the s BA-15-s Cm /s cathode (solid symbols). released from the s BA-15 at the end of discharge. n o glassy sulphide phase (b) Comparison of galvanostatic cycling of s Cm /s (black) and Cm K- − 1 − 2 is evident on the surface of the particles even after 40 cycles. 3/s (red) at a current rate of 1C (1,672 mA g or 2 mA cm ), discharge s cale bar = 500 nm. capacity: open symbols; charge capacity: solid symbols. and without the SBA-15 additive in this large-pore carbonaceous over 10 cycles, the bimodal carbon at low sulphur/carbon loading in electrode. Less than 23% of sulphur is found in electrolyte at 30th cycle the composite (11 wt%) exhibited more stable capacity retention of in the former case, and 54% of sulphur for the latter case, as shown about 50% over 30 cycles. Coulombic efficiency was not provided. in Figure 6a. This result confirms the electrochemical results. Sulphur/carbon ratios in the bimodal carbon up to 52 wt% increased To confirm the expected good rate performance of SCM/S, we the fade rate to 60% however, which is about double of that we find −1 −2 also examined cells at high rates of 1C (1,672 mA g or 2 mA cm ) with the SBA-15 additive. By using a microporous ( < 0.7 nm) car- to compare the SCM/S electrode and a CMK-3/S electrode with bon to form a C/S composite, Gao et al. reported very unusual − 1 the same sulphur loading in the composites (70 wt%), but with- stable cycling performance at a capacity of 800 mAh g with 100% out the SBA-15 additive. SCM/S shows higher capacity than its coulombic efficiency. This was based on a very low sulphur loading CMK-3 counterpart, as shown in Figure 6b. e Th larger pore size of 29 wt% of the electrode mass . Higher loading led to extremely of the carbon structure for C/S electrode is clearly advantageous in low utilization of the active sulphur. e Th flat, low potential of 1.8 V high power applications. Interestingly, both cells show no shut- on the first cycle, the use of carbonate electrolytes and highly slop - tle phenomenon at this C rate and a stable cycling performance. ing subsequent discharge profiles are not characteristic of Li–S cells, By comparison to Figure 4b (blue curve), it is evident that the however. It suggests that treatment of the S/C composite at 300 °C coulombic efficiency is improved to about 98% at high current may have resulted in formation of a C–S polymer or that other fac- rates. It is likely that dissolution/diffusion is diminished under these tors are at play. In another interesting study, Cui et al. studied conditions. the fully discharged form of a S electrode (Li S/CMK-3), with no Our results can be compared with other recent studies on Li/S control on polysulphide diffusion. A clear shuttle phenomenon cells where some new approaches have been explored, from two was observed and the capacity and cycling stability were also poor. aspects: cycling stability and coulombic efficiency. A good summary Important progress was recently made by Hassoun and Scrosati , of the effect of carbon pore structure is provided by the work of who showed that by combining S/C cathodes with lithium sulfide- Liang et al., where a bimodal micro-mesoporous carbon was uti- saturated polymer electrolytes, and Sn/C anodes, a good cycling sta- lized to increase the carbon absorption capacity of polysulphide bility (about a 30% fade rate over 30 cycles), similar to that reported ions . Compared with a mesoporous (7 nm) carbon studied in their here, could be obtained at low rates (C/20). Comparison is made work, which displayed dramatically rapid capacity fading of 80% difficult by the large differences in cell configuration. n ATu RE Commun ICATIons | 2:325 | Do I: 10.1038/ncomms1293 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. –1 Specific capacity (mA h g s) Sulphur in electrolyte/total sulphur ARTICLE nATuRE C ommunICATIons | DoI: 10.1038/ncomms1293 concentration and the total volume of the added electrolyte, which was recorded In summary, we observe that stabilized cycling with a high cou- when assembling the cell. lombic efficiency has not been reported for Li–S cells at practical S/C loadings at high rates. Our results fill this void and represent Characterization. Nitrogen adsorption and desorption isotherms were obtained the best level achieved so far to the best of our knowledge. This was using a Quantachrome Autosorb-1 system at − 196 °C. Before measurement of made possible by utilizing a mesoporous silica that is shown to be SCM, the sample was degassed at 150 °C on a vacuum line following a standard protocol. It was not possible to carry this out for SCM/S owing to the volatility a highly effective internal polysulphide reservoir for the cathode in of the sulphur, and so no pretreatment was used. The BET method was used to Li–S batteries. This not only greatly improves the cycling stability calculate the surface area. The total pore volumes were calculated from the amount but also eliminates the polysulphide shuttle mechanism to a large adsorbed at a relative pressure of 0.99. The pore size distributions were calculated degree and allows large-pore carbons to be utilized. A mesoporous by means of the Barrett–Joyner–Halenda method applied to the desorption branch. monolithic carbon with a large particle size around 10 µm and a uni- e m Th orphology of the SCM/S-SBA-15 composites was examined by a LEO 1530 field-emission SEM instrument equipped with an energy-dispersive X-ray form large mesopore size of 12.5 nm was prepared and investigated spectroscopy attachment. TEM images in this study were obtained with a Hitachi as an encapsulating chamber for the sulphur active mass. e Th C/S HD-2000 scanning transmission electron microscope. composite exhibits larger tap density, less need for binder and very − 2 good rate performance. Although we have established proof-of-con- Calculation of adsorption of polysulphides in SBA-15. The S molecule was used as the prototype polysulphide anion. Gaussian calculations determined the cept with SBA-15, we expect this approach to be widely applicable to length of the molecule as 7.33 Å and the width as ~3 Å×3 Å. other silicas with suitable mesopores of dimensions large enough to − 2 Surface adsorption. e Th maximum surface area of S in the flat configuration accommodate the polysulphide ions. Key to the polysulphide reser- − 19 2 is 2.19×10 m /molecule. u Th s, compared with the surface area of SBA-15 (ref. voir concept is that the material permits absorption/desorption in a 26 2 − 1 − 3 − 2 ) of ~850 m g , 6.44×10 mol S can be adsorbed per gram of SBA-15; that is, − 6 − 2 − 6 reversible manner during the electrochemical reaction. 1.28×10 mol S , or 7.7×10 mol of sulphur, can be adsorbed in the as-pre- pared electrodes, which contain ~0.2 mg of SBA-15 (that is, 10 wt%). Compared with the total mass of sulphur in the electrode (1.2 mg), the SBA-15 can therefore Methods accommodate about 20% of the sulphur mass in the extreme lower limiting case Preparation of SCM. Silica colloid (LUDOX HS-40 40 wt%, Sigma-Aldrich; 5 g) − 2 of surface adsorption—where the S molecules only adsorb on the surface of was dried in a petri dish to form a semi-transparent silica monolith template SBA-15 utilizing the maximum surface area of their structure, and only monolayer (2g), which was impregnated for 10 min with an isopropyl alcohol solution (5 ml) adsorption occurs. ‘End’ adsorption or bi/trilayer adsorption would double or containing 80 mg of oxalic acid (97% Fluka), as a catalyst for the polymerization of triple this value. the carbon precursors. The isopropyl alcohol was then removed by evaporation at − 2 Surface absorption. The volume spanned by the S molecule is approximately 85 °C. The oxalic acid-loaded silica monolith was impregnated in a mixture of 2 g of − 29 3 3 − 1 6.57×10 m /molecule, and the typical pore volume of SBA-15 is ~1.2 cm g . resorcinol (98%, Sigma-Aldrich) and 1.7 g of crotonaldehyde (98%, Sigma-Aldrich) − 2 − 2 − 6 u Th s, 3.03×10 mol S can be absorbed per gram of SBA-15, or 6.06×10 mol for 1 h. Filtration was applied to the soaked silica monolith to remove excessive 6 − 2 S per electrode given the mass of SBA-15 present (see above). This is equivalent precursor. The mixture was then subjected to polymerization through a series of − 5 to 3.6×10 mol of sulphur , which, compared with the total mass of sulphur in heat treatments in air under the following conditions: 60 °C for 30 min, 120 °C for the electrode, means that the SBA-15 can accommodate about 97% of the sulphur 10 h, 200 °C for 5 h. The resultant polymer was carbonized at 900 °C under an argon mass in the extreme upper limiting case of pore absorption if complete pore atmosphere. The silica/carbon composite monolith was ground into a powder filling occurs. The expected co-adsorption of Li ions and solvent molecules before the silica template was removed by HF (15%) etching. would decrease this value by perhaps a factor of two or three. Preparation of CMK-3 and the CMK-3/S composite. We have followed the same procedures described in our previous paper . CMK-3/S contains 70 wt% of sulphur in the composite. References 1. Herbert, D. & Ulam, J. Electric dry cells and storage batteries. US Patent No. Preparation of SCM/S. A mixture of SCM (0.2 g) and sulphur (0.47 g) was 3043896 (1962). ground, and melt diffusion was carried out in an oven at 155 °C. The composite 2. Peramunage, D. & Licht, S. A solid sulphur cathode for aqueous batteries. contains 70 wt% of sulphur. Science 261, 1029 (1993). 3. Rauh, R. D., Shuker, F. S., Marston, J. M. & Brummer, S. B. Formation of Synthesis of SBA-15. Pluronic P123 (EO PPO EO ), 2 g, was dissolved in 60 ml lithium polysulfides in aprotic media. J. Inorg. Nucl. Chem. 39, 1761 (1977). 20 70 20 of 2 M HCl solution at 38 °C. Tetraethylorthosilicate (4.2 g) was added to the above 4. Rauh, R. D., Abraham, K. M., Pearson, G. F., Surprenant, J. K. & Brummer, S. solution under vigorous stirring. The mixture was stirred for 6 min and remained B. Lithium dissolved sulphur battery with an organic electrolyte. J. Electrochem. quiescent for 24 h at 38 °C. The mixture was subsequently heated at 100 °C for Soc. 126, 523 (1979). another 24 h in an autoclave. The as-synthesized SBA-15 with platelet morphology 5. Rao, B. M. L. & Shropshire, J. A. Effect of sulphur impurities on Li/TiS cells. was collected by filtration, dried and calcined at 550 °C in air. J. Electrochem. Soc. 128, 942 (1981). 6. Marmorstein, D., Yu, T. H., Striebel, K. A., McLarnon, F. R., Hou, J. & Cairns, Preparation of SBA-15–SCM/S composites. A mixture of SBA-15 (50 mg) and E. J. Electrochemical performance of lithium/sulphur cells with three different SCM/S (450 mg) was dispersed in water (5 ml), and sonicated for 30 min and polymer electrolytes. J. Power Sources 89, 219 (2000). stirred for 2 h. The mixture was then filtered and dried at 80 °C overnight to remove 7. Obrovac, M. N. & Dahn, J. R. Electrochemically active lithia/metal and lithium residual water. sulfide/metal composites. Electrochem. Solid-State Lett. 5, A70 (2002). 8. He, X., Ren, J., Wang, L., Pu, W., Wan, C. & Jiang, C. Electrochemical Electrochemical measurements. Positive electrodes were constructed from characteristics of a sulphur composite cathode for reversible lithium storage. SCM/S-SBA-15 (95 wt%), and polyvinylidene u fl oride (PVDF) binder (5 wt%). Ionics 15, 477 (2009). The cathode material, ready for electrochemical studies, contained 60 wt% of sul- 9. Hassoun, J. & Scrosati, B. A high-performance polymer tin sulphur lithium ion phur as active mass. The cathode material was well dispersed in cyclopentanone battery. Angew. Chem. Int. Ed. 49, 1 (2010). by sonication and slurry-cast onto a carbon-coated aluminium current collector 10. Yang, Y., McDowell, M. T., Jackson, A., Cha, J. J., Hong, S. S. & Cui, Y. New (Intelicoat), and 2,025 coin cells were constructed using an electrolyte composed nanostructured Li S/silicon rechargeable battery with high specific energy. of a 1.2 M LiPF solution in ethyl methyl sulphone. Lithium metal foil was used Nano Lett. 10, 1486 (2010). as the counter electrode. For the electrode containing CMK-3/S, 8 wt% PVDF 11. Gorenshtein, A., Segal, M. & Peled, E. Characterization of CoS (x=2.1–4.5) as binder was employed. Only in the comparison study with CMK-3/S was the cathode materials for rechargeable lithium batteries. 35th IEEE Power Sources SCM/S electrode also mixed with 8 wt% of PVDF binder. According to the active Symposium 182 (1992). − 2 − 1 mass loading (1.2 mg cm ), the equivalent current density for the 334 mA g 12. Peled, E., Gorenshtein, A., Segal, M. & Sternberg, Y. Rechargeable lithium- − 1 − 2 rate is 0.4 and that for the 1,672 mA g is 2 mA cm . To measure the degree sulphur battery. J. Power Sources 26, 269 (1989). of sulphur retention in the cathode, a 1.0 M LiPF solution in tetraethylene 13. Aurbach, D., Pollak, E., Elazari, R., Salitra, G., Kelley, C. S. & Affinito, J. On the glycol dimethyl ether was used as the electrolyte. SCM/S-SBA-15 cathodes were surface chemical aspects of very high energy density, rechargeable Li–Sulphur compared with SCM/S cathodes containing no SBA-15 at the exact same S/C batteries. J. Electrochem. Soc. 156, A694 (2009). ratio. We used large Swagelok-type cells that accommodate a sufficient excess of 14. Liang, C., Dudney, N. J. & Howe, J. Y. Hierarchically structured sulphur/carbon the electrolyte to dissolve sulphur species. Swagelok cells were disassembled and nanocomposite material for high-energy lithium battery. Chem. Mater. 21, a portion of the electrolyte with the volume accurately measured was taken and 4347 (2009). diluted for sulphur concentration measurement, which was carried out by Gal- 15. Ji, X., Lee, K. T. & Nazar, L. F. A highly ordered nanostructured carbon–sulphur braith Laboratories. The total dissolution can be calculated based on measured cathode for lithium–sulphur batteries. Nature Mater. 8, 500 (2009). nATuRE C ommunICATIons | 2:325 | DoI: 10.1038/ncomms1293 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. nATuRE C ommunICATIons | DoI: 10.1038/ncomms1293 ARTICLE 16. Song, M. S. et al. Effects of nanosized adsorbing material on electrochemical 26. Garneau, A. et al. True microporosity and surface area of mesoporous SBA-15 properties of sulphur cathodes for Li/S secondary batteries. J. Electrochem. Soc. silicas as a function of synthesis temperature. Langmuir 17, 8328 (2001). 151, A791 (2004). 27. Zhang, B., Qin, X., Li, G. R. & Gao, X. P. Enhancement of long stability of sulfur 17. Choi, Y. J. et al. Electrochemical properties of sulphur electrode containing cathode by encapsulating sulfur into micropores of carbon spheres. Energy nano Al O for lithium/sulphur cell. Phys. Scr. 62, T129 (2007). Environ. Sci. 3, 1531 (2010). 2 3 18. Gorkovenko, A., Skotheim, T. A. & Xu, Z.-S. Cathodes comprising electroactive sulfur materials and secondary batteries using same. US Patent No. 6878488 Acknowledgments (2005). e N Th atural Sciences and Engineering Research Council of Canada is gratefully 19. Zhao, D. et al. Triblock copolymer syntheses of mesoporous silica with periodic acknowledged for financial support. We thank N. Coombs, University of Toronto, for 50–300 angstrom pores. Science 279, 548 (1998). help with acquisition of the high-resolution SEM images. (Supporting Information is 20. Song, S. W., Hidajat, K. & Kawi, S. Functionalized SBA-15 materials as carriers available.) for controlled drug delivery: influence of surface properties on matrix-drug interactions. Langmuir 21, 9568 (2005). 21. Lai, C. Y., Trewyn, B. G., Jeftinija, D. M., Jeftinija, K., Jeftinija, S. & Lin, V. Author contributions S. Y. A mesoporous silica nanosphere-based carrier system with chemically X.J. and L.N. designed and conducted the research. Electrochemical experiments were removable nanoparticle caps for stimuli-responsive controlled release of performed by S.E. and X.J., and R.B. prepared the SCM carbon. TEM/SEM experiments neurotransmitters and drug molecules. J. Am. Chem. Soc. 125, 4451 (2003). were performed by S.E. and X.J., L.N. and X.J. wrote the paper. 22. Gierszal, K. P. & Jaroniec, M. Carbons with extremely large volume of uniform mesopores synthesized by carbonization of phenolic resin film formed on Additional Information colloidal silica template. J. Am. Chem. Soc. 128, 10026 (2006). Competing financial interests: The authors declare no competing financial interests. 23. Jaroniec, M., Choma, J., Gorka, J. & Zawislak, A. KOH activation of mesoporous carbons obtained by so- ft templating. Chem. Mater. 20, 1069 (2008). Reprints and permission information is available online at http://npg.nature.com/ 24. Bruce, P. G. Energy storage beyond the horizon: rechargeable lithium batteries. reprintsandpermissions/ Solid State Ion. 179, 752 (2008). 25. Badway, F. et al. Metal oxides as negative electrode materials in Li-ion cells. How to cite this article: Ji, X. et al. Stabilizing lithium-sulphur cathodes using Electrochem. Solid-State Lett. 5, A115 (2002). polysulphide reservoirs. Nat. Commun. 2:325 doi: 10.1038/ncomms1293 (2011). nATuRE C ommunICATIons | 2:325 | DoI: 10.1038/ncomms1293 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Nature Communications Springer Journals

Stabilizing lithium–sulphur cathodes using polysulphide reservoirs

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ARTICLE DOI: 10.1038/ncomms1293 Received 28 Dec 2010 | Accepted 28 mar 2011 | Published 24 may 2011 stabilizing lithium–sulphur cathodes using polysulphide reservoirs 1 1 1 1 Xiulei Ji , scott Evers , Robert Black & Linda F. nazar The possibility of achieving high-energy, long-life storage batteries has tremendous scientific and technological significance. An example is the Li– s cell, which can offer a 3–5-fold increase in energy density compared with conventional Li-ion cells, at lower cost. Despite significant advances, there are challenges to its wide-scale implementation, which include dissolution of intermediate polysulphide reaction species into the electrolyte. Here we report a new concept to mitigate the problem, which relies on the design principles of drug delivery. our strategy employs absorption of the intermediate polysulphides by a porous silica embedded within the carbon–sulphur composite that not only absorbs the polysulphides by means of weak binding, but also permits reversible desorption and release. It functions as an internal polysulphide reservoir during the reversible electrochemical process to give rise to long-term stabilization and improved coulombic efficiency. The reservoir mechanism is general and applicable to Li/s cathodes of any nature. Department of Chemistry and Department of Electrical Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1. Correspondence and requests for materials should be addressed to L.F.N. (email: [email protected]). nATuRE C ommunICATIons | 2:325 | DoI: 10.1038/ncomms1293 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. ARTICLE nATuRE C ommunICATIons | DoI: 10.1038/ncomms1293 ithium–sulphur (Li–S) batteries are one of the most promis- desorption of the polysulphide anions. Herein, as a significant ing candidates to couple renewable energy sources for green step towards solving the challenge of polysulphide dissolution, we Ltransportation and large-scale energy storage owing to their present a new approach to sulphur cathode composites that com- various desirable characteristics including competitive cost, and prise a carbon–sulphur nanocomposite together with a small frac- low environmental impact. Moreover, their high theoretical energy tion of a mesoporous silica additive. We were inspired by studies density is over 5 times larger than that of conventional Li-ion bat- using triblock copolymer-templated SBA-15 as a reversible drug teries based on intercalation electrodes . Despite these advantages, delivery system. SBA-15 is a well-developed mesoporous silica that massive implementation of Li–S batteries remains hindered by exhibits high surface area, large pore volume, bi-connected porous various challenges that mainly arise from the cathode. e Th major structure and highly hydrophilic surface properties . es Th e char - problem is rapid capacity fading, which is mainly due to dissolu- acteristics allow it to adsorb certain kinds of drugs and release 2 − tion of polysulphide anions (S )—intermediate reaction species them in a highly reproducible and predictable manner. It has been formed on charge and discharge—from the cathode into the elec- shown that both small and large molecular drugs can be entrapped 3,4 trolyte . Once the polysulphide ions diffuse out from the nano- within the mesopores by an impregnation process and liberated via structured cathode, not only will their reaction with the Li anode a diffusion-controlled mechanism . Although the Si–O groups that cause active mass loss, but also the redeposition of sulphide species cover its internal surface are too weak to engage in molecular drug from the electrolyte back onto the cathode surface generates large interaction without being first functionalized, they serve as ideal agglomerates at the end of discharge. e Th polysulphide anions also binding sites for the sulphur cathode. e Th cavities not only accom - act as an internal redox shuttle, which gives rise to low coulombic modate polysulphide anions through weak binding via the posi- efficiency: namely, a charge capacity larger than the corresponding tively charged silica surface, but also permit reversible desorption. discharge capacity . Various electrolyte systems have been employed u Th s, they function as an internal sulphide reservoir to absorb and to solve the problem, including polymers . A physical barrier release the active material for electrochemical reduction–oxidation that prevents diffusion of the polysulphide species could solve the during cycling. We employed SBA-15 with a platelet morphology, dissolution problem, but in the long term this could be compro- that not only exhibits good absorption properties but can also be mised. A fast-responding sulphur battery requires facile transport easily dispersed with carbon–sulphur particles. Other silicas with of electrolyte/Li into and out of the sulphur electrode, but eventu- suitable pore sizes, such as zeolite beta or MCM-41—which have ally some soluble polysulphide ions will diffuse over the physical also been employed for targeted drug delivery —should be equally barrier, which initiates the shuttle phenomenon. effective. Furthermore, we have developed and utilized a new large- Recently, there has been some promising progress made on new pore ( > 10 nm) porous carbon for the sulphur/carbon nanocompo- sulphur battery configurations. Li S has been employed as an ini- site to illustrate the efficacy of the SBA-15 additive in binding the tial active material in the sulphur electrode, to couple with non- polysulphide anions. Polysulphide diffusion is more of a concern in 8 9 15 metallic lithium anodes, that is, graphite , a carbon/tin alloy and highly open pore systems than in CMK-3/S , which was examined Si nanowires . es Th e sulphur batteries still suffer capacity loss, previously, and hence it necessitates an effective absorption addi - however, due to the untackled polysulphide dissolution problem. tive. An additional benefit is that this combination can oer ff better Cobalt polysulphides have been investigated as cathode materials to rate performance owing to faster kinetics as we show herein. reduce the solubility of polysulphide ions in the electrolyte . Efforts devoted to electrolytes include rational selection of solvents and Results introduction of additives such as LiNO (ref. 13), which diminishes Synthesis and characterization of the mesoporous carbon the effect of the polysulphide shuttle mechanism as a result of com - (SCM). e Th mesoporous carbon was prepared by replicating a silica plex reactions on the Li metal anode. Ordered mesoporous carbons monolith. This methodology was originally reported by Jaroniec 22,23 have been employed as nanosized reaction chamber assemblies et al. and is modified in this study. e Th monolith is formed by for the sulphur electrode . This approach overcomes the low con - drying a commercialized silica colloid (LUDOX HS-40 40wt%, ductivity of the sulphur electrode, but falls short of completely Sigma-Aldrich). e Th mesoporous carbon prepared from it by the controlling polysulphide dissolution. Our group has developed a replica technique is referred to as SCM (silica colloidal monolith), polymer-decorated mesoporous carbon/sulphur-interwoven nano- which exhibits the N absorption and desorption isotherm shown composite as a sulphur cathode, where the surface polymer chains in Figure 1a. SCM exhibits a Brunauer-Emmett-Teller (BET)- 15 2 − 1 can effectively retard the polysulphide anion dissolution . Impor- specific surface area of 1,100 m g , and a very narrow pore size tantly, the agglomeration of precipitated sulphides on the cathode distribution centered at 12.5 nm, as determined by the BJH method. 3 −1 is greatly reduced. This is important because such a build-up on the This carbon exhibits a specific pore volume of 2.3 cm g . As shown surface results in capacity fading owing to poor contact with the in a representative high resolution SEM image of a fractured surface conductive carbon surface underneath, and the barrier to Li-ion (Fig. 1b), the pores (~12 nm in diameter; in good accord with the diffusion that is created. BET data) are distributed with no strict long-range order, and are With all these considerations in mind, it is still vital to retain the inter-connected. e Th porous structure can also be observed in the polysulphide anions within the cathode layer by using additional corresponding dark-field STEM image ( Fig. 1c). methods. On-site adsorption is one promising concept. Oxide An advantage of SCM as the carbon framework for the sulphur additives, including Mg Ni O (ref. 16) and Al O (ref. 17) nano- electrode is that the particle size can be controlled by varying the 0.6 0.4 2 3 particles (~50 nm), were employed as adsorbents in the sulphur grinding force and duration on the carbon monolith. e Th SCM used cathode and they showed some ee ff ct to increase an otherwise low here for the cathode exhibits an irregular morphology and an aver- coulombic ec ffi iency; however, the ee ff ct of these additives can only age particle size of ~10 µm, as shown in the low-resolution SEM be observed at a low level of sulphur loading in the cathode, which image in Figure 2a. e Th SCM/S electrode will exhibit a higher tap is probably due to their limited adsorption surface area. Additives density than counterparts with smaller carbon particle sizes. Impor- based on porous carbons have also been described . However, elec- tantly, the micron-sized SCM/S structures still preserve all the bene- tronically conductive additives will facilitate the reduction of the ts fi of nano-dimensions due to their fine porous structure. As shown polysulphides to insoluble sulphides onto their surface during the by the high resolution SEM image in Figure 2b, which can be com- charge cycle. Once the adsorbents are covered, their function ceases. pared to empty SCM shown in Figure 1c, the surface morphology of A successful additive should not only be inert to redox reac- SCM is altered aer ft the melt-diffusion process for sulphur impreg - tivity and show strong absorption capacity, but also allow facile nation. e Th corresponding STEM image ( Fig. 2c) shows much less nATuRE C ommunICATIons | 2:325 | DoI: 10.1038/ncomms1293 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. n ATu RE Commun ICATIons | Do I: 10.1038/ncomms1293 ARTICLE a a b 1,500 0.015 1,200 0.010 0.005 0.0 0.2 0.4 0.6 0.8 1.0 P/P d c 0.000 0 10 20 30 40 50 Pore size (nm) Figure 2 | Electron micrographs of the SCM/sulphur and SBA-15/SCM/ sulphur electrode materials. (a) Low-resolution s Em of s Cm , scale bar = 20 µm; (b) high-resolution s Em and (c) dark-field s TEm images of s Cm /s (sulphur 70 wt%). (d) s Em of s Cm /s with s BA-15 additive (the white platelets highlighted by the red box), scale bar = 1 µm; inset: s BA-15 for reference (scale bar = 1 µm). (b, c) s cale bar = 300 nm. a b c Figure 3 | Schematic diagram illustrating the concept of the ‘polysulphide reservoir’ afforded by the SBA-15 platelets in the SCM/S electrode. (a) Before discharge: the black area represents carbon with pores infiltrated by sulphur and grey particles on the cube surface are s BA-15. (b) Discharge to 2 − 2.15 V: the green-coloured area denotes polysulphide ions (s , 3 ≤ n ≤8), Figure 1 | Porosity characterization of the ‘silica colloidal monolith’- which are ‘concentrated’ in the s BA-15. (c) Discharge to 1.5 V: polysulphide derived mesoporous carbon (SCM). (a) Pore size distribution for s Cm ions diffuse out of s BA-15 platelets and are further reduced into solid carbon. Inset: adsorption/desorption isotherm; (b) high-resolution sulphides (Li s /Li s ) within the s Cm carbon framework. 2 2 2 s Em image of s Cm ; (c) dark-field s TEm image of s Cm . (b, c) s cale bar = 300 nm. aggregated particles by the mixing process; they are also visible on porosity aer ft sulphur-filling than before ( Fig. 1d), which is con- the surface as shown in the SEM image in Figure 2d. eir Th charac - firmed by pore volume measurements of the SCM/S composite teristic shape makes them easy to identify, which is important for 3 −1 (0.31 cm g ). e Th particle size of the SCM/S has benefits for elec - the EDX(Energy dispersive X-ray spectroscopy) studies to verify the trode preparation as well. Extensive efforts have been devoted to sulphur reservoir concept (vide infra). A schematic that illustrates fabricating electrode materials with decreased particle sizes but it the incorporation of the SBA-15 into the SCM/S is illustrated in has been shown that the superior performance of nanoparticles can Figure 3a, along with the concept of the function and benefit of the come at the expense of necessity of binder overuse, lowered tap den- polysulphide reservoirs (Fig. 3b,c) that will be discussed in detail sity and potential safety concerns . e Th large particle size of SCM/S later. e Th electrical conductivity of the electrode materials both with −1 means that the amount of the polymer binder necessary to prepare and without the SBA-15 additive was equivalent, ~6 S cm showing electrodes is reduced to 5 wt% (vide infra) compared to the typical that the silica has no effect owing to its low overall concentration. content of 20–28 wt% for electrode materials comprising nanoparti- cles . u Th s the composite exhibits the advantage of bulk-sized elec - Electrochemistry. Electrochemical measurements of SCM/S elec- trode materials but with internal nanostructure. trodes were carried out to investigate the influence of the SBA-15 incorporation. Figure 4a shows the galvanostatic discharge/charge −1 −1 Incorporation of the polysulphide ‘reservoirs’ into the SCM. profiles recorded at a current rate of C/5 (334 mA g or 0.4 mA cm ). −1 To homogeneously incorporate the silica SBA-15 (10 wt%) within e Th initial discharge capacity of the cell with SBA-15 is 960 mA h g , the SCM/S (90 wt%), the solids were well dispersed and mixed by where the mass (g) refers to the active sulphur component, follow- −1 sonication (see Methods). e Th platelets are incorporated within the ing convention. This is greater than the capacity of 920 mA h g n ATu RE Commun ICATIons | 2:325 | Do I: 10.1038/ncomms1293 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. 3 –1 –1 dV/dr (cm g angstrom ) Volume adsorbed 3 –1 (cm g ) ARTICLE nATuRE C ommunICATIons | DoI: 10.1038/ncomms1293 3.5 e Th theoretical sorption capacity is dependent on whether sur - face adsorption or pore absorption (or a combination of the two) governs the process. We based our estimate on the assumption of 3.0 complete dissolution of all of the sulphur in the cathode as polysul- phide, which we know does not occur but serves as a boundary 2.5 point. For the former case—adsorption—a lower limit of about 20% adsorption is predicted, based on the surface area of the SBA-15 2.0 2 −1 26 2 − (850 m g ) and that of the intermediate polysulphide S species − 19 2 2 − (2.2×10 m /molecule S ; see Methods). In the latter case— 1.5 absorption—nearly 100% of the polysulphide can be absorbed into the volume in the upper limit of complete pore filling (based on the 2 − estimated volume of the intermediate polysulphide S species of 0 200 400 600 800 1,000 − 29 3 2 − –1 6.6×10 m /molecule S , and a typical pore volume of SBA-15 of Specific capacity (mA h g s) 6 −1 1.2 cc g (ref. 26). A reasonable estimate of the sorption capacity lies somewhere between these two extremes. Of course, it is not neces- 1,000 sary for SBA-15 to hold all the polysulphide ions at any stage owing to dynamic equilibrium. er Th efore, we believe that the fraction of SBA-15 employed in this study is not only valid to demonstrate the concept but adequate to effectively prevent polysulphide ions from diffusing away from the cathode side. To confirm the role of the SBA-15 additive in stabilizing the cycling and its mechanism, we carried out analytical measurements on the electrodes at various stages of cycling. EDX was used to inves- tigate whether electrochemically generated polysulphide anions are absorbed by SBA-15 platelets and desorbed when necessary, that is, 0 10 20 30 40 near the end of discharge. We employed tetraethylene glycol dime- Cycle number thyl ether as the electrolyte solvent (containing 1 M LiPF ) in the cells for this EDX study and for the analysis of the sulphur concentration Figure 4 | Electrochemical data for SCM/S and SBA-15-SCM/S in the electrolyte. Elemental analysis could not be performed using demonstrating the positive effect of silica incorporation. Data were the electrolyte based on ethyl methyl sulfone, as it contains sulphur. obtained on galvanostatic cycling at a C/5 rate (corresponding to a current − 1 − 2 e Th concentration of LiPF should be a constant value within SBA- density of 334 mA g or 0.4 mA cm ). (a) Comparison of the discharge– 6 15 particles in the sulphur electrode throughout cycling. e Th phos - charge profiles of the first cycle of s Cm/s (black solid line) and s Cm/s phorus signal acts as an internal reference via determination of the with the sBA-15 additive (red dotted line). ( b) Comparison of the cycling S/P ratio. To determine the absorption capacity of SBA-15 additive stability of s Cm/s (blue) and s Cm/s with sBA-15 additive showing for polysulphide anions, the electrode material was extracted (in an the capacity stabilization in the latter case. Discharge capacity: empty Ar filled glovebox) from a cell which was discharged to 2.15 V in its symbols; charge capacity: solid symbols. −1 −2 40th cycle at a current rate of C/5 (334 mA g or 0.4 mA cm ). At this potential, elemental sulphur is completely converted to soluble 2 − + polysulphide species, that is, S ·2Li . e Th cathode was investigated exhibited by the cell without SBA-15. Both cells exhibit some irre- by SEM and EDX. As shown in Figure 5a, EDX signals collected versible capacity in the first cycle, but it is less with the SBA-15 from an SBA-15 particle (marked by the red square) show a very additive. It is also evident that cell polarization with SBA-15 is only high sulphur/phosphorus (S/P) atomic ratio of 3.4 averaged from slightly greater than in the case without it. Overall, the presence of 20 spots. er Th efore, one can expect that the polysulphide anion con - SBA-15 in the sulphur electrode greatly improves the overall elec- centration in the electrolyte will be much lower in the presence of trochemical performance. As Figure 4b (blue curve) shows, with- SBA-15 in the cathode layer, as conceptualized in Figure 3b. This out SBA-15, the cell suffers both capacity fading and an increasing will greatly hinder the redox shuttle in the electrolyte and, in turn, divergence between the charge and discharge capacity as a result of prevent active mass loss on both electrodes. the polysulphide shuttle mechanism. It is likely that the large pore To determine whether the absorbed polysulphide can be size of SCM carbon permits significantly more polysulphide dis - desorbed on demand, as schematically illustrated in Figure 3c, elec- solution than CMK-3, for example . With the addition of SBA-15, trode material was obtained from another cell which was discharged as Figure 4b (black curve) illustrates, although the cell experiences to 1.5 V at the end of the 40th discharge. A much lower average S/P some initial capacity fading (~30%), from the tenth cycle onward, ratio of 0.2 in the SBA-15 was measured (30 spots), as shown in this is almost completely curtailed. A discharge capacity well above Figure 5b. By comparing the S/P ratio at 2.15 V and 1.5 V, it is esti- −1 650 mA·h g is steadily maintained aer ft 40 cycles. Both cells exhibit mated that ~94% of the sulphur mass in the SBA-15 particles was slightly uc fl tuating cycling behaviour, which is not yet well under - desorbed and participated in electrochemical reactions even during stood. We speculate that the release rate of the polysulphide ions the 40th cycle. Most importantly, we did not observe glassy sulphide from the silica may not be constant, and sudden expulsion may give agglomeration phase on either electrode surface. Due to the fact rise to higher capacity. Further investigation is needed to study this that the SBA-15 polysulphide nano-reservoirs reside on the surface phenomenon. Importantly, the coulombic efficiency is maintained of SCM/S particles in addition to being contained within the bulk, at above 95% for 30 cycles, which indicates the effective suppression polysulphide ions can easily diffuse back within the pores of SCM of a polysulphide shuttle mechanism in the presence of the SBA-15 instead of being reduced on the surface to form agglomerates. e Th additive. reversible absorption and desorption of polysulphide anions is also facilitated by the insulating properties of the silica. If the absorb- Discussion ent is electronically conductive, we believe that sulphide agglom- Back-of-the-envelope calculations suggest, in fact, that a large frac- eration will rapidly occur on the surface of the absorbent. We also tion of polysulphide can be accommodated in the SBA-15 particles. measured the sulphur electrolyte concentration in the cells with nATuRE C ommunICATIons | 2:325 | DoI: 10.1038/ncomms1293 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. –1 Specific capacity (mA h g s) Voltage (V) versus Li*/Li n ATu RE Commun ICATIons | Do I: 10.1038/ncomms1293 ARTICLE a a 0.6 0.5 0.4 0.3 0.2 SiK S K 0.1 P K 0.0 2.00 2.50 0 5 10 15 20 25 30 Cycle number b 600 SiK P K S K 1.60 2.40 Figure 5 | SEM and EDX results of the SBA-15-SCM/S electrode at 0 10 20 30 40 different cell voltages. The EDX results are collected from the area Cycle number marked by the red boxes shown at the left bottom corner of the images. (a) Discharge to 2.15 V at the 40th cycle reveals an s /P ratio of 3.4, Figure 6 | Effect of SBA-15 incorporation on sulphur dissolution in indicating that a high concentration of residual polysulphide is absorbed in electrochemical cells and high-rate cycling behaviour of cathodes. the s BA-5 particles. s cale bar=200 nm; (b) Discharge to 1.5 V at the 40th (a) Percentage of sulphur dissolution into the electrolyte from the s Cm /s cycle reveals an s /P ratio of 0.2, demonstrating that 94% of the sulphur is cathode (open symbols); from the s BA-15-s Cm /s cathode (solid symbols). released from the s BA-15 at the end of discharge. n o glassy sulphide phase (b) Comparison of galvanostatic cycling of s Cm /s (black) and Cm K- − 1 − 2 is evident on the surface of the particles even after 40 cycles. 3/s (red) at a current rate of 1C (1,672 mA g or 2 mA cm ), discharge s cale bar = 500 nm. capacity: open symbols; charge capacity: solid symbols. and without the SBA-15 additive in this large-pore carbonaceous over 10 cycles, the bimodal carbon at low sulphur/carbon loading in electrode. Less than 23% of sulphur is found in electrolyte at 30th cycle the composite (11 wt%) exhibited more stable capacity retention of in the former case, and 54% of sulphur for the latter case, as shown about 50% over 30 cycles. Coulombic efficiency was not provided. in Figure 6a. This result confirms the electrochemical results. Sulphur/carbon ratios in the bimodal carbon up to 52 wt% increased To confirm the expected good rate performance of SCM/S, we the fade rate to 60% however, which is about double of that we find −1 −2 also examined cells at high rates of 1C (1,672 mA g or 2 mA cm ) with the SBA-15 additive. By using a microporous ( < 0.7 nm) car- to compare the SCM/S electrode and a CMK-3/S electrode with bon to form a C/S composite, Gao et al. reported very unusual − 1 the same sulphur loading in the composites (70 wt%), but with- stable cycling performance at a capacity of 800 mAh g with 100% out the SBA-15 additive. SCM/S shows higher capacity than its coulombic efficiency. This was based on a very low sulphur loading CMK-3 counterpart, as shown in Figure 6b. e Th larger pore size of 29 wt% of the electrode mass . Higher loading led to extremely of the carbon structure for C/S electrode is clearly advantageous in low utilization of the active sulphur. e Th flat, low potential of 1.8 V high power applications. Interestingly, both cells show no shut- on the first cycle, the use of carbonate electrolytes and highly slop - tle phenomenon at this C rate and a stable cycling performance. ing subsequent discharge profiles are not characteristic of Li–S cells, By comparison to Figure 4b (blue curve), it is evident that the however. It suggests that treatment of the S/C composite at 300 °C coulombic efficiency is improved to about 98% at high current may have resulted in formation of a C–S polymer or that other fac- rates. It is likely that dissolution/diffusion is diminished under these tors are at play. In another interesting study, Cui et al. studied conditions. the fully discharged form of a S electrode (Li S/CMK-3), with no Our results can be compared with other recent studies on Li/S control on polysulphide diffusion. A clear shuttle phenomenon cells where some new approaches have been explored, from two was observed and the capacity and cycling stability were also poor. aspects: cycling stability and coulombic efficiency. A good summary Important progress was recently made by Hassoun and Scrosati , of the effect of carbon pore structure is provided by the work of who showed that by combining S/C cathodes with lithium sulfide- Liang et al., where a bimodal micro-mesoporous carbon was uti- saturated polymer electrolytes, and Sn/C anodes, a good cycling sta- lized to increase the carbon absorption capacity of polysulphide bility (about a 30% fade rate over 30 cycles), similar to that reported ions . Compared with a mesoporous (7 nm) carbon studied in their here, could be obtained at low rates (C/20). Comparison is made work, which displayed dramatically rapid capacity fading of 80% difficult by the large differences in cell configuration. n ATu RE Commun ICATIons | 2:325 | Do I: 10.1038/ncomms1293 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. –1 Specific capacity (mA h g s) Sulphur in electrolyte/total sulphur ARTICLE nATuRE C ommunICATIons | DoI: 10.1038/ncomms1293 concentration and the total volume of the added electrolyte, which was recorded In summary, we observe that stabilized cycling with a high cou- when assembling the cell. lombic efficiency has not been reported for Li–S cells at practical S/C loadings at high rates. Our results fill this void and represent Characterization. Nitrogen adsorption and desorption isotherms were obtained the best level achieved so far to the best of our knowledge. This was using a Quantachrome Autosorb-1 system at − 196 °C. Before measurement of made possible by utilizing a mesoporous silica that is shown to be SCM, the sample was degassed at 150 °C on a vacuum line following a standard protocol. It was not possible to carry this out for SCM/S owing to the volatility a highly effective internal polysulphide reservoir for the cathode in of the sulphur, and so no pretreatment was used. The BET method was used to Li–S batteries. This not only greatly improves the cycling stability calculate the surface area. The total pore volumes were calculated from the amount but also eliminates the polysulphide shuttle mechanism to a large adsorbed at a relative pressure of 0.99. The pore size distributions were calculated degree and allows large-pore carbons to be utilized. A mesoporous by means of the Barrett–Joyner–Halenda method applied to the desorption branch. monolithic carbon with a large particle size around 10 µm and a uni- e m Th orphology of the SCM/S-SBA-15 composites was examined by a LEO 1530 field-emission SEM instrument equipped with an energy-dispersive X-ray form large mesopore size of 12.5 nm was prepared and investigated spectroscopy attachment. TEM images in this study were obtained with a Hitachi as an encapsulating chamber for the sulphur active mass. e Th C/S HD-2000 scanning transmission electron microscope. composite exhibits larger tap density, less need for binder and very − 2 good rate performance. Although we have established proof-of-con- Calculation of adsorption of polysulphides in SBA-15. The S molecule was used as the prototype polysulphide anion. Gaussian calculations determined the cept with SBA-15, we expect this approach to be widely applicable to length of the molecule as 7.33 Å and the width as ~3 Å×3 Å. other silicas with suitable mesopores of dimensions large enough to − 2 Surface adsorption. e Th maximum surface area of S in the flat configuration accommodate the polysulphide ions. Key to the polysulphide reser- − 19 2 is 2.19×10 m /molecule. u Th s, compared with the surface area of SBA-15 (ref. voir concept is that the material permits absorption/desorption in a 26 2 − 1 − 3 − 2 ) of ~850 m g , 6.44×10 mol S can be adsorbed per gram of SBA-15; that is, − 6 − 2 − 6 reversible manner during the electrochemical reaction. 1.28×10 mol S , or 7.7×10 mol of sulphur, can be adsorbed in the as-pre- pared electrodes, which contain ~0.2 mg of SBA-15 (that is, 10 wt%). Compared with the total mass of sulphur in the electrode (1.2 mg), the SBA-15 can therefore Methods accommodate about 20% of the sulphur mass in the extreme lower limiting case Preparation of SCM. Silica colloid (LUDOX HS-40 40 wt%, Sigma-Aldrich; 5 g) − 2 of surface adsorption—where the S molecules only adsorb on the surface of was dried in a petri dish to form a semi-transparent silica monolith template SBA-15 utilizing the maximum surface area of their structure, and only monolayer (2g), which was impregnated for 10 min with an isopropyl alcohol solution (5 ml) adsorption occurs. ‘End’ adsorption or bi/trilayer adsorption would double or containing 80 mg of oxalic acid (97% Fluka), as a catalyst for the polymerization of triple this value. the carbon precursors. The isopropyl alcohol was then removed by evaporation at − 2 Surface absorption. The volume spanned by the S molecule is approximately 85 °C. The oxalic acid-loaded silica monolith was impregnated in a mixture of 2 g of − 29 3 3 − 1 6.57×10 m /molecule, and the typical pore volume of SBA-15 is ~1.2 cm g . resorcinol (98%, Sigma-Aldrich) and 1.7 g of crotonaldehyde (98%, Sigma-Aldrich) − 2 − 2 − 6 u Th s, 3.03×10 mol S can be absorbed per gram of SBA-15, or 6.06×10 mol for 1 h. Filtration was applied to the soaked silica monolith to remove excessive 6 − 2 S per electrode given the mass of SBA-15 present (see above). This is equivalent precursor. The mixture was then subjected to polymerization through a series of − 5 to 3.6×10 mol of sulphur , which, compared with the total mass of sulphur in heat treatments in air under the following conditions: 60 °C for 30 min, 120 °C for the electrode, means that the SBA-15 can accommodate about 97% of the sulphur 10 h, 200 °C for 5 h. The resultant polymer was carbonized at 900 °C under an argon mass in the extreme upper limiting case of pore absorption if complete pore atmosphere. The silica/carbon composite monolith was ground into a powder filling occurs. The expected co-adsorption of Li ions and solvent molecules before the silica template was removed by HF (15%) etching. would decrease this value by perhaps a factor of two or three. Preparation of CMK-3 and the CMK-3/S composite. We have followed the same procedures described in our previous paper . CMK-3/S contains 70 wt% of sulphur in the composite. References 1. Herbert, D. & Ulam, J. Electric dry cells and storage batteries. US Patent No. Preparation of SCM/S. A mixture of SCM (0.2 g) and sulphur (0.47 g) was 3043896 (1962). ground, and melt diffusion was carried out in an oven at 155 °C. The composite 2. Peramunage, D. & Licht, S. A solid sulphur cathode for aqueous batteries. contains 70 wt% of sulphur. Science 261, 1029 (1993). 3. Rauh, R. D., Shuker, F. S., Marston, J. M. & Brummer, S. B. Formation of Synthesis of SBA-15. Pluronic P123 (EO PPO EO ), 2 g, was dissolved in 60 ml lithium polysulfides in aprotic media. J. Inorg. Nucl. Chem. 39, 1761 (1977). 20 70 20 of 2 M HCl solution at 38 °C. Tetraethylorthosilicate (4.2 g) was added to the above 4. Rauh, R. D., Abraham, K. M., Pearson, G. F., Surprenant, J. K. & Brummer, S. solution under vigorous stirring. The mixture was stirred for 6 min and remained B. Lithium dissolved sulphur battery with an organic electrolyte. J. Electrochem. quiescent for 24 h at 38 °C. The mixture was subsequently heated at 100 °C for Soc. 126, 523 (1979). another 24 h in an autoclave. The as-synthesized SBA-15 with platelet morphology 5. Rao, B. M. L. & Shropshire, J. A. Effect of sulphur impurities on Li/TiS cells. was collected by filtration, dried and calcined at 550 °C in air. J. Electrochem. Soc. 128, 942 (1981). 6. Marmorstein, D., Yu, T. H., Striebel, K. A., McLarnon, F. R., Hou, J. & Cairns, Preparation of SBA-15–SCM/S composites. A mixture of SBA-15 (50 mg) and E. J. Electrochemical performance of lithium/sulphur cells with three different SCM/S (450 mg) was dispersed in water (5 ml), and sonicated for 30 min and polymer electrolytes. J. Power Sources 89, 219 (2000). stirred for 2 h. The mixture was then filtered and dried at 80 °C overnight to remove 7. Obrovac, M. N. & Dahn, J. R. Electrochemically active lithia/metal and lithium residual water. sulfide/metal composites. Electrochem. Solid-State Lett. 5, A70 (2002). 8. He, X., Ren, J., Wang, L., Pu, W., Wan, C. & Jiang, C. Electrochemical Electrochemical measurements. Positive electrodes were constructed from characteristics of a sulphur composite cathode for reversible lithium storage. SCM/S-SBA-15 (95 wt%), and polyvinylidene u fl oride (PVDF) binder (5 wt%). Ionics 15, 477 (2009). The cathode material, ready for electrochemical studies, contained 60 wt% of sul- 9. Hassoun, J. & Scrosati, B. A high-performance polymer tin sulphur lithium ion phur as active mass. The cathode material was well dispersed in cyclopentanone battery. Angew. Chem. Int. Ed. 49, 1 (2010). by sonication and slurry-cast onto a carbon-coated aluminium current collector 10. Yang, Y., McDowell, M. T., Jackson, A., Cha, J. J., Hong, S. S. & Cui, Y. New (Intelicoat), and 2,025 coin cells were constructed using an electrolyte composed nanostructured Li S/silicon rechargeable battery with high specific energy. of a 1.2 M LiPF solution in ethyl methyl sulphone. Lithium metal foil was used Nano Lett. 10, 1486 (2010). as the counter electrode. For the electrode containing CMK-3/S, 8 wt% PVDF 11. Gorenshtein, A., Segal, M. & Peled, E. Characterization of CoS (x=2.1–4.5) as binder was employed. Only in the comparison study with CMK-3/S was the cathode materials for rechargeable lithium batteries. 35th IEEE Power Sources SCM/S electrode also mixed with 8 wt% of PVDF binder. According to the active Symposium 182 (1992). − 2 − 1 mass loading (1.2 mg cm ), the equivalent current density for the 334 mA g 12. Peled, E., Gorenshtein, A., Segal, M. & Sternberg, Y. Rechargeable lithium- − 1 − 2 rate is 0.4 and that for the 1,672 mA g is 2 mA cm . To measure the degree sulphur battery. J. Power Sources 26, 269 (1989). of sulphur retention in the cathode, a 1.0 M LiPF solution in tetraethylene 13. Aurbach, D., Pollak, E., Elazari, R., Salitra, G., Kelley, C. S. & Affinito, J. On the glycol dimethyl ether was used as the electrolyte. SCM/S-SBA-15 cathodes were surface chemical aspects of very high energy density, rechargeable Li–Sulphur compared with SCM/S cathodes containing no SBA-15 at the exact same S/C batteries. J. Electrochem. Soc. 156, A694 (2009). ratio. We used large Swagelok-type cells that accommodate a sufficient excess of 14. Liang, C., Dudney, N. J. & Howe, J. Y. Hierarchically structured sulphur/carbon the electrolyte to dissolve sulphur species. Swagelok cells were disassembled and nanocomposite material for high-energy lithium battery. Chem. Mater. 21, a portion of the electrolyte with the volume accurately measured was taken and 4347 (2009). diluted for sulphur concentration measurement, which was carried out by Gal- 15. Ji, X., Lee, K. T. & Nazar, L. F. A highly ordered nanostructured carbon–sulphur braith Laboratories. The total dissolution can be calculated based on measured cathode for lithium–sulphur batteries. Nature Mater. 8, 500 (2009). nATuRE C ommunICATIons | 2:325 | DoI: 10.1038/ncomms1293 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. nATuRE C ommunICATIons | DoI: 10.1038/ncomms1293 ARTICLE 16. Song, M. S. et al. Effects of nanosized adsorbing material on electrochemical 26. Garneau, A. et al. True microporosity and surface area of mesoporous SBA-15 properties of sulphur cathodes for Li/S secondary batteries. J. Electrochem. Soc. silicas as a function of synthesis temperature. Langmuir 17, 8328 (2001). 151, A791 (2004). 27. Zhang, B., Qin, X., Li, G. R. & Gao, X. P. Enhancement of long stability of sulfur 17. Choi, Y. J. et al. Electrochemical properties of sulphur electrode containing cathode by encapsulating sulfur into micropores of carbon spheres. Energy nano Al O for lithium/sulphur cell. Phys. Scr. 62, T129 (2007). Environ. Sci. 3, 1531 (2010). 2 3 18. Gorkovenko, A., Skotheim, T. A. & Xu, Z.-S. Cathodes comprising electroactive sulfur materials and secondary batteries using same. US Patent No. 6878488 Acknowledgments (2005). e N Th atural Sciences and Engineering Research Council of Canada is gratefully 19. Zhao, D. et al. Triblock copolymer syntheses of mesoporous silica with periodic acknowledged for financial support. We thank N. Coombs, University of Toronto, for 50–300 angstrom pores. Science 279, 548 (1998). help with acquisition of the high-resolution SEM images. (Supporting Information is 20. Song, S. W., Hidajat, K. & Kawi, S. Functionalized SBA-15 materials as carriers available.) for controlled drug delivery: influence of surface properties on matrix-drug interactions. Langmuir 21, 9568 (2005). 21. Lai, C. Y., Trewyn, B. G., Jeftinija, D. M., Jeftinija, K., Jeftinija, S. & Lin, V. Author contributions S. Y. A mesoporous silica nanosphere-based carrier system with chemically X.J. and L.N. designed and conducted the research. Electrochemical experiments were removable nanoparticle caps for stimuli-responsive controlled release of performed by S.E. and X.J., and R.B. prepared the SCM carbon. TEM/SEM experiments neurotransmitters and drug molecules. J. Am. Chem. Soc. 125, 4451 (2003). were performed by S.E. and X.J., L.N. and X.J. wrote the paper. 22. Gierszal, K. P. & Jaroniec, M. Carbons with extremely large volume of uniform mesopores synthesized by carbonization of phenolic resin film formed on Additional Information colloidal silica template. J. Am. Chem. Soc. 128, 10026 (2006). Competing financial interests: The authors declare no competing financial interests. 23. Jaroniec, M., Choma, J., Gorka, J. & Zawislak, A. KOH activation of mesoporous carbons obtained by so- ft templating. Chem. Mater. 20, 1069 (2008). Reprints and permission information is available online at http://npg.nature.com/ 24. Bruce, P. G. Energy storage beyond the horizon: rechargeable lithium batteries. reprintsandpermissions/ Solid State Ion. 179, 752 (2008). 25. Badway, F. et al. Metal oxides as negative electrode materials in Li-ion cells. How to cite this article: Ji, X. et al. Stabilizing lithium-sulphur cathodes using Electrochem. Solid-State Lett. 5, A115 (2002). polysulphide reservoirs. Nat. Commun. 2:325 doi: 10.1038/ncomms1293 (2011). nATuRE C ommunICATIons | 2:325 | DoI: 10.1038/ncomms1293 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved.

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Published: May 24, 2011

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