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In-situ Reactive Assembly of Scalable CoreShell Sulfur-MnO2 Composite Cathodes Xiao Liang, and Linda F. Nazar ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b07458 • Publication Date (Web): 24 Feb 2016 Downloaded from http://pubs.acs.org on February 26, 2016

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In-situ Reactive Assembly of Scalable Core-Shell Sulfur-MnO2 Composite Cathodes Xiao Liang and Linda F. Nazar* University of Waterloo, Department of Chemistry, Waterloo, Ontario, Canada N2L 3G1 *

Corresponding Author Email: [email protected].

ABSTRACT The lithium-sulfur battery is the subject of much recent attention, but the polysulfide shuttle remains problematic owing to dissolution of intermediate polysulfide species in the electrolyte. Despite much effort in limiting such dissolution via physical confinement or chemical binding to the sulfur host materials, the high cost and complicated preparation of the related materials present an impediment to their practical application. Here we demonstrate a simple methodology to fabricate an effective nanometric MnO2 shell on sulfur particles, which is realized by an in-situ redox reaction between sulfur and KMnO4 under ambient conditions. The bi-functional MnO2 shell provides physical confinement and chemical interaction, and shows excellent efficiency for trapping the polysulfides. MnO2 sheets crystallized onto nano-sized sulfur particles result in cathodes with a very low fading rate of 0.039 % per cycle over 1700 cycles in Li-S cells. Moreover, directly crystallizing nanometric shells of MnO2 on micron-sized sublimed sulfur delivers stable Li-S cycling performance over 800 cycles. Since both sulfur and KMnO4 are inexpensive and widely used, the production of MnO2 coated sulfur composites can be easily scaled-up for practical applications of Li/S batteries in light of the very simple reaction processes involved.

Keywords: Lithium sulfur battery; MnO2; core- shell; scalable

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Lithium-ion batteries (LIBs) have been used in a variety of portable electronic devices since 1990s because of their stable electrochemistry and long lifespan. However, current LIBs may soon no longer be able to meet the ever-increasing demands of affordable electric vehicles and large scale energy storage because of cost factors, and their energy density which is approaching its limits.1 Lithium sulfur batteries are a promising alternative owing to their high theoretical energy density (2500 Wh kg-1) based on the reaction between Li and S to form Li2S.2 In addition, S is inexpensive, abundant, and nontoxic. However, the commercialization of Li/S batteries has not yet been achieved due to problematic issues that remain even after 30 years of research,3 although recent studies are starting to break important new ground. The main challenges facing the practical application of lithium sulfur batteries are capacity degradation and low charging efficiency. 4 - 6 These are both related to the “polysulfide shuttle”, attributed to the dissolution of lithium polysulfides (Li2Sn, the series of sulfur reduction intermediates) into liquid electrolytes,7 leading to the loss of active material from the positive electrode during operation. On cycling, the dissolved polysulfides diffuse to the negative electrode, where they are chemically reduced to form thick insoluble/insulating layers of Li2S2-x (where 1 < x ~ 0) on the surface of the metallic lithium, resulting in high surface impedance. Electrochemical oxidation of this layer on the charge cycle leads to the “shuttle” effect, and low coulombic efficiency;8 whereas fairly thick layers cannot be removed by electrochemical oxidation, thus shutting down the cell. Precipitation of highly insulating Li2S2-x can also occur on the surface of the cathode at deep discharge, leading to impediment of ion transfer and an increased overpotential.4 Although the polysulfide shuttle can be alleviated by employing LiNO3 in the electrolyte as an additive to passivate the Li anode surface,9-11 both the cathode and anode consume LiNO3 during cell operation.12 Therefore, trapping polysulfides in the cathode is crucial to address the polysulfide shuttle issue. In the past decade, the approach to suppress the polysulfide shuttle was primarily by physical confinement. The most popular strategy was to absorb polysulfides within high surface area micro/mesoporous carbonaceous frameworks.13-15 However, the synthesis of mesoporous carbon host materials often requires convoluted procedures, including surfactant templating. Seminal studies have shown that sulfur can be encapsulated in a core-shell

16-18

or yolk-core shell structure,19,

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in which a 2


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polymer shell or a titanium oxide shell acts as a physical barrier to hinder the polysulfide diffusion. Alternatively, chemical adsorption has been explored, which is highly reliable because it does not depend on the structure of the host matrix. This approach has been reported for metal oxides based on their polar interaction with polysulfides;21,22 N-doped graphene via the nitrogen functional groups;23 and MXene nanosheets and metal-organic frameworks owing to Lewis acid-base interactions. 24,25 Recently, we uncovered a different chemical approach for polysulfide retention in the sulfur cathode. It relies on mediating polysulfide redox through in-situ formed insoluble thiosulfate species on the surface of MnO2, where the thiosulfate functions by catenating sulfur chains of polysulfides to form a polythionate complex. This process curtails active mass loss during the discharge/charge process and supresses the polysulfide shuttle, resulting in high performance cathodes.26 We have extended this work to explore graphene oxide, and many other metal oxides. We demonstrated that thiosulfate formation is dictated by the redox potential of the host structure: only those oxides with a redox potential between 2.4 V < Eº ≤ 3.05 V can trigger such a reaction.

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Correlation of the existence of the

thiosulfate/polythionate surface mediator-host with excellent cycling stability of the resultant cathodes employed in Li-S cells provided proof of the concept. However, the fraction of the host material in the sulfur composite should be < 30 wt% in order to obtain high energy density. Such a small fraction of host material puts limitations on polysulfide absorptivity. Herein, we present a combination of physical and chemical adsorption in a sulfur-core MnO2 shell structured composite as the cathode in Li-S batteries. The physical adsorption is created by the core-shell architecture and chemical adsorption is affected by the MnO2 shell, which also acts as a semiconductor to transfer electrons to the sulfur core. The chemically active MnO2 shell is grown in-situ onto the sulfur particles by a simple redox reaction between sulfur and potassium permanganate. This is the first report of such a reaction under such mild conditions to the best of our knowledge. To reserve space to accommodate sulfur volumetric expansion to Li2S upon charge/discharge, solvent extraction was used to partially remove some sulfur from the core. Both nano-sized sulfur and the micron-sized sulfur particles coated with MnO2 showed excellent cycling performance. The latter material is especially suitable for mass production due to the simple and efficient fabrication procedure involved, 3


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and exhibits a very low capacity fade rate of 0.048% per cycle over 800 cycles at 2C. This represents the best performance for a Li-S battery using commercial sulfur particles as the active material to date.

RESULTS AND DISCUSSION 1. Reaction of KMnO4 and sulfur The reaction between KMnO4 and sulfur has been widely studied for applications in explosives, where sulfur is oxidized by flame initiation in a narrow space, resulting in extremely high temperatures and pressures.28 This process is destructive and uncontrollable, in contrast to our case. Our results show that the reaction of KMnO4 and sulfur can be controlled under ambient conditions in aqueous solution, leading to a highly efficient method to construct a core shell structure with a nanometric coating of MnO2. The reaction of sulfur with KMnO4 in H2O yields a dark-brown powder (Figure 1a, b). Nano-sized (300-400 nm) or micron-sized sulfur particles (sublimed) were used in this study. To demonstrate the effectiveness of the reaction, sulfur was completely removed from the core by toluene washing at 60 oC. A shell composed of self-assembled nanosheets is clearly visible in the TEM images

Figure 1. Comparison of the sublimed sulfur (a) and after reaction with KMnO4 (b). SEM images of the MnO2 shell, after the sulfur core was removed. (c) MnO2 shell formed on nano-sized sulfur,(d) MnO2 shell formed on sublimed sulfur. (e) X-ray diffraction pattern of the MnO2 shell, (f) XRD pattern of the powder recovered from the dried filtrate from the S-KMnO4 reaction system. 4


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(Figure 1c and d). The X-ray diffraction pattern of the shell reveals that only monoclinic birnessite (δ-MnO2, JCPDS-01-080-1098) is present as a crystalline phase (Figure 1e). The selected area electron diffraction (SAED) pattern detected from the shell reveals weak diffraction rings, indexed to the 0.71 nm interlayer d-spacing of δ-MnO2 projected along the [001] zone axis (Figure 1c, insert). X-ray photoelectron spectroscopy (XPS) analysis confirms the birnessite nature of the shell, exhibiting a majority Mn4+ component on the surface, and ~5% Mn3+ which is characteristic of δ-MnO2 (Figure S1).26 XRD pattern of the white powder recovered by drying the filtrate of the S-KMnO4 reaction shows the co-existence of K2SO4 (00-005-0613) and K3H(SO4)2 (98-107-4402) (Figure 1f). This demonstrates that KMnO4 can oxidize elemental sulfur to its highest oxidation state (VI), with MnO2 as the reduction product. The reaction can be simply described by the redox Equation 1: 6 + 3 + → 6 + + [( ) ] +

Equation 1

In birnessite, the sheets of MnO6 octahedra are in fact, slightly reduced to form KxMnO2-δ (x ~ 0.15) and the potassium ions reside between the sheets (along with residual water) to counterbalance the charge.29 This redox reaction is the first report of the S-KMnO4 reaction under ambient conditions. It results in an in-situ formed high surface area shell composed of self-assembled MnO2 nanosheets, which provides physical confinement to entrap the soluble polysulfides within the shell. Moreover, polysulfides are chemically bound by thiosulfate-polythionate conversion – initiated by their redox reaction with MnO2 – which has been detailed in our recent publications.26, 27 Given the fact that both sulfur and KMnO4 are inexpensive and widely used - in addition to the extremely simple reaction process involved - we believe that production of the MnO2 coated sulfur composite can be easily scaled-up at low cost. This approach was used to generate two sulfur-MnO2 core-shell composites with a core comprised of either nano-sized sulfur (NS-core/MnO2), or sulfur particles (SS-core/MnO2), as discussed below.

2. MnO2 core-shell materials with a nano-sized sulfur particle core Nano-sized sulfur spheres were synthesized via the disproportionation of Na2S2O3 in the presence of acid and polyvinylpyrrolidone (PVP), as reported elsewhere.19, 20 Scanning electron microscopy (SEM; Figure 2a) revealed uniform sulfur spherical nanoparticles with particle sizes of 300-400 nm. The sulfur 5


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nanoparticles were coated with MnO2 via Equation 1, resulting in the formation of a sulfur-MnO2 core shell structure shown in Figure 2b. The thin MnO2 shell is composed of irregularly shaped MnO2 nanosheets (Figure 1c). As sulfur has a volume expansion up to 80% when fully converted to Li2S during discharge, it is necessary to provide a buffer void in the core-shell composite in order to maintain the integrity of the electrode. To achieve this, toluene was used to partially extract the sulfur core, resulting in a “yolk-shell” structure as first elegantly demonstrated for [email protected] Sulfur has moderate solubility in toluene at 20 oC,30 allowing for tuning the space in the core shell structure by simply adjusting the amount of solvent. The toluene rinsed NS-core/MnO2 retained its core shell structure, as shown by the SEM image in Figure 2c. Sulfur leaching from the interior was demonstrated by thermogravimetric analysis (TGA) in Figure 2d, clearly showing that the sulfur content decreased from 85 wt.% for the pristine NS-core/MnO2 to 80 wt.% for the rinsed sample.

Figure 2. SEM images of (a) nano-sized sulfur, (b) NS-core/MnO2, (c) the rinsed NS-core/MnO2. (d) TGA curves in air of the pristine NS-core/MnO2 (black) and rinsed NS-core/MnO2 composite (red).

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Electrochemical performance of NS-core/MnO2. Voltage profiles collected at different rates in 2325 coin cells (sulfur loading between 1.5 – 1.7 mg cm-2) show two plateaus (Figure 3a), which correspond to the two-step process of the Li-S electrochemistry.14 The rinsed NS-core/MnO2 delivered a specific capacity of 1380 mAh g-1 at C/20 (defined as discharge or charge of 2 Li in 20 hours), corresponding to 82% of the theoretical capacity. Even at a high rate of 2C (3350 mA g-1), the rinsed composite with 80 wt% sulfur still delivered 1000 mAh g-1 capacity. The MnO2 shell host itself does not contribute significantly to the capacity in the 1.8 - 3.0 V window, as it only accounts for 30 mAh g-1 for the initial discharge and less than 5 mAh g-1 after 20 cycles (Figure S2).

Figure 3. Electrochemical performance of NS-core/MnO2. (a) Voltage profile of the rinsed NS-core/MnO2 at C/20, C/2 and 2C, collected from different cells. The pristine NS-core/MnO2 is shown as the dashed line. (b) Comparison of the cycling performance of the pristine and rinsed NS-core/MnO2 at C/2. (c) Long term cycling performance of the rinsed NS-core/MnO2 at 2C. 7


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Figure 3b compares the cycling performance and coulombic efficiency of the cells with pristine and rinsed NS-core/MnO2 cathode materials at C/2. Significant improvement in capacity retention was achieved for the latter. It stabilized at 950 mAh g-1 after 300 cycles, corresponding to 88% capacity retention. The coulombic efficiency was above 99.5% throughout, suggesting the polysulfide shuttle was significantly suppressed by the MnO2 shell. On the other hand, under identical conditions, the capacity of the pristine sample decreased more rapidly on cycling, reaching 680 mAh g-1 after 300 cycles and achieving much lower coulombic efficiency. We note that even this capacity retention (65%) of the pristine NS-core/MnO2 is better than other “fully-filled” core-shell composites such as S@PANi19 that are subject to damage of the protective shell during sulfur expansion, and is still comparable to many reports using mesoporous carbon hosts.14, 15 This is attributed to the intrinsic polysulfide adsorption by the MnO2 shell, which provides chemical bonding to the polysulfides through in situ formed thiosulfate/polythionate groups.26,27 However, although our detailed study of polysulfide adsorption shows that MnO2 nanosheets have the best properties among the most commonly used host materials,31 15 wt% MnO2 in the pristine (85 wt%) sulfur NS-core/MnO2 material is clearly not sufficient enough to adsorb all of the polysulfides generated on reduction. The combination of chemical adsorption and physical confinement provided by the core shell architecture is thus necessary to trap polysulfides in high sulfur-content composites. Variable-current density and long term cycling studies of NS-core/MnO2. The voltage window for rinsed NS-core/MnO2 cells run at a 2C current density was adjusted to 1.7-3.0V to fully allow the cell to discharge. After 1700 cycles, the cell exhibited a discharge capacity of 315 mAh g-1 with an excellent average coulombic efficiency of ﹥99% and an ultra-low fading rate of 0.039 % per cycle (Figure 2c). After 1700 cycles at 2C, the NS-core/MnO2 still delivered a reversible capacity of 690 mA h g-1 when the rate was switched back to C/20. The cell was also subjected to cycling at five different C-rates to evaluate its current density robustness (Figure S3). The specific capacity measured between C/5 and 3C ranged from 1120 to 860 mAh g-1, respectively. On lowering the rate from 3C back to C/5, the specific capacity recovered to 1100 mAh g-1, i.e., 98% of the original. Similar behaviour was exhibited for 4C and 5C rate studies after 10 8


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cycles at C/5: 96% of the discharge capacity could be regained. Even at ultra-high rates of 8C (13.4 A g-1), a reversible discharge capacity of 720 mAh g-1 was obtained and yet most of the discharge capacity could be recovered after the rate was switched back to C/5. This behaviour demonstrates exceptional rate performance owing to the nature of the shell. As mentioned above, birnessite “δ-MnO2” is actually a layered structure best described as K0.15MnO2-δ, with a 7.1 Å interlayer gap containing large, immobile K+ ions and a fraction of strongly bound H2O. Such an architecture is unlikely to facilitate the passage of solvated Li+ ions. The nanodimensions of the δ-MnO2 crystallites are more likely responsible for intergrain boundary transport of Li+/electrolyte.

3. MnO2 core-shell materials with a sublimed sulfur-particle core Since the synthesis of sulfur nanoparticles is not suitable for mass production, commercial sulfur is the most promising choice for practical applications of Li-S batteries. However, due to its large particle size (typically around 10um) and low specific surface area, chemical and electrochemical activity is limited. Therefore, a common strategy to utilize sulfur as an active material involves its melt diffusion at 155 oC in order to achieve a homogenous and intimate contact with the host material. However, as we show below, the redox reaction between sulfur and KMnO4 described above can also be used to construct an effective MnO2 shell on sublimed sulfur particles (SS-core/MnO2). This core-shell structured composite can be used directly as an electrode without additional heat treatment, and benefits from the MnO2-polythionate surface mediator effect of the shell, and potentially controlled Li2S precipitation within the particles. This highly economical and efficient route is suitable for mass production of practical lithium sulfur batteries. The sublimed sulfur particles used here are irregular in shape and about 5-15 um in diameter (Figure 4a, insert). To facilitate their redox chemistry with KMnO4, the reaction was maintained at 60 o

C for 24 h (see Experimental).

EDS confirmed that the composite particles are composed of S, Mn

and O, (Figure S4). The uniform shell surrounding the underlying sulfur particle exhibits the characteristic lamellar birnessite type δ-MnO2 morphology comprised of nanoleaflets (Figure 1d; Figure 4a, b). The thickness of the shell can be determined owing to its relatively high contrast in the 9


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(Figure 1d), which is 50 ± 30 nm. The layer is thicker than that on nano-sized sulfur

(Figure 1c), owing to the more forcing reaction conditions. The crinkled morphology of the shell lends it inherent flexibility, which can be further improved by the incorporation of polymeric components (work underway in our laboratory). Figure S5 shows the XRD pattern of the SS-core/MnO2 composite that reveals reflections of orthorhombic elemental sulfur.

The diffraction signature from the birnessite

shell (i.e. a weak broad peak at ca. 12.5 degrees (2θ)) is more visible upon full extraction of sulfur from the composite (Figure S5). To prepare electrodes, the same toluene-rinsing procedure was employed for the SS-core/MnO2 composites as for the NS-core/MnO2 material. The intact MnO2 shell was well preserved (Figure 4c). The TGA curve shows a decrease in sulfur content from 80 to 74 wt.% (Figure 4d) after rinsing.

Figure 4. Morphology of the SS-core/MnO2. (a) Low- and (b) high- magnification SEM images of the pristine SS-core/MnO2, the insert in (a) is the SEM image of the sublimed sulfur. (c) rinsed SS-core/MnO2, (d) TGA curves of the pristine SS-core/MnO2 (black) and rinsed SS-core/MnO2 composite (red). The TG analysis was conducted in air. 10


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Electrochemical performance of SS-core/MnO2. Cathodes prepared of the micron-sized SS-core/MnO2 also exhibited a typical two-plateau sulfur discharge profile (Figure S6) in coin cell studies (see Experimental for details). These studies were first carried out at sulfur loadings of ~1.5 mg cm-2 to enable comparison with the NN-core/MnO2 materials, and to compare with results in the literature for other core-shell materials (higher sulfur loading is described below). The capacity (1040 mAh g-1) was only slightly lower than the NS-core/MnO2 (1080 mAh g-1) at the same C/2 rate, surprisingly. As shown in Figure 5, capacity retention for the pristine SS-core/MnO2 was only 60% after 300 cycles, whereas the rinsed composite exhibited much better cycling performance with > 98%

c

Figure 5. Electrochemical performance of the SS-core/MnO2. (a) Comparison of the cycling performance of the pristine and rinsed SS-core/MnO2 at C/2 with a sulfur loading of 1.5 mg cm-2, (b) Long term cycling performance of the rinsed SS-core/MnO2 at 2C, with a sulfur loading of 1.5 mg cm-2. (c) Cycling performance of the rinsed SS-core/MnO2 at C/5, with a sulfur loading of 2.8 mg cm-2 (red) and 4.1 mg cm-2 (black). 11


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coulombic efficiency, and 820 mAh g-1 capacity after 300 cycles. The difference is due to a rupture of the core-shell morphology for the pristine material, whereas the shell structure is largely preserved for the rinsed material on cycling (Figure S7). Voids in the core are clearly essential to preserve structural integrity and thus mitigate capacity fading. The rinsed SS-core/MnO2 also showed excellent performance at high rates (2C) after being activated at C/20 for one cycle (Figure 5b). It delivered an initial capacity of 780 mAh g-1-sulfur (or 460 mAh g-1 considering the entire electrode mass). After 800 cycles, 480 mAh g-1 of discharge capacity was maintained, corresponding to a fade rate of 0.048% per cycle. Most importantly, the rinsed SS-core/MnO2 composite also exhibits very good performance with higher sulfur loading, i.e. at 2.8 and 4.1 mg cm-2, as shown in Figure 5c. To the best of our knowledge, these areal sulfur loadings are the highest among the recently reported core-shell (yolk-shell) materials for Li/S batteries. The remarkable cycling stability of these cells with practical-high sulfur loading demonstrates effective polysulfide confinement via the combination of chemical and physical entrapment provided by the efficient core-shell nanostructure.

CONCLUSIONS We demonstrate that a nanometric “crinkled” MnO2 protective shell is formed in situ on sulfur particles by their reaction with KMnO4. The benefits are the ease of the synthesis, the nature of the materials and the scalability. In our previous studies,26,27 sulfur was coated on MnO2 nanosheets that were harvested via ultrasonication. The work demonstrated the effect of the “Wackenroder” thiosulfate mechanism in chemically trapping the polysulfides. In the current work however, the MnO2 is formed in-situ as a coating on the sulfur particles (ie, exactly the opposite of the former). The advantage is that the sulfur particles are completely enclosed in an MnO2 shell, providing another barrier to polysulfide transport. The concept echoes previous inorganic “yolk-shell” work, but represents an advance in several important respects. First, the reaction does not require delicate hydrolysis control of metal alkoxide precursors to achieve a good, reproducible coating - the coating forms spontaneously through the redox reaction in a one-step, easy process. Second, MnO2 is a black semiconductor - thus enabling electron transport through the shell. Not only is the first report of the aqueous KMnO4-sulfur redox process at mild conditions, but both 12


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nano-size and micron-sized sulfur can be encapsulated by this method. In both cases, the physical MnO2 core shell structure and the intrinsic polysulfide binding by the MnO2 leads to excellent polysulfide confinement and substantially improves the cycling stability of the Li-S battery. The approach is particularly efficient and economical as it can utilize commercially available sulfur particles and an aqueous reaction medium, making it amenable to the mass production of high performance sulfur cathodes. The composite material exhibits a very low capacity fade rate of 0.048% per cycle over 800 cycles at 2C, providing a final useable capacity of 480 mAh g-1. This represents the best performance for a Li-S battery using commercial, bulk sulfur particles as the active material to date.

METHODS Synthesis of nano-sized sulfur core/MnO2 (NS-core/MnO2). Na2S2O3 (2.37 g) in 50 mL water was slowly added into a dilute sulfuric acid solution (500 mL, 14.4 mM) containing 1g of polyvinylpyrrolidone (PVP, Mw~40,000). After stirring for 2 hours at room temperature, the sulfur nanoparticles were collected by centrifugation. The as-prepared sulfur nanoparticles (83.8 mg) were re-dispersed into 100 mL aqueous solution, and 50 mL KMnO4 (2.5 mM) was added dropwise to the nano-sized sulfur suspension. The mixture was stirred for 2 hours at room temperature and then was filtered. Synthesis of the sublimed sulfur core/MnO2 (SS-core/MnO2). Commercial sulfur (Alfa Aesar, >99.5%, 100 mesh) was used as received. Commercial sulfur (1 g) was dispersed in 170 mL of deionized water by sonication for 1 hour. KMnO4 (250 mg) was dissolved in 50 mL DI water and poured into the sulfur suspension. The mixture was stirred at 60 oC for 24 hours. The dark-brown powder was collected by filtration. Toluene rinsing. Sulfur has moderate solubility in toluene (17 mg/cc at 20 oC).30 A calculated amount of toluene was mixed with ethanol (1:5 volume ratio). The NS-core/MnO2 composite was stirred in the solution for 30 minutes at room temperature. To obtain a pure MnO2 shell devoid of sulfur, the composite was soaked in pure toluene at 60 oC for at least 3 times. The desired products were obtained by filtration and dried at 60 oC overnight. 13


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Characterization. XRD measurements were carried out on a Bruker D8-Advance powder X-ray diffractometer operating at 40 kV and 30 mA and using Cu-Kα radiation (λ=0.15405 nm). SEM studies were carried out on a Zeiss Ultra field emission SEM instrument. TEM was performed on a Jeol 2010F TEM/STEM operating at 200 KeV. Thermogravimetric analysis (TGA) was used to determine the sulfur content of the material, on a TA Instruments SDT Q600 employing a heating rate of 10 °C/min from room temperature to 700 °C under an air flow. Nitrogen adsorption-desorption analysis were performed at 77 K on a Quantachrome AUTOSORB-1. The surface area of the sample was calculated by the Brunauer Emmett Teller (BET) method by taking 5 data points where P/P0 < 0.3. These data showed that the nano-sized yolk shell composite has almost double the surface area (~ 32 m2/g) of the micron-sized composite (~ 14 m2/g ), whereas the recovered MnO2 shells (after removal of the sulfur) show a similar surface area ( 123 vs. 103 m2/g). Electrochemical measurements. The MnO2 coated sulfur composite was mixed with super P and PVDF in a weight ratio of 8: 1: 1 in DMF. Cathodes were prepared by drop casting the DMF slurry on P50 carbon paper current collectors and dried at 60 oC overnight. For high sulfur loading electrodes, the slurry was made from rinsed SS-core/MnO2, Super P and PAA/PVA (9:1 weight) in a weight ratio of 8: 1: 1 in water. Lithium foil was employed as the anode with a Celgard 3051 separator in a 2325 coin cell. The electrolyte was 1 M bis-(trifluoroethanesulfony)imide lithium (LiTFSI) in 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) (v/v= 1:1), with 2 wt% of LiNO3. Different cut-off voltages were selected to ensure full discharge at different rates, i.e.: 1.8- 3.0 V for 0.05C, 0.2C, 0.5C and 1C; 1.7- 3.0 V for 2C, 3C, 1.6- 3.0 V for 4C and 1.5-3.0 V for 6C, 7C, 8C (1C = 1675 mAh g-1).

AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected] Notes: The authors declare no competing financial interest.

ACKNOWLEDGMENT The research was supported by the BASF International Scientific Network for Electrochemistry and 14


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Batteries. LFN thanks NSERC for underlying support through their Discovery Grant and Canada Research Chair programs.

We gratefully acknowledge the Canadian Facility for Electron Microscopy

and Dr. Carmen Andrei, McMaster University, for help with acquisition of the TEM images.

ASSOCIATED CONTENT Supporting Information Available: The Supporting Information is available free of charge via the internet at http://pubs.acs.org. Additional information on the characterization of the MnO2 shell; rate capability studies (at variable current densities) of the NS@MnO2 composite; XRD patterns of the SS@MnO2 composite.

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Sulfur TiO2 Yolk-Shell

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Figure for the TOC

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