Mechanism for the Stable Performance of Sulfur-Copolymer Cathode

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Mechanism for the stable performance of sulfur-copolymer cathode in lithium–sulfur battery studied by solid-state NMR spectroscopy Alexander Hoefling, Dan Thien Nguyen, Pouya Partovi-Azar, Daniel Sebastiani, Patrick Theato, Seung-Wan Song, and Young Joo Lee Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b05105 • Publication Date (Web): 12 Apr 2018 Downloaded from http://pubs.acs.org on April 12, 2018

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Chemistry of Materials

Mechanism for the stable performance of sulfur-copolymer cathode in lithium− −sulfur battery studied by solid-state NMR spectroscopy Alexander Hoefling,§,† Dan Thien Nguyen,#,† Pouya Partovi-Azar,⊥ Daniel Sebastiani,⊥ Patrick Theato,§ Seung-Wan Song,*,# and Young Joo Lee*,‡ §

Institute for Technical and Macromolecular Chemistry, Department of Chemistry, University of Hamburg, Bundesstrasse 45, 20146 Hamburg, Germany # Department of Chemical Engineering and Applied Chemistry, Chungnam National University, Daejeon 34134, Republic of Korea ⊥

Institute of Chemistry, Martin-Luther-University Halle-Wittenberg, Von-Danckelmann-Platz 4, 06120 Halle (Saale), Germany ‡ Institute of Inorganic and Applied Chemistry, Department of Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, 20146 Hamburg, Germany ABSTRACT: Rechargeable lithium−sulfur (Li−S) batteries have drawn significant attention as next-generation energy storage systems. Sulfur-copolymers are promising alternative cathode materials to elemental sulfur in Li−S batteries as they provide high reversible capacity. However, the redox mechanisms of these materials are not well understood owing to the difficulty in characterizing amorphous structures and identifying individual ionic species. Here, we use solid-state NMR techniques together with electrochemistry experiments and quantum calculations to investigate the structural evolution of the prototype S-copolymer cathodes, sulfur-diisopropenylbenzene copolymers (poly(S-co-DIB)) during cycling. We demonstrate that polysulfides with different chain lengths can be distinguished by 13C and 7Li NMR spectroscopy, revealing that the structure of the copolymers can be tuned in terms of polysulfide chain lengths and resulting reaction pathways during electrochemical cycling. Our results show that the improved cyclability of these cathodes originates from the role of organic moieties acting as anchors that fixate polysulfides to the polymeric network during cycling, thus preventing their diffusion into the electrolyte. We provide a new methodological concept for the mechanistic studies to track the intermediate species and phase transition in Li−S batteries.

1. INTRODUCTION Lithium−sulfur (Li−S) batteries have been considered promising alternatives to the current lithium ion batteries (LIB) due to their advantages of using elemental sulfur, including earth abundance, low cost, high theoretical specific capacity (1672 mAh g−1), and high energy density (~2600 Wh kg−1).1 Unfortunately, cycle-life of Li−S cells with elemental sulfur cathodes is limited owing to the several issues such as low conductivity of elemental sulfur and of its reduction products, dissolution and the parasitic internal shuttle phenomenon of polysulfide intermediates in the electrolyte, and large volumetric expansion upon reduction (up to ∼80%).2 Various methods have been suggested to overcome these drawbacks of elemental sulfur, such as forming S composite materials with carbon nanotubes and conducting polymers and encapsulating sulfur in the pores of macro-, meso-, and microporous carbon.3–11 Since the first introduction by Chung et al,12 Scopolymers prepared by copolymerizing molten sulfur with molecular dienes and diynes via the formation of carbon−sulfur (C−S) bonds, have opened new research avenues because of good processibility and a facile, solvent-free route enabling for mass production at low cost.13–16 These Scopolymer cathodes display an increase in capacity retention and an extension of cycle-life, exhibiting compositional dependence, even though their conductivity is still poor. Their

battery performance is enhanced as the ratio of organic group to sulfur decreases and the highest initial capacity and capacity retention are obtained for S-copolymers with a 5−15 wt% concentration of organic moieties.17 However, the reaction mechanisms contributing to the improved cyclability are not well understood. The discharge process of Li−S batteries is known to proceed in a stepwise fashion, with two plateaus at ∼2.3 and ∼2.0 V appearing in the voltage profile. These plateaus correspond to (1) the conversion of solid sulfur to soluble long-chain polysulfides (S8 → Li2S4) at high potential and (2) the transformation of Li2S4 into insoluble lithium sulfides (Li2S2, Li2S) at lower potential.18–20 The charge process, on the other hand, follows a much simpler route, yielding one oxidation plateau in the voltage profile.19,21 It has been suggested that Li2S is transformed to the same intermediate polysulfide (plausibly S82−) by a slow chemical reaction and this intermediate is oxidized to elemental sulfur following a fast charge-transfer step. In order to improve the cycle stability of Li−S batteries, it is crucial to understand their reaction mechanisms. However, the nature of the electrochemical redox reactions that occur in Li−S cells is still controversial because they consist of complex multistep reactions, accompanied by solid-liquid phase transition and the formation/cleavage of bonds involving versatile polysulfide species. Various techniques, such as X-ray

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diffraction, microscopy, Raman spectroscopy, UV/Vis spectroscopy, X-ray photoelectron spectroscopy (XPS), and liquid chromatography have been utilized to investigate the reaction mechanisms in Li−S batteries,22–30 however, these approaches are not suitable to monitor the structural changes occurring in amorphous materials, such as S-copolymer cathodes. In particular, characterization methods that enable to distinguish between different chain lengths of polysulfides in solid as well as in liquid phases are extremely limited.27,31 Solid-state NMR spectroscopy is an ideal tool to study the molecular structure, local ordering, and molecular motion of soft matter and energy materials even when long-range order is lacking.32–37 In the present study, utilizing prototype S-copolymer cathodes, poly(S-co-DIB), synthesized by inverse vulcanization,12 we will explore the feasibility of using 13C and 7Li magic angle spinning (MAS) NMR spectroscopy to distinguish individual species associated with different polysulfide chain lengths and to monitor the structural changes that the S-copolymer cathodes undergo during the electrochemical cycling. The influence of polysulfide chain length on the redox behavior and on the reaction pathway during electrochemical cycling will be examined. Finally, reaction mechanisms contributing to the improved cycling performance of S-copolymer cathodes in comparison to elemental sulfur will be proposed. 2. EXPERIMENTAL SECTION Materials. Poly(S-co-DIB) copolymers with various compositions were synthesized by inverse vulcanization method as reported before.12 The samples are labeled as S-DIB-X where X denotes weight percentage of DIB in the starting materials. Elemental sulfur and 1,3-diisopropenylbenzene (DIB, MW = 158.24 g mol−1) were combined in a 15 mL glass vial equipped with a magnetic stirring bar at a 2 g scale. Under vigorous stirring, the mixture was heated at 180 °C in an oil bath for maximum 10 minutes, while the color changed from yellow to dark red. In addition, viscosity of the mixture increased and stirring stopped. After the gel point was reached the glass vial was transferred into a cold water bath in order to stop any further reaction. The dark red colored and partially transparent materials were hardly soluble in common organic solvents. The composition of resulting polymers was confirmed by elemental analysis. Electrochemistry. The poly(S-co-DIB) cathodes, S-DIB-50 and S-DIB-10, were prepared by casting a slurry, which was composed of poly(S-co-DIB) active material (70 wt%), carbon black (Super-P, 20 wt%) and binder (10 wt%), onto an aluminum foil. The binder consisted of sodium carboxymethyl cellulose (CMC, MW 250,000, Aldrich) and polyacrylic acid (PAA, MW 450,000, Aldrich) at the equal weight ratio in deionized water. S-DIB-10 cathodes containing carbon fiber (SDIB-10-CF) were prepared in the same method but with carbon black (20 wt%), carbon fiber (5 wt%, Aldrich, 100 nm in diameter and 20−200 µm in length) and CMC-PAA binder (5 wt%). The loading level of sulfur was approximately 0.75 mg cm−2. The prepared cathodes were dried under vacuum at 45 o C for 24 hours. Cyclic voltammetry (CV) was conducted with three-electrodes cells, which consist of poly(S-co-DIB) cathodes as a working electrode, lithium metal as a reference and a counter electrode, separator (Celgard C210), and the electrolyte at a scan rate of 20 µV s−1 between 1.5 and 2.8 V vs. Li/Li+, using a potentiostat (VPS SP 150, Bio-Logic). The electrolyte is composed of 2M lithium

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bis(trifluoromethane)sulfonimide (LiTFSI; Aldrich) and 0.31M lithium nitrate (LiNO3; Aldrich) in 1,3-dioxolane (DOL; Aldrich) and 1,2-dimethoxyethane (DME; Aldrich) at the equal volume ratio. The current density of CV was normalized to the weight of sulfur. Electrochemical charge/discharge cycling performance was tested with 2032 coin cells consisting of poly(S-co-DIB) cathodes as a working electrode and lithium metal foil as a counter electrode at the rate of 0.1 C (167 mA g−1) between 1.5 and 2.6 V after two formation cycles, using a multichannel cycler (WBCS3000, Wonatech). We used the optimized ratio of electrolyte volume to the weight of sulfur, 25 µL mg−1 for coin cells. The current rate is defined as 1 C equals to 1672 mA g−1 based on the theoretical capacity (1672 mA h g−1) of sulfur. The specific gravimetric capacities of the cathodes were calculated based on the weight of sulfur. All the cells were assembled in an argon-filled glove box. Computations. The geometries of phenyl−Cq(CH3)(CH2CH3)−Sx−CH3 with x = 1−5 and Sx (terminated with a methyl group for the simplicity.) were first optimized in the gas phase using density-functional theory. In the geometry optimizations, an accurate TZV2P−MOLOPT basis set38 was used along with the BLYP exchange−correlation energy functional39,40 and GTH pseudopotentials39 as implemented in the CP2K software package.38 Convergence criteria for maximum geometry change and maximum force component were set to 1.6×10−3 Å and 0.02 eV Å−1, respectively. A damped interatomic potential was also added to account for the van der Waals interactions (DFT-D3).40 The NMR chemical shift calculations were carried out on the optimized structures again using BLYP exchange-correlation functional, but at the all-electron level with IGLO-III41 and pcSseg-242 basis sets using the Gaussian Augmented Plane Wave (GAPW) method.43 The values for the chemical shifts are referenced with respect to the x = 1 system, at 0 ppm. Solid-state NMR spectroscopy. Solid-state NMR experiments were performed on a Bruker Avance II 400 spectrometer, equipped with 4 and 2.5 mm double resonance MAS probes. 13C{1H} cross-polarization (CP) MAS NMR spectra were acquired using ramped polarization transfer from proton to carbon at a 13C frequency of 100.66 MHz. The experimental conditions were 1H 90° pulse length of 4.0 µs, contact time of 1 ms, repetition delay of 4 s and spinning frequency of 13 kHz. Two-pulse phase-modulated (TPPM) decoupling was used during the acquisition. 13C T1 (spin−lattice) relaxation time was measured by CP-T1 pulse sequence.44 7Li MAS NMR spectra were acquired with direct excitation using 45° pulse length of 1.5 µs and repetition delay of 20 s at a spinning frequency of 20 kHz. For ex situ 7Li MAS NMR, the cathodes were prepared by the same methods as electrochemistry measurements with carbon black (25 wt%) and CMC-PAA binder (5 wt%). For ex situ 13C{1H} CP MAS NMR measurements, the cathodes were prepared by mixing poly(S-co-DIB) active material (85 wt%), carbon black (Super-P, 10 wt%) and polytetrafluoroethylene (PTFE; Aldrich, 5 wt%) binder, and pressing the mixture into a disk with a diameter of 13 mm under the pressure of 10 MPa. The cathodes were assembled in 2032 coin cells that include Li metal foil as a counter electrode and the electrolyte of 2M LiTFSI + 0.31M LiNO3/DME:DOL. After a formation cycle, the cells were discharged to the given states of discharge (SOD) at the constant current of 70 µA cm−2 (C/50). When the cell voltage reached to each state of


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discharge, the cell was kept at that constant voltage until the current flowed was lowered down to 5% of discharge current, which allows reaching equilibrium. Afterwards, the cathodes were separated from the coin cells, dried at room temperature in vacuum for 24 hours and then packed into MAS rotors under N2 condition. To remove soluble components, the cathodes were also prepared after washing with DOL. 3. RESULTS AND DISCUSSION 13 C{ {1H} } CP MAS NMR spectroscopy of poly(S-co-DIB) copolymers. Poly(S-co-DIB) copolymers12 with various compositions were characterized by 13C{1H} cross-polarization (CP) MAS NMR spectroscopy. The presence of a signal at ∼110 ppm is not significant, suggesting that only a minor amount of unreacted C=C remains and a conversion of C=C into C−S bonds is almost complete (Figure 1). Intriguingly, the chemical shifts and intensities of the resonances due to quaternary (Cq, α-position to one S linkage and β-position to another S linkage) and aromatic carbons (Car, β- and γ-position to the S linkages) vary as a function of S content. A Cq signal at 55 ppm is dominant for S-DIB-50, whereas another signal at 59 ppm gradually increases in intensity as the S content increases. Additionally, a weak shoulder at 53 ppm is present (these signals can be clearly identified in spectral deconvolution shown in Figure S1). In previously published liquid NMR studies, chemical shifts were shown to depend on the number of S atoms adjacent to the C−S linkage.45–48 For example, the 1 H NMR signals of α-methylene and α-methyl units bridged by a polysulfide chain shift to higher frequencies as the number of S atoms increases so that compounds containing between one and six S atoms in the polysulfide chains could be distinguished from each other using 1H NMR data.45,46 Hence, in this study, we assign the individual 13C NMR signals of poly(S-co-DIB) based on the length of the polysulfide chains bound to the carbon centers. In S-DIB-50, there are 5 sulfur atoms per DIB molecule, resulting in a copolymer with an average chain length of 2.5 S atoms. Thus, S-DIB-50 is predominantly composed of polysulfide chains involving 2 or 3 S atoms (C−Sx−C, x = 2 or 3) and we tentatively assign the 13C NMR signals at 53 and 55 ppm to the Cq carbons bound to the polysulfide chain with two and three S atoms, respectively. Moreover, it is expected that polysulfide chains with x = 1 and 4 are present to minor extent resulting from a distribution in the local structure. However, no additional discrete signals are observed, which is probably due to overlaps of broad signals whose chemical shift values are similar to each other. Therefore, the signals at 55 and 53 ppm are assigned to a Cq bound to a medium-chain polysulfide (C−Sx−C, x ≈ 3−4) and a Cq bound to a short-chain sulfide (x ≈ 1−2; this signal is clearly visible in S-DIB-70 containing higher amount of short-chain sulfides in comparison to other composition), respectively. Likewise, the signal at 59 ppm is attributed to a Cq linked to a long-chain polysulfide (x ≥ ∼5). No additional signals at higher frequency can be seen with further increase in S, indicating that the chemical shifts of 13C NMR are sensitive up to a limited length of polysulfide chain. The 13C NMR signals of aromatic, non-protonated carbons (Car) shift in the opposite direction (to lower frequencies) in response to an increase in S content. Thus, we assign the signals at 144 and 140 ppm to Car associated with medium- and long-chain polysulfides, respectively.47 Our assignment is further confirmed by 13C spin−lattice relaxation time T1(13C) data, which suggest that

the long-chain polysulfide linkage exhibits lower segmental motion in comparison to the short-chain sulfide linkage (details in Supporting Information).49 Remarkably, the chemical shifts of 13C NMR signals are sensitive to polysulfide chain length. Individual moieties containing polysulfide chains of different lengths can be distinguished from each other by the presence of discrete 13C NMR signals. This characteristic will be exploited to follow the reactions occurring during electrochemical cycling.

Figure 1. 13C{1H} CP MAS NMR spectra of poly(S-co-DIB) copolymers with varying ratio of DIB vs S. The samples are denoted as S-DIB-X where X = wt% of DIB (10−70%).

First-principles calculations of 13C NMR chemical shifts. First-principles calculations have been performed to examine the effect of polysulfide chain length on the chemical shifts of α- and β-carbons. First, we have performed 13C NMR chemical shift calculations on simple model systems containing a single polysulfide chain, phenyl−Cq(CH3)(CH2CH3)−Sx−CH3 with x = 1−5, as shown in Figure 2. The 13C chemical shifts are referenced internally to the molecule with the shortest sulfide chain, i.e. the one with x = 1. We have verified that different basis sets result in almost identical chemical shifts. The isotropic chemical shifts of Cq (α-position to the S linkage, black solid circles) increase as the length of the polysulfide chain increases, up to an offset of 8 ppm for x = 4 and 5. In the case of aromatic carbon (Car, β-position to the S linkage, black hollow circles), in turn, the 13C NMR chemical shifts decrease by about −2 ppm for polysulfide chains longer than x = 3. Furthermore, in order to have a more realistic model of the poly(S-co-DIB) copolymers, we have considered the same molecule with an additional polysulfide chain of the same length attached, i.e. phenyl−Cq(CH3)(CH2−Sx−CH3)−Sx−CH3 (see the inlet in Figure 1). For this second series of molecules, we obtained a similar trend for the 13C NMR chemical shifts with increasing length of polysulfide chain (Figure S2). In summary, our quantum chemical calculations of simplified Scopolymer models show that the 13C NMR chemical shifts of Cq depend characteristically on the polysulfide chain length,



with a variation of about 8 ppm between short (x = 1) and long (x = 5) chains. The microstructure of real poly(S-co-DIB) copolymers is of course more complex than our models since it is composed of macrocycles with multiple branching points and polysulfides chains of various lengths can be bound to one DIB linker. In order to obtain a full picture of the influence of polysulfide chains on the NMR lines, a more complete ensemble of morphologies and spatial conformations would have to be considered. Nonetheless, our quantum chemical calculations confirm the NMR assignment where the peaks of Cq carbons at higher frequencies and the signals of Car carbons at lower frequencies are assigned to carbons linked with longer polysulfide chains.

Figure 2. (a) Optimized gas phase structures of phenyl− Cq(CH3)(CH2CH3)−Sx−CH3 with x = 1 and 5. The sulfur atoms in the polysulfide chain (Sx) are represented as yellow spheres. (b) Calculated isotropic NMR chemical shifts of quaternary (Cq, α to the S linkage) and aromatic carbons (Car, β to the S linkage) for phenyl−Cq(CH3)(CH2CH3)−Sx−CH3 as a function of x.

Electrochemistry of poly(S-co-DIB) copolymers. Now, we study the influence of polysulfide chain length on electrochemical redox behavior, focusing on S-DIB-50 (containing short-/medium-chain polysulfides) and S-DIB-10 (containing long-chain polysulfides). In cyclic voltammograms at slow sweep rate representing more thermodynamic effect than kinetic influence, two cathodic peaks near 2.35 and 2.15 V are observed, which are attributed to the stepwise reduction to long-chain polysulfides (S82−, S62−) and short-chain sulfides (S22−, S2−), respectively (Figure 3a−b).18,21 For S-DIB-50, the cathodic peak current at 2.15 V is prominent (its intensity diminishing with the number of cycles), whereas the one at 2.35 V is negligible, implying the production of mainly short-chain sulfides. By contrast, in the case of S-DIB-10, the cathodic process yields higher peak currents at 2.35 V than that of the 2.15 V, indicating the predominant generation of long-chain polysulfides. These reduction behaviors are clearly different from that displayed by elemental sulfur. An additional weak, yet distinct, cathodic peak is observed at 2.20 V for S-DIB-10, which is absent in the cyclic voltammogram of S-DIB-50. Even though many studies have discussed a discharge reaction of Li−S cells associated with two potential plateaus, more

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complex mechanisms involving three reduction potentials have been reported as well, where the additional second plateau is attributed to the reduction of S62− to S42−.2,21,27,50,51 Thus, we ascribe the peak at 2.20 V to the formation of mediumchain polysulfides for S-DIB-10. The fact that the cathodic peak due to a redox process involving the long-chain polysulfides is maintained throughout three CV cycles indicates the reversibility of the reaction and stability of the long polysulfide chains linked to the DIB polymeric framework. Moreover, significantly higher redox peak current of S-DIB-10 compared to S-DIB-50 suggests an increased utilization of active S in SDIB-10. CV results demonstrate that the redox reactions of SDIB-10 and S-DIB-50 take place through different pathways and intermediates, involving different extent of short- vs. longchain polysulfide generation and reflecting their diverse molecular structures. We optimized multiple factors, such as the concentration of lithium salt in the electrolyte, the ratio of electrolyte to sulfur, and the type and amount of binder materials and carbon conductor, which affect the cyclability of Li//poly(S-co-DIB) cells (details in Supporting Information, Figure S4). The resulting discharge/charge (lithiation/delithiation) voltage profiles and cycling performance of S-DIB-50 and S-DIB-10 cathodes are compared. The discharge curves of the S-DIB-50 cathode (Figure 3c) exhibit hardly any plateau at 2.3 V but a long plateau at 2.1 V, implying that the step leading to the formation of long-chain polysulfides is absent in S-DIB-50. A similar potential profile has been observed in small sulfur allotropes of S2−4 confined in microporous carbon; in these cases, the single discharge plateau at low voltage has been assigned to the reduction process of short polysulfide to S2−, suggesting that sulfur confinement in micropores inhibits its growth to form long polysulfides (S5−8), and thus the charge/discharge reaction involves the S2−4 and S2− species only.9 On the contrary, discharge profiles of S-DIB-10 (Figure 3d) display two plateaus, which are attributed to the production of long- and short-chain polysulfides, respectively. This difference in discharging behavior between the two cathodes is in agreement with CV and NMR results. Higher reversible capacity and good coulombic efficiency are observed for S-DIB-10 in comparison to S-DIB-50 (Figure S5), which is consistent with previous reports.17 Moreover, the cycling performance of SDIB-10 is further improved by the addition of carbon fibers (Figure 3e-f), indicating that the drawbacks of the poor conductivity and volume change of S-copolymer materials can be alleviated by optimizing the cathode design.52,53 7 Li MAS NMR spectroscopy of cycled poly(S-co-DIB) copolymer cathodes. Investigation of the reaction mechanisms involving S-copolymers is challenging since different from elemental S, two types of sulfur species can be generated during the electrochemical cycling of S-copolymers: sulfur species bound to DIB via C−S bonds (organic polysulfides) and sulfur species dissociated from DIB (inorganic polysulfides). Thus, a probe that can distinguish these two species is required. 13C and 7Li NMR spectroscopies are ideal techniques to follow the structural evolution of organic and inorganic polysulfides, selectively, during cycling.


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Figure 3. Cyclic voltammograms of (a) S-DIB-50 and (b) S-DIB-10 cathodes at 20 µVs−1. Electrochemical charge/discharge cycling performance of (c) S-DIB-50, (d) S-DIB-10, and (e)−(f) S-DIB-10-CF (5 wt% carbon fiber added) cathodes. The current densities of CV and the specific gravimetric capacities of the cathodes were calculated based on the weight of sulfur.

Ex situ 7Li MAS NMR spectra of S-DIB-50 and S-DIB-10 subjected to varying states of discharge (SOD) are shown in Figure 4. First, the NMR spectra of the electrodes were acquired without them being washed to capture both soluble and insoluble species. Hence, Li ions in residual electrolyte are also shown as a very sharp signal in both electrodes. Due to the narrow range of 7Li NMR frequencies of diamagnetic compounds, spectral broadening, and lack of well-defined polysulfide reference compounds, it is hard to identify each lithium polysulfide species characterized by a different chain length unambiguously. However, clear differences are noted between the two cathodes. In the case of S-DIB-50 (Figure 4a), a broad 7Li signal centered at ∼ −0.4 ppm appears from the beginning of discharge, and it grows in intensity as the discharge process advances. Since the reduction of S-DIB-50 yields short-chain organic sulfides, we assign the signal at ∼ −0.4 ppm to a C−S−Li unit, which is a major reduction product of S-DIB-50. In the case of S-DIB-10, two distinct signals at 1.0 and ∼2.3 ppm are observed, whose intensities vary at different SOD (Figure 4b). The signal at 1.0 ppm, which appears in the sloping region (i.e., right after the upper plateau in discharge profile, SOD = 25%), undergoes an increase in intensity, followed by a reduction as the discharge proceeds. The signal at 2.4 ppm is seen at the beginning of the lower plateau of discharge curve (SOD = 30%) and becomes dominant upon discharge. Polysulfides are well known to be metastable, and various intermediate species with different chain lengths are co-present through the disproportionation reaction. Different isolable intermediate species have been reported by 7Li NMR, which shows the complexity of the polysulfide system.54–56 Cuisinier et al. reported that Li2S6 and Li2S were the only intermediate species which could be isolated, giving rise to discrete 7Li NMR signals.54 Patel et al. showed distinctive 7Li NMR signals only for Li2S8 and Li2S.55 All other polysulfide compositions between S8 and Li2S yielded spectra consisting of linear combinations of these two isolable signals. Nonetheless, it is clear that 7Li MAS NMR can distinguish between Li in the tetrahedral environment of the Li2S phase (2.3−2.5 ppm)

and Li in the chain structure of Li2Sx polysulfides (1.0−1.2 ppm).54,55 7Li NMR chemical shift values of our cycled electrodes are similar to previously reported ones. Thus, we can assign the signal at 1.0 ppm to the C(or Li)−Sx−Li unit (where x ≥3)  long/medium-chain organic or inorganic lithium polysulfides  and the one at ∼2.3 ppm to Li2S.54,55 Whether the formation of Li2S in Li−S cells occurs at the upper plateau57 or in the beginning,23,58 in the middle24,54 or at the end of the lower voltage plateau,56 is controversial. In some published XRD studies, the authors have even claimed that crystalline Li2S does not form at all.22 This conflicting observation by various XRD studies is probably due to the lack of long-range order in reduction products since sensitivity of XRD technique is limited by the crystallinity and the particle size. On the contrary, our 7Li NMR results provide clear evidence that Li2S is already formed at the beginning of the lower voltage plateau for S-DIB-10, demonstrating that the reduction of polysulfides occurs simultaneously rather than successively. To distinguish between soluble and insoluble components, 7 Li MAS NMR experiments were performed after washing the cathodes with the electrolyte solvent (Figure 5). As expected, the sharp signal corresponding to the soluble Li ions of the residual electrolyte disappears after washing. In the case of SDIB-50, the washing step causes the signal at −0.4 ppm to lose its sharp component, leaving only a broad component. This evidence implies that a small fraction of materials is fully reduced to the monomeric unit which is then only slightly soluble in the electrolyte. In the case of S-DIB-10, both 7Li signals at ∼2.3 and ∼1.0 ppm remain, demonstrating that Li2S and medium-to-long-chain polysulfides are not soluble in the electrolyte. Insolubility of the polysulfides is intriguing since this behavior is clearly distinct from the reduction behavior of elemental S. In an in situ 7Li NMR study of an Li−S cell with an elemental S cathode, See et al. have reported that the solid product formed during discharge is only Li2S, and the other two signals can be assigned to the dissolved Li species associated with Sx2−.57 By contrast, our 7Li NMR data on the S-DIB10 cathode indicate that the organic polysulfides bound to DIB



(C−Sx−Li), which are insoluble in the electrolyte, are mainly formed during discharge in addition to Li2S. This anchoring of polysulfides to the polymeric network can mitigate or prevent the dissolution of long-chain polysulfides, and thus, improves battery performance.

length: the Li NMR chemical shift does not vary by more than 2 ppm. Instead, preliminary calculations show that the Li NMR chemical shift is indeed very sensitive to the chemical environment beyond the directly bound sulfur chain, i.e. the neighboring C−Sx molecules. Presently, a comprehensive study with focus on the lithium ions is in progress which is taking into account a realistic representation of their chemical environment as well as their high mobility at ambient temperatures. 13

Figure 4. Ex situ 7Li MAS NMR spectra of (a) S-DIB-50 and (b) S-DIB-10 cathodes discharged to various states after one formation cycle. The spectra were acquired without washing the electrodes to capture both soluble and insoluble species. (c)−(d) Second discharge profiles of Li//poly(S-co-DIB) cells cycled at constant current density (C/50). State of discharge (SOD) for ex situ NMR measurements is denoted.

Figure 5. Ex situ 7Li MAS NMR spectra of (a) S-DIB-50 (SOD = 100%), (b) S-DIB-10 (SOD = 30%), and (c) S-DIB-10 (SOD = 100%) cathodes discharged to various states and subsequently washed with solvent to remove soluble components. Corresponding spectra of the electrodes without washing step and difference spectra (spectrum without washing minus spectrum with washing) are plotted together for a comparison.

We have also computed the Li NMR chemical shift values for the C−Sx−Li systems, but our quantum chemical calculations indicate no clear trend as a function of sulfur chain

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C{ {1H} } CP MAS NMR spectroscopy of cycled poly(Sco-DIB) copolymer cathodes. To monitor exclusively organic polysulfides, 13C{1H} CP MAS NMR spectroscopy was utilized, exploiting the dependence of the 13C NMR frequencies on polysulfide chain length. Because electrodes contain various sources of 13C, such as conducting carbon and binders, a special design of the electrodes with polytetrafluoroethylene (PTFE) binders in combination with the CP technique was developed to selectively observe DIB units while avoiding interference from conducting carbon and binders. Since CP depends on the polarization transfer from 1H to 13C via dipolar interaction, the CP signal can be selectively enhanced for carbon centers chemically bound to protons.59 Using PTFE binders containing no 1H sources and thick electrodes with high loading of cathode materials enabled us to selectively detect 13 C NMR signals from DIB. The amount of carbon conductor was also minimized to reduce the effect of eddy current during sample rotation of MAS. Notably, unless complete analysis of CP kinetics is performed, CP experiments do not provide quantitative information since signal intensity of CP depends on functional groups and mobility of molecules. The signal build-up at relatively short contact time is governed by the immediate local environment of nuclei (particular functional groups under study), whereas the signal decay at relatively long time scale reflects more molecules as a whole. Assuming that CP signal at short contact time is more sensitive to the number of protons and mobility of the functional groups involved in CP rather than dynamic changes of the DIB molecule, we follow the variation in intensity to obtain qualitative (or semi-quantitative) insight into the reaction as a function of SOD. In the discussion below, we will focus on Cq signals due to higher resolution in comparison to Car signals. In the case of SDIB-50 (Figure 6a), the Cq signal assigned to the short-chain organic sulfide is clearly discernable in the middle of the lower plateau (SOD = 60%) and its intensity increases gradually at the expense of the one assigned to the medium-chain organic polysulfide upon discharge (Figure 6c). This observation implies a shortening of organic polysulfides to form C−S−Li in correspondence of the lower voltage plateau. The spectrum of S-DIB-10 (Figure 6b) obtained after one formation cycle is characterized by a broader Cq signal than the pristine compound, which suggests a rearrangement of the organic polysulfides, leading to a broader distribution in local structures. The Cq signal can be deconvoluted into individual signals corresponding to various lengths of organic polysulfides and the changes of their relative intensity at different SOD can be followed (Figure 6d). Long-chain organic polysulfides are gradually transformed into medium- and short-chain organic polysulfides during the course of the discharge. The presence of long- and medium-chain organic polysulfides in the cathode reveals that the polysulfides remain linked to the polymer network rather than being dissolved into the electrolyte, which


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is consistent with 7Li NMR results. This conclusion is further supported by spectra recorded after washing the cathodes (Figure S8).

solid interface, forming two tethered polysulfides (eg. C−Sx−C + Sy2− → C−Sn− + C−Sm−). This re-networking of polysulfides to the polymeric matrix will be more specific to the S-polymer cathodes with higher content of sulfur. Overall, it is likely that the better capacity retention of S-DIB-10 in comparison with that of the S-DIB-50 results from the higher insolubility of SDIB-10 reduction products due to anchored long polysulfides of large molecular weight (Figure 5, S3, and S8).12 The redox mechanisms of S-copolymers in comparison to those typical of elemental S are proposed in Figure 7. It is worthwhile to mention that not only the short-chain but also medium- or longchain organic polysulfides are present in S-DIB-10 at the end of discharge, which can be explained by incomplete reduction of the electrodes, as indicated by a measured total capacity that is smaller than the theoretical value. Obviously, the cathodes investigated in the present study suffered from low conductivity and poor accessibility of their core sections, leaving residual organic polysulfides without undergoing reduction.25,61 This issue can be improved by using 13Cenriched DIB to increase NMR signal sensitivity, which enables to use thinner electrodes and increase the utilization of sulfur.62

Figure 6. Ex situ 13C{1H} CP MAS NMR spectra of (a) S-DIB50 and (b) S-DIB-10 cathodes discharged to various states (SOD in Figure 4) after one formation cycle. The spectra were acquired without washing the electrodes. (c)−(d) 13C NMR signal intensity corresponding to various polysulfides chain lengths as a function of SOD. The signal intensity is obtained from deconvolution of spectra (a) and (b) as shown in Figure S6−7.

After multiple cycles (Figure 6a−b), the spectra of both SDIB-10 and S-DIB-50 are comparable to those of the initial cathodes, which illustrates that the DIB unit plays a critical role in maintaining C−S bonds and S-copolymer structure, contributing to the reversibility. In addition, XPS data of SDIB-10 at charged (delithiated) state after multiple cycles do not show clear signature of elemental sulfur, Li2S2 and Li2S, again verifying that formation of polysulfide network is electrochemically reversible (Figure S9). Contrary to elemental S, which forms soluble long-chain inorganic polysulfide ions that diffuse into the electrolyte, thus causing capacity loss, S-DIB10 enables immobilization of long- and medium-chain polysulfides through formation of C−S bonds, thereby hindering the dissolution of polysulfides. Certainly, the possibility of generation of long-chain inorganic polysulfides in Scopolymer cathodes cannot be ruled out. However, these inorganic polysulfides, even if they are formed, appear to rebind with polymer-bound sulfides through disproportionation reaction within short time scale,27,60 yielding mainly anchored polysulfides (organic polysulfides). Since some inorganic polysulfide ions are not stable in solution phase, they can disproportionate with fully oxidized sulfur loop quickly at the liquid-

Figure 7. Schematic representation of the reaction mechanisms during discharge/charge cycle. (a) Elemental sulfur: soluble longchain polysulfides are formed from the initial stage of discharge, which diffuse into the electrolyte. (b) S-DIB-50 cathode: formation of Li2S is minimal; hyperbranched structure can be easily fragmented into the monomeric units due to short sulfide chains. (c) S-DIB-10 cathode: long-chain lithium polysulfides are anchored to the polymeric network, preventing dissolution into the electrolyte.

4. CONCLUSION In conclusion, we have demonstrated that MAS NMR spectroscopy is capable of monitoring the reduction and growth of polysulfide chain of different lengths and their phase transitions. Moreover, we have proven that it enables to distinguish between organic and inorganic polysulfides as well as between electrolyte-soluble and insoluble components, thus providing valuable information on the intermediate species formed and the redox mechanisms of Li//poly(S-co-DIB) batteries. The



studies on poly(S-co-DIB) cathode materials with various compositions reveal that the structure of the copolymers can be tuned in terms of polysulfide chain lengths by the choice of composition, and their redox reactions follow different reaction pathways depending on the polysulfide chain lengths. Our results show that the covalently bound organic moieties serve as anchors that stabilize long-chain polysulfides, leading to improved electrochemical cycling performance. The improved understanding regarding the correlation between molecular structure and the reaction pathway of Li−S batteries obtained from this study will further help designing more stable and efficient sulfur-based cathode materials. Furthermore, the new methodology described herein can be used to study various S-copolymer structures of technological importance as well as reactions that take place in battery systems, such as solid−electrolyte interphase formation.

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ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Deconvolution of 13C{1H} CP MAS NMR spectra of poly(S-coDIB) copolymers, T1(13C) relaxation time, first principles calculations of the systems bound to two polysulfide chains, solubility test, battery optimization procedure, deconvolution of 13C{1H} CP MAS NMR spectra of S-DIB-50 and S-DIB-10 cathodes at various SOD, 13C{1H} CP MAS NMR spectra of the electrodes after washing step, ex situ X-ray photoelectron spectroscopy (XPS).

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AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] (13)

ORCID Patrick Theato: 0000-0002-4562-9254 Daniel Sebastiani: 0000-0003-2240-3938 Seung-Wan Song: 0000-0003-4208-206X Young Joo Lee: 0000-0002-5782-6431

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Author Contributions Alexander Hoefling,† Dan Thien Nguyen,† †These authors contributed equally.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENT This research was supported under the framework of international cooperation program managed by National Research Foundation of Korea (2015K2A5A3000068) and German Academic Exchange Service (DAAD, ID 57141898). PPA and DS acknowledge support by the DFG SFB/TRR 102 (project B2).

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