Following the Transient Reactions in LithiumâSulfur Batteries Using

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Following the Transient Reactions in Lithium-Sulfur Batteries Using an In Situ Nuclear Magnetic Resonance (NMR) Technique Jie Xiao, Jian Zhi Hu, honghao chen, M Vijayakumar, Jianming Zheng, Huilin Pan, Eric D Walter, Mary Y. Hu, Xuchu Deng, ju Feng, Bor Yann Liaw, Meng Gu, Zhiqun Daniel Deng, Dongping Lu, Suochang Xu, Chongming Wang, and Jun Liu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b00521 • Publication Date (Web): 18 Mar 2015 Downloaded from http://pubs.acs.org on March 23, 2015

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Following the Transient Reactions in Lithium-Sulfur Batteries Using an In Situ Nuclear Magnetic Resonance (NMR) Technique Jie Xiao1, Jian Zhi Hu1, Honghao Chen1, M. Vijayakumar1, Jianming Zheng1, Huilin Pan1, Eric D. Walter1, Mary Hu1, Xuchu Deng1, Ju Feng1, Bor Yann Liaw2, Meng Gu1, Zhiqun Daniel Deng1, Dongping Lu1, Suochang Xu1, Chongmin Wang1 & Jun Liu1 1

Joint Center for Energy Storage Research, Pacific Northwest National Laboratory, Richland,

WA 99352. 2 Electrochemical Power Systems Laboratory, Hawaii Natural Energy Institute, School of Ocean and Earth Science and Technology, University of Hawaii-Manoa, Honolulu, HI 96822, USA. Correspondence should be addressed to J.H ([email protected]) or J.L. ([email protected]).

Abstract

A fundamental understanding of electrochemical reaction pathways is critical to improving the performance of Li-S batteries, but few techniques can be used to directly identify and quantify the reaction species during disharge/charge cycling processes in real time. Here, an in situ 7Li NMR technique employing a specially designed cylindrical microbattery was used to probe the transient electrochemical and chemical reactions occurring during the cycling of a Li-S system. In situ NMR provides real time, semi-quantitative information related to the temporal evolution

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of lithium polysulfide allotropes during both discharge/charge processes. This technique uniquely reveals that the polysulfide redox reactions involve charged free radicals as intermediate species that are difficult to detect in ex situ NMR studies. Additionally, it also uncovers vital information about the 7Li chemical environments during the electrochemical and parasitic reactions on the Li metal anode. These new molecular-level insights about transient species and the associated anode failure mechanism are crucial to delineating effective strategies to accelerate the development of Li-S battery technologies.

Keywords: in situ NMR, Li-S batteries, radicals, energy storage,

Compared with conventional Li-ion batteries (LIBs), Li-S chemistry has the attractive potential to afford transformational improvements in specific energy and cost reductions for next-generation energy storage systems1-7. Theoretically, the energy conversion of sulfur is based on the electrochemical reactions with lithium resulting in phase transitions between S8 and Li2S (corresponding to a specific energy of 2,500 Wh kg-1)8. The formation of soluble lithium polysulfides as intermediates in the Li-S chemistry is a critical factor in harnessing the high energy stored in the system, although the dissolved species are also an initiator of cell failure. Various nanostructured materials have been identified to effectively mitigate the diffusion of soluble polysulfides at the molecular level9, alleviate the volume expansion of sulfur during the conversion from S to Li2S10, as well as improving the electronic conductivity of the sulfur composite cathode11. Promising advances have been observed by employing nanostructured materials, which also provide a good platform to further understand the fundamental mechanism underlying polysulfide species and their evolution under the electric field.12


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For the characterization techniques, much effort has been devoted to the direct observation and analysis of the Li-S kinetic reactions13-15, highlighting the importance of in operando studies of electrochemical cell reactions. These in situ techniques have focused on the identification of discrete species related to sulfur or lithium polysulfides. To date, there have been no reports of in situ techniques that can simultaneously identify the different reaction species and provide quantitative information about how these species change with time during discharge/charge processes in a functioning Li-S battery. It is generally accepted that during discharge, soluble long-chain lithium polysulfides (i.e., Li2Sx with 8 ≥ x > 4) are initially generated at an upper voltage around 2.3 V, followed by the formation of short-chain Li2Sx (4 ≥ x ≥ 1) at ca. 2.1 V, which are much less soluble. While some of these soluble species redeposit on the sulfur cathode at 2.1 V, a significant portion of the dissolved polysulfides diffuse gradually to the Li metal anode, constituting the well documented “shuttle process,” which contributes to an internal leakage current and lowers the cell efficiency16-17. It is also possible for the polysulfides to irreversibly accumulate on other cell components such as the separator in the form of various Scontaining products18. The collective effects lead to a gradual loss of active sulfur from the cathode and increased cell impedance, both of which are detrimental to the performance and cycle life of Li-S batteries19. However, the electrochemical reaction of elemental S8 with Li is complex, involving a series of ring-opening reactions, both electrochemically20 and chemically21. The details of this discharge mechanism are still being intensely studied17,20-25. Here, we take advantage of the fact that Li+ cations ubiquitously exist in the cathode, electrolyte and anode to quantitatively monitor the electrochemical and chemical reactions incurring in the whole Li-S system by using an in situ NMR technique with an in-house cylindrical microbattery26. The transient chemical environments around the Li+ cations in the


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discharge/charge process of the polysulfides are captured, which provides previously unattainable information and complements ex situ NMR observations. The microstructural evolution of the Li metal anode entangled with parasitic reactions has also been recorded concurrently in the functioning Li-S microbattery. The new finding in this work is also applicable to explain many phenomenon in nano-structured sulfur cathode materials, providing vital guidance about future research direction for Li-S battery technologies.

Results and Discussions The in situ NMR measurement cell and probe for the Li-S microbattery are displayed in Fig. 1a. Fig. 1b shows a stacked plot of 500 consecutive individual 7Li NMR spectra as a function of time for a working Li-S cylindrical microbattery with each spectrum acquired using an experimental time of 259 s. The NMR peaks of interest within the range of -260 to 300 ppm were extracted at different times by fitting the spectra as shown in Fig. 1c. Two sets of broad peaks are observed with one set around 0 ppm ranging from -260 to 100 ppm and the other set between 100 and 300 ppm. In addition, we synthesized specific lithium polysulfide compositions (Li2Sx with x = 2, 4, 6 and 8) dissolved in DOL/DME and performed extensive NMR measurements on these “standard” samples. All of these samples show a resonance around 0 ppm (Fig. S1a), consistent with literature reports that Li+ cations in Li salts, electrolytes and the SEI all resonates around 0 ppm27-29. Ex situ NMR measurements on a similar Li-S cell (Supplementary Information Fig. S1b) at different depth of discharge show chemical shifts spanning from -5 to 5 ppm, similar to the ex situ NMR results reported in the literature13. These results (Figure S1) indicate that ex situ NMR is difficult to differentiate various Li2Sx species,


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produced chemically or electrochemically, not mentioning each of these nominally stoichiometric polysulfides may be a mixture. The static 7Li NMR spectra obtained on commercial Li2S solids show a broad peak centered around 0 ppm and spanning from about -80 ppm to +80 ppm (bottom right tail to bottom left tail of the peak: see Fig. S1a for details). Therefore, the broad peaks centered around 0 ppm (-260 to 100 pm) are attributed to the Li+ cation environments in various polysulfide species. The charge-discharge voltage profiles of the Li-S cell used in this in situ NMR study are displayed in Fig. 2a. The areas under the two sets of broad peaks (Fig. 1c), i.e., from -260 to 100 ppm and from 100 to 300 ppm, are integrated and plotted versus the discharge/charge time in Fig. 2b and 2c, respectively. These areas reflect the relative populations of the particular species with a periodic temporal variation. During discharge, the total area under the first broad peak from -260 to 100 ppm in Fig. 1c gradually increases, reaching a maximum at about 2.0 V, and then gradually decreases and reaches a minimum at nearly the fully discharged state (Fig. 2b). Note that the devonvoluted contribution in Fig.2b starts at 0.5 and in Fig.2c at 1 because of the partial dissolution of sulfur in the beginning. Unlike Li-ion batteries, the OCV of Li-S cell is usually at ca. 2.2-2.4 V due to the partial dissolution of sulfur. Once the sulfur electrode is immersed in the electrolyte, it was unavoidable for some of the sulfur to dissolve into the electrolyte. To understand the significance of the in situ NMR data, each of the two broad peaks in Fig. 1c were fitted by subpeaks. The broad peak from -260 to 100 ppm can be deconvoluted into four hhpeaks with the upshift in the descending order: A1 (-128 ppm) > A2 (-71 ppm) > A3 (-34 ppm) > A4 (-7 ppm). During discharge, the peak area under Peak 1, as represented by A1 in Fig. 2d, increases rapidly when the potential decreases from 2.3 to 2.1 V. According to the literature,


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before reaching 2.1 V, S8 is reduced to soluble long-chain polysulfide species (at 2.3 V) and less soluble species such as Li2S4 before reaching 2.1 V. A1 in Fig. 2d always reaches a maximum when the 2.1 V plateau occurs and sharply decreases over the entire plateau region at about 2.0 V, where the soluble species are further reduced to insoluble short-chain lithium sulfides. At the end of the discharge, therefore, A1 reaches a local minimum where the insoluble Li2S2/Li2S species dominate. Based upon these observations, A1 is largely reflective of the soluble longchain polysulfides. Interestingly, when the battery is charged, A1 reaches a maximum at approximately 2.3 V instead of the end of charge. Since A1 is related to long-chain polysulfides, the peak concentration of these dissolved polysulfides (maximum A1) is reached at an earlier stage than expected, which will be discussed later. The changes of area under Peak 2 (Fig. 1c), represented by A2 in Fig. 2d, follows the same trend as A1. From these discussions, it is reasonable to assume that both Peaks 1 and 2 are correlated with the appearance and disappearance of the soluble species, and are therefore associated with the generation and consumption of the longer polysulfides such as Li2S8 and Li2S6. The area under Peak 4 (A4 in Fig. 2d) follows the opposite trend. During discharge, the A4 peak area starts to grow only at the beginning of the 2.1 V plateau, where the soluble polysulfides are transformed into less soluble short-chain species. A4 always reaches a maximum close to the end of discharge where insoluble polysulfides are the major products. Therefore, Peak 4 reflects the formation and disappearance of the insoluble lithium sulfides such as Li2S2 and Li2S. The signals from Peak 4 are consistent with those measured by using ex-situ NMR (Fig. S1), although a slight chemical shift is evident. A closer inspection reveals that A4 also reaches a local maximum value repeatedly even during charge. This is because NMR has no spatial resolution and the increase of A4 during charge is indicative of the generation of


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insoluble Li2S2/Li2S species on the Li metal anode, consistent with the documented “shuttle mechanism” 17. The trend for the area under Peak 3 (A3 in Fig. 2d) is between those of A1/A2 and A4, and therefore may reflect the intermediate Li2S4 species or mixed soluble polysulfide species. Peak 3 also contains a narrow peak that may be related to Li+ cations in the LiTFSI-based electrolyte. In fact, the global minimum of A3 does not reach zero during the whole charge/discharge process suggesting a contribution from the electrolyte. The in situ NMR study provides semi-quantitative data that tracks the temporal fluctuations of both the soluble and insoluble polysulfide concentrations in a functioning cell. However, it is not possible to directly assign specific NMR peaks to individual polysulfide species. This is because the environments surrounding the Li+ cations in the Li2Sx series during charge and discharge are complex and differ somewhat from the “standard” polysulfide samples (for which mixtures of several polysulfide species coexist due to the fast dispropotionation/dissociation reactions as demonstrated in chemically synthesized Li2Sx compositions with specified stoichiometries13,21). The chemically synthesized Li2Sx samples (Fig. S2) prepared in the present study obtained by mixing stoichiometric amount of Li metal and sulfur indicate the existence of S3˙- radicals, produced from S62- dissociation, in each case, further confirming that disproportionation reactions are a general occurence. Each nominal stoichiometric Li2Sx composition is in fact a mixture of several polysulfide species in equilibria. Therefore, importantly, ex situ NMR studies on lithium polysulfides cannot accurately determine the populations of the Li2Sx species with differing chain lengths present during the Li-S cell reactions13,30. Based on the above in situ NMR discussion, some of the anomalies attributed to the Li-S system can be explained. For example, in the literature almost all reports assign the 2.3 and 2.1 V


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plateaus during discharge to the generation of soluble Li2Sx (8 ≥ x > 4) and the deposition of insoluble Li2S2/Li2S, respectively. The accurate characterization of lithium polysulfide species is difficult, however, for the reasons specified earlier even though various in operando characterization techniques have been employed13-15. But the in situ NMR measurements reported here reveal that the ring cleavage mechanism of cyclic S8 may not occur in a discrete step-by-step manner and mixed species are always present at any stage making the exact differentiation challenging. Another phenomenon worth noting is that the reaction pathway during charge of a Li-S cell is different from that in the discharge regime, as implied by the differences in the plateau lengths between the two plateaus in the charge curve, in contrast to the proportions for the discharge curve13,31-34. Based on the in situ NMR results, it is probable that during the charge process, once intermediate polysulfides such as Li2S4 are generated, they quickly transforms into soluble long-chain polysulfides instead of waiting for more Li2S4 to be accumulated and then converting together into longer chains. This hypothesis is well supported by the fact that, during charge, A1 and A2 (Fig. 2d) corresponding to the soluble Li2Sx (8 ≥ x > 4) species always reach peak values earlier than A3 and A4 (which are derived from insoluble species). For the anode side, to assign the different 7Li peaks in various chemical environments, ex situ 7

Li NMR is firstly conducted on cycled lithium surface layers containing both Li-dendrite and

the SEI layers(Figure S3) which was carefully scraped from the Li-metal electrode after different cycles. The ex situ NMR of their corresponding sulfur cathodes are plotted in Figure S3 as reference. The broad peak corresponding to Li in SEI and shuttled Li-polysulfide in Fig.S3 was observed with peak centered at about 0 ppm and half peak heights ranged up to about ±50 ppm. However, there are no peaks observed beyond -100 ppm. Li-dendrite signal is located at about


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240 to 270 ppm depending on whether a powder sample is used or an intact Li-anode is used for measurement. There are no signal observed between 100 and 240 ppm. Thus, ex situ 7Li NMR is unable to regenerate the in situ 7Li NMR signals located between about 100 and 240 ppm and therefore cannot help to assign the different peaks for anode side. Likely, because of the considerable reduction of sulfur radical concentration after the withdrawn of the electrical field, the radical influence on the chemical shifts of the porous Li-dendrite signal is also minimized, leading to the disappearance of signal between 100 and 240 ppm. In other words, the in situ NMR signal in Figure 1c between 100 and 300 ppm (anode side signals) can be assigned as the following two parts. The first portion (240-270 ppm) of the anode signal in Figure 1c arises from both a skin layer of Li-metal and Li-dendrite depending on the orientation of the Li-metal foil relative to the magnetic field. The second portion (100-240 ppm) of the in situ NMR signal from the anode side (Figure 1c) is, in fact, the highly porous Li-dendrite interacting with sulfide radicals. The mechanisms of causing such an upfield shifts from 270-240 ppm to 240-100 ppm are similar to the Li-polysulfides interaction with S-radicals discussed extensively in the manuscript. In addition, it is known that 7Li interacting with paramagnetic species varies between -500 and +3000 ppm as a result of the hyperfine interaction with the unpaired electrons27-29. Based on these established principles, the peak labeled as “8” (254 ppm) can be assigned to dendrite or mossy Li environments, including a skin depth signal from the Li-metal anode strip. The assignments of Peaks 5-7 are not straightforward. For the cylindrical battery with the axis of the battery placed perpendicular to B0 as occurs in this study (Fig. 1a), the orientation of the Li metal surface with respect to B0 varies between 0° and 90°. If the only contribution is from pure metallic Li metal and porous Li metal microstructures, the peaks for Li metal would appear


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between about 250-270 ppm—it is well known that the chemical shift of 7Li in metallic Li is at approximately 250 ppm as a result of Knight shift due to the interaction of the nuclear spin with the unpaired electrons located at the Fermi level of the conduction band of the metal. In addition, porous dendritic or mossy Li formed on the Li metal anode, including the 7Li signal from the Li metal strip (up to a skin depth defined by the radio frequency and the conductivity of the metal), can vary from around 250 ppm (metal surface perpendicular to the magnetic field) to about 270 ppm (metal surface parallel to the magnetic field), depending on the orientation of the metallic metal surface27. However, from Fig. 1c, the peaks appear between 100-300 ppm with the peak center for the dominant peak at about 200 ppm. Thus, Peaks 5-7 can only be assigned to quasimetallic Li structures with their exact identity requiring further study. The most likely candidates would be very fine sized, i.e., micron- or submicron-sized Li metallic clusters that are separated by the electrolyte or SEI, and/or lithium polysulfide species, including species containing unpaired electrons. Accordingly, the set of broad peaks located between 100-300 ppm reflects the various 7Li environments present when building the microstructures of the cycled Li anode (i.e., porous or mossy depositions of Li metal, SEI components, etc.). Notably, Peaks 5-8 (Fig. 2e) follow the opposite trend during charge and discharge to those from the lithium polysulfide species (Fig. 2d). During charge, Li metal is plated on the anode generating a strong signal in Fig. 2e because the microstructure of the newly deposited Li (highly porous) is different from bulk Li35 and observable by NMR27. During the first discharge, Li+ cations are stripped from the anode so the signal from the microstructure is rduced. The peak area under Peak 5 (A5 in Fig. 2e) reaches a maximum when the cell is fully charged (Li deposition) and decreases during the discharge process (Li stripping), thus Peak 5 in Fig. 1c is principally attributed to a portion of reversible Li that continuously participated in the


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electrochemical reactions. The 7Li signal quantified from Peaks 6-8 (A6-A8 in Fig. 2e) climbs to the highest value at about 50% state of charge, followed by a decrease to a local minimum at the end of charge. This finding suggests that part of the redeposited/nucleated porous Li undergoes a growth process in the later deposition process, which is reasonable given that the SEI is conductive to Li+ cations. This “densification” process leads to the reduction in the areas of A6A8. Of note, these peak areas also reach a maximum (Fig. 2e) during discharge (Li stripping) in the beginning of the 2.1 V plateau, indicating that the Li stripping process (except in the first discharge) also enriches the Li environments within the anode. This can be explained by the nonuniform stripping from the bulk Li metal due to the uneven distribution of the electrical field, which may temporarily form a “loose” structure enriching the microstructure. Therefore, Peaks 6-8 in Fig. 2c are more related with the Li coordination environment participated in creating the SEI and isolated Li, both of which build up the evolving Li metal anode microstructure during charging. It is noted in Fig. 2c that although Peaks 5-8 show periodic variations, the total 7Li signal continuously accumulates, indicating a buildup of the porous Li microstructure and SEI layer growth on the Li anode side upon cycling. For Li-S batteries, the corrosion of the Li anode is indeed a great concern, especially in the presence of dissolved lithium polysulfides continuously exposed to the newly formed Li metal surface upon charging. The formation of the Li microstructure on the Li anode can be attributed to a combination of Li corrosion resulting in SEI formation and the deposition of insulating lithium sulfides. A series of S-containing species including short-chain polysulfides Li2S2/Li2S and sulfonate byproducts have been identified as part of the anode SEI components36. Some of these may still be electrochemically active and thus reversibly stripped in the subsequent electrochemical reactions, while the remainder may reside


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permanently on the Li anode surface, resulting in a resistive layer which quickly increases the cell impedance. Fig. 3a is the cross-sectional SEM image of a Li anode harvested from Li-S cell after 50 cycles. A thick passivation film on the Li metal anode is clearly distinguishable with a highly porous structure. Energy dispersive spectroscopic (EDS) elemental mapping revealed a large amount of sulfur-containing species deeply penetrating into the interior of the Li anode (Fig. 3b). The continuous contamination of the Li anode surface by insulating and inactive sulfides leads to the rapid increase in the cell impedance and loss of active material, both of which are directly reflected by the swift capacity degradation of Li-S batteries upon cycling (Fig. S4). The most convenient way to verify the side reactions on the anode is to raise the cutoff voltage during discharge, which reduces the depth of the Li stripping from the anode. Fig. 3c shows the sulfur elemental content left on the cycled Li anode surface for different discharge cutoff voltages. The detected sulfur content on the Li metal drastically decreases with increasing discharge cut-off voltage, validating our conclusions from in situ NMR about the anode. The corrosion of the Li metal (insets of Fig. 3c) has been somewhat alleviated when the cut-off voltage is increased from 1.0 V to 2.0 V leading to much more stable cycling and improved Coulombic efficiency (Fig. S4). Similar electrochemical control strategies can be achieved by limiting the discharge31 or charge capacities34 as well. One issue that needs further explanation is the large line shift, line broadening and peak splitting from -260 to 100 ppm related to the polysulfide species. Such line broadening and peak shifting is not observed in “standard” polysulfide samples prepared ex situ or in other ex situ NMR measurements. A possible reason for the induced large negative chemical shifts of the Li signals arises from the bulk magnetic susceptibility from the aluminum mesh. To rule out this possibility, lithium polysulfides were dip-coated onto the surface of the aluminum mesh for 7Li


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NMR measurements. It was found that the bulk magnetic susceptibility only induces less than ±10 ppm chemical shifts (data not shown here). Likewise, it is known that a bulk Li metal strip can only induce about 20 ppm shifts27,28. Therefore, the large negative chemical shifts are most likely associated with Li+ cations interacting with unpaired electrons. The peak shift can be also related to the binding of the Li+ cations with the surrounding ligands. In some cases, a strong Li+ cation binding to anionic ligands in the electrolyte can produce more than 10 ppm shifts37. In LiS chemistry, the Li+ cation environment depends upon the polysulfide anions and the solvent molecules and other anions present within the electrolyte. The long-chain soluble polysulfides are well dispersed in the solvent, which provides a good opportunity for the Li+ cations to bind strongly with electrolyte components. Density functional theory (DFT) analysis predicts that the Li+ cations have strong interactions with these polysulfide anions, along with the DOL and DME solvent molecules (see Fig. 4a). However, the chemical shifts caused by these anions and solvent molecules are limited to traditional 7Li chemical shift ranges (±2 ppm) (Fig. 4a). Therefore, other charged species may exist and induce the observed large chemical shifts. For example, paramagnetic polysulfide free radicals such as S3˙- and S2˙- have been detected in our ex situ EPR measurements (Fig. S5), as well as in other reports21,38-40. 7Li paramagnetic NMR chemical shifts due to interaction with S2˙- and S3˙- free radicals are predicted at -98 ± 10 ppm and -58 ± 10 ppm, respectively (Fig. 4b). Although we only calculated the effects from concentrated S3˙-/S2˙radicals (Fig. 4b), the most stable form of sulfide radicals in nature41, other kinds of polysulfide radicals such as S4˙-40,42 or even longer-chain radicals may also be present concurrently in the transient states via the cleavage of the polysulfide rings or chains under the electrical field, considering the highly insulating nature of sulfur. However, the structural stability of other longer-chain radicals is poor and it is likely that they immediately transform to the more stable


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forms, e.g., S3˙- and S2˙-, that have been identified by ex situ EPR experiments43,44. In fact, a recent work using in situ XAS measurements reported the existence of S3˙- radicals generated during the electrochemical reactions, further confirming the critical effects of sulfur radicals in the system45. It should also be noted that the free radical concentrations that we measured in the ex situ experiments are low (on the order of mM). Due to the sensitivity limit of NMR experiments, the effect of free radicals on the 7Li NMR chemical shifts cannot really be detected in ex situ experiments. However, in a functioning cell, the unique electrochemical environment, the local electrical field and the charged surfaces may induce a larger concentration of the free radicals during charge and discharge. It is likely that during discharge, a large amount of free radicals are generated and these free radicals could be a key determinant for the observed line shifts and line shapes of the in situ NMR spectra. In particular, the polysulfide reactions likely involve the formation of transient free radicals initially, followed by their relatively rapid transformation to the different polysulfide dianion Li2Sx species—processes which are poorly captured by ex situ evaluations. In summary, in situ

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Li NMR spectroscopy has been used to study the transient

electrochemical and chemical reactions initiated on the cathode, anode and electrolyte in a functioning Li-S cell. These studies provide detailed semi-quantitative information related to the polysulfide reactions and the Li anode microstructural evolution during the charge and discharge processes. The in situ experiments not only provide detailed information about the reaction sequences, but also reveal reaction pathways composed of mixed species rather than discrete step-by-step reactions. The large line shifts and line broadening noted suggest a complex chemical environment, and potentially the important role of intermediate species that are difficult


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to study using ex situ tools. For the anode side, the continuous evolution of the Li microstructure, SEI accumulation and polysulfide contamination are also simultaneously captured by 7Li in situ NMR which are well correlated with the Li-S cell degradation mechanisms. This work provides new insights to understand the electrochemistry and chemistry in Li-S battery systems during electrochemical transformations that may help gauge new approaches to overcome the longstanding challenges of Li-S battery technologies.

Methods Materials preparation. Sulfur powder (Sigma-Aldrich) was dissolved in carbon disulfide (CS2) to form a 10 wt% solution. Ketjen black (KB) (Akzo Nobel Corp.) was added into the sulfur-CS2 solution, followed by sonication for 15 min to form homogeneous slurry. This was then stirred at ambient temperature to evaporate CS2. The powder obtained was ground and heated at 155 °C for 12 h in a Teflon-lined stainless steel autoclave to improve the sulfur distribution inside the carbon framework via capillary action. The accurate sulfur content (79.3%) was determined by thermo gravimetric analysis (TGA) with a Netzsch STA 449C thermal analysis system (Netzsch, Germany).

Cylindrical

micro

battery

preparation.

For

the

in

situ

NMR

measurements,

polytetrafluoroethylene (PTFE) was used as the binder, and the cathode composite was made into a freestanding film first. The film with thickness of about 175 m was then laminated onto an aluminum mesh to finish the cathode preparation. A cylindrical cell was made by the aforementioned cathode, Celgard 2500 separator (thickness:25 µm) and Li metal (thickness:100


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µm). The detailed assembly process can be found in our recent publication26. The battery (flattened view in Fig. 1a) was inserted into a plastic in situ NMR capsule case as detailed below.

In situ NMR. The in situ 7Li single pulse (SP) NMR experiments were performed on a VarianAgilent 300 MHz NMR spectrometer, corresponding to 7Li Larmor frequencies of 116.58 MHz. A

specially

designed

in

situ

NMR

battery

capsule

case

made

of

PCTFE

(polychlorotrifluoroethylene) was used to hold the cylindrical Li-S battery. This cylindrical shape battery cell was used to effectively fit the solenoid coil of the probe for best sensitivity due to its good filling factor (Fig. 1a). The PCTFE battery capsule case contains two parts: a cylinder holder with OD of 7.5 mm, ID of 5.4 mm and length of 8 mm and a cap that fits into the cylinder holder. A center hole of about 1 mm ID was drilled for guiding the electric wires to the outside of the battery for charging and discharging. The cap was glued to the cylinder and the center hole was sealed using epoxy to prevent the leakage of the electrolytes, as well as to prevent exposure of the battery to air. A home-made in situ 7Li NMR probe with a 5-turn solenoid RF coil (ID of 8 mm), with the excitation magnetic field (B1) perpendicular to the main magnetic field (Bo), was used for the measurements (Fig. 1a). 1 M LiCl in H2O was used as the reference (0 ppm). Each spectrum was acquired using a single pulse with tip angle of 45°, an acquisition time of 10 ms, a recycle time of 1 s and a total accumulation number of 256.

EPR. EPR spectra were recorded at a temperature of 120 K on a Bruker EPR apparatus. The quartz tube was filled with about 0.3 mL sample in the glove box, dipped in liquid N2 to quickly cool down the sample, and then inserted into the center of the cavity for testing.


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Electrochemical tests. The electrolyte used in the batteries for the in situ NMR tests is 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved in a mixture of 1,3-dioxolane (DOL) and dimethoxyethane (DME) (1:1 in volume). All of the cells were tested at a C/5 rate (1C = 1680 mA g-1) between 1.0 and 3.0 V at room temperature.

Modeling. Molecular structure and NMR chemical shift are modelled using DFT based home build NWChem 6.3 package46. For both geometry and open shell pNMR chemical shift calculations, a B3LYP function was used along with an all electrons Valence Quadruple Zeta + Polarization basis set (cc-pVQZ) under relativistic zeroth-order regular approximation implemented in NWChem. As a first step, we considered gas phase molecular structures of Li+ cations bonded with free radical without any geometrical constraints. These were subsequently used as seed structures to construct a supramolecular structure surrounded by explicit DME and DOL solvent molecules. This supramolecular structure was again fully optimized using the COSMO model (εs = 7.0) to capture the solvent induced structural conformations. Finally, the 7

Li paramagnetic chemical shifts were calculated from the g-tensor and A-tensor values, as

reported in the literature47,48. The calculated 7Li chemical shifts were referenced with respect to fully hydrated Li+ cations, i.e., [Li(4H2O)]+Cl-·6H2O, under the COSMO (water) model to represent the aqueous LiCl solution used as a reference in the experimental NMR analysis.

ASSOCIATED CONTENT


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Supporting information 7

Li NMR spectra of Li2Sx prepared chemically or electrochemically; Chemically synthesized Li2Sx series; Ex situ 300 MHz static 7Li NMR on lithium anode and sulfur cathode after different cycles; Comparison of the performance of Li-S batteries discharged to different voltages for 400 cycles; EPR measurement of Li2S6 solutions. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding author * [email protected] or [email protected] Author Contributions J.L., J.H. and J.X. conceived and designed this work. J.X., B.Y.L. J. H and J.L. proposed the mechanism and develop the manuscript. H.C., J.F. and D.Z.D. helped assemble the microbattery for NMR. M.V. did the modeling work. H.P., D.L. and E.W. did the EPR measurement. J.Z. performed the electrochemical testing. M.G. and C.W. did the SEM characterization. J. H., M. Hu, J.F., X. D. and S. Xu established the in situ NMR capability and did the NMR measurement, data collection and analysis. Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This work was supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (BES). The NMR, EPR and computational studies were conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility


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sponsored by DOE’s Office of Biological and Environmental Research (BER) and located at PNNL. The micro battery design used for NMR measurement is partially based upon previous work supported by the U.S. Army Corps of Engineers, Portland District. PNNL is operated by Battelle for the Department of Energy under Contract DE-AC05-76RLO1830. Mr. Jason Skouson and Mr. Hardeep S. Mehta are acknowledged for their assistance of establishing the in situ NMR device.

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Figure 1 In situ NMR probe and the acquired spectra as a function of discharge/charge time. (a) Cylindrical micro battery, fitted into the in situ NMR probe. (b) Stacked plot of the in situ 7Li NMR spectra in a functioning Li-S cell vs. time during discharge/charge. (c) Major peaks of interest within the range of -260 to 300 ppm extracted at different times by fitting the spectra. Peak assignments are labeled by the numbers and explained in the text.


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Figure 2 Changes of individual integrated peak areas at different potentials. (a) Voltage profile of Li-S cell used for in situ NMR measurement. Evolution of the integrated areas at different discharge/charge times: (b) under the broad peak from -260 to 100 ppm in Fig.1c, (c) under the broad peak from 100 to 300 ppm in Fig.1c, (d) for individual Peaks 1 (A1), 2 (A2), 3 (A3) and 4 (A4) and (e) for individual Peaks 5 (A5), 6 (A6), 7(A7) and 8(A8). Peaks 1-4 are deconvoluted peaks from the broad peak spanning from -260 to 100 pm, while Peak 5-8 are deconvoluted from the second broad peak spanning from 100 to 300 ppm, all of which are labeled in Fig.1c.

(a)

(b)

Cross sectional Mapping bulk

surface

(c)


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Figure 3 Morphology evolution of lithium metal anode. (a) SEM image showing the cross-sectional view of the Li metal electrode—the surface that is in contact with electrolyte is marked as Li surface. (b) EDS elemental mapping on the cross-sectioned sample reveals the depth distribution of S within the Li metal anode after 50 cycles (S is colored as blue). (c) Sulfur elemental content left on the cycled Li anode surface after different discharge cut-off voltages—insets are the morphologies observed from the cycled Li anodes.

(a)

Li+ – S62- interactions w/ DOL/DME 7

Li+ – S42- interactions w/ DOL/DME 7

Li NMR shift = -2.05 ppm

Li NMR shift = -1.70 ppm

(b)

Li+ – S3˙- interactions w/ DOL/DME 7

Li+ – S2˙- interactions w/ DOL/DME 7

Li NMR shift = -58 ppm

Li NMR shift = -98 ppm

+

Figure 4 Simulation on the interactions between Li and other species in the electrolyte. (a) Molecular structure of polysulfide dianions (S62- and S42-) and the predicted 7Li chemical shifts caused by these anions and DOL/DME solvent molecules. (b) Interactions between polysulfide free radicals (S2˙and S3˙-) and Li+ cations solvated by DOL/DME molecules. The predicted chemical shifts caused by these radicals are greatly increased.


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TOC Vital information of lithium polysulfide reactions and lithium metal microstructural evolution on the cathode and anode side, respectively, has been collected simultaneously in a functioning Li-S cell by using in situ NMR characterization, providing new insights to this new energy storage system.

TOC graphic


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Following the Transient Reactions in LithiumâSulfur Batteries Using

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