In Situ NMR Observation of the Temporal Speciation of Lithium Sulfur

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In Situ NMR Observation of the Temporal Speciation of Lithium Sulfur Batteries During Electrochemical Cycling Hao Wang, Niya Sa, Meinan He, Xiao Liang, Linda F. Nazar, Mahalingam Balasubramanian, Kevin G. Gallagher, and Baris Key J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01922 • Publication Date (Web): 03 Mar 2017 Downloaded from http://pubs.acs.org on March 5, 2017

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The Journal of Physical Chemistry

In Situ NMR Observation of the Temporal Speciation of Lithium Sulfur Batteries During Electrochemical Cycling 1,2

Hao Wang*, 1,2Niya Sa, 2Meinan He, 1,3Xiao Liang, 1,3Linda F. Nazar, 1,4Mahalingam Balasubramanian, 1,2Kevin G. Gallagher, 1,2Baris Key 1

Joint Center for Energy Storage Research, Argonne National Laboratory, Lemont, IL 60439, United States

2

Chemical Sciences and Engineering, Argonne National Laboratory, Lemont, IL 60439, United States

3

Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada

4

X-ray Science Division, Argonne National Laboratory, Lemont, IL 60439, United States

ABSTRACT: The understanding of the reaction mechanism and temporal speciation of the lithium-sulfur batteries is challenged by complex polysulfide disproportionation chemistry coupled with the precipitation and dissolution of species. In this report, for the first time, we present a comprehensive method to investigate lithium sulfur electrochemistry using in situ 7Li NMR spectroscopy, a technique that is capable of quantitatively capturing the evolution of the soluble and precipitated lithium(poly)sulfides during electrochemical cycling. Through deconvolution and quantification, every lithium bearing species was closely tracked and a four-step soluble lithium polysulfide mediated lithium sulfur electrochemistry was demonstrated in a never seen before detail. Significant irreversible accumulation of Li2S is observed on the Li metal anode after 4 cycles due to sulfur shuttling. The application of the method presented here to study electrolyte/additives development and lithium protection research can be readily envisaged.

1. Introduction Since its introduction in the 1990s, lithium-ion batteries (LIBs) have been dominating the portable electronic energy 1 storage market. Decades later, LIBs still play a critical role in the portable energy storage market, as well as the nascent but expanding transportation and stationary storage applications. Lithium-sulfur (Li-S) battery is one potential alternative to LIBs with the promise of achieving high specific ener2 gy on the pack level with attractive cost. High energy density and low cost at the pack level is also possible if low excesses of lithium metal and electrolyte are utilized while extend2-4 ing lifetimes to those required by consumer. The attractive system level promise is built upon the foundation of the low equivalent weight (i.e. 1675 mAh/g) and low cost of sulfur. However, Li-S batteries suffer from a number of drawbacks 5that are limiting their development and commercialization. 7 The low conductivity of sulfur requires a relatively high amount of conducting agent in the electrode, such as carbon

black, and more importantly a large excess of electrolyte to dissolve the polysulfide intermediate, which limits the volumetric/areal capacity of Li-S batteries. It has been reported that a duel-layer lithium polysulfides cathode was used to enhance the sulfur utilization and carbon nano framework to 8-10 However, electrolyte to sulfur increase the conductivity. 3 ratios less than 4 mL/g is not reported in the literature. Values of 1 mL/g are required to supersede LIBs on an energy density basis.2 Perhaps more importantly, the capacity fade of Li-S batteries was observed in nearly all Li-S batteries. This fade is attributed to the consumption of electrolyte and the shuttle effect - long chain lithium polysulfides were shuttled from the cathode to the anode forming inactive Li2S solid 11-12 species on the anode. The shuttle effect is the main reason that causes Li-S batteries to have capacity fade, but has been 5 mitigated to some degree through the use of LiNO3. Researchers have been investigating novel methods of inhibiting the shuttle effect, but a complete understanding of the 13-17 mechanism of the shuttle effect is clearly missing. Particularly, a quantitative study of the evolution of the solid species formation and the lithium polysulfides mediation is lacking in the literature. In order for Li-S battery technology to succeed and gain more commercial attention, it is crucial to have a thorough understanding of how a Li-S battery works and a clear explanation of its failure mechanism, i.e. the formation of solid species which leads to capacity fade and cell failure. A number of characterization techniques have been used to investigate the mechanism of Li-S batteries. Canas et al. and others used in situ X-ray diffraction (XRD) to investigate this lithium-sulfur chemistry and found out that Li2S formation starts in the second discharge plateau at 1.8 V and during charge Li2S reacts entirely and sulfur recrystallizes to 18-20 a different structure and particle size. Nelson et al. discovered that the recrystallization of sulfur occurs only to the sulfur cathode and their transmission X-ray microscopy (TXM) results showed that lithium polysulfide can be suc21 cessfully restrained on the cathode. Barchasz et al. reported that via UV-vis spectroscopy and HPLC data a three steps 22reaction: (1) long chain lithium polysulfide (S8 and S6 ) formation during the first reduction step (2.4 - 2.2 V); (2)

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medium chain polysulfide (S4 ) formation during the second 2reduction step (2.15 – 2.1 V); (3) short chain polysulfide (S3 , 22 2S2 , and S2-) formation at the end of discharge. Zou et al. reported their findings on the speciation intermediate with 23 operando UV-vis spectroscopy. Cuisinier et al. employed operando X-ray absorption spectroscopy (XANES) to study the speciation process, and they have shown that the first discharge plateau is governed by the formation of long chain lithium polysulfide (Li2S8, Li2S6 and Li2S4) and the second plateau by the formation of the short chain polysulfide 21 (Li2S). Long chain lithium polysulfide species (Li2S4, Li2S6 and Li2S8) dissolve in the most commonly used solvent while the solid species (Li2S) does not. Even though many characterization techniques were applied to study Li-S batteries, most approaches are only capable of monitoring either solution or crystalline species, but not both. Clearly, a method that can track and quantify the evolution of the entire electrochemical speciation process would be very informative. NMR spectroscopy serves as a useful tool in studying reaction mechanisms. With in situ NMR spectroscopy, See et al. showed Li2S formation presumably on the cathode and proposed electrolyte as a third phase in the reaction mechanism. However, only the first discharge was reported in this study, and the electrochemical profile showed a non-typical discharge profile (one slope throughout the entire discharge process rather than two discharge plateaus: 2.3 and 2.1 V). It is believed that the cell suffered from a cycling issue, and the short cycling life and the convoluted electrochemistry profile cast much doubt on the conclusions drawn from this study. Notably, the authors claimed that the solid species formation started at the beginning of 24 the first discharge plateau. Xiao et al. also reported their findings with NMR spectros25 copy. However, the observation of a solid species at the pristine state brings significant concerns about the cell design and the spectroscopic resolution. In a pristine Li-S battery, the observation of a solid species other than lithium metal and sulfur should never be expected, and the authors fail to give a reasonable explanation for the appearance of the solid species in their pristine Li-S battery and the basis of their peak assignments. Though attempted to employ in situ NMR spectroscopy to study Li-S batteries, neither See et al. nor Xiao et al. reported a convincing method to study the entire chemistry of Li-S batteries. Here, contrary to the undesired electrochemical performance and peak assignment, we present a comprehensive method that can quantitatively monitor the changes of all species in lithium-sulfur batteries during electrochemical cycling. Through quantification of the solid and the soluble species, the correlation between the formation of the soluble and the solid species is revealed in this study, where a clear four-step soluble species mediated electrochemical process is observed and described. Finally, the NMR spectroscopy method presented in this report toward researching Li-S battery speciation process opens up the opportunity to study Li-S batteries speciation process with novel electrolyte/additive systems and will provide tremendous assistance to electrolyte development, sulfur cathode research, and lithium metal protection schemes. 2. Experiment: 2.1 Cathode preparation

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Sulfur loaded (70% sulfur) Porous Carbon Nanospheres 26 (PCNS) were used as previously reported. Strips of carbon coated aluminum current collectors were weighed before and after loading any cathode material to determine the cathode loading amount. A recipe of PCNS: Timcal 45 carbon: PVDF (6:2:2 by weight) was used to prepare the slurry. NMP was used to dissolve the binder for a homogeneous mix. The slurry was then brushed onto the carbon coated aluminum curo rent collectors and dried at 50 C under vacuum overnight. The effective sulfur loading is 42 wt%. 2.2 Electrolyte preparation The electrolyte consisted of 1.0 M of lithium bis(trifluoromethane sulfonimidate) (LiTFSI, 99.95%, Aldrich) in a binary mixture of 1:1 (v:v) DOL:DME (anhydrous, 99.5%, Aldrich), with 2 wt% lithium nitrate (>99.0%, Aldrich) as additive was prepared. LiTFSI and LiNO3 were dried in a vacuum oven at 150°C overnight. The as-prepared electrolyte was then stirred for over 12 hours before use. All experiments were performed in an argon atmosphere glovebox with H2O and O2 levels less than 1 ppm. 2.3 Cell fabrication and testing The plastic pouch cells were assembled in an argon atmosphere glovebox (O2/H2O less than 2 ppm). A packaging film (3M Scotchpak LF400M) was used to contain the cell parts. The cells were hermetically sealed using a heat sealer. All cells were saturated with electrolyte. 2.4 In situ NMR spectroscopy 7

In situ Li NMR experiments were performed with an HX static probe in a Bruker Avance 300 wide bore magnet. Cells were cycled between 1.7 and 2.7 V using a portable Maccor electrochemical test station (model 4400). A slow cycle rate (C/30) was used to allow sufficient time for NMR spectra acquisition. NMR acquisition was started simultaneously with the electrochemical cycling, and each NMR spectrum with an acquisition time of 42 minutes. A one pulse excitation with a sufficiently long repetition interval (10s) was used to allow quantitative analysis, and a pulse length of 4.25 μs was used. A total of 164 spectra were collected. 2.5 Ex situ NMR spectroscopy At the end of the fourth cycle, the cell was disassembled in argon atmosphere glove box, and the electrodes were sealed in separate pouches individually. For precise quantification purposes, exact same NMR acquisitions (same parameters as one spectrum of the in situ NMR experiment) were performed on the cathode and the anode. 3. Results and discussion: 3.1 Peak deconvolution A representative spectrum (1.7 V) of a Li-S battery is shown in Figure 1a, where two groups of peaks were used to deconvolute the spectrum: three peaks at around 0 ppm and two peaks at around 250 ppm. Features at around 0 ppm (Figure 1b, 1c) are ascribed to the soluble species and the solid spe24 cies that are formed during the electrochemical process. Features at around 250 ppm are ascribed to the metallic lith27 ium and the large shift is due to the Knight shift. In a functional Li-S battery, soluble lithium polysulfide 18, 21 species coexist (Li2S8, Li2S6 and Li2S4) together with the electrolyte (LiTFSI) and additive (LiNO3). The shifts of these 7 Li-bearing species overlap with each other in Li NMR spectra.


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Figure 1: (a) deconvoluted spectrum of a cell at the end of discharge (1.7V) (b) three peaks were used to deconvoluted the region at around 0 ppm; (c) a closer view of the region at around 0 ppm; (d) metallic lithium region deconvolution. The overlap and similarity between these species make it difficult to separate individual peaks in NMR spectra. Attempts have been made to deconvolute the spectra with multiple peaks (more than three) in the diamagnetic region, however results do not follow any logical explanation. It is believed that peaks more than three will cause misinterpretation of the electrolyte/additive resonance with the lithium polysulfide species making it difficult to track the changes. Therefore, an approximation is used during deconvolution as 24 previously employed in See’s work : soluble species are grouped to high-frequency and low-frequency feature. In Figure 1b, three Li environments were used to deconvolute the spectrum in 0 ppm region. These three environments are: one at around -0.06 ppm which is ascribed to the solid species formed during the electrochemical cycling process and two sharp features at around 3.67 and -0.16 ppm which are ascribed to the soluble lithium bearing species. Each soluble species NMR feature represents a group of lithium environment which contains lithium bearing soluble species that resonates at a very similar frequency. Figure 1c shows a closer view of how these two soluble species peaks are assigned.

trolyte (E/S > 15 µL/mg), and the electrochemical profile of the in situ NMR cells is consistent with conventional Li-S 2 cells. The cell configuration and testing parameters can be found in the experimental section. The first multi-cycle pouch cell was tested and the electrochemical cycling profile was shown in Figure 2. Significant improvement has been made in the electrochemical cycling performance compared with cell employed by See et al., where the cell died after the 24 first discharge. The insertion of the multi-layer separators is believed to be a contributor to the performance improvement. In addition to the application of the multi-layer separator, pressure was applied to the pouch cell so that the contact between the cathode and the anode can be well maintained during data acquisition. We have found that the amount of electrolyte between the cathode and the anode plays a critical role, and the increased distance between the cathode and the anode allows more solvent readily available to dissolve lithium polysulfides.

In Figure 1d, two features were used to deconvolute the spectrum in this region at around 250 ppm: lithium metal at around 247 ppm and dendritic/mossy lithium at around 257 ppm. Due to the bulk magnetic susceptibility difference between these two sites, dendritic/mossy lithium signal appears at a slightly different chemical shift, enabling the sepa27 ration from the bulk lithium metal signal. 3.2 Cell performance improvement A cell with 1 M LiTFSI in DOL/DME with 2 wt% LiNO3 was employed as a model system. The cell was flooded with elec-



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Figure 2: Electrochemical cycling profile and the discharge/charge capacity of the cell with LiNO3, and four full cycles are shown.

Figure 3: (a) Solid species formation upon cycling; (b) soluble species at around -0.16 ppm evolutions with time. 3.3 Solid/soluble species evolution A total of 164 NMR spectra were acquired during electrochemical cycling, and carefully deconvoluted with the method described in the experimental section. The integral area of each deconvoluted feature was recorded and plotted together with the electrochemical profile as shown in Figure 3. Figure 3a shows the evolution of the solid species during the electrochemical cycling of the cell, and Figure 3b shows the evolution of the soluble species (-0.16 ppm resonance). During

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the entire first discharge plateau of the first discharge, no solid species formation was observed (region I in Figure 3a with blue highlight), and a large increase in the soluble species intensity was noticed (region I in Figure 3b with blue highlight). These observations suggest that during the first discharge plateau at 2.3 V, elemental sulfur (S8) dissolves in the solvent and reacts with lithium metal forming long chain soluble lithium polysulfides, such as Li2S8, Li2S6, and Li2S4. In the following discharge plateau (2.1 V), the solid species (short chain lithium polysulfide, e.g. Li2S) was observed to have formed (region II in Figure 3a with yellow highlight) and the long chain lithium polysulfides were observed to be consumed and their intensity continued to decrease till the end of the first discharge (Figure 3b region II with yellow highlight). Once the current was reversed and the charging process began, two charge plateaus were observed: 2.3 and 2.45 V. The intensity of the solid species decreases continuously throughout the entire charge process (region III in Figure 3a with green highlight). However, the soluble species intensity recovered during the first charge plateau (2.3 V, region III in Figure 3b with red highlight) and decreased during the second discharge plateau (2.45 V, region IV in Figure 3b with red highlight). It is believed that, upon charge, the lithium-sulfur chemistry was reversed where short chain lithium polysulfide became long chain lithium polysulfide during the first charge plateau. At the end of the first charge plateau, elemental lithium sulfur started to form till the end of the entire charge process accompanied by the decreased intensity of the soluble long-chain lithium polysulfide. The percentage of the reversible solid species was calculated to be 52% during the first cycle. In the subsequent cycles, the solid species formation and consumption repeated themselves upon charge and discharge. However, the irreversible solid species accumulated because of the lithium-sulfur chemistry cannot be completely reversed during the charging process. At the end of the fourth cycle, the solid species intensity increased by eight folds. To summarize, the soluble species shown in Figure 3b undergoes a four-step evolution in a complete electrochemical cycle: (I) formation from reaction between elemental sulfur and lithium; (II) consumption because of solid species formation; (III) formation through solid species decomposition; (IIIV) consumption due to elemental sulfur formation. These four stages repeated themselves in each of all four cycles. Soluble species mediated lithium-sulfur chemistry is clearly demonstrated in Figure 3a and 3b, and the soluble long-chain lithium polysulfide plays a significant role in lithium-sulfur chemistry. In addition, the capacity fade was clearly observed (Figure 2) while the irreversible Li2S continues to accumulate as the cell was cycled. The accumulation of the Li2S solid species is the main contributor of the capacity fade while the Li2S solid species is stranded in the cell losing contact with the electrochemical reaction. Moreover, the slow cycling rate (C/30) utilized in this study also facilitates the shuttling effect accelerating the accumulation of the Li2S solid species. For better visualization of the process, all 164 spectra were stacked and plotted (Figure 4). In this figure, the formation of the solid species (green/yellow intensity) and its evolution can be clearly seen, where the solid species gains intensity on discharging and loses intensity on charging (the appearance and disappearance of the green/yellow feature).


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The Journal of Physical Chemistry magnetic susceptibility enables peak separation between the two species: the lithium metal signal appears at around 247 ppm and the dendritic/mossy lithium signal appears at around 259 ppm.

Figure 4: Stacked plot of all 164 spectra acquire for the cell.

In Figure 5a, an increase in lithium metal intensity during the first discharge process and a continuous decrease in the subsequent cycles were observed. The increase in lithium metal density during the first discharge process is attributed to the skin depth effect, where radio frequency can only pen27 etrate tens of micrometers of the metal chunk. Therefore, the intensity of the lithium metal signal is proportional to the surface area of the lithium metal anode. During discharge, lithium consumption is not evenly distributed on the lithium metal surface thus causing a coarsening process of the lithium metal anode. Upon charge, the lithium metal intensity started to decrease due to the formation of dendritic/mossy lithium on the surface of the lithium metal anode. Since the density of the dendritic/mossy lithium differs from the lithium metal, the accumulated dendritic/mossy lithium compromises the ability of the radio frequency to penetrate the lithium metal anode due to the dendritic/mossy lithium on the surface. In the following cycles, the lithium metal anode intensity continued to decrease due to the continuous formation of inactive dendritic/mossy lithium. In Figure 5b, the dendritic/mossy lithium did not form till the beginning of the first charge process and continued to grow through the charging process. During the second discharge, interestingly, the signal intensity did not decrease but plateaued upon discharge and kept increasing in the following discharge process. In the third and fourth cycle, dendritic/mossy lithium formation slowed down during charge and was observed reversible during the discharge process. A continuous formation of dendritic/mossy lithium on the surface of the lithium metal and consumption of lithium metal are observed. The fast accumulation of the dendritic/mossy lithium is ascribed to the quick formation in the first two cycles until a thick layer of dendritic/mossy lithium was formed on the surface of the lithium metal anode. The slowed down formation process during the third and the fourth cycle can be explained by the dendritic/mossy lithium’s participation in the electrochemical process after the lithium metal surface was completely covered by a thick layer of dendritic/mossy lithium. 3.5 Ex situ NMR spectroscopy

Figure 5: (a) Metallic lithium evolution/consumption upon cycling; (b) Dendritic lithium formation with time. 3.4 Lithium metal/dendritic lithium evolution Lithium metal and dendritic/mossy lithium NMR feature were also deconvoluted and shown in Figure 5. The bulk

After in situ NMR spectroscopy data acquisition, the cell was discharged to 1.7 V and disassembled. The cathode and the anode were sealed separately in plastic pouches. The normalized ex situ NMR spectra of the cathode and the anode are shown in Figure 6, which decoupled the cathode reaction and the anode reaction. In this figure, the formation of a solid species was clearly seen both on the cathode and the anode. However, a surprisingly small amount of the solid species is observed on the cathode while a significant solid species accumulation is observed on the anode. This solid species on the anode is ascribed to the lithium sulfide (Li2S) formed via the sulfur shuttled to the anode and reacted with the lithium metal. The observation is consistent with continuous solid species intensity increase as a function of cycle number in Figure 3a and highlights the sulfur shuttling to the anode. This phenomenon is occurring despite the presence of LiNO3 additive and is more pronounced in a cell without LiNO3 (suggesting that the majority of the signal in



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Figure 6 anode is due to Li2S, not LiNO3 decomposition, see supplemental information) where overall reversibility of the cell reaction is poor.

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sulfur shuttling and side reactions in Li-S batteries with conventional electrolytes. 4. Conclusion In conclusion, a comprehensive and highly quantitative in situ NMR method is presented in this report to monitor the electrochemical process in Li-S batteries, and this method is a vital complementary tool to other in situ techniques. The intensity of the soluble species and the solid species were carefully deconvoluted, and the evolution is correlated with the electrochemical process nicely (the four-step soluble lithium polysulfide mediated Li-S electrochemistry). More importantly, simultaneous quantitative detection of both the solid and the soluble species in real time is critical in understanding the lithium-sulfur battery electrochemistry processes and side reactions such as the shuttle effect. The most important outcome of this study is that this method can be readily employed to study a variety of novel Li-S battery electrolyte, assist sulfur cathode design, and guide lithium metal protection research via quantitative real-time monitoring of the changes in the speciation process.

Figure 6: The disassembled cell cathode and anode spectra after in situ analysis.

AUTHOR INFORMATION Corresponding Author *[email protected]

Supporting Information The species evolution of a lithium-sulfur battery without the LiNO3 additive.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported as part of 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.

REFERENCES

Figure 7: Demonstration of the four-step soluble species based Li-S electrochemistry 3.5 Mechanism revelation Via the detailed in situ NMR characterization and the quantitative analysis of solid/soluble species evolution, we report the four-step reaction mechanism of a Li-S battery in a never before seen detail as summarized in Figure 7: (I) formation from the reduction of elemental sulfur; (II) consumption due to solid species formation; (III) formation through solid species decomposition; and (IV) consumption owing to the conversion to elemental sulfur. It is clear that the soluble species in the electrolyte plays a critically important part in this chemistry. On the anode side, the evolution of the lithium metal can be accurately correlated with the electrochemical process, where significant irreversible Li2S formation is also observed in in situ cells highlighting the magnitude of

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In Situ NMR Observation of the Temporal Speciation of Lithium Sulfur

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