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Easily Accessible, Textile Fiber-Based Sulfurized Poly(acrylonitrile) as Li/S Cathode Material: Correlating Electrochemical Performance with Morphology and Structure Martin Frey,†,‡,⊥,∥ Roland Krisp Zenn,†,§,∥ Sven Warneke,†,‡,⊥ Kathrin Müller,§ Andreas Hintennach,† Robert Ernst Dinnebier,§ and Michael Rudolf Buchmeiser*,‡,⊥ †

Daimler AG, RD/EKB, HPC G012-BB, 71034 Böblingen, Germany Max Planck Institute for Solid State Research, 70569 Stuttgart, Germany ‡ Institute of Polymer Chemistry, University of Stuttgart, 70569 Stuttgart, Germany ⊥ Institute of Textile Chemistry and Chemical Fibers (ITCF) Denkendorf, 73770 Denkendorf, Germany §

S Supporting Information *

ABSTRACT: Lithium−sulfur (Li/S) technology holds great promise for efficient, safe, and economic next-generation batteries. However, commercialization is limited by some issues, which are related to the fast degradation of Li/S cells and poor rate capability. Existing strategies addressing these issues are often unsuitable for commercialization because of their complexity and lack of scalability. This Letter presents a simple, cheap, and scalable synthesis of a sulfur-based cathode material from commercially available poly(methyl methacrylate)/poly(acrylonitrile) (PMMA/PAN) fibers. Thermal conversion of PMMA/PAN fibers with elemental sulfur yields sulfurized poly(acrylonitrile) (SPAN) with up to 46 wt % covalently bound sulfur. The fibrous morphology with cylindrical macropores helps to form electronic conduction networks in the cathode and provides directed diffusion pathways for ions. Consequently, these Li/SPAN cells show low internal resistances, high initial capacities up to 1672 mAh·g−1sulfur, high rate capabilities up to 8C, and excellent cycle stabilities over 1200 cycles. In addition, structure and postmortem analysis allow the correlation of electrochemical performance with SPAN’s chemical structure.

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well as during rest. Polysulfides dissolve readily in standard electrolytes such as ethers or glymes and can then diffuse to the anode and form a passivating surface layer mostly composed of Li2S and Li2S2. This causes a dramatic capacity loss and results in poor cycle stability of the battery.1,3 Several approaches that aim at avoiding this loss of polysulfides have been reported. Most of them entail the encapsulation of sulfur in more or less complex entities, such as in porous organic frameworks;4 silicon−carbon5−7 or silicon− thiophene hybrid materials;8 nitrogen-doped carbon-based composite materials;9,10 microporous,11 mesoporous,12 or porous (hollow) carbonaceous materials;13−15 and hierarchically (nano-) structured carbonaceous materials,16−18 to name

he viability of any battery-based concept strongly depends on a few issues, which are, apart from price, safety of the device, energy density, longevity, cycle stability, and rate capability. A promising combination of elements for the next generation of batteries is lithium and sulfur. The reversible total reaction in Li/S batteries is S8 + 16 Li ⇄ 8 Li2S and provides the basis for the high theoretical specific energy of approximately 2600 Wh·kg−1, which exceeds the theoretical specific energies of Li-ion-batteries many times over.1 The specific energy of Li/S batteries results from the high specific capacities of the cathode (elemental sulfur) with 1672 mAh·g−1 for the reaction S + 2 e− → S2− and the anode (metallic lithium) with 3860 mAh·g−1 for 2 Li → 2 Li+ + 2 e−. However, classical Li/S batteries face several challenges, among which are low sulfur utilization and low rate capability caused by the insulating nature of cyclo-S8 and its sluggish redoxkinetics.2 Another issue is the formation of polysulfides (Sx2−, x = 3−8) as intermediate products during battery operation as © 2017 American Chemical Society

Received: January 4, 2017 Accepted: February 4, 2017 Published: February 5, 2017 595

DOI: 10.1021/acsenergylett.7b00009 ACS Energy Lett. 2017, 2, 595−604

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ACS Energy Letters just a few. What all these approaches have in common is that the unifying concept relies on a retardation of the polysulfides inside the porous structures by nanoscaled confinements. However, scalability and costs of these sophisticated cathode materials often impose a problem. A very appealing alternative addressing the aforementioned issues of Li/S batteries has been described by Wang et al.19,20 Taking advantage of the cyclization chemistry of poly(acrylonitrile) (PAN), PAN was mixed with excess elemental sulfur and heated to a temperature >300 °C. In the course of this procedure, sulfur-assisted dehydrocyclization yielded a π-conjugated, sulfur-containing composite, referred to as “SPAN”. We were able to elucidate the structure of SPAN and show that sulfur is covalently bound to the carbonaceous matrix in the form of organic di- and polysulfides and thioamides. We could also correlate the cathode’s thermal stability and sulfur content with synthesis conditions and cycle stability.21,22 On the basis of that knowledge, the specific capacity of the composite could be enhanced from 430 to 630 mAh·g−1composite.23 Nonetheless, despite the ease of preparation, this optimized SPAN material still suffered from insufficient cycle stability and rate capability. However, recent reports on high cycle stabilities, high capacities, and high C-rates up to 10C in Li/SPAN cells showcased the reduced polysulfide dissolution, the high sulfur utilization, and the fast redox kinetics of SPAN-based cathodes, which outrival the cyclo-S8-based cathodes of classical Li/S cells by far.24−26 Interestingly, most works entail the use of PAN with spherical primary particle morphology for the synthesis of particulate SPAN. We developed a porous, fibrous version of SPAN, which reinforces the cathode coating mechanically, provides directed diffusion pathways for ions, and thus improves the electrical and ionic conductivity of the cathode. Herein, we report on this material, which in combination with a fluoroethylene carbonate-based electrolyte exhibits superior rate capability and cycle stability in Li/SPAN cells compared to its particulate counterpart. Synthesis and Structure. Fibrous SPAN was synthesized via thermal conversion of PMMA/PAN-based nonwovens (75 wt % PAN, 25 wt % PMMA, degree of stretching of 500%) at 550 °C for 3 h in the presence of excess elemental sulfur. The idea behind using PMMA as blend polymer is related to the high crystallinity of pure PAN-fibers, which was 45 ± 1% for a degree of stretching of 500% (Figure S6, Table S3). This high crystallinity impedes the incorporation of sulfur into the compact PAN-fibers, resulting in SPAN materials with low sulfur contents, typically 1000 mAh·g−1 was reached at 4C in the initial discharge. The high capacity in the initial discharge is followed by a substantial capacity loss. This is a multifactorial peculiarity of Li/SPAN cells caused by electrochemically irreversible processes like the reduction of SPANs backbone, dissolution and/or reactions of active materials with the electrolyte, formation of the SEI and insulating bulk Li2S and Li2S2, etc.26,29,31,47,54,55 Cathodes that contained fibrous SPAN surpassed reference cathodes that contained particulate SPAN by far, both in terms of capacity and rate capability (Figure 5b). At C-rates ≤1C, both systems provided capacities in a range of 800−1000 mAh· g−1. However, at a discharge current of 2C, particulate SPAN showed a pronounced capacity drop to ∼400 mAh·g−1. By contrast, fibrous SPAN still provided ∼800 mAh·g−1. At rates >4C, the particulate system could not be discharged anymore, whereas fibrous SPAN still provided 380 mAh·g−1, even at 8C. After high C-rate discharges, both systems almost reached their previous capacities when the C-rate was lowered again. Very recent reports have highlighted the importance of conductive networks in sulfur cathodes for battery performance.56 The formation of conductive networks depends strongly on the morphology of the used cathode materials. Van der Pauw measurements revealed that both fibrous and

(EDX) mapping revealed a homogeneous distribution of sulfur throughout the fibrous SPAN material (Figure 4d). Specific surface areas of PMMA/PAN and particulate and fibrous SPAN were determined by N2-adsorption measurements (Table S2). Despite the elimination of the PMMA fraction, fibrous SPAN exhibited a smaller surface area than particulate SPAN (22 m2·g−1 vs 37 m2·g−1, respectively). Obviously, the elimination of PMMA produces preferably macropores that contribute little to the specific surface area of fibrous SPAN. However, these macropores still increase the porosity of the material and provide directed channels for diffusion and mass transport. Generally, higher specific surface areas are usually beneficial for rate capability, because it allows the liquid electrolyte to access more of the active materials surface. However, Li/SPAN cells containing particulate SPAN have rate capabilities that are far lower than those of Li/SPAN cells based on fibrous SPAN, although particulate SPAN possesses a higher specific surface area. This suggests that the rate capability depends more on the porous, fibrous morphology than on the specific surface area for SPAN-based cathodes (vide infra). Electrochemical Characterization. Short-chain polysulfides (Li2Sx; x ≤ 3) have a significantly lower solubility in carbonates than in ethers.21,31,47−49 However, because long-chain polysulfides are soluble in both electrolytes and carbonates can react with solubilized polysulfides via nucleophilic addition, carbonates are incompatible with classical Li/S batteries containing cyclo-S8. In such systems, soluble, long-chain polysulfides emerge inevitably during reduction of cyclo-S8. By contrast, the covalently bound short-chain sulfur units in SPAN react directly to short-chain polysulfides,31,50 which are soluble in ethers but not in carbonates. In this case, employing carbonates is beneficial, not only to suppress polysulfide shuttling but also because carbonates are known to form stable solid electrolyte 599

DOI: 10.1021/acsenergylett.7b00009 ACS Energy Lett. 2017, 2, 595−604

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Figure 6. SEM images of cathodes containing fibrous (left) and particulate SPAN (right).

Figure 7. High-resolution X-ray photoelectron spectra of an aged cathode and an aged anode. The cathode contained 70 wt % fibrous SPAN, 15 wt % PVDF Solef 5130, and 15 wt % Super C65. The cell was aged with 1000 cycles between 3 and 1 V at 1C. Spectra in the energy regions of (a) C 1s, (b) S 2p, (c) O 1s, and (d) F 1s are shown.

particulate SPAN have electronic conductivities below 10−8 S· cm−1. Because particulate and fibrous SPAN are chemically very similar, differences in the electrochemical performance must be related to their distinct morphologies. SEM investigations on both types of SPAN cathodes shed light on this issue (Figure 6). As becomes evident, the linked and interwoven fibers (aspect ratio 1.5 < r < 15) of fibrous SPAN reinforce the cathode materials mechanically and provide a crack-free coating. This enables the conductive additive to form percolating conductive networks for electrons throughout the cathode. In contrast, the spherical particles of particulate SPAN cannot bridge and link

the cathode materials. Cracks in the coating form more easily in the particulate system. Consequently, isolated active material, which is electrochemically no longer addressable, is much more likely to occur in the particulate system. These differences with regards to the electronic conductivity were further supported by van der Pauw measurements of the two cathode materials, which were coated on an insulating surface. Thus, the specific electronic conductivity of a fibrous SPAN cathode was more than twice as high as that of a particulate SPAN cathode, 0.38 S· cm−2 vs 0.15 S·cm−2. Moreover, the cylindrical macropores of fibrous SPAN (vide supra) provide directed diffusion pathways for lithium ions. To shed more light on conductivity and 600

DOI: 10.1021/acsenergylett.7b00009 ACS Energy Lett. 2017, 2, 595−604

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charged fibrous SPAN cathode in XPS measurements (Figure 7). Most of these atoms are part of the SEI, which contains diverse decomposition products of the electrolyte. When FECcontaining electrolytes are used, the SEI typically contains LiF, diverse fluoroorganic compounds, organic and inorganic carbonates (Li2CO3, ROCO2Li, ROCO2R), lithium alkoxides, poly(ethylene oxide), and other organic oligo- and polymers.24,51,53,60,62−67 An intense peak of LiF appears at ∼685.5 eV in the F 1s spectra of the anode and the cathode (Figure 7d). In the case of the cathode, another F 1s peak at ∼687.7 eV stems from the binder PVDF, which also generates signals at ∼290.6 and ∼286.5 eV in the C 1s spectrum (Figure 7a).68 In the case of the anode, in addition to the LiF peak two more peaks appear in the F 1s spectrum at 687.7 and 690.3 eV from fluoroorganic compounds and the electrolyte salt LiN(SO2CF3)2, respectively.58,59,69 Peaks appearing on the anode and cathode between 286.5 and 290.5 eV in the C 1s spectra in combination with a peak at ∼531.5 eV in the O 1s spectra (Figure 7c) can be ascribed to the OCO2 group, and C 1s peaks between 285.5 and 287 eV in combination with the O 1s at ∼533.2 eV and can be ascribed to the C−O group.51,53,58,59,63,65,66 The former peak combination indicates the existence of lithium carbonates, alkyllithium carbonates, dialkylcarbonates, or polycarbonates. The latter peak combination indicates the existence of lithium alkoxides and/or poly(ethylene oxide)s. Regarding the cathode, carbon species with binding energies below 285.5 eV are related to the polyaromatic C−N backbone of SPAN (vide supra) and the conductive additive. In the case of the anode, these species probably stem from aliphatic polymeric decomposition products or from the polyolefine separator.63 Thus, both the cathode and the anode surfaces contain typical decomposition products of the FEC-based electrolyte. Interestingly, the S 2p spectra of the charged cathode and fresh fibrous SPAN are quite similar (compare Figures 7b and 2c). Peaks at ∼163.6 eV from C−S and S−S species and at ∼161.7 eV from CS species indicate that sulfur can be reversibly bound to the SPAN backbone with a restoration of the previous binding situation. In contrast to the cathode that was investigated by means of XRPD, no Li2S could be detected on the cathode that was investigated by means of XPS. Thus, in this cathode, even in the 1000th cycle most of the sulfur was still accessible and electrochemically active. The aged cathode displays a weak S 2p3/2 signal at ∼169.4 eV, which is related to SOx species. Different groups reported on similar findings in cycled sulfur-based cathodes, but there is no consensus on their origin.31,48,70−73 However, the cathode surface contains only ∼1.1 atom % of sulfur, which means that the surface is effectively covered by the SEI. Traces of sulfur were identified on the surface of the aged lithium anode. The S 2p3/2 signals at ∼169.4 eV and ∼167.6 eV are related to the sulfonyl groups of the electrolyte salt LiN(SO2CF3)2 or decomposition products thereof.71,72 No Li2Sx deposits, which usually are formed as a consequence of the polysulfide shuttle, were found on the anodes surface. These results clearly suggest that a strongly reduced polysulfide shuttle is the main reason for the high cycle stability of the Li/SPAN cells. The interplay of SPAN’s particular structure, which binds and releases short-chained polysulfides reversibly, and the carbonate-based electrolyte, in which these short-chained polysulfides are not soluble, prevents polysulfide dissolution and the concomitant polysulfide shuttle.

diffusion properties, electrochemical impedance spectroscopy was performed on two assembled cells that contained fibrous and particulate SPAN, respectively. The Nyquist plots of both Li/SPAN cells and the corresponding equivalent circuit (EC) are shown in Figure S3. The resistance of the electrolyte to ion transport and the resistance of the electrodes to electron transport contribute to the ohmic resistance R1 of the cell, which is given by the intersection of the impedance data at very high frequencies with the x-axis. At medium frequencies, two semicircles appear in the Nyquist plot, which are attributable to charge-transfer processes at the anode and/or cathode. The semicircles have been fitted by a series circuit of two nonideal RC-elements, which in turn are composed of a resistor parallel to a constant phase element (R2//CPE2 and R3//CPE3). The straight rise of the Nyquist plot at low frequencies for both cells is related to diffusion and mass transport and has been fitted by the open Warburg element Wo1. Fitted values for the different EC elements are given Table S1. The ohmic resistance as well as charge-transfer resistances and Warburg impedance are smaller in the Li/SPAN cell containing fibrous SPAN. This indicates a better electronic conductivity of the fiber-based cathode as well as superior diffusion properties. To test for both rate capability and cycle stability of fibrous SPAN, cycling between 1 and 3 V at symmetrically alternating C-rates between 0.5C and 8C was conducted (Figure 5c). After 1000 cycles, an average capacity decay of 0.25, 0.23, 0.23, and 0.28 mAh·g−1 per cycle was observed at 0.5, 1, 2, and 4C, respectively. Notably, no significant decrease in capacity was observed after the 500th cycles for the cycling experiments performed at 0.5, 1, 2, and 3C. This suggests that the system is in a stationary state between 500 and at least 1000 cycles, during which no further parasitic reactions, e.g., between the polysulfides and the electrolyte, occur. By contrast, the capacity fade at 4 and 6C is linear, though with shallow inclination, resulting in still remarkable 352 and 89 mAh·g−1, respectively, after 1000 cycles. Even at 8C, 67 mAh·g−1 was found after 500 cycles. Figure 5d displays the discharge capacity of the cell after 1000 continuous cycles. A good capacity retention of >800 mAh·g−1 at 0.5C was observed after 1200 cycles. Postmortem Analysis. Li/SPAN cells containing solely ethers as electrolytes displayed poor cycle stabilities. However, the cycle stability improved significantly when FEC or other carbonates were used as (co-) solvents in the electrolyte. This is related to the strongly reduced dissolution and shuttling of polysulfides in carbonates. Polysulfide dissolution can usually easily be detected in postmortem investigations of Li/SPAN cells by a yellowish color of the electrolyte or separator and degradation signs on the reference lithium anode. FECcontaining cells showed none of these signs (Figures S4 and S5a). More rigorous postmortem examinations of Li/SPAN cells after 1000 cycles by means of XRPD and XPS shed more light on this matter. In XRPD measurements (Figure S5b), Li2S and LiF were detected on an aged, charged cathode. LiF is probably a decomposition product of FEC and/or the electrolyte salt LiN(SO2CF3)2.57−60 The presence of Li2S on the cathode indicates an incomplete charge process due to an increased internal resistance of the aged cell or the formation of electrochemically inaccessible sulfur species. However, the absence of diffraction peaks from α-S8 suggests a reduced polysulfide dissolution, which otherwise usually is connected to the formation of crystalline sulfur.49,61 High concentrations of lithium, fluorine, oxygen, and carbon were detected on the surface of both the lithium anode and a 601

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ACS Energy Letters In summary, we have presented a PMMA/PAN fiber-based cathode material for use in Li/SPAN batteries, which can be readily prepared from commercial textile fibers. The fibrous SPAN composite is available via simple reaction of PAN/ PMMA fibers with sulfur at 550 °C. It contains up to 46 wt % of covalently bound sulfur. Quantitative structure analysis by means of elemental analysis, IR, Raman, NMR, ToF-SIMS, XRPD, and XPS allowed different sulfur-containing functional groups to be identified in fibrous SPAN. Possible structure motifs contain thioethers, organic di- and polysulfides, thiols, (thio-)amides, thioketones, and pyridyl (poly-)sulfides. Electrochemical characterization revealed superior rate capability up to 8C, which outperforms particulate SPAN by far. In combination with a FEC-containing electrolyte, long-term cycling was successfully demonstrated with >1000 cycles at C-rates alternating between 0.5 and 8C. The rate capability of the novel material is attributed to the porous, fibrous morphology, which provides a crack-free cathode coating and thus helps to form percolating conduction networks for both electrons and lithium ions throughout the cathode. Moreover, this percolating conduction network may also help to improve the cycle stability by providing electrical contacting, i.e., the electrochemical accessibility of sulfur species throughout the entire cathode. In addition, the special chemical structure of SPAN, which releases only short-chain polysulfides, and the low solubility of short-chain polysulfides in the FEC-based electrolyte mitigate the dissolution and shuttling of polysulfides and thus enable the outstanding cycle stability. Thus, the quantitative structure determination and postmortem analysis elucidated the superior behavior of SPAN-based cathodes compared to cyclo-S8-based cathodes. The comparative study of particulate and fibrous SPAN revealed that the performance of Li/SPAN systems can be further improved by tweaking the morphology of the active material.

capillary, and sealed hermetically. For XRPD measurements on the separator, a custom-built XRPD cell was used, which possessed kapton windows for X-ray transmission. For XPS measurements, the anode and cathode were fixed on a sample holder with conductive carbon tape and transferred in a vacuum vessel directly to the airlock of the X-ray photoelectron spectroscope to avoid any contact with air or moisture.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00009. Additional experimental details, details on TOF-SIMS, Nyquist plots, postmortem analysis, XRPD, N2-adsorption and crystallinity measurements (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Michael Rudolf Buchmeiser: 0000-0001-6472-5156 Author Contributions ∥

M.F. and R.K.Z. contributed equally to this work. M.F. developed and synthesized the SPAN composites and the electrolyte, prepared the Li/SPAN cells, and performed the electrochemical characterization. R.K.Z. conducted and/or evaluated elemental analysis, ATR-FTIR, Raman, XRPD, ToF-SIMS, 13C mCP-NMR, EIS, XPS, and N2-adsorption measurements and DFT calculations for structure determination and postmortem analysis.

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Notes

The authors declare no competing financial interest.

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EXPERIMENTAL METHODS Synthesis of SPAN. Fibrous and particulate SPAN were prepared from PMMA/PAN-based nonwovens (1:3) and PAN powder, respectively, in the presence of excess elemental sulfur at 550 °C for 3 h in a nitrogen atmosphere.19 Excess sulfur was removed via Soxhlet extraction with toluene overnight. Electrochemical Characterization. Electrodes were prepared by coating a 70:15:15 mixture of fibrous SPAN/particulate SPAN:Super C65:PVDF in NMP on aluminum foil (200 μmwet). After drying of the electrode sheet at 75 °C, electrode coins 12 mm in diameter were punched out and transferred to Swagelok T-type cells using a Freudenberg FS 2190 membrane and freshly prepared electrolyte comprising 3 M bis(trifluoromethylsulfonimide) (LiN(SO2CF3)2, LiTFSI) in fluoroethylene carbonate (FEC) and 1,3-dioxolane (DOL) in a volume ratio of 1:1. The sulfur loading of the cathode was 0.672 mg·cm−2 (areal capacity = 1 mAh·cm−2). Electrochemical testing was performed on a BasyTec XCTS-LAB in a voltage range of 3−1 V. Current density and specific capacity were calculated based on the mass of sulfur in the cathode (1C = 1672 mAh·g−1 = 1 mA·cm−2). Sample Preparation for Postmortem Analysis. After 1000 cycles in the range of 3−1 V, charged cells were opened in an argon filled glovebox. The electrodes and the separator were withdrawn, rinsed with fresh 1,3-dioxolane, and then dried in vacuo to remove the electrolyte and unbound species. For XRPD measurements on the cathode, the coating was scraped off from the aluminum current collector, filled into a glass

ACKNOWLEDGMENTS We thank U. Hageroth and Dr. J. Spörl (Institute of Textile Chemistry and Chemical Fibers (ITCF) Denkendorf, Kö r schtalstraße 26, 73770 Denkendorf, Germany) for assistance with scanning electron microscopy and for XRD measurements. We thank T. Acartürk and U. Starke (Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany) for assistance with X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry. We thank A. Schulz, M.-L. Schreiber, I. Moudrakovski, J. Chen, and R. Kremer (Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany) for assistance with Raman spectroscopy, IR spectroscopy, NMR spectroscopy, density functional theory calculations, and conductivity measurements, respectively. Dralon GmbH (Alte Heerstr. 2, 41540 Dormagen, Germany) is gratefully acknowledged for providing PAN-based fibers.

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REFERENCES

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on February 9, 2017, with the incorrect graphic in Figure 3. The corrected version was reposted on February 17, 2017.

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DOI: 10.1021/acsenergylett.7b00009 ACS Energy Lett. 2017, 2, 595−604