Inverse vulcanization of elemental sulfur with 1,4-diphenylbutadiyne for cathode materials in Li–S batteries

Abstract

High sulfur content copolymers were prepared via inverse vulcanization of sulfur with 1,4-diphenylbutadiyne (DiPhDY) for use as the active cathode material in lithium–sulfur batteries. These sulfur-rich polymers exhibited excellent capacity retention (800 mA h g⁻¹ at 300 cycles) and extended battery lifetimes of over 850 cycles at C/5 rate.

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Sustainable exploitation of inherently sporadic renewable energy sources such as solar, wind, and tidal necessitates the development of electrical energy storage systems constructed with earth abundant materials.¹ State-of-the-art electrochemical storage based upon Li-ion technology typically relies on materials with diminishing abundance such as cobalt-based oxides for the electroactive cathode material which are unlikely to be able to meet the comprehensive requirements needed for electrified transportation or stationary base load requirements.² Thus, the challenge remains to develop electroactive components for battery electrodes which utilize sustainable, earth abundant materials that also possess improved electrochemical properties. Next generation battery technologies which utilize electroactive polymers as the electrode materials are a promising route to developing sustainable electrochemical energy storage devices capable of facilitating the use of intermittent renewable energy sources.^3–11

High sulfur content copolymers have recently been of particular interest as a class electroactive polymeric cathode materials for secondary lithium batteries due to their high specific capacity and exceptional cycle life.^12–16 The analogous electrochemical reactivity of the high S–S bond content of these copolymers compared with elemental sulfur (S₈) enables their use as the active material in lithium–sulfur (Li–S) batteries.¹² Preparation of these sulfur-rich polymers have been achieved via the polymerization methodology termed, inverse vulcanization, where molten elemental sulfur is directly utilized as a comonomer in a free radical copolymerization. Furthermore, sulfur is an economical, earth abundant material making its direct use with inverse vulcanization attractive for development of sustainable next generation electrochemical storage technology. Inverse vulcanization of S₈ has thus far been demonstrated with a few comonomers such as 1,3-diisopropenylbenzene (DIB),^{12,13,15,17–21} 1,3-diethynylbenzene (DEB),¹⁴ oleylamine,²² and a styrenic propylenedioxythiophene.²³ In an effort to further expand the scope of inverse vulcanization and produce a copolymer with additional functionality capable of enhancing performance as the active cathode material for Li–S batteries we have investigated 1,4-diphenylbutadiyne (DiPhDY) as a new comonomer.

The choice of using a butadiyne comonomer was motivated by previous reports of preparing small molecule multiple-sulfur containing heterocycles e.g. dithiolodithioles and particularly thiophenes such as the thieno-3,4-pentathiepin reported by Swager et al. upon treating diarylbutadiynes with sulfur in solution.^24,25 By applying the inverse vulcanization methodology to the reaction of S₈ and DiPhDY we aimed to prepare a copolymer with a high content of S–S bonds in place of the intramolecular polysulfide rings (i.e. pentathiepin) reported for small molecule analogs. Incorporation of heterocycles such as thiophene into the sulfur-rich copolymer was desirable as Li–S cathodes formulated with polar and aromatic heterocycle containing materials have been shown to increase device performance by promoting adsorption of the soluble lithium polysulfides generated during battery cycling and thus mitigating loss of active material.^26–28

Herein we report the inverse vulcanization of elemental sulfur with 1,4-diphenylbutadiyne for the first time to yield poly(sulfur-co-1,4-diphenylbutadiyne) (poly(S-co-DiPhDY)) and establish its utility as an active cathode material in Li–S batteries. We demonstrate that the sulfur content of poly(S-co-DiPhDY) could be easily modified by simply varying the feed ratio of S₈ and DiPhDY comonomers. It was found that copolymers synthesized with ≥50 wt% DiPhDY exhibited solubility in common polar organic solvents (e.g. CHCl₃, THF, CH₂Cl₂) allowing for conventional solution based characterization such as NMR, size exclusion chromatography (SEC), and cyclic voltammetry (CV) to be completed. Additionally, the effect of poly(S-co-DiPhDY) composition on thermal properties were investigated where copolymers prepared with increasing DiPhDY loadings were found to have higher glass transition temperatures (T_g) respectively. Finally, very high sulfur content copolymers (90 wt% sulfur) were prepared for formulating Li–S battery cathodes where a maximum sulfur content is desirable. Utilizing these electroactive polymers with a high content of S–S bonds, Li–S batteries were fabricated that demonstrated exceptional capacity (800 mA h g⁻¹ at 300 cycles, C/5 rate) and the longest cycle lifetime (850 cycles) reported to date of a Li–S battery employing an electroactive polymer.

To prepare poly(S-co-DiPhDY) via inverse vulcanization a mixture of elemental sulfur and 1,4-diphenylbutadiyne was heated above the melting point of S₈ to yield a homogeneous solution of the two comonomers and facilitate bulk free radical copolymerization. We propose that the copolymerization proceeded with the initial formation of a dithiolodithiole (1, Scheme 1) that subsequently rearranges to form a 2,5-diphenylthiophene with bridging S–S connectivity at the 3 and 4 positions (Scheme 1). Elucidating the structure of the resulting copolymer and gaining insight into the course of the copolymerization was challenging due to the broad range of possible products and isomers that have been alluded to in the literature upon sulfurization of (bis)alkynes,^29–33 along with broad resonances observed in both ¹H and ¹³C NMR spectroscopies. Initial NMR experiments revealed the presence of diphenyldithiolodithiole as a component of the reaction mixture generated upon inverse vulcanization of S₈ with low loadings of DiPhDY (i.e. 5 wt%). To interrogate the role of the dithiolodithiole species in the copolymerization compound 1 was synthesized and isolated according to the method of Blum et al.²⁴ and subjected to a series of control experiments examining homopolymerization and copolymerization of 1 with S₈ and other intermediates. It was found that the dithiolodithiole 1 did not homopolymerize in the melt (T = 175 °C) nor was it reactive with additional S₈ as determined by size exclusion chromatography (see ESI, Fig. S2†). In both cases however a second small molecule was observed via NMR with signals characteristic of a 2,5-diphenylthiophene presumably formed via rearrangement of the dithiolodithiole 1 to trithiolethiophene 2. Such a rearrangement would be in agreement with earlier related reports on multi-sulfur containing heterocycles such as dithiins which form thiophenes by the extrusion of a sulphur atom upon thermal treatment.^34–38 In other control experiments we observed that diphenyldithiolodithole 1 afforded low molar mass copolymer when reacted with another equivalent of DiPhDY as observed from SEC (Fig. S2†).

Scheme 1 Inverse vulcanization of elemental sulfur with 1,4-diphenylbutadiyne (DiPhDY) yielding poly(sulfur-co-1,4-diphenylbutadiyne).

To further support the proposed structure of poly(S-co-DiPhDY), fully soluble copolymer samples were prepared with high DiPhDY loadings (60 wt%) and the higher molecular weight components of the mixture were isolated via a series of precipitations and flash chromatography. Cyclic voltammetry of the isolated higher molecular weight poly(S-co-DiPhDY) (Fig. 1) revealed a largely irreversible oxidation with an onset at 0.5 V and peak current at a potential of 1.3 V corresponding to oxidation of the diphenylthiophene units which exhibited similar voltammetry to those of 2,5-arylthiophene derivatives as reported previously by Swager et al.²⁵ The minor redox couple at E_1/2 = −0.2 V was attributed to residual small molecule impurity in the copolymer sample. The presence of cantenated S–S connectivity was supported by laser desorption mass spectrometry (see ESI, Fig. S6†) where mass distributions were observed with periodic variances equivalent to the mass of sulfur. These collective findings pointed to the formation of diphenylthiophene and S–S containing moieties in the repeating units of poly(S-co-DiPhDY) as proposed in Scheme 1.

Fig. 1 Cyclic voltammogram (2^nd scan) of poly(S-co-DiPhDY) (2 mg mL⁻¹ in 100 mM TBAFP/CH₂Cl₂, scan rate: 50 mV s⁻¹).

Thermal analysis was also conducted on a series of poly(S-co-DiPhDY) prepared with varying feed ratios of S₈ and DiPhDY to confirm the formation of a copolymer (as noted by the presence of distinct and tunable glass transition temperatures) and examine the effect of composition on the thermal properties of the material. Differential scanning calorimetry (DSC) thermograms (Fig. 2a) of poly(S-co-DiPhDY) prepared with DiPhDY feed ratios ranging from 10 to 60 wt% revealed that the T_g of poly(S-co-DiPhDY) increased respectively with DiPhDY content. The lowest DiPhDY content copolymer (i.e. 10 wt%) exhibited a T_g of −26 °C poly(S-co-DiPhDY) and increases in T_g with increasing DiPhDY loadings to a maximum of 47 °C for 60 wt% DiPhDY were observed. Thermogravimetric analysis (TGA) was also conducted on the series of poly(S-co-DiPhDY) (Fig. 2b). The TGA thermograms show an initial mass loss with an onset at 200 °C attributed to the volatilization of sulfur with a subsequent plateau in mass loss beginning at 300 °C corresponding to the overall sulfur content. A clear correlation between this mass loss with the S₈ feed ratio utilized for inverse vulcanization was observed confirming that the sulfur content of the S₈ : DiPhDY ratio was retained in the final copolymer product.

Fig. 2 (a) DSC thermograms highlighting glass transition temperatures (T_g) (curves staggered for clarity) and (b) TGA thermograms of elemental sulfur and poly(S-co-DiPhDY) prepared with varied sulfur : 1,4-diphenylbutadiyne (DiPhDY) feed ratios.

To evaluate the electrochemical properties of these sulfur copolymers (and to structurally confirm the presence of S–S bonds) poly(S-co-DiPhDY) was utilized as the active cathode material in Li–S batteries. Cycling performance of coin cells fabricated with poly(S-co-DiPhDY) of various S : DiPhDY compositions revealed higher capacity was exhibited with increasing sulfur content of the copolymer (Fig. S10†). Poly(S-co-DiPhDY) prepared with 90 wt% sulfur and 10 wt% DiPhDY was therefore selected for long term Li–S cycling studies. The high sulfur content copolymer exhibited a notable initial capacity of 1050 mA h g⁻¹ and low capacity loss was observed as indicated by capacities of ≥800 mA h g⁻¹ for the first 300 charge/discharge cycles (Fig. 3a). Additionally the batteries were found to have exceptional lifetimes with over 850 total cycles at a rate of C/5 while retaining high Coulombic efficiency (>97%) throughout long term testing. Plots of cell potential versus capacity at 100 cycle intervals (Fig. 3b) show electrochemical behaviour consistent with more conventional Li–S cathode materials^39,40 suggesting that the polysulfide S–S connectivity between the organo-sulfur components of poly(S-co-DiPhDY) participates in analogous reactions to those of S₈ during charge/discharge. No appreciable polarization was observed with extended cycling as the high voltage plateau during discharge at ca. 2.3 V corresponding to the lithiation of higher order polysulfides (e.g. S_6–8 to Li₂S_6–8) followed by further reduction of these lithiated polysulfides lower order lithium polysulfides (e.g. Li₂S_2–5) from 2.25 V to the plateau at ca. 2.05 V was consistently observed during extended battery cycling. These processes were found to be highly reversible as indicated by the charging profile with oxidation of the lower order lithiated polysulfides observed at ca. 2.25 V and their subsequent conversion to higher order polysulfides at potentials ≥2.3 V. A rate study conducted to evaluate performance of the polymeric cathode material at various current densities (Fig. 3c) found that poly(S-co-DiPhDY) was able to facilitate high capacities of over 800 mA h g⁻¹ even at a faster, practical rate of 1 C (1 C = 1672 mA h). When cycled at a more rapid rate of 2 C significantly reduced capacity (ca. 400 mA h g⁻¹) was observed which was attributed to the limited rate of diffusion of electroactive species, however high capacity was recovered upon returning to a less aggressive charge/discharge rate of C/5.

Fig. 3 (a) Cycling performance at C/5 of Li–S battery fabricated with poly(S-co-DiPhDY) prepared with a 10 wt% DiPhDY, 90 wt% sulfur feed ratio. (b) Plot of potential versus charge/discharge capacity for the Li–S cell shown in (a) at 100 cycle intervals. (c) Charge/discharge rate performance of Li–S battery with poly(S-co-DiPhDY) (10 wt% DiPhDY) at various current densities (1 C = 1672 mA h). All capacities are specific capacity based on sulfur loading.

In conclusion we report the first example of inverse vulcanization of elemental sulfur with 1,4-diphenylbutadiyne. The facile bulk copolymerization was shown to yield poly(sulfur-co-1,4-diphenylbutadiyne) as a high sulfur content copolymeric material with easily tunable sulfur content ranging from 90 wt% to 40 wt% sulfur. Copolymers prepared with up to 90 wt% sulfur were utilized as the active cathode material in Li–S batteries which demonstrated high capacities of >800 mA h g⁻¹ at a rate of C/5 for 300 cycles and the longest lifetime of a Li–S battery employing a sulfur copolymer prepared with inverse vulcanization yet reported.

Acknowledgements

We acknowledge the NSF (CHE-1305773), the University of Arizona Renewable Energy Network, the WCU Program through the NRF of Korea funded by the Ministry of Education, Science and Technology (R31-10013), and the University of Delaware Department of Materials Science and Engineering for support of this work. KC acknowledges the support from NRF for the National Creative Research Initiative Center for Intelligent Hybrids (2010-0018290).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details for preparation of sulfur copolymers via inverse vulcanization and detailed characterization of these materials. See DOI: 10.1039/c5ra01188d

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