Improved capacity retention of low cost sulfur cathodes enabled by a novel starch binder derived from food

Abstract

Rechargeable lithium sulfur batteries are attracting great attention in recent years due to their high theoretical energy density, but they still suffer from poor cycle performance and other drawbacks. Up to now, most researchers have focused on the design of the state of the art S-containing composites or new and complex electrode architectures to improve their electrochemical performances. However, simple and low cost sulfur cathodes usually exhibit poor cycle performances. Here, we propose gelatinized starch, a natural biological macromolecule from the food industry as a binder for low cost sulfur cathodes to improve their capacity retention. Compared with PVDF, the starch binder is able to reduce the polarization of the sulfur cathode and enhance its capacity retention. The sulfur cathode with the starch binder exhibits a specific capacity retention of ∼90% at 0.2 C after 200 cycles, which corresponds to a small capacity decay of 0.05% per cycle. It is believed that these improvements result from the high stability of the starch binder in a liquid electrolyte, which can stabilize the structure of the cathode and reduce the irreversible accumulation of the discharge products.

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1. Introduction

Recently, when many researchers realized that commercial lithium ion batteries had almost reached their limit in terms of energy density and could not meet the requirements of intelligent portable electronic devices and electrical vehicles, rechargeable lithium sulfur batteries attracted more attention due to their high theoretical energy density, low cost, environmental benignity and unlimited sulfur supply.^1,2 However, some drawbacks, such as rapid capacity fading, low coulombic efficiency and low sulfur utilization, are still hindering their commercial application. Rapid capacity fading, which mainly results from the dissolution of polysulfides and the large volume change in active materials during the discharge/charge process, may be the most severe problem at present. Up to now, various approaches including entrapping sulfur in porous carbon or hollow conductive matrices,^3–7 covering cathodes with carbon cloths or other films,^8–11 adding polysulfide absorbers^12–15 and employing new binders^16–30 were employed to improve the cycle performance of the sulfur cathode. Aimed at the dissolution of polysulfides, many researchers have developed various state of the art conductive hosts for sulfur encapsulation and these composites usually exhibit good cycling performances and coulombic efficiencies.^3–7 However, they also usually mean both more conductive agents and less active materials, which probably reduce the overall energy density of commercial batteries. With regard to the large volume change during the electrochemical process, recent studies on electrode materials undergoing large volume changes in electrochemical reactions, including Si or Sn, have shown that using new effective binders rather than the most commonly used binder, polyvinylidene fluoride (PVDF), can usually provide a profoundly positive effect on the long term cycling performance and kinetic properties resulting from the structural stability of the cathode. Thus, in the case of the sulfur cathode, to enhance its cycling performance, a more effective binder is required to create a more robust conductive network for enduring the volume change in active materials during the discharge and charge process. Some new binders recently developed by researchers, such as gelatine,¹⁶ SBR + CMC,¹⁸ carbonyl-β-cyclodextrin,¹⁷ polydopamine + poly(acrylic acid),²⁹ can usually improve the strength and structural stability of the entire sulfur electrode, and thus enhance its cycling performance as well as its kinetics. Some other binders, such as PVP (polyvinylpyrrolidone), act as polysulfide absorbers besides being good binders for Li₂S due to their strong affinity with Li₂S, and thus also obviously enhance the cycling performances of the electrodes.²¹

In this article, we present a novel binder, gelatinized starch, for low cost sulfur cathodes using commercial sulfur powder and carbon black. Gelatinized starch is the most common carbohydrate and can be contained in large amounts in our food. It is abundant, low cost and environmentally friendly compared with PVDF. Even compared with other new binders derived from starch, such as β-cyclodextrin and carbonyl-β-cyclodextrin, it is still better due to the simplicity of its production process and its cost. The typical production of β-cyclodextrin usually needs to gelatinize the starch first, then hydrolyze it with α-amylase, and convert it into β-cyclodextrin with a ring-like structure in the presence of cyclodextrin glycosyltransferase, and finally purify it.³¹ However, β-cyclodextrin has low solubility in water and poor bonding strength, thus it is unsuitable for adoption as an aqueous binder in batteries.¹⁷ Carbonyl-β-cyclodextrin exhibits enough solubility in water and bonding strength but needs an extra process of carbonylation besides the preparation process of β-cyclodextrin mentioned above.¹⁷ So, gelatinized starch prepared by a one step gelatinization process from raw starch is more suitable for low cost cathodes. Undoubtedly, some state of the art S/C composites composed of graphene or carbon nanotubes can exhibit good performances readily, but according to the cost calculation by M. Hagen,³² the application of carbon nanotubes will increase the cost of 18650 lithium sulfur batteries dramatically and thus make them impractical commercially compared with lithium ion batteries. To retain the cost competitiveness of rechargeable lithium sulfur batteries, only commercial carbon black and commercial sulfur powder and binder were used here. The sulfur cathode with the starch binder exhibits a specific capacity retention of ∼90% at 0.2 C after 200 cycles, which corresponds to a small capacity decay of 0.05% per cycle, much higher than that of the sulfur cathode with the PVDF binder. Finally, a promising lithium sulfur battery with good capacity retention was fabricated using the starch binder.

2. Experimental

2.1 Gelatinization of the starch binder

5 g starch (AR, Kelong Chemical Co., Ltd.), 15 g distilled water and 0.05 g LiOH were placed in a Teflon sealed container and heated for 2 h at 58 °C with a magnetic stirring speed of 500 rpm. Then, 0.45 g LiOH was added and the Teflon sealed container continued to be heated for 2 h under the conditions mentioned above.

2.2 Preparation of the cathodes and electrochemical measurements

Commercial sublimed sulfur (AR, Kelong Chemical Co., Ltd.) and Super P were mixed in a weight ratio of 70 : 15 using a mortar and pestle, followed by heating in a sealed can at 160 °C for 1 h. The obtained S/C composite, Super P and the starch binder with a weight ratio of 85 : 5 : 10 were stirred for 0.5 h to form a slurry (the weight of the water in the binder solution was not included). After stirring, the mushy slurry was coated onto Al foil by a doctor blade technique and dried at 60 °C for 24 h. To demonstrate the effect of the starch binder, sulfur cathodes using polyvinylidene fluoride as a binder were also fabricated in which N-methyl pyrrolidone (NMP) was used as the solvent instead of water. The sulfur loading of all cathodes was controlled to 1 mg cm⁻².

2025-type coin half cells were assembled in an argon-filled glove box using lithium foil as the counter electrode and Celgard 2325 as the separator. The electrolyte used in this work was 0.65 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in a mixed solvent of dimethoxy ethane (DME) and 1,3-dioxolane (DOL) (1 : 1 by volume). After assembled, the cells were stood for 3 hours before testing to ensure the immersion of electrolyte into the electrodes. The coin cells were tested in galvanostatic mode at room temperature within a voltage window of 1.0–3.0 V using a LAND 2001A battery tester. Cyclic voltammetry (CV) measurements were carried out with the coin cell at a scan rate of 0.1 mV s⁻¹ by using a Parstate 2273 electrochemical workstation.

2.3 Characterization of the materials and cathodes

The morphology and composition of the electrodes were examined by scanning transmission electron microscopy (SEM, JSM-5900LV) and energy dispersive X-ray spectroscopy (EDS, X-MAX SDD), respectively. Crystal structure characterization was carried out with an X-ray diffractometer (XRD, DX-2600) with Cu-Kα radiation at a scan rate of 0.04° s⁻¹. A plastic film was used to protect the cycled electrode from the moisture in the air. FTIR spectra for the binders (powder) were recorded using an FTIR spectrometer (Nikolet 6700). Transmission spectra were obtained with a KBr pallet. To measure the swelling ratio of the binders, PVDF sheets and the starch sheet were prepared and weighed accurately, and then were immersed in the electrolyte. The carbonate electrolyte used here was composed of ethylene carbonate and dimethyl carbonate mixed in a 1 : 1 ratio with one molar of lithium hexafluorophosphate salt. The ether electrolyte used was 0.65 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in a mixed solvent of dimethoxy ethane (DME) and 1,3-dioxolane (DOL) (1 : 1 by volume). At regular intervals, the sheets were washed in DME and wiped with filter paper to remove the residual liquid on the surface, then weighed. The swelling ratio was described as the percentage of weight change of the binder sheets in the electrolyte. The dielectric constant of the dry starch binder sheet was measured using a TH2816 LCR meter at 10 kHz after Au deposition (at room temperature).

3. Results and discussion

The long molecular chains of starch consist of hundreds to several thousands of monosaccharide repeated units, in which each monosaccharide contains three hydroxyl groups.³³ Despite the abundance of hydroxyl groups, native starch usually has limited solubility in water at room temperature, hindering its application as a binder. After gelatinization in an alkali solution, the solubility of the starch in water increases obviously. It is believed that the breaking of the hydrogen bonds among these long molecule chains in a hot alkali solution should be responsible for the increased solubility.³³ During gelatinization, the types and quantity of functional groups in the molecule chains are not changed greatly, which is proven by the FTIR spectra presented in Fig. 1a. However, as Fig. 1b shows, the sharp peaks in the native starch between 15°and 40° almost disappeared after gelatinization, indicating that the crystalline region decreased dramatically during the gelatinization and most of the starch binder existed in the amorphous form in the sulfur cathodes in this experiment. According to pioneering research by many researchers,^33–36 at room temperature, only the amorphous regions can absorb water reversibly. However, when aqueous suspensions of starch granules are heated above the gelatinization temperature gradually, the intermolecular hydrogen bonds which occur between –OH and the adjacent O atom of the D-glucosyl residues on different molecules weaken. So, the crystalline regions become diffuse and also begin to absorb water. On further heating of the aqueous suspension, the hydrogen bonds continue to be disrupted and water molecules become attached to the hydroxyl groups, resulting in greater swelling of the granules and melting of the crystallites. The starch granules swell irreversibly and rupture severly. Then, the macromolecular chains dissolve in water and the crystallinity of the starch granules is destroyed. Even after removing the water, most of the macromolecular chains can not self-assemble in an orderly fashion and form the crystalline regions seen previously at room temperature.

Fig. 1 Chemical structure evolution of the starch binder before and after gelatinization. (a) FTIR spectra (b) XRD patterns.

The electrochemical stability of the binder in the operating voltage range of the lithium sulfur battery is also a factor of importance. To evaluate it, cyclic voltammetry curves of the starch binder and the PVDF binder were measured in the voltage range of 1–4 V. It is believed that PVDF, the binder widely used in commercial lithium ion batteries, is quite stable in the voltage range of 0–5 V, so it can also be used as a reference sample.¹⁷ Fig. S1† presents the cyclic voltammetry curves of these cells during the first three cycles. In the sulfur/starch cathode and the sulfur/PVDF cathode, no obvious peaks are found between 1–3 V after the 1^st cycle, quite similar to that of a typical supercapacitor, which also usually consists of conductive carbon with a large specific surface area and a binder.^37–39 The electrochemical stability of the starch binder is enough good for the lithium sulfur battery.

The discharge and charge curves of the cathodes with PVDF and the starch binder are shown in Fig. 2a and b, respectively. Although the initial specific capacity of the sulfur/starch cathode is a little lower than that of the sulfur/PVDF cathode, the smaller polarization of the sulfur/starch cathode during the discharge and charge process can probably compensate for the loss of energy density, and thus the energy density is similar for the sulfur cathodes with the PVDF and starch binders. Most importantly, different from that of the sulfur/PVDF cathode, the discharge and charge voltage plateaus of the sulfur/starch cathode remain constant during cycling, meaning less loss of energy in the sulfur/starch cathode. The CV curves of the sulfur cathodes with the two binders are presented in Fig. S2.† In accordance with the typical CV curves of the sulfur/carbon black cathodes, both curves show two distinct peaks and one oxidation peak in the voltage range of 1–3 V.^40–42 The two reduction peaks are assigned to the reduction of sulfur to soluble high-order polysulfides and the subsequent change from high-order lithium polysulfides to insoluble low-order polysulfides (Li₂S₂ or Li₂S), respectively. The oxidation peak is related to the oxidation of low-order polysulfides and often splits into two peaks when a cathode with excellent reaction kinetics or a very slow scan rate is used. These results have been discussed by some researchers in detail.^{16,18,40–44} Here, what we are concerned about most is the locations and shapes of the peaks. For the sulfur/starch cathode, two reduction peaks appear at the potentials of 2.3 and 2.0 V and one oxidation peak appears at 2.5 V, whereas the three potentials of the corresponding peaks of the sulfur/PVDF cathode are at about 2.1, 1.6 and 3.3 V. Moreover, the reduction and oxidation peaks for the sulfur/starch cathode appear much sharper than those for the sulfur/PVDF cathode. The lower polarization and sharper peaks both demonstrate that the electrochemical redox reactions in the sulfur/starch cathode are more likely to occur. These improvements are also found in cathodes using other water-soluble binders, such as SBR + CMC and gelatin.^16,18

Fig. 2 The discharge/charge curves and cycle performances of (a and c) the sulfur/PVDF cathodes and (b and d) the sulfur/starch cathodes at a rate of 0.2 C.

The cycle performances of the cathodes with the different binders are shown in Fig. 2c and d. After 100 cycles, a capacity retention as high as 94% was obtained for the sulfur/starch cathode, much higher that of 35% for the sulfur/PVDF cathode. Even after prolonged cycling over 200 cycles, the sulfur/starch cathode retained ∼90% of the initial capacity, which corresponds to a small capacity decay of 0.05% per cycle, much higher than that of the sulfur cathode with the PVDF binder. The average specific capacity of the sulfur/starch cathode within 100 cycles is 594.3 mA h g⁻¹, 46% higher than that of the sulfur/PVDF cathode. The stable discharge voltage plateaus and slower capacity fading of the sulfur/starch cathode both mean less energy loss during cycling. Compared with commercial cathode materials for lithium ion batteries, such as LiCoO₂, the sulfur cathode using starch as a binder exhibits a remarkable advantage in gravimetric energy density while maintaining a similar capacity retention. The coulombic efficiency of the sulfur/starch cathodes is close to that of the sulfur/PVDF cathodes, indicating that the starch binder is incapable of inhibiting the migration of polysulfides. Therefore, the improvement in the cycle performance might not result from the mitigation of the shuttle effect; what play an important role needs further investigation.

In commercial lithium ion batteries, the active materials undergoing large volume changes in the discharge/charge process usually suffer from fast capacity fading usually due to the deteriorating integrity of the electrodes during cycling. Different from active materials such as LiCoO₂, LiFePO₄ and graphite, the cycle performances of these active materials which undergo large volume changes usually exhibit a high sensitivity to the binders.^45,46 To reveal the reason why the starch binder can improve the cycle performance, the phase changes and morphology of the sulfur cathodes during cycling were examined by SEM and XRD, as shown in Fig. 3 and 4, respectively.

Fig. 3 Surface morphology of the sulfur cathodes at the charged state during cycling. (a) The pristine sulfur/starch cathode. (b) The pristine sulfur/PVDF cathode. (c) The sulfur/starch cathode after the 1^st charge. (d) The sulfur/PVDF cathode after the 1^st charge. (e) The sulfur/starch cathode after the 100^th charge. (f) The sulfur/PVDF cathode after the 100^th charge.

Fig. 4 XRD patterns of the cathodes with the different binders after (a) the 1^st charge and (b) the 100^th charge.

In Fig. 3a and b, larger granular aggregates are observed in the sulfur/starch cathode, which can explain why the sulfur/starch cathode exhibits a lower polarization but a relatively lower initial specific capacity. More severe agglomeration may result from the greater wettability of carbon black and sulfur particles in water than in NMP, suggesting that more vigorous stirring or other more effective dispersion methods are needed when the starch binder is used. In Fig. 3f, many more cracks or ruptures are observed on the surface of the sulfur/PVDF cathode after the 100^th cycle compared with those after the 1^st cycle, whereas only a few are observed for the sulfur/starch cathode. The exfoliation of the carbon matrix was also observed on the surface of the sulfur/PVDF cathode after 100 cycles, as zones A and B show. When a part of the carbon matrix exfoliates from the current collector, the active particles in the exfoliated part, sulfur or Li₂S, become electrochemically active, which results in a loss of capacity. Besides, the cathodes also lose a part of the carbon matrix which can provide a conductive area and reaction space for the deposition of the discharge products. Finally, the exfoliation of the carbon matrix leads to the capacity fading of the sulfur/PVDF cathode. By contrast, in Fig. 3e, almost no differences are observed on the surface of the sulfur/starch cathode during cycling. The starch binder seems to be beneficial to the integrity of the sulfur cathode, making it tougher and more resistant to the stress induced by the volume change during cycling. This integrity undoubtedly benefits the cycle performance of the sulfur cathode.

From Fig. 4, it can be seen that for the sulfur/PVDF cathodes, three peaks centred at 2θ = 26.9°, 31.3°and 53.1° appear after the 100^th charge. Although they have a low intensity, they can still be identified as the characteristic peaks of Li₂S. This result is surprising because Li₂S, as the discharge product of sulfur, should not have remained on the charged cathodes if the charge process had been finished completely. Meanwhile, more sulfur element is detected on the surface of the sulfur/PVDF cathode after the 100^th charge (Fig. S3†). Undoubtedly, these inactive discharge products will also cause the loss of active materials and further lead to the capacity fading of the sulfur/PVDF cathode. As is well known, the full utilization of active materials depends on whether there are enough electron and ion migration paths near the reactive sites. For irreversible Li₂S, the migration of ions is not the limiting step, since the residual Li₂S is always in contact with the electrolyte. Thus, the accumulated Li₂S may be related to the decreasing electrical conductivity of the cathode during cycling, which has been observed by Aurbach.⁴⁷

Now, we focus on the reason why the electrical conductivity decreases during cycling. Considering the remarkable effects of the binder on the cycle performance in our experiment, the properties of the binders in the sulfur cathodes, especially those related to stability, should be paid more attention. Perhaps we need to consider the swelling of the binders in the electrolyte used. Generally speaking, if a binder swells or even dissolves in the electrolyte, its strength and adhesion performances will degrade seriously. On the one hand, with this degradation of the binder, the structure of the cathodes weakens and becomes incapable of enduring the volume change in the active materials and particles during cycling, even making the active materials or conductive agents exfoliate, just as shown in Fig. 3f. On the other hand, it also weakens the electrical connection of the active particles remaining on the surface of the current collector, and thus negatively affects the completion of the electrochemical reactions. In particular, in lithium sulfur batteries, the incompletion of the reoxidation of solid Li₂S will lead to the accumulation of inactive Li₂S in the fully charged cathode, just as shown in Fig. 4. Fig. 5 shows the swelling ratio of PVDF and the starch binder in a typical ether electrolyte used in lithium sulfur batteries and a carbonate electrolyte used in commercial lithium ion batteries. Compared with the starch binder, PVDF exhibits a much larger amount of swell in the ether liquid. Empirically speaking, it may result from a similar polarity (ε_DME = 7.075,⁴⁸ ε_DOL = 7.13,⁴⁸ ε_PVDF = 4.5–9.5,⁴⁹ ε_starch = 1.68, ε = dielectric constant) to the ether electrolyte. From Fig. 5b, it can be seen that PVDF suffers from a much milder swell in the carbonate electrolyte than in the ether electrolyte. Meanwhile, considering that the active materials used in commercial lithium ion batteries, such as LiCoO₂, LiFePO₄ and graphite, usually undergo very small volume changes (<10%) and never inhibit the binders commonly used in lithium ion batteries,^50–53 the mild swell of PVDF in the carbonate liquid will not deteriorate the cycle performance of the commercial electrodes in lithium ion batteries. However, in lithium sulfur batteries, a serious swell of PVDF in the ether electrolyte and a large volume change (∼80%) in sulfur during the electrochemical reaction make it difficult for PVDF to keep the integrity of the electrodes.⁵⁴ Even after 24 h in the coin cell filled with the ether electrolyte, obvious exfoliation of the active materials was observed on the sulfur/PVDF cathode, but no similar phenomenon was found on the sulfur/starch cathode soaked in the ether liquid or on the sulfur/PVDF cathode soaked in the carbonate liquid (Fig. S4†), in agreement with the swelling results of these binders shown in Fig. 5. Although the swelling rates of the binders in the cathodes are not precisely equivalent to those of the binder sheets used due to different shapes, the swelling results can still qualitatively explain why the starch binder can stabilize the sulfur cathode and enhance its cycle performance. This phenomenon, that the swelling of the binders greatly affects the cycle performances of the electrodes, is also found in other rechargeable batteries. For example, too much hydrophilic binder will deteriorate the cycle performance of Ni–H batteries because of the high swelling rates of the hydrophilic binders in an alkali liquor. Furthermore, the reason why binder-free sulfur cathodes usually have good cycle performances can also be understood easily.^55–59

Fig. 5 Weight change of the starch binder and PVDF in liquid electrolytes. (a) Ether electrolyte. (b) Carbonate electrolyte.

4. Conclusions

In summary, a starch binder was developed to improve the cycle performance of a low cost sulfur cathode with a high sulfur content. By using the starch binder, the sulfur cathode exhibits a specific capacity retention of ∼90% at 0.2 C after 200 cycles, corresponding to a small capacity decay of 0.05% per cycle. It is believed that these improvements result from the lower amount of swell of the starch binder in ether electrolytes compared with PVDF. The starch binder can thus stabilize the structure of the cathodes and avoid the exfoliation of the carbon matrix and the accumulation of irreversible Li₂S. Besides, compared with traditional PVDF, the starch binder exhibits additional advantages, such as abundant sources, environmental benignity and low cost. Meanwhile, understanding the relationship between the cycle performances of the sulfur electrodes and the binders will provide a guide to designing high cycle performance electrodes for not only lithium sulfur batteries but also other electrochemical batteries using liquid electrolytes. In the future, more binders with high stabilities in ether electrolytes, such as polyvinyl alcohol and other water soluble binders,⁶⁰ can be considered as the potential binders for lithium sulfur batteries.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra10953h

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