Elsevier

Electrochimica Acta

Volume 53, Issue 24, 15 October 2008, Pages 7084-7088
Electrochimica Acta

Application of gelatin as a binder for the sulfur cathode in lithium–sulfur batteries

https://doi.org/10.1016/j.electacta.2008.05.022Get rights and content

Abstract

Gelatin, a natural biological macromolecule, was successfully used as a new binder in place of poly(ethylene oxide) (PEO) in the fabrication of the sulfur cathode in lithium–sulfur batteries. The structure and electrochemical performance of the two types of sulfur cathodes, with gelatin and PEO as binders, respectively, were compared in 1 M LiClO4 DME/DOL (V/V = 1/1) electrolyte. The results showed that the gelatin binder had multifunctional effects on the sulfur cathode: it not only functioned as a highly adhesive agent and an effective dispersion agent for the cathode materials, but also an electrochemically stable binder. The gelatin binder-sulfur cathode achieved a high initial capacity of 1132 mAh g−1, and remained at a reversible capacity of 408 mAh g−1 after 50 cycles, all of which were better than with the PEO binder-sulfur cathode under the same conditions.

Introduction

The use of elemental sulfur as a potential active material for rechargeable lithium–sulfur batteries has attracted more attention with the rapid development of secondary rechargeable batteries. Lithium–sulfur batteries, which are composed of a composite positive electrode (cathode), a polymer or liquid electrolyte, and a lithium negative electrode (anode), are expected to exhibit a high theoretical specific capacity of 1675 mAh g−1 based on the lithium/(elemental) sulfur redox couple [1], [2]. In addition, elemental sulfur benefits from advantages such as natural abundance, low cost and non-toxicity [3], [4].

However, the development of a rechargeable lithium–sulfur battery using a liquid electrolyte has a number of problems to overcome, such as low-active material utilization due to the insulating nature of sulfur, a poor cycle performance with agglomeration, and poor electrical contact between the sulfur and conductive carbon after the charge–discharge process [2], [5], [6], [7], [8], [9], [10], [11], [12], [13]. To mitigate the blockages, Wang et al. reported that the Li/S battery has a high specific capacity at room temperature when particles size of sulfur less than 10 nm [8]. Visco et al. proposed the concept of mixed ionic electronic conductor (MIEC) coating on the surface of the sulfur cathode [14] and Han et al. used multi-walled carbon nanotubes (MWNTs) to improve electrical contact between active mass and electrical conductor [15]. Besides, Jeon et al. indicated that the morphology changes of sulfur cathode, which occurs during the charge–discharge cycles, also causes poor cycle life and sulfur utilization [9]. In addition, the uniform combination of active sulfur and conductive carbon is very important for a high performance sulfur cathode.

Among the components in the sulfur cathode, the binder plays an important role in improving cell performance, especially in regards to the cycle life. A high performance binder should have high adhesion ability for the electrode materials to the current collector, as well as the ability to form a good electric network between the active material and conductive carbon, to facilitate the electron transport as well as the diffusion of the lithium ion. Some research works on the binder have been previously carried out, such as Kim et al. used PTFE + CMC as binder to improve the capacity and sulfur utilization of sulfur cathode [16], and Jung et al. developed a mixed polymer binder system of PVP and PEI to improve the cycle performance [17]. Choi et al. studied the cycling property of poly(ethylene oxide) (PEO) and poly(vinylidene fluoride) (PVDF) binder with carbon nano-fiber [18]. Based all the studies of lithium–sulfur battery system, PEO and PVDF are the most commonly used binder, but both of them still have revealed some problems. The PEO-based sulfur cathode usually presents poor adhesion properties [19], [20] and low ionic conductivity at low temperature [21]. The PVDF binder is readily dissolved in the organic electrolytes, especially at elevated temperatures, resulting in an increase in the interfacial resistance [22]. Furthermore, the binder must be dispersed in some high boiling solvent such as N-methyl-2-pyrrolidone (NMP), which tends to be difficult to vaporize, so the drying process of the cathode might lead to the loss of active sulfur at temperatures higher than 80 °C under vacuum.

In this work, a water-soluble gelatin macromolecule was adopted as the binder for the elemental sulfur cathode. This gelatin was formed by the covalent linkage of several amino acids into a stable peptide, where the sequence of the peptide chain was determined by the protocol used in its production. As a biological macromolecule, gelatin has some advantages for the fabrication of the sulfur cathode. First, as an ampholytic polymer with ionizable groups such as COOH and NH2, gelatin shows great hydrophilic properties and is substantially insoluble in commonly used organic electrolyte solvents, such as carbonic acid esters and ether, which can keep the electrode stable. Its aqueous solution has a high viscosity, which makes it suitable as an adhesion agent for bonding different types of small particles onto substrates [23], [24], [25]. Second, gelatin serves as a strong dispersion agent, and as such is widely used in the food, photographic and pharmaceutical industries. Therefore, we expect it to promote the dispersion of the cathode materials [26]. In addition, gelatin is a cheap and environmentally friendly material that can be obtained through collagen hydrolysis.

In this study, we used gelatin as a new binder in the preparation of a well-dispersed sulfur cathode, and investigated its electrochemical performance in detail. A PEO binder-sulfur cathode was also made for comparison purposes.

Section snippets

Preparation and analysis of the cathode

Elemental sulfur (99.5%, analytically grade, Beijing, China) and acetylene black (AB, Jinpu. Corp., China) were dried at 60 and 120 °C for 5 h under vacuum before use, respectively. Gelatin (160Bloomg, type B, derived from bovine bones) and PEO (Mw = 5 × 106, Aldrich) were used as binders for the gelatin binder-sulfur cathode and the PEO binder-sulfur cathode, respectively. Both cathodes were composed of the same proportion of elemental sulfur, acetylene black and binder as shown in Table 1. However,

XRD analysis of the cathode

The XRD patterns observed for the SGA cathode and its compositions are shown in Fig. 1. It is clearly seen that the XRD pattern of gelatin shows the characteristic of an amorphous structure, with a broad peak centered at 2θ = 20°, while the acetylene black, which is also in an amorphous form, shows a broad peak at 2θ = 25°. Analysis of the elemental sulfur provides two prominent peaks at 23.4° and 28.0°, which correspond to Fddd orthorhombic structure [27], [28]. The pattern of the SGA cathode

Conclusions

We have successfully used gelatin as a binder in the design and development of a sulfur cathode. It was found that the gelatin had multifunctional effects on the sulfur cathode in the lithium–sulfur cell: it was not only a high adhesion agent and a strong dispersion agent for the cathode materials, but also an electrochemically stable binder. The gelatin binder-sulfur cathode achieved a high initial capacity of 1132 mAh g−1, and remained at a reversible capacity of 408 mAh g−1 after 50 cycles, all

Acknowledgement

The authors are grateful for funding from National 863 projects (no. 2007AA03Z223).

References (31)

  • D. Marmorstein et al.

    J. Power Sources

    (2000)
  • H. Yamin et al.

    J. Power Sources

    (1983)
  • Y.M. Lee et al.

    J. Power Sources

    (2003)
  • B. Jin et al.

    J. Power Sources

    (2003)
  • A. Hayashi et al.

    Electrochem. Commun.

    (2003)
  • J.L. Wang et al.

    Electrochim. Acta

    (2003)
  • B.H. Jeon et al.

    J. Power Sources

    (2002)
  • J.H. Shin et al.

    Mater. Sci. Eng. B

    (2002)
  • N.-I. Kim et al.

    J. Power Sources

    (2004)
  • Y. Jung et al.

    Electrochem. Commun.

    (2007)
  • F. Croce et al.

    Solid State Ionics

    (2000)
  • S.S. Zhang et al.

    J. Power Sources

    (2002)
  • M. Bele et al.

    Colloids Surf. A

    (1998)
  • M. Bele et al.

    Colloids Surf. A

    (2000)
  • X.J. Zhu et al.

    J. Power Sources.

    (2005)
  • Cited by (0)

    View full text