With specific energies of 400–500 Wh kg^-1 Li-Sulphur (Li-S) batteries are promising candidates for energy storage applications. Challenges that still impede their commercialization are the expansion of the cathode during discharge, low conductivity, operational instability due to side reactions and slow Li⁺ transport [1]. In the presence of Li⁺, sulphur undergoes a multistep reduction, yielding Li₂S as an end product. Practically, however, the highly reactive and soluble sulphur intermediates (polysulfides) inevitably form and start their activity as a redox shuttle or/and participate in side reactions. These processes follow a complicated mechanism, which clarification requires a comprehensive analysis [1-3].

This paper will discuss recent results from the authors' labs on the multi-electron transfer of sulfur as a function of the electrolyte composition. Additionally, we have investigated the influence of cathode binder type (e.g. Polyvinylidene difluoride-PVDF, Carboxymethylcellulose-CMC, Guar gum - GG) on the cycling performance of the cathodes.

Our methodology involves a combination of electrochemical (galvanostatic cycling, cyclic voltammetry, impedance spectroscopy- EIS) and spectroscopic methods (X-ray Photoelectron Spectroscopy, XPS; Electron Paramagnetic Resonance, EPR), useful for analysis of electrolyte and interface stability. We will show that the increase of lithium bis(trifluoromethylsulfonyl)imide (Li[Tf₂N]) concentration displays a valuable impact on the cycling stability(Fig1A,B), suppressing the shuttle effect and improving capacity retention of the sulfur-lithium cells. Moreover, the polysaccharide type binders present better cycling performance in terms of capacity retention and current efficiency (Fig1A,B).

To understand the origin of this improvement we characterized the chemical composition of the cycled electrodes by means of XPS. Additionally, the role of small amount 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][Tf₂N]) was also studied. Fig.1C,D compares the Li1s and S2p experimental core level peaks from three different anode samples. Sulphur displays multicomponent spectra, in which Li₂S (S2p_3/2~160eV), disulfides (S2p_3/2~162eV), sulfites (S2p_3/2~167eV) and sulfates (S2p_3/2~169eV) can be distinguished [4]. Their comparison reveals difference in the composition on the surface and in the bulk of the passivating layer, when lower concentration of Li[Tf₂N] was used, independent on the presence of [BMP][Tf₂N] (Spectra 1 and 3). The later however, facilitate the overall decrease of sulphur quantity in the bulk (Spectra 3). Contrary, higher Li[Tf₂N] concentration shows nearly identical spectral features before and after sputtering (Spectra 2), suggesting homogeneous in-depth elemental distribution in the interfacial layer. We anticipate that the Li1s core level peaks originate from at least three Li-containing species (Fig.1C), which close proximity in the binding energy scale leads to spectral broadening (FWHM ~3eV, Fig1, Li1s-Panel).

The influence of the electrolyte composition on the electrical properties of Li-electrolyte interface was studied by impedance spectroscopy using a symmetric cell [5]. This method allows an individual assessment of the Li|electrolyte interface without interferences from the reference electrode position and the presence of cathode. The properties of the anode interface are correlated with their surface chemical composition, analyzed by XPS.

By using EPR we studied the paramagnetic species formed at the early lithiation stage, where typically just long chain polysulfide anions are present. Our studies demonstrate generation of at least one sulphur-centered radical, which suggests that disproportionation accompany the sulfur lithiation reaction [6]. We will discuss the quantity, the life-time and the coordination geometry of the detected radicals.

References:

1. Y. Yin, S. Xin, Y.Guo, L. Wan, Angewandte Chemie, International Edition 52 (2013) 13186-13200.

2. C. Barchasz, F. Molton, C. Duboc, J. Leprêtre, S. Patoux, F. Alloin, Anal. Chem. 84 (2012) 3973-3980.

3. Y. Lu, Q. He, H. Gasteiger, J. Phys. Chem. C, 118, (2014) 5733-5741.

4. C.D.Wagner, A.V. Naumkin, A. Kraut-Vass, J.W.Allison, C.J.Powell and J.R.Rumple, Jt., NIST X-Ray Photoelectron Spectroscopy Database, NIST Standard Reference Database 20, Version 3.5, http://srdata.nist.gov/xps/

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6. Q. Wang, J. Zheng, E. Walter, H. Pan, D. Lv, P. Zuo, H. Chen, Z. Deng, B. Liaw, X. Yu, X. Yang, J. Zhang, J. Liu, J. Xiao, J. Electrochem. Soc., 162 (2015) A474-A478