Tisnsb, a Promising Anode Material for Li-Ion Batteries? the Role of the Electrode/Electrolyte Interphase

Conversion materials for Li-ion batteries, such as Sb¹ and Sn-based² compounds, have attracted much intense scientific attention for their high storage capacities. Among conversion materials, TiSnSb has been developed as a negative electrode for Li-ion batteries.³ This material can reversibly take up 6.5 Li per formula unit which corresponds to a specific capacity of 580 mAh/g with noteworthy high rate capabilities.⁴ As the working potential of this active material is out of the electrochemical window of classical electrolytes like alkylcarbonates mixtures, the formation of a protective and stable passivation film (the solid electrolyte interphase or SEI)⁵ is required. As a large volume change occurs during lithiation/de-lithiation of a conversion material, a dynamic and unstable behavior of the SEI layer is expected upon cycling⁶, which will drastically limit the cycle life of the electrode.

At this time, little information can be found about the formation and composition of the SEI layer on conversion electrodes. With the aim to study the electrode/electrolyte interphase, X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS) were performed on TiSnSb electrodes in association with electrochemical studies. In order to improve the performances of this material, SEI builder additives have been added to the standard electrolyte (1M LiPF₆in EC/PC/3DMC). As vinylene carbonate (VC) and fluoroethylene carbonate (FEC) have been already recognized as efficient SEI builders for graphite and silicon negative electrodes, they have been employed in this study to modify the SEI composition and morphology.

XPS quantitative analysis and EIS results, lead first to the conclusion that the thickness and the resistance of the SEI layer formed at the TiSnSb surface was lower at high cycling rate (4C) than low cycling rate (C/2). Adding additives, like FEC to the standard electrolyte containing already VC, has also a strong impact on the SEI formation: FEC/VC-containing electrolyte form a thinner SEI layer and the corresponding impedance measurements supported the same conclusion. Moreover, the partial dissolution of the SEI layer occurs during the electrode de-lithiation⁷, and the continuously growing in thickness of the SEI layer upon cycling confirms the dynamic and unstable behavior of the SEI layer already reported for other conversion materials. Nevertheless, the use of both VC and FEC additives permits to limit the SEI growth and  to perform extensive cycling before failure (500 cycle).

In order to overcome the SEI growth drawback, a pyrrolidinium based ionic liquid (IL) has been used as electrolyte instead of the alkylcarbonates mixture. As a matter of fact, the IL-containing electrolyte was found not only to improve the cycling performances of TiSnSb electrode but also to decrease the cumulative capacity losses upon cycling. XPS studies reveal that the nature of the SEI species which are responsible of the enhanced cycleability.

In conclusion, an efficient way to improve the cycling performances of TiSnSb, and possibly other conversion electrodes, is to modify the electrolyte formulation. The use of a more stable IL as solvent or co-solvent and adding efficient additives are the best means to stabilize the SEI layer which is strongly related to the specific capacity and cycleability of this type of active material.

References :

1. L. Monconduit et al., J. Power Sources, 107 (2002) 74.

2. J. Wolfenstine et al., J. Power Sources, 109 (2002) 230.

3. H. A. Wilhelm et al., Electrochem Commun24 (2012) 89.

4. Peled, E. J. Electrochem. Soc.126 (1979) 2047.

5. C. Marino et al., J. Phys. Chem. C117 (2013) 19302.

6. M. Stjerndahl et al., Electrochim. Acta 52 (2007) 4947.