The demand for developing next generation Li-ion batteries (LIBs) with high energy density and long cycle life drives the research toward alternative electrode materials with larger specific capacities. Among the several materials proposed to replace graphite (372 mAh g^-1) as an anode material, antimony (Sb) and tin (Sn) have long been considered as an attractive replacement due to their superior lithium storage capacities of 660 mAh g⁻¹ and 994 mAh g⁻¹, respectively ^{1, 2}. Unfortunately, the electrochemical performance of these materials fades during cycling because of the electrical contact loss induced by large volume variations combined with the continuous growth of a solid electrolyte interphase (SEI) layer at the surface hindering the electron transfer. The use of multi-phase active materials, such as SnSb, instead of pure metals (Sn or Sb) has been used to limit the large volume change by reacting with Li⁺ at different stages during the charge and discharge process ¹. Unfortunately, the alloying/dealloying reaction is still accompanied by a volume change which results in the loss of electrical contact with the active material, Sn agglomeration, and the formation of SEI film on the electrode surface leading to poor electrochemical performance ^{3, 4}.

In the present work, micron-sized SnSb coated with a thin thermoplastic elastomer (TPE) is investigated as anode electrode for LIBs. The high failure strain of TPE is a very desirable feature for rechargeable LIBs as it improves the lifetime of high specific capacity anode materials that undergo mechanical fractures induced by large volume variations. Accordingly, an elastomer showing high strain property and good adhesion to the surface of the active material has been designed. Highly rubbery PS-b-PHEA was synthesized by the nitroxide meditated polymerization (NMP) method using a specific polystyrene macroinitiator allowing the presence of long hydroxyethyl acrylate blocks.

Figure 1a shows the discharge capacity vs cycle number for the non-coated SnSb and the PS-b-PHEA-coated SnSb cycled at C/10 (i.e. 0.0827 A g^–1). It is clearly apparent that the PS-b-PHEA-coated SnSb has superior cyclability than the non-coated SnSb with a reversible capacity of 720 mAh g^–1 obtained, whereas only 356 mAh g^–1 was retained after 50 cycles for the non-coated SnSb electrode. The remarkable stable capacity is attributed to the elastomer polymer film at the surface of SnSb. Indeed, highly stretchable PS-b-PHEA is able to bear the mechanical stress caused by the large volume variations and thus maintains good quality at the electrode/electrolyte interface and the inter-grains electronic percolation pathways during successive charging and discharging of the battery as well as the limiting the continuous growth of the SEI. Moreover, excellent capacity reversibility was achieved when cycled at fast kinetics (Figure 1b) and multiple C-rates (Figure 1c) confirming the strong protection role of the polymer. The advanced chemical and mechanical properties of PS-b-PHEA open up promising perspectives to significantly improve the electrochemical performance of all electrodes that are known to suffer from large volume variations.

Figure 1. Discharge capacity versus the cycle number for PS-b-PHEA-coated SnSb and non-coated SnSb at a rate of (a) C/10 over 50 cycles and (b) 1C over 100 cycles, (c) cycling performance of PS-b-PHEA-coated SnSb at multiple C-rates.

References

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2. P. Antitomaso, B. Fraisse, M. Sougrati, F. Morato-Lallemand, S. Biscaglia, D. Aymé-Perrot, P. Girard and L. Monconduit, J. Power Sources, 2016, 325, 346-350.
3. A. T. Tesfaye, Y. D. Yücel, M. K. S. Barr, L. Santinacci, F. Vacandio, F. Dumur, S. Maria, L. Monconduit and T. Djenizian, Electrochimica Acta, 2017, 256, 155-161.
4. L. Ji, M. Gu, Y. Shao, X. Li, M. H. Engelhard, B. W. Arey, W. Wang, Z. Nie, J. Xiao and C. Wang, Adv. Mater. , 2014, 26, 2901-2908.