High energy and power densities are required for lithium-ion batteries (LIBs) for electric vehicle (EV) applications.^{1, 2} To meet the above mentioned requirements, silicon (Si), as an alloy-type material, has attracted great attention due to its extremely high theoretical gravimetric capacity (~4200 mAh g^-1), relative low discharging potential (0.2 V versus Li/Li⁺), environmental friendliness, and natural abundance.^{3, 4} However, Si-based anodes undergo a huge volume variation upon lithiation and delithiation (~400%) that would lead to pulverization of active materials, loss of electrical contact, unstable electrolyte interphase on silicon surface and consequently, capacity fading.⁵ Previous studies have revealed that poly(vinylidene fluoride) (PVDF), the most common binder in making the electrode, is not good enough for Si-based anode due to its very weak wan der Waals interaction with Si.^{6, 7} Other functionalized binders are highly demended to improve the perforance and stability of Si-based electrodes.

In this study, polyaniline (PANI) based conductive binder were developed by in-situ polymerization in acidic solution at ~ 0 ^oC in the presence of poly(acrylic acid) (PAA). The structure of obtained composites were characterized by Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy. Si nanoparticles and PANI/PVA composite were mixed together in a weight ratio of 6:4. The electrode was fabricated by coating the mixture onto a copper foil (current collector) and dried under vacuum at 100 ^oC for 10 h. To measure the electrochemical performance, coin cells were assembled inside an Ar-filled glove box by using a lithium-metal plate as the counter and reference electrode, microporous polypropylene as the separator and 1 M LiPF₆in ethylene carbonate (EC)–diethyl carbonate (DEC) solvent (1:1 v/v) with FEC additives as electrolyte.

The assembled coin cells were activated at a current density of 200 mA/g and cycled at a current density of 4000 mA/g (1 C). The electrode showed a stable cycle performance for 100 cycles at a current density of 1 C (Figure 1a). This great improvement can be attributed to the enhanced strain compatibility (Figure 1b), great adhesive property and high elasticity of this novel binder.

References

1. Midilli, A.; Dincer, I.; Ay M. Energy Policy 2006, 34, (18), 3623-3633.
2. Armand, M.; Tarascon, J. M. Nature 2008, 451, (7179), 652-657.
3. Chan, C. K.; Peng, H.; Liu, G.; McIlwrath, K.; Zhang, X. F.; Huggins, R. A.; Cui, Y. Nature nanotechnology 2008, 3, (1), 31-35.
4. Reece, S. Y.; Hamel, J. A.; Sung, K.; Jarvi, T. D.; Esswein, A. J.; Pijpers, J. J.; Nocera, D. G. Science 2011, 334, (6056), 645-648.
5. Wu, H.; Cui, Y. Nano Today 2012, 7, (5), 414-429.
6. Su, X.; Wu, Q.; Li, J.; Xiao, X.; Lott, A.; Lu, W.; Sheldon W. B.; Wu, J. Advanced Energy Materials 2014, 4, (1).
7. Erk, C.; Brezesinski, T.; Sommer, H.; Schneider, R.; Janek, J. ACS applied materials & interfaces 2013, 5, (15), 7299-7307.

Figure 1. (a) Cycle performance of silicon nano-particles using PANI based conductive polymer composites as binder. (b) Illustration of a possible reason why conductive binder can improve the cycle performance of silicon.