Polyaniline/Poly(acrylic acid) Composite Binder for Si Anode in Lithium Ion Batteries
Authors: Yao Yuze , Shao Minhua 
Published: February 2020

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.

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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.

Figure 1