Controllable Preparation Of Conductive Polymer-coated Silicon-based Composites And Their Electrochemical Properties

Driven by the rapid growing field of hybrid and electric vehicles, there is an increasing demand for lithium-ion batteries(LIBs). Silicon is regarded as one of the most promising anodes materials for Li-ion batteries due to its high theoretical specific capacity(3579 m Ah·g-1), low discharging potential, eco-friendly properties and safety. However, the Si-based anode suffers from the huge volume change during lithium ion insertion and extraction process, which results in the pulverization and unstable growth of solid-electrolyte interface(SEI) film on the surface of Si. Moreover, the poor electrical conductivity of Si is another challenge for the practical application of Si-based anodes. Recent works we made is to overcome two disadvantages of poor electrical conductivity and severe volume variation of Si. The article is mainly focus on the preparation and performance of Si-G, Si-G-PANI and Si-SA-PANI composites. X-ray diffraction(XRD), X-ray photoelectron spectroscopy(XPS), Raman spectrum, scanning electron microscope(SEM) and transmission electron microscope(TEM) are used to characterize the structure of the various composites. In addition, the electrochemical performances of the composites are characterized by cyclic voltammtery(CV), electrochemical impedance spectroscopy and galvanostatic charge-discharge measurements. The main thesis is consisted of the following parts:(1) Synthesis of graphene-like grafted Si nanoparticles(Si-G) composites: First, the Si nanoparticles were uniformly dispersed in liquid-polyacrylonitrile(LPAN) and the mixture was transformed from liquid to solid by stabilization at 220 ?. Then, a thin layer of graphene sheets was formed over the surface of Si nanoparticles by the pyrolysis of LPAN, which not only improve conductivity but also constrains the serious mechanical stress caused by the volume expansion of Si. The ratio of Si and LPAN is consisted of 2:1, 2:2 and 2:3, and the corresponding Si-G composites are referred to 2Si-1G, 2Si-2G and 2Si-3G. These three composites show much better cycle performance than the pure Si. What's more, the 2Si-3G electrode maintained a specific capacity of ~800 m Ah·g-1 after 50 cycles at the density of 0.5 A·g-1.(2) A novel approach to fabricate three-dimensional(3D) network structure of silicon/graphene-like/polyaniline(Si-G-PANI) has been developed. First, the graphene-like sheet was first coated on the surface of Si from a pyrolysis reaction of LPAN. Subsequently, Si-G-PANI composite was obtained through an in-situ polymerization, the PANI layer is tightly bounded to graphene-like owing to an enhancement of ? conjugation between the PANI and graphene-like nanosheets. Herein, the graphene-like nanosheets improve the dispersion and conductivity of composites, and also constrain the serious mechanical stress caused by the volume expansion of Si. PANI layer offers a 3D conductive and protective network to allow the volume variation of Si during cycling, as well as facilitate the electron and lithium ion transfer processes. With the dual protection of graphene-like nanosheets and PANI, the Si-G-PANI electrode can deliver a reversible specific capacity of 1001.8 m Ah·g-1 at the current density of 1.5 A·g-1 after 50 cycles.(3) Preparation of Si-SA-PANI composite: A known amount of SA, aniline and Si nanoparticles were added to deionized water under stirring. By using the in situ polymerization of aniline in an aqueous of sodium alginate, we prepared Si-SA-PANI composite. In this structure, SA and PANI fabricate the reinforced conducting 3D network structure to tolerate the volume swelling of Si nanoparticles. The conducting SA-PANI hydrogels were reinforced by the entanglement of SA and PANI molecular chains and inter/intramolecular groups in the SA structure, because of the large number of hydroxyl and carboxyl groups in SA. Thus, the SA-PANI composite not only improve the conductivity, but also fabricate the 3D network to accommodate volume expansion. The Si-SA-PANI composites can keep a stable cycling performance at high current density,delivering a high capacity of 1031.9 m Ah·g-1 at the current density of 1.0 A·g-1 after 50 cycles.