Hydrogel-Derived Three-Dimensional Porous Si-CNT@G Nanocomposite with High-Performance Lithium Storage

Silicon is a promising anode material for lithium-ion batteries (LIBs) because of its natural abundance, high theoretical capacity, and relatively low working potential for lithium storage. However, two main obstacles exist that hinder its commercial application. One is the large volume variation during prolonged cycling, which causes irreversible cracking and disconnection of the active mass from the current collector and subsequently rapid decay of capacity of the electrode. The other is its poor intrinsic electronic conductivity, which seriously restricts its rate performance. To date, strategies to improve its cycling stability and rate capability include rational designs of different Si nanostructures and the incorporation of conductive agents. In this study, we present a novel and effective method to fabricate a Si/C composite. Through hydrogen bonding and the electrostatic interaction between graphene oxides (GO) and acidized chitosans (Cs), a hybrid hydrogel was fabricated in which silicon nanoparticles and carbon nanotubes were encapsulated in situ. Following freeze-drying and subsequent calcination, a three-dimensional porous silicon/carbon nanotube/graphene (Si-CNT@G) nanocomposite was obtained. The phase, structure, and morphology of the sample were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA). The results show that the silicon nanoparticles were uniformly distributed in the graphene network, which was interwoven with carbon nanotubes. The resultant Si-CNT@G nanocomposite featured a porous three-dimensional conductive carbonaceous support, providing short pathways for electrons, conductive transport highways for lithium ions, a sufficient interface for contact of the electrolyte and electrode, and an effective buffer matrix to alleviate structural change during discharge/charge cycling. Benefiting from these particular features, the as-prepared SiCNT@G nanocomposite exhibited superior lithium storage performance with high specific capacity and excellent long-term cycling stability when evaluated as an anode material for LIBs. For example, a high discharge capacity of 673.7 mAh.g(-1) can be retained after 200 discharge/charge cycles at a current density of 500 mA.g(-1) in the potential range of 0.01-1.20 V, with a decent capacity retention of 97%. Even when at a current density of 2000 mA.g(-1) , a high discharge capacity of 566.9 mAh.g(-1) can still be retained. In contrast, the discharge capacity of pure silicon nanoparticles, when tested under the same conditions, was practically nil. These results suggest that the Si-CNT@G nanocomposite is a promising anode material for high-performance LIBs.