Si has been considered as one of the most promising anode materials for the next-generation Li-ion batteries (LIBs) because of its ultrahigh theoretical capacity and low lithiation potential. However, the industrial application of Si anode is hampered by its huge volume change during the lithiation/delithiation process. The huge volume variation leads to the pulverization of Si particles, the over-growth of the solid electrolyte interphase (SEI) layer, loss of electrochemical contact, thus yields fast deterioration of capacity. In addition, the low intrinsic electrical conductivity is also an obstacle for Si anode with excellent electrochemical performances. To improve the cycling stability and rate performance, Si is often combined with advanced carbon materials such as carbon nanotubes (CNT), graphene (G), and so on to construct Si/C composites. However, these advanced carbon materials are limited by their high cost and low output and cannot meet the requirements of industrial applications. In this work, we presented a novel Si/C composite for lithium storage, using polyvinyl alcohol (PVA) and two-dimensional (2D) chitin nanosheets (CNs) as carbon precursors. CNs were exfoliated from commercial chitin powders through a simple and scalable hydrothermal process. Then CNs, PVA and nano Si were mixed in deionized water at 90 ℃, followed by continuously stirring at this temperature to evaporate the water and obtain the robust intermediate CNs-Si-PVA. Benefiting from the strong hydrogen bonding effect between CNs, PVA and Si nanoparticles, Si nanoparticles were tightly encapsulated by CNs and PVA. After carbonization at 850 ℃ for 3 h in Ar atmosphere, 2DC-Si-C composite was obtained. The phase, structure, morphology and elemental composition of the sample were studied by X-ray diffraction (XRD), thermogravimetric (TG) analysis, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM and TEM results demonstrated that Si nanoparticles were dual-confined by ~12 nm amorphous carbon layer derived from PVA and 2DC derived from CNs, forming a robust and moderate compact structure. Selected area electron diffraction (SAED) and XRD results indicated that Si nanoparticles remained original crystal structure after high temperature carbonization process. Energy dispersive spectroscopy (EDS) mapping and XPS analysis results revealed that 2DC-Si-C contained Si, O, C and N elements. EDS mapping results also demonstrated that Si, O, C and N were distributed uniformly in 2DC-Si-C composite, implying that Si nanoparticles were also evenly dispersed in carbon matrix. TGA results revealed that Si content of 2DC-Si-C was 72.9%. According to N2 adsorption and desorption isotherms, Brunauer-Emmett-Teller (BET) specific surface of 2DC-Si-C was determined to be 175 m2·g-1. The pore size mainly varied between 1.5 and 5 nm. Such structure could effectively buffer the volume variation of Si and lead to the formation of a stable SEI layer, thus enhance the cycling performance. The lithium storage performance of 2DC-Si-C was assessed in CR2032 coin cells assembled in glove-box filled with Ar. The working electrode was prepared by mixing 2DC-Si-C, Super P and sodium alginate in a weight ratio of 70:15:15. The areal loading mass of 2DC-Si-C was 1~1.5 mg·cm-2. A Celgard 2400 polypropylene membrane was used as the separator. The used electrolyte LX025 was purchased from DoDoChem. The electrochemical tests were carried out after letting the assembled cells standed for 8 h to ensure full infiltration of the electrolyte. The galvanostatic charge-discharge measurements were carried out on a Neware battery testing system within the voltage range from 0.01 to 2.5 V. 2DC-Si-C manifested a discharge capacity of 2220, 1502, 1174 and 865 mAh·g-1 at 0.2, 0.5, 1.0 and 2.0 A·g-1, respectively, demonstrating much better rate performance than pure Si anode. The initial Coulombic efficiency of 2DC-Si-C was 78.2%. The relatively low initial Coulombic efficiency could be ascribed to the formation of SEI layer in the first cycle. At a current density of 1.0 A·g-1, 2DC-Si-C composite maintains a discharge capacity 1510, 975, 815 and 765 mAh·g-1, after 10, 50, 200 and 300 cycles, respectively, displaying a much better cycling performance than pure Si and Si-C anodes. The fast capacity decay in the initial 50 cycles of 2DC-Si-C could be attributed to the pulverization of the micron-sized particles obtained by sieving. After 50 cycles, Coulombic efficiency of 2DC-Si-C became higher than 99.5% which could be attributed to the dual-confining of the carbon layers that prevented the direct contact of Si and the electrolyte and stabilized the SEI layer. The highly improved cycling performance and rate capability of 2DC-Si-C could be attributed to the dual-confining effect of 2DC and amorphous carbon layer derived from PVA. The dual-confining of carbon layers could not only buffer the volume change of Si, leading to the formation of a stable SEI layer, but also effectively improve the electronic conductivity. This work demonstrated that 2DC-Si-C was a potential anode material for next-generation LIBs, and low-cost and green biomass could be used as carbon precursor to build high performance Si/C composite. This work also paved a new way for the for the industrial application of seafood waste derived chitin. © 2022, Youke Publishing Co., Ltd. All right reserved.
