Functionalized graphene-based materials for li-ion batteries : from anode to cathode
Date of Issue2018
School of Physical and Mathematical Sciences
Owing to the 2D geometrical structure and extraordinary physicochemical properties, graphene and its derivatives have gained intense attentions in application as the promising alternatives of current graphite anode for rechargeable Li-ion batteries (LIBs). However, the Li storage performance of graphene-based materials is extremely sensitive to their morphology and microstructure. In this PhD project, using chemical approaches, we focus on these two aspects for rational design and fabrication of functionalized graphene-based materials, including both anodes and cathodes, with boosted performance for Li storage. In the meantime, we also aim to contribute to the understanding and exploration of graphene-based materials for next generation LIBs. Firstly, heteroatom doped graphene anodes include N- and S-codoped graphene (NS-G) and N-doped porous graphene (NPG) are fabricated via organic chemistry enabled in situ doping strategies. For the preparation of NS-G, 2-aminothiophenol, functions as both N and S sources, was firstly covalently bonded to graphene oxide (GO) through a polyphosphoric acid catalyzed cyclization reaction. The resultant product was subsequently annealed in Ar atmosphere to achieve NS-G with a doping content of 1.76 at% for N and 0.86 at% for S. While in the case of NPG, a hydrogen-bonded supramolecular polymer consisting of N-enriched molecules, namely, melamine and cyanuric acid, was applied to modify GO. This polymer could be completely decomposed and release a myriad of N-containing gases during the later thermal treatment, which results in N doping and the generation of porous structures in graphene simultaneously. With the merits of unique structure and valuable dopants (N,S-codoping or high N doping level), both NS-G and NPG display excellent anodic performance for LIBs, that is, stable cycling, high reversible Li storage capacity and good rate charge-discharge capability. Secondly, ultra-small SnO2 nanoparticles (~5 nm) decorated graphene was produced via a facile one-step redox reaction between Sn2+ and GO in aqueous solution. The nanoparticles are found to be uniformly deposited on graphene with a homogeneous size distribution in the presence of a cationic surfactant — hexadecyltrimethylammonium bromide (HTMAB), whereas inhomogeneously and randomly deposited in the case of without HTMAB. In sharp contrast to the previous method for constructing SnO2/graphene hybrid, our newly developed one is more appealing and effective because of its simplicity and does not require any further thermal treatment to obtain SnO2 with high crystallinity. Impressively, as an anode for LIB, the hybrid shows remarkable performance with good capacity retention (~88%) in 120 consecutive cycles. Thirdly, chemical modulation of the oxygen-based groups on GO are realized via chemoselective removal of the specific functional groups. For example, carbonyl/ hydroxyl functionalized GO (C/HGO) was prepared by reacting GO with n-Butyllithium, and decarboxylated GO (DCGO) was obtained through silver(I)-catalyzed decarboxylation of GO. The resultant C/HGO and DCGO both exhibit better Li storage performance than GO when evaluated as the cathodes for LIBs in the voltage between 1.5 and 4.5 V (vs. Li/Li+). Specifically, C/HGO shows the best cathodic performance with a stable capacity of ~170 mAh g-1 (current rate: 100 mA g-1), which documents the engineering of oxygen functional groups can significantly decrease the irreversible reaction between Li ions and the oxygen-based groups. Lastly, a general and versatile method involving ultrasonication and hydrothermal treatment was developed for scalable preparation of various vat dye/graphene composites. The vat dyes, i.e., Vat Green 8 (VB 8), Vat Brown BR (VB BR) and Vat Olive T (VO T), are typical carbonyl compounds with abundant electroactive carbonyl groups bonded to condensed aromatic rings, however, their densely stacked structure stemming from strong intermolecular interactions, associated with low electrical conductivity result in sluggish kinetics for electrochemical reactions. After sonication-induced disassembly and then hydrothermal-promoted reassembly processes, a series of compatible hybrids with vat dye molecules absorbed on graphene sheets were produced, leading to considerably increased conductivity and accessibility for electrolyte as compared to the pristine vat dyes. Due to the novel structure, the hybrids, namely, VB 8/graphene, VB BR/graphene and VO T/graphene, all show exceptional cathodic performance for Li storage with long cyclic durability and high specific capacity.