Microgel-assisted assembly of hierarchical porous reduced graphene oxide for high-performance lithium-ion battery anodes

Abstract

Graphene has emerged as one of the foremost candidates for replacing graphite anodes in lithium-ion batteries (LIBs) due to its unique physical and electrochemical properties. Most techniques for synthesis of graphene-based electrode materials utilize graphene oxide (GO). However, restacking of GO sheets during common fabrication processes usually results in significant loss of usable Li-insertion sites, and consequently, a substantial decrease in cycling performance of the electrode. In this work, we demonstrate a facile and scalable approach for fabrication of 3D, hierarchical macro/mesoporous reduced graphene oxide (RGO) anodes for LIBs using a polymer sphere (PS) microgel as a template. The synthesis process involves controlled encapsulation of GO sheets on the surface of thermal degradable PS microgels, followed by shrinkage of PS microgels to generate GO wrinkles. The GO-wrapped cross-linked PS swells to a microgel in N-methyl-2-pyrrolidone (NMP), while it shrinks after replacing the NMP with distilled water. The overall specific surface area of the resulting porous/wrinkled RGO with mesopores and macropores, obtained by annealing the wrinkled GO@shrunk PS, increases from 96 m² g⁻¹ to 276 m² g⁻¹; the highly porous structure also shortens the transport length of Li ions. The porous/wrinkled RGO anode material achieves a high reversible capacity and durability (∼720 mA h g⁻¹ at 0.2C after 200 cycles), and a high rate capability (∼160 mA h g⁻¹ at 20C). The electrode performance is comparable to the best RGO anodes. The microgel-assisted method opens up a promising route for potentially controlling the properties of 3D graphene-based electrodes.