image: Schematic illustration of Li+ in (001) facet of NSTiO2/rGO.
The design of materials with high capacity, excellent rate capability and long cycle life is one of the major challenges for the rechargeable lithium ion batteries (LIBs). Among the virous anode materials, TiO2 stands out due to its nontoxicity, high abundance, high activity, safe operation, electrochemical and structural stability. However, the kinetics of Li+ insertion-extraction and the electrochemical performance of TiO2 are always limited by slow Li+ diffusion and charge transport as well as the low electrode/electrolyte contact area. Various efforts have been made to solve these problems in recent reports, such as assembling low-dimensional nanostructures with (001) facets and combining TiO2 with conductive supports. However, the high energy of (001) facets easily results in a tight aggregation of the TiO2 nanosheets and the internal electrochemical processes have not been studied in detail. In this work, it reported an unprecedented lithium storage and electrochemical performance of a nanosheet-constructed hierarchically porous TiO2/rGO hybrid architecture.
In the synthesis, TiF4 was used as a structure-directing agent to ensure the formation and exposure of (001) faceted TiO2 nanosheets and the flexible graphene oxide could regulate assembly of the nanosheets. The density functional theory (DFT) calculations evidenced that the energy barrier of Li+ entering into the (001) surface of anatase TiO2 was the lowest and the Li+ was easier to migrate across the (001) surface. The resulting hierarchically porous NSTiO2/rGO hybrid structure offered a high and stable surface area (304.5 m2 g-1) and exhibited an excellent reversible capacity of 250 mAh g-1 at 1 C (1 C = 335 mA g-1). On one hand, it could intensively increase the active sites for Li+ insertion-extraction, its porous structure would significantly facilitate electrolyte permeation and largely endurance volume expansion. On the other hand, its thinner nanosheets constructed porous network could notably shorten the path length for Li+ and increase the charge transfer, leading to a highly enhanced capacity. The discharge-charge profiles at various current densities demonstrated that the (001) facets of TiO2 facilitate and dominate the Li+ insertion-extraction at low current densities ( 1 C), the high specific surface area and porosity in the hierarchically porous architecture dominated the Li+ insertion-extraction process. Electrochemical impedance spectroscopy (EIS) was used to confirm stable surface layer resistance Rs at the surface layer and inverse charge-transfer resistance Rct in the electrode/electrolyte interface during the discharge-charge cycling process. In addition, in-situ XRD, SEM and TEM measurements were also adopted to detect the electrochemical transitions of NSTiO2/rGO anode material, which further demonstrated structural stability and the formation of cubic Li2Ti2O4 nanocrystallites.
This work was reported in National Science Review, entitled "Unprecedented and highly stable lithium storage capacity of (001) faceted nanosheet-constructed hierarchically porous TiO2/rGO hybrid architecture for high performance Li ions battery" by Prof. Bao-Lian Su and Prof. Yu Li in State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology. The researchers envision that this hierarchically porous NSTiO2/rGO micro/nanostructure hybrid can be used as a very promising anode material for industrial application in high-performance LIBs and may be employed in other applications.