A three-dimensional TiO2-Graphene architecture with superior Li ion and Na ion storage performance

In this work, a three-dimensional TiO2-graphene composite with large specific surface area is designed by freeze drying. In this architecture, primary TiO2 nanoparticles (less than 10 nm in size) are wrapped with graphene homogeneously, forming spherical secondary particles (approximate to 100 nm), and the spherical TiO2 particles further agglomerate into platelet-like particles with several micrometers in size. The TiO2-graphene composite delivers high de-lithiation and de-sodiation capacities of 312 mAh g(-1) and 280 mAh g(-1) at 17 mA g(-1) as negative electrode materials in lithium and sodium cells, respectively, and excellent cycling performance with negligible capacity loss after 500 cycles at a specific current of 85 mA g(-1). When coupled with an activated carbon positive electrode, it demonstrates high capacitance and long cycle life in sodium ion capacitors (81% after 2000 cycles at 1 A g(-1)) and lithium ion capacitors (95% after 5000 cycles at 1 A g(-1)). Its superior performance benefits from the designed 3D architecture, which combines the advantages of small primary particle size, a homogeneous carbon coating and an adequate contact area with the liquid electrolyte. The storage mechanisms of the superior TiO2-graphene composite in lithium and sodium cells are investigated by operando X-ray diffraction studies.