Design, Synthesis And Properties Research Of Transition Metal Oxides-graphene Composite Energy Storage Material

Lithium- ion batteries have been widely applied in portable electronic products, power or energy storage batteries due to their high energy/power density, long cycle life, and environmental friendly. Currently, commercial carbonaceous material such as graphite, are the most commonly used anode materials in lithium- ion batteries. However, they suffer from a the lower capacity, poor rate performance and serious safety problems during repeated charge-discharge cycling. Therefore, researchers devote to looking for new electrode materials to replace carbonaceous anode materials. Among variety of novel candidates, titanium dioxides（TiO₂） and iron oxides（Fe₂O₃） have attracted great interest as anode materials for lithium- ion batteries, due to their high safety, non-pollution etc. However, the electronic conductivity of oxides material is too low, limiting its application. it is considerably effective to prepare nanostructured oxides combining with highly conductive material, such as carbon nanotubes（CNT）, carbon fibers（CNF）, and graphene etc., in order to improve the high rate charge and discharge performance and cycle stability of the materials. To imporove the conductivity of oxides, we prepared all-doped anode materials in order to achieve excel ent electrochemical properties.First,we apply a one-step solvothermal approach, followed by calcination in H₂/Ar to prepare a hierarchical composites of ultrathin 2D carbon-self-coated TiO₂ nanosheet arrays on the reduced graphene oxide nanosheets（TiO₂@C/RGO）. Ultrathin TiO₂@C nanosheets with a thickness of less than 10 nm are homogeneously anchored on surface of RGO nanosheets to form a hierarchical nanostructure. Importantly, the ternary TiO₂@C/RGO composites exhibit excellent electrochemical performance as negative electrode material for Li- ion batteries:with the current density up to 2500 m A g^-1, the TiO₂@C/RGO composites can still provide capacities of 126.5 m Ah g^-1. at a current density of 800 m A g^-1,TiO₂@C/RGO give discharge capacities of 173.1 m A h g^-1 at the 200 th cycle.Secondly, we apply a one-step solvothermal approach, followed by calcination at 500 °C for 2 h in H₂/Ar to prepare a hierarchical nanocomposite, in which carbon-coated mixed-phase porous TiO₂ nanosheets are grown directly on graphene sheets. The as- formed nanocomposite has a hierarchical porous structure, involving an average pore size of 28.56 nm, a large pore volume of 0.589 cm3g^-1 and a desired surface area. Importantly, the material composed of TiO₂（B）/anatase mixed-phase exhibits a satisfactory electrochemical performance, which is attributed to the synergistic effects of the nanosized TiO₂（B）/anatase, the conductive reduced graphene oxide nanosheets and carbon layer with a nano-porous structure, thus facilitating the electrolyte-electrode contact and accommodating the strain of Li+ ion intercalation/deintercalation. Therefore, it delivers the high reversible capacity, excellent cycling stability and high rate performance. With the current density up to 1500 m A g^-1, the TiO₂@C/RGO composites can still provide capacities of 96.9 m Ah g^-1;and at a current density of 800 m A g^-1,it shows discharge capacities of 158.6 m Ah g^-1 at the 500 th cycle.Then, combining the merits of reduced graphene oxide, carbon coatings, N-all-doping and nanostructures, we design a unique three-dimensional（3D） N-all-doped hierarchical composites comprising carbon self-coated TiO₂ nanoflakes on reduced graphene oxide（termed: NTCG）. Ti-complex/RGO precursor was firstly synthesized by a facile one-top sovothermal method, and followed by calcination in NH₃/N₂ to achieve in situ formation of NTCG nanocomposites. As expected, the 3D hierarchical structure with high electrical conductivity delivers high reversible capacity, superior cycle stability, and remarkable rate capability in lithium ion batteries. The improved lithium storage properties are demonstrated by N TCG,with a high reversible discharge capacity of 164.4 m Ah g^-1 at a current density of 800 m A g^-1 after 500 cycles and an excellent rate capability of 115.1 m Ah g^-1 at 3000 m A g^-1. The excellent lithium storage performance could be ascribed to the 3D hierarchical structure with high specific surface area and higher electronic conductivity.Finally,we apply a one-step solvothermal approach, followed by calcination in N2 to prepare a hierarchical nanocomposite, in which Fe₂O₃ nanoparticles grown directly on graphene sheets. Fe₂O₃/RGO composites exhibit excellent electrochemical performance as negative electrode material for Li- ion batteries:the improved lithium storage properties are demonstrated,with a high reversible discharge capacity of 585.2 m Ah g^-1 at a current density of 100 m A g^-1 after 100 cycles and an excellent rate capability of 139.9 m Ah g^-1 at 1000 m A g^-1.Above all, combining the merits of oxides and carbonaceous material, the electrochemical performance of nanocomposites is adjusted by change various kinds of factors which includes morphologies, structures, conductivity of nanocomposites. This works give us a theoretical a nd technical guidance for basic research and practical applications of nanocomposites anode materials in lithium-ion batteries.