Controllable Synthesis Of Graphene/transition Metal Oxide Composites And Their Application In Lithium Ion Batteries

With the booming of electric vehicle market,it is urgent to develop a new generation of traction lithium-ion batteries?LIBs?with high specific capacity,outstanding rate capability and long service life.Due to the low theoretical capacity?372 mAh g-1?,traditional graphite anode material has limited space for energy density improvement.In contrast,transition metal oxides?TMOs?have high theoretical capacity??1000 mAh g-1?,environmental friendliness,and relatively low cost.In this paper,we focus on the nanostructured design of TMOs,construction of graphene-based composite system and introduction of hierarchical porous structure to improve the conductivity,structural stability and Li+ diffusion efficiency of the electrode material.Meanwhile,these strategies are very effective to promote the specific capacity,rate capability and cycle life of the electrode materials.In addition,the large-scale production of high-quality graphene is a major bottleneck limiting its use as anode material.The non-oxidation intercalation technology is used to develop scale-production of high-quality graphene.The main research contents and results of this thesis are as follows:?1?TMOs nanoparticles/N-doped graphene?NG?hybrids were synthesized by hydrothermal method using graphene oxide,metal salt precursor and ammonia as raw materials.The nanosized TMOs provide abundant active sites for insertion/desertion of Li+.The flexible graphene substrates effectively mitigate the volume change of nanoparticles during cycling.Nitrogen doping treatment further improves the conductivity of reduced graphene oxides.When packaged in CR2025 button half cell and experienced 150 cycles,the Mn3O4/NG and Fe2O3/NG reached high reversible specific capacities of 1208.4 and 650 mAh g₁.Even at high current density of 5.0 C,the specific capacities of Mn3O4/NG and Fe2O3/NG hybrids still maintained at 284 and 313 mAh g-1.The EIS results indicated that N-doped graphene in hybrids provided a highly conductive matrix for the fast transfer of Li+ and electron.?2?On the basis of metal-amine complex chemistry,a series of hollow TMOs?Co3O4,NiO,CuO-Cu2O and ZnO?/NG were synthesized.The TMOs hollow spheres are uniformly anchored on N-doped graphene surface,and the shell layer had a hierarchical porous structure consisting of tiny nanocrystals.The integrated conductive network of NG and the unique hollow porous structure of TMOs can effectively shorten the transport path of electrons and ions,thereby greatly enhancing their rate performance.The hybrids had a higher N/C atomic ratio between 9.1-17.3 at.%than that?4-8.3 at.%?reported in the literatures.When used as anode materials for LIBs,the H-Co3O4/NG and H-NiO/NG hybrids exhibited high reversible specific capacities of 825 and 1046 mAh g-1 after 200 successive cycles at the current density of 0.1 C and excellent rate capacities of 446 and 422 mAh g-1 at 5.O C.?3?The novel NiO nanocrystals bonded on 3D graphene framework were synthesized by two-step strategy involving hydrothermal treatment followed by thermal annealing.The 3D-GF interconnected network not only prevented the re-stacking of graphene,but also provided hierarchical porous structure for electrolyte and Li+ transport.The ultrafine NiO nanocrystals further increase the active sites for electrode reaction,and the presence of bonding effect prevented the shedding and agglomeration of the NiO.The as-prepared NiO/3D-GF electrode exhibits ultra-high reversible capacity,superior rate capability and excellent capacity retention.After 250 successive cycles at 0.2 C,the NiO/3D-GF electrode maintained a reversible capacity of 1104 mAh g-1,which was far better than those of the bare counterparts and other NiO-based anode materials reported in the previous literatures.?4?To meet the application requirements,the large-scale production of high-quality graphene was explored.The traditional preparation process of reduced graphene oxide was complicated as well as environmentally harmful,and the resulting graphene has poor electrical conductivity.When used as anode material,the charge-discharge curve showed no voltage platform,limiting its increase in energy density.When used as anode material,the reversible capacity remained as high as 499 mAh g-1 under the current density of 200 mA g-1 after 100 cycles.The reversible capacity maintained at 384 mAh g-1 after continuous 300 cycles under high current density of 500 mA g-1.When used PVP as surfactant,high-quality graphene could still be produced by repeating the shear stripping process in water system.Using PVP-G as raw material,the MoS2/G hybrids could be prepared,which further expand the application of high-quality graphene anode.