Ni/Co And Ti-based Oxides Mesoscale Assembly Structures:Controllable Construction And Energy Storage Properites

The oxide materials based on the conversion mechanism are widely used in the research of anode materials for lithium ion batteries due to their high theoretical specific capacity and low cost and availability.In particular,complex metal oxides can compensate for the disadvantages of a single component,while also having excellent synergy,and thus exhibit excellent lithium-ion battery performance.However,in the process of lithium ion intercalation and deintercalation,the severe volume change of metal oxide materials hinders its practical application.The titanium-based oxide materials,especially titanium dioxide,based on the intercalation mechanism have been extensively studied due to their excellent sodium storage properties.The titanium-based oxide material is favored for its low cost,rich source and small change of volume in the process of de/intercalation of sodium ions.However,the intrinsic low conductivity of titanium-based oxide materials is not conducive to their rapid charge and discharge capabilities.Structural control methods,such as micro-nanonization of structures,dimensional control,and compounding with conductive carbon materials,can significantly improve the electrochemical performance of materials.Based on the above discussion,the research work of this paper is as follows.（1）Mesoporous single-crystalline NiCo₂O₄ nanoribbons have been fabricated via a hexamethylenetetramine（HMT）-assisted hydrothermal method applied to Ni-Co precursor nanobelts,followed by annealing in an air atmosphere.The mesoporous single-crystalline feature was demonstrated by a series of characterization methods.The porous profiles of the as-obtained spinel NiCo₂O₄ products could be tuned feasibly by changing the post-treatment temperature of the precursors in terms of specific surface area and pore size distribution.As expected,when applied as an anode material in lithium-ion batteries,the mesoporous single-crystalline NiCo₂O₄ nanoribbons annealed at lower temperatures delivered a high specific capacity,excellent rate capability,and outstanding cycling performance.The reversible discharge capacity could reach 1198 mA h g^-1 after 60 discharge-charge cycles at a current density of 200 mA g^-1.After being tested at a high rate of 1000 mA g^-1,it can still circulate at a current density of 100 mA g^-1 for 100 cycles without significant attenuation.The superior electrochemical performance can be attributed to the unique structure,including the suitable pore size distribution,mesoporous single-crystalline feature,enlarged specific surface area,and highly textured nanostructure,as well as synergetic effects of the different metal ions.（2）We synthesized a novel hierarchical porous hybrid nanosheet composed of interconnected uniform TiO₂ nanoparticles and nitrogen-doped graphene layer networks（TiO₂@NFG HPHNSs）by using dual-functional C₃N₄ nanosheets as both the self-sacrificial template and nitrogen-containing carbon source.In the as-synthesized structure,interconnected uniform TiO₂ nanoparticles are confined in the continuous ultrathin graphitic carbon network（<3 layers）and preferentially assemble into hierarchical porous 2D nanosheets.The hybrid nanosheets have excellent sodium storage properties.Specifically,these HPHNSs deliver high reversible capacities of 146 mA h g^-1 at 5 C for 8000 cycles,129 mA h g^-1 at 10 C for 20000 cycles,and 116 mA h g^-1 at 20 C for 10000 cycles,as well as an ultrahigh rate capability up to 60 C with a capacity of 101 mA h g^-1.The unprecedented sodium storage performance of the TiO₂@NFG HPHNSs is assigned to their unique composition,hierarchical porous 2D structure and apparent capacitive effect.（3）When two-dimensional TiO₂ nanosheets are used as anode materials for sodium ion batteries,the thickness of the nanosheets has a great influence on the sodium storage properties.We also used dual-functional C₃N₄ nanosheets as self-sacrificial templates and nitrogen-containing carbon sources to prepare two-dimensional carbon-coated TiO₂ hierarchical porous hybrid nanosheets.By changing the amount of titanium source added,three kinds of hybrid nanosheets with different thicknesses（TiO₂@NC-1,TiO₂@NC-2 and TiO₂@NC-3）were controllably synthesized.With resepct to them,we carried out the analyses through a series of morphological and structural characterization methods and conducted electrochemical sodium storage tests.The results show that the hybrid nanosheets,TiO₂@NC-2,with medium thickness has the best sodium storage performance,showing excellent rate performance and long cycle stability.The reversible specific capacity is 145 mAh g^-1 at 5C(1C = 335 mA g^-1)after 5000 cycles,and the capacity retention is close to 100%.A reversible specific capacitance of 130 mA h g^-1 can be obtained for even 10000 cycles at 10 C.What is more surprising is that when the rate reaches 40 C,the material still has a reversible specific capacity of 106 mAh g^-1.（4）In this study,we used a simple solvothermal method to synthesize Ti-polymer precursor nanosheets composed of nanosheets in a one-step process,and then annealed under inert atmosphere to form TiO₂ nanoparticles embedded in nitrogen-doped carbon nanosheets composed of clusters（Ti02@NCSs）.The 3D cluster structure composed of such 2D nanosheets facilitates the penetration of the electrolyte,while the TiO₂ nanocrystals shorten the length of sodium ions.In addition,the carbon skeleton also enhances the conductivity and structural stability of the material.TiO₂@NCSs have excellent sodium storage properties owing to its advantages.A high capacity of 168.1 mA h g^-1 can be obtained at 0.5 C after 800 cycles.It is worth noting that the reversible specific capacity can reach 151.5 mA h g^-1 at 2 C after 3000 cycles.Even after 10000 cycles at 10 C,a reversible capacity of up to 114.1 mA h g^-1 is obtained,showing excellent long-term cycle life.At the same time,the TiO₂@NCSs electrode has a 50 C rate capability,which makes it an embarrassing application prospect.