Design Of Hybrid Anode Materials For Sodium Ion Batteries

Sodium ion batteries are generally considered as promising next-generation alternative energy storage device due to the large abundance of sodium and similar chemistry to that of Lithium ion batteries.However,the radius of sodium ion is much larger than that of lithium ion,which results in slow mass/ion transfer kinetics in the electrode.Therefore,it is of great significance to develop electrode materials with superior electrochemical performance.In this thesis,high-performance anode materials based on MoS₂ and TiO₂ have been designed and synthesized.Several strategies have been applied to improve the electronic conductivity and structural stability of the electrode,including the decrease of the particle size to nanoscale to shorten the diffusion distance of Na⁺,the incorporation with conductive substrates to enhance the electron conductivity,and the introduction of oxygen vacancies in the crystal lattice to improve the intrinsic ionic/electronic conductivity.The main research results are summarized as follows:(1)Neuron typically composed of a soma and neuritis is the basic structure and functional unit of the nervous system.With the help of axons,electrical and chemical signals can be transmitted efficiently from a soma to other neurons or effector cells.Inspired by the signal transfer mode of a neuron,we designed a composite by stringing MoS₂ nanoflower(soma)with multiwall carbon nanotubes(MWCNTs)(axons).The hierarchical flower morphology together with MWCNTs could accommodate the volume change of MoS₂ during the charge/discharge process.The expanded interlayer spacing of MoS₂ and lattice matching between MoS₂ and MWCNT revealed by the TEM observation would facilitate the diffusion and reversible intercalation/extraction of sodium ions and reduce interface impedance,respectively.When used as anode materials in SIBs,the MoS₂/MWCNT composites with a nervous-system-like conductive network exhibit excellent electrochemical performance,such as high specific capacity and improved cycling and structural stability.A stable capacity of 527.7 m Ah g^-1 can be achieved after 110 cycles at a current density of 0.1 A g^-1.The neuron-inspired design proposed is a promising and efficient strategy for the development of electrode materials for SIBs with high mass transport kinetics and structural stability.(2)A Ti₃C₂/TiO₂/MoS₂ sandwich structure was synthesized by a feasible hydrothermal method.The surface of Ti₃C₂ was oxidized to TiO₂ by the trace oxygen in the water and then the MoS₂ nanosheet was growing vertically on the surface of Ti₃C₂/TiO₂.Ti₃C₂ has excellent conductivity,which can greatly improve the electronic conductivity of the electrode.MoS₂ vertical grows on the Ti₃C₂,effectively preventing the aggregation of MoS₂ during the charge/discharge process and improving the cycling stability of the electrode material.In addition,the lattice spacing of the MoS₂ layer was expanded to 0.635 nm which can further promote the diffusion kinetics and reversible intercalation/extraction of sodium ions and reduce interface impedance.When tested as the anode material for sodium ion battery,the Ti₃C₂/TiO₂/MoS₂ composite shows excellent sodium storage performance.A discharge capacity of 403.6 m A h g^-1 is retained after cycled for 110 cycles at a current density of 0.1 A g^-1.At a current density of 8.0 A g^-1,a rate capacity of 302.2m A h g^-1 is obtained,indicating the high rate capability.(3)Inspired by the structure of the dandelion clocks,TiO₂@MoS₂ composites with TiO₂ nanospheres as the receptacle cores and MoS₂ nanosheets as the pappi shell were synthesized through a hydrothermal process.The TiO₂ nanospheres with high electrochemical activity and structural stability can not only contribute to the specific capacity of the composite,but also ensure the structural integrity of the composites upon cycling.MoS₂ nanosheets with high theoretic capacity and fast ion diffusion efficiency guarantee the high specific capacity and good rate capability of the composites.Owning to the synergistic effects between TiO₂ cores and MoS₂nanosheets and the core-shell heterostructure feature,the TiO₂@MoS₂ composites show excellent energy storage properties when used as anode materials of SIBs.A capacity of as high as 375.2 m Ah g^-1 is maintained even after 350 cycles at a current density of 0.5 A g^-1 with a capacity retention rate of approximately 100%.This work indicates that the consideration of the morphology and structure is the key for the rational design and preparation of high-performance electrode materials.(4)Core–shell anatase TiO₂ spheres with plenty of oxygen vacancies and surface nitrogen-doped carbon coating,denoted as TiO_2-x@NC,have been prepared through the calcination of polydopamine(PDA)coated core–shell anatase TiO₂ spheres prepared through a solvothermal process.Being used as an anode material for SIBs,TiO_2-x@NC spheres showed excellent specific capacity and superior rate capability due to the synergistic effect of the core–shell structure,the oxygen vacancies and nitrogen-doped carbon coating.At a high current density of 1.0 A g^-1,a high reversible capacity of 167.3 m A h g^-1 is retained even after 2000 cycles.This work opens a new avenue for improving the electrochemical performance of metal oxides in sodium-ion batteries.