Three-Dimensional Self-Supported Oxide Electrodes For Sodium Storage

Sodium-ion batteries(SIBs)are regarded as the most promising substitute for lithium-ion batteries(LIBs),because of their analogous working principle,the low price and abundant reserves of sodium,and similar chemical properties between Na and Li.However,the substantial volume swelling and shrinking upon Na uptake and release disturbs the structural integrity of electrode,leading to poor electrochemical durability and low Coulombic efficiency.And slow kinetics of SIBs is also the bottleneck for the commercialization because of the larger radius of Na+ ion(102 pm)than that of Li ion(76 pm).As a result,the development of electrode materials with high energy density,high power density and excellent stability is the key for SIBs.In this paper,three-dimensional(3D)self-supported nanoarrays have been designed and prepared,this structure ensures great mechanical adhesion and electrical connection between active materials and the current collector without binder and carbon additive.And it breaks out the limits of two-dimensional planar structure,which shortens the transport paths of ion and electron.It also enables full exposure of active materials to the electrolyte,because of the large specific surface area.Moreover,3D self-supported nanoarrays contribute to relieve structural collapse caused by volume expansion and improve electrochemical stability of electrode.In this paper,high-capacity metal oxide materials with conversion mechanism are used as the main research object.We employed various methods to regulate their structure and electrochemical sodium storage.The specific researches are as follows:(1)The surface phosphorylation strategy effectively improves the sodium storage performance of the molybdenum trioxide(MoO3)nanotube array.3D self-supported MoO3 nanotube arrays were directly synthesized on the Mo foil by anodization.Then the surface of MoO3 was phosphorylated,which simultaneously introduced an appropriate amount of oxygen vacancies in MoO3.Surface phosphorylation increases the surface adsorption of Na+ion of MoO3,and the introduction of oxygen vacancies increases the conductivity of MoO3.Their synergism combined with the advantages of nanotube arrays results in the excellent sodium storage of MoO3,which was higher than that of other MoO3 or Mo2.(2)The heterostructure films of molybdenum chalcogenide were transformed from 3D self-supported weave-like MoO3 using a simple in situ sulfurization.The synthesis of the films involves two sequential thermal annealing of a Mo foil in air and in sulfur vapor,respectively,leading to a heterostructure of MoO2 and MoS2.Benefit from its high specific surface area,short ion diffusion distance and inherent advantages of heterostructure,when it used as Na electrode,it exhibits a high reversible capacity and a superior rate performance.(3)A structural strategy was proposed to regulate the breathing of copper oxide(CuO)nanowire arrays using an atomic layer deposition of titanium dioxide(TiO2)film to improve the stability of sodium storage.The 3D self-supported CuO array was directly synthesized on Cu foam by anodization.TiO2 acts as a structural elastic reinforcement to relieve volume expansion of CuO.Moreover,TiO2 improves the solid electrolyte interface layers,which promoted the Coulombic efficiency of CuO.As a result,the regulated CuO electrode arrays enable a large reversible capacity,a high cycle efficiency,and an excellent cycling stability,which are some of the best sodium storage performance values reported for CuO systems.