High-Performance Transition Metal Compounds As Anode Materials For Sodium-Ion Batteries

In recent years,sodium ion batteries have been regarded as one of the most promising alternatives to lithium-ion batteries among various new energy storage devices.They are regaining increasing attention,but lacking high-performance anode material is a major factor that restricts the development of sodium-ion batteries.In this study,on the basis of a thorough understanding of different anodes' working mechanisms and a series of severe application challenges,we developed highly-efficient and feasible methods to alleviate those problems.We also designed and prepared novel nanostructured transition metal compounds and studied their sodium storage properties.The main research contents and results are as follows:1.Phosphorus(P)owns an ultra-high theoretical specific capacity to store Na+ ions,but it is suffering from poor conductivity and sereve volume expansion during charging/discharging process.We prepared new copper phosphide/carbon composites(CUP2/C)successfully to relieve the problem.The carbon layer can not only improve the conductivity,but also suppress the volume change.Our CuP2/C composites can exhibit a reversible specific capacity of>500 mAh/g,and the rate performance is also decent.In addition,ex-situ XRD and TEM characterizations revealed that the working mechanism of CuP2 electrode materials is based on the conversion reaction.2.For alleviating the problem of poor conductivity,unstable structure and loss of active materials in sulfides,we chose nickel sulfides(NiSx)as the research object and developed an effective strategy.Firstly,we selected carbon nanotubes(CNTs)as the skeleton for growth of NiSx nanoparticles.Then,an amorphous carbon layer was coated on the outermost layer of the material through high temperature carbonation of polydopamine on the surface.The final NiSx/CNT@C composite can exhibit a specific capacity of>500 mAh/g and an excellent cyclic stability of more than 200 cycles.3.We prepared iron selenide quanutum dots(FeSe2QDs)via a high temperature solution method.We then explored and found the excellent sodium storage properties of FeSe2 QDs.At a current density of 100 mA/g,it has a specific capacity of>500 mAh/g.A specific capacity of 250 mAh/g at 10A/g shows us an excellent rate performance.FeSe2 QDs also exhibit very good cyclic stability.Additionally,we confirmed that FeSe2 QDs can be paired with Na3V2(PO4)3(NVP)to form high-performance sodium-ion full cells.