Rational Design And Properties Of Selenium-Based Anode Materials For High-Performance Sodium Ion Batteries

Sodium-ion batteries?SIBs?have been unanimously considered as one of the most promising alternatives to the market-dominant lithium-ion battery?LIBs?,owing to the highly abundant,low-cost and geographically even-distributed sodium resources in the Earth's crust,as well as its environmental friendliness and the identical working mechanism as LIBs.Unfortunately for the anode of SIBs,majority of the traditional anode materials of LIBs cannot deliver the expected Na-storage performance,which is mainly originated from the larger ionic radius and more sluggish diffusion kinetics of Na ion compared to those of the Li counterpart.For instance,the commercially available carbon materials with high graphitization can barely store sodium with practicable capacity in common Na⁺electrolyte,although they dominate the market of LIBs anode with a share of above 90%.The silicon anode with the highest Li-storage capacity(theoretically 4200 mA h g^-1)is almost impossible to alloy with Na when used for SIBs.Therefore,there is a great challenge to prepare superior Na-storage anode materials and hence assemble the advanced Na-ion full batteries.This paper mainly focuses on the design of structure and morphology,synthesis and electrochemical properties of selenium-based anode materials.The major contents and research results are listed below:?1?Two kinds of continuous carbon coated with double double metal selenide,namely Ni_1.5.5 CoSe₅@rGO and Ni_1.8Co_1.2Se₄@C?C from DAC?,was successfully prepared by through a simple annealing response,in which NiCo-MOF as precursor and graphene and dopamine?DAC?as two kinds of common carbon source.Graphene as a carbon source,the Ni_1.5CoSe₅@rGO composite with three-dimensional network structure was successfully prepared by freeze-drying and simple annealing treatment.In the prepared composite,all Ni_1.5CoSe₅ nanospheres are wrapped by nitrogen doped carbonlayers,further assembly form Ni_1.5CoSe₅ cube,and finally anchored on both sides of few-layer graphene?FLG?nanosheets,forming the Ni_1.5CoSe₅@rGO composite.It delivers outstanding high specific capacity and excellent cycle life,when used as anode for SIBs in half cells.The other is using DAC as the carbon source.We have successfully prepared a double nitrogen doped carbon coated double double metal selenide Ni_1.8Co_1.2Se₄@C.The results shows that the electrochemical properties of the materials were improved significantly due to the introduction of polydopamine.?2?However,these two composites have poor cycle performance,in order to further improve the cycle stability of the material.The 3DSG nanocomposite was prepared via a facile melting-diffusion process,in which amorphous selenium?a-Se?is evenly coated on the surface of three-dimensional?3D?conductive network with open channels composed of reduced graphene oxide?rGO?nanosheets.The rGO-constructed 3D interwoven conductive networks can not only enable the highly efficient and ultrafast transports of both electrons and sodium ions,but also work as an effective buffer to accommodate volume changes during the successive de-/sodiation processes.The 3DSG electrode delivers outstanding high-rate capability and ultralong cycle life,when used as anode for SIBs in half cells.The3DSG anode was further assembled as sodium-ion full battery by coupling it with NVPOF cathode.The fabricated 3DSG//NVPOF batteries exhibit not only superior high-rate performance but also ultralong cycling stability.For instance,the capacity retention is still up to 86.3%after even 15000 cycles at 1 A g^-1.As the testing temperature decreases gradually from 25 ^oC to 15,5,-5,-15 and-25 ^oC,the 3DSG//NVPOF full cells exhibit the capacity retention values of around 96%,95%,90%,94%,89%and 75%,respectively,over 1000 cycles at 0.4 A g^-1.?3?Nevertheless,the 3DSG has low specific capacity,in order to further improve the specific capacity of the material,SnO₂ with high specific capacity is mixed with Se by us.The h-SSG nanocomposite was prepared via facile in-situ co-reduce and melting-diffusion processes.The h-SSG electrode delivers much improved electrochemical properties in terms of high Na-storage capacity(643.8 m A h g^-1 at 0.05 A g^-1),ultralong cycle life(70%capacity retention over even 5000 cycles at 5 A g^-1)and outstanding high-rate capability(e.g.,a specific capacity of 320.5 mA h g^-1 at an ultrahigh current density of 20 A g^-1),when used as anode for SIBs in half cells.More significantly,the fabricated h-SSG//NVPOF batteries exhibit superior energy storage performance and low-temperature performance,e.g.,a high energy density of 263.8 W h kg^-1 and power density up to 7410 W kg^-1,calculated according to the total weight of both cathode and anode materials.This initially proves its practicability in actual Na-ion full batteries as a promising anode.