Structural Modification Of Cobalt Selenides Based On Element Doping And Their Applications On Sodium Storage Performance

With the increasing concern about environmental pollution and energy crisis,the search for reliable and suitable energy storage technology has become an urgent challenge.Lithium-ion batteries(LIBs)have gained significant interest for their remarkable cycle life and high energy density,and have emerged as a crucial technology in the field of energy storage.However,the further development of LIBs has been hindered by the scarcity of lithium resources and the high cost associated with their production.In contrast,sodium-ion batteries(SIBs)have become the focus of energy storage due to abundant sodium resources and relatively low cost,especially in the field of large-scale energy storage in the power grid.However,compared with LIBs,the large ion radius of sodium ions slows down the diffusion rate of sodium ion insertion and extraction processes,resulting in relatively slow ion kinetics and significant capacity decay issues for SIBs.Therefore,it is crucial to develop electrode materials that can accommodate fast shuttle of sodium ions for current SIBs research.Currently,researchers have synthesized various materials for SIBs anode materials such as carbon,alloy,metallic oxides and selenides.Among these,cobalt selenide materials exhibit higher specific capacity and rate performance due to high bulk density.However,the low conductivity of cobalt selenide and its significant volume expansion during the cycling process lead to severe pulverization and poor cycling stability,which in turn limits its applications.Therefore,it is of great significance to reduce the volume change of cobalt selenide materials during cycling and improve their electronic conductivity.In this paper,three anode materials based on modified cobalt selenide were designed to address the issues of poor conductivity and significant volume expansion.The sodium storage performance and mechanism of these materials were thoroughly analyzed,as follows:(1)In order to alleviate the large volume change of cobalt selenide during electrochemical cycle,we prepared nitrogen-doped cobalt selenide yolk-shell spherical materials(N-CoSe2 yss)with an average diameter of 500 nm by solvothermal method as well as calcination.The effect of etching on the formation of core-shell structure was investigated.As anode material for SIBs,the N-CoSe2 yss electrode exhibited excellent cycling stability and rate performance,with a high reversible capacity of 530.5 mAh g1 after 300 cycles.Even after 1000 cycles at a high current density of 10.0 A g-1,it still maintained a high capacity of 500.5 mAh g-1 with 94.6%capacity retention.Experimental and density functional theory calculations show that the excellent electrochemical performance can be attributed to the synergistic effect of nitrogen doping and structural engineering.This can not only enhance the electron/ion transport dynamics,but also improve the structural stability.(2)In order to improve the conductivity of cobalt selenide,we prepared 2D copperdoped cobalt selenide/nitrogen doped carbon composite nanosheets(Cu-CoSe@NC)using a conventional hydrothermal and calcination method.Electrochemical testing showed that the material exhibited excellent cycling performance,with a specific capacity of 422.9 mAh g-1 after 800 charge-discharge cycles at a high current density of 5.0 A g-1,and capacity retention of 98%.The structure is a large-size 2D porous nanoplate,and the kinetic analysis proves that the 2D layered material has high pseudocapacitance contribution.The high porosity as well as large specific surface area can ensure the fast electron and electrolyte transport.Meanwhile,the doping of Cu elements effectively improves the electrical conductivity of the material,thus significantly enhancing the rate performance of the electrode.In addition,in SIBs fullcell testing,Cu-CoSe@NC maintained a cycling capacity of 242.3 mAh g-1(at 0.1 A g1).(3)A three-dimensional network structure CoSe2/N-doped carbon heterojunction(CoSe2@NC)was synthesized by topological transformation to mitigate the volume change of cobalt selenide and improve its conductivity.The structural features of the heterojunction are as follows:cobalt selenide nanoparticles are attached to the 3D network of nitrogen-doped carbon substrate,and the 3D structure of the nitrogen-doped carbon substrate can effectively relieve the volume strain of the material during cycling,thus improving the cycling stability of the electrode.Meanwhile,the interface between CoSe2 and nitrogen-doped carbon is produced more favorable for electron transport.CoSe2@NC retains a capacity of 354.1 mAh g-1 after 800 cycles at 5.0 A g-1 and its capacity retention can reach 98%.Based on this anode material,a sodium-ion full battery has a high energy density of 136.6 Wh kg-1.By doping elements and modifying the structure of cobalt selenide materials,this study provides new ideas for the widespread application of cobalt selenide electrode in sodium storage.