The Preparation Of Iron And Copper-based Selenides As Anode Materials For Sodium-ion Batteries

The rapid development of lithium-ion batteries in the field of secondary battery makes lithium resources in short supply,leading to the increase of battery cost.Recently,sodium-ion batteries have become one of the most anticipated candidates for large-scale energy storage devices with their low cost,plentiful sodium reserves and similar working principles to lithium-ion batteries.Among them,the suitable electrode material is the core component of the successful development of sodium-ion batteries.Among various anode materials,metal selenides with high theoretical capacity and unique structure have attracted wide interest.Among selenide electrode materials,iron-base and copper-base selenides are potential candidates for sodium-ion batteries because of their low price,abundant reserves and excellent electronic conductivity.However,the large volume variation of iron and copper selenides during charging and discharging leads to poor cycle stability.In this thesis,the sodium storage properties of iron selenide and copper selenide electrode materials as anode materials for sodium-ion batteries were studied,and the modification of iron selenide and copper selenide electrode materials by using carbon composite,adjusting the nano-morphology and the cut-off voltage,so as to achieve the purpose of improving the electrochemical performance.Specific work contents are as follows:（1）The morphology and structure of Fe Se₂ were controlled by the strategy of glycol solvent-induced synthesis.We found that the growth of Fe Se₂ on（110）crystal plane was promoted by glycol solvent,thus forming a unique three-dimensional（3D）flower structure.The low migration energy barrier of Na⁺on Fe Se₂（110）crystal plane were determined by density functional theory（DFT）.At the same time,reduced GO was added to the reaction system to limit the volume change of Fe Se₂ and improve its cyclic stability.Fe Se₂/r GO-EG has good sodium storage properties,especially cyclic stability.At 1 A g^-1,the Fe Se₂/r GO-EG electrode can provide a high capacity of 400 m A h g^-1 after 1000 cycles,and the capacity retention rate is～100%.In addition,the reaction mechanism of Fe Se₂was analyzed by CV,GITT and in situ XRD.This work reveals the influence of dominant crystal surface and morphology on the electrochemical performance of Fe Se₂ electrode,and provides guidance for the subsequent research of TMSes as anode materials for sodium ion batteries.（2）Fe₃Se₄ nanoparticles（20～50 nm）were inserted onto nitrogen-doped carbon nanotubes through a simple method,and a Fe₃Se₄-modified nitrogen-doped carbon nanotubes composite（Fe₃Se₄/CNTs）was obtained.On the one hand,the presence of CNT can effectively inhibit the aggregation of metal particles and make Fe₃Se₄ nanoparticles evenly distributed.On the other hand,the good conductivity of CNT can effectively improve the electrochemical performance of electrode materials.Due to the above advantages,Fe₃Se₄/CNTs showed excellent electrochemical performance.At the current density of 1 A g^-1,the reversible specific capacity of 424 m A h g^-1 can be provided after500 cycles,and the coulomb efficiency is close to 100%.The excellent performance of sodium storage makes Fe₃Se₄/CNTs composites an ideal anode material for sodium-ion batteries.（3）Fe₃Se₄ quantum dot modified graphene composite was synthesized through a simple hydrothermal method.The graphene containing oxygen functional groups on the surface was used as the conductive matrix,and the surface defect site was used as the initial nucleation site for the controlled growth of Fe₃Se₄.Thanks to the above advantages,Fe₃Se₄/r GO shows excellent electrochemical performance.When tested at the current density of 1 A g^-1,the Fe₃Se₄/r GO electrode can remain a stable specific discharge capacity of 293 m A h g^-1 after 300 cycles.Fe₃Se₄/r GO composite has great application potential as the negative electrode of sodium ion battery.（4）Cu_1.8Se/r GO composites were synthesized by solvothermal method using ethanol as solvent.As an anode material of sodium-ion battery,it delivers excellent electrochemical performance when the cut-off voltage is adjusted to 0.3～3.0 V.When tested at 5 A g^-1,the Cu_1.8Se/r GO electrode can maintain the specific discharge capacity of 293 m A h g^-1 after 2000 cycles,and the Coulomb efficiency was close to 100%during the cycle.The excellent cycling performance makes Cu_1.8Se/r GO electrode material one of the candidates for anode materials of sodium-ion batteries.