Design,Preparation And Electrochemical Performance Of Bimetallic Sulfides

Electrochemical energy storage has attracted tremendous attention to relieve the resouce shortage and environmental pollution.The performance of energy storage systems is determined by the physical and chemical properties of electrodes.Rational design of novel materials with high conductivity,activity and stability is propitious to improve energy utilization.Bimetallic sulfides are considered as potential electrode materials due to the merits of the controllable structure,high theoretical specific capacity,and rich valence states.However,the low reactivity and slow reaction kinetics hamper its rate performance and cycle stability.In this papers,we aimed to design the bimetallic sulfides and enhance its electrochemical performance through regulating the elemental composition and atomic coordination environment.The electronic structure,crystal structure and surface properties were supposed to be precisely modulated,optimizing the charge transfer and ion transport behaviors and exploring the intrinsic mechanism.The asymmetric supercapacitors were fabricated to investigate the electrochemical performance.The specific researches are as follows:（1）The electrochemical performance of bimetallic sulfides was tuned by cation substitution.The effect of metal compositions on the crystal structure,electron transfer and ion transport was studied by adjusting the metal species.The MCo₂S₄（M=Co,Ni and Cu）nanosheets arrays were synthesized by a hydrothermal method using the nickel foam as the substrate.The Co²⁺ions on the tetrahedral sites of Co₃S₄ were substituted by the Ni²⁺or Cu²⁺to form Ni Co₂S₄ or Cu Co₂S₄.The lattice structures were induced to strain owing to the ionic radius and electronegativity differences,reinforcing the reactivity of the metal sites.Compared with Co₃S₄,the electron transfer and ion transport resistances of Ni Co₂S₄ decreased by 48.6%and 28.7%,exhibiting better electrochemical properties.The specific capacity of Ni Co₂S₄ was451.5 C g^-1 at 1 A g^-1 and can maintain 59.8%at 10 A g^-1.The capacity retention was 60.2%at10 A g^-1 after 8000 cycles.The results indicate that the regulation of elemental compositions can effectively tune the specific capacity and rate performance of bimetallic sulfides.（2）The electronic structure and surface activity of bimetallic sulfides were modulated by anion doping.The Ni Co₂S₄ was seleted as the research object to construct the substitutional boron(B_sub)and interstitial boron(B_int)co-doped Ni Co₂S₄（B-Ni Co₂S₄）.The density of states,charge density distribution and OHˉadsorption energy were calculated to investigate the effect of B_sub and B_int on the electronic structure and reactivity of host material.The density of states of B-Ni Co₂S₄(26.7 e V^-1)was much higher than that of Ni Co₂S₄(6.2 e V^-1)and B_sub doping possessed higher partical density of states near the Fermi level.The results implied that B_submade the contribution to modulate the electronic structure and ameliorate the intrinsic conductivity.B_int species induced the redistribution of the surface charge and obtained a high OHˉadsorption energy（-4.21 e V）,indicting the increased active sites and enhanced surface absorption.The theoretical analysis revealed that the synergistic effect of B_sub and B_int modes can improve the charge transfer and surface reactivity of B-Ni Co₂S₄.B-Ni Co₂S₄ was synthesized by a hydrothermal process.In a three-electrode cell,the specific capacity of B-Ni Co₂S₄ was 738.0 C g^-1 at 1 A g^-1,and the specific capacity retention reached 72.9%at 10 A g^-1.The two-electrode cell exhibited a high energy density of 32.9 Wh kg^-1 at a power density of 804 W kg^-1.The capacitance retention can be maintained 98.5%after 5000 charge-discharge cycles.This research manifests the ccuracy and reliability of the strategy of theoretical prediction and experimental verification.