Design And Synthesis Of Transition Metal Sulfides And Their Energy Storage Mechanism In Sodium/Magnesium Secondary Batteries

In recent years,thanks to the successful commercial application of lithium ion battery?LIBs?,portable electronic equipment and electric vehicles with LIBs as power source have rapidly developed.However,with the large-scale application of electric vehicles and energy storage grid,the cost of LIBs and lithium raw material prices increasd year by year.Faced with the problem that the limited lithium resources in the world cannot meet the rapidly developing energy market,sodium/magnesium ion battery?S/MIBs?is proposed as a new type of secondary battery system for its advantages of being environmental friendly,cheap and abundant.In order to build sodium ion batteries with higher energy density,designing and synthesizing anode materials with excellent electrochemical properties are of particularly importance to improve the overall performance of SIBs.For being in infancy of the MIBs,developing stable magnesium ion battery electrolyte and cathode materials are the current development focus.Transition metal sulfides?TMSs?are considered as a promising negative electrode material for sodium ion batteries due to their high theoretical capacity,environmental friendliness and controllable morphology.However,the low electronic conductivity,large volume expansion,poor dynamic performance and high dissolubility during the electrochemical cycling limit their practical applications.To address these issues,this paper chooses the appropriate micro-nano structure construction method to improve their electrochemical properties,and systematically studies the TMSs in the electrochemical process of sodium storage mechanism.Based on above,we selected three different TMSs?CoS,Co₉S₈ and VS₂?as the research objects,designed and synthesized four micro-nano structures,and obtained excellent performance of SIBs anode materials.Combining with the discussion of the mechanism of sodium storage and the structural phase transition in the electrochemical process,the reasons for the improvement of the electrochemical properties of the materials were analyzed and verified.Finally,the experience in the research of the negative electrode material of SIBs was successfully applied to the design and synthesis of the cathode material of MIBs.The cathode electrode material with good cycling stability and high energy density was obtained.The obtained main research results were as follows:?1?CoS nanoparticles wrapped with RGO were successfully synthesized by hydrothermal method.The electrochemical properties and sodium storage mechanism as an andoe of SIBs were studied.The material showed the reversible specific capacity of 450 mAh g^-11 after 50 cycles under the current density of 100 mA g^-11 and good rate performance.The results of DSCV showed that the nano-sized CoS particles have obvious pseudo capacitance storage characteristics,which ensures the fast and efficient storage of sodium ions and the further development of electrochemical reaction.Meanwhile,evidenced bu the ex-situ SEM images and EIS,the presence of RGO in the composites not only improved the dynamic properties of the electrode materials,but also effectively alleviated the volume expansion of charge and discharge.The above results indicate that CoS@RGO composites with GO as carrier is an ideal anode material for SIBs.?2?Co₉S₈/Co composites were synthesized by solid phase method.Its electrochemical properties and sodium storage mechanism in ether-based electrolyte?EBE?were studied.The reversible discharge specific capacity of 528 mAh g^-11 can be obtained under the current density of 100 mA g^-11 in EBE system,the capacity retention rate is 90%after 50 cycles.At the high current density of 10 A g^-1.The material still showed the high discharge specific capacity of 173 mAh g^-1.This is because the non-active elemental Co nanoparticles in the Co₉S₈/Co composite improve the electronic conductivity of the material and inhibit the volume expansion during the cycling process.The physical and chemical properties of EBD and carbonate-based electrolyte?CBE?showed that ether electrolyte had lower surface activation energy and over potential,which was beneficial to improve the dynamic properties of the material.At the same time,EBE could effectively inhibite the side reactions between the electrode and the electrolyte interface and the dissolute of S^2-ions in the electrochemical reaction process.Therefore,under the synergistic effect of Co composite and EBE,the material as a anode electrode material of SIBs showed excellent cycling stability and rate performance.In addition,ex-situ XRD,TEM and HRTEM preliminarily verified the conversion reaction mechanism of Co₉S₈.?3?Co₉S₈@CYSNs composites with a Yolk-shell hierarchical structure were prepared,using ZIF-67 as the precuisor system.The sodium storage mechanism and electrochemical properties of Co₉S₈ as an anode of SIBs are systematically studied.In situ XRD,XAS and ex situ XPS,HRTEM and first principles calculations showed Co₉S₈ as an anode for SIB is a direct conversion reaction mechanism with“shrinking core mode”.In addition,the nano-sized Co₉S₈ particles and the unique Yolk-shell hierarchical structure in the Co₉S₈@CYSNs composites could accelerate the generation of conversion reaction,shorten the diffusion path of sodium ions and provide a fast electronic outlet channel,which cleverly solved the problem of large volume change and poor electrode dynamic performance of Co₉S₈ as a direct conversion reaction material.Therefore,the composite exhibited excellent long-cycle stability,with a capacity retention rate of up to 84.0%for 400 cycles at 1 A g^-11 and up to 84.5%for 800 cycles at 10 A g^-1.?4?Since the structural defects and doping of materials play a significant role in improving the kinetics of SIBs,we discussed the influence of S defects on the sodium storage mechanism and properties of two-dimensional TMSs VS₂.We successfully prepared VS₂ with disorder and ordered S defects by a simple solid phase method.Among them,the disordered defect stateVS_2-x?x=0.1?as an anode of SIBs showed excellent rate performance.At the current density of 0.5 C(1 C=200 mAh g^-1),the materials showed the discharge specific capacity of 297 mAh g^-1.At 100 C the material still dilivered the reversible specific capacity of 150 mAh g^-1.The resuls of the DSCV,GITT and EIS showed that the disordered S defects in the VS_2-x-x structure can improve the dynamic properties of the materials.The electrochemical energy storage mechanism could also be changed to more pseudocapacitance storage mechanism.Therefore,the introduction of some defects into the materials is a promising method to improve the sodium storage performance of the anode materials.If combined with other micro-nano structure construction methods,the anode materials of SIBs with better performance will be developed.?5?PP₁₄Cl was used as the additive of APC/THF electrolyte in MIBs to improve the difficulty of dissolvation of chloride electrolyte and the poor dynamic performance of Mg²⁺in anode materials.The magnesium storage mechanism and electrochemical properties of VS₂ were studied systematically.Ex situ XRD,FTIR and XPS confirmed that PP₁₄⁺could inserted into the VS₂ layers during the electrochemical process,and the spacing was increased from 5.7?to 11.3?.The interlaminar expansion of VS₂ effectively improved the diffusion coefficient of Mg²⁺.At the same time,due to the reduction of solvation energy of the Mg²⁺and the improvement of diffusion coefficient in the VS₂?APC-PP₁₄Cl/THF?Mg system,the material showed 348 mAh g^-11 reversible specific capacity and excellent rate performance.This work provided a new direction for the development of electrolyte of the MIBs.The method of macromolecular intercalation was expected to further improve the energy density and power density of MIBs.Above all,this paper successfully designed and developed five new synthesis methods of nano-composite materials through the construction of micro-nano structure of TMSs and applied them to the anode materials of SIBs and cathode materials of MIBs.The mechanism of sodium storage and structural phase transition of several kinds of TMSs during the electrochemical process were investigated.The reasons for the improvement in material properties were analyzed and verified at the molecular and atomic scales.It provides theoretical and experimental guidance for the construction of nano-micro structure of TMSs as anode material of SIBs and the development of cathode material of MIBs.