Preparation And Energy Storage Of Transition Metal Sulfides

With the rapid development of new energy electric vehicles and portable electronic devices,the development of low-cost,safe,reliable,and high-energy-density secondary batteries has become a top priority.Since the commercialization of lithium ion batteries(LIBs),they have caused explosive academic progress worldwide due to their light weight,long life and excellent electrochemical performance.However,considering the current supply restrictions and rising costs of lithium salts make it difficult for LIBs to meet energy storage needs.Sodium and potassium salts are not only rich in resources and low in price,but their chemical environment and working principle are similar to LIBs.Sodium ion batteries(SIBs)and potassium ion batteries(PIBs)built on this basis have the advantages of low cost and good sustainability,and are expected to achieve commercial applications.However,Na⁺/K⁺has a larger mass and radius than Li⁺,which increases the internal diffusion barrier during the insertion/extraction process between the positive and negative electrodes of SIBs/PIBs.Therefore,it is difficult for many LIBs anode materials to exhibit good performance in SIBs/PIBs.In view of this,the development of a negative electrode material capable of quickly and stably inserting/extracting Na⁺/K⁺is the key to improving the performance of SIBs/PIBs.Based on the above considerations,transition metal sulfides were selected as the research object.The structure design,synthesis method,energy storage mechanism and performance control of active materials are introduced in detail,and the electrochemical performance as a battery anode material was deeply studied.The research contents are as follows:(1)Based on the high theoretical capacity of transition metal sulfides,we constructed a one-dimensional spiral VS₄ nanomaterial by a template-free hydrothermal method,and studied the performance and mechanism of storing Na⁺.In Chapter 2,the study of growth mechanism revealed the Oswald ripening process of VS₄.During the growth process,it first nucleated into a spherical shape of about 3?m,and then gradually ripened and split into a spherical structure composed of one-dimensional spiral VS₄.One-dimensional spiral VS₄ as a negative electrode material for SIBs can obtain excellent reversible capacities of 660 and 589 mAh g^–1 when the current density is 1 and 3 A g^–1 at room temperature.In addition,the VS₄ electrode also exhibits excellent discharge capacity and cycle stability at a low temperature of 0?.At the same time,the storing Na⁺mechanism of VS₄ was studied based on ex-situ characterization,cyclic voltammetry and electrochemical kinetic analysis.The research results show that the storage mechanism of Na⁺can be divided into three stages:(?)In the first ten cycles,the transformation of VS ₄ to Na₂S and V is partially reversible,which makes the discharge capacity decay in the initial stage;(?)In the second stage of subsequent capacity increase,the reversible conversion reaction between Na₂S and S lead to the increase of the storage capacity of Na⁺;(?)In the final stable third stage,the reversible conversion reaction between Na ₂S and S became stable.This is similar to the main reaction mechanism of Na-S batteries.The research on the one-dimensional spiral VS₄ in this chapter can provide basic ideas for the research of other chalcogenides in the fields of batteries,catalysis and electronic devices.(2)On the basis of Chapter 2,we invented a simple method for regulating and preparing VS₄ ultra-thin nanosheets and applied it to SIBs system in Chapter 3.Using deionized water as the solvent,the thickness of VS ₄ nanosheets can be controlled by adjusting the molar ratio of sodium orthovanadate an d thioacetamide in the solution.The study found that the ultra-thin VS₄ nanosheet has a larger specific surface area(157.1 m² g^–1),which is conducive to the contact between the electrode material and the electrolyte and provides more active sites for th e storage of Na⁺,and its ultra-thin structure can effectively shorten the transmission path of Na⁺during charge and discharge.In addition,VS₄ nanosheets have a rich mesoporous structure,which can buffer the volume expansion caused by Na⁺storage while providing a transmission path for Na⁺transmission.The optimized VS₄ ultra-thin nanosheet has a storage Na⁺capacity of 584 mAh g^–1 after 900 cycles under current density of 3 A g^–1.In addition,the VS₄electrode also showed excellent electrochemical pe rformance at a low temperature of0?,indicating that it has good temperature adaptability.This achievement not only provides a certain research basis for the morphology control of VS ₄,but also shows that VS₄ ultra-thin nanosheets are promising active materials for high-performance energy storage devices.(3)A method for preparing high crystallinity(NH₄)₂Mo₃S₁₃ material was established,and it was first used as a negative electrode material in the SIBs/PIBs system.In Chapter 4,(NH₄)₂Mo₃S₁₃ was successfully prepared by one-step hydrothermal method,and the morphology and crystallization properties of(NH₄)₂Mo₃S₁₃ were controlled by adjusting the molar ratio of thioacetamide and sodium molybdate in the solution.When the optimized(NH ₄)₂Mo₃S₁₃ is used as the negative electrode material of SIBs/PIBs,it exhibits excellent discharge capacity and capacity retention.In the room temperature environment,the Na⁺storage capacity of(NH₄)₂Mo₃S₁₃ after 1000 cycles at a high current density of 10 A g^–1 is 165.2 mAh g^–1.In the room temperature potassium ion half-cell test,the capacity remains at 120.7 mAh g^–1 after 500 cycles under the current density of 1 A g^–1.In addition,the(NH₄)₂Mo₃S₁₃electrode also shows excellent electrochemical performance under low temperature test environment.In addition,a combination of electrochemical kinetic analysis and a series of ex-situ characterization tests revealed the storage mechanism of Na⁺in the(NH₄)₂Mo₃S₁₃ electrode material.Therefore,(NH₄)₂Mo₃S₁₃ has the potential to become a low-cost,high-performance and safe SIBs/PIBs anode material,which will also arouse widespread interest in the research of sulfur-rich transition metal sulfide engineering and energy storage devices.(4)Optimized and improved the preparation method of(NH ₄)₂Mo₃S₁₃.In Chapter4,an excess of thioacetamide with a molar ratio of 14 times that of sodium molybdate was added to provide a sufficient source of sulfur.However,it generates a large amount of highly toxic H₂S gas during the hydrothermal reaction,which not only pollutes the environment but also poses a great threat to the personal safety of researchers.In Chapter 5,the introduction of ammonium hydroxide greatly reduces the difficulty of preparing(NH₄)₂Mo₃S₁₃(the molar ratio of sulfur to molybdenum is 4.3:1),and high crystallinity of(NH₄)₂Mo₃S₁₃ can be obtained by thioacetamide with a molar ratio of 5times that of sodium molybdate.This method not only greatl y reduces the release of H₂S,but also reduces the amount of thioacetamide by 64.3 wt%,which effectively reduces manufacturing costs.The prepared(NH₄)₂Mo₃S₁₃ has excellent discharge capacity and long cycle performance at room temperature when used as th e negative electrode material of SIBs.In addition,the(NH₄)₂Mo₃S₁₃ electrode also showed excellent electrochemical performance at a low temperature of 0?,which shows that the(NH₄)₂Mo₃S₁₃ electrode has good temperature adaptability.This work provides the basis for the subsequent application of(NH₄)₂Mo₃S₁₃ in the field of energy storage.