Research And Application Of Transition Metal Selenium In Anode Materials Of Lithium/Potassium Ion Batteries

Since the first lithium-ion battery was introduced in 1991s,after more than 20 years of development,lithium-ion batteries have been widely used in mobile power supplies such as mobile phones and notebook computers.However,graphite,which is the anode material of commonly used commercial LIBs,has a only theoretical capacity of only 326 mAh g'1.This theoretical capacity value can barely meet the requirements of current small-capacity energy storage device,and is the limiting factor for LIBs to become the electric vehicle energy supply devices.Metal selenides has attracted wide attention from researchers because of its high capacity,abundant resources and low price.However,it has problems such as low conductivity and large volume expansion in the cycle tests.At present,material properties are usually improved by constructing nanostructures and carbon doping.The increasing price of lithium sources and the safety problems caused by lithium dendrites since 2000s lead to the research and development of potassium ion batteries.Potassium ion batteries have the advantages of high,low reduction potential,high potassium ion conductivity,etc.However,the larger ionic radius of potassium ions makes it difficult to select suitable electrode materials.Conventional carbon materials have high electrical conductivity and are relatively stable in cycling testing,but have the disadvantage of lower specific capacity.The capacity of alloy-based materials is relatively high,but the volume expansion produced by them is more serious than that of lithium-ion batteries.When the metal oxide is used as a cathode material,the cycle capacity is higher than that of the carbon material,but there is also a disadvantage that the structure is unstable in a long cycle test.In order to find better ways to develop high-performance electrode materials,this work focuses on the rational design of microstructures and composite nanomaterials for Li+/K+ storage,in which hydrothermal methods are used.Combined with some phase characterizations and corresponding performance tests,the electrochemical properties of the materials were studied and the effects of material structure on electrochemical performance were explored.In view of the above problems,the research work of this paper are mainly in the following two aspects:(1)Research on the synthesis and modification of two-dimensional metal selenide which can be controlled by internal structure,the electrocatalytic hydrogen evolution performances and lithium ion batteries performances of two-dimensional metal selenide are improved by the regulation of internal structure.(2)Controllable synthesis of metal selenide and metal oxide composites and performance study of potassium ion batteries.The optimum molar ratio of the heterojunction was determined by adjusting the molar ratio of metal selenide to metal oxide and studying its performances when used as negative electrodes of potassium ion batteries.The specific introduction are as follows:Using a simple two-step hydrothermal method,MoSe2 nanospheres with different internal structures and morphologies can be obtained only by adjusting the temperature of the second step reaction.The results show that when the reaction temperature is 100 ?,the products are MoSe2 solid spheres with smooth surface.When the reaction temperature is 140 ?,nanospheres having a hollow core-shell structure and having shallow short nanosheets on its surface are obtained.When the reaction temperature is 180?,the MoSe2 nanospheres have a hollow structure with many ultra-thin nanosheets on the surface.When the reaction temperature is raised to 220?,the MoSe2 nanospheres still showed a hollow structure,and the nanosheets constituting the nanospheres became larger and thicker than that in 180 ?.Electrocatalytic hydrogen evolution performance and lithium ion battery performance test results show that the MoSe2 sample prepared at 180 ? has the best activity compared with other samples when the reaction temperature is 180 ?.The MoSe2@MoO2 heterojunction in which MoO2 nanoparticles embedded on MoSe2 nanosheets was synthesized by hydrothermal method and ethylene glycol as reducing agent.The optimum ratio of MoSe2@MoO2 heterojunction was determined by adjusting the ratio of added MoSe2 and MoO2.The performance of the heterojunction potassium ion battery was studied.Compared with the pure phase MoSe2 and pure phase MoO2,the electrochemical performance of the heterojunction was greatly improved compared with the phase-pure material.After 100 cycles of current density of 500 mAh g-1,the heterojunction still maintained a capacity of 255 mAh g-1 with the coulombic efficiency being approximately 100%.