Synthesis Of Transition Metal(Co,Mn)Selenide Nanocomposites For Performance Of Potassium Storage

With the worldwide popularization and promotion of new energy vehicles,lithium-ion batteries with high energy density,excellent energy storage performance and high safety performance have been used as electrical energy providers.However,lithium resources on the earth are scarce,accounting for only 0.017%,expensive and not sustainable.Therefore,it is necessary to find alternative lithium elements.Potassium metal,which has the same main group,high abundance,low price,and similar chemical properties to metal lithium,has received widespread attention.There are also a large number of researchers involved in the research of metal potassium ion batteries.However,the large radius of potassium ions results in the slower migration of potassium ions in the active material,which makes it difficult to perform rapid charge and discharge processes.Seriously,during the charge and discharge process,it can cause huge volume expansion until the material structure is destroyed,resulting in a sharp capacity attenuation.Therefore,the search for suitable electrode materials is still the key to the development of anode materials for potassium ion batteries.In this thesis,transition metal(Co?Mn)chalcogenide is used as the anode material for potassium ion batteries.Transition metal chalcogenides have a typical crystal structure.Each metal atom and chalcogen atom form a six-coordinate octahedron in the SMS(M=transition metal atom,S=chalcogen atom such as S,Se,etc.)It can rely on the weak van der Waals force link between the layer and the layer to allow foreign ions to freely insert and disengage in the layer gap.Therefore,as a anode material for potassium ion batteries,transition metal chalcogenides have great advantages such as high theoretical specific capacity,high energy density,non-toxicity,and low cost.Compared with transition metal oxides,transition metal chalcogenides have higher reversibility during electrochemical reactions.Furthermore,if combined with a conductive carbon material,the K+transmission rate of the electrode material can be increased.The metallic Co0.85Se hollow nanostructures encapsulated in graphen layers(Co0.85Se/G)is successfully synthesized by selenization of Co3[Co(CN)6]2 prussian blue analogue(Co-Co PBAs)in graphene suspensions through hydrothermal method.That is a unique hollow metallic Co0.85Se cubes encapsulated in graphene(Co0.85Se/G)with enhanced potassium storage performance.The unique hollow structure has high surface that guarantees Co0.85Se enough contact with electrolyte.The graphene wrapping blocks the agglomeration and relieves volume expansion of Co0.85Se.The metallic property of Co0.85Se and high conductivity of graphene can boost the rate performance.As a consequence,Co0.85Se/G delivers a high capacity of 324 mAh g-1 at 50 mA g-1 over 200 cycles,a high capacity of 213 mAh g-1 can be kept even at 1000 mA g-1 over 1000 cycles.Firstly,pure phase CoSe2 cubes have successfully synthesized byprecursor Co-Co PABs and selenium source.Then CoSe2 nanocubes was doped with Mo.The doping of metallic molybdenum is beneficial to increase the lattice spacing of CoSe2 while reducing the resistance of potassium ions.Morever,the addition of metal ions can greatly increase the carrier adsorption of the material.As a consequence,Mo-CoSe2 nanocubes as anode materials for potassium ion batteries exhibit high reversible capacity(861 mAh g-1)at a current density of 50 mA g-1 over 200 cycles,a high capacity of 241 mAh g-1 at a current density 1000 mA g-1 over the 1000 cycles.Hollow MnO2 nanotubes in the precursor were coated with organic dopamine to obtain hollow PDA@?-MnO2 nanotubes(PDA@?-MnO2 NT),then carbonized to obtain hollow C@?-MnO nanotubes(C@?-MnO NT),and finally selenized at high temperature to obtain hollow C@?-MnSe nanotubes(C@?-MnSe NT).The unique hollow structure of C@?-MnSe NT can increase the area in contact with the electrolyte and increase the potassium storage capacity.Carbon materials can increase the electrical conductivity of materials and increase the conductivity of electrons.As a anode materials for potassium ion batteries,at a current density of 50 mA g-1,C@?-MnSe NT material has a higher first charge and discharge specific capacity(672 and 822 mAh g-1)than C@?-MnO NT material,and CE is 82%.After 300 cycles,the specific capacity is 245 mAh g-1.The specific capacity of the C@?-MnO NT electrode after 300 cycles is only 142 mAh g-1.When the current density is increased to 1000 mA g-1,the specific capacity is stabilized at 221 mAh g-1.