The Synthesis And Potassium Storage Mechanism Study Of Manganese-based Oxides

In recent years,with the explosive growth of the lithium-ion battery market,global lithium reserves are expected to be depleted in the near future.Potassium ion energy storage devices are emering as a large-scale energy storage systems due to their wide sources,high safety,and similar working principles compared with lithium ion energy storage devices.Manganese-based oxides are widely used as electrode materials for potassium ion energy storage devices due to their simple synthesis process,low cost,and high theoretical capacity.This thesis focues on manganese-based oxide materials,and the layered electrode materials with good performance are obtained through morphological design and component doping.By taking advantage of the pseudocapacitive properties,the manganese-based oxides are used as anodes for potassium ion capacitors.Meanwhile,the manganese-based oxides are applied as the positive electrode of potassium ion batteries and is further improved by the element doping and surface modification.X-ray diffraction and transmission electron microscopy techniques were used to characterize the potassium storage mechanism of manganese-based oxides electrode materials.The details are as follows:（1）The potassium permanganate and manganese sulfate were used as the manganese source and the reduced graphene oxide is used as the carbon source.MnO₂@rGO anode material with nanowire morphology was prepared by a simple hydrothermal method and used for potassium ions battery anode materials.The MnO₂@rGO anode shows excellent rate performance due to its one-dimensional structure and the presence of graphene coating(the specific capacity is 252 m A h g^-1at 0.02 A g^-1;and the specific capacity is 100 m A h g^-1at 0.5 A g^-1)and excellent cycle stability(400 cycles at 0.2 A g^-1,the capacity retention is 81.7%).The cyclic voltammetry was used to test the pseudocapacitive behavior of MnO₂@rGO.The specific capacitance at 1m V s^-1can reaches 389 F g^-1.In addition,using ex-situ XRD,TEM and other advanced characterization methods,the potassium storage mechanism of MnO₂anode materials was explored.（2）K_0.5MnO₂material and a series of K_0.5(Mn_1-x-yNi_xCo_y)O₂（x=1/6,y=1/6;y=0,x=1/3）cathode materials were synthesized by solid-phase sintering method.XRD results shows that Ni and Co elements successfully replaced part of the Mn atoms and formed P3-type solid solution structure.Electrochemical tests show that the rate performance and cycle stability of the electrode material after element doping are better than those of K_0.5MnO₂material.P3-type K_0.5Mn_2/3Ni_1/6Co_1/6O₂demonstrates the best electrochemical performance.The discharge capacity of K_0.5Mn_2/3Ni_1/6Co_1/6O₂is 75 m A h g^-1at a current density of 0.1C.At 0.2C,the capacity retention is 67%over 100 cycles.Ex-situ XRD analysis illuminate that K_0.5Mn_2/3Ni_1/6Co_1/6O₂is a single-phase transition during the charge and discharge process,which is conducive to improving the cycle stability of the material.The AC impedance test results show that K_0.5Mn_2/3Ni_1/6Co_1/6O₂materials have a higher potassium ion diffusion coefficient than K_0.5MnO₂materials,which explains the phenomenon that the rate performance of K_0.5Mn_2/3Ni_1/6Co_1/6O₂is improved after element doping.（3）On the basis of the above work,K_0.5Mn_2/3Ni_1/6Co_1/6O₂material is modified to improve the stability of the material.First,tetrabutyl titanate was fully mixed with K_0.5Mn_2/3Ni_1/6Co_1/6O₂material under the protection of argon gas,and TiO₂coating layer is formed on the K_0.5Mn_2/3Ni_1/6Co_1/6O₂material through a low temperature solid-phase sintering method.Electrochemical test results show that the TiO₂coating improves the electrochemical performance of the material,which shows a capacity of 83 m A h g^-1at0.1C.At 0.5C,the capacity retention of the TiO₂coated material can remain 60%after 100cycles.In addition,the AC impedance test shows that the potassium ion diffusion coefficient of the coated material is higher than K_0.5Mn_2/3Ni_1/6Co_1/6O₂,which is conducive to the improvement of electrochemical performance.