The Controllable Preparation And Electrochemical Potassium Storage Properties Of Hollow Carbon Spheres

Potassium is abundant in nature and cheap.In electrochemical systems,potassium has a lower redox potential and faster ionic conductivity;thus,potassium-ion batteries（PIBs）are expected to be a promising alternative to lithium-ion batteries（LIBs）in the future.However,because the radius of potassium ion（1.38?）is considerably larger than that of lithium ion（0.76?）,the electrode materials always undergo huge volume expansion and structural damage during the potassiation/depotassiation process,resulting in severe capacity fading.Therefore,constructing structurally stable potassium storage electrode materials is the focus of PIB research.In this dissertation,a carbon nanosphere with hollow structure is proposed.The potassium storage performance and structural stability are effectively enhanced through carbon shell optimization and heteroatom doping.The specific works are as follows:（1）Taking Si O₂ microspheres as the template and metal polyphenol complexes as the carbon source,hollow porous carbon spheres（HPCS）with different shell thicknesses are obtained through interfacial complexation and high temperature carbonization.The electrochemical performance of HPCS with different shell thickness in potassium ion batteries is systematically investigated to determine the shell thickness of hollow carbon spheres suitable for potassium storage.Among them,HPCS-3 has a hollow cavity structure,suitable wall thickness,large layer spacing,high specific surface area and high porosity,which is conducive to alleviating the volume expansion caused by the embedding and exiting process of potassium ions,as well as introducing more potassium active sites to increase the surface capacitance for potassium storage,thus exhibiting excellent potassium storage performance.Specifically,HPCS-3 exhibits a reversible specific capacity of 231.1 m Ah g^-1 at a current density of 50 m A g^-1 and a reversible specific capacity of 201.8 m Ah g^-1 after 1200 cycles,demonstrating a high potassium storage performance and electrode stability.（2）Sulfur doping has been demonstrated an effective strategy for improving the potassium storage performance of carbon anode materials.However,the poor thermal stability of sulfur restricts the development of sulfur-doped carbon anode materials.Here,a series of sulfur-doped hollow porous carbon spheres（SHPCS）with different S-doping amounts are facilely prepared via direct doping at high temperature,using sulfur powder as the sulfur source and selecting carbon spheres with appropriate wall thickness on the basis of the previous work.It is found that various features of SHPCS exhibited different trends with the change of S-doping amounts.In addition,the depth of the K-ion insertion reaction,the additional oxidation-reduction reaction,and the adsorption process of K ions on active sites can be enhanced by regulating the S-doping amount and defect level.Notably,the optimized SHPCS can achieve a prominent reversible specific capacity of 454 m Ah g^-1 at a current density of 0.05 A g^-1,with an initial Coulomb efficiency of 62.3%.Moreover,after1000 cycles at a current density of 1 A g^-1,the capacity retention rate is as high as 93.7%,demonstrating outstanding cycling stability.Meanwhile,the density functional theory calculation shows that the doped sulfur atoms provide abundant active sites for the adsorption of potassium ions,thereby increasing the reversible capacity of PIBs.