Design And Preparation Of High Performance Anode Materials For Sodium/Potassium-ion Batteries

The sodium/potassium ion batteries work in a similar way to lithium ion batteries,and the richer reserves and lower prices make sodium/potassium ion batteries have been regarded as the greatly promising alternatives to lithium ion batteries for large-scale energy storage fields.The performances of sodium/potassium ion batteries depend on the electrode materials and other factors.So,it is a great challenge to develop suitable electrode materials.Because the radius of sodium and potassium ion is even larger than that of lithium ion,resulting in the lower diffusion coefficient of sodium/potassium ion and huge volume expansion during the reaction process.This thesis focuses on improving the electrochemistry performance of several kind of anode materials by the multiple strategies,including designing the structure of materials and optimizing the electrolyte.Moreover,the electrochemical reaction mechanisms are revealed.In Chapter 1,we briefly introduce the composition,working principle and electrode materials of sodium/potassium ion batteries.Besides,the research progress of the materials in the thesis are discussed in detail,which consist of Nb2O5,hard carbon and Bi.In Chapter 2,we give a summary of raw materials,characterization methods and equipments used for the experiment in the thesis.In Chapter 3,we achieved the superior sodium storage performance of nano-sized Nb2O5 encapsulated in the three-dimensional porous carbon matrix self-supported thin film by a facial electrostatic spray deposition(ESD)technique.It can still deliver a specific capacity of 130 mAh g-1 after 7500 cycles at high rate of 10 C.The kinetics analysis reveals the fast sodium storage mechanism of the pseudocapacitance.Furthermore,the sodium ion full batteries are assembled to verify its potential application value.In Chapter 4,we realize free-standing and binder-free carbon nanofiber electrode by simply carbonization bacterial cellulose.The high initial Coulombic efficiency(93%)and long cycle life at high rate(105 mA h g-1 at 10 A g-1 after 10,000 cycles)of anode materials were obtained by tailoring the structure and optimizing the electrolyte.Furthermore,the study shows sodium ions could adsorption-co-intercalation of solvent and sodium ions in ether-based electrolytes.In Chapter 5,based on previous chapter,the MoS2 nanoparticles with high theoretical capacity(660 mAh g-1)are introduced in the porous carbon nanofibers,and the obtained MoS2/Carbon composites possess 617 mAh g-1 specific capacity at 0.1 A g-1 current density.In Chapter 6,the nanosized Bi spheres are encapsulated by a conductive porous N-doped carbon shell to form a core shell structure Bi/Carbon composites.The carbon shell prevents the volume expansion and the optimized electrolyte reduces the impedance.The obtained Bi/Carbon composite exhibits unprecedented rate capability in both sodium ion batteries(178 mAh g-1 at 100 A g-1)and potassium ion batteries(152 mAhg-1 at 100 A g-1).In Chapter 7,we design optimized Bi@Void@C nanospheres with yolk-shell structure that exhibit the best cycling performance(198 mA h g-1 at 20 A g-1 over 10000 cycles)and enhanced volumetric energy density.The in-situ X-ray diffraction(XRD)and in-situ transmission electron microscope(TEM)reveal the electrochemical reaction mechanisms between the Bi and Na.Moreover,the theoretical calculation reveals the fast kinetics process at the atomic level.In Chapter 8,we give a summary and perspective of this thesis.