Preparation Of Carbon-based Anode Materials And Their Potassium Storage Properties

Potassium-ion batteries?PIBs?have been considered to be a promising alternative to lithium-ion batteries?LIBs?for large scale applications due to the abundant potassium resources and the lower redox potential of potassium.However,K ion has a larger radius,which inevitably results in huge volume expansion and structural damage of anode material during cycling,causing inferior cycling stability.Therefore,exploring appropriate anode material is needed to promote the development of PIBs.In this work,carbon materials and carbon modified composites have been investigated by designing efficient nanostructures to explore anode materials with high performance for PIBs.This thesis contains seven parts,and the results are specifically illustrated as below:1)The first chapter of the thesis introduces the key materials for PIBs and their research status.Particularly,the cathode materials and anode materials,and their unsolved scientific problems are summarized.2)The second chapter introduces the raw materials,instruments,and characterization methods of samples.3)In chapter 3,in order to combine the advantages of graphite showing an obvious voltage plateau during potassiation/depotassiation and amorphous carbon with more flexible structure to tolerate the volume expansion during cycling,we fabricated a nitrogen-doped bamboo like carbon nanotubes?NBCNTs?with a suitable graphitization degree by controlling the pyrolysis conditions.The electrochemical performance results suggest that the NBCNTs annealed at 800^ofor 1h exhibits superior K-storage performance(204 m Ah g^-1 after 1000 cycles at 500 m A g^-1).The reasons for the excellent electrochemical performance are as follows:First,the NBCNTs is composed of short range carbon layers and amorphous carbon,thus creating an open structure on the surface.The K⁺can insert/extract into/from carbon layers from both the open end and the surface;Second,the short range carbon layers possess more flexible structure,which can effectively accommodate the volume change during cycling;Third,the incorporation of N can also provide active sites to enhance the K⁺storage.Additionally,the discharge/charge curves consist of two sections called plateau and slop originating from the K⁺insertion/extraction process and surface-driven K-storage process.Therefore,the capacity from surface-driven capacitive contribution and diffusion-controlled insertion processes can be reasonably combined by regulating the graphitization degree of carbon materials,which is conducive to improving rate performance and energy density when they are assembled into full PIBs.4)Due to the limitation of potassium storage mechanism,the specific capacity of carbon-based materials is lower.However,the potassium storage performance of anode materials with high theoretical specific capacity can be effectively improved through the carbon modification and structural design.In chapter 4,urchinlike Bi microspheres coated with nitrogen-doped carbon composite?Bi@NC?was prepared by solvent thermal method,PPy coating and calcining reduction process.The urchinlike structure have the following advantages:First,the urchinlike Bi@NC microspheres have abundant fluffy needles,which can greatly augment the contact surface between the electrodes and electrolyte,as well as acting as an obstacle to aggregation of the Bi@NC microspheres.Second,PPy-derived N-doped carbon coating can improve the electronic conductivity and accommodate volume expansion during cycling,resulting in excellent structure stability and electrochemical performance.The in situ X-ray diffraction results successfully reveal the phase evolution of Bi@NC composite,which is Bi?Bi K_x?Bi₂K?Bi₂K₃?Bi K₃,suggesting high reversibility.As a result,Bi@NC composite electrode exhibits superior rate capability(331.9 m Ah g^-1 at 4.0 A g^-1)and cycle stability(272m Ah g^-1 at 2.0 A g^-1 after 500 cycles).5)In chapter 5,in order to explore the anode material with high-performance,we investigated the K-storage performance of Fe Se₂.A composite composed of Fe Se₂nanoparticles?NPs?and N-doped carbon with a structure of carbon-coated Fe Se₂ NPs anchored on thin carbon nanosheets?Fe Se₂/NC?was prepared via a simple in situ chemical transformation method through pyrolyzing a mixture of Fe?NO₃?₃,urea,and citric followed by selenization process.The nanoscale particles can shorten the K⁺diffusion path,the in situ grown carbon coating acts as a protective layer to alleviate the volume change of Fe Se₂ during cycling,and the carbon nanosheets serve as a matrix offering the required conductivity to individual Fe Se₂ NPs,and stabilizing the integral structure.As a result,Fe Se₂/NC composite delivered high capacities of 434 m A h g^-1 after 70 cycles at 0.1 A g^-1,and 301 m A h g^-1 after 250 cycles at 1.0 A g^-1.The facile synthesis and superior electrochemical performance of Fe Se₂/NC composite render it a promising anode material for PIBs.6)The potassium storage performance of metal chalcogenide compounds?MCs?can further improved by changing the composition of electrolyte.In chapter 6,the composites of CoSe nanoparticles dispersed in nitrogen-doped carbon nanotubes?CoSe@NCNTs?were synthesized by wet chemistry and solid-phase selenization process,and their electrochemical properties were investigated in different electrolytes.The results demonstrates that the CoSe@NCNTs anode shows the best cycling performance in KPF₆-DME electrolyte and a high initial Coulombic efficiency?ICE?of 95%in the voltage window of 0.01-3.0 V vs.K/K⁺.The electrochemical performance combination with the quantum chemistry?QC?calculations indicate that KPF₆ play a catalytic role for the polymerization of DME in the initial activation process,forming a polymer-like film on CoSe@NCNTs surface.This in-situ formed polymer-like film can effectively prevent the dissolution of polyselenide intermediates into the electrolyte,stabilize the Co⁰/K₂Se interface and enhance the Co⁰/K₂Se conversion reaction reversibility.Moreover,the one-dimensional?1D?NCNT shell acts as a protection layer can effectively adapt the volumetric expansion during intense?de?potassiation process,shorten the electron transportation path and improve electronic conductivity.Consequently,CoSe@NCNTs exhibit a high ICE,excellent rate capability and cycling stability in 1 M KPF₆ in DME electrolyte.The characteristic of KPF₆-DME electrolyte might be extended to other metal chalcogenide compounds?MCs?for performance-enhanced PIBs.7)Finally,the main research achievements and future research plans are given.