| With the rapid development of consumer electronics,electric vehicles and renewable energy sources,electrochemical energy storage（EES）system has become an important part of new energy.As an integral part of the EES system,electrode materials have an important effect on system performance.Among these,graphite has been used as the anode for lithium-ion batteries（LIBs）for nearly 30 years and is being widely used in the emerging potassium-ion batteries（PIBs）.However,the conventional layered graphite has high anisotropy,which is prone to slippage and flaking during alkali metal storage,leading to degradation of cycling stability.Therefore,the modulation of microstructure and morphology is one of the main research directions to improve the electrochemical performance for LIBs/PIBs.In our laboratory,micron-sized graphitized onion-like carbon（GOC）with specially arranged carbon layers has been prepared by a simple injection pyrolysis method using pyridine as the carbon source.This structure can effectively avoid interlayer slippage and has a higher cycle stability than the common used artificial or natural graphite with high anisotropy.However,the restricted multi-shell structure leads to sluggish kinetics and low capacity of GOC,limiting its application in high performance batteries.Based on this,this thesis focuses on the structural control and modification of GOC,mainly including the construct pore structure,heteroatom doping,expanding interlayer spacing,and combining structural engineering with electrolyte engineering to solve the problems of low rate and capacity.It provides a new idea for GOC to replace the traditional layered graphite material as the anode of LIBs/PIBs.Specific works are as follows:（1）The GOC with suitable edge and pore structure was prepared by using high energy laser beam to peel the carbon layer from the surface of GOC.On the one hand,the edge structure and porous structure generated by laser etching provide more active sites and ion diffusion channels,thus improving the reversible capacity and rate performance in both LIBs and PIBs.On the other hand,the laser-etched graphitic onion-like carbon（LGOC）still maintains concentric multi-shell structure,which facilitates the structural integrity during lithium/potassium storage and thus has excellent cycling stability.When used as a LIBs anode,the LGOC has a high reversible capacity of 436 m A h g-1and 89 m A h g-1at 0.05 A g-1and 10 A g-1,respectively,and maintains a high reversible capacity of 312 m A h g-1after 300 cycles at 1 A g-1.LGOC also exhibits excellent potassium storage capacity of 289 m A h g-1.Laser etching provides a simple and universal strategy for the controlled introduction of structural defects in graphite materials.（2）GOC with pore structure and nitrogen doping was prepared from GOC with definite structural morphology and crystallinity by air oxidation and ammonia water reaction at high temperature.On the one hand,nitrogen doping improves the wettability of electrode materials,providing more ion diffusion channels and shortening diffusion paths.On the other hand,the interconnected pores and the restricted polyhedral structure can effectively buffer the bulk effect during charging and discharging preocesses.In addition,the single factor effect of nitrogen doping on electrochemical energy storage of graphite was investigated using GOC as a model.The modified GOC shows excellent electrochemical properties of potassium storage with a high specific capacity of 220 m A h g-1at 2 C and a reversible specific capacity of 263 m A h g-1after 200 cycles at 1 C.In addition,XPS confirmed that only 1.14 at.%pyridine nitrogen was found in GOC reacted with ammonia at high temperature.Moreover,GOC treated with oxygen and ammonia contained1.61 at.%pyridine nitrogen.It demonstrates that pyridine nitrogen in graphitic carbon can provide more reactive sites and improve the wettability of the electrode materials.（3）Expanded graphitic onion-like carbon（EGOC）with larger layer spacing was prepared by the rapid thermal expansion method using GOC as raw material.Comparing with the expanded natural flake graphite（ENFG）,we systematically revealed the formation mechanism and the evolution of the morphological structure of GOC with restricted structure during rapid thermal expansion.The results show that the GOC covered by carbon base plane presents a restricted polyhedral structure,which makes the GOC present a different expansion mode from the NFG,but dissociates into bamboo shoot-like expanded graphite along the different directions of the polyhedron.Different from the planar structure of ENFG lamellar,EGOC possesses a spiral structure with a large number of wavy wrinkles,and exhibits better cyclic stability(275 m A h g-1after 1000 cycles at 2 C),which has higher structural stability than ENFG.In addition,the increased layer spacing and interconnected mesopores of EGOC provide fast electron migration pathways.The EGOC exhibits excellent potassium ion storage performance with high reversible capacity(404 m A h g-1at 0.2 C)and excellent rate performance(200 m A h g-1at 5 C).This work proves that pyridine nitrogen can provide more reaction sites and improve the wettability of graphite electrode.（4）In response to the shortcomings of the low initial Coulomb efficiency of expanded graphite prepared by rapid thermal expansion,and the conventional ester/ether-based electrolyte with high melting point,volatility and flammability,structural design and electrolyte engineering strategies are proposed to improve ion/electron diffusion dynamics and battery safety and low temperature operating performance.Mild-expanded spherical graphitic onion-like carbon（MEGOC）with extended layer distances and low specific surface area was prepared by a slow thermal expansion method,which can accommodate volume changes and promote potassium diffusion,achieving excellent stability and rate performance.In addition,using a non-flammable and low-melting electrolyte,triethyl phosphate,ensures safety and low-temperature operating performance.In a non-flammable potassium ion electrolyte,the MEGOC exhibits a long cycle life(175 m A h g-1after 2000cycles at 2 C),excellent low-temperature performance(165 m A h g-1at-40°C)and high initial Coulomb efficiency（53%）close to that of the graphite anode.At the same time,the assembled non-flammable potassium ion capacitor（PIC）has excellent electrochemical performance,which can meet the energy and power density requirements of EES devices at room temperature and low temperature.At 0℃,the PIC has high specific capacity of 142 m A h g-1at 2 A g-1,which is 51%of its room temperature capacity. |
PDF Full Text Request