Design Of Hard Carbon Anode Materials And Binders And Their Sodium-storage Performance

Sodium-ion batteries（SIBs）,endowed with abundant resources,low cost,and high safety,are expected to solve the consumption,instability,discontinuity,and potential safety hazards of power generation and grid connection in clean energy,which has become an important research direction of large-scale energy storage technologies.Since the anode is the core component of SIBs,the development of anode materials with high sodium storage performance will contribute to promoting the commercialization of SIBs.Hard carbon has great potential as an anode material for SIBs due to its high specific capacity,low sodium storage potential,and good cycle stability.However,due to the large ion radius and atomic mass of sodium ions（Na⁺）,the diffusion kinetics of Na⁺is sluggish,resulting in a poor rate performance of hard carbon anodes.Simultaneously,the abundant pores and defects of hard carbon often result in the formation of massive solid electrolyte interphase（SEI）films during the first charge-discharge process,and the irreversible reaction of Na⁺will occur at some defective sites of hard carbon,leading to the loss of Na⁺,which both lead to a low initial Coulombic efficiency（ICE）of hard carbon anodes.Currently,the sodium storage mechanism of hard carbon is still not clear,so the preparation of high-performance hard carbon anodes lacks scientifically theoretical guidance.This thesis focuses on the structural design of hard carbon,the study of the sodium storage mechanism of hard carbon,and the optimization of binders for hard carbon anodes to solve the problems mentioned above.The specific contents are as follows:（1）A kind of phenolic resin-based@glucose-based hard carbon composite material was synthesized by the glucose melting-coating strategy using the phenolic resin-based microporous hard carbon as the coating substrate.The phenolic resin-based microporous hard carbon has a lower graphitization degree and richer defects,which can improve the capacitive capacity of sodium storage,while glucose-based hard carbon has a higher graphitization degree and a lower specific surface area,which can improve the graphitization degree and reduce the specific surface area of composite materials.Through the synergistic effect of the two hard carbons,the ICE and rate performance of the composites are improved.Compared with the microporous hard carbon,the ICE of the composite material increases from 76.4%to 88.6%,and the reversible specific capacity increases from 195.4 to 310.3 m A h g^-1 at 0.02 A g^-1 when the coating amount of glucose-based hard carbon is 30 wt.%.The reversible specific capacity of the composite material is 97.1 m A h g^-1 at 5.0 A g^-1.In addition,the glucose melting-coating method can also effectively improve the ICE and rate performance of phenolic resin-based mesoporous hard carbon anodes.（2）Using graphene oxide as a structure-directing agent,a kind of carbon nanosheets were synthesized via in-situ polycondensation of multi-component（resorcinol,formaldehyde,and aniline）on both sides of graphene oxide and a carbonization process.The carbon nanosheet has extremely concentrated ultramicropores（～0.52 nm）and C=O/-OH groups.When the carbon nanosheet is used as an anode material for SIBs,it delivers a capacity of 318 m A h g^-1 at 0.02A g^-1,a rate capability of 145 m A h g^-1 at 5.0 A g^-1,and approximately 95%of reversible capacity below 1.0 V.These results show that carbon nanosheet anode has high specific capacity,superior rate capability,and high reversible capacity below 1.0 V.Additionally,the carbon nanosheet anode exhibits a new charge model with dual potential plateaus.The deintercalation of Na⁺from graphitic layers is manifested as the low-potential plateau region（0.01～0.1 V）,contributing to stable insertion capacity.Meanwhile,the surface desodiation process of the C=O and-OH groups corresponds to the high-potential plateau region（0.4～0.7V）,contributing to a fast capacitive sodium storage.The sodium-storage mechanism of“two potential plateaus”provides a new insight into the design and regulation of the microstructure of hard carbon anodes for high-performance SIBs.（3）A multi-functional sodium alginate（SA）/polyethylene oxide（PEO）binder with massive-COOH/-OH groups and abundant Na⁺was synthesized via a feasible esterification reaction.The specific functions mainly include that:1)the binder forms a passivation film on glucose-derived carbon（GC）to inhibit the decomposition of the electrolyte and provide stronger adhesion strength;2)the Na⁺conduction is improved via sufficient ionic transfer channels provided by ionic PEO;3)The abundant Na⁺in the binder can compensate for the irreversible Na⁺consumption of the hard carbon anode during the first charge-discharge process and generate the function of pre-sodiation to promote the interfacial ionic transport.The ICE and rate capability of the GC anode using the SA/PEO binder are improved due to the multiple functions of the SA/PEO binder,compared with the GC anode using the PVDF binder.The ICE of the GC anode increases from 77.1%to 87.1%at 0.02 A g^-1,and the capacity increases from98.3 to 193.0 m A h g^-1 at 0.2 A g^-1.The ICE and capacity of GC anode using SA/PEO binder increase by 10%and 100 m A h g^-1,respectively.Furthermore,SA/PEO binder is also suitable for coal-based and polymer-based hard carbon anodes,which can also improve the ICE and rate capability of the anode materials.