| Recently,sodium-ion batteries(SIBs)as an alternative of lithium-ion batteries,have been a significant electrochemical technology for energy storage owing to their abundant Na resource and low cost.Developing high-performance electrode materials and further improving the energy density and power density of SIBs as well as cycling life are crucial for their practical applications.The hard carbon materials have been considered as a promising candidate,which suffer from hardly volumetric expansion during charge-discharge process because of their large carbon interlayer distances and stable structures.However,the explored hard carbon materials as SIBs anode with low capacity and unstable cyclability as well as sluggish kinetics are unable to meet the requirements of high energy/power density,which hinders their practical applications.Optimizing the structure of hard carbon to improve the Na-storage performance is crucial for solving the problems above.As for the chanllenges above,the chemical modification and structural design are applied to tune the microstructure of hard carbon,such as interlayer distances,defects,active functional groups and specific surface area,to enhance electrochemical sodium-storage performances by taking the exploration of the structure-activity relationships of hard carbon anode materials as main line,which favors and provides theory and methods for high power density and high energy density of SIBs anode.This paper reports several studies and innovative achievements on the hard carbon as anode of SIBs.(1)The tunable microstructure and sodium storage mechanism of hard carbon near-sphere microsphere were investigated.To provide guidance for the design and development of hard carbon,hard carbon near microspheres(HCNSs)were prepared through spray dry and carbonization to explore the correlation between microstructure of hard carbon and electrochemical behaviors.Based on the comprehensive analysis,the storage mechanism of hard carbon was determined as“adsorption-insertion”:the plateau region corresponded to the insertion of Na+into carbon interlayer;the slope curve corresponded to the adsorption of Na+on the surface of carbon materials.Improving the ordered carbon layers degree and decreasing heteroatoms content could contribute to the increased plateau capacity when the carbon interlayer spacing was above 0.364 nm.Changing specific surface area can tune the electrochemical behaviors of“adsorption-insertion”of hard carbon anode.The optimized HCNSs exhibited a high reversible capacity of 305 m Ah g-1 at 0.02 A g-1,preserving a stable cyclability with 210m Ah g-1 for 1100 cycles at 1 A g-1.Meanwhile,it presented an excellent rate performance with 139 m Ah g-1 at 10 A g-1.(2)Hybrid hard carbon nanomaterials were designed and tested as SIBs anode.To build high specific surface area of hard carbon materials,turf-like hybrid carbon nanomaterials(TNCs)were prepared.The obtained TNCs were composed of carbon nanotubes and carbon nanosheets,where carbon nanotubes tightly connected with carbon nanosheets and grew in carbon nanosheets stack.The TNCs exhibited larger specific surface area,abundant defects and loose structures owing to the combined the conbined advantages derived from both parts.As a result,the TNCs could display better storage capability,which could deliver an initial charge capacity of 393 m Ah g-1 at 0.05 A g-1 and remain 75%of storage capacity after 100 cycles.Additionally,at a high current density of 1 A g-1,TNCs presented a reversible capacity of 120 m Ah g-1 for 2000 cycles.(3)Sulfur-doped carbon nanofibers were designed and tested as SIBs anode.To enlarge carbon interlayer spacing,increase active surface functional groups and discrease electron/ion transfer distance of hard carbon,sulfur doped carbon nanofibers(S-CNFs)were constructed by combining nanosizing technology and heteroatom doping.The results showed that S-CNFs delivered a capacity as high as 460 m Ah g-1 at 0.05 A g-1,which was higher than that of CNFs(150 m Ah g-1).Also,it presented a superior cycling stability with310 m Ah g-1 for 1100 cycles at 1 A g-1 and an excellent rate performance with 255 m Ah g-1at 10 A g-1.The nanosizing structure could decrease the electron/ion transfer distance while sulfur doping could increase surface area,enlarge carbon layer distance and create active covalent-C-S-C-bonds,which led to the remarkable electrochemical performances of hard carbon.(4)N,S co-doping carbon nanoparticles were designed and tested as SIBs anode.To improve the cycling stability of sulfur doped hard carbon materials,N,S co-doping carbon nanoparticles(NSCs)were synthetized using N atoms to partly replace S stoms.The optimized NSCs presented good rate performances and excellent cycling stability,which remained 280 m Ah g-1 for 200 cycles at 0.05 A g-1 and 223 m Ah g-1 for 2000 cycles at 1 A g-1.Moreover,the optimized NSCs were proved to be surface-dominated storage mechanism for improved Na-storage capacity and stable cycles,since N,S co-doping could create more defects,generate rich active-C-S-C-covalent bonds and enhance electron conductivity.(5)N,B co-doping carbon nanosheets were designed and tested as SIBs anode.To improve electrochemical kinetics and increase the Na-storage capacity,N,B co-doping carbon nanosheets(NBTs)with rich active sites were designed based on pseudocapacitive-dominated storage mechanism using self-assembled method by hydrogen bond reaction.The optimized NBTs showed a capacity of 309 m Ah g-1 at a current density of 0.2 A g-1,preserving a capacity of 225 m Ah g-1 for 2000 cycles at 1 A g-1,which was far higher that of micro-sized N doping carbon materials(45 m Ah g-1).The superior Na-storage performance and enhanced reaction kinetics could be ascribed to that ultrathin nanosheets were beneficial to decreasing the electron and ion transfer distance,while the larger surface area could promote the contact between electrode and electrolyte for decreased interfacial resistance and more active sites,and that the synergy of N,B co-doping could significantly improve the electronegativity and defects. |
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