Preparation And Sodium-storage Performance Of Pitch-based Anode Materials

Sodium-ion battery（SIBs）is a low-cost alternative energy-storage device to replace the lithium-ion battery in the fields of large-scale power-storage systems and electric vehicles.Anode material plays a vital role in the commercialization of SIBs.Mesocarbon microbeads and mesophase pitch,as the main phase products in the thermal polycondensation of pitch,are rich in graphite microcrystals,showing good electrical conductivity,but the shortcomings of small interlayer distance and less surface defects inevitably result in the limited sodium-storage capacity of pitch-based soft carbon anodes.The limited sodium-storage capacity can be enhanced by means of surface chemical modification,pore-structure control and soft/hard carbon composites,which can not only obtain considerable intercalation capacity by expanding the interlayer distance of carbon materials,but also create abundant sodium-storage sites to achieve a large proportion of surface-capacitive contribution.The effects of sulfur（S）species,S content and pore structure on the microstructure and electrochemical properties of S-doped pitch-based carbon materials have been studied,and the sodium-storage mechanisms of S-doped pitch-based carbon anodes are also discussed.（1）S doping induces the hybridization of sp²carbon,enhancing the sodium-storage performance.S-doped mesocarbon microbeads were prepared from mesocarbon microbeads by H₂S etching.The controlled S doping can be realized by adjusting the etching temperature and etching time,resulting in the expanded interlayer distance of graphite microcrystalline by S atom occupation and the formation of C-S-C/C=C covalent bonds by reactions between H₂S and C-C/O-C=O,in which the content of C=C bonds increases from 38.25 to 43.70 at%.C-S-C can provide abundant sodium-storage sites,and C=C can significantly improve the electrical conductivity of S-doped mesocarbon microbeads.Influenced by the capacitive charge-dominated fast kinetics behavior,the S-doped mesocarbon microbead anodes display an increase in reversible capacity from 76 to 170 m Ah g^-1at 0.2 A g^-1.After 800 cycles at 1 A g^-1,the sodium-storage capacity is stabilized at 120 m Ah g^-1with a capacity retention of 97%.（2）Mesophase pitch-based hierarchical porous carbons with high C=C content prepared by using metal acetate as hard template facilitate the increase of sodium-storage capacity and significant improvement of cyclic stability.The mesopore-dominated hierarchical porous carbons were obtained by the volatilization of acetate in the pyrolysis process and the removal of hard templates.Acetate contributes additional sp²carbon,improving the electrical conductivity of soft carbons,which effectively compensates for the decreased graphitization degree of soft carbons due to the template occupation.Under the synergetic effect of hierarchical porous structure,high C=C content and moderate surface functional groups,the pitch-based hierarchical porous carbon anode delivers a high specific capacity of 270 m Ah g^-1at 0.05 A g^-1,and a charging capacity of 144 m Ah g^-1after 1000 cycles at 1 A g^-1with a capacity retention of 92.2%.（3）S-doped fiber-like soft carbons with 3D network structure can achieve the high reversible capacity at a large rate.Pitch-based hierarchical porous carbons with fiber-like 3D network structure were synthesized using calcium acetate as the hard template.The fibrous carbon skeletons provide a large number of adsorption sites,expressways for electron conduction and electrolyte-diffusion channels.Thiophene-type C-S-C covalent bonds are introduced to the surface of soft carbon materials by H₂S etching,which can contribute abundant active sites,further improve the sodium-storage performance,including the increased reversible capacity from 210 to 300 m Ah g^-1at 0.2 A g^-1,and the significantly improved rate performance(167 m Ah g^-1at 10 A g^-1).（4）Highly S-doped mesophase pitch-based hierarchical porous carbon anodes by one-step carbonization can realize the high sodium-storage capacity and long cycle life at a large rate.S-doped mesophase pitch-based hierarchical porous carbons were prepared by one-step template carbonization,in which Mg SO₄was employed as hard templates and S sources.By adjusting the carbonization temperature,the S content is controlled in the range of6.85-12.19 at%,and mostly exists in C-S-C groups（77.9-95.3%）,which contributes to rich pseudocapacitance and maintains the structural stability.Under the synergistic function of hierarchical porous structure,high C=C content（35.49 at%）and S doping（12.19 at%）,the soft carbon anodes exhibit a superior sodium-storage capacity(430 m Ah g^-1at 0.2 A g^-1)and long cycle life at a large rate(150 m Ah g^-1after 800 cycles at 10 A g^-1).（5）Construction of core-shell structure soft/hard carbon composites facilitates the significant improvement in the rate performance and cyclic stability of hard carbon anodes.The composites of soft and hard carbons have a core-shell structure,in which S-doped mesocarbon microbeads are the core and PEDOT-based hard carbons are the shell.The core with abundant graphite microcrystals provides the excellent electrical conductivity,and the shell featuring the cross-linked structure and rich S-containing functional groups is conductive to the sodium-ion storage and electrolyte transport.By adjusting the amount of soft carbon and carbonization temperature,the precise regulation of the carbon layer spacing（0.383-0.410 nm）,the S-doped amount（4.86-8.84 at%）and the active S-containing functional groups（94-100%）is realized.Under the mixed sodium-storage mechanism dominated by capacitive charge,the obtained composite anodes have excellent rate performance with a capacity retention of 28.6%from 0.2 to 5 A g^-1.The anode delivers an initial reversible capacity of 187 m Ah g^-1at 1 A g^-1,and cycles 800 times at 1 A g^-1with a high capacity retention rate of 99.4%.（6）Surface capacitive charge-dominated mixed sodium-storage mechanism promotes high-speed reversible sodium-ion storage.The surface capacitive-charge behavior includes pseudocapacitive behavior and surface absorption,and the diffusion behavior mainly refers to interlayer intercalation.The difference in sodium-storage capacity origins from the different contribution for the capacitive-charge behavior and diffusion behavior.For the S-doped mesocarbon microbeads,S-doped mesophase pitch-based hierarchical porous carbons and soft/hard carbon composites,most of sodium-storage capacity comes from Faraday reaction between sodium ions and C-S-C covalent bonds,showing a pair of redox peaks near1.0V/1.8V in the CV curves,which is the key to improving the rate performance and cyclic stability.Surface adsorption,as one of the fast kinetic behaviors,is closely related to the specific surface area of carbon materials,manifesting the reversible-capacity storage in the high-voltage region above～1.25V.For both S-doped mesocarbon microbeads and soft/hard carbon composites,the expanded interlayer distance contributes the considerable intercalation capacity.