Structural Regulation Of Lignin-derived Carbon Materials By Oxalates For Lithium/Potassium Ions Storage

Lignin is an aromatic macromolecule with the content next to cellulose in biomass.The high-value development and utilization of lignin is of great significance to the promotion of the“double carbon”strategy of carbon neutralization and carbon emissions peak.Industrial lignin mainly comes from the papermaing and biorefining industries,with the advantages such as low cost,easy availability,renewability,high carbon content and enriched functional groups.Due to the various advantages,lignin has been considered as the ideal carbon source to prepare carbon-based materials.The high-value application of lignin-derived carbon materials to electrochemical energy storage not only promotes the development of energy storage,but also expands the high-value utilization of lignin,which has important research significance and practical value.The lithium/potassium ion capacitors with both high energy density and power density have broad development prospects in the background of carbon neutralization and carbon emissions peak.The lignin-derived carbon materials have adjustable pore structure,surface structure and low cost characteristics,making them highly potential lithium/potassium anode materials.However,lignin-derived carbon materials exhibit severe aggregation,disordered structure and abundant micropores.When used as the anode materials for lithium/potassium storage,there exists the issues of poor rate and cycling performance.To address these issures,the microstructures of lignosulphonate sodium-derived carbon materials（LSCMs）is firstly controlled by the gas exfoliation from the thermal decomposition of oxalates to significantly improve the rate and cycling performance for lithium storage.Simultaneously,the formation mechanism of the microstructure of the LSCMs is further revealed.Secondly,the edge-nitrogen-doped LSCMs are prepared by coupling gas exfoliation with edge-nitrogen doping strategy,exhibiting good rate and cycling performance for potassium storage.Simultaneously,the mechanism of edge-nitrogen dopant is further revealed.Finally,the LSCMs with gradient large interlayer spacing are prepared by coupling gas exfoliation and selective etcing strategy,exhibiting excellent rate and cycling performance for potassium storage.The main research contents and conclusions are listed as follows:（1）In order to solve the poor rate and cycling performances for lithium storage resulted from the excessive micropores and easy collapsion of the LSCMs prepared with traditional corrosive activators,the oxalates（Zn C₂O₄,Mg C₂O₄ and Ca C₂O₄）with gas exfoliation effect are used as activators to prepare the LSCMs through pyrolysis at 700°C,which also reveals the influence of gas exfoliation on the microstructure and lithium storage performances.The gas exfoliation promotes the demethylation of methoxyl groups and the cleavage of carboxylic groups in lignin,followed by converted into C=O groups.The heteroatom contents are increased by the generated CO₂ etching the carbon atoms,expanding the interlayer spacing,which improves the lithium storage performance.The pyrolytic characteristics of oxalates are key factors in affecting the microstructure of the LSCMs.Zn C₂O₄ is decomposed at 400°C,generating CO and CO₂ simultaneously,which constructs the LSCM with a high content of C=O groups and large interlayer spacing,delivering excellent lithium storage performance（555m Ah/g）.Mg C₂O₄ is decomposed at 450 and 480°C,generating CO and CO₂ staged-weakly,which constructs the LSCM with more C=O groups content and larger interlayer spacing,delivering superior lithium storage performance（575 m Ah/g）.Ca C₂O₄ is decomposed at 480and 700°C,generating CO and CO₂ staged-uniformly,which constructs the LSCM with abundant C=O groups and largest interlayer spacing,delivering outstanding lithium storage performance（673 m Ah/g）.The high C=O groups content and large interlayer spacing are the key to improve the lithium storage performance of the LSCMs.The C=O groups can serve as active sites for the reversible storage of lithium ions,and the expanded interlayer spacing can accelerate diffusion kinetics.（2）In order to further reveal the effect mechanism of gas exfoliation on the microstructure,the LSCMs are prepared by the multi-stage gas exfoliation from the thermal decomposition of Ca C₂O₄ at different pyrolysis temperature（600,700,and 800°C）.Ca CO₃ and Ca O generated by the staged decomposition of Ca C₂O₄ play the role of in-situ template in forming mesoporous structure.The multi-stage gas exfoliation constructs the nanosheet framework and interconnected mesoporous structure.The LSCM with less nanosheets and micropore-dominanted porous structure is prepared by the one-stage gas exfoliation caused by the decomposition of Ca C₂O₄ releasing CO at 600°C,which only shows a lithium storage capacity of 543 m Ah/g.The LSCM with more nanosheets and interconnected mesoporous structure（LSCM-700）is prepared by the two-stage gas exfoliation caused by the decomposition of Ca C₂O₄ releasing CO and partial CO₂ at 700°C,which shows an enhanced lithium storage performance（706 m Ah/g）.The LSCM with thin nanosheets and interconnected mesoporous structure is prepared by the three-stage gas exfoliation caused by the decomposition of Ca C₂O₄releasing CO and whole CO₂ at 800°C.However,the excessive CO₂ constructs more micropores,resulting in a decreased lithium storage performance（619 m Ah/g）.The lithium ion capacitor device assembled with LSCM-700 anode and commercial activated carbon（YP-80F）cathode exhibits a high energy density of 78 Wh/kg and excellent cycling stability.（3）In order to solve the insufficient storage sites and poor potassium storage performance of the LSCMs,the edge-nitrogen-doped LSCM（EN-LSCM）is prepared by two-step carbonization at 700°C,and the edge-nitrogen doping mechanism is revealed.Ca O reacts with the cyanamide units of g-C₃N₄ to form edge-nitrogen-enriched skeleton,which is then linked into the defects in mesoporous or macroporous structure through sp³ C-N bonds.EN-LSCM shows an edge-nitrogen content of 7.0 at.%,which is much higher than that of the undoped LSCM（1.7 at.%）.EN-LSCM exhibits a high potassium storage capacity of 310 m Ah/g at 50m A/g,good rate performance（126 m Ah/g at 5 A/g）and cycling stability,which is far superior to that of the undoped LSCM.The superior potassium storage performance of EN-LSCM is attributed to that high-content edge nitrogen can provide a large number of active sites and fast reaction kinetics for potassium ion storage.The potassium ion capacitor device assembled with EN-LSCM anode and YP-80F cathode exhibits a high energy density of 111 Wh/kg.（4）In order to solve the inferior rate performance for potassium storage of the LSCMs,the LSCM with gradient large interlayer spacing is prepared by CO₂ selective etching,and the formation mechanism of gradient large interlayer spacing is revealed.The opened mesoporous and macroporous channels facilitate the entry of outer CO₂ into the interior of the LSCM to selectively etch carbon atoms,which increases the content of oxygen atoms,forming the gradient large interlayer spacing.The LSCM with gradient large interlayer spacing（O-LSCM-800）is prepared by CO₂ selective etching at 700-800°C,exhibiting a high potassium storage capacity of 330 m Ah/g at 50 m A/g,outstanding rate performance（204 m Ah/g at 5 A/g）and excellent cycling stability.The superior potassium storage performance of O-LSCM-800 is attributed to that the gradient large interlayer spacing improves the accessibility of active sites and accelerates the diffusion kinetics.The potassium ion capacitor device assembled with O-LSCM-800 anode and YP-80F cathode displays an energy density of 80 Wh/kg,and excellent cycling stability（capacity retention rate of 99.6%after 10000 cycles）.