Construction And Lithium-storage Performance Of Lignin-based Porous Carbon Materials

Lignin is the second most abundant natural polymer in the world,and it is a kind of aromatic polymer with a high carbon content.At present,industrial lignin mainly comes from papermaking and bio-refining industries,and the way of its main utilization is combustion to recover heat energy.As the yield of lignin pyrolytic carbon is more than 50%,converting lignin into advanced functional materials is one of the important methods for high-value utilization of lignin.The rapid development of today’s social economy and the improvement of people’s living standards have put forward higher requirements for electrochemical energy storage equipment.Lithium-ion battery is the leader of modern power batteries,its specific capacity still can not meet the increasing social demand.Lignin-based porous carbon as an anode material for lithium-ion batteries not only has a high theoretical specific capacity（about 2-3 times that of graphite）,but also has a broad development prospect because of its easy-to-regulate pore structure.In this dissertation,to improve the lithium-ion storage performance of lignin-based porous carbon,the effects of lignin structure and nitrogen doping on the microstructure and lithium storage performance of porous carbon were explored.The fractionated lignin was used as a carbon source to construct the silicon-carbon composite material with higher lithium-ion storage performance,and the response of lithium storage performance to carbon loading were analyzed.Firstly,lignin-based porous carbon materials were constructed by evaporation-induced self-assembly and activated carbonization using different industrial lignins.It can be found that there was a significant difference in the morphology and lithium-ion storage performance of the lignin-based porous carbon prepared from different industrial lignins.Among them,the lignin-based porous carbon constructed from autohydrolyzed lignin had an uniform three-dimensional（3D）structure and the most stable long cycling performance when storing lithium-ion.To investigate the effect of lignin structure on the microstructure and lithium-ion storage performance of lignin-based porous carbon,autohydrolyzed lignin was firstly fractionated into three fractions by an ethanol fractionation method,and the fractions were then used to construct porous carbon.The results show that a high β-O-4 linkage content,low molecular weight and low glass transition temperature was conducive for lignin to the formation of porous carbon with 3D flower-like structure.The β-O-4 linkage content and the glass transition temperature of lignin are usually high due to its high molecular weight.It is necessary to find a balance between the three elements to construct the 3D flower-like structure of lignin-based porous carbon.The 3D flower-like LPC_E70 can be obtained from the lignin fraction L_E70 by activation and carbonization,which had the largest layer spacing,graphitization degree and specific surface area,and also had a high reversible specific capacity of 528 mAh g^-1 when used as the anode material for lithium-ion batteries.Secondly,to further improve the specific capacity of lignin-based porous carbon,melamine was selected and used for nitrogen doping of the lignin-based porous carbon combined with the activation process of lignin,and the influence of melamine on the activation process was explored.Melamine can achieve effective nitrogen doping of lignin-based porous carbon.The decomposition process of melamine and the activation process of lignin were carried out at the same time,which enlarged the layer spacing of crystal structure in lignin-based porous carbon,accelerating the transport of lithium-ions.Therefore,the lignin-based carbon sample N-33 obtained with a nitrogen doping of 11.07%had the highest charge-discharge capacity(691 mAh g^-1 at a current density of 0.2 A g^-1after 200 cycles)and initial coulomb efficiency.Finally,to solve the problem that the improvement of the specific capacity with pure lignin-based porous carbon or nitrogen-doped lignin-based porous carbon was limited,silicon-based materials with high theoretical specific capacity and lignin were used to construct silicon-carbon composites by one-pot method.The effects of carbon loading on the structure and lithium-ion storage performance of the prepared SiO_X@C composites were investigated.It can be found that as the carbon loading increased,the nanosheet structure of the composite increased significantly,and the modified nano-silicon was completely wrapped in the carbon.Among them,the sample SiO_X@C-80 with 80%carbon loading had the highest specific capacity.Its specific capacity of SiO_X@C-80 is 1080 mAh g^-1 when the current density is 0.2 A g^-1 after 200 cycles,that is,more than 3827 mAh g^-1in silicon.