Microstructure Construction And Storage Lithium Performance Of Lignin-based Porous Carbon

Lignin is a kind of natural high molecular polymer rich in aromatic structure and its reserves are second behind to cellulose with a regenerate's rate of 70 billion tons per year.Technical lignin mainly comes from pulping black liquor and bio-refining residue.Owning to its high carbon content,high aromaticity and high calorific value,lignin is an ideal carbon precursor to produce porous carbon materials.Lithium-ion batteries?LIBs?are considered as classical energy sources in modern materials and new energy science because of their high open-circuit voltage,high energy density and low self-discharge.Porous carbon materials have a theoretical capacity 2³ times higher than graphite,and their well-developed pore structure is more conducive to the transport of Li⁺,which is expected to replace graphite electrodes to meet the needs of high-power battery.However,the amorphous structure and high porosity lead to low initial Coulomb efficiency,low voltage plateau and poor cycling performance.Therefore,how to use lignin to construct special micro-structure lignin-based porous carbon?LPC?and apply in the negative electrode of LIBs has important theoretical significance and practical value for realizing high value utilization of technical lignin.In this dissertation,technical lignin was used as raw material to construct LPC with different microstructures by chemical activation method,and its electrochemical performance as anode material for LIBs was studied.Firstly,the high graphitization LPC was constructed and represented well lithium storage performance using common potassium compounds as activators,and the carbonization activation mechanism of potassium compounds on lignin was clarified.Secondly,different kinds of technical lignin were selected as raw materials to reveal the mechanism of lignin structural characteristics?molecular weight,functional group type and content,etc.?affecting its carbonization activation process and microstructures/lithium storage performance of LPC by means of TG,N₂ adsorption/desorption and Raman.On this basis,in order to reduce the specific surface area and microporosity of LPC and increase the ratio of mesoporous,an activator with low temperature vapor phase stripping and self-template was designed to construct a hierarchical porous LPC,which improved the inital Coulomb efficiency and volume specific capacity of LPC.Finally,in order to further improve the problems of inital Coulomb efficiency,high irreversible capacity and poor cyclic stability of LPC negative electrode and expand its application,lignin carbon/carbon nanotube composites?LPC/CNTs?and lignin carbon/tin dioxide?LPC/SnO₂?composites were prepared according to the amphiphilic and aromatic characteristics of lignin and the porous characteristics of LPC,respectively.The main conclusions are as follows:?1?In order to study the effect of activators on the microstructure and lithium storage performance of LPC,the activators,KOH,K₂CO₃,K₂C₂O₄ and K₃PO₄,were selected to carbonize and activate the enzymatic hydrolysis lignin.The carbonization and activation process of lignin and LPC microstructures were measured by TG,N₂adsorption/desorption,Raman,XRD and TEM.The results showed that the activation of activators to lignin was as follows:K₂CO₃>KOH>K₂C₂O₄>K₃PO₄.Compared with the other three activators,K₂CO₃ exhibits a unique activation effect due to the synergistic activation effect of K₂CO₃ and lignin.There are gas phase peeling and carbon layer rearrangement,when K₂CO₃ reaches the initial decomposition temperature and lignin reaches the activation temperature at about 900 ^oC.So the layered LPC with high specific surface area and high graphitization degree was obtained after K₂CO₃ activation at 900 ^oC.Compared with common KOH activation,the special microstructures of LPC activated by K₂CO₃ were beneficial to the transport and storage of lithium ions,showing excellent lithium storage capacity,including high specific capacity(494 mAh·g^-1 for 200 cycles at 200 mA·g^-1),high rate(249 mAh·g^-1 for 600 cycles at 1 A·g^-1)and high cycle stability,which were 1.5 times higher than those of commercial graphite anode.?2?The effects of lignin structure characteristics on LPC microstructures and electrochemical properties were studied with the TG,N₂ adsorption/desorption and Raman using four kinds of techinical lignin as raw materials by K₂CO₃ activation.The results show that the high oxygen/carbon value of lignin was beneficial to the activation and pore-forming of lignin by K₂CO₃,but not to the improvement of graphitization degree;high molecular weight and low oxygen/carbon value are beneficial to the rearrangement of potassium atoms to the carbon layer of LPC,which makes LPC have high graphitization degree and keep porosity.Furthermore,alkyl bridging modification of enzymatic hydrolysis lignin improves the molecular weight,which significantly promoted the increase of graphitization degree of carbonized LPC.The higher graphitized LPC obtained by alkyl bridge modification can provide more active sites for lithium ion storage and its inital Coulomb efficiency and the specific capacity of reversible discharge as negative electrode material were increased by 14%and 5%,respectively compared with the LPC from non modification.?3?In order to develop non-corrosive activator for preparing hierarchical LPC,a low-temperature,green,recyclable,and vapor-phase peeling-self-template activator-ZnCO₃ was developed by thermal analysis technology according to the pyrolysis characteristics of lignin.Unlike other low-temperature decomposing activators,ZnCO₃ can generate CO₂ as a vapor-phase peeling agent for lignin particles during the early stage of lignin pyrolysis about 300 ^oC and in-situ generated nano ZnO particles?8¹0 nm?can provide templates for the formation of mesoporous structure.So the hierarchical LPC was obtained at 600 ^oC with ZnCO₃ carbonization and the particle size was about 200 nm,which is beneficial to reduce the contact area between the material and electrolyte,increase the transmission rate of Li⁺and electron,and increase the active sites of lithium storage.Compared with K₂CO₃ activated,the inital Coulomb efficiency and reversible discharge capacity of LPC activated by ZnCO₃increased by 17%and 4.5%respectively,and the volume specific capacity of LPC activated by ZnCO₃ was 730 mAh·cm^-3,which was much higher than LPC activated by K₂CO₃(445 mAh·cm^-3)or ZnCl₂(162 mAh·cm^-3).Compared with common ZnCl₂and K₂CO₃ activators,ZnCO₃ is non-toxic,non-corrosive,recyclable and suitable for soluble carbon precursors.?4?In order to overcome the problems of high specific surface area,high microporosity and poor structural stability of LPC activated by K₂CO₃,a simple,green,low-cost,hydrophobic self-assembly and in-situ carbonization activation method was used to introduce carbon nanotubes?CNTs?into LPC to obtain self-supporting LPC/CNTs composites with layered structure.The effects of CNTs on the morphology and pore structure of LPC were studied by N₂ adsorption/desorption and TEM and a series of LPC/CNTs composites with different proportions were designed and constructed.The pore structure was transformd from microporous to mesoporous and the high graphitization degree was retained.CNTs can enhance the structural stability of LPC,reduce the microporosity,facilitate the rapid transmission of lithium ions and shorten the transmission distance.After 300 cycles,the composite material with a ratio of LPC to CNTS of 5:5 can maintain 614 mAh·g^-1 discharge capacity and excellent rate performance,which are 24%and 192%higher than LPC and CNT_S,respectively.?5?In order to alleviate the volume expansion of metal oxide anode materials during charging and discharging,a series of LPC/SnO₂ composites were prepared by simple ultrasonic dispersion-ball milling method using SnO₂ as active material.The effects of different microstructures of LPC on the microstructures and electrochemical properties of the composites were studied by using K₂CO₃-activated high graphitized LPC and ZnCO₃-activated hierarchical LPC as carbon carriers.The results of XRD,N₂ adsorption/desorption and SEM showed that the composites had higher composite degree and uniform particle size when the ratio of LPC to SnO₂ was 7:3.So,hierarchical LPC was more beneficial to reduce the particle size of composites.The reversible discharge capacity of the composites with hierarchical LPC was 574mAh·g^-1 after 100 cycles,which was higher than 388 mAh·g^-1 of the composites prepared by high graphitized LPC.The hierarchical porous structure can make SnO₂enter into the carbon carrier channel or even the carbon layer,which is beneficial to alleviate the volume change during charging and discharging,reduce the growth of SEI film and inhibit the agglomeration of tin particles.This work has important guiding significance for widening the application field of LPC and realizing industrialization of LPC negative electrode.