Preparation Of Lignin-Based Carbon Materials And Their Properties Of Energy Storage

Energy storge and conversion have become crucial links in the adjustment of energy structure and governance of environmental issues.Therefore,it is important to carry out structural design,performance optimization,controllable preparation and structure-effective relationship of active materials for energy storage devices which have significant academic and application value.Carbon materials have become the main electrode materials for energy storage devices such as electric double layer capaciotrs（EDLCs）,lithium ion batteries（LIBs）,and sodium ion batteries（SIBs）due to their variable structure and stable chemical properties.However,the structural characteristics of application devices are different,so higher requirements are put forward for the source and molecular characteristics of carbon precursors.Lignin,a precursor of biomass charcoal material,has a wide range of sources and is easy to realize the regulation and modification of molecular structure.Although lignin has natural three-dimensional network molecular structure and abundant oxygen-containing functional groups,it is usually regarded as solid waste or low-grade fuel in practical applications.Therefore,in order to realize the high value-added utilization of resources,lignin can be used as precursor for high-performance activated carbon,hard carbon and composite carbon materials through chemical modification and other methods,which not only improves the performance of energy storage devices,but also shows good application potential.This thesis takes lignin-based carbon materials as the research object,and focuses on the structure design,preparation process,structure-effective relationship and electrochemical energy storage mechanism of carbon materials used in different energy storage devices.In chapter 3-4,two kinds of lignin-based spherical porous carbon materials were prepared.A hierarchical porous carbon microspheres material with highly connected three-dimensional pore structure was obtained by inverse phase forming with pre-adding activator.In addition,a size-controllable nanospheres material was obtained by solvent controlling.The physical and chemical properties were measured by a variety of characterization methods,and the electrochemical performance were evaluated,and the influence of the pore structure on the electrochemical reaction kinetics was investigated.In chapter 5,lignin-based hard carbon materials with large carbon interlayer space and crystal size were prepared by phenolic resin-modified lignin nanospheres.The physical and chemical properties of the materials were measured by various characterization methods,and the electrochemical performance of the materials were evaluated as the anode materials in sodium ion batteries.In addition,the mechanism of sodium storage of the hard carbon materials was explored.In chapter 6,the lignin-based carbon/Fe Se₂ materials were prepared by ball milling mixing method and high temperature pyrolysis method.The physical and chemical properties of these composite materials were measured by various characterization methods.The electrochemical performance of composite materials as anode materials for sodium ion batteries were evaluated.In addition,the evolution mechanism of carbon formation under the action of iron compounds,and the structure-effective relationship between the carbon structure and the electrochemical properties of materials were explored.The research results are as follows:（1）An improved chemical activation method of inverse phase forming with pre-addition of activator was established to obtain lignin-based hierarchical pore carbon microspheres with high connectivity and three-dimensional channel structure.The optimum preparation conditions of the material were as follows:the mass ratio of K₂CO₃ to lignin was 0.75,the evaporation temperature of inverse phase forming was85°C,and the activation reaction temperature was 850°C.The mass specific capacitance and volume specific capacitance of the obtained porous carbon materials can reach 140 F g^–1 and 65 F cm^–3,which were related to the large specific surface area(1529 m² g^–1),total pore volume(1.02 cm³ g^–1)and excellent pore structure.In addition,the capacitance retention was 89%after 10000 cycles test.It was found that the K₂CO₃ has successively played the role of p H regulator,template and activator in the preparation process of porous carbon.Firstly,mesopores were formed by the position occupied effect of K₂CO₃,and then the microspheres were activated from the inside out.While forming microporous,the internal mesopores were connected to form a channel for rapid ion diffusion.In this method,pre-addition of activator greatly improved the efficiency of activation reaction and reduced the ratio of alkali to carbon,which expanded the application prospect of the improved chemical activation method.（2）Lignin-based colloidal particles whose average particle size can be adjusted freely in the range of 0.15–1.4μm were prepared by using solvent control method.After activation reaction,lignin-based hierarchical porous carbon nanospheres achieved a high specific surface area of 2566 m² g^–1,and a total pore volume of 1.53cm³ g^–1.The specific capacities of the materials obtained in the organic and ionic liquid electrolyte of EDLCs are 147 and 160 F g^–1 at a current density of 0.05 A g^–1,and 117 and 108 F g^–1 at a current densities of 10 A g^–1.In addition,the capacitance retention was 81%after 10000 cycles test.The results indicated that the binary solvent of acetone and water can interact with the hydrophobic and hydrophilic groups on the lignin molecule to form colloidal particles under the effect of van der Waals force andπ-πconjugation,and the size of colloidal particles was determined by the relative content of acetone.The study also found that porous carbon nanospheres with small particle size can effectively increase the surface energy of the sample,enhance its electrochemical adsorption capacity,and shorten the ion diffusion path,which were conducive to the improvement of the material capacitance and rate performance.（3）Lignin-based carbon nanospheres with both large carbon interlayer spacing(d₀₀₂=0.370 nm)and crystallite size（L_a=6.13 nm）were prepared by the phenolic resin modification method.It exhibited a high specitic capacity of 345 m Ah g^–1,and a high initial coulomb efficiency of 74%in the sodium ion half-cell system.The specific capacity based on the anode electrode in the full battery reached 279 m Ah g^–1,and it had good cycle stability.The optimum preparation conditions of the material were as follows:the resin reaction temperature under acidic conditions was 90°C,the reaction time was 12 h,and the optimal carbonization temperature was 1300°C.The results indicated that the reaction of formaldehyde in phenolic resin can make lignin molecules/monomer tend to linear polycondensation,thus maintaining the spherical morphology of the lignin-based nanospheres and enhancing their thermal stability.In the subsequent heat treatment,the larger size of graphite-like crystallites were formed,which can improve the capacity performance.Combining the microcrystalline structure of the material and its electrochemical performance,it can be found that the capacity of the low-potential plateau regon corresponded to the behavior of sodium ions intercalating into the graphite-like interlayers,and the capacity of the high-potential slope regon corresponded to the adsorption-type of sodium storage behavior at the defect site,which was consistent with the“adsorption-insertion”mechanism of the hard carbon materials.（4）Using ball milling and high-temperature pyrolysis,the high performance C/Fe Se₂ composite materials was prepared,which lignin-based carbon material used as the supporting material and Fe Se₂ used as the active material.These composite materials had the best capacity performance of 433 m Ah g^–1 for anode electrode in sodium ion batteries,and the maximum capacity was 213 m Ah g^–1 at a current density of 10 A g^–1.In addiditon,all samples shown excellent cycle stability.The mechanism of carbon structure evolution of lignin catalyzed by iron-containing compounds was as follows:at low temperature,small carbon-containing molecules escaped from lignin entered the iron citrate crystal and formed small graphite-like microcrystalline structures;when the temperature increased,the reduced Fe species can further catalyze the surrounding carbon atoms to form graphite carbon structure.In addition,the study suggested that the active material Fe Se₂ content of the composite material was higher at low pyrolysis temperature,showing better capacity characteristics;while at high temperature pyrolysis,the carbon load of the composite material has been highly graphitized,and the electronic conductivity of the material has been greatly improved,which can provide an electron transport channel for the Fe Se₂ that stores sodium in a conversion reaction mechanism,thereby achieving a significant improvement in its rate performance.