Preparation Of Biomass-derived Composite Carbon Materials And Their Properties Of Sodium Storage

In the past decade,energy storage and conversion technologies have become a crucial part of energy restructuring and environmental governance.Manufacture of active materials is one of the key technologies of energy storage devices.It is of great scientific significance and application value to probe into the structural design,performance optimization,controllable preparation and structure-function relationship of active materials for energy storage devices.Due to the similar chemical properties of sodium to lithium,abundance and low cost,sodium ion batteries are considered a promising replacement to lithium-ion batteries in large-scale grid energy storage.Biomass-derived carbon,as an efficient,cheap and environmentally friendly active material,has received a lot of research attention in the application as anode material in sodium ion batteries.According to different energy storage mechanisms of sodium ion batteries,the energy storage performance of carbon materials is related to specific surface area,pore structure,crystal structure and other factors.Biomass-derived carbon materials display the characteristics of graphite-like microcrystalline structure with large layer-spacing,appropriate specific surface area and pore structure,and heteroatom self-doping,which make them potential anode materials for promoting the commercialization of sodium ion batteries.This thesis takes biomass-based carbon materials as the research object,focusing on the structural design,synthetic strategy,structure-function relationship and electrochemical sodium storage mechanism of biomass-based carbon materials.The research results are as follows:Low-cost lignin was selected as hard carbon precursor and coal tar pitch based amphiphilic carbonaceous material（ACM）was used as soft carbon precursor.The composite carbon microspheres were controllably prepared by emulsion-solvent evaporation method.The influence of composite ratio on the structural evolution of materials was investigated.The chemical bonds generated between ACM and lignin in the synthesis process improved the carbonization yield of the material,which is beneficial to reduce the preparation cost of the material.Through the study of the structure-function relationship of the materials,the sodium storage mechanism of as-obtained hard/soft carbon composite materials accorded with electrochemical adsorption-insertion mechanism.With appropriate graphite-like microcrystalline interlayer spacing,less surface defects,the sodium storage performance of the hard/soft carbon composite materials brought out the combination properties of high conductivity and initial Coulombic efficiency（82.0%）from soft carbon and high reversible capacity(296.5 m Ah g^–1)from hard carbon.At the same time,the cyclic stability of the materials has been improved to some extent.With lignin as carbon precursor and（NH₄）₂HPO₄ as N/P sources,a preparation technology was formed to achieve nanoporous N,P-codoped hard carbon microspheres.The addition of（NH₄）₂HPO₄ can effectively enhance the thermal stability,optimize microstructure and composition of the materials.As-obtained x NPL-1300（x=0.05,0.10 and 0.20）samples are ideal anode material for sodium ion batteries（SIBs）due to their amorphous carbon structure,enlarged d₀₀₂ interlayer spacing,appropriate particle size,porous structure,and increased local conductivity by in situ N,P heteroatom co-doping.The x NPL-1300 electrodes accorded with electrochemical adsorption-insertion mechanism.The sample 0.20NPL-1300exhibited the best rate performance and cycling stability,with reversible capacity of307.0 m Ah g^–1 and corresponding initial Coulombic efficiency of 79.0%.Faced up with the problem of low specific capacity of hard carbon anode materials in SIBs,sorghum stalk with a loose structure was selected as vector to prepare high-performance metal selenide（MSe,M=Fe,Co and Ni）composite materials.The best performance of the composite materials as anode for sodium ion battery showed a high reversible specific capacity of 481.0 m Ah g^–1,good rate performance as 249.5 m Ah g^–1 at 5 A g^–1 as well as excellent cycle stability.By analyzing the structure and phase evolution of composites at different pyrolysis temperatures,the carbon structure evolution mechanism of sorghum stalk derived carbon in the composites was revealed.This method is liable to produce carbon materials with elevated degree of order at low temperature.The carbon matrix increased the overall electronic conductivity of the composite materials,which can provide more electron transport channels for MSe,thus realizing the improvement of their rate performance.This work provides theoretical and experimental basis for the fabrication of short-range ordered low temperature pyrolytic carbon and plays a positive role in the practical application of biomass-derived hard carbon anode materials for sodium ion batteries.