Preparation And Energy Storage Performance Of Biomass Carbons And Their Composites

Biomass carbons are green and low-cost with a wide range of sources.Biomass carbons and their composites have been widely applied in electrochemical energy storage because of their high specific surface area,tunable pore structure,heteroatom doping,multiple morphologies,and physicochemical stability.However,the low specific capacity,complex preparation methods,and slow reaction kinetics limit their further applications.Therefore,it is significant to study simple and efficient preparation methods to adjust their microstructure and improve their electrochemical energy storage performance.This work applied cotton cloth as the precursor to prepare biomass carbon materials.Structural regulation,active site introduction,and composite construction enhanced their energy storage capacity.Dual-system activation,liquid membrane in-situ polymerization,and vapor-phase deposition methods were developed to prepare various biomass carbon-based materials and their composites.The effects of microstructural changes on their electrochemical energy storage performance were explored using structure characterization,electrochemical tests,and Density Functional Theory（DFT）calculations.This research will provide new ideas and references for developing new biomass carbons and their composites and applications in energy storage.The main results are as follows:（1）A simple and efficient dual-system（molten salt system+activation system）activation method was developed,and the preparation temperature was optimized to prepare a hierarchical porous nitrogen-doped biomass carbon（HNBC）.HNBC consists of a three-dimensional porous network connected by carbon nanorods and lamellar edges constructed by carbon nanosheets,displaying a high specific surface area(1771.78 m² g^–1)and high nitrogen content（9.18 at.%）.HNBC exhibits a high lithium storage capacity of 1392 m A h g^–1 and excellent cycling stability at 0.1 A g^–1.The assembled Li-ion capacitor（HNBC//HNBC）provides a high energy density of 186 W h kg^–1 at 225 W kg^–1.In HNBC,high nitrogen doping promotes charge transfer during charging/discharging.Meanwhile,the hierarchical porous structure can increase the BET surface area and reduce the diffusion path for lithium-ion transport,speeding up its reaction kinetics.Thus,HNBC displays excellent lithium storage performance.（2）Using the dual-system secondary activation method,a graphitic microcrystalline carbon（GMC）was prepared to adjust the biomass carbon’s internal crystal structure.GMC has a porous graphite microcrystalline structure and abundant defects,displaying a high specific surface area(2191.50 m² g^–1)and low nitrogen content（2.44 at.%）.GMC displays a high capacity of 1195 m A h g^–1 at 0.1 A g^–1,and the assembled Li-ion capacitors（GMC//GMC）provide a high energy density of 191 W h kg^–1 at 225 W kg^–1.Density functional theory（DFT）calculations show that the nitrogen vacancies in graphite microcrystals provide abundant reaction sites and induce excess electrons around local nitrogen atoms to form negative charge centers.These effects improve the storage of Li⁺and accelerate Li⁺transport to make GMC show high lithium storage performance under low nitrogen doping content.（3）Using a liquid membrane in-situ polymerization method,a PPy layer was grown on porous biomass carbon particles to prevent PPy agglomeration from blocking the carbon particles’internal pores and prepare a porous PPy/C composite.By adjusting the mass ratio of the carbon matrix to pyrrole monomer(m_c/m_py),the PPy/C composite’s structure and sodium storage performance can be regulated.Under m_c/m_py=10:5,PPy/C exhibits a high sodium storage capacity(545 m A h g^–1)and excellent cycling stability.The sodium ion cells assembled from PPy/C and Na₃V₂（PO₄）₃ show a high capacity of 410 m A h g^–1 at 0.1 A g^–1.DFT calculations show that PPy enhances the adsorption of sodium ions on the composite and improves the material’s conductivity.Meanwhile,PPy provides abundant active sites and Faraday capacity to improve the sodium storage property of the PPy/C composite.（4）Flexible PPy/Mn O/C composites were prepared by hydrothermal method,vapor-phase deposition,and in-situ polymerization method using biomass-activated carbon cloth as a flexible conductive framework.PPy/Mn O/C exhibits a discharge capacity of 918 m A h g^–1 at 0.1 A g^–1 after 120 cycles.The Li-ion cells assembled with Li Fe PO₄ as the cathode provide a high energy density of 465 W h kg^–1 at a power density of 125 W kg^–1.Mn O increases the lithium storage active sites and plays a vital role in increasing the specific capacity of the material.Meanwhile,PPy can buffer the structural damage caused by the volume expansion and shrinkage of Mn O.These effects make the flexible PPy/Mn O/C materials exhibit excellent lithium storage capacity and good cycling stability.