Preparation Of Bamboo-based Porous Carbon Composite Electrode Materials And Their Application Study In Lithium-ion Capacitors

Lithium-ion capacitor is a high-performance battery-capacitor hybrid energy storage device with the advantages of high power density and energy density and excellent cycling performance,which is considered as a new generation energy storage device with great development potential.Lithium-ion capacitors with commercial activated carbon as the cathode and graphite as the anode have been commercially applied.However,due to the low capacity of commercial activated carbon and poor rate performance of graphite,the energy density and power density of the devices are increasingly unable to meet the market demand.The development of porous carbon cathode materials with high capacitance performance and inserted lithium anode materials with high rate capacity is the key to improve the energy density and power density of lithium-ion capacitors.In this work,the widely sourced,low cost and renewable biomass bamboo is used as the raw material,and after liquefaction,a series of bamboo liquid derived porous carbons and their composites were prepared by using the method of alkali activation coupled with templating and heteroatom doping modification technique for the construction of high performance lithium-ion capacitors.The main research contents and results are as follows:（1）The effects of alkali concentration,hydrothermal temperature and hydrothermal time on the degree of liquefaction of bamboo powder was systematically investigated by using alkali hydrothermal liquefaction technology.The preparation conditions of KOH concentration of 3 mol L^-1,hydrothermal temperature of 200℃and hydrothermal time of 6 h could achieve up to 98.6%liquefaction rate of bamboo material,while impurities were also deeply removed.During the liquefaction process,lignin and hemicellulose of bamboo were more easily dissolved,and cellulose underwent dissolution and transformation from cellulose I to the more easily hydrolyzed cellulose II.（2）Zinc nitrate as a hard template agent and potassium hydroxide as an activator were added to the hydrothermally liquefied precursor of bamboo.Acid/water washing after heat treatment to obtain bamboo liquid-derived porous carbon cathodes and their Zn O-loaded anode materials.Thanks to the high specific surface area of 2303.9 m² g^-1,BLZAC2 cathode has good capacitance and excellent cycling stability,with a specific capacity of 94.3 m Ah g^-1 at 0.2 A g^-1and 88.4%capacity retention after2000 cycles at 5 A g^-1.The bamboo liquid-based porous carbon loaded Zn O anode CZ2 has a high specific capacity and long cycle life,The specific capacity was 709.3 m Ah g^-1 at 0.2 A g^-1.The capacity retention rate reached80.9%after 1000 cycles at 5 A g^-1.The highest energy and power densities of the assembled CZ2//BLZAC2 lithium-ion capacitors were 135 Wh kg^-1and 9.8 k W kg^-1.The capacity retention was 83.9%after 8000 cycles at 5A g^-1.（3）Thiourea was added as the nitrogen and sulfur source in the preparation of bamboo liquid-derived porous carbon.The prepared nitrogen and sulfur co-doped bamboo liquid-based porous carbon BLZSC1.Thanks to the doping of nitrogen and sulfur heteroatoms,BLZSC1 cathode exhibits excellent capacitive performance and outstanding cycling stability.It has a high specific capacity of 111.8 m Ah g^-1 at 0.2 A g^-1.The capacity retention rate reaches 85.8%after 2000 cycles at 5 A g^-1.The nitrogen and sulfur co-doped bamboo liquid-based carbon-loaded zinc sulfide anode CZS1 was prepared by the sulfide method.Due to the heteroatom doping and lower inherent conductivity of zinc sulfide than zinc oxide,CZS1anode has excellent rate performance and good cycling stability.The specific capacity at 0.2A g^-1 is 740.5 m Ah g^-1.The capacity retention rate reaches 86.9%after 1000 cycles at 5 A g^-1.The highest energy and power densities of the assembled CZS1//BLZSC1 lithium-ion capacitors are 158Wh kg^-1 and 10 k W kg^-1.The capacity retention rate is 79.2%after 8,000cycles at 5 A g^-1.（4）The hard template magnesium hydroxide was added to the hydrothermal liquefaction precursor of bamboo.The bamboo liquid-based two-dimensional hierarchically porous carbon FPC2 was prepared by using the method of alkali activation coupled with the two-dimensional template.The total pore volume of FPC2 is 2.84 cm³ g^-1 and the mesoporous rate is up to 90%.Due to the high mesoporous volume and two-dimensional flake structure of FPC2,FPC2 cathode exhibits excellent rate performance and outstanding cycling stability.There is still a specific capacity of 48.5 m Ah g^-1 at 10 A g^-1 and the capacity retention rate is 92.6%after 2000 cycles.Then,the two-dimensional hierarchically porous carbon loaded FPC-Si material based on bamboo liquid was prepared by the in-situ growth of nanosilica on FPC2.With the structural advantage of porous flake carbon substrate,the specific capacity of FPC-Si anode is 1385.1 m Ah g^-1 at 0.2 A g^-1,the capacity retention rate is 70.1%after 500 cycles at 5 A g^-1.The highest energy and power densities of the assembled FPC-Si/FPC2 lithium-ion capacitors are 179 Wh kg^-1 and 27.5 k W kg^-1.The capacity retention rate is 82.3%after 8,000 cycles at 5 A g^-1.