Preparation And Electrochemical Properties Of Biomass-Derived Porous Carbon Electrodes

Lithium-sulfur batteries are currently the most promising high-energy batteries due to their high theoretical specific capacity(1675 m Ah·g^-1),high specific energy(2600 m Ah·g^-1),low price,and environmental friendliness.secondary battery.However,the industrialization of lithium-sulfur batteries faces many problems,such as the electronic insulation of sulfur-based reaction products,the volume expansion during charging and discharging,and the shuttle effect.To solve these problems,designing new cathode materials has become an important means to improve battery performance.Biomass porous carbon materials have the characteristics of good electrical conductivity,large specific surface area,and rich pore structure.The composite of sulfur and porous carbon materials can effectively improve the electrochemical performance of lithium-sulfur batteries and become an excellent composite cathode material for lithium-sulfur batteries.Therefore,this thesis attempts to use activated sludge obtained from landfill leachate treatment as a precursor of biocarbon,and prepare biomass-derived porous carbon by a simple chemical method,which is loaded with active sulfur as a cathode material for lithium-sulfur batteries and studies the energy storage performance.The details are as follows:1、Using activated sludge as carbon source and sodium hydroxide as activator,two biomass porous carbons（C and C1）were prepared by one-step carbonization and secondary high-temperature annealing activation,respectively.The prepared biomass porous carbon C has a specific surface area of 624.13 m~2g^-1and a pore capacity of 0.082 cm~3g^-1.C1 has a specific surface area of 780.24 m~2g^-1and a pore volume of 0.231 cm~3g^-1.The C/S electrode is better than the C1/S electrode.When the sulfur loading is 70%,the discharge specific capacity of C/S is as high as 1150 m Ah·g^-1at 0.1 C,and the initial discharge specific capacity is 837 m Ah·g^-1at 0.2 C,and still has a discharge specific capacity of 589m Ah·g^-1after 200 cycles,showing relatively excellent rate performance and cycle performance.The initial discharge specific capacity at C is 645 m Ah·g^-1,and the material has a capacity of 317.19 m Ah·g^-1after 200 cycles.The material exhibits super-strong sulfur-fixing ability,and the Coulombic efficiency is maintained at about 99%.2、Using activated sludge as carbon source,phosphoric acid as phosphorus source,and secondary high-temperature activation,different phosphorus-doped porous bio-carbon materials were prepared,which were used as sulfur cathodes.The effects of different H₃PO₄additions（1 m L/3 m L/6 m L）on the structure and properties of porous carbon materials were investigated.The results show that the porous carbon material 3-HP-C after 3 m L phosphoric acid treatment has a larger specific surface area(891.93 m~2g^-1)and a pore volume(0.871cm~3g^-1).Electrochemical tests show that when the sulfur loading density is 0.51 mg cm^-2,the initial discharge specific capacity of 1310 m Ah·g^-1is achieved at the current density of 0.1C,and the initial discharge specific capacity of 0.2 C is 1204.84 m Ah·g^-1,after 200 cycles,the discharge specific capacity of the battery remains 696.99 m Ah·g^-1.Under the high areal loading of 5.44 mg cm^-2,the discharge specific capacity is as high as 1236.40 m Ah·g^-1at the rate of 0.05 C,which is the theoretical ratio 73.81%of the capacity,achieving high capacity at high sulfur loading density.The experimental results show that the P element doped porous carbon material can significantly improve the overall performance of the battery.3、Using activated sludge as the carbon source,adjusting different ratios of thiourea（0.3 g/0.9 g/1.5 g）,and secondary high-temperature roasting,different ratios of nitrogen and sulfur co-doped biomass-derived carbon were prepared.Used as a sulfur cathode,and its electrochemical performance was tested and studied.The I_D/I_Gof the composite material（2-NS-C/S）with a thiourea ratio of 0.9 g was 1.37,the degree of graphitization was the highest,and it had large specific surface area pores(517.07 m~2g^-1)and pore volume(0.634 cm~3g^-1).Electrochemical tests show that when the sulfur content is about 70%,the 2-NS-C/S composite cathode material has an initial discharge specific capacity of 1584.52 m Ah·g^-1at0.1 C under low areal loading,accounting for the theoretical specific capacity 94.59%,showing high capacity at low magnification.Composites with thiourea ratio of 0.3 g（1-NS-C/S）,the discharge specific capacity decays from 965.43 m Ah·g^-1to 660.67 m Ah·g^-1after200 cycles at 0.2 C,and the decay rate per cycle is only 0.090%,showing good cycling stability.The results show that N and S diatomic doping is beneficial to improve the electrochemical performance of porous carbon materials.