| With the increasing global demand for energy and the reduction in the supply of fossil fuel have prompted researchers to shift the focus from nomal energy research to sustainable and renewable energy,such as solar,wind,and tidal energy.In order to meet the demand for emerging energy applications,the selection of suitable and efficient energy storage devices had also became an urgent problem to be solved.Lithium-ion batteries,supercapacitors and lithium-sulfur batteries had became the most widely used electrochemical energy storage devices because of their own advantages.It became a crucial issue that how to improve the energy density and cycle life of energy storage devices.Active materials which as an important part of electrochemical energy storage devices,it has became the focus of research.This work selected the Juncus effusus L.,a biomass which rich in natural reserves and renewable.Here we made full use of Juncus effusus L.as precursor,because of its good adsorption of salt solution,to prepare a series of biomass carbon and its composite materials by a variety of treatment methods,explore their microstructure and electrochemical properties.The specific research content as follows:1.Juncus effsus L.as a precursor,activated in K2CO3 solution and carbonizationed at a high-temperature to synthesize biomass carbon.The characterization results showed that the morphology and structure of biomass carbon changed significantly with the change of activator's ration.With the increase of activator ration,the biomass carbon from a carbon block that had not been completely activated to cheese-like biomass carbon with appropriate porosity and large specific surface area,and finally the structure collapsed.The experiment had prepared cheese-like graded porous biomass carbon with large specific surface area(2338 m2·g-1).When applied to supercapacitors,the specific capacity can reach 357 F·g-1 at a current density of 1 A·g-1.Even when the current density reached 20 A·g-1,it could still maintain a high specific capacity of 280 F·g-1.Also it had showed the excellent cycle stability,the capacitance retention rate was 98%after 2000 cycles.This method synthesized a large specific surface area biocarbon that combines micropores,mesopores and macropores,which could provide more active sites for reduce transmission resistance and improve rate performance,so that it had a higher electricity chemical activity.XPS results showed that the biomass contained the carbon nitrogen,which was beneficial to improve the the active material's utilization rate.When modified the Celgard 2300 separator used in a lithium-sulfur battery,the initial discharge capacity could reach 1359 mAh·g-1 at a current density of 0.1A·g-1?and remained 493 mAh·g-1 after 500 cycles at 2 A·g-1.The porous carbon with the large specific surface area had a stronger adsorption for polysulfide,can intercepted polysulfide and fixed it in the biomass porous carbon layer,which greatly relieved the puncture effect on Celgard 2300 separator.In addition,the biomass porous carbon layer could also promote electron transport fastly in the insulated sulfur cathode.Due to its porous space and conductive frame,it could greatly improve the sulfur load and enhance the conductivity of the electrode material.2.A series of SnO2/C composites with different morphologies and structures were successfully prepared by using Juncus effusus L.as biological hard template,which by immering+heating directly,adjusted the concentration and temperature.As an electrode material assembled into lithium-ion batteries for various electrochemical tests,samples 300-0.5-SnO2 showed the excellent capacitive and electrochemical stability.At a current density of 0.07 A·g-1?sample 300-0.5-SnO2 had a first discharge capacity of 1226 mAh·g-1.Also,The discharge capacity of the electrode was kept at 550 mAh.g-1 after 200 cycles at 2.4 A·g-1.Various characterization results showed that the interconnected large-network sponge biocarbon framework not only played a important role of confining SnO2 nanoparticles but also increased the contact area between the active material and the electrolyte,which could provide more active sites.At the same time,biomass carbon could also increase the conductivity of electrode materials and increase the battery's capacitance.3.In the same way,Juncus effusus L.was a precursor and another three biomass(Tremella and Wood)was a comparative.Three kinds of SnO2/C composite materials with novel and unique structure,J-SnO2(Juncus effusus L.as the precursor),T-SnO2(Tremella as the precursor)and W-SnO2(Wood as the precursor),which were prepared by impregnation with the molten salt assisted.The active materials were prepared into an electrode assembled in lithium-ion battery for various electrochemical performance tests.At a current density of 0.07 A·g-1,the first discharge specific capacities of the J-SnO2,T-SnO2 and W-SnO2 could reach 1439,1387 and 1055 mAh·g-1,respectively.Even at a high current density of 6 A·g-1,the discharge capacities of J-SnO2 and T-SnO2 could maintain at 325 and 330 mAh·g-1,and when the current density was restored to a lower level of 0.8 A·g-1,the discharge capacity could still be restored to 642 and 676 mAh.g-1,respectively.The characterization results showed that due to plant diversity and the metal ions' selective absorption of of plant cell walls,the prepared samples also had their own characteristics.Such as J-SnO2,as the temperature of the carbonization treatment increases,the molten liquid medium directly cracked the carbon matrix,and finally a fluffy,randomly stacked bird nest-like SnO2 nanowires was formed.However,the nanowires of T-SnO2 and W-SnO2 were thicker and shorter,and the stacking was very regular.It just like the nanowires SnO2 growing around the nucleus.The molten liquid medium promoted the distribution of SnO2 and biomass carbon uniformly,built a more effective carbon framework,improved the conductivity of the composite material,and gave the material excellent lithium storage performance. |
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