Studies On The Synthesis And Application Of Biomass-based Porous Carbon Materials For Lithium-selenium Battery Cathodes

Lithium-selenium?Li-Se?batteries,with a high gravimetric capacity of 675 mAh g^-1 or a high volumetric capacity of 3253 mAh cm-3 in theory,are appealing for one kind of high-capacity energy storage devices owing to the multi-electron redox reaction mechanism of elemental selenium and lithium.However,in practice Li-Se batteries generally suffer from fast capacity fading,poor cyclability and low Coulombic efficiency because of low utilization ratio and volume change of cathode active material Se.In this regard,combining Se with porous carbon has been proved an effective method to improve the specific capacity and rate capability of Li-Se batteries,and therein porous carbon can encapsulate Se into its micro-/meso-pores,adsorb the active substances by its high specific surface area,and can act as a conductive architecture.Using naturally abundant and unexpensive biomass rather than traditional chemical reagents as source materials to prepare the Li-Se battery based porous carbons is a relatively facile process route,which could extend the practice application of biomass and satisfy the concept of environment-friendly.In this paper,two kinds of novel biomass-based porous carbon have been prepared using the inner spongy layers of pomelo pericarp and commercial alkaline lignin as raw materials through a facile KOH activation route.Subsequently,the loading of elemental Se is adopted to fabricate biomass-based porous carbon-Se composites for high-performance Li-Se batteries.A possible structure-function relationship between the nanostructure of biomass-based porous carbon and the electrochemical performance of corresponding selenium-carbon composite.has been extensively discussed in detail in context.In a word,this dissertation mainly deals with two aspects,shown as below.?1?A new-type porous material of macro-/micro-porous biochar?MMPBc?framework derived from the inner spongy layer of pomelo pericarp has been prepared by the initial carbonization and subsequent KOH activation,which is utilized as porous matrix for selenium loading to form Se/MMPBc composite.The influence of weight ratio between KOH and the pre-carbonized inner sponge of pomelo on the porous properties of MMPBc samples and electrochemical performance of corresponding Se/MMPBc composites was explored.It is the appropriate balance between the micropores and macropores of MMPBc resulting from KOH ratio that predominantly determines the excellent electrochemical durability of corresponding Se/MMPBc composite.Due to an optimal weight ratio of KOH to pre-carbonized sponge at 2:1,the resulting Se/MMPBc composite acquire a high specific capacity and good cycling stability.Serial tests were applied to examine the electrochemical performance of Se/MMPBc composite cathode.When the weight ratio of KOH to pre-carbonized sponge is 2:1,at 135 mA g^-1 the Se/MMPBc cathode achieves an initial discharge capacity of 848.5 mAh g^-1 and a reversible capacity of 577.8 mAh g^-1 in the 2nd cycle.Even in the 300th galvanostatic charge-discharge cycle,a high reversible value of 451.5 mAh g^-1 can be maintained,being equal to a retention rate of 78.1%according to the capacity in the 2nd cycle.These,together with a good rate performance of the Se/MMPBc composite,indicate a high electrochemical activity and a stable electrode structure.The superior cycling stability and rate capability of the selected composite Se/MMPBc should be attributed to both the structural parameters of pristine MMPBc,such as the porous carbon framework with high conductivity,a functional groups-rich surface,and a synergistic effect between macropores and micropore and the effective capture and homogeneous distribution of elemental selenium within the porous nanostructures.The micropores of MMPBc frameworks serve as firm immobilizers for selenium species,while the macropores with infiltrated electrolyte could cut down the pathway for Li-ion diffusion and thus improve the electrochemical performance of Se/MMPBc composites.?2?An one-step carbonization/activation of commercial alkaline lignin is adopted to fabricate lignin-derived porous carbon?LPC?with high specific surface area(1810.8 m2 g^-1)and large micropore volume(0.75 cm3 g^-1),since this method is easier than the two step route of initial carbonization and subsequent KOH activation.LPC and elemental selenium were used to fabricate composite through heating and applied as Li-Se battery cathode,and the electrochemical performance of Se/LPC cathode was investigated systematically.At a current density of 0.5 C(1 C = 675 mA g^-1),Se/LPC composite electrode exhibits a reversible capacity of 596.4 mAh g 1 in the 2nd cycle and a capacity retention of 453.1 mAh g^-1 over 300 cycles with an average decay of 0.08%per cycle and the corresponding Coulombic efficiencies reach nearly 100%from the 2nd to 300th cycle.A working electrode of Se/LPC composites also shows good rate performance,indicating a stable porous frame structure even at a high current density.Only one pair of redox peaks is observed in cyclic voltammetry profiles,assigned to the Se???Li2Se one-step transformation with almost non-formation of intermediate lithium polyselenides.When different mass ratios of KOH to lignin are applied to distinguish the effect of KOH amount on the structural parameters of LPC samples and on the electrochemical properties of Se/LPC composites,serial Se/LPC composites show high specific capacities and cycling performances at a KOH:lignin mass ratio ranging from 0.5 to 2.0.This may be due to the fact that:commercial alkaline lignin can be easily dissolved into KOH aqueous solution,which leads to a well-distributed pore-forming effectiveness.As for the intergral formation of Se/LPC composites,the high specific surface area,rich micropores and oxygen-doping of porous carbon frameworks should be emphasized for the surface adsorption and micropore-loading of elemental Se.Thereafter,the good electron conductivity of LPC scaffolds facilitates the reversible electrochemical reaction of selenium towards metallic Li and thus Se/LPC cathode displays low charge-transfer resistances before and after the charging-discharging processes.Therefore,all these factors co-attribute to the superior electrochemical performance of Se/LPC cathodes.In summary,this research illustrates the feasibility of using natural biomass?i.e.,the inner spongy layers of pomelo pericarp or the commercial alkaline lignin?as raw material to prepare porous carbon framework and then to fabricate selenium-carbon composite for high-performance Li-Se batteries.This may provide a low cost and environmentally friendly approach to developing renewable green energy sources for high performance Li-Se battery cathode materials.