Synthesis Of Porous Carbon And Its Application In Lithium Selenium Batteries

As the same main group element to sulfur?S?,selenium?Se?has a high volumetric capacity?3253 mAh/cm³?and natural abundance,when it was employed as the novel cathode active materials of lithium battery.Thus,Li-Se battery was considered as one of the next-generation Li secondary batteries.However,Se cathode faces some challenges toward practical application:?1?Se suffers volumetric expansion/shrinkage during the charge/discharge process,which results in the pulverization of active materials and thus the capacity decay;?2?Se is a semiconductor which hinders the electron conduction;?3?The lithium polyselenides are soluble in electrolyte,the resulted shuttle effect may degrade the coloumbic efficiency and cycling durability.To address these serious issues,the most effective method is loading Se into porous carbon to form Se/C composite materials.Porous carbon materials are promising host for the composite cathodes because of their both large surface area and high electric conductivity.For example,the porous structure of carbon materials could restrain the volume expansion and suppress the diffusion of polyselenide into electrolyte.In this work,a series of carbon materials with variable pore structure were prepared and worked as Se hosts,to investigate the influence of pore structure on the performance of Li-Se battery.More specifically,mesopores carbon CMK-3 with pore size of 4.5 nm was prepared by using SBA-15 as template.Another mesoporous carbon with pore size of 10.3 nm was also prepared by direct carbonization of a protic salt[pPDA][2HSO₄]?PMeC?.Two microporous carbons with pore size of 0.8 nm and0.58 nm were obtained by direct carbonization of potassium sodium tartrate tetrahydrate?TCNS?and potassium citrate?CCNS?,respectively.The Se/C composite materials were prepared by a melt impregnation method.The electrochemical test indicated that the rate capability and cycling durability of microporous carbon/Se is obviously better than that of mesoporous carbon/Se composite.The specific capacity of CMK-3/Se electrode only retains 286.9 mAh/g after 50 cycles at a rate of 0.5C and150.1 mAh/g at 5C.The specific capacity of PMeC/Se electrode only retains 162.3mAh/g after 100 cycles at a rate of 0.1C,and 19.7 mAh/g at 5C.In contrast,the CCNS/Se composite electrode could remain a very high capacity of 409.9 mAh/g after100 cycles at a rate of 0.1C,and 288 mAh/g at 5C.Furthermore,the TCNS/Se composite electrode could remain a very high capacity of 501 mAh/g after 100 cycles at a rate of 0.1C,and 320 mAh/g at 5C.Based on the results mentioned above,we specifically designed and synthesized a nitrogen doped zeolite-templated microporous carbon?N-ZTC?by an easy sacrificial template method.N-ZTC possesses a super high specific area of 1757 m²/g,a pore volume of 0.85 cm³/g,and a pore diameter of 1.12 nm.When serving as Se host for Li-Se battery,the electrochemical tests indicate that the cell possess a high initial capacity of 671 mAh/g and excellent cyclic stability of about 400 mAh/g after 500cycle at 0.5C,which corresponds to the capacity decay of 0.081%per cycle.By comparing the surface morphology and chemistry of the electrode before and after cycling,it was found that the good electrochemical performance of microporous carbon/Se composite materials may be attributed to the enhanced electric conductivity and ionic conductivity.The Li⁺diffusion coefficient of microporous carbon/Se composite materials was five times of that of the mesoporous carbon/Se composite materials.At the same time,the micropores facilitate the formation of stable solid electrolyte interphase?SEI?which could enhance the ionic conductivity.The theoretical calculation further suggested that the Li₂Se is likely to turn into electronic conductor because of the emergence of itinerant electron,and the improved electric conductivity enhances not only the kinetics of the whole electrode but also the utilization of active materials.We believe that this work provides a new insight into the mechanism of electrochemical behavior and a new direction of designing C/Se composite materials.