Construction And Performance Of Li₂S@porous Carbon@carbon Nanofiber Aerogel Electrode

The energy density of lithium-sulfur battery is more than 5 times that of traditional Li-ion battery.Its theoretical specific capacity and specific energy are 1675 mAh g-1 and 2600 Wh kg-1 respectively,and its cost is low,and it is environmentally friendly.Therefore,lithium-sulfur battery is considered to be the next generation of Li-ion battery.However,there are some problems in lithium-sulfur battery,such as insufficient conductivity of cathode material,the shuttle effect of polysulfide,volume expansion of electrode in charging and discharging process,growth of lithium dendrites and so on,which lead to poor cycle stability and low safety index of lithium-sulfur battery at present.Li2S acting as a cathode can be paired with lithium-free anode to avoid the growth of lithium dendrites,and will not cause volume expansion of electrode,but there are still problems of poor conductivity of the electrode and the shuttle effect of polysulfide.In view of the above problems,the researchers have made many improvements on the Li2S cathode,but there are still some shortcomings such as complex preparation process of electrode,low content of active substance and low Li2S load per unit area.Therefore,starting from the structure of the electrode,Li2S@porous carbon@carbon nanofiber aerogel(Li2S@PC@CNF)self-supporting electrode has been fabricated in a simple and convenient way with low-cost raw materials.The details are as follows:(1)The Li2S@PC@CNF self-supporting electrode,which is similar to the reinforced concrete structure,is constructed by in-situ reaction at high temperature with low-cost bacterial cellulose,Li2SO4 and glucose as raw materials.Carbon nanofiber aerogel(CNF)derived from bacterial cellulose not only serves as the support framework of electrodes,but also constructs a three-dimensional conductive network.Porous carbon(PC)derived from glucose is embedded with Li2S particles,which is firmly connected to CNF and promotes the charge transfer between electrolyte and active substance.PC and CNF work together to weaken the shuttle effect and provide space for the loading of active substances.Under the Li2S load of up to 3.82 mg cm-2,the Li2S@PC@CNF electrode has the initial discharge specific capacity of 612 mAh g-1 at the current density of 0.2 C.After 700 cycles,it still has the discharge specific capacity of 386 mAh g-1.The capacity decay rate of each cycle is about 0.05%.In addition to the first two cycles,the coulombic efficiency of Li2S@PC@CNF electrode can be kept at about 99%in 300 cycles.(2)The carbon nanofiber aerogel(CNF)interlayer is prepared by freeze drying and high-temperature carbonization using bacterial cellulose as raw material,and the CNF interlayer is added to the Li2S@PC@CNF electrode to form a Li2S@PC@CNF/CNF electrode to further improve the electrochemical performance of the electrode.The CNF interlayer has a three-dimensional porous network structure.The addition of CNF interlayer further improves the conductivity of the electrode,provides a larger reaction space,limits lithium polysulfide more effectively,and does not reduce Li2S load per unit area.The Li2S@PC@CNF/CNF electrode has an initial discharge specific capacity of 700 mAh g-1 at a current density of 0.2 C.After 700 cycles,the discharge specific capacity is 431 mAh g-1.The capacity decay rate of each cycle is about 0.05%,and the coulombic efficiency is close to 100%except the first cycle.In the rate performance test,the average discharge specific capacity of Li2S@PC@CNF/CNF electrode is 695 mAh g-1,626 mAh g-1,555 mAh g-1 and 463 mAh g-1 respectively at the current density of 0.2 C,0.5 C,1 C and 2 C.