Synthesis And Electrochemical Study Of High-Energy-Density Lithium Sulfur Batteries Based On Ionic Regulation Strategy

Lithium-sulfur(Li-S)batteries are considered to be one of the most promising candidates for next-generation energy storage system due to their ultra-high theoretical capacity of 1675 m Ah g^-1 and other advantages such as environmental friendliness,naturally abundant and low cost.Many efforts have been devoted to improving the electrochemical performance of Li-S batteries and realizing their commercialization.However,many challenges hinder the actual application of Li-S batteries.For instance,the transport of Li ions is blocked in high-loading cathode due to the poor electrolyte wettability,enlargeing polarization and resulting poor rate performance and cycling stability of Li-S batteries.The strong solvation of Li ions and uneven distribution of Li ions in electrolyte cause the growth of lithium dendrite,resulting low coulombic efficiency and safe issue.To solve above problems,in this dissertation,strategies based on Li ion regulation,such as constructing fast Li ions transfer paths to enhance electrochemical kinetics,regulating the solvation of Li ions to increase their concentration at the interface of electrodes and regulating the concentration gradient of Li ions,are adopted to improve the electrochemical performance of Li-S batteries.The researches are as follows:To improve the electrolyte wettability of thick electrodes with high-loading,a porous high load lithium sulfide cathode was prepared by the combination of 3D printing technology and argon atmosphere sintering(3DP@Li₂S).The in-situ sintering process helps to promote the growth of lithium sulfide on the surface of carbon skeleton,yielding fast Li ion transfer paths to the electrolyte/active material/current collector triple-phase-boundaries.Fast Li ion transport to reaction site is critical for enhancing the electrochemical kinetics,improving rate performance of Li-S batteries and developing electrode with loadind-independent properties.As a result,the 3DP@Li₂S cathode delivers area capacity of 6.29 m A h cm^-2 and capacity of 629 m A h g^-2 with a Li₂S loading as high as 10 mg cm^-2.To solve the problem of lithium uneven deposition caused by the instability of SEI,PP separator decorated by PVDF electrospinning layer with Li-MMT(PP@Li-MMT)is designed to weaken the solvation intensity of Li ions,weakens the coordination between lithium ions and solvent molecules,thus increasing the proportion of inorganic components in SEI and improving the stability of SEI.As a result,the growth of Li dendrite is suppressed and the interface stability and cycle performance of the battery are enhanced.According to the experimental results,PP@Li-MMT based Li||Cu half cell stably cycles more than 100 cycles at a current density of 2 m A cm^-2 with a capacity of 1m Ah cm^-2,with an average coulomb efficiency of 97.5%per cycle.To regulate the distribution of Li ion,PP separator decorated with ferroelectric Bi Fe O₃ is designed to promote the uniform distribution of electric field at the electrode interface,regulating the Li ion flux and reducing Li ion concentration gradient.As a result,the uniform deposition of Li metal is realized and the interface stability of battery is improved.The experimental results show that even under the high current density of 10m A cm^-2,the PP@BFO allow the uniform deposition of Li metal in Li||Cu half cell.Moreover,PP@BFO contributes to promote the uniform deposition of Li metal and improve the electrochemical performance of the battery even at low temperature.PP@BFO based Li||Cu half cell stably cycles more than 180 cycles at a current density of 0.5 m A cm^-2 with a capacity of 0.5 m Ah cm^-2,with an average coulomb efficiency of95%per cycle,which delivers good cyclical stability and provide an idea for the design of low-temperature battery.