Design And Performance Study Of Functional Modification Layer Of Lithium-sulfur Battery Separator

In order to alleviate the greenhouse effect and energy shortage problems,the development of high-efficiency energy storage systems has received extensive attention.Due to its ultra-high theoretical specific capacity（1675 m Ah g-1）with their cathode sulfur,lithium-sulfur（Li-S）batteries are expected to replace lithium-ion batteries and become the next generation of high-efficiency energy storage battery systems.However,limited by the problems of low specific capacity and fast capacity decay in practical use,the industrial application of Li-S batteries still faces a lot of challenges.Among them,the“shuttle effect”of polysulfides is the biggest challenge of Li-S batteries,causing the loss of active material sulfur,which leads to the rapid decline of the actual capacity of the battery,but also leads to irreversible corrosion to the lithium negative electrode,bringing security risks.To alleviate this problem,two zwitterionic microporous polymer-functionalized PP separators were designed and synthesized.The zwitterionic microporous polymers can selectively allow the transport of lithium-ions（Li⁺）and block the diffusion of polysulfide anions,effectively suppressing the“shuttle effect”.The main work is as follows:（1）A zwitterionic microporous polymer（QTB-SA）containing sulfonic acid anions and quaternary ammonium cations was successfully synthesized by post-modification of the intrinsic microporous polymer（TB）containing Tr?ger’s base units.Then,the QTB-SA/PP functionalized separator was constructed by a coating method.The batteries based on the QTB-SA/PP separator possess excellent electrochemical performance.Its capacity retention rate reaches 82.9%after 100 cycles at a current density of 0.2 C.Meanwhile,the rate performance of the QTB-SA/PP battery is significantly better than other batteries,with a capacity of 845.8 m Ah g^-1at 2 C,and the capacity recovers to 1071.3 m Ah g^-1when the current is switched back to 0.2 C.The battery also exhibits good long-cycle stability,with a good capacity retention of 722.3 m Ah g^-1after 500 cycles at a current density of 1 C,with an average capacity fade of 0.042%per cycle.（2）First,the PP separator was modified with SWCNT to block its macropores.Then,the synthesized zwitterionic microporous polymer（TB-BAA）containing carboxylate cations and quaternary ammonium cations was spin-coated on the surface of SWCNT to construct TB-BAA/SWCNT/PP functionalized separators.The batteries based on TB-BAA/SWCNT/PP separator possess good cycling stability and rate capability.At a current density of 0.2 C,the battery achieves a capacity retention rate of 81.0%after 100cycles.Long-cycle performance test was carried out at a current density of 1 C,and a good capacity retention of 686.1 m Ah g^-1remained after 500 cycles,with an average capacity fading of only 0.055%per cycle.In addition,the battery has a capacity of 736.6m Ah g^-1at a current density of 3 C,which recovers to 1129.8 m Ah g^-1when the current is switched back to 0.2 C.When the cathode sulfur loadings were 3.67 and 5.45 m Ah cm^-2,batteries based on this separator achieves capacity retention rates of 88.7%and 92.3%,respectively,after 50 cycles at 0.2 C.