Polysulfides Shuttle Inhibition Separator For Lithium-sulfur Batteries

As the demand for new energy electric vehicles,small household appliances and portable electronic devices increases,the requirements for rechargeable batteries that quickly convert and store electrochemical energy are also increasing.Lithium-sulfur battery is a new type of secondary battery with lithium metal as the negative electrode and sulfur as the positive electrode.It is expected to replace the lithium-ion battery which has touched the energy density ceiling and become the next generation of energy storage system in the future.Different from lithium-ion battery based on the principle of Li ion intercalation-deintercalation,lithium-sulfur battery will experience multiple reaction intermediates during the charging and discharging process.The dissolution and diffusion of long-chain lithium polysulfides in the electrolyte will lead to the loss of sulfur,which will lead to the decline of capacity and coulombic efficiency.In addition,the volume of sulfur species will change greatly during the charging and discharging process（S₈molecule will eventually be transformed into eight Li₂S）,and the sulfur electrode will constantly update the micro interface.In addition to the design of sulfur carrier materials for positive electrode,it is a feasible and efficient way to control the loss of polysulfides by using separator,which is far away from the electrochemical reaction interface and has relatively stable structure,so as to regulate the micro interface behavior of sulfur electrode.In view of the above problems,from the perspective of molecular design,organic molecules interacting with lithium polysulfides were introduced into the separator,which is expected to achieve the effect of capturing lithium polysulfide without blocking the channel.The main results are as follows:（1）The commercial polypropylene separator（PP）for lithium sulfur battery was studied.The PP-C-St-DA separator was prepared by polymerizing a layer of nano scale polymer materials on the PP separator,and then introducing dopamine（DA）into the reaction site of polymer materials.The charging and discharging performance,rate performance and electrochemical reversibility of the lithium sulfur battery assembled with PP separator and PP-C-St-DA separator were investigated.The results show that the initial discharge capacity of lithium sulfur battery assembled with PP-C-St-DA separator is 1123 m Ah×g^-1 at the current density of 0.5 C,and the capacity decay rate of each cycle is 0.113%,which shows good electrochemical performance.The physical and chemical properties of the separator were characterized,and the pore size,hydrophilic and hydrophobic properties and the behavior of capturing lithium polysulfide before and after modification were investigated.The characterization results show that the pore size of the PP-C-St-DA separator is reduced,the hydrophilic performance is improved,and it can effectively adsorb lithium polysulfides,which is attributed to the PP separator coated with a layer of polymer and the introduction of DA molecules,played a role in reducing the pore size and increasing reaction sites.（2）Also based on the idea of molecular design material,hemin,which is used as the electroactive center of hemoglobin,was introduced into PP separator by chemical grafting method to prepare PP-C-St-Hemin separator,which can effectively regulate the transport behavior of lithium ion and lithium polysulfides in meso scale pores of the separator.The PP-C-St-Hemin separator was assembled into a lithium sulfur battery and the electrochemical test was carried out.The results show that at a current density of 0.5 C,the initial discharge specific capacity of the battery with PP-C-St-Hemin separator is1244.7 m Ah×g^-1.Even at high current density of 2.0 C,it can stably cycle for 2000 cycles,corresponding to a capacity decay rate of 0.036%per cycle,showing excellent long-term cycle stability.