Research On Structure Design, Electrochemical Performance And Mechanism Of Lithium-sulfur Battery Separato

Lithium-sulfur（Li-S）batteries have attracted widespread attention,due to its high theoretical specific capacity(1675 m Ah g^-1),low cost and environmental friendliness of elemental sulfur,etc.Unfortunately,the commercialization of Li-S batteries is hindered by inherent drawbacks.The severe“shuttle effect”of lithium polysulfide（Li PSs）and the slow reaction kinetics of sulfur cathodes are more serious.In order to solve the two problems,three kinds of composites comprised porous carbon nanofibers（PCNFs）and MoP-based composites and a Co-doped MoP nanofiber-modified CeCoO₃@PI-PVDF nanofiber membrane were fabricated and applied to modify separators of Li-S batteries through the strategies of nanoscale and composite.And the MoP-based composites exhibited various morphologies and structures（nanorods,nanoflowers and nanofibers）and components（thin carbon layer wrapping,heterojunction and Co-doped）.The mechanism of battery performance improvement is studied by using a variety of characterization techniques and theoretical calculations.The research contents are as follows:（1）Mo-based MOFs are prepared as precursors though hydrothermal reaction,and then converted into MoP nanorods decorated N-doped thin carbon layer（MoP@NC）during the annealing and phosphorization process.Among the MoP@NC composite,the NC can effectively limit the“shuttle effect”of Li PSs through physical barriers,and the MoP nanorods with abundant active sites can effectively facilitate the conversion of adsorbed Li PSs.In addition,the NC can also avoid MoP oxidation.The Li-S batteries with the MoP@NC/PCNFs modified separators exhibit excellent initial specific capacity of 1063.1 m Ah g^-1 and achieve a discharge capacity of 828.2 m Ah g^-1over 300cycles at 1 C(1 C=1675 m Ah g^-1)with 0.08%fading rate.（2）Heterojuncted MoP-Mo S₂ nanoflowers are synthesized though hydrothermal reactions and phosphorization process.The nanoflowers manifest outstanding traits:1)2 D nanosheets possess larger specific surface area and numerous edges sites;2)The heterojuncted structure can effectively reduce the interface impedance and accelerate the transfer of electrons and ions at the interface;3)Among various heterojuncted MoP-Mo S₂,MoP-50-Mo S₂ enables the most complete and satisfying“capture-diffusion-conversion”process of Li PSs.The Li-S batteries assembled with MoP-50-Mo S₂/PCNFs modified separators show an initial specific capacity of 1090.0 m Ah g^-1and high discharge capacity of 886.2 m Ah g^-1 after 300 cycles at 1 C with 0.06%fading rate.（3）The process parameters of MoP nanofibers are explored by electrostatic solution blowing technology,and then Co doped MoP nanofibers（Co-MoP）are exquisitely fabricated based on the basis of the optimal process parameters.The continuous and convenient network structure of Co-MoP nanofibers can provide efficient channels for ion transport and electron migration,resulting in lower interfacial impedance.The Co-MoP nanofibers are able to form van der Waals attraction with PCNFs,thereby constructing stable interfaces.The Li-S batteries with Co-MoP/PCNFs-modified separators exhibit the outstanding Li⁺diffusion coefficient up to6.47×10^-8 cm² s^-1 and excellent cycling performance with satisfactory fading rate of0.05%per cycle（1 C,300 cycles）.（4）The CeCoO₃ doped PI-PVDF nanofiber membranes（CeCoO₃@PI-PVDF）are prepared by electrospinning technology,and then the synthesized Co-MoP nanofibers were loaded onto CeCoO₃@PI-PVDF membranes to constitute a novel type of separator（CMC@PI-PVDF）.The CMC@PI-PVDF nanofiber membranes possess excellent dimensional stability.And the gel electrolyte is formed after absorbing the electrolyte,which was furtherly beneficial to inhibit the shuttle of Li PSs without affecting the transport of lithium ions.The CeCoO₃and Co-MoP nanofibers provide a synergistic effect on the adsorption and conversion of Li PSs.Benefiting from the above merits,the Li-S batteries with CMC@PI-PVDF nanofiber membranes exhibit excellent rate performance,and the corresponding reversible specific capacity is 1236.6,1135.4,996.8,938.2 and 878.8 m Ah g^-1,when the current density is 0.2 C,0.5 C,1.0 C,2.0 C,and 5.0 C,respectively.It is worth noting that when the current density returns to 0.2C,the Li-S batteries still display a high capacity of 1142.1 m Ah g^-1.