Study On The Separator Of Poly-m-phenylene Isophthalamide Nanofiber Lithium Secondary Battery

As one of the core components of lithium secondary batteries,the performance of separator directly determines the safety and electrochemical performances of the batteries.However,the current commercial polyolefin-based separators have some disadvantages,such as poor hydrophilicity,low heat resistance,and can not effectively inhibit the growth of lithium dendrites and the shuttle of polysulfides.The electrospun nanofiber membranes show the characteristics of large length-diameter ratio,high specific surface area and macro preparation,which have become the research hotspot of separator materials for lithium secondary batteries.Based on the electrospinning technology,the high-performance poly(m-phenyleneisophthalamide)(PMIA)polymer was used as the main substrate,and with the help of structural design and functional doping,the nanofiber separators with excellent comprehensive performances for lithium secondary batteries were developed.The main research contents are as follows:(1)The tree-like F-PMIA(T/F-P)nanofiber gel membrane was prepared through one-step electrospinning process.It showed outstanding hydrophilicity,heat resistance and mechanical strength as well as excellent interface compatibility,which can effectively promote the uniform stripping/deposition behavior of lithium ions at the interface between separator and lithium electrode.And its small pore size and gelation were favorable for inhibiting the“shuttle effect”of lithium polysulfides.The capacity retention rate of the LCO/Li battery using the T/F-P separator was 89.22%after 200cycles.Moreover,the discharge capacity of the lithium-sulfur cell can still remain at1012 m Ah g^–1 after 200 cycles.(2)A core-shell structured PI@F-PMIA nanofiber gel separator coupled hardness with softness was prepared by coaxial electrospinning technique.The PI@F-PMIA separator exhibited laudable thermal stability,favorable wetting mechanical flexibility and excellent electrolyte uptake,which had the capability to promote the efficient migration and homogeneous deposition of lithium ions and enhance the interfacial stability,thus inhibiting the formation and growth of lithium dendrites.The PI@F-PMIA separator possessed high ionic conductivity(1.30×10^–3 S cm^–1)and wide electrochemical stability window(?5.0 V).And both the PI@F-PMIA-based LCO/Li and lithium-sulfur cells delivered superior cycle stability.(3)A novel OAPS anchored F-PMIA(Oli/OAPS-PMIA)nanofiber gel separator with ion-selective functionalization was prepared by electrospinning technology,which can effectively promote lithium ions migration while blocking the negative polysulfide anions transfer.The Oli/OAPS-PMIA membrane showed small fiber diameter and pore size,good liquid electrolyte affinity and high mechanical strength,and the novel PMIA membrane had no significant size shrinkage after heat-treatment at 240?.The lithium-sulfur cell with the Oli/OAPS-PMIA separator delivered a low average capacity decay rate of 0.057%per cycle within 800 charge-discharge cycles.(4)A multi-scale F-PMIA and F/Ti co-doped PMIA(T-PMIA)composite gel separator was successfully prepared by electrospinning technique.The T-PMIA separator exhibited higher ionic conductivity(2.36×10^–3 S cm^–1),larger lithium ion migration number(t _Li+=0.534)and wider electrochemical stability window(?5.2 V),which can effectively inhibit the growth of lithium dendrites and block the“shuttle effect”of polysulfides.The capacity retention rate of the LCO/Li battery using the multiscale PMIA separator can reach 91.7%after 200 cycles,and the average capacity decay rate of the assembled lithium-sulfur cell was only 0.043%per cycle during 200cycles.