Study On Preparation,characterization,and Application In Lithium Batteries Of Lithium Sulfonated Polyoxadiazole Nanofiber Membranes

Lithium batteries（LBs）can be divided into two major categories,that is,lithium-ion batteries（LIBs）and lithium metal batteries（LMBs）.As the most widely used battery today due to their numbers of advantages,for example,high energy density and working voltage,and long cycle life,the LIBs fail,however,in meeting the increasing requirements of the rapidly developing industries,such as electric vehicles,consumer electronics,etc.,on the electrochemical properties and security of LBs.Therefore,the LMBs that use lithium metal as the anode have become one of the most concerned research hotspots of energy storage in recent years due to the extremely high theoretical specific capacity(3860 m Ah g^-1)and lowest redox potential（-3.04 V vs.standard hydrogen electrode）of lithium metal.However,the lithium dendrites formed during the charging process due to lithium’s uneven deposition seriously hinder their commercial application.The dendritic lithium can cause rapid degradation of battery capacity and pierce the separator,leading to short circuits and thermal runaway.The separator is one of the core components of LBs.The popular polyolefin-based microporous separators for LBs possess the disadvantages of poor electrolyte wettability,inferior resistance to high temperature and fire,low porosity and electrolyte uptake,and inability to inhibit lithium dendrites,etc.,which restrict the improvement of energy densities and increase the safety risks of the batteries.Although the separators made of heat-resistant polymers can overcome most of the polyolefin separators’defects mentioned above,the rigid molecular segments generally make the heat-resistant polymers difficult to process.Given the problems mentioned above of traditional polyolefin and heat-resistant polymer separators for LBs,this paper proposes a high-performance aromatic heterocyclic polymer,lithium sulfonated polyoxadiazole（Li-SPOD）,from the perspective of molecular design,to prepare separators for LBs.In the molecular structure of Li-SPOD,a large number of-SO₃Li groups and weakly basic N and O heteroatoms endow the separator with excellent electrolyte wettability and ionic conductivity;-SO₃Li groups can significantly improve the processability at no expense of heat resistance of the material.Electrospinning was used to process the Li-SPOD into nanofiber porous membranes,and then their application performances in LBs were studied.Firstly,a series of soluble Li-SPODs with different sulfonation degrees were synthesized using the hydrazine sulfate,4,4’-diphenyl ether dicarboxylic acid（DPEA）,and isophthalic acid（IPA）as monomers,considering the asymmetry of the IPA and the flexibility and sulfonability of the DPEA.The molecular structure,aggregate structure,thermal stability,mechanical properties,and electrolyte wettability of these polymers were comprehensively studied through spectral analysis,thermal weight loss analysis,and tensile test,et al.The results show that the relative molecular masses of Li-SPODs are higher than 10⁵,and the cyclization degree of the hydrazide groups in their molecular chains are close to 100%.Only the benzene rings of diphenyl ether structural units containing electron-donating ether oxygen groups can be sulfonated,and generally,a benzene ring can only undergo one sulfonation reaction.The Li-SPOD membranes exhibit amorphous aggregate structures,and the ratios of diacid monomers or the degree of sulfonation exert little effect on the orders of molecular arrangements.Li-SPOD membranes’mechanical strengths vary regularly with the IPA/DPE ratios,among which the 3/7 Li-SPOD membrane possesses the highest breaking strength（47.2 MPa）.Li-SPODs display excellent thermal stability,and their initial decomposition temperatures in the N₂ atmosphere are higher than 460°C.As the degree of sulfonation decreases,Li-SPOD molecules’hydrophilicity gets worse and worse,while the affinity with organic electrolytes firstly increases and then decreases.The 2/8 Li-SPOD exhibits the best electrolyte affinity;Li-SPODs show better affinity with Li TFSI/ether than with Li PF₆/ester liquid electrolyte.Secondly,the lithium sulfonated poly-4,4’-diphenyl ether oxadiazole（Li-SPEOD）,which has the highest sulfonation degree and excellent spinnability,was used to prepare the nanofiber separator（es-Li-SPEOD）by electrospinning.The aggregate structure,mechanical and combustion properties,dimensional stability in the liquid electrolyte,the interaction between polar groups and Li⁺s,electrochemical and battery performances,etc.,of the es-Li-SPEOD separator were comprehensively studied through X-ray diffraction,tensile test,molecular simulation,and constant current charge/discharge test,etc.The results show that the es-Li-SPEOD separator has a pore structure with high porosity（65.5%）,large pore sizes（the average is 1.378μm）,and a low tortuosity（2.15）,and a typical amorphous aggregate structure.Its tensile breaking strength（9.4 MPa）reaches 87%of that of the commercial Celgard-2500 separator.The es-Li-SPEOD separator exhibits excellent heat and flame resistance,does not shrink and deform in the air at 400°C.The liquid electrolyte uptake of the es-Li-SPEOD separator is 4 times that of the Celgard-2500,while the Li-SPEOD nanofibers can only be slightly swollen by Li PF₆/ester liquid electrolyte.Due to weakly alkaline N atoms and sulfone groups in the oxadiazole and phenoxathiine rings,the es-Li-SPEOD separator’s interaction with Li⁺s is not only much stronger than that of the Celgard-2500 separator but also more robust than that of the carbonate solvents.The ionic conductivity(σ,3.57 m S cm^-1)and lithium-ion transference number(t_Li+,0.37)of the es-Li-SPEOD separator are 628.6%and 32.1%higher than those of the Celgard-2500 separator,respectively.The two separators exhibit the same electrochemical stability window（ESW）,which is around 4.75 V（vs.Li⁺/Li）,while the es-Li-SPEOD separator displays better compatibility with Li metal electrodes.The es-Li-SPEOD separator also shows some ability to inhibit lithium dendrites,and its cycle stability and rate performance are better than those of the Celgard-2500 separator.At a rate of 5 C,the discharge capacity of the Li Fe PO₄-Li cell using the es-Li-SPEOD separator is 30.0%higher than that of the cell using the Celgard-2500 separator.Finally,in order to enhance the interaction between the es-Li-SPEOD separator and liquid electrolyte,so that more facilitating effect on the electrochemical properties can be exerted by the N,O heteroatoms and-SO₃Li groups in the Li-SPEOD molecular structure,the es-Li-SPEOD/Li TFSI/ether hybrid gel polymer electrolyte（GPE）separator was prepared by reducing the molecular weight of Li-SPEOD and using Li TFSI/ether liquid electrolyte with a better affinity towards Li-SPEOD molecular chains.The molecular chain structure,aggregate structure,microscopic morphology,electrochemical and battery performances of the es-Li-SPEOD GPE were comprehensively studied via spectral analysis,scanning electron microscopy,molecular simulation,and constant current charge/discharge test,etc.The results show that the immersion treatment in Li TFSI/ether electrolyte for 72 h gives rise to a more disorderly arrangement of the Li-SPEOD molecular chains,adhesion of the nanofibers to each other,and an increase of tensile breaking strength（14.2 MPa）by 39.2%.The electron-withdrawing oxadiazole ring can promote the dissociation of the-SO₃Li groups.Moreover,when the oxadiazole ring is complexed with a Li⁺,this promotion effect becomes much more significant.Theσof es-Li-SPEOD single-lithium-ion conducting gel polymer electrolyte（SLIC-GPE）swelled with DOL/DME mixed ether solvent is higher than those of some other lithium polysulfonate and lithium polysulfimide based SLIC-GPEs.The es-Li-SPEOD GPE separator’sσ(2.05 m S cm^-1)and t_Li+（0.64）are 454.1%and 56.1%higher than those of the Celgard-2500 separator,respectively.The ESW（4.75 V vs.Li⁺/Li）of the es-Li-SPEOD GPE separator is also slightly higher than that of the Celgard-2500separator,while the es-Li-SPEOD GPE separator displays better compatibility with Li metal electrodes as well.es-Li-SPEOD GPE separator has a strong lithium dendrite inhibition ability,endowing Li Fe PO₄-Li,Li-Li,and Cu-Li cells with a better rate and cycle performances.The remaining discharge capacity(125.4 m Ah g^-1)of the Li Fe PO₄-Li cell using the es-Li-SPEOD GPE separator is 23.9%higher than using the Celgard-2500 separator after operating for 300 cycles at a current of 2 C.The Li-Li symmetrical cell using the es-Li-SPEOD GPE separator can keep stable cycling for 400 hours with lower hysteresis voltages（～50 m V）,with a fixed lithium capacity of 1 m Ah cm^-2 and a current density of 2 m A cm^-2.With the es-Li-SPEOD GPE separator,the Cu-Li cell can steadily operate for 58 cycles with a Coulombic efficiency of higher than 97%,with a fixed lithium capacity of 1 m Ah cm^-2 and a current density of 2 m A cm^-2.