Study On Preparation And Properties Of Polyvinylidene Fluoride Composite Separator For Lithium Metal Batteries

Lithium metal batteries with high energy density are considered to be one of the best choices for the next generation of energy storage devices,however,their commercialization is severely restricted by the safety issues caused by the growth of lithium dendrites and the volume expansion of lithium metal anode.To solve this problem,scholars proposed to improve the safety performance of lithium metal battery from the separator design and modification perspective.The separator serves as a physical barrier within the battery,avoiding direct contact of the cathode and anode,storing electrolyte,and promoting the shuttle of lithium ions in the battery.Therefore,it is closely related to the safety performance of lithium metal batteries.Compared with strategies such as direct modification of lithium anode and construction of artificial solid electrolyte interface（SEI）,modification on separator is more efficient in improving the safety performance of the battery.The further development of commercial polyolefin separators is restricted by the inferior thermal stability and poor wettability with electrolyte.Based on this,this paper focuses on polyvinylidene fluoride（PVDF）,which owns superior thermal stability and wettability with electrolyte.The composite separators were prepared by utilizing flexible ethyl cyanoacrylate,rigid talcum to modify PVDF separator,as well as flexible PVDF and rigid polysulfone impregnated polyethylene separator together.The density functional theory and molecular dynamics simulation were applied to analyze the mechanism of the dopant on the separator,the physical and chemical properties of the composite separators as well as the electrochemical performance of the battery assembled with the above composite separators were also investigated.The main research contents are as follows:PVDF was used as a separator substrate while the ethyl cyanoacrylate（ECA）introduced as an additive to improve the mechanical strength of the separator.PVDF/ECA composite separators with different mass ratios of ECA were prepared by phase inversion method.The ester additive（ECA）undergoes anionic polymerization with trace amounts of water molecules to form flexible organic polymers and entangles with the chains in PVDF,resulting in the production of a large number of hydrogen bonds and improving the mechanical strength of the PVDF separators.The stress-strain curves of the above composite separators showed that the ECA-15 composite separator with a 15%ECA mass ratio in PVDF/ECA composite separators reaches a mechanical strength of 70 MPa（2.3 times that of the PVDF separator）.Molecular dynamics simulation（MD）was applied to simulate the fracture modes of the PVDF separator and ECA-15 composite separator,revealing that the PVDF separator exhibits rapid ductile fracture,while the ECA-15 composite separator alleviates the slip between chain segments and enhances their mechanical strength due to the presence of hydrogen bonds.The contact angle test results show that that the ECA additive significantly improves the mechanical strength of the PVDF while not affecting the wettability between the separator and the electrolyte.The in-situ Raman test results show that the characteristic peaks of ECA at 358 cm^-1 exist stably throughout the entire charge-discharge process of the battery,indicating that ECA can exist stably in the ECA-15composite separator.The Li Fe PO₄/Li battery assembled with the ECA-15 composite separator delivers a specific discharge capacity of 125 m Ah g^-1 after 800 cycles at 1 C and room temperature.Based on ensuring the mechanical strength of the separator,it is crucial to improve the thermal stability and the wettability of the separator for further upgrading the safety performance of lithium metal batteries.On account of the thermal stability and calculation results of density functional theory（DFT）on the adsorption energy between talcum powder and electrolyte,rigid inorganic talcum powder as an additive can effectively improve the thermal stability and electrolyte wettability of PVDF,thus upgrade the safety at high temperature and rate performance of the battery.The LFP/Li lithium metal battery assembled with the T-10（talcum mass ratio 10%）composite separator still owns a discharge capacity of 90 m Ah g^-1 at 10 C,which is higher than that of the battery assembled with other composite separators with different talcum mass ratios.The electrochemical performance of LFP/Li lithium metal battery was analyzed by reducing the thickness of the T-10 composite separator to 16μm.It was found that batteries assembled with thinner separators under the same conditions have lower lithium ion transfer impedance and better rate performance.Due to the excellent thermal stability of talcum,the electrochemical cycling performance test results at high-temperature show that the LFP/Li lithium metal battery assembled with the T-10composite separator can cycle stably for 300 cycles at 1 C and 80℃,which is better than the battery assembled with the PVDF separator and PP separator（120 cycles）.Based on ensuring high mechanical strength,thermal stability and wettability of the separator,the composite separator was prepared by impregnating polyethylene（PE）into flexible PVDF and rigid polysulfone（PSU）to improve the cycle stability of the battery.Theoretical research shows that the high binding energy between PSU and lithium ions is positively related to the uniform distribution of lithium ions,which can inhibit the growth of lithium dendrites,thus improving the cycle stability of lithium metal batteries.The morphology test shows that PSU and PVDF are filled in the pores of the PE separator,and the obtained PESV composite separator owns a uniform pore size distribution.Forward Looking Infrared Radiometer（FLIR）tests show that the PESV composite separator remains intact at 150℃,while the PE separator shrinks significantly at 60℃.The stress-strain curve obtained by tensile testing shows that the mechanical strength of the PESV composite separator is 145 MPa,while that of the PE separator is only 78 MPa.DFT calculation shows that the maximum binding energy between PSU and lithium ion is-1.40 e V,which is superior to the binding energy of that between PE and lithium ion（-0.22 e V）.Li/Li batteries were assembled with the PE separator and PESV separator,the battery assembled with the PESV composite separator could cycle stably for nearly 1000 h at 0.50 m A cm^-2 without obvious short circuit.However,Li/Li battery with the PE separator cycles for only 400 h with an obvious short circuit.The LFP/Li lithium metal battery assembled with the PESV composite separator delivers a discharge specific capacity of 91 m Ah g^-1 after 1000cycles at 1 C with a mass loading of 4.00 mg cm^-2.