Modification Of PVDF-based Polymer Single Ion Conducting Separator And Its Application In Lithium-Oxygen Batteries

The theoretical specific energy of lithium-oxygen batteries is as high as11429 Wh kg^-1,which has the potential to replace the current lithium-ion batteries,so it has been widely studied.Although some important progress has been made,there are still some problems,such as metal lithium dendrite and corrosion,the consumption rate of organic electrolyte is fast,and the decomposition rate of discharge products is slow,which leads to their accumulation in the cathode,which affects the mass transfer of lithium ions and oxygen.transmission,etc.In this paper,redox intermediates（RMs）are used to enhance the decomposition process of discharge products.Modified polyvinylidene fluoride（PVDF）with ion selectivity is used as a separator to conduct lithium ions,prevent oxidized RM⁺,dissolved oxygen and other harmful substances from entering the negative electrode chamber,which prevents Li anode corrosion.The modified PVDF separator also contributes to the uniform deposition and dissolution of metallic lithium,thereby significantly improving the performance of lithium-oxygen batteries.The main contents include:1.The PVDF-based single ion conducting separator is ion-selective through alkalization treatment,polystyrene grafting and sulfonation treatment of the PVDF-based membrane,and lithium single ion conduction characteristics are formed through lithiation treatment,and block the transmembrane conduction of anions.FTIR and XPS analysis showed that the sulfonic acid groups were successfully grafted on the PVDF backbone.In order to improve its ionic conductivity,BMIMCl ionic liquid was used for modification.SEM,XRD and EIS analysis showed that the ionic liquid modification formed denser grooves inside the PVDF membrane,but no through holes were formed,the ionic conductivity of the membrane was also significantly improved,the ion exchange capacity（IEC）remained unchanged,and it can effectively block the permeation of redox intermediate I^-and its oxidation state I₃^-,and has excellent ion selectivity.In order to improve the mechanical properties of PVDF-based separator,polyvinylidene fluoride-co-polyhexafluoropropylene（PVDF-HFP）was used to make ionic liquid-modified PVDF-HFP single ion conducting separators.SEM,XRD and EIS analysis show that the modified PVDF-HFP single ion conducting separators have dense grooves,good ionic conductivity,IEC and mechanical properties,and can block the permeation of I^-and I₃^-,with good ion selectivity.2.The ionic liquid-modified PVDF single ion conducting separator was used as the battery separator,in which 1.0 M Li Cl O₄/DMSO electrolyte was used in the negative electrode chamber,and with 0.05 M Li I（RM）in 1.0 M Li Cl O₄/DMSO electrolyte was used in the positive electrode chamber.Compared with the lithium-oxygen battery using glass fiber（GF）membrane,the cycle life is increased from 60 cycles to 210 cycles,and the rate capability and full discharge capacity are also greatly increased.SEM,XRD and Raman analysis found that the use of PVDF single ion conducting separator helps to strengthen the decomposition of the discharge product lithium peroxide（Li₂O₂）,preventing electrolyte decomposition and lithium corrosion In addition,Li|Li symmetric cell analysis shows that the use of PVDF single ion conducting separator facilitates the uniform deposition and dissolution of lithium,which can effectively improve the Li anode process and prevent lithium dendrites even in the presence of I^-in the electrolyte.The effect of Li I concentration was further discussed in this paper,where 1.0 M Li Cl O₄/DMSO electrolyte was used in the negative electrode chamber and 1.0 M Li I/DMSO electrolyte was used in the positive electrode chamber.Comparing the operation of the battery under the conditions of argon and oxygen,it shows that the reaction under this condition is still a lithium-oxygen battery process.Through SEM analysis,it was found that the decomposition ability of the discharge products was significantly enhanced,the charging voltage of the battery was always stable below 3.7 V,and the number of cycles reached 147cycles.In contrast,the battery can only 3 cycles when using the GF separator.On the one hand,it shows that the high concentration of Li I in the electrolyte can further strengthen the reaction process of the cathode,and at the same time has a more serious negative impact on the charging and discharging process of the Li anode;on the other hand,it also shows that the modified PVDF single ion conducting separator can block the diffusion of high concentration Li I,effectively protect the Li anode.3.The modified PVDF-HFP single ion conducting separator was used as the battery separator,in which the negative electrode chamber used 1.0 M Li Cl O₄/DMSO electrolyte,and the positive electrode chamber used with 0.05M Li I in 1.0 M Li Cl O₄/DMSO electrolyte.Compared with the GF separator,the cycle life of the battery increased from 60 cycles to 160 cycles,and the rate capability and full discharge capacity also improved.SEM,XRD and Raman analysis found that the use of PVDF-HFP single ion conducting separator helps to strengthen the decomposition of Li₂O₂and alleviated the electrolyte decomposition and lithium corrosion.Futhermore,Li|Li symmetrical battery analysis shows that the use of PVDF-HFP The single ion conducting separator facilitates the uniform deposition and dissolution of lithium,preventing the effect of I^-in the electrolyte on the Li anode.In this paper,the modified PVDF-HFP single ion conducting separator is used as the separator,in which the negative electrode chamber uses 1.0 M Li Cl O₄/DMSO electrolyte,and the positive electrode chamber uses 1.0 M Li I/DMSO electrolyte.Through SEM analysis,it was found that the decomposition ability of the discharge products was significantly enhanced,the charging voltage of the battery was always stable below 3.65 V,and the number of cycles reached 93 cycles.This indicates that the modified PVDF-HFP single ion conducting separator can block the diffusion of high-concentration Li I and effectively protect the Li anode.