Modification Of Polyolefin Membranes And Alginate Excipients For Lithium Ion Batteries

With the continuous development of society,the high-rate,high-security,environmentally friendly lithium-ion battery systems is the constantly pursuing goal.The property of the separator can directly affect the important performances of the lithium ion battery such as rate,cycle,safety and internal resistance.Now,commercial polyolefin separators are the most widely used separators,but their electrolyte properties,heat resistance and barrier to lithium dendrites are not fully satisfied due to the properties and structures of original materials.Moreover,polyolefin materials cannot be degenerated easily in the natural environment and thus have a negative impact on the ecological environment.Based on the review on relative literature,this paper carried out the following work:(1)Surface modification of commercial polypropylene/polyethylene/polypropylene(PP/PE/PP)separator.The surface hydrophilic functional groups connected on the surface of separatore.Therefore,a hydrogen bond network can be formed with the polyacrylic acid and polyethylene oxide which added in the positive electrode material.Based on the hydrogen bond network,it was successfully realized the flexibility of the lithium ion battery electrode.Moreover the long cycle life and high rate performance were achieved in this lithium ion battery.(2)Surface modification of graphene oxide and nanocellulose composites on commercial PP/PE/PP separators.To improve the liquid retention rate and heat resistance,the composite of GO and NFC was grafted on the surface of separator,and finally the purpose of improving the safety of lithium-ion batteries was achieved.(3)Alginate dressings as a new kind of separator for Lithium-ion battery.In order to realize the environmentally-friendly Li-ion battery's separator,,a new membrane---alginate dressings were proposed as lithium ion battery separator.Here the alginate dressing was modified to improve the cycle stabilities of the assembled lithium ion battery..stability.The details are as follows:(1)Preparation of Polyolefin Membrane Modified and Flexible,High-Resolution Lithium Ion Battery Electrode: The surface of PP/PE/PP separator was pretreated by chemical oxidation method to graft some hydroxyl and carboxyl polar functional groups on the surface of PP/PE/PP separator.On the one hand,it improves the wettability of the electrolyte;on the other hand,it provides a bonding point for the flexible electrode,and the ion conductive polymer having a hydrogen bond functional group is substituted for a part of the binder,and is directly coated.On the surface-pretreated PP/PE/PP separator,a hydrogen bond is formed between the polar functional group on the surface of the pretreated membrane and the ion conductive polymer,and a hydrogen bond network is formed inside the flexible electrode material.Therefore,the flexibility of the lithium ion electrode is ensured,and the transmission performance of lithium ions inside the electrode is increased(increasing the rate performance).The result of infrared spectroscopy is that the absorption peak of PAA:PEO=4:1 has the highest red shift,indicating the formation of hydrogen bonds inside the electrode;the scanning electron microscopy of the cross section of the electrode shows that there is no obvious interface between the separator and the electrode material.,indicating that the electrode material is firmly bonded to the surface of the diaphragm,and the flexible electrode is assembled into a half-cell to further test its electrochemical performance,and it is found that the rate performance is good,and the capacity retention rate is still 65.8% at the 10 C discharge rate;In the same condition,the half-cell assembled by the conventional method has a capacity retention rate of only 59.8% at a discharge rate of 10 C.(2)Surface modification of polyolefin separators to prepare high-safety lithium-ion batteries: in order to improve the high-temperature anti-shrinkage performance,puncture prevention ability,and liquid absorption and liquid retention rate of the separator.The surface-modified membrane was coated with graphene oxide and nano-cellulose.The surface modification of the separator was the same as that of the content(1),so that the separator could be coated with graphene oxide with hydroxyl and carboxyl groups.and nanocellulose are better combined,and the graphene nanosheets and nanocellulose filaments self-assemble into a multilayer structure as the water evaporates.The battery of Li/separator/Li “sandwich” structure assembled by graphene oxide/nanocellulose composite membrane was subjected to continuous charging and charge-discharge cycle test.The experimental results show that the composite layer is obvious to lithium dendrites.Inhibition.The composite membrane was subjected to heat shrinkage,liquid absorption and liquid retention rate test.The experimental results show that the high temperature shrinkage resistance and liquid absorption and liquid retention ratio of the composite membrane are significantly improved.(3)Alginate excipients are used as modification of new membranes: Alginate excipients have the characteristics of degradable and high temperature resistance,capable of absorbing 18 times their own weight and 9 times their own weight,and have good liquid absorption.Rate,fluid retention rate.This chapter modifies medical alginate dressings for use as a new lithium ion battery separator.It is modified with a mixed solution of polyacrylic acid and lithium hydroxide monohydrate(LiOH·H2O)(solvent anhydrous ethanol)to remove impurities contained in the alginate excipient and improve it as a lithium ion battery separator.Ion transmission performance.It was assembled into a ternary/modified alginate adjuvant/lithium plate half-cell.The test results show that the modified alginate adjuvant can ensure the lithium ion transport and cycle stability in the half-cell.The heat shrinkage and liquid absorption and liquid retention rate tests show that the modified alginate adjuvant has good high temperature anti-shrinkage performance and liquid absorption and liquid retention rate.The first two systems in this paper are modified PP/PE/PP separators to obtain flexible lithium ion high-magnification electrodes and lithium ions with good liquid retention and heat resistance.Battery separators,considering the non-degradable nature of polyolefin-based membranes,modify the degradable alginate excipients to provide an alternative possibility for the selection of lithium-ion battery separators.By improving the electrochemical performance and safety performance of lithium ion batteries by modifying different separators,the lithium-ion battery system is flexible and environmentally friendly.It is hoped that a complete green,safe and efficient flexible lithium-ion battery system can be constructed in the future..