Preparation And Properties Of Fluorine Containing Ionic Binders And Polymer Electrolyte For Lithium Ion Batteries

Lithium-ion batteries,comprised of electrodes,separators and electrolytes,are widely used for their advantage of high specific energy,no memory effect,large capacity,small volume and long cycle ability.Focusing on the needs of achieving higher power density,larger capacity,and improved safety,works have been conducted in the development of high performance lithium batteries,including the development of silicon anode batteries,lithium-sulfur batteries,high-performance separators and new electrolytes.However,issues associated with these developments are still bottlenecking their wider application.For example,the large volume change of Si particles during battery cycles caused by Li⁺ion insertion and extraction had caused the deterioration of the electrodes;the"shuttling"of the dissolved polysulfide anions in lithium sulfur batteries had resulted in the decrease of battery capacity;the use of current organic liquid based electrolytes and polymer based separators had limited the improvement in voltage,capacity and safety indices.To deal with above mentioned issues,fluorine containing single-ion conducting polymers were synthesized in this work.They were proposed as flexible ionic conducing binders for the Si-particle based electrodes,binders for ceramic coating on polymeric separators,and key components for solid state or gel based electrolytes.The electrochemically stable ion conductive materials are investigated as components for binders and separators to improve the lithium ion conductivity,the high rate cycling performance,and safety property of the lithium ion batteries,and to suppress the shuttling of the dissolved polysulfide anion in lithium sulfur batteries.Series of novel ionomers based on sulfonated polyether ether ketones backbones,bearing pendant lithiated fluorinated sulfonimide groups?SPEEK-PSI-Li,SPEEK-PFSI-Li?were synthesized and used as the binders and electrolytes of the battery.The properties of the batteries were investigated.The main contents of the dissertation include:?1?Novel ionic binders based on sulfonated polyether ether ketones and polysulfone backbones,bearing pendant lithiated fluorinated sulfonimide groups?SPEEK-PSI-Li and PSU-PSI-Li?were developed.Their performance in high performance silicon?Si?nano-particles anodes were studied in comparison with conventional binders such as lithiated sulfonated poly?ether-ether-ketone?containing fluorinated sulfonic acid side chains?SPEEK-PSA-Li?,poly?vinylidene?difluoride?PVDF?,sodium carboxymethyl cellulose?CMC?and alginate.Much superior to other anode binders,the ionic conducting SPEEK-PSI-Li binders could not only provide good adhesion property,but also effectively reduce the over-potential during rapid charge and discharge cycles.When cycled at a current of 400 mA g^-1,these anodes could maintain above 2000 mAh g^-1 after 50 cycles.Even cycled at a high current rate of 2000 mA g^-1,the anodes with SPEEK-PSI-Li binders could be cycled above 500 mAh g^-1 in 50 cycles.?2?To improve the thermal stability,electrolyte wetting and absorption rate of common PP based separator,novel fluorine containing ionic conductive binders were used in preparing ceramic coatings for the ceramic/PP composite separators.It was found that compared with traditional ceramic/PP composite separators with nonionic adhesive,novel ceramic/PP composite separators containing ion conductive binders exhibits higher liquid absorption rate and higher conductivity.It was also found that in the process of charging and discharging,using ion conductive binder could effectively improve the formation of the SEI film.After a long cycle,the impedance of the battery with ionic conductive binder is much lower than that of using ordinary binder.High temperature tests showed that the novel ceramic/PP composite separators had high melt-down temperature of 120 ^oC,high heat shrinkage temperature above140 ^oC,providing ways to form safer batteries.?3?Novel comb shape ionomers with sulfonated polyether ether ketone backbone,bearing pendant lithiated poly?perfluoroalkylsulfonyl?imide groups?SPEEK-PFSI-Li?were developed and studied.Novel gel polymer electrolytes?GPEs?based on these comb shape ionomers were studied,whose room temperature ionic conductivity swollen by PC and G4 reached 1.3×10^-4 and 4.4×10^-5 S cm^-1 respectively.The battery with G4 GPE could maintain a reversible discharge capacity of 130 mAh g^-1 at 70 ^oC for 50 cycles.The superior performance of the novel SPEEK-PFSI-Li gel electrolyte could be attributed to their high lithium ion transference number,high compatibility with electrode,high electrochemical stability and high ionic conductivity,resulting from of the co-existence of flexible PEEK backbones,flexible fluorinated ether side chains,ionic transporting sulfonated groups,and pent-dentate super ionic sulfonimide groups in the molecule structures.?4?The cycling performance of lithium-sulfur battery using the PP/SPEEK-PFSI-Li composite separator,regular PP separator was compared.The results indicated that at 0.5C,the cycle process of the battery with the PP/SPEEK-PFSI-Li composite separator was much more stable.Combining with the lithium transfer number and diffusion experiment,it could be inferred that the SPEEK-PFSI-Li was much better at suppressing the active material diffusion than a pure PP separator and could efficiently improve the performance of the lithium-sulfur batteries.