Study Of The Li-ion Battery Separator Based On Zirconia Fiber

Lithium ion battery separator is one of the key inner components of the battery,which determines the capacity,cycle and battery life of the battery.Currently,the market of lithium ion battery separator is occupied by polyolefin microporous film because of its good mechanical properties,but problems do exist with its low porosity,low absorption rate and poor thermal stability.Polyvinylidene fluoride（PVDF）membrane with its high porosity and absorption rate become a hot spot,while its poor mechanical strength,low melting point and easy deformation have limited its using scope.To solve these defects of the separators,researches focused on polyolefin film or PVDF membrane as substrate combined with the inorganic particles（Al2O3,Si O2,ZrO₂ etc.）.But the complicated method and high cost are real teaser.In this paper,we adopt a novel preparation method of lithium ion battery separator based on inorganic fiber.Study on the selection of raw materials of PVDF polymer membrane,and synthesis and properties of PVDF-HFP/ ZrO₂ fiber membranes,and the electrochemical performance evaluation and testing,and the commercial Celgard2400 and 75%PVDF-HFP/ZrO₂ powder separator prepared by the same method were studied for comparison.The following conclusions were reached:（1）The structure and properties of microporous membranes were studied under different concentrations of PVDF and the copolymer of PVDF（PVDF-HFP）.The results show that the mechanical strength increased and pore size,porosity,absorption rate decreased with the increase of polymer concentration in a certain range.The mechanical strength,porosity,and absorption rate of PVDF-HFP were better than that of PVDF.（2）The PVDF-HFP/ZrO₂ microporous membranes were prepared.Inorganic zirconia,either in the form of fiber or powder,can be dispersed evenly in the PVDF-HFP microporous membrane.With the increase of fiber ratio,the mechanical performance of microporous membrane increased both in dry state or fracture in the wet state strength because of the increasing of friction and cohesion between the fibers,and the mechanical performance is better compare in wet with the dry condition.（3）Compared with Celgard2400,PVDF-HFP/ ZrO₂ fiber membrane has better electrolyte wettability and thermal stability.The absorption height of 75% ZrO₂ fiber membrane is about 4 times of Celgard2400;The electrolyte drops on surface of fiber membrane spread rapidly while on Celgard2400 is almost spherical;There was little change in the quality of fiber film at 450 ℃,while Celgard2400 began to break down sharply at 400 ℃,and almost decomposed completely at 495 ℃;75% fiber membrane has no shrinkage in the high temperature oven,while Celgard2400 start to shrink in CD at 160 ℃ and seriously at 170℃.Fiber membrane shows good flame retardant,and Celgard2400 quickly shrink into a ball.Compared with 75%PVDF-HFP/ZrO₂ powder membrane,75%PVDF-HFP/ZrO₂ fiber microporous membrane showed excellent mechanical properties and structural stability,and the powder on powder separator is easy to be scattered.Whether in the electrolyte or in the flame,the fiber can provide a good structural stability for the microporous membrane.In contrast,the powder membrane electrolyte is easy to curl in electrolyte and lose stable structure in the fire.（4）Interface resistance PVDF-HFP/ ZrO₂ separators were less than Celgard2400.The interfacial resistance of the cell decreases and the ionic conductivity increases gradually with the addition of the fiber ratio.After charging and discharging at 0.2C for 50 times,the capacity retention rate of 75% ZrO₂ fiber membrane is about 93%,which is 3% higher than that of Celgard 2400,and 45% higher than that of 75% ZrO₂ powder separator.The capacity retention of 75% ZrO₂ powder cell was only 24% at 8C,deeply lower than that of Celgard 2400（35%）and 75% ZrO₂ fiber cells（38%）.The 75% fiber membrane has the maximum energy density,and proves that the fiber plays an important role in maintaining the structure of the microporous membrane.（5）The cycle and rate performance of the cells based on Celgard 2400 and the different ZrO₂ fiber ratio at 60 ℃ show no interference,but cells based on 75%ZrO₂ powder membrane decrease slightly,which once again reflects the advantage of using fibers as the reinforcement for separators to keep the chemical stability at high temperature.The OCV at 140 ℃ of the cells using Celgard 2400 drops to 2.39 V in less than 10 minutes,and only 0.26 V after 210 minutes.The cells composed of 75%ZrO₂ powder separator and the 75%ZrO₂ fiber separator are able to maintain the original OCV throughout the testing time.Taken together,the method of using inorganic fibers as substrates to fabricate separators for lithium-ion batteries may become a platform strategy for the manufacture of battery separators.