Study On Ceramic Coated Polyolefin And PSA/PET/PSA Composite Nonwoven Separators For High Power Lithium-ion Battery

Lithium-ion batteries(LIBs)have rapidly expanded into industrial application fields such as portable device,automotive industry,aerospace and military equipment due to their high energy density,high operational voltage,low self-discharge rate and long cycle life.Separator is a critical component with two main functions including preventing physical contact of positive and negative electrodes and maintaining fast ionic transport within the cell.The selection of a proper separator is very important for achieving excellent battery performance such as cycle life,energy density,power density,high rate capacity and safety.Currently,the polyolefin separators such as polyethylene(PE)or polypropylene(PP)are the most widely used in lithium-ion batteries because of their excellent electrochemical and chemical stability against the liquid electrolytes,considerable mechanical strength and low price.However,two main disadvantages of polyolefin separators should be overcome for their usage in power lithium ion battery.One is their large thermal shrinkage,which is difficult to ensure electrical isolation between electrodes at an elevated temperature due to their low softening and melting temperature.Another one is their low porosity and large difference of polarity against the polar liquid electrolyte,which lead to high cell resistance and therefore restrict the performance of LIBs.The ceramic nanoparticles which possess high thermal stability,high specific surface area and a lots of polar functional groups were used to prepare ceramic composite separator.In this paper,the dip-coating method was firstly performed on the porous PP separator,the effects of component ratio of coating slurry on the microstructure,thermal stability,and electrochemical properties of the separators were studied.In order to improve the adhensive property between ceramic powder and PP matrix,magnetron sputtering deposition treatment were used to form a thin uniform ceramic layer on the PP substrate without using binders.The effects of magnetron sputtering treatment time on the properties of composite separators were studied.In order to obtain a nonwoven-based separator with excellent properties,porous separator with multi-scale was designed.It was developed by electrospinning polysulfonamide fibrous layers on both sides of poly(ethylene terephthalate)nonwoven.Various properties of composite separator were studied.The electrolyte penetration and absorption experiments were designed,the results indicated that the porous structure had a great impact on the wettability of separators.The major contents and results were showed as follow:Firstly,a series of ceramic coating slurry with different ceramic/binder ratio were prepared,the PP separator was used as the substrate and Ba SO4/PP ceramic composite separator was prepared by dip-coating method.The dispersion property of the coating slurry was studied and the optium coating slurry was selected.The results showed that the introduction of the ceramic coating improved the thermal stability,the wettability toward liquid electrolyte and electrochemical properties of the separator.The ceramic slurry had the best dispertion property when the the ratio of ceramic powder / binder was 70/30,the particle size distribution of the ceramics was between 300-800 nm,mainly distributed at 600 nm.Nano Ba SO4 formed a continuous and uniform distribution on the PP surface.The thermal shrinkage of CSS-1 at 160℃ was 15%,afforded a 60% increase in thermal stability compared to PP separator.The water contact angle and electrolyte uptake for the separator was about 59.5±5.5° and 211.5±6.2%.After 100 cycles,the capacity of the cell with the separator was 123.0 m Ah/g,the capacity retention was 90.5%.The discharge capacity was 67.8 m Ah/g at 8 C rate.Magnetron sputtering deposition(MSD)approach was developed to modify the porous PP separators,a composite ceramic separator was prepared.The effects of magnetron sputtering time on the properties of the separators were studied.Compared with the bare PP separator,the thermal stability,the wettability toward liquid electrolyte and the electrochemical performance of the modified separator were improved obviously.The microstructure results showed that the deposition rate of nano Ti O2 coating was 2.2 nm/min,the ceramic layer(nanoparticle size was around 5 nm)had little effect on the pore structure of the separator when the MSD time was less than 30 mins.The uniform pore structure of the bare PP separator was greatly preserved.The Ti O2 coating tightly and continuously attached on the nonporous area of PP separator,the Ti O2 coating worked as a skeleton.After MSD treatment,the thermal shrinkage was effectively suppressed in the whole measured temperature window.For instance,the thermal shrinkage of 60 MSD sample at 160℃ was 17.8%,afforded a 53% increase in thermal stability compared to PP separator.The tightly coated Ti O2 thin layer prevented the direct exposure of the polymeric component to oxygen which greatly improved the thermal decomposition temperature of modified separator.The water contact angle decreased obviously after the separators were subjected to MSD treatment.Due to the polar nature of the deposited particles,the Ti O2 coating effectively changed the hydrophobic surface of PP separators to the hydrophilic one,the water contact angle for 60 MSD sample was about 59.3±2.5°.The Ti O2 coated separators presented a higher electrolyte uptake than the bare PP separator.For instance,compared to 144.3 % for bare PP separator the electrolyte uptake for 60 MSD sample were 184.6%.Although the 60 MSD had the best thermal stability and wettability,a blocked pore structure was observed when the MSD time was longer than 30 mins.It seriously affected the electrochemical properties of separator.The 30 MSD sample showed the best electrochemical properties.The ionic conductivity was 0.48?10-3 S/cm.After 100 cycles,the capacity of the cell with the 30 MSD sample was 124.6 m Ah/g,the capacity retention was 91.9%.The discharge capacity was 73 m Ah/g at 8 C rate.The electrolytes/30 MSD separator systems showed improved electrochemical stability,the electrochemical stability window was up to 5.5 V(vs Li/Li+).The electrospinning technology was successfully applied to modify the PET nonwoven supporter,a composite separator with a multi-scale porous structure was obtained.The microstructure results showed that a nanofibrous structure in the PSA/PET/PSA separator was realized by electrospinning of ultrafine PSA fibers on PET nonwoven.The PSA nanofibers were attached with the PET supporter without binders,the pore size of PSA/PET/PSA separator was around 150 nm to 200 nm.Due to the high thermal stability of PET and PSA,the shrinkage of PSA/PET/PSA separator was very small over the whole measuring temperature range.The PP separator was highly shrunk and its thermal shrinkage was observed to be 75 % after being exposed to 170 °C for 0.5 h.The thermal decomposition temperature of PSA/PET/PSA separator was 450 °C,a 192 °C increase was obtained compared to PP separator.The electrolyte uptake of PSA/PET/PSA separator was 338%,a 136% increase was obtained compared to PP separator.This improvement was attributed to the high porosity(64%)and the interaction between the polar groups of liquid electrolyte molecules and polar groups(such as-C-N-/-C-O-,-CONH-,O=S=O,and-O=C-OH)in the PSA molecular chains.The tensile strength increased 240% compared to pure electrospun PSA separator,the PET nonwoven worked as an efficient mechanical supporter.The composite separator showed an excellent electrochemical properties.After 100 cycles,the capacity of the cell with the separator was 124.0 m Ah/g,the capacity retention was 90.1%.The discharge capacity retention was 45.4% at 8 C rate.The lithium ion conductivity behavior of PSA/PET/PSA,PSA and PP separator obeyed Arrehenius equation.The tortuosity of PSA/PET/PSA,PSA and PP separator were 2.30,3.25,7.10 respectively.PET layer had a low tortuosity and the lithium ion conduction in the PET layer was not hindered.The penetration and adsorption experiments were designed to determine the relationship between pore structure parameters and electrolyte penetration and adsorption properties.The PP,PSA/PET/PSA and PSA separator were chosen as the research separators.The general law of electrolyte flow in the porous structure of separator is revealed by the penetration model.The breakthrough pressure and the flow rate under breakthrough pressure were obtained.Penetration pressure for PP separator is 826.8 Pa which was 6.5 times greater than that for PSA/PET/PSA separator(127.2 Pa),and 1.2 times greater than that for PSA separator.The calculated breakthrough radius is 286 μm for PSA/PET/PSA separator,95.4 μm for PSA separator and 21.2 μm for PP separator.The effective length of the aperture for PSA/PET/PSA,PSA and PP separator were 0.24 μm,0.91 μm,5.89 μm respectively.The diffusion coefficients of the PSA/PET/PSA(0.22611)and PSA separator(0.27673)were much higher than that of PP separator(0.0034).The calculated radius was 22.9 nm for the PSA/PET/PSA separator,27.9 nm for PSA separator,while the radius for PP separator was 1045 nm.The pore structure of nonwoven separators were beneficial for the penetration and absorption of electrolyte in nonwoven-based separators.