Xiao Wang ,Gaojie Xu ,Qingfu Wang ,Chenglong Lu ,Chengzhong Zong *,Jianjun Zhang ,Liping Yue ,Guanglei Cui ,*

1 School of Polymer Science and Engineering,Qingdao University of Science and Technology,Qingdao 266042,China

2 Qingdao Industrial Energy Storage Research Institute,Qingdao Institute of Bioenergy and Bioprocess Technology,Chinese Academy of Sciences,Qingdao 266101,China

Keywords:Polysulfonamide/SiO2 composite Phase inversion method Separator Performance enhancement Lithium-ion battery

A B S T R A C T In this work,a sponge-like polysulfonamide(PSA)/SiO2 composite membrane is unprecedentedly prepared by the phase inversion method,and successfully demonstrated as a novel separator of lithium-ion batteries(LIBs).Compared to the commercial polypropylene(PP)separator,the sponge-like PSA/SiO2 composite possesses better physical and electrochemical properties,such as higher porosity,ionic conductivity,thermal stability and flame retarding ability.The LiCoO2/Li half-cells using the sponge-like composite separator demonstrate superior rate capability and cyclability over those using the commercial PP separator.Moreover,the sponge-like composite separator can ensure the normal operation of LiCoO2/Li half-cell at an extremely high temperature of 90°C,while the commercial PP separator cannot.All these encouraging results suggest that this phase inversion based sponge-like PSA/SiO2 composite separator is really a promising separator for high performance LIBs.

1.Introduction

Due to their intrinsic high energy density,low self-discharge rate,and long lifetime/shelf life,rechargeable lithium-ion batteries(LIBs)have been widely used as main power source in portable electronic devices and electric transportation tools,and are expected to play a prominent role in accessing(converting–storing–transporting)renewable energy,such as wind,solar and tidal pow er[1,2].As one of the most important components in rechargeable LIBs,the separator not only provides a physical barrier between the positive and negative electrodes to prevent electrical short circuits,but also serves as the electrolyte reservoir to enable ionic transport during battery operation[3,4].Despite their good chemical/electrochemical stability,excellent mechanical strength,and proper thermal shutdown properties,the most widely commercialized polyolefin-based(such as polyethylene(PE),polypropylene(PP)and PP/PE/PP sandwich composite)microporous separators always suffer from severe thermal shrinkage,poor electrolyte wettability,low porosity,and highly flammable,which will inevitably cause safety concern and deteriorate the electrochemical performances(especially the rate capability)of LIBs[5–10].Therefore,tremendous efforts have been paid to select alternative materials(such as poly(vinylidene fluoride-co-hexa fluoropropylene)(PVd FHFP)[11–14],polyimide(PI)[15],polyacrylonitrile(PAN)[16],polyetherimide(PEI)[17],poly(acrylonitrile–methyl methacrylate)(P(AN-MMA)[18])and pore-structure making technologies(such as electrospinning[11,19],phase inversion[20–22])for advanced separators with better performances than conventional polyolefin-based separators[3].

With excellent thermal,mechanical,dielectric properties,as well as favorable chemical resistance,the rigid polymer of polysulfonamide(PSA)has been investigated asakey component in advanced separators(composite of PP nonwoven membrane and PSA polymer,composite of PSA nonwoven membrane and silicananoparticles,nonwoven composite membrane with PSA core/PVDF-HFP shell structured micro fiber,micro fibrillar composite membrane of both cellulose and PSA)by our group[9,10,23,24].As an effective way to prepare sponge-like porous membranes(separators),the phase inversion method(based on the dissolution of polymer in a precisely controlled mixture of solvents)has received attention[9,18,25].How ever,to the best of our know ledge,the phase inversion based PSA-based porous membrane and its application as separator in LIBs is not reported yet.

It is proved that ceramic materials(such as silica(SiO2),Al2O3)can endow the separators with excellent thermal/interfacial stability and high thermal conductivity,which are beneficial for suppressing micro-short circuiting and ensuring battery safety at abuse conditions(such as crush/penetration,overcharge,overheat)[26–31].Among these ceramic fillers,SiO2particles can most efficiently reduce the interfacial resistance between separator and electrodes[30,31].Compared to the Al2O3particles,SiO2particles have a better affinity with organic solvents[31].In this work,a self-standing sponge-like PSA/SiO2composite membrane,which possesses excellent thermal and electrochemical stability,is unprecedentedly prepared by the phase inversion method.LiCoO2/Li half-cells using this novel composite separator demonstrate superior rate capability and cyclability,suggesting that this novel composite separator is promising for future high performance LIBs.

2.Experimental

2.1.Materials

Polysulfonamide(PSA)short fiber was purchased from Du Pont Co,USA.N,N-dimethylacetamide(DMAc,99.5%)and ethylene glycol(EG,99.0%)was purchased from Sinopharm Chemical Reagent Co.,China.Deionized water was commercially available.Fumed silica powder(SiO2,CAB-O-SIL TS530)was purchased from Cabot Co.,USA.Polypropylene(PP)separator(Celgard 2500)was purchased from Celgard Co.,USA and used as comparative analysis.Lithium cobalt oxidate(LiCoO2)was used as cathode and offered by Citic GuoAn Power Technology Co.,China.Solution of lithium hexafluorophosphate(LiPF6,1 mol·L−1)in ethylene carbonate(EC)/dimethyl carbonate(DMC)(1/1,V/V)was supplied by Long Power Systems(Suzhou)Co.,China.Other chemical reagents were all purchased commercially and used without further purification.

2.2.Sponge-like PSA/SiO2 composite membrane by phase inversion method

The PSA short fiber was ultrasonically washed with ethanol and dried for 12 h at 80°C before usage.The casting solution was prepared by dissolving PSA,EG,SiO2in DMAc(with amassratio of 10:9:2:90)homogeneously at 40°C.Then the obtained homogeneous solution was casted on a glass plate with a thickness of 200 μm.The semi- finished membrane was treated by a dual-bath coagulation method to make porous structure.In the first coagulation bath,the membrane was washed with mixtures of deionized water and DMAc(3:1,by mass)for 10 min.After this,the membrane was washed with deionized water for 10 h to get rid of all solvent.The final PSA/SiO2composite membrane was dried under vacuum at 100°Cto remove residuals,and later was investigated as the separator in LIBs.

2.3.Characterization of separators

The thickness of the separators was measured by a micrometer caliper.The surface morphology of the separators and cross-section image of the PSA/SiO2composite separator was obtained using field emission scanning electron microscopy(SEM,Hitachi S-4800,Japan).The separators were immersed in n-butanol for 2 h,and then the porosity of separators was calculated by the equation:

where mband mais the mass of n-butanol and the separator,ρband ρais the density of n-butanol and the separator,respectively.To evaluate the air permeability of separators,the Gurley value(defined as the time of 100 cm3air to pass through the separator)was measured using the Gurley-type densometer(4110 N,Teledyne Gurley,USA).The electrolyte uptake(EU)was estimated by the following equation:

w here Wiand Wfis the mass of the separator before and after soaking in electrolyte for 2 h,respectively.Thermal dimensional change of separators(original size is 2 cm×2 cm)was measured after half an hour storage at various temperatures(100 °C,110 °C,120 °C,130 °C,140 °C and 150 °C).And the thermal shrinkage ratio(Rts)was calculated using the following formula:

w here S0and S1indicate the area of separator before and after thermal treatment,respectively.The pore sizedistribution and poresize was calculated by the BJH method using a pore size analyzer(Quantachrome instruments,USA)[32].Contact angle measurements were performed using a contact angle goniometer(Shanghai Zhongchen Digtai Technology Apparatus Co.,China).A differential scanning calorimeter(Diamond DSC,PerkinElmer DSC,USA)ranging from 50 °C to 300 °C at 10 °C·min−1under N2atmosphere was used to evaluate the thermal stability of separators.

The ionic conductivity(σ)of the separators with electrolyte between two stainless steel(SS)plates was calculated by the equation:σ =L/(A·R）

where L and A was the thickness and geometric area of the separator,respectively,while R was the total bulk resistance,which was obtained through analysis of electrochemical impedance spectroscopy(EIS)(VMP3,Bio-Logic Science Instruments SAS,France,frequency range:100 mHz–1 MHz,amplitude:10 m V).The electrochemical stability window of the electrolyte-soaked separators was obtained by alinear sweep voltammograms(LSV)experiment(VMP3,Bio-Logic Science Instruments SAS,France)performed on SS/Separator/Li cells ranging from 2.5 V to 6.0 V at a 10 m V·s−1scan rate.

The charge–discharge tests of LiCoO2/Li 2032-coin type half cells including cyclability and rate capability were carried out using a LAND battery testing system(LANHE CT2001A,Wuhan LAND electronics Co.,China).The LiCoO2cathode was composed of LiCoO2powder(90 wt%),carbon black(5 wt%)and PVDF binder(5 wt%).The cycling performance was evaluated at 0.5 C rate at room temperature and at 1 C rate at 90 °C,respectively,with a potential range of 4.2–3 V.The rate capability test was performed at 0.2 C,0.3 C,0.5 C,1 C,2 C,3 C,5 C,10 C and then reversed back to 0.2 C,successively.The EIS(VMP3,Bio-Logic Science Instruments SAS,France)of the cells was performed over frequencies ranging from 1 MHz to 100 m Hz with a voltage amplitude of 10 m V.

3.Results and Discussion

Fig.1.Typical SEM images of(a)the surfacemorphology of phase inversion based PSA/SiO2 composite separator and(b)its partial cross-section morphology after being freeze-fractured in liquid nitrogen.The pore size distribution of(c)the PSA/SiO2 composite separator and(d)the commercial PP separator,the inset of(d)is the surface morphology of the commercial PP separator.Inset of(a)is a photograph of homogeneous PSA/DMAc/EG solution with and without SiO2.

The typical SEM images and pore size distribution of the PSA/SiO2composite separator and commercial PP separator are demonstrated in Fig.1.It is clearly observed(Fig.1a and b)that the phase inversion based PSA/SiO2composite separator is “sponge-like”and has homogeneously distributed largepores.The as-prepared PSA/DMAc/EG solution with SiO2is transparent and homogeneous,indicating the fumed SiO2particles are well dispersed in the polymer matrix(inset of Fig.1a).Here,the model of the binodal liquid–liquid phase separation is used to explain the formation mechanism of sponge-like pore structure[33–35].When the casted membrane is immersed in the first coagulation bath,the solvent DMAc in the casted membrane and the nonsolvent H2O in the first coagulation bath will inter-diffuse into each other immediately.As a result,the binodal liquid–liquid phase separation happens,leading to the formation of interconnected(spongelike)pore structure[33–35].In the initial process of the binodal liquid–liquid phase separation,the small droplets of the polymer-poor phase(containing large amount of solvent DMAc,non-solvent H2O,and small amount of polymer PSA)are well dispersed in the continuous polymer-rich phase(containing small amount of solvent DMAc,nonsolvent H2O,and large amount of polymer PSA).Then,under the driven of concentration gradient,these small polymer-poor phase droplets become larger until the surrounding continuous polymer-rich phase is solidified by phase transformation of polymer PSA.Just before the solidification of continuous polymer-rich phase,the polymer-poor phase droplets coalesce with each other to facilitate the interconnected pore structure formation.In the second coagulation bath,the continuous polymer-rich phase is totally solidified,and all solvents are removed.

The pore size distribution of the PSA/SiO2composite separator is discontinuous(Fig.1c).About 90%of its pore dimension ranges from 1000 nm to 1500 nm,confirming that the sponge-like PSA/SiO2composite separator possessed relatively uniform pore distribution.As a comparison,the commercial uniaxial stretched PP separator has smaller elliptic pores and narrower pore size distribution(Fig.1d).The homogeneous incorporation of fumed SiO2(with no obvious agglomeration of silica particles in Fig.1a and b)will not only impart the mechanical strength to the composite separator,but also enhance the thermal/interfacial stability of the composite separator.Thus,this unique sponge-like PSA/SiO2composite separator is anticipated to obtain high electrolyte uptake,uniform Li-ion transportation at high charge/discharge rates and excellent ability of suppressing grow th of lithium dendrites,which are beneficial for battery power and safety[9,10,23,24].

The thickness,porosity,Gurley value and electrolyte uptake of PSA/SiO2composite separator and the commercial PP separator are listed in Table 1.It is well known that high porosity and low Gurley value can result in high electrolyte uptake,which is advantageous to rapid ionic transportation.The sponge-like PSA/SiO2composite separator also has excellent wettability(Fig.2).As clearly shown in Fig.2a and b the contact angle of the sponge-like PSA/SiO2composite separator is 68.5°,which is lower than that of PP separator 105.5°.The lower contact angle implies that the sponge-like PSA/SiO2composite separator can be quickly wetted by organic liquid electrolyte,which is confirmed in Fig.2c and d.Inferior electrolyte wettability of the separators will inevitably result in dry-zone during battery operation,which can deteriorate the cyclability of battery and possibly cause hazard.These excellent properties of the sponge-like PSA/SiO2composite separator are favorable to enhance the electrochemical performances(especially rate capability)of LIBs.

Thermal shrinkage of separators plays a key role in the safety of LIBs[9,10,23,24].The thermal shrinkage of the sponge-like PSA/SiO2composite separator and the commercial PP separator is obtained by recording their size change after being stored for 0.5 h at varied temperatures.As it is clearly shown in Fig.3,the sponge-like PSA/SiO2composite separator demonstrates slightly thermal shrinkage over temperatures ranging from 100 °C to 150 °C,while the commercial PP separator shrinks seriously and the shrinkage at 150°C is about 47%at the uniaxial-stretched direction.According to DSC curves in Fig.4,the commercial PP separator begins to melt at 150°C and has an endothermic peak at 165°C.Encouragingly,the sponge-like PSA/SiO2composite separator shows no obvious endothermic peak until 245.87°C(with the peak width of 7.28°C).The slightly thermal shrinkage of the spongelike PSA/SiO2composite separator is mainly ascribed to thehigh melting point of both PSA and SiO2[36–38].Therefore,during high temperature operation of LIBs,the sponge-like PSA/SiO2composite separator can ensure battery safety,better than the commercial PP separator,by efficiently preventing internal short circuit problem caused by thermal shrinkage of separators.

Table 1Brief physical properties of the sponge-like PSA/SiO2 composite separator and the commercial PP separator

Fig.2.Contact angle images between organic liquid electrolyte droplet and(a)PSA/SiO2 composite separator,(b)PP separator,respectively.Photographs showing liquid electrolyte wetting behavior((c)before and(d)after 10 s droplet of electrolyte)of the sponge-like PSA/SiO2 composite separator and the PP separator.

The flame retarding capability of separators is also important for guaranteeing the safety of LIBs at abusive conditions(such as crush/penetration,short circuit,overcharge,and overheat).Combustion test of the sponge-like PSA/SiO2composite separator and the commercial PP separator in air is clearly shown in Fig.5.The sponge-like PSA/SiO2composite separator demonstrates excellent flame retarding ability(self-extinguishing immediately after ignition),while the commercial PP separator shrinks and keeps combusting after ignited.The excellent flame retarding ability of the composite separator is mainly ascribed to the high limiting oxygen index(LOI)of PSA material(the LOI of PSA material is as high as 33%,while the LOI of PP material is only 18%)[39,40].In general,concerning the safety issues of LIBs,the sponge-like PSA/SiO2composite separator is a promising alternative to the commercial PP separator.

Fig.3.(a)The thermal shrinkage of the sponge-like PSA/SiO2 composite separator and the commercial PP separator over temperatures ranging from 100 °C to 150 °C,for 0.5 h;(b)The photographs of the sponge-like PSA/SiO2 composite separator and the commercial PP separator before and after thermal treatment at 150°C for 0.5 h.

Fig.4.DSC curves of the sponge-like PSA/SiO2 composite separator and the commercial PP separator.

Fig.6.Nyquist plots of the liquid electrolyte soaked sponge-like PSA/SiO2 composite separator and commercial PP separator at room temperature,frequency range:100 mHz–1 MHz,amplitude:10 mV.

The ionic conductivity(σ)of the liquid electrolyte soaked separator sandwiched between two stainless steel(SS)plate electrodes is calculated by the equation:σ =L/(A·R),where L and A is the thickness and geometric area of the separator,respectively,while R is the total bulk resistance,which is represented by the Z′-axis intercept in Nyquist plot(Fig.6).The ionic conductivity of the sponge-like PSA/SiO2composite separator and commercial PP separator is 0.748×10−3S·cm−1and 0.312× 10−3S·cm−1,respectively.The high conductivity of the composite separator is greatly associated with its high porosity(spongelike pore structure),low Gurley value,and high electrolyte uptake,and will contribute to the improvement of rate capability of LIBs.The electrochemical stability of liquid electrolyte soaked separators is normally measured by linear sweep voltammograms(LSV)of asymmetric stainless steel plate/Li cell.As clearly shown by Fig.7,the oxidation peak of the asymmetric SS/Li cell with the liquid electrolyte soaked sponge-like PSA/SiO2composite separator appears above 5.0 V,suggesting that this separator is suitable for most of LIB systems with different working potential window s.

Fig.5.Combustion test of(a-before ignition,b-after ignition)the sponge-like PSA/SiO2 composite separator and(c-before ignition,d-after ignition)the commercial PP separator.

Fig.7.Linear sweep voltammograms(LSV)of the liquid electrolyte soaked sponge-like PSA/SiO2 composite separator and commercial PP separator ranging from 2.5 V to 6.0 V at room temperature.

For practical applications,in the follow ing,the sponge-like PSA/SiO2composite separator was preliminarily used for LiCoO2/Lihalf-cell ranging from 3 V to 4.2 V.The rate capability of the LiCoO2/Li half-cells using the sponge-like PSA/SiO2composite separator and commercial PP separator is demonstrated by Fig.8.Encouragingly,at high testing rates,the LiCoO2/Li half-cell with the composite separator has a significantly higher specific capacity than the cell with the commercial PP separator.Specifically,at the rate of 10 C,the specific capacity of the LiCoO2/Lihalfcell with the composite separator can be kept high at 72.1 m A·h·g−1,while that of the LiCoO2/Li half-cell with the commercial PP separator is only 45 m A·h·g−1.

The cycle performance of the LiCoO2/Li half-cells using the spongelike PSA/SiO2composite separator and commercial PP separator is displayed(Fig.9a and b).After 200 cycles at 0.5 C rate,the capacity retention of the LiCoO2/Li half-cell using the sponge-like PSA/SiO2composite separator is 94.2%(decreasing from 130.5 m A·h·g−1to 122.9 m A·h·g−1),while the commercial PP separator is only 87.6%(decreasing from 130.9 m A·h·g−1to 115.4 m A·h·g−1)(Fig.9a).Furthermore,upon cycling,the Coulombic efficiency of the LiCoO2/Li half-cell using the sponge-like PSA/SiO2composite separator(99.6%on average)is slightly higher and more stable than that of the half-cell using commercial PP separator(99.4%on average)(Fig.9b),indicating that the parasitic reactions happened in the half-cells are possibly suppressed because of the usage of the composite separator.

AC impedance measurement was carried out to investigate the variation of cell impedance during cycle performance test.It is well known that the high-medium frequency semicircle corresponds to the interfacial resistances(including the charge-transfer resistance at electrode/electrolyte interface)and the linear tail at low-frequency region is always called Warburg resistance(W)related to the solid-state diffusion of lithium ions in the electrode materials.As it is clearly shown(Fig.9c and d)that,at both the 1st cycle and the200th cycle,the LiCoO2/Li halfcell using the sponge-like PSA/SiO2composite separator demonstrates significantly lower interfacial resistance than the cell using commercial PP separator.The superior ratecapability and cyclability of the LiCoO2/Li half-cells using the sponge-like PSA/SiO2composite separator can be attributed to the high ionic conductivity and excellent interface compatibility of the composite separator.

Fig.8.(a)The discharge capacity of LiCoO2/Li half-cells using the sponge-like PSA/SiO2 composite separator and commercial PP separator at various C-rates,and the corresponding discharge voltage curves of half cells with(b)composite separator and(c)and commercial PP separator,at room temperature with a voltage range of 3.0–4.2 V.

Moreover,it is clearly demonstrated(Fig.10)that the sponge-like PSA/SiO2composite separator can ensure the normal operation of LiCoO2/Li half-cell at extremely high temperature testing condition(at 1 C rate at 90°C)because of its slight thermal shrinkage and high thermal stability.In summary of all the above mentioned encouraging results,the phase inversion based sponge-like PSA/SiO2composite separator is really a promising alternative to the commercial PP separator.

Fig.9.(a)Capacity retention and(b)the corresponding Coulombic efficiency of the LiCoO2/Li half-cells using the sponge-like PSA/SiO2 composite separator and commercial PP separator,upon cycling at 0.5 C rate at room temperature with a voltage range of 3.0–4.2 V.Corresponding Nyquist plots for the LiCoO2/Li half-cells with different separators measured after(c)the 1st cycle and(d)the 200th cycle at room temperature.

Fig.10.Cycle performance of the LiCoO2/Li half-cells using the sponge-like PSA/SiO2 composite separator and commercial PP separator,upon cycling at 1 C rate at 90°C with a voltage range of 3.0–4.2 V.

4.Conclusions

In this work,a sponge-like PSA/SiO2composite membrane is unprecedentedly prepared by phase inversion method,and successfully investigated as a novel separator of LIBs.Compared to the commercial PP separator,the sponge-like PSA/SiO2composite separator possesses better physical and electrochemical properties,such as higher porosity,ionic conductivity,thermal stability and flame retarding ability.The LiCoO2/Li half-cells using the sponge-like composite separator demonstrate superior rate capability and cyclability than those using the commercial PP separator.Moreover,the sponge-like composite separator can ensure the normal operation of LiCo O2/Li half-cell at an extreme high temperature of 90°C.All these encouraging results suggest that this phase inversion based sponge-like PSA/SiO2composite separator is really a promising separator for high performance LIBs.