Polyvinyl formal based single-ion conductor membranes as polymerelectrolytes for lithium ion batteries

Fang Lian n, Hong-yan Guan, Yan Wen, Xiao-rong PanSchool of Materials Science and Engineering, University of Science and Technology Beijing, No 30, Xueyuan Road, Haidian District, Beijing 100083, China

a r t i c l e i n f o

Article history:Received 8 January 2014Received in revised form20 April 2014Accepted 31 May 2014Available online 24 June 2014

Keywords:Lithium-ion batteriesSingle-ion conductor polymerMembraneElectrolytePolyvinyl formal

a b s t r a c t

Novel single-ion conductor polymer (SCP) membranes based on oxalate-chelated borate structuregrafted polyvinyl formal (PVFM) were developed herein. The membranes and their application aspolymer electrolytes for lithium-ion batteries were characterized by Fourier transformation infraredspectroscopy (FT-IR), X-ray diffraction (XRD), thermogravimetry (TG), differential scanning calorimetry(DSC), stress–strain test, electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV)and charge/discharge test. Results show that the novel SCP membranes exhibit good thermal stabilityand excellent mechanical strength of 11–34 MPa; the obtained gel polymer electrolytes (GPEs) frompropylene carbonate (PC) plasticized SCP membranes show ionic conductivity of 10�6–10�5 S cm�1

between 15 and 60 1C, wide electrochemical stability window of 45 V (vs. Liþ/Li) and good matchingproperties with Li/LiFePO4 cells. The PVFM based SCP membrane shows promising characteristics aspolymer electrolytes for lithium-ion batteries.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

A combination of high fuel costs, concerns about petroleumavailability and air quality issues related to fossil fuel based vehiclesare driving interests in hybrid electric vehicles (HEVs) and plug-inhybrids (PHEVs) [1]. Lithium-ion battery is fast emerging as the longterm winner for these EVs compared with other electrochemicalpower sources, because of its lightweight and high energy density[2,3]. Safety issues of lithium-ion batteries using the liquid electrolyte,arising from potential leakage and combustion of organic liquids, areof primary concern in their practical applications [4–6]. While solidpolymer electrolytes (SPEs) consisting of a polar polymer matrix andlithium salts have attracted great interest [7] due to its much betterstability, safety, flexibility in the design of the cells, and also its doublefunction as an ionic conductor electrolyte and separator in theelectrochemical devices [8,9]. However, SPEs have dual ion con-ductivity of the anion X� and the lithium ions, which is contributedto the dissolution of a discrete lithium salt LiX into a dissociatingmedium. Therefore the problems such as salt overconcentrationand/or salt depletion in the polymer matrix often lead to anincrease of the polarization and an appearance of strong osmoticforce in the electrolyte system [10].

Recently single-ion conductor polymer electrolytes were proposedto solve these issues, in which the anionic functions were tetheredonto a rigid polymer chain to increase cation transport number in thesolid electrolyte membranes. Bouchet et al. prepared the single-iontriblock copolymers P(STFSILi)–PEO–P(STFSILi) by grafting styrenesul-phonyl(trifluoromethanesulphonyl)imide lithium salt (STFSILi) to neu-tral polystyrene–PEO–polystyrene (PS–PEO–PS) copolymer chain [7].Wu et al. proposed single-ion polymers with oxalate-chelated boratestructure tethering to polyvinyl alcohol (PVA) [11] and polyacrylic acid(PAA) [12] main chain, respectively. The previous studies make anapproach to a novel macromolecular electrolyte for batteries includinglithium-metal based ones.

Polyvinyl formal (PVFM) has been widely noted for its highconductivity, unique film-forming properties and in particularexcellent adhesion on many surfaces [13,14]. In our recent study,novel PVFM based gel polymer electrolytes with high ionic con-ductivity and wide electrochemical stability have been preparedby means of initiator-free thermal polymerization method [15].In the paper, we reported the synthesis and unique electrochemicalproperties of novel oxalate-chelated borate grafted PVFM basedsingle-ion conductor polymer (SCP) membranes. The membraneswere characterized by X-ray diffraction (XRD), FT-IR, stress–straintest, thermogravimetric (TGA) and differential scanning calorimetry(DSC) analysis. Then their performance as separators and ionicconductors for lithium-ion batteries is studied by AC impedance,and linear sweep voltammetry (LSV).

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/memsci

Journal of Membrane Science

http://dx.doi.org/10.1016/j.memsci.2014.05.0650376-7388/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel./fax: þ86 10 82377985.E-mail address: lianfang@mater.ustb.edu.cn (F. Lian).

Journal of Membrane Science 469 (2014) 67–72

2. Experimental

2.1. Preparation of single-ion conductor polymer membranes

Polyvinyl formal (PVFM, Aldrich) with a molecular weight of70,000 was used as raw material, in which the molar percentagesof vinyl acetal, vinyl hydroxyl and vinyl acetate groups in PVFM are62.3%, 10.5%, and 27.2%, respectively. Boric acid (H3BO3), lithiumcarbonate (Li2CO3), oxalic acid (H2C2O4) and dimethylsulfoxide(DMSO) were purchased from Sinopharm Chemical Reagent Co., Ltd.

The single-ion conductor polymer membranes were prepared asshown in Fig. 1, which is similar to the reported processes [11,12].PVFM was dissolved into DMSO (1:10 by weight) to get a lightyellow homogeneous and transparent solution. A certain amount ofH3BO3 was added to the above solution and magnetically stirred at80 1C for 4 h to get a homogeneous mixture. Subsequently Li2CO3

and H2C2O4 were added in turn and then the mixture wasmagnetically stirred at 100 1C for 8 h. The molar ratio of H3BO3,Li2CO3 and H2C2O4 is fixed at 2.0:1.0:2.0. The solution was finallycooled to room temperature, casted on a neat glass plate by doctorblade method and dried at 70 1C for 12 h to remove the solvent. Theresulting membranes with thicknesses of 30–50 mm were punchedinto circular pieces (d¼16 mm), further dried in a vacuum oven at50 1C for 24 h and transferred into a dry-argon filled glove box (H2Oand O2o0.5 ppm) before any experiment. The molar ratio of –OH inPVFM and B is fixed at 2:0.8, 2:0.9, 2:1.0, 2:1.1 and 2:1.2 to obtainthe corresponding SCP samples, which are marked as SCP-0.8, SCP-0.9, SCP-1.0, SCP-1.1 and SCP-1.2, respectively. Measurement ofmolar ratio of –OH in PVFM was described in detail as Supplemen-tary information.

2.2. Sample analysis

FTIR spectra were recorded on NEXUS FT-IR670 spectrometer(USA) in the range of 400–4000 cm�1. XRD patterns of polymermembranes were performed on an XD2618N X-ray diffraction analy-zer at room temperature. TGA and DSC data were collected through a

NETZSCH STA449F3 analyzer (Germany) from room temperature to600 1C at a heating rate of 10 1C min�1 in argon atmosphere. For thestudies on mechanical properties of the polymer electrolytes, themembranes were prepared in the size of 1 cm�3 cm and weremeasured from stress–strain tests at room temperature using Instron4465 instrument with a tensile speed of 5 mmmin�1. The morphol-ogy of the membranes was observed by field emission scanningelectron microscope (FESEM) (Carl Zeiss, SUPRA55, Germany).

2.3. Electrochemical measurements

For the electrochemical measurements, PC about 10 wt% of therelated membrane was dropped to the surface of polymer mem-branes to form the GPEs by means of a pipettor (Nichipet EX,Nichiryo, Japan). The ionic conductivities (σ) of the GPEs weremeasured by the AC impedance method. The samples weremeasured in blocking-type cells which were fabricated by sand-wiching the membranes between two stainless steel electrodes.Impedance data were obtained with an electrochemical workingstation CHI660a (Shanghai, China) in the frequency range of 1 Hz–100 kHz. The ionic conductivity was calculated from

σ ¼ L=ðRSÞðS cm�1Þ ð1Þwhere σ is the ionic conductivity, R is the bulk resistance, L is thethickness of the polymer membrane and S is the area of thestainless steel electrode.

The electrochemical stability window of the as-prepared GPEswas measured by means of linear sweep voltammetry (LSV) on thecells, in which stainless steel was used as the working electrodeand lithium foil was used as the counter and reference electrode,respectively. The measurement was done between 2 and 7 V (vs.Liþ/Li) at a scanning rate of 5 mV S�1 on an electrochemical workstation (Chi660a, Shanghai, China). LiFePO4 based cathode plateswere prepared by coating a slurry consisting of LiFePO4, acetyleneblack and poly(vinylidienefluoride) (PVdF) in N-methyl pyrroli-done (NMP) in the ratio of 80:15:5 by weight, on an aluminum foilby the doctor blade process. 2032-type Li/LiFePO4 coin cells wereassembled with GPEs as the separators and electrolyte. Charge and

Fig. 1. Preparation schematic of oxalate-chelated borate structure grafted PVFM based SCP membranes.

F. Lian et al. / Journal of Membrane Science 469 (2014) 67–7268

discharge tests for the batteries were performed galvanostaticallyat room temperature over a voltage range of 2.5–4.25 V at 1/10Crate (corresponding to 0.14 mA cm�2) using Land Battery TestSystem (Wuhan Land Electronic Co. Ltd., China).

3. Results and discussion

FT-IR spectra of pure PVFM and SCPs are shown in Fig. 2a. Forpristine PVFM, peaks at 1020, 1070, 1135, 1180 and 1241 cm�1

represent the C�O�C�O�C bonds in the ether ring and the peakat 1735 cm�1 is ascribed to the C¼O bonds in vinyl acetate groupsof polymer matrix [13]. The small peak at 3480 cm�1 for SCPsrepresents the ν(–OH) caused by water absorbed by Liþ ions inSCP structure after exposure to air. The peaks at 1778 and1648 cm�1 are assigned to C¼O groups in oxalate-chelated boratestructure in SCP as shown in Fig. 1. It can be seen from Fig. 2b thatall the PVFM and SCP membranes show a large diffusion peakbetween 121 and 191 indicating their amorphous state. Thecrystallinity of the polymer chains generally depend on theirthermal history and molecular weight [16]. The oxalate-chelatedborate structure can enlarge the distance between the C–C chainsin PVFM and decrease the amount of hydroxyl groups as well,which is favorable for forming intermolecular hydrogen bonds. Incompared with pure PVFM, SCPs show much wider peak at 191with reduced intensity, implying that the crystallinity of thepolymer is reduced after the craft of oxalate-chelated boratestructure. Additionally, no LiBOB phase [17] can be observed inthe XRD patterns, which indicates that there is few LiBOBgenerated during the synthesis process of SCP membranes.

Fig. 3 depicts the TG/DSC curves of PVFM and various SCPmembranes. Two small exothermic peaks at 110 1C and 145 1C forPVFM in Fig. 3b represent the glass transition temperature andcrystallization peak of the polymer main chain [15], respectively. Itcan be observed from Fig. 3a that pure PVFM shows weight loss upto 322 1C and the SCP membranes can keep thermally stable up to

106 1C. Fig. 3a and b shows that the SCP membranes decomposeslightly between 106 1C and 214 1C attributing to the slightdecomposition of oxalate-chelated borate structure. Additionally,the weight loss of oxalate-chelated borate structure grafted PVFMbased SCP membranes at 330 1C is about 20% while the weight lossof LiPVAOB [11] and LiPAAOB [12] is 35–40% at 330 1C, whichindicates that the thermal stability of as-prepared membranes ismuch better than competitive systems. Besides the decompositionfragments of polymer main chain, the evolved gas in the hightemperature range is mainly CO2 according to the in-situ thermo-gravimetry/mass spectrum analysis (Supplementary Fig. S3),which is helpful to prevent the batteries from fire.

Mechanical performance is a key point determining whetherSCP membranes can be practically applied as both the batteryseparators and ionic conductors, since they should withstand thestress during cell packaging and charge/discharge cycling [17].Stress–strain tests at room temperature on SCPs are demonstratedin Fig. 4. All the SCPs have a good mechanical strength due to theirsemi-crystalline property shown in Fig. 2b. It can be observed fromTable 1 that all the membranes display high tensile strengthranged from 11 to 34 MPa, and the membrane SCP-1.0 (n�OH:nB¼2:1.0) exhibits the highest tensile strength of 34 MPa. The highmechanical strength is due to the unique film forming propertiesof PVFM and the strong crosslinking between oxalate-chelatedborate structure and PVFM domains, which will be of great benefitfor the long-time operation of batteries. Besides, the excellentmechanical properties of the membranes are also expected tomitigate the effect of dendritic growth of lithium metal. Thoughthe oxalate-chelated borate structure grafted PVFM display

Fig. 2. FT-IR (a) and XRD (b) spectra of pure PVFM and various oxalate-chelatedborate structure grafted PVFM based SCP membranes.

Fig. 3. Thermogravimetry (a) and differential scanning calorimetry (b) curves ofPVFM and SCP membranes under argon atmosphere at a scanning rate of 10 1Cmin−1.

F. Lian et al. / Journal of Membrane Science 469 (2014) 67–72 69

excellent tensile strength, low elasticity of elongation at roomtemperature should be overcome in the follow-on work.

The morphology of the oxalate-chelated borate structure graftedPVFM based SCP-1.0 membrane was observed by field emission

scanning electron microscope (FESEM) and the results are shown inFig. 5. It can be obtained from Fig. 5a and c that the surface of theSCP-1.0 membrane is dense, and some precipitates appear in themembrane. The cross-section in Fig. 5d demonstrates its densesurface and uniform inner structure, which contribute to highmechanical strength of the membrane. However, the existingprecipitates and thin edge as shown in Fig. 5b may lead to thelow elasticity of elongation of the membrane, which will be over-come in the next further study.

The temperature dependence of GPEs on ionic conductivity σ isshown in Fig. 6. As expected, the curvatures of the Arrhenius-typeplots indicate that the conductivity–temperature relationship ofGPEs obeys the Vogel–Tamman–Fulcher (VTF) relation, whichdescribes the transport properties in a viscous matrix [18]. Fig. 5shows that all the GPEs exhibit the conductivity of 10�6–

10�5 S cm�1 in the temperature range of 15–60 1C, which is asuperior result for a single-ion polymer conductor [11,12]. It shouldbe noted that the ionic conductivity of single-ion conductor

Fig. 4. Stress–strain curves for oxalate-chelated borate structure grafted PVFMbased SCP membranes at room temperature.

Table 1The main relevant characteristics of PVFM based SCP membranes.

Sample Mechanicalstrength (MPa)

Conductivity at60 1C/(�10�5 S cm�1)

Electrochemical stabilitywindow/V (vs. Liþ/Li)

SCP-0.8 21 1.397 6.9SCP-0.9 16 4.239 6.0SCP-1.0 33 3.371 6.4SCP-1.1 20 3.245 5.7SCP-1.2 25 3.159 6.3

Fig. 5. Micrographs of (a) macroscopic surface, (b) precipitates, (c) membrane matrix and (d) cross-section of oxalate-chelated borate structure grafted PVFM based SCP-1.0membrane.

Fig. 6. Conductivity and the fitted results of oxalate-chelated borate structuregrafted PVFM based GPEs between 15 and 60 1C.

F. Lian et al. / Journal of Membrane Science 469 (2014) 67–7270

polymer gel electrolytes is generally lower than that of dual ionsconducting system because of the lost conductivity contribution ofanion transference. Additionally, the conductivity of single-ionconductor GPEs depends on the two competing factors includingsegmental mobility and the mobile charge carrier concentration[19,20]. Segmental mobility influences the mobility of chargecarriers, while the mobile charge carrier concentration dependson interaction between the Li ions and the polymer chain. Thereforethe highest conductivity of GPE from SCP-1.0 can be explained bythe fact that the segmental motion dominates ionic conductivity atlow temperatures, while the high concentration of charge carrier isprior to the segmental motion at high temperatures.

The electrochemical stability windows of single-ion conductorGPEs were analyzed by linear sweep voltammograms (LSV) usingLi/GPE/stainless steel cells at room temperature between 2 V and7 V vs. Li/Liþ at a scanning rate of 5 mV s�1 as shown in Fig. 7. Noobvious current peak is found through the working electrode fromopen circuit potential to 5 V vs. Li/Liþ for all the GPEs. Thephenomena are consistent with the fact that anions contribute tothe instability of the electrolyte at high potential [21], which aretethered to the polymer chain and only lose their negative charge atinterface in the oxalate-chelated borate structure grafted PVFMbased single-ion GPEs. While the applied voltage is larger than 5 V,the single-ion conductor GPEs will be oxidized slowly due to theoxidation of plasticizer PC. The as-obtained oxalate-chelated boratestructure grafted PVFM based GPEs show wide electrochemicalstability window, which is accessible to the application in lithiumpolymer battery especially for high-voltage lithium ion batteries.

Li/LiFePO4 coin cells were constructed to investigate the appli-cation of the single-ion conductor based GPEs as polymer electro-lytes in lithium-ion batteries using charge/discharge testing atroom temperature. Fig. 8 shows the cycling performance at C/10rate between 2.5 and 4.25 V of Li/LiFePO4 cells assembled withSCP-1.0 (n–OH:nB¼2:1). Results show that the cell exhibits aninitial capacity of 139 mAh g�1 and initial coulombic efficiencyof 82.7%. After 20 cycles the cell still reveal discharge capacity of137 mAh g�1 and coulombic efficiency of 99.7%. Consequently, itcan be concluded that the oxalate-chelated borate structuregrafted PVFM based GPE provides good electrochemical reversi-bility as polymer electrolytes for the cells.

4. Conclusions

In the paper, we propose a series of oxalate-chelated borategrafted polyvinyl formal (PVFM) as single-ion conductor polymers(SCPs), which are prepared through the reaction of PVFM, H3BO3,Li2CO3 and H2C2O4 in the presence of DMSO. The SCP membranesexhibit high thermal stability compared with competitive systemsand remarkable tensile strength ranged from 11 to 34 MPa at roomtemperature. The membrane with the molar ratio –OH: B¼2:1.0exhibits the highest tensile strength of 34 MPa. The correspondinggel polymer electrolytes (GPEs) after swelling a small amount ofplasticizer propylene carbonate (PC) show ionic conductivity rangedfrom 10�6 to 10�5 S cm�1 between 15 and 60 1C, wide electroche-mical stability window of 45 V (vs. Liþ/Li) and good electrochemicalreversibility as polymer electrolytes for cells. All these positive resultsindicate that PVFM-based single-ion conductor polymers are acces-sible to the high safety of lithium ion batteries.

Acknowledgments

This work was financially supported by the National 863Program of China (No. 2013AA050901).

Appendix A. Supplementary information

Supplementary data associated with this article can be found in theonline version at http://dx.doi.org/10.1016/j.memsci.2014.05.065.

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