A Novel PVB Based Polymer Membrane and Its Application in Gel Polymer Electrolytes for Lithium-Ion Batteries

Journal of Membrane Science 456 (2014) 42–48

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Journal of Membrane Science

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

A novel PVB based polymer membrane and its application in gel polymer electrolytes for lithium-ion batteries

Fang Lian n, Yan Wen, Yan Ren, HongYan Guan

School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China article info abstract

Article history: The polyvinyl acetal based polymer membranes have been prepared via a phase inversion method using Received 25 October 2013 our own synthetic polyvinyl butyral (PVB) from polyvinyl alcohol (PVA) as the polymer matrix. The Received in revised form proportion of hydroxyl group in PVB is controlled using acid modiﬁcation initially, and the appropriate 2 January 2014 polyurethane reaction occurs between the residual hydroxyl group of PVB and the crosslinking agent Accepted 4 January 2014 4,40-diphenylmethane diisocyanate (MDI) to approach three-dimensional network structure, which Available online 17 January 2014 contributes to the chemical stability of PVB based membrane in LiFP6-based liquid electrolyte. The dense Keywords: polymer membranes show a high liquid uptake of 288% and a high mechanical strength of 16.33 MPa. Polyvinyl butyral The membranes adsorb and swell electrolyte to form stable membrane supporting gel polymer Membrane electrolytes (GPEs), which delivers excellent performance including high ionic conductivity of Gel polymer electrolyte 4.09 10 4 Scm1 at room temperature, wide electrochemical window of 1.6–5.0 V (vs. Li/Liþ ) and Acid-modiﬁcation good compatibility with a LiFePO electrode. Lithium-ion batteries 4 & 2014 Elsevier B.V. All rights reserved.

1. Introduction batteries via an in-situ polymerization process. Novel PVFM based GPEs are developed using an initiator-free thermal polymerization Safety becomes particularly important for the application of method [13]. lithium-ion batteries as the power source for vehicles and also as Polyvinyl butyral (PVB), one of the most common series of energy storage device for distributed generation. Gel polymer polyvinyl acetal will dissolve in LiFP6-based liquid electrolyte electrolytes (GPEs) have recently received much attention owing according to the principle of the solubility parameters similarity to their high ionic conductivity at ambient temperature and [14]. The instability of PVB based membranes in its supporting absence of risk for leakage of electrolyte [1–6]. Previous studies electrolyte will probably cause an internal short-circuits of the of GPEs have proposed numerous systems based upon polymers lithium-ion batteries. So very few techniques about PVB based such as polyethylene oxide (PEO) [7], polyacrylonitrile (PAN) [8], membranes and GPEs for Li-ion batteries are reported in detail to polymethyl methacrylate (PMMA) [9] and polyvinylidene fluoride our knowledge. Chemical crosslinking polymerization or thermal (PVDF) [10] etc. The works are mainly focused on the exploration initiator polymerization [15] is accepted as an approach to of polymer electrolyte with further improved performance in ionic improve its stability. In the paper, the PVB based polymer was conductivity, chemical-, thermal- and electrochemical-stability, stabilized through polyurethane reaction using 4,40-diphenyl- and mechanical strength to realize its wide application in methane diisocyanate (MDI) as the chemical crosslinking agent. lithium-ion batteries. Moreover, the formation of PVB based membrane, which can keep Polyvinyl acetal and its derivatives have been widely noted for stable in the liquid electrolyte, depends on the acetalizing degree their advantages such as excellent thin-film forming ability, good of PVB. Herein, PVB was prepared from polyvinyl alcohol (PVA), heat- and water-resistance and relatively stable chemical struc- which is a cheap and easily available starting material. Addition- ture. Polyvinyl acetal based gel polymer electrolytes have recently ally, the proportion of hydroxyl group in the as-obtained PVB can drawn increased attention, some companies like Japan SONY [11] represent the concentration of residual PVA and the degree of and Mitsui Chemicals [12] have applied for patents successively acetalation. In our work, the proportion of hydroxyl group in PVB about the polyvinyl formal (PVFM) based gel polymer lithium-ion was determined via the catalyzed acylation method and adjusted by acid modification. The PVB based polymer membranes were prepared via the phase inversion method using the acid-modified Abbreviations: PVB, polyvinyl butyral; PVA, polyvinyl alcohol; GPEs, gel polymer PVB as the polymer matrix, N-methyl pyrrolidone (NMP) as the electrolytes n Corresponding author. Tel./fax: þ86 10 8237 7985. solvent and deionized water as the nonsolvent, respectively. The E-mail address: [email protected] (F. Lian). obtained PVB based polymer membranes with three-dimensional

network structure adsorbed and swelled liquid electrolyte to form where V1 is the volume of the sodium hydroxide solution required stable membrane supporting GPEs. The membranes and their for titration of the sample (mL); V0 is the volume of the sodium performances as GPEs were characterized by FESEM, FT-IR, liquid hydroxide solution required for titration of the sample blank uptake test, mechanical strength test, electrochemical impedance solution (mL); c is the normality of the sodium hydroxide solution spectroscopy, cyclic voltammetry and charge–discharge test. (mol L1); M is the molar mass of potassium hydroxide (M¼56.1); m is the weight of PVB sample (g); x represents the deviation in pH value of the test solution from 7 (x¼[(pH value of the test 2. Experimental solution)7], in detail x is a positive value as pH47; or x is a negative value as pHo7). In our experiments, x refers to a 2.1. Raw material difference between the pH value of PVB dissolved in NMP solution and 7, so x is zero (i.e. x¼0) herein. The related raw materials were listed as follows: polyvinyl The as-obtained PVB was performed by acid-modification to alcohol (PVA, Sinopharm Chemical Reagent Co., Ltd.), reduce the proportion of hydroxyl group in PVB. PVB powder was n-butyraldehyde (98.5%, Sinopharm Chemical Reagent Co., Ltd.), dissolved in NMP, 1 wt% acetic acid as catalyst was added into the acetic anhydride (98.5%, Sinopharm Chemical Reagent Co., Ltd.), above solution under heating for 24 h at 100 1C. The acid-modified petroleum ether (Sinopharm Chemical Reagent Co., Ltd.), product was separated from the mixture via reprecipitation. The p-toluenesulfonic acid (p-TSA, 99%, Sinopharm Chemical Reagent above mixture was added dropwise into deionized water to Co., Ltd.), polyvinyl butyral (PVB, Aldrich), NMP (98%, Sinopharm precipitate PVB, and finally dried under vacuum at 25 1C for 48 h. Chemical Reagent Co., Ltd.), MDI (Alfa Aesar), and liquid electrolyte

(1 mol/L LiPF6 in EC:DMC¼3:7(v/v), Beijing Institute of Chemical 2.3. Preparation of PVB based polymer membranes and their GPEs Reagent). The synthetic PVB powder was dissolved in NMP (PVB: ¼ 2.2. Preparation and acid-modification of PVB NMP 1:15, m/m) to obtain a homogenous solution with the aid of magnetic stirrer. MDI (PVB:MDI¼5:1, m/m) was subsequently 1 Polyvinyl alcohol was used as the starting material for pre- added into the above solution under stirring for 30 min at 75 C. ¼ paration of PVB. 7.005 g PVA was dissolved in 70 ml deionized The deionized water (PVB:H2O 1:1, m/m) as the nonsolvent was fi water at 90 1C with vigorous stir for 2 h. The flask was immersed nally added to precipitate a white micelles, which was continu- in the water bath at 10 1C; 3.995 g n-butyraldehyde was subse- ously stirred until it became a clear and viscous solution. The quently added into the above solution under stirring for 15 min. resulting slurry was coated onto a glass plate using a doctor blade The pH value in the flask was adjusted at 2–3 by adding a few with a gap of 0.02 mm, immediately followed by immersion into a drops of 36–38 wt% HCl solution. The mixture in the flask was coagulation bath to precipitate the polymer membrane. Porous continually stirred for 2 h at room temperature, and then stirred membrane can be obtained using the coagulation bath only with for 3 h at 50 1C. The as-obtained acetal product was washed deionized water, while dense membrane can be obtained in the – several times with deionized water and finally dried under NMP water system by adding a small amount of NMP in coagula- fi ¼ vacuum at 25 1C for 120 h, which was compared with the com- tion bath lled with deionized water (NMP:H2O 1:10, v/v). The mercial reagent of PVB. obtained membranes were rinsed with deionized water, and the fi The proportion of hydroxyl group in PVB was determined via residual solution on the surface was removed with lter paper and fi 1 the catalyzed acylation method using acetic anhydride–petroleum nally dried under vacuum at 25 C for 24 h. ether as main reagent and p-toluenesulfonic acid (p-TSA) as The PVB based polymer membranes were punched into disks of catalyst. At first, acetic anhydride and petroleum ether (1:24,V:V) 16 mm in diameter. Dropping a small amount of liquid electrolyte fi were mixed in the iodine volumetric flask, and p-toluenesulfonic to the surface of polymer membranes in the argon- lled glove box acid (p-TSA:acetic anhydride¼3%, m/m) was then added into the in which the content of water and oxygen are less than 0.5 ppm, above solution, then shaken well and stored in a brown bottle for and then the liquid electrolyte penetrated into pores of mem- 24 h to obtain acetic anhydride–petroleum ether–p-TSA as the branes and swelled the polymer chains to form the GPEs. acetylation reagent. The sample blank solution titration was as follows: 5 mL acetylation reagent was added into the iodine 2.4. Test and characterization volumetric flask, and then an excess of deionized water was added to hydrolyze acetic anhydride. Subsequently 5–6 drops of 1 wt% Polymer functional groups were analyzed by fourier transform phenolphthalein solution was added as an indicator, and the infrared spectroscopy (FT-IR) (NEXUS, FT-IR670, USA) in the range of – 1 fi mixture was titrated with NaOH solution until a faint pink color 4000 400 cm . The surface morphology was observed by a eld was obtained. In the PVB sample solution titration, 0.2 g PVB was emission scanning electron microscope (FESEM) (Carl Zeiss, SUPRA55, dissolved in 2 g NMP and 5 ml acetylation reagent was added Germany). Mechanical strength measurements were carried out on a under stirring for 0.5 h at 45 1C, subsequently followed the afore- tensile testing apparatus (INSTRON, 5567, USA) at a tensile speed of 1 mentioned procedures. The formulas of the reactions were shown 10 mm min ,usingthesizeofsamplesof1cm 3 cm at room in Eqs. (a)–(c). The hydroxyl value corresponding to the sample temperature. The electrochemical stability window of GPE was (g) is defined as the equivalent hydroxyl content of potassium measured by means of cyclic voltammetry (CV) with the three- ∅¼ hydroxide (in mg), which was calculated by the following Eq. (d): electrode cell containing stainless steel (SS, 16 mm) as working electrode, metallic lithium as counter and reference electrode, respec- ð Þ þ CH3⟹PhSO3H þ ð Þ tively. The CV measurement was conducted on an electrochemical CH3CO 2O ROH CH3COOR CH3COOH a work station (CH Instruments, Chi660a, China) with the potential ð Þ þ - ð Þ scanning rate of 5 mV s1 from0V to5V (vs.Li/Liþ ). The ionic CH3CO 2O H2O 2CH3COOH b conductivity of the GPE was determined using impedance measure-

CH3COOHþNaOH-CH3COONaþH2O ðcÞ ment with potential amplitude of 5 mV over a frequency range of 100 kHz–0.01 Hz. For study on the performance of GPEs matching ðV V ÞcM with electrode, Li/LiFePO coin cells were assembled containing PVB IðOHÞ¼ 0 1 þx ðdÞ 4 m based GPEs, LiFePO4 as cathode, and metallic lithium as anode. 44 F. Lian et al. / Journal of Membrane Science 456 (2014) 42–48

Thepositiveelectrodewasfabricatedfrom85wt%LiFePO4 powder as to –OH asymmetric stretching vibration shifts toward low wavenum- the active material, 10 wt% acetylene black as a conductive additive, ber near 3397 cm1,andthepeaksat1136cm1 and 1055 cm1 and 5 wt% polyvinylidene fluoride as a binder. The mixture was corresponding to C–O–C–O–C stretching vibrations of cyclic acetal stirred, dried and then rolled into a film with a thickness of 50 μm. groups (2) and (3) become sharper than that of the synthetic sample The electrode film was dried under vacuum for 24 h at 120 1Cto without acid modification, which reveals an increase of acetalizing remove any solvent on the electrode surface. Coin cells (CR2032) degree in the acid-modified PVB samples. The hydroxyl value of as- without separators were assembled and sealed in an argon-filled glove obtained PVB decreases from 390.3 mg KOH g1 to 124.0 mg KOH g1 box. For comparison, the coin cells were also assembled using 1 M after acid-modification, which is in agreement with FTIR analysis.

LiPF6 in EC/DMC (3:7 in volume) and PP/PE/PP membrane (Celgard Fig. 3 shows SEM of self-made PVB and commercial PVB. The 2325) as electrolyte and separator, respectively. Charge and discharge particle size of self-made PVB is 4–20 μm and larger than that of tests were performed galvanostatically between 2.5 and 4.25 V at commercial PVB. Moreover self-made PVB shows an aggregate room temperature at 0.2 C with a battery test system (Jinnuo Wuhan structure. Corp., LAND CT2001A, China). To characterize their rate capability, the It is found that the mixture of the synthetic PVB without acid cells were discharged at 0.2 C, and then discharged at 0.5 C, 1 C, 2 C modification and MDI becomes a polymer micelles irreversibly and 0.2 C in subsequent cycles after being charged at 0.2 C. and cannot convert into a clear and viscous solution under stirring, so the homogeneous membrane cannot be obtained from the 3. Results and discussion synthetic PVB via the phase inversion method. The excessive crosslinking reaction of PVB containing high concentration of 3.1. Synthetic PVB and PVB based polymer hydroxyl with MDI contributes to an instantaneous formation of the stable micelles. So it is vital to control hydroxyl value and PVB is obtained via the acetalization reaction between aldehydes acetalizing degree of PVB for preparing PVB based polymer and PVA reagent, which is industrially produced through hydrolyzing membrane via the phase inversion method. polyvinyl acetate under acidic conditions. As a result, the synthetic PVB FT-IR spectra of MDI and acid modified PVB based polymer contains several structural units, i.e. acetal products including five- after chemical crosslinking is shown in Fig. 4. The peak at membered cyclic acetal group (2) and hexatomic cyclic acetal group 2282 cm1 ascribed to –NQCQO stretching vibration of MDI (3) as well as residual polyvinyl alcohol (1) and acetate group (4) due cannot be seen [18,19] in FT-IR spectra of PVB based polymer, to incomplete acetalization reactions, whose chemical formula is which confirms the crosslinking effect of MDI in the PVB based shown in Fig. 1. R represents a propyl in the formula. The synthetic polymer. The chemical crosslinking reaction occurs between the PVB and commercial PVB exhibit the similar main peaks in FT-IR hydroxyl of structural unit (1) and isocyanate groups (–NQCQO) spectra as shown in Fig. 2. For self-made PVB, the peaks at 2853– of MDI [18,19], as shown in the given equation 1 2920 cm are ascribed to stretching vibration of saturated –CH, –CH2 1 1 1 and –CH3.Thepeaksat1439cm , 1382 cm and 1348 cm are assigned to C–H bending vibration. The peaks at 1133 cm1 and 1054 cm 1 are assumed to C–O–C–O–C stretching vibrations of cyclic acetal groups (2) and (3), which confirms the presence of acetalization reaction between polyvinyl alcohol and n-butyraldehyde. The peak at 1727 cm 1 relates to CQO stretching vibration of acetate group (4). The peaks at 999 cm1 and 1245 cm1 are ascribed to C–O–C stretching vibration of acetate group (4) [16,17].Thepeakat 3430 cm1 is assigned to –OH asymmetric stretching of polyvinyl alcohol (1), which shows a reduction in magnitude compared with the commercial PVB samples. The self-made PVB was confirmed from catalyzed acylation test to have hydroxyl concentration of 390.3 mg KOH g1, which is a little bit lower than 437.8 mg KOH g1 of the commercial samples. While in the FTIR results of acid-modified PVB, the peak at 1674 cm1 is also ascribed to CQOstretching vibration of acetate group (4), and the shift could be contributed to its overlap with the bending vibration peaks of residual crystal water from the reprecipitation procedure to prepare acid-modified PVB, which cannot be found in FT-IR spectra of the PVB sample after chemically crosslinked (as shown in Fig. 4). Moreover, the peak Fig. 2. FT-IR spectra of commercial PVB, self-made PVB and acid-modified corresponding self-made PVB.

Fig. 1. Structural units in PVB. F. Lian et al. / Journal of Membrane Science 456 (2014) 42–48 45

(e)

Moreover, the presence of band at 3355 cm1 corresponding to In electrolyte uptake measurement, the polymer membrane N–H stretching vibration and three characteristic bands of the amide with 16 mm in diameter was weighted and then immersed in bond including stretching vibration of CQO(amideI)at1665cm1, liquid electrolyte at 25 1C for 30 min. After taking out and slightly bending vibration of N–H (amide II) at 1540 cm1 [18] and stretching removing the excessive electrolyte on the surface using ﬁlter vibration of C–N (amide III) at 1310 cm1 suggest that the amide bond paper, the GPE was weighted again for calculating the electrolyte forms via the polyurethane reaction between the hydroxyl of PVB and uptake as equation (f)[21] MDI. Therefore, it is concluded that the desired chemical crosslinking is realized via the polyurethane reaction between PVB and MDI, which W2 W1 A ð%Þ¼ 100 ðfÞ contributes to the improved stability of PVB based polymer membrane W1 in the liquid electrolyte.

3.2. PVB based membranes and their GPEs

A membrane with different morphology will be obtained by controlling the proportion of the polymer, solvent and nonsolvent and the components of coagulation bath in the process of the phase inversion technique. A high solubility system using NMP as a solvent and deionized water as a nonsolvent is favorable to a porous membrane [20]. While dense membrane can be obtained in NMP–water system by adding a small amount of NMP in coagulation bath ﬁlled with deionized water. The PVB based porous and dense membranes are formed, whose morphologies are shown in Fig. 5. A few pores can be observed with homogeneous distribution on the surface of PVB based dense membranes. By contrast, porous membranes show interconnected uniform pores with diameter of about 200–800 nm. Fig. 4. FT-IR spectra of PVB based polymer after chemical crosslinking.

Fig. 3. SEM images of self-made PVB (a) and commercial PVB (b). 46 F. Lian et al. / Journal of Membrane Science 456 (2014) 42–48

Fig. 5. SEM images of PVB based polymer membranes (a) (b) porous and (c) (d) dense.

Fig. 7. Nyquist plot of PVB based gel polymer electrolyte and liquid electrolyte. Fig. 6. The cyclic voltammetry curves of PVB based GPE.

where, W1 and W2 are the mass of the membrane and GPE, to aldehydes through electrolytic oxidation [12]. Additionally, the respectively. Soaked in the electrolyte for 30 min, the porous PVB acetalization reaction between parts of the hydroxyls without based membrane could absorb 575% of liquid electrolyte vs. the electrolytic oxidation only appears in the first scan and improves weight of the dried membrane. The dense membrane exhibits low the adhesion of PVB based gel polymer electrolyte on the elec- porosity, but its liquid uptake is as high as 288%, which is trode. In particular, no significant anodic current can be observed attributable to the penetration of the liquid electrolyte into up to 5 V in the first three cycles. With the potential scanning rate three-dimensional networks of membrane. Moreover, the porous of 5 mV s1, the corresponding potential range as the current membrane shows the low ultimate tensile strength of 4.52 MPa, between 0.1 and þ0.1 mA is considered to be an electrochemical while the ultimate tensile strength of the PVB based dense stability window. PVB based GPEs exhibit good electrochemical polymer membrane reaches 16.33 MPa, which is close to that of stability in the potential range from 1.6 V to 5.0 V, which is commercial PP/PE/PP membranes [22]. The cell can be assembled accessible to the application in the lithium-ion batteries. without separator to keep safety using GPE from PVB based dense The electrochemical impedance spectroscopy of PVB based GPE is polymer membrane in view of its chemical stability and mechan- shown in Fig. 7. The ionic conductivity was calculated from the bulk ical strength. Therefore, the dense membrane and its GPE are electrolyte resistance using the following equation (Eq. (g)) [23]: focused in the paper. L The cyclic voltammetry curves of the PVB based GPE is shown s ¼ ðgÞ RS in Fig. 6, the responses of current to potential scan show a hysteresis loop onset at 2.5 V and a peak at about 1.5 V during where L is the thickness of the GPE, S is the contact area between the reverse cathodic scan of the first cycle, whose magnitude GPE and SS disc, the bulk electrolyte resistance was obtained from sharply decrease in the subsequent scans. The obvious current the Nyquist plot at room temperature. The obtained ionic conductiv- peak at about 1.5 V in the first cycle might be attributed to the ity of PVB based GPE from dense membrane is 4.09 10 4 Scm1, transformation of part hydroxyls in the structural unit (1) of PVB while the ionic conductivity of liquid electrolyte with the PP/PE/PP F. Lian et al. / Journal of Membrane Science 456 (2014) 42–48 47 membrane (Celgard 2325) is 4.15 10 4 Scm1.ThePVBbasedGPE first 15 cycles. The charge and discharge efficiency improves to shows the comparable ionic conductivity with the commercial liquid higher than 99% in the subsequent cycles. The phenomenon has electrolyte system. been also observed in other GPEs. The cycling performance of the

Fig. 8 shows charge/discharge voltage proﬁles of the Li/LiFePO4 Li/LiFePO4 cell is shown in Fig. 9. Fig. 10 presents the rate cell using the GPE at room temperature. The cells deliver a performance of the Li/LiFePO4 cell. The cell using PVB based GPE discharge capacity of 134 mAhg1 and Coulombic efﬁciency of shows the comparable cycling stability and rate capability to the 77% in the initial cycle. The polarization gradually decreases with cell with liquid electrolyte. an increase of the cycle number and the charge/discharge potential reaches stability in the 15th cycle, which is close related with the gradual reduction of internal resistance of the cell during the 4. Conclusions

Polyvinyl butyral (PVB) was prepared from low-cost polyvinyl alcohol (PVA) and n-butyraldehyde via acetalization reaction. The concentration of hydroxyl group in PVB can be obviously reduced from 390.34 mg KOH g1 to 123.95 mg KOH g1 via the acid modiﬁcation method, which is also favorable to the increase of acetalizing degree in PVB. The chemically and structurally stable membranes formed through the polyurethane reaction between PVB and MDI via the phase inversion method. The dense PVB based membrane shows high mechanical strength of 16.33 MPa and high liquid uptake of 288%. Its GPE has good chemical stability, good ionic conductivity of 4.09 10 4 Scm1 at room temperature and wide electrochemical window of 1.6–5.0 V (vs. Li/Li þ). In addition, the PVB based gel polymer electrolyte matched

with LiFePO4 shows comparable electrochemical performance, which – Fig. 8. Charge discharge curves of Li/PVB based GPE/LiFePO4 at 0.2 C between suggests PVB based GPE to be a promising candidate for the polymer 2.5 V and 4.25 V. electrolyte of lithium-ion batteries.

Acknowledgments

This work was ﬁnancially supported by Johnson Controls Battery Group, INC. and the Fundamental Research Funds for the Central Universities in China (No. FRF-MP-12-005B).

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