Facile Synthesis of Unique Cellulose Triacetate Based Flexible and

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Facile Synthesis of Unique Cellulose Triacetate Based Flexible and High Performance Gel Polymer Electrolyte for Lithium Ion Batteries Trupti C. Nirmale,† Indrapal Karbhal,‡ Ramchandra S. Kalubarme,†,§ Manjusha V. Shelke,‡ Anjani J. Varma,‡,∥ and Bharat B. Kale*,† †

Centre for Materials for Electronics Technology (C-MET), Ministry of Electronics and Information Technology (MeitY), Panchavati, Pune 411008,India ‡ CSIR-National Chemical Laboratory, Homi Bhabha Road, Pune 411008, India § Department of Physics, Savitribai Phule Pune University, Ganeshkhind, Pune 411007, India ∥ School of Chemical Sciences, Central University of Haryana, Mahendragarh, Haryana 123031, India S Supporting Information *

ABSTRACT: Lithium ion batteries (LIBs) with polymer based electrolytes have attracted enormous attention due to the possibility of fabricating intrinsically safer and flexible devices. However, economical and eco-friendly sustainable technology is an oncoming challenge to fulfill the ever increasing demand. To circumvent this issue, we have developed a gel polymer electrolyte (GPE) based on renewable polymers like cellulose triacetate and poly(polyethylene glycol methacrylate) p(PEGMA) using a photo polymerization technique. Cellulose triacetate offers good mechanical strength with improved ionic conductivity, owing to its ether and carbonyl functional groups. It is observed that the presence of an open network has a critical impact on lithium ion transport. At room temperature, GPE PC exhibits an optimal ionic conductivity of 1.8 × 10−3 S cm−1 and transference number of 0.7. Interestingly, it affords an excellent electrochemical stability window up to 5.0 V vs Li/Li+. GPE PC shows a discharge capacity of 164 mAhg−1 after the first cycle when evaluated in a Li/GPE/LiFePO4 cell at 0.5 C-rate. Interfacial compatibility of GPE PC with lithium metal improves the overall cycling performance. This system provides a guiding principle toward a future renewable and flexible electrolyte design for flexible LIBs (FLIBs). KEYWORDS: cellulose triacetate, polymer electrolyte, Li-ion battery, flexible, stability

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INTRODUCTION Recently, LIBs have dominated the consumer electronics market especially in portable electronics such as laptops, cell phones, etc. LIBs are also the most promising candidates for the rising electric-vehicle market.1 Considering the large volume of production and wide application, it is increasingly imperative to design these batteries keeping in mind sustainability issues. In this context, the crucial areas for further development of these batteries are low production costs and life cycle enabled by the use of environmentally friendly renewable polymers like cellulose and their derivatives, lignin, etc. The conventional LIBs use liquid electrolytes such as carbonate based organic solvents due to their high ionic conductivity. However, reactions of volatile, flammable organic solvents, reactivity toward lithium electrode, and the leakage of electrolytes creates safety related problems with liquid electrolytes. Polymer electrolytes are the most promising replacement of liquid electrolytes due to their improved safety hazards, increased tenacity to the variable electrodes volume during charge/discharge process, ample options in design, and excellent flexibility.2−5 Solid state polymer electrolytes have © 2017 American Chemical Society

achieved great advancement in recent years. However, polymer electrolytes face many challenges such as lower conductivity, electrochemical instability, and lithium metal interfacial incompatibility.6,7 Hence, some amount of plasticizer or solvent is incorporated into the solid polymer electrolytes which leads to the gel polymer electrolytes (GPE). Their distinctive hybrid network provides GPEs properties like cohesive nature and diffusive transportation.8,9 Although GPEs are good candidates for making LIBs with improved safety, they usually suffer from poor mechanical strength and are environmentally non benign, complicated, and tedious processes. Therefore, it is highly encouraging to develop a facile and reliable strategy by using renewable polymers which are sustainable, biodegradable, and much cheaper than the synthetic polymers they replace.10,11 Cellulose triacetate (CTA) is a cellulose derivative with ether and ester functional groups possessing interesting properties such as high melting point, mechanical strength, exceptional Received: May 18, 2017 Accepted: September 19, 2017 Published: September 19, 2017 34773

DOI: 10.1021/acsami.7b07020 ACS Appl. Mater. Interfaces 2017, 9, 34773−34782

Research Article

ACS Applied Materials & Interfaces Table 1. Electrochemical Performance of Recent GPEs Based on Different Cellulose Derivatives sr. no.

GPE composition (polymer/electrolyte)a

1 2 3 4 5 6 7

CMC/LiPF6EC-DMC-DEC cellulose/TPU/LiTFSI/TEGDME methyl cellulose CA/PLLA/Halloysitenanotube/LiPF6-EC-DMC methacrylate/MFC/LiPF6EC-DEC HEC/LiPF6EC-DMC-EMC CTA/P(PEGMA)/LiPF6-EC-DMC

conductivity (S cm−1) 4.8 2.3 2.0 1.52 6 1.8 1.8

× × × × × × ×

10−4 10−4 10−4 10−3 10−4 10−4 10−3

Tg (°C)

at 25 °C

−65.8

at room temp. at 25 °C at 30 °C

−53.4 −46.0

transference no. (t+) 0.46 0.68 0.29 0.45 0.48 0.7

anode/cathodeb

cell capacity (mAhg−1)

ref.

Li/LiFePO4 Li/LiFePO4 Li/LiFePO4 Li/LiCoO2 Li/LiFePO4 Li/LiFePO4 Li/LiFePO4

128 (0.5 C) 128.2 (2C) 130 (0.2 C) 125.2 (0.1 C) 120 (1 C) 110 (0.2 C) 164(0.5 C)

22 23 24 25 26 27 this work

a

EC, ethylene carbonate; EMC, ethyl methyl carbonate; DMC, dimethyl carbonate; DEC, diethyl carbonate; LiTFSI, Bis(trifluoromethanesulfonyl)imide lithium salt; LiPF6, lithium hexafluorophosphate; CA, cellulose acetate; PLLA, poly-L-lactic acid; CMC, carboxymethyl cellulose; TPU, thermoplastic polyurethane; TEGDME, tetraethylene glycol dimethyl ether; MFC, microfibrillated cellulose; and HEC, hydroxyethyl cellulose. b LiFePO4, lithium iron phosphate; and LiCoO2, lithium cobalt oxide.

film-forming properties, and biodegradability which makes it a potential material in the areas of optical devices, separation membranes, and in thermoplastics.12−15 However, CTA is hardly considered as a GPE for LIBs. It is well-known that in the case of polymers, lithium ions are coordinated and transported mainly through the amorphous region. Since CTA is highly crystalline it cannot be used alone to prepare polymer electrolytes.5 To circumvent this issue, polyethylene glycol methacrylate (PEGMA) was polymerized and blended with CTA. PEGMA has an ethylene oxide chain and shows similar properties to poly(ethylene glycol) (PEG) such as nontoxicity and biocompatibility.16 Moreover, it has an acrylate end group which can be polymerized and ether groups able to solvate and entrap carbonate solvents in the polymer matrix.1−4 Such entrapping not only suppresses the evaporation and leakage of solvent but also creates room for ionic motion.17,18 Combination of CTA and p(PEGMA) allows greater availability of ether and ester functional groups where oxygen atoms form temporary ionic interaction with lithium ions (Li+) and assist them in transport by a hopping mechanism which ultimately improves ionic conductivity.5,19 In the present work, we have used a cost-effective and simple technique like UV irradiation for film fabrication. Photoinduced free radical polymerization has many advantages over thermal curing such as rapid processing time and development of welldefined networks,17 although it is rarely exposed to the field of batteries. Recently, a few groups have used UV polymerization to prepare an electrolyte.20,21 However, their work depends only on cross-linking with more synthetic polymers. In our system, a self-standing polymer film based on biocompatible polymers is formed at ambient room condition and a glovebox is needed only at the end of the process, which makes this process industrially feasible. As compared to earlier reports (Table 1), an Li/GPE/LiFePO4 cell shows better performance in the form of high storage capacity, high transference number, and good cycle life. We believe that the use of cellulose derivatives (i.e., CTA) and a simple fabrication process will make this system eco-friendly and economical. More significantly it has good potential in FLIBs.

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distilled it before use. Lithium hexafluorophosphate solution, i.e., 1.0 M LiPF6 in ethyl carbonate (EC) and dimethyl carbonate (DMC) (1:1 v/v) was received from JK impex and kept in a glovebox for further use. The lithium hexafluorophosphate (LiFePO4), polyvinyldifluoride (PVDF), and N-methyl pyrrolidone (NMP) were purchased from Aldrich. Carbon black and graphite were purchased from Alfa Aesar. Celgard of a 25 μm trilayer polypropylene-polyethylene-polypropylene membrane was supplied by MTI corporation. Structures of oligomers PEGMA, PEGDMA, and cellulose triacetate(CTA) are depicted in Figure 1.

Figure 1. Chemical structure of (a) poly(ethylene glycol)methyl ether methacrylate (PEGMA), (b) poly(ethylene glycol)dimethacrylate (PEGDMA), and (c) cellulose triacetate (CTA). Synthesis of GPE Films. The monomers were mixed together with chloroform in a round-bottom flask and stirred well on a magnetic stirrer. Further, 2 wt % of photoinitiator was added to the mixture. In a separate vial, 1.6 g of CTA was dissolved in the chloroform with continuous stirring. Then the above two solutions were put together in molar ratio as shown in Table 2. The flask was sealed and kept overnight for continuous stirring, allowing the monomers, cellulose triacetate, and photoinitiator to mix and dissolve completely. The resultant homogeneous and clear mixture was then exposed to UV irradiation for 30 min with continuous stirring at 15 cm distance. The light source used for reaction was a medium pressure mercury vapor lamp of 450 W (supplied by ACE Glass Inc., U.S.A.). Further, the reaction mixture was poured into a Petri dish to allow solvent evaporation and kept under vacuum drying at 80 °C overnight. Finally, mechanically stable, free-standing films were peeled from the Petri dish. The obtained films were swelled in 1.0 M LiPF6 in EC/ DMC (1:1 v/v) solution for 2 h in a glovebox and then used for further characterization. Characterization. Morphological characterization of the GPE films and electrodes was performed by Quanta 200 3D FEI scanning

EXPERIMENTAL SECTION

Materials. Poly(ethylene glycol)methyl ether methacrylate (PEGMA, average Mn 500), poly(ethylene glycol)dimethacrylate (PEGDMA, average Mn 550), and molecular sieves (4 Å), 2-hydroxy2-methylpropiophenone were obtained from Aldrich. PEGMA and PEGDMA were treated with molecular sieves before use. Cellulose triacetate (CTA) (degree of substitution 2.8) was obtained from Eastman. Chloroform was purchased from Fischer scientific and 34774


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ACS Applied Materials & Interfaces Table 2. Composition of PC and PDC Mixture with Their Gel Content and Swelling Percentage sr. no.

name

composition

molar ratio

active swelling percentage (%)

gel content after 48 h in hexane (%)

1 2

PC PDC

PEGMA/CTA PEGMA/PEGDMA/CTA

1.2:1 1.2:0.06:1

47 55

91.2 93.3

Figure 2. Preparation of cellulose derived electrolyte films and chemical characterization. (a) Reaction scheme of GPE PC synthesis. (b) FTIR spectra of Polymer mixture before and after UV irradiation. (A) PEGMA monomer, (B) PC mixture before UV exposure, and (C) PC mixture after UV exposure. (c) Lithium ions transfer through the coordination sites of ether and carbonyl functional groups of blend polymers. (d) Photo of a LiFePO4/GPE PC/graphite flexible Li ion battery. Photographs of flexible and transparent polymer films. GPE films are highly bendable and can be twisted around the narrow rod (diameter 3 mm). electron microscopy (SEM). To observe morphological changes by SEM after battery cycling, a half cell with lithium and LiFePO4 was opened, and the polymer film and lithium electrode were analyzed. TA DSC Model Q100 was used for differential scanning calorimetry (DSC) from −80 to 120 °C at 10 °C min−1 scan rate under nitrogen atmosphere. Thermal stability was tested by thermogravimetric analysis (TGA) with a TG-DTA Setsys instrument by Setaram from 25−700 °C under N2 flux at 10 °C min−1 heating rate. PerkinElmer Fourier transform infrared (FT-IR) spectrophotometer was used to record the FT-IR spectra of the samples in the range of 500−4000 cm−1 in transmittance mode. The gel content (insoluble fraction) of the samples was determined by soaking them in hexane for 48 h and then dried, weighed, and calculated according to eq 1: gel content (%) =

Wf × 100 Wi

with impedance data of Li/GPE/Li cell with time. Cyclic voltammetry (CV) curves were recorded from −1.0 to 6.0 V for the Li/GPE/SS cell and from 2.0 to 4.5 V for the LiFePO4/GPE/Li cell at a scanning rate of 0.1 mV s−1 using an Autolab electrochemical workstation. The galvanostatic charge (constant current) and discharge (constant current) cycling tests were performed at room temperature on a Li/ GPE/LiFePO4 half-cell in the potential window of 2.0 to 4.5 V at a 0.5 C rate, using a battery cycler (MTI-Corp., U.S.A.). The rate performance was also checked at different C rates. The cathode used for testing, comprised 80 wt % LiFePO4, 5 wt % PVDF, and 15 wt % super-P carbon black. All these materials were mixed with NMP solvent to prepare a slurry which was then coated on aluminum foil and vacuum-dried for 24 h. Then afterward, disks of 16 mm diameter were cut from the coated foil and again vacuum-dried overnight. For better particulate contact and adhesion between the cathode material and the foil the disks were roll pressed. To prepare a half-cell, GPE was sandwiched between the above prepared LiFePO4 cathode and lithium (disk of diameter 16 mm) as anode material followed by vacuum sealing a coin cell. Cell fabrication was carried out in a glovebox filled with argon gas where the oxygen and moisture level is lower than 0.1 ppm. Electrochemical measurements, except for the temperature dependent conductivity, were carried out at room temperature.

(1)

where, Wf and Wi are the weights of the film after soaking and before soaking. An X-ray powder diffraction (XRD) technique (XRD-D8, Advance, Bruker-AXS) with Cu Ka radiation, over the range of 2θ: 5°−80° at ambient temperature was performed. Mechanical measurements on polymer films were performed using a universal testing machine STS 248, India on dumbbell shaped samples for tensile stress strain testing. Electrochemical Characterization. Electrochemical impedance spectroscopy (EIS) was used to measure ionic conductivity of GPEs over a temperature range from 30 °C to 100 °C. GPEs were sandwiched between two electrodes made up of stainless-steel (SS) for measurement of impedance, which were conducted within a frequency range from 0.1 Hz to 3 MHz at an AC potential amplitude of 10 mV. The stability of GPE in contact with lithium metal electrode was tested

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RESULTS AND DISCUSSION The synthesis of GPE has been carried out as per the procedure mentioned in the Experimental Section and graphical illustration shown in Figure 2. In-situ polymerization of PEGmethacrylates along with CTA has been performed under UV light. A homogeneous blend is formed by two polymers to give 34775


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ACS Applied Materials & Interfaces Table 3. Tensile Stress Strain Testing of Polymer Films sr. no.

samples

yield strength (MPa)

tensile strength (MPa)

strain at break (%)

modulus of elasticity (MPa)

1 2

PC PDC

8.23 13.06

13.97 20.10

34.0 12.92

643 1416

Figure 3. (a) TGA data and (b) DSC thermograms of polymer films before swelling (PC and PDC) and after swelling (GPE PC and GPE PDC) in LiPF6 solution.

transparent films, which were then swelled in 1.0 M LiPF6 in EC/DMC (1:1 v/v) solution. Two types of GPEs were synthesized, named GPE PC and GPE PDC (Table 2). GPE PC is prepared by using PEGMA and CTA and GPE PDC is prepared using PEGMA, CTA, and PEGDMA (cross-linker) to observe the effect of cross-linking. The films obtained were transparent, smooth, flexible, and easy to handle free-standing films (Figure 2). Both membranes exhibit the highest gel content (Table 2). This shows that such a large amount of cellulose triacetate does not interfere with the PEGMA/ PEGDMA polymerization. The FTIR spectra of pure PEGMA and PEGMA-CTA composite based electrolytes before and after UV irradiation are shown in Figure 2. Previous studies described that UV curing is well suited for PEG-methacrylates to make electrolyte films.4,17,20−23,26−28 Disappearance of peaks at 1637 and 815 cm−1 corresponding to vinyl bonds in methacrylate groups of monomers PEGMA and PEGDMA confirms the success of UV curing. The films were then swelled after soaking in 1.0 M LiPF6 in EC/DMC (1:1 v/v) for 2 h to form a GPE. The electrolyte uptake, i.e., the active swelling percentage (AS%), was calculated by eq 2: active swelling percentage (AS%) =

Wf − Wi × 100 Wf

ions will move along the polymer chain through the coordination site of CO which exerts an ionic conductivity.14,29 Previously, it has been reported that carboxyl groups form an ionic bond with Li+ cations while carbonyl and ether groups coordinate.30,31 Further, carbonate solvent molecules present in the polymer matrix dissociate Li+ ions from the LiPF6 salt and provide an alternate pathway for Li+ mobility.32 By virtue of it is low lattice energy, LiPF6 can be easily dissociated into Li+ and PF6− upon contact with carbonates,33 ester functionality of CTA, and ether groups of PEGmethacrylates of polymer electrolyte. Such a unique polymer network assures fast and reliable Li+ ion transfer which ultimately improves the ionic conductivity.34 The phase nature of the blended film was investigated by XRD analysis and was performed before and after swelling (Figure S1 of the Supporting Information, SI). The XRD pattern highlights the amorphous nature of the polymer electrolyte. It can be noted from the XRD patterns of the samples that the sharp crystalline peaks corresponding to LiPF6 are absent in the polymeric blends when mixed with lithium salt solution. It confirms that there is no separate phase of the lithium salt that exists in the polymer blends. This shows a complete complexation between the polymer blend and the lithium salt. The pure cellulose acetate exhibited typical crystalline peaks at 2θ = 8.5° and 13.5°, corresponding to the (110) and (200) reflections.5 Sample PC and PDC show similar peaks pointing out the presence of cellulose acetate. However, it is observed that upon swelling in 1.0 M LiPF6 in EC/DMC solution, the specific peaks of cellulose acetate disappear and a more broadened peak with lowered intensity is observed in the blend system. It confirms that the addition of salt disrupts the ordering of polymer chains, due to the Li+ ion interaction with carbonyl as well as ether functional groups of CTA and p(PEGMA), respectively. This suggests an increase of amorphicity of the polymeric blend films and the occurrence of complexation between CTA, PC, or PDC with lithium salts, which is crucial for GPE. However, good mechanical properties are essential for GPEs. Here, mechanical properties were analyzed by tensile stress

(2)

where Wf and Wi are the weights of the membrane after and before swelling. Even after swelling for 2 h, the membranes exhibited better mechanical and dimensional stability. The greater percentage of swelling in PDC (Table 2) confirms that the greater the number of ethoxy groups, the greater the binding with carbonate molecules will become. Soft and flexible polymers can make good contact with electrodes easily. However, the polymer matrix is required to solvate the Li+ ions. One of the most studied polymers, PEO, is well-known for its capability to form complexes with lithium ions. Hence, PEG-methacrylates together with cellulose triacetate form a polymer network that provides coordination sites and vacant space for Li+ ions transportation. With the given ratio both polymers make a completely homogeneous phase. CTA contains polar functional groups containing CO, where Li+ 34776


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ACS Applied Materials & Interfaces

Figure 4. (a) Temperature dependent ionic conductivities of the GPE PC and GPE PDC obtained from impedance response. (b) Impedance plots of Li/GPEPC/Li and Li/GPEPDC/Li at room temperature.

molecules. According to the free volume theory, the PC has more free volumes than PDC making polymer chains tangled and folded more easily, confirming its elastic nature (see Figure S2). Also, the Tm of PDC is depressed after swelling, while disappearing in PC, highlighting its amorphous nature. As a result, the lower crystallinity and glass-transition temperature favorably enhance the ionic conductivity and improve the rate capability. Temperature dependent ionic conductivities of GPEs were measured from 30 °C to 100 °C range by AC impedance spectroscopy. The ionic conductivity of electrolytes was calculated by the following equation:

strain testing. Figure S2 shows the tensile stress strain curves for both PC and PDC polymer films. The average modulus of elasticity was observed to be 643 MPa for PC and 1416 MPa for PDC, while the tensile strength was 13.97 MPa for PC and 20.10 MPa for PDC (Table 3). This is as expected, since the presence of cross-linking in the polymer matrix increases the strength and modulus and at the same time decreases the elongation to break.35,36 Both GPEs exhibited a rubbery mechanical state which is necessary for structural battery applications.11,17 As shown in Figure S2, PC shows a typical flexible polymer curve that reveals its amorphous and elastic nature. One of the premium features of the film is its mechanical bendability. The GPE PC (after swelling in liquid electrolyte) is very bendable and twistable and remains intact even after being rolled around a 3 mm diameter rod (Figure 2). Such exceptional mechanical behavior of the cellulose derived polymer helps to improve the safety of the GPE. Moreover, GPE PC has shown enhanced mechanical strength, with which we have assembled a FLIB LiFePO4/GPE PC/graphite (Figure S3), showing its robustness by turning on a blue LED even after several bending times. The first decomposition step as shown in Figure 3a (below 200 °C) was linked to the presence of organic solvents that evaporated at a relatively low temperature. The T5 temperature (the temperature at which 5% of material is lost from initial weight) was reached at around 90 °C because of highly volatile carbonate molecules. The residues remaining at high temperature before swelling were found to be 5.5% and 6.5%, after which the swelling increased to 12.6% and 15.4%, confirming the presence of the lithium salt.26 It confirms that lithium is absorbed by the functional groups of polymers after 2 h soaking time. The stability is high and as evidenced by this test, GPEs can be safely used in LIBs. Increased Li+ ion conductivity is usually observed in polymers with low glass transition temperatures (Tg). Both GPEs show very low Tg values and are still extremely flexible, self-standing, and easy to handle. Elevation in Tg after swelling for GPE PC (−46 °C) and GPE PDC (−52 °C) (Figure 3b), as compared to films before swelling implies the formation of a network formed by interaction between dissociated Li+ and oxygen atoms of carbonyl and ether functional groups of polymer chains. Reduction and broadening of Tg after swelling shows that polymer chain relaxation is afforded by effective plasticization of solvents which is very important for conductivity. It is noted that, although PDC contains crosslinking, its Tg is low, which indicates that a greater number of ethoxy groups of PEGDMA binds more carbonate solvent

σ=

l RA

(3)

where σ is the ionic conductivity, R is the intercept at the real axis in Nyquist plot, A is the area of the electrode−electrolyte interface, and l is the thickness of the polymer film. Figure 4a shows ionic conductivities of GPE PC and GPE PDC at various temperature ranges in the Arrhenius expression. The conductivity of both membranes increases with temperature especially at high temperature, showing a typical Arrhenius behavior by GPEs. At higher temperature, the conductivity increases because of increase in mobile carriers and polymer chains segmental motion. Vibrational modes of the polymer segment increase with temperature causing the formation of voids and free volume, which promotes charge conduction and confers enhanced conductivity.37,38 No turning points are observed confirming the temperatureinsensitive behavior of the cellulose hosted electrolytes which are able to maintain battery capacities in a wide temperature range.39 Introduction of CTA creates new anchoring points to Li+ ions and also segregates the ether chains to create more free volume for ions mobility as shown in Figure 2. It was observed that blending two different polymers provides interstitial space increasing the polymer-chain motion, thereby facilitating transport of ions.40−42 Observed conductivity at room temperature is higher for GPE PC (1.8× 10−3 S cm−1) than GPE PDC (6.37× 10−4 S cm−1). Low conductivity of GPE PDC is attributed to cross-linking present in polymer film, which reduces the free volume availability and limits mobility of Li+ ions and polymer chains, and therefore had a controlling effect on ionic conductivity.43,44 Also, compared to GPE PC, GPE PDC shows higher activation energy in the Arrhenius equation, this may be due to the combination of both transient physical (Li+ ion-dipole complexation) and chemical crosslinking, synergistically suppressing the segmental relaxation of 34777


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and liquid electrolyte (0.2−0.4).40 Such a high t+ value minimizes polarization and the concentration gradient, eventually improving Li+ ion diffusion and cell cyclability.48,49 Mono functionalized ethoxy chains of GPE PC creates a more flexible and open network. Li+ ions which are weakly bound to chains can be easily dissociated by an applied electric field. These cations accelerate the conduction when they hop through the coordinating site of carbonyl and ether oxygen of host polymer segmental motion.38 The formation of PF6−··· (δ+)CCO(δ−) complexes in the polymer matrices restrict the PF6− anion movement. Electrochemical stability above 4 V is necessary for practical applications to LIBs. Therefore, the interfacial compatibility of GPE PC with lithium electrode was further elaborated. Figure S6 illustrates a continuous expansion of the middle frequency semicircle in a time dependent impedance analysis. It is generally attributed to a continuously growing resistive layer on the lithium metal surface (Figure 6). It is seen that the resistance values remain more or less the same even after 30 days. However, one may notice that the interfacial resistance reached a stable plateau after a sharp initial increase (Figure 7a). This shows that the lithium metal surface is passivated when in contact with the GPE. Generally, this behavior shows the deposition of an SEI layer on the electrode with time.50,53 It can be clearly seen in SEM images of the lithium electrode (Figure 6) with some depositions that are caused due to reaction between electrolyte components (i.e., LiPF6) and the lithium metal electrode. Decomposition products such as LiF, Li2CO3, etc. forms an SEI layer on the electrode surface (see the EDS in Figure 6c). This LiF rich SEI shed the solvent molecules and helps to deposit Li+ ions uniformly.54 This indicates that the lithium anode was passivated to form a stable SEI favorable for a long-term cycling. CV measurements were conducted with SS as the working electrode and lithium as reference and counter electrodes. It is observed that GPE PC is stable up to 5.0 V which is far superior to that of commercial liquid organic electrolyte (∼4.2 V). The higher anodic stability makes GPE more compatible with high-voltage cathode materials such as LiCoO2 and LiMn2O4.39 A single lithium stripping peak at 0.43 V and single cathodic deposition peak at −0.58 V reveal a reversible Li plating/stripping process on the SS electrode (Figure 7b). The ideal peak definition suggests that the kinetics of the lithium plating/stripping process is fast and its efficiency is high.51 Therefore, the GPE PC makes good contact with the lithium electrode and can be used effectively for high capacity and high potential cathodes. Further, GPE PC was evaluated where the LiFePO4 material is carefully chosen as a cathode to fabricate half-cell with Limetal due to its favorable properties such as high theoretical capacity (170 mAh g−1), operating voltage of 3.4 V vs Li/Li+, environmental friendliness, high safety, low cost, and excellent cycling properties.25,51−53 Figure 8a shows the galvanostatic charge−discharge profile of Li/GPE PC/LiFePO4 cell recorded at 0.5 C rate within 2.0−4.5 V (Li/Li+) potential window. During the charge/discharge process, a voltage plateau near 3.45 and 3.6 V, shows a typical biphasic Li+ insertion/extraction mechanism in the LiFePO4 cathode. Slanted charge−discharge profiles are caused due to ion diffusion within the electrolyte, the electrodes, and across their interface. Tilting in profile can happen principally due to the low ionic conductivity and higher thickness (typically 260 μm) of GPE.40 Moreover, we have investigated the electrochemical performance of the LiFePO4 cathode material using a

polymer chains in GPE PDC. Such ion dipole interaction restricts the segmental motion of the polymer chain, causing the average chain length to shorten and become less mobile between the cross-linked points.33 The polymer pristine structure with more open network makes easier and faster diffusion of Li+ ions. Interfacial resistance of GPEs with Li electrode in Li/GPE/Li cell was monitored for 3 weeks. Figure 4b shows the initial resistance for GPE PC is 620 Ω which is considerably lower than that of GPE PDC, i.e., 2400 Ω. With elapsed time GPE PC shows slow growth (Z = 1160 Ω) than GPE PDC (Z = 5100Ω). It reveals a more stable electrochemical interface of GPE PC with the lithium metal electrode.37,45 To test the stability of the films, cells were open after cycling, and the surface of the films (facing lithium metal) were studied by SEM (Figure S4). It was observed that the surface of GPE PC is uniform while some deposition observed on GPE PDC surface. Furthermore, the GPE PDC surface was rough compared to GPE PC, and few cracks were also observed which results in higher resistance. At the same time cyclic voltammetry was performed (Figure S5), where GPE is sandwiched between LiFePO4 cathode and Li metal anode. The cyclic voltammogram curve (CV) of GPE PC shows oxidation (3.35 V) and reduction (3.8 V) peaks with a potential difference of 0.45 V. However, GPE PDC shows oxidation (3.15 V) and reduction (3.95 V) peaks with a potential interval of 0.8 V, which is larger than that of GPE PC. This difference clearly explains the large impedance observed with GPE PDC.24 GPE PC was subjected to further characterizations due to its optimal ionic conductivity and stability with electrodes. Transference number (t+) of GPEs were determined using DC polarization and impedance analysis.18,46,47 For measurement, 5 mV DC voltage was applied to Li/GPE/Li cell and then measured initial (Io) and steady state currents (Iss). At the same time initial (Rint,o) and final (Rint,ss) resistances were

Figure 5. Room temperature current−time curves of the Li/GPE PC/ Li cell. Corresponding AC impedance plots of the cell before polarization and after the steady-state current.

measured by AC impedance spectroscopy (Figure 5). Finally, the t+ value was calculated by the following equation,46 t Li + =

Iss(ΔV − IoR int,o) Io(ΔV − IssR int,ss)

(4)

+

The average t obtained is observed to be 0.7 for GPE PC, which is higher than common polymer electrolytes (t+ ≤ 0.5) 34778


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Figure 6. SEM images of (a) lithium foil, (b) salt clusters on lithium foil (on Li/GPE interface), and (c) EDS spectrum of a lithium foil showing C, O, F, and P elements.

Figure 7. (a)The interfacial resistance within the Li/GPEPC/Li cell with time and (b) cyclic voltammetry curve of the Li/GPE PC/SS cell (scan rate of 0.1 mV s−1).

GPE could afford an excellent cycle performance even at 25 °C.50 The GPE based Li-ion half-cell shows capacity of 136 mAh g−1 after 100 cycles at 25 °C together with capacity retention of 83%. However, the capacity fading of the cell is heavily retarded after 50 cycles, signifying a better reversibility and stabilization of the system as a whole after a few cycles. The performance and the capacity fade of GPE PC is comparable with the liquid electrolyte, due to improved transport of ions and the compatibility between electrolyte and electrode.55 Further, to demonstrate the unique performance of the cell Li/GPE PC/LiFePO4, its rate performance was evaluated at diverse current densities as shown in Figure 8b. The observed capacity of 164 mAhg−1at 0.5 C drops to 90

standard Celgard separator soaked in LiPF6 (EC/DMC) (Figure S7). It is fascinating that the polarization observed for the standard Celgard separator is nearly equal to that of GPE, suggesting the superiority of GPE. The higher reversible capacity for GPE is because of the higher Li+ ion conductivity and higher t+ in the GPE PC than Celgard membrane. The cell with the LiFePO4 cathode shows the first discharge capacity of 164 mAhg−1, which is 96% to that of the theoretical capacity (170 mAhg−1). The cell retains 83% of initial discharge capacities after 100 cycles (Figure 8c), which corresponds to 80% of the theoretical capacity. Despite the capacity fading due to the reactions between Li metal and electrolyte, the obtained Coulombic efficiency was 96% after 100 cycles, manifesting that 34779


Research Article


Figure 8. (a) Galvanostatic charge−discharge profiles, (b) rate performance at various C rates, and (c) discharge capacities and Coulombic efficiency at 0.5 C-rate at a voltage range between 2.0 and 4.5 V of the Li/GPE PC/LiFePO4 half-cell.

mAhg−1at 1.5 C. The capacity value is noticed to decrease further at higher rates at 3, 4.5, and 6.0 C. It exhibits 28 mAhg−1at 6.0 C, showing sustainability and robustness of GPE at a high current density which can be applicable for high power LIBs. It confirms the better cycling and rate performance of GPE PC. To demonstrate the feasibility of the flexible battery, we have fabricated a pouch cell with LiFePO4 and graphite as cathode and anode, respectively, and GPE PC as both an electrolyte and separator (see Figure S3). This evaluation demonstrates the further suitability of GPE PC for FLIB applications which is under study.

mobility of lithium ions. The Li/LiFePO4 half-cell with the GPE PC exhibits a better capacity retention of 83% with 136 mAh g−1 capacity after 100 cycles. It also shows good rate performance even at 6 C. It is noteworthy that these important results contribute significantly in an economical and scalable process. The approach presented here has the potential to produce sustainable energy storage systems including FLIBs.

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ASSOCIATED CONTENT

* Supporting Information S

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b07020. Tensile testing graphs; XRD pattern of PC and PDC films before and after swelling; cyclic voltammetric curves of the LiFePO4/GPE/Li cell of GPE PC and GPE PDC; the interfacial resistance of GPE PC with time; SEM of surface morphology of GPE PC and PDC films interfacing the lithium electrode after cycling; the discharge/charge profile of the LiFePO4 cathode with the GPE PC film; and the celgard and flexible battery fabrication method with photographs (PDF)

CONCLUSIONS In a nutshell, a unique combination of cellulose triacetate and PEG-methacrylates was used to prepare gel polymer electrolytes. The prima facie characteristics are as follows: (i) a new approach of combination of biocompatible green polymers as cellulose triacetate and PEG-methacrylate; (ii) a facile photopolymerization synthesis technique; and (iii) a higher transference number and electrochemical stability window with flexible battery potential. Prepared polymer films exhibited improved thermal and mechanical stability even after swelling in the liquid electrolyte solution. The ether abundant GPE PC exhibited conductivity up to 1.8 × 10−3 S cm−1 at ambient conditions and demonstrated a higher lithium transference number of 0.7. This specific polymer framework along with carbonate solvent provides free volume and anchoring groups which assist Li+ ion transportation through matrix. The higher conductivity of GPE PC compared to GPE PDC indicates that the free volume availability in the polymer matrix controls the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (B.B.K.). ORCID

Ramchandra S. Kalubarme: 0000-0002-3327-5110 Bharat B. Kale: 0000-0002-3211-717X 34780


Research Article

ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.C.N. acknowledges the Senior Research Fellowship from the University Grant Commission (UGC). The athors acknowledge financial support from C-MET, Pune and MeitY, New Delhi. The authors would like to thank Dr. J. Nithyanandhan (NCL, Pune) for providing the UV lamp facility and Dr. K.D. Trimukhe (NCL, Pune) for analytical support. T.C.N. is also thankful to the nanocrystalline materials group of C-MET, Pune.

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Facile Synthesis of Unique Cellulose Triacetate Based Flexible and

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