Electroactive poly(vinylidene fluoride) fluoride

Solid State Ionics 292 (2016) 130–135

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Electroactive poly(vinylidene fluoride) fluoride separator for sodium ion battery with high coulombic efficiency S. Janakiraman a, Abhijith Surendran b, Sudipto Ghosh a, S. Anandhan b, A. Venimadhav c,⁎ a b c

Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur,721302, India Metallurgical & Materials Engineering, N.I.T.K., Srinivasnagar, Mangaluru, 575025, Karnataka, India Cryogenic Engineering Center, I.I.T Kharagpur, Kharagpur, 721302, West Bengal, India

a r t i c l e

i n f o

Article history: Received 18 April 2016 Received in revised form 26 May 2016 Accepted 27 May 2016 Available online 9 June 2016 Keywords: Electrospinning Polyvinylidene fluoride Electroactive separator Sodium ion battery

a b s t r a c t Electroactive separators are recent interest in self-charging rechargeable batteries. In this study, electrospun polyvinylidene fluoride (PVDF) is characterized as an electroactive separator for Na-ion batteries. The intrinsic β-phase with high porosity of the separator is confirmed from X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Field emission scanning electron microscopy (FESEM) and Atomic Force Microscopy (AFM) studies. The electroactive separator is immersed in 1M NaClO4-ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 by weight) solution. The physicochemical characteristics of electroactive separator electrolyte (EaSE) were investigated using sodium ion conductivity, ion transference number and contact angle measurements. Linear and cyclic voltammetry studies were also carried out for the electrolyte system to evaluate oxidation stability window. The inherent β-phases of the separator as obtained by electrospinning has an ionic conductivity of ~7.38 × 10−4 S cm−1 under ambient condition. Sodium ion cell made from EaSE with Na0·66Fe0.5Mn0·5O2 as cathode and Na metal as anode has displayed a stable cycle performance with a coulombic efficiency of 92% after 90 cycles. © 2016 Published by Elsevier B.V.

1. Introduction Sodium ion batteries (SIB) are among the emerging rechargeable batteries for portable electronics in recent years. Sodium (Na) based compounds have made a comeback due to increased cost of Li metal. Apart from the low cost, natural abundance of sodium and its satisfactory electrochemical potential (−2.7 V) on standard hydrogen electrode makes it attractive for the rechargeable batteries [1–3]. Some cathode and anode materials have been identified and tested for Na battery application [2,4]. The electrolyte separator system (ESS) is also one of the critical aspects of the galvanostatic charging-discharging processes [4]. The primary function of the ESS is to insulate the electrodes while maintaining the reservoir of electrolyte and control of Na ions transport. Though this area is not explored extensively for Na-ion batteries, separators like polypropylene, glass fiber [5] polyethylene oxide and polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP) are being regularly used [6]. The electroactive polyvinylidene fluoride (PVDF) are piezoelectric in nature and have numerous applications in actuators, sensors and energy harvesters etc. Electroactive separator electrolyte (EaSE) based on PVDF have attracted considerable attention in Li-ion batteries for self⁎ Corresponding author. E-mail address: [email protected] (A. Venimadhav).

http://dx.doi.org/10.1016/j.ssi.2016.05.020 0167-2738/© 2016 Published by Elsevier B.V.

charging power cell that hybridizes the mechanical and chemical energy for self-charging of the battery [7–10]. The EaSE system consists of ionic salt as the electrolyte in the electroactive polymer matrix separator. PVDF is a semi-crystalline polymer with a good affinity for polar organic electrolytes. It mainly exists in five different crystalline phases namely, α, β, and γ where β and γ are polar lattices. The β phase is the most electroactive one with a dipole moment of 8 × 10− 30 cm electroactive in nature. The electroactive phases are inherent due to the spatial arrangement of electric dipoles contributed by the polar C–F bonds in the polymer chain. In recent years, ionic conductivity and oxidation stability of polyvinylidene fluoride and its copolymers poly(vinylidene fluoride-co-tri-fluoroethylene), PVDF-TrFE, polyvinylidene fluoride-co-hexa-fluoroporpylene, PVDF-HFP [9–12] have been improved in the organic solvent electrolyte. These are widely used as polymer separator for lithium ion battery due to their high dielectric constant and strong electron withdrawing functional groups, which can also be advantageous to the dissociation of sodium salts; however only a few reports are available based on the study of ionic conductivity of Na with the PVDF ESS [6,13]. The safety of the ESS can also been improved using PVdF-glass fiber mats composite [14]. Though the β phase is electroactive, it is relatively more crystalline and hence it is believed that the high crystallinity could cause low ionic conductivity and low stability due to the reduced migration rate

S. Janakiraman et al. / Solid State Ionics 292 (2016) 130–135

of lithium ions in the crystalline phase. Efforts were directed towards lowering the crystallinity of non-woven mats by suitable PVDF copolymers or modified PVDF [10]. Alternatively one can improve the porosity and the wettability of the separator to increase the ionic conductivity and to reduce the cell resistance [15]. The electroactive phases of PVDF are obtained by several techniques such as solution casting, spin coating and Langmuir–Blodgett deposition. But these microporous membranes are handicapped by low porosity and poor wettability [10, 16–18]. Electrospinning is another useful technique to obtain nonwoven mats with fiber diameters ranging from micrometers to a few nanometers. In electrospinning, the β phase is formed through in-situ poling and stretching of the polymer solution [19]. Electrospun fibrous membranes have advantages of high porosity, interconnected open pore web network structure, large surface area, which are essential for proper electrolyte uptake and ionic conductivity [9]. The crystalline electroactive and highly porous PVDF membrane produced by electrospinning are reported in the current work. Above 90 cycles are observed retaining coulombic efficiency of more than 90% of the sodium cells composed of Na0·66Fe0.5Mn0·5O2 cathode and Na metal anode. 2. Experimental 2.1. Optimization of the electrospinning process Several experiments are performed by optimizing the process parameters to get bead free and uniform, but smaller average fiber diameter with electroactive phase. Various parameters like polymer concentration, voltage, flow rate, needle to collector distance, temperature etc. influence the physical features of the polymer membrane [20–22]. Dimethyl sulfoxide (DMSO) is used as the major solvent in our experiment. Guo et al. [23] reported that DMSO can cause instability as its high dielectric constant leads to misalignment of fibers. But the solvents with lower dielectric constants contain less free charge available at the surface and hence, diminishes the electrostatic repulsion. The addition of acetone, which has a dielectric constant of 21, decreased the instability and helped in the formation of aligned fibers. Acetone can also make the solvent more volatile that can stabilize the jet for longer distances from the spinneret to the drum collector fixed at 1200 rpm and thereby can improve the fiber alignment. PVDF is dissolved in DMSO/acetone mixtures optimized to 8:2 by weight ratio. In this study, we varied polymer concentration and voltage for the optimization are varied while keeping flow rate, needle to collector distance, temperature and humidity at a fixed value. The critical parameter to get a bead free structure with electroactive phase is the voltage, and it was varied from 10 kV, 15 kV, 20 kV and 25 kV respectively. The increase in solution concentration further improves the viscosity of the solution. Higher viscosity aids in the formation of smooth and uniform fibers as it attains enough entanglement concentration [21]. Thus, it also helps in reducing the formation of beads. The fiber diameter is decreased with an increase in voltage at all the concentration levels and, bead formation is also suppressed with the applied voltage. With increase in net charge built up on the surface, the electrostatic repulsion is increased. Considering the high electrostatic field and PVDF concentration of the electrospinning process, the electroactive phase is formed through in-situ poling and mechanical stretching of the polymer solution. 2.2. Preparation of EaSE and cathode The PVDF powder (average Mw ~ 534,000), DMSO and Sodium perchlorate (NaClO4) are purchased from Alfa Aesar; EC and DEC purchased from Sigma-Aldrich was used without further purification. The polymer solution for electrospinning is prepared by adding 19% by weight of PVDF to 10 mL of DMSO. Considering the high boiling point of DMSO, the temperature during electrospinning is increased to a higher 44 ± 1 °C using an IR light source and the temperature continuously monitored using a thermometer placed inside the electrospinning chamber.

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The humidity, needle to collector (drum collector) distance, applied voltage, spinning time and the flow rate were 30 ± 2%, 10 cm, 25 kV, 6 h and 0.5 mlph respectively. The microporous membranes obtained after electrospinning are swollen by immersing in 1M NaClO4-EC: DEC (1:1 by weight) solution to form EaSE. The immersion is carried out inside an argon filled glovebox. Na0.66Fe0.5Mn0.5O2 is prepared by a solid-state reaction using high purity starting precursors of Na2CO3, Fe2O3, and Mn2O3.·The appropriate stoichiometric amounts of these starting materials were ground using a mortar and pestle and pressed into pellets. These pellets are heat-treated at 900 °C for 12 h in air. The pellets are quenched to room temperature by using a copper plate and stored in the glovebox. The cathode electrode is prepared by mixing active material, acetylene black and PVDF binder (8:1:1) in the weight ratio and pasted onto the aluminium current collector [24]. 2.3. Physico-chemical characterization of EaSE X-ray diffraction (XRD) pattern of the PVDF membrane is recorded using a Bruker diffractometer with Cu Kα radiation (λ = 1.54 A).The crystallinity of the membrane is found by peak deconvolution method. The percentage of β crystallinity of membrane is calculated using Eq. (1) [25]. Crystallinity of β ð%Þ ¼ Aβ = Aα þ Aβ

100%

ð1Þ

where Aβ, Aα are respectively the area of the β-phase and α-phases. FTIR spectra (Nexus-870) in ATR mode in the region of 4000–600 cm − 1 and 32 scans are collected with a spectral resolution of 0.5 cm−1. An average of three scans for each sample is taken for the measurement. Morphology of the polymer membrane is then observed using Field emission scanning electron microscope (Merlin, Germany). Atomic Force Microscope (Agilent 5500) in tapping mode is used to obtain the 3-dimensional topography image of the sample. The porosity is estimated from Eq. (2) [9]. Porosity ð%Þ ¼ ððMw−MdÞ=ðρb V m ÞÞ 100%

ð2Þ

where Mw and Md, ρb and are Vm masses of the wet and dry membrane, the density of butanol and the geometric volume of the membrane, respectively. The static contact angle of 5 μL electrolyte droplet is measured using Contact Angle Goniometer (Rame-Hart instrument & Co. Model no90F2). It is measured between the electrolyte and the separator to describe the features of electrolyte wettability. Electrolyte uptake by the membrane is assessed by soaking the membrane (1.4 × 1.4 cm2) in the liquid electrolyte, 1 M NaClO4 in EC/DEC solution. The weight of the wet membrane is measured after removing the excess electrolyte on the surface by using a tissue paper. Electrolyte uptake is calculated using the Eq. (3). Electrolyte uptake; Sw ð%Þ ¼

Mwet −M dry =M dry 100%

ð3Þ

where Mwet and Mdry are the masses of liquid electrolyte soaked and dry membrane respectively. The ionic conductivity of EaSE is measured by AC impedance spectroscopy using electrochemical workstation (Hioki 3532–50 LCR tester) with in a frequency range of 50 Hz to 1 MHz with 10 mV of AC amplitude at various temperatures. The soaked separator is sandwiched between two stainless steel (SS/EaSE/SS system) and sealed in CR2032 coin cell shells. The ionic conductivity of the separator (σ) is calculated using the equation: σ ¼ t=ðREaSE AÞ

ð4Þ

where REaSE is the resistance obtained from the intercept of the Nyquist plot with the real axis, the membrane thickness (t), and the electrode

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Fig. 1. a) & b) XRD & FTIR patterns of the PVDF powder and membrane.

area (A).The activation energy is obtained from ionic conductivities over a temperature range from the Eq. (5) [26]. ln σ ¼ ln ðAÞslopeð1000=T Þ

ð5Þ

The term σ is ionic conductivity; A is the intercept and Ea/R is the slope of the plot ln σ Vs 1000/T, and the activation energy is calculated by multiplying the negative slope and the universal gas constant (R = 8.314 J K−1 mol−1).Sodium ion transference number (It) of the membrane is evaluated by analyzing current vs. time plot using dc polarization technique [27]. By biasing the voltage at 1.5 V across the cell (SS/ EaSE/SS) for 15 min. It is calculated using Eq. (6): t ion ¼ ðI t −Ie Þ=It

ð6Þ

where It and Ie are the total and electronic currents. Comparative cyclic voltammetry is then carried out for two identical cells with one containing EaSE between steel electrodes while the other with Na electrodes. A scan rate of 5 mV s−1 over a range of −4 to 4 V is cycled to ensure sodium ion conductivity. The electrochemical stability window (SS/EaSE/SS) is evaluated by linear sweep voltammetry from 2 to 5 V at a scan rate of 5 mV s−1. 2.4. Electrochemical characterization The electrochemical performance of Na0·66Fe0.5Mn0·5O2 against Na metal anode cell containing liquid electrolyte-soaked fibrous nonwoven membranes is evaluated using CR2032 coin cells fabricated inside an argon-filled glove box. The charge/discharge behavior and cycling performance of coin cells is studied using Arbin automatic battery

cycler (BT-2000) in a potential window of 2–3.5 V. The battery was charged using a constant current (C/10) until the battery reached 3.5 V after which constant voltage (3.5 V) charging is employed until the charging current reached 1/10th of that in the constant current mode. Discharging was done in constant current mode at a rate of C/10. 3. Results and discussion 3.1. XRD & FTIR analysis Fig. 1 a) represents the XRD pattern of semi-crystalline PVDF powder and electrospun membrane. There are three different polymorphs of PVDF (α, β & γ).The purchased polymer powder has been indexed to phase α and electrospun membrane is indexed to a mixture of both α and β phases. For the PVDF membrane, the primary peak at 20.26° corresponds to the convoluted diffraction intensities from β (200) and β (110) planes. The weaker peaks at 18.30° and 36.03° are assigned to α (020) and β (201) respectively [25] [28]. The sharp peak indicates that the fiber mat contains large amount of crystalline β-phase owing to the high electric field accelerated stretching of the polymer jet while electrospinning. The percentage β crystallinity has been measured to be 38.22%. Infrared spectroscopy analyzes the crystalline phases of PVDF powder and membrane. The bands at 840, 1074 and 1279 cm− 1 can be assigned to polar β phase whereas the bands at 761, 798, 880, 975, 1186, 1401 cm−1 correspond to non-polar α phase. The peak of the βphase at 840 cm−1 is enhanced in the membrane as the peaks corresponding to α phase at 761, 795, 975 and 1186, 1401 cm−1 are absent. The peaks centered at 880 cm− 1 and 1401 cm− 1 are due to the C–F stretching vibration [25] and CH2 bending. The band at 1184 cm−1 is

Fig. 2. a) SEM image showing ultra-fine fibers, Inset: Thickness of the membrane and Fiber diameter distribution, b) AFM image of 3D web structure.


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10− 3 S cm− 1. In the following sections, we discuss high porosity, βphase and reasonable conductivity of the PVDF membranes with β phase. 3.3. Ionic transference number, liquid electrolyte uptake and contact angle

Fig. 3. The variation of current vs. time of the EaSE, Inset: The Static Contact Angle images between the PVDF membrane and NaClO4 electrolyte.

due to C–C bond. Peaks at 1279 cm−1 due to the C–F out of plane deformation [29] and at 1074 cm−1 are also characteristic to β-phase [30]. The bands at 840 and 1279 cm−1 corresponding to the electroactive dipoles perpendicular to the polymer backbone at a higher electric field. During this process the stretching and alignment of the fiber elude. And, the bands at 1074 and 1401 cm−1 correlate with the molecular chain orientation that favors the fiber alignment along the longitudinal axis with electroactive dipoles perpendicular to it [31]. 3.2. Morphology analysis The SEM image (Fig. 2a) shows cylindrical PVDF nanofibers in the electrospun PVDF mat. The electrospun membrane reveals an interrelated porous structure with narrow channels. The average thickness of the membrane is investigated by SEM (Fig. 2a) and is found to be around 79.4 μm. The fiber diameters are found to vary from 160 to 720 nm (Gaussian curve in Fig. 2a). The average fiber diameter is determined to be around 389 ± 30 nm for a population of 36 threads, measured at several places. Further investigation under AFM has revealed the formation of interstices by the 3-D interconnected nanofibers. Fig. 2b shows the presence of pores within the scanned area of 10 × 10 μm2. The high porosity is a desirable quality for a separator membrane because it allows the easy transport of ions across the separator. The voids observed here can aid in the transfer of the liquid electrolyte during charge/discharge process for fast diffusion of Na+ ions. The porosity of the nanofiber mat is determined using n-butanol uptake and is found to be around 81%. In fact, electrospun PVDF fibers in general have high porosity but reported mostly in α-phase. While, Gao et al. [18] have been reported the electroactive (γ phase) PVDF membrane for lithium battery with 80% porosity and ionic conductivity was of the order of

DC polarization technique can obtain the total ionic transference number (tion) of the EaSE. Fig. 3 shows the variation of current versus time at a dc bias potential of 1.5 V. The electronic current (Ie) can be obtained by extrapolating the saturation region to the Y-axis. The value of tion was calculated to be 98.30% which is higher than other reported system of separator and electrolyte combination for SIB [6]. This implied that the ionic current was significantly larger than the electronic counterpart. Very high transference number supports the migration of sodium ions, and considerably reduces the effects of concentration polarization. The liquid electrolyte uptake was calculated to be around 341%. The higher absorption is due to the very high porosity, small pore size and the increased surface area of the electrospun membrane. It has a greater affinity towards the liquid electrolyte and is held in the pores formed by the interstices between the 3D interconnected fibers. Further, the wettability of the ionic liquid is tested with the β-PVDF nanofibrous membrane. Inset of Fig. 3 shows the static contact angle of PVDF separator (25.3°) that confirms that separator exhibited better electrolyte wettability. 3.4. Sodium ionic conductivity of EaSE The effect of temperature on the ionic conductivity over 29 °C to 80 °C is shown in Fig. 4a & b. The ionic conductivity at each temperature is found by calculating the bulk resistance (REaSE) determined by taking the x-intercept. The ionic conductivity at 29 °C is 7.38 × 10−4 S cm−1 that gets increased to 1.10 × 10−3 S cm−1 at 80 °C. The ambient value is indeed higher than commercially available Celgard and of the order of other P(VDF-HFP) gel electrolyte [13]. In the high-frequency region, the semicircle is not visible because the total conductivity is all by ionic conductivity in the EaSE [32]. At the low-frequency part, the steeper slope of the curve indicates the resistance due to steel electrodes. High conductivity at high temperature indicates the phase transition from semi-crystalline to an amorphous phase [33]. For the semi-crystalline EaSE, the ionic conductivity is described by Arrhenius equation. The consequence of segmental motion allows the ion movements from one site to another inter-chain or intra-chains. This leads to the charge separation in the polymer membrane and aids in the transference of Na + ions through the liquid electrolyte filled in the interstices of the interconnected fibers [33]. The activation energy of ion transport in the EaSE is found to be 3.02 kJ mol− 1 which is lower than the activation energies of PVDF copolymers in the literature [6]. In the present EaSE, the high dielectric constant of the β-phase

Fig. 4. a) Nyquist plot of EaSE at various temperatures, b) Arrhenius plot of log σ vs. inverse absolute temperature.

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Fig. 5. a) Linear sweep voltammetry of SS/EaSE/SS at 5 mV s−1, b) Cyclic Voltammetry of symmetric cell-1(SS/EaSE/SS) & cell-2 (Na/EaSE /Na).

separator promotes the dissociation of ion pairs that enhance the concentration of ionic charge carriers. 3.5. Electrochemical stability of EaSE The electrochemical stability of EaSE is investigated by linear sweep voltammetry with stainless steel and sodium metal as the electrodes. From the Fig. 5a there is no significant increase in current up to 4.3 V, over this potential the polymer electrolyte breaks down. Electrochemical equilibrium of the EaSE is verified by comparing the cyclic voltammogram by inserting it between Na electrodes and SS electrodes. Fig. 5b shows the cyclic voltammogram of cell-1(SS/EaSE/SS) and cell2 (Na/EaSE/Na) in the same potential window at 5 mV s−1 for five cycles. The reversible cathodic (reduction) and anodic (oxidation) peaks of the sodium electrodes and the Na+ ions in the EaSE were observed in each cycle for cell-2 but not for cell-1. This reveals the electrochemical activity in cell-2 [34]. One other observation is the increase in current up to 5 cycles. Previously with PVDF-HFP system, Kumar et al. have observed a decrease in the current with more number of cycles hinting the formation of Na plating [35]. This purely an interface phenomenon and depends on the interface energies and porosity, the increase of current in the present case suggest the increase in ion concentration with cycling probably due to better porosity of the EaSE.

the 89th cycle from an initial value of 97% after the first cycle. The charge-discharge capacities and coulombic efficiencies versus cycle index are given in Fig. 6b. Interestingly, the voltage profiles in the battery electrodes decline marginally as shown by the next charge/discharge process. This kind of small voltage results from faster chemical kinetics that corresponds to the improved coulombic efficiency [4,36, 37]. The overall performance of the cell was found to be good considering the very little capacity loss per cycle. The high porosity and wetting of the EaSE seem to enhance the kinetics and retention of efficiency. The capacity loss occurring can be due to the entrapment of the Na+ ions at the separator membrane or the surface modification of the cathode due to cycling. It is also concluded that the testing membrane with liquid electrolyte 1M NaClO4-EC: DEC (1:1 by weight) changes to yellow color after prolonged cycling. Electrochemical decomposition of the liquid electrolyte to sodium alkoxide and alkyl carbonate which is yellow in color can be influential in the gradual increase of potential difference in charge/discharge cycling leading to fade in capacity [4]. The gradual decrease of Columbic efficiency with cycles has many possible origins and often, parasitic reactions as mentioned above or overhung in anode are cited as the reasons. In the present case, apart from decomposition of liquid electrolyte, the crystalline nature of the separator can also reduce the efficiency. 4. Conclusions

3.6. The galvanostatic charging/discharging Charge-discharge profile of the Na-ion battery carried for 89 cycles at room temperature is shown in Fig. 6a. The initial discharge capacity is found to be 153 mAhg−1 after the first cycle that eventually retains 110 mAhg−1 after the 89th cycle at an average discharge capacity loss of 0.4 mAhg−1 per cycle. The charge capacity of 158 mAhg−1 for the initial cycle is of 77% of the theoretical value and changes to 120 mAhg−1 after the 89th cycle. The coulombic efficiency was reduced to 92% after

PVDF electroactive separator is successfully synthesized by electrospinning and demonstrated sodium, ion batteries with high coulombic efficiency. The Presence of crystalline β-phase with high porosity is confirmed by XRD, FTIR and FESEM analysis. With good wettability, anionic conductivity of the order of 10−4 S cm−1 is obtained in EaSE with 1 M NaClO4-EC/DEC (1:1 by weight) electrolyte solution. The discharge capacity of sodium cell fabricated to Na0·66Fe0.5Mn0·5O2 as the cathode has retained the initial value with a little loss after 90 cycles.

Fig. 6. a) Galvanostatic charge-discharge profile of the cell at different cycles, b) cycle performance and coulombic efficiency of the cell.


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