Influence of Hydrothermally Synthesized Cubic-Structured BaTiO3

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C: Energy Conversion and Storage; Energy and Charge Transport

Influence of Hydrothermally Synthesized Cubic Structured BaTiO Ceramic Fillers on Ionic Conductivity, Mechanical Integrity and Thermal Behavior of P(VdF-HFP):PVAc Based Composite Solid Polymer Electrolytes for Lithium Ion Batteries 3

Moorthy Sasikumar, Murugan Raja, Hari Krishna R, A Jagadeesan, Periyasamy Sivakumar, and Somasundaram Rajendran J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03952 • Publication Date (Web): 24 Oct 2018 Downloaded from http://pubs.acs.org on October 25, 2018

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The Journal of Physical Chemistry

Influence of Hydrothermally Synthesized Cubic Structured BaTiO3 Ceramic Fillers on Ionic Conductivity, Mechanical Integrity and Thermal Behavior of P(VdF-HFP):PVAc Based Composite Solid Polymer Electrolytes for Lithium Ion Batteries M. Sasikumara, M. Rajab, R. Hari Krishnac, A. Jagadeesand, P. Sivakumare*, S. Rajendranf a

PG and Researc Department of Physics, Bishop Heber College, Tiruchirappalli 620 017,Tamil Nadu, India.

b

Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, Tamil Nadu, India.

c

Department of Chemistry, M.S. Ramaiah Institute of Technology, Bangalore – 560 054, India

d

PG and Research Department of Physics, Nehru Memorial College, Puthanampatti, Trichy 621 007, Tamil Nadu, India.

e

PG and Research Department of Physics, Periyar EVR College, ,Tiruchirappalli 620 023, Tamil Nadu, India.

f

School of Physics,, Alagappa University, Karaikudi, Tamil Nadu 630 003, India.

*Corresponding author. Email: [email protected]

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Abstract Solid polymer electrolytes (SPEs) with high ionic conductivity and wide electrochemical window are highly desirable for all solid state rechargeable lithium batteries. Herein, we report the use of hydrothermally derived nano BaTiO3 as nano-filler in PVAc/PVdF-HFP [poly (vinyl acetate) / poly (vinylidene fluoride-hexafluoro propylene)] and its use as composite solid polymer electrolyte (CSPE) for Li ion batteries. The CSPE were prepared by solution casting technique and lithium bis-trifluoromethanesulfonylimide (LiTFSI) is used as salt. The molecular interaction among the various constituents and the surface morphology of the CSPEs were characterized by FT-IR and FE-SEM analysis respectively. 7.5 wt % of BaTiO3 (BT) in CSPE was found to be the optimum composition to obtain a high ion conductivity of 2 x 10-3 Scm-1 at ambient temperature. The CSPE exhibits better mechanical strength (6.9 MPa), wider electrochemical window (5.4 V) and higher lithium transference number (0.48) than that of SPEs. Solid-state-lithium cell was demonstrated as a proof of concept using lithium as an anode, LiFePO4 and SPE / CSPE (7.5 wt. % BT) as cathode and electrolyte respectively. The CPES ell shows an enhanced specific discharge capacity of 132 mAh g-1 at 0.1 C, cycling performance up to 40 cycles and 99 % coulombic efficiency. The properties above well support the CSPE as a potential electrolyte cum separator for Li ion batteries couple with high voltage cathode material.


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1. Introduction Lithium ion battery is considered to be one of the most promising power sources for portable electronic consumer devices and electronic vehicles.1 This is primarily due to long cyclic life, small size, light weight, fast response, high energy density, environmental safe compatibility, low cost and worldwide distribution.2 Present lithium ion batteries rely on the use of organic liquid electrolyte with lithiated metallic oxide (LiCoO2) and metallic lithium as the cathode and anode, respectively. The organic liquid electrolyte is a main source of electrochemical side reactions, such as undesirable Li dendrite formation at electrode-electrolyte interfaces. These lead to low stability and safety issues, and hence seriously limit their use in large scale devices.3-8 One promising solution to these issues, solid polymer electrolytes (SPEs), is emerged as one of the best candidate for Li ion batteries. Armand et al.9 first proposed the utilization of PEO and alkali metal salts as SPEs for Li batteries, which led to extensive studies on the SPEs by various researchers. SPEs can be accommodated as both electrolytes and separators between cathode and anode. The most striking features of SPEs are easy scale up, non leakage, flame resistance, flexible geometry, long durability, good mechanical integrity, wide electrochemical stability,10,

11

suppress the anode roughening and effectively impede from Li

dendrite formation.12 Despite excellent properties, SPEs exhibit a relatively low ionic conductivity at room temperature.13 To bypass this problem, various techniques have been attempted such as increasing the amount of salt and the addition of plasticizers such as ethylene carbonate and propylene carbonate.14-16 Although these efforts impart high ionic conductivity, the mechanical property of the polymer electrolyte matrix will significantly reduce. Therefore, it is still a great challenge to merge high ionic conductivity and good mechanical stabillity for Li ion battery



applications. To address these concerns, dispersing nanoscale ceramic fillers in polymer electrolyte matrix to increase the conductivity, together with improving mechanical strength and electrochemical stability has been attempted.17-24 In general, the ceramic fillers are mainly classified into two categories namely, inactive and active fillers. Inactive fillers do not participate in Li ion conduction (TiO2, SiO2, MnO2, Al2O3 and etc.,) whereas active fillers are essentially involved in Li ion conduction (Li3N, LiA12O3, LLZO, LLTO, LATP and etc.,). The incorporation of such ceramic fillers can act as charge carriers and prevents local reorganization of the polymer chains, and subsequently reduce the degree of polymer crystallinity, which favours high Li ion transport.25 Moreover, the free volume space created at the interfaces between the ceramic fillers and the polymer matrix provide additional ion transport pathways, gradually enhancing the ionic conductivity.26 BaTiO3 is one of the most important perovskite material with unique properties and versatile applications. It exhibits very high dielectric constant, which is desirable especially for promoting the Li salt dissociation to increase the charge carrier concentration in polymer electrolyte matrix.27,28 This provides that the BaTiO3 grains are large enough to produce ferroelectric domains. Generally grain size of the fillers is known to affect the properties of polymeric electrolyte. When the polymer melts, the fillers would induce the rapid growth of poorly developed spherulites. This result in reduction of crystallinity of the polymeric electrolyte, eventually leading to high ionic conductivity with enhanced mechanical strength.29 In this regard, synthesis of well dispersed BaTiO3 with small-sized particles would be appropriate to study the ionic conductivity of polymeric electrolyte. From the literature it is evident that use of pervoskite based nano oxide fillers in polymer electrolytes for Li ion battery application is not explored in detail.25, 26 The influence of nano


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BaTiO3 as ceramic filler in P(VDF-HFP) was investigated by Raghav P et al.

29

These results

suggested that the CPE in presence of BaTiO3 exhibited high ionic conductivity (4. 21 x 10-3 S cm-1) with enhanced electrochemical stability (up to 4.9 V). In spite of these promising results the CPE reported is based upon liquid electrolyte system. This limits the widespread application of the polymer systems in Li ion batteries. Therefore, it is need of the hour in battery technological research to design flexible free standing polymer electrolytes. In addition to the higher ionic conductivity, improved mechanical strength and enhanced electrochemical stability are desired characteristics for polymer electrolytes. Hence, in this work, for the first time we present the systematic study involving preparation of nano BaTiO3 as filler in P(VDF-HFP) polymer with PVAc as host polymer. This is unique in the sense that addition of BaTiO3 filler fulfils the requirements for solid polymer electrolytes for Li ion batteries. Herein, cubic BaTiO3 nanoparticles (BT) were synthesized via a CTAB assisted hydrothermal route and studied the influence of the BaTiO3 nanoparticles in the development of the CSPEs. The CSPE developed herein is expected to exhibit better ionic conductivity, lithium transfer number, compatibility, cycle stability, and thermal as well as mechanical stability. The prepared CSPE was characterization by various physicochemical techniques, such as FTIR, DSC, TGA, Tensile, FESEM. Also, the electrochemical properties of the CSPE were analysed by chronoamperometry, chronopotentiometry, linear sweep voltammetry, and electrochemical impedance spectroscopy. 2. Experimental Methods 2.1 Materials The starting materials, Ba(OH)2.8H2O (Purity 98%), Titanium tetra isopropoxide (purity 95%) and hexadecylcetyltrimethylammonium bromide (Purity 98%) were supplied by Alfa



Aesar. PVdF-HFP (Kynar, Japan, PVdF : HFP 88 : 12) with an average molecular weight of 400 K and PVAc with an average molecular weight of 140 K were obtained from Sigma Aldrich. Lithium bis-(trifluoro methanesulfonyl)imide salt (LiTFSI) [LiN(CF3SO2)2], was obtained from Alfa Aesar. Polymers and Li salt were dried under vacuum at 80°C and 100 °C respectively, for 5 hours prior to the preparation of composite solid polymer blend electrolytes. Plasticizer EC and acetone were purchased from Alfa Aesar and solvents were used as received. Poly (vinylidene fluoride) (PVDF) binder (Sigma Aldrich), super P (Alfa Aesar), LiFePO4 (Sigma Aldrich), 1methyl-2-pyrrolidone (Aldrich, USA) and Lithium metal foil (Alfa Aesar). 2.2 Preparation of cubic structured BaTiO3 nanoparticles Cubic structured BaTiO3 nanoparticles were synthesized by a conventional hydrothermal method using Ba(OH)2.8H2O as a barium precursor and C12H28O4Ti as a titanium source. Barium and titanium precursors in the ratio of 1:5 were dissolved in a 50 ml of double distilled water. To this mixture, varying concentration of CTAB (5, 10, and 15 mM) was added and stirred for 30 min. The resulting mixture was then transferred into a 100 mL Teflon lined stainless steel autoclave and maintained at 200 ˚C for 12 h. After the reaction, the solution was washed several times with double distilled water and ethanol to eliminate the impurities. Finally precipitate was collected by centrifugation and dried at room temperature. 2.3 Preparation of composite solid polymer electrolytes A free standing composite solid polymer blend electrolytes were prepared via solutioncasting technique. The detailed optimization for the preparation of SPE is discussed elsewhere in our previous work.31 In the present study, the protocol for the preparation of CSPEs is as follows; 40 wt % P(VdF-HFP)/10 wt % PVAc and 10 wt % LiTFSI- 40 wt % EC were dissolved in acetone separately. The obtained solutions were mixed together and stirred continuously for 24 h. Then the cubic structured BaTiO3 nanoparticles were added to the above mixtures and stirred


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at 60 °C to form a homogenous phase, followed by casting on a well cleaned teflon covered glass plate. Further, the samples were kept in a vacuum oven at 50 ˚C for 4 h to remove any trace of the residual solvent in the electrolytes. Atlast, free standing composite solid polymer blend electrolytes obtained were stored in a globe box until further use (Fig. 1).

Fig. 1. Photograph of free-standing and flexible CSPE. 2.4 Morphology and structural characterization The crystallinity and morphology of the samples were identified by X-ray diffraction (PanalyticalX’pert-Pro powder diffractometer) and field emission scanning electron microscopy (JEOL (JSM-840A)), respectively. The microstructure of the sample was analysed by transmission electron microscopy (Hitachi H-8100 (LaB6 filament, accelerating voltage up to 200 kV) equipped with an energy dispersive spectrometer (EDS; Keney Sigma TM Quasar, USA)). The surface functional groups of the samples were studied by Fourier transform infrared spectroscopy (PerkinElmer, spectrum two). The thermal property and safety performance of the samples were performed by differential scanning calorimetry (Pyris 6 DSC 6000) and thermogravimetric analyser (TA instruments, SDT Q600), respectively. The mechanical strength of prepared CSPEs were carried out by Universal Testing Machine (UTM, Instron Instruments).



2.5 Electrochemical measurements All the electrochemical properties of as prepared CSPEs were determined by Biologic SP 150 unit and ionic conductivity were determined by Electrochemical impedance spectroscopy (EIS) with a voltage amplitude of 10 mV in a frequency range from 1x106 Hz to 1Hz using SS/CSPEs/SS (stainless steel). The electrochemical stability of CSPE was analysed by Linear sweep voltammetry (LSV) in the voltage range from 2 to 6 V vs. Li/Li+ using cell composed of SS/CSPE/Li. In addition, Li/CSPE/Li symmetric cells were fabricated to investigate the interfacial resistance at the electrolyte/electrode interface (as time dependent impedance by EIS) and Li transference number, respectively. The Li transference number was determined by the combination of chronoamperometry (CA) with a dc pulse of 10 mV and EIS technique, according to the method proposed by Vincent and co workers.32 The commercial LiFePO4 cathode material was prepared in the form of thin film (thickness ~ 70 micron) by thoroughly mixing a 10 wt% of poly(vinylidene fluoride) binder, 20 wt. % super P electronic conductivity enhancer and 70 wt% of LiFePO4/C active material in 1-methyl-2pyrrolidone. The slurry was then coated onto an aluminium foil current collector. After drying, the electrode was punched by 15 mm dia (3-4 mg) to make a coin cell. All the coin cells were prepared in an argon-filled glove box (MBraun Labstar, Germany) with oxygen and humidity content below 1 ppm. Lithium metal was used as an anode. SPE/CSPE (7.5 wt. % BT) as a separator cum electrolyte for assembling a coin cell. The cycling of the cell was performed at different C-rate (0.1 C, 0.2 C and 0.5 C) by an Arbin Instrument Testing System under 40 ºC

.


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3. Results and Discussion 3.1. Ceramic structural investigations: XRD, FE-SEM and TEM analysis: Fig. 2 shows the X-ray diffraction pattern of BaTiO3 nanoparticles prepared with different concentrations of CTAB. The diffraction peaks at 2θ of 22.09°, 31.56°, 38.95°, 45.19°, 50.86°, 56.17°, 65.77°, 70.46° and 74.89° are respectively indexed as (100), (110), (111), (200), (210), (211), (220), (300), and (310) planes and they are ascribed to the cubic BaTiO3 with a = 4.03 Å, space group Pm 3m (JCPDS card No. 31-0174). The intensity and sharpness of (110) peak increase with increase in CTAB concentration, which clearly indicates that addition of CTAB improves the crystallinity of BaTiO3.33 This is because CTAB could selectively attach onto the specific absorption of (110) crystal facet. No peak splitting in the (200) and (201) diffraction peaks are observed which indicate that the BaTiO3 phase is cubic. Based on XRD patterns, the crystal cell parameter, crystal cell volume and crystallite size of all the samples are calculated and listed in Table 1. Furthermore, the crystal cell parameter and cell volume of CTAB assisted BaTiO3 are lower than those of surfactant free BaTiO3. This indicates that the unit cell of the crystal lattice is slightly contracted along x, y, and z directions owing to the augmentation of CTAB concentrations. The decrease in unit cell volume of ~ 1.11% may be ascribed to crystal defect in the lattice due to the gradual changes of lattice parameter.34 From Table 1 is it is evident that the hkl values(for 110 plane) of the sample prepared with CTAB are found to be less than that of the surfactant free BaTiO3. The average crystallite size was calculated using Scherrer’s equation as follows, τ = Cλ/(β cosθ) where τ is the average size of the crystalline domains, C is the dimensionless shape factor (0.941 used in this work), β is the full width at half-maximum (FWHM) and θ is the Bragg angle. Since



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the crystallite size of the BaTiO3.nanoparticle is unchanged remarkably with increase in CTAB concentration, these results confirm that CTAB does not have significant influence on crystallite size.

Fig. 2. XRD patterns of BaTiO3 with CTAB concentration (a) 0 mM (b) 5 mM (c) 10 mM (d) 15 mM. Table 1. Preparation conditions and crystallites size of the prepared BaTiO3 nano particles sample CTAB (mM)

(a)

0

(b)

5

(c)

10

(d)

15

cell parameter (Å) and unit cell volume V(Å3)

a=4.01272 V=64.6127 a=4.00932 V=64.4483 a=4.00911 V=64.4384 a=3.95568 V=63.8961

average crystallite size (nm)

hkl plane

level of agglomeration

(Å)

15

2.8374

19

2.8350

heavily agglomerated agglomerated

22

2.8348

agglomerated

23

2.7971

well dispersed


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Fig. 3(a-d) shows the scanning electron micrographs of BaTiO3 nano-powders prepared in presence of different concentrations of CTAB surfactant. From Fig 2 (a), it is evident that the surface morphology of the BaTiO3 prepared in absence of CTAB shows near spherical and irregular particle agglomerations. The particles form agglomeration clusters in order to decrease the surface free energy. In general, the surface energy is owing to size of the particles in nano regime. This is typical of most of the nano metal oxides prepared by hydrothermal synthesis without the surfactant.35 BaTiO3 prepared with 5 mM CTAB shows similar morphology and agglomeration as one prepared without CTAB. However, it is worth noticing that the particles synthesised in the presence of 10 and 15 mM CTAB are also agglomerated. Notably, the extent of agglomeration clusters is decreased compared to the sample prepared without CTAB. The weak agglomerations in samples using CTAB can be attributed to the weak interaction between particle clusters in presence of surfactant. Careful observation of SEM micrographs at same magnification also reveals very small decrease in the size of the particle clusters. However, since the variation is not significant, it can be concluded that CTAB surfactant has no noticeable effect on the surface morphology among the samples. Transmission electron micrographs (TEM) and selected area electron diffraction SAED images are shown in Fig. 4(a-c) and 3(d) respectively. TEM image revealed that the synthesized compounds exhibit irregular and a few cubic shaped particle agglomerates with size ranging from 50-70 nm. The average particle size from the TEM images is calculated as ~60 nm and is consistent with XRD results. SAED pattern reveals clear circular bright spots suggesting the particles are nanocrystalline in nature. The d-spacing for the lattice fringes are calculated and the corresponding (hkl) values are labelled that exactly matches with the PXRD diffraction pattern. Therefore, these results again confirm that the prepared samples possess cubic structured



nanocrystalline particles. Owing to the weak agglomeration with noticeably small size , 15mM CTAB assisted BaTiO3 (BT) nanoparticles were selected as the ceramic fillers dispersed into P(VdF-HFP)-PVAc-LiTFSI-EC SPE for further study.

Fig. 3. FE-SEM images of BaTiO3 with CTAB concentration (a) 0 mM (b) 5 mM (c) 10 mM (d) 15 mM.


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Fig. 4. TEM images of 15mM CTAB assisted BaTiO3 cubic structured nanoparticles with corresponding SAED pattern. 3.2 FTIR analysis of CSPEs: FTIR is the general analytical tool to identify the functional groups of polymer blends and the interaction of Li ions in the solid polymer blend electrolyte. Fig. 5(a) presents the FT-IR spectra of pure PVdF-HFP, PVAc, EC, LiTFSI and as prepared cubic structured BaTiO3 nanoparticles. For PVdF-HFP, the absorption peak at 513 cm-1 is ascribed to the -CF2 bending vibration of PVdF. The band corresponding to the CH2 bending vibration of PVdF is observed at 847 cm-1. The peak at 1057 cm-1 is assigned to the symmetric stretching vibration of CF and the vibration appearing at 1138 cm-1 can be ascribed to the symmetric stretching vibration of -CF2- in PVdF.



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The absorption bands at 1200 and 1350 cm-1 are attributed to the stretching band of C-C and wagging vibration band of -CH2- in PVdF respectively.

The bands due to CH stretching

vibration of PVdF-HFP are observed at 2920 and 2997 cm-1.36-38 From the spectrum of PVdFHFP, the vibrational bands of 796 and 607 cm−1can be identified as the crystalline phase of VdF unit.39

Fig. 5(a). FTIR spectra of (a) P(VdF-HFP) (b) PVAc (c)EC (d) LiTFSI and (e) cubic structured BT.

.

The characteristic peaks of PVAc are observed at 628 cm-1 for bending vibration of C=O in CH3 group, 802 cm-1 for wagging vibration of CH group, 946 cm-1 for bending vibration of CH


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group, 1022-1117 cm-1 for stretching vibration of C-O group, 1224

cm-1 for symmetric

stretching vibration band of C-O-C group, 1374 cm-1 for symmetric bending vibration of C=O in CH2 group, 1736 cm-1 for stretching vibration of C=O group, and 2934 cm-1 for asymmetric stretching vibration band of O-CH3 group, respectively.40-42 For the FTIR spectrum of LiTFSI, the vibrational bands at 646 cm-1 and 510 cm-1 are ascribed to SO3 symmetric and asymmetric bending vibrations, respectively. The band at about 570 cm-1 can be attributed to asymmetric bending vibration of CF3 in LiTFSI. The bands at 748 cm-1, 798 cm-1 and 1056 cm-1 are attributed to symmetric and asymmetric stretching vibration of S-N-S, respectively. Furthermore, the observed band at 1135 cm-1 is attributed to C-SO2-N bonding of LiTFSI. The absorption peaks at 1192 cm-1 and 1320 cm-1 are assigned to the asymmetric stretching vibration of SO3 and CF3 group of LiTFSI, respectively, The peak is identified at 1636 cm-1, indicates the less aggregation of LiTFSI.43-45 FTIR spectrum of EC, the C-H stretching vibration band is evident at 1064 - 1153 cm-1. The vibrational bands at 1390 and 1480 cm-1 are ascribed to CH2 scissoring bending vibrations, respectively. The bands corresponding to the C=O stretching vibration mode of the EC molecules are observed at 1763 cm-1 and 1793 cm-1.46 FTIR spectrum of 15 mM CTAB assisted BaTiO3 nanoparticles, the broad absorption band at 590−670 cm−1 is assigned to TiOVI stretching vibration connected to the barium component.47 The band situated at 850−860 cm−1 can be ascribed to metal−oxygen stretching vibrations.48 The band appearing at 3800-3600 cm-1 is indexed to symmetric and asymmetric vibration of O-H. The band at 1600 cm-1 is attributed to the bending vibration of H-O-H. Upon increasing the dispersion of cubic structured BaTiO3 nanoparticles into PVAc/PVdF-HFP/EC/LiTFSI to make the composite solid polymer electrolytes resulting in an observable changes in the wavenumber, shape and intensity of vibrational bands are observed..



Some vibrational bands are also disappeared in the composite solid polymer electrolytes. These results clearly suggest that composite solid polymer electrolytes are successfully prepared (Fig. 5(b)).

Fig. 5(b). FTIR spectra of P(VdF-HFP)-PVAc-LiTFSI-EC based CSPEs as a function of weight percentage of BaTiO3 (BT). 3.3. FE-SEM analysis The surface morphology of the prepared cubic structured BaTiO3 nanoparticles incorporated in PVdF-HFP-PVAc-LiTFSI-EC complexes is depicted in Fig. 6(a). The FESEM observations vividly demonstrate that, after the introduction of various amounts of BaTiO3 in PVdF-HFP-PVAc-LiTFSI-EC SPEs, some significant morphological changes are


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Fig. 6(a). FE-SEM images of composite solid polymer electrolytes containing different amounts cubic structured BaTiO3 nanoparticles (a) 2.5 wt % (b) 5 wt % (c) 7.5 wt % and (d) 10 wt %.

Fig. 6(b). Cubic structured BaTiO3 elemental distribution in the SPE.



observed. With incorporation of 2.5 wt % cubic structured BaTiO3, CSPE exhibits closed pores structure of coagulated spherical grains around 3 µm in size. As for the 5 wt% cubic structured BaTiO3, the size of the spherical grain and pore size substantially increases with a diameter of 15 µm. Upon further increasing the BaTiO3, that is 7.5 wt%, CSPE provide a large number of pores on its surface and enormous number of interconnected pores under the surface with typical pore sizes of 1-20 µm, which is expected to be beneficial to provide direct pathways for lithium ion transport. Further, addition of 10 wt% BaTiO3, leads to strong aggregation of BaTiO3 nanoparticles at the pores of CSPE surface, as shown in Figure 6(d). This might be due to their higher concentration and nanosize effect.49 The increase in pore structures not only indicate that the very strong interfacial interaction between BaTiO3 and PVdF-HFP-PVAc-LiTFSI-EC SPEs but also their solvent evaporation, substantial decrement in crystallinity and ability of solvent retention of CSPE. The elemental distribution of BaTiO3 in SPE is shown in Fig. 6(b). The existence of C element can be attributed to the polymer matrix. The result clearly confirms that the cubic structured BaTiO3 is uniformly distributed in SPE. The even distribution of BaTiO3 in the polymer matrix confirms the successful preparation of CSPE by solution casting method. 3.4. Impedance Analysis Fig. 7 shows the ionic conductivity of CSPEs measured at ambient temperature using AC impedance spectroscopy. The complete disappearance of semicircle portion of high and intermediate frequencies region is characteristic of the parallel combination of bulk resistance and bulk capacitance, which is mainly related to the bulk resistance and properties of the polymer electrolytes. The inclined spike in the low frequency region corresponds to the double layer capacitance formed at the interface between electrode and electrolyte.50 The ionic


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conductivity can be calculated by the equation

, where Rb is the bulk resistance and L

and A are the thickness and effective area of CSPEs, respectively. In addition to that the conductivity of the ceramic free CSPE is 2.3 x 10-3 S cm-1. With increase in the weight concentration of cubic structured BaTiO3, the ionic conductivities of CSPEs increased initially, and attained maximum value with 7.5 wt% and then reduced. Reduction of ionic conductivity at higher concentration of cubic structured BaTiO3 is probably due to BaTiO3 aggregation in the polymer electrolyte, which leads to increase in the crystallinity that block the conduction path. The CSPEs with 7.5 wt% cubic structured BaTiO3 exhibit the highest ionic conductivity of 2.3 x 10-3 S cm-1 at 30 °C and 6 x 10-3 S cm-1 at 80 °C, respectively, which is doubled (1 x 10-3 S cm-1) at 30°C and 3 times higher (2.2 x 10-3 S cm-1) at 80 °C compared to PVDF-HFP-PVAc-LiTFESIEC SPE.

Fig. 7. Nyquist plots of 7.5 wt % cubic structured BaTiO3 nanoparticles in P(VdF-HFP)PVAc-LiTFSI-EC CSPEs at room temperature.



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The temperature dependent ionic conductivity of P(VdF-HFP)-PVAc-LiTFSI-EC SPE with different amounts of cubic structured BaTiO3 nanoparticles (2.5 to 10 wt%) from 30 °C to 80 °C is shown in Fig 8. It is observed that the ionic conductivity of CSPEs increases with increase in temperature and obeys the Arrhenius equation exp

where A is the

pre

exponential factor, T is the absolute temperature, k is the Boltzmann constant, Ea is the activation energy. With increase in temperature, CSPEs expand, creating more free volume phase, resulting in increased mobility of the polymer chain that significantly enhances the transfer of ionic charge carriers,51 which is the main reason for the higher ionic conduction in CSPEs. Such results can be explained as follows. First, the conductivity is found to increase with increasing cubic structured BaTiO3 loading. These results confirm that SPBEs are strongly influenced by incorporation of cubic structured BaTiO3. This is because the interaction between Li cation and bulky anion TFSI on the BaTiO3 surface may decrease the ion pair interaction in SPEs matrix and hinder the movement of TFSI anion, leading to a higher ionic conduction in CSPEs.52 In addition, the cubic structure of BaTiO3, which has a basic centre can react with the Lewis acid centres of polymer chains and these interaction leads to the reduction in the crystallinity of CSPEs, that further enhances Li+ ion mobility.53 Particularly, when the filler content of BaTiO3 is beyond the critical level, the void fraction of CSPEs is decreased. This could be due to aggregation of BaTiO3 nanoparticles in the PVdF-HFP-PVAc-LiTFESI-EC SPE, which results in decreased miscibility between polymer electrolyte and ceramic filler, consequently, reducing the salt dissociation and ionic mobility.54,55 Second, the Li-blend polymer matrix interactions play a vital role in improving the conductivity of CSPEs. Generally, the polymers which act as a solvent can effectively solvate Li ion in the blend polymer matrix. However, the electro negativity of C-F in PVdF and C=O in PVAc can separate the LiTFSI salt and efficiently restrict the movement of


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Fig. 8. Arrhenius plots of composite solid polymer electrolytes at different temperature.

LiTFSI anion. Further, it prevents the reconnection between Li and TFSI, which in turn, results in creation of more Li ion in the blend polymer matrix. Third, EC species in the polymer matrix would be a hard barrier for the complexation of TFSI anion and to Li ion with carbonyl oxygen and fluoride atom,56 creating free volume phase for the polymer chain movement, which is also useful for enhancing ionic conduction in CSPEs. This work optimized 7.5 wt% incorporation of cubic structured BaTiO3 in P(VdF-HFP)-PVAc-LiTFSI-EC SPE to achieve the best performance for desirable Li ion batteries. Therefore, this sample is selected for further electrochemical tests. The ionic conductivity of the CSPE is comparable to that of the pure polymer electrolyte and still higher than that reported for other CSPEs.19, 20, 57, 58 3.5 DSC analysis DSC can be utilised as an effective method to study the thermal properties of the polymer electrolytes. Valuable information about glass transition temperature (Tg), and melting temperature (Tm) are obtainable, when the CSPEs are subjected to temperature analysis. The



DSC studies of CSPEs are depicted in Fig. 9. A miscible blend polymer electrolyte would exhibit a single glass-transition (Tg) between Tg’s of the two polymers. It can be seen that pure PVAc has g of approximately 30 °C. It is very difficult to determine the Tg of PVdF-HFP polymer. This is because PVdF-HFP is semicrystalline in nature. Notably, a single Tg was observed in all the samples. With increase in cubic structured BaTiO3 concentrations in the PVAc/PVdF-HFP-ECLiTFSI matrix, it is interesting to note from the DSC trace (Fig 9(a)) that, the glass transition (Tg) shifts towards the low temperature side, which in turn gives the direct evidence for the improvement of Li ion conduction in the polymer matrix. The addition of cubic structured BaTiO3 fillers can further reduce the interaction between CO groups in the polymer matrix and Li cation. Hence, there is a drop in Tg due to weakening of the polymer-ion effect when ceramic filler is incorporated. Generally, it is believed that the lowering of the Tg significantly enhances the segmental motion of polymer chains, resulting in higher conductivity. Furthermore, a general reduction of melting endotherm of semicrystalline phase PVdF can be observed as the ceramic filler concentration is increased in the polymer electrolyte samples as shown in Fig. 9(b). This clearly indicates that the PVdF-HFP semicrystalline phase is almost getting amorphous, which plays a vital role in improving the ionic conductivity of CSPEs. In addition, the melting endothermic peaks at 224, 218, 216, 190 and 210 °C represented as ceramic free, 2.5, 5, 7.5 and 10 wt% of cubic structured BaTiO3 respectively are ascribed to the complete melting of electrolytes. This is, however, in contrast to the observations reported by Namrata Shukla et.al, 58 and Zhang et.al. 59,60 These trends may be attributed to prevention of reorganization of polymer chain due to the cross linking centres formed by the interaction of the Lewis acid group ceramic fillers with polar group and fluorine atom present in the polymers. The influence of addition of cubic structured BaTiO3 is to stabilize the amorphous phase of polymer matrix that increases the


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segmental chain motion of polymer matrix. This highly enhances the Li ion species in blend polymer matrix.61 Beyond 7.5 wt% of cubic structured BaTiO3, the melting point slightly increases. The increase in the melting point is due to the fact that BaTiO3 in polymer matrix results in immobilization of PVdF-HFP in PVAc that leads to stearic hinderence and restriction of the segmental mobility in polymer chain.62 Notably, 7.5 wt% of cubic structured BaTiO3 nanoparticles incorporated in the polymer matrix has been considered as an optimized composition for Li ion batteries.

Fig. 9(a). Change in the Tg profile of PVAc due to dispersion of cubic structured BaTiO3 nanoparticles and (b) DSC curves of composite solid polymer electrolytes.

3.6 Thermal stability High Temperature of the polymer electrolyte is a crucial characteristic for the safety concern of a Li ion battery. To investigate the temperature stability of the prepared composite solid polymer electrolytes, thermogravimetric analysis (TGA) of PVdF-HFP-PVAc-EC- LiTFSI with different contents of cubic structured BaTiO3 (2.5 wt%, 5 wt%, 7.5 wt% and 10 wt%) was performed under a nitrogen (N2) flow. From TGA curves (Fig. 10), the thermal decomposition of



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ceramic free SPE is observed at about 250°C. It can be observed that all the polymer electrolytes shows lower thermal stability with the increase of cubic structured BaTiO3. The weight loss of the polymer membrane before 200 °C corresponds to the melting of polymer blends and gradual degradation of all samples. Such results can be explained as follows: (i) the longer polymer chain PVAc can significantly reduce the crystallinity in PVdF, and (ii) the polymer blend electrolyte can anchor on to the surface of the cubic structured BaTiO3 thereby further reducing the PVdF crystallinity, which are consistent with the DSC results. The complete thermal decomposition of all CSPEs begins at 310 °C as reflected by the TGA analysis, as shown in Figure 10. It was found form the EIS that a 7.5 wt% of cubic structured BaTiO3 in polymer electrolyte has higher ionic conductivity than ceramic free polymer electrolyte with a better thermal stability, which is good enough for the application of the CSPE in polymer Li ion battery. 3.7 Mechanical strength Good mechanical properties are essential factor for the CSPEs used in Li ion batteries. The stress-strain curves of the CSPEs are shown in Fig. 11. It can be seen that the ceramic free SPE is plastic in nature and its tensile strength is 3.2 MPa with an elongation at break value of 79%. With increase of BaTiO3, the tensile strength of CSPEs are significantly enhanced and their elongation at break value is notably reduced. This could be due to the adhesion effect between inorganic BaTiO3 and polymer matrix. Remarkably, 7.5 wt% cubic structured BaTiO3 in P(VdFHFP)-PVAc-LiTFSI-EC CSPE possesses higher ionic conductivity and also exhibits

the

favourable mechanical properties with the tensile strength of 6.93 MPa. This is very essential for facile cell assembly


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Fig. 10. TGA curves of composite solid polymer electrolytes..

Fig. 11. Stress-strain curves of composite solid polymer electrolytes.



3.8 Li interfacial stability Compatibility with Li electrode is the key factor for improving the safety concern and cyclic performance at high rates in Li ion battery. The stabilization of SPE and CSPE with lithium metal electrode are understood by their interfacial resistance. This could be due to formation of passivation layer leading to charge transfer resistance on the lithium metal electrode. Fig. 12 represents the variation of interfacial resistance (Rb) as a function of time for the symmetric cell Li/SPE/CSPE/Li at room temperature. It can be seen from Fig. 12(a) that the interfacial resistance of ceramic free SPE (0 wt % BT) increases from 210 Ω on the first day and changes to 1900 Ω after 30 days. As illustrated in Fig. 12 (b), the interfacial resistance of 7.5 wt % cubic structured BaTiO3 in P(VdF-HFP)-PVAc-LiTFSI-EC SPE increased from 160 Ω to 1350 Ω during the 30 days of storage time. These clearly indicate the formation of passive layer on the lithium electrode. Beyond 20 days, the Rb value of CSPE shows slight fluctuation with time, indicating that the prepared CSPE is stable at the lithium metal electrode interface compared with ceramic free SPE. Thus, CSPE exhibits good compatibility with lithium electrode. This is due to the dispersed BaTiO3 in SPE that reduce the growth rate of passive layer on the electrode surface.


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Fig. 12. The AC impedance spectra of cell (a) Li/SPE/ Li and (b) Li/CSPE/Li with 7.5 wt % cubic structured BaTiO3 nanoparticles at various storage time.

3.9 Li ion transference number For practical applications, electrolyte lithium transference number (Li+) is an important parameter used to evaluate the performance and rate capability of the Lithium batteries. Thus, polymer electrolytes with high Lithium transference number as well as high electrochemical stability window are highly desirable. Lithium transference number can be calculated by the following equation, Li

−

−

where Io and Iss are initial and steady state values of the current flowing through the cell during the dc polarization measurement, respectively. Ro and Rss represent the resistance value before and after perturbation of the system. AC impedance spectroscopic analysis was measured before and after perturbation. Fig. 13 presents chromoamperometic curve of the Li/SPE/Li and Li/CSPE/Li cell after applying dc voltage of 10 mV to the cell and insert shows the corresponding Nyquist plot before and after perturbation, respectively. More importantly, after dc polarization, there is no distinct deviation between the initial Ro (156 Ω) and final resistance Rss (198 Ω) of the two Li interfaces. These results further confirm the stability of the Li electrode with our prepared polymer electrolytes.63 The value of Li have been calculated as 0.29 for SPE and 0.48 for CSPE, respectively The small variations of Lit+ indicate that (PVdF-HFP-PVAC) polymer chains strongly solvate anions and restrict their motion in solvent channels. A high Lit+ value decreases the electrode polarization caused by anion accumulation and suppresses the concentration gradient to facilitate lithium ion transport.64



Fig. 13. The chronoamperommetric profile for (a) SPE and (b) CSPE with 7.5 wt % cubic structured

BaTiO3 nanoparticles. Insets: Nyquist plot of symmetric cell before and after

perturbation. 3.10. Electrochemical stability The electrochemical stability window of the electrolyte is an important criterion for high energy lithium batteries. Recently, high potential cathode materials have been identified, such as LiCoPO4, LiNiPO4. These electrode material exhibits redox reactions at 4.8 and 5.2 V Vs Li, respectively, which are higher than the safer operational limit of traditional electrolyte. However, the traditional electrolytes are normally stable up to 4.6V vs Li.65 Therefore, high electrochemical stability of the electrolytes is highly desired. The LSV curve of ceramic free


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Fig. 14 (a). The linear sweep voltammogram of SPE and CSPE with 7.5 wt % cubic structured BaTiO3 nanoparticles at room temperature (b) cycling stability of Li|CSPE|Li symmetric cell (at room temperature , current density 0.1 mA/cm2), inset (enlarged view of cycling profile for better view). SPE exhibits anodic decomposition at a potential of 4.8 V versus Li/Li electrodes. Notably, highly extended electrochemical stability window of 5.4 V can be observed with incorporation of 7.5 wt% BaTiO3 in SPE, as shown in Fig. 14(a). This result implies that the incorporation of cubic structured BaTiO3 is more effective on improvement of electrochemical stability. Such enhanced electrochemical stability is mainly attributed to the presence of acidic sites on the surface of cubic structured BaTiO3. More pertinently, the hydrogens of acidic site surface on the BaTiO3 constitute hydrogen bond with TFSI- anion of lithium salt. As a result, strong surface interactions are anticipated/formed between cubic structured BaTiO3 and TFSI-anion which retards the decomposition of lithium salt anion. Thus, CSPE shows the enhanced electrochemical stability than ceramic free SPE. In general, the electrochemical instability mainly originates from anodic decomposition of anion at high potential.66, 67 Therefore, the electrochemical stability of CSPE toward lithium metal was examined through ‘strip-plate test’ method with Li/7.5 wt%



BaTiO3 in SPE/Li symmetric cell. As shown in Fig 16(b), the symmetric cell can be charged and discharged sequentially for more than 350 min without significant change in cell voltage at 0.1 mA cm−2current density. The symmetric cell exhibits stable voltage profile with increase in cycling time that indicates the formation of stable SEI layer on lithium surface during cycling. The electrochemical stability results show the suppression of decomposition of anode material and formation of lithium dendrite. The stability studies revels that a stable CSPE to form good interfacecial nature to Li metal and excellent dielectric properties.

3.11. Galvanostatic charge-discharge studies The galvanostatic charge-discharge (cycling studies) studies were carried out to make a 2032-type coin cell and compared with the SPE to understand the utility of these CSPEs. Fig. 15 depicts the discharge capacity vs. the cycle number plot of the (Li/SPEs/LiFePO4)/ (Li/CSPEs/LiFePO4) cell at 40 ˚C performed at different C-rate. LiFePO4 is commonly considered as one of the better cathode materials for nanocomposite polymer electrolyte systems as reflected by its flat operating voltage of 3.45 V vs. Li.1 The experimental cell delivered an initial (2nd cycle) discharge capacity of 132 mA h g-1 at 0.1 C-rate for CSPE where as SPE shows 118 mA h g-1 at 0.1C rate with negligible fade in capacity for 25 cycles in both the cases. It is worth noting that CSPE shows the stable discharge capacity as well as higher coulombic efficiency at higher C-rate (0.2 and 0.5 C) compared to the SPE. This behavior might be attributed to the enhancement of ionic conductivity, better compatibility and Li-ion transference number of CSPE (7.5 wt. % BT).


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Fig. 15. The discharge capacity of cell (LiFePO4/CSPEs/Li) cell vs. cycle number for different C-rates at 40º C. Insert figure shows the initial cycling profile of the cell at 0.1 C. 4

Conclusion In summary, cubic structured BaTiO3 nanoparticles were successfully prepared by hydrothermal method and is used as filler in PVdF-HFP-PVAc-LiTFSI-EC complex for the preparation of composite solid polymer electrolytes for Li ion batteries. Impedance measurements revealed that the ionic conductivity of all the CSPEs was significantly increased, when compared to the ceramic free SPE. The glass transition temperature and degree of crystallinity of the CSPEs were largely reduced with increase in the concentration of cubic structured BaTiO3 ceramic filler. It is worth noting that the CSPE (PVdf-HFP: PVAc-7.5 wt% BT) exhibits a significant enhancement in its discharge capacity, cycling stability as well as coulombic efficiency at various C-rates (0.2 and 0.5 C) compared with



ceramic free SPE.. The enhancements of these characteristics are attributed to the increased Li-ion transference number. Meanwhile, CSPE prepared with 7.5wt % cubic structured BaTiO3 in SPE displayed larger electrochemical stability of 5.4V versus Li/Li+, which indicates no anodic decomposition of any components in this potential range. Moreover, CSPE have good compatibility with lithium electrode. Which can effectively inhibits the formation of resistive layer on the lithium electrode surface. Thus, the CSPE containing 7.5 wt% BaTiO3 exhibits all the desired characteristics that make it a potential solid polymer electrolyte for high voltage cathode material in Li ion batteries. Acknowledgement M. Sasikumar gratefully acknowledges the financial support from the University Grants Commission (No. F. MRP-5633/15 (SERO/UGC) of India. P. Sivakumar would like to acknowledge the financial support from the University Grants Commission (UGC –MRP NO. F. 42-807/2013 (SR)), New Delhi, India. M. Raja sincerely acknowledges funding through DST-SERB-NPDF (No. PDF/2017/001756) and IIT Madras for the facility. Authors are grateful to Dr. R. Kothandaraman, Department of Chemistry, Indian Institute of Technology Madras, Chennai, Helen Annal Therese (NRC, SRM Chennai) for her help in lithium ion cell fabrication and Dr. M. N. Chandra Prabha (Ramaiah Institute of Technology, Bangalore) for helpful discussion. Conflicts of interest There are no conflicts to declare


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Influence of Hydrothermally Synthesized Cubic-Structured BaTiO3

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