Design of porous calcium phosphate based gel polymer electrolyte for Quasi-solid state sodium ion battery

Journal Pre-proof Design of porous calcium phosphate based gel polymer electrolyte for Quasi-solid state sodium ion battery

Ajay Piriya Vijaya Kumar Saroja, R. Arunkumar, Bhaskar Chandra Moharana, M. Kamaraj, S. Ramaprabhu PII:

S1572-6657(20)30047-3

DOI:

https://doi.org/10.1016/j.jelechem.2020.113864

Reference:

JEAC 113864

To appear in:

Journal of Electroanalytical Chemistry

Received date:

5 November 2019

Revised date:

2 January 2020

Accepted date:

15 January 2020

Please cite this article as: A.P.V.K. Saroja, R. Arunkumar, B.C. Moharana, et al., Design of porous calcium phosphate based gel polymer electrolyte for Quasi-solid state sodium ion battery, Journal of Electroanalytical Chemistry(2020), https://doi.org/10.1016/ j.jelechem.2020.113864

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© 2020 Published by Elsevier.

Journal Pre-proof Design of porous calcium phosphate based gel polymer electrolyte for Quasi-solid state sodium ion battery Ajay Piriya Vijaya Kumar Saroja†, ‡, R. Arunkumar†, Bhaskar Chandra Moharana†, M. Kamaraj‡, and S. Ramaprabhu†* †

Alternative Energy and Nanotechnology Laboratory (AENL), Nano Functional Materials

Technology Centre (NFMTC), Department of Physics, ‡Department of Metallurgical and Materials Engineering; Indian Institute of Technology Madras, Chennai 600036, India

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*Corresponding Author (Email: - [email protected])

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Abstract

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The design of a suitable separator is an effective approach to enhance the performance as well as the safety of a rechargeable battery. The conventional glass fiber separator has

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electrolyte leakage due to the random distribution of pores in the structure. The design of a

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gel polymer electrolyte with phosphorus containing compound is considered to be safer for the operation of a rechargeable sodium ion battery. Hence, we have developed a gel polymer using hydroxyapatite, a calcium phosphate-based compound in poly (vinylidene fluoride-

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hexafluoropropylene)-poly (butyl methacrylate) blend membrane by a simple solution casting technique. The developed membrane has an ionic conductivity of 1.086 x 10-3 S cm-1 with an

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electrochemical stability of up to 4.9 V, good porosity and electrolyte uptake, thereby making it a promising electrolyte to be used in a rechargeable sodium ion battery. To demonstrate its feasibility, the electrochemical properties of Na3V2(PO4)3/C are investigated using the prepared gel polymer electrolyte. The sodium ion cell using gel polymer electrolyte exhibits a specific capacity of 97 mAh g-1 at 4 C which is about 33.5 % enhancement in specific capacity when compared to the cell with the conventional glass fiber membrane. This study illustrates the feasibility of using gel polymer electrolyte as a replacement to the existing glass fiber separator. Key words: calcium phosphate, sodium ion battery, gel polymer electrolyte, sodium vanadium phosphate.

Journal Pre-proof 1. Introduction Sodium ion batteries are considered as low cost energy storage devices and have attracted great interest in the field of large scale energy storage applications[1–4]. However, the larger size of the sodium ions when compared to lithium ions poses problems such as huge volume expansion leading to poor cycle life. The higher redox potential of sodium (-3.01 V for Li/Li+, -2.7 V for Na/Na+ vs. Standard hydrogen electrode (SHE)) reduces the energy density of the battery. This has inhibited the commercialisation of sodium ion batteries [5,6]. Recently, research interest in the development of electrode materials leads to the emergence

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of high capacity sodium ion batteries [7,8]. Other than the development of electrode materials, electrolytes also play a major role in improving the performance, cycle life as well

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as safety of the battery[9–12]. So far, carbonate based liquid electrolytes are used in sodiumbased batteries due to their higher ionic conductivity (10-3 S cm-1). But the use of liquid

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electrolyte soaked in glass fiber separator poses safety concerns due to the electrolyte

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leakage. The liquid electrolyte possess low boiling point and flash point that lead to high volatility and inflammability[13]. So, the battery with liquid electrolytes causes fire hazard

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during overcharging and also leads to the formation of a gas due to the decomposition of electrolyte. Other than safety, reaction of liquid electrolyte with the electrode surface also

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leads to degradation in electrochemical performance. Due to the above mentioned problems, focusing on other types of electrolytes as well as electrolyte additives can lead to the

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development of a high performance and safer sodium ion battery[14]. Recently, research and development (R&D) is focused on the development of efficient electrolytes for room temperature rechargeable metal ion batteries which can be used as separator cum electrolytes [15,16]. In this scenario, solid electrolytes are considered as an alternative to the liquid electrolytes in rechargeable batteries due to their leakage free nature and safety[17]. But the major drawback lies in the poor migration of metal ions at room temperature as well as its mechanical stability. Other than solid electrolyte, the use of polymer membrane is beneficial for obtaining higher ionic conductivity at room temperature as well as mechanical stability. In addition, polymer electrolytes have lower electrolyte leakage, low flammability and possess good flexibility that can be applied in practical batteries and are called as quasi- solid-state electrolytes. The liquid electrolyte absorbed in pores and cavities facilitates the migration of ions and leads to good electrochemical performance. The solid phase helps to enhance the mechanical, thermal and interfacial stability of the electrolyte with the electrodes. Thus, the gel polymer electrolyte has overall good physical and electrochemical properties than its

Journal Pre-proof contemporaries (solid and liquid electrolytes) [18]. The most investigated polymer hosts that are used as separator are polyethylene (PE), polypropylene (PP), polyethylene oxide (PEO), polyacrylonitrile (PAN), polymethylmethacrylate (PMMA), polyvinylidene fluoride (PVDF) and its copolymers [19–22]. Among the various polymers, copolymer poly(vinylidene fluoride-hexafluoro propylene((PVDF-HFP) is widely studied as a polymer matrix for gel polymer electrolytes due to its high ionic conductivity, mechanical and electrochemical stability. Also, the high dielectric constant and lower crystallinity help to absorb large amount of liquid electrolyte when compared to other polymers [23–29]. Further enhancement in thermal stability and ionic conductivity of polymer membranes can be achieved by adding

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inorganic fillers into the polymer matrix. Different types of inorganic materials like SiO2, membrane in lithium ion battery

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Al2O3 and TiO2 have been exploited for enhancing the ionic conductivity of polymer [30–33]. The conductivity of sodium ions in solid

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electrolyte like -Al2O3 occurs at a temperature greater than 300 C. So, designing a polymer electrolyte membrane that can conducts sodium ions at room temperature will be crucial for

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the normal operation of the battery. Till date, PVDF-HFP polymer is used as a coating layer

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on the surface of glass fiber membrane to control the porosity of glass fiber membrane [34,35]. So, the development of a separator with controlled porous structure, high mechanical stability, ionic conductivity, low electrolyte leakage and electrochemical stability is crucial

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for the further advancement of a safer sodium ion battery. Hence, we have attempted to

dendrites.

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utilise an inorganic filler with sufficient mechanical stability and to prevent the formation of

Herein, we have used hydroxyapatite as an inorganic filler incorporated in PVDFHFP/PBMA blend as a gel polymer electrolyte for sodium metal battery. Hydroxyapatite (Ca10(PO4)6(OH)2) is an environmentally friendly inorganic biomaterial which is the major component present in bones. This has higher mechanical stability and the presence of functional groups in hydroxyapatite helps to form a composite structure within the polymer network. This aids in enhancing the safety as well as life time of the battery. To utilise these benefits, hydroxyapatite incorporated in poly (vinylidene fluoride-co-hexafluoropropylene) [PVDF-HFP] blend poly (butyl methacrylate) [PBMA] (HAP-GPE) is used as the gel polymer electrolyte. The polymer blends-based gel polymer electrolytes have been prepared by a simple solution casting technique. The operation of sodium ion battery using the gel polymer electrolyte is illustrated using carbon coated sodium vanadium phosphate (Na3V2(PO4)3) /C as the cathode and sodium metal as the anode. The cell exhibits a specific

Journal Pre-proof capacity of 97 mAh g-1 at a higher current density of 4C. This is ~ 33 % enhancement in specific capacity when compared to the cell with the conventional glass fiber separator. The better performance with HAP-GPE membrane at higher current density is attributed to the better interfacial stability, low electrolyte leakage and good ionic conductivity. The developed gel polymer electrolyte will be a better replacement to the existing liquid electrolyte using glass fiber membrane as the separator and can lead to further advancement in sodium ion battery technology. 2. Experimental section

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2.1 Preparation of HAP-GPE membrane The polymer membrane was prepared by simple solution casting technique. The PVDF-HFP

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and PBMA were used as polymer and hydroxyapatite was used as ceramics. All the

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chemicals were purchased from sigma Aldrich for synthesising membrane; the chemicals are annealed at 60 oC for 12 h by using vacuum oven before use. Dimethylformamide (DMF) and

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acetone were procured from Merck and used as solvent as received (without any further purifications). The appropriate amount of PVDF-HFP and PBMA are dissolved in the mixed

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solvent of DMF: Acetone (3:7) and stirred for 8 h to obtain homogeneous solutions. Later, the required amount of hydroxyapatite was added into the homogeneous polymer matrix and

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stirred continuously for 6 h. The homogeneous solution was casted on a glass plate and left for 1 day at room temperature. Finally, the obtained free-standing polymer membrane was

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stored in the glove box and used for further physical and electrochemical characterizations. 2.2 Preparation of Na3V2(PO4)3/C

Na3V2(PO4)3/C is prepared by solution evaporation technique. Na2CO3, V2O5, NH4H2PO4 and citric acid were mixed in the molar ratio of 1:0.67:2:1 in 100 ml of DI water and the mixture was stirred at 80C overnight. The dried mixture is then heated at 120 C for 12h. The dried mixture was pre-calcined at 350 C for 4h under argon atmosphere and then heat treated at 800 C for 12 h in the presence of argon and hydrogen to obtain NVP/C. 2.3 Physical Characterization The scanning electron microscopy images were recorded using field-emission SEM Inspect F50 at acceleration voltage of 200 V to 30 kV. The samples were mounted via conductive carbon sticky tape. Powder X-ray diffraction (XRD) patterns were recorded using a Rigaku

Journal Pre-proof Smartlab X-ray diffractometer with Cu Kα radiation (λ= 1.5418 Å). All measurements were taken in the range of 10° to 90° with the step size of 0.02. Thermogravimetric analysis was carried out in SDTQ600 analyzer from TA instruments in air and nitrogen atmosphere from 30 C to 800 C at a heating rate of 20 °C /min with a flow rate of 160 ml/min. Transmission Electron Microscopy (HRTEM) was employed for obtaining the micrographs.The samples preparation of HRTEM by ultrasonic dispersion of powder carbon samples in ethanol, then one drop of sample deposited on a centre of carbon film on a copper supported grid. The electrolyte uptake, porosity, tortuosity for the membrane were tested in an argon- filled glove

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box with the moisture level maintained below 0.1 ppm.

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2.4 Electrochemical characterization

The ionic conductivity of HAP-GPE and GF was studied using electrochemical impedance

various temperatures ranging from

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spectroscopy technique (Biologic SP 300) in the frequency range of 10 mHz to 10 kHz at 303-373 K. Sodium ion transference number was

the

membrane

between

the

non-blocking

sodium

electrodes.

The

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sandwiching

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determined using chronoamperometric technique at a step voltage of 10 mV s-1 by

electrochemical stability of the membrane was studied using linear sweep voltammetry technique at the scan rate of 1 mV s-1. For this measurement, the cell was assembled by

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sandwiching the membrane between sodium and stainless steel.

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For the preparation of cathode, the electrode was fabricated by slurry coating process. The slurry was prepared by mixing 75% of Na3V2(PO4)3/C, 10% of acetylene black and with 15% of Poly vinylidene difluoride (PVDF) as binder in N-methyl-2-pyrolidone (NMP) solvent. The working electrodes were prepared by coating the slurry on the aluminium foil and then dried at 80° C overnight in a vacuum oven. The dried electrodes were cut in the form of circular disc of 12mm. 2032-coin cells were assembled in an argon filled glove box where the oxygen and moisture level were maintained below 0.1ppm using sodium metal as the counter electrode and glass fiber (Whatman GF/C) / HAP-GPE membrane as the separator. The electrolyte used was 1M NaClO4 dissolved in Ethylene carbonate: Propylene carbonate (EC: PC). The cells were galvanostatically cycled at various current densities between 2.2 V and 3.8 V on Biologic BCS 810 battery cycler. 3.Results and discussion 3.1 Membrane Morphology and structure

Journal Pre-proof The FESEM images shown in Fig. 1 indicate the surface morphological image of the porous gel polymer membrane. The distribution of pores is uniform throughout the membrane as shown in Fig. 1a. The higher magnification image indicates that the pores formed are in the range of 10 m and the presence of highly porous structure enables to absorb more amount of the electrolyte without leakage. The pores are observed to be interconnected inside the membrane which provides an efficient path for the fast transportation of the ions. The formation of pores in the polymer membrane can be explained by breath-Fig. preparation method [36–38]. After casting the polymer solution on the glass slide, the evaporation of acetone solvent in high humidity atmosphere causes the formation of water droplets on the

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surface of polymer membrane. The polymer membrane forms like a shield around the water

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droplets which acts like a template for the formation of pores in the membrane. The presence of HAP particles in the polymer membrane are not observed in the SEM image since HAP

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particles are embedded inside the polymer membrane without any agglomeration. This is verified from the EDAX mapping of PVDF-HAP membrane wherein the presence of

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hydroxyapatite within the polymer membrane is confirmed by the presence of Ca, P and O.

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Also, the HAP particles are distributed throughout the membrane without any agglomeration as evident from the distribution of Ca in the EDAX mapping of the membrane (Fig. S1). The well embedded HAP particles in the polymer membrane provide a networking path and

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thereby enhances the ionic conductivity of the gel polymer membrane. The presence of HAP particles does not affect the porosity of the polymer membrane. This is further verified from

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the morphological image of PVDF-HFP/PBMA membrane without the addition of HAP as shown in Fig. S2. The distribution of pores is observed to be uniform and after the addition of HAP particles into the polymer membrane does not affect the distribution of pores as these particles does not block the pores but are distributed uniformly within the polymer matrix (Fig. 1). Thermal stability is an important parameter for the battery which decides its safety. The thermal stability of the HAP incorporated PVDF-HFP/PBMA blend membrane is determined using thermogravimetric analysis technique and is shown in Fig. S3. The membrane has an initial weight loss of less than 1% at a temperature below 373 K indicating the presence of trace amounts of moisture absorbed by sample during loading. Further, the weight loss starts gradually from 505 K and 30% weight loss occurs at a temperature of 730 K which implies the beginning of degradation of the polymers. Further increase in temperature causes rapid weight loss and it signifies the complete decomposition of the gel polymer electrolyte membrane. The residue obtained after 730 K is due to the presence of

Journal Pre-proof hydroxyapatite and the weight percentage is observed to be around 20 %.

This result

suggests that HAP based polymer membrane is thermally stable up to 500 K. The good thermal stability of PVDF HAP suggests that the prepared membrane can provide better thermal resistance than polypropylene and polyethylene oxide based separators. The thermal stability of the membrane is also visualised by heating the membrane at various temperatures. It is observed from Fig. S4 that the membrane heated at different temperatures remains to be stable and begins to shrink beyond 473 K. This again proves the thermal stability of the

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membrane.

Fig. 1.a) &b) SEM images of HAP-GPE membrane

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The XRD pattern of pristine polymer (PVDF-HFP & PBMA) indicate peaks at 18o and 20o indexed to (020) and (100) planes which implies the amorphous nature of the polymer (Fig. S5a). The appearance of a broad halo peak at ~18o for pure PBMA denotes the amorphous phase of PBMA (Fig. S5b). The X-ray diffraction (XRD) pattern of pristine HAP and HAP incorporated PVDF-HFP/PBMA blend composite membrane is shown in Fig. 2(a & b). In the pristine HAP (Fig. 2a), the appearance of sharp diffraction peaks at 21.5o, 22.7o, 25.6o , 27.8o , 28.7o , 31.6o , 32.7o , 34.1o, 35.4o , 39.8o , 41.8o , 43.6o , 45.2 , 46.6o , 47.7o, 49.3o , 50.4o , 51.9o , 53.2o , 55.6o and 57o correspond to the hexagonal phase of hydroxyapatite. The HAP incorporated composite polymer membrane shows the diffraction peaks of HAP along with a hump at ~ 18o which signifies the presence of both PVDF-HFP and PBMA blend polymers and HAP in the membrane (Fig. 2b). The intensity of the diffraction peak at 20o for the composite polymer membrane is lower when compared to the pristine polymer, this implies the lower crystallinity of the composite polymer membrane. Also, the intensity of diffraction

Journal Pre-proof peaks of HAP in the composite polymer membrane is decreased when compared to pristine HAP which signifies the well embedded nature of HAP inside the polymer membrane as seen from the SEM image shown in Fig. 1a.The reduction in the crystallinity is due to the formation of a weak acid-base interaction between C-F bond in the polymer and ceramic filler which restricts the formation of crystalline nature in the composite polymer membrane[39,40]. The amorphous nature of the composite polymer membrane provides a homogeneous porous structure as well as acts as a framework for immobilising liquid

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electrolytes and thereby increases the ionic conductivity.

Fig. 2. XRD pattern of (a) HAP incorporated polymer membrane and (b) pristine HAP 3.2 Ionic transport properties For a membrane to be used as a gel polymer electrolyte in a rechargeable battery, ionic conductivity of the membrane is a critical factor that decides the electrochemical performance of the battery. The ionic conductivity of the HAP incorporated PVDF-HFP/PBMA blend polymer membrane and glass fibre membrane is determined using electrochemical impedance spectroscopy (EIS) technique and their respective plots are shown in Fig. 3 (a, b). The ColeCole impedance plot of HAP incorporated membrane and glass fibre membrane at various temperatures (303-373K) are shown in Fig. 3 (a, b). The plot indicates an inclined straight

Journal Pre-proof line in the low frequency region and the absence of a semicircle in the higher frequency region signifies that conduction occurs by only ionic migration [41,42]. The bulk resistance of both the membranes (HAP-GPE and GF) are found to be decreasing with increase in temperature which is due to the enhancement of free volume at higher temperatures. The obtained bulk resistance of HAP-GPE membrane varies from 5.387 Ω at 303 K to 2.473 Ω at 373 K. The bulk resistance of glass fibre membrane varies from 7.766 Ω at 303 K to 3.867 Ω at 373 K. Fig. 3c. shows the temperature dependent ionic conductivity of HAP-GPE membrane and glass fibre membrane. Room temperature ionic conductivity of the HAP incorporated polymer membrane and glass fibre membranes are found to be 1.086 m S cm-1

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and 1.674m S cm-1 respectively. The detailed variations in ionic conductivity of the

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membranes at different temperatures (303-373 K) are given in Table S1. The higher ionic

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conductivity of HAP-GPE membrane is due to the high porosity of the HAP-GPE membrane.

Journal Pre-proof Fig. 3. Cole-Cole impedance plot of membranes (a) HAP-GPE and (b) GF and (c) Ionic conductivity of the both membranes at different temperatures (303-373K)

3.3 Porosity and electrolyte uptake It is well known that the polymer matrix acts as the support for the gel polymer electrolyte and the presence of liquid electrolyte is responsible for the ionic conductivity of the

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membrane. So, the porosity of the polymer membrane decides the electrolyte uptake and influences the ionic conductivity of the membrane. To confirm the reason behind the high

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ionic conductivity of HAP-GPE membrane, the porosity of the membrane is quantified by n-

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butanol uptake method. The porosity of HAP-GPE membrane is found to be 64 % and the high porosity is responsible for the better ionic conductivity of the membrane. Also, the high

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porosity demonstrates that the developed membrane can effectively enhance the electrolyte uptake with less leakage which in turn can lead to the development of a safer sodium ion

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battery. The high porosity associated with the electrolyte uptake facilitates the transport of sodium ions because of increase in the number of ion transport channels. The tortuosity of the

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membrane is a measurable parameter which describes the formation of a porous network and if the value of tortuosity is nearly one, it indicates the formation of porous network with

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cylindrical and parallel pores. To provide efficient batteries, the tortuosity of the membrane should be lower. High tortuosity of the membrane results in poor pore connectivity and leads to poor ionic migrations. The tortuosity of the HAP incorporated polymer membrane and glass fibre membrane are found to be 17.16 and 14.04 respectively. This value of tortuosity is much lower than the values of the membrane reported for Zeolite based polymer membrane and MgAl2O4 based porous ceramic membrane [43,44]. This value of tortuosity is also suitable for preventing the dendrite formation in batteries. So, HAP-GPE membrane has the benefits of better pore connectivity as well as the property of hindering the dendrite formation. The detailed physical properties of the membranes (HAP-GPE& GF) are listed in Table. 1. Due to the better pore connectivity and porosity of HAP-GPE membrane, it is expected that the electrolyte uptake of the membrane will be sufficient for the better ionic conductivity. So, the electrolyte uptake of HAP-GPE membrane is measured at different intervals of time (Fig. 4). The measurement was done by using circular piece of free-standing HAP-GPE membrane soaked in liquid electrolytes [1M NaClO4 in EC: PC (1:1)] at different

Journal Pre-proof intervals of time. The electrolyte uptake of membrane increases with increase in time and reaches a saturation level after a certain interval of time. The electrolyte uptake of HAP-GPE membrane is found to be ~350 % or 15.06 mg/cm2. The obtained uptake of electrolyte in the membrane is due to the high porosity as well as low tortuosity of the membranes. Due to the high porosity and electrolyte uptake, the ionic conductivity of HAP-GPE membrane is more which helps in the better ionic movement and leads to better electrochemical performance of

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the battery.

Fig. 4. Electrolyte uptake vs time for HAP incorporated polymer membrane Table 1. Physical parameters of the membranes

Membrane

Porosity (%)

Electrolyte uptake (%)

Tortuosity

HAP-GPE

64

~350

17.16

GF

66 [45]

360[45]

14.04

3.4 Electrochemical properties As observed in Fig. 3c., the HAP-GPE membrane exhibits a higher ionic conductivity compared to the liquid electrolyte. In polymer-based electrolytes, the ionic conductivity can be due to the presence of both cations and anions. So, it is necessary to determine the ionic

Journal Pre-proof conductivity contributed due to sodium ions. In this regard, sodium ion transference number of HAP-GPE and GF membranes are calculated using chronoamperometric and electrochemical impedance spectroscopy (EIS) techniques. For the measurement, a 2032-coin cell is assembled by sandwiching the membrane in between symmetric sodium metal electrodes. It is observed that both the membranes HAP-GPE and GF are polarized at 10 mV at room temperature. The initial and final (saturated) current values are also noted from the polarization curve. The initial current arises due to the migration of both cations and anions. After the cell polarisation occurs due to the applied potential, the movement of anions is opposed and the final current is due to the movement of cations [46–49] The sodium-ion

ISS (V−RO IO ) IO (V−RSS ISS )

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tNa+ =

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transference number of the membrane is calculated by using equation (1), (1)

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where, V is the applied voltage, IO and ISS are the initial and steady state current and RO and

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RSS are the resistances before and after polarization. Fig. 5 shows the chronoamperometric analysis of the membranes and their respective impedance plots before and after polarisation.

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It is observed from the plot that the current decreases with increase in time and finally a steady state current is obtained after a certain period of time. The values of initial and steady

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state current and their respective impedance values before and after polarization for both the membranes are tabulated in Table 2. The calculated values of sodium transference numbers

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of the membranes are found to be 0.83 and 0.91 for HAP-GPE and GF respectively. This value of ion transference number suggests that charge transport in the membrane is predominantly due to the transport of ions rather than electrons. The sodium ion transference number of HAP incorporated gel polymer electrolytes is slightly lower than the glass fiber membrane but the obtained transference number of HAP-GPE is sufficient to be used as a gel polymer electrolyte in a rechargeable sodium ion battery. Furthermore, HAP-GPE has a slightly higher transference number than the previously reported BaTiO3 based porous ceramic membrane for sodium ion battery[50].

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Fig. 5. Chronoamperometric analysis of (a) HAP-GPE and (b) GF (Inserted: respective

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impedance plot of before and after polarizations)

Table 2. Sodium ion transference number of HAP-GPE and their respective measured values Iss (mA)

HAP-GPE

0.1521

0.1277

GF

0.0318

0.0290

Rin (Ω)

Rss (Ω)

tNa+

81

135

0.8398

205

290

0.9136

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Iin (mA)

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Membrane

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In addition to the better physical properties of the polymer membrane, the electrochemical stability of the membrane is an inevitable parameter that determines the stable operation of

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the battery. So, the electrochemical stability of the membranes is studied using linear sweep voltammetry technique (Fig. 5). The measurement is carried out by sandwiching the glass

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fiber/polymer membrane between stainless steel electrodes. It is observed from the plot that there are no reduction peaks observed up to 4.8 V. This implies that both glass fiber and gel polymer electrolyte membranes can be operated at even with a high voltage electrode material of up to 4.8 V. Normally, a voltage stability of up to 4.5 V is sufficient for the operation of rechargeable sodium ion batteries. Both the membranes have stability of up to 4.8 V due to good affinity with liquid electrolytes (carbonate based plasticizers) [51]. The electrochemical stability of these membranes is found to be more compatible with most of the higher voltage cathodes and can find suitable application in the development of full cell sodium ion battery. Further, gel polymer electrolyte membrane has a slightly higher anodic stability than the glass fibre membrane. Also, a suitable electrolyte should have less reactivity towards the electrode and the interfacial stability plays a major role in obtaining high performance over the cycles. The interfacial stability measurement of HAP-GPE membrane is carried out by sandwiching the HAP-GPE membrane between sodium metal and the electrochemical impedance spectroscopy technique is carried out over different intervals of

Journal Pre-proof time. The Nyquist plot shown in Fig. 6b from the electrochemical impedance spectroscopy measurement as a function of time gives information about the interfacial stability of the polymer membrane. The formation of a stable electrolyte interface layer on the electrode surface prevents unwanted side reactions at the electrode-electrolyte interface layer and thus reduces the capacity fading of the cell. The resistance from the high frequency region to the intercept of the semicircle on x-axis contributes to the solution resistance and the semicircle from the high frequency to medium frequency region corresponds to the resistance due to SEI layer formation [52,53]. It is observed from Fig. 6b that interfacial resistance is high after the assembly and then decreases after a certain period of time which implies the formation of SEI

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layer. After the 4th day, a constant interfacial resistance is observed which implies the stable

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formation of a solid electrolyte interface (SEI) layer. The better compatibility of HAP-GPE membrane is responsible for the formation of a stable SEI which in turn is responsible for the

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long-term cyclic stability and good coulombic efficiency.

Fig. 6. a) Linear sweep voltammetry analysis of the membranes (GF and HAP-GPE), b) Nyquist plot of symmetrical Na/HAP-GPE/Na at different intervals of time 3.5 Sodium ion storage property of NVP/C using PVDF-HAP membrane Due to these advantageous properties of HAP-GPE membrane, the electrochemical performance of gel polymer electrolyte is evaluated with coin cells using carbon coated Na3V2(PO4)3(NVP/C) as the cathode. The XRD pattern of NVP/C is shown in Fig. S6 and the peak indexed corresponds to the rhombohedral crystal structure. The morphological image of NVP/C is shown in Fig.S7 (a, b). and it indicates that the NVP particles are covered with the carbon. In the TEM image, it is seen that the NVP particles of about 100 nm are covered by the carbon sheets. This coating of carbon sheets over the surface of NVP provides sufficient

Journal Pre-proof conductivity and helps in enhancing the rate capability of the battery Fig. S7 (c, d). In order to further confirm the presence of carbon in Na3V2(PO4)3

/C structure, the Raman

spectroscopy is carried out for the sample. The Raman spectrum of Na3V2(PO4)3/C as shown in Fig. S8 indicates the presence of D-band and G-band centered at 1364 cm-1 and 1592 cm-1 respectively. The presence of G band confirms the presence of carbon and the high intensity D band indicates the existence of non-graphitic nature of carbon. The amount of carbon in NVP/C is quantified from thermogravimetric analysis carried out in air atmosphere. The weight of the carbon in NVP/C is observed to be around 1.8 wt.%. Intrigued by the structural properties of NVP/C as well as the HAP-GPE membrane the electrochemical properties are

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investigated using galvanostatic charge discharge measurements. The galvanostatic charge

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discharge measurements are carried out in CR2032 coin cells for NVP/C electrodes using HAP-GPE as well as GF membrane as separator at room temperature. Fig.7 shows the

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galvanostatic charge discharge profiles of NVP/C in the potential window of 2.2 to 3.8 V at different C-rates. The distinct plateau observed in the charge discharge profile indicates the

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conversion reaction from Na3V2(PO4)3 to NaV2(PO4)3 corresponding to V4+/V3+ redox reactions. From the charge discharge studies, the specific capacity of NVP/C using HAP-GPE

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membrane as the separator is observed to be 136 mAh g-1 at 0.1 C (1C= 117 mA g-1). The cell with GF as the separator exhibits a specific capacity of 110 mAh g-1 at the same current

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density. This value of specific capacity obtained for NVP/C cathode using HAP-GPE membrane is about 23.6 % enhancement in specific capacity when compared to GF

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membrane. Also, it is observed that polarisation of the cell is higher when glass fiber membrane is used. The overpotential for the cell with glass fiber membrane is about 24, 36, 71,131,238 and 443 mV respectively for the different C-rates 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C and 4 C.

The respective overpotential for the cell with HAP-GPE membrane is

21,30,48,80,146 and 265 mV. It is clearly evident that using liquid electrolytes in glass fiber separator increases the overpotential of the cell due to less interfacial stability. At higher Crate, the overvoltage increases to much higher values when compared to HAP-GPE membrane which signifies the better compatibility of HAP-GPE membrane with sodium metal. The rate capability of the cell is also evaluated at different C-rates and reversible discharge specific capacities of 115, 115, 113, 109, 104 and 97 mAh g-1 at 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C and 5 C are obtained respectively. These values are higher than those cells using glass fiber with liquid electrolyte where the specific capacity decreases to 62 mAh g-1 at a higher C-rate of 4 C. Even when the cell is cycled back to higher current density of 4 C, the capacity is about 91.8 % of the initial capacity.

Journal Pre-proof The cyclic performance of sodium ion cells with gel polymer electrolyte and glass fiber membrane is investigated at a particular current density of 1 C. The cell with HAP-GPE membrane exhibits an excellent cyclic stability of 92.7 % at the end of 100 cycles whereas the glass fiber membrane exhibits a capacity retention of 88.3 %. At the end of 500 cycles, the capacity retained by the cell with HAP-GPE membrane is about 71.7 %. The low capacity retention for the cell with glass fiber membrane is due to polarisation of the cell which is consistent with the charge discharge profile with larger overpotential value (Fig. 7b).In addition, good electrochemical stability, lower sodium ion transference number and ionic conductivity favours the better electrochemical performance of NVP/C cathode with HAP-

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GPE membrane over the glass fiber separator. The better electrochemical performance of

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development of high performance and safer battery.

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sodium ion battery using HAP-GPE membrane at room temperature can thus leads to the

Fig. 7. Galvanostatic charge discharge curves of NVP/C using a) HAP-GPE, b) GF, c) rate capability, d) cyclic stability of NVP/C cathode using GF and HAP-GPE The morphological image of sodium metal of the cycled cell is analysed in order to understand the role of PVDF HAP membrane in preventing the formation of dendrite

Journal Pre-proof structure. It is observed from Fig. S9 that the surface of sodium metal with glass fiber membrane shows the non-uniform distribution of sodium metal with the formation of sharp needles like structure. This is because the SEI layer formed on the sodium metal in the carbonate electrolyte cannot effectively passivate sodium metal and the continuous formation of SEI layer during cycling leads to form fibrous structure which further can leads to the formation of dendrite structure. The surface of the sodium metal with PVDF HAP membrane indicates the smooth surface without the formation of surface deposit. This signifies that PVDF HAP membrane helps in the uniform deposition/stripping process.

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Further, the electrochemical behaviour of the sodium metal with glass fiber separator and PVDF HAP membrane is evaluated by galvanostatic cyclic performances of symmetric

Na//GF//Na

with a constant polarisation voltage of

cell exhibits continuous

increase in

8 mV. But

polarisation even after 20 h. The

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a constant cycling performance

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Na/Na cells at 1 mA cm-2. It is observed from Fig. S10 that Na//PVDF HAP//Na cell exhibits

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continuous SEI layer formation and non-uniform electroplating and stripping in the cell with glass fiber membrane results in increase in polarisation of the cell. This demonstrate that the

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stability of the sodium metal is significantly improved using PVDF HAP membrane and

4. Conclusion

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helps in preventing the dendrite formation when compared to glass fiber separator.

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A calcium phosphate based porous gel polymer electrolyte is developed using a simple solution casting technique and employed for sodium ion battery. The developed gel polymer electrolyte exhibits an ionic conductivity of 1.086 x10-3 S cm-1at room temperature which is of the same order of ionic conductivity as that of glass fiber separator with liquid electrolyte, high porosity (64 %) and high electrolyte uptake with an electrochemical stability of up to 4.9 V which makes it interesting and advantageous to be utilised as separator cum electrolyte in a sodium ion battery. Due to these better physical and electrochemical properties of the membrane, the electrochemical performance of carbon coated sodium vanadium phosphate exhibits better specific capacity at higher current density when compared to the cell operated using liquid electrolyte with the conventional glass fiber separator. The enhancement in specific capacity is attributed to the good interfacial stability due to the formation of stable SEI layer which is evident from the lower overpotential in the galvanostatic charge discharge studies. Also, the dendrite formation is prevented due to the stable electrode-electrolyte

Journal Pre-proof interface and the presence of phosphate group in the gel polymer electrolyte leads to safer and high-performance sodium ion battery. ACKNOWLEDGEMENT The authors acknowledge Indian Institute of Technology Madras (IITM), Chennai, India and DST, Ministry of Human Resource Development (MHRD), Government of India for the financial support. One of the authors thanks Department of Science and Technology (DST), India for the financial support to establish Nano Functional Materials Technology Centre (NFMTC) through SR/NM/NAT/02−2005 project.

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Conflict of Interest The authors declare no conflict of interest.

H. Pan, Y.-S. Hu, L. Chen, Room-temperature stationary sodium-ion batteries for

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[1]

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Journal Pre-proof

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Technol. 6 (2018) 1214–1219.

Journal Pre-proof Credit author statement Ajay Piriya Vijaya Kumar Saroja has done the experiments and analyzed the results. R. Arunkumar has performed the analysis of the results. Bhaskar Chandra Moharana has performed the synthesis of materials.

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M. Kamaraj, and S. Ramaprabhu have guided the work and corrected the manuscript.

Journal Pre-proof Declaration of competing interests

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Journal Pre-proof Highlights 

A hydroxyapatite based gel polymer electrolyte is prepared by a simple solution casting technique.



The gel polymer electrolyte has been used as the separator cum electrolyte for room temperature sodium ion battery. The gel polymer electrolyte exhibits a superior cell performance with 23 %

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enhancement in capacity and durability.

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