A high ionic conductive glass fiber-based ceramic electrolyte system for magnesium‒ion battery application

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A high ionic conductive glass fiber-based ceramic electrolyte system for magnesium‒ion battery application Rupali Singh∗, S. Janakiraman, Ashutosh Agrawal, Debasis Nayak, Sudipto Ghosh, K. Biswas Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, 721302, West Bengal, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Batteries Glass-ceramics Ionic conductivity Thermal properties

The rechargeable magnesium-ion batteries are one of the emerging alternatives of lithium-ion batteries as it has a high volumetric capacity, non-toxic nature and a divalent charge of Mg-ions. The design of an excellent performing magnesium-ion battery requires a stable electrolyte system with high ionic conductivity. However, there is a lack of understanding of how different materials affect the properties of separators in terms of ionic conductivity and stability. In the present study, an attempt has been made to compare the physical and electrochemical characteristics of glass-ceramic and polypropylene membranes as separators in the magnesium-ion battery, using magnesium bis(trifluoromethanesulfonimide) and propylene carbonate as an organic electrolyte. The characterization like X-ray diffraction, field emission electron microscopy, electrolyte uptake, ionic conductivity, voltage stability, thermal stability and transference number are thoroughly examined for both the membranes. The glass-ceramic electrolyte system showed significantly higher ionic conductivity of 9.22 mS cm−1 at room temperature as compared to the polypropylene membrane. Additionally, the glassceramic electrolyte system showed higher thermal and voltage stability.

1. Introduction In the recent past, the energy storage devices are increasing energy demand to develop new materials for high performance batteries. Among all the energy storage devices like lithium, sodium, potassium ion batteries, lithium-ion batteries (LIBs) are prevalent [1–3]. The growing demand for batteries in the market increases the LIBs production, which further creates stress on limited lithium reserves. Consequently, researchers have started finding other alternatives for LIBs. Magnesium-ion batteries (MIBs) are one of the most suitable options in this regard. The Mg has two electrons in its outer cell, hence has a high volumetric capacity than LIBs [4]. Besides, it is a non-toxic and 5th most abundant material on earth's crust [4–6]. Also, it has a high theoretical volumetric capacity (3832 mAh cm−3), energy and charge density, no dendrite formation during cycling and is inexpensive [7]. Due to these properties, MIBs may be the potential for next-generation battery technology. However, the development of MIBs is at a very nascent stage [8]. The components in MIBs are similar to LIBs; i.e., they have cathode, anode, electrolyte, and separator [9]. The main focus is to design high-performance electrode cathode and anode materials. The intensive research on the cathode, anode, and the electrolyte is going on, whereas only limited researchers are working on the separator electrolyte system of MIBs [10,11]. ∗

The separator plays a crucial role in battery performance and the main function of separator is to stop an electrical short circuit by avoiding physical contact between cathode and anode [12,13]. The other essential requirements of porous separators are high chemical resistance, high porosity, excellent wettability, high mechanical, and dimensional stability [1,13–15]. The microporous polyolefin membranes (polypropylene (PP) Celgard) are very commonly used in LIBs as a separator due to its superior mechanical strength and high chemical stability. However, it has poor wettability and low porosity increases the cell resistance, thereby affecting ionic conductivity and thus reducing the performance of battery [16]. Also, polyolefin membranes have low thermal stability and high thermal shrinkage at high temperatures. Thus, the chances of internal short circuits are high due to overcharging or overheating that may lead to fire, followed by an explosion [17]. The battery safety is an important parameter; thus the high temperature stable porous membrane may be a better option for separators [18]. The other polymer films like polyvinylidene fluoride (PVDF), poly (vinylidene fluoride-co-hexafluoropropylene) P(VDF-co-HFP), polyethylene oxide (PEO), poly (ethylene carbonate) (PEC), polyacrylonitrile (PAN), and poly (methyl methacrylate) (PMMA) are now under review [11,19]. Although they show better results than PP, high cost, low thermal stability, complicated manufacturing process, relatively low conductivity, and low dimensional stability limit their usages [20–22].

Corresponding author. E-mail address: [email protected] (R. Singh).

https://doi.org/10.1016/j.ceramint.2020.02.154 Received 30 November 2019; Received in revised form 30 January 2020; Accepted 16 February 2020 0272-8842/ © 2020 Published by Elsevier Ltd.

Please cite this article as: Rupali Singh, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2020.02.154

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Electrolyte uptake (%) = ((Mwet − Mdry) / Mdry) × 100

The separator materials used in MIBs are made of either nonwoven fabrics like glass-ceramics (fibers) or microporous polymeric films like polypropylene (PP) [23], PVDF [23,24], P(VDF-co-HFP) [25–28], PAN [29,30], PEO [31,32], PEC [33] and PMMA [34]. In contrast, nonwoven glass-ceramic mat (GCM) having outstanding properties like high-temperature stability, excellent resilience to sustain pressure, high wettability, high porosity (90–95%), simple manufacture process, and low cost [12,35,36]. The nonwoven glass fiber-based ceramic membranes are mainly used as a separator in lead-acid batteries. The high wettability and highly porous structure enhance the electrolyte uptake, thereby increasing ionic conductivity [16]. These exceptional properties of GCM make it an attractive material for separator applications [18]. In present work, an effort is made to understand the electrochemical properties of GCM and PP Celgard with 0.5 M Mg (TFIS)2-PC electrolyte for MIBs. Mg (TFIS)2 is used as salt owing to its (1) high ionic conductivity, (2) natural dissociation in organic solvents due to weak interaction between ions, and (3) high anodic stability [4,37]. The PC is used as a solvent because of high polarity, and it also enhances the flexibility of the membrane [38]. The experimental results show that the glass-ceramics electrolyte system (GCES) provides high ionic conductivity, high thermal stability, as well as high voltage stability than PP Celgard. Hence, GCES may offer better battery performance than PP Celgard.

(2)

where, Mdry and Mwet are masses of the membrane before (dry) and after (wet) soaking in an electrolyte, respectively. The AC impedance spectroscopy (HIOKI 3532-50 LCR meter, Japan) was used to measure the ionic conductivity. The membranes were placed between two SS electrodes and scan over the frequency range 42 Hz to 5 MHz from room temperature to 80 °C at 10.0 mV amplitude. The ionic conductivity is calculated by using the following Eq. (3): σ = t / (RM × A)

(3)

where, RM is a bulk resistance of membrane and is calculated by the intercept on the real axis in impedance spectra. The t and A are the thickness and area of the membrane, respectively. The activation energy of GCES and soaked PP were calculated by using Eq. (5) as given below: σ = σο × e(−E/kT)

(4)

where, σ is ionic conductivity, T is absolute temperature, σο is a preexponential factor, E is activation energy, and k is Boltzmann constant (8.617 × 10−5 eV K-1). The linear sweep voltammetry (LSV) was obtained on the BioLogic SP-200 (France) instrument. The three-electrode cell (working electrode (SS), a counter electrode (Mg metal), and a reference electrode (Mg metal) were used for both membranes, and the cells were swept between 0.0 to 6.0 V with a scan speed of 0.1 mV s−1. The total transference number (tion) of GCES and soaked PP membranes was calculated by the DC polarization technique [40]. The membrane was placed between two stainless steel (SS) electrodes like cell A (SS/ GCES/SS), and cell B (SS/PP/SS) biased at 1.5 V for 15 min. The tion is calculated by using the following Eq. (5):

2. Experimental 2.1. Materials and synthesis Magnesium bis(trifluoromethanesulfonimide) (Mg (TFSI)2) (Alfa Aesar) and propylene carbonate (PC) (Sigma Aldrich) were used in as a received condition without any further purification. The glass-ceramics mat (GCM) microfiber membrane (Whatman grade C, thickness 0.26 mm) was purchased from Sigma Aldrich, and polypropylene (PP) membrane (Celgard 2400, thickness 0.025 mm) was purchased from Hohsen Corporation (Japan). The GCM and PP were used as separator membranes. The electrolyte was prepared by dissolving 0.5 M Mg (TFSI)2 in the PC solution inside the glove box. Afterward, the separator membranes were soaked overnight in a liquid electrolyte for further characterization.

tion = It - Is / It

(5)

where, It is total current, and Is is electronic current. To study the interfacial stability of GCM and PP with Mg metal the electrochemical impedance spectroscopy (EIS) measurement was done. The two symmetric cell Cell A (Mg/GCES/Mg) and Cell B (Mg/PP/Mg) were prepared for the EIS study. The measurement was done on the BioLogic SP200 (France) instrument in the frequency range of 7 MHz -100 mHz with a scan speed of 10 mV/s (see Fig. 1). 3. Results and discussion

2.2. Characterization techniques

3.1. Phase and thermal analysis

The phases present in the pristine PP, GCM, and GCES membranes were identified by the X-ray diffraction (XRD) technique (XRD Bruker D8 Advance) in the range of 2θ = 10°–60°, with step size 0.3 and Cu-Kα radiation (λ = 0.154 nm). The thermogravimetric analysis (TGA) (PerkinElmer Pyris Diamond) of pristine PP and GCM membranes was done to examine thermal stability. The TGA was carried out under a nitrogen atmosphere up to 600 °C with a heating rate of 10 °C min−1. The morphology and elemental analysis of membranes were done by field emission scanning electron microscopy (FESEM), Zessis Merlin (Germany), instrument after gold coating under vacuum. The porosity was calculated by using n-butanol uptake method. In this method, the pristine membrane was immersed in n-butanol for 2 h and allow the solvent to fill in the open voids and pores without hampering the structure of the membrane. The porosity of GCM and PP was calculated by using the following Eq. (1) [39]: Porosity (%) = (Mw − Md) / (ρb × Vm)) × 100

The XRD results of pristine PP, GCM, and GCES membranes are shown in Fig. 2 (a). The pristine PP shows semi-crystalline nature the peaks at 2θ = 13.8°, 16.8°, and 18.4°, that is characteristic peaks of isotactic polypropylene [41]. Whereas, in glass-ceramics (both pristine and soaked) membranes, the broad hump is observed which indicates the presence of the amorphous phase in the system. The high amorphous phase helps in ion movement, as it has less hindrance in the path compared to the crystalline system. Due to the addition of electrolyte, the GCES membrane shows a shift as well as narrowing in the XRD peak. Besides, no additional XRD peaks are found in the soaked membrane, confirming the complete dissolution of Mg salt [23]. The TGA curve of pristine PP and GCM membranes is shown in Fig. 2 (b). The pristine PP membrane starts decomposing from 60 °C, and at 400 °C, it completely evaporates. In contrast, the pristine GCM is stable up to 600 °C, and very less amount of weight loss occurs (less than 2%). Hence, the results confirm that the GCM is thermally stable at high temperatures in comparison to PP. The high thermal stability membranes reduce thermal shrinkage and enhance battery safety [10].

(1)

where, Mw and Md represent the masses of wet and dry membranes. ρb and Vm are the density of the n-butanol and geometric volume of the membrane, respectively. The electrolyte uptake of membranes was calculated by submerging the pristine GCM and PP in liquid electrolyte for 2 h, by using following Eq. (2):

3.2. Morphology, porosity, electrolyte uptake and wettability The FESEM images of pristine GCM and GCES are shown in Fig. 3(a) 2

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Fig. 1. (a) XRD spectra of Pristine PP, Pristine GCM, and GCES membrane, (b) TGA spectra of pristine PP and GCM.

confirmed by SEM images, which enhances the electrolyte uptake; hence, the number of charge carriers increases. The liquid electrolyte uptake is found to be 758% for electrolyte soaked GCM and 30% for PP, using Eq. (2). The high electrolyte uptake of electrolyte soaked GCM is due to high porosity, high surface area and high retention capacity than PP. Consequently, the number of charge carriers is more in GCES than PP.

and (b). The pristine GCM image shows a highly porous structure, and the fibers are loosely bonded with less than 2 μm diameters. Whereas in GCES, the electrolyte is held in the pores of the fibrous membrane, shown in Fig. 3 (b). The EDX analysis is done to confirm the presence of electrolytes in the system and shown in Fig. 3 (c). The presence of Mg, C, F, O, N and S confirms that electrolyte is present in the GCES membrane. For comparison, the FESEM of pristine PP is shown in Fig. 3 (d). The PP has an open, and tortuous structure with uniformly distributed slit-like pores, which may provide high density and also elude internal short circuits [14,16]. However, it has low porosity than the GCM, which reduces the ion movement. The porosity of the GCM and PP membrane is calculated by using Eq. (1) and found to be 93 ± 2% and 41 ± 2% [42], respectively. The high porosity of GCM is also

3.3. Ionic conductivity The ionic conductivity is an essential parameter for battery performance as it influences the cell resistance. The ionic conductivity is dependent on the following parameters; thickness of the membrane, the

Fig. 2. FESEM images of GF (a) Pristine (b) GCES membrane (c) EDX graph of GCES (d) FESEM image of pristine PP. 3

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Fig. 3. Ionic conductivity of (a) GCES (b) soaked PP; activation energy of (c) GCES (d) soaked PP.

Fig. 4. (a) AC ionic conductivity, (b) Dielectric constant vs. frequency for GCES and PP.

follows the Arrhenius law. By using Eq. (4), the activation energy is calculated and found to be 0.051 and 0.140 eV for GCES and soaked PP, respectively. The lower activation energy in GCES is attributed due to higher ionic conductivity, resulting in the faster Mg2+ ion migration in the system.

porosity of membrane, wettability of membrane, and ionic conductivity of the liquid electrolyte. The ionic conductivity of GCES and soaked PP is shown in Fig. 4 (a). Usually, the impedance spectra consist of a semicircle in higher frequency and a spike in the lower frequency region. However, in this study semi-circle is disappeared, and only spike is present in GCES, indicating that the charge carriers are purely ions and the total conductivity is due to ion conduction [23]. Although, soaked PP shows a high-frequency semi-circle followed by a low-frequency spike. The semi-circle in higher frequency region is due to bulk resistance, and a spike in a lower frequency region is due to the blocking electrodes [43]. As the temperature increases, the semi-circle decreases; hence, it confirms that ionic conductivity increases with an increase in temperature [43]. The RM value of PP is calculated by fitting the curve using Zsimp view software. The ionic conductivity is calculated for GCES and soaked PP using Eq. (3) and found to be 9.22 × 10−3 and 6.55 × 10−6 S cm−1 at room temperature. The high conductivity is achieved in GCES, due to high electrolyte uptake, and good wettability. The temperature dependence of ionic conductivity of GCES and soaked PP for magnesium ion batteries is shown in Fig. 4(a) and (b). The ionic conductivity curves are linearly fitted, which confirms the conductivity

3.4. Conductivity and dielectric properties The AC ionic conductivity vs. frequency plot of GCES and soaked PP at different temperatures is shown in Fig. 4 (a). The conductivity increases as the frequency and temperature increase. This is due to the charge accumulation occurs at low frequency because of the interfacial reaction of electrode and electrolyte. This may decrease the number of mobile ions, and hence, the conductivity is low at a lower frequency, whereas at higher frequency, the mobility of charge carriers increases. As a result, the conductivity is high at higher frequency. The AC ionic conductivity is higher in GCES than soaked PP, which proves the number of mobile ions is higher in the GCES due to higher electrolyte uptake and retention. Therefore, GCES provides high ionic conductivity and shows better results than PP. Further to study the ionic transport 4

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Fig. 5. (a) Voltage stability of GCES and PP; transference number of (b) GCES (c) PP.

Fig. 6. (a) Wettability of pristine PP and GCM membrane, (b) Digital photographs of pristine PP and GCM membrane before and after heating.

3.5. Voltage stability and transference number

phenomena, the dielectric property is also investigated. The dielectric constant, ε* = εr - jεi where, εr is a real part of dielectric constant, and εi is an imaginary part of dielectric constant. The dielectric constant value is high at a lower frequency, and as the frequency increases, the dielectric constant values decrease and finally become constant. The reason for this behavior is that the electrode polarization occurs at low frequency, i.e., the dipoles have sufficient time for orientation. Whereas at a higher frequency, the charge accumulation occurs due to inadequate time for dipole orientation; hence, the dielectric constant value decreased [38]. The dielectric constant value also increases with temperature because of space-charge polarization. The dielectric constant value of GCES is higher than PP. It shows that glass-ceramics have high insulating properties compared to PP, which enhances battery safety.

Fig. 5 (a) shows the voltage stability curve of GCES and soaked PP. The GCES is stable up to 6.0 V; beyond that, the onset current is not increasing significantly, which indicates it is stable at high voltage. In contrast, the PP is stable up to 3.5 V, and the current onset increases significantly above this voltage. The main reason was due to the large pores, high surface area of glass fibers and superior interfacial compatibility of GCM with electrodes [44–48]. The total (cation and anionic) transference number for the porous separator membranes by DC polarization technique to confirm how good they conduct ions. The tion is calculated by using Eq. (5) and found to be 99% for GCES and 42% for PP, shown in Fig. 5 (b). This high value of GCES shows that electronic conductivity is minimum in glass ceramics and, ionic conductivity is predominant. While in PP, the ionic conductivity is trivial.

5

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Table. 1 Comparison table of PP Celgard and glass ceramics mat. Parameters

PP Celgard

Glass ceramic mat (GCM)

Porosity (%) Pore size (μm) Electrolyte uptake (%) Ionic conductivity (mS cm−1) Activation energy (eV) Voltage stability (V) Total transference number (%) Wettability Thermal Stability (°C)

41 ± 2 0.043 30 6.55 × 10−3 0.140 3.5 42 Not absorbing Low ~ 60

93 ± 2 1.2 758 9.22 0.051 6.0 99 Very rapid High < 600

Table. 2 The values of bulk and interfacial resistance for Cell-A and Cell-B. Time (Hour)

Cell-A Rb (Ω)

0 2 24 30 36 42 48

8.78 7.7 6.66 6.65 6.62 6.50 6.44

Cell-B Rct (Ω) 3

15 × 10 49 × 103 500 497 482 498 5448

Rb (Ω)

Rct (Ω)

129 187 124 124 128 127 232

132 × 103 102 × 103 775 622 720 623 23 × 103

electrochemistry in Cell A and Cell B with the EIS data is modelled using Zsimpwin software with electrochemical equivalent circuit element [(R (QR) (QR))]. The bulk and interfacial resistance of both cells are tabulated in Table 2. In Cell-A, the interfacial resistance of electrode-electrolyte is decreasing with storage time (shown in fig 7 (a)). This behavior confirms that with time the charge transfer process becomes faster at the electrode-electrolyte interface [49]. The interfacial resistance decreases from 15 kΩ to 391 Ω with storage time which is better than reported electrolyte system (MgTFSI-DME) ≈ 275 kΩ [50]. Hence, the GCES shows better interfacial reaction kinetics for deposition/stripping [50]. After 48h, the interfacial resistance of the Cell-A starts increasing which is attributed to the growth of the passivation layer on the Mg metal surface [49]. On the contrary, cell B shows high bulk and surface resistance (shown in Table 2) due to poor wettability and low ionic conductivity of PP membrane compare to GCES. Therefore, the GCES shows improved interfacial properties with Mg electrodes relative to PP due to high porosity and interconnected network structure which helps in faster migration of Mg ions [51,52].

3.6. Wettability and thermal stability The wettability images of both PP and GCM membrane is shown in Fig. 6 (a). By using micropipette, 30 μl of liquid electrolyte is dropped on the pristine PP and GCM membrane. It is observed that the GCM absorbed the electrolyte quickly in less than a minute, whereas in PP, the electrolyte could not wet the membrane completely. This result shows the GCM has higher wettability than the PP attributed to high porosity and pore size. The thermal shrinkage is observed by exposing the membrane at high temperatures and analyze the dimensional changes after heating. The membranes are heated up to 150 °C for 2 h in a vacuum oven, and the thermal shrinkage images of pristine PP and GCM are shown in Fig. 6 (b). The PP membrane is completely shrunk after heating due to its poor thermal stability at high temperature and low melting point. In contrast, the GCM membrane does not show any dimensional changes after exposing in high temperatures due to high thermal stability and high melting point of borosilicate structure. The high thermal stability is also confirmed by TGA analysis (discussed in above section 3.1). The high thermal stability of glass ceramics is favorable for battery safety when it is operated at a high rate of charging and discharging. Table 1 shows the comparison of PP and GCM membranes.

4. Conclusions The glass-ceramic membrane has high porosity, wettability, and high thermal stability than the PP Celgard membrane. Due to the highly porous structure, the GCM has high electrolyte uptake and retention, which leads to improving the ionic conductivity. The ionic conductivity of GCES is 9.22 mS cm−1, which is much higher than PP (6.55 × 10−3 mS cm−1). The activation energy of PP is two times higher than GCES; thus, GCES requires very less amount of energy for ion transfer. The voltage stability of GCES is more than 6.0 V, whereas the PP starts degrading at 3.5 V. The low ionic conductivity and voltage stability of PP membrane make it unsuitable for MIB application. This availability of a better understanding of different material as separator for MIB will help researchers to choose and develop better separator material for MIB application. Hence the results indicate that the GCM

3.7. Interfacial stability of separator electrolyte system The AC impedance of GCES and PP was measured to study the interfacial stability in the system. The impedance spectra of Cell-A (Mg/ GCES/Mg) and Cell-B (Mg/PP/Mg) with time are shown in Fig. 7(a) and (b). The impedance spectra of Cell-A shows a well-defined semi-circle, confirming the electrochemical equilibrium between the electrode (Mg metal) and electrolyte (GCES) [28]. The large semi-circle is a combination of two or more physical parameters like electrical double-layer capacitance and charge transfer resistance occurring at the electrodeelectrolyte interface. For better understanding, the physical

Fig. 7. Impedance spectra of (a) Cell A (Mg/GCES/Mg) (b) Cell B (Mg/PP/Mg). 6

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membrane is a promising battery separator for MIB application.

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