Preparation of LAGP/P(VDF-HFP) polymer electrolytes for Li-ion batteries

Abstract

LAGP (Li_1.5Al_0.5Ge_1.5(PO₄)₃) as a lithium ion conducting filler, was added into P(VDF-HFP) based gel polymer electrolytes to study its effects on the morphology, structure, ionic conductivity and battery performances. The composite membranes show an enormous number of micropores, and exhibit high swelling when activated with liquid electrolyte. The addition of a glass ceramic filler increases the amorphous phase of the polymer by acting as facile gel centers and enhances the ionic conductivity. The electrochemical stability window is up to 5.0 V versus Li⁺/Li. The discharge capacity of cells assembled with LAGP/P(VDF-HFP) membranes reaches 169 mA h g⁻¹ and retains over 96% of its initial capacity after 100 cycles, and its rate performance improves after the addition of LAGP. The results indicate that the LAGP/P(VDF-HFP) composite materials can be developed as excellent polymer electrolyte membrane for lithium ion batteries.

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1. Introduction

With the widespread application of lithium-ion batteries in our daily life, more and more attention has been focused on the accompanying safety demands. It is worthy to note that the safety performance of the lithium ion battery with gel polymer electrolytes (GPEs) gains much improvement due to few free-flowing liquid electrolytes, which can cause combustion and explosions in some extreme conditions such as under overcharge, abnormal heating or mechanical rupture conditions,¹ and thus intensive research has been conducted on GPEs-based batteries.

GPEs, formed by a significant amount of a liquid solution entrapped in a polymer host, are widely regarded as promising electrolytes for advanced lithium-ion rechargeable batteries owing to their mechanical flexibility and stability, good interfacial compatibility with electrodes, cost competitiveness and safety in combination with ionic conductivity comparable to that of liquid electrolytes.^2–6 Among GPEs, poly(vinylidene fluoride-co-hexefluoropropylene) (P(VDF-HFP)) is considered as a promising candidate polymer matrix material because it has a low degree of crystallinity, excellent chemical stability and plasticity as well as strong electron-withdrawing fluorine groups.^7–11

Composite polymer electrolytes based on P(VDF-HFP) with the addition of ceramic fillers such as SiO₂,^12–14 Al₂O₃,^15–17 and TiO₂,^15,18–20 can improve not only the mechanical properties, but also the ionic conductivity and interfacial compatibility between electrolyte and electrode.²¹ These inert ceramic fillers enhance the performance of P(VDF-HFP) electrolyte based on the grain boundary effect and Lewis acid–base principles.²² The conductivity of P(VDF-HFP)-based composite GPEs with different kind of fillers listed in Table 1 are smaller than 2 mS cm⁻¹. Recently, few attempts have been made to investigate the roles of the lithium ionic conductor lithium aluminum germanium phosphate glass-ceramic Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) in P(VDF-HFP)-based composite polymer electrolytes systems.

Table 1 Comparison of conductivity of P(VDF-HFP)-based composite GPEs with different kind of fillers

No.	The kind of filler	Conductivity (mS cm^−1)	Temperature (°C)	Ref.
1	SiO2	1.0	Room temperature	12
2	SiO2	1.12	25	13
3	SiO2	0.9	Room temperature	14
4	Al2O3	0.5	30	15
5	Al2O3	0.33	30	16
6	TiO2	0.26	25	18
7	TiO2	1.4	25	19
8	TiO2	0.35	30	15

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LAGP, which is the lithium analogue of NASICON type glass ceramics,²³ is considered as a potential candidate filler material, owing to its excellent lithium ionic conductivity, high mechanical strength and superior stability in contact with lithium metal.^24–30 In previous studies, LAGP was always used as the solid electrolyte layer in lithium/air,^31,32 lithium/sulfur³³ and all-solid-state lithium batteries.^34–36 Recently, Shubha and coworkers incorporate LAGP into GPEs to improve thermal and mechanical stability,³⁷ but there is no study of its effect on ionic conductivity and battery performance.

In the present work, LAGP was added into the P(VDF-HFP) polymer electrolyte, and porous membranes were prepared by using the Bellcore technique. The influences of LAGP material as an active filler on the structure and the electrochemical performance of P(VDF-HFP) composite polymer electrolyte are investigated. The morphologies and structures of the resultant electrolyte were characterized by scanning electron microscopy (SEM), thermo gravimetric analysis (TGA), and X-ray diffraction (XRD), respectively. The cell performance such as discharge capacity, discharge C-rate capability, and cyclability are studied in 4.4 V LiCoO₂/Li cells. The results indicate that LAGP can be an excellent filler to fabricate the polymer electrolyte membrane for a lithium ion battery.

2. Experimental

2.1. Materials

Poly(vinylidene difluoride-co-hexafluoropropylene) (P(VDF-HFP), average M_n ∼ 130 000) was purchased from Sigma-Aldrich. An electrolyte solution with 1.0 mol L⁻¹ LiPF₆ solution in a mixture of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and ethylene carbonate (EC) (DMC/EMC/EC, volume ratio 1 : 1 : 1) was supplied by Guotai-Huarong Chemical. N,N-Dimethylforamide (DMF), dibutyl phthalate (DBP) and ethanol were analytical purity and used as received without further treatment.

2.2. Preparation of LAGP

In this work, LAGP was synthesized by reagent grade chemicals, including Li₂CO₃, Al₂O₃, GeO₂ and NH₄H₂PO₄.³⁸ The four raw materials were weighed and mixed by the stoichiometry. To begin with, the mixed raw materials were milled in a milling machine. Then they were heated slowly to 450 °C and kept for 1.5 h to release gaseous production. Subsequently, the mixture was heated up to 1450 °C and melted for 2 h. Then the homogeneous, viscous glass melt was poured onto a flat stainless steel (SS) plate at room temperature and pressed by another steel plate to yield transparent glass sheets. The pressed glass sheets were annealed at 500 °C for 2 h to release the thermal stresses and then allowed to cool to room temperature. They were heated at 850 °C for 12 h to obtain the resultant materials with high ionic conductivity. Finally LAGP experienced high-energy ball milling for 24 h to reduce its particle size and obtain as fine powder. Then they were dried under vacuum at 80 °C for 24 h prior to use.

2.3. Preparation of LAGP/P(VDF-HFP) electrolytes

The P(VDF-HFP)-based polymer electrolyte membranes were prepared by the standard Bellcore technique in this work. DBP and DMF were used as organic plasticizer and solvent, respectively. For preparation of the composite, an appropriate amount of LAGP was added to the solvent and stirred for 48 h at room temperature. The required amount of P(VDF-HFP) (the LAGP content in the composite was kept at 5 wt% with respect to P(VDF-HFP)) was added and the mixed solution was continuously stirred at 50 °C to obtain a homogeneous casting solution. After 2 h, DBP with weight ratio of 1.5 : 1 to P(VDF-HFP), was added accordingly and dissolved in DMF. The solution was cast onto a glass plate to form the wet membrane and dried in an air oven at 40 °C. Then the dry and porous membrane was obtained by immersing it in ethanol to extract DBP. After being dried in a vacuum oven at 60 °C for 12 h. The desired polymer electrolytes were achieved by soaking the as-prepared polymer electrolyte membranes, with a thickness about 100 μm, in a 1.0 M EC/DMC/EMC liquid electrolytes solution at room temperature for 4 h in an argon-filled glove box. For comparison, the pure P(VDF-HFP) GPEs were also synthesized through the same procedure without the presence of LAGP. The weights of the active materials were measured by a precision electronic balance (AL104, Mettler Toledo).

2.4. Characterization

The microstructures of the polymer electrolytes before soaking the liquid electrolyte were observed by SEM (Hatchi S-4800). Thermal properties were evaluated by TGA at a heating rate 10 °C min⁻¹ under Ar atmosphere from room temperature to 700 °C. The XRD patterns were obtained using X-ray powder diffraction (Rigaku D/max-2500) with Cu Kα radiation in the range of 10–90° to identify the crystalline phase of the as-prepared polymer electrolyte membranes.

Liquid uptake is measured by weighting method. The dry as-prepared membranes are cut into identical round pieces and the weight is W₀. After being submerged in the electrolyte solution, the excess liquid is removed by filter papers. The polymer electrolytes are activated and then weighed as W. The liquid uptake Q is calculated using the following formula:³⁹

The ionic conductivity of the as-prepared GPEs was determined at various temperatures (27–80 °C) between two blocking SS by electrochemical impedance spectroscope (EIS) on Solartron analytical instrument (1400 cell test system), with Ac amplitude of 5 mV from 10⁵ to 0.01 Hz. The SS/GPEs/SS cells were kept at each temperature for 30 minutes prior to ionic conductivity measurement to ensure thermal equilibrium. Ionic conductivity (σ) was calculated as σ = d/(R_bA), where d and A denote the thickness and available area, respectively, and R_b is the bulk resistance obtained from the X-axis intercept of the complex AC impedance response with frequency.⁴⁰

The electrochemical stability was determined by linear sweep voltammetry (LSV) on a Princeton applied research electrochemical workstation (PARSTAT 2273) with a scanning rate of 1 mV s⁻¹ from 2.5 to 6 V at room temperature, using SS as the blocking working electrode, and lithium foil as both the counter and the reference electrode.

The charge/discharge tests of the Li/GPEs/LiCoO₂ coin cells were carried out using Land Battery Test System (Wuhan Land Electronic Co., Ltd China). The electrode formulation consisted of 85 wt% LiCoO₂ (Dongguan Shanshan Battery Materials Co., Ltd), 10 wt% carbon black, and 5 wt% PVDF. The charge–discharge cycling was conducted from 2.75 to 4.4 V at a stable testing temperature of 25 °C. The C-rate performances of LiCoO₂/Li coin cells with different electrolytes were galvanostatically measured in the voltage range of 2.75–4.4 V at various charge/discharge current densities ranging from 0.5 to 7C. The interface chemistry between the electrolyte and electrode was analyzed by EIS tests of the LiCoO₂/Li symmetrical cells after their 100^th charge–discharge cycles at room temperature in the frequency range of 10 mHz to 1000 kHz with an AC oscillation of 5 mV on the electrochemical workstation. The activation of membranes to prepare GPEs and the fabrication of test cells were carried out in an argon-filled glove box.

3. Results and discussion

3.1. Microstructures of the samples

The surface images of P(VDF-HFP) and LAGP/P(VDF-HFP) complexes are shown in Fig. 1. The two polymer membranes present both spherical grains, which are mainly due to the monomer units of P(VDF-HFP), and a porous structure enhancing the ion hopping conduction.⁴¹ The morphology of the composite sample seems to be the same as that of pristine membranes, indicating that LAGP particles are homogeneously dispersed in the polymeric matrix. What’s more, as shown in Fig. 1a, P(VDF-HFP) presents a two-phase structure, one of which is spherical and the other is yarn-like, due to the insolubility of the units of hexafluoropropylene. However, there are only spherical particles in the composite (Fig. 1b). It indicates that LAGP can increase the compatibility of the units in P(VDF-HFP) owing to its capacity of dispersing in the two constituents in the copolymer.

Fig. 1 SEM of dry P(VDF-HFP) and LAGP/P(VDF-HFP) polymer electrolyte membranes.

Compared with pure P(VDF-HFP), the composite membranes with LAGP show an enormous number of micropores, which helps with entrapping or storing a large amount of liquid solution (salt and plasticizer mixtures). Besides, the grain sizes of LAGP/P(VDF-HFP) membranes increase, probably because the addition of LAGP, the active filler, can serve as gel cores in the process of solvent evaporation. The higher porosity ratio that can be attained is attributed to the formation of larger particles. Thus more electrolytes are absorbed in the composite GPEs, and enlarge the contact area between the polymer and liquid electrolyte, which ensures that the electrolyte is well retained in the polymer membranes. This is favorable to improve the ionic conductivity and other electrochemical performance.

3.2. Crystalline and thermal properties of the samples

The XRD patterns of the polymer electrolyte membranes with or without LAGP are shown in Fig. 2. It is well known that LAGP is a crystal with perfect crystalline form.⁴² As displayed in Fig. 2, P(VDF-HFP) is a semicrystalline copolymer with characteristic diffraction peaks at 2θ = 20, 26.6 and 40°, which correspond well with the (100), (021) and (131) planes of VdF phase crystals, respectively.^43,44 The polymer electrolyte membranes with LAGP have similar diffraction patterns, in which the diffraction peaks at 2θ = 20 and 40° become broader and weaker compared with the plot of pure P(VDF-HFP) polymer electrolyte membrane arising from the inhibition of crystallization by the LAGP glass ceramic during the solidification process. The increased intensity of the peak at 26.6° demonstrates the existing of LAGP in the composite GPEs. The results suggest that adding LAGP into P(VDF-HFP) can reduce the crystallinity and increase the amorphous areas of polymer matrix, which can enhance ionic conductivity of the polymer electrolytes.

Fig. 2 XRD patterns of P(VDF-HFP) and LAGP/P(VDF-HFP) polymer electrolyte membranes.

Fig. 3 presents the TGA curves of the polymer electrolyte membranes. From this it can be derived that the thermal decomposing temperature of the pure P(VDF-HFP) polymer membrane is about 390 °C and that of the composite GPEs with LAGP is up to 405 °C. With the incorporation of LAGP, the thermal decomposing temperature increases, demonstrating that the polymer electrolyte membranes with LAGP become more thermally stable. The results indicate that the thermal stability of the GPEs can meet the practical demands of lithium ion batteries in terms of the thermal safety well.

Fig. 3 TGA curves of P(VDF-HFP) and LAGP/P(VDF-HFP) polymer electrolyte membranes.

3.3. Electrochemical properties of the samples

Fig. 4 exhibits the ionic conductivity dependence of the temperature ranging from 27 to 80 °C for polymer electrolytes. The dots are experimental data, and the two lines are their Arrehnius fitting curves. The ionic conductivity of the membrane electrolyte is highly increased over the temperature range, and is enhanced after combination with LAGP. These fitting curves appear linear so that the activation energy for ions transport E_a can be further obtained by using the Arrehnius mode [σ = σ₀ exp(−E_a/RT)], where R is gas content, σ the conductivity of polymer electrolyte, σ₀ the pre-exponential index and T the testing temperature, respectively. According to the Arrehnius equation the activation energy E_a for ions transport can be calculated from the slope of the straight line. The E_a of the pristine electrolyte is higher than that with LAGP, suggesting that the energy barrier of Li⁺ transport is lowered when P(VDF-HFP) is combined with LAGP, leading to higher ionic conductivity. The two GPEs have a room-temperature ionic conductivity higher than 10⁻³ S cm⁻¹ as shown in Table 2 which is an appropriate value for an electrolyte medium of lithium rechargeable battery.

Fig. 4 Variation of the ionic conductivity of the P(VDF-HFP) and LAGP/P(VDF-HFP) GPEs with temperature, as well as their VTF fitting curves.

Table 2 Liquid uptake and ionic conductivity of P(VDF-HFP) membranes with or without LAGP at room temperature

	P(VDF-HFP)	LAGP/P(VDF-HFP)
Liquid uptake (wt%)	197.7%	242.7%
Ionic conductivity (10^−4 S cm^−1)	12.2	20.1

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However, compared to ionic conduction in liquid electrolyte, the mechanism of ionic conduction in PVDF-HFP GPEs is obviously more complicated. In addition to the conventional ionic transport in the bulk of liquid electrolyte filling the pores of GPEs, there exists ionic mobility in the swelling gel polymer and LAGP particles based on its excellent lithium ionic conductivity, as well as grain boundary effect and Lewis acid–base principles closely connected to LAGP,^22,37 which may affect the ion transport processes by creating favorable conduction pathways through or around the two components. In the composite GPEs, lithium ion conduction mainly occurs through the entrapped liquid electrolyte and the swelling gel P(VDF-HFP). Besides, the active filler acting as a source of lithium ions thereby increasing the number of active charge carries plays a vital role. Therefore the data slightly deviate from the fitting lines.

The liquid uptake and ionic conductivity of P(VDF-HFP) and LAGP/P(VDF-HFP) are demonstrated in Table 2. The composite polymer membranes has 65% higher Li⁺ migration rate at room temperature than that of pristine P(VDF-HFP) due to a greater amount of entrapped liquid electrolyte. This is related to the fact that LAGP dispersing in the P(VDF-HFP) matrix creates favorable lithium ion conduction pathways in the vicinity of the particles. The conductivity of the composite membrane with LAGP is 2.01 mS cm⁻¹, larger than that with the addition of inert fillers (Table 1). This is the same as in the above discussion.

One of the important parameters in the characterization of P(VDF-HFP) based GPEs is the electrochemical stability window. As shown in Fig. 5, there is no obvious current through the working electrode from 2.5 to 4.5 V versus Li⁺/Li for the liquid electrolyte and pristine P(VDF-HFP) GPEs, which may be associated with the decomposition of the electrolytes, while for the composite GPEs a onset current can be observed at the potential higher than 5 V versus Li⁺/Li. The result shows that the electrochemical stability window is at least 4.5 V for the PVDF-HFP GPEs, and LAGP improve the electrochemical stability of P(VDF-HFP) electrolytes. This is the same as the report that LAGP particles can act as a surface stabilizer and enhance the electrochemical oxidation stability of the composite polymer electrolytes.^37,45,46 The stability of the GPEs renders them suitable for use in a lithium ion polymer battery.

Fig. 5 LSV curves of P(VDF-HFP) and LAGP/P(VDF-HFP) polymer electrolyte membranes.

Fig. 6 compares the cyclability of the LiCoO₂/Li cells with pristine PE separator and GPEs separators under a voltage range between 2.75 and 4.4 V at a constant charge/discharge current density (0.5C/0.5C). The batteries with liquid electrolyte present serious capacity fading especially after 20 cycles which may be due to the progressive decomposition and leakage of the electrolyte.³⁷ An intriguing discovery is that the P(VDF-HFP) based GPEs show more favorable discharge capacity up to 100 cycles than that of the pristine PE separator, especially after combination with LAGP, the cycling stability appears to be higher than that of P(VDF-HFP). Besides, discharge capacities of the GPEs both demonstrate slight decay during the first few cycles, but keep almost constant afterward. This is due to the fact that GPEs have superior ability to retain liquid electrolytes than a hydrophobic commercial PE separator, and thus helps to prevent leakage of liquid electrolyte during cycling, resulting in improved cyclability.⁴⁷ What’s more, the addition of LAGP into P(VDF-HFP) can hinder the reaction of LiCoO₂ with electrolytes greatly, and thus show better cycling stability than pristine P(VDF-HFP).⁴⁸

Fig. 6 (a) Comparison of the cycle performance of cells assembled with three different electrolytes. Discharge profiles of cells assembled with: (b) liquid electrolytes, (c) P(VDF-HFP) separator, (d) LAGP/P(VDF-HFP) composite separator at a constant charge current density of 0.5C.

Discharge profiles of cells with three different electrolytes are shown in Fig. 6b–d, respectively. During the first cycle, all present a typical voltage plateau. The batteries with LAGP/P(VDF-HFP) GPEs demonstrate discharge capacity of 169 mA h g⁻¹. After 100 cycles, the capacity retention is found to be 55% for liquid electrolyte, 92% for P(VDF-HFP) GPEs and 96% for LAGP/P(VDF-HFP) GPEs. Possibly because LAGP fillers are able to retain a large amount of electrolyte and, therefore, achieve higher capacity even after 100 cycles.

The C-rate discharge performance of LiCoO₂/Li cells assembled with three different electrolytes are evaluated, where the cells are charged under a voltage range of 2.75–4.4 V at various charge and discharge current densities ranging from 0.5 to 7C. As shown in Fig. 7, the difference in the discharge C-rate capacities among the three separators become more pronounced at the higher discharge currents. It is obvious that the C-rate discharge performance of cells with P(VDF-HFP) GPEs is improved after addition of LAGP. The higher ionic conductivity of polymer electrolytes and better electrolyte wettability suggest an improved rate performance for battery.⁴⁹ In brief, the cell with the LAGP/P(VDF-HFP) composite electrolyte demonstrates the optimal electrochemical performance among the three cells.

Fig. 7 The C-rate discharge performance of cells assembled with three different electrolytes.

In order to compare the LiCoO₂/Li cells with liquid electrolyte and LAGP/P(VDF-HFP) composite membranes directly. The AC impedance spectra of cells before and after 100 cycles were analyzed in this study. Fig. 8a shows that the cell impedance of the battery assembled with LAGP/P(VDF-HFP) GPEs is almost the same as, yet a little better than that of pristine polyethylene electrolyte, with a charge transfer resistance of 200 ohm. However, as shown in Fig. 8b, after 100 cycles two overlapped semicircles are presented, in which the high and low frequency semicircle is caused by solid electrolyte interface impedance (R_SEI) and charge-transfer impedance (R_ct), respectively.⁵⁰ This is consistent with reports from Cao.⁵¹ The ohmic resistance, which can be estimated by the intercept of the high frequency semicircle with real axis, is 15 and 5 ohm for liquid electrolyte and LAGP/P(VDF-HFP) composite membranes. Compared to that of pristine polyethylene membranes, R_SEI of LAGP/P(VDF-HFP) GPEs is much smaller. However, the R_ct increases possibly due to thickness of composite GPEs. The control of GPEs membranes by optimizing the thickness of GPEs to improve cell performance will be definitely one of our major research interests in future studies.

Fig. 8 AC impedance spectra of LiCoO₂/Li cells assembled with polyethylene and LAGP/P(VDF-HFP) polymer electrolytes, measured before (a) and after (b) 100 cycles. The insets are the corresponding details with enlarged scale.

4. Conclusions

The micro-porous composite polymer electrolyte membranes with the addition of LAGP into P(VdF-HFP) has high ionic conductivity of 20.1 × 10⁻⁴ S cm⁻¹ at room temperature and its corresponding electrolyte uptake was about 242.7%. The electrochemical stability window of LAGP/P(VdF-HFP) based GPEs is about 5.0 V versus Li⁺/Li. The active glass ceramic filler can improve the porosity, the amorphous portion and the thermal stability. The cells assembled with LAGP/P(VDF-HFP) membranes exhibit a discharge capacity of 169 mA h g⁻¹ and capacity retention with over 96% after 100 cycles, and its rate performance improve after the addition of LAGP. To conclude, the LAGP/P(VdF-HFP) composite polymer membranes can be used as a promising candidate for lithium-ion polymer batteries.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra09837h

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