High thermal and electrochemical stability of PVDF-graft-PAN copolymer hybrid PEO membrane for safety reinforced lithium-ion battery

Abstract

A polyvinylidene fluoride-graft-polyacrylonitrile (PVDF-g-PAN) copolymer was prepared by ozone polymerization and characterized by ¹H-NMR. The as-prepared copolymer is a hybrid with polyethylene oxide (PEO), named m-PVDF, and was applied as a conductive gel–polymer electrolyte for lithium-ion batteries. According to morphology analysis, the m-PVDF membrane has less microphase separation than the PVDF blending with the PAN and PEO system (b-PVDF) which means that the PVDF-g-PAN copolymer can increase the compatibility of PVDF and PEO polymers. From the DSC analysis, introduction of PVDF-g-PAN effectively decreases the crystallinity of the PEO polymer in the m-PVDF membrane, which assists in lithium-ion transport. Moreover, m-PVDF shows high thermal stability up to 400 °C and good dimensional-stability under 150 °C, which can prevent the batteries from short-cutting and burning as well as other safety problems at high temperature. For battery application, the membrane shows good electrochemical stability up to 5 V (vs. Li/Li⁺). Furthermore, cells incorporating the m-PVDF membrane demonstrated remarkably excellent capacity retention with a capacity decay of only 9.1% after 300 cycles. Accordingly, these results suggest that the introduced PVDF-g-PAN significantly improved the electrolyte compatibility, thermal properties and wettability of the membrane, yielding a high-performance and high-safety electrolyte.

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1. Introduction

Lithium-ion batteries (LIBs) are widely used in today's digital products. Except for high working voltages, LIBs have high energy density on either weight or volume.^1–5 Nevertheless, there are safety concerns for LIBs because of the flammable liquid electrolyte inside. If the batteries are operating at high temperature or if the electrolyte leaks, a fire or explosion will likely occur. To mitigate this safety concern, gel electrolytes prepared with a polymer that supports ion transport are been studied and developed.^6–12 Gel polymer electrolytes trap the liquid electrolyte in the polymer network, so liquid electrolyte leaks can be reduced to avoid the potential safety risks.^13–15 The mostly commonly used polymer for gel–polymer electrolytes is polyethylene oxide (PEO), because it assists in lithium ion transport.^16–20 Moreover, in reducing lithium-ion transport resistance, the performance of LIBs can be improved. However, PEO also has concerns namely high crystallinity, poor chemical stability and solving in the liquid electrolytes.

In order to overcome these three problems, we introduce polyvinylidene fluoride (PVDF), which has good mechanical strength and chemical stability.^21–27 In addition, we also incorporate polyacrylonitrile (PAN) to improve compatibility between PVDF and PEO polymers. In addition, PAN-based electrolytes have attractive characteristics, such as high ionic conductivity, good thermal stability, superior morphology for electrolyte uptake, and compatibility with lithium electrodes.^28,29 Accordingly, we hypothesize that polymer electrolytes prepared from the composites of PVDF and PAN may have the synergistic advantages of both polymers. However, preparing a miscible blend of PVDF and PAN is problematic thereby restricting their combined use for electrolyte applications.

In 2002, E. T. Kang et al. reported ozone polymerization to synthesize a PVDF-based graft copolymer.^30–32 In this article, we present the synthesis of PVDF-graft-PAN copolymer (PVDF-g-PAN) by ozone polymerization and its blending with PEO polymer to form modified-PVDF (m-PVDF), which is then applied as the gel–polymer electrolyte in an LIB. The incorporated PVDF-g-PAN improves electrolyte compatibility, thermal properties and membrane wettability, which leads to a high-performance and high-safety electrolyte. Moreover, results show that the as prepared m-PVDF membrane features a stable electrochemical window and good thermal stability. More specifically, in the cycling stability test, cells with the m-PVDF membrane exhibited up to 91% capacity retention after 300 cycles at 0.5C.

2. Experimental

2.1 Materials

Acrylonitrile (99%), N-methyl-2-pyrrolidone (anhydride, NMP) and γ-butyrolactone (99%) solution were purchased from Acros. PVDF (MW ∼ 534 000), PEO (MW ∼ 400 000) and PAN (MW ∼ 150 000) powder were purchased from Aldrich. Polypropylene (PP) separator (Celgard 2400) was acquired from Celgard company.

2.2 Ozone treatment/graft copolymerization of PVDF with PAN: the PVDF-g-PAN copolymer

The graft copolymers of PVDF and PAN were prepared by ozone treatment and radical polymerization as follows. PVDF was first dissolved in NMP, after which a continuous stream of O₂–O₃ mixture was bubbled through the solution at room temperature. Subsequent to the ozone treatment, the polymer solution was purged with N_2(g) for 2 h and placed in a vacuum chamber to remove residual O₂–O₃. The polymer solution was then added to acrylonitrile and NMP. After adding acrylonitrile, polymerization took place at 70 °C for 24 h. Then, the reactor flask was cooled and the PVDF-g-PAN copolymer precipitated in excess water/ethanol solution. After filtration, the PVDF-g-PAN was re-dissolved in γ-butyrolactone to remove residual PAN homo-polymers, and again subjected to filtration and then drying. The entire processes of the ozone treatment and thermal graft copolymerization of PVDF with PAN is shown schematically in Scheme 1.

Scheme 1 Schematic representation of the synthesis procedures for PVDF-g-PAN copolymer, which are described in more details in the Experimental section.

2.3 Preparation of m-PVDF and b-PVDF membranes

To prepare the m-PVDF membrane, the PVDF-g-PAN copolymers were mixed with PEO polymer under a copolymer/EO weight ratio of 1 : 1. Then, the mixture was cured at 80 °C for 12 h to obtain the membranes. For comparison, a simple of directly blended PVDF, PAN, and PEO, named b-PVDF, was prepared according to the same weight ratio as m-PVDF. The thicknesses of the m-PVDF and b-PVDF membranes are 70 ± 10 μm.

2.4 Methods of characterization

Fourier transform infrared spectroscopy (FT-IR) was recorded with a Nicolet Magna II 550 spectrometer. The ¹H-NMR spectra of the monomers and polymers were recorded on a Bruker AMX500 spectrometer using tetramethylsilane (TMS) as an internal standard in CDCl₃. The morphology of the membrane was investigated using field emission scanning electron microscopy (FE-SEM, JEOL, JSM-6380LV). Further, a weight loss temperature value was determined by thermogravimetric analysis (TGA7, Perkin Elmer) at a heating rate of 20 °C min⁻¹ under a nitrogen atmosphere. Differential scanning calorimetry (DSC) measurements were carried out using a TA Instruments Q100 DSC. Under the curve of a thermal event gives its enthalpy. Direct integration will give this enthalpy as energy per sample mass (joule per gram, J g⁻¹). In the solution uptake test, the original weight of the polymer electrolyte was fixed at 100%, then, after soaking in the liquid electrolyte for 12 hours to ensure the maxima uptake, the weight was recorded again.

2.5 Electrochemical measurements

For the battery test, the membranes were soaked in organic electrolyte solution, 1.0 M LiPF₆ in ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate (DEC) (1 : 1 : 1 in volume, from Formosa Plastics Corporation) over 12 hours in an argon-filled glove box to obtain the gel–polymer electrolyte. The electrochemical stability window of the gel–polymer electrolytes were determined by linear sweep voltammetry (LSV) using a stainless steel working electrode and lithium foil as the counter electrode at a scanning rate of 5 mV s⁻¹. The cathodes of the test cells were prepared by slurry coating a mixture of 80 wt% LiFePO₄/C powder (Aleees, Advanced Lithium Electrochemistry Co. Ltd., Taiwan), 10 wt% Super P (TIMCAL), 10 wt% PVDF (Solvay) and NMP onto high purity aluminum foil which was then dried at 100 °C for 24 h in vacuum and then pressed. The anode used was high purity lithium metal. The test cells were assembled in a dry, oxygen free glove box. Finally, the charge–discharge testing was performed galvanostatically between 2.5 and 4.2 V at room temperature on a Battery Automation Test System (Acu Tech Systems, BAT-750B).

3. Results and discussions

3.1 Preparation and characterization of PVDF-g-PAN copolymer, m-PVDF and b-PVDF membrane

The PVDF-g-PAN was fabricated by ozone polymerization according to the method described in E. T. Kang and shown in Scheme 1. The graft copolymers of PVDF and PAN were prepared as aforementioned in Section 2.1. The PVDF-g-PAN was characterized by FT-IR and ¹H-NMR. Fig. 1 shows the FT-IR spectrum of PVDF and PVDF-g-PAN copolymer. The existence of absorption peaks at 2245 cm⁻¹ is attributed to –CN groups. Fig. 2 shows the ¹H-NMR spectra of PVDF-g-PAN. As can be seen, the chemical shift values at 2.08 (–CH₂) and 3.15 ppm (–CH) for PAN and 2.9 ppm (–CH₂) for PVDF confirm the existence of the PVDF-g-PAN copolymer.

Fig. 1 FT-IR spectra for PVDF and PVDF-g-PAN copolymers.

Fig. 2 ¹H-NMR spectra for PVDF-g-PAN copolymer.

For morphology analysis, the FE-SEM images of the m-PVDF and b-PVDF membranes are presented in Fig. 3. Both the m-PVDF (Fig. 3a and b) and b-PVDF (Fig. 3c and d) membranes have particles with a diameter of about 2 μm spreading on the surface, which indicates the microphase separation in the membrane. Coupled with the EDS result, the F element has a relatively high concentration in the non-particle region. This implies that the non-particle region is comprised mainly of PVDF and PEO, while the particle region is primarily constitutes PEO and PAN. As seen, the m-PVDF membrane consists of relatively few particles on the surface, which means a less phase-separated microstructure. Therefore, the compatibility of PVDF and PEO polymers was successfully increased by introducing the PVDF-g-PAN copolymer. The DSC results are provided to further discuss (Section 3.2) the phase-mixing of the m-PVDF membrane.

Fig. 3 SEM images of (a and b) m-PVDF and (c and d) b-PVDF membranes.

3.2 Thermal properties analysis

Crystallinity is a key polymer–electrolyte factor influencing LIB performance. If a high proportion of amorphous morphology exists in the polymer electrolyte, better performance will likely result. Analysis of the DSC curves shown in Fig. 4 reveals that both the m-PVDF and b-PVDF membranes feature a sharp endothermic peak at 60 °C corresponding to the crystalline peak of PEO. In addition, the activated energy for the PEO, m-PVDF and b-PVDF membranes were 137.2, 55.2 and 95.7 J g⁻¹, respectively. The m-PVDF shows the lowest activated energy, which indicates that m-PVDF is amorphous with the least amount of PEO crystallinity. The amorphous region can vibration chain with a relatively lower energy, which supports ion transport.

Fig. 4 DSC curves for m-PVDF, b-PVDF and PEO polymers.

The TGA curves in Fig. 5 show that the as-prepared m-PVDF membrane undergoes a decomposition weight loss in a single step up to 400 °C. Thus, the PVDF skeleton of the m-PVDF membrane leads to an electrolyte with higher thermal stability. Furthermore, the dimensional stability of the separators and GPE membranes at high temperature is an important factor for battery safety. Accordingly, shrinkage of the separator was evaluated during heating at temperatures up to 150 °C. To this end, samples were stored in an oven for 30 minutes at 150 °C, after which their dimensional change was noted. Fig. 6 show the image of the m-PVDF, b-PVDF and commercial PE/PP membrane before (Fig. 6a–c) and after (Fig. 6d–f) the dimensional stability test, respectively. As can be seen, the dimensional change of m-PVDF (Fig. 6d) and b-PVDF (Fig. 6e) membranes were negligible; in contrast, the PE/PP separator (Fig. 6f) curled and shrank when the temperatures reached 150 °C, because the PE melted at around 120 °C. Hence, the PE/PP encounters significant dimensional reduction when temperatures exceed 120 °C. However, the m-PVDF membrane exhibited superior thermal stability, which is attributed to the high melting temperature of the PVDF-g-PAN skeleton, and so is ideal for LIB safety.

Fig. 5 Thermograms for m-PVDF and b-PVDF membranes.

Fig. 6 Images of dimensional test before (a) m-PVDF, (b) b-PVDF and (c) commercial PP separator and after heating (d) m-PVDF, (e) b-PVDF and (f) commercial PP/PE separator.

3.3 Wettability and electrolyte uptake of m-PVDF membranes

The wettability and electrolyte uptake of the membranes indicate an attraction between the polymer and liquid electrolyte, which is another important factor of LIB performance. The wettability of the membranes can be characterized by measuring their contact angles, as shown in Fig. 7. During the dynamic process, the contact angle of the m-PVDF membrane decreased more rapidly than that of the b-PVDF membrane, which means that the impregnation properties of the m-PVDF membrane were greatly improved compared to the b-PVDF membrane. Moreover, the electrolyte contact angles on the m-PVDF and b-PVDF membranes were 18.7° and 54.5°, respectively and since the m-PVDF membrane's contact angle is small, it has better attraction to liquid electrolyte than the b-PVDF membrane. Furthermore, the high electrolyte uptake of 223 wt% was observed for m-PVDF membrane, as compared to b-PVDF membrane at 129 wt%. This indicates that the ionic conductivity of the m-PVDF membrane, which is mainly dependent on the amount of organic electrolyte, is higher than the b-PVDF.

Fig. 7 Contact angle test for (a) m-PVDF and (b) b-PVDF membranes.

3.4 Electrochemical stability, specific conductivity and long-term stability

The electrochemical stability window of the m-PVDF membranes was determined with Li/polymer electrolyte/SS cells by linear sweep voltammetry in the range of 3–6.0 V at 25 °C (Fig. 8). The electrochemical stability window of the neat PAN electrolytes membrane is around 4.0 V. However, with the introduction of the PVDF-g-PAN polymer into the PEO matrix, the stability window of the electrolyte membrane was significantly increased to 5.5 V. Compared to the neat PAN electrolyte, the composite electrolytes presented much improved electrochemical stability, which may be ascribed to the utilization of PVDF-g-PAN as the polymer backbone in the composite membranes.

Fig. 8 Linear sweep voltammograms of m-PVDF and b-PVDF membranes at a scan rate of 1 mV s⁻¹.

The charge–discharge curves of the coin cells incorporating the m-PVDF and b-PVDF membranes are shown in Fig. 9a and b, respectively, while their specific discharge capacities are summarized in Table 1. As can be seen, the respective specific capacities of the coin cells with the m-PVDF and b-PVDF membranes are 147 and 156 mA h g⁻¹ at 0.1C, while at 5C, they are 114 and 105 mA h g⁻¹. In an LiFePO₄ half-cell system, and under the charge–discharge procedure, the curve has a flat platform due to the Fe²⁺/Fe³⁺ oxidation–reduction reaction however, at higher charge–discharge rates the discharge curve decreases earlier due to the poor conductivity of the material caused by polarization. These results demonstrate that cells incorporating the m-PVDF membranes have good performance at 5C, which suggests the high compatibility of the PEO–polymer hybrid PVDF-g-PAN polymer of the m-PVDF membrane leads to an electrolyte with uniform structure, thereby improving the rate performance. Moreover, the most beneficial feature of the cell incorporating the m-PVDF membrane is that the potential drop related to high C rates is insignificant compared with the b-PVDF-membrane cell, indicating that higher energy densities can be obtained (Table 2).

Fig. 9 Rate performance test for (a) m-PVDF and (b) b-PVDF cells; cycle stability test for (c) m-PVDF and (d) b-PVDF cells.

Table 1 Capacity value and retention from 0.1C to 5C and retention of the cell made by m-PVDF and b-PVDF membranes

Samples	0.1C	0.5C	1C	3C	5C	Capacity retention (5C/0.1C)
m-PVDF	147	144	139	128	114	77.6%
b-PVDF	156	149	147	128	105	67.3%

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Table 2 Capacity retention of the cell made by m-PVDF and b-PVDF membranes for cycling test under 0.5C charge–discharge rate

Sample	Capacity at 3^rd discharge	Capacity at 300^th discharge	Capacity retention (%)
m-PVDF	132	120	91%
b-PVDF	139	80	57%

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For the long term stability test, the cells made with m-PVDF and b-PVDF membranes were tested under charge and discharge at 0.5C for 300 charge–discharge cycles, as shown in Fig. 9c and d, respectively. The m-PVDF-membrane cell had an initial discharge capacity of 132 mA h g⁻¹, and after 300 charge–discharge cycles, a capacitance value of 120 mA h g⁻¹ remained. This means that the capacity retention reached 91%, indicating a very low level of capacitance-values degradation. By comparison, there was a significant unstable charge and discharge current in the b-PVDF LIB when the charge–discharge cycles reached 150. Further, the capacity decay of the b-PVDF-membrane cell was up to 59% after 300 cycles. Overall, these results indicate that the cell featuring m-PVDF membrane had excellent reversible charge–discharge cycle performance and battery stability.

4. Conclusions

In this article, a high-safety m-PVDF membrane was fabricated and characterized. This membrane has high thermal stability, dimensional stability (under 150 °C test) and excellent electrochemical stability up to 5.0 V. The introduction of PVDF-g-PAN greatly increased the amorphous-region proportion in the PEO gel–polymer electrolyte, which can support ion transport in the LIBs and electrolyte uptake. The cells tested exhibited an outstanding capacity retention of 91% under 0.5C charge/discharge for up to 300 cycles, suggesting that the novel composite electrolytes offer excellent properties, promising great potential for safe and extended cycle-life LIBs.

Acknowledgements

The authors would like to thank the Ministry of Science and Technology, Taipei, R. O. C. for their generous financial support of this research.

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