High thermal and electrochemical stability of a SiO₂ nanoparticle hybird–polyether cross-linked membrane for safety reinforced lithium-ion batteries

Abstract

A phenolic-resin cross-linked polyoxyethylene (PEO) network (named NX) was synthesized to simultaneously act as both the separator and gel-polymer electrolyte in a lithium ion battery (LIB). To improve the rate performance, as well as the thermal and electrochemical stability, SiO₂ nanoparticles with a diameter of 40 nm were hybridized with the above polymer (named NX-S). SEM images confirm that the surfaces of both the NX and NX-S membranes are nonporous, as compared to the porous surfaces of commercial separators. In addition, the hybrid composite has higher thermal and electrochemical stability up to 400 °C and 5 V, respectively, and higher electrolyte compatibility than the pristine NX polymer. For battery application, a high-rate performance test demonstrates that the specific half-cell capacities of the NX-S cell composed of the NX-S membrane are all higher than those of the aforementioned NX separator. Moreover, the NX-S membrane fabrication process is very simple and low cost.

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1. Introduction

Recently, high-performance and high-safety lithium-ion batteries (LIBs) have been considered an attractive power source for a wide variety of applications, such as energy storage systems and electric vehicles.^1–4 However, large capacity batteries require higher safety standards due to the existence of highly flammable organic liquid electrolytes, which increase the possibility of explosions.^5–9 Gel-polymer electrolytes, which feature the characteristics of both solid and liquid electrolytes, are receiving increased attention due to their wide electrochemical window, high ionic conductivity, good electrode compatibility and high thermal stability.^10–16 Nevertheless, the application of gel-polymer electrolytes in large-scale systems is currently limited because of low stability, poor mechanical strength and high cost.^17–19

Polyoxyethylene (PEO) is the most commonly used host polymer for preparing gel-polymer electrolytes, however, the organic solvents plasticize the resulting polymers, causing loss of mechanical strength. Thus, the gel-polymer electrolytes are always coated on polypropylene (PP) separators as supporters to retain mechanical strength. One shortcoming, however, is that this porous PP film only provides holes to adopt electrolyte solution, but cannot adsorb the electrolyte solution to transfer Li-ions. One solution is to use phenolic resin, which has been used as an alternative polymer composite due to its superior electrical and mechanical properties and low cost.^20–23

In this work, we prepared a phenolic-resin cross-linked-PEO polymer, denoted as NX, to function as a separator. This composite can adsorb, uptake and retain a great amount of electrolyte solution to transfer lithium ions through the PEO segment. Subsequently, we added SiO₂ nanoparticles, thereby creating the NX-S membrane, to improve the rate performance, as well as the thermal and electrochemical stability. Results show that prepared NX-S features high ionic conductivity, good mechanical properties, a stable electrochemical window and good thermal stability. Unlike typical liquid electrolytes, which are impossible to assemble without a conventional separator in a cell, the NX-S membrane can be conveniently assembled without a separator in an LIB.

2. Experimental

2.1 Preparation of SiO₂ nanoparticles and the NX and NX-S membranes

Tetraethyloxysilane (TEOS, 9.18 g) was added to 150 ml of ethanol and 25 ml of distilled water with 9.8 g of ammonium hydroxide solution (28 wt%) under boiling temperature. The reaction was allowed to continue for 4 h, after which the resulting precipitate was centrifuged and washed three times with ethanol.

Polyetheramine (JEFFAMINE® ED 2003, MW ∼ 2000, 39 EO repeat segments) and phenolic epoxy resin (Nanya plastic) were well-stirred at 80 °C for 48 h to prepare the NX membrane. For the NX-S samples, the SiO₂ nanoparticles, polyetheramine and phenolic epoxy resin were mixed under SiO₂ content ratios of 2%, 5% 10%, 15%, and 20%, which denoted as NX-2S, NX-5S, NX-10S, NX-15S and NX-20S, respectively. Then, the mixtures were cured at 80 °C for 24 h to form the membranes. Finally, the films were soaked in organic electrolyte solution (1.0 M LiPF₆ in EC/DEC (1 : 1 in wt%), from UBIQ TECHNOLOGY CO., LTD) over 12 h in an argon-filled glove box to produce to the gel-polymer electrolyte for further measurement.

2.2 Methods of characterization

FT-IR spectra were obtained with a Nicolet Magna II 550 spectrometer. Differential scanning calorimetry (DSC) measurements carried out using a TA Instruments Q100 DSC. Further, transmission electron microscopy (TEM) was conducted using a Hitachi-H7500 with an accelerating voltage of 80 kV, while membrane morphology was investigated using field emission scanning electron microscopy (FE-SEM, JEOL, JSM-6380LV). Weight loss temperature values were determined by thermogravimetric analysis (TGA7, Perkin Elmer) at a heating rate of 20 °C min⁻¹ under a nitrogen atmosphere.

2.3 Electrochemical measurements

The ionic conductivities of the gel-polymer electrolytes were determined by electrochemical impedance spectroscopy on an electrochemical instrument (CHI604A, CH Instrument, Inc.) using an alternative current signal with a potential amplitude of 10 mV at frequencies from 100 kHz to 10 Hz. In the determination of the ionic conductivities, the gel-polymer electrolytes were sandwiched between the two parallel stainless steel discs (diameter Φ = 16 mm) of coin cells. Moreover, ionic conductivities were calculated from the bulk electrolyte resistance (R) according to:

σ = l/RS

where l is the thickness of the membrane and S is the contact area between the membrane and stainless steel discs. The bulk electrolyte resistance (R) was obtained from the complex impedance diagram. The electrochemical stability windows of the gel-polymer electrolytes were determined by linear sweep voltammetry 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, 10 wt% polyvinylidene fluoride (PVDF) and N-methyl-2-pyrrolidone (NMP) onto high purity aluminum foil, which was subsequently dried at 100 °C for 24 h under vacuum and then pressed. The anode used was high-purity lithium metal. The resulting test cells were assembled in a dry, oxygen free glove box. Finally, 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 NX polyelectrolyte

As aforementioned, the NX membrane was fabricated by cross-linking polyetheramine and phenolic epoxy resin. Fig. 1a–c show the FT-IR spectra of the (a) polyetheramine, (b) phenolic epoxy resin, and (c) NX membrane, respectively. The peak at 2850 cm⁻¹ was due to the CH₂ stretching vibration of the EO chain of polyetheramine and, the bands at 1450 and 1349 cm⁻¹ were due to the CH₂ scissoring and wagging vibration of the as-prepared polymer. The strong peaks at 1100 and 1500 cm⁻¹ respectively correspond to the (C–O–C) stretching vibration of polyetheramine and (C=C stretching vibration) phenolic epoxy resin. Further, from the DSC curve shown in Fig. 2, the T_m of the –CH₂CH₂O– segment of polyetheramine is at about 35 °C, however, there is no T_m peak for phenolic resin epoxy. After polyetheramine was cross-linked with phenolic epoxy resin, no T_m peak for the NX membrane existed. Accordingly, this result confirms that NX copolymer was successfully prepared.

Fig. 1 FT-IR curves for (a) polyetherdiamine, (b) phenolic epoxy resin, and (c) the as-prepared NX membrane.

Fig. 2 DSC thermograms of (a) polyetherdiamine, (b) phenolic epoxy resin, and (c) the as-prepared NX membrane.

3.2 Morphology and corresponding electrolyte uptake and retention

Here, we added the SiO₂ nanoparticles to improve the thermal and electrochemical stability. Fig. 3a shows the TEM images of the SiO₂ nanoparticles, which have an average particle size of 40 nm. For the NX-S samples, the SiO₂ nanoparticles, polyetheramine and phenolic epoxy resin were as well-mixed and then cross-linked at high temperature. This ensured that all components were well-dispersed in the membrane, including up to 20% SiO₂ nanoparticle addition, as shown in Fig. 3b. The images presented in Fig. 4 display the SEM images for the cross-sections of NX and all varieties of the NX-S membranes. It can be seen that the surfaces of all the NX-S membranes are nonporous, which is a highly desirable characteristic since electrolyte retention is crucial to the safety of LIBs. In comparison, the surface of the commercial separator has a pore diameter ranging from 50 to 200 nm.²⁴ This property results in a comparable electrolyte uptake as the commercial separator (200 wt%), moreover, the uptake amount of the NX, NX-2S, NX-5S, NX-10S, NX-15S and NX-20S membranes can respectively reach 180, 145, 144, 148, 142 and 144 wt%, respectively. Furthermore, the SiO₂ nanoparticles were well-dispersed without agglomeration in all of the NX-S membranes.

Fig. 3 (a) TEM and (b) SEM images of the SiO₂ nanoparticles and (c) photograph of the NX-20S membrane.

Fig. 4 SEM images of the cross-sections of the NX and NX-S membranes.

3.3 Thermal and electrochemical stability of the membranes, and compatibility between the NX-S membranes and electrode

Thermal stability is a critical property for electrolytes comprising polymers and lithium salts for application in LIBs. After adding the SiO₂ nanoparticles in the cross-linked polyetheramine and phenolic epoxy resin, the TGA curves in Fig. 5 show that the as-prepared NX, NX-2S, NX-5S, NX-10S, NX-15S and NX-20S membranes had a 5 wt% loss at 366, 379, 391, 397, 401 and 404 °C, respectively. Thus, it appears that incorporating SiO₂ nanoparticles into the cross-linked polymer network leads to an electrolyte with higher thermal stability. Moreover, the process of preparing the NX-S membranes is very simple and low cost.

Fig. 5 Thermal stability tests for the NX and NX-S membranes.

As aforementioned, dimensional stability of the separators and GPE membranes at high temperature is an important factor for battery safety. To verify this requirement, shrinkage of the separator was evaluated after heating at temperatures up to 150 °C. Samples were stored in an oven for 30 min at a set temperature, after which their dimensional change was noted. Fig. 6a–c show the dimensional change of the commercial PE/PP, NX and NX-S membranes under RT, 150 °C, and 250 °C, respectively. As can be seen in Fig. 6b, the dimensional change of the NX and NX-S membranes were negligible during the test; in contrast, the PE/PP separator ripped and disintegrated at the temperatures of 150 °C for 30 min because the PE melted at around 120 °C. Hence, the PE/PP separator encounters significant dimensional reduction when temperatures exceed 120 °C. Even after being subjected to a temperature of 250 °C for 30 min, the NX and NX-S membranes still had high-dimensional stability, as presented in Fig. 6c. Moreover, the improved thermal stability of the NX and NX-S membranes is attributed to the high melting temperature of the NX skeleton, which is ideal for LIB safety.

Fig. 6 Dimensional stability tests for the NX, NX-S and PP membranes (a) room temperature, (b) after 150 °C for 30 min, and (c) after 250 °C for 30 min.

The electrochemical stability of the energy-storage device provides essential information for assessing success. Voltage stability was tested by linear sweep voltammetry (LSV) under a scanning range between 2.0 and 6.0 V as, shown in Fig. 7. The LSV curve of the NX membrane indicates it had an onset oxidation voltage at 4.5 V, while the NX-2S–NX-20S membranes well all above 5.0 V. These results indicate an increase in the stability of the NX-S membranes due to the SiO₂ nanoparticles enhancing chemically stable structures via high thermal stability. These results demonstrate that the NX-S gel electrolyte has good oxidation stability in the 2.0–5.0 V operating voltage environment, which is an important issue for battery performance.

Fig. 7 Electrochemical oxidation limits of the NX and NX-S membranes.

The wettabilities of the membranes illustrate the attraction between the polymer and liquid electrolyte, which is an important factor of LIB performance. The wettabilities of the membranes can be characterized by measuring the contact angle. During the dynamic process, the contact angles of the NX-S membranes decreased more rapidly than that of the NX membrane, which means that the impregnation properties of the NX-S membranes are greatly improved compared to the NX membrane. In addition, the electrolyte contact angles of the NX, NX-2S, NX-5S, NX-10S, NX-15S, and NX-20S membranes had values of 32, 26, 25, 26, 23, and 25°, respectively. Accordingly, all NX-S membranes show smaller contact angles than that of the NX membrane, indicating their superior attraction to liquid electrolyte.

3.4 Cell performance

The charge–discharge curves of coin cells incorporating with the NX, NX-2S, NX-5S, NX-10S, NX-15S, and NX-20S membranes are shown in Fig. 8a–f, respectively. Accordingly, the cell-specific capacities of the NX, NX-2S, NX-5S, NX-10S, NX-15S, and NX-20S membrane-integrated coin cells are 151, 155, 164, 163, 154 and 152 mA h g⁻¹, respectively, at 0.1C, while at 5C, they are 41, 62, 64, 72, 59, and 42 mA h g⁻¹. In the LiFePO₄ half-cell system, under the charge–discharge procedure, the curve can be seen as having a flat plateau due to the Fe²⁺/Fe³⁺ oxidation–reduction reaction. However, at a higher charge–discharge rate, the discharge curve starts to decrease earlier because of the material's poor conductivity caused by the polarization. Further, the capacity retentions of 0.1C/5C for the NX, NX-2S, NX-5S, NX-10S, NX-15S, and NX-20S membranes are 27, 40, 42, 47, 32, and 28%, respectively, as shown in Table 1. These results show that the NX cells without SiO₂ nanoparticles had the worst performance at 5C, which suggests that the existence of SiO₂ in the cross-linked polymer networks of the NX-S membranes improved the rate performance. Moreover, the capacity of cells composed with NX-10S was the highest, especially at a high C rate.

Fig. 8 Charge–discharge curves of the Li/LFP cells containing the NX and NX-S membranes at 0.1C charge rate.

Table 1 The capacities value and capacity retention of NX and NX-S membrane cell

Samples	NX	NX-2S	NX-5S	NX-10S	NX-15S	NX-20S
0.1C (discharge capacity, mA h g^−1)	150.7	154.9	153.5	152.5	153.9	152.1
5C (discharge capacity, mA h g^−1)	40.6	62.2	64.1	72.1	49.6	41.8
Capacity retention (%)	26.9	40.2	41.8	47.3	32.2	27.5

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In addition, the interfacial compatibility (Fig. 9) between the electrolyte and the lithium metal electrode was further characterized in the coin cells by measuring the interfacial resistance between the electrode and the NX-S membranes by using AC-impedance spectroscopy. The AC-impedance responses comprised a distorted semicircle at the high frequency region followed by a small spike, both of which are attributed to the presence of a diffusion process. Before charge–discharge cycling, the magnitudes of the interfacial resistances of the cells composed with the NX-2S, NX-5S, NX-10S, NX-15S, and NX-20S membranes had values of 50, 62, 45, 62, and 105 ohm, respectively. However, after 3 charge–discharge cycles at 0.1C, the resistance values of the NX-2S, NX-5S, NX-10S, NX-15S, and NX-20S (Fig. S1†) membranes were 264, 142, 110, 420, and 1976 ohm, respectively. These results indicate that the NX-10S had the lowest resistance among the NX-S membranes. In addition, the resistance value of the NX-20S membrane rapidly increased to 1976 ohm. Thus, we can conclude that the NX-10S membrane does not produce a significant increase in electrolyte–electrode interface resistance after charge–discharge cycling due to the superior compatibility between the electrode and membrane.^25–28

Fig. 9 Comparison of resistance values of the Li/LFP cells containing the NX-S membranes (a) before and (b) after cycling.

4. Conclusions

In this study, the SiO₂ nanoparticles were well-dispersed into a cross-linked polymer network to form a series of NX-S membranes. Results showed that these membranes feature high thermal stability up to 400 °C and high dimensional stability under 150 °C, as well as electrochemical stability up to 5 V. The existence of SiO₂ in the NX-S membrane increased the compatibility of the membranes and electrode, thereby leading to an electrolyte with improved rate performance. Moreover, the cell incorporating the NX-10S membrane demonstrated excellent reversible charge–discharge cycle performance, suggesting that the novel composite electrolyte offers excellent properties and holds great potential for safe and high cycle-life LIBs. Further, the process of the preparing the NX-S membrane is very simple and low cost.

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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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra26694g

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