Study on cellulose/nylon 6 lithium battery separators modification by polyacrylonitrile in ionic liquid [Emim]Ac

With the popularization of consumer electronics and the development of electric vehicle industry, lithium battery technology has become a major bottleneck which restricting the further development of the lithium battery industry. As the key inner component to expand the life time and efficiency of the lithium battery, the separator has attracted extensive attention.^1,2 The separator is mainly used to separate the positive and negative poles of lithium battery to prevent short circuit. In addition, it has the function of allowing ions in the electrolyte to pass freely between the positive and negative poles, while the electrons cannot pass freely.^3,4 The physicochemical property of separator determines the capacity, safety and cycling performance of lithium battery.² At present, the market-oriented lithium battery separators are mainly made of polyolefin materials, but the polyolefin separator has defects in heat resistance and affinity with electrolyte, which has become a safety hazard for lithium batteries.^3,4

Cellulose is a natural renewable resource with wide distribution and abundant content, which characterized by good film-forming, hydrophilic, thermal stability, biocompatibility and biodegradability.^5,6 Cellulose is an ideal material for lithium battery separators, but there are some defects in the aspect of mechanical properties and the pore size, which making the safety performance cannot be guaranteed in the process of using.^7,8 Swatloski et al. shown that some ionic liquids have good solubility to cellulose.⁹ Turner and his research group obtained regenerated cellulose separator by dissolving cellulose in ionic liquid [C₄mim]Cl.¹⁰ Nylon 6 has favorable impact resistance, and the amide groups are more likely to form hydrogen bonds between molecular chains. Polyacrylonitrile (PAN) is a kind of polymer with high permittivity, which synthesized from acrylonitrile by free radical polymerization. PAN is often used to make lithium battery separators, because it has great mechanical properties and excellent performance with respect to electrode compatibility and thermal stability. Related studies have shown that PAN separator has higher dimensional stability at a higher temperature.^11–13 In this study, the advantages and disadvantages of cellulose and PAN were combined in the manufacture of lithium battery separators.

At the early stage, our research group have successfully manufactured cellulose/nylon 6 lithium battery separator in ionic liquid [Emim]Ac, and found that the optimal composition of the casting solution in the ionic liquid was: 4wt% (cellulose), 3wt% (nylon 6), 4wt% (PVP K30).¹⁴ On this basis, a separator (pCN) was prepared via PAN modified cellulose/nylon 6 (CN). The effect of PAN concentration on the comprehensive performance of the separator was investigated by corresponding tests and characterizations.

Hardwood pulp (with α-cellulose content of approximately 90%, DP=700) was purchased from Shandong Sun Paper Industry Joint Stock Co., Ltd. (Shandong, China). Nylon 6 (density: 1.13g/cm³) was provided by Dongguan huangjiang shengbang plastic material business department (Guangdong, China). Polyacrylonitrile (PAN, Mw ∼5.0×10⁴ g·mol^-1) was supplied by Dongguan yingsheng plastic chemical Co., Ltd. (Guangdong, China). Hardwood pulp, nylon 6 and PAN were dried in a vacuum oven at 80°C for 8h before using. Polyvinylpyrrolidone (PVP K30, Mw∼5.0×10⁴ g·mol^-1) was analytical grade and purchased from Shanghai shanpu chemical Co., Ltd. (Shanghai, China). Ionic liquid [Emim]Ac (1-ethyl-3-methylimidazole-3-ium, acetate, Mw ∼170.21g·mol^-1) was supplied by Shanghai chengjie chemical Co., Ltd. (Shanghai, China).

Before the separators were prepared, the optimal dissolved time (2h) and temperature (140°C) of PAN in the ionic liquid were determined based on the preliminary experiments. In the next step, blend casting solution was prepared. First, hardwood pulp was crushed by high speed universal pulverizer (FW80, Tianjin tester instrument Co., Ltd., China). Second, a certain amount of ionic liquid (30g) and nylon 6 (0.9g) were placed in a three-necked flask, and dissolved in an oil bath at 180°C for 2h. Third, when the temperature dropped to 140°C, PAN (1wt%, 2wt%, 3wt%, 4wt% and 5wt%) was added into the casting solution and stirred for 2h. Finally, cellulose and PVP K30 were mixed in it and stirred for 1h as the temperature reduced to 100°C. Then the homogeneous and stable blend casting solution was obtained.

The casting solution was scraped onto the glass plate through a glass rod which wound with copper wire (0.5mm in diameter). After that, the separator was prepared by the phase inversion method. In the next step, the separators were soaked in deionized water for 24h to extract the solvent completely, and then removed and dried it for later use.

The mechanical properties of separators were measured on an electronic tensile testing machine (XLW, Labthink, China) at a strain rate of 25 mm/min with 20*150mm samples under room temperature (25±2°C, 50±5%RH). The samples were presoaked in the liquid electrolyte (LiPF₆:EC:DMC=1:1:1(mol/g/g)) for 2h.

The separators were cut into 20mm*20mm samples and dried at 60°C for 3h. The samples were immersed in the liquid electrolyte (LiPF₆:EC:DMC=1:1:1(mol/g/g)) for 2h to test electrolyte uptake, retention rate and porosity of the separators. Five samples were selected from each test group, and the average value of the data was calculated as the final result.

The microstructure of the separators in surface and cross-section were observed by cold field emission scanning electron microscope (SEM, SIGMA, Carl Zeiss AG, Germany).

The thermal shrinkage of the separators was determined by a thermal shrinkage tester (FST-02, Labthink, China) on 15*130mm samples at 200 °C.¹⁵ Thermogravimetric analysis (TG) was carried out using a thermogravimetric analyzer (STA2500, NETZSCH, Germany) under a nitrogen atmosphere from room temperature to 600°C at a 10°C min^-1 heating rate.

The pore structure of the separators was measured through a specific surface area and porosity analyzer (NOVA, Quantachrome Instruments, American) with the relative pressure range p/p₀ 0.01-0.996.¹⁵ The contact angle between separators and electrolyte was measured by a contact angle measuring instrument (SDC-100, Dongguan shengding precision instrument Co., Ltd.).

The electrochemical stability of the separator was tested by the electrochemical workstation (CHI760E, Shanghai chenhua instrument co., Ltd.). Linear scanning voltammetry (LSV) was used to measure the electrochemical window of the separators under the system SS/separator/Li. The scanning range was 3V∼ 6.5V and the scanning rate was 2mV/s.

The morphology of the separator was closely related to electrolyte uptake and conductivity, the surface and cross-sectional morphologies of the separators were studied with SEM images, an example was shown in Fig. 1. It could be seen from Fig. 1a that the surface of the CN separator was rough and there were some micropores in it, while the pCN separator (Fig. 1c) was smoother, and the micro-holes were more uniform in size and distribution. It could be seen from Fig. 1b and Fig. 1d that the holes in the cross-section of the pCN separator were compact than CN. It could be concluded that the dense micropores structure in the surface and cross-section of the pCN separator was more conducive to forming the ion channels, so as to ensure the rapid migration of ions during charging and discharging. The result was consistent with the analysis in 3.3.

The effects of PAN concentration on the mechanical properties of the separators were shown in Fig. 2. It could be found from Fig. 2 that the tensile strength and elongation of pCN separators first increased and then decreased with the increase of the PAN concentration. When the content of PAN was 3wt%, the tensile strength and elongation of pCN separators reached the maximum value, which was 71.24MPa and 33.7%, respectively, higher than that of CN and other similar published separators in Table II.^15,17–20 This result might be due to the limited thermodynamic compatibility between PAN and cellulose.^13,16 When the concentration of PAN was less than 3wt%, the interaction force between -CN in PAN was greater than that between -OH in cellulose. The addition of PAN enhanced the entanglement degrees between cellulose and nylon 6 molecular chains, a small amount of PAN acts as a cross-linking point between cellulose and nylon 6, achieving the function of reinforcing and toughening.^13,16 Thus, the tensile strength and elongation of the modified separator were increased. However, when the content of PAN was higher than 3wt%, the dispersion of cellulose and nylon 6 were decreased due to the excessive effect of -CN in PAN, and increased the viscosity of the system, thus affecting the homogeneity of the separator formation. Meanwhile, the rigidity of the molecular chain was increased, which led to the tensile strength and elongation of the separators decreased.

The electrolyte uptake, retention rate and porosity of the separator with different PAN contents were shown in Table I. According to the data in the table, the liquid absorption rate, retention rate and porosity of the separator presented a trend of increased firstly and then decreased as the increasing of PAN. When the contents of PAN in casting solution was 3wt%, the liquid absorption rate and retention rate reached the maximum value, which was 225.3% and 66.7% respectively, and the porosity of the separator was 55.7%. Compared with the similar published separators (Table II), the electrolyte uptake and porosity are in the middle level,^15,17–20 and meet the requirements of lithium battery industry for separator. It could be ascribed that the strongly polarity of PAN. On the one hand, the separator exhibited better affinity to the electrolyte after the addition of PAN. On the other hand, the phase transformation process of the casting solution was accelerated to form a dense network structure, so the separator could be better wetted and penetrated by electrolyte, and improving the hydrophilicity, liquid retention and porosity of the separator.¹¹ As the concentration of PAN continued to increase, the molecular chains of various substances were aggregated, which could easily block the micropore of the separator, thus reducing the electrolyte uptake, retention rate and porosity of the separators.

The thermal dimensional stability of separators is vital for the battery safety because separator can prevent short-circuits between anode and cathode. The separator expanded and contracted after heated, and produced the corresponding thermal shrinkage stress. Fig. 3 shows the thermal shrinkage stress of separators before and after exposure to 200 °C.¹⁵ It could be seen from the Fig. 3 that the thermal shrinkage stress of the separators increased first, then decreased, and finally tended to be stable. Compared the curves of each samples, it could be concluded that the thermal shrinkage stress of pCN separators was less than CN. When the concentration of PAN was 3wt%, the thermal shrinkage stress of the separator reached the minimum value (1.27N), and gradually decreased to 0.94N at 11s and tended to be stable. This result could be attributed to the existence of PAN increased the crosslinking point, and produced a certain bonding between cellulose and nylon 6 molecular chains, which hindered the movement of the molecular chain, thus improving the thermal stability of the separators.

In order to evaluate the separators’ thermal stabilities, the dimensional changes of the separators before and after heating can be reflected by thermal shrinkage rate.¹⁵ The smaller the thermal shrinkage rate is, the better the size stability of the separator. According to the data in Table III, the maximum thermal shrinkage stress and thermal shrinkage rate of the separator decreased firstly and then increased with the increasing of PAN concentration. The maximum thermal shrinkage stress of CN separator was 2.28N while the pCN (PAN= 3wt%) decreased to 1.27N. Thermal shrinkage rate was reduced from 3.1% to 2% with the increase of PAN from 0wt% to 5wt% (Table III). Compared with the CN and similar published separator (Table II), thermal shrinkage rate of pCN were decreased in different degree.^15,17–20 These results suggest that PAN had a positive effect on the thermal stability of separators, and the data met the requirements of the lithium battery industry for thermal shrinkage rate (< 6%).

Fig. 4 showed the thermogravimetric analysis of CN and pCN separators in the range of 50-600°C. It could be seen from curve (a) that the weight loss of CN separator was about 15% when the temperature reached 130°C, which was due to the evaporation of residual moisture in the separator sample. The degradation onset temperature of the CN separator was around 240°C, and the weight-loss was 48.4% between 240°C and 320°C. At 600°C, the residual decomposition mass of separator samples was 12.5%. According to the curve (b), the weight-loss of the pCN separator was about 7% when the temperature reached 130°C, which was caused by the removal of water molecules from separator samples. The degradation onset temperature of the pCN separator occurred around 330°C, and the weight loss was about 56% between 330°C and 435°C. At 600°C, the residual decomposition mass of the pCN was 15.4%. The degradation onset temperature and residual mass of pCN separator was increased compared to CN. These results suggested that the addition of PAN had a positive effect on improving the thermal stability of separators, similar to the data reported recently.^21,22 This could be attributed to the molecular chain of PAN was bound to cellulose and nylon 6 in a certain degree, which increased the binding strength between cellulose and nylon 6 molecular chains, thus delaying the decomposition and improving the thermal stability of the separators.

N₂-adsorption isotherms of CN separator and pCN separator were shown in Fig. 5. It could be seen from the figure that the curves were convex and downward in the whole pressure range, and there was no inflection point B in the curves, which was classified as type III isotherm according to BDDT. The isotherms of the two samples deviated from the X-axis in the low pressure region (0.0-0.1) with less adsorption and no inflection point B, which indicated that the interaction between adsorbate and adsorbent was weak. However, with the increase of relative pressure, the adsorption increased gradually, and displayed as porous filling. At the same pressure, the adsorption capacity of pCN separator was less than CN, which indicated that the pore size of the modified separator was diminished.

Fig. 6 showed the contact angle between the separators and the electrolyte, where Fig. 6a was CN separator and Fig. 6b was pCN separator. It could be seen from the figure that the contact angles of CN and pCN were 41.940° and 29.330°, respectively. The contact angle of pCN separator was reduced by 30.1% compared with CN, and the hydrophilic property of the separator was improved, so that the liquid absorption and retention properties of the separator were raised, and the transport capacity of the separator for lithium ions in the electrolyte was improved.

Fig. 7 showed the electrochemical window of the separators. It could be seen from the figure that the current value of the separator presented a stable state within a certain range of scanning voltage. The electrochemical window of the CN separator was about 4.2 V, while the pCN was reached about 4.5 V. The electrochemical stability was improved compared to CN separator, this may be due to the PAN has excellent chemical resistance and could exist stably in the electrolyte. In addition, the improvement of the liquid absorption rate and the micropores distribution of the pCN separator increased the affinity between separators and electrolyte, thus the electrochemical window of the pCN were improved, and could be better met the work of the lithium battery.

A new type of separator was prepared by phase inversion using PAN modified cellulose/nylon 6 membrane in ionic liquid [Emim]Ac. The results showed that the separator had the best comprehensive performance when the concentration of PAN, cellulose, nylon 6, PVP K30 were 3wt%, 4wt%, 3wt%, 4wt%, respectively. Compared with the separator before modification, the performance of the separator has been improved in varying degrees. The addition of PAN could improve the tensile strength, wettability, pore size distribution, thermal stability and the electrochemical properties of the separators. SEM results showed that there were irregular mesh-like structures in the surface and cross-section of pCN separators. TG curves showed that the thermal stability of pCN separator was improved. N₂-adsorption isotherm indicated that the pore size of pCN separator was decreased. The contact angle test indicated that the wettability of pCN separator was improved. The electrochemical window of the separator was improved, too. These characteristics provided a guarantee for the safe use of lithium battery. The prospect of pCN separator is promising.

The authors would like to acknowledge the financial support from Shandong Provincial Natural Science Foundation, China (Grant No. ZR2017MB040) and A Project of Shandong Province Higher Educational Science and Technology Program, China (Grant No. J15LC03).