A thermal resistant and flame retardant separator reinforced by attapulgite for lithium-ion batteries via multilayer coextrusion

Highlights

• A robust multilayer separator of polypropylene and polyethylene reinforced by poly(methyl methacrylate) modified attapulgite (ATPM) is prepared via multilayer coextrusion without multiple stretching processes, then can avoid serious separator shrinkage at elevated temperature.

• The addition of hydrous and heat-resistant ATPM can not only enhance thermal integrity of separator, but also produce water vapor/oxide anti-flaming isolation layers at high temperature, thus improve the flame retardancy of separator.

• This preparation method can be extended to fabricate other multilayer separators by choosing other suitable polymeric matrix and reinforce filler (inorganic nanowire/fibre, flame retardant powder, etc).

Abstract

Multilayer separators are widely used due to their wide shutdown window by combining lower melting temperature and higher melting temperature of different layers. With the development of high power lithium-ion batteries, multilayer separators equipped with effective thermal stability and flame retardancy are highly required. Herein, the poly(methyl methacrylate) modified attapulgite (ATPM) is selected as the heat resistant reinforcing component and blended with polypropylene (PP)/polyethylene (PE) respectively. Then we prepare PP(ATPM)/PE(ATPM) separators via multilayer coextrusion efficiently without multiple stretching processes, which can avoid serious separator shrinkage at elevated temperature. The intertwined ATPM could not only enhance the separator integrity, but also produce water vapor and oxide anti-flaming isolation layers at high temperatures. The as-prepared separators, referred to as MC-TIPS PP/PE/ATPM, exhibit higher thermal stability (with negligible dimensional shrinkage up to 180 °C), better flame retardancy and wider shutdown temperature window (124–183 °C) than the commercial multilayer separators. Moreover, the introduction of ester and hydroxyl groups could improve the wettability and electrolyte uptake of the separators. These properties, as well as the potential for large-scale production of multilayer coextrusion, make MC-TIPS PP/PE/ATPM an ideal choice for high-power battery separators.

Introduction

During the past decades, high power lithium-ion battery (LIB) has been vigorously developed, with applications expanding from portable electronic devices, electric vehicles to energy storage systems, which brings forward higher request for the safety performance of LIB [1]. Once LIBs are subjected to extreme conditions such as short-circuiting or overcharging [2], the exothermic chemical reactions will be initiated, which will raise internal pressure and release heat rapidly, eventually lead to thermal runaway [3,4]. Functional LIB separators or solid/gel electrolytes, which act as physical barriers between electrodes and provide pathway for lithium ions migration, have been considered to be important self-activating protection components [5,6] to prevent LIBs from thermal runaway. For example, thermotolerant biopolymer-based electrolytes [7], flame-resistant separator [8,9], thermotolerant separator [9], flame-resistant solid state organic electrolytes with superior thermal stability [10], hybrid ceramic-polymer/ionic liquid electrolytes with high thermal stability as well as excellent Li ion-conductivity [[11], [12], [13], [14], [15], [16]], etc. Especially, the application of polypropylene (PP)/polyethylene (PE) multilayer separators with shutdown function is the widely used strategy to protect LIBs safety due to the low cost and rich resources. Once the temperature rises up to the melting temperature of PE, the PE layer melts to close off pores, prevents ionic conduction and terminates electrochemical reaction [17]. Meanwhile, the PP layer with higher melting temperature can act as physical barrier between electrodes. However, they are flammable and their shrinkage is serious at high temperatures due to the residual stress caused by the multiple stretching process [18,19].

In order to improve the thermal stability and flame retardancy of polyolefin separators, many methods have been developed [20,21]. Separators coated with high temperature resistant organic polymers or flame-retardant additives [22], such as cellulose nanofiber [23], phenolformaldehyde resin [24], hyperbranched polybenzimidazole [25], and melamine [26], are proved to have better thermal safety performance. In addition, coated by inorganic oxide layer is another solution to improve the thermal safety of separators [20,27,28], especially silicon dioxides with high-temperature stability and rich in resources. For instance, Liao and co-authors coated SiO₂ nanoparticles/ammonium polyphosphate composites on both sides of polyolefin separators, the obtained sandwich-like separator exhibited self-extinguishing performance and had no visibly shape changes until 150 °C [29]. Likewise, Cho and co-authors coated the amino-functionalized SiO₂ particles on the PE separators, the coated separators displayed good thermal stability at 130 °C [30]. Interestingly, compared to zero-dimensional particles, one-dimensional nanostructured materials (nanowires, nanorods, nanowhisker) are proved to be better enhancing components, since the interconnected one-dimensional materials could form robust networks and exist long range interaction inside, which can prevent the mechanical deformation [31,32]. Zhang and co-authors found that, after the thermal treatment at 150 °C, the bare PE separator, the silica particle coated PE and silica tube coated PE shrink about 70%, 54%, and 19%, respectively. Moreover, the tensile strength of silica tube coated separator is 8% higher than the silica particle coated separator [33]. Although layer coated commercial separator shown significantly improved thermal safety performance, the coated layers are easy to fall off and lead to blocked porous structure or increased thickness [34,35]. In order to overcome this problem, Yue and co-authors developed a heat resistant and flame-retardant polysulfonamide/polypropylene composite nonwoven separator via melt-blown spinning, which shown remarkably improved resistance against thermal shrinkage at elevated temperatures and excellent flame retarding ability [36]. Shao and co-authors incorporated ethylenediamine (EDA) modified ammonium polyphosphate (APP) into PP via melt blending to improve its flame retardation. The results showed that the oxygen index (OI) value of PP/EDAAPP reached 29.5%, and at the loading of 30 wt % EDA-APP the V-0 rating was achieved, while it is still necessary to develop some more efficient APP derivatives to improve the flame retardancy of PP separators [37]. Later, they improved their previous work by developing a highly-efficient mono-component polymeric intumescent flame retardant-piperazine-modified ammonium polyphosphate (PA-APP), the vertical burning test could pass V-0 rating at the loading of 22 wt % PA-APP [38]. The above methods can produce separators with significantly improved thermal stability and flame retardancy, and most of which focused on modifying or reinforcing the existing commercial polyolefin separators and obviously maintained the mechanical strength of separators [39]. Nevertheless, the ralatively cumbersome production process clearly decreases the production efficiency and makes the separators more expensive [40]. Thus it is essential to find new solutions that can optimize the thermal stability, flame retardancy and shutdown property, without sacrificing the convenient and cost-effective preparation process.

Multilayer coextrusion (MC) represents an advanced polymer processing technique which is capable of economically and continuously producing multilayer separators without multiple stretching processes and can avoid serious separator shrinkage at elevated temperature [17]. In our previous paper, PP/PE multilayer separators with higher thermal stability (no visibly shape changes until 160 °C) are mass-produced by multilayer coextrusion [41]. While when the temperature reached above 160 °C, the shrinkage increased a lot due to the melting of PP. Besides, this separator did not have flame retardancy. Hence, it is extremely necessary to extend our previous work and improve the thermal stability and flame retardancy of this separator further. Attapulgite (ATP) is a crystalline hydrated magnesium aluminum silicate with excellent thermal stability, inexpensive price, one-dimensional fibrous nanostructure. It generally has three kinds of water (zeolitic water, crystal water and constitution water) at room temperature [42,43]. Besides, most of aluminum and magnesium included in ATP exists in the form of hydroxide (Al(OH₃), Mg (OH₂)), which are typical inorganic metal hydroxide flame retardant [44]. ATP can inhibits combustion by decomposing into water vapor and inorganic oxide isolation layers (MgO, Al₂O₃) during burning [45]. Moreover, there are a plenty of polar hydroxyl on the surface of ATP, which is suitable for its application in polar electrolyte systems [10,46]. Consequently, ATP was selected as the enhancing additive to improving the safety properties of multilayer separator. Herein, the dispersion of ATP in polyolefin was improved by grafting poly(methyl methacrylate) (PMMA), which also possess high affinity to the electrolyte due to the ester groups [47]. The PMMA modified ATP (ATPM) was firstly added in the PP/PE matrix, then the PP/PE/ATPM multilayer separators were prepared by multilayer coextrusion. Both heat-resistance and flame retarding experiments stated that PP/PE/ATPM separators present better thermal safety than commercial multilayer separator (Celgard® 2325), this would shed light on the development of high safety LIBs.