Functional double-layer membrane as separator for lithium-sulfur battery with strong catalytic conversion and excellent polysulfide-blocking
Graphical abstract
Introduction
Continuously increasing demand for energy has driven the development of energy-storage technologies to overstep the conventional lithium-ion batteries. The safe, low cost and environmentally benign materials, and easy preparation processes are also pursued to meet the requirement of manufacturing and global sustainability [1]. Simultaneously, Li-S battery has received wide concerns and attentions owing to its high theoretical energy density of 2600 Wh kg−1 and theoretical specific capacity of 1675 mAh g−1 [2]. In addition, elemental sulfur is resourceful, nontoxic and cost-effective. However, the commercialization process of Li-S cell is hindered by several difficulties and challenges including insulating nature of elemental sulfur and low order lithium polysulfides, considerable volume change and serious “shuttle effect” caused by the dissolved polysulfide (PS) [3]. Recently, several approaches have been applied to address these challenges including the application of conductive additives or absorbers, and the design of new electrodes and innovation in cell configuration [4], [5]. For effectively reducing the “shuttle effect” of PS, some novel sulfur host materials (such as nano-conductive porous carbon materials) or special structures of cathode materials (such as various bionic structures) based on the principle of “physical confinement” or “chemical adsorption” to PS were applied in Li-S cells. At the same time, the growth of lithium dendrites was inhibited by the nano crystallization of lithium metal or the stabilization of the solid electrolyte interface on the surface of lithium metal. In addition, separator, which affects the safety and electrochemical performances of the battery, is an important component of Li-S battery. Conventional polyolefin separators are extensively applied in Li-S batteries because of their low cost, proper mechanical strength, good chemical stability and electro-chemical stability. However, these polyolefin separators are incapable of effectively retarding “shuttle effect” of PS due to the large-pore framework and lack of strong chemisorption to PS. More seriously, the charge-discharge performances of these assembled Li-S batteries using polyolefin separators are extremely poor when they work in rather “bad” conditions such as high temperature or large current density leading to the short circuit and potential safety hazard even explosion.
For effectively inhibiting the “shuttle effect” of PS and significantly improving the safety of the battery, many efforts in the battery separators have been made to solve these thorny problems. One approach is to develop novel polyolefin separators with functional surface coating including carbon modified separators [6], [7], [8], polymer modified separators [9], [10], [11], [12], inorganic and metallic compounds substance modified separators [13], [14], [15], [16], [17], [18] and their combination of these modified separators [19], [20]. Another method is to explore alternative separators or some membranes based on the heat-resistant materials and ceramic fibers for Li-S batteries [21], [22], [23], [24], [25], [26], [27]. Similarly, poly-m-phenyleneisophthalamide (PMIA) fiber membrane has been reported by many groups regarded as the separator in lithium ion batteries due to the advantages of excellent thermal resistivity, chemical resistance, self-extinguishing characteristics, good mechanical properties and electrical insulation ability [28], [29]. However, it is difficult for these pure heat-resistant membranes including pure PMIA membrane to obtain high electrolyte affinity or excellent electrolyte retention capacity.
Nowadays, the preparation and application of gel polymer electrolyte (GPE) has been considered as one of the most promising methods to reduce the “shuttle effect” of PS and suppress the formation of lithium dendrite on the surface of lithium metal for Li-S batteries [30]. This is because that a flexible and stable passivation layer on the carbon-sulfur electrode can be formed through using GPE in Li-S cells. Meanwhile, GPE such as PVDF membrane [31], PEO membrane [32], PETEA membrane [33] and their composite membranes [34] also can effectively inhibit the PS diffusion because of the strong integrated electrolyte/electrode structure. Nowadays, many researches also have presented that the formed C-F chemical bonds with high bond energy and short bond length can lead to make some F-doped membranes like PVDF membrane, PTFE membrane and other membranes including F element have high dielectric constant, excellent resistance to chemical degradation, strong electron withdrawing function and low surface energy, which are helpful to provide the prepared separators with outstanding affinity to electrolyte solutions and excellent gelation degree [35]. It is generally known that electrospinning, chemical self-assembly method or other similar methods are rather effective ways to fabricate nanofiber membrane for various batteries. Because of high specific surface area, numerous interconnected pores, these nanofiber membranes are advantageous to form gelation for the reasonable separators in the charge-discharge processes [36], [37], [38], [39], [40], [41], [42]. But up to now, few researches have been reported on thermostable gel polymer electrolyte based on the nanofiber membrane with excellent physical and chemical adsorption to PS for Li-S battery. Although our group have once prepared one-layer F-doped tree-like structural PMIA nanofiber membrane for the Li-S batteries through structure design to realize the solely physical adsorption to PS, the long cycle performance of the battery, especially at high current density, is still unsatisfactory due to rather non-uniform pore size distribution and no chemical adsorption and catalysis [43].
As early as 2000, Poizot et al. reported for the first time that transition metal oxide nanomaterials (MOx, x = Cr, Mn, Fe, Co, Ni, Cu and so on) can be used as anode materials for Li-ion batteries [44]. Afterwards, some researchers also applied various modified Mn-based materials as anodes for Li-ion batteries because of high theoretical specific capacity. Different from the layer-type structure of MnO2, Mn3O4 is a spinel ion structure with Mn2+(Mn3+)2O4, in which Mn2+ and Mn3+ ions are distributed at two different lattice positions and the special structure can form different chains bonded each other through strong Van der Waals' force (vdW, which is the term applied to describe a general attractive force among neutral atoms or molecules) interactions. The polar PS can potentially be trapped by the polar Mn3O4 based on the strong vdW interactions [45]. Meanwhile, the chemisorption and catalysis roles of MnOx have been reported in some literatures [46], [47], [48], [49], [50], which also have presented high performance Li-S batteries through applying the MnOx doped cathode materials can be obtained. However, Mn3O4 doped separators and GPEs both have not been reported for improving the electrochemical performance of Li-S cells [51].
On the basis of the above considerations, we prepared a novel thermostable double-layer membrane including F-doped PMIA membrane and F-Mn3O4-co-doped PMIA membrane by electrospinning technology for Li-S battery exhibiting excellent discharge capacity and outstanding cycling stability. The F-doped PMIA membrane and F-Mn3O4-co-doped PMIA membrane endow the prepared separator with extraordinary high electrolyte uptake, good preserving liquid electrolyte and excellent thermal stability. Simultaneously, the doping F and Mn3O4 can endow the modified PMIA membranes with strongly chemical adsorption and effectively physical blocking to PS due to the Lewis base role of Mn3O4 with the applied electrolytes during battery cycling, reduced pore size based on the nanoparticle doping and gel forming, and the strong catalysis for promoting the oxidation-reduction reaction of the battery. The batteries with such a composite separator show an ultra-high rate performance and excellent cycling performance, leading to a great potential for use in next generation energy storage.
Section snippets
Experimental section
The detailed information about the preparation procedures of the double-layer membrane based on the F-doped PMIA membrane and F-Mn3O4-co-doped PMIA membrane, materials characterizations, and cell assembly process and electrochemical performance measurements of the Li-S battery assembled with the separators are presented in the Supporting Information.
Results and discussion
The phase structure of the as-prepared nanoparticles was characterized by X-ray diffraction (XRD) testing firstly. As shown in Fig. 1(a), the main diffraction peaks can be well indexed to hausmannite Mn3O4 phase (JCPDS No. 08-0382). The morphology and structure characterizations of the as-synthesized Mn3O4 were presented in Fig. 1(b ~ d). As shown in Transmission Electron Microscope (TEM) images (Fig. 1(b ~ c)), the particle size of the obtained sample was about 20 nm. The HRTEM images of the
Conclusions
In conclusion, we successfully developed a novel thermostable double-layer membrane based on F-doped PMIA membrane and F-Mn3O4-co-doped PMIA membrane for Li-S batteries by the electrospinning technique. The F-doped PMIA membrane, in combination with F-Mn3O4-codoped PMIA membrane, endowed the functional separator with extraordinary high electrolyte uptake, preserving liquid electrolyte, thermal stability and forcefully physical and chemical adsorption to polysulfides. In addition, the F-doped
Acknowledgments
The author would like to thank the National Natural Science Foundation of China (51673148, 51678411), China Postdoctoral Science Foundation Grant (2019M651047) and The Science and Technology Plans of Tianjin (No. 17PTSYJC00040 and 18PTSYJC00180) for their financial support.
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