Al2O3-coated porous separator for enhanced electrochemical performance of lithium sulfur batteries
Graphical abstract
Al2O3-coated separator with developed porous channels is prepared by coating Al2O3 polymer solution on routine separator. The batteries with Al2O3-coated separator exhibited a reversible capacity of as high as 593 mAh g-1 at the rate of 0.2Ā C after 50th charge/discharge cycle. The enhancement in the electrochemical performance could be attributed to the reduced charge transfer resistance after the introduction of Al2O3 coating layer. Besides, the Al2O3 coating layer, acting as a physical barrier for polysulfides, can effectively prevent polysulfides shuttling between the cathode and the anode. We believe that the Al2O3-coated separator is promising in the lithium sulfur battery applications.
Introduction
Lithium sulfur (Li-S) batteries have been studied as one of the most promising systems for the next generation high-energy rechargeable lithium batteries because of their high theoretical specific capacity (1675 mAh gā1) and energy density (2600 Wh kgā1). As a cathode active material, sulfur also has advantages of non-toxicity and abundance in nature [1], [2]. However, there are still many problems hindering the practical application of Li-S batteries. Sulfur and its final discharge products (Li2S2, Li2S) are electrical insulators, which can cause poor electrochemical accessibility, leading to a low utilization of active materials. In addition, polysulfides (Li2Sx, 4Ā ā¤Ā xĀ ā¤Ā 8) produced in discharge/charge processes can dissolve into organic electrolyte and be reduced to lower-order polysulfides at the interface of the lithium anode. These reduced products will migrate back to the cathode where they may be reoxidized. This process takes place repeatedly, creating polysulfides shuttle, which can cause loss of active materials and the low coulombic efficiency of Li-S batteries, eventually resulting in rapid capacity fading [3], [4], [5].
In order to prevent polysulfides shuttling in organic electrolyte, various approaches have been proposed by research teams over the latest three decades. One of the most effective strategy is to confine polysulfides in the cathode by using adsorbing material, including porous carbon materials and porous metal oxides. Various types of porous carbon materials have been employed in the sulfur cathode, resulting in higher active mass utilization [6], [7], [8], [9]. Besides, metal oxides for polysulfides entrapment also have attracted significant attention, such as Mg0.6Ni0.4O [10], Mg0.8Cu0.2O [11], and TiO2 [12]. Al2O3 has also been used as an adsorbing material to trap polysulfides by Dong [13] and Choi [14]. This electrodes containing alumina show an obviously superior cycle performance. Another effective strategy to restrain polysulfides from diffusing is the modification of cell configuration, for instance, building a physical barrier for polysulfides between the cathode and the separator. Several kinds of carbon interlayer have been used to enhance the cycle performance and capacity retention of lithium sulfur cell, including porous multiwalled carbon nanotube paper [15], microporous carbon paper [16], treated carbon paper [17]. A free standing acetylene black interlayer was prepared to capture dissolved polysulfide by Kim [18]. Separator, the basic component of the lithium-sulfur battery, has also great impact on the performance of Li-S batteries. However, modification of the separator is rarely reported in Li-S batteries. We proposed a new method to prevent polysulfides diffusing by separator modification, namely coating an oxide absorbent layer on the surface of routine separator, which may act as a physical obstacle to block polysulfide migrations.
Herein, a novel modified separator was prepared by coating Al2O3-polymer solutions on the surface of routine separator. The electrochemical performance of lithium sulfur batteries with Al2O3-coated separator can be obviously improved, with a specific capacity of 593.4 mAh gā1 after 50 cycles at 0.2Ā C, which is higher than that of lithium sulfur batteries with routine separator. These results indicate that the Al2O3-coated separator is promising in the lithium sulfur battery applications.
Section snippets
Preparation and characterization of Al2O3-coated separator
Commercial aluminum oxide powder (average particle sizeĀ =Ā 200Ā nm, Aldrich) was added to 8 wt% polyvinylidene fluoride (PVDF 6020 Solef) solution with N-methyl-2-pyrrolidinone solution (NMP) as solvent where a ratio of Al2O3/PVDF was fixed at 90/10 (wt%/wt%). And the solution was ultrasonically dispersed for 2Ā hour. This solution was coated on separator (Celgard 2320) and then was dried at 55Ā Ā°C under vacuum for 24Ā h. The thickness of the aluminum oxide layer was approximately 4Ā Ī¼m. A schematic
Result and discussion
Scanning electron micrographs of the surface of the routine separator and the Al2O3-coated separator are presented in Fig. 2. The routine separator (Fig. 2a) exhibits a uniformly interconnected submicron pore structure, which maintains the ionic pathway and blocks the transfer of electrons between the cathode and anode [20]. And the size of the porous is around 100Ā nm. In contrast to the routine separator, the Al2O3-coated separator (Fig. 2b) has unique coating layers comprising close-packed Al2O
Conclusions
In summary, separator modification method was proposed in this paper to improve the electrochemical performance of Li-S battery. An Al2O3-coated separator was fabricated by coating Al2O3/PVDF slurry on the routine separator. The unique structure of Al2O3-coated separator allows the free transportation of lithium cation while blocks the transportation of the polysulfide anions by physical absorption and electrochemical deposition, which could effectively decrease the shuttle effect and the loss
Acknowledgements
The authors thank the financial support of the Strategic Emerging Industries Program of Shenzhen, China (JCYJ20120618164543322) and National Natural Science Foundation of China (20803095). We also thank the support of the Engineering Research Center of Advanced Battery Materials, the Ministry of Education, China.
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