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A Trimethylsilyl Chloride Modified Li Anode for Enhanced Performance of Li-S Cells Meifen Wu, Zhaoyin Wen, Jun Jin, and Bobba V.R. Chowdari ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02612 • Publication Date (Web): 08 Jun 2016 Downloaded from http://pubs.acs.org on June 16, 2016
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ACS Applied Materials & Interfaces
A Trimethylsilyl Chloride Modified Li Anode for Enhanced Performance of Li-S Cells Meifen Wu,† Zhaoyin Wen,*,†Jun Jin,† Bobba V.R. Chowdari ‡ †CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China ‡ School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798 KEYWORDS Li anode, Li-S cell, surface modification, trimethylsilyl chloride, shuttle effect
ABSTRACT A facile and effective method to modify Li anode for Li-S cells by exposing Li foils to tetrahydrofuran (THF) solvent, oxygen atmosphere and trimethylsilyl chloride ((CH3)3SiCl) liquid in sequence is proposed. The results of SEM and XPS show the formation of a homogenous and dense film with a thickness of 84 nm on Li metal surface. AC impedance and polarization test results show the improved interfacial stability. The interfacial resistances as well as polarization potential difference have obviously decreased as compared with that of a pristine Li anode. CV and charge-discharge test results demonstrate that more reversible discharge capacity and higher coulombic efficiency can be achieved. Specific capacity of 760 mAh g-1 and an average coulombic efficiency of 98% are retained after 100 cycles at 0.5C without LiNO3 additive. Additionally, the Li-S cell with a modified Li anode displays a greatly improved rate
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performance with ~425 mAh g-1 at 5C, making it more attractive and competitive in the applications of high-power supply.
INTRODUCTION Li-S battery is considered as one of the most promising candidates for electric vehicle applications due to its high specific capacity (1675 Ah kg-1) and energy density (2650 Wh kg-1), low cost, abundant sulfur resource, and less environmental pollution.1-4 However, in order to realize its commercial applications in full, several challenges such as insulating nature of sulfur, large volume expansion, rapid capacity decay, poor stability, low coulombic efficiency, high self-discharge rate, etc. are yet to be solved.5-7 To overcome the drawbacks mentioned above, different kinds of hosting materials, e.g. porous carbons (nanotubes, nanofibers and nanospheres)8-10 and conducting polymers,11-12 are adopted for the fabrication of composite cathode to enhance its conductivity and avoid soluble polysulfides. The electrolyte composition is also optimized by selecting suitable ether solvents, introducing functional additives or ionic liquids,13-14 and even using solid electrolytes.15-16 For instance, LiNO3 is proved to be a useful functional additive to improve the coulombic efficiency by forming a passivation film on pristine Li surface.17-20 However, a few researches on the improvement of Li anode were reported for Li-S cells, owing to the complex and severe side reactions between Li metal and polysulfides as well as active species in the electrolyte, and hence the modification of Li metal anode has been adopted as a new strategy. In a Li-S battery, the Li metal anode suffers from several problems. The excellent reactivity of Li metal makes it easily reducible by active species and impurities in the electrolyte, leading to the formation of a solid electrolyte interphase (SEI) layer on the surface. This causes an
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irreversible consumption of electrolyte and the degradation of Li anode, and eventual battery failure.21-22 In practical applications of Li metal batteries, the accumulation of this highly resistive SEI layer will accelerate the increase of the overall cell impedance, and even lead to early failure of the cell before “dendritic” Li forms, especially at high current density.23 The second problem is ascribed to the non-uniform Li deposition, which can produce large Li dendrites, resulting in the short circuiting of the battery and safety issues.24-25 Especially in a polysulfide-rich electrolyte, the SEI layer is so loose and rigid that it cannot totally prevent the shuttle effect of lithium polysulfides. Thus, the fresh Li source under the SEI layer is continuously corroded, leading to the pulverization of Li anode and the termination of the cell’s cycle life.26 The third problem arises from the reduction of the poflysulfides by Li metal to form an insoluble and insulating Li2S2 and/or a Li2S film on its surface, which can block Li+ diffusion and deteriorate the cell performance.27-28 In addition, a large amount of heat will be released when the Li metal reacts with the electrolytes, which can pose risks of overheating.29 Many in-situ/ex-situ attempts, such as reactive organic/inorganic additives,30-32 Li alloy,33-34 polymer coatings,35-36 sputtered solid electrolytes,37 have been applied to passivate Li metal. Some alternative nanostructured Li anodes such as 3D skeleton-based Li anode,38 graphene framework-based Li anode39 as well as Li7B6 framework-based Li anode,40-41 have also been used, which can effectively prohibit the Li dendrite formation and extend the lifespan of Li-metal anodes. They can greatly enhance the electrochemical performance and the safety of Li-S cells. Although some success has been achieved by these attempts, there is still a distance from practical application. Chlorosilanes were first proposed by Marchioni, et al.42 to functionalize Li metal surface through a nucleophile substitution reaction between trimethylchlorosilane and lithium hydroxide
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on a clean Li surface. It was found that the coated Li metal can enhance the interfacial stability in the electrolyte (1MLiPF6/EC+DMC) as judged by AC impedance spectra. It has also been demonstrated that chlorosilane based coatings can improve the cycling performance of electrochemical cells against Li4Ti5O12.43 Further studies44 indicate that the cycle life of a clean Li anode can be prolonged by surface modifications with big R-groups, e.g. bulkier than triphenyl, and very small R-groups, e.g. trimethlysilyl. As the clean Li anode has finite thickness of the hydroxide-terminated layer, and also the surface modification with silane molecules is self-terminating, the passivation coating on Li surface should be very thin. However, the serious corrosion of Li metal by polysulfides species shuttling in the electrolyte of Li-S cells requires a more uniform and denser coating on Li surface. Hence, in this work, we not only took advantage of a chemical cleaning agent to reduce surface defects of Li metal, which is helpful to enhance the homogeneity of the coating, but also artificially designed a certain thickness of hydroxylcontaining layer before modification, which will make the thickness of the protective film controllable. Therefore, the coating is no longer thin, which is different from the work mentioned above, and moreover, its density can be easily improved. We used the Li-S cell as a test system to systematically investigate the electrochemical behaviors of the protected Li metal anode, which shows effective prevention of Li corrosion and highly improved cell performances. We also explored and discussed the possible mechanism in the text.
RESULTS AND DISCUSSION It is widely known that Li metal has a native film on its surface because of its unavoidable reactions with air components during the preparation process. The components of the native film mainly include Li2CO3, LiOH, and Li2O.45 Hence, various attempts were made to remove them.
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The simplest way is to clean the Li surface with an inert liquid. According to our previous research,46 we found that THF is a good cleaning agent because it does not evaporate quickly when it is used to treat Li surface. The Li surface can also be dried quickly when removed from the liquid. Figure 1 shows the SEM morphologies of Li foil surfaces at low (a-d) and high magnification (e-h) after each step of treatment in sequence. Figure 1(a, e) show a typical surface morphology of a pristine Li foil. Substantial white debris is displayed on the surface, resulting from heterogeneous deposition of Li salts. Figure 1(b, f) show the surface morphology of the Li foil after being stirred in THF for 3 min. Less debris is observed, indicating that the surface is clean and smooth. Figure 1(c, g) show the surface of THF treated Li after exposure to oxygen atmosphere for 0.5 h. A few of white boundaries are observed indicating that a Li2O/LiOH layer is covered on Li surface, causing the Li surface uneven again. As the oxygen glove-box contains a limited amount of H2O (
