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Letter
Enhancing Stability of Sulfur Cathodes in Li-S Cells via in-situ Formation of a Solid Electrolyte Layer Jung Tae Lee, KwangSup Eom, Feixiang Wu, Hyea Kim, Dong-Chan Lee, Bogdan Zdyrko, and Gleb Yushin ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00163 • Publication Date (Web): 30 Jun 2016 Downloaded from http://pubs.acs.org on July 2, 2016
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ACS Energy Letters
Enhancing Stability of Sulfur Cathodes in Li-S Cells via in-situ Formation of a Solid Electrolyte Layer Jung Tae Lee a, Dr. Kwang-Sup Eom b, Dr. Feixiang Wu a, Dr. Hyea Kim a, Dong Chan Lee a, Dr. Bogdan Zdyrko c, and Prof. Gleb Yushina,* *
[email protected] a
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA b
School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA c
Sila Nanotechnologies, Inc., Alameda, CA, 94501, USA
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Abstract Enhancing performance of rechargeable lithium (Li) – sulfur (S) batteries is one of most popular topics in a battery field because of their low cost and high specific energy. However, S experiences dissolution during its electrochemical reactions hence maintaining its initial capacity is challenging. Protecting S cathode with a Li ion conducting layer that acts as a barrier for polysulfide transport is an attractive strategy but formation of such protective layers typically involves significant efforts and cost. Here, we report a facile route to form a conformal solid electrolyte layer on S cathodes in-situ using a carbonate solvent.
The chemically and
mechanically stable and Li ion conducting protective layer is formed by inducing electrolyte reduction and polymerization reactions on the cathode surface. The layer serves as a polysulfides’ barrier, successfully helping to retain S active material in the carbon pores. In addition, it helps to improve performance of Li anodes.
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The needs of higher energy storage capability are increasing at fast rate due to the prolonged usage of portable electronics and application of storage in transportation and electrical grids.1-3 The current state-of-art and prevalent battery technology, a Li-ion battery system, has not sufficiently fulfilled these rising demands. Hence we are obliged to find alternative battery materials and chemistries. The sulfur (S) is considered to be one of the most promising candidates for lighter, cheaper and safer next generation batteries.4 The beauty of S is attributed to its very low cost, low density, low toxicity and most of all, high theoretical specific capacity of 1672 mAh g-1. However, the dissolution of intermediate species formed during electrochemical reactions of S limits its practical applications. The dissolution of polysulfides can result in multiple problems. First of all, the cell capacity is reduced due to active material loss. Besides, dissolved species are able to move back and forth between the anode and cathode which results in low Coulombic efficiency and over-charge of the Li-S cells. Additionally, both S and final product, Li2S, are electrically isolative and thus typically require addition conductive media and size reduction.4-7 To suppress polysulfides dissolution, researchers introduced multiple strategies, such as application of conductive scaffolds8,9, physical/chemical barrier layer on sulfur10-12, modified separator
13
, new electrolytes14-17 and combination of multiple techniques stated
above.4,18,19 Although many promising results are reported, further improvements are needed. In lithium-chalcogen batteries, formation of a cathode surface layer, which acts as a polychalcogenides barrier and Li ion conductor, is an effective and intuitive method to suppress migration of such intermediate species. Indeed multiple oxide materials, such as SiOx, VOx, and TiO2, have shown significantly improved cycling performance.10,16 However, these processes and materials typically require substantial efforts and result in an increased final battery cost. Therefore, from an economic perspective in-situ protective layer formation might be a more preferable technique because it is more facile and scalable. Many carbonate solvents form electrochemically stable and Li ionic conductive solid electrolyte interphase (SEI) in-situ on graphite and other conventional anodes. Vinylene carbonate (VC),20,21 ethylene carbonate(EC),22 and fluoroethylene carbonate (FEC)23,24 have shown significant performance improvement and now widely used in battery industry. Besides, LiBOB25 and 1-fluorodecane26 have been shown to induce a stable SEI layer on such anodes. It is known that cathode SEI is typically extremely thin because the electrolyte reduction typically takes place in the operating potential of anode materials (around 0-1 V vs. Li/Li+). At very high potentials (e.g., > 4 V vs. Li/Li+), passivating 4
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surface film may also be formed by electrolyte oxidation.
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The operating potential of an S
cathode, however, is within 1-3 V vs. LiLi+, where most electrolytes are electrochemically stable. In one study, we found that FEC was used in a Li-S cell, however, the capacity improvement was limited (40 cycles) and the improvement was originated from the Li metal passivation rather than S cathode protection.28 In this study, we were interested to investigate if a solid electrolyte interphase (SEI) layer, formed on a S cathode by reducing its potential close to 0.1 V vs. Li/Li+ on the initial cycle, may effectively protect this cathode against polysulfide dissolution during the subsequent operation. Since a high (particularly > 4.5 V vs. Li/Li+) cathode potential may induce current collector corrosion, since main solvent oxidation may induce undesirable gassing and since electrolyte additives forming a passivating layer at high potentials are expensive, we selected a broadly utilized, low-cost fluoroethylene carbonate (FEC) as an SEI forming additive utilized at low potentials. Our studies demonstrated greatly improved stability of the SEI-coated S cathode. The proposed technique may also be applicable for stabilization of other conversion type cathode materials, such as those based on LiI or I, LiCl, LiF and others, against dissolution, thus offering a viable solution for the broad range of cell chemistries. Scheme 1 shows a simplified schematic of the proposed technique, where a protective SEI layer is formed around S cathode particles in-situ using FEC as an SEI forming electrolyte component. Sulfur cathode is prepared via S infiltration into activated carbon (AC) particles at 120oC and followed by 200oC heat treatment to remove excess of S from the exterior surface. The protective SEI is formed by the electrochemical reduction of FEC during the first formation cycle at potentials down to 0.1V vs. Li/Li+. This layer is formed on the outer surface of the particles and additionally fills the residual pores after full volume expansion of active material took place by conversion from S into Li2S. The electron microscopy study of the AC powder shows diameter of the particles in the range of 100-300 nm (Fig 1a). The surface of the particle is not smooth owing to the existence of mesopores.
The
morphology
of
S-AC
composite
does
not
show
evident
S
residues/agglomerations on the outer surface of the composite after introduction of S into the AC and the S and C mapping of the representative S-AC particles are exactly matched, implying uniform S distribution within the pores of S-AC (Fig 1b). The N2 isotherm curve of the AC reveals bimodal pore size distribution containing both micropores and mesopores. (Fig 1c) Large 5
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specific surface area (~2900 m2/g) and large total volume (1.85 cm3/g) of AC are beneficial for accommodating high S content and volume change in the composites. Pore size distribution confirms that synthesized AC have micropores (
