Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 24554−24563
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Electrochemically Controlled Solid Electrolyte Interphase Layers Enable Superior Li−S Batteries Yang Wang,† Chuan-Fu Lin,*,‡ Jiancun Rao,§ Karen Gaskell,† Gary Rubloff,‡ and Sang Bok Lee*,†,‡,∥ †
Department of Chemistry and Biochemistry, ‡Department of Materials Science and Engineering, and §NanoCenter AIMLab, University of Maryland, College Park, Maryland 20742, United States ∥ Graduate School of Nanoscience and Technology, KAIST, Daejeon 305-701, South Korea
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ABSTRACT: Lithium−sulfur (Li−S) batteries suffer from shuttle reactions during electrochemical cycling, which cause the loss of active material sulfur from sulfur−carbon cathodes, and simultaneously incur the corrosion and degradation of the lithium metal anode by forming passivation layers on its surface. These unwanted reactions therefore lead to the fast failure of batteries. The preservation of the highly reactive lithium metal anode in sulfur-containing electrolytes has been one of the main challenges for Li−S batteries. In this study, we systematically controlled and optimized the formation of a smooth and uniform solid electrolyte interphase (SEI) layer through electrochemical pretreatment of the Li metal anode under controlled current densities. A distinct improvement of battery performance in terms of specific capacity and power capability was achieved in charge−discharge cycling for Li−S cells with pretreated Li anodes compared to pristine untreated ones. Importantly, at a higher power density (1 C rate, 3 mA cm−2), the Li−S cells with pretreated Li anodes protected by a controlled elastomer (LiProtected-by-Elastomer, LPE)) show the suppression of the Li dendrite growth and exhibit 3−4 times higher specific capacity than the untreated ones after 100 electrochemical cycles. The formation of such a controlled uniform SEI was confirmed, and its surface chemistry, morphology, and electrochemical properties were characterized by X-ray photoelectron spectroscopy, focused-ion beam cross sectioning, and scanning electron microscopy. Adequate pretreatment current density and time are critical in order to form a continuous and uniform SEI, along with good Li-ion transport property. KEYWORDS: Li−S batteries, Li metal anodes, anode protection, elastomer, solid electrolyte interphase layer, electropolymerization, electrochemical pretreatment capacity of 1675 mA h g−1.1,2,4−6 In addition, the cathode material, sulfur, is low-cost and more abundant than the current Li-ion battery cathodes.1,2,4−6 However, the Li−S system faces its own challenges that prevent it from being widely commercialized. First of all, the active material sulfur is electrically insulating, so it requires the addition of conductive materials as the electrical connecting network or substrates, and often the substrates fail during cycling because of the drastic volume change of sulfur.1,4−7 Second, Li−S batteries suffer from “shuttle reactions”during the discharging process, sulfur in the cathode is reduced and dissolved in the electrolyte in the form of long-chain polysulfides, and then the long-chain polysulfides migrate to the anode side and react with Li to form Li2S and Li2S2, which are ionically insulating
1. INTRODUCTION With the depletion of fossil fuels, the exploitation of new energy sources and the development of new energy storage devices are becoming increasingly important. Since the commercialization of the first lithium-ion (Li-ion) battery in the 1990s by Sony Corp., this technology has achieved considerable success and become the predominant energy storage system powering portable devices.1,2 However, even the state-of-the-art Li-ion batteries so far cannot deliver enough energy and power to approach those generated from the combustion of gasoline in automobile markets.1 Currently, the best commercialized Li-ion batteries only have a gravimetric energy density up to 240 W h kg−1 and a specific capacity up to 200 mA h g−1.1,3 Therefore, energy storage devices beyond Li-ion batteries are required. Lithium−sulfur (Li−S) chemistry is one of the promising studies beyond Li-ion technologies, and it has a theoretical specific energy of 2500 W h kg−1 and a theoretical specific © 2018 American Chemical Society
Received: May 3, 2018 Accepted: June 29, 2018 Published: June 29, 2018 24554
DOI: 10.1021/acsami.8b07248 ACS Appl. Mater. Interfaces 2018, 10, 24554−24563
Research Article
ACS Applied Materials & Interfaces and insoluble products.1,2,4,7,8 During the charging process, Li2S and Li2S2 are oxidized to short-chain polysulfide intermediates and would migrate back to the cathode side and be further oxidized.4,6 These shuttle reactions can cause severe cell self-discharge, loss of active materials, low Coulombic efficiency, and fast decay of Li anodes, all of which strictly limited Li−S batteries’ commercialization.4,6 Extensive work has been successfully done on the cathode side to contain the polysulfides from dissolution and block the migration of polysulfides to the anode side.9−17 However, much work still remains to be done on the anode side in comparison with that on the cathode side. Li metal has a specific capacity of 3860 mA h g−1, which is nearly 10 times larger than the commercialized graphite.1,5 Though the use of Li metal as anodes has long been a goal, its practical application has been hindered by its high reactivity with the liquid electrolyte, dendrite growth, and theoretically infinite volume change during electrochemical cycling, which can cause catastrophic safety issues.5,6,8,18 One way to address concerns of Li metal anodes is modifying the electrolyte components to generate a stable passivation layer upon cycling, which can reduce further side reactions between the electrolyte and the anode and also minimize the generation of electrochemical hotspots that trigger the growth of dendrites.6,18,19 According to the research done by Mikhaylik et al., LiNO3 can oxidize Li metal and solvent molecules to form a passivation layer on the surface to protect the anode from further erosion by components of the electrolyte.20−23 Moreover, 1,3-dioxolane (DOL), which has been used as an electrolyte solvent, can form a preferable solid electrolyte interphase (SEI) with improved mechanical properties that can also protect the Li metal from the polysulfide reaction and suppress dendrite growth.7,8,19,24 However, with only DOL as the solvent and LiNO3 as the additive, the improvement of Li− S cycling is still limited. When the cycling current is beyond a threshold, this SEI layer cannot maintain its uniformity and good mechanical property, thus causing fast capacity decay (
