Electrochemically Controlled Solid Electrolyte Interphase Layers

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Electrochemically Controlled Solid Electrolyte Interphase Layers Enable Superior Li-S Batteries Yang Wang, Chuan-Fu Lin, Jiancun Rao, Karen J. Gaskell, Gary W. Rubloff, and Sang Bok Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07248 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on June 30, 2018

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ACS Applied Materials & Interfaces

Title Electrochemically Controlled Solid Electrolyte Interphase Layers Enable Superior Li-S Batteries Authors Yang Wang,† Chuan-Fu Lin,*,‡ Jiancun Rao,§ Karen Gaskell,† Gary Rubloff,‡ and Sang Bok Lee*,†,‡,¶ Affiliations †

Department of Chemistry and Biochemistry, ‡Department of Materials Science and Engineering, § NanoCenter AIMLab, University of Maryland, College Park, Maryland, 20742 USA and ¶ Graduate School of Nanoscience and Technology, KAIST, Daejeon 305-701 South Korea Abstract Lithium-sulfur (Li-S) batteries suffer from shuttle reactions during electrochemical cycling, which cause the loss of active material sulfur from sulfur-carbon cathode, 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 electrolyte has been one of the main challenges for Li-S batteries. In this study, we systematically controlled and optimized the formation of 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 higher power density (1 C rate, 3mA cm-2), the Li-S cells with pretreated Li anodes protected by controlled 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 controlled uniform SEI was confirmed and its surface chemistry, morphology and electrochemical properties were characterized by X-ray photoelectron spectroscopy (XPS), focused-ion beam (FIB) cross-sectioning, and scanning electron microscopy (SEM). Adequate pretreatment current density and time are critical in order to form the 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

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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 Sony Corp. commercialized the first lithium-ion (Li-ion) battery in 1990s, this technology has achieved considerable success and become the predominant energy storage system powering portable devices.1,

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However, even the state-of-the-art Li-ion batteries so far cannot deliver enough

energy and power to approach those generated from combustion of gasoline in automobile markets.1 Currently, the best commercialized Li-ion batteries only have a gravimetric energy density up to 240 Wh kg-1 and specific capacity up to 200 mAh g-1.1, 3 Therefore, energy storage devices beyond Li-ion batteries are required. Lithium-sulfur (Li-S) chemistry is one of the promising beyond Li-ion technologies, and it has a theoretical specific energy of 2500 Wh kg-1 and theoretical specific capacity of 1675 mAh g-1.1, 2, 4-6

In addition, the cathode material, sulfur, is low-cost and more abundant than current Li-ion

battery cathodes.1, 2, 4-6 However, 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 due to the drastic volume change of sulfur.1,

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Second, Li-S batteries suffer from “shuttle reactions” - during the discharging process, sulfur in cathode is reduced and dissolved in electrolyte in the form of long-chain polysulfides, then the long-chain polysulfides migrate to the anode side and react with Li to form Li2S and Li2S2, which are ionically insulating and insoluble products.1, 2, 4, 7, 8 During the charging process, Li2S and Li2S2 are oxidized to short-chain polysulfides intermediates and would migrate back to the cathode side and be further oxidized.4,

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These shuttle reactions can cause severe cell self2


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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. Li metal has a specific capacity of 3860 mAh g-1 which is nearly ten 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 liquid electrolyte, dendrite growth, and theoretically infinite volume change during electrochemical cycling, which can cause catastrophic safety issues5, 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 electrolyte and the anode and also minimize the generation of electrochemical hotspots that trigger the growth of dendrites.6, 18, 19 According to research done by Mikhaylik et al., LiNO3 can oxidize Li metal and solvent molecules to form a passivation layer onto the surface to protect the anode from further erosion by components of electrolyte.20-23 Moreover, DOL (1,3-dioxolane), which has been used as an electrolyte solvent can form preferable SEI with improved mechanical properties that can also protect Li metal form polysulfide reaction and suppress dendrite growth7, 8, 19, 24. However, with only DOL as solvent and LiNO3 as 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 cause fast capacity decay (< 150 mAh g-1, after 100 cycles) or Li dendrites formation at high power.6, 25 Most of the Li-S battery anode research has been done by forming the desired SEIs through adding new additives or lithium salts,26-34 exploiting new solvents35 and by limiting the 3



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electrolyte decomposition through maintaining high concentration of lithium salts.36-38 Busche et al developed a solid-liquid hybrid electrolyte interphase.39 Other protective layers on Li anodes have been deposited via surface coatings with conductive polymers,40-46 porous membrane,47 carbon materials,37, 48, 49 glassy fiber,50 thin film methods,51-57 ionic liquid,58 inorganic salts59, 60 and metal oxide coatings.61, 62 Hybrid anodes were also developed as an alternative.63, 64 Here, our research focuses on controlling the electropolymerization of DOL to electrochemically grow -well-defined SEI layer on Li metal anodes that serves as a good protection layer of Li metal that possesses good mechanical properties for high power cycling.24 Several groups including our own have employed electrochemically grown SEI layer to protect Li metal anodes.53, 55, 57 Nonetheless, in this work we have focused on the systematic study of the effects of electrochemical pretreatment under various conditions (e.g. current density and the total charge). Additionally, we identified the chemical variations of the artificial SEIs prepared by different current densities, which allows us to connect the chemical composition of the SEIs, and the electrochemical performance to the pretreatment conditions. . The Li anodes protected by the controlled elastomer (LPE) demonstrated much better Li-S battery performance compared with the cells that have untreated Li with uncontrolled formation of SEI, in terms of specific capacity, rate capability and Li dendrites formation. Therefore, this controlled SEI layer in Li-S system can better accommodate the volume change and suppress dendrite growth during electrochemical cycling, which may in result greatly expand its cycle life and alleviated safety risk. We also conducted extensive characterizations and testing to study the surface chemistry and morphology of the layer, to understand how setting of electrochemical pretreatment affected its properties which ultimately determined the battery performance of the Li metal anodes.

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Methods and Materials 1. Pretreatment The 9.5 mm in diameter Li metal anodes were punched from 0.75 mm thick Li ribbon (SigmaAldrich) stored inside an Ar filled glovebox (MBraun LabStar 20) and pressed onto 304 stainless steel spacers (15.5 mm diameter x 0.2 mm). Those Li metal anodes were assembled into symmetric coin cells (CR2013, MTI Corp) with a Celgard separator and 80 µL of 0.35 M LiTFSI (Lithium bis(trifluoromethane)sulfonamide, Sigma-Aldrich) in DME (1,2-dimethoxyethane, Sigma-Aldrich):DOL (1,3-dioxolane, Sigma-Aldrich) (1:1, v/v) electrolyte with 1% w/w LiNO3 (Alfa Aesar) as additive. During pretreatment, these symmetric coin cells were cycled for different discharge/charge cycles (i.e. 25, 50, 100 and 200, annotated as LPE-25, LPE-50, LPE100 and LPE-200 respectively) of 1 h per discharge or charge process at a low current density of 0.03 mA cm-2 to form the polymeric SEI layer. Separately, symmetric coin cells were also cycled at a higher current density of 0.3 mA cm-2 to study the effect of pretreatment current density on the formation of the polymeric layer and the subsequent battery performance, which were annotated as LPE-50-x10. After the Li metal anode pretreatment, the coin cells were disassembled, and the anodes were removed in glove box. Those anodes were washed with DME then vacuum dried for 30 min in a vacuum transfer chamber loaded directly from the glove box. Later the pretreated anodes were separately stored in dry packs inside the glove box. Li metal anodes freshly cut from Li ribbon without any pretreatments were used as control for all characterizations and electrochemical testing which were annotated as LPE-0. 2. Characterizations The pretreated Li anode samples and untreated control samples were transferred via an air-tight glove bag with dry nitrogen atmosphere to an XPS system for surface chemical analysis. The samples were exposed to the dry nitrogen atmosphere for less than 1 minute. XPS data were 5



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collected on a Kratos axis 165 X-ray photoelectron spectrometer operating in hybrid mode, using monochromatic Al Ka x-rays (240 W). Charge neutralization was required to minimize sample charging, the working pressure of the instrument was 5 x 10-8 Torr or better throughout data collection. Survey spectra and high-resolution spectra were collected with pass energies of 160 eV and 40 eV respectively. Peak fitting was done using Casa XPS software after application of a Shirley background, using peaks with a 30 % Lorentzian, 70% Gaussian product function. All peaks within a region were fixed to have peaks of equal FWHM (full width at half maximum), the spin-orbit split components of the S 2p were fixed to have spin-orbit splitting of 1.18 eV and area ratios of 2:1 for the 3/2, 1/2 components respectively, the O-C-O, RCOOLi and CO32- were fixed to have separations of 1.0, 2.0, 3.4 eV separation from the R-C-O peak. All spectra were calibrated to the C-C/C-H peak at 285.0 eV. The samples for FIB-SEM and EDS characterizations were sealed in glove bag with Ar atmosphere in the glove box. The glove bag was not opened until the samples were ready to be loaded onto an SEM stage, which ensured minimal (

Electrochemically Controlled Solid Electrolyte Interphase Layers

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