Highly reversible lithium metal anode and lithium-sulfur batteries

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Surfaces, Interfaces, and Applications

Highly reversible lithium metal anode and lithiumsulfur batteries enabled by an intrinsic safe electrolyte Jiahang Chen, Huijun Yang, Xuan Zhang, Jingyu Lei, Huiming Zhang, Huanhuan Yuan, Jun Yang, Yanna NuLi, and Jiulin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09215 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 19, 2019

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ACS Applied Materials & Interfaces

Highly reversible lithium metal anode and lithium-sulfur batteries enabled by an intrinsic safe electrolyte Jiahang Chen, Huijun Yang, Xuan Zhang, Jingyu Lei, Huiming Zhang, Huanhuan Yuan, Jun Yang, Yanna Nuli, Jiulin Wang* (Department of Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China, [email protected])

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Abstract: Rechargeable lithium-metal batteries (LMBs) have gained significant attention as potential candidates of energy storage systems, but severe safety issues including flammable electrolyte and dendritic lithium formation hinder their further practical application. In this work, we develop a novel intrinsic flame-retardant electrolyte, which enables stable and dendrite-free cycling with lithium plating/stripping Coulombic efficiency (CE) up to 99.1% over 500 cycles. Raman spectra indicates no free molecular solvent existence and X-ray photoelectron spectroscopy (XPS) reveals the LiF-rich interphase on Li metal anode. When coupled with sulfurized pyrolyzed poly(acrylonitrile) (S@pPAN) cathode, it shows benign electrochemical reversibility with areal-capacity up to 3.41 mAh cm-2 after 70 cycles. To further check its compatibility with sulfur cathode, higher sulfur content (51.6%) is examined with areal capacity of 3.92 mAh cm-2 and sulfur utilization of 81.7%. This work provides an alternative for safe and high-performance Li-S batteries via novel electrolyte strategy. Keywords: flame-retardant electrolyte; lithium metal anode; Coulombic efficiency; dendrite-free; lithium-sulfur battery;


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1. Introduction Nowadays, lithium-ion batteries (LIBs) have been widely used in portable electronic products and hybrid electric vehicles (HEVs).1,2 However, the development of LIBs gradually reaches the eventual maturation limited by the energy density (300 Wh kg-1).3,4 Accordingly, rechargeable lithium-metal batteries (LMBs) have attracted extensive attention as the next generation of high-energy storage system due to the ultrahigh specific capacity (3,860 mAh g-1) and the lowest redox potential (-3.04 V vs. standard hydrogen electrode (SHE)) of Li, which is regarded as the “holy grail” in the battery community.5 Numerous strategies have been implemented to modify the electrochemical behavior of LMBs.6-8 However, safety issues remain to restrain their further development and practical application,9,10 because 1) the volatile and flammable nature of the conventional electrolyte solvents like esters or carbonates are easily ignited for thermal runaway and explosion; 2) the Li metal is highly reactive and undesirable lithium dendrite forms to induce short-circle. Besides, detrimental side reactions occur in the instable interface between Li and electrolyte during the repeated cycles, which induce a low Coulombic efficiency (CE).11,12 Electrolytes play one of the vital roles to affect the performance of the LMBs,13,14 but the conventional electrolytes such as ether-based electrolytes and carbonate electrolytes are highly volatile and flammable, and can be ignited at room temperature, attributed to the low flash points and boiling points.15 Under extreme conditions, like over voltage, over current, over temperature, the electrolyte may break down to release flammable gas and the build-up pressure induces possible rupture. Once



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encountering the oxygen released from cathode breakdown or in air, the flammable gas is likely to burn and even explode.16-18 Many different strategies have been employed to address the safety problems and the research on safe electrolytes appears to be effective.19,20 Solid-state electrolytes have been proved nonflammable whereas suffer the native limitations of solid-state electrolytes, such as the low ionic conductivity at room temperature and high interface resistance between electrolyte and electrode.21-24 The ionic liquid electrolytes also attract abundant works for their thermal stability as well as excellent ionic conductivity, but they show high viscosity and poor compatibility with conventional electrode materials.25,26 Meanwhile, numerous phosphate solvents are studied as additives and co-solvents to render a flame-retardant electrolyte, such as triethyl phosphate (TEP),27,28 trimethyl phosphate (TMP),29,30 which

prove efficient and

effective to render a flame-retardant electrolyte. Aimed at the unstable interphase, it has been demonstrated that high-concentration electrolytes can improve the interfacial stability, attributed to their unique solvation structures and functionalities.31-33 In such electrolytes, almost all solvents are solvated with Li+ and no free solvents exist, which differ greatly from that of dilute electrolytes. More importantly, the high-concentration electrolytes can efficiently suppress the growth of Li dendrite in LMBs. Moreover, inert solvents which have minimal effect on Li+ solvation were applied as co-solvent to improve the viscosity and ionic conductivity.34-36


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According to reported literature, artificial Li3PO4 layer possesses high Young’s modulus and enhanced Li+ transport capacity while employing phosphate solvents directly

may

achieve

“concentrated+diluted”

the

same

effect.37

Yamada’s

lithiumbis(ﬂuorosulfonyl)amide

group38

reported

(LiFSA)/TMP-HFE

to

achieve both non-flammability and excellent cycle performance of carbonaceous negative electrodes. Yang et al.39 employed TEP solvent to form an intrinsic flame-retardant electrolyte system for lithium-sulfur (Li-S) batteries. Comparing two flame-retardant solvent TEP and TMP, TMP has higher dielectric constant (21.6),40 which guarantees better physical property like higher ionic conductivity of electrolyte. Moreover, TMP may react preferentially with lithium based on the molecular structure. Inspired by this idea, herein we reported a novel flame-retardant electrolyte, consisting of saturated lithium bis(fluorosulfonyl)imide (LiFSI) in a mixture of TMP and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl (HFE) in which the LiFSI concentration ranged from 3.6 to 1.3 M diluted by HFE as the co-solvent (corresponding to TMP/HFE volume ratios of 1:1, 1:2, 1:3, and 1:4). Particularly, the 1.6 M LiFSI/TMP-HFE (1:3) electrolyte can enable high CE (>99%), dendrites-free and stable cycling of lithium metal anodes. Furthermore, coupled with common and high-loading cathode respectively, it exhibited excellent cycling stability and rate performance. Especially the high-loading cathodes with higher sulfur content displayed remarkable areal capacity, which work profoundly for the practical application of Li-S battery.



2. Materials and Methods 2.1 Electrolyte preparation and characterization The high-concentration electrolytes and proposed electrolytes were prepared through dissolving LiFSI into the solvent mixtures of TMP and HFE at the volume ratios of 1:0 (pure TMP), 1:1, 1:2, 1:3, and 1:4 respectively until they were saturated. Then the corresponding molar concentrations were calculated as 5.0 M, 3.6 M, 2.3 M, 1.6 M and 1.3 M. All the electrolytes were stored in glass bottles. The pure trimethyl phosphate (TMP) was made by distillation from a lower concentrated TMP commercially purchased from Aladdin Shanghai China and then was quickly transferred to argon-filled glove box. The 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl (HFE) (purity>99.9%) was kindly provided by Braunway Technology Co., Ltd. The TMP and HFE were dried using 4 Å molecular sieves prior to use. The conventional battery-grade electrolyte of 1.0 M lithium hexafluorophosphate (LiPF6) in a mixture of ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 by volume) was purchased directly from Company Xiaoyuan. Furthermore, all chemicals were stored in and experiments were conducted in an Ar-filled glove box (MB-10 compact, MBRAUN) with almost no O2 and H2O (

Highly reversible lithium metal anode and lithium-sulfur batteries

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