Subscriber access provided by UNIV OF DURHAM
Article
High conductivity solvates with unsymmetrical glymes as new electrolytes Shanmukaraj Devaraj, Sandrine Lois, Sébastien Fantini, Francois Malbosc, and Michel Armand Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04270 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 18, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
High conductivity solvates with unsymmetrical glymes as new electrolytes Devaraj Shanmukaraj†, Sandrine Lois‡, Sebastien Fantini‡, Francois Malbosc‡ and Michel Armand†* † ‡
CIC EnergiGUNE, Parque Tecnológico de Álava, 48, 01510 Miñano, Álava, Spain. Solvionic, Site Bioparc Sanofi, 195 Route d’Espagne, Toulouse 31036, France.
Abstract A designed custom glyme ether with unsymmetrical (ethyl and butyl) end groups (UG) have been shown to give highly conducting liquid solvate electrolytes. Solvates prepared with UG glyme: salt ratio of 1:1, 2:1 and 3:1 using Lithium bis(trifluoromethane sulfonyl) imide (LiTFSI) and Lithium bis(fluoro sulfonyl) imide (LiFSI) lithium salts have been characterized with emphasis on the 2:1 composition. Conductivity in the order of 10-3 S/cm was observed at RT with an electrochemical stability window of 4.5 V. DSC studies indicate a low crystallisation temperature between -60°C and -75°C for LiFSI based solvate electrolyte. Galvanostatic cycling studies (C/10) with LiFePO4 electrodes indicate a capacity deliverance of 145mAh/g and good C-rate capability up to 2C at RT. Preliminary tests using unsymmetrical glyme-based solvate electrolytes with sulphur and LiNi1/3Mn1/3Co1/3O2 (NMC) electrodes reveal a low degree of polysulphide dissolution with sulphur electrodes and stable voltage profiles with NMC electrodes suggesting the use of these electrolytes for Li/S batteries and high-voltage Li-ion batteries.
1. Introduction Lithium batteries, whose success increases by the day, face the double challenge of increasing the energy density and improving safety1-5. A lithium metal negative electrode is seen as one of the most effective strategy for the former6. A change in 1 ACS Paragon Plus Environment
Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
electrolyte from the volatile/flammable mixtures of organic carbonate solvents is actively searched for, with ceramic, polymer and ultra-concentrated salt solutions7-12. To now, only PEO based polymer electrolytes have shown, in commercial cells, to be able to cycle well with a Li° electrode, but operate at 60-80°C13 due to the low conductivity of the system at RT. Angell has studied extensively concentrated solution especially in water or in polymers (“polymer in salt”) and studied their fragility which gives an indication of the decoupling of conductivity from viscosity 14-17. In the recent years, Watanabe and co-workers18-21 have shown that the so-called glymes i.e. dimethyl ether of oligo-ethylene glycols, in this case G3 and G4 having four respectively five coordinating oxygen, gave 1:1 complexes with Li salts of the imide family, namely Li[(CF3SO2)2N] (LiTFSI) which were liquid at ambient temperature with conductivities in the range of 1.1 and 1.6 mS.cm-1 for G3 and G4. An added advantage was found in terms of stability window, as all the ether oxygens are engaged in electron doublet donation with lithium, making the electrolyte much less prone to get oxidized, and in practice, up to +4.5 volts have been suggested from CV experiments. Though appealing, such materials, when used in an operating battery with lithium-exchanging electrodes have problems of solvent balance at the electrodes. At the anode (negative electrode on discharge, positive on charge) when lithium is injected in the electrolyte, there is no free glyme available to solvate the corresponding salt, and this will lead to its precipitation. Conversely, at the cathode the desolvation process frees some glyme, not coordinated to Li+ and this lowers the potential at which the electrolytes get oxidized. Also, there is now strong indications that the glymes with their terminal O–CH3 ether group are mutagenic and should be phased out in the future (EPA norms22). We thus decided to address both concerns by redesigning the PEG ethers used for the preparation of solvates with the unsymmetrical end groups, ethyl and butyl in this 2 ACS Paragon Plus Environment
Page 2 of 16
Page 3 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
study. Diethylene glycol was chosen as the PEG base. The lack of symmetry was expected to allow a wider liquid range, in an approach similar to that of ionic liquids, and lithium salt would be able to coordinate to form a mono solvate (3 oxygens) and a di-solvate (6 oxygens) the latter satisfying all the coordination requirement of Li+. Further, the solvates would likely be miscible into excess solvent, due again to the low symmetry of the solvate.
2. Experimental Diethylene glycol ethyl-n-butyl ether (Unsymmetric glyme-UG) was custom synthesized through a Williamson coupling from diethylene glycol monoethyl ether and butyl chloride. The solvent was further dried by adding NaH followed by distillation in a bulb-to-bulb apparatus (Büchi®-B585). All handling was carried out inside a dry-box under Ar atmosphere (H2O/O2 < 0.1ppm). LiTFSI (Li(CF3SO2)2N) came from Solvay and LiFSI (Li(SO2F)2N) was received from Fluolyte® (Suzhou, China). Solvates of UG/ lithium salt (LiFSI/LiTFSI) was prepared inside a glove box under Argon atmosphere (H2O and O2
