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Electrolyte Additive Enabling ConditioningFree Electrolytes for Magnesium Batteries Sung-Jin Kang, Hyeonji Kim, Sunwook Hwang, Minsang Jo, Minchul Jang, Changhun Park, Seung-Tae Hong, and Hochun Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13588 • Publication Date (Web): 07 Dec 2018 Downloaded from http://pubs.acs.org on December 11, 2018
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ACS Applied Materials & Interfaces
Electrolyte Additive Enabling ConditioningFree Electrolytes for Magnesium Batteries Sung-Jin Kang,† Hyeonji Kim,† Sunwook Hwang,† Minsang Jo,† Minchul Jang,‡ Changhun Park, ‡ Seung-Tae Hong,† and Hochun Lee*,†
†Department ‡Batteries
of Energy Science and Engineering, DGIST, Daegu 42988, Republic of Korea
R&D, LG Chem Ltd., Daejeon 34122, Republic of Korea
*Corresponding author: Tel.: +82-53-785-6411 Fax: +82-53-785-6409 E-mail:
[email protected] 1
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ABSTRACT: Most electrolytes for rechargeable Mg batteries require time-consuming conditioning or pre-cycling process to achieve fully reversible Mg deposition/dissolution, which hinders the normal operation of Mg batteries. This study details a simple and effective method for eliminating this conditioning behavior using heptamethyldisilazane (HpMS) as an electrolyte additive. It was found that the HpMS additive greatly increases the current density and Coulombic efficiency of Mg deposition/dissolution from the initial cycles in various sulfone and glyme solutions containing MgCl2 or Mg(TFSI)2. The beneficial effect of HpMS was ascribed to its ability to scavenge trace water in the electrolytes and remove Mg(OH)2 and Mg(TFSI)2-decomposition products from the Mg surface. Considering its applicability for a wide range of Mg electrolytes, the use of HpMS is expected to accelerate the development of practical Mg batteries.
Keywords: Magnesium battery, Electrolyte, Additive, Heptamethyldisilazane, Conditioning behavior, MgCl2, Mg(TFSI)2
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ACS Applied Materials & Interfaces
1. INTRODUCTION Rechargeable Mg batteries are promising candidates to replace Li-ion batteries due to the innate advantages of the Mg anode such as low electrode potential, high theoretical volumetric capacity (3800 mAh cm−3), environmental inertness, and relative abundance.1−5 However, the development of practical Mg batteries has stagnated as a result of the spontaneous formation of a passivation layer on the Mg surface in most conventional electrolyte systems, which hinders the interfacial charge transfer reaction.1,6,7 Although mixtures of Grignard reagents and strong Lewis acids enable reversible Mg deposition/dissolution, these systems suffer from low anodic stability, high chemical reactivity, and high flammability.8−11 To date, various Mg electrolytes free from Grignard reagents and Lewis acids have been reported, including glymes, ethers, sulfones, and ionic liquids containing magnesium bis(trifluoromethanesulfonyl)amide (Mg(TFSI)2), or MgCl2.12−19 Unfortunately, these nonGrignard electrolytes commonly suffer from “conditioning” or “pre-cycling” behavior.12,18,20 During conditioning, the electrochemical reversibility of Mg deposition/dissolution gradually improves, the current density and Coulombic efficiency (CE) increase, and the overpotential diminishes over consecutive cycles. The adverse influence of conditioning is two-fold. (1) It is time consuming, requiring dozens or hundreds of cycles depending on the experimental conditions (e.g., nature of the electrolyte and the amount of electrolyte relative to the electrode material). (2) The huge overpotential during oxidation of the Mg anode hampers the normal operation of Mg batteries. Since most Mg cathode materials developed to date are Mg-deficient in their pristine state, the first discharge reaction, i.e., the magnesiation of the cathode, can proceed only when Mg-ions are provided by the dissolution of the Mg anode. Conditioning behavior is commonly ascribed to changes in the bulk electrolyte properties or in the surface chemistry of the Mg electrode.21−23 Recently, it was reported that the presence of 3
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trace water impurity forms a Mg-ion blocking surface layer composed of MgO and Mg(OH)2, greatly slowing the kinetics of Mg deposition.21−23 To accelerate or eliminate conditioning, Mg powder treatment of Mg and Al chloride complex (MACC) electrolytes has been proposed.24,25 However, these methods rely on the use of flammable Mg powder and toxic CrCl3. Meanwhile, Mg-alkoxides have been reported as an electrolyte additive for promoting the reversible Mg deposition in a Mg(TFSI)2 triglyme solution.26,27 Nonetheless, the presence of Mg-alkoxides leads to complex ionic speciation and poor anodic stability of the electrolyte. In addition, the performance improvement upon alkoxide addition proved insufficient to ensure a reversible Mg redox reaction. More recently, the addition of Mg(BH4)2 has been reported to facilitate Mg deposition/dissolution by scavenging water impurities in the Mg(TFSI)2 tetraglyme electrolyte.28 However, the poor anodic stability of Mg(BH4)2 impairs the electrochemical stability window of the electrolyte. Although considerable progress has been made regarding suitable electrolytes for Mg/S batteries, further improvements are sorely needed.29−32 To address the aforementioned conditioning issue when using non-Grignard Mg electrolytes, in this study, heptamethyldisilazane ((CH3)3SiN(CH3)Si(CH3)3, HpMS) was used as an electrolyte additive. It was found that HpMS markedly enhances the reversible Mg deposition and dissolution reactions in various non-Grignard Mg electrolytes without compromising other properties of the pristine electrolytes, i.e., electrochemical stability and ionic conductivity. In addition, the origin of the beneficial effects of HpMS was examined in terms of bulk electrolyte properties and surface chemistry of the Mg electrode by in-depth chemical, electrochemical, and spectroscopic analysis.
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2. EXPERIMENTAL SECTION 2. 1. Chemicals and solutions. Dipropyl sulfone (DPSO, >99.0%) was purchased from TCI. Heptamethyldisilazane (HpMS, >97.0%), 1,2-dimethoxyethane (1G, 99.5%), diglyme (2G, 99.5%), triglyme (3G, 99%), tetrahydrofuran (THF, >99.9%), aluminum chloride (AlCl3, 99.99%), phenyl magnesium chloride (PhMgCl, 2 M solution in THF) were obtained from Aldrich. MgCl2 (99.9%, Alfa) and magnesium(II) bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2, 99.5%, Solvionic) were used as-received. All solvents were treated with molecular-sieves to minimize the water content (
