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Highly Safe Ionic Liquid Electrolytes for Sodium-Ion Battery: Wide Electrochemical Window and Good Thermal Stability Feng Wu, Na Zhu, Ying Bai, Libin Liu, Hang Zhou, and Chuan Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07054 • Publication Date (Web): 25 Jul 2016 Downloaded from http://pubs.acs.org on August 1, 2016
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ACS Applied Materials & Interfaces
Highly Safe Ionic Liquid Electrolytes for Sodium-Ion Battery: Wide Electrochemical Window and Good Thermal Stability Feng Wu†,‡, Na Zhu†, Ying Bai†,*, Libin Liu†, Hang Zhou†, Chuan Wu†,‡ † Beijing Key Laboratory of Environmental Science and Engineering, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, PR China ‡ Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, PR China KEYWORDS: ionic liquid; electrolyte; NaBF4; quantum chemical theoretical calculation; sodium- ion battery ABSTRACT: Novel ionic liquid (IL) electrolytes are prepared by mixing 1-ethyl-3-methylimidazolium-bis
tetrafluorobroate
(EMIBF4)
with
different
concentrations of sodium salt NaBF4. The as-prepared IL electrolytes displays wide electrochemical windows around 4V (1～5V), which are consistent with the quantum chemical theoretical calculation. The IL electrolyte with 0.1M NaBF4 shows excellent ionic conductivity, namely, 9.833×10-3 S·cm-1 at 20℃. In addition, non-flammability and good thermal stability are exhibited by combustion test and thermo-gravimetric analysis (TGA), which indicate highly safety of the IL electrolyte. 1. INTRODUCTION 1
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Lithium-ion batteries (LIBs) have been considered as the dominant charge storage units for high energy and power electrical application due to their high energy and power densities.1-5 However the application of LIBs for the widespread use faces important challenges linked with the high cost and availability of Li.6-9 These concerns have pushed to develop novel alternative energy storage devices which could replace LIBs. Sodium-ion batteries (SIBs) have rapidly captured much attention because of the abundant resources and low cost of sodium.10-11 The element sodium is easy to recycle.12-14 What’s more, sodium shares similar electrochemical properties and intercalation chemistry with lithium.15-16 The redox potentials of sodium and lithium differ of only 300mV.13, 17 An electrolyte plays an important role in secondary batteries, which governs the electrochemistry in a battery. At present, organic electrolytes such as NaClO4/ PC, NaPF6/EC-DEC and NaClO4/EC-DEC have been widely used.18 However, the organic solvent safety has always been a noticeable problem and restricted the development owing to high flammability, poor thermal stability, and low heat capacity19. RTILs characterized by wide electrochemical window, non-flammability, high ionic conductivity, high thermal stability improve safety for secondary batteries. 20-26 Ionic liquid 1-ethyl-3-methylimidazolium bis-(trifluoromethanesulfonyl) imide (EMITFSI), 1-butyl-3-methylimidazolium-bis (trifluoromethylsulfonyl) imide (BMITFSI) and 1-butyl-3-methylimidazolium tetraflu orobroate (BMIMBF4) were adopted as the electrolytes in the sodium-ion batteries in recent studies.26-27 However, BMIBF4 with 2
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ACS Applied Materials & Interfaces
high viscosity showed low conductivity and inferior cell cycle performance.27 EMITFSI and BMITFSI with the corresponding sodium salt (NaTFSI) exhibited excellent ionic conductivities, up to 5.5 mS. cm-1 at room temperature, and a useful thermal window of - 86 to 150 ℃.26 As shown in Table 1, the issues of RTILs with EMI+
BMI+
and
(EMIBF4,
EMIPF6:
1-ethyl-3-methylimidazolium-bis
hexafluorophosphate, EMITFSI, BMIBF4, BMIPF6: 1-butyl-3-methylimidazolium-bis hexafluorophosphate, BMITFSI) are listed. Table 1. The comparison of primary imidazole-based RTILs (EMIBF4, EMIPF6, EMITFSI, BMIBF4, BMIPF6, BMITFSI) used in SIBs Anion Cation BF4-
PF-6
TFSI-
BMI+
Low conductivity
Low conductivity
Low conductivity
+
Show liquid state at
Shows solid state at
room temperature;
room temperature
High conductivity;
EMI
High cost
Low cost;
Therefore, considering the low conductivity of ILs with BMI+, high cost of EMITFSI and the solid state of EMIPF6 at room temperature, herein, we investigated the molecular structure and electrochemical performance of EMIBF4 and did the quantum chemical theoretical calculation. . Quantum chemical theoretical calculation can help design the composition of solution in secondary batteries.28-31 Quantum chemical theoretical calculation based on density functional theory (DFT) is used to evaluate the stability of a solvent molecule 3
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due to its ability of gaining or losing electrons.20 The ability depends on frontier molecular orbital, including the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In theory, the lower HOMO energy is, the better oxidation resistance；The higher LUMO energy is, the better resistance to reduction.32 The RTIL electrochemical window is tested and speculated according to the results of quantum chemical theoretical calculation. In this work, the electrochemical properties of a promising RTIL EMIBF4 in combination with different concentrations (from 0.1 to 0.75M) of NaBF4 are investigated. With increasing concentration of the sodium salt, lower ionic conductivity and wider electrochemical window (>4V) are found, which has been confirmed by quantum chemical theoretical calculation. The IL electrolytes are proved to be non-flammable and stable under 380 ℃ . The GF/C and GF/D membranes are compatible with the IL electrolytes, which makes NaBF4/EMIBF4 electrolyte available in sodium-ion batteries. 2. EXPERIMENTAL SECTION 2.1. Preparation of IL Electrolytes
The pure IL EMIBF4 (99%) was purchased from Shanghai Cheng Jie Chemical Co.LTD, which was used as received. The water content of the electrolyte was measured using Karl-Fisher titration. It was found to be
