Ionic liquid doped PEO-based solid polymer electrolytes for lithium-ion polymer batteries
Graphical abstract
Introduction
Rechargeable lithium-ion batteries represent an excellent choice for electrochemical power sources described by high energy density, good cyclability and reliability [1]. One of the major challenges in the development of Li-ion batteries is finding a suitable electrolyte [2]. Lithium batteries based on solid polymer electrolytes (SPEs) have many advantages over their liquid counterparts such as high energy density, flexible geometry, no-leakage of electrolyte, thus higher safety [3]. Additionally, the SPEs are the materials of choice in upcoming devices, including Li-sulfur or Li-O2 batteries, due to their solid-state nature which inhibits dendritic growth of Li metal, as well as dissolution of electrode materials and gas permeation [4]. Unfortunately, solid polymer electrolytes (SPEs) offer very low ionic conductivity of the order of 10−7 S/cm at ambient temperature that excludes their practical applications [5]. Many improved methods have been reported to overcome this problem, for example polymer blending, using single ionic conductive polymers as polymer electrolyte membrane [6] adding organic/inorganic nanoparticles or ionic liquids and so on [7], [8].
In this work, poly(ethylene oxide) (PEO) was selected as a host polymer for an SPE system. This polymer has been widely investigated so far because of its efficiency in coordinating metal ions, due to the finest distance and alignment of the ether oxygen atoms in polymer chains [9]. The weak coordination in presence of the salt makes possible the incorporation of ion with the polymer PEO and reduces the PEO degree of crystallinity. The increased amorphous phase favors the inter- and intrachain hoping of ion and develops ionic conduction [10]. Nowadays, there has been increasing interest to replace the conventional lithium salt LiPF6 in lithium-ion batteries. LiPF6 is thermally unstable and may decompose into LiF and PF5 while absorbing the room temperature moisture [11], [12]. From last two decades, many salts have been proposed for the replacement of LiPF6, but so far none of the proposed salts have been met necessary requirements for commercial Li-ion batteries. Lithium difluoro(oxalato)borate (LiDFOB) have the many better unique properties which possesses the combined advantages of LiBOB and LiBF4 [12].
The incorporation of room-temperature ionic liquids (RTILs) as plasticizers is regarded as a most promising approach to improve the ionic conductivity and electrochemical properties of the SPEs [13]. The RTILs are molten salts at room temperature that generally consist of large organic cations and inorganic anions. The RTILs benefit by numerous favorable properties, including their good chemical and electrochemical stability, non-flammability, negligible vapor pressure and high ionic conductivity [14]. In addition, the incorporation of ionic liquid leads to a significant enhancement in the contribution of Li+ ions to the overall conductivity of the PEO-Li salt complexes [13]. In this work, we selected 1-ethyl-3-methyllimidazolium bis(trifluoromethyl-sulfonyl)imide (EMImTFSI) ionic liquid, because of its high conductivity, low viscosity and wide electrochemical stability window [13].
The purpose of this work is to prepare PEO-LiDFOB-EMImTFSI solid polymer electrolyte and evidence a potential electrolyte material for batteries. The electrolyte was characterized in terms of structural, thermal, electrical and electrochemical properties. Finally, the best performing sample is assembled in Li polymer “coin” cell with a LiFePO4 cathode and Li-foil anode was assembled to demonstrate a possibility as an electrolyte material for the future concepts of all solid-state Li polymer batteries.
Section snippets
Materials
Poly(ethylene oxide) (MW = 4 × 106), anhydrous acetonitrile were obtained from Sigma–Aldrich. The lithium salt, lithium difluoro(oxalato)borate (LiDFOB, MW = 143.77 g/mol), was purchased from Suzhou Fosai New Material Co., Ltd., China. The ionic liquid 1-ethyl-3-methyllimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI) was purchased from C-TRI, South Korea.
Preparation of solid polymer electrolyte membranes
The solid polymer electrolyte membranes have been prepared by using solution cast technique. The high molecular weight PEO and LiDFOB
XRD analysis
XRD studies have been carried out to investigate the possible changes in the semi-crystalline nature of PEO due to the addition of lithium salt (LiDFOB) and ionic liquid (EMImTFSI). Fig. 2 shows the recorded XRD patterns of the pure PEO, pure LiDFOB and PEO-LiDFOB-EMImTFSI solid polymer electrolytes at different concentrations of ionic liquid. From the XRD pattern of pure PEO, it has sharp well-defined peaks at 19.36° and 23.72° with a halo at the base indicating the semi-crystalline nature.
Conclusions
In the present study, solid polymer electrolyte membranes comprising different concentrations of EMImTFSI ionic liquid were prepared by standard solution casting technique and thoroughly studied. XRD and DSC studies revealed the amorphous nature of the SPE membranes that produce greater ionic conductivity. From impedance spectroscopy, it has been shown that the ionic conductivities of the SPE membranes increased with increase in IL loading and exhibited a maximum value of 1.85 × 10−4 S/cm at
Acknowledgments
This work was supported by Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future Planning, Korea (2011-0031570) and by the Korea Center for Artificial Photosynthesis (KCAP) located in Sogang University funded by the Minister of Science, ICT and Future Planning (MSIP) through the National Research Foundation of Korea (No. 2009-0093883) and also supported by the Human Resources Development program
References (41)
- et al.
A quaternary poly(ethylene carbonate)-lithium bis (trifluoromethanesulfonyl)imide-ionic liquid-silica fiber composite polymer electrolyte for lithium batteries
Electrochim Acta
(2015) - et al.
Stabilizing lithium/sulfur batteries by a composite polymer electrolyte containing mesoporous silica particles
J Power Sources
(2014) - et al.
Single-ion-conducting nanocomposite polymer electrolytes based on PEG400 and anionic nanoparticles: part 2. Electrical characterization
Int J Hydrogen Energy
(2014) - et al.
Nanocomposite solid polymer electrolytes based on poly(ethylene oxide)/POSS-PEG (n = 13.3) hybrid nanoparticles for lithium ion batteries
J Ind Eng Chem
(2015) - et al.
Ionic conductivity enhancement in PEO: CuSCN solid polymer electrolyte by the incorporation of nickel-chloride
Solid State Ion
(2015) - et al.
Lithium ion transport and ion–polymer interaction in PEO based polymer electrolyte plasticized with ionic liquid
Solid State Ion
(2011) - et al.
Plasticizer doped ionic liquid incorporated solid polymer electrolytes for photovoltaic application
Curr Appl Phys
(2011) - et al.
Ionic conductivity studies in poly(methyl methacrylate)-polyethylene oxide hybrid polymer electrolytes with lithium salts
J Power Sources
(2001) - et al.
Conductivity and FTIR studies on PEO-LiX [X: CF3SO3−, SO42−] polymer electrolytes
Spectrochim Acta Part A
(2008) - et al.
A novel PEO-based composite polymer electrolyte with absorptive glass mat for Li-ion batteries
Electrochim Acta
(2007)