Ionic liquid doped PEO-based solid polymer electrolytes for lithium-ion polymer batteries

Highlights

• Ionic liquid doped solid polymer electrolytes were synthesized for Li-ion batteries.

• Strong interaction among PEO, LiDFOB and EMImTFSI was confirmed by FTIR studies.

• High total ionic conductivity; 1.85 × 10⁻⁴ S/cm at 30 °C.

• EMImTFSI ionic liquid gave significant enhancement in ionic conductivity and electrochemical performance.

• Good cycling capabilities for LiFePO₄ based Li metal polymer batteries.

Abstract

The influence of adding the room-temperature ionic liquid 1-ethyl-3-methyllimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI) to poly(ethylene oxide) (PEO)–lithium difluoro(oxalato)borate (LiDFOB) solid polymer electrolyte and the use of these electrolytes in solid-state Li/LiFePO₄ batteries has been investigated. Different structural, thermal, electrical and electrochemical studies exhibit promising characteristics of these polymer electrolyte membranes, suitable as electrolytes in rechargeable lithium-ion batteries. The crystallinity decreased significantly due to the incorporation of ionic liquid, investigated by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The ion–polymer interaction, particularly the interaction of cations in LiDFOB and ionic liquid with ether oxygen atom of PEO chains, has been evidenced by FT-IR studies. The polymer electrolyte with ∼40 wt% of ionic liquid offers a maximum ionic conductivity of ∼1.85 × 10⁻⁴ S/cm at 30 °C with improved electrochemical stabilities. The Li/PEO-LiDFOB-40 wt% EMImTFSI/LiFePO₄ coin-typed cell cycled at 0.1 C shows the 1st discharge capacity about 155 mAh g⁻¹, and remains 134.2 mAh g⁻¹ on the 50th cycle. The addition of the ionic liquid to PEO₂₀-LiDFOB polymer electrolyte has resulted in a very promising improvement in performance of the lithium polymer batteries.

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.