Li-ion battery with poly(acrylonitrile-methyl methacrylate)-based microporous gel electrolyte

Abstract

This paper describes the fabrication and performance of microporous gel electrolyte (MGE) Li-ion batteries. The MGE battery was prepared through three steps: (1) making microporous polymer membrane as battery separator by the phase-inversion method, (2) making the battery assembly and activating it with liquid electrolyte, and (3) forming MGE in situ by warming the battery. Depending on liquid electrolyte uptake and warming conditions of the microporous membrane, the resulting MGE may contain three phases: liquid electrolyte, gel electrolyte, and polymer matrix. Therefore, the MGE combines many advantages such as high ionic conductivity, good adhesion to the electrodes, and good mechanical strength. In this work, we used poly(acrylonitrile-methyl methacrylate) (AMMA, AN:MMA=94:6) as the polymer matrix, and a solution of 1.0 m LiBF₄ dissolved in a 1:3 (wt.) mixture of ethylene carbonate (EC) and γ-butyrolactone (GBL) as the liquid electrolyte. Typically, an MGE gelled with 390 wt.% of liquid electrolyte vs. the dried membrane has an ionic conductivity of 2.2 mS/cm at 20 °C and the resulting Li-ion battery shows good cycling performance.

Introduction

Gel polymer electrolyte (GPE) has been attractive for developing plastic Li-ion batteries due to its combined advantages of liquid electrolyte (high ionic conductivity) and solid electrolyte (free of leaking). In general, the GPE is fabricated in an anhydrous environment by dissolving host polymer into a highly hydroscopic liquid electrolyte and casting the polymer solution at elevated temperatures (120–140 °C), followed by cooling the cast thin solution to form the gel electrolyte film [1], [2], [3], [4]. In addition, the highly viscous feature of the GPE makes it inconvenient for assembling batteries, especially for large sizes. To overcome the above difficulties, scientists at Bellcore (currently Telcordia) developed a two-step procedure for the fabrication of plastic Li-ion battery, which allows fabrication to be carried out in a nonanhydrous environment until the battery is finally activated [5], [6]. In this technology, a plasticizer with high boiling point is first used to create pores for the battery assembly by dissolving it into the polymer matrix and then extracting it out of the polymer, and finally, a liquid electrolyte is introduced into the porous assembly to activate battery. Alternatively, porous polymer membranes have been proposed to make gel electrolyte Li-ion batteries [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. In this approach, the GPE is formed by immersing the porous polymer membrane into a liquid electrolyte and leaving it in an anhydrous environment until being gelled. The key to this approach is successful fabrication of the porous polymer membrane. Among many polymer pore-making technologies, the phase-inversion method has been the most widely adopted. In this method, the porous structure is formed through solvent exchange when a polymer thin solution is immersed into a nonsolvent coagulation bath [19], [20].

In order to form the gel electrolyte, the polymer of the porous membrane must be swellable or be soluble in liquid electrolyte. Depending on the degree of swelling and liquid uptake of the polymer, the resulting electrolyte may contain three phases of liquid electrolyte, gel electrolyte, and polymer matrix, showing a liquid–gel–solid multiphase structure [7], [11], [15], [17], [18]. Currently, most of the studies have been focused on poly(vinylidene fluoride) (PVdF)-based polymers due to their excellent film-forming property and solubility in many electrolyte solvents. However, PVdF based polymers are potentially unstable against the negative electrode of Li-ion batteries. It has been reported [21], [22] that the –CF₂– groups in PVdF-based polymers may react with lithiated graphite to form more stable LiF and >CCF– unsaturated bonds, which not only decay battery performance but also raise safety concerns due to thermal runaway caused by the highly exothermic reactions. To replace PVdF with a more stable polymer, we have found that poly(acrylonitrile-methyl methacrylate) (AMMA) not only has excellent miscibility with nonaqueous liquid electrolyte to form GPE [3] but is also a very stable binder for both anode and cathode of Li-ion batteries [23]. Therefore, in this work, we will use AMMA to make microporous gel electrolyte Li-ion batteries, and evaluate cycling performance of such batteries.