Li-ion battery with poly(acrylonitrile-methyl methacrylate)-based microporous gel electrolyte
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.
Section snippets
Experimental
Poly(acrylonitrile-methyl methacrylate) (AMMA; AN:MMA=94:6, MW=100,000) was purchased from Polysciences and was used as received. Microporous membrane was prepared by the phase-inversion method as follows. AMMA powder was dissolved into dried N,N-dimethylformamide (DMF) to make a 10 wt.% solution by stirring and heating at 80 °C. The resulting solution was cast onto an Al foil using a doctor blade with a gap of 0.24 mm, immediately followed by immersing the polymer dope into a coagulation bath
Morphology of the porous membrane
Fig. 1 shows micrographs of the surface and the cross-section of a typical porous AMMA membrane prepared by the phase-inversion method. It is shown that textures of the surface and body (cross-section) of the membrane are significantly different. The membrane is composed of a very dense skin and a body containing numbers of finger-like macrovoids. Formation of the above asymmetric structure can be understood in terms of the kinetics of the mass transport in the phase-inversion process. In the
Conclusions
The present work shows that MGE Li-ion batteries can be fabricated in situ by gelling a pre-prepared microporous polymer separator. Because the separator is porous and the polymer matrix is swellable, three phases of liquid electrolyte, gel electrolyte, and polymer matrix may be present in the MGE. Therefore, the MGE combines the advantages of the liquid electrolyte (high conductivity), gel electrolyte (good adhesion to electrodes), and solid electrolyte (stable dimensions over a wide
Acknowledgment
Receipt of the cathode and graphite anode films from SAFT America, Inc. is gratefully acknowledged.
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