Short communicationApplication of a new acrylonitrile/butylacrylate water-based binder for negative electrodes of lithium-ion batteries
Introduction
Generally, polyvinylidene fluoride (PVDF) has been used as a conventional binder for lithium-ion battery (LIB) due to its superior chemical and electrochemical stabilities, however it suffers from problems such as the use of toxic organic solvents (e.g. n-methyl-pyrrolidone), and heat stability problems [1], [2]. As a result, water-soluble or water-dispersed binders, such as carboxymethyl cellulose (CMC) [3], [4], styrene-butadiene rubber (SBR) [5], [6], and other water-based polymers [7], [8] were successfully adopted. Much attention has been given to the synthetic SBR binder due to its high flexibility, good adhesion strength, and electrochemical stability with environmentally-friendly manufacturing process. Additionally, the SBR binder can connect the active material by a point-to-point contact mechanism such that the amount of binder used can be reduced compared to conventional PVDF binders [6].
Polyacrylonitrile (PAN) is a good choice for a binder due to its high polarity, electrochemical stability and ability to participate in Li+ transport [9], [10], [11]. Unfortunately, PAN is a semicrystalline polymer with a high glass transition temperature (Tg) of 96.5 °C such that its application as a binder can produce a rigid electrode leading to cracking during the manufacturing process. To resolve this problem, modifying PAN with a flexible polymer through copolymerization in a water medium can be acceptable. In this work, butylacrylate (BA) is chosen as it has a very low Tg of − 45 °C. The copolymerization of acrylonitrile (AN) with BA can improve the adhesion strength, electrical and electrochemical properties of electrodes [8]. Furthermore, the copolymer, poly(acrylonitrile-butylacrylate) (P(AN-BA)) has a mechanical behavior similar to synthetic acrylonitrile-butadiene rubber [12]. This study was directed to optimize AN/BA ratio in the P(AN-BA) binder and prove that P(AN-BA) is a promising binder for LIB anodes compared to commercial SBR binders.
Section snippets
Experimental
The copolymers (Fig. 1a) of AN and BA (Junsei Chemical Co., Japan) were prepared by emulsion polymerization at 70 °C. Potassium persulfate (KPS, Sigma–Aldrich) and sodium dodecyl sulfate (SDS, Tokyo Chemical Industry Co.) were used as an initiator and emulsifier, respectively. A mixture of AN/BA with different weight ratios (2/1, 1/2, 0/3) was examined as monomers for the emulsion polymerization. The 3/0 AN/BA sample, PAN, was not considered since the PAN was neither made by the emulsion
Results and discussion
The results in Table 1 indicate that there was not much difference in particle size between the samples synthesized via a batch process. The average size of the BA series was from 127 to 146 nm. Similarly, the total solid content and viscosity were nearly the same for these emulsions. Tg of the polymers are also in Table 1, with the lowest Tg belonging to the lowest AN/BA ratio polymer and the highest Tg belonging to the highest AN/BA ratio polymer. As expected, the flexibility of the polymer
Conclusion
Water-based P(AN-BA) copolymers synthesized from AN and BA, especially the BA2 sample, may serve as a new efficient binder for negative electrodes. The BA2 binder synthesized by the 1/2 weight ratio of AN/BA was more flexible and thermally stable than the other BAs, including conventional SBR. Additionally, the adhesion was sufficient for maintaining the connection with active materials and current collectors. Such superior properties contribute to an increase in the diffusion coefficient and a
Acknowledgment
The work was supported by a grant from the National Research Foundation of Korea, funded by the Korean Government (MEST) (NRF-2009-C1AAA001-2009-0093307).
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