Elsevier

Electrochimica Acta

Volume 196, 1 April 2016, Pages 41-47
Electrochimica Acta

Ionic Conducting and Surface Active Binder of Poly (ethylene oxide)-block-poly(acrylonitrile) for High Power Lithium-ion Battery

https://doi.org/10.1016/j.electacta.2016.02.154Get rights and content

Highlights

  • PEO-b-PAN binder conducts Li+ inside and acts as a dispersant to LiFePO4 cathode.

  • PEO-b-PAN binder shows low polarization and less interfacial resistance.

  • The PEO-b-PAN binder delivers extraordinary capacities of 101 mAh g−1at 10C-rate.

Abstract

In this work, poly(ethylene oxide)-block-poly(acrylonitrile) (PEO-b-PAN) copolymer is used as a binder for LiFePO4 cathodes, where PEO-b-PAN not only conducts Li+ inside the cathode but also acts as a dispersant to disperse LiFePO4. This binder significantly increases the capacity under high discharge rate and overcome the limitation of LiFePO4 for high power density application. By XPS analysis, the incorporation of the PEO-b-PAN binder to the active materials of the LiFePO4 cathodes can be clearly observed from the binding energy of the nitrogen atom of the PEO-b-PAN. Due to the surface active properties of the PEO and PAN, PEO-b-PAN obviously increases the effective contact area and reduces electronical resistance. In addition to the surface active properties, this binder provides Li+ pathway; thus, it features low polarization, less interfacial resistance and good activity for electrochemical reaction. Consequently, these properties enable the PEO-b-PAN binder to have a higher discharge plateau potential at 3.10 V, while it is only 2.86 V for the PVDF binder at a 5C rate. Moreover, even at a 10C rate, the PEO-b-PAN binder still delivers extraordinary discharge capacities of 101 mAh g−1, significantly higher than that of the PVDF binder (32 mAh g−1). Overall, this ionic conducting and surface active binder exhibits good electrochemical properties and excellent high rate performance.

Introduction

Lithium-ion batteries provides high energy and power density, as well as long cyclic life; for these reasons they are widely used as power sources for portable electronic devices and electric vehicles. [1], [2], [3], [4] In such batteries, polymer binders for preparing the electrode play an important role affecting cell performances. Poly(vinylidene fluoride) (PVDF) is the most common binder for electrodes, because it exhibits good electrochemical stability and high adhesivity to electrode materials. [5], [6], [7], [8] However, PVDF binders hinder Li+ transport and reduce ion conductivity; consequently, such binders show high concentration polarization at high charge/discharge rates. To overcome these drawbacks, some studies have focused on lithiated ionomer binders to enhance the Li+ transport inside the active materials. In references, a lithiated sulfonated polymer and a lithiated carboxylate polymer were use as ionic conducting binders improve battery performance. Nevertheless, the charge–discharge performance at high C-rates was still limited. [9], [10], [11], [12]

Poly(acrylonitrile) (PAN) and Poly(ethylene oxide) (PEO) and their copolymers are known to be polymer electrolytes due to their high Li+ conductivity, electrochemical stability, and formation of a stable interface between electrode and electrolytes. [13], [14], [15], [16] Further, PEO is well known as a component of a wide range of surface active materials due to its affinity to polar surface or media, and has been used as a dispersants to stabilize metal oxide nanoparticles. [17], [18] Moreover, PAN is another good polymer binder candidate because the electronegative nitrogen of the nitrile group has very strong polarity and good solvent resistance. [19] This polar nitrile group can interact with Li+ and active material due to the dipole–dipole interaction; hence, it can not only transfer Li+ but also assist in active materials contacting more efficiently.

In this work, poly(ethylene oxide)-block-poly(acrylonitrile) (PEO-b-PAN) is used as an ionic conducting binder for LiFePO4 electrode, where LiFePO4 is well dispersed due to the presence of PEO-b-PAN. Compared with the PVDF binder, a cell with the PEO-b-PAN binder provides an effective diffusion pathway for Li+ transport and the contact area of the active materials. Most importantly, it greatly improves the rate performance and shows excellent reversible charge–discharge cycle performance because of the high discharge platform voltage and lower polarization. As a result, the PEO-b-PAN binder can be used in LiFePO4 cathodes for high-power applications.

Section snippets

Preparation of PEO-b-PAN copolymer

Poly(ethylene oxide)-block-poly(acrylonitrile) was prepared by redox copolymerization of acrylonitrile monomer (≥99%, Sigma-Aldrich) with polyetheramine (JEFFAMINE® ED2003), the process of which performed according to our previously published procedures [20], [21], [22].

Preparation of electrodes

The LiFePO4 cathode with PVDF binder and PEO-b-PAN binder were fabricated and tested. The LiFePO4 powder (Aleees, Taiwan), Super P, and polymer binder (weight ratio of 8:1:1) were mixed with N-methyl-2-pyrrolidone (NMP). The

Results and discussion

As aforementioned, the ion conducting binder, PEO-b-PAN copolymer was prepared by redox polymerization, the synthesis of which is detailed in our previous work [20], [21]. Fig. S1 shows the [1] H NMR spectra of the PEO-b-PAN; all signals depicted in the figure are assigned to PEO-b-PAN. The average molecular weight is close to 43,000 g mol−1, as calculated by the integrated areas of the PEO and PAN segments. This polymer is considered to be a polymer electrolyte due to its satisfactory ionic

Conclusion

In conclusion, this study presented the PEO-b-PAN copolymer as an ionic conducting binder, providing an excellent rate performance for LiFePO4 cathodes. The electrode with the PEO-b-PAN binder enhanced the diffusion pathway for Li+ transport and electrical conductivity, and effectively increased the contact area; further, it exhibited a low interfacial resistance and low polarization. The cell made with the PEO-b-PAN binder maintains a plateau potential of 3.10 V at 5C, and reaches the specific

Acknowledgment

The authors would like to thank the Ministry of Science and Technology, Taipei, R. O. C. within the project “Development of High-Performance, High Voltage and High Safety Lithium Ion Electrolytes” for their generous financial support of this research.

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