Elsevier

Electrochimica Acta

Volume 56, Issue 14, 30 May 2011, Pages 5201-5204
Electrochimica Acta

Potential application of microporous structured poly(vinylidene fluoride-hexafluoropropylene)/poly(ethylene terephthalate) composite nonwoven separators to high-voltage and high-power lithium-ion batteries

https://doi.org/10.1016/j.electacta.2011.03.040Get rights and content

Abstract

We demonstrate potential application of a new composite non-woven separator, which is comprised of a phase inversion-controlled, microporous polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP) gel polymer electrolyte and a polyethylene terephthalate (PET) non-woven support, to high-voltage and high-power lithium-ion batteries. In comparison to a commercialized polyethylene (PE) separator, the composite non-woven separator exhibits distinct improvements in microporous structure and liquid electrolyte wettability. Based on the understanding of the composite non-woven separator, cell performances of the separator at challenging charge/discharge conditions are investigated and discussed in terms of ion transport of the separator and AC impedance of the cell. The aforementioned advantageous features of the composite non-woven separator play a key role in providing facile ion transport and suppressing growth of cell impedance during cycling, which in turn contribute to superior cell performances at harsh charge/discharge conditions such as high voltages and high current densities.

Highlights

► Microporous-structured PVdF-HFP/PET composite nonwoven separators for Li-batteries. ► Well-developed microporous structure and liquid electrolyte wettability. ► Provision of facile ion transport and suppressed growth of cell impedance. ► Superior cell performance at high-voltages/high-current densities.

Introduction

From the viewpoint of preventing internal short-circuit failure in a lithium-ion battery, a separator is considered a critical component to secure battery safety, because its primary function is to maintain physical separation between the cathode and anode of the battery [1], [2], [3]. Currently widespread separators in lithium-ion batteries are typically made of polyolefins, predominantly polyethylene (PE) or polypropylene (PP). These polyolefin-based separators have many advantages, however, their poor thermal shrinkage and mechanical strength have raised serious concerns over their ability to ensure the necessary electrical isolation between electrodes. In addition to this safety-reinforcing function, a crucial requirement for separators is improvement of their electrochemical performances. Conventional polyolefin-based separators are known to have low porosity of less than 50% and poor wettability with polar liquid electrolytes [2], [3], [4], which may often cause strong resistance to ionic transport, resulting in deterioration of cell performances.

Among numerous approaches to overcome these shortcomings of polyolefin-based separators, the use of non-wovens comprising multi-fibrous layers has drawn substantial attention owing to their superior thermal properties, high porosity, and cost competitiveness [3], [4], [5], [6], [7], [8]. In our previous study [8], we developed a new non-woven-based composite separator that consists of a microporous PVdF-HFP gel polymer electrolyte and a PET non-woven support. The microporous structure of the composite non-woven separator was determined by controlling the phase inversion of coating solutions (PVdF-HFP/acetone (solvent)/water (non-solvent)). The effects of phase inversion-governed, microporous morphology on the thermal stability and electrochemical performances of the composite non-woven separators were elucidated.

In this study, in a bid to explore the feasibility of applying the previously proposed composite non-woven separator to lithium-ion batteries targeting harsh applications such as high voltages and high current densities, cell performances of the separator at challenging charge/discharge conditions are investigated and discussed in terms of ion transport of separators and AC impedance of cells.

Section snippets

Experimental

A wet-laid PET non-woven (weight = 10 g m−2, thickness = 17 μm) was immersed in a coating solution comprising PVdF-HFP (HFP content = 6 mol%)/acetone/water. A predetermined amount (= 5 wt%) of water was added into the solution to induce phase inversion. The water content of 5 wt% was reported to provide a highly developed microporous structure [8]. The final thickness of the composite non-woven separator was observed to be 35 μm. The detailed preparation procedure of the composite non-woven separator has

Results and discussion

Structural characterization of the composite non-woven separator was conducted. In contrast to the PE separator having pore diameter of less than 0.1 μm (Fig. 1(a)), the composite non-woven separator (Fig. 1(b) and (c)) shows a number of large-sized and well-connected micropores (pore diameter < approximately 2.0 μm), wherein the insets demonstrate the pore size distributions of the separators. In the previous study [8], we reported that the microporous structure of the composite non-woven

Conclusions

The composite non-woven separator comprising a PVdF-HFP gel polymer electrolyte and a PET non-woven support provided a highly developed microporous structure and better wettability of liquid electrolyte, in comparison to a PE separator. These advantages of the composite non-woven separator allowed for facile ion transport and suppressed growth of cell impedance during cycling, which consequently contributed to superior cell performances, i.e. higher discharge capacities, discharge C-rate

Acknowledgments

This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF-2009-C1AAA001-2009-0093307).

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