Abstract
Li-ion battery, separator, multicoreshell structure, thermal stability, long-term stability. A nanofibrous membrane with multiple cores of polyimide (PI) in the shell of polyvinylidene fluoride (PVdF) was prepared using a facile one-pot electrospinning technique with a single nozzle. Unique multicore-shell (MCS) structure of the electrospun composite fibers was obtained, which resulted from electrospinning a phase-separated polymer composite solution. Multiple PI core fibrils with high molecular orientation were well-embedded across the cross-section and contributed remarkable thermal stabilities to the MCS membrane. Thus, no outbreaks were found in its dimension and ionic resistance up to 200 and 250 °C, respectively. Moreover, the MCS membrane (at ~200 °C), as a lithium ion battery (LIB) separator, showed superior thermal and electrochemical stabilities compared with a widely used commercial separator (~120 °C). The average capacity decay rate of LIB for 500 cycles was calculated to be approximately 0.030 mAh/g/cycle. This value demonstrated exceptional long-term stability compared with commercial LIBs and with two other types (single core-shell and co-electrospun separators incorporating with functionalized TiO2) of PI/PVdF composite separators. The proper architecture and synergy effects of multiple PI nanofibrils as a thermally stable polymer in the PVdF shell as electrolyte compatible polymers are responsible for the superior thermal performance and long-term stability of the LIB.
Highlights
The lithium ion battery (LIB) is the most popular energy storage device
In the case of the MCS0.5 (PI/polyvinylidene fluoride (PVdF) = 1/2), a smooth fiber morphology was obtained, which was similar to the pure PVdF membrane case reported in our previous studies[22,23] (Fig. 1c)
The PVdF part within the MCS membranes was dissolved in acetone during the extraction, and the PI part remained without any loss due to its insolubility in acetone
Summary
Based on the fact that there was no significant increase in ionic resistance of the conductivity cell up to 250 °C (Fig. 2e), an unstable interface was formed via electrode decomposition, which contributed to the sharp increase in the AC impedance at 1 kHz and to the steep drop in the OCV (Fig. 3b) These results confirmed that the MCS1 separator had a high thermal stability in the LIB at temperatures greater than 200 °C. The average capacity decay rate of the two electrospun PI/PVdF composite separators with two different architectures (single core-shell and co-electrospun incorporating functionalized TiO2)[13,26] and other two thermally stable separators of different materials (polyaniline/ polyimide composite with hierarchical 3D micro/nano-architecture, PANI/PI_3D, and partially oxidized polyacrylonitrile, Oxidized PAN)[32,33] are plotted with the average capacity decay rate of the MCS1 separator in Fig. 4e to compare the long-term cycle performance. It was concluded that the MCS separator was more beneficial in suppressing the growth of dendrites and in maintaining its uniform permeability and thickness[5]
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