Structural, thermal, and dielectric properties of porous PVDF/Li4Ti5O12 nanocomposite membranes for high-power lithium-polymer batteries

Abstract

Polymer nanocomposites consisting of materials such as ionic polymers and nanoceramic fillers are widely used in high-power lithium-polymer batteries because of their high ionic conductivity, good mechanical strength, electrothermal stability, and better compatibility with electrodes. Nanocomposite films of polyvinylidene fluoride (PVDF)/lithium titanium oxide Li4Ti5O12 (LTO) with different volume fractions of LTO nanoparticles (NPs) are prepared via casting method. The DSC thermograms revealed a slight decrease in the melting temperature Tm and a noticeable reduction in the degree of crystallinity with increasing the volume fraction of LTO. This decrease is confirmed by the increment in the relative fraction of β-phase in the PVDF matrix calculated from both XRD and FTIR. SEM images indicated the growth of porous globular structures in the presence of LTO NPs. Besides, the hydrophilicity of PVDF is improved by incorporating LTO NPs. The dielectric constant ε0(ω), loss ε00(ω), ac conductivity σac(ω), complex impedance Z*(ω), and Nyquist plots of PVDF/ LTO nanocomposites are investigated in the temperature range from 303 to 413 K and frequency range from 100 Hz to 1 MHz. The σac(ω) and frequency exponent s are found to obey the correlated barrier hopping model. Values of the frequency exponent s and the charge carriers binding energy Wm for the studied nanocomposite films decrease with rising temperature and LTO addition. Furthermore, dielectric constant ε0(ω), loss ε00(ω), and ac electrical conductivity σac(ω) of films are found to be strongly frequency and temperature dependent. The localized states density N(EF) at the Fermi level increase with increasing temperature and LTO NPs volume fraction, resulting in the Wm decrease and the enhancement of the ac electrical conductivity σac(ω). The impedance spectrum and Nyquist plots provide an insight into the influence of LTO vol% in the resistive and capacitive characteristics of PVDF/LTO films. These results recommend the choice of LTO NPs as dopants to enhance the electrical properties of the PVDF matrix to be used in high-power lithium-ion batteries and electronic devices.