Governing mechanism of ion transport in lithium-iron-phosphate glasses

Abstract

We show a fundamental relationship between network structural attributes and ionic transport of lithium iron phosphate glasses xLi2O–2Fe2O3–3 P2O5 (xLFPO; x = 0.375, 0.75, 1.5, 3.0) used for a cathode in lithium-ion batteries. The glass samples have been synthesized using a solid-state reaction method. We have investigated the electrical characteristics of the glass by combinational measurements of electrical impedance in the frequency 100 Hz – 30 MHz and temperature from ambient to 240 °C, differential thermal analysis, X-ray diffraction pattern, and Raman spectroscopy. The ionic hopping and relaxation processes in xLFPO glasses are analyzed by using Cole-Cole, power law, and modulus representations. We reveal that the exponential increase in dc conductivity with increasing Li2O in xLFPO glasses is caused by the combination of an increase in non-bridging oxygen sites and a change in the bonding forces of iron valence surrounding the iron phosphate clusters inside the network. The power law conductivity, which is driven by the fractional Rayleigh equation, exhibits that the rate of increase in dc ionic conductivity is the same as the rate of increasing numbers of the ionic charge carriers. Raman spectra show that the increase of depolymerization of the iron phosphate network with the addition of Li2O content is due to the conversion of Q2 to Q1 and Q0 species.