Small polaron hopping conduction mechanism in LiFePO4 glass and crystal

Abstract

The optimization of a cathode material is the most important criterion of lithium ion battery technology, which decides the power density. In order to improve the rate capability, a cathode material must possess high electronic and ionic conductivities. Therefore, it is important to understand the charge transport mechanism in such an advanced cathode material in its intrinsic state before modifying it by various means. In this work, we report the thermal, structural, and electrical conductivity studies on lithium iron phosphate, LiFePO4, both in its polycrystalline (LFPC) and glassy (LFPG) counterpart states. The vibrational spectroscopic measurements reveal the characteristic vibrational modes, which are the intrinsic part of LFPC, whereas in LFPG, the phonon modes become broader and overlap with each other due to the lattice disorder. The electrical conductivity measurements reveal that LFPG exhibits a higher polaronic conductivity of 1.6 orders than the LFPC sample. The temperature dependent dc conductivity has been analyzed with the Mott model of polarons and reveals the origin of enhanced polaronic conductivity in LFPG. Based on the analysis, the enhanced polaronic conductivity in LFPG has been attributed to the combined effect of reduced hopping length, decreased activation energy, and enhanced polaron concentration.

Polaronic Conductivity in Iron Phosphate Glasses Containing B2O3

“…It is well-known that above room temperature conduction in iron phosphate-based glasses is due to phonon-assisted hopping of polarons between nearest neighboring sites [1][2][3][4][5][6][7] and the DC conductivity is thermally activated with characteristic activation energy. The temperature dependence of DC conductivity, σ DC , in this temperature range is usually expressed by Austin-Mott's relation [1,2,5]:…”

“…Iron phosphate-based glasses belong to a family of electronically conducting materials in which the conduction mechanism follows the small polaron hopping theory [1][2][3][4][5][6][7]. The polaronic conductivity in these glasses originates from the electron transfer between ferric and ferrous ions and, consequently, shows a strong dependence on the total amount of iron oxide, fraction of ferrous/ferric ions and average distance between them.…”

Electrical Transport in Iron Phosphate-Based Glass-(Ceramics): Insights into the Role of B2O3 and HfO2 from Model-Free Scaling Procedures

“…Austin and Mott proposed a detailed theoretical approach to the conduction process and activation energy of transition metal oxide (TMO) glasses with their model [24,25]. As is well known, conduction in IPGbased glasses at temperatures above RT is considered as phonon-assisted small polaron hopping (SPH) between local neighboring sites [30,[36][37][38]57,58] and the DC conductivity exhibits an Arrhenius temperature dependence with characteristic activation energy. The temperature dependence of DC conductivity, σ DC , is usually expressed by the Austin-Mott relation [36,37,57]:…”

A review on first principles based studies for improvement of cathode material of lithium ion batteries

“…The electric conductivity showed that LFPG exhibited a higher polaronic conductivity of 1.6 orders than the LFPC sample. The investigations revealed that the enhancement of polaronic conductivity in LFPG is attributed to the combined effect of the reduced hopping length, decreased activation energy, and the enhanced polaron concentration [154] .…”