Effect of Particle Size on Thermodynamics and Lithium Ion Transport in Ti2Nb2O9 Electrodes Synthesized By Solid State or Sol-Gel Method

Abstract

Ti2Nb2O9 has been identified as a promising lithium-ion battery anode material with large specific capacity, small cycling degradation, and good capacity retention at large currents. This study aims to gain insight into the charging mechanisms as well as the thermodynamics and ion transport in Ti2Nb2O9 synthesized by the solid state or the sol-gel method and formed by particles of different sizes using potentiometric entropy and operando isothermal calorimetric measurements. First, electrochemical testing showed that Ti2Nb2O9 electrodes made by sol-gel synthesis exhibited larger specific capacity, smaller polarization between lithiation/delithiation, and better capacity retention at large currents compared to those made by solid state synthesis. The measured open-circuit voltage and entropic potential revealed that the same solid solution charging mechanism prevailed and was independent of particle size, as confirmed by in situ XRD measurements. In other words, particle size had no influence on the quasi-equilibrium thermodynamics behavior of Ti2Nb2O9. However, Ti2Nb2O9 electrodes made by sol-gel synthesis featured smaller overpotential and faster lithium diffusion. In fact, operando isothermal calorimetry revealed smaller instantaneous heat generation rates and smaller time-averaged irreversible heat generation rates at Ti2Nb2O9 electrodes made by sol-gel synthesis compared to those made by solid state synthesis at any given C-rate. These observations highlight the smaller resistive losses and the larger electrical conductivity of Ti2Nb2O9 synthesized by the sol-gel method. Furthermore, time-averaged reversible heat generation rates at Ti2Nb2O9 electrodes made by both synthesis methods featured significant contributions from entropic changes, ion adsorption/desorption, and ion solvation/desolvation accompanied by ion-pairing.