Effect of particle size on thermodynamics and lithium ion transport in electrodes made of Ti2Nb2O9 microparticles or nanoparticles

Abstract

This study compares the charging mechanisms, thermodynamics, lithium ion transport, and operando isothermal calorimetry in lithium-ion battery electrodes made of Ti2Nb2O9 microparticles or nanoparticles synthesized by solid-state or sol-gel methods, respectively. First, electrochemical testing showed that electrodes made of Ti2Nb2O9 nanoparticles exhibited larger specific capacity, smaller polarization, and better capacity retention at large currents than those made of Ti2Nb2O9 microparticles. Furthermore, potentiometric entropy measurements revealed that electrodes made of either Ti2Nb2O9 microparticles or nanoparticles showed similar thermodynamics behavior governed by lithium intercalation in solid solution, as confirmed by in situ XRD measurements. However, electrodes made of Ti2Nb2O9 nanoparticles featured smaller overpotential and faster lithium ion transport than those made of Ti2Nb2O9 microparticles. In fact, operando isothermal calorimetry revealed smaller instantaneous and time-averaged irreversible heat generation rates at electrodes made of Ti2Nb2O9 nanoparticles, highlighting their smaller resistive losses and larger electrical conductivity. Finally, the measured total heat generation over a charging/discharging cycle matched the measured net electrical energy loss. Overall, Ti2Nb2O9 nanoparticles synthesized by the novel sol-gel method displayed excellent cycling performance and reduced heat generation as a fast-charging lithium-ion battery anode material. These features present major advantages for actual battery systems including larger energy and power densities, simpler thermal management, and enhanced safety.