Operando Calorimetry Informs the Origin of Rapid Rate Performance in Microwave-Prepared TiNb2O7 Electrodes

Abstract

<div>The shear-phase compound TiNb<sub>2</sub>O<sub>7</sub> has recently emerged as a safe and high-volumetric density replacement for graphite anodes in lithium ion batteries. An appealing feature of TiNb<sub>2</sub>O<sub>7</sub> is that it retains capacity even at high cycling rates. Here we demonstrate that phase pure and crystalline TiNb<sub>2</sub>O<sub>7</sub> can be rapidly prepared using a high-temperature microwave synthesis method. Studies of the charging and discharging of this material, including through operando calorimetry, permit key thermodynamic parameters to be revealed. The nature of heat generation is dominated by Joule heating, which sensitively changes as the conductivity of the electrode increases with increasing lithiation. The enthalpy of mixing, obtained from operando calorimetry, is found to be small across the different degrees of lithiation pointing to the high rate of lithium ion diffusion at the origin of rapid rate performance.</div>

Potentiometric entropy and operando calorimetric measurements reveal fast charging mechanisms in PNb9O25

“…However, under high Crates, QJ (x, T ) dominates [30] and Li concentration gradients form within the electrode due to diffusion limitation. The latter results in additional irreversible heat generation due to enthalpy of mixing [19,[22][23][24], , as discussed later.…”

“…The total heat generation rate QT (x, T ) (in W) in a battery can be attributed to four main phenomena including (i) Joule heating QJ (x, T ), (ii) reversible entropic heat generation Qrev (x, T ), (iii) enthalpy of mixing Qmix (x, T ), and (iv) undesirable side reactions Qsr (x, T ). Thus, QT (x, T ) can be expressed as [19,[22][23][24],…”

“…Assuming uniform temperature and constant volume, the exothermic Joule heating QJ (x, T ) owing to irreversible resistive losses caused by the deviation of the cell operating voltage V (x, T ) from its equilibrium potential can be expressed as [19,[22][23][24],…”

“…In LIB cells, the open circuit voltage U ocv (x, T ) is directly related to the change in the Gibbs free energy of reaction of the system [18]. In addition, the derivative of U ocv (x, T ) with respect to temperature T for a given lithium composition x, ∂U ocv (x, T )/∂T , also known as the entropic potential, can be used to estimate the partial molar entropy of the cell [17,19]. The entropic potential measurement is particularly insightful for understanding the structural stability upon phase transitions in the electrode material [17].…”

“…In addition, our recent operando calorimetry can directly measure the instantaneous heat generation rate in each electrode of a battery cell during charging/discharging [20]. The measurements can enable a complete energy balance of the cell and can provide insights into the different phenomena contributing to energy dissipation upon charging/discharging including resistive losses, enthalpy of mixing, and reversible entropic process [19].…”