Isothermal calorimetry, a tool too long neglected for the study of anionic redox
For 7 years, RS2E researchers have been publishing several papers on the phenomenon of anionic redox. This oxidation-reduction phenomenon is added to the “classical” cationic redox, in particular in the “Li-rich” materials and leads theoretically to higher energy density batteries. Their lifetime could be increaded by 20%.
Unfortunately, this phenomenon is often responsible for a large voltage hysteresis (voltage difference between charge/discharge), which means that some of the energy is lost through heat dissipation. A main disadvantage for batteries because this dissipation poses a risk for safety. In a running battery, the dissipation causes overheating that could lead to a thermic runaway.
Which mechanism and what thermochemical conditions lead to this hysteresis? When is the energy lost precisely?
A RS2E team, led by Prof. Jean-Marie Tarascon, attempted to answer these questions in a new paper published in the journal Nature Energy.
Isothermal calorimetry to probe the batteries
Until now, the isothermal calorimetry have been little used to analyze the thermal behavior of the storage systems. This method permits to probe and evaluate enthalpy and entropy variations to deduce the mechanisms that happen around the cathode during the charge and the discharge.
RS2E researchers decided to use this analytical method on a “Li-rich” model, LRSO or Li2Ru0.75Sn0.25O3, to understand the phenomena that occur when we use anionic redox in a battery.
The observations are at the origin of two conclusions: 1) The heat dissipation reflects a voltage hysteresis and the associated energy loss. 2) The electrochemical oxidation/reduction of the anions is immediately followed by a structural rearrangement to stabilize it. The waste heat is due to these rearrangements and entropy they are the cause because the mechanism is non-equilibrium.
Figure 1: Above, single-step model for cationic redox; At the middle, multistep « square-scheme » model for anionic redox, similar the one of some molecular machines, below.
With these results, this study paves the way for new approaches to lower voltage hysteresis, minimize heat dissipation in the materials and achieve 100% energy efficiency.
The authors of the paper hope that their new characterization method will encourage the electrochemical community to appropriate the isothermal calorimetry to study other storage systems than batteries.
Gaurav Assat, Stephen L. Glazier, Charles Delacourt, Jean-Marie Tarascon
Nature Energy, 01/07/19, DOI : 10.1038/s41560-019-0410-6