Seeking for Electrochemical Instabilities in Lithium-Oxygen Batteries Using Halides As Redox Mediator
Authors: Costa Gabriel , Machado Eduardo , Carpanedo Thayane , Anchieta Chayene , Doubek Gustavo , Nagao Raphael 
Published: July 2021

One of the greatest challenges of contemporary civilization is the need for batteries that supply the global energy demand because of the increasing use of mobile devices and electric vehicles. Lithium-oxygen batteries are promising alternatives to meet this energy demand, due to its high theoretical energy capacity since its operating mechanism is based on the anodic dissolution of the Li metal combined with the cathodic oxygen reduction reaction (ORR) that culminates in the formation of Li2O2..1 However, there are still some challenges to overcome for the improvement of this technology, such as charging voltage which is considerably larger than the discharging one and the poor lifetime and cyclability, demanding efforts to improve the capacity retention during cycling.
Recently, spontaneous potential oscillations have been identified during the process of discharging in lithium-oxygen batteries.2 Nonlinear dynamics in these systems are known to occur under conditions far from thermodynamic equilibrium as a result of the interaction between kinetic instabilities and mass transport properties.3 Understanding, therefore, the reaction mechanism can provide important clues to the birth and evolution of this temporal complexity, thereby enabling precise control and prevention of battery efficiency collapse.
In this work, it is proposed the use of lithium halides salts acting as redox mediator in the electrolyte composition. During the charging process, that is, the oxidation of Li, the X- of LiX can be oxidized to X3
- leading to a decrease in the charge overpotential.4 Besides, the halides can act as a diffusional component that could help the emergence of electrochemical instabilities. Thus, it was evaluated the role of LiBr in the electrolyte composition in different gavanostatic regimes, through the assembly of Li-O2 batteries cells with and without LiBr which was submitted to different galvanostatic charge/discharge conditions to comprise the effect of this species in the electrolyte. It has been observed that the presence of bromide reduced the charge overpotential and improved the cyclability (Figures 1a, 1b). Besides, the role of applied current was evaluated and higher it was, less completed cycles were obtained.
Figure 1 presents the results obtained in cyclability of Li-O2 batteries tests with and without LiBr as redox mediator and using different galvanostatic conditions. Figure 1a shows cycling tests conducted with (black line) and without (red line) LiBr in the electrolyte composition. With this result, it was possible to conclude the improvement in battery lifetime given by the presence of the redox mediator. Figure 1b highlights the first and fifth cycles, showing the overpotential reduction with LiBr. It was also evaluated the evolution of the maximum capacity of each cycle (Figure 1c) and the results reinforce the LiBr performance. Figure 1d presents the evaluation of different galvanostatic conditions, with lithium bromide, and it was observed that lower the operating current, more cycles have been completed. Finally, Figure 1e highlights from a preliminary study an oscillatory behavior in the battery potential during the first charge of the cell with LiBr in the electrolyte composition, in galvanostatic condition. This is a promising result that indicates the system susceptibility for electrochemical instabilities.
After determining an interval of current operation, galvanodynamic experiments will be conducted to obtain the conditions under which spontaneous potential oscillations emerges. Thereby, it will be designed appropriated cells in which in operando experiments will be conducted to monitor spontaneous emergence of patterns with temporal resolution. Then, it is hoped to understand how experimental parameters can impact the nonlinear dynamics observed during the battery operation.

Figure
1: (a) Cyclability Li-O2 batteries tests in galvanostatic conditions (±50 µA) with different electrolytes: with and without LiBr in its composition. (b) 1st and 5th cycles for cyclability Li-O2 batteries tests with different electrolytes: with and without LiBr in its composition. (c) Evolution of Li-O2 battery capacity with and without LiBr in the electrolyte composition. (d) Cyclability Li-O2 batteries tests in different galvanostatic conditions with LiBr in the electolyte. (e) Oscillatory behavior during the charge in a galvanostatic regime at (±50 µA) in a battery assembled with LiBr in electrolyte composition.
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Figure 1