(693d) The Influence of Tetrabutylammonium (TBA) Salts on the Discharge and Charge Behavior of Li-O2 Batteries

The rise of carbon dioxide (CO₂) emissions from fossil fuel use due to rapid industrial growth has exacerbated changes in the climate. Alternative energy sources such as solar and wind have been developed, but extensive commercial utilization has been hindered by their intermittency. To complement these energy conversion methods, energy storage media such as batteries and fuel cells are being developed. Of the battery chemistries, Li-air (O₂) appears promising because it can allow for an order of magnitude greater gravimetric energy density than current commercial Li-ion batteries. However, numerous challenges face Liâ??O₂ batteries that stem from electrolyte, electrode, and salt degradation, and the limited oxygen reduction and evolution kinetics. These challenges lead to limited current rates, poor capacity retention, and lower cycle life.

Additives such as water, fluorinated solvents, and salts like LiNO₃ have been explored to modify the intermediate (superoxide) and O₂ solubility to increase discharge capacities. During charging, catalysts or redox mediators have also been incorporated to aid in the decomposition of the discharge product â?? Li₂O₂; a bulk insulator that requires charging potentials above 4 V_Li.

Ammonium-based salts such as tetrabutylammonium (TBA) have been shown to support oxygen reduction and evolution reactions using cyclic voltammetry, and TBA-superoxide complexes are known to form during reduction and the complexes are easily removed during oxidation. However, these salts have not been studied in actual Liâ??O₂ batteries.

In this work, we studied the influence of TBA on the discharge and charge behavior of Liâ??O₂ batteries. We fabricate Liâ??O₂ cells using TBA as the only salt present in the electrolyte. Despite the lack of lithium salt, a typical discharge plateau (~2.6 V_Li) is observed â?? similar to the voltage obtained with lithium salt â?? and lithium peroxide (Li₂O₂) is formed as the primary discharge product. The mechanism for discharge was studied, and the influence of solvent on the discharge process was also explored. The presence of TBA also affects the charging behavior, leading to lower charging overpotentials (compared to lithium salts) and Li₂O₂ oxidation. We investigated the charging mechanism and show that oxidation of TBA during charge is responsible for the subsequent Li₂O₂ oxidation. Knowledge gained from this work can spur greater understanding of the discharge and charge mechanisms of Liâ??O₂ batteries.