464841 The Influence of Tetrabutylammonium (TBA) Salts on the Discharge and Charge Behavior of Li-O2 Batteries

Thursday, November 17, 2016: 1:40 PM
Golden Gate 5 (Hilton San Francisco Union Square)
Chibueze Amanchukwu, Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, Yang Shao-Horn, Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA and Paula Hammond, Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA

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 Li–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 – Li2O2; a bulk insulator that requires charging potentials above 4 VLi.

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 Li–O2 batteries.

In this work, we studied the influence of TBA on the discharge and charge behavior of Li–O2 batteries. We fabricate Li–O2 cells using TBA as the only salt present in the electrolyte. Despite the lack of lithium salt, a typical discharge plateau (~2.6 VLi) is observed – similar to the voltage obtained with lithium salt – 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 Li–O2 batteries.

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See more of this Session: Materials for Electrochemical Energy Storage II
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