Operando detection and suppression of spurious singlet oxygen in Li–O 2 batteries

Abstract

The rechargeable lithium air (oxygen) battery (Li-O2) has very high energy density comparable to fossil fuels (∽3,600 Wh/kg). However, the parasitic reactions of the O2- reduction products with solvent and electrolyte lead to capacity fading and poor cyclability. During the oxygen reduction reactions (ORR) in aprotic solvents superoxide radical anion (O2-.) is the main one-electron reaction product, which in the presence of Li+ ions undergoes disproportionation to yield Li2O2 and O2, a fraction of which results in singlet oxygen (1O2). Very reactive (1O2). is responsible for the spurious reactions that lead to high charging overpotential and low cycle life due to solvent and electrolyte degradation. Several techniques have been used for the detection and suppression of (1O2).inside a Li-O2 battery under operation and test the efficiency and electrochemical stability of different physical quenchers of 1O2: azide anion, 1,4-diazabicyclo[2.2.2]octane (DABCO) and triphenylamine (TPA) in different solvents (DMSO, diglyme and tetraglyme). In-operando detection of 1O2 inside the battery was accomplished by following dimethyl anthracene fluorescence quenching using a bifurcated optical fibre in front-face mode through a quartz window in the battery. Differential oxygen pressure measurements during charge-discharge cycles vs. charge during battery operation showed that the number of electrons per oxygen molecule was n > 2 in the absence of (1O2). physical quenchers due to spurious reactions and n = 2 in the presence of (1O2). physical quenchers proving the suppression of spurious reactions. Battery cycling at limited specific capacity of 500 mAh/gC of MWCNTs cathode and 250 mA/gC current density in the absence and presence of physical quenchers and physical quencher plus the redox mediator I3-/I- (with lithiated Nafion ® membrane) showed increasing cyclability from coulombic efficiency and cell voltage data over 100 cycles. In-operando Raman studies with a quartz window at the bottom of the battery allowed detection of Li2O2 and excess I3- redox mediator during discharge and charge respectively.