Oxygen Reduction Reaction Mechanism in Highly Concentrated Lithium Nitrate-Dimethyl Sulfoxide: Effect of Lithium Nitrate Concentration

The use of high-concentration electrolytes in a nonaqueous lithium–oxygen (Li–O2) battery can effectively improve the cycle stability of the battery because a stable solid electrolyte interphase is formed on the surface of the lithium metal anode. However, the effect of lithium salt concentration on the oxygen reduction reaction (ORR) at the cathode of the Li–O2 battery remains to be studied. In particular, the use of the in situ technique can better reveal the real situation of the electrode surface reaction. In this work, in situ surface-enhanced Raman spectroscopy (SERS) and electrochemical quartz crystal microbalance (EQCM) were used to investigate the influence of LiNO3 concentration on the ORR mechanism in LiNO3-DMSO solutions. O2 was proved to be electrochemically reduced into Li2O2 with O2(electrode)– (O2– adsorbed on the cathode) as an intermediate. It turns out that the effect of LiNO3 concentration on the ORR mechanism attributes to affect the dissolution reaction of O2(electrode)– into solution (O2(electrode)– + Li+ → LiO2(solution)) and further alter the formation of Li2O2 on the cathode. In low LiNO3 concentration, the proportion of O2(electrode)– that dissolves into solution is lower than that in medium concentration due to the higher O2 solubility, resulting in more Li2O2 formation electrochemically in low concentrations. In medium concentrations, the least Li2O2 was formed among the investigated electrolyte because most of O2(electrode)– would dissolve into the solution under higher Li+ concentration and lower O2 solubility. At high concentrations, due to the extremely low LiO2 diffusion, the dissolution of O2(electrode)– tends to be prohibited and accumulated on the electrode surface, resulting in more O2 being electrochemically converted into Li2O2 compared with medium concentrations.