The use of a low-polarity (weakly solvating) electrolyte solution in stabilizing lithium-air (Li-O2) batteries largely limits the battery performance due to product formation on the cathode surface. Researchers from the University of Oxford added a new component into the solution and found that this prevented the formation of deposits on the cathode surface. The added component DBBQ (2,5-di-tert-butyl-1,4-benzoquinone), LiTFSI in ether, prompted promising improvements into the solution such as a higher energy storage for the battery, as well as an increased reaction rate.
When lithium-air batteries are discharged, O2 at the cathode is reduced to Li2O2 (porous carbon fiber that creates a film), which can grow on the cathode surface. If Li2O2 could form in solution rather on the cathode surface, this would overcome this problem. DBBQ, in particular, enhances oxygen reduction, and also can serve as more than an electrocatalyst or electron shuttle. Addition of DBBQ creates a completely different reaction mechanism from the typical mechanism in a Li-O2 cell.
The oxidation-reduction reaction of lithium metal and oxygen proceeds through a lithium oxide high-energy intermediate, LiO2, thus requiring high thermodynamic overpotential; this lowers the battery voltage. It is also soluble in polar solvents; this determines whether Li2O2 forms in solution or on the cathode surface.
The addition of DBBQ involves a completely different mechanism that bypasses the LiO2 intermediate. Instead, DBBQ is reduced at the electrode surface, to form a lithium-DBBQ complex. In the presence of DBBQ, O2 reduction occurs at the same reduction potential for DBBQ, therefore at a higher potential (lower overpotential) than the reduction of O2 through the LiO2 intermediate; this is due to the formation of a LiDBBQO2 intermediate complex.
The LiDBBQO2 intermediate reacts with another LiDBBQ complex to form Li2O2 in solution, which was confirmed using a TiOSO4 titration test. The composition of the lithium oxide product was confirmed using powder x-ray diffraction, infrared and Raman spectroscopy, and in situ differential electrochemical mass spectroscopy, which showed Li2O2 was predominantly formed. SEM studies confirmed that Li2O2 particles were formed from solution rather than at the cathode surface.
The addition of DBBQ to a weakly solvating electrolyte results in a completely different reaction mechanism that involves the reduction of O2 via a LiDBBQ complex. This mechanism results in the formation of Li2O2 product insolution. By eliminating the LiO2 intermediate, the team eliminated the source of cathode deposits, and were able to increase the cell’s capacity by 80-to-100-fold. Their mechanism results in higher current densities while halving the overpotential.
This study provides important insights that will help make lithium-air batteries a practical reality.