It is an open question and difficult to access experimentally [3] which properties an optimal redox mediator should have. In this work we systematically investigate the diffusion and reaction properties of redox mediators using a mathematical model and provide guidelines for a knowledge-driven search for better redox mediators.
Our model incorporates the redox mediator regeneration on the electrode surface, transport of redox mediator as well as the reaction between redox mediator and Li2O2. Depending on the reaction and diffusion properties of the redox mediator, cylindrical particles on the cathode surface are dissolved uniformly or not over the particle height (see Fig. 1 a)). We suggest to use a quotient that describes the relationship between redox mediator diffusion coefficient, electrochemical regeneration of the mediator as well as the chemical reaction with the particle as an additional measure besides the Nernst potential to characterize redox mediators. As an example the chemical reaction rate constant directly influences the charge potential (see Fig. 1 b)), this observation is currently often neglected in the selection of redox mediators. We show that there is an optimum between the above mentioned parameters. In addition we recommend redox mediator properties that allow to dissolve Li2O2 particles homogeneously to reduce the necessary redox mediator concentrations which helps to decrease undesired degradation reactions. Preventing inhomogeneous Li2O2 particle dissolution and loss of contact to the electrode has the advantage that long diffusion paths are prevented. Especially if the diffusion process has a determining effect, this is of great importance. Furthermore, we investigate the influence of the discharge product morphology on the desirable properties of a redox mediator.
Our work shows that besides the Nernst potential also the diffusion coefficient and the chemical reaction rate constant between Li2O2 and redox mediator play a crucial role for the observed oxygen evolution overpotentials. The presented simulation model can predict desirable redox mediator properties depending on the discharge product particle sizes and might guide the way towards tailoring more efficient redox mediators.
References:
[1] J. Christensen, P. Albertus, R. S. Sanchez-Carrera, T. Lohmann, B. Kozinsky, R. Liedtke, J. Ahmed and A. Kojic, „A Critical Review of Li∕Air Batteries“, J. Electrochem. Soc., vol. 159, no. 2, pp. 1-30, 2012.
[2] J. B. Park, S. H. Lee, H. G. Jung, D. Aurbach and Y. K. Sun, „Redox Mediators for Li-O2 Batteries: Status and Perspective“, Adv. Mater., vol. 30, no. 1, pp. 1-13, 2018.
[3] Y. Chen, X. Gao, L. R. Johnson, and P. G. Bruce, “Kinetics of lithium peroxide oxidation by redox mediators and consequences for the lithium-oxygen cell”, Nat. Commun., vol. 9, no. 1, 2018.