Communications: Elementary oxygen electrode reactions in the aprotic Li-air battery

Abstract

We discuss the electrochemical reactions at the oxygen electrode of an aprotic Li-air battery. Using density functional theory to estimate the free energy of intermediates during the discharge and charge of the battery, we introduce a reaction free energy diagram and identify possible origins of the overpotential for both processes. We also address the question of electron conductivity through the Li2O2 electrode and show that in the presence of Li vacancies Li2O2 becomes a conductor.

Rate-Dependent Morphology of Li2O2 Growth in Li–O2 Batteries

“…Note that in our model I 0 depends on activity, which is a complicated function of the height profile h(x). We assume that the first charge transfer step in the ORR (1) is rate limiting and symmetric (α 1 = 1 2 ), so the overall charge transfer coefficient is α = 1 4 (see also [40,45]), which is consistent with the Tafel slope measured on glassy carbon [49]. The activity coefficient of the transition state γ ‡ is approximately constant and can be estimated by Marcus theory [25] because it is dominated by desolvation.…”

“…Cell-level models propose pore blocking due to reaction products [34][35][36][37][38] and surface passivation [24,39]. Atomistic models discuss the surface structure of Li 2 O 2 crystals [40][41][42][43][44], the kinetics of the oxygen reduction/evolution in aprotic electrolytes [40,44,45], and the electron conductivity of Li 2 O 2 [24,41,46]. Here, we develop a nanoscale continuum model based on these atomistic studies, which bridges the gap to macroscopic models by predicting morphological selection in the early stages of surface growth.…”

“…The parameters E 0 and E 1 are taken from galvanostatic discharge measurements. We find the open circuit voltage E 0 = 2.96 V and the typical overpotential E 1 = 0.2 V, at which all reaction steps are downhill in energy [24,45]. We add Gaussian noise with standard deviation 0.004 V = 0.15 k B T /e to E 1 to model molecular fluctuations.…”

Electrical conductivity in Li2O2 and its role in determining capacity limitations in non-aqueous Li-O2 batteries

“…It is generally assumed that crystal growth occurs principally at kink and step sites since these allow the most stabilization of adspecies. 8 This, of course, implies that intermediates are somewhat mobile on the Li 2 O 2 surface. Since Li-O 2 discharges form Li 2 O 2 crystallites of ∼15 nm dimensions (from x-ray analysis), 6 we take this mechanism to be dominant in the growth during discharge.…”

“…Since Li-O 2 discharges form Li 2 O 2 crystallites of ∼15 nm dimensions (from x-ray analysis), 6 we take this mechanism to be dominant in the growth during discharge. In particular, the thermodynamics for Li 2 O 2 electrochemistry on Li 2 O 2 steps has been discussed previously 8 so that we use this mechanism to investigate charge transport during electrochemistry. This mechanism is given in terms of the following steps of charge transfer: (1)- (3) above need to run twice before the initial and final states of the surface are the same.…”