Intrinsic Conductivity in Magnesium-Oxygen Battery Discharge Products: MgO and MgO₂

Abstract

Non-aqueous magnesium-oxygen (or 'Mg-air') batteries are attractive next generation energy storage devices due to their high theoretical energy densities, projected low cost, and potential for rechargeability. Prior experiments identified magnesium oxide, MgO, and magnesium peroxide, MgO2, as the primary discharge products in a Mg/O2 cell. Charge transport within these nominally-insulating compounds is expected to limit battery performance; nevertheless, these transport mechanisms are either incompletely understood (in MgO2) or remain a matter of debate (in MgO). The present study characterizes the equilibrium conductivity associated with intrinsic (point) defects within both compounds using first-principles calculations. For MgO, negative Mg vacancies and hole polarons -the latter localized on oxygen anions -were identified as the dominant charge carriers. However, the large formation energies associated with these carriers suggest low equilibrium concentrations. A large asymmetry in the carrier mobility is predicted: hole polarons are highly mobile at room temperature, while Mg vacancies are essentially immobile. Accounting for non-equilibrium effects such as frozen-in defects, the calculated conductivity data for MgO is shown to be in remarkable agreement with the three "Arrhenius branches" observed in experiments, thus clarifying the longdebated transport mechanisms within these regimes. In the case of MgO2, electronic charge carriers alone -electron and hole polarons -are the most prevalent. Similar to MgO, the equilibrium concentration of carriers in MgO2 is low, and moderate-to-poor mobility further limits conductivity. If equilibrium behavior is realized, then we conclude that: (i.) sluggish charge transport in MgO or MgO2 will limit battery performance when these compounds cover the cathode support, and (ii.) what little conductivity exists in these phases is primarily electronic in nature (i.e., polaron hopping). Artificially increasing the carrier concentration via mono-valent substitutions is suggested as a strategy for overcoming transport limitations.

Challenges and prospects of Mg-air batteries: a review

“…In 2016, Smith et al further studied the electrochemical processes on the surfaces of possible discharge products (MgO and MgO 2 ) through DFT calculations and the results showed that pathways involving oxygen intermediates were preferred [16] . They pointed out that even though the maximum energy density could be achieved when MgO served as the discharge product, the surface-mediated reactions should be avoided due to its inefficiency.…”

“…Modeling and calculation can predict the experimental results and guide the experiments effectively. Previous studies have been conducted to model and calculate the cathode discharge reaction paths [120] , initial nucleation of discharge products (MgO and MgO 2 ) [16] , and their equilibrium conductivity associated with the intrinsic ionic (point) defects [17] . Furthermore, DFT studies can calculate the composition and electronic conductivity of passivation film on the cathodes and predict the catalytic activity towards the ORR and OER of activity sites on the catalyst surface [129] .…”

“…In recent years, significant progress has been made by the team of Shiga, who first reported rechargeable Mg-air batteries in 2013 [13] . Other outstanding work by Siegel's group included analyzing of the discharge products and reaction pathways for rechargeable Mg-air batteries through a combination of theoretical calculations and experimental observations [12,16,17] .…”

“…(B) SEM images of first discharge products (a), higher magnification of the first discharge products (b) and SEM images of the air electrode after its first charge process (c), higher magnification of the residual product after the first charge process (d)[12] . (C) Calculated free energy diagram of different reaction paths of Mg-air batteries[16] . Reproduced from Refs [12,16].…”

“…(C) Calculated free energy diagram of different reaction paths of Mg-air batteries[16] . Reproduced from Refs [12,16]. with permission from the American Chemical Society.…”