TY - JOUR
T1 - Intrinsic Conductivity in Magnesium-Oxygen Battery Discharge Products: MgO and MgO2
AU - Smith, Jeffrey G.
AU - Naruse, Junichi
AU - Hiramatsu, Hidehiko
AU - Siegel, Donald Jason
PY - 2017
Y1 - 2017
N2 - Nonaqueous 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 either are 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 nonequilibrium
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 long-debated
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 monovalent substitutions is
suggested as a strategy for overcoming transport limitations.
AB - Nonaqueous 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 either are 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 nonequilibrium
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 long-debated
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 monovalent substitutions is
suggested as a strategy for overcoming transport limitations.
U2 - 10.1021/acs.chemmater.7b00217
DO - 10.1021/acs.chemmater.7b00217
M3 - Journal article
VL - 29
SP - 3152
EP - 3163
JO - Chemistry of Materials
JF - Chemistry of Materials
SN - 0897-4756
IS - 7
ER -