TY - JOUR
T1 - Adiabatic and Nonadiabatic Charge Transport in Li-S Batteries
AU - Park, Haesun
AU - Kumar, Nitin
AU - Melander, Marko
AU - Vegge, Tejs
AU - García Lastra, Juan Maria
AU - Siegel, Donald Jason
PY - 2018
Y1 - 2018
N2 - The insulating nature of the redox end members in Li-S batteries, -S and
Li2S, has the potential to limit the capacity and efficiency of this
emerging energy storage system. Nevertheless, the mechanisms responsible
for ionic and electronic transport in these materials remain a matter
of debate. The present study clarifies these mechanisms – in both the
adiabatic and nonadiabatic charge transfer regimes – by employing a
combination of hybrid-functional-based and constrained density
functional theory calculations. Charge transfer in Li2S is predicted to
be adiabatic, and thus is well described by conventional DFT
methodologies. In sulfur, however, transitions between S8 rings are
nonadiabatic. In this case, conventional DFT overestimates charge
transfer rates by up to 2 orders of magnitude. Delocalized holes, and to
a lesser extent, localized electron polarons, are predicted to be the
most mobile electronic charge carriers in -S; in Li2S hole polarons
dominate. Although all carriers exhibit extremely low equilibrium
concentrations, and thus yield negligible contributions to the
conductivity, their mobilities are sufficient to enable the sulfur
loading targets necessary for high energy densities. Our results
highlight the value of methods capable of capturing nonadiabadicty, such
as constrained DFT. These techniques are especially important for
molecular crystals such as -S, where longer-range charge transfer events
are expected. Combining the present computational results with prior
experimental studies, we conclude that low equilibrium carrier
concentrations are responsible for sluggish charge transport in -S and
Li2S. Thus, a potential strategy for improving the performance of Li-S
batteries is to increase the concentrations of holes in these redox end
members.
AB - The insulating nature of the redox end members in Li-S batteries, -S and
Li2S, has the potential to limit the capacity and efficiency of this
emerging energy storage system. Nevertheless, the mechanisms responsible
for ionic and electronic transport in these materials remain a matter
of debate. The present study clarifies these mechanisms – in both the
adiabatic and nonadiabatic charge transfer regimes – by employing a
combination of hybrid-functional-based and constrained density
functional theory calculations. Charge transfer in Li2S is predicted to
be adiabatic, and thus is well described by conventional DFT
methodologies. In sulfur, however, transitions between S8 rings are
nonadiabatic. In this case, conventional DFT overestimates charge
transfer rates by up to 2 orders of magnitude. Delocalized holes, and to
a lesser extent, localized electron polarons, are predicted to be the
most mobile electronic charge carriers in -S; in Li2S hole polarons
dominate. Although all carriers exhibit extremely low equilibrium
concentrations, and thus yield negligible contributions to the
conductivity, their mobilities are sufficient to enable the sulfur
loading targets necessary for high energy densities. Our results
highlight the value of methods capable of capturing nonadiabadicty, such
as constrained DFT. These techniques are especially important for
molecular crystals such as -S, where longer-range charge transfer events
are expected. Combining the present computational results with prior
experimental studies, we conclude that low equilibrium carrier
concentrations are responsible for sluggish charge transport in -S and
Li2S. Thus, a potential strategy for improving the performance of Li-S
batteries is to increase the concentrations of holes in these redox end
members.
U2 - 10.1021/acs.chemmater.7b04618
DO - 10.1021/acs.chemmater.7b04618
M3 - Journal article
VL - 30
SP - 915
EP - 928
JO - Chemistry of Materials
JF - Chemistry of Materials
SN - 0897-4756
IS - 3
ER -