Factors Contributing to Path Hysteresis of Displacement
and Conversion Reactions in Li Ion Batteries
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Abstract
We
investigate the thermodynamic and kinetic attributes of electrode
materials that are necessary to suppress path hysteresis during displacement
and conversion reactions in Li ion batteries. We focus on compounds
in the Li–Cu–Sb ternary composition space, as the displacement
reaction between Li<sub>1+ϵ</sub>Cu<sub>1+δ</sub>Sb and
Li<sub>3</sub>Sb can be cycled reversibly. A first-principles analysis
of migration barriers indicates that Cu, while not as mobile as Li
in the discharged phase (Li<sub>3</sub>Sb), nevertheless should exhibit
mobilities similar to that of Li in common intercalation compounds.
A calculation of phase stability in the ternary Li–Cu–Sb
system predicts that the intermediate phases along the reversible
charge/discharge path are stable in a large Cu chemical potential
window. This ensures that intermediate phases are not bypassed upon
Li extraction even when large thermodynamic driving forces are needed
to reinsert Cu into the discharged electrode. Our study suggests that
the suppression of path hysteresis during displacement reactions requires
(i) a high mobility of the displaced metal and (ii) the thermodynamic
stability of intermediate phases along the reversible path in a wide
metal chemical potential window. Even in the absence of path hysteresis,
displacement and conversion reactions suffer from polarization needed
to set up thermodynamic driving forces for metal extrusion and reinsertion.
This polarization can be estimated with a Clausius–Clapeyron
analysis