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Thermodynamic Origin of Irreversible Magnesium Trapping in Chevrel Phase Mo6S8: Importance of Magnesium and Vacancy Ordering
journal contribution
posted on 2017-04-04, 00:00 authored by Chen Ling, Koji SutoThe rechargeable magnesium battery
is an alternative to current
Li-ion technologies, potentially offering a higher energy density
by employing metal magnesium as an anode material. The realization
of a magnesium battery, however, is hampered by the barrier to finding
a cathode material that reversibly stores and releases Mg ions through
electrochemical intercalation. Understanding the underlying mechanism
that prevents successful Mg intercalation is now crucial for the design
and exploration of new cathode candidates. Our work reports the critical
effect of a thermodynamic factor, the ordering of Mg and interstitial
vacancies, on the activity of a Chevrel phase Mo6S8 cathode. Specifically, we demonstrate that the irreversible
trapping in the Chevrel phase at low Mg concentrations is thermodynamically
driven by the ordering of Mg and vacancies instead of being kinetic
in origin. Through a combination of nudged elastic band, geometry-confined
static calculation, and ab initio molecular dynamics methods, we show
that the kinetic diffusion of Mg in a Chevrel phase cathode depends
little on the composition. On the other hand, the formation of ordered
Mg and vacancy structure along the ⟨100⟩ direction at
low Mg concentrations greatly decreases the population of mobile ions,
reducing the chemical diffusivity by 3–5 orders of magnitude
and consequently causing the irreversible trapping. At high concentrations,
the ordering is disrupted by the repulsion between neighboring Mg–Mg
pairs, which enhances Mg mobility and leads to fully reversible intercalation.
Our results not only provide mechanistic knowledge about the irreversibility
of a prototype Mg battery cathode but also highlight the apparent
importance of the ordering effect in Mg intercalation for the future
exploration of cathode materials.