Toward Establishing Electronic and Phononic Signatures
of Reversible Lattice Oxygen Oxidation in Lithium Transition Metal
Oxides For Li-Ion Batteries
Posted on 2020-06-19 - 14:11
The
prospect of accessing anionic (oxygen) oxidation and reduction
reversibly in lithium transition metal oxides for the positive electrode
offers exciting opportunities to greatly boost the energy density
of Li-ion batteries. Unfortunately, the physical mechanisms governing
oxygen redox in these oxides remain under debate. In this article,
density functional theory studies using maximally localized Wannier
functions revealed that deintercalation of both lithium ions from
Li2–xRuO3 as well as
Li1–xNiO2 (Li3/2–xNi3/2O3) was dominated by the
oxidation of nonbonding states of oxygen or bonding states of oxygen
from metal–oxygen bonds, which was accompanied by moderate
Ru/Ni oxidation and reduction in the O–O bond distance, facilitated
by high metal–oxygen covalency in oxides. In contrast, deintercalation
of lithium ions from Li2–xMnO3 as well as Li2–xTiO3 and Li2–xSnO3 was dominated by the oxidation of nonbonding states of oxygen to
form O–O p sigma (σ) and π states with accompanied
distinct O–O peroxo-like bond formation but without Mn oxidation,
which is facilitated by relatively low metal–oxygen covalency.
Remarkably, the average oxygen phonon density of states (phonon DOS)
of oxides with high metal–oxygen covalency like Li2–xRuO3, Li2–xIrO3, and Li1–xNiO2 was moved to higher frequencies while that of those
with low covalency like Li2–xMnO3, Li2–xTiO3,
and Li2–xSnO3 was moved
to lower frequencies, which could promote oxygen and metal migration
and structural instability, leading to irreversible oxygen redox.
It is postulated that high metal–oxygen covalency is essential
to enable reversible access of oxygen redox along with metal redox
in transition metal oxides, which bridges different schools of thoughts
for oxygen redox and provide new insights into design of new oxygen-redox
capable positive electrodes.
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Charles, Nenian; Yu, Yang; Giordano, Livia; Jung, Roland; Maglia, Filippo; Shao-Horn, Yang (2020). Toward Establishing Electronic and Phononic Signatures
of Reversible Lattice Oxygen Oxidation in Lithium Transition Metal
Oxides For Li-Ion Batteries. ACS Publications. Collection. https://doi.org/10.1021/acs.chemmater.0c00245