V. Focused Fundamental Research - EERE - U.S. Department of ...

V.B.9 Layered Cathode Materials (ANL)
Thackeray – ANL
uncoated and coated) carried out at (a) RT, and (b) 55C.
Cells were cycled between 4.6 and 2.0 V at various current
densities of 15, 30, 75, 150, 300 and 750 mA/g. At RT, the
average capacity of coated electrodes is higher than
uncoated electrodes (Fig. 2a); furthermore, the first cycle
coulombic efficiency at 15 mA/g improves from 71% for
the uncoated electrode to 82% for the coated electrode. At
55°C, the performance of uncoated and coated electrodes
were similar at the lower current rates (15-75 mA/g); at
higher rates (150-750 mA/g), the coated electrodes clearly
provide significantly higher capacities. Because of this
promising result, this work will be extended in FY2012 to
other sonicated metal oxide coatings, particularly those
containing lithium that can act as protective solid
electrolyte, specifically to obtain detailed information
about surface structure and transport mechanisms that
occur at the electrode/electrolyte interface.
Figure V - 49: Top: Comparative rate study of uncoated and TiO2-coated
NMC electrodes vs. Li metal at 15, 30, 75, 150, 300 and 750 mA/g at RT;
Bottom: Corresponding rate study at 55°C.
Theory: Surface Structure of LiMn 2 O 4 . Chemical
and electrochemical reactions of lithium manganate spinel,
LiMn 2 O 4 , are strongly influenced by surface and interface
atomic structure. To help elucidate that structure,
simulations were performed of (a) bare LiMn 2 O 4 surfaces,
both pristine and Ni-doped, (b) LiMn 2 O 4 coated with
AlF 3 , and (c) LiMn 2 O 4 slabs immersed in H 2 O and acid (to
simulate acid-promoted dissolution) at the GGA+U level
of density functional theory. The most extensive results are
for bare surfaces. In Figure V - 50a, coordination numbers of
surface ions at surfaces of stoichiometric spinel slabs with
different orientations and terminations are shown. (All of
the surfaces contain vacancies introduced to cancel the
long-range dipole moments that would be present in bulkterminated
surfaces, and thereby avoid electrostatic
instabilities.) The relative stabilities of the different surface
orientations agree, for the most part, with predictions for
the prototype spinel MgAl 2 O 4 . Some of the surfaces show
Mn with coordination numbers as small as 3 (compared to
6 in the bulk). The deficient surface ion coordination
numbers result in lower Mn oxidation states, on average,
than the bulk, as illustrated in Figure V - 50b. Most Mn ions
near the surfaces exhibit trivalent oxidation states, whereas
the mean oxidation state in the bulk is 3.5. Mn ions with
coordination number of 3 are found to be divalent, a state
that does not occur in the bulk, which would enhance
vulnerability to dissolution. The preferred trivalent
oxidation state of surface Mn influences the near surface
chemistry of LiMn 2 O 4 . This was illustrated in simulations
of Ni ion substitution for Mn near the surface of a spinel
slab or wire. In the bulk, a Ni ion that substitutes for a
trivalent Mn ion adopts a divalent oxidation state, and
results in the oxidation of a neighboring Mn from a
trivalent to a tetravalent state. Since near-surface Mn
strongly prefers the trivalent state, however, suitable Mnion
candidates for oxidation to 4+ are unavailable, which
explains why energies of substitution near the surface are
positive, and substitution is unfavorable. At a distance of
about 1 nm from the surface, the structure is sufficiently
bulk-like so that energies of substitution are close to the
bulk limit. These results suggest that the near surface
region of a Ni-doped spinel specimen would be depleted of
Ni. As the Ni-depleted surface layer is eroded in a lithiumion
battery cathode (for example, by acid promoted
dissolution), however, Ni-rich regions would be
uncovered, which would be expected to give some
protection against further dissolution, because the presence
of Ni would diminish the number of Mn-ion candidates for
oxidation to the tetravalent state, a necessary
accompaniment to the dissolution of a surface trivalent Mn
ion that would go into solution in a divalent state by
disproportionation.
Energy Storage R &D 510 FY 2011 Annual Progress Report