Upon discharging and subsequent charging,
the pre-peak S remained at nearly the same
position. However, a new pre-peak emerged
at ~58.0 eV (labeled by“M”), which is absent
in the spectra of the two end-members (Fig. 2D)
as well as the partially lithiated LTO electrodes
in ex situ measurements [section 3 in ( 22 )]
(fig. S7). Pre-peak M was commonly observed
across the electrode, and its intensity was
found to be strongly rate-dependent (fig. S8).
The ratio of integrated intensity of the two
pre-peaks M and S (defined asIM/IS)asa
function of Li concentration (x) was plotted
in Fig. 2E. At low rates (1 C and 2 C),IM/IS
values are small (only ~0.2) but increase
abruptly (to ~2.0) at high rates (3 C and 8 C)
[quantitative analysis in section 4 of ( 22 )]. As
shown below, the evolution of features in the
Li-EELS spectra (e.g.,IM/ISratio) provides key
information about Li occupancy and migration
in the metastable intermediates (Li4+xTi 5 O 12 )
and its rate-dependent behaviors.
To understand the origin of the pre-edge
features in the Li-EELS spectra and their
evolution during charge and discharge, we
studied the local configurations of Li+ions in
the two end-members (Li 4 Ti 5 O 12 and Li 7 Ti 5 O 12 )
and the intermediates (Li4+xTi 5 O 12 ,x= 1 and
2) using density functional theory (DFT) ( 22 ).
In the low-energy structures of Li4+xTi 5 O 12 ,
one of the Li(8a)tetrahedra shares a three-
coordinated oxygen face with a Li(16c)octahe-
dron at the domain boundary between Li 4 Ti 5 O 12
and Li 7 Ti 5 O 12 (fig.S9,CandD).Theface-
sharing Li(8a)and Li(16c)polyhedra are highly
distorted compared to the unperturbed Li(8a)
and Li(16c)polyhedra in Li 4 Ti 5 O 12 and Li 7 Ti 5 O 12
and are stabilized by neighboring Li(16d)
octahedra [section 5 in ( 22 )], which is con-
sistent with recent DFT calculations ( 12 ).
Within 100 meV/O 4 , a large number of rele-
vant atomic configurations exist with differ-
ent local environments, especially local motifs
of Li(8a)-Li(16c)face-sharing polyhedra with
various levels of distortion (fig. S10). The
DFT-based sampling of the Li4+xTi 5 O 12 po-
tential landscape reveals that a very large
configuration space can be accessed, even
within the formation energy of 100 meV/O 4
[the nature of these intermediate configura-
tionsisdescribedfurtherinsections5and
8in( 22 )].
After establishing thestructural models of
Li4+xTi 5 O 12 at different Li concentrations (x=
0, 1, 2, and 3), we calculated the correspond-
ing Li-EELS spectra for Li at the 8a, 16c, and
16d sites for the most stable configurations of
Li4+xTi 5 O 12 using the Z + 1 approach (where Z
refers to the atomic number) to model the
core-hole effect [computational details in ( 22 )].
Becausethepre-edgepeaksoriginatepredom-
inantly from the intra-atomic Li 1s to 2p tran-
sition, this level of description is adequate. The
good agreement in the near-edge region of the
end-members between the Z + 1 method and
the Bethe-Salpeter equation–based method
supports our choice of method (fig. S11). The
energy positions of pre-peak S in the com-
puted spectra and their relative intensities
compared with those of the main peak for
both Li 4 Ti 5 O 12 and Li 7 Ti 5 O 12 are in good
agreement with the experimental results (fig.
S12A), validating the Z + 1 method used for
computing the Li-EELS spectra. The Li-EELS
spectra of Li(8a)in the most stable Li 5 Ti 5 O 12
and Li 6 Ti 5 O 12 configurations are almost the
same as those of Li(8a)in Li 4 Ti 5 O 12 ,especially
near the pre-peak region (Fig. 3A). However,
a new pre-peak appears at ~58 eV in the EELS
spectra of face-sharing Li(16c)that is absent in
the computed spectra of Li(16c)in Li 7 Ti 5 O 12
and Li(16d)at any composition (Fig. 3B; fig.
S12, B and C; and fig. S14). Because this new
pre-peak coincides in energy with pre-peak M
observed in the operando EELS measurements(Fig. 2B), we assign pre-peak M in the Li-EELS
spectra at low rates to the inelastic scattering
from distorted face-sharing Li(16c)in the meta-
stable configurations. Owing to the distortion
of face-sharing Li(16c),severalLi–O bonds are
elongated, which breaks the degeneracy of
Li–O coupling. A representative configuration
is shown in Fig. 3C that demonstrates elon-
gated Li–O bond lengths of 2.33 to 2.50 Å in
Li 5 Ti 5 O 12 , considerably longer than those in
Li 7 Ti 5 O 12 (2.06 to 2.20 Å). The weakened Li–O
bonds effectively pull the antibonding Li–O
states to lower energy and cause the pre-peak
to split, giving rise to pre-peak M in the EELS
spectra. The corresponding partial charge den-
sities are shown in the isosurface plot in Fig. 3C.
Whereas the partial charge density associated
with pre-peak M concentrates more heavily on
O atoms with longer Li–O bonds, the partial
charge density of pre-peak S is distributed
evenly on all O atoms regardless of Li–ObondZhanget al.,Science 367 , 1030–1034 (2020) 28 February 2020 3of5
Fig. 3. Identification of Li-EELS fingerprints for Li-polyhedral configurations in Li4+xTi 5 O 12 (0≤x≤3)
by DFT calculations.(AandB) Calculated Li-EELS spectra of Li4+xTi 5 O 12 (x= 0, 1, and 2) for Li at
8a sites and Li4+xTi 5 O 12 (x= 1, 2 and 3) for Li at 16c sites, respectively. The dashed lines in (A) and
(B) mark the energy positions of the main peaks and pre-peak S. The black arrow in (B) indicates the
pre-peak M from face-sharing Li(16c)in Li 5 Ti 5 O 12 and Li 6 Ti 5 O 12 , which is not observed for Li(16c)in Li 7 Ti 5 O 12.
arb. unit, arbitrary unit. (C) Isosurface of partial charge density around face-sharing Li(16c)in Li 5 Ti 5 O 12
in the energy range of pre-peak M and pre-peak S, as marked with dashed red and blue boxes, respectively,
in (B), with 0.015 of isovalue. Bond lengths between Li(16c)and O ions are labeled. (D) Intensity of the
pre-peak M as a function of distortion index (d) of face-sharing Li(16c)for various Li configurations
in Li 5 Ti 5 O 12 and Li 6 Ti 5 O 12 .(E)dof face-sharing Li(16c)in thenth stable configuration (ordered by
formation energy) of Li 5 Ti 5 O 12. The horizontal and vertical dashed lines indicate the approximate value of
dabove which pre-peak M appears and the formation energy of thenth stable configuration at 50, 100,
and 150 meV/O 4. When a single configuration contains multiple face-sharing Li(16c), the different face-sharing
Li(16c)are labeled as colored points according to theirdvalues in the ascending order: gray (first lowest
distortion index), black (second), red (third), and blue (fourth).RESEARCH | REPORT