order modes to circumvent this issue. We envision
that this concept may open new directions in acoustics research, including advances in noise control,
transducer technologies, energy harvesting systems,
acoustic imaging, and sensing.
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Acknowledgments: This work has been supported by the
Defense Threat Reduction Agency Young Investigator
Program (YIP) award HDTRA1-12 1-0022 and the Air Force
Office of Scientific Research YIP award FA9550-11-1-0009.
A provisional U.S. patent has been filed with title “
Nonreciprocal acoustic devices based on angular momentum
bias” (61/868,178). R.F., D.L.S., and A.A. developed the
concept presented in this paper. R.F. and D.L.S. carried out the
analytical and numerical modeling and built the device.
R.F. and C.F.S. conducted the measurements. M.R.H. contributed
to the design and realization of the experimental set-up.
A.A. supervised the entire project. All authors discussed the
results and commented on the article.
Materials and Methods
8 October 2013; accepted 3 December 2013
Unlocking the Potential of
Cation-Disordered Oxides for
Rechargeable Lithium Batteries
Jinhyuk Lee,1 Alexander Urban,1 Xin Li,1 Dong Su,2 Geoffroy Hautier,1 Gerbrand Ceder1†
Nearly all high–energy density cathodes for rechargeable lithium batteries are well-ordered
materials in which lithium and other cations occupy distinct sites. Cation-disordered materials
are generally disregarded as cathodes because lithium diffusion tends to be limited by their
structures. The performance of Li1.211Mo0.467Cr0.3O2 shows that lithium diffusion can be facile
in disordered materials. Using ab initio computations, we demonstrate that this unexpected
behavior is due to percolation of a certain type of active diffusion channels in disordered Li-excess
materials. A unified understanding of high performance in both layered and Li-excess materials may
enable the design of disordered-electrode materials with high capacity and high energy density.
Rechargeable lithium-ion batteries enable increasingly capable portable electronics and are the crucial factor in the deployment of electric vehicles. Cathodes with high energy density are desirable for high-performance
lithium batteries, as they make up a substantial
part of the cost, weight, and volume of a battery.
Cathode compounds operate by reversibly releas-
ing (de-intercalation) and reinserting (intercalation)
lithium ions during charge and discharge, respec-
tively. This process must occur without causing
permanent change to the crystal structure be-
cause the battery must endure hundreds of charge-
discharge cycles. Traditionally, cathodes have been
sought from well-ordered close-packed oxides—in
particular, layered rocksalt-type lithium–transition
metal oxides (Li-TM oxides) (1–3) and ordered
spinels (4, 5)—whereas nonordered materials have
received limited attention (6–9). In these ordered
compounds, Li sites and pathways (a 2D slab in
the layered oxides and a 3D network of tetra-
hedral sites in the spinels) are separated from the
TM sublattice, which provides stability and elec-
tron storage capacity. Having well-ordered struc-
tures where there is little or no intermixing between
the Li and the TM sublattice is generally con-
sidered important for obtaining high-capacity
cathode materials with good cycle life (10, 11).
In some cases, improvements in ordering have
led to notable increases in power or energy den-
sity (3, 12–14). Here, we show that this “ordering
paradigm” may have led the community to over-
look a large class of cathode materials in which
Li and TM share the same sublattice in a random
(disordered) fashion; some of these materials may
offer higher capacity and better stability relative
to the layered oxides.
We chose the Li1.211Mo0.467Cr0.3O2 (LMCO)
compound because of our interest in metals that
can exchange multiple electrons, such as Mo and
Cr. In addition, both Mo and Cr have been shown
to migrate in layered materials (15, 16). LMCO
was synthesized through standard solid-state procedures as described (17). The material forms as
a layered rocksalt but transforms to a disordered
rocksalt after just a few charge-discharge cycles,
as seen in the x-ray diffraction (XRD) patterns in
Fig. 1A. The (003) reflection, characteristic of the
layered structure, starts to disappear after one cycle and is essentially gone at the 10th cycle. From
Rietveld refinement, we estimate 34 to 52% of
the TM ions to be in Li layers after 10 cycles, indicating substantial cation mixing in LMCO (17).
The evolution of LMCO to a disordered structure
was confirmed in real space with scanning transmission electron microscopy (STEM) (Fig. 1B).
The bright and dark columns in the “before”
image correspond to atomic columns of mixed
Li-Mo-Cr ions and Li ions, respectively. The
Z-contrast decreases after one cycle and is very
weak after 10 cycles, indicating increased cation
mixing. This substantial structural evolution is consistent with the change in voltage profile (Fig. 1C)
between the first charge and all subsequent cycles.
The reversible Li capacity of carbon-coated
LMCO (LMCO/C) is remarkably high, even after
disordering (17). As seen in Fig. 1C, approximately one lithium (= 265.6 mAh g−1) per formula unit can be reversibly cycled at C/20 rate
[= 16.4 mA g−1; the C/n rate denotes the rate
of cycling the theoretical capacity of LMCO
(327.5 mAh g−1) in n hours], delivering an energy density of ~660 Wh/kg (~3100 Wh/liter) at
~2.5 V. Such high capacity is rarely achieved even
in layered Li-TM oxides (18–20) and is counter-intuitive because cation mixing has been argued
to markedly degrade the cyclability of layered
oxides, primarily by reducing the Li layer spacing
1Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
2Center for Functional Nanomaterials, Brookhaven National
Laboratory, Upton, NY 11973, USA.
*These authors contributed equally to this work.
†Corresponding author. E-mail: firstname.lastname@example.org