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

V.B.4 Stabilized Spinels and Nano Olivines (U. Texas)
Manthiram – U. Texas
state or solution-based synthesis approaches are
characterized by a variety of chemical and physical
techniques: wet-chemical analysis by inductively coupled
plasma (ICP) analysis, X-ray diffraction (XRD), scanning
electron microscopy (SEM), transmission electron
microscopy (TEM), thermal analysis, Fourier transform
infrared (FTIR) spectroscopy, Raman spectroscopy, and
X-ray photoelectron spectroscopy (XPS). The
electrochemical performances are evaluated with coin cells
with charge-discharge, cyclability, rate capability, and
impedance spectroscopic measurements. Based on the
chemical, structural, morphological, and electrochemical
data collected with the synthesized samples, an in-depth
structure-property-performance relationship is established.
The understanding gained is utilized to refine the
compositions and develop new materials.
Results
Stabilized High-voltage Spinel Cathodes: We
previously showed that the Fe-substituted high-voltage
spinel cathodes exhibit much improved electrochemical
performances (cyclability and rate capability) compared to
the pristine LiMn 1.5 Ni 0.5 O 4 . In order to develop further
understanding, we have focused on investigating various
cationic substitutions in LiMn 1.5 Ni 0.5-y M y O 4 (M = Al, Cr,
Fe, Co, Zn, and Ga) and the self-surface segregation of
cations by time-of-flight – secondary ion mass
spectroscopy (TOF-SIMS).
The above cationic substitutions eliminate the Li x Ni 1­
xO impurity phase, increase the Mn 3+ content, and suppress
the ordering between Mn 4+ and Ni 2+ , resulting in improved
electrochemical performance. Figure V - 19 compares the
TOF-SIMS depth profiles of Mn, Ni, and Fe in
LiMn 1.5 Ni 0.42 Fe 0.08 O 4 . The data show a much higher
concentration of Fe on the surface compared to that in the
bulk, indicating a segregation of Fe to the surface during
the synthesis process. This observation is consistent with
our earlier findings with XPS. The segregation of Fe to the
surface seems to provide a more stable interface with the
electrolyte, offering superior cycle life at 55°C and high
rate capability. Similar TOF-SIMS investigations of other
LiMn 1.5 Ni 0.42 M 0.08 O 4 also reveals a segregation of M = Al,
Cr, and Ga to the surface, but not M = Co and Zn.
With an aim to understand the origin of variations in
the performance of undoped LiMn 1.5 Ni 0.5 O 4 and to increase
the tap density of the high-voltage spinels, we have also
focused on the synthesis of the undoped LiMn 1.5 Ni 0.5 O 4 by
various methods such as a hydroxide precipitation, a
carbonate precipitation, an ammonium bicarbonatemediated
process, and a urea-mediated process. These
processes were used to prepare materials with varying
morphologies.
Figure V - 20 shows the first charge-discharge profiles
and the corresponding SEM images of the LiMn 1.5 Ni 0.5 O 4
samples prepared with various morphologies. The
commercial and spherical 2 samples composed of
relatively smaller particles (1 – 2 m) with a lower tap
density of 1.3 – 1.5 g/cm 3 show lower discharge capacities
of ~ 120 mAh/g. On the other hand, the polyhedral 1,
polyhedral 2, cubic, spherical 1, and spherical 3 samples
with a larger particle size of 5 to 15 m exhibit larger
discharge capacities of ~ 130 mAh/g. The larger particle
size also leads to a high tap density of 1.8 – 2.0 g/cm 3 for
the polyhedral 1, cubic, and spherical 1 samples, which is
beneficial to increase the volumetric energy density.
Among the seven samples in Figure V - 20, the spherical 3
sample shows the lowest coulombic efficiency of 77 % due
to the high surface area caused by the loosely packed,
submicron-size crystallites, lowering the tap density to 1.6
g/cm 3 , and the increased solid-electrolyte interfacial (SEI)
layer formation. On the other hand, the cubic and spherical
1 samples show higher coulombic efficiencies of,
respectively, 80 and 84 % due to a larger particle size and
smaller surface area.
Figure V - 21 compares the cyclability of the various
LiMn 1.5 Ni 0.5 O 4 samples. All the samples show better
cyclability than the commercial sample. Among the six
samples synthesized by us, the spherical 3 sample shows
the largest capacity fade due to the larger surface area and
increased reaction with the electrolyte. Also, the
commercial and the spherical 3 samples show a smaller 4
V plateau, implying a lower Mn 3+ content, compared to the
cubic, polyhedral 2, spherical 1, and spherical 2 samples,
so the better cyclability of the latter samples could be
related to the higher Mn 3+ content. However, the
polyhedral 1 sample with a smaller 4 V plateau also
exhibits better cyclability. Thus, particle size, surface area,
morphology, Mn 3+ content, and degree of cation disorder
in the 16d octahedral site all seem to play a role in
controlling the electrochemical properties and our future
work will focus on developing a further understanding of
the influence of these parameters.
Figure V - 19: TOF-SIMS depth profile of the LiMn1.5Ni0.42Fe0.08O4 sample.
Energy Storage R &D 484 FY 2011 Annual Progress Report