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V.B.11 Crystal Studies on High-energy Density Cathodes (LBNL)
Chen – LBNL
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Figure V - 58: XRD patterns of the LiNi0.5Mn1.5O4 crystals synthesized at
indicated temperatures. Arrows indicate peaks from the rock-salt type
impurity phase.
The choices of both reaction precursors and molten
salt were found to have a large impact on crystal size and
morphology. With the oxide precursors, octahedronshaped
crystals with an average size of 30 nm were
obtained from a eutectic mixture of 0.58LiCl-0.42KCl, as
shown in the SEM image in Figure V - 59a. When the nitrates
were used, plate-shaped crystals, averaged at 1 m in size
and 100 nm in thickness, precipitated in LiCl flux at 750°C
(Figure V - 59b), whereas octahedron-shaped crystals with an
average size of 2 m formed in the eutectic flux of LiCl-
KCl at 700°C (Figure V - 59c). TEM and SAED analyses on
the large crystals suggested that the octahedrons were
predominantly enclosed by the (111) crystal planes, while
the main surface facets on the plates were (112), as shown
on the insets in Figure V - 59b and Figure V - 59c.
(space group of P4 3 32) appeared strongly in the large
crystals made from the nitrates (Figure V - 60). The peak
intensity ratio of 590/620, which has been used as a
qualitative measure of transition-metal ordering in the
structure, suggested that the octahedrons are most ordered
among three samples. Superlattice peaks from the ordered
structure, however, were not discernible in the XRD
patterns.
The redox behavior and rate capabilities of the
crystals are compared in Figure V - 61. The main peaks on
the integrated capacity and voltage profiles are known to
be associated with the Ni 2+ /Ni 3+ and Ni 3+ /Ni 4+ redox
couples. For LiNi 0.5 Mn 1.5 O 4 plates with (112) surface
facets, the charging peaks were separated by 55 mV, close
to the reported value of 60 mV in the spinels with a
disordered structure (space group of Fd3ത m). The peaks
also had a significant shift toward higher voltage,
suggesting increased resistance for Li extraction from
(112) crystal plane. Broad peaks at 4.1 V that is
characteristic of the Mn 3+ /Mn 4+ transitions were observed,
with the peak area integration estimating 18% Mn 3+ in the
sample. The amount decreased to 1.5% in the octahedronshaped
crystals with (111) surface facets, where the
charging peaks were separated by a much smaller amount
of 25 mV, close to the reported value of 20 mV for the
ordered spinel. Despite the higher degree of ordering in
the crystal structure and larger size, the octahedral crystals
demonstrated superior rate capability, delivering a capacity
of 138 mAh/g at C/5 rate. The smaller-sized crystals, on
the other hand, had the worst performance among the three
samples. The main surface facets on this sample remain to
be analyzed, but the study clearly illustrates the importance
of particle morphology on the performance of this cathode
material.
Figure V - 59: SEM images of LiNi0.5Mn1.5O4 crystals prepared from a) oxide
precursors in eutectic LiCl-KCl mixture, b) nitrate precursors in a LiCl flux,
and c) nitrate precursors in eutectic LiCl-KCl mixture.
On the FTIR spectra, the bands at 430, 560 and 650
cm -1 that are characteristic of the cation ordered structure
Figure V - 60: FTIR spectra of the LiNi0.5Mn1.5O4 crystals. Arrows indicate
peaks from the ordered structure.
Work was also initiated to investigate the effect of
Mn 3+ content on rate performance and stability of this
high-voltage cathode. LiNi x Mn 2-x O 4 (0.3x0.5) single
Energy Storage R &D 520 FY 2011 Annual Progress Report