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

Whittingham – Binghamton U.
V.B.5 Synthesis and Characterization of Substituted Olivines and Mn Oxides (Binghamton U.)
materials. To meet the DOE cost targets, we are looking at
reducing high-cost components and for power and energy
targets at modifying the chemical composition and
morphology of the cathode compounds.
Approach
Our cathode approach is to place emphasis on low
cost materials, predominantly oxides and phosphates, both
pure and modified with other transition metals, using a
range of practical synthesis approaches. These materials
will be synthesized, and characterized both structurally,
including defects and morphology, and for thermal and
chemical stability. All will be evaluated electrochemically
in a range of cell configurations.
For the modified layered dioxides, we are determining
the role of each of the transition metals, with the goal of
minimizing expensive components such as cobalt. To that
end, we are studying the layered compositions,
LiNi y Mn y Co 1-2y O 2 , with a close to stoichiometric Li to
transition metal ratio and comparing them to the Li-rich
Mn-rich compounds.
We are also searching for new classes of materials
that might react with more than one lithium ion per redox
center. Phosphates are one area for our search and
vanadium containing materials another area.
Results
Layered Transition Metal Oxides. We formed a
range of transition metal oxides of formula LiNi y Mn y Co 1­
2yO 2 to determine the optimum composition for both
energy density and power density. The voltage/capacity
curves were determined and found to depend little on the
chemical composition except at low values of the lithium
content, when the redox of Co becomes involved and
pushed the potential higher. The lower cobalt material with
y = 0.4 showed as good electrochemical behavior as the
commonly used LiNi 1/3 Mn 1/3 Co 1/3 O 2 material, except at the
highest discharge rates. In collaboration with C. Ban and
A. Dillon at NREL, the rate behavior was determined and
shown to be excellent as indicated in Figure V - 23. Full
details are given in publication #3 listed below.
Figure V - 23: High-rate cycling of LiNi0.4Mn0.4Co0.22O2, using carbon
nanotube mesh grid.
The results, shown in Figure V - 23, clearly show that the
layered materials can deliver high rates when the electrode
is configured optimally.
Our earlier work showed that a manganese content
exceeding 0.5 in the formula Li[Ni 1-y-z Mn y Co z ]O 2 is
detrimental to the rate capability of these layered oxides
with a Li/M ratio of unity. This year we expanded the
study to the lithium-rich oxides described by the mixture
yLi 2 MnO 3 •(1-y)LiNi 0.4 Mn 0.4 Co 0.2 O 2 , where y=0.5 giving
an overall composition of Li 1.2 Ni 0.16 Mn 0.56 Co 0.08 O 2 . The
results were somewhat surprising, with the rate capacities
being worse than the Li/M stoichiometric material at
higher rates, but this material also showed very poor
thermal stability, no better than NCA. Although the
thermal stability could be improved by substitution of
some of the cobalt by aluminum, it is still much worse than
LiNi 1/3 Mn 1/3 Co 1/3 O 2 and LiNi 0.4 Mn 0.4 Co 0.4 O 2 as shown in
Figure V - 24. Aluminum contents above 5% do not seem to
further stabilize the compound. As earlier work from
Argonne National Lab has shown good stability for the Lirich
Li 1+y [Ni 1/3 Mn 1/3 Co 1/3 ] 1-y O 2 the instability is probably
associated with the high manganese content. Thus, our
conclusions of this layered oxide study are that the
manganese content should probably be kept at less than
50% of the transition metal and also no higher than the
nickel content in order to achieve high rates in a safe
cathode.
FY 2011 Annual Progress Report 489 Energy Storage R&D