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

V.E.7 Predicting/Understanding New Li-ion Materials Using Ab Initio Atomistic Computational Methods (LBNL)
Persson – LBNL
results in a different response of the material when
synthesized under different conditions.
Approach
The Persson group uses atomistic modeling to study
the relevant thermodynamic and kinetic processes. The
calculations are performed on the Lawrencium cluster at
LBNL and at NERSC. In the case of the high voltage
spinel we have used first-principles zero-temperature
calculations and a coupled cluster expansion to establish
the relationship between the cation order and the
electrochemical signature of the materials.
For the investigation on the Li-carbon system we are
using a combination of DFT to describe the low
temperature characteristics of the material as well as
statistical mechanics to calculate the phase diagram and
the Li chemical diffusivity and phenomenological models
to capture the van der Waals interactions.
Results
Li x Ni 0.5 Mn 1.5 O 4 Spinel. As part of the high-voltage
spinel focus group effort we have investigated the cation
ordering influence on the stable ground states as a function
of Li content in Li x (Ni 0.5 Mn 1.5 )O 4 . Previous work has
shown that the voltage profile is generally much more flat
in the spinel where Ni (Mn) sits exclusively at 4b (12d)
sites compared to the structure where both sites are
occupied randomly by Ni or Mn. Our study unequivocally
shows how the preferred Li-Vac arrangement is
incommensurate with the cation ordering in the ordered
spinel, which results in an absence of intermediate Li
concentration ground states. However, for uniformly
disordered spinel the cation arrangement accommodates
the preferred Li-Vac arrangement, which is exhibited in a
strong ground state at half lithiated composition. Figure V -
207 shows the predicted voltage profiles for several cation
orderings. The uniformly disordered (blue) corresponds to
the highest entropy achievable and exhibits a strong
ground state at x = 0.5. A perfectly ordered spinel (red) is
expected to exhibit only a combination of the two end
states at any Li content, devoid of any intermediate ground
state as a function of lithium content.
Li-Carbon. We have concluded our study of Li
absorption on graphite surfaces. We have found, in
agreement with experimental evidence, that the 0001
surface is the most thermodynamically stable surface.
More surprisingly we also found that Li does not absorb on
the predominant 0001 surface, if that surface is defect-free.
Furthermore, we have studied the Li absorption and
intercalation as a function of increasing number of layers
of graphene and Li content. The challenge has been to
obtain the correct van der Waals interaction as a function
of Li content, especially in the low lithiation regime. We
have been exploring the Grimme formulation 24 of the van
der Waals potential, which requires tuning to obtain the
correct order of magnitude and dissipation, see Figure V - 208.
Figure V - 207: The voltage profiles for ordered (red) and 4 different
disordered’ (yellow, green, blue and magenta) versions of Lix(Ni0.5Mn1.5)O4.
Figure V - 208: Tuning the Li-C absorption energy as a function of Li content
using the Grimme vdW formulation.
The Materials Project. To the conclusion of a
milestone for 2011, we successfully launched the first
Materials Project web site and database. The web site
provides free searchable access to general materials
properties covering >15,000 inorganic compounds and the
number of compounds increases continuously. The site
contains tools (‘apps’) designed to aid in materials design
for specific application areas such as Li-ion battery
technology. Figure V - 209 shows the announcement featuring
many of the possible search capabilities of the web site –
among them an example electrode material with its
computed voltage profile and oxygen evolution as a
24 S. Grimme, J. Comp. Chem. 27, 1787 (2006).
Energy Storage R &D 652 FY 2011 Annual Progress Report