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

Smith, Borodin – U. Utah
V.D.3 Molecular Dynamics Simulation Studies of Electrolytes and Interfaces (U. Utah)
properties, good mechanical properties and
electrochemical stability is of paramount importance.
Particularly challenging is development of electrolytes
and/or additives that allow use of high voltage cathode
materials.
Approach
Our approach to simulation of bulk electrolytes,
SEI layers, and electrode/electrolyte interfaces is multifold.
First, we carry out high-level quantum-chemistry
(DFT and correlated methods) to study the structure of
electrode materials, their surfaces, interactions between
electrodes and electrolytes (including reactions), and the
interaction between electrolyte components. Second,
where possible and appropriate, we utilize quantumchemistry
based force fields and non-reactive simulation
methods. These studies include bulk electrolytes, model
SEI layers and electrode/electrolyte interfaces. Third, we
utilize an electroactive interface model to study electrolyte
structure and charge transfer processes at
electrode/electrolyte interfaces where control of electrode
potential is paramount. Finally, we utilized atomistic MD
simulations with ReaxFF, a reactive force field developed
by Adri van Duin at Penn State and William Goddard at
Caltech. This simulation method combines accuracy in
modeling reaction energies and barriers with the capability
to simulate systems large enough and over sufficiently
long time to capture the diffusive properties of molecules
necessary to adequately capture important chemical and
structural reorganization during SEI formation. The
ReaxFF is an empirical potential that is parameterized to
reproduce results from quantum chemistry calculations.
Results
Quantum chemistry studies of the influence of anion
(PF 6 - ) on the solvent oxidative stability and oxidative
decomposition pathways were performed in collaboration
with the Army Research Laboratory. Oxidative stability of
PC/PF 6 - complex was found to be reduced compared to the
intrinsic oxidative stability of PC solvent due to the
formation of HF upon oxidation of PC/PF 6
-
. Importantly,
the presence of the PF 6 - anion also altered the PC oxidative
decomposition pathways.
In collaboration with van Duin (Penn State), the
reactive molecular dynamics simulation methods utilizing
ReaxFF has been extended to accurately treat both
oxidation and reduction at electrode interfaces. This effort
required modification of the ReaxFF code to allow for
control of electrode charge. The new ReaxFF code (q-
ReaxFF) will handle inert (carbon, metal) electrodes and
active (graphite, LiNi 0.5 Mn 1.5 O 4 ) electrode materials.
For the purpose of parameterizing both reactive
(ReaxFF) and non-reactive (Lucretius) molecular
dynamics simulation models for metal and LiNi 0.5 Mn 1.5 O 4
electrodes we have carried out DFT studies of bulk crystal
and surfaces utilizing the Vienna ab-initio simulation
package (VASP). These calculations have allowed us to
determine interactions between metal substrates and
electrode materials and will allow us to parameterize
partial charges, polarizabilities, dispersion/repulsion
interactions and reactivity with LiNi 0.5 Mn 1.5 O 4 . We have
calculated basic properties (including energies, internal
stresses, and unit cell geometries) for components of the
LiNi 0.5 Mn 1.5 O 4 high voltage spinel as well as for the spinel
directly in order to aid parameterization of the forcefield
(ReaxFF) for reactive molecular dynamics simulations of
the high voltage spinel. We have also investigated β-MnO 2
(pyrolusite) under a number of compression conditions
ranging between 50 and 120 percent of the original
volume. For investigations of the LiNi 0.5 Mn 1.5 O 4 spinel, we
have begun by reproducing published results for the bulk
spinel, including the predicted average voltage of the cell
of approximately 4.7 V. The relaxed geometries and
energies which will serve, in conjunction with the MnO 2
data, for parameterization of ReaxFF for the spinel. We
have also utilized DFT to identify the lowest energy
surfaces of LiNi 0.5 Mn 1.5 O 4.
Our reactive MD simulations have demonstrated that
various alkyl carbonates are formed in the outer SEI layer
at the anode due to single electron reduction. To improve
our understanding of Li + transport through the
electrolyte/SEI interface and through the SEI itself
extensive simulations of model SEIs comprised of
dilithium alkylcarbonates (ethylene and butylene) have
been initiated using a new polarizable force field. The
force field has been re-parameterized against higher-level
quantum chemistry calculations and fit to accurately
reproduce conformational properties of alkylcarbonates,
electrostatic field around these molecules in vacuum, and
molecular dipole moments.
Conclusions and Future Directions
We hope to continue investigating low energy
surfaces of LiNi 0.5 Mn 1.5 O 4 by including oxygen in a nonstoichiometric
ratio as necessary to cap unterminated
transition metal bonds, or direct calculation of the reaction
of elemental oxygen with the ideal surfaces to determine
whether or not oxygen termination of the exposed surface
plays an important role. These studies will be extended to
include reactions with electrolyte components, including
candidates for high-voltage electrolytes and additives for
formation of stable cathode SEI layers. Oxidative
decomposition pathways for these materials will also be
determined. ReaxFF simulations of oxidation/reduction of
electrolytes as a function of electrode potential will also be
carried out, allowing us to see initial reactions and
ultimately formation of SEI layers.
FY 2011 Annual Progress Report 603 Energy Storage R&D