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

Chan, Miller – SwRI
V.C.12 Silicon Clathrates for Anode Applications in Lithium-Ion Batteries (SwRI)
Figure V - 122: Raman spectropic analysis of the Si46 produced by PEMS deposition into an ionic liquid.
Batch Synthesis via Soft Oxidation of BaSi 2. A 2
synthetic pathway was designed and the necessary
hardware (manifolds and glass reactor) was set up to 1
conduct direct, batch synthesis of guest-free Type I
silicon clathrates at elevated temperature. First, the
0
thermochemical stability of candidate ionic liquid (IL)
solvents needed to carry out the synthesis at
temperatures at or slightly above 300C for periods in
excess of 48 hours was evaluated. Differential scanning
-1
-2
calorimetry coupled with thermal gravimetric analysis -3
(DSC-TGA) showed that the PF - 6 salt of 1-butyl-3­
methylimidazolium was not sufficiently stable for the
reaction due to its degradation onset near 300C.
Subsequently, an alternate, proprietary IL, consisting of
the bis(trifluoromethylsulfonyl)imide anion was
discovered to show excellent thermochemical stability
up to 400C over 48 hours with negligible mass loss.
This IL was incorporated into the reactor setup and
guest-free silicon clathrates were synthesized via soft
oxidation of BaSi 2 . Several reaction schemes are
currently being attempted.
Molecular Modeling of Silicon Clathrates. The
possible routes for extracting Ba atoms from Ba 8 Si 46 and
Ba 8 Al 6 Si 40 were investigated by performing firstprinciple
energetic computations using the Car-
Parrinello molecular dynamics code. The energy change
due to the presence of a guest Ba atom residing in a Si 46
cage or an alloyed Al 6 Si 40 cage was computed as a
function of the number of Ba atoms. The results, shown
in Figure V - 123, indicate that the energy required to
extract a Ba atom from the Si 46 or Al 6 Si 40 cage ranges
from 2.43 eV to 3.66 eV at 0K. The energy change was
also computed as a function of temperature and Li
content to identify potential pathways and processing
windows for extracting guest Ba atoms from the
clathrates.
E/Ba atom, eV
Si 46
Ba 2
Si 46
Ba 6
Si 46
Ba 8
Si 46
Ba 8
Al 6
Si 40
-4
9 10 11 12 13
Lattice Constant, A
Lattice Constant,
Figure V - 123: Energy change of Si46 due to Ba guest atoms or Al
substitution of the Si framework.
Half-Cell Electrochemical Characterization. A
method was devised to fabricate anodes consisting of
previously synthesized Ba 8 M 8 Si 38 conformed to ~1 cm
diameter thin disks. The anode material was pulverized
to a very small particle size using a simple mini-ball and
capsule anvil system. The fine powders were then
pressed into a thin disk in a mini-press apparatus.
Mechanically stable anode-disks were successfully
prepared without the use of any binder material. A
method was also devised to bond the pressed disk to a
current collector (metal foil) for use in an
electrochemical half-cell apparatus. A three-electrode
bulk-electrolysis cell was first used to deintercalate Ba 2+
from the electrode material. Subsequently, a threeelectrode
split-cell was used to intercalate Li + into the
clathrate anode and measure the capacity. These tests
are still on-going. Preliminary results, shown in Figure V -
124, indicated that Ba 2+ ions were extracted during
oxidation and Li + ions were inserted into the anode
during reduction.
FY 2011 Annual Progress Report 583 Energy Storage R&D