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)
inserted into and extracted from the cage structure of
silicon clathrates without substantial volume changes or
pulverization of the cage structure. Theoretical
computations of the total volume, occupiable volume,
and accessible volume within the Type I silicon
clathrate structures indicate that the empty spaces within
the cage structure are accessible to Li and amenable to
Li intercalation through electrochemical means, thus
making silicon clathrate a potential anode material for
Li-ion battery applications.
Approach
SwRI is working with LBNL to develop silicon
clathrate anodes for PHEV and EV applications. The
approach is to synthesize guest-free Type I silicon
clathrate (Si 46 ) using a number of high-temperature
processing methods, while concurrently exploring an
investigational route for direct synthesis of guest-free
clathrate and performing ab initio and classical
molecular dynamics (MD) computations to identify
lithiation pathways. Silicon clathrates will be utilized to
fabricate prototype anodes. Electrochemical
characterization will be performed to evaluate and
improve, if necessary, anode performance including
cyclic stability. The final year of the program will be
directed at the design, assembly, and characterization of
a complete (anode/cathode) small-scale, prototype
battery suitable for concept demonstration.
Results
Arc-Melt Synthesis. An arc-melting technique
was utilized to synthesize metal-substituted clathrate
(Type I) structures of the form Ba 8 M 8 Si 38 (M = Al, Cu),
starting from pellets of pure Si, Al or Cu, and Ba and
melting the admixture of pellets under an argon
atmosphere. The product – either containing Al or Cu –
was brittle and grey-silver in color. After forming
powders of each product, powder X-ray diffractometry
(PXRD) confirmed that Al- or Cu-substituted silicon
clathrate (Type I) was indeed formed. The formation of
the clathrate I structure requires the addition of Al or Cu
in the starting admixture; namely, Ba 8 Si 46 is not formed
without metal addition. Twelve grams of Ba 8 M 8 Si 38
Type I clathrates were produced. Figure V - 117 shows the
PXRD pattern for Ba 8 Al 8 Si 38 produced by the arcmelting
technique. These materials were utilized to
fabricate half-cells for characterization of
electrochemical performance.
Counts
300
250
200
150
100
50
Sample B1
Measured Spectrum
Theoretical Ba 8 Si 46
= 1.5418
0
0 10 20 30 40 50 60 70
2
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Normalized Counts (Theory)
Figure V - 117: PXRD pattern of Ba8Al8Si36.
Multi-Anvil Synthesis. High-pressure, hightemperature
Walker-type multi-anvil techniques were
used to synthesize small quantities (~100 mg) of
barium-intercalated silicon clathrate (Type I, Ba 8 Si 46 ) by
structurally converting barium silicide (BaSi 2 ) to the
clathrate I structure at 800°C with 3, 4, and 5 GPa of
mechanical pressure. An image of the products is shown
in Figure V - 118 for each of the three pressure points.
Initial PXRD results (Figure V - 119) suggested that
conversion of BaSi 2 to the clathrate (Type I) structure
(Ba 8 Si 46 ) was incomplete. However, upon close
inspection, it was subsequently discovered that the
spectrum was partially obscured by the presence of h-
BN, indicating that the technique’s conversion yield
was, in fact, very good.
Figure V - 118: Image of barium-intercalated silicon clathrate
(Type I, Ba8Si46) pellets formed from the high-pressure, high-temperature
multi-anvil structural conversion of barium silicide (BaSi2) at three
different pressure regimes.
FY 2011 Annual Progress Report 581 Energy Storage R&D