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

V.B.1 First Principles Calculations and NMR Spectroscopy (MIT, Stony Brook U.) Ceder – MIT, Grey – Stony Brook U.
These experiments, along with NMR experiments designed
to measure Li-Si connectivities are being employed to
investigate structure in cycled 29 Si-enriched battery
materials. In parallel to this method development, we
have now (i) established a method to perform in situ NMR
experiments of carboxymethyl-cellulose coated Si
nanoparticles, so that we can now study this chemistry in
situ and (ii) performed detailed ex situ NMR studies of Si
nanoparticles.
Figure V - 4: (top) Energy barriers for Li migration in LiNi0.5Mn1.5O4.
(bottom): Capacity at high rates for cathodes made with micron sized
LiNi0.5Mn1.5O4
Structure and Function of High Capacity Anodes:
Silicon has attracted considerable interest as an anode
material for high-energy lithium ion batteries due to its
very high specific capacity (3579 mAh/g). Efforts in this
reporting period have focused on developing methods to
establish how Si’s operating behavior is affected by the
size and morphology of the Si particles, making use of new
experiments involving 29 Si NMR spectroscopy.
Unfortunately, analysis of the silicon substructure by the
direct probe 29 Si, has been hampered by the low signalnoise
ratio and the difficulty in interpreting the usually
very broad and poorly resolved spectra of lithiated Si
particles. Thus, we have developed a number of suitable
NMR techniques for the determination of both Si-Si and
Li-Si connectivities, which make use of covalent
interactions between Si atoms (J-coupling), and through
space (dipolar) interactions between nuclei. The approach
is demonstrated with the crystalline lithium silicide Li 12 Si 7 .
Its structure includes 5-membered Si rings which are
stacked in one-dimensional chains, as well as 4-membered
Si “Y” stars located in the planes between the Si 5 rings, as
shown in Figure V - 5.
Figure V - 6 shows dimensional (2D) NMR experiment
that investigates covalent interactions (bonds) between
atoms ( 29 Si INADEQUATE NMR). In this spectrum, two
bonded atoms giving rise to resonances with frequencies ν 1
and ν 2 result in a correlation at ν 1 +ν 2 in the indirect
dimension of the 2D spectrum. No correlations are seen
for non-bonded atoms. Consistent with this, separate sets
of connectivities are seen between the Si atoms in the rings
(resonances b, c, d, e, and g) and the stars (a, f, h and i).
Figure V - 5: Fragment of the crystal structure of Li12Si7 showing the Si4
stars and the Si5 rings (black) and Li atoms in light/dark blue.
Figure V - 6: Experimental one-pulse spectrum (top) and 29 Si 2D
INADEQUATE NMR spectrum of Li12Si7 at 233 K.
The dashed line represents the 2- line. Solid lines
are added so as to follow the Si-Si connectivities.
Materials discovery: Using ab initio highthroughput
computations, we performed a search for new
Li-ion cathode materials with high specific energy,
electrochemical activity in a voltage range compatible with
current commercial electrolytes (< 4.5V) and good
Energy Storage R &D 472 FY 2011 Annual Progress Report