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

Kumta – U. Pitts
V.C.1 Nanoscale Composite Hetero-structures: Novel High Capacity Reversible Anodes (U. Pitts)
layers have been coated on the Si/C composite anode
derived from HEMM using electro less coating processes.
Two new types of polymers of different molecular weight
have been developed as a suitable binder for Si/C
composite anode. Attempts were also made to synthesize
amorphous Si films directly on copper foil by
electrochemical reduction of silicon salt based electrolyte.
Typical electrolyte comprises 0.5 M the Si-salt dissolved
in propylene carbonate (PC) and tetrabutylammonium
chloride (TBACL) used as supporting electrolyte to
improve the ionic conductivity. A three electrode set up
was used utilizing Cu foil of 11mm diameter as the
working electrode. A Pt foil and wire served as counter
and reference electrodes, respectively.
These promising systems were tested in half cells
using metallic lithium as both counter and reference
electrodes. Rate capability, long term cyclability, including
origin and state of the SEI layers were investigated.
Results
Binder free Si/CNT hybrid nanostructures
synthesized by CVD techniques on INCONEL 600. The
long term cycling data (Figure V - 74) of binder free nc-
Si/VACNT electrodes, comprising 54wt. %
nanocrystalline Si deposited on VACNTs grown in
INCONEL, shows a 1 st discharge capacity of ~1870
mAh/g with a low irreversible loss (~16%). The rate
capability study of nc-Si/VACNT performed at 100 mA/g
(C/15), 200 mA/g (C/7) and 400 mA/g (C/3.5) shows a
capacity retention of ~1350 mAh/g after 30 C/15 cycles
~1000 mAh/g after 60 C/7 cycles and ~700 mAh/g after 90
C/3.5 cycles. The capacity fade is a reflection of poor
interface between Si and CNT which can be improved by
engineering the interface. These studies are currently
ongoing in order to improve the stability of the electrode.
Synthesis of Si/C/CA nanocomposites by HEMM.
The cycling response of the C/Si/CA nano-composite,
obtained by HEMM followed by thermal treatment at
773K, cycled for 30 cycles at ~C/5 rate, displayed in Figure
V - 75, shows a 1 st cycle discharge capacity of ~1020 mAh/g
and a 1 st cycle charge capacity of ~800 mAh/g with an
irreversible loss ~20%. The composite shows excellent
capacity retention with a 0.1% loss per cycle up to 30
cycles and an coulombic efficiency of ~99.84%. These
results indicate the beneficial influence of the conducting
additive (CA) to reduce the irreversible loss and improve
the coulombic efficiency in contrast to microcrystalline
Si/C which display higher ICL ( 30%) and lower
coulombic efficiency (~99.4-99.5%). In order to
understand the effect of the additive, preliminary
electrochemical impedance spectroscopy (EIS)
measurements have been conducted. It has been identified
that the charge impedance decreases with addition of the
additive which indicates a reduction in the charge transfer
resistance of the composite electrode and a relatively
stable SEI. The results will be fitted to an equivalent
circuit model in the near future to understand the variation
of SEI film resistance, charge transfer resistance and
interphase electronic contact resistance with voltage and
cycle number. The effect of additives to lower the
irreversible loss and improve the coulombic efficiency will
be studied in detail and reported in the near future.
Figure V - 75: Variation of specific capacity vs. cycle number of C/Si/CA
composite cycled at C/5 rate.
Novel Thermoplastic binders. New polymeric high
strength binders denoted as Binder-1 and Binder-2 were
developed which exhibit better electrode stability (Figure V -
76) than PVDF for the Si/C system. In the case of polymer
1, a citric acid and KOH based buffer (pH=3) was used as
the solvent to enhance coupling of the hydroxyl ions
present on Si to the polymer.
Figure V - 74: Variation of specific capacity vs. cycle numbers of nc-Si/CNT
on INCONEL 600 cycled at a current rates of 100 mA/g, 200 mA/g and 400
mA/g.
FY 2011 Annual Progress Report 537 Energy Storage R&D