Design and Optimization of Lithium Titanate Spinel has shown that the technology delivers 10 to 15 Wh/kg at
Paired with Activated Carbon and Comparison with a 1000 to 2000 W/kg for over one million full discharge
Lithium Iron Phosphate Battery cycles.2 The asymmetric-hybrid supercapacitor consists
of an activated-carbon positive electrode and a lithium
Sarah Stewart,*,** Paul Albertus,* Venkat Srinivasan,** titanate spinel negative electrode. Activated carbon is a
Irene Plitz,*** Nathalie Pereira,*** Glenn Amatucci,*** and capacitive electrode that adsorbs and desorbs anions in the
John Newman*,** double layer during cycling, similar to an electrochemical
double layer capacitor (EDLC), while lithium titanate
Department of Chemical Engineering spinel reversibly intercalates lithium.
University of California, Berkeley, CA 94720-1462
Here we optimize the design of an asymmetric hybrid
Environmental Energy Technologies Division supercapacitor and compare its performance to a typical
Lawrence Berkeley National Laboratory 2.7 V EDLC available today as well as an optimized
lithium titanate spinel/lithium iron phosphate battery. We
Energy Storage Research Group chose this battery chemistry for comparison because of its
Rutgers, The State University of New Jersey high power capability and stability.
North Brunswick, NJ 08902
The lithium titanate spinel electrode of the asymmetric
hybrid supercapacitor was characterized in a coin cell
Here we describe a model for lithium titanate spinel with a lithium anode. Lithium titanate spinel and
paired with an activated carbon electrode, an asymmetric activated carbon models were then compared to constant-
hybrid supercapacitor. The model is compared with current experiments in a Li4Ti5O12/activated carbon cell.
experimental results. The performance of this system is
compared with a lithium titanate spinel/lithium iron Figure 1 shows that, for discharge times less than 3
phosphate battery. The model is used to study the minutes, the capacitive electrode outperforms the
performance of these chemistries and to assist in cell intercalative iron phosphate electrode. The top solid line
optimization. A Ragone plot is generated for various cell indicates the improved performance from optimizing the
designs in order to assess the ability of the chemistries to capacity of the lithium iron phosphate and lithium titanate
achieve the U.S. Department of Energy goals for hybrid- spinel electrodes separately rather than using a fixed
electric vehicles. The specific energy of a cell is capacity ratio for the two electrodes (a 1:1 ratio is plotted
maximized by optimizing the design for a fixed time of with the top dashed line).
discharge. The thickness and porosity of both electrodes
are varied, while holding constant the capacity ratio for The design of the asymmetric hybrid supercapacitor as
the two electrodes, as well as the properties of the well as the design of a lithium titanate spinel/lithium iron
separator. The capacity ratio can also be optimized for phosphate battery was optimized by varying electrode
each time of discharge. A 41% increase in specific power thickness and porosity for various discharge times of
is seen when one optimizes the capacity ratio of a lithium interest. From these results, recommendations are made
titanate spinel/iron phosphate battery (top solid line in on how to improve further the performance of the hybrid
Figure 1). The optimized designs derived here can be supercapacitor. For the experimentalist to optimize a
used as a starting point for battery manufacturers and to system empirically is very challenging when faced with
help decrease the time to commercialization. numerous variables such as electrode kinetics, transport
limitations in the solid and liquid phase, and the effects of
3h 1h 30 min 15 min 5 min 2 min a changing electrolytic concentration. A model assists in
identifying limiting cell properties. Once identified, the
Specific energy (Wh/kg)
Li4+xTi5O12/LiyFePO4 1 min experimentalist can then design around these limitations.
3 Hybrid Supercap Acknowledgements
We would like to thank X. Song and V. Battaglia
for their help in obtaining SEM and TEM images, and G.
Chen for her assistance in obtaining XRD results.
6 This work was supported by the Assistant
2 3 4 5 6 78
2 3 4 5 6 78
2 3 4 5 Secretary for Energy Efficiency and Renewable Energy,
Specific power (W/kg)
Office of Vehicle Technologies of the U.S. Department of
Energy under Contract No. DE-AC02-05CH11231.
GGA would like to thank the Office of Naval
Figure 1. Ragone plot comparing the performance of a Research for support (under contract #N00014-05-1-
lithium iron phosphate cathode and an activated 0503)
carbon cathode when paired with lithium titanate
spinel. Values are plotted without correction for References
packaging weight (correcting for this would decrease
energy and power densities by about a factor of 2).
1. G. G. Amatucci, F. Badway, A. Du Pasquier, and T.
The plotted goals for a power-assist HEV include
Zheng, J. Electrochem. Soc., 2001. 148(8): A930-
packaging weight. LiPF6 in acetonitrile was used as
2. I. Plitz, A. DuPasquier, F. Badway, J. Gural, N.
The asymmetric-hybrid supercapacitor was developed in Pereira, A. Gmitter, and G. G. Amatucci, Appl. Phys.
order to increase the specific energy of the supercapacitor A-Mater. Sci. Process., 2006. 82(4): 615-626.
while maintaining power and robustness.1 Previous work