Design and Optimization of Lithium Titanate Spinel Paired with

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					 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
                                                                                                         30 s

                                                                                                                                  We would like to thank X. Song and V. Battaglia
                                                              Power Assist
                                                                                                         10 s
                                                                                                                        for their help in obtaining SEM and TEM images, and G.
                                           EDLC                  HEV
                                                                                                                        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
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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
the electrolyte.
                                                                                                                        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