Development of High Energy Li-ion Capacitors

Project Objective: We will design and demonstrate a new class of electrochemical (EC) capacitors with the energy density being comparable to advanced rechargeable batteries. The charge storage and transferring mechanisms in new EC capacitors are different from conventional EC capacitors, but similar with Li-ion batteries, in which ions shuttle between two electrodes.

The biggest challenge facing EC capacitors is how to significantly increase the energy density of the cell. For a long period of time, a great amount of work has been done in understanding the relationship of the pore size to the ionic accessibility from the electrolyte and developing various pseudocapacitance materials in order to maximize the charge storage capability; however, there are limited studies on charge storage mechanisms such as the active role the electrolyte plays during the charge and discharge process: ion separation or ion shuttle. In double-layer capacitors (Fig. 1(a))[1] and conventional asymmetrical cells (Fig. 1(b)),[2] the ion concentration in the electrolyte increases and decreases during charge and discharge cycles, respectively. The energy density theory guide clearly shows that the energy densities for both double-layer capacitors and asymmetrical cells are mainly limited by how many ions are available in the electrolyte or the salt concentration in the electrolyte, because the minimum amount of required ions in the electrolyte equals the maximum charge capacity of the electrode in a capacitor. In contrast, for Li-ion batteries, the Li ions shuttle between two electrodes and the concentration keeps a constant value during charge and discharge cycles; therefore, a high energy density cell can be obtained.[3]

We have demonstrated lithium-ion capacitors using the Li intercalated carbon/stabilized Li metal powder (SLMP) as anode and activated carbon as cathode electrodes. An energy density above 100 Wh/kg was obtained based on the weight of electrode materials as shown in Fig. 2.


Fig. 1 Schematic potential changes of (a) symmetrical cell with two double-layer electrodes, (b) asymmetrical cell with double-layer and intercalation electrodes, and (c) proposed asymmetrical cell with double-layer and Li pre-inserted electrodes during the charge-discharge process.

Fig. 2 The voltage profiles of Li-ion capacitors during charge and discharge cycling at a constant density of 0.8 mA/cm2.

References

  1. J.P. Zheng, "The Limitation of Energy Density of Battery/Double-Layer Capacitor Asymmetrical Cells", J. Electrochem. Soc. 150, A484 (2003).
  2. J.P. Zheng, "Theoretical Limitation of Energy Density for Electrochemical Capacitors with Intercalation Electrodes", J. Electrochem. Soc. 152, A1864 (2005).
  3. J.P. Zheng, "High energy density electrochemical capacitors without consumption of electrolyte", J. Electrochem. Soc. 156, A500 (2009).

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