Lifetime and Cycle Charging

Life cycle profile of electrochemical capacitor operating within moderate parameters where time is 10 cycles. The horizontal line indicates acceptable loss in relative capacitance. 

Supercapacitor maintained at 2.7 V to evaluate aging affects at constant temperatures of 65 and 7(fC. Source Gualous, H. et at 2010. Microelectronics Reliability, 50,1783-1788. With permission.) 

The Coulombic efficiency of ESs, defined as the ratio of the discharge current to the charging current, can be expected to approach 100% even at high currents. In practical use, the charge and discharge current values are controlled to be the same, so the Columbic efficiency equals 100%. This efficiency applies to fhe currenf, and does not directly correspond to high energy efficiency. 

The lifetime of an EC will be shortened considerably when the cell connected to the positively marked terminal is connected to the negative terminal of the charging source. Reversing the voltage may not show any ill effects on performance, but the cell will show degraded lifetime and performance characteristics after it is reconnected correctly. The negative effects on lifetime and performance can be avoided if cells are properly connected based on their polarities. 

For asymmetric ESs, as discussed in Chapter 2, polarity is very important because the anode material is different from fhe cathode material. Particularly, if an electrochemically active material is used as the electrode material, the corresponding electrochemical reaction or process on that electrode may be different from the activities of the other electrode, resulting in different electrode potentials and different polarities. Therefore, the polarity must be marked on each electrode in an asymmetric ES to indicate how each material should perform within a specific operating potential. 

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