In the context of batteries, impedance is an important property that affects both the performance and efficiency of the system. The lower the impedance, the more efficiently the battery can deliver its stored energy. An increase in impedance, on the other hand, can lead to a decrease in overall performance, including decreased power output, decreased energy efficiency, and accelerated aging.
At its core, impedance (Z) is a fundamental concept in electrical engineering that refers to the measure of opposition a circuit presents to a current when a voltage is applied. It is a complex quantity that combines the effects of resistance (R), inductance (L), and capacitance (C) in a single measurement.
✔️The impedance in lithium-ion batteries can be broadly divided into two categories:
1) Ohmic Impedance:
The Ohmic or DC impedance is the initial voltage drop observed when a load is suddenly applied to the battery. It's primarily caused by the resistance of the electrolyte, the separator, and the battery contacts. This kind of impedance does not significantly change with the state of charge (SOC) or aging but can be affected by temperature.
2) Non-ohmic impedance (Charge Transfer, diffusion, double-layer charging):
Non-ohmic impedance, on the other hand, is more complex. It relates to the electrochemical reactions that occur at the electrode/electrolyte interfaces. This impedance depends on the nature of the active material, the electrode microstructure, and the electrolyte properties, and it significantly changes with the state of charge, temperature, and aging.
As a battery charges and discharges, lithium ions move back and forth between the anode and the cathode. This intercalation process is not entirely efficient, leading to a loss of lithium ions and the creation of a solid electrolyte interface (SEI) layer. The formation of this SEI layer contributes to the charge transfer impedance and is a leading cause of battery aging.
✔️Mitigating the Impact of Impedance:
Since impedance directly impacts a battery's performance, researchers and engineers are continuously seeking ways to mitigate its effects. Techniques for minimizing impedance include improving the electrode materials and design, optimizing the electrolyte composition, and implementing intelligent battery management systems.
It's worth noting that while reducing impedance is generally desirable for improving battery performance, very low impedance can also lead to issues, such as short-circuiting or thermal runaway. Therefore, there is a delicate balance that must be struck to ensure optimal battery operation.