Overpotential

When a battery is fully at rest with no current flowing, the voltage measured is very close to its equilibrium voltage (also called the thermodynamic voltage). This is the theoretically expected voltage based on the chemistry of the system. 

But once a load or charger is connected, things change. The battery is now required to either supply or accept current, which means the redox reactions at the electrode surfaces have to proceed at a net rate in one direction rather than remaining at equilibrium.

These reactions, along with ion and electron transport inside the battery, have limitations and are not perfectly efficient. As such, the battery needs additional driving force to overcome its internal limitations and sustain the flow of current. This causes the measured battery voltage to shift away from the equilibrium value. The difference between these two voltages is the overpotential. 

In concise terms: Overpotential is the additional voltage required to push a battery away from its equilibrium so it can supply or accept current. 

The equations for the measured voltage of a battery are:

Where: V_actual = measured voltage, E_eq = equilibrium voltage, η_total = sum of overpotential contributors 

During discharge, overpotential lowers the measured voltage because voltage is lost overcoming the internal limitations to generate current. During charge, overpotential increases the measured voltage because additional driving force is required to push current into the battery against those same internal limitations. 

Sources of Overpotential

There are three main sources of overpotential in batteries:

1. Activation (kinetic) overpotential

2. Ohmic overpotential

3. Concentration (mass transport) overpotential

All three generally become more significant as current increases, and most of the energy associated with overpotential is ultimately released as heat. 

1. Activation Overpotential

   There is a kinetic barrier associated with the rate of charge transfer reactions at the electrode surface. Activation overpotential is the additional voltage needed to make these reactions proceed faster than they naturally would at equilibrium. 

   
   

   At higher currents, faster kinetics are required, which increases the activation overpotential. The relationship between current and activation overpotential is mathematically described by the Butler–Volmer equation.

   
   

   Typical causes of high activation overpotential include: 

   • Slow charge transfer rate

   • Poor catalytic activity

   • Low exchange current density

   • Few reaction sites

   • Low temperature

     
     

   These factors slow down the rate of redox reactions, meaning the battery must deviate even further from equilibrium to sustain the desired current, resulting in higher activation overpotential.

2. Ohmic Overpotential

   For a battery to operate, both ions and electrons must move. Ions move through the electrolyte and separator, and electrons move through the electrodes and external circuit.

   
   

   None of these pathways are perfectly conductive so resist current flow to some extent, resulting in voltage loss. This voltage loss has a linear relationship with current in accordance with Ohm's law (V = IR) . At higher currents, more voltage is required to push current through these resistive pathways. The additional voltage needed is the ohmic overpotential.

   
   

   Some factors that increase ohmic overpotential are: 

   Ionic resistance: Conditions that make ion travel through the electrolyte and separator more difficult increase ionic resistance. Some of these are:

   • Low electrolyte conductivity 

   • Thick separators

   • Low electrode porosity 

   • Low temperature 

   
   

   Electronic resistance: Electronic resistance increases when electron movement through the electrode and external circuit is impeded. Some factors that increase this are:

   • Low electrode electronic conductivity

   • Poor conductive networks

   • Thin current collectors

   • Weak tab welds or electrical connections

   
   

   Interfacial resistance: Resistance can also occur at the electrode-electrolyte interface. For instance, if the SEI  (solid electrolyte interphase) gets too thick or is poorly formed, it impedes ion movement and contributes to ohmic overpotential.

3. Concentration Overpotential

   As a battery operates, redox reactions continuously consume and generate ions near the electrode surfaces. Ideally, ions from the bulk electrolyte should replenish those used at the electrode surfaces quickly enough to sustain the reactions. However, at higher currents, the reactions consume ions faster than they can be replenished, creating concentration gradients between the electrode-electrolyte interface and the bulk electrolyte. This situation is referred to as a mass transport limitation.

   
   

   As the concentration gradients grow, each electrode's potential moves further away from its equilibrium value, and additional voltage is required to sustain current. The higher the current, the larger the concentration gradients, and the greater the transport overpotential. 

   
   

   The factors that worsen concentration overpotential are very similar to those that increase ohmic overpotential through ionic resistance because factors that increase the resistance of a path also contribute to slowing down mass transport.

   
   

   The core difference is in the effect on the battery. Ohmic overpotential appears almost immediately because electrical resistance exists as soon as current begins to flow. Concentration overpotential, on the other hand, takes time to develop. Concentration gradients don’t form from the onset, but as the battery continues operating, ions gradually become depleted in some regions and accumulate in others.  

In Conclusion

When a battery is operating, all three overpotentials occur simultaneously. Activation overpotential is related to reaction kinetics, ohmic overpotential is related to electrical resistance, and concentration overpotential is related to mass transport limitations.

Overpotential has major effects on battery performance, efficiency, and safety. High overpotential can reduce energy efficiency, lower power capability, and cause faster degradation. 

As such, it is important to understand which type of overpotential dominates in a battery in order to effectively engineer targeted solutions and improvements. For instance, improving catalyst activity can reduce activation overpotential but will not be a fix for high ohmic overpotential, and so on.