Intercalation Potential Shifts in Large Format Lithium Cells
Local intercalation overpotentials in large format cells force anode potentials below 0V vs Li/Li+, causing lithium plating long before terminal voltage limits.
Tab Overpotential
Meaning
Voltage drop measurement quantifies the loss of electrical potential that occurs across the physical connection between the internal electrode and the external terminal. This tab overpotential is caused by the electrical resistance of the tab material and the contact resistance of the welds. In high power applications, this loss can be significant, leading to a reduction in the energy delivered to the load and the generation of excess heat.
It is a critical design factor for cells used in electric vehicles and power tools, where high currents are common. Reducing this value is a major goal for engineers looking to improve battery efficiency and thermal management.
Heat Generation
Joule heating occurs at the tab connection whenever current flows through the battery. Because the tab is a relatively small and resistive component, the local temperature can rise much faster than the rest of the cell. This tab overpotential results in a localized hot spot that can damage the nearby separator or the cell casing.
If the heat is not properly dissipated, it can lead to thermal runaway or the premature failure of the terminal connection. Designers use thicker tabs or multiple connection points to spread the current and reduce the heat generated by this resistance.
Welding Quality
Contact resistance at the junction of the foil and the tab is a major contributor to the total voltage loss. If the ultrasonic or laser welds are not perfect, the tab overpotential will increase, leading to poor performance and potential safety issues. Manufacturers use high resolution inspection tools to ensure that every weld meets strict quality standards.
A weak weld can also be a source of mechanical failure, as the vibrations in an electric vehicle can cause the connection to break over time. Improving the weld consistency is essential for high volume battery production.
Design Optimization
Geometric configuration of the tabs plays a major role in determining the total electrical resistance of the cell. Engineers use simulations to find the best placement and shape for the terminals to minimize the tab overpotential. This often involves moving the tabs to the center of the electrode or using a tabless design where the entire edge of the foil acts as the current collector.
These advanced designs are becoming more popular in the latest generation of large format cells, such as the 4680 cylindrical format. By reducing the distance that electrons must travel, these innovations significantly lower the internal resistance and improve the fast charging capability.