Electrical Breakdown - Battery Design

Electrical breakdown is a transient physical process in which an insulating medium, such as a gas or a solid dielectric, loses its insulating properties and rapidly transitions into an electrical conductor. This phenomenon occurs when an applied voltage or electric field across a gap becomes sufficiently intense to cause a massive multiplication of charge carriers.

Breakdown in a Gas

In a gaseous medium, this process is primarily driven by the Townsend discharge mechanism. The breakdown sequence generally unfolds in the following stages:

Initiation: Free electrons—which may initially originate from natural radioactivity, cosmic rays, or field emission from an electrode—are driven and accelerated by the applied electric field across the gap.
Impact Ionization (Electron Avalanche): As these electrons accelerate, they gain substantial kinetic energy. If their energy exceeds the ionization threshold of the surrounding neutral gas molecules, they collide with these molecules and dislodge additional electrons. Each collision produces a positive ion and a new free electron. These new electrons are also accelerated, causing further collisions in a chain reaction that results in an exponential multiplication of charge carriers known as an electron avalanche.
Self-Sustaining Discharge: The newly formed positive ions are relatively slow-moving and drift back toward the cathode. When they bombard the cathode surface, they release “secondary electrons”. When the rate of secondary electron emission perfectly balances and supports the avalanche process, the discharge becomes self-sustaining. This results in plasma ignition, forming a highly conductive electric arc or spark.

Externally, electrical breakdown in a gas is characterized by a sudden, precipitous drop in the gap voltage and an abrupt, massive surge in electrical current flowing through the newly formed plasma channel. The specific voltage required to trigger this event in a uniform electric field is governed by Paschen’s Law, which determines the breakdown threshold based on the product of the gas pressure and the distance between the electrodes.

schematic of cells venting and then going into thermal runaway

Thermal Runaway in High-Voltage Systems

When a cell goes into thermal runaway, it acts like a pressure vessel blowing its top. Data highlights the staggering volume of material released during these events.

Solid and Surface Breakdown

While gaseous breakdown occurs through air or a gas void, electrical breakdown also critically manifests on or within solid insulating materials (such as those on a printed circuit board):

Dielectric Breakdown (Clearance Failure): When continuous high-voltage stress is too great for the straight-line air gap (clearance) or solid insulation, the electric field surpasses the material’s insulating capacity, ionizing the medium and forcing a permanent conductive path.
Surface Tracking (Creepage Failure): This form of breakdown occurs when a large voltage difference gradually creates a conductive leakage path along the surface of an insulating material. Contaminants like moisture, salts, or dust lower the surface’s insulation resistance and promote partial discharges. The heat from these discharges carbonizes the insulating surface, creating a permanent, highly conductive track that leads to a short circuit.