Designing High Energy Density Batteries for Abuse Tolerance
Authors: Armstrong Michael 
Published: November 2022

Every month or so there is a news article announcing a recall of a product due to overheating and thermal runaway of the battery pack. This has occurred more frequently as the energy density of battery cells has increased. Here we present a method to greatly improve the tolerance of a cell to internal shorting and nail penetration that is uniquely enabled by Enovix LiB cells.

A higher energy density results in less mass to absorb the energy released and less surface area to dissipate the heat. Frequently the failure has been identified as being initiated by an internal short between anode and cathode current collectors within a cell. These internal shorts have been associated with mechanical damage to the cell container, anode-separator-cathode alignment, and lithium dendrite breaches of the separator.

External short circuits are most likely to be controlled by the battery management system before damage to the cell. In the event of an external short circuit that bypasses the BMS the power released uniformly heats the entire cell. A safe landing of the cell is reliant on the separator or electrolyte increasing resistance to the point that heat transfer from the surface of the cell can maintain the temperature below thermal run away.

Shorts created by an internal short or mechanical penetration of a cell are more dangerous than external shorts. The short typically occurs at a point such that power is dissipated non-uniformly in a very small volume of material. This volume rapidly heats beyond thermal run away before the remaining cell increases significantly in temperature such that separator shutdown or electrolyte shutdown can not be relied upon to prevent thermal runaway. A thermal runaway event due to an internal short can be prevented by reducing the energy released at the point of the short.

Internal shorts are difficult to create purposefully without modifying the cell. Frequently an internal short is simulated by mechanically penetrating the cell with a nail. The nail breaches the cell container and penetrates the current collector(s) and separator. Penetration of the cell is typically done perpendicular to the plane of the current collector which produces a parallel connection between all layers of anode and cathode current collectors with out much mixing of anode and cathode active materials. Penetration parallel to the current collectors tends to crush the separator and current collectors, mixes a larger volume of active materials, and connects few layers of the current collectors. The larger volume of active material mixing and fewer parallel short locations results in a more challenging test of the cell.

Enovix battery architecture is designed for those systems that require a significant pressure between anode and cathode layers. A feature of this design is the segmentation of the cell into small area sub cells that are connected to a bus. Each sub cell bus connection can be designed to limit the current from the remainder of the cell in case of a short in the sub cell. We present abuse testing results of 3.4 AH cells with sub-cell protection that enable survival of nail penetration testing even when penetrated parallel to the current collector surface.