When we look at the triggers of thermal runaway we have the nail test, overcharge and external heater. All of these are reasonably easy to define and apply. However, they don’t truly replicate the failure that they are meant to and that is a rogue metallic particle inside the cell.
Cell Internal Short Circuits
Although very rare, cell internal short circuits are a leading cause of battery thermal runaway. They are a major safety issue for any application of a battery pack. Hence there is a requirement to prevent them and to detect them.
Steve Grodt’s white paper from Chroma Systems Solutions shows that the temperature versus time graph is very dependent on the type of short-circuit within the cell.
The worst case is shown to be for the aluminium current collector to the graphite anode. This could be caused by a foreign particle in the cathode layer or by a burr on the edge of the aluminium current collector.
Hence we wanted to look at the NREL/NASA Cell Internal Short Circuit Device invented in 2010 by: Matthew Keyser, Dirk Long, Ahmed Pesaran and Eric Darcy.
In simple terms this is a metallic particle embedded in wax and inserted into the cell during the manufacturing stage:
The device or switch is used in a test method to simulate latent flaws for triggering internal short circuit in energy storage cells. In this test method, the device is implanted in a cell in a specific and user-defined location between specific and user-defined internal layers of the cell. The cell is formed, or charged and discharged, or aged according to user-defined specifications. Then the device is activated by the appropriate stimulus such as heating, sonication, radiation, etc. in a test facility. After activation, the device creates a conductive path between the positive and negative layers of the cell. The response of the cell will be measured as the results of this internal short circuit.
US9142829B2, United States
When you look at the device in detail you see it has four layers and five parts:
- Copper Pad
- Battery Separator with Copper Puck
- Wax – this is a phase change material
- Aluminium Pad
Looking at this in cross-section in context of the cell active layers shows how it sits and how when triggered it will form an electrical short.
Finally we see the device inside an actually cell.
The trigger method has been used in a number of studies:
- Type of Separators
- Non-flammable electrolytes
- Electrolyte AdditivesFusible Tabs
- Propagation Studies
- Top and bottom vents
- Gas generation within a cell
This trigger method is interesting as it so closely replicates the failure mode that is of concern when we nail test a cell or pack.
KULR has exclusively licensed the internal short circuit (ISC) technology, which was co-developed between the National Renewable Energy Laboratory (NREL) and NASA.
This post has been built based on the support and sponsorship from: Thermo Fisher Scientific, Eatron Technologies, About:Energy, Quarto Technical Services and Alvatek.
References:
- Eric Darcy, Safe, High Power / Voltage Battery Design Challenges, NASA
- Passive safety device and internal short tested method for energy storage cells and systems, US9142829B2, United States
- Matt Keyser, Eric Darcy, NREL/NASA Internal Short-Circuit Instigator in Lithium Ion Cells, JRC Lithium Ion Safety Workshop Petten, Netherlands, March 2018
- Short-Circuiting on Purpose: How NREL-Licensed Tech Continues To Make a Difference in Space, NREL