66 battery cell and the water-glycol coolant, another expert says. One approach to increasing the amount of surface area is a ‘3D’ cold plate or multiple cold plates. “Rather than cool the batteries just on one side, there could be cold plates on the top and bottom of the cells, or cooling ribbons and cold plates could be used in conjunction. The drawback to this is the complexity, weight and cost of implementation. Adding a second cold plate will double the weight and cost of the thermal management system, as well as increase the manufacturing complexity,” he says. Thermal insulation Radical use of thermal insulation is another mitigation approach, the expert adds, elaborating that cells could be thermally isolated from one another by using insulating foams, sheets or resins. “If the cells are thermally isolated from one another, the risk of propagation disappears. The consequence, of course, is that a thermally isolated cell is equally difficult to cool since it is isolated from the coolant system as well. This will minimise the amount of heat that can be removed from the cells, which will ultimately limit the rate at which the cells can be charged. “This type of system will certainly not be implemented in any sort of fast charge or discharge system.” Mitigation options While it might not be possible, for the moment, to stop a thermal runaway in a cell, it is practical to isolate the problem to the cell in which it started, or at least to a small group of cells immediately around it. In the earliest stage of a developing thermal runaway, the electrical aspects of the cell are most important, another expert says. An immediate stop to charging or discharging can reduce the risk of an uncontrolled release of heat inside the cell. Commonly used technologies include thermal fuses, positive temperature coefficient (PTC) current-limiting devices and fail-safe separator membranes. A thermal fuse, as the name suggests, is a fuse that melts and stops the flow of current to and from the cell permanently when the temperature reaches a set limit. For redundancy, these can be used in conjunction with metallised film current collectors, which essentially work in the same way. PTC devices are made from materials that exhibit a sharp increase in resistance when the temperature rises above an engineered-in threshold, limiting the current flow and helping to prevent a further rise in temperature. In a cylindrical cell, such as an 18650, the PTC device is built into the top cap and positioned in the cell’s main current path. It consists of a non-conducting, crystalline polymer material with a granular, conductive material such as carbon black embedded in it. Current passes through the device in normal cell operation, but if it gets too high the polymer expands in response to ohmic heating, progressively moving the conductive particles apart, which increases resistance and eventually disconnects the conductive path. Unlike a thermal fuse, the action of a PTC device reverses as the material cools down. Typically made from polymers such as polyethylene or polypropylene from composites that combine polymers with ceramic and other additives, and even from structured nanomaterials, the separator membrane prevents electrons from flowing directly between the anode and the cathode, thereby forcing the current to pass through the external electric circuit and the load, while allowing the ions to pass through. Increasingly, separators are designed with in-built thermal, mechanical or chemical shutdown mechanisms for safety. For example, some separator membranes are designed to undergo a phase change at high temperatures, physically blocking the flow of ions through the cell, and consequently stopping the flow of current May/June 2024 | E-Mobility Engineering Focus | Thermal runaway prevention Metallised polymer films can be used to make current collectors that open the circuit by localised melting as an electrical fail-safe (Image courtesy of Soteria) A typical PTC fuse. Its transparent barrel reveals the granular, conductive material that reduces/cuts off conductivity in response to ohmic heating (Image courtesy of EVs Enhanced)
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