Lithium-Ion and Other Cell Chemistries 83 production. The solid electrolytes are generally being deposited onto either a separator layer or directly onto the cathode material. Safety Features Like many other technologies, lithium-ion cells have some inherent safety challenges. In essence, you have a container of energy that under certain conditions can be released. The amount of energy released will depend on the size (capacity) of the lithium-ion cell. Addition- ally, there is always the potential for contamination or errors to occur during the cell manu- facturing process, which could result in an early failure or energy release.
In order to help mitigate these risks, there are several safety features that many cell manufac- turers have integrated into their designs. The first and most common among a wide variety of cell types is the vent. The cell vent is basically designed as an engineered failure point within the cell in the event of a buildup of pressure inside the cell. The vent is designed to open up in the event that there is a pressure buildup within the cell. An engineered vent is generally only included in cells that have a hard can-type container; pouch-type cells use a slightly different variation of the vent.
The pouch cell is more likely to be designed with a notch or other point in order to attempt to direct the failure in a specific area. Without this notch, the pouch type cell is much more difficult to forecast where the cell will fail. The CID is a type of nonresettable fuse that is often integrated into the internal cell compo- nents. The CID is an engineered fuse that is most often pressure-based. So the CID is inte- grated into hard can-type cells as it is much more difficult to use a pressure-based fuse in a pouch-type cell.
In essence, the CID is a two-part mechanism that is designed to separate and break the flow of current to the terminals if the pressure builds up beyond a certain point. Another type of safety feature that is included in some cell designs is a thermal fuse, referred to as a PTC device. The PTC is essentially a resettable thermal fuse that is designed to open if the temperature cell rises above a predetermined point. If this PTC opens, the current flow between the cell terminals and the jellyroll is temporarily broken. If the temperature of the cell falls back down to normal operating temperatures, the PTC will reset and the cell will again be usable.
The challenge with the PTC is that it will generally limit the maximum voltage that can flow from a cell and is therefore limited to small cells in small portable-type applications, typically below 26 V or so. The other safety feature is one that we have already actually talked about, the separator. In lithium-ion cells, the three types of separators PP, PE, and ceramic-coated separators also perform a safety function. In the plastic-based (PP, PE) separators, they are engineered to fail (melt) at a predefined temperature. In fact, some cells use a three-layer PP/PE/PP cell which allows the center layer to melt at a temperature about 20 °C lower than the PP layers.
This prevents the lithium-ions from flowing between anode and cathode and is intended to slow/