82 Chapter 7 flow of lithium-ions. However, for many separators the plastic begins to shrink and entirely melt at somewhat lower temperatures causing an internal short circuit in the cell and thereby permanent failure above temperatures of about 90°–110 °C. Some lithium-ion cell manufacturers are beginning to integrate ceramic-layered separators in their cells as it enables higher temperatures and increases the safety of the cell, especially during nail penetration testing events. This also tends to improve the power rate capability of the cells by reducing the internal resistance.
Electrolytes The electrolyte is usually a liquid- or gel-based solution that the anode and cathodes are emerged in which acts as a conductor allowing the lithium-ion cells to move between the anode and the cathode. The electrolyte is typically a HC-based mixture that includes multiple additives which provide different functionality within the cell. The use of additives is the cell manufacturer’s “secret sauce” and is one of the key areas of intellectual property of the cell makers. Typical electrolytes may include a mixture of alkyl carbonates such as ethylene carbonate, dimethyl, diethyl, and ethylmethyl carbonates and lithium salts (LiPF6) (Aurbach, et al., 2004).
These additives offer various different benefits during operation. The main functions of the additives are to facilitate the formation of the solid electrolyte interphase (SEI) layer, to reduce the irreversible capacity and gas generation of the SEI formation and long-term cycling, and to enhance the thermal stability of the cell (Zhang, 2006). For example, in cylindrical- and can-type cells, an additive may enable “gassing” or gas generation at pre- defined temperatures in order to engage the CID, a resettable fuse internal to the cell. SEI layer formation is another important function performed by the electrolyte.
As the first electrochemical reactions begin to occur inside the cell, the electrolyte begins to react with the graphite or carbon anodes. Some of the lithium begins to form a passivation layer on the surface of the anode; this is referred to as the SEI layer (Dahn & Ehrlich, 2011). It results in an irreversible loss of capacity during the formation of the cells and generally during the very first cycle. However, this nonactive layer of lithium now begins to protect the graphite or carbon surface from further chemical reactions with the electrolyte. One of the challenges associated with the electrolyte is during the failure and cell venting.
As an HC-based liquid, the electrolyte will burn when ejected from the cell during a thermal runaway event. Due to this, there has been much interest in both gel electrolytes and solid electrolytes as well as in aqueous-based electrolyte solutions that are nonflammable. Gel electrolytes have many of the same features as a liquid but as they are a gel-type material they have less potential to “flame” up during a failure event. Solid electrolytes are being evaluated by many manufacturers as well, but most are not yet ready for high-volume