How Electrolytes Influence Battery Safety

45 (continued on next page) Lithium batteries use organic electrolytes because of the wide operating voltage. For lithium ion rechargeable batteries, these electrolytes are almost universally based on combinations of linear and cyclic alkyl carbonates. These electrolytes make possible the use of Li as the anodic active component and results in the high power and energy densities characteristic of the Li-ion chemistries. However, these organic electrolytes have high volatility and flammability that pose a serious safety issue for their use in the consumer and transportation markets. If exposed to extreme conditions of elevated voltage and temperature, these electrolytes can react with the active electrode materials to release significant heat and gas. The formulation of electrolytes is developed to meet performance criteria such as conductivity, temperature range (high and low), and voltage range stability. There are many studies that correlate relationship between performance criteria to selection of solvent species, solvent ratios, electrolyte salts, and additives. The choice of electrolyte can also have a significant impact on the safety, thermal stability, and abuse tolerance of the cell. Some materials that have superior performance properties, such as LiAsF6, cannot be used because of high toxicity.1 Some solvent species, such as propylene carbonate (PC), are limited in concentration because they cause disruption of the anode graphite grains. However, there are few studies that correlate the influence of electrolyte on the cell response during an abuse event. For example, gas generation in Li-ion cells under abuse conditions has an effect on safety because gas production, if generated at sufficient pressure, will vent flammable solvent vapor into the surrounding environment. The resulting fuel–air mixture can be quite explosive and only requires an ignition source to ignite the vapors. The heat generation of the reactive cell components is often sufficient to self-ignite this mixture as shown in Fig. 1 for a Li-ion cell undergoing runaway and venting into an air-containing enclosure. In higher voltage modules and packs, there are often sparks that are generated during an abuse condition which can ignite these vapors. Accelerating Rate Calorimetry (ARC) has been used as a sensitive analytical tool to determine the self-generated heat and gas evolution from cells and electrolytes from room temperature up through a full thermal runaway (up to 450°C). Special highpressure fixtures are required to contain the vented gases and allow adiabatic temperature control of 18650 size cells.2 Figure 2a shows an ARC thermal runaway profile of an 18650 cell showing this heat and gas volume How Electrolytes Influence Battery Safety