Lithium-imide bests Li-ion in mobile devices

Battery manufacturers are hard-pressed to pack any more run-time into their products, and the energy density of lithium-ion batteries isn’t high enough to power the next generation of super-svelte mobile devices. Worse, Li-ion chemistry lacks durability; energy density declines rapidly as the cells age, especially in the elevated temperatures typical of consumer use. Run-time shortens, and cells can balloon and damage a device.

The cells in today’s Li-ion batteries are made of an anode (+) and a cathode (–), with a liquid electrolyte contained between them. The salt most commonly used in the electrolyte—lithium hexafluorophosphate (LiPF₆)—tends to react with residual moisture in the cell to produce hydrofluoric acid, one of the most chemically reactive substances imaginable.

When the hydrofluoric acid corrodes the cathode, cell impedance increases, capacity falls and run-time diminishes. In multicell batteries, the stress of a single degrading cell can disable the entire battery pack and can generate a gas that causes swelling of the cell, which can damage the device and even pose a safety hazard.

The process accelerates as temperature rises. Li-ion cell data sheets generally show results for cells and batteries at “room temperature” of 20°C (68°F), but in-device temperature is much higher. That holds particularly true for today’s thinner, tighter enclosures, which typically pack more heat-generating semiconductors than earlier device generations did.

An alternative Li-ion electrolyte chemistry known as lithium imide (Li-imide) clears the energy density and durability hurdles. Li-imide batteries not only provide up to 25 percent more energy for a given battery size, but they also are virtually impervious to temperature and water impurities inside the cell, thereby offering greater durability.

The Li-imide chemistry can offer 500 watt-hours/liter with a cycle life comparable to that of ordinary Li-ion batteries. Using a lower, 80 percent depth of discharge, Li-imide provides 450 or 400 Wh/l, with enormously extended cycle life. Even at temperatures higher than 40°C (104°F), the chemistry enables more than 750 cycles—over 2.5 years of real calendar life with daily recharging.

Li-imide represents a new lease on life for existing devices. The alternative chemistry costs no more than traditional Li-ion, is available in standard formats and can be multisourced.

Greater thermal stability also means greater design flexibility, because device designers need worry less about the juxtaposition of the battery and heat-generating components.

About the author

Marc Juzkow is vice president of R&D and engineering at Leyden Energy. He holds an executive MBA and an MS in chemistry from Simon Fraser University (Vancouver, B.C.).