IPS' largest rechargeable, thin-film lithium micro-energy cell is about the size of two postage stamps (50 mm x 25 mm), and some 170 µm thin, but it claims 2.5mAh capacity and continuous current output of 100mA.
Tim Bradow, VP of business development at IPS, says that's enough for a wide range of products. This cell can provide backup power for real time clocks, memory devices, and solid-state drives, and can store the ambient energy collected by solar, piezoelectric, or thermoelectric energy harvesters to power wireless sensors, powered cards, active RFID tags, watches, consumer electronics and medical devices. Bradow also describes products in development that include remote controls that replace infrared diodes with low power RF signals and micro energy cells continuously trickle charged by solar cells, and wireless automotive switches that look to replace the cost and weight of copper wiring with RF signals and micro energy cells continually recharged with vibrational energy harvesting.
IPS uses lithium phosphorus oxynitride (LiPON) for the solid electrolyte, which provides good mobility of Li ions across the very thin electrolyte film, enabling high continuous discharge currents. The solid electrolyte also prevents electrons from leaking across the cell, so the unit does not lose charge in storage. The films are deposited on metal foil substrates in large chambers with conventional PVD tools from the flat panel industry, but with unique target materials and proprietary hardware and processes. The metal substrate also serves as the positive terminal to simplify the architecture and to eliminate the need for expensive metal deposition such as platinum. The foil also serves as a moisture-resistant encapsulant.
The small batteries can be combined for more power and capacity, but there's also still headroom for process improvement with better target materials and deposition processes, notes Bradow. The company has doubled the product's capacity in the last several years, and has demonstrated up to 4mAh on a single 25 mm x 25 mm cell in the lab.
"Solid state always wins", argues Bradow, pointing to the history of vacuum tubes, records, tapes, cameras, and, perhaps next, lighting. An additional push towards solid state may well come from environmental regulations, which now prevent the common trash disposal of a variety of wet chemical batteries in many locations around the world, and which may eventually lead to the banning of their use in certain consumer electronics.
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