Energy Storage Sector Looks to Solid-State Solutions
By Paula Doe
May 27, 2010 – Chip-sized and stamp-sized solid-state batteries are showing up in chip- and board-level backup power and for storing harvested energy to enable wireless systems. Larger capacity versions may be coming next.
“Bringing energy storage into the IC realm should provide a path to increasing process efficiency and decreasing costs,” says Planar Energy CEO Scott Faris. “We picked energy storage as a market that’s been underinvested and is ripe for a sea change. It’s like going from vacuum tubes to the transistor era. Further scaling in liquid battery technology doesn’t even make sense to think about—it’s like making bigger vacuum tubes.”
Planar Energy (Orlando, FL) is targeting large capacity automotive batteries, but other companies like Infinite Power Solutions (Littleton, CO) and Cymbet Corp. (Elk River, MN) are applying solid-state technology to make small batteries that can be integrated with electronics and energy harvesters to back up systems or to run sensors or wireless systems almost indefinitely.
IPS gets 100mA from stamp-sized cells
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.
Cymbet makes ‘batteries-in-a-chip’
Cymbet Corp. uses a similar LiPON solid electrolyte, but in an even smaller form factor, for a battery-in-a-chip package that aims to make local energy storage just another electronic component on the board or in the SiP. The chip-scale batteries are finding traction for embedded backup power to replace coin cells or supercapacitors in backing up memory, microcontrollers, and real time clocks in electronic systems.
These chip-like rechargeable lithium-based batteries, with nominal capacity of 50µAh in an 8x8 mm package, are made on silicon wafers with conventional deposition and etch tools, though unconventional materials. The chips withstand up to 260°C, so they can be reflow soldered in normal board assembly. They are sold as a bare die, or packaged with a power management ASIC in a SiP. Cymbet’s power management IC converts and regulates input ranging from 2.5V to 5.5V and a steady 3.3V output.
Cymbet also sees a big market developing for storing harvested energy for wireless sensor networks, to control and reduce energy usage in smart buildings, and to monitor and control industrial processes, where there is immediate ROI. Cymbet’s vice-president of marketing, Steve Grady, also notes there are a lot of medical applications in the pipeline, from external devices like neurostimulators and smart patches, to internal systems that monitor processes in the body, all powered by RF induction. “There are still ecosystem issues,” he notes. “But there is big potential in powered sensors, using energy storage at the point of load. The market has moved from the stage of ‘that sounds interesting,’ to the stage of actually shipping product in volume.’”
Planar Energy aims at R2R deposition of large-area thin films with 200mAh/g
Planar Energy, meanwhile, intends to extend solid-state battery technology to larger capacity batteries with a roll-to-roll spray coating process that it claims challenges vacuum deposition in quality.
The company claims its solid electrolyte improves LiPON’s ionic conductivity by orders of magnitude, and the new cathode material enables reversible capacities of more than 200mAh/g.
The company started specifically with the idea of trying to leverage the materials and processes of the IC, PV and FPD industries to energy storage. Solid-state technology is harder for large capacity batteries because solid electrolyte conductivity rates are limited in thicker films, and because large batteries need thick 10 micron films deposited at rates of tens of meters per minute to be economical.
Planar uses a stack architecture developed at the National Renewable Energy Laboratory, which uses some layers as both packaging and active layer simultaneously, and reduces the number of deposition steps for simpler manufacture. The original work deposited a lithium cathode layer over a LiPON electrolyte, and then applied a charge, which sent the lithium through the LiPON to form a metal anode on the other side, thus eliminating the separate deposition of the anode and protecting the buried anode from the atmosphere and corrosion.
Also key is a high speed roll-to-roll solution coating process that originated in Bell Labs’ work on copper interconnect and was later taken up again at the University of Central Florida for deposition of compound semiconductor films for solar cells. The process sprays aqueous elements directly on the surface and works through nanoparticle self assembly, enabling deposition of multiple materials with precise gradient control. Planar’s Faris says this process enables a new solid electrolyte material that matches liquid in ionic conductivity, creating ionic pathways across thick film cathode layers by grading dopant concentration to change conductivity as the film gets thicker.
“We’ve done 52 different materials systems in both this process and vacuum deposition, and get equal or better quality,” claims Faris. The company purports the technology could potentially enable batteries with 2X to 3X the energy density of current lithium ion batteries at lower cost.
The company received $4 million in funding from DOE in April to accelerate commercialization. Faris reports that the company has picked its material of choice, demonstrated the performance of the film, and is now building an integrated device and optimizing the stack. It’s in the process of moving to an R2R tool, targeting first samples in the next quarter or two, and then will start a demo/pilot line.
These speakers will update SEMICON West attendees on this recent progress in solid-state energy storage technology in the session on micromanufacturing technology for energy applications on Tuesday, July 13, at the Extreme Electronics stage. That program also features Texas Instruments’ Mark Buccini’s overview of ultra-low-power wireless and energy harvesting technology, updates from IMEC and Microgen Systems on their progress on more efficient MEMS energy harvesting, and Nextreme Thermal Solutions’ update on using thermal energy harvesting systems.