Ilika CEO Graeme Purdy (Source Ilika)
Batteries go unnoticed until they run out, catch fire or explode. None of which are good scenarios for IoT solution providers, so the emergence of thin-film chemistries that are more stable, can be recharged easily and that are more in line with current manufacturing processes, are long awaited and timely.
The much-publicized explosion of a Li-ion battery on a Samsung Galaxy Note 7 said less about the Samsung design team and more about the nature of Li-ion batteries: they are inherently unsafe. Battery and systems designers can mitigate the risk, but it’s always there. The risks increase as temperatures rise and environments become unpredictable, such as in automotive, industrial or military applications. In medical devices, especially implantables, classic Li-ion or any battery based on a liquid electrolyte, becomes particularly liable to scrutiny.
Despite the inherent risks, Li-ion batteries in various forms, from large, multi-cell implementations for electric vehicles, down to miniature coin cells for remote IoT sensors, remain popular. Manufacturers and designers like their energy density, fast recharge capabilities and falling cost. However, system and sensor designers need to be certain the battery will remain stable in specific cases.
One application is automotive, where longer range is also a requirement. Toyota, for example, announced it is spending $35 million with universities to study how to make better vehicle batteries – using artificial intelligence techniques to sift through the research.
Vehicles, over time, are becoming one big IoT device, on wheels. Smaller devices, such as individual sensors, need smaller batteries. Companies like Ilika plc have a strong play here. Ilika is a spin-off from the University of Southampton (UK) and has developed a thin-film chemistry that uses a solid electrolyte instead of a semi-liquid solvent (Figure 1).
Figure 1. Ilika’s Stereax is a solid, thin-film battery chemistry that is more stable than Li-ion, with higher energy density. (Image source: Ilika plc)
Called Stereax, its energy density is 2x volumetrically compared to Li-ion, but “gravimetrically it’s the same,” said Ilika CEO Graeme Purdy. Essentially, it can hold twice the charge for a given volume, while weighing the same. This combination of energy density and light weight is good, but it has other advantages to consider. Add in longer life and lower manufacturing cost due to its solid electrolyte, and Ilika’s Stereax battery needs to be given serious consideration for future IoT deployments.
Energy harvesting with Stereax
The long life of Stereax technology has yet to be proven fully, but according to Purdy, accelerated testing shows it will hold up past three or four years at least, and to date it has shown no leakage at ambient temperatures. Super-capacitors or Li-ion rechargeables are commonly used in sensor applications, but the liquid electrolyte chemistry breaks down after two years. Also, Li-ion batteries can handle 500 to 1,000 recharge cycles, while Stereax batteries can handle 5,000, he said.
The number of recharge cycles and its ability to last a long time in the field makes Stereax a good solution for remote IoT devices that can use photovoltaic cells to gather ambient light energy to recharge the cells. Other ambient energy sources are vibration, temperature differentials and pressure. All are being refined in an effort to provide the optimum solution for any application. Photo-voltaic-based charging, for example, is of no use when sensing structural stress in a tunnel. In that scenario, vibration or pressure energy may be a better option.
It’s important to keep in mind that solid batteries are more conducive to large-scale semiconductor manufacturing processes, which leads to gains in cost and integration capabilities. Ilika has already demonstrated Stereax in what it calls a Perpetual Beacon for Smart Homes (Figure 2).
Figure 2. Ilika’s Stereax demonstration combines its M250 battery with a solar cell, temperature sensor and Bluetooth transceiver to enable a “place and forget” room-temperature monitor. (Image source: Ilika)
The demonstration combines a Stereax-based M250 battery with a photovoltaic cell, temperature sensor and a Bluetooth transceiver to implement a temperature monitor that can be placed – and forgotten.
The M250 implementation of Stereax is less than 750 µm thick and is rated at 250 µAh. The other advantage of thin film is flexibility in layout in that the footprint can be adapted to the application’s constraints.
Figure 3. Comparison of Stereax to supercapacitors and conventional Li-ion batteries.
Ilika’s technology shows promise and it’s being promoted as having a life of 10 years, though that is unproven, as it’s not that old as a technology yet. For IoT solution providers looking to avoid replacing batteries in thousands of end-node sensing devices, or who are looking to replace cables with wireless connections in weight-sensitive applications such as automobiles, it’s worth a serious look.