­Printed rechargeable batteries for the IoT are disposable

Technology News |
By Nick Flaherty

Researchers at InnovationLab in Germany have developed printed rechargeable batteries for sensor nodes in the Internet of things (IoT).

Several companies have been producing and selling printed batteries, but no rechargeable printed battery solution has been commercialized until now, say Jean-Nicolas Tisserant and Florian Ullrich at Innovation Labs.

The TAeTTOOz printed battery technology was developed by Evonik and has now been acquired by InnovationLab for mass production.

The rechargeable printed battery technology is based on redox-active polymers and conventional printing methods can be used to produce thin, flexible batteries that can store electrical energy without requiring metals or metallic compounds in their storage system. Importantly, battery cells produced using TAeTTOOz technology do not require a liquid electrolyte to function, which inherently eliminates the risk of leakage and subsequent hazards.

The polymer battery technology relies on redox-active organic molecules (polymers) whose redox states can be reversibly changed during the charging and discharging phases. Essentially this means that the redox polymer can undergo both a loss of electrons (oxidation) and a gain of electrons (reduction), with both processes being reversible.

TAeTTOOz batteries have two polymer-based conductive materials that are used as cathode and anode inside the battery, and a third ionic material that functions as a solid-state electrolyte.

The inks are water-based and can be formulated to meet customers’ specific needs, and do not require the use of toxic or carcinogenic solvents. Due to the non-toxic nature of these organic inks, the printed products are compatible with common waste, making the rechargeable batteries disposable.

To match specific printing needs, these inks are characterized in terms of their particle size, stability and flow characteristics. Combining these inks with printed conductive traces on a substrate allows both the batteries and the associated charging and discharging circuitry to be printed in a relatively small number of printing steps. Flexible substrates that can be used include foils made from polyimide (PI), polyesters (PET, PEN) or thermoplastic polyurethanes (TPU).

The lateral dimensions of printed batteries typically range from 1 to 20 cm, and the overall thickness does not exceed 0.5 mm. These batteries can, of course, also be stacked, folded, or rolled to design 3D objects for integration into existing systems.

The use of universal printing techniques enables customized batteries in different sizes to be fabricated. The size of printed batteries can vary from a few cm² to several m² with certain performance limitations in the case of extreme sizes.

Printed battery applications and customization

One of the major applications for the TAeTTOOz battery technology is its combination with a sensor or sensor array coupled with one of various energy harvesting components to create a fully autonomous, self-powered unit for IoT applications. The same principle can be utilized in signage or other similar devices.

Due to the interesting fact that the battery does not actually hold any voltage prior to its first charge, subsequent production processes such as picking and placing of components are possible without any risk of overvoltage damage. These batteries can be printed from “start-to-finish” on standard screen-printing presses either in a full roll-to-roll continuous production mode or in sheet-to-sheet mode.

As a result InnovationLab is working with several customers on ‘self-powered autonomous sensor unit’ projects, where a printed rechargeable battery is combined with a temperature or moisture sensor is combined with a solar cell, a printed RF-harvesting antenna or a piezoelectric material. This technological concept has already been successfully applied and proven with the use of printed organic photovoltaic (OPV) solar cells.

The specifications are defined by the chosen layout. Battery capacities from 0.1 to 0.2 mAh/cm² at an operating voltage of 1.2 V are typically achieved. The mentioned capacity and voltage determine the target applications. Obviously, they are too low to drive bright LEDs or heaters but optimal for low-power applications such as the powering of sensors in smart labels and patches.

Due to the screen-printing process, there is full freedom of cell design, with vertical or coplanar designs entirely possible. To achieve higher voltages, the printing technology enables several cells to be connected in series. For example, connecting two cells in series provides 2.4 V. The number of printing steps remains constant for a coplanar layout and increases in vertically stacked designs.

Until now, these batteries have been primarily used in demonstration setups and for R&D purposes. The batteries have now been fully characterized in terms of their performance under various loading conditions, with self-discharge and cyclical measurements recorded. The achieved battery capacity is approaching the theoretical maximum achievable with the materials used – currently, it sits at about 70 % to 80 % with printed batteries.

The performance and reliability of printed batteries depends on a multitude of factors, including the printing geometry, the printing machine used, the resulting printed layer thickness and reliability, battery configuration, substrate, encapsulation, etc.

The cycling stability of the anode and cathode materials exceeds 500 cycles for above 80 % capacity retention, when used as individual materials in button cell batteries. Long term stability crucially depends on the utilized encapsulation technology. InnovationLab will be providing more information shortly, following the full completion of the technology transfer from Evonik.

The TAeTTOOz technology enables flexible, rechargeable solid-state batteries to be printed on industrial scale, with Heidelberg Printed Electronics as the manufacturing partner. Soon, InnovationLab will supply both the printing materials and the know-how for the design, printing, and characterization of printed batteries. The company will also produce and sell its proprietary range of printed batteries to its customers, enabling the design-in and use of printed rechargeable batteries across industry.

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