A fundamental element of the industrial internet of things (IIoT) is continuous data collection from sensors over time. While some sensors are connected to powered or wired networks, some need to be located without access to grid power. Those have to transmit their collected data via wireless connectivity.
This creates an important challenge: How do you power such wireless industrial sensors over a long period of time? So, there is a need for a power source that is reliable and able to remain operational for extended periods without any maintenance or human intervention, ideally for several years. Rechargeable energy sources, including batteries, are appealing in this context to make up for self-discharge over time. Such a power source must typically be low-cost, compact, and able to fit into the form factor of the sensor.
Solar cells have an important role to play as an energy source for IoT sensors, but the electricity they generate needs to be stored to handle variations in power output resulting from fluctuating light intensity. Similarly, other energy harvesting devices, such as piezoelectric or thermoelectric generators, also require a battery for a stable and continuous power supply.
An appealing solution to store energy from various intermittent or discontinuous energy sources is provided by printed batteries. Printed batteries offer benefits such as mechanical flexibility, compact dimensions, and low cost. Several companies have been producing printed batteries for some time, but no rechargeable printed battery solution had been commercialized until now. In this article, we will present a novel printed battery solution that directly addresses this challenge, and we will discuss multiple possible applications for rechargeable printed batteries.
Printed Batteries 101
Printed electronics is an innovative production method offering numerous advantages. It is simple, cost-effective, and environmentally friendly. Components are printed using organic inks made from soluble polymers. Printing processes such as inkjet or screen printing enable high-volume production at low cost.
Different organic polymers can be formulated as printable inks, enabling a myriad of different functions to be achieved. These polymers can be printed on a large scale, deposited on a variety of flexible or rigid substrates at relatively low temperatures, and subsequently integrated into industrial or consumer products. Printed electronics is a technology ideally suited for the manufacturing of sensors on flexible substrates. This enables them to be used to great effect in situations lacking the space for conventional sensors.
This same printing technology can be applied in the area of battery production. However, it is only recently that it has become possible to print rechargeable batteries. Printable battery technology was developed by Evonik Industries AG (Essen, Germany) in partnership with InnovationLab GmbH (Heidelberg, Germany). The technology is called TAeTTOOz and has now been acquired by InnovationLab for upscaling and mass production, working with Heidelberg Printed Electronics (Weisloch, Germany) as InnovationLab’s manufacturing partner.
How it Works
This rechargeable printed battery technology is based on ‘redox-active’ polymers. Conventional printing methods can use these polymers 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.
In conventional Li-ion batteries, only the small Li+ cations move in and out of the electrodes on either side of the battery, in a process known as ‘intercalation’. A polymer-based battery works differently. Here, both anions and cations move within the electrolyte during cycling.
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 cathode and anode materials are optimized to chemically store electric charges supplied from an external power source. This is achieved by a change of their redox state under a given loading voltage during the charging process.
When a discharge voltage is applied, the initial redox state is reversibly restored, and power can be drawn from the battery. The solid-state electrolyte ensures electrical charge compensation in the battery using ionic mobility.
The inks are water-based and can be formulated to meet users’ specific needs — they do not require the use of toxic or CMR (carcinogenic, mutagenic, or toxic for reproduction) solvents. Due to the non-toxic nature of these organic inks, the printed products are compatible with common waste-disposable rechargeable batteries.
To match specific printing needs, the inks are characterized in terms of their particle size, stability, and rheology (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).
Due to the interesting fact that the battery does not hold any voltage before its first charge, subsequent production processes such as picking and placing 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.
The lateral dimensions of printed batteries typically range from 1 to 20 cm, and the overall thickness does not exceed 0.5 mm. The batteries can 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 the printed batteries can vary from a few cm2 to several m2 with certain performance limitations in the case of extreme sizes.
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 IIoT applications. The same principle can be utilized in signage or other similar devices.
Several ‘self-powered autonomous sensor unit’ projects are ongoing with our customers and partners, both from industry and academia, in which a printed rechargeable battery is combined with a temperature or moisture sensor, together with a solar cell, a printed RF-harvesting antenna, or a piezoelectric material. This concept has already been successfully applied and proven with the use of printed organic photovoltaic (OPV) solar cells.
The battery’s technical 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. This capacity and voltage determine the target applications. They are too low to drive bright LEDs or heaters but are 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 possible. To achieve higher voltages, the 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 but increases in vertically stacked designs.
Reliability and General Characteristics
Until now, these batteries have been primarily used in demonstration setups and for R&D purposes. They have now been fully characterized in terms of their performance under various loading conditions, with self-discharge and cyclical measurements recorded. The actual battery capacity is approaching the theoretical maximum achievable with the materials used.
The 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, and encapsulation.
The cycling stability of the anode and cathode materials exceeds 500 cycles for above 80 percent capacity retention when used as individual materials in button cell batteries. Long-term stability crucially depends on the encapsulation technology. InnovationLab will be providing more information shortly, following the full completion of the technology transfer from Evonik.
Printed batteries are thin, lightweight, and flexible. They can provide a cost-effective solution for industrial wireless sensors and other IoT applications. The new TAeTTOOz technology enables flexible, rechargeable solid-state batteries to be printed on an industrial scale. These printed batteries are also notably safer and environmentally friendlier than traditional metal-based batteries.
Shortly, 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 thin, flexible printed rechargeable batteries across many industries.
This article was written by Dr. Florian Ullrich, Head of Business Development at InnovationLab. For more information, visit here .