Zinc-carbon primary battery printed on paper and rolled up

Researchers at Fraunhofer ENAS are developing a degradable zinc-carbon primary battery printed on paper for a sustainable sensor application in the agricultural sector. The screen-printed and subsequently rolled-up battery system offers an optimal form factor and supplies the sensor system with energy on demand.

Researchers at Fraunhofer ENAS are developing a degradable zinc-carbon primary battery printed on paper for a sustainable sensor application in the agricultural sector. The screen-printed and subsequently rolled-up battery system offers an optimal form factor and supplies the sensor system with the energy it needs.

1. introduction

Printed batteries, especially environmentally friendly systems based on the well-known zinc-carbon material system, have been realized for more than a decade. While the standardized D, C and AA batteries are rigid and batch produced, the advantages of a printed battery are e.g. flatness, bendability, thin form factors, shape variability, scalability in voltage and capacity, to highlight a few and the reason for the research on this type of battery [1-7]. Typical applications are sensor systems and advertising. In this paper, the focus is on a zinc-carbon battery application. For a broader overview of the different material types of printed battery technology, see e.g. [8-13]. This approach is fundamentally different from rechargeable and 3D microbatteries, which aim for a small area of less than 1 ^cm2 [14].

In modern printed batteries, a flat battery design that can be bent is common [8]. Typical energy densities of printed zinc-carbon batteries are in the range of <1 ^mA/cm2 up to 5 ^mA/cm2 for the active area. Additional surface area is required for the encapsulation of the aqueous electrolyte, which enables a chemical reaction inside the battery. Material structures, layouts and applications are described in [8], for example. The advantage of this primary battery is that it is fully charged after production. It can be scaled up to a multiple of 1.5 _Vnom in the operating voltage. The energy content depends on the surface area. The possible currents that can be driven by the battery are determined by its internal resistance.

The intended application of this newly developed battery is described in the following section Sensor system for agriculture. There is no publication to date on a printed battery that is rolled up for application, with the inner core open. The approach of using mainly paper instead of polymer film or so-called coffee bag material is also innovative in order to achieve a higher proportion of degradable materials.

Sensor system for agriculture

Global agriculture is undergoing a profound transformation due to the increasing demand for food from the growing population, which is predicted to reach 8 billion by 2025 and 9.6 billion by 2050, requiring a 70% increase in food production by 2050 [15]. However, natural resources such as arable land and water for irrigation are limited and will be further strained by climate change. Intensive farming methods that rely on fertilizers and pesticides exacerbate the degradation of ecosystems.

Smart farming, which uses sensor technology, data processing and telematics, has proven to be a solution for increasing crop yields, reducing pollution and attracting skilled labor. Sensor technology in arable farming must meet strict criteria, including affordability (10 to 25 EUR/ha), wireless data transmission of over 300 m to a base station and meeting farmers' expectations for higher yields, a more consistent harvest and lower costs. In addition, the sensor technology should not be a burden on the arable land in the event of damage. The most important sensor measurements include soil moisture for efficient irrigation, soil nitrate content for optimal fertilization and leaf moisture/temperature for timely fungicide application to combat infections such as Phytophthora in crops. In addition to functionality, environmentally friendly disposal of the sensors, which ideally decompose during plowing, is also a growing concern. These advances in sensor technology aim to sustainably increase agricultural productivity amid resource constraints and environmental concerns.

The EU project 'PLANtAR' (www.plantar-project.eu) aimed to develop such cost-effective, miniaturized, networked and partially biodegradable monitoring sensor technology [16]. Fig. 2 shows one of the sensors developed in the project, consisting of a miniaturized electronic module with a single-chip radio system, sensors for measuring temperature, soil tension and nitrate [17]. The sensors and the electronic module are made of biodegradable or inert materials with a minimum of metal and ceramic components and can remain in the field after the harvest. The device is powered by a biodegradable zinc-carbon battery. An antenna printed on paper is used for wireless communication. All conductive paths within the sensor are also printed. Finally, a gateway collects the data transmitted by the distributed sensor devices and forwards it via the Internet to a central server running a corresponding expert system.

Fig. 2: Partially biodegradable sensor developed in the EU project 'PLANtAR' - (left) sensor assembly with all components for a wireless intelligent sensor system; (right) technology demonstrator of partially compostable sensors for agriculture

2. materials and methods

2.1 Configuration of the battery

The basic idea for the realization of a printed zinc-carbon flat battery is shown in Fig. 3a. The required components are stacked in layers: silver grid (optional), carbon current collector, anode, electrolyte with separator and cathode. A functional description can be found in [18,19]. The advantage of this pressure approach is that the series connection of batteries can also be easily adapted by adjusting the layout. Fig. 3b shows a series connection of three cells with a total battery voltage of 4.5 Vnom. Only one side of two different substrates is used for this battery.

Following the basic concept shown in Fig. 3, the layout can be slightly modified by printing all layers on only one substrate, using the front and back sides (see Fig. 4a). The corresponding anode and cathode layers are sufficiently overlapped when the substrate is rolled up (see Fig. 4b).

Fig. 3: Schematic of a printed zinc-carbon battery structure: (a) stacked structure of a single cell; (b) series connection of three single cells, realized by printing arrangement

Fig. 4: Schematic of a printed zinc-carbon battery structure: (a) stacked structure of a 4.5 Vnom battery consisting of three cells (top); (b) start of the rolling-up process, including electrolytes and separator layer (left); (c) rolled-up 4.5 Vnom battery from Fig. 4a, pictorial diagram (right)

Two different arrangements were used in the experiments. Figs. 5a and 5b show the arrangement of three batteries; they are arranged next to each other in the winding direction ('crosswise'). This means that the seal between the three cells is perpendicular to the winding direction. Each battery has an active area of 55 ^cm2. The second layout is shown in Fig. 4c and 4d. Each battery has an active area of 57 ^cm2. The main difference in the layout is that there is no longer a seal between the batteries transverse to the winding direction ('in longitudinal direction'). Instead, all seals between the individual battery cells run in the winding direction. The effects of this difference are described in the Results section. Photographs of the different layers of the longitudinal battery are shown in Fig. 1 (see beginning of this article).

Fig. 5: Schematic of two different rolled-up 4.5 Vnom battery layouts: (a,c) battery cells lying transverse to the winding direction - (a) shows the arrangement on the front side, while (c) shows the overlap during rolling up - (b,d) battery cells in the longitudinal direction parallel to the winding direction - (b) shows the arrangement on the front side, while (d) shows the overlap during rolling up

The reaction scheme of zinc, zinc chloride and manganese dioxide is shown in the following equation:

Environmental aspects were discussed for different battery systems in [20]. Source [21] discussed the leaching of sulphur dioxide from spent zinc-carbon battery scrap. This was caused by steel encapsulation, especially in alkaline battery cells. Neither NaOH nor steel is present in the printed batteries.

2.2 Experimental setup

For both layouts (see Fig. 5), a set of three or four screens is made for printing the silver (optional), carbon, zinc and manganese dioxide layers. Three or four layers are printed using a screen printing machine (EKRA E1 XL):

1. a silver grid (DuPont PV410) to reduce the internal resistance of the battery (optional)
2. a carbon layer (Henkel Electrodag) to cover the silver and prevent any chemical reaction with the battery cell
3. a zinc layer as the anode of the battery;
4. a manganese dioxide layer as the cathode of the battery. After each printing step, the ink is completely dried in a convection oven (3D Micromac microDRY, 110 °C, 10 min) before the next layer is applied

A circuit developed in-house is used to discharge the finished battery. The circuit has a stand-by current of 15 µA. This is about 1/667 of the current load of 10 mA (20 ms) for wireless data transmission. The firmware of the electronic circuit is modified so that the wireless data transmission, which causes the highest current load on the battery, is performed at a repetition rate of almost once per second (1.0064 s) instead of the application frequency of once every 30 minutes. The voltage is monitored during discharge with a potentiostat (BioLogic VMP 3). This setup was chosen so that the discharge is not also controlled by the potentiostat, as mechanical relays are used in the machine to switch the discharge on and off. If this is done thousands of times, the service life of the device will be considerably shortened.

2.3 Materials and assembly methods

The focus of this work is on rolling up a flat material system encapsulating an aqueous layer. To reproduce and build upon the published results, only the substrate and encapsulation materials and methods are required. Material systems used for the production of printed batteries are hardly described in the literature. Since the ingredients and additives of the inks used are part of the background knowledge of every player in this field, we cannot disclose our recipes in this article.

The substrate used for the experiments is Felix Schoeller 'P_E:SMART Paper Type 1'. The main component is a raw paper coated with a resin coating that prevents the aqueous electrolyte from drying out. Therefore, only a small amount of this material remains with very slow degradation. By changing the substrate from 150 µm PET to P_E:SMART paper, the weight of the polymer encapsulation could be reduced by more than 80 %.

After the layers of silver (optional), carbon, zinc and manganese dioxide have been printed and dried, the electrolyte (gelled aqueous zinc chloride) and the separator (to prevent a short circuit between the anode and cathode inside the battery) as well as the encapsulation are still missing to complete and functionalize the primary battery system.

For the manual sealing of the cells, a 680 µm spacer made of 3M 467MP 200MP is used, which is provided with an adhesive layer on each side. Alternatively, a UV-curable adhesive (KIWOPRINT-UV 94) was applied to the flat substrate by screen printing. The mounting process was not successful, so these activities are not reported further in this paper.

After the sealant was applied, a pre-cut porous paper was placed on the active battery areas and the electrolyte was applied with a syringe, which was spread over this area.

To roll up the battery layers, a rod was used to pinch one end of the substrate so that the inner core is open after the procedure and removal of the rod.

3. results

This chapter presents the observations made during the construction of the rolled-up batteries and the electrical performance data.

3.1 Cross-wound battery 3.1.1 Production of the battery

When the cross-wound battery structure is wound up, the electrolyte in each battery cell is pressed in the direction of winding due to the resulting pressure. This leads to wetting of the adhesive layer between the individual cells. If the adhesive layer is wetted by the electrolyte, it loses its encapsulation properties. As a result, the electrolyte of two cells is not separated - as intended - but connected. The result is a short circuit between the two battery cells through the electrolyte.

After many tests, it was realized that the transverse structure of the battery causes such major problems with regard to encapsulation that a new approach was developed for the longitudinal structure of the battery, which is described in "Longitudinal battery".

3.1.2 Electrical power

To get an idea of the battery performance, the non-coiled battery was simply made by encapsulating two flat substrates that had already been prepared for the coiling experiments. Using two of these substrates, a battery with three cells could be realized. Fig. 6a shows the discharge setup with the external electronics.

Two areas can be distinguished in the discharge diagram of the potentiostat, which is shown in Fig. 6b: Up to cycle 78,000, the battery voltage fluctuates between an upper (4.2-3.7 V) and a lower voltage level (2.4-1.9 V) depending on the load level (15 µA vs. 10 mA). At the higher voltage level, the power consumption of the electronics is low. During wireless data transmission, the current requirement is highest, which leads to a lower voltage level due to the internal resistance of the battery. At approx. 78,000 cycles (i.e. 21.8 h), there is a significant drop in voltage from 3.7 V to 2.2 V. This drop indicates that one of the battery cells has reached the end of its service life. With >70,000 discharge cycles, the battery remains within the requirements of the application, which is defined as at least 20,000 cycles.

3.2 Longitudinal battery 3.2.1 Preparation of the battery

When the longitudinal battery is wound up, the electrolyte in each battery cell is pressed in the winding direction by the pressure in this process - as in the transverse arrangement. However, the movement of the electrolyte can be controlled much better than in the first approach. By precisely dosing the electrolyte during the winding of the battery, two problems can be solved: 1. no wetting of the adhesive layer between the individual battery cells; 2. no wetting of the adhesive layer perpendicular to the winding direction at the closing end of the batteries. With this design, it is possible to produce coiled batteries without internal short circuits of the electrolyte.

3.2.2 Electrical performance

To get an idea of the performance of the battery, the non-coiled battery was produced simply by encapsulating two flat substrates that had already been prepared for the coiling experiments. With two of them, a battery with three cells could be realized.

Two zones can be distinguished in the discharge diagram recorded by the potentiostat(Fig. 7): From 0 to 90,000 cycles, the voltage fluctuated between an upper level (4.3-4.0 V) and a lower level (2.9-2.0 V). At the higher voltage level, the power consumption of the electronics is low. During wireless data transmission, the current requirement is highest, which leads to a lower voltage level due to the internal resistance of the battery. The electronics require a minimum voltage level of 2.5 V. Therefore, reliable operation is only in the range of 0 to 44,000 cycles. For the intended operation, the internal battery voltage should be lower. This can be realized by a lower resistance in the current collector by means of an underlying silver grid. This modification is described in section 3.3.

At 90,000 cycles, this battery also shows a significant voltage drop from 3.9 V to 2.5 V. This drop also indicates that one of the battery cells has reached the end of its service life.

With more than 44,000 discharge cycles, the battery essentially remains within the requirement of the application, which is defined as at least 20,000 cycles.

3.3 Longitudinal battery plus silver grid 3.3.1 Production of the battery

The battery is manufactured in a similar way to that described in section 3.2.1. The only difference is that the layer stack is underlaid with a silver grid, which is already shown in Fig. 1 at the beginning of the article.

3.3.2 Electrical power

Fig. 8b: Longitudinally rolled-up, three-cell 4.5 _Vnom battery with silver grid: discharge setup with electronicsInthis case, the battery performance was determined with a rolled-up battery, as shown in Fig. 8a. The measurement results are shown in Fig. 8b. Due to the lower electrolyte content in the battery compared to the flat setup from section 3.2, the total discharge cycles drop from 44,000 to 34,000. This still meets the requirement of at least 20,000 cycles.

Two ranges can be distinguished in the discharge diagram recorded by the potentiostat in Fig. 8b: From 0 to 34,000 cycles, the voltage fluctuates between an upper level (4.3-3.3 V) and a lower level (4.1-2.5 V). At the higher voltage level, the power consumption of the electronics is low. During wireless data transmission, the power requirement is at its highest, which leads to a lower voltage level due to the internal resistance of the battery. The electronics require a minimum voltage level of 2.5 V. Reliable operation is therefore only in the range of 0 to 34,000 cycles.

At 38,000 cycles, this battery also shows a significant voltage drop from 3.1 V to 2.3 V. This drop also indicates that one of the battery cells has reached the end of its service life.

With more than 34,000 discharge cycles, the battery essentially remains within the requirement of the application, which is defined as at least 20,000 cycles.

With this accelerated discharge, the battery powering an application with less frequent discharge pulses is expected to last longer, i.e. allowing more cycles as the battery has time to recover through ion reorganization. Therefore, the battery life will be sufficient for the application.

4. discussion

The research and development goal of providing a coiled battery with an open inner core as a power supply for a sensor system for a specific agricultural application has been successfully achieved. The main challenges, such as the layout or the leakage of the battery encapsulation, were solved.

Comparing a printed battery in the flat and rolled-up form factor, the flat version has a higher capacity than the rolled-up version. The main reason for this is the smaller amount of electrolyte in the battery that is required for the chemical reaction. The amount of electrolyte is limited by the thickness of the seal of 100 µm and the paper separator in between. The volume was 500 µL. With the flat form factor, there is no problem of bulging the paper substrate of the battery due to excessive electrolyte volume. The electrolyte volume was about 2 mL in each cell. This is not possible with the rolled-up battery due to the superimposed layers in each layer.

The biggest challenge in rolling up a flat substrate is the mechanical stress created in each stacked material system due to the different bending radii on the inside and outside of the substrate. When using a relatively stiff encapsulation material, these limitations become even more pronounced due to small wrinkles, which leads to electrolyte leakage. This occurs preferably in the adhesive layer, which is weaker than any paper or polymer film layer.

For agricultural applications, there are currently two disadvantages with regard to the biodegradability of the battery described in this article:

1. The chemical system requires _H2Oin the electrolyte for the chemical reaction. Therefore, the electrolyte requires a hermetic seal that must withstand the aqueous electrolyte. This is realized by a polymer coating of the paper and a polymer frame with adhesive layers for encapsulation. These polymers must not be water-degradable and therefore remain in the soil for a long time.
2. The resistance of the carbon electron carrier itself is too high. In order to supply the current required to operate the electronics, an additional silver grid is required to reduce the overall resistance of the current-conducting layer. Silver is not degradable. In addition, silver ions are sometimes used to prevent the growth of bio-organic cells, e.g. as dopants in sportswear. A review of the discussion on silver in soil was written by [22].

5 Conclusions

In this paper, three important steps for the development and construction of a rolled-up printed primary battery were selected and described. For the first time, printed zinc-carbon batteries were scaled not only in terms of voltage and capacity, but also in terms of their three-dimensional shape. A primary battery printed on a paper substrate was rolled up to fulfill the application's requirement for an open inner core.

6. summary

There are several established form factors in battery systems aimed at mass market applications, such as D, C, AA, AAA, lithium round cells and button cells. In addition to these standardized batteries, there are several approaches in printed electronics for the realization of flat batteries from different material systems, which include primary and secondary battery types. For a special application in agriculture, a sensor system requires a degradable primary battery. This paper describes the development of a special zinc-carbon battery that supplies the sensor application with 4.5 Vnom. The battery has a length of 170 mm and an outer diameter of 23 mm, while the inner core is open for the application's antenna system. The active battery area is up to 161 ^cm2. The design and manufacturing aspects are described. The coiled battery system is fully charged and ready for operation after manufacturing. It can remain inside the degradable sensor system after use in the field.

7. acknowledgements

We would like to thank the German Federal Ministry of Education and Research (BMBF) for funding the project (16ME0160).

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