Don’t ignore the need for small, low-capacity batteries

printMuch of the media attention about batteries. as well as significant R&D efforts and business investment, is focused on high-capacity, power-dense rechargeable (secondary) cells. That perspective makes a lot of sense, of course, as these are used in electric vehicles (EVs) and other higher-power, often mobile situations.

Still, there are also countless cases where a low-power, limited-energy, non-rechargeable (primary) source is all that is needed, especially for one-shot single-use applications. These include point-of-care diagnostic devices, smart packaging and tags, and environmental sensing.

Many of these needs are presently being met by small button or coin cell batteries, often using silver-oxide chemistry, as well as lithium, alkaline, zinc-air, and other chemistries, Figure 1. These certainly do the job but may actually be “too much” for the modest application.

Figure 1 Button and coin-cell batteries come a large range of sizes (diameter and thickness) as well as voltage/energy rating; most are tossed out when depleted. Source: Wikipedia

Further, they create a significant waste stream and environmental challenge since they are almost always just tossed in the nearby trash. Recycling them, even if they make it to such a facility, is costly and complex. (Yes, AA, A, C, and D-size batteries should also be recycled, but that’s another story.)

One obvious question is: “How many of these batteries are used, and how many are then tossed?” I did extensive on-line research and came up blank. The few numbers I found for button/coin-cell usage were in dollars rather than units, which told me very little.

As for recycling, there are lots of estimates for lead-acid car batteries (80% to 90% was the consensus) and for larger lithium battery packs such as used in EVs (the numbers were all over the place). Further, most of the estimates (except for the lead-acid) focused of dollar figures for recycling as a business rather than the percentage of button and coin batteries recycled, which is what I wanted to know. For recycling of coin and button cells, one source of unknown credibility put that at 3%—I suppose that could be.

Seeing the need for a one-shot and relatively low-energy/low-power disposable battery, a team at the highly respected Swiss Federal Laboratories for Materials Science and Technology known as EMPA (the German acronym for Eidgenössische Materialprüfungs und Forschungsanstalt), has devised a disposable paper battery which reduces the environmental impact of batteries in such single-use applications. Their basic battery, approximately 1 cm2, uses zinc as the metal anode, graphite as a nontoxic cathode material, and paper as a biodegradable substrate.

The battery remains inactive and thus retains its full energy capacity until water is added and absorbed by the paper substrate, taking advantage of its natural wicking behavior. Once activated, a single cell provides an open-circuit potential (OCP) of 1.2 V and a peak power density of 150 µW/cm2 at 0.5 mA.

The materials are both simple at first glance yet sophisticated, Figure 2. The anode and cathode materials they developed are compatible with additive manufacturing techniques and can be stencil printed in a wide range of shapes and sizes. The paper serves as a separator between the anode and the cathode and is infused with the dry electrolyte which requires only a few drops of water to be activated.

Figure 2 (a) The battery’s electrochemical (EC) cell is composed of a paper membrane sandwiched between a zinc-based cathode and a graphite-based air cathode. The device remains inactive until water (the electrolyte), is added and permeates the membrane. (b) Picture of single cell battery fabricated by stencil printing on filter paper. At the battery terminals, the filter paper is impregnated with wax to avoid electrochemical reactions of the lead wires and to provide mechanical stability. (c) Photograph of a stencil-printed paper battery with a design that spells the name of the research institution (Empa); also powering a small LCD clock. (d) The device is composed of two electrochemical cells that are separated by a water barrier and connected in series as illustrated in the (e) schematic cross-section of the battery with its overlaid equivalent circuit (for ideal voltage sources).

The battery consists of three inks printed onto a rectangular strip of paper. Standard salt (sodium chloride) is dispersed throughout the strip of paper and one of its shorter ends is dipped in wax. An ink containing graphite flakes is printed onto one of the flat sides of the paper and functions as the positive end of the battery (the cathode), while an ink containing zinc powder is printed onto the reverse side of the paper as the negative end of the battery (the anode).

Another ink containing graphite flakes and carbon black is printed on both sides of the paper, on top of the other two inks. This ink makes up the current collectors connecting the positive and negative ends of the battery to two wires, which are located at the wax-dipped end of the paper. The role of the current collector is to connect the cathode and anode to external circuitry. All the inks were specially developed and tested to ensure that they exhibited shear-thinning gel properties which were compatible with additive manufacturing techniques such as stencil printing and extrusion-based 3D printing.

When a small amount of water is added, the salts within the paper dissolve and charged ions are released, thus making the electrolyte ionically conductive. These ions activate the battery by dispersing through the paper, resulting in zinc in the ink at the anode being oxidized thereby releasing electrons. By closing an external circuit, these electrons are transferred from the zinc-containing anode—via the graphite- and carbon black-containing ink, the wires, and the device—to the graphite cathode where they are transferred to—and hence reduce—oxygen from ambient air. These redox reactions (reduction and oxidation) thus generate an electrical current that can be used to power an external electrical device.

Analysis of the performance of a one-cell battery showed that when just two drops of water were added, the battery activated within 20 seconds, Figure 3. After one hour, the one-cell battery’s performance decreased significantly as the paper dried. However, after the researchers added just two extra drops of water, the battery maintained a stable operating voltage of 0.5 volts for more than one additional hour. As a demonstration, the team combined two cells into one battery to increase the operating voltage and used it to power an LCD alarm clock.

Figure 3 Various perspectives on performance of the paper-based, water-initiated battery.

The work is detailed in their readable paper “Water activated disposable paper battery” published in Nature; there’s also a two-page Supplementary Information posting with graphical views of the single-cell battery fabrication process at each step.

Do you see a role for these low-capacity, single-use, easily disposable batteries? Or are they just a clever idea but with limited real-world application?

Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.

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