When designing a device, it is important to plan for the appropriate size of the battery. Without this advanced planning, the battery development team may face unrealistic expectations when it comes to how much space they have. The device developers must have an understanding of thin film batteries and what dimensions are reasonable to expect for the final product.
As a clarification, the term thin film in this blog post is not a reference to batteries made using a thin film deposition process like PECVD or thermal evaporation. The definition that this article will be using is simply batteries that are thinner than traditional batteries and made using conventional methods like printing and coated.
Active vs. Inactive Volume
The volume usage of a battery can be divided into two categories: active and inactive. The active portion of the battery consists of materials and parts that allow the battery to function chemically. The inactive portion is not essential for the chemical function of the battery. Instead, it allows for the battery to function properly as a power source.
Elements of the battery such as the container or packaging, empty space (void volume) within the battery, and safety features including ring-shaped seals on opposing terminals are all considered inactive. While they are crucial to the function of the final product, these elements do not necessarily contribute to the chemical reactions within the battery.
The active volume includes the active chemicals needed to supply power, the electrolyte, the conductive additive(s), the electrode current collectors, and the separator. Each of these parts is necessary for a functioning battery on electrical and chemical levels, and sacrifices in the thickness of any of these elements will generally result in tradeoffs when it comes to actual battery performance.
Thickness by Internal Battery Design
This battery configuration involves stacking the two electrodes vertically, thus increasing the thickness of the total battery. This method can be helpful, however to increase the energy density of the battery and minimize the inactive volume. It is worth considering if a battery has more thickness to work with.
The other configuration does not stack the electrodes. Instead, it places them side by side on the same film. This configuration allows the battery developer to reduce the thickness of the finished product by the thickness of one electrode and sometimes the separator as well, as they are no longer stacked vertically but on the same plane. There is a loss of energy density with this configuration because the space between the electrodes is greater, but it is worth considering this configuration as a means of cutting down the battery’s thickness if this is a key parameter.
Coplanar (top) and cofacial (bottom) battery configurations.
Thickness by Battery Part Design
For each part of the battery, there is an engineering frame for an expected thickness. These engineering frames are in no way set in stone; it may be possible to build certain parts thinner that what is commonly accepted, but these alterations may offset part of the battery function or potentially limit the thickness of other battery parts. Furthermore, pushing the limits of a component’s thickness could make the design cost prohibitive or difficult to scale. The battery needs to be able to handle the manufacturing process as well, so the design must account for the shock and movement that the battery will undergo. Establishing a minimum abuse tolerance and tailoring the design to meet it is a necessity.
Starting with the parts that account for active volume, the electrodes are going to be 25-50 microns each. The anode and cathode current collectors each take up 10-15 microns worth of thickness; this restraint is mostly due to the increased difficulty in handling and coating thinner components. The separator is 25-50 microns because anything thinner will compromise the mechanical integrity of the battery. The only inactive volume that needs to be considered in the thickness of the battery is the packaging, which ranges from 20-100 microns in total, accounting for each side. Anything less than this and the packaging becomes difficult to handle.
The total thickness range comes out to be 115-280 microns, accounting for two electrodes, if the battery uses a cofacial configuration. If the battery, however, is created using a coplanar configuration, the range is 90-230 microns.
Thickness by Assembly Process Selection
The approach that a designer takes for battery construction has a significant impact on the final product in terms of thickness and function. A general tradeoff is that the greater the emphasis placed on reduction in size, the greater the sacrifice of capacity will have to be. While this post does not go into detail about this process, vacuum processed thin film batteries can be deposited onto flexible substrates at thicknesses of under 10 microns.
Each of the following four methods is a way to combine the chemically active elements of the electrode with the current collector.
These materials that provide the chemically active portion of an electrode are a few nanometers for at least one of their dimensions. To clarify, 1,000 nanometers are equal to a micron, and 1,000 microns equal a millimeter, so the parts could be, for example, .000005 millimeters thick. These materials are the smallest and thinnest options, and often come in the form of ultrafine powders such as carbon-nanotubes.
This method involves firing ions at a target material that will coat the electrode’s collector. This material is then released as a result of the ions and it travels onto the collector to create a thin film of chemically active materials. The conditions for this method are very precise, and the resulting layer provides a chemical function to the non-chemically active collector. A film created from sputtering will be less than one micron and in the tens of nanometers. In millimeters, it could measure at .00005, for example.
Printing is a method of deploying a film on the electrodes’ collectors and is a middle ground of the four methods listed, at a range of 10-20 microns. There are several methods under this category, including screen printing and stencil.
Screen printing involves masking certain areas of a woven material to create a specific pattern and forcing ink through the fabric onto the substrate. Another method is stencil printing, which is a process where stencil materials are pre-cut to form(s). The stencils are laid down onto the electrode substrate and are formed by filling the gaps with ink. Battery developers can also utilize a few other methods, including flexographic and rotogravure printing.
4. Coated Laminated Electrode
This means of providing the chemical elements of the electrode involves a lamination process on the substrate, in this case the collector with a film created from a formulation of active chemical. The doctor blade method is often chosen for large-scale manufacturing. It uses a blade to uniformly control the size of the gap between the substrate and the pool of ink being used as a film. The methods to coat also include slot-die, a precise and adjustable metering of the chemical onto the substrate at a uniform height by pushing a reservoir of your active material through a predefined slot. This coating on the metallic substrate is an increase in capacity for the battery, but the cost is greater thickness, ranging from 70-400 microns.
The answer to exactly how thin a thin-film battery can be is not a simple one. It involves considering tradeoffs of capacity, volume, and power and consideration of different configurations and manufacturing processes and assembly methods. With each set of desired characteristics of a thin film battery comes a different answer to how thin it can be, but the uncertainty highlights the in-depth customization that thin film batteries allow.