Stretchable, Breathable Wearable Batteries using a Holey Design

Category: Energy
Student: Christine Wu
Table: ENERG1
Experimentation location: Reseach Institution
Regulated Research (Form 1c): No
Project continuation (Form 7): No

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Abstract:

Bibliography/Citations:

Bai, L, et al. “Stretchable microbatteries and microsupercapacitors for next-generation wearable electronics.” Energy Mater, vol. 47, Oct. 2023, p. 386.  DOI.org (Crossref), https://doi.org/10.20517/energymater.2023.31 

 

Cardiovascular Diseases. https://www.who.int/health-topics/cardiovascular-diseases. 

 

Kumar, Rajan, et al. “All‐Printed, Stretchable Zn‐Ag 2 O Rechargeable Battery via Hyperelastic Binder for Self‐Powering Wearable Electronics.” Advanced Energy Materials, vol. 7, no. 8, Apr. 2017, p. 1602096. DOI.org (Crossref), https://doi.org/10.1002/aenm.201602096.

 

Meng, Qinghai, et al. “Combining Electrode Flexibility and Wave‐Like Device Architecture for Highly Flexible Li‐Ion Batteries.” Advanced Materials Technologies, vol. 2, no. 7, July 2017, p. 1700032. DOI.org (Crossref), https://doi.org/10.1002/admt.201700032.

 

Mo, Funian, et al. “An Overview of Fiber‐Shaped Batteries with a Focus on Multifunctionality, Scalability, and Technical Difficulties.” Advanced Materials, vol. 32, no. 5, Feb. 2020, p. 1902151. DOI.org (Crossref), https://doi.org/10.1002/adma.201902151.

 

Mustapha, Rasha, et al. “Modified Upright Cup Method for Testing Water Vapor Permeability in Porous Membranes.” Energy, vol. 195, Mar. 2020, p. 117057. DOI.org (Crossref), https://doi.org/10.1016/j.energy.2020.117057.

 

Pu, Xiong, et al. “A Self-Charging Power Unit by Integration of a Textile Triboelectric Nanogenerator and a Flexible Lithium-Ion Battery for Wearable Electronics.” Advanced Materials (Deerfield Beach, Fla.), vol. 27, no. 15, Apr. 2015, pp. 2472–78. PubMed, https://doi.org/10.1002/adma.201500311.

 

Song, Woo‐Jin, et al. “Recent Progress in Stretchable Batteries for Wearable Electronics.” Batteries & Supercaps, vol. 2, no. 3, Mar. 2019, pp. 181–99. DOI.org (Crossref), https://doi.org/10.1002/batt.201800140.


Additional Project Information

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Research paper:
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Research Plan:

Rationale 

Wearable batteries are batteries specifically tailored to power wearable electronics—devices integrated into clothing. Wearable batteries are typically embedded within wearable electronics, which are embedded in garments. These garments, called e-textiles, have capabilities such as sensory feedback, health monitoring, and illumination abilities. Wearable batteries have enabled the use of electronic devices without being tethered to stationary power sources, enhancing user mobility and expanding the potential for innovative, interactive clothing. 

 

Applications of wearable batteries are vast and diverse. For example, medical monitoring devices such as heart rate monitors, whether in the form of patches or belt devices, require wearable batteries to power. As heart diseases are the leading cause of death globally, taking an estimated 17.9 million lives each year (Cardiovascular Diseases), wearable electronic heart monitors can provide significant benefits for individuals with, recovering from, or at risk of heart conditions. Another innovative application of the wearable battery is clothes with embedded heating. These garments help keep individuals warm in cold climates and extreme physical activity, provide relief for medical conditions, or simply offer added comfort and versatility. Heated clothing can even be engineered to respond dynamically to body temperature, ensuring the body remains at an appropriate level of warmth.

 

Below are some more applications of wearable electronics:

Medical devices (blood sugar monitors, sleep trackers, temperature monitors, posture monitors, knee-cap stimulators)

Fitness trackers (muscle-activity monitors, body-temp monitors, hydration level monitors)

LED embedded clothing (nighttime cycling clothing, halloween costumes) 

Gaming devices (motion trackers for immersive experience)

 

A major problem with current wearable batteries that power e-textiles is their non-stretchable, non-breathable nature. As the wearable battery is meant to be integrated in clothing, a lack of stretchability and breathability can cause discomfort and sweat trapping, which is especially problematic in applications that involve a lot of movement. The goal of this research is to design a wearable battery prototype that is stretchable and breathable, while retaining functionality, high energy density, or ease of fabrication. The resulting design, termed the “holey” battery, incorporates a strategic array of holes within a standard pouch cell battery framework. The edges of the holes are properly sealed to prevent leakage. Pouch cells are a type of battery that have a foil casing, allowing for lighter weight and more versatile shapes compared to rigid-cased cells. 

 

The basic architecture of the battery follows that of a standard lithium-ion pouch cell battery to maintain energy density and composition from common battery materials. However, a standard pouch cell battery would offer no stretchability nor breathability, and therefore would not be ideal for integration into clothing. In order to improve the stretchability and breathability of the battery, holes are cut into the battery material. Specifically, holes are cut into the lithium-ion pouch cell battery’s five layers: cathode, anode, separator, and two packaging layers. The inclusion of holes in the battery's layers allows air circulation, mitigating heat accumulation and moisture retention, thus improving breathability. The presence of holes also improves the battery's stretchability and flexibility, contributing to increased comfort for the wearer. 

 

Using the holey battery to power wearable electronics integrated into clothing will allow for greater comfort while maintaining high battery performance. 

 

Procedure Summary 

  • Design a battery prototype
    • Use 3D modeling to model multiple battery designs 
    • Use FEM to choose the design with the most stretchability
  • Create samples of the battery
    • Make the cathode and anode slurries and coat onto foils
    • Align the five layers of the pouch cell battery and seal
  • Test the breathability of the battery
    • Use an upright cup test
  • Test the functionality and stretchability of the battery
    • Test battery capacity after charge-discharge cycles, with and without deformation
    • Test battery functionality after stretching and folding
    • Test battery functionality under physical activity

 

Procedure Details and Results

Design

3D modeling is used to model a multitude of potential hole structure designs for comparisons. Finite Element Method (FEM) is used to determine the battery design that would produce the most stretchability. All modeled battery pouches have a side length of 10 cm, while the areas of the holes are kept the same for holes with different shapes and patterns. 

 

First, pouch cell designs with a single centered hole in the shape of a circle, square and cross are studied. A 3% horizontal edge stretch is applied, and the distribution of the maximum principal strain after stretching, shown with help of FEM, is displayed in figure 1 in the presentation file. Single-hole battery designs exhibit too much strain for 3% stretching. 

 

The edge stretch simulation is therefore extended to a standard pouch cell, a battery pouch with a circular array, a square array, and a designed array. A 7% horizontal edge stretch is applied, and the results are shown in figure 2. As the diagrams show, the designed array exhibits the lowest strain when an edge stretch is performed. The no-hole pouch and the three array battery pouches are further studied. A 10% diagonal stretch is applied to the four designs, and the results are shown in figure 3A. The other three cases start to exhibit tensile instability to accommodate such large stretching, with the no-hole pouch twisting and the circular and square arrayed hole battery pouches undergoing complex bending. However, the designed pouch shows a strain of less than 2% in most regions—a five times reduction compared to the applied stretch. This shows an excellent stretchability for the designed holey battery. Furthermore, figures 3B and 3C show a quantitative comparison of the percentage of area with the maximum principal strain greater than the applied stretch in the four battery designs. For both horizontal and diagonal stretching, the designed pouch shows the lowest percentage area with great strain. After testing a multitude of designs, FEM validates the designed holey battery as the most stretchable. 

 

Fabrication

Creating samples of the holey battery for testing is the part of the project I mainly contribute to. The fabrication process is very similar to the regular fabrication process of a pouch cell, as our design maintains the basic structure and materials of a pouch cell. The battery includes a cathode and anode interspersed with a separator saturated in electrolyte, all contained in sealed packaging. Figure 4 illustrates the design. 

 

To create a sample battery, the electrodes are first prepared using a conventional slurry method. The cathode material LiFePo4 (LFP; 15 mg/cm2) powder is mixed with the conductive additive Ketjenblack powder (<50 nm), and Polyvinylidene fluoride (PVDF) powder as binder, with a mass ratio of 90:5:5. These materials are dispersed in the solvent N-methyl-2-pyrrolidone (NMP, 99.5%) to help liquefy the cathode material into a slurry substance. Similar to the production of the cathode, The anode material uses graphite powder (7 mg/cm2), mixed with Ketjenblack powder and PVDF powder, with a mass ratio of 90:5:5, dispersed in NMP. After stirring the cathode and anode slurries overnight, doctor blade is used to coat the cathode slurry onto an aluminum foil and the anode slurry onto a copper foil. The two electrodes are then dried at 120°C in a vacuum oven for 12 hours. 

 

The separator used is Celgard 2500 membrane. The electrolyte used is 1.0 M Lithium hexafluorophosphate (LiPF6) solution, in 50/50 (v/v) ethylene carbonate (EC) and dimethyl carbonate (DMC) (Battery grade). Laminated aluminum foils are used as the packaging layer. Thus the electrodes, electrolyte, separator, and packaging material are all of traditional battery design and use commercially available materials. 

 

The hole designs are marked and cut through the material-coated foils, separator, and packaging layers using a razor blade. Figure 5 shows a packaging layer being marked and cut. The holes in the packaging layer are 1.5mm smaller on each side than those in the separator, which are again smaller than the holes of the electrodes. This allows for proper sealing of packaging layers around the hole area, and also for the proper isolation of the electrodes by the separator. 

 

The five layers of the battery are aligned. The separator layer is sandwiched between the anode and cathode foils, and these three sheets are sandwiched between two aluminum packaging layers. The edges of the holes are sealed with a modified hot-pressing technique, involving two heat bars rather than one like the standard hot-presser. Figures 4 and 7 show this hot-press technique. The dimensions of the heating bar are slightly bigger than the hole areas so that only the edges of the hole areas are sealed. To avoid electrolyte leakage, the cell is first sealed along three edges, and then the electrolyte is injected. Finally, the fourth side of the cell is sealed in a vacuum sealer hot-press in a glovebox. A sample of the holey battery has been created.

 

Test breathability

 

A breathability test helps ensure the holey wearable battery can be integrated into clothing without causing discomfort through heat accumulation and moisture retention. Breathability test is conducted using an upright cup test, as illustrated in figure 8(Mustapha et al., 2020). Silica gel desiccator is placed inside a jar, and battery samples are sealed to the top of the jars. The jars are placed in an environmental chamber with controlled relative humidity of 70% and temperature of 25°C. The results of the breathability test are summarized in figure 9. The breathability of the holey battery outperforms that of the conventional cotton cloth, with the rate of water absorption of the holey battery being twice as much as that of cotton cloth. Therefore, integrating holey batteries into textile should not compromise the clothing’s breathability and will not cause heat accumulation or moisture retention. On the other hand, conventional pouch cells with no holes exhibit no breathability.  

 

Test stretchability and functionality

 

Functionality tests are conducted to demonstrate the holey battery’s ability to maintain performance under various conditions. Stretchability and flexibility are also tested by assessing the holey battery’s functionality during and after stretching and folding. 

 

First, the holey battery is subjugated to charge-discharge cycles to assess its cycling stability, or ability to maintain energy capacity. Capacity retention exceeds 95% over the span of 100 cycles, indicating that the holey design will not compromise the battery’s ability to supply power for wearable electronics (Figure 9A). Next, the performance of holey batteries under deformation modes are assessed. The holey battery is subjugated to alternating cycles between a free-standing mode and a 10% diagonal stretch mode, each comprising 20 charge-discharge cycles. As shown in figure 9F, even when the battery is stretched, it maintains over 90% of its capacity, indicating that stretching does not hinder the holey battery’s performance. Similarly, the functionality of the holey battery when folded by 180° is assessed. As shown in figure 9G, more than 95% of the battery’s capacity is retained when folded, indicating that folding does not hinder the holey battery’s performance. It should also be noted that the capacity of the battery recovered upon the release of the folding. The battery’s capacity retention can be attributed to the strategic hole patterns chosen by FEM, which minimizes the portion of the battery subjected to significant strain. 

 

Additionally, after 100 stretch/release or fold/release cycles, when the holey battery is stretched diagonally by 10% and folded by 180°, the battery is still able to continuously power LED light bulbs. This demonstrates the battery’s functional capability to provide power even after being stretched or folded many times. 

 

I also help test the functionality of the battery by sewing the holey battery onto a lab coat using cotton yarn. A simple circuit connects the battery to a button control and LED lights so that when the wearer of the lab coat presses the button, the battery is able to power the LEDs. As the wearer of the lab coat tests various physical activities such as running, the battery is again able to continuously power the LEDs (Figure 10). The ability for the holey battery to power LEDs in static and dynamic states suggests its suitability for use in wearable devices. 

 

Risk and Safety

The procedure contains the usage of dangerous machinery and chemicals, such as the hot press and graphite powder. All of the research is done in Professor Liangbing Hu’s Center for Materials Innovation Lab at University of Maryland College Park. Lab coats, goggles, and if appropriate masks are mandated in the lab. Prior to entering the lab, I went through required safety training and testing. Safety meetings are also held by the lab’s safety manager for reminders and feedback.

Questions and Answers

1. What was the major objective of your project and what was your plan to achieve it? 

    a. Was that goal the result of any specific situation, experience, or problem you encountered?  

    b. Were you trying to solve a problem, answer a question, or test a hypothesis?

The problem my project solves is the discomfort of e-textiles, due to a lack of stretchable and breathable wearable batteries. The comfort of e-textiles is especially crucial for medical electronic devices that need to be worn consistently. To allow for further development and innovation in e-textiles, it is imperative to ensure the comfort of these textiles by ensuring the comfort of wearable batteries. 

2. What were the major tasks you had to perform in order to complete your project?

    a. For teams, describe what each member worked on.

There were a total of ten contributors to this project. I mainly worked with a postdoc, phD, and another intern on the fabrication of the pouch cells. Others completed tasks such as designing the experiments, 3D modeling the batteries, conducting electrochemical and physical tests, and writing the research paper that has been submitted. Though I focused on the fabrication of pouch cells, I was lucky enough to learn about and try a bit of each of these steps in the research process. I completed tasks in the other steps in the wholistic research process as well, such as researching other wearable battery models, using 3D modeling to compare the stretchability of the holey design with other pouch cell designs, sewing the pouch cell onto the lab coat for a functionality test, photographing the functionality tests, designing certain figures in the paper, and contributing to the editing of the paper. 

3. What is new or novel about your project?

    a. Is there some aspect of your project's objective, or how you achieved it that you haven't done before?

    b. Is your project's objective, or the way you implemented it, different from anything you have seen?

    c. If you believe your work to be unique in some way, what research have you done to confirm that it is?

Other designs have been developed with aims of addressing challenges for wearable battery technology. Initial studies focused on using flexible substrates in battery fabrication. For lithium-ion batteries, substrates refer to the surface on which the cathode materials and anode materials sit. The work of Meng et al. utilized a Cu-deposited conductive nonwoven cloth as the substrate material with a wave-like device architecture to achieve electrode flexibility sufficient for use on curved surfaces like watch straps (Meng et al.). The work of Pu et al. achieved a flexible lithium-ion battery belt using Ni-cloth textile as a substrate (Pu et al.). The battery belt exhibited decent electrochemical performance even when bent, ensuring the feasibility of wearing it as a waist belt, etc. However, these materials, while flexible, are not inherently stretchable, limiting their use in situations involving significant strain, such as active sports. 

To overcome this, elastic materials for substrates and battery packaging have been explored. Kumar et al. utilized poly-styrene-block-polyisoprene-block-polystyrene (SIS) as a hyper-elastic binder, enabling the battery to accommodate for substantial elastic strain experienced during vigorous activities (Kumar et al.). However, despite this improvement in stretchability, the use of elastic substrates often requires inactive additives in the electrode materials to maintain performance under stretching, which reduces the overall energy density of the battery (Bai et al.). Moreover, both flexible and elastic battery designs require sealing of the entire surface, which compromises their breathability. 

A promising solution has been the development of fiber-shaped batteries. This innovative design typically uses conductive wires like metal as substrates for electrode materials. Anode and cathode wires intertwine and are sealed with polymer packaging (Song et al.). Since these batteries mimic the geometry of yarns, they can be directly woven or knitted into clothing, offering similar deformability and breathability. However, this design is not without its drawbacks. The inclusion of numerous inactive materials leads to a lower energy density. Furthermore, while these fiber-shaped batteries are bendable and twistable, their stretchability is limited due to the rigidity of the metal core, posing a challenge for certain applications. Finally, because the fabrication of fiber-shaped batteries is complex, requiring unique manufacturing procedures and equipment, large-scale commercialization is compromised (Mo et al.). 

The FEM chosen holey battery, which is able to have stretchability and breathability while maintaining energy density and ease of fabrication, is a novel invention. 

On a personal note, this is the first time I’ve gotten real lab research experience. I’m very grateful to Professor Hu for having me as an intern and Dr. Lin Xu for guiding me through a complete science research process, and also for answering the many questions I’ve had about this project.

4. What was the most challenging part of completing your project?

   a. What problems did you encounter, and how did you overcome them?

   b. What did you learn from overcoming these problems?

Cutting holes in the materials was tedious. For each pouch cell, there were five layers we had to align: two packaging layers, one cathode layer, one anode layer, and one separator layer. For each layer, I first used a template to trace the edges of where the holes should be, then use a razor to cut out the holes. Because the holes are about 0.5cm in width, I had to be very precise when cutting. Additionally, since there were five layers to the pouch cell, the cutting had to be extra precise so that when the layers are stacked, the holes align and the pouch seal can be properly sealed. There were many times where I dragged the razor too far, cutting too much of the material and needing to restart. The first time successfully cutting all the layers took me an entire morning and afternoon. A trick I discovered to minimize errors is to start the razor stroke at the middle of the hole, dragging it towards each end, then slowly angle the razor to cut the ends. This helps minimize error, since starting and ending the razor path is the most unstable. 

Additionally, stacking the five layers and sealing them with the hot press was similarly difficult and required great precision. Many batteries that survived the razor-cutting perished at the hot press.

5. If you were going to do this project again, are there any things you would you do differently the next time?

I would have to brainstorm a smarter way to cut and seal the batteries. Perhaps purchasing automated machinery would help. 

6. Did working on this project give you any ideas for other projects? 

Something to try in the future is scaling up the holey battery so that it covers a bigger surface area. For example, creating a wearable battery that covers the entire body. Something else to try is stacking multiple layers of electrodes in the battery to further increase energy density. 

7. How did COVID-19 affect the completion of your project?

My project was not affected by COVID-19.