Thermally drawn rechargeable battery fiber enables pervasive power

Abstract

The increasing demand for mobile computing, communications, and robotics presents a growing need for suitable portable power solutions in non-flat customized electronic devices. Fibers as fundamental building blocks of fabrics and 3D-printed objects provide unique opportunities for developing pervasive multidimensional power systems. The characteristic small diameter (<10⁻³ m) and high aspect ratios (>10⁶) of fibers and expansion of fibers into 2D and 3D power systems necessitate ultra-long lengths to meet the energy specifications of portable electronic systems. Here, we present a Li-ion battery fiber, fabricated for the first time using a thermal drawing method which occurs with simultaneous flows of multiple complex electroactive gels, particles, and polymers within protective flexible cladding. This top-down approach allows for the production of fully-functional and arbitrarily long lithium-ion fiber batteries. The continuous 140 m fiber battery demonstrates a discharge capacity of ∼123 mAh and discharge energy of ∼217 mWh. The scalability and material tunability of these fibers position them for use in varied non-planar electronic systems, including a 1D-flexible electronic fiber, a 2D-large-scale machine woven electronic fabric (∼1.6 m²), and a 3D-printed structural electronic system. The fiber battery satisfies the requirements of portable electronics systems as it is machine washable, flexible, usable underwater, and fire/rupture-safe. We have demonstrated the powering of a submarine drone, LiFi fabric, and flying drone communication through different rechargeable fiber battery schemes, which paves the way for the emergence of the pervasive battery-powered electronics.

Introduction

Innovation in the form factor of electronics continues to push the limits of applications for electronic systems. Currently, advanced battery-powered electronics can be foldable, flexible, and rollable but the battery component is still rigid and bulky. Unconventional batteries that would be mechanically flexible, compact, and mass-produced without sacrificing energy storage capabilities are needed for the realization of innovative electronics in new form factors. Fibers offer a unique platform for building these advanced compliant power systems in a bottom-up manner. They are ubiquitous and fundamental building blocks in our daily life; we find them in fabrics we wear, in the vehicles we drive, and in the homes we live in. Long lengths of fibers can be integrated into two-dimensional fabrics or be the feeding material to construct three-dimensional objects. These opportunities motivate the development of a fiber battery that can power a large suite of non-planar electronics of varying architectures spanning from 1D to 3D power sources.

The goal of realizing energy storage in fibers has been the focus of a number of previous publications [1], [2], [3], [4], [5], [6], [7], [8], [9]. The small diameter (<10⁻³ m) of fibers requires extended fiber battery lengths (>10² m) in order to provide similar capacities to conventional batteries. However, outstanding fundamental challenges have limited the lengths of the functional fiber devices from previous works to the centimeter-scale [10]. Reliable fiber battery fabrication should involve materials and an architecture that promote fast ionic transport in the transverse direction and electronic transport in the axial direction so that the electrochemical performance is not hindered at narrow fiber thicknesses or long fiber lengths. In the transverse direction, the presence of interfacial gaps, especially between the electrolyte and electrode, reduces the rate of ionic transport across the fiber. The use of carbon-active material composites as electrodes in previous work lead to high electronic resistance in the axial direction, causing the length-normalized performance to decrease as fiber length increases [10]. In addition, the fiber battery should exhibit environmental stability, water resistance, and robust meachanical properties in order to realize its variety of applications.

The preform-to-fiber thermal drawing approach for fiber production stands out as an intrinsically scalable fabrication method – with more than 100 million kilometers of optical fiber produced every year using the same method [11]– while offering the ability to combine multiple materials into complex fiber architectures [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. In this method, the constituent materials are assembled into a preform of the desired architecture. The cross-sectional geometry of the preform is preserved as it is heated above its melting point and scaled down into fiber. Recently, an ultra-long electric double layer capacitor (EDLC) fiber with promising electrochemical performance was successfully prepared by thermal drawing of multiple gels [22]. Although EDLC enables high power densities, the energy storage mechanism, which is based on the physical separation of charges, results in inherently low energy densities. Therefore, the use of battery fiber is preferred in situations where high energy densities are needed. Thermal drawing of battery components presents a unique set of challenges. In order to achieve laminar flow so that the cross-section geometry is preserved during the preform to fiber transition, the materials must be carefully selected to have similar viscosities that allow them to flow without mixing. The rheological and thermomechanical properties of active battery materials do not initially appear to be compatible with thermal drawing. For example, commonly produced composite-based electrodes have high particle loadings that increase viscosity and thus inhibit flow. Even with a low-viscosity electrode material, co-flowing all the components of a battery while maintaining physical separation of anode and cathode and guaranteeing intimate contact of both electrodes with the electrolyte is a significant challenge. Furthermore, typical battery active materials either have very high melting points or degrade prior to melting. Lastly, high-temperature processing could trigger chemical reactions and material degradation that would render the battery electrochemically deactivated.

This study describes how these challenges are addressed to produce the first thermally-drawn Li-ion fiber battery which is fully operational across hundreds of meters to power multiple electronics in new form factors. Three different gel components in which each gel is composed of four to six different materials are drawn to achieve segregated domains of anode, cathode, and electrolyte (Fig. 1). The gels are prepared via thermally-induced phase separation (TIPS) whereby the PVDF matrix solidifies from the EC:PC solvent [24]. TIPS is a well-known procedure for producing interconnected porous structures based on the temperature-dependent solubility of the polymer [25], [26]. At an elevated temperature, a homogenous polymer solution is formed, and the polymer and solvent separate upon cooling. The thermal drawing method enables interfacial bonding with minimal gaps between the different domains over the entire length of the fiber as the domains interface in the liquid phase during the draw. Also, the thermally drawn fiber battery incorporates metallic microwires integrated into a conductive polymer in order to preserve axial conductivity while achieving structural integrity and full contact of battery electrode with the current collector. The fiber battery is then utilized for the realization of compliant power sources from 1D to 3D via bottom-up approaches (i.e. fiber-drawing, machine weaving, 3D printing) (Fig. 1). This approach introduces the concept of a pervasive fiber battery for next-generation mobile electronics.