A 3D-printed flexible potassium-ion microbattery with a conductive polymer coating delivers ultrahigh power density and thousands of stable cycles in a stick-and-use form factor.
(Nanowerk Spotlight) Miniaturized electronics demand miniaturized power sources, but shrinking a battery to the scale of a microchip creates problems that go far beyond geometry. Planar electrode designs lose usable surface area as dimensions decrease, limiting both the energy a device can store and the rate at which it can release it.
The chemistries most commonly used in microbatteries add further complications: lithium, sodium, and zinc anodes suffer from uneven metal deposition, continuous side reactions, and the growth of needle-like dendrites that can short-circuit a cell.
One way to sidestep these failure modes is the rocking-chair configuration, in which ions shuttle between two host electrodes rather than plating onto a metal surface. Among candidate ions for such a system in aqueous electrolytes, potassium stands out. It has the smallest hydrated radius and the highest electrical conductivity of any common alkali cation, properties that favor fast charge transport. Yet building a practical potassium-ion microbattery has remained difficult because suitable electrode architectures and microscale fabrication methods have been lacking.
The work pairs direct ink writing, a form of additive manufacturing in which functional inks are extruded through a fine nozzle to build structures layer by layer, with electrochemical deposition of a conductive polymer coating. The result is a three-dimensional electrode architecture that delivers both high energy and high power in a working area smaller than a fingernail.
(a) Fabrication processes of the microbatteries, along with the digital photographs of its (b) single-layer and (c) multi-layer. (d) TEM images of discharged CoHCF@PEDOT. (e) DFT calculation on the efficacy of PEDOT coating layer and its charge density difference plots. (Image: Reproduced with permission from Wiley-VCH Verlag) (click on image to enlarge)
The device combines two complementary electrode materials chosen for their different potassium storage mechanisms. The cathode uses cobalt hexacyanoferrate, a compound whose open, cage-like crystal framework contains pockets where potassium ions can be inserted and extracted. The anode uses PTCDA, a commercially available organic pigment whose layered molecular structure stores potassium ions both by forming chemical bonds at its oxygen-containing reactive sites and by trapping ions between its molecular planes through electrostatic attraction.
Carbon nanotubes are woven into the cathode ink during synthesis to form an internal conductive network, and a biopolymer gel electrolyte based on xanthan gum seals the cell. Because the electrolyte is aqueous, the device operates safely even when fully exposed to air, eliminating the need for elaborate sealed packaging.
The critical innovation is a thin coating of PEDOT, a conductive polymer, applied to the cathode by electropolymerization, a process in which an electric current drives polymer formation directly on the electrode surface. This coating serves two purposes at once. It shields the cathode from water damage by halving the energy with which water molecules bind to the surface, creating a hydrophobic barrier that preserves the crystal structure.
It also boosts electrical conductivity: electrons transfer from the sulfur atoms in the polymer to the nitrogen atoms in the cathode’s cyanide bridges, establishing continuous conduction pathways absent in the bare material.
The consequences for durability were stark. Without the coating, cathode particles fused and aggregated after repeated cycling, a sign that cobalt ions were dissolving from the electrode. The coated version maintained a dispersed particle structure and showed far less volume fluctuation. In situ Raman and infrared spectroscopy tracked the electrochemical reactions in real time, revealing highly regular, periodic spectral changes that confirmed excellent reversibility of potassium-ion insertion and extraction. The coated cathode retained more than 76% of its capacity after 10,000 charge-discharge cycles.
Assembled into a complete single-layer microbattery, the device delivered an areal capacity of 1.65 mAh cm⁻² and a power density of 50.2 mW cm⁻², several orders of magnitude above many previously reported aqueous microbatteries and micro supercapacitors. It sustained 3,000 cycles while retaining over 95% of its capacity.
The three-dimensional printing approach also enables vertical stacking to boost energy output. Because the working unit occupies a square just 3.5 mm on a side, multiple electrode layers can be printed on top of one another within the same footprint. A five-layer stack reached an energy density of 5.9 mWh cm⁻², more than sixteen times that of commercial thin-film batteries. Eight devices connected in series continued to perform under bending, confirming the system’s mechanical resilience.
To demonstrate practical integration, the researchers attached their microbatteries to a miniature car. Coated with the bio-gel electrolyte and activated, the car ran with stable, sustained power without complex wiring or enclosures.
The team also built a skin-mounted patch combining the microbattery with a micro pressure sensor. When pressed, the patch produced periodic current signals, illustrating potential use in wearable health monitoring for applications such as heart rate or blood glucose tracking.
The stick-and-use form factor removes the need for custom integration hardware, while the stackable architecture lets designers tune energy output without changing the device footprint. Remaining challenges include scaling the direct ink writing process for high-volume manufacturing and extending cycle life at the full-device level to match the cathode’s standalone durability. If those gaps can be closed, these potassium-ion microbatteries could serve as a practical power platform for medical implants, wearable sensors, and autonomous microrobots.
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