Ryan Wartena

Stamped microbattery electrodes based on self-assembled M13 viruses.

The fabrication and spatial positioning of electrodes are becoming central issues in battery technology because of emerging needs for small scale power sources, including those embedded in flexible substrates and textiles. More generally, novel electrode positioning methods could enable the use of nanostructured electrodes and multidimensional architectures in new battery designs having improved electrochemical performance. Here, we demonstrate the synergistic use of biological and nonbiological assembly methods for fabricating and positioning small battery components that may enable high performance microbatteries with complex architectures. A self-assembled layer of virus-templated cobalt oxide nanowires serving as the active anode material in the battery anode was formed on top of microscale islands of polyelectrolyte multilayers serving as the battery electrolyte, and this assembly was stamped onto platinum microband current collectors. The resulting electrode arrays exhibit full electrochemical functionality. This versatile approach for fabricating and positioning electrodes may provide greater flexibility for implementing advanced battery designs such as those with interdigitated microelectrodes or 3D architectures.

Ultrahigh‐Energy‐Density Microbatteries Enabled by New Electrode Architecture and Micropackaging Design

Progress in microfabrication technology has enabled increasingly compact autonomous microsystems for applications ranging from distributed sensing and communications networks to implantable medical devices. Yet, power sources to enable their widespread adoption have not advanced nearly as rapidly. While batteries remain the most practical form of portable power given their simplicity and reliability, the energy density of commercial technologies decreases rapidly with size, reaching impractically low values well before volumes of interest to microtechnology, for example, <0.1 cm, are reached. This trend corresponds to a rapidly diminishing volume fraction of active materials with decreasing cell size due to internal inactive components and external packaging. Consequently, microsystems can be dwarfed by the battery attached. These wellrecognized limitations have spurred research into alternatives for storing or generating power in small volumes, including miniaturized fuel cells and combustion turbines. Several microbattery technologies have also emerged, with thin-film solid-state batteries being the most developed. While thin-film, solid-state batteries deliver excellent capacity retention over thousands of cycles, they possess very low volumetric energy densities. Electrode thicknesses limited to several micrometers by room-temperature values of the lithium diffusion coefficient result in a 2D geometry dominated by the substrate and other inactive cell components, thus, the performance of thin-film batteries is typically characterized in terms of energy or power per area, rather than per volume or mass. Although areal metrics are appropriate for some uses, many developing applications (e.g., wireless transmission) require small batteries with high volumetric energy and power in nonplanar form factors. Progress in this direction includes reduction of the footprint of thin-film batteries through 3D architectures enabled by advanced microfabrication. However, to our knowledge, fully packaged batteries with demonstrated energy densities exceeding 100WhL 1 in packaged volumes <100mm have not been reported. Microbattery technologies remain undermatched in form factor, performance, and manufacturability to most of the devices they are meant to power. Here, we demonstrate lithium rechargeable microbatteries at the cubic millimeter scale providing energy densities normally achieved only in batteries more than 100 times larger in volume. Two specific materials innovations enable this achievement. The first originates from the surprising discovery that densely sintered electrodes fabricated from brittle intercalation oxides can be repeatedly cycled to full capacity utilization, a result that is wholly unexpected given the well-known cycling-induced fracture (‘‘electrochemical grinding’’) that occurs in intercalation compounds electrodes due to the Vegard’s stresses. This discovery allows us to develop thick, sintered 3D cathodes with areal active materials loadings 4–10 times larger than in conventional lithium ion battery electrodes, yet the electrodes can still be cycled at practical rates. The second innovation is an electroformed packaging approach, which further maximizes active material volume and yields cells with a desirable low-aspect-ratio form factor (e.g., 3mm 3mm 0.7mm) similar to existing miniaturized surface-mount components. Using LiCoO2/Li cell chemistry, we produced microbatteries of 6mm volume with unprecedented energy densities for batteries of this size range. For a charge voltage of 4.6 V, these cells deliver up to 675WhL 1 energy density at C/3 discharge rates (150–200WL 1 power density). At this charge voltage, the cycle life of the oxide is limited, but even comparing to existing primary Li-metal batteries, energy densities are achieved that previously were available only at cell volumes more than 100 times larger (>600mm). Using a lower charge voltage of 4.5 V, more stable cycling of the cells was obtained, while producing energy densities of 400WhL 1 that were previously available in existing rechargeable Li cells only at >1000mm volume. Some measured metrics for our microbatteries, all delivered at C/3 or higher rates, are included in the Supporting Information, Table S1. The theoretical storage energy density of an electrochemical couple is obtained by integrating the thermodynamic voltage capacity curve for capacity-matched electrodes and dividing by the maximum volume that the couple reaches during cycling. Intrinsic values for existing lithium-based systems are universally high (>1000WhL 1 for LiCoO2/graphite, LiMn2O4/graphite, and LiFePO4/graphite, and >2000WhL 1 for LiCoO2/Li and MnO2/Li). [13] It is the amount of inactive components, exceeding two-thirds the total volume and half the total weight even in ‘‘large’’ lithium ion cells, that results in poor historical energy densities. These limitations arise from compromises in electrode