Microbatteries much smaller than a grain of rice for a smart dust future
(Nanowerk Spotlight) 'Smart dust' is a vision of the networked future where intelligent networks of trillions of minuscule sensors continuously feel, taste, smell, see, and hear what is going on in their surrounding environment, communicate with each other and exchange information. Smart dust networks are the ultimate Internet-of-Things (IoT) devices (read more about what smart dust is and what it does here in our primer).
One of the challenges of realizing smart dust concepts, as well as nano- and microrobotics in general, is a lack of equally small on-chip power sources for ubiquitous anytime anywhere operation.
One solution would be energy conversion systems harvesting external energy such as micro-thermoelectric or triboelectric nanogenerators, or on-chip photovoltaics (read more: "Solar-powered smart dust"). However, these systems tend to be dependent on specific times and locations, greatly limiting on-demand operation of smart dust and microrobots in many environments.
Another solution would be to provide the smart dust chip with on-board energy storage, i.e., a battery. Already, researchers have demonstrated that microbatteries that are smaller than a grain of salt can be produced in large quantities on a wafer surface and are suitable for powering dust-sized computers.
"Current limitations in fabrication strategies mean that on-chip microbatteries cannot achieve high energy density and small footprint simultaneously," Dr. Minshen Zhu from Prof. Dr. Oliver G. Schmidt's research group at TU Chemnitz, tells Nanowerk. "In contrast, the most successful design in the bulky battery world is to comprise many layers of the electrode material into a limited volume. For instance, Tesla is using so-called Swiss roll cylinder batteries for its electric cars."
However, Tesla's cylinder batteries have a diameter of 1.8 cm – much too large for being integrated with microscale systems.
"So far, no techniques have been available for the realization of a Swiss roll battery with a diameter of hundreds of micrometers on a chip" Zhu points out.
In their latest work, recently reported in Advanced Energy Materials ("A Sub-Square-Millimeter Microbattery with Milliampere-Hour-Level Footprint Capacity"), Zhu and his colleagues from Schmidt's group report on the creation of an on-chip micro-Swiss-roll battery by using a self-assembly process known as micro-origami. In this process, a flat actuator layer stack made of a non-swellable polyimide film and swellable hydrogel layer rolls up a thin metal layer current collector (illustrated below).
Swiss rolls in cylinder batteries and concept/realization of a micro-Swiss roll. Schematic illustrations of a) the cross-section of a cylinder battery, b) a centimeter Swiss roll and micrometer Swiss roll (micro-Swiss roll) used for battery electrodes, and c) parallel fabrication of micro-Swiss rolls on a wafer by the micro-origami technique. The inset shows the interdigital pattern of the current collector in the micro-Swiss roll. d) Image of a wafer with six micro-Swiss rolls. Size comparisons of the micro-Swiss roll with e) a grain of rice and f) a millimeter-scale resistor. (Reprinted with permission by Wiley-VCH Verlag)
The self-assembly mechanism allows for the parallel fabrication of multiple micro-Swiss rolls on the wafer in a single run. One roll is 3 mm in length and about 178 µm in diameter – much smaller than a grain of rice.
"One major factor restricting the energy density of microbatteries is the limited choice of electrode materials because materials used for on-chip microbatteries are mostly obtained by deposition tools," explains Zhu. "In contrast, electrode slurries containing high-capacity materials, binders and conductive additives offer good stability, high conductivity and excellent energy storage, and therefore are used for energy-dense bulky batteries."
A problem with conventional electrode slurries is that they require long drying periods (more than 10 hours) at temperatures above 120 °C and in vacuum – which tends to destroy the microstructure of the microfabricated material layers.
The team got around this problem by creating a fast drying (1 hour) electrode slurry by dispersing MnO2 nanowires in a zincophilic binder (polyimide). Polyimide forms an active electrode???electrolyte interface that improves the zinc ion transportability and prevents MnO2 dissolution.
MnO2 Swiss-roll based microbattery. a) Scheme of a microbattery with a Zn wire and a MnO2 Swiss-roll microcathode. SEM images of b) a bare Swiss-roll microelectrode, c) a MnO2 Swiss-roll microelectrode, and d) its cross-section. (Scale bar, 300 µm (b), (c); 1 µm (d)). (Reprinted with permission by Wiley-VCH Verlag)
Paired with a 250 µm long zinc wire, the electrode footprint of this on-chip microbattery enters the sub-millimeter regime (∼0.75 mm2). The footprint capacity reaches up to 3.3 mAh cm???2. Zhu notes that 150 stable cycles with a footprint capacity of more than 1 mAh cm???2 are attainable – "this performance outperforms microbatteries with an electrode footprint of less than 10 mm2 to date."
"Our work offers a new technology to create on-chip microbatteries and is compatible with both on-chip processes (lithography, etching, etc) and battery fabrication protocols (synthesis of high-performance electrode materials, making electrode slurries and uniform coating on the current collector)," he concludes. "The next stage of this work is to develop a parallel fabrication process for Swiss-roll microbatteries, which is an essential step towards their commercialization for actual applications."