The recent explosion in technology has led to the development of devices that would have been unfathomable just a few decades ago. Many of these technological advancements are enabled by the miniaturization of electronic components. Microscale devices can be used for a host of applications ranging from portable and implantable medical devices to wireless sensors.

Unfortunately, the development of functional microscale devices has been stalled by difficulties in miniaturizing energy storage to match. The high energy and high power density required for most applications is difficult to achieve in microbatteries due to their size and footprint restrictions. Though scientists have been researching a variety of possible workarounds, few functional microbatteries have been developed; the majority of the existing microbatteries designs simply cannot be manufactured easily.

A team of researchers has now fabricated microbatteries containing microelectromechanical and complementary metal-oxide-seminconductor (CMOS) devices using a futuristic 3D fabrication route. By combining 3D holographic lithography with conventional photolithography, the scientists demonstrated increased control of the electrode structure and spatial arrangement.

In general, the term "lithography" denotes a process that creates defined patterns from a variety of starting materials. Holographic lithography allows the construction of 3D lattices with a defined periodicity; the photolithography outlines a set of 2D solid structures that divide the 3D lattices, forming an interdigitated pattern.

For the new batteries, 3D holographic lithography was used to define a lattice with a typical periodicity of ~1 µm and ~40 percent porosity. This lattice was created on glass coated with indium tin oxide using four interfering laser beams that were arranged in an umbrella geometry. The 3D holographic structure was then infiltrated with a positive photoresist using conventional 2D photolithography, which defined the microbattery electrode distribution.

This second resist of the electrodes consisted of solid straight walls inside the 3D lattice. Nickel was then deposited through the porous lattice. After that, oxygen was used to remove the photoresist template; finally, active materials for the cathode and anode were independently plated onto the battery construct.

These active materials are critical to the function of a battery; when a device is hooked up to both ends of a battery, a current is established within the device through the transfer of electrons from the anode to the cathode via a series of chemical reactions. Lithiated MnO 2 was used as the cathode and Ni-Sn was used as the anode. The anode was independently cycled versus lithium metal six times before fresh electrolyte was cast on the micro battery, and it was capped using a silicon polymer. Using this method, the batteries were stable in air for several days, although they weren't tested for longer periods.

The battery itself was only 2 mm on a side and roughly 10 µm thick, and all its components were tiny. It contained a current collector consisting of a 4-mm2 area interdigitated 3D porous nickel scaffold. Individual electrodes had a rectangular cross-section that was about 10 µm high with a porosity of 60 percent. The electrodes were 35 µm wide with 15 µm spacing.

The packaged microbattery was tested between 3.2 and 1.4 volts. The microbattery was charged in an hour and discharged for various times. The batteries exhibited energy and power densities up to 6.5µWh cm-2 µm-1 and 3,600µWh cm-2 µm-1. (Given the radically different sizes, these numbers aren't really comparable to those of larger batteries currently on the market.)

Practical microbattery applications would require a cycle life of several hundred charge-discharge cycles. In this construct, the capacities of the cathode and anode are carefully matched. The careful control of the Ni-Sn cycling range, when added to the 3D Ni scaffold, improves the cyclability. The scientists think that the scaffold relieves the cycling-induced stress in the film. The microbatteries retained 80 percent or more of their initial capacity after cycling 100 times at varied rates; they also retained almost 90 percent of the initial capacity after driving an LED for 200 cycles.

These studies demonstrate that this new fabrication technique shows promise for the future development of functional microbatteries. Once put into practice, it could open the door for a new generation of electronics.

PNAS, 2015. DOI: 10.1073/pnas.1423889112 (About DOIs).