(606f) Picoliter-Sized Zn-Air Batteries for Releasable Microscopic Sensors and Robots

The past decade has witnessed a surge of interest in microscopic sensors, robots and computers. With the linear dimensions smaller than 100 μm and the volume entering the picoliter (10^-6 mm³) regime, these devices could be fabricated in enormous quantities, and unlocked applications that were inaccessible for larger devices. To date, the majority of sub-millimeter smart dust and picoliter sensors were either powered by solar cells or did not have internal power sources. However, photovoltaics does not work without light, thus limiting the application in dark environment, during night time or on foggy days. Thus there is a pressing need for integrating batteries with the tiny sensors and robots.

Unlike large-sized batteries that may be produced separately, picoliter batteries must be fabricated together with the electronic parts through photolithography, since the manual assembly is not scalable. Unfortunately, conventional wet chemistry method for battery manufacturing is fundamentally incompatible with microelectronic fabrication process. Furthermore, the usage of liquid organic electrolyte, the high sensitivity of anode material to air and moisture create huge challenges for fabrication.. As a result, picoliter-sized Li-ion batteries have not been integrated with microscopic sensors so far.

In contrast, Zn-based batteries offer unique advantages for integration with photolithography process, such as high stability under ambient conditions, fast rate of deposition, and perfect compatibility with physiological environment. Among all types of Zn-based batteries, Zn-air batteries provide the highest specific energy (~1350 Wh kg^-1) and energy density (>6000 Wh L^-1) in theory. In practice, primary Zn-air batteries also provide an impressive volumetric energy density of 1100 Wh L^-1, much higher than rechargeable Li-ion batteries.

In this work, we have invented a facile route for fabricating and subsequently releasing microscopic Zn-air batteries with cleanroom techniques in massive parallel (10,000 per wafer). Both the Pt cathode and the Zn anode were patterned by conventional photolithography, and deposited by thermal/e-beam evaporation. With linear dimensions ranging from 10 to 100 μm, they represent the first class of full-cell batteries capable of providing electricity to picoliter-sized machines. Picoliter Zn-air batteries working in a neutral aqueous solution (phosphate buffered saline) provided energy density of 760 Wh L^-1 to 1000 Wh L^-1 (3.9 μJ pL^-1) and areal power of 0.15 mW cm^-2, with an open circuit voltage of 1.2 V. Picoliter batteries of different electrode patterns were created, and the in situ observation of discharge process verified the performance acquired with probe station. The simple structure of the cell and the stable chemical properties of Zn make them easy to be integrated with microelectronics. Such on-board energy storage units would greatly extend the accessible operation space and time of microscopic electronic devices.