High-Energy-Density High-Power-Density Micro Aluminum-Air Batteries for Small Scale Quadcopters
Authors: Huang Yanghang , Lyu Haoxuan , Allen Mark George , Bidstrup Allen Sue Ann 
Published: August 2024

Small-scale quadcopters are increasingly used in various fields, including rescue, surveillance and precision agriculture. The predominant energy storage system for such quadcopters has been the micro lithium-ion battery (mLIB). Two of the most important design metrics for power sources used in quadcopters are specific power and specific energy. The specific energy determines the flight duration of the quadcopters, while specific power is necessary for maneuverability and takeoff. The mLIB can easily meet the power requirement of small scale quadcopters; however, the limited energy density of mLIBs (exacerbated by the packaging overhead of these batteries as scales shrink) constrains their flight duration. To increase the flight time, aluminum-air batteries (AABs), with a theoretical energy density twenty times greater than conventional lithium-ion batteries, are being explored. [1] Despite AAB’s high theoretical energy density, the conventional AAB lacks the high-power performance of LIBs. [2] To fulfill the demanding power requirements of small scale quadcopters, the AAB cell has been re-engineered to enhance power capability while maintaining high energy density.
To maximize AAB energy and power density at the cell level, it is imperative to address the issue of battery packages that add weight without contributing to electrochemical performance. A light-weight battery packaging approach is developed for the micro-AAB (mAAB). In the context of AABs, the primary function of the battery package is to retain the aqueous electrolyte within the cell. Typically, the cathode of an AAB is constructed using a carbon cloth, doped with a layer of polytetrafluoroethylene (PTFE). This hydrophobic carbon cloth cathode effectively retains the aqueous electrolyte. In order to increase the mechanical integrity of mAAB pouch cells, 3D printed polypropylene is utilized as the body frame of mAABs, providing a robust and lightweight structure. The carbon cloth cathode is affixed to the 3D printed polypropylene using epoxy, as shown in Figure 1A. An image of an assembled 2-gram mAAB pouch cell is shown in Fig 1B. Remarkably, the electrochemically inactive packaging materials contribute to less than 13% of the total cell weight, achieving the goal of minimizing packaging material to enhance battery performance. As a comparison, the packaging material of gram-scale mLIBs can contribute to over 30% of total cell weight. Furthermore, in contrast to traditional aluminum-air battery designs, the pouch cell architecture optimizes cathode area exposure, which is advantageous for high-power applications.
The anode structure is also engineered for high power capability. Figure 2A shows that increasing the anode/cathode area ratio can augment areal power density. Therefore, a multi-layer anode structure, supported by 3D printed polypropylene, is developed for mAAB pouch cells. This design effectively increases the anode/cathode surface area ratio and enables rapid ion and electron transport within the multi-layer anode, optimizing power performance. A schematic of a 3D printed polypropylene supported multi-layer anode structure is shown in Figure 2B.
The electrochemical performance of assembled mAAB pouch cells is evaluated to assess their suitability for the demanding requirements of small scale quadcopter applications. Figure 3A provides a comparative analysis of power densities between mAABs and commercial micro-quadcopter mLIBs. Notably, mAABs exhibit a peak power density exceeding 1500 Wh/kgbattery at 4 Amp, surpassing that of mLIBs. Battery weight encompasses anode, cathode, electrolyte and all necessary battery packaging, confirming that mAABs can meet the power demands of micro-quadcopters. In Figure 3B, the energy density of mAABs is compared to commercial mLIBs. When the power density is above 500 W/kgbattery, a threshold sufficient to lift a quadrotor drone, mAABs exhibit an energy density of 320 Wh/kgbattery, which is multiple times higher than that of mLIBs. [3] Notably, the anode utilization of the mAAB exceeds 85%, underscoring the effectiveness of the multi-layer anode design. This superior electrochemical performance emphasizes the superior capability of mAABs, indicating their potential as an advanced power source for long flight duration small scale quadcopters.
References:

Li, Yanguang, and Jun Lu. "Metal–air batteries: will they be the future electrochemical energy storage device of choice?." ACS Energy Letters 2.6 (2017): 1370-1377.

Cao, Ruiguo, et al. "Recent progress in non‐precious catalysts for metal‐air batteries." Advanced Energy Materials 2.7 (2012): 816-829.

Mulgaonkar, Yash, et al. "Power and weight considerations in small, agile quadrotors." Micro-and Nanotechnology Sensors, Systems, and Applications VI. Vol. 9083. SPIE, 2014.

Figure 1