Another battery story.
A battery made with urea, commonly found in fertilizers and mammal urine, could provide a low-cost way of storing energy produced through solar power or other forms of renewable energy for consumption during off hours.
Developed by Stanford chemistry Professor Hongjie Dai and doctoral candidate Michael Angell, the battery is nonflammable and contains electrodes made from abundant aluminum and graphite. Its electrolyte’s main ingredient, urea, is already industrially produced by the ton for plant fertilizers.
“So essentially, what you have is a battery made with some of the cheapest and most abundant materials you can find on Earth. And it actually has good performance,” said Dai. “Who would have thought you could take graphite, aluminum, urea, and actually make a battery that can cycle for a pretty long time?”
In 2015, Dai’s lab was the first to make a rechargeable aluminum battery. This system charged in less than a minute and lasted thousands of charge-discharge cycles. The lab collaborated with Taiwan’s Industrial Technology Research Institute (ITRI) to power a motorbike with this older version, earning Dai’s group and ITRI a 2016 R&D 100 Award. However, that version of the battery had one major drawback: it involved an expensive electrolyte.
The newest version includes a urea-based electrolyte and is about 100 times cheaper than the 2015 model, with higher efficiency and a charging time of 45 minutes. It’s the first time urea has been used in a battery. According to Dai, the cost difference between the two batteries is “like night and day.” The team recently reported its work in Proceedings of the National Academy of Sciences.
Renewable energy storage
Unlike energy derived from fossil fuels, solar energy can essentially be harnessed only when the sun is shining. A solar panel pumps energy into the electrical grid during daylight hours. If that energy isn’t consumed right away, it is lost as heat. As the demand for renewable technologies grows, so does the need for cheap, efficient batteries to store the energy for release at night. Today’s batteries, like lithium-ion or lead acid batteries, are costly and have limited lifespans.
Dai and Angell’s battery could provide a solution to the grid’s storage problem.
“It’s cheap. It’s efficient. Grid storage is the main goal,” Angell said.
99.7 % efficient
According to Angell, grid storage is also the most realistic goal, because of the battery’s low cost, high efficiency and long cycle life. One kind of efficiency, called Coulombic efficiency, is a measurement of how much charge exits the battery per unit of charge that it takes in during charging. The Coulombic efficiency for this battery is high – 99.7 percent.
Though also efficient, lithium-ion batteries commonly found in small electronics and other devices can be flammable. By contrast, Dai’s urea battery is not flammable and therefore less risky.
“I would feel safe if my backup battery in my house is made of urea with little chance of causing fire,” Dai said.
The group has licensed the battery patents to AB Systems, founded by Dai. A commercial version of the battery is currently in development.
To meet the demands of grid storage, a commercial battery will need to last at least ten years. By investigating the chemical processes inside the battery, Angell hopes to extend its lifetime. The outlook is promising. In the lab, these urea-based aluminum ion batteries can go through about 1,500 charge cycles with a 45-minute charging time.
According to Dai, there is plenty of demand for a grid-suitable battery; he receives numerous emails from firms or individuals interested in developing aluminum batteries. And with the battery now in development, its success rests on the interest of companies and consumers.
“With this battery, the dream is for solar energy to be stored in every building and every home,” Dai said. “Maybe it will change everyday life. We don’t know.”
This research was supported by the Department of Energy, The Global Networking Talent 3.0 Plan, the Ministry of Education of Taiwan and the Taishan Scholar Project. Additional co-authors include Chun-Jern Pan, Youmin Rong, Chunze Yuan, Meng-Chang Lin and Bing-Joe Hwang.
Cyclic Voltammetry and Galvanostatic Charge/Discharge of Al Battery.
The battery cathode was constructed using a graphite powder/polymer binder pasted onto a carbon fiber paper substrate, and the anode was free-standing, high-purity Al foil. AlCl3/urea electrolyte was kept below 40 °C during mixing to avoid electrolyte decomposition. Residual HCl impurities were removed by adding Al foil under heat and vacuum, followed by the addition of ethylaluminum dichloride (SI Materials and Methods). Fig. 1 shows the cyclic voltammogram (CV) of the Al and graphite electrodes in the AlCl3/urea (by mole) = 1.3 electrolyte; the ratio we found yielded the battery with the highest capacity with good cycling stability. We observed several graphite oxidation peaks in the 1.6–2.0-V (vs. Al) range, while another well-defined peak appeared at ∼2.05 V (Fig. 1A). These processes, as well as the corresponding reduction events on the negative sweep, were easily correlated with the galvanostatic charge–discharge curve (Fig. 1C) for a battery with ∼5-mg cm−2 loading of active graphitic material. The redox processes are largely reversible but somewhat kinetically hindered, showing relatively wide peaks (Fig. 1A), most likely as a result of the high viscosity of the electrolyte (3). The deposition/dissolution of aluminum (Fig. 1B) was also quite reversible, but required some cycling to stabilize (Fig. S1). Based on the aluminum stripping/dissolution reaction and chloroaluminate anion intercalation in graphite, battery mechanisms are suggested and illustrated schematically in Fig. 1D.
Fig. 2 shows galvanostatic charge–discharge data for the Al-graphite cell using the AlCl3/urea ILA electrolyte. Initial cycling at 100 mA g−1 required ∼5–10 cycles for stabilization of the capacity and CE, suggesting side reactions during this time. The CE during first cycle was consistently around 90%, and then (during the first 5–10 cycles) increased above 100% until a stable capacity was reached (at which point CE was stabilized at ∼99.7%) (Fig. 2A). The phenomenon of CE >100% is unknown to the EMIC-AlCl3 electrolyte system (7) and therefore might involve side reactions with the cationic aluminum species or unbound urea, a topic requiring further investigation. The boxed region of Fig. 2A (enlarged in Fig. 2B) demonstrates the capacity at varied charge–discharge rate using two different cutoff voltages (2.2 and 2.15 V—chosen based on Fig. 1A CV results), after which cycling at 100 mA g−1 was resumed until ∼180 cycles. A slight decay in CE was observed over this time but it remained >99%. Despite the slight decay in Coulombic efficiency, energy efficiency increased slightly over cycling (due to increasing voltage efficiency), yielding values of 87.8% and 90.0% at specific currents of 100 mA g−1 or 50 mA g−1, respectively. The effects of rate on the galvanostatic charge–discharge curves are shown in Fig. 2C, and reasonable capacities of ∼75 mAh g−1, 73 mAh g−1, and 64 mAh g−1 were obtained at 50 mA g−1 (0.67 C), 100 mA g−1 (1.4 C), and 200 mA g−1 (3.1 C) specific currents, respectively.