U.S., Chinese Scientists Team on Nanomaterials for Rechargeable Batteries
A team of U.S. and Chinese researchers have developed a method to improve the electrical capacity and recharge lifetime of sodium ion rechargeable batteries. The work could present a dramatically cheaper alternative for connecting solar and wind energy sources to the electrical grid.
Heat-treated manganese oxide provides tunnels for sodium ions to flow through, improving the performance of the electrodes.
It was done by a team of scientists from the U.S. Department of Energy's Pacific Northwest National Laboratory and visiting Chinese researchers from Wuhan University in Wuhan. EMSL, the Environmental Molecular Sciences Laboratory located at Pacific Northwest National Laboratory, is a national scientific user facility sponsored by the Department of Energy's Office of Science.
"The sodium-ion battery works at room temperature and uses sodium ions, an ingredient in cooking salt. So it will be much cheaper and safer," said PNNL chemist Jun Liu, who co-led the study with Wuhan University chemist Yuliang Cao.
Researchers wanted to mimic several characteristics of manganese oxide electrodes found within lithium rechargeable batteries. Atoms in manganese oxide form many holes and tunnels that lithium ions travel through when batteries are being charged or are in use. This free movement of lithium ions allows batteries to hold electricity or release it in a current. That said, simply replacing the lithium ions with sodium ions was problematic, mainly because sodium ions are 70 percent larger than lithium ions.
The solution was found in nanomaterials. The team found that the short distances that sodium ions have to travel in nanowires might make the manganese oxide a better electrode in ways unrelated to the size of the tunnels.
To explore this possibility, the team went looking for atomic shapes that might promote ion movement. They mixed two different kinds of manganese oxide atomic “building blocks” one with atoms that arrange themselves in pyramids and another with octahedron-shared atoms.
After mixing, the team treated the materials with temperatures ranging from 450 to 900 degrees Celsius, then examined the materials and tested which treatment worked best.
Using an electron microscope, the team saw that manganese oxide heated to 600 degrees had pockmarks in the nanowires that could impede the sodium ions, but the 750 degree-treated wires looked uniform and very crystalline. In testing the nanowires, the team recorded peak capacity at 128 milliAmp hours per gram of electrode material as the experimental battery cell discharged.
The 750 degree-treated sample also held up well to cycles of charging and discharging; after 100 cycles of charging-discharging, it lost only 7 percent of its capacity. Even after 1,000 cycles, the capacity of the electrodes only dropped about 23 percent.
The researchers thought the material performed very well, retaining 77 percent of its initial capacity.
This work was funded by the Department of Energy's Office of Science and Office of Electricity Delivery & Energy Reliability.
The team reported their work in the online journal Advanced Materials on June 3. Team members contributing to the article, Reversible Sodium Ion Insertion in Single Crystalline Manganese Oxide Nanowires with Long Cycle Life, include Yuliang Cao, Lifen Xiao, Wei Wang, Daiwon Choi, Zimin Nie, Jianguo Yu, Laxmikant V. Saraf, Zhenguo Yang and Jun Liu,