They found that graphite in KIBs suffers fast capacity fading and moderate rate capability, which may be due to the large volume change over cycling, they sugested. To further improve the performance, they explored the used of a low-density soft carbon as a KIB anode, which exhibited much improved cycling life and very high rate capability.

The soft carbon also exhibited a high depotassiation capacity of 273 mAh/g at C/40—the same as graphite. At C/1 and C/2, soft carbon exhibited very high capacities of 210 and 185 mAh/g, respectively, compared with 264 mAh/g at C/10. Even at 5C (1395 mA/g), the soft carbon retained a capacity of 140 mA/g. In contrast to the fast capacity fading of the graphite anode, soft carbon exhibited much improved cyclability with a capacity retention of 81.4% after 50 cycles at 2C. Its CE value increased from 56.4% in the first cycle to 92.4% in the second cycle and eventually stabilized at ∼99%.

(Left) Rate performance of soft carbon and (Right) Cycling performance of soft carbon at 2C. Credit: ACS, Ji et al. Click to enlarge.

The findings open some new alternatives to batteries that can work with well-established and inexpensive graphite as the anode. Lithium can do that, as the charge carrier whose ions migrate into the graphite and create an electrical current. Aside from its ability to work well with a carbon anode, however, lithium is quite rare, found in only 0.0017 percent, by weight, of the Earth’s crust. Because of that it’s comparatively expensive, and it’s difficult to recycle. Researchers have yet to duplicate its performance with less costly and more readily available materials, such as sodium, magnesium, or potassium.

For decades, people have assumed that potassium couldn’t work with graphite or other bulk carbon anodes in a battery. That assumption is incorrect. It’s really shocking that no one ever reported on this issue for 83 years.

The cost-related problems with lithium are sufficient that you won’t really gain much with economies of scale. With most products, as you make more of them, the cost goes down. With lithium the reverse may be true in the near future. So we have to find alternatives.

—Xiulei (David) Ji, lead author

Potassium is 880 times more abundant in the Earth’s crust than lithium. The new findings show that it can work effectively with graphite or soft carbon in the anode of an electrochemical battery. Right now, batteries based on this approach don’t have performance that equals those of lithium-ion batteries, but improvements in technology should narrow the gap, Ji said.

It’s safe to say that the energy density of a potassium-ion battery may never exceed that of lithium-ion batteries. But they may provide a long cycling life, a high power density, a lot lower cost, and be ready to take the advantage of the existing manufacturing processes of carbon anode materials.

—Xiulei Ji

OSU officials say they are seeking support for further research and to help commercialize the new technology, through the OSU Office of Commercialization and Corporate Development.

Resources

• Zelang Jian, Wei Luo, and Xiulei Ji (2015) “Carbon Electrodes for K-Ion Batteries” J. Am. Chem. Soc. 137 (36), pp 11566–11569 doi: 10.1021/jacs.5b06809