Performance of battery materials depends on density: Research

Performance of battery materials depends on density: Research
Better batteries may be possible with zinc, which is cheap, plentiful, and environmentally benign, but there is a major problem.

Aqueous zinc ion batteries (AZIBs) are unable to match lithium-ion batteries in terms of power output. A Chinese research team constructed two organic frameworks with the same elements but different orderings to see what electrode material composition could improve AZIBs.

The structure that performed better in the test had an appropriate density of active sites, which are where zinc ions pick up electrons to recharge the battery. The organic compounds that make up the cathode are molecules arranged in a crystalline form. During charge and discharge reactions, which correspond to the absorption and release of zinc ions, functional groups undergo alternating oxidation and reduction processes.

Ions react and pick up electrons at the active sites found in these molecules. But the number of reactions that inorganic molecules can support and the length of time they can last before breaking down are both limited.

"In contrast to inorganic materials, organic materials exhibit superior redox properties, high specific capacity and structural flexibility," Li stated.

"In this paper, we designed two covalent organic framework (COF) materials with the same structure and number of energy groups to investigate the correlation between the densities of active sites and electrochemical performance."

nd that appears to be the key to better electrochemical performance, according to Li.
"Although TB-COF boasts a commendable initial specific capacity stemming from its densely packed functional groups, its susceptibility to capacity deterioration due to potent interactions places it at a disadvantage when vying for the role of an AZIBs cathode material," Li said.

"BB-COF, on the other hand, demonstrated stable cycling even at - 40 degrees Celsius for 2,000 cycles -- meaning it could remain stable even as a battery charges and discharges 2,000 times."

The researchers analyzed the chemical composition and the morphology of the COFs, determining that the larger diameter of BB-COF enabled rapid ion transport and more effective utilization of the active sites.
At room temperature, after 10,000 cycles, BB-COF retained a specific capacity -- the measurement of charge per weight -- of 72 milliampere-hours per gram mass. TB-COF's specific capacity dropped to 40 milliampere-hours per gram mass.

"Regulating and controlling pore dimensions to obtain the optimal densities of active sites are believed to not only enhance the zinc ion transport rate in COF pathways but also to help maintain the stability of COF structures," Li said.

"Therefore, this approach is a direction for future efforts."