Modern batteries rely mainly on metals, but researchers are looking for ways to involve other elements as well. A new study published in the journal Small shows that chlorine - a negatively charged ion (anion) - can contribute to the operation of the cathode (positive electrode) of a lithium battery.
A group led by Professor Ru-Shi Liu of National Taiwan University, together with colleagues from the National Synchrotron Radiation Research Center (Taiwan) and Nagoya Institute of Technology (Japan), studied the compound Li2FeCl4. They assembled a fully solid-state cell in which this compound served as the cathode, and carried out electrochemical measurements.
During charging, the scientists found two oxidation peaks - at about 3.8 and 4.2 volts relative to the lithium electrode. The first process is associated mainly with the oxidation of iron, while the second indicates the involvement of chlorine. To understand the nature of the second peak, the researchers used X-ray photoelectron and X-ray absorption spectroscopy, as well as calculations using density functional theory (computer modeling of electronic structure).
It turned out that the electronic state of chlorine changes during charging. The calculations revealed so-called non-bonding chlorine orbitals - electronic states that can donate charge when electrons are removed from the material. In addition, an effect of "reductive binding" was found: in the first stage of charging, iron is oxidized, and at a higher voltage, when chlorine begins to work, iron is partially reduced. This helps the material redistribute charge and partially restore symmetry.
Additional X-ray diffraction experiments showed that when lithium is extracted, different phases coexist in the cathode, and changes within individual grains of the material occur unevenly. This suggests that the process involving chlorine is connected not only with electron transfer, but also with local bonds and structural deformations.
The authors emphasize that practical application is still far off. The high-voltage process involving chlorine was only partially reversible, and the researchers do not rule out the release of chlorine gas under the experimental conditions. The work rather explains mechanisms that will help create new battery materials in the future than offers a ready-made technology.