A new role for chlorine: Uncovering anion redox in solid-state battery cathodes

As demand grows for safer and more capable rechargeable batteries, scientists are looking beyond the familiar metal-centered chemistry of conventional cathodes. A promising route is to draw additional charge from anions—the negatively charged atoms in a material—rather than relying on transition metals alone. Oxygen and sulfur have attracted substantial attention in this area. Whether chlorine can play a comparable role in chloride-based battery cathodes has been less clear.

Now, a research team led by professor Ru-Shi Liu in the Department of Chemistry at National Taiwan University, working with researchers at the National Synchrotron Radiation Research Center in Taiwan and Nagoya Institute of Technology in Japan, has found converging experimental and computational evidence for chlorine anion redox in lithium iron chloride (Li₂FeCl₄). Their study, published in the journal Small, helps explain the structural and electronic changes that accompany charging in this halide cathode.

The researchers assembled an all-solid-state cell using Li₂FeCl₄ as the cathode, a Li₃InCl₆ solid electrolyte, a Li₁₀GeP₂S₁₂ interlayer and a lithium–indium alloy anode. Electrochemical measurements identified two oxidation features, near 3.8 and 4.2 V versus Li⁺/Li. The lower-voltage process was associated primarily with Fe²⁺/Fe³⁺ oxidation, while the higher-voltage feature indicated an additional contribution involving chlorine.

To investigate the origin of this second process, the team combined X-ray photoelectron spectroscopy and X-ray absorption spectroscopy with density functional theory calculations. These measurements showed changes in chlorine's electronic state upon charging. Calculations of the partial density of states and crystal orbital Hamilton population further identified occupied, chlorine-derived nonbonding orbitals—electronic states capable of contributing to charge compensation when electrons are removed.

The study also uncovered an intriguing interaction between the iron and chlorine atoms. During the first charging stage, iron is oxidized, and its surrounding Fe–Cl octahedra become distorted. At higher voltage, as chlorine-related oxidation becomes important, the iron centers show signs of partial reduction rather than continued oxidation. This behavior, known as "reductive coupling," helps explain how the material redistributes electronic charge while partially recovering its local structural symmetry.

Further diffraction experiments supported this picture. X-ray diffraction detected phase coexistence and lattice evolution during lithium extraction, while nanoscale X-ray diffraction mapped spatially uneven changes within individual cathode grains. Together, these observations indicate that chlorine-related redox is linked not only to electron transfer but also to local bonding, structural distortion and the way different parts of a particle respond during cycling.

The findings extend the discussion of anion redox beyond the better-known oxygen- and sulfur-based systems and suggest new design principles for halide cathodes. In particular, controlling chlorine nonbonding states, metal–chlorine interactions and structural strain may help researchers explore higher-capacity all-solid-state battery materials.

Important challenges remain. The chlorine-associated high-voltage process was only partially reversible in the reported cells, and the authors could not rule out the evolution of chlorine gas under the measurement conditions. The work therefore provides a mechanistic foundation for further materials development rather than a demonstration of a ready-to-deploy battery technology.