Chlorine may help carry charge in a solid-state battery cathode, according to a study combining electrochemical measurements, spectroscopy and calculations. The work examines lithium iron chloride, Li₂FeCl₄, and reports evidence that chlorine atoms participate in oxidation as the material is charged. The finding extends research into anion redox—the removal of electrons from negatively charged atoms—an approach that could broaden the chemistry considered for future cathodes.
The research team was led by professor Ru-Shi Liu of the Department of Chemistry at National Taiwan University, with researchers at Taiwan’s National Synchrotron Radiation Research Center and Japan’s Nagoya Institute of Technology. The report says the study was published in the journal Small. Oxygen- and sulfur-based anion redox have attracted more attention, while chlorine’s potential role in chloride cathodes has been less clear.
To investigate, the researchers assembled an all-solid-state cell with Li₂FeCl₄ as its cathode. The cell also contained a Li₃InCl₆ solid electrolyte, a Li₁₀GeP₂S₁₂ interlayer and a lithium–indium alloy anode. Electrochemical measurements showed two oxidation features, near 3.8 and 4.2 volts relative to Li⁺/Li. The lower-voltage feature was associated mainly with iron changing from Fe²⁺ to Fe³⁺. The higher-voltage feature indicated an additional contribution involving chlorine.
The team then examined what was happening to the material’s electronic structure. X-ray photoelectron and X-ray absorption spectroscopy showed changes in chlorine’s electronic state during charging. Density functional theory calculations identified occupied, chlorine-derived nonbonding orbitals—electronic states that can contribute to charge compensation when electrons are removed. Together, the measurements and calculations support the interpretation that chlorine participates in the higher-voltage process.
The report also describes a changing relationship between iron and chlorine. During the first charging stage, iron is oxidized and the surrounding Fe–Cl octahedra become distorted. At higher voltage, as chlorine-related oxidation becomes important, the iron centers show signs of partial reduction instead of continuing to oxidize. The researchers describe this behavior as “reductive coupling.” They propose that it helps explain how charge is redistributed as the local structure partially recovers its symmetry.
Structural measurements add another part to the picture. X-ray diffraction detected phases coexisting and changes in the lattice as lithium was extracted. Nanoscale X-ray diffraction mapped uneven changes within individual cathode grains. These observations link the proposed chlorine-related redox to changes in local bonding and structure, as well as to differences in how regions of a particle respond during charging.
The results broaden the discussion of anion redox beyond oxygen and sulfur and point to questions for future work on halide cathodes. The report suggests that controlling chlorine’s nonbonding electronic states, interactions between metal and chlorine, and structural strain could help researchers explore higher-capacity all-solid-state battery materials. It does not report a ready-to-use battery technology: in the tested cells, the chlorine-associated high-voltage process was only partially reversible, and chlorine gas could not be ruled out under the measurement conditions. The study therefore offers a proposed mechanistic foundation for materials research, with performance and reversibility still important open issues.
The study is listed as Yun-Ping Chang et al., “Structural and Electronic Origins Enabling Chlorine Anion Redox in Halide Cathodes,” Small (2026).