CuFeS₂ as a Cathode Active Material in All Solid-State Batteries

One bottleneck in the development of solid-state battery (SSB) technologies is the cathode active material (CAM). Layered oxide materials commonly utilized in other lithium-ion battery technologies are quickly approaching their limits in terms of capacity. It is also clear that realizing the benefits of lithium metal anodes in SSBs necessitates the use of high-capacity conversion-type cathodes. Sulfides offer an intriguing direction for further research because they can reversibly contribute to charge storage through stable anion redox (2S²⁻ → (S₂)²⁻ + 2e⁻). In this talk I will discuss our recent work on copper iron disulfide (Chalcopyrite, CuFeS₂) as a conversion-type CAM for SSBs (CuFeS₂ + 4Li⁺ + 4e^- → Cu + Fe + 2Li₂S). CuFeS₂ has a theoretical capacity of 587 mAh/g and it is further attractive because it is a naturally occurring mineral that is mechanically soft and has high electronic and ionic conductivities. These properties enable stable electrochemical cycling without conductive additive and despite a large volume expansion during discharge of ~63%. Our group previously reported on a macroscopic displacement mechanism that occurs during lithiation of CuS in SSBs, whereby micron-sized Cu networks were observed with lithiation, the process of which was also followed by in-situ synchrotron-based X-ray tomography.[1,2] We hypothesize that the inclusion of a second metal center (e.g. Fe in combination with Cu) in the CAM will help to promote increased stability by hindering the macroscopic displacement mechanism. In this talk I will discuss our recent work applying CuFeS₂ as CAM in Li-SSBs. Through a combination of X-Ray diffraction, X-Ray absorption spectroscopy, resonant inelastic X-ray scattering, constant pressure experiments, and electrochemical characterization we are able to identify the complete reaction mechanism and better understand the roles of anion redox for boosting capacity and metastable states in improving stability in CuFeS₂-based Li-SSBs.<br/><br/><b>References</b><br/>1. A. L. Santhosha, N. Nazer, R. Koerver, S. Randau, F. H. Richter, D. A. Weber, J. Kulisch, T. Adermann, J. Janek, P. Adelhelm “Macroscopic displacement reaction of copper sulfide in lithium solid-state batteries” <i>Advanced Energy Materials,</i> 2002394 (2020).<br/>2. Z. Zhang, K. Dong, K. A. Mazzio, A. Hilger, H. Markötter, F. Wilde, T. Heinemann, I. Manke, P. Adelhelm “Phase Transformation and Microstructural Evolution of CuS Electrodes in Solid-State Batteries Probed by in-situ 3D X-ray Tomography” <i>Advanced Energy Materials</i> 2203143 (2023).