Copper Sulfides as Cathode Active Materials 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 metal sulfide-based conversion reactions as CAMs for SSBs, including the dynamic microstructural evolution of CuS using <i>in-situ</i> synchrotron X-ray tomography and the use of ternary Cu₃PS₄ with β-Li₃PS₄ solid electrolyte.[1,2] Copper sulfide (CuS) is a naturally occurring mineral that reacts with lithium via a conversion reaction to form Li₂S and Cu with a theoretical energy density of 961 Wh/kg. CuS demonstrates a peculiar feature of Cu displacement into large (µm-sized) domains during cycling that are highly reversible during electrochemical cycling. We were able to follow this dynamic microstructural evolution <i>in-situ</i> by synchrotron X-Ray tomography.[1] In addition to these phase transformations, we also observe significant crack formation in the cathode that we found to be dependent on the stacking pressure of the cell. When replacing the binary compound CuS with the ternary compound Cu₃PS₄, the theoretical energy density increases to 1301.5 Wh/kg and at the same time we anticipate greater compatibility with the β-Li₃PS₄ solid electrolyte due to similarities in chemical composition and structure.[2] We studied the reaction mechanism of Cu₃PS₄ in SSBs and found that it undergoes an irreversible conversion reaction in the 1st cycle. The subsequent redox is largely dominated by Cu₂S and S₈ which form finely dispersed redox centers that promote stable cycling behavior.<br/><br/><br/>[1] Zhenggang Zhang, Kang Dong, Katherine A Mazzio, André Hilger, Henning Markötter, Fabian Wilde, Tobias Heinemann, Ingo Manke, Philipp 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> (<b>2023</b>) 2203143.<br/><br/>[2] Zhenggang Zhang, Katherine A. Mazzio, Luise M. Riegger, Wolfgang Brehm, Jürgen Janek, Joachim Sann, Philipp Adelhelm “Copper Thiophosphate (Cu₃PS₄) as an Electrode Material for Lithium Solid-state Batteries with Lithium Thiophosphate (β-Li₃PS₄) Electrolyte” <i>Energy Technology</i> (<b>2023</b>) 2300553.