Are 2D materials a good choice for K-ion batteries? Assessing CrSe₂ as an emerging cathode material

Back in the 1970s, Whittingham demonstrated the first rechargeable Li‑ion cell with Li metal and TiS₂. Since then, oxides displaced chalcogenides in Li systems because of their higher operating voltages, yet for K‑ion storage 2D chalcogenide hosts again make sense. In this lecture I describe my team’s work that lead to the discovery of the 2D CrSe₂ as a potential cathode for potassium‑ion batteries (PIBs) and compare it with the state-of-the-art PIB analogues.The efficient Cr⁴⁺/Cr³⁺ redox helps CrSe₂ to readily accept one potassium to form KCrSe2, giving the experimental capacity close to its theoretical capacity of 128 mAh g^–1. This capacity is retained during cycling at low C rate with modest polarisation, despite a sequence of staging and phase transitions as confirmed by in operando synchrotron X‑ray diffraction at ESRF. As in any battery, at higher rates the capacity drops. In this context, I will provide insight from in operando synchrotron PXRD experiments at 5C and discuss the results how incomplete depotassiation could carry over a capacity penalty into the next cycle.Moreover, I will present iodine‑driven chemical deintercalation experiments and show how they can be used to probe fast‑rate capacity loss and determine whether it is dominated by kinetic trapping rather than irreversible collapse of the 2D framework. I will close by presenting follow‑up work on CrSe₂ in Li‑ion and Na‑ion systems, and by outlining potential routes to stabilise 2D chalcogenide cathodes under fast charging.