Superstoichiometric reversible and manipulable copper-ion intercalation in niobium selenide

Few-layer stacked niobium selenide (NbSe₂) has evoked great interest owing to its intrinsically exotic properties and accessible manipulation by controlled ion intercalation for superconductivity physics and advanced device applications. However, attempts to extend the range of reversible intercalation stoichiometries are often hindered by overexpanded bond rupture and intrinsic-limit transition metal redox centres in selenides when proceeding towards deep intercalation. Here, we report that reversible unconventional superstoichiometric controlled intercalation in NbSe₂ with up to two copper-ions per unit cell can be realized by triggering anionic redox, a fivefold improvement over previous report. Synergistic charge transfer of the transition metal and selenium framework inhibited the disorder of bonds and lattice structures to avoid falling into conversion, which is essential for obtaining superstoichiometric intercalation products, enabling tunable copper-ion de/intercalation repeatable for 11,000 cycles. Moreover, deep copper-ion intercalation and its derived intercalation compound family demonstrate milestone performance in capacity and cycling stability for extended electrochemical energy storage applications such as copper batteries, hybrid-ion zinc batteries, and nonaqueous potassium batteries. Our findings broaden the realm of intercalation compounds and offers appealing possibilities for tailoring on-demand physicochemical properties of materials towards the envisioned functional applications.