Revealing the Valence Evolution of Metal Element in Heterostructures for Ultra-High Power Li-Ion Capacitors

The difficulty in matching cathode and anode kinetics due to slow ion transport in anodes constrains the development of lithium-ion capacitors. Heterogeneous structures with built-in electric field can promote lithium-ion migration and improve the anode reaction kinetics. However, the valence evolution of metal elements in heterostructures during charging/discharging processes is often overlooked. Inspired by theoretical calculations, transition metal selenides heterostructures (FeSe2/CoSe2) with low migration energy barriers (E-b = 0.35 eV) are successfully engineered and fabricated. As expected, the designed heterostructure material exhibits outstanding rate performance (512 mAh g(-1) at 30 A g(-1)) and ultra-high pseudocapacitance contribution (98.02% at 1.0 mV s(-1)), exceeding the state-of-the-art values for anodes. Impressively, synchrotron X-ray absorption spectroscopy (SXAS) and ex situ XPS find that the valence states of the Fe and Co elements in the heterogeneous structure gradually increase as charging and discharging proceeds, inducing a continuous climb in reversible specific capacity, while further reducing the migration energy barrier in the heterogeneous structure (E-b = 0.20 eV). This work reveals that the valence states of iron and cobalt elements increase as charging and discharging proceeds, providing theoretical guidance for improving the anode kinetics and revealing the capacity rise mechanism of other transition metal compounds.