Nb-Se-C Thin Films Deposited by a 1064 nm Laser as Model Electrodes for Li⁺ and Na⁺ Storage

Chalcogen elements, typically sulfur and selenium, could be used for energy storage either in elemental form based on Q/Q(2-) redox or in layered transition metal chalcogenides, which allow insertion/extraction of Li+/Na+/K+/Mg2+. These two types of chalcogen-containing materials were rarely discussed together because of their significantly different synthetic routes, except for being mentioned in regard to the over-reduction of chalcogenides into elemental chalcogens. Recently, in attempts to deposit Nb2Se2C MXene by ablating a Nb-Se-C target using a 1064 nm fiber laser, a series of selenium-containing thin films were obtained as model electrodes. It was discovered that substrate temperature strongly affected the composition, structure, and accordingly the electrochemical performance of these thin films. From room temperature (RT) to 100 degrees C, the thin films were amorphous and selenium-abundant. Niobium mainly aggregated in metallic form. Both the Raman scattering and Li+ storage performances were close to those of a pure selenium cathode. With increasing substrate temperature, selenium partially evaporated to vacuum and partially reacted with Nb to form layered NbSe2 at similar to 300 degrees C and layered Nb2Se3 at above 400 degrees C. All these thin films exhibited Li+ storage capacity of over 600 mAh g(-1) and Na+ storage capacity of over 350 mAh g(-1) (except for the RT-Na sample) at the charge/discharge rate of 0.1 A g(-1). These discoveries reveal the structural evolution from near elemental selenium to layered niobium selenides and their Li+/Na+ storage capability. It also demonstrates the feasibility of the 1064 nm laser for depositing high-performance and binder-free thin film electrodes.