Investigating Composites of 2D NbSe 2 and 1D Nb 2 Se 9 as Sodium Ion Hosts

The success of lithium-ion batteries has undoubtedly paved a path forward in the climate crisis; however, concern is emerging over depletion and unethical extraction of critical minerals utilized in popular batteries. Na-ion battery technology provides possibility for an earth-abundant replacement for lithium, especially in applications where high-voltage (≥ 4 V) batteries are not necessary. Low-dimensional transition metal chalcogenides are a potential class of anode materials that exhibit both high capacity and high initial rate performance. The most common structures contain TX 2 stoichiometry where T is typically an early row transition metal like Mo, Nb, Ti, and V and X is S, Se, or Te. They are distinguished by strong intralayer T-X bonds in stacked 2D layers, in which the stacking is stabilized by weak Van der Waals (vdW) forces with ~ 3 Å interlayer distances. These unique structures, somewhat analogous to graphite in Li-ion systems, permit fast intercalation of alkali ions into the vdW gap, leading to a “pseudocapacitive” (A x MX 2 where A = Li, Na, K) charging without disrupting the intralayer bonds within an allowed voltage range. 1 Beyond this voltage range, the anode materials undergo phase conversion, resulting in A 2 X + T products. 2 This dual-charging capability leads to high specific capacities, but the resulting chemical conversions commonly result in irreversible cracking of the material during discharge. Managing detrimental reactions is often addressed by adding other stabilizing structures into the material via heterostructure or composite formation, which can conversely increase chemical complexity in synthesis and increase mass loading. Still, transition metal chalcogenides demonstrate promise as next generation anode materials but require synthetic ingenuity to tackle structural degradation. NbSe 2 is a well-studied, electronically conductive 2D transition metal dichalcogenide with numerous applications. The combination of its metallic character with fast ionic conductivity and reasonable specific capacity positions NbSe 2 as an attractive anode for alkali batteries; however, as previously discussed, the conversion reaction A x NbSe 2 à Nb + A 2 Se is partially irreversible and adversely affects cyclability. 2 Additionally, Nb 2 Se 9 , the most selenium rich phase in the Nb-Se system, is a 1D chain-like structure with a very high specific capacity from conversion reactions to A 2 Se. 3 Conversely, this structure has low electronic conductivity which leads to sluggish electrode kinetics. It was recently shown that crystalline Nb 2 Se 9 can be modified to convert to NbSe 2 via selenium evaporation during H 2 annealing 4 , suggesting that composites of Nb 2 Se 9 /NbSe 2 are synthetically possible. Therefore, exploiting the electronic conductivity of NbSe 2 and high specific capacity of Nb 2 Se 9 at different Nb 2 Se 9 /NbSe 2 ratios should provide insight into the mixed 1D/2D electrode system. Our work investigates structural and electrochemical behavior of thin films with varying Nb 2 Se 9 /NbSe 2 ratios. The films were synthesized via the modulated elemental reactants method which consists of depositing repeating ultrathin (≤ 10Å) Nb|Se units to target multiple amorphous ~Nb 2 Se 9 layers, followed by post-deposition annealing in a Se-rich, inert atmosphere to form crystalline Nb 2 Se 9 . The films were then annealed in N 2 atmosphere over short time periods to form NbSe 2 /Nb 2 Se 9 composites via Se evaporation. The specular X-ray diffraction (XRD) patterns show strong (h00)/(0k0) alignment to the silicon substrate for 1D Nb 2 Se 9 , followed by formation of strong (00l) alignment of 2D NbSe 2 after further annealing.. The in-plane XRD patterns show that after annealing the Nb 2 Se 9 film for 9.5 minutes at 400 °C, a composite of NbSe 2 and Nb 2 Se 9 is observed. From a structural perspective, the in-plane and out-of-plane diffraction probes provide understanding of the 1D chain to 2D layer phase transition in this system, and the thin film morphology offers a model system to study intercalation and conversion. The structural transitions that occur in these films will be investigated via ex-situ XRD at varying potentials to understand the structural phase transitions associated with Na-ion charging and discharging. Additionally, galvanostatic cycling and impedance measurements will be analyzed to observe how specific capacity/electrode kinetics are affected by varying the ratios of the 1D semiconductor and 2D metal. To summarize, this investigation presents structural and mechanistic insight into how varying dimensionality and electrical conductivity in a composite impacts electrochemical properties. Wei, X. et al. Three-dimensional hierarchically porous MoS2 foam as high-rate and stable lithium-ion battery anode. Nat. Commun. 13 , 6006 (2022). Liu, Z. et al. Are sodiation/de-sodiation reactions reversible in two-dimensional metallic NbSe 2 ? Energy Environ. Sci. 17 , 2173–2181 (2024). Wu, X. et al. Synthesis of Flower-Like Nb2Se9 as High-Performance Anode Materials for Lithium-Ion and Sodium-Ion Batteries. Energy Fuels 35 , 11563–11571 (2021). Lee, S. H. et al. Conversion of 1D Nb 2 Se 9 Nanowires to 2D Conductive NbS