Inducing [001]-orientation in Nb2O5 capsule-nanostructure for promoted Li+ diffusion process

Highly [001]-oriented N-doped orthorhombic Nb2O5 microflowers with intercalation pseudocapacitance for lithium-ion storage

Orthorhombic Nb₂O₅ (T-Nb₂O₅), a typical intercalation pseudocapacitor, is favorable for realizing high power and energy density for lithium-ion batteries; furthermore, the 2D layered channels perpendicular to the [001] direction facilitate fast Li⁺ intercalation in T-Nb₂O₅. Herein, N-doped T-Nb₂O₅ microflowers (N-Nb₂O₅) assembled from highly [001]-oriented nanoflakes are rationally synthesized using NH₄F as the nitrogen source and capping agent. It is found that NH₄⁺ can adsorb on the O-terminated (010) plane of T-Nb₂O₅via N-H⋯O hydrogen bonds, which is highly conducive to the generation of 1D nanorods and the subsequent fusion of the nanorods into highly [001]-oriented nanoflakes. The special growth orientation of the T-Nb₂O₅ nanoflakes endows them with abundant available Li⁺ intercalation channels; moreover, the bandgap of N-Nb₂O₅ is narrowed (∼2.91 eV) owing to the doping of N atoms, and the intrinsic electronic conductivity is improved. Accordingly, the intercalation pseudocapacitive behavior of N-Nb₂O₅ is notably promoted and N-Nb₂O₅ exhibits superior Li⁺ storage properties, including large discharge capacity (214.7 mA h g^-1 at 1C), excellent rate capability (203.7 and 174.6 mA h g^-1 at 1 and 20C), and superior cyclic stability (150.7 mA h g^-1 at 10C after 1000 cycles). In addition, the LiNi_0.5Mn_0.3Co_0.2O₂//N-Nb₂O₅ full cell delivers outstanding Li⁺ storage performance, especially in terms of long-term cycling (126.2 mA h g^-1 at 10C after 3500 cycles).

Constructing a Micrometer-Sized Structure through an Initial Electrochemical Process for Ultrahigh-Performance Li+ Storage

Orthorhombic niobium pentoxide (T-Nb₂O₅) is a promising anode to fulfill the requirements for high-rate Li-ion batteries (LIBs). However, its low electric conductivity and indistinct electrochemical mechanism hinder further applications. Herein, we develop a novel method to obtain a micrometer-sized layer structure of S-doped Nb₂O₅ on an S-doped graphene (SG) surface (the composite is denoted S-Nb₂O₅/SG) after the initial cycle, which we call "in situ electrochemically induced aggregation". In situ and ex situ characterizations and theoretical calculations were carried out to reveal the aggregation process and Li⁺ storage process. The unique merits of the composite with a micrometer-sized layer structure increased the reaction degree, structural stability, and electrochemical kinetics. As a result, the electrode exhibited a large capacity (∼598 mAh g^-1 at 0.1 A g^-1), outstanding cycling stability (∼313 mAh g^-1 at 5 A g^-1 and remains at ∼313 mAh g^-1 after 1000 cycles), and a high Coulombic efficiency and has a high fast-charging performance and excellent cycling stability.

Precisely Tunable T-Nb2O5 Nanotubes via Atomic Layer Deposition for Fast-Charging Lithium-Ion Batteries

The demand for fast-charging of lithium-ion batteries (LIBs) in modern electric transportation and wearable electronics is rapidly growing. However, commercially available graphite anodes still suffer from slow kinetics of lithium-ion diffusion and severe safety concerns of lithium plating when achieving the fast-charging goal. Here, it is demonstrated that the Li-ion diffusion kinetics of orthorhombic Nb₂O₅ nanotubes (T-Nb₂O₅ NTs) is enhanced by atomically precise manufacturing of nanoarchitectures. The controlled fabrication of T-Nb₂O₅ NTs with wall thicknesses from 24 to 43 nm is realized via atomic layer deposition (ALD) using electrospun polyacrylonitrile nanofibers as a sacrificing template. The wall thickness of T-Nb₂O₅ NTs can be precisely tuned by adjusting the number of ALD cycles. The relationship between the wall thicknesses and electrochemical performances is investigated in detail. The electrochemical kinetic analysis suggests that the lithium storage in T-Nb₂O₅ NTs is dominated by surface and intercalation pseudocapacitance. The morphology of T-Nb₂O₅ crystallites is found to have significant effects on the Li-ion insertion/extraction kinetics and the performance of the electrodes in LIBs. The resulting T-Nb₂O₅ NTs exhibit fast charge-storage kinetics and enable highly reversible insertion/extraction of Li ions without a phase change. This work may open up a new avenue for further development of intercalation-pseudocapacitive nanostructured materials for high-rate and ultrastable energy-storage devices.