Copper niobate nanowires boosted by a N, S co-doped carbon coating for superior lithium storage

In-depth insight into the effects of oxygen vacancies on the excellent Li+-storage performances of Cu2Nb34O87-x/N-doped carbon composite

Wadsley-Roth phase niobates demonstrate significant Li+-storage advantages. Even though their Nb5+ can fully transform into Nb4+ during lithiation process, however, only partial Nb4+ can further convert to Nb3+, leading to much lower practical capacities than the theoretical values according to the two-electron transfer per Nb5+. The specific mechanism to improve their conversion ratio of Nb4+ to Nb3+ during lithiation process has rarely been reported so far. Herein, the ultrafine oxygen-deficient Cu2Nb34O87-x nanoparticles are closely connected by the N-doped carbon-based 3D conductive framework to form a cloud-like Cu2Nb34O87-x/N-doped carbon composite (denoted as VU-CNO-NC) with nanoaggregate structure and porous structure. Based on density functional theory (DFT) calculations and ex situ X-ray photoelectron spectrometer (XPS), the oxygen vacancies in VU-CNO-NC can catalyze the conversion of Nb4+ to Nb3+ during lithiation process, which significantly enhance the conversion ratio of Nb4+ to Nb3+ to generate much higher capacity. This effect of oxygen vacancies has rarely been reported so far. Moreover, the oxygen vacancies, ultrafine primary nanoparticles, 3D conductive framework, porous structure, and nanoaggregate structure synergistically endow VU-CNO-NC with fast Li+-storage kinetics and highly stable structure. Consequently, VU-CNO-NC not only shows high capacity (287 mAh g-1 after 500 cycles at 1 C and 181 mAh g-1 after 1000 cycles at 10 C) and excellent rate performance as anode material of lithium-ion batteries, but also endows hybrid lithium-ion capacitor with high energy density (126 Wh kg-1 at 175 W kg-1) and remarkable capacity retention (87.3 % after 9000 cycles at 2 A g-1), demonstrating great application prospect.

Mixed-phase enabled high-rate copper niobate anodes for lithium-ion batteries

The advancement of rapid-response grid energy storage systems and the widespread adoption of electric vehicles are significantly hindered by the charging times and energy densities associated with current lithium-ion battery technology. In state-of-the-art lithium-ion batteries, graphite is employed as the standard negative electrode material. However, graphite suffers from polarization and deteriorating side-reactions at the high currents needed for fast charging. Transition metal-oxide anodes are attractive alternatives due to their enhanced power density. However, often these anodes make use of toxic or scarce elements, significantly limiting their future potential. In this work, we propose a new, facile solid-state synthesis method to obtain non-toxic, abundant, mixed-phase copper niobate (Cu x Nb y O z ) anodes for lithium-ion batteries. The material consists of various phases working synergistically to deliver high electrochemical capacities at exceptional cycling rates (167 mA h g-1 at 1C, 95 mA h g-1 at 10C, 65 mA h g-1 at 60C and 37 mA h g-1 at 250C), large pseudocapacitive response (up to 90%), and high Li+ diffusion coefficient (1.8 × 10-12 cm2 s-1), at a stable capacity retention (99.98%) between cycles. Compared to graphite, at a comparable energy density (470 W h L-1), the composite material exhibits a 70 times higher power density (27 000 W L-1). These results provide a new perspective on the role of non-toxic and abundant elements for realizing ultrafast anode materials for future energy storage devices.

Polyvinylpyrrolidone-assisted synthesis of ultrathin multi-nanolayered Cu2Nb34O87-x for advanced Li+ storage

The ultrathin multi-nanolayered structure with ultrathin monolayer thickness (<10 nm) and certain interlayer spacing can significantly shorten Li+ paths and alleviate the volume effect for Li+-storage materials. However, unlike layered materials such as MXene and MoS2, shear ReO3-type niobates have difficulty forming ultrathin multi-nanolayered structures due to their crystal structures, which still remains a challenge. Herein, by a polyvinylpyrrolidone (PVP)-assisted solvothermal method, we first synthesize ultrathin multi-nanolayered Cu2Nb34O87-x with oxygen vacancies composed of ultrathin nanolayers (2-10 nm in thickness) and interlayer spacing (1-5 nm). Oxygen vacancies can radically enhance the inherent electronic/ionic conductivity and Li+ diffusion coefficient of this material. The PVP-induced formation mechanism of this material is expounded in detail. The well-preserved ultrathin multi-nanolayered structure and excellent multi-electron electrochemical reversibility (Nb5+ ↔ Nb4+ ↔N b3+ and Cu2+ ↔ Cu+) of this material during cycling are fully verified. Based on an ultrathin multi-nanolayered structure and oxygen vacancies, this material as the anode of lithium-ion batteries is highly competitive among reported shear ReO3-type Cu-Nb-O anodes, displaying a high reversible capacity (315.3 mAh g-1 after 300 cycles at 1 C), durable cycling stability (85.7 % capacity retention after 1000 cycles at 10 C), and outstanding rate performance. Moreover, the application of this material to lithium-ion capacitors generates a large energy density (97.9 Wh kg-1 at 87.5 W kg-1) and a high power density (17,500 W kg-1 at 12.6 Wh kg-1), thus further indicating its fast faradaic pseudocapacitive behavior for practical applications. The results of this work indicate a breakthrough in synthesizing ultrathin multi-nanolayered shear ReO3-type niobates.

Green carbon-carbon homocoupling of terminal alkynes by a silica supported Cu(II)-hydrazone coordination compound

A Cu(II) complex, [Cu(HL)(NO₃)(CH₃OH)]·CH₃OH (1), was obtained by the reaction of Cu(NO₃)₂·3H₂O and H2L in methanol solvent (H2L is (E)-4-amino-N'-(2-hydroxy-3-methoxybenzylidene)benzohydrazide). H2L and compound 1 were characterized by various spectroscopic analyses and the molecular structure of [Cu(HL)(NO₃)(CH₃OH)]·CH₃OH was determined by single-crystal X-ray analysis. The results indicated the product is a mononuclear Cu(II) complex and contains a free NH₂ functional group on the structure of the ligand. [Cu(HL)(NO₃)(CH₃OH)]·CH₃OH was used for the preparation of a heterogeneous catalyst by supporting it on functionalized silica gel. The heterogeneous catalyst (Si-Cu) was prepared by an amidification reaction of [Cu(HL)(NO₃)(CH₃OH)]·CH₃OH with functionalized silica gel. The resulting silica-supported catalyst (Si-Cu) was characterized by TGA, FT-IR, EPR, DRS, EDS, XRD, SEM and XPS analyses. Si-Cu was employed in a carbon-carbon coupling reaction and the effects of the amount of Si-Cu and temperature were investigated in the catalytic coupling. The structure of one of the products of the catalytic reactions (C₁₆H₂₂O₂, CP1) was determined by single-crystal X-ray analysis, which proved the formation of a C-C bond and the production of di-acetylene by homocoupling of terminal alkyne. This catalytic system is stable and it can be reused for a coupling reaction without a significant change in its catalytic activity.