Aliovalent Ni in MoO₂Lattice— Probing the Structure and Valence of Ni and Its Implication on the Electrochemical Performance

Abstract

Here, we present a synthesis of MoO 2 nanoparticles doped with 2 at% of Ni in a mixture of acetophenone and benzyl alcohol at 200 °C. Based on in situ X-ray absorption nearedge structure (XANES) and ex situ extended X-ray absorption fine structure (EXAFS) measurements at Ni K-edge and Mo Kedge, we discuss scenarios on how the "doping" reaction, that is, the incorporation of Ni in the MoO 2 , proceeds. We can clearly exclude the formation of NiO or Ni nanoparticles. Moreover, within the resolution of our in situ XANES experiments, we observe that the ternary compound Ni:MoO 2 nucleates directly in the final composition. Although the local structure around the Ni ion adopts the MoO 2 crystal structure pointing at the substitution of tetravalent Mo by Ni, we find that Ni remains divalent. This aliovalent substitution results in the relaxation of the local structure, which is additionally reflected in the slight shrinking of the total volume of the unit cell of Ni:MoO 2 . Interestingly, such a small amount of divalent Ni has a tremendous effect on the performance of the material as anode in Li-ion batteries. The initial discharge capacity of Ni:MoO 2 based anodes almost doubles from 370 mAh/g for MoO 2 to 754 mAh/g for Ni:MoO 2 at 0.1 C (1 C = 300 mA/g). Additionally, we observed an atypical increase of capacity for both MoO 2 and Ni:MoO 2 anodes upon cycling with increasing cycling rate.

Rock Salt Ni/Co Oxides with Unusual Nanoscale‐Stabilized Composition as Water Splitting Electrocatalysts

“…The exceptionally high solubility of Co and Ni in Ni x Co 1− x O may be attributed to the extremely small size of the nanoparticles showing the great influence of nanoscaling on the formation of new phases. Larger particles usually do not show this behavior, being able to incorporate only a few percent of the dopant without losing structural integrity . The structure of the nanoparticles was analyzed by XRD, Raman, XPS, and TEM measurements that indicate substitutional incorporation of the metal ions in the lattice.…”

“…The discovery of novel nanoscale‐specific compositions has the potential to emerge as a new direction in materials research. Besides the academic interest, the composition control on the nanoscale can become an additional tool to tune the performance of materials in different energy‐related fields . Electrocatalysis is one of the applications that is extremely sensitive to any changes in stoichiometry and phase composition of catalysts, and thus could benefit from the discovery of novel material libraries …”

Fabrication and application of hierarchical mesoporous MoO2/Mo2C/C microspheres

“…Although it has excellent physicochemical properties, MoO 2 still possesses two crucial drawbacks before practical application. One weakness is sluggish kinetics performance resulting in limited redox conversion process of bulk MoO 2 [20][21][22] . The other weakness is huge volume change of MoO 2 during lithiation and delithiation processes leading to electrode pulverization and rapid capacity fading [23] .…”

Matching the organic and inorganic counterparts during nucleation and growth of copper-based nanoparticles – in situ spectroscopic studies

“…It has been previously successfully used to determine the concentration profile and pure XANES spectra of individual species during in situ studies of Cu-catalysts, Cu-doped V 2 O 3 batteries and to follow the synthesis of Co, TiO 2 and Ni-doped MoO 2 nanoparticles. 18,[37][38][39][40][41][42][43][44][45] Additionally, we track the formation of organic compounds by Fourier Transform Infrared spectroscopy (FTIR) studies that are recorded simultaneously with UV-vis measurements. In the control experiments without copper precursor, we: (a) evaluate the decomposition of benzyl alcohol upon heating, (b) determine the lower detection limit and (c) calibrate the curve for benzaldehyde, benzyl acetate and dibenzyl ether, compounds that are considered the main organic counterparts of copper-based nanoparticle nucleation.…”