Electrochemical intercalation of lithium into the perovskite-type NbO2F: influence of the NbO2F particle size

Abstract

Niobium(V) oxy¯uoride, NbO 2 F, has a perovskite structure and presents the property of lithium intercalation by topotactic chemical reaction either with n-butyllithium dissolved in n-hexane or by electrochemical reaction. The intercalation leads to the reduction of the transition metal from the oxidation state Nb(V) to the oxidation state Nb(III). This allows a theoretical Li/NbO 2 F intercalation ratio of 2. In this paper we will show that this theoretical value can be approached by using micron the sized active material particles. Moreover, the electrical properties of the cathode studied by the galvanostatic intermittent titration technique and a.c. impedance spectroscopy are explained in terms of structural and grain size considerations. Results of cycling experiments are also described.

An Electron Diffraction and Crystal Chemical Investigation of Oxygen/Fluorine Ordering in Niobium Oxyfluoride, NbO2F

“…1). They have been of interest in recent years as host compounds for Li-insertion (3,4) but also because of a pressure-induced structural phase transformation arising from a condensed Rigid Unit Mode (RUM) distortion of the simple cubic parent structure type (5,6). The average structure (space group Pm3m) consists of corner-sharing MX (X"O, F) octahedra (see Fig.…”

Synthesis of Nb2O5 Nanoplates and their Conversion into NbO2F Nanoparticles by Controlled Fluorination with Molecular Fluorine

“…As already mentioned by Bohnke et al,[3a] decreasing the particle size of NbO 2 F through grinding benefits to the lithium insertion mechanism. It was also mentioned that insertion of lithium is easily possible up to a stoichiometry of 0.5 because of the maintaining of the structure.…”

“…In the literature, very few examples of NbO 2 F nanostructuration are reported. Bohnke et al tune the particles size by ball‐milling but only reach mean particles diameter of about 1.2 µm,[3a] while Tu et al obtained particles with average size of about 50–80 nm using PVA in HF solution . The present work appears as a new and effective method to obtain small nanoparticles of NbO 2 F. In addition, the presented method allows to obtain white compound unlike grey NbO 2 F samples obtained in HF solutions due to the presence of –OH or HF moieties (see Figure S3b, Supporting Information), although the ReO 3 type crystalline structure is the same (see Figure S3c, Supporting Information).…”

“…Niobium(V) oxyfluoride (NbO 2 F) is one of the promising technological materials owing to its wide application prospects, including but not limited to electrochromic devices, photocatalysis, lithium ion batteries (LIBs), and negative thermal expansion . Existing in the cubic ReO 3 type crystallographic phase, the wide band gap NbO 2 F has shown fair enough lithium (Li) storage capacity (180 mAh g –1 ) and good reversibility as an electrode material in LIBs .…”

“…Searching through the literature, it is revealed that nearly all the synthesis attempts for preparation of NbO 2 F are based upon the direct action of hydrofluoric (HF) acid over Nb 2 O 5 and/or Nb ,. [3a], , These solution based methods have environmental implications on one hand, and on the other hand, give resulting materials which are high HF contaminated (HF as high as 14 atomic %) and lacking control over the nanostructured particles . It is worth noting that recent work of Hirai et al, devoted to the fluorination of Nb 2 O 5 by reaction with PTFE (polytetrafluoroethylene) above 500 °C showed the formation of two niobiumoxyfluorides i.e., Nb 5 O 12 F and Nb 3 O 7 F .…”