Abstract
A novel oxynitride Li0.94FePO3.84N0.16 with olivine structure (space group Pnma, no. 62) has been synthesized by heating a parent LiFePO4 precursor obtained by citrate chemistry in flowing ammonia at 650 °C. The polycrystalline sample has been characterized by X-ray and neutron powder diffraction (NPD), elemental and thermal analysis, scanning electron microscopy (SEM) and electrochemical measurements. Based on the existing contrast between the scattering lengths of the N and O species, a Rietveld refinement of the structure from NPD data revealed that N preferentially occupies the O2 positions, as likely required to fulfil the bonding power of N ions. The refined crystallographic formula implies an oxidation state of 2.2+ for Fe cations. The differential thermal analysis, in still air, shows a strong exothermic peak at 520–540 °C due to the combustion of C contents, which are embedding the olivine particles, as observed by SEM. The electrochemical measurements suggest a better performance for the nitrided sample relative to the unnitrided LiFePO4 material, as far as capacity and cyclability are concerned. A bond-valence energy landscape study reveals a decrease in the percolation activation energy of about 6% upon nitridation, concomitant with the better electrochemical properties of the oxynitride compound. Additionally, ceramic samples prepared under NH3 flow could be obtained as pure and well-crystallized olivine phases at milder temperatures (650 °C) than those usually described in literature.
Highlights
In order to store chemical energy, rechargeable lithium batteries[1] (LIBs) are of particular interest since they have a high degree of portability together with high conversion efficiency
In an inert or reducing atmosphere, there is no combustion of the organic precursor resin; an important amount of carbon is present in both samples in the nanostructured form, which sizably would contribute to the electronic conductivity of the samples but rather hinder the active Fe(III) availability and the reversible capacity of the cathode
Olivine particle sizes and their distributions proved to rely on the synthesis procedures used to obtain the samples: (i) even submicrometric LFP crystallites were obtained by sol–gel approaches, while (ii) ceramic methods led to micrometric-sized materials with uneven particle size distribution
Summary
In order to store chemical energy, rechargeable lithium batteries[1] (LIBs) are of particular interest since they have a high degree of portability together with high conversion efficiency. Over the past 40 years, intensive research on transition metal oxides suitable as hosts for a rapid insertion/ extraction of alkali ions has led to the development of different materials in which parameters such as structure, chemistry, morphology, synthesis method, and texture (powdered, nanoparticulated, nanostructured or mesoporous phases) have been tuned Among this rather broad variety of materials, NASICON-related framework structures based on the Ti4+/Ti3+ or Fe3+/Fe2+ redox couples and presenting tetrahedral polyanions (XO4)2À or (XO4)3À, with X 1⁄4 Mo, W, S, P, and Si, have been largely studied.[3,4,5,6,7,8,9,10,11,12,13,14,15,16,17] Besides possessing high redox potentials and promising Li+ transport, polyanion frameworks can exhibit outstanding stability for daily-use batteries,[18,19,20,21] appropriate for applications where safety and durability are a concern. Treatments over the polyanions, such as uorination, have received much attention due to their appealing properties; for instance, the incorporation of uoride fosters a charge difference and variations in the lattice parameters along with an increase in the redox potential at which lithium insertion takes place.[22,23] More recently, studies over boron-doped phosphates presented enhanced electrochemical properties and cyclability up to a certain B content, above which oxygen vacancies are generated and the performance is hindered.[24]
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