Mastering of Particle Size and Morphology of the Puckered Layer γ’-V2O5 Polymorph for Enhanced Na Electrochemical Properties
Authors: Baddour-Hadjean Rita , Safrany-Renard Marianne , Emery Nicolas , Laïk Barbara , Batyrbekuly Dauren , Bakenov Zhumabay , Pereira-Ramos Jean-Pierre 
Published: February 2020

Due to the cost and low availability of Li sources, Na-ion batteries (NIBs) are attracting considerable interest as tomorrow’s world batteries. Compared to LIBs, the number of electrode materials for NIBs are limited but progress in Na intercalation grows very rapidly [1]. Layered materials with Van der Waals interlayer spacing constitute ideal frameworks for intercalation reactions of guest cationic species from which high discharge-charge rate and minimum structural distortions can be expected. While orthorhombic α-V2O5 was identified in the 70’s as a promising cathode material for secondary Li batteries [2], it is only very recently that Na insertion was addressed in this oxide at room temperature [3].
An alternative way to identify new attractive stable V2O5 polymorphs consists in considering the chemical removal of metallic species from vanadium pentoxide bronzes MxV2O5. Such approach allows taking advantage of the availability of new types of structure with various original layer stacking. This strategy was successfully applied in the 90s’ to obtain the puckered layer γ’-V2O5 polymorph synthesized from the topotactic chemical removal of Li from γ-LiV2O5 by means of strong oxidizing agent [4]. The electrochemical behavior of γ’-V2O5 was shown to explain the enhancement of the cell potential of Li//V2O5 [4] and a recent work has demonstrated the potentialities of this polymorph toward Li insertion [5].
Here we report the interesting capability of γ’-V2O5 toward electrochemical sodium insertion. Nearly one Na+/ mole involving the V5 +/V4 + redox couple can be inserted between the puckered layers of the γ’-V2O5 structure (see inset in Figure 1). As a result, an attractive initial discharge capacity of 145 mAh g-1 is obtained at a high working potential of 3.3 V vs. Na+/Na (Fig.1 curve a). Nevertheless, strong kinetic limitations are evidenced during the first charge process, with a 50% efficiency at RT (Fig.1 curve a). Further cycles exhibit an excellent capacity retention (stable value of 70 mAh g− 1 available after 70 cycles at C/10) [6].
To solve the charge efficiency issue, a downsizing approach was performed using planetary ball milling on the as prepared γ’-V2O5 platelets-like powder. While the platelet morphology is kept, the mean crystallite size is reduced by a factor 3 (90 nm for the as prepared sample vs. 35 nm for the ball-milled γ’-V2O5-BM). As shown in Fig.1 curve b, a strong effect of the crystallite size reduction is observed both on the shape of the voltage-composition curve and on the first charge efficiency that is strongly improved for the ball-milled powder. Indeed, a charge capacity of 127 mAh g-1 corresponding to a 90% efficiency is recovered for γ’-V2O5-BM.
A detailed structural study by X-ray diffraction (XRD) and Raman spectroscopy reveals a greatly modified phase diagram by reducing the dimensions of the particle. The nanosize effect promotes a wide single phase domain at the expense of diphasic region and is also responsible for an easier sodium extraction process in the ball milled compound, leading to a genuine electrochemical and structural reversibility.
A mastering of the particle morphology has also been conducted : γ’-V2O5 was synthesized from a home-made α-V2O5 precursor obtained through polyol process, leading to pure, fine and coral-like porous powders with homogeneous grain size distribution [7]. This peculiar morphology drastically increases the available surface for sodium diffusion, leading to a quantitative charge process at a high working voltage of 3.4 V vs. Na+/Na (Fig.1 curve c). Furthermore, enhanced rate capability performance and excellent cycle life are achieved, with 130 mAh g-1 still available after 60 cycles at C/10. Raman and XRD measurements demonstrate the high structural reversibility of the sodium insertion-extraction reaction in γ’-V2O5.

References

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Figure 1. First discharge-charge curve of a γ’-V2O5 composite electrode made of the following active material: (a) as-prepared platelet-like powder/ crystallite size 90 nm (b) ball-milled powder / crystallite size 30 nm (c) coral-like powder. C/10 rate. NaClO4 1M/PC electrolyte, 2% vol. FEC. Inset: crystal structure of γ’-V2O5.

Figure 1