Sodium intercalation into α- and β-VOSO4

Na-ion battery is one of the best alternatives to Li-ion battery. Abundance of sodium on earth is three orders of magnitude higher than lithium, which should make Na-ion battery technology cheaper. But alkaline-ion battery prices, which tend to increase because of the massive world demand, also depend on the choice of electrode materials. Therefore, cost-effective electrode development remains an important subject of research because this will allow Na-ion battery to be even more competitive. Electrochemical performances of anhydrous VOSO4 as electrode for Na-ion battery are reported in this letter. Two anhydrous phases of vanadyl sulfate have been studied. The first one, α-VOSO4, shows that up to 0.8 sodium per formula unit (Na / f.u.) can be intercalated in this phase, and a reversible intercalation of 0.4 Na / f.u. has been observed with a strong polarization. The second one, β-VOSO4, can intercalate up to 0.9 Na / f.u. with a reversible intercalation of 0.4 Na / f.u. leading to a reversible capacity of 64 mAh / g.


Introduction
The search for new materials that could be used as electrode material for the Na-ion batteries is one of today's most challenging issues.Many families of transition metal oxides as well as transition metal polyanionic frameworks have been proposed these last five years.Among them, Na-Super-Ionic-Conductors (NaSICON) are one the most popular materials due to their good cycling ability and Na + mobility.However, sulfates represent an interesting and low-cost class with only few reported members.Therefore, few sodiated iron sulfates [1][2][3][4] can be found in the literature and only one example of sodiated vanadate sulfate has been reported up to date (Na 2 VO(SO 4 ) 2 ) as an electrode material for Na-ion battery [5].This material delivers a reversible capacity of 60 mAh / g at 4.5 V vs Na + / Na.
In this work, we report the use of anhydrous vanadyl sulfate as an electrode material for Na-ion battery.Anhydrous VOSO 4 exists in two forms at room temperature: α-VOSO 4 is tetragonal and is formed by dehydration of its hydrate below 280 °C [6], β-VOSO 4 is orthorhombic and may be prepared either from the reaction of H 2 SO 4 and V 2 O 5 [7] or by dehydration above 280 °C, although decomposition occurs when using this last method [8].The charge-discharge profile of both known phases, α-and β-VOSO 4 , will be discussed.

Experimental
The alpha form, α-VOSO 4 , was prepared by a simple dehydration of VOSO 4 •xH 2 O (5 g, Sigma Aldrich) at 260 °C for 2 days, then stored in a glove-box to prevent rehydration from air moisture.On the other hand, β-VOSO 4 was prepared by a precipitation reaction starting from stoichiometric amounts of hydrated vanadium oxysulfide VOSO 4 •xH 2 O (1.8 g) heated at 140 °C in 100 mL of sulphuric acid solution (0.1M H 2 SO 4 ) for 2 hours.The resulted green mixture was then filtered and washed with water.The obtained powder is then left overnight at 160 °C in an oven before being stored in argon-filled glove-box.The compounds were characterized by X-ray powder diffraction (XRD) using a Philips X'Pert 2 diffractometer with Bragg-Brentano geometry (Cu Kα radiation).Note that due to their instability in air, the reduced phases' XRD patterns were registered under vacuum using a chamber attached to the XRD instrument.The electrochemical characterization was performed in cells build in Swagelok compression tube fitting with a solution 1M NaClO 4 in propylene carbonate (PC) as electrolyte and metallic sodium as counter electrode.The working electrode was prepared from a mixture of active material with acetylene black in a weight ratio of 50:50.The electrochemical cells were cycled at constant current between 1.0-3.0V at different galvanostatic rates on a VMP III potentiostat / galvanostat (Biologic SA, Claix, France) at room temperature.

Results and discussion
First report on preparation of the phase alpha of anhydrous vanadyl sulfate was published in 1965 by J. Tudo [6].Its crystal structure was optimized and its magnetic properties studied by R. J. Arnott and J. M. Longo in 1970 [9].They suggest that trace of water was present in Tudo's sample.This phase crystallizes within a tetragonal structure (space group: P4 / n) with a = 6.258Å, c = 4.122 Å and a volume of V = 161.42(3)Å 3 .Along the c-axis, we can observe continuous chains of corner-shared VO 6 octahedra, as shown in Fig. 1.All these chains are corner-shared with SO 4 tetrahedra forming a three-dimensional network.
First report on the phase beta of anhydrous vanadyl sulfate was published in 1927 by A. Sieverts and E. L. Müller [7].In 1970, its crystal structure and its magnetic properties have been studied in the same paper than α-VOSO 4 [9].This phase crystallizes within an orthorhombic structure (space group: Pnma) with a = 7.384 Å, b = 6.275Å, c = 7.078 Å and a volume of V = 327.92(3)Å 3 .β-VOSO 4 is described by Gaubicher et al. as chains of corner-sharing distorded vanadium oxygen octahedra along the aaxis.Those chains are linked to sulphate groups which alternately point in opposite directions along the c-axis [10].
Interestingly, Gaubicher et al. published the reversible intercalation of 0.6 lithium ions into β-VOSO 4 at 2.84 V vs Li + / Li.After a first intercalation of 0.9 lithium through a biphasic process at 1.75 V, a solid solution reaction takes place.The structure of the reduced phase Li 0.9 VOSO 4 has not been solved [10].
We investigated the charge-discharge profile of α-VOSO 4 carried out at C / 20 between 1.0 and 3.0 V (Fig. 2a).Th e slope of the curve suggests that a solid solution process occurs during both charge and discharge.Th e theoretical capacity for the intercalation of 1 sodium per VOSO 4 is 160 mAh / g.Th e fi rst discharge allows the intercalation of 0.8 Na / f.u. at an average voltage of 1.58 V with an average of 0.6 Na / f.u.reversibly deintercalated aft er 4 cycles.Th is corresponds to a reversible capacity of 96 mAh / g.Th e intercalation and deintercalation of sodium occur in two distinct processes centered respectively at 1.45 then 1.15 V for the intercalation and 2.42 then 2.68 V for the deintercalation, as observed on Fig. 2b.
The charge-discharge profile of β-VOSO 4 carried out at C / 20 between 1.0 and 3.0 V is depicted in Fig. 2c.Th e slope of the curve suggests also that a solid solution process occurs during both charge and discharge.Th e fi rst discharge allows the intercalation of 0.9 Na / f.u. at an average voltage of 1.58 V, but only 0.4 Na / f.u. were reversibly deintercalated, corresponding to a reversible capacity of 64 mAh / g.Th is potential characterizes the V 4+ / V 3+ redox potential.and deintercalation of sodium occur in two distinct processes centered respectively at 1.90 and 2.40 V for the intercalation and 2.30 and 2.85 V for the deintercalation process, as observed in Fig. 2d.
According to the electrochemical study (lower polarization and almost no shift on capacity after few cycles), β-VOSO 4 seems more suitable for the intercalation of Na and therefore should be more deeply investigated.Best performance of β-VOSO 4 can be explained by the channels observed in α-VOSO 4 structure (1.5 Å) being smaller than in b-VOSO 4 structure (2 Å) (see Fig. 1).The difference in channel sizes comes from a difference of configuration of SO 4 tetrahedra in these structures.In the α-VOSO 4 structure, SO 4 tetrahedra are linked to four channels of VO 6 octahedra.In contrast, only three channels of VO 6 octahedra are connected to the SO 4 channels in the β-VOSO 4 structure.Consequently, the structure is more constrained with less space between VO 6 octahedra chains in a-VOSO 4 than in β-VOSO 4 .
To complete our study, we decreased the size of the particles of α-VOSO 4 by using a mechanochemical process (250 rpm / 1.5 hrs).Although this ball milling process effectively nanostructured our material, as shown on the following X-ray pattern (Fig. 3, middle line), this did not improve the electrochemical properties of our material.Attempts to chemically reduce either αor β-VOSO 4 using sodium naphthalenide in THF have been unsuccessful due to the dissolution of the material in THF.
Finally, ex situ XRD pattern has been obtained after the first reduction of α-VOSO 4 .This shows that an amorphization process occurred during the intercalation of sodium into α-VOSO 4 phase (Fig. 3, upper line).

Conclusions
In this work, we demonstrated that αand β-VOSO 4 can be used as an electrode material in Na-ion battery.To the best of our knowledge, this is only the second vanadyl sulfate based material used in Naion battery.The β phase exhibits smaller polarization than the α phase.Intercalation and deintercalation of 0.4 Na / f.u.have been observed, which correspond to a ca-pacity of 65 mAh / g.This reversible capacity is quite low, but could be improved by playing with the particle size as well as carbon coating, even though nanosizing has been unsuccessful on the α phase.Then, due to its attractive price and its cycling capability, further investigations on the intercalation of sodium in β-VOSO 4 are in progress.

Fig. 1 .
Fig. 1.(a) Rietveld refi nement of the XRD pattern for α-VOSO 4 and its structure along the c-axis; (b) Rietveld refi nement of the XRD pattern for β-VOSO 4 and its structure along the a-axis

Fig. 3 .
Fig.3.Powder X-ray diffraction pattern of as prepared a-VOSO 4 phase (lower curve); powder X-ray diffraction pattern of a-VOSO 4 phase after ball milling (middle curve); powder X-ray diffraction pattern of a-VOSO 4 phase after Na intercalation (upper curve)