Sodium intercalation into α- and β-VOSO4 31 D O I: 1 0. 15 82 6/ ch im te ch .2 01 9. 6. 1. 04 Deriouche W., Anger E., Amdouni N., Pralong V. Chimica Techno Acta. 2019. Vol. 6, No. 1. P. 31–36. ISSN 2409–5613 W. Deriouchea,b, E. Angera, N. Amdounib, V. Pralonga,* a Normandie Univ, ENSICAEN, UNICAEN, CNRS, CRISMAT, 14000 Caen, France b Unite de Recherche Physico-chimique des Materiaux Condenses, Universite Tunis El Manar, Faculte des Sciences de Tunis, Campus Universitaire, 2092 El Manar Tunis, Tunisia e-mail: valerie.pralong@ensicaen.fr 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 inter- calation of 0.4 Na / f.u. leading to a reversible capacity of 64 mAh / g. Keywords: VOSO4; Na-ion Batteries; cathode; vanadium sulfate. Received: 12.02.2019. Accepted: 19.03.2019. Published: 29.03.2019. © Deriouche W., Anger E., Amdouni N., Pralong V., 2019 Introduction The search for new materials that could be used as electrode material for the Na-ion batteries is one of today’s most chal- lenging issues. Many families of transition metal oxides as well as transition metal poly- anionic 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. How- ever, sulfates represent an interesting and low-cost class with only few reported mem- bers. Therefore, few sodiated iron sulfates [1–4] can be found in the literature and only one example of sodiated vanadate sulfate has been reported up to date (Na2VO(SO4)2) as an electrode material for Na-ion battery [5]. This material delivers a reversible capac- ity of 60 mAh / g at 4.5 V vs Na+ / Na. In this work, we report the use of an- hydrous vanadyl sulfate as  an electrode material for  Na-ion battery. Anhydrous VOSO4 exists in two forms at room tem- perature: α-VOSO4 is tetragonal and is formed by dehydration of its hydrate below 280 °C [6], β-VOSO4 is orthorhombic and may be prepared either from the reaction of H2SO4 and V2O5 [7] or by dehydration 32 above 280 °C, although decomposition oc- curs when using this last method [8]. The charge-discharge profile of  both known phases, α- and β-VOSO4, will be discussed. Experimental The alpha form, α-VOSO4, was prepared by a simple dehydration of VOSO4·xH2O (5 g, Sigma Aldrich) at 260 °C for 2 days, then stored in a glove-box to prevent re- hydration from air moisture. On the other hand, β-VOSO4 was prepared by  a  pre- cipitation reaction starting from stoichi- ometric amounts of  hydrated vanadium oxysulfide VOSO4·xH2O (1.8 g) heated at  140  °C in  100 mL of  sulphuric acid solution (0.1M H2SO4) for  2 hours. The resulted green mixture was then filtered and washed with water. The obtained pow- der is then left overnight at 160 °C in an oven before being stored in  argon-filled glove-box. The compounds were characte- rized 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 instabil- ity in  air, the reduced phases’ XRD pat- terns were registered under vacuum using a  chamber attached to  the XRD instru- ment. The electrochemical characterization was performed in cells build in Swagelok compression tube fitting with a  solution 1M NaClO4 in propylene carbonate (PC) as electrolyte and metallic sodium as coun- ter electrode. The working electrode was prepared from a mixture of active mate- rial with acetylene black in a weight ratio of 50:50. The electrochemical cells were cy- cled at constant current between 1.0–3.0 V 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 crys- tal structure was optimized and its mag- netic 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  = 6.258 Å, c = 4.122 Å and a volume of V = 161.42(3) Å3. Along the c-axis, we can ob- serve continuous chains of corner-shared VO6 octahedra, as shown in Fig. 1. All these chains are corner-shared with SO4 tetrahe- dra forming a three-dimensional network. First report on the phase beta of anhy- drous vanadyl sulfate was published in 1927 by A. Sieverts and E. L. Müller [7]. In 1970, its crystal structure and its magnetic pro- perties have been studied in the same paper than α-VOSO4 [9]. This phase crystallizes within an orthorhombic structure (space group: Pnma) with a = 7.384 Å, b = 6.275 Å, c = 7.078 Å and a volume of V = 327.92(3) Å3. β-VOSO4 is described by  Gaubicher et al. as chains of corner-sharing distorded vanadium oxygen octahedra along the a- axis. 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-VOSO4 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 Li0.9VOSO4 has not been solved [10]. We investigated the charge-discharge profile of  α-VOSO4 carried out at  C / 20 33 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 VOSO4 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 deintercala- tion, as observed on Fig. 2b. T h e c h a r g e - d i s c h a r g e p r o f i l e of β-VOSO4 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 aver- age voltage of 1.58 V, but only 0.4 Na / f.u. were reversibly deintercalated, correspond- ing to a reversible capacity of 64 mAh / g. Th is potential characterizes the V4+ / V3+ re- dox potential. Th e capacity remains almost unchanged aft er 4 cycles. Th e intercalation 0 20 40 60 80 -800 -600 -400 -200 0 200 400 600 800 1000 1200 1400 C ou nt s 2θ 2θ P4/n a = 6.2673(2) c = 4.1068(2) V = 161.313(9) χ2 = 2.36 α-VOSO4 β-VOSO4 a 0 50 100 -4000 -2000 0 2000 4000 6000 8000 10000 C ou nt s Pnma a = 7.3837(2) b = 6.2750(2) c = 7.0776(2) V = 327.925(9) χ2 = 4.81 b Fig. 1. (a) Rietveld refi nement of the XRD pattern for α-VOSO4 and its structure along the c-axis; (b) Rietveld refi nement of the XRD pattern for β-VOSO4 and its structure along the a-axis 34 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 pro- cess, as observed in Fig. 2d. According to the electrochemical study (lower polarization and almost no shift on  capacity after few cycles), β-VOSO4 seems more suitable for the intercalation of Na and therefore should be more deeply investigated. Best performance of β-VOSO4 can be explained by the channels observed in α-VOSO4 structure (1.5 Å) being smal- ler than in b-VOSO4 structure (2 Å) (see Fig.  1). The difference in  channel sizes comes from a difference of configuration of  SO4 tetrahedra in  these structures. In the α-VOSO4 structure, SO4 tetrahedra are linked to four channels of VO6 octahedra. In contrast, only three channels of VO6 oc- tahedra are connected to the SO4 channels in the β-VOSO4 structure. Consequently, the structure is more constrained with less space between VO6 octahedra chains in a- VOSO4 than in β-VOSO4. To complete our study, we decreased the size of  the particles of  α-VOSO4 by  using a  mechanochemical process (250 rpm / 1.5 hrs). Although this ball mill- ing process effectively nanostructured our 0.0 0.2 0.4 0.6 0.8 1.0 0.5 1.0 1.5 2.0 2.5 3.0 x in NaxVOSO4 V ol ts v s N a+ /N a a 0 50 100 150 Capacity (mAh/g) 1.0 1.5 2.0 2.5 3.0 -2 -1 0 1 2 Derivative curve 1.451.15 2.68 2.42 d( Q –Q 0) /d E (m A h/ V ) E (V vs Na+/Na) b 0.0 0.2 0.4 0.6 0.8 1.0 1.0 1.5 2.0 2.5 3.0 V ol ts v s N a+ /N a 0 50 100 150 x in NaxVOSO4 Capacity (mAh/g) c 1.0 1.5 2.0 2.5 3.0 -5 -4 -3 -2 -1 0 1 2 Derivative curve 1.90 2.40 1.58 2.852.30 d( Q –Q 0) /d E (m A h/ V ) E (V vs Na+/Na) d Fig. 2. (a) Potential-capacity curves of α-VOSO4 at the galvanostatic rate of C / 20 between 3.0 and 1.0 V; (b) corresponding derivative curves; (c) potential-capacity curves of β-VOSO4 at the galvanostatic rate of C / 20 between 3.0 and 1.0 V; (d) corresponding derivative curves 35 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 α- or β-VOSO4 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-VOSO4. This shows that an amorphi- zation process occurred during the inter- calation of  sodium into α-VOSO4 phase (Fig. 3, upper line). Conclusions In this work, we demonstrated that α- and β-VOSO4 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 Na- ion battery. The β phase exhibits smaller polarization than the α phase. Intercala- tion and deintercalation of 0.4 Na / f.u. have been observed, which correspond to a ca- pacity of  65  mAh / g. This reversible ca- pacity is quite low, but could be improved by  playing with the particle size as  well as carbon coating, even though nanosiz- ing 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-VOSO4 are in progress. Acknowledgements The authors gratefully acknowledge the CNRS, the Minister of Education and Research, the Region of Normandy and Labex “Energy Materials & Clean Combustion Center” for funding this work. We also thank J. Thuillier, S. Duffour and S. Gascoin for technical help. References 1. Singh P, Shiva K, Celio H, Goodenough JB. 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