Electrochem Commun 2011 NaxVO2 as possible electrode for Na-ion batteries

Na x VO 2as possible electrode for Na-ion batteriesDavid Hamani a ,Mohamed Ati b ,Jean-Marie Tarascon b ,Patrick Rozier a ,⁎a CEMES/CNRS,Universitéde Toulouse,BP 94347,31055Toulouse Cedex,FrancebLaboratoire de Réactivitéet Chimie des Solides,CNRS UMR 6007,Universitéde Picardie Jules Verne,80039Amiens,Francea b s t r a c ta r t i c l e i n f o Article history:Received 17May 2011Received in revised form 2June 2011Accepted 3June 2011Available online 12June 2011Keywords:Na batteriesElectrode materials Insertion compound NaxVO 2phasesElectrochemical processWe report the electrochemical properties vs.Na of the layered Na x VO 2phases having either octahedral or trigonal prismatic symmetries.Both phases can reversibly insert 0.5Na atom per unit formula leading to sustained reversible capacities of nearly 120mAh/g.Their voltage pro file shows numerous voltage steps being mainly associated to subtle ordering of the Na ions within the layers while each phase preserves its initial structure through the entire Na insertion –deinsertion process.©2011Elsevier B.V.All rights reserved.1.IntroductionLi-ion batteries are now setting as the best contenders for automotive applications.Therefore fears of lithium shortage [1]drive researchers to investigate alternative battery technologies;one of them is the sodium technology [2]with its positive attributes like its abundance,ready availability,easy recovery and its non alloying reaction toward Al.Sodium has already been implemented in high temperature system cells,but the development of Na-ion cells has remained very sporadic.A few positive oxide insertion compounds (Na x CoO 2[3],NaMo 2O 4[4],Na x MnO 2[5–7],Na x Mo 6X 8[8])have been studied while so far no negative insertion ones have been reported with the exception of carbonaceous materials.Low potential Li insertion compounds are also very scarce owing to the competition between insertion vs.conversion reactions [9],which appears in favor of the conversion for the right 3d metal elements (MnO,CoO,NiO)and of the insertion for the left ones (TiO 2,V 2O 5).Supporting this statement is the early reported reversible insertion of Li into LiVO 2at 0.2V vs.Li +/Li°by Samsung for which the complete mechanism was recently elaborated [10].As sodium and lithium have somewhat related intercalation chemistry,we decided to embark into the electrochemical reactivity of Na x VO 2compounds toward Na,as the literature was free of published studies at the time we launched the investigation.Among the several announced Na x VO 2compounds only two with compositions x=1.0and x=0.7have been clearly characterized [11,12].They both belong to the well-known lamellar AMO 2phaseswith VO 6octahedra sharing edges to form [VO 2]−1layers with stacking sequences governing the oxygenated surrounding of Na ions lying in the interlayer space (octahedral for x=1and trigonal prismatic for x=0.7)(Fig.1).Using the nomenclature proposed by Delmas et al.[13]NaVO 2is O3type (Fig.1a)and Na 0.7VO 2is P2type (Fig.1b).While no speci fic reactivity for the Na 0.7VO 2(P2)phase is reported,NaVO 2(O3)reacts spontaneously with air (moisture)to form de-intercalated Na 1−x VO 2phases [12].Recently it has been shown that,even though leading in both cases to monoclinic distortion,chemical oxidation of NaVO 2(O3)induces a switch from octahedral to prismatic for Na +site [14]while electrochemical oxidation preserves the octahedral site [15].We report herein an attractive electrochemical activity of both (O3)and (P2)type phases toward Na.2.ExperimentalThe phases are prepared using appropriate amounts of Na 2CO 3(Aldrich)and V 2O 3(reduction of V 2O 5under H 2flow at 800°C)heated under slightly reductive (Ar/H 2:95/5)atmosphere at 800°C for 10h.The sample identification,which results from the comparison between the reported XRD patterns and the experimental ones collected using a Seifert 3000TT (λCuK α),shows that the x=0.7sample corresponds to the Na 0.7VO 2(P2)phase [11]while the sample with x =1is a mixture of NaVO 2(O3)and its monoclinic form.To prevent moisture reactivity,both phases were always handled in an inert atmosphere (Ar dry box)once recovered from their reacting vessel.Electrochemical tests were done using Swagelok®-type cells,assembled in an argon-filled glove box.The cells comprised a 5toElectrochemistry Communications 13(2011)938–941⁎Corresponding author.E-mail address:rozier@cemes.fr (P.Rozier).1388-2481/$–see front matter ©2011Elsevier B.V.All rights reserved.doi:10.1016/j.elecom.2011.06.005Contents lists available at ScienceDirectElectrochemistry Communicationsj o u r n a l h o m e p a g e :w ww.e l s ev i e r.c o m /l o c a t e /e l e c o m6mg composite positive electrode made by hand mixing powders of Na 0.7VO 2with 15%in mass of carbon Ketjen black and separated from the 1cm 2Na metal disc negative electrode by a Whatman GF/D borosilicate glass fiber sheet saturated with 1M NaClO 4in propylene carbonate as the electrolyte;they were cycled (rate C/20)in the galvanostatic mode using a Mac-Pile system.The insertion/deinsertion mechanism was studied by in situ X-ray diffraction using a home designed electrochemical cell,assembled similarly to our Swagelok cell,but with a beryllium window as current collector.XRPD patterns were collected using a Bruker D8diffractometer (CuK α)for every exchange of 0.05Na ions.3.Results and discussionThe electrochemical behavior of NaVO 2(O3)vs.Na +/Na 0(Fig.2a)shows a perfect reversibility of the Na insertion/de-insertion processes in the 1.2–2.4V (vs.Na +/Na)domain.It also shows Na de-intercalation process to induce i)the reversible formation of two de fined compounds Na 1/2VO 2and Na 2/3VO 2and ii)for oxidizing potentials greater than 2.4V the irreversible formation of an electrochemically inactive phase.As our experimental data fully agree with previous ones [15],they will solely be used for the sake of comparison fromhereafter.Fig.1.Structure representation of a)NaVO 2(O3)and b)Na 0.7VO 2(P2)with indication of anionic (red)layers stacking sequences;Na ion (green)surroundings and interlayerdistance.Fig.2.Voltage-composition curves for a)NaVO 2(O3);c)Na 0.7VO 2(P2)and associated respectively b)and d)derivative curves.Insets indicate the capacity evolution vs.number of cycles.The cells using 5to 6mg of positive electrode composites were cycled at C/20.The small polarization between charge and discharge for the P2phase suggests a high rate capability.This was con firmed by power rate measurements (e.g.signature curves)as this electrode can deliver 90%of its capacity at C rate (1Na +in one hour).939D.Hamani et al./Electrochemistry Communications 13(2011)938–941Fig.2c shows the electrochemical behavior of Na 0.7VO 2(P2)vs.Na +/Na 0.The charge –discharge curves nicely track each other indicating a fully and sustainable (Fig.2c inset)reversible Na intercalation /de-intercalation process over the voltage range 1.2–2.4V vs.Na +/Na.Besides,whether the cell was started on oxidation (e.g.,charge)or in reduction (e.g.,discharge),a staircase type variation of the voltage suggests the appearance of subsequent two-phase and single-phase domains,which occurs over a very narrow composition range especially for the highest potential domain (1.6–2.4V).From the starting material nominal composition and the amount of transferred charge we could deduce that the Na intercalation process occurs in the range 0.47b x b 0.92within the accuracy of the experiment.The sodium insertion/deinsertion mechanism in Na 0.7VO 2(P2)was examined by in situ X-ray diffraction with patterns collected at every exchange of 0.05Na ions.For reasons of space we solely report i)the evolution of the main (002)Bragg peak (Fig.3b)upon cycling in order to nicely convey the reversibility of the process and ii)the patterns collected during the second charge,but only showing the most relevant 2Θrange (30−45°)for phase identification (Fig.3c).We identified four single-phase domains denoted A,B,C and D whose composition is indicated on the voltage composition curve (Inset Fig.3c).They were successfully indexed (full profile matching with conventional R p and R wp ranging between 7and 10%),on the basis of a hexagonal cell (SG:P63/mmc)as for the precursor Na 0.7VO 2(P2)suggesting that there is no drastic structural change when varying the Na content.The lattice parameters variation reported along one charge in Table 1and as a function of the sodium content upon cycling in Fig.4con firms the reversibility of the structural evolutions.Due to the observed reversibility,we next focused on solely describing the structural evolution of the Na x VO 2(P2)system during one charge starting start with the discharged compound D and thenconsidering the subsequent oxidized phases (C,B,A).Through this description,we must keep in mind (Fig.1)the interplay between cell parameters and both the interlayer and the shortest in-plane V –V interatomic distance.Along that line,compound D exhibits the highest V –V distance (2.99Ǻ)identical to the ones reported for the phases with high V 3+content [12]and the lowest interlayer distance (5.45Ǻ)which,although shorter than usually observed for P-type phases (5.70Ǻ)[11],con firms the large attractive effect between Na +ions and VO 2δ−layer (e.g.a large screening effect).Upon Na removal,in agreement with the lowering of the Na ion screening effect,the transformation of D into C is associated to a discontinuous increase in the interlayer distance (5.45Ǻup to 5.54Ǻ);this is followed through successive C to B then B to A phase transformations by an almost linear increase up to 5.73Ǻfor the highest value obtained in the A domain.Worth noting is the evolution of the V –V interatomic distance during sodium removal with namely a continuous decrease in the C solution domain (2.99Ǻdown to 2.90Ǻ)as opposed to a discontinuous one for the C to B transformation with the end distance being about 2.87Ǻ.This value,in agreement with the one reported for low Na content phases [11],remains constant over the A and B solution domains.Though the overall evolution is explained by the decrease in the V ionic radii due to V 3+to V 4+oxidation,as the amount of V 4+linearly increases with Na removal,such irregular variation of the V –V distance could be reminiscent of a more complex phenomenon enlisting,among others,a strong V 3+–V 3+coupling [12].As part of this complexity,turning to the B →A phase transition,one should note that it is accompanied by several step voltage changes centered around 2.2,2and 1.8V that we failed to interpret.One belief:this is due either to subtle phase transitions undetectable when using laboratory XRD sources or most likely to the presence of density of states singularities in their electronic band structure,which should serve as an impetus for initiating theoreticalstudies.Fig.3.a)Voltage –composition curve during the two first cycles together in b)the evolution of the (002)Bragg peak upon cycling,and in c)the XRD patterns collected during the first charge within the (30–45°)2θrange domain,with indications of single and biphasic domains reported on the cycling curve d).The cell using a 12mg composite electrode was cycled at C/20.940 D.Hamani et al./Electrochemistry Communications 13(2011)938–941At this stage,a comparison of the electrochemical behavior between Na 0.7VO 2(P2)and Na 1VO 2(O3)phases with respect to Na +/Na 0shows similarities among which are 1)a reversible process involving an overall uptake or removal of ~0.5Na +ions and 2)the maintenance of the original type of structure (including oxygenated surroundings of Na ions)whatever the amount of exchanged Na.However,the charge –discharge polarization is amazingly small (b 50mV)for Na 0.7VO 2(P2)as compared to 200mV for the NaVO 2(O3)phase using a similar testing procedure (e.g.equal amounts of electrode,carbon additive and current rates).This can be interpreted on the basis of reported electronic properties showing that NaVO 2(O3)is an insulator [12]while Na x VO 2phases (both O ′3and P2)are semiconductors with the latter Na 0.7VO 2(P2)phase presenting the highest electronic conductivity [11].Additionally,the shape of the charge –discharge curves and their corresponding derivatives (Fig.2)indicate that the reactivity mecha-nisms are different.Five reversible oxidation –reduction peaks are present for the Na 0.7VO 2(P2)phases as opposed to four for the NaVO 2(O3);moreover none of them occurs at the same voltage.The well defined biphasic domain evidenced by the plateau located at low voltage and high Na content for NaVO 2(O3)is substituted for a more complex voltage composition curve for Na 0.7VO 2(P2)as confirmed by in situ XRD analysis with namely the presence of a de fined D compound and two C and D phases with varying Na contents (e.g.;not single line phase).The upper part of the voltage composition curves,whatever the starting compound,presents a succession of staircase voltage variation indicating sequences of biphasic and solution domains with,in both cases,the high oxidation plateau (above 2.4V)corresponding to the irreversible formation of an inactive phase.Dealing with structural features,whatever the starting material,the evolution of the interlayer distance upon Na removal is roughly identical ranging from 5.4Ǻ,for low voltage and high Na content phases,up to 5.7Ǻfor high voltage and low Na content phases.These similarities indicate that despite different stacking sequences of the anionic planes,the interlayer distance is driven by the attractive effect due to bridging Na ions rather than their respective site.Along that line,the discontinuities in the evolution of the c cell parameter for almost the same Na contents suggest the existence of successive orderings of the remaining Na ions in the interlayer space.The evolution of the shortest V –V distance appears here again roughly identical meaning close to 2.87Ǻand 3.00Ǻfor low and high Na content phases respectively.Note however that the monoclinic distortion observed upon Na removal from NaVO 2(O3)[15],is not evidenced at first for (P2)starting phase.4.ConclusionThrough the present study we show the two Na x VO 2polymorphs (having either Na located in trigonal prismatic or octahedral environments)capable of i)reversibly reacting with 0.5Na ions per unit formula,and ii)having more than 50%of their capacity centered near 1.6to 1.7V,while preserving their initial prismatic or octahedral sites throughout the entire Na uptake and release process.Moreover,despite their rocky voltage profiles such electrodes show outstanding capacity retention.Nevertheless,a thorough investigation of physical properties of these Na x VO 2phases as well as band structure calculations will be necessary to fully interpret their voltage pro file variation upon cycling.Such results provide a further impetus to consider V or Ti-based compounds for our search of low voltage sodium insertion compounds.AcknowledgmentsMany useful discussions with 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