Synthesis, Structure, and Na-Ion Migration in Na4NiP2O7F2: AProspective High Voltage Positive Electrode Material for the Na-IonBatteryDipan Kundu, Rajesh Tripathi, Guerman Popov, W. R. M. Makahnouk, and Linda F. Nazar*

Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L3G1

*S Supporting Information

ABSTRACT: In the recent hunt for novel Na-ion cathodehosts, a variety of sodium analogues of classic Li-ion structureshave been thoroughly explored. However, Na-ion analoguesgenerally possess modied structures and dissimilar Na-ionenergetics compared to their Li-ion analogues due to the largesize of Na+ (102 pm) vs Li+ (76 pm), often resulting insluggish Na+ kinetics. Materials development targeted towardnew and dierent specic host structures possessing optimumproperties for Na-ion migration is crucial. Here, we report therst sodium metal uoropyrophosphate Na-ion host with athree-dimensional frameworkNa4NiP2O7F2which is predicted to have a high voltage (5 V) based on its Ni2+/3+/4+ redoxcouple and composition. Structure solution from single crystal diraction data combined with atomistic simulation computationsuggests the presence of low activation energy Na-ion migration pathways (

shown much promise to date.23,24 Another NASICON typematerial, Na3Ti2(PO4)3, is a potential anode material.

25

Incorporating F into the polyanionic structure further givesrise to a new class of structural hosts and compositions. Avariety of favorable candidates have emerged, includingNaVPO4F,

26 Na3V2(PO4)2F3,27 Na1.5VOPO4F0.5,

28 andNa2FePO4F.

29 The presence of electronegative uorine ensuresa relatively high average voltage for the V3+/V4+ redox couple inNa-ion cells. Reversible de(intercalation) of Na+/Li+ intolayered Na2FePO4F was rst demonstrated by Ellis et al.

29 Arobust framework structure along with a very small volumechange (3.7%) upon Na+ de(insertion) results in goodelectrochemical reversibility.30,31 Later, Tripathi et al. usedatomistic simulation to predict high Na+ mobility (0.29 eV)along the [100] direction in the 2D network in the ac plane ofPnma Na2FePO4F.

32 Very recently, Park et al. demonstratedexcellent electrochemical cyclability for Na1.5VPO4.8F0.7,

33

which utilizes the V3.8+/V5+ redox range at an average voltageof 3.8 V against Na+/Na. They ascribed the outstanding cyclingperformance of this compound to its structural rigidity and verylow volume change (2.9%) upon Na+ de(intercalation), alongwith rapid Na+ diusion (activation barrier: 0.30 eV) in thetwo-dimensional ab plane of the tetragonal P42/mnm structure.In this work, these concepts have been integrated to

synthesize a novel prospective high voltage NIB positiveelectrode material, Na4NiP2O7F2 which possesses a 3D openframework structure and low energy pathways for Na-ionmigration in all three dimensions. To gain a clear understandingof Na+ mobility, interatomic potential based atomisticsimulation methods were applied. They suggest a very high3D Na+ ion mobility in the structure with a lowest energybarrier of 0.36 eV. This is similar to the values predicted forNa2FePO4F and Na1.5VPO4.8F0.7 (see above), although thesematerials are restricted to two dimensions in ion mobility. Thecomputational prediction is validated by temperature depend-ent ionic conductivity measurements that provide a lowactivation energy of 0.32 eV/atom for Na+ ion migration. Bycomparison to olivine LiNiPO4

34where high voltage of 5.2 Vvs Li is predictedand considering the electron withdrawingeect of P2O7 and uorine on the NiO bond,

35 a redoxvoltage of 5 V vs Na is anticipated for Na4NiP2O7F2 andconrmed by preliminary electrochemical studies. This wouldmake it suitable for high energy density batteries preferablycombined with a solid state electrolyte.

EXPERIMENTAL SECTIONSingle Crystal Synthesis. Single crystals of the target compound

Na4NiP2O7F2 were recovered from a ux reaction of NiO andNa2PO3F. A similar method was used to produce single crystals ofNa2FePO4F earlier by our group.

29 In a typical procedure, NiO wasmixed and ground with an excess of Na2PO3F, poured into a goldcrucible, and placed in a silica glass tube. The reaction mixture washeld at 675 C for 8 h under ambient atmosphere and was cooledslowly to 400 C over a period of 10 h followed by a rapid cooling toroom temperature. The ux was dissolved in water by sonication, andcrystallites were collected by ltration. Pale green crystallites ofNa4NiP2O7F2 were obtained mixed with a second phase, identied asNa2NiPO4F.Preparation of Single Phase Polycrystalline Powder. For the

synthesis of polycrystalline Na4NiP2O7F2, Na2PO3F (vacuum-dried at90 C over a period of 1 week) was mixed with as-synthesized NiOnanopowder in a 2.5:1 molar ratio. The mixture was thoroughlyground and pelletized and then heat treated under an argon ow at575 C for 4 h. As-obtained green pellets were ground and washedthoroughly with deionized water to obtain phase-pure Na4NiP2O7F2.

Single Crystal and Powder X-ray Diraction. A needle shaped0.170 0.060 0.040 mm pale green crystal grown in a Na2PO3F uxwas mounted on a nylon ber with peruoropolyether oil. Data werecollected at 295 K using a Bruker APEX CCD (charge-coupled device)platform diractometer with monochromated Mo K ( = 0.7107 )radiation. Data collection and reduction were carried out with theBruker AXS SAINT and SMART5.0 programs, respectively. The nalunit-cell parameters were determined from 4,064 reections. A total of989 independent reections were used for the solution by least-squaresstructural renement (F2) using the Bruker SHELXTL package.

Powder X-ray diraction was performed on a Bruker D8-Advancepowder diractometer equipped with a Vantec-1 detector, using CuK radiation ( = 1.5405 ) in the range from 10 to 80 (2) at astep size of 0.025 using BraggBrentano geometry. X-ray datarenement was carried out by conventional Rietveld renementmethod using the Bruker-AXS TOPAS 4.2 software (Bruker-AXS,2008). The background, scale factor, zero point, lattice parameters,atomic positions, and coecients for the peak shape function wereiteratively rened until convergence was achieved.Scanning Electron Microscopy and Thermal Analysis.

Powder samples were gold coated and examined in a LEO 1530eld emission scanning electron microscope (FESEM) equipped withan energy dispersive X-ray spectroscopy (EDX) attachment. Imageswere recorded at 15 kV with a backscattered electron detector.Thermogravimetric analysis (TGA) was performed using a TAInstruments SDT Q600 in air at a heating rate of 5 C min1.Ionic Conductivity Measurements. The Na4NiP2O7F2 powder

was pressed into a pellet of 2.16 mm in thickness and 11.38 mm indiameter using an uniaxial press (Carver) under a pressure of 4 ton/cm2. The pellet was then sintered at 400 C under an argon ow for 12h. Silver conductive paste was applied to both sides of the pellet beforemounting it in a symmetric Swagelok-type cell, sandwiched betweentwo stainless steel rods. The assembled cell was then heated at 100 Cfor 1 day and tightened to seal it before cooling it down. The bulkresistance of the pellet was measured using two probe ac impedancespectroscopy (VMP3, Biologic) in a frequency range of 200 kHz to100 mHz using a potentiostatic signal perturbation of 300 mV.Impedance spectroscopy data were recorded in a temperature range of50 to 200 C, in a temperature controlled oven. Bulk ionicconductivities at dierent temperatures were obtained by tting theNyquist plot using EC-lab Z-t analysis software. The activationenergy for Na-ion diusion was obtained by linear tting of the ionicconductivity values at dierent temperatures by applying the Arrheniusequation, modied to include a temperature dependent pre-exponential: T = 0 exp(Ea/kbT), where is the temperaturedependent ionic conductivity, 0 is the ionic conductivity at absolutezero temperature, Ea is the activation energy of ion migration, and kband T have their usual meanings.Electrochemistry. Electrochemical properties of the material were

evaluated in Swagelok type cells against a Na metal anode using 0.5 MNaTFSI in P13-TFSI ((N-methyl-N-propylpyrrolidinium bis-(triuoromethanesulfonyl) imide) as the electrolyte. A Teon treatedcarbon paper (Toray, TGP-H-030, Fuel Cell Store) was used as thecathode current collector instead of aluminum to avoid electro-chemical corrosion. For electrode fabrication, Na4NiP2O7F2 micro-crystalline powder was mixed with Super P by ball milling at 400 rpmfor 5 h. The obtained mixture was mixed with PVDF as a binder inNMP (N-methyl-2-pyrrolidone) to obtain a slurry which was doctorbladed onto the Toray carbon paper. The weight ratio of activematerial:carbon:binder in the cathode was 70:20:10. Electrodes of 1cm2 geometric area were punched and dried at 120 C in a vacuumoven for 12 h prior to assembly in the cell.Computational Modeling. Atomistic scale simulations used in

the current study employ the Born model, where the ionic interactionsare divided into long-range Coulombic forces and short-range forcesrepresenting electronelectron repulsion and van der Waalsinteractions.36 Short range interactions were modeled using a twobody Buckingham potential. To account for the angle dependence ofthe OPO bond in PO43, a three body potential term was used.The well-known shell model used to account for the polarizability of

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the charged defects, lattice relaxation around defects, and migratingions was calculated by implementation of the Mott-Littleton schemeincorporated in the GULP code.37 These methods have beendescribed in detail elsewhere and have been extensively used forpredicting defects and ion migration in a range of oxides, phosphates,silicates, and sulfates.3841 In the current work, all the interatomicpotentials except NiO and NiF were obtained from our earlierwork on Na2FePO4F.

32 Interatomic potentials for NiO and NiFwere obtained by recursively changing the FeO and FeF potentialsused in Na2FePO4F to match the lattice parameters obtained from thediraction pattern of Na4NiP2O7F2 until excellent agreement(

0.62 eV for long-range Na ion diusion. These results suggest ahigh Na ion mobility within the b axis channels and goodmobility within the a and c axis channels leading to 3D Na-ionic diusion, which is expected to be benecial for highpower energy storage. The existence of low energy ionicdiusion pathways along three dimensions further signies thatantisite defects will be of lesser signicance for Na ionmigration in Na4NiP2O7F2 than they are for 1D diusionmaterials like olivine, LiNiPO4. These results also demonstrate

that the Na ion diusion in Na4NiP2O7F2 is not impeded owingto bottlenecks.Synthesis of Polycrystalline Powder and Ionic

Conductivity Measurements. Single crystal synthesis ofthe title compound yielded needle shaped Na4NiP2O7F2crystals as described in the Experimental Section, along withNa2NiPO4F as the major phase. The preparation of phase purepolycrystalline material is vital to the exploration of physicaland electrochemical properties relevant to Na-ion batteryapplications. Attempts to synthesize this material starting fromcommercial microcrystalline NiO and Na2PO3F at a temper-ature of 550600 C resulted in incomplete reaction, where theproducts consisted of Na4NiP2O7F2 and unreacted NiO.Prolonging the reaction time or attempting the synthesis athigher temperature (>600 C) resulted in Na2NiPO4F as asecondary phase. This illustrates the thermodynamic meta-stability of Na4NiP2O7F2 and its tendency to convert toNa2NiPO4F at elevated temperature. Apart from reactiontemperature and time, the moisture content of the precursorswas also found to have an inuence on the productcomposition. In order to achieve a complete reaction of apure phase material in a shorter reaction time, NiOnanopowder was synthesized and employed as the precursor(Supporting Information Figure S1).Successful synthesis of Na4NiP2O7F2 was conrmed by

powder X-ray diraction and Rietveld renement (Figure 3a).The details of the data collection parameters, rened latticeparameters, goodness of t, and comparison with the single

Figure 2. (a) Visualization of all unique NaNa distances (marked L1L5) and actual migration trajectory (as calculated) along L1L5 hopsrepresented by colored spheres. Na ion migration pathways in Na4NiP2O7F2 viewed along (b) (010), (c) (001), and (d) (100) directions. Colorscheme: L1, olive; L2, wine; L3, light green; L4, dark green; L5, golden.

Table 2. (a) Hops (as described in Figure 2a) for Na-IonMigration: Lengths and Corresponding Activation EnergyBarrier and (b) Activation Energy Barrier for Long RangeNa-Ion Migration in Na4NiP2O7F2

(a)

hop length () activation energy (eV)

LI 3.28 0.53L2 3.54 0.61L3 3.51 0.36L4 3.89 0.57L5 3.24 0.52

(b)

direction combination of hops activation energy (eV)

[100] L1L2L5 0.61[010] L3 0.36[001] L4 0.57

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crystal data are provided in the Supporting Information. Thetypical crystallite morphology and dimensions are shown in therepresentative SEM image (Figure 3b). The crystals are small,rod-like, and faceted, with lateral dimensions ranging from 2 to5 m. Thermogravimetric analysis was used to assess thethermal stability of the Na4NiP2O7F2 powder sample underambient atmosphere (air) in a temperature window between 20and 800 C (Supporting Information Figure S2). The materialis thermally stable below 600 C, above which a phasetransition takes place indicated by the small peak in the DTAprole (blue curve). The DTA peak is most likely associatedwith conversion of the Na4NiP2O7F2 to Na2NiPO4F, asobserved during the (unwanted) production of the former atlonger reaction times or at higher synthesis temperatures. Theobserved thermal stability in air suggests a good oxidativestability for the framework structure, which is crucial for thestability of a high voltage cathode under overcharge. In largepart, this owes to the fact that pyrophosphates are much morestable to oxygen loss compared to phosphates.The activation energy for Na ion migration was exper-

imentally determined from linear tting of the Arrhenius plot ofthe bulk conductivity of a pressed powder pellet sample.Complex plane impedance plots of the polycrystalline powdersample were obtained at dierent temperatures ranging from 50to 200 C. The same equivalent circuit model (SupportingInformation Figure S3) was used to t the impedance plots forthe entire temperature range, consisting of two conventionalcharge transfer (Rct)/constant phase element (CPE) in series.These two R/C components correspond to the bulk ionicconduction and that arising from grain boundary contributions,which are distinguished based on their frequency dependence.43

Bulk and grain boundary conductivities, denoted by b andgb, respectively (summarized in Supporting Information), areobtained from tting the impedance data (SupportingInformation Figure S4). As shown in Figure 4b (inset), thebulk conductivity value increases with rise in temperature,which is typical of a fast ion conductor. A bulk conductivityvalue of 2.03 108 S cm1 is extrapolated at 298 K based onthe parameters obtained from the linear tting of the Arrheniusplot. This is comparable to the ion conductivity of undopedpolyanion materials such as 107 S cm1 for NASICON typeNa3V2(PO4)3

44 and 108 S cm1 for LiZr2(PO4)3,45 or 107

S cm1 for Na3TiP3O9N,46 a recently explored Na ion cathode

compound. The bulk conductivity increases 1000-fold at 200C. A very low activation energy of 0.32 eV/atom for bulk Naion migration is also obtained (Figure 4b), in very close

agreement with the energy of the least hindered Na ionmigration along the [010] crystallographic direction, obtainedby computational modeling. This provides direct experimentalevidence of a low barrier to Na ion mobility, facilitated by the3D open framework structure. The electrical conductivity valueof Na4NiP2O7F2 is expected to be comparable to the Ni

2+ basedpolyanionic compound like LiNiPO4 (>10

9 S cm1).47

Therefore, the conductivity obtained from the ac impedancespectroscopy measurement, which exceeds that of electricalconductivity, is expected to be predominantly ionic in nature.This ionic conductivity is quite substantial and can potentiallybe enhanced by introducing Na ion vacancies or by substitutionat the transition metal site.Na+/Li+-Ion Exchange of Na4NiP2O7F2. Ion exchange is

an eective way of preparing novel metastable phases that areotherwise inaccessible via traditional synthetic routes. More-over, successful ion exchange clearly demonstrates the presenceof mobile and replaceable ions in the structure. In this work,Na+/Li+ ion exchange was carried out in 1 M LiBr/acetonitrileat 70 C for 2436 h. This leads to the expulsion of about 3Na+ for Li+ per formula unit as revealed by energy dispersive X-ray spectroscopy in the SEM (Supporting Information FiguresS5 and S6). The exchange of more than two Na+ ions directlyindicates the ability to exchange both Na1 and Na2 sodium in

Figure 3. Rietveld renement of the powder X-ray diraction pattern of the synthesized polycrystalline Na4NiP2O7F2. Data points (red circles);calculated prole (black line); dierence prole (blue line); Bragg positions (magenta lines). (b) A typical SEM image showing the morphology ofthe polycrystalline Na4NiP2O7F2.

Figure 4. Arrhenius plot of ionic conductivity (using a temperature-dependent pre-exponential factor). Black dots represent the datapoints. The activation energy for Na+ ion migration is calculated fromthe slope of the tted line (red). The variation of bulk ionicconductivity vs temperature obtained from the impedance data ttingis shown in the inset.

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Na4NiP2O7F2, pointing to their mobile nature in agreementwith the computational studies. The morphology of the ionexchanged sample remained similar to that of the pristine Naphase, proving the physically nondestructive nature of theprocess. Structural investigation of the lithiated phase wasperformed using XRD and time-of-ight neutron diraction(ND). Details are presented in the Supporting Information. Inboth cases, the reections were indexed to a noncentrosym-metric space group Pna21. Although structure determination ofthe lithiated phase was not possible, the change in the spacegroup together with the EDX data clearly indicates ionexchange under mild conditions, conrming the presence ofmobile Na+ ions with a low activation barrier to migration. Thisis a prerequisite for both Na-ion battery cathode and solidelectrolyte applications.Electrochemical Performance of Na4NiP2O7F2. This

material is expected to undergo Ni2+/Ni4+ redox at highpotential based on its polyanionic framework as explainedabove. Its electrochemical performance was therefore evaluatedagainst a Na metal anode using 0.5 M NaTFSI in P13-TFSI( ( N - m e t h y l - N - p r o p y l p y r r o l i d i n i u m b i s -(triuoromethanesulfonyl) imide) as the electrolyte. Theionic liquid (IL) based electrolyte was chosen because ILs aretheoretically predicted48 to have much higher anodic stability(>6 V against Li+/Li) by comparison to organic carbonatebased electrolytes or even sulfones. A Teon treated carbonpaper was used as the cathode current collector to preventelectrochemical corrosion by TFSI based salts or ionic liquids(see details in the Experimental Section).49 Information onoverall electrolyte stability in the absence of a cathode wasdeduced from cyclic voltammetry measurement in a potentialwindow of 3 and 5.5 V, as shown in Figure 5a. A high anodicstability is observed without any major decomposition currentboth in the oxidation and in the reduction sweep. The samefeature is reproducible over consecutive CV scans, conrminglong-term electrochemical stability of the chosen electrolyte.Galvanostatic chargedischarge protocols were used tomeasure electrochemical sodium (de)intercalation fromNa4NiP2O7F2 as shown in Figure 5b. On the basis of anexpected 2 e redox for Ni2+/Ni4+ a theoretical capacity of 150mA h g1 is expected. In the rst electrochemical charge,however, only about 0.35 Na+ is extracted at an average voltageof 55.2 V. Such a high average voltage is anticipated fromthe theoretically calculated voltage of LiNiPO4, namely, 5.2 Vvs lithium34 (hence 4.9 V vs sodium, accounting for a 0.3 Vdierence in the redox potential between Na+/Na and Li+/Li).This is the only predicted value for Ni2+/Ni3+ couple in a

phosphate polyanionic compound. Prior to our work, no Nipyrophosphate systems have been reported nor computation-ally studied. In a uoro-pyrophosphate material, the stronginductive eect exerted by the PO moieties and uorineatoms would aect the Ni2+/Ni4+ redox energy,35 thuspredictably increasing the average redox voltage to >5 V vsNa. Following the initial charge region between 4.7 and 5.2 V,the prole exhibits a steady slope which most likely representspredominantly electrolyte decomposition catalyzed by thecathode. Furthermore, an insignicant amount of Na+ reinsertsback in the subsequent discharge. Clearly, even for anoxidatively stable ionic liquid electrolyte, oxidation at thecathode surface is quite probable at high voltages50,51 and likelyaggravated by interfacial parasitic reactions at the conductivecarbon interface.52

The electrochemical performance of this material is mostlikely also limited by kinetic factors, such as poor electricalconductivity and large particle dimensions which can becountered by synthesizing nanocrystals and/or better con-ductive composite formation. However, realizing the fullpotential of this compound would require electrochemistry inconjunction with electrochemically and thermally stable solidionic conductors in an all-solid-state battery, thus eliminatingany possibilities of cathode catalyzed liquid electrolytedecompositions.

CONCLUSIONSThe surge of NIB research has been mainly dominated byexploration of structural chemistries that are analogous to thewell-established Li-ion systems. Although this approach isundoubtedly advantageous to speed up materials development,targeted discovery of novel structural chemistries is crucial totap into the prospects of highly demanding applications such asgrid storage or electried transportation. In this regard, thenovel open framework Na4NiP2O7F2 with its low energy Na-ion conduction pathways, good sodium ionic conductivities,and impressive thermal stability is particularly attractive as aprospective rechargeable NIB cathode. Although investigationswith ionic liquid based electrolytes are preliminary, we note thatelectrochemical activity is observed at the highest reportedoperating voltage ever, inviting further exploration of thiscompound in an all solid state NIB. This work is expected toprovide the impetus for the discovery of a family of new sodiummetal uoropyrophosphate compounds as novel high voltagecathode host materials for NIBs.

Figure 5. (a) Cyclic voltammogram of 0.5 M NaTFSI in P13-TFSI with Toray paper as the working electrode at a scan rate of 1 mV/S againstmetallic sodium as the counter electrode. (b) Galvanostatic chargedischarge prole of Na4NiP2O7F2 cathode against Na anode at a current densityof 5 mA/g with 0.5 M NaTFSI in P13-TFSI as the electrolyte.

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ASSOCIATED CONTENT*S Supporting InformationDetailed single crystal data collection, powder diractionrenement results, thermogravimetric analysis, ac impedancespectroscopy, SEM-EDX, XRD, and neutron diraction data ofthe Li-exchanged sample. This material is available free ofcharge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATIONNotesThe authors declare no competing nancial interest.

ACKNOWLEDGMENTSWe gratefully acknowledge NSERC for generous fundingthrough its Discovery Grant and Canada Research Chairprograms. Dr. Jalil Assoud in the UWaterloo CrystallographicLaboratories is warmly acknowledged for assistance withcollection of the single crystal data and structure solution.The neutron diraction data was collected at the Oak RidgeNational Laboratorys Spallation Neutron Source; researchsponsored by the Scientic User Facilities Division, Oce ofBasic Energy Sciences, U.S. Department of Energy.

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Chemistry of Materials Article

DOI: 10.1021/cm504058kChem. Mater. 2015, 27, 885891

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