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Journal of Power Sources 247 (2014) 377e383

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Journal of Power Sources

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Unexpected performance of layered sodium-ion cathode materialin ionic liquid-based electrolyte

Luciana Gomes Chagas, Daniel Buchholz, Liming Wu, Britta Vortmann, Stefano Passerini*

Institute of Physical Chemistry, MEET Battery Research Center, University of Muenster, Corrensstrasse 28, 48149 Muenster, Germany

h i g h l i g h t s

� Performance of layered NaNCM cathode in different electrolytes.� NaNCM/Na cells offer a specific energy of 550 Wh kg�1 for the positive electrode active material and average voltage of 2.7 V.� Reduced Mn2þ dissolution and good capacity retention in ionic liquid-based electrolyte.

a r t i c l e i n f o

Article history:Received 9 July 2013Received in revised form22 August 2013Accepted 26 August 2013Available online 5 September 2013

Keywords:SodiumCathode materialLayered compoundIonic liquid-based electrolyteSodium battery

* Corresponding author.E-mail address: stefano.passerini@uni-muenster.d

0378-7753/$ e see front matter � 2013 Elsevier B.V.http://dx.doi.org/10.1016/j.jpowsour.2013.08.118

a b s t r a c t

The electrochemical performance of Na0.45Ni0.22Co0.11Mn0.66O2 in an ionic liquid-based electrolytic so-lutions is reported and compared with that obtained in a conventional, carbonate-based electrolyte. Evenat ambient temperature, the Na-ion intercalation material reveals a much better electrochemical per-formance in 10 mol% NaTFSI (or 0.45 M) in PYR14FSI electrolyte than 0.5 M NaPF6 in PC electrolyte interms of specific capacity and cycling stability. In particular, the electrodes cycled in the IL-based elec-trolyte combine a capacity retention of about 80% after 100 cycles with high specific capacities (about200 mAh g�1) and high average voltage (2.7 V vs. Na/Naþ), demonstrating that Na0.45Ni0.22Co0.11Mn0.66O2

is a promising cathode material for sodium-ion batteries.� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Nowadays, the lithium-ion technology is proposed as the mostpromising battery chemistry for large scale applications, e.g. elec-tric vehicles, although serious concerns about availability and pricestability of lithium resources exist [1e3]. The low cost, high abun-dance and easy mining of sodium minerals as well as the feasibleuse of aluminum as anode current collector and aqueous electro-lytes [4] promote the interest in sodium-based electrochemicalsystems as an alternative for lithium-ion batteries for energy stor-age devices [5].

Lithium and sodium share neighboring positions in the periodictable and the fundamental principles of sodium- and lithium-basedsystems are indeed very similar [2,3]. On the other hand sodiumhas a larger ionic radius than lithium, it is about three times heavier

e (S. Passerini).

All rights reserved.

and its redox potential is 300 mV above that of lithium [4e6]. Thus,sodium-based systems have a lower energy density compared tolithium-based systems [7]. The development and improvement ofnew high-performance sodium intercalation materials is necessaryto overcome the energy density limitation and to forward theirimplementation [4,8].

Layered LiMO2 materials (M ¼ transition metal) have beenthoroughly studied for lithium-ion batteries due to their excellentelectrochemical characteristics, e.g., high average voltage and ca-pacities [9]. Hence, similar layered NaMO2 cathode materials havebeen considered promising for sodium-ion batteries [4,5] and someexamples are showed below.

Caballero et al. [10] synthesized Na0.6MnO2 to investigate theion-exchange mechanism of Naþ and Liþ ions in 1M LiPF6 inethylene carbonate (EC)/dimethyl carbonate (DMC). This systemdelivered an initial capacity of 150 mAh g�1 vs. Li, but only a ca-pacity retention of 50% after 24 cycles could be achieved. Kim et al.[11] reported the electrochemical performance of layeredNa0.85Li0.17Ni0.21Mn0.64O2 exhibiting an average voltage of 3.4 V vs.

L.G. Chagas et al. / Journal of Power Sources 247 (2014) 377e383378

Na/Naþ in 1M NaClO4 in propylene carbonate (PC) as electrolyticsolution. At 0.2C the cell exhibited a reversible capacity of about100 mAh g�1 for 50 cycles.

Following the idea of the successful compound LiNCM [9],layered NaNi1/3Mn1/3Co1/3O2 (NaNMC) was synthesized and elec-trochemically tested using 1 M NaClO4 in EC/DMC as electrolyte, bySathiya et al. [7]. The reversible intercalation of 0.5 eq. Naþ wasachieved leading to specific capacities of about 120 mAh g�1 vs. Na(3.75e2.0 V). However, cycling this material up to 4.20 V wasleading to a high capacity loss. A sodium-ion battery was presentedby Kim et al. [8] using the layered NaNi1/3Fe1/3Mn1/3O2as cathodematerial and hard carbon as anode (NayC) with 1M NaClO4 in PC aselectrolyte. The cell revealed a specific capacity of about100mAhg�1with high coulombic efficiencies (>99%) for 150 cycles.

Yabuuchi et al. [12] synthesized and characterized P2-typeNax[Fe0.5Mn0.5]O2 in sodium half cells using 1M NaClO4 in PC aselectrolyte. This compound, using earth-abundant elements,delivered 190 mAh g�1 in the first discharge with an averagevoltage of 2.75 V, unfortunately accompanied by a low capacityretention of only 75% after 30 cycles.

Summarizing, in all layered compounds reported so far eitheronly relatively low specific capacities could be obtained or strongcapacity fade was present, limiting the application of these cath-ode materials in sodium-ion batteries. Furthermore, all thesematerials were electrochemically characterized in typical organiccarbonate-based electrolytes, which can favor the manganesedissolution into the electrolyte, as a result of JahneTeller distor-tion [13]. In this work we explore the effect of alternative elec-trolytes based on ionic liquids and their influence on thereversibility of Naþ ion intercalation in a layered material. For sucha reason we compare the electrochemical performance of layeredNa0.45Ni0.22Co0.11Mn0.66O2 in two different electrolytic solutions,organic carbonate (PC)- and ionic liquid (IL)-based. The in-vestigations reveals that the electrochemical performance ofNa0.45Ni0.22Co0.11Mn0.66O2 in the IL-based electrolyte is superior tothat in PC-based electrolyte in terms of specific capacity andcycling stability as well, although both cell systems were oper-ating at about 20 �C. Very high initial specific discharge capacities(225 mAh g�1) with an average voltage of about 2.9 V could beobtained for both cell systems. However, the capacity retention ofabout 80% after 100 cycles was obtained only with the ionic liquid-based system.

2. Experimental section

Thematerial was synthesized by a solid-state reaction of sodiumhydroxide (NaOH, Aldrich, >98%) and a manganeseenickelecobalthydroxide precursor. The latter was prepared by co-precipitating anaqueous solution of the three metal acetate salts (Mn, Ni and Co;Aldrich, >98%, weight ratio of 66:22:11, respectively) with sodiumhydroxide (50% excess). After extensive rinsing with distilled water,the precipitate was dried under vacuum at 120 �C overnight. Thedriedmaterial was then dispersed in an aqueous solution of sodiumhydroxide (0.76 eq. of NaOH per mole of Ni0.22Co0.11Mn0.66(OH)2)and the water was slowly removed by a rotary evaporator. Afterdrying and grinding, the mixture was annealed in air at 500 �C for5 h, and then at 750 �C or 800 �C for 6 h, using an open-air muffleoven. For the water treatment about 1 g of the as-preparedmaterialwas stirred in 20 mL of distilled water (at 25 �C) for 5 min. Thesuspension was then filtered and washed with 80 mL of distilledwater, dried at 120 �C in air for 24 h. Afterward the material wasground by hand in a mortar, screened over a 45 mm sieve and finallystored under inert atmosphere.

For the electrode preparation a slurry was made by mixing85wt.% of activematerial, 10 wt.% carbon black Super C65 (TIMCAL)

and 5 wt.% polyvinylidene di-fluoride (PVDF 6020 Solef�, ArkemaGroup) binder dissolved in N-methyl-2-pyrrolidinone (NMP), viaball milling. Following, the slurry was casted on aluminum foilusing doctor blade and dried at 80 �C overnight. Afterward, rounddiscs electrodes were cut with 12 mm diameter, pressed and finallydried at 120 �C under vacuum for 24 h. The active material massloading in the electrodes was about 2.5 mg cm�2. The electrodeswere assembled into three-electrode cells with a glass fiber sepa-rator (Whatman) and sodium metal (99.8%, Acros Organics) ascounter and reference electrode in an argon filled glove box. Thesodium metal was cut from sodium chunks, then rolled and finallypressed on the current collector.

As electrolytic solution either a 0.5 M NaPF6 (sodium hexa-fluorophosphate, 98% Aldrich) in PC (propylene carbonate, UBE) orionic liquid electrolyte (IL), 10 mol% NaTFSI (sodium bis(tri-fluoromethanesulfonyl)imide, Solvionic, 99.5%) in PYR14FSI (N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide), corre-sponding to a 0.45 M solution, were used. The synthesis of PYR14FSIis already reported in literature [14]. Karl Fischer titrations wereperformed with a Mettler Toledo C30 Compact coulometer toevaluate the water content in the salts used to prepare the elec-trolytes. The coulometer was allowed to stabilize until a steady driftvalue close to 0 mg min�1 of water was achieved. Metallic dispos-able syringes were used to inject approximately 1 mL of the elec-trolytic solutions.

Cyclic voltammetry (CV) experiments were performed at a scanrate of 0.1 mV s�1 between 1.5 and 4.6 V vs. Na/Naþ, using a VMPmultichannel potentiostaticegalvanostatic system (Biologic Sci-ence Instrument, France). The cells were galvanostatically cycledat a current rate of 0.1 C (nominal capacity ¼ 123 mAh g�1,1 C¼ 123mA g�1) between 4.6 V and 1.5 V (vs. Na/Naþ) at 20� 2 �Cusing Maccor series 4000 battery tester (USA).

The manganese content in the electrolyte after cycling wasdetermined by inductively coupled plasma optical emission spec-trometry with a Spectro ARCOS ICP-OES (Spectro Analytical In-struments, Kleve, Germany) instrument with axial plasma viewing,with a 0.09 ppb limit of detection for manganese. The crystallinestructure was characterized by X-ray diffraction (XRD) using the CuKa radiation on the Bruker D8 Advance (Germany) in the 2q rangefrom 10� to 90�. Lattice parameters were determined by Rietveldrefinement with TOPAS software. The particle morphology wasevaluated via a high resolution scanning electron microscopy (FE-SEM, Zeiss Auriga).

3. Results and discussion

Details of the structural and electrochemical characterization ofthe layered P2-Na0.45Ni0.22Co0.11Mn0.66O2 as well as a comparisonof its performance upon sodium and lithium intercalation can befound in previous works [15,16].

Cyclic voltammetry was used to investigate the electrochemicalperformance of Na0.45Ni0.22Co0.11Mn0.66O2 (synthesized at 750 �C)in the organic carbonate (PC)- and ionic liquid (IL)-based systems.The cyclovoltammograms are depicted in Fig. 1.

In the cyclic voltammograms of the organic carbonate-basedsystem (Fig. 1a) six anodic peaks located at about 1.85, 2.35, 3.40,3.72 and 4.30 V and seven cathodic peaks at 1.77, 1.91, 2.30, 3.25,3.40, 3.60 and 4.10 V are detected in the first cycle, indicating theinitial reversibility of sodium (de-)insertion process into the ma-terial structure. Additionally, a peak shoulder is seen at lower po-tentials (<2.2 V) for the anodic sweep. From the second cycle thepeak shoulder disappears and only one broad peak is observed.Upon the consecutive voltammetric cycles a strong potential shiftand intensity decrease is observed for the lower (<2.2 V) andhigher (>3.8 V) potential features, besides the appearing of a

Fig. 2. Specific charge and discharge capacities of Na0.45Ni0.22Co0.11Mn0.66O2 (anneal-ing temperature: 750 �C) in the organic carbonate-based (circles) and the ionic-liquidbased system (squares) during galvanostatic cycling at 0.1 C (12 mA g�1). Cut-off limits:4.6e1.5 V (vs. Na/Naþ). Reference and counter electrode: Na. Electrolytic solution:0.5 M NaPF6 in PC (circles) or 10 mol% of NaTFSI in PYR14FSI (squares). Temperature:20 �C � 2 �C.

Fig. 1. Cyclovoltammograms of Na0.45Ni0.22Co0.11Mn0.66O2 (annealing temperature:750 �C) cycled in the a) organic carbonate-based and b) ionic liquid-based electrolytes.Cut-off limits: 4.6e1.5 V (vs. Na/Naþ). Reference and counter electrode: Na. Electrolyticsolution: a) 0.5 M NaPF6 in PC and b) 10 mol% of NaTFSI in PYR14FSI. Scan rate of0.1 mV s�1. Temperature: 20 �C � 2 �C. The arrows indicate the peak evolution duringcycling.

Fig. 3. Potential vs. capacity profiles of Na0.45Ni0.22Co0.11Mn0.66O2 (annealing temper-ature: 750 �C) during the 2nd, 5th, 10th, 50th, 75th and 100th cycle at 0.1 C (12 mA g�1)for a) the organic carbonate-based and b) the ionic liquid-based system. Cut-off limits:4.6e1.5 V (vs. Na/Naþ). Electrolytic solutions: 0.5 M NaPF6 in PC (a) or 10 mol% ofNaTFSI in PYR14FSI (b). Temperature: 20 �C � 2 �C. The arrows indicate the peakevolution during cycling.

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current peak at about 3.52 V in the anodic sweep. After that, thecurrent peaks between 2.2 and 3.8 V exhibit only minor changes inintensity and potential.

Fig. 1b shows the cyclovoltammograms of the ionic liquid-basedsystem. Seven anodic peaks located at about 1.90, 2.12, 2.45, 3.30,3.62, 3.72 and 4.35 V and six cathodic peaks located at about 1.90,2.35, 3.20, 3.48, 3.58 and 4.13 V can be distinguished, where by thebroad cathodic peak at 1.90 V originates from two overlappingcurrent peaks. Also, the overlapping in a single current peak atabout 3.55 V of the 3.48 and 3.58 V cathodic peaks is observed fromthe second cycle and forward. In contrast to the PC-based system,only minor potential shift is seen to affect the peaks during eitheranodic or cathodic sweep. Additionally, no major change in thepeaks intensity is observed. In both cell systems, the anodic andcathodic current peaks located at higher potentials (>3.8 V vs. Na)are correlated with the phase transition from prismatic P2-type tooctahedral O2-type coordination, which occurs at low sodiumcontents in the cathode material [15e18]. The decrease and shift ofthe peaks indicate the P2/O2 phase transition as the least reversibleprocess in both electrolyte systems, but, especially, in the PC-basedone. All the current peaks between 3.8 and 1.5 V relate to thedifferent accommodation and ordering of Na cations into the so-dium layers during the regular (de-)intercalation process [19,20]. Inaddition, the current peaks located at potentials lower than 2.5 Vcan further be related to a Mn3þ/Mn4þ redox process, [15,16]whereas it occurs at high sodium contents where manganesemust be redox activated since no further redox process of Ni and Co

can take place as it can be calculated on the base of the compoundstoichiometry.

Summarizing it can be stated that the minor changes of the IL-based cell clearly indicates for its more stable cycling behavior incomparison to the PC-based cell. To validate the assumption bothcell systems were galvanostatically cycled at 0.1 C (see Fig. 2).

At a first glance it is seen that the IL-based system has a muchbetter cycling performance in terms of cycling stability and specific

L.G. Chagas et al. / Journal of Power Sources 247 (2014) 377e383380

capacity compared to the PC-based system. The first cycle chargecapacity in both electrolyte systems correspond to the full des-odiation of the material, although somewhat higher than thetheoretical value of 123 mAh g�1, presumably due to electrolytedecomposition at higher potentials, especially, once more, for thePC-based system. The consecutive discharge delivers a capacity of225 mAh g�1 for both cells. The coulombic efficiency of both elec-trolyte systems increase in the initial cycles probably due to a betterwetting of the electrodes, but, unfortunately, decrease upon furthercycling.

The PC-based cell (circles) exhibits a higher capacity fade thanIL-based cell (squares). The former cell delivers discharge capacityof about 90 mAh g�1 at the 100th cycle, which means a capacityretention of only 40%. Furthermore, the coulombic efficiency in the100th cycle is only 97.8%. The IL-based cell exhibits a much bettercycling performance finally leading to a discharge capacity of177 mAh g�1 at the 100th cycle, which corresponds to a capacity

Fig. 4. SEM images of the pristine and after cycling electrodes in IL (10 mo

retention of about 80%. The coulombic efficiency slightly decreasesto about 98.7% in the 100th cycle.

The low coulombic efficiency in both systems can be explainedby a combination of different reasons. First, at lower sodium con-tents in the layered Na0.45Ni0.22Co0.11Mn0.66O2 a P2eO2 phasetransition takes place, as aforementioned. Since the O2 phase ismore energetically stable at low sodium contents, a driving forcefavoring the octahedral structure exists [21]. Thus, upon consecu-tives cycles the phase transition is not fully reversible. In particularthe oxygen framework is strongly affected, thus leading to a loweroverall efficiency. The second contribution to the capacity fading inboth electrolyte systems might result from a different solubility ofmanganese-ions, which are formed as a result of the dispropor-tionation reaction of Mn3þ, present at very high sodium contents inthe material [13,22]. Additionally, the low coulombic efficiencyobserved for the organic carbonate-based electrolyte might be dueto the continuous reactionwith the sodium electrode leading to the

l% of NaTFSI in PYR14FSI)-based or PC (0.5M NaPF6 in PC)-based cells.

Fig. 5. Potential vs. capacity profiles for the 5th cycle at 0.1 C (12 mA g�1) ofNa0.45Ni0.22Co0.11Mn0.66O2 annealed either at 750 �C or 800 �C (see label inside theplots). Electrolytic solutions: a) 0.5 M NaPF6 in PC and b) 10 mol% of NaTFSI in PYR14FSI.Cut-off limits: 4.6e1.5 V (vs. Na/Naþ). Temperature: 20 �C � 2 �C.

L.G. Chagas et al. / Journal of Power Sources 247 (2014) 377e383 381

formation of reduced products which move to the cathode andreoxidize [3].

The different cycling performance can be better understood byobserving the potential profiles depicted for the 2nd, 5th, 10th, 50th,75th and 100th cycle for the PC-based and IL-based systems shownin Fig. 3. Both cells exhibit six potential plateaus during the chargeand discharge in accordance with the cyclic voltammetry results.Two long plateaus are located at about 4.2 V and 2.0 V and four shortplateaus are located at approximately 3.6, 3.3, 2.4 and 1.8 V. In moredetail, however, the PC-based system (Fig. 3a) shows a fading of thehigher potential plateau (at both charge and discharge), whichirreversibly shortens to, eventually, disappear. This behavior can beexplained by the irreversibility of the P2eO2 phase transition,which takes place at high potentials, when the Na content is lowerthan 0.33 eq. per formula unit [18]. The potential plateaus locatedbetween 3.8 and 2.2 V exhibit only a minor shortening and a minorcapacity fading. The lower potential plateaus (<2.2 V) are short-ening, either for the charge and discharge process. The irrevers-ibility of such process at lower potential plateaus can be explainedconsidering the Naþ content (de-)intercalated from or into thematerial. For instance, experimentally, it was determinate that at100% depth of discharge the cathode structure is able to accom-modate 0.82 eq. Naþ, which clearly indicates that not only cobaltand nickel are redox active (max. 0.55 eq. Naþ), but also the Mn3þ/Mn4þ redox process has to take place to allow their accommodation.

It is known that Mn3þ is a JahneTeller distorted ion [22e25]which can undergo a disproportionation reaction leading to theformation of Mn4þ and Mn2þ. The latter ion is known to dissolveinto the organic carbonate-based electrolyte [13,26,27]. To evaluatethe extent of manganese dissolution we performed ICP-OES mea-surements of the organic carbonate- and ionic liquid-based elec-trolytes, both after 20 cycles at 0.1C. However, in order to excludemanganese dissolution by reaction of the electrode with water inthe electrolytes, Karl- Fischer titration was performed on the pris-tine electrolytes. The results indicated that less than 1 ppm of waterwas present in both the IL- or PC-based electrolytes, which is thelimit of detection of the device. Then it is reasonable to assume thatsuch value of water content into the electrolytes is negligible andwe can exclude any reaction between the cell compounds andwater present into the electrolytes solution.

ICP-OES measurements detected about 1 ppm of manganeseions dissolved into the PC-based electrolyte. Such a low value isonly due to the fact that dissolved manganese ions spontaneouslyplate onto the sodium metal anode in the cell. However, practicallynoMnwas detected into the IL-based electrolyte, although the limitof detection of the technique is 0.09 ppb, thus indicating that Mnions do not dissolve into the IL-based electrolyte.

The absence of manganese dissolution in the IL-based electro-lyte is one of the reason the potential profiles of the IL-based cell(Fig. 3b) exhibiting a much higher reversibility of all electro-chemical processes. It is, in fact, evident that the P2eO2 phasetransition at higher potential is more reversible for this cell. Thecharge and discharge plateaus at higher potential shorten muchless in the IL-based system and are still able to deliver a substantialcapacity after 100 cycles. Once more, the potential plateaus locatedbetween 3.8 and 2.2 V exhibit minor shifting. Nevertheless, thenegative effect of manganese dissolution into the electrolyte alsoaffects this cell system at lower potential plateaus (<2.2 V), espe-cially at charge where the potential plateaus almost disappear,however, also here it is less prejudicial than for PC-based system.

The morphology of pristine and cycled electrodes was investi-gated by SEM and the SEM images of are shown in Fig. 4. The topSEM images show as in the pristine electrode the active materialparticles are homogeneously distributed and surrounded by thecarbon conductive additive aggregates. A well-defined porosity

among the particles is also observed. After the electrochemicalcycling, the electrode extracted from the IL-based cell (SEM imagesin the center of Fig. 5) shows no major changes of the active ma-terial particles size and the preservation of the electrode surfaceporosity. In the higher magnification SEM, however, a thin butuniform film covering the active material and carbon conductivematerial particles is seen. The formation of a Solid ElectrolyteInterphase (SEI) film on the positive electrode surface of a similarmaterial (LiNi1/3Mn1/3Co1/3O2) in presence of IL-based electrolytewas already reported [28,29]. The SEI film was seen to protect theelectrode particles from undesirable reactions with the electrolyte.

The electrode extracted from the PC-based cell exhibitedagglomeration of particles (see Fig. 5, bottom left). This effect,resulting in a decreased electrode surface porosity, is due to thecontinuous formation of the SEI, which is known to take place uponlong-term electrochemical tests in organic carbonate-based elec-trolytes [28]. The high magnification SEM (Fig. 5, bottom right)shows the sharp change of the active material particle surface. Eventhe carbon additive particle morphology appears to be changed bythe continuous formation of a non-homogeneous, thus not pro-tective, SEI layer as already reported for LiNCM in carbonate-basedelectrolytes [30].

The poor quality of the formed SEI in the PC-based electrolyte isanother reason for the poorer electrochemical performance ofNa0.45Ni0.22Co0.11Mn0.66O2 when compared with the IL-basedelectrolyte in which a thin and uniform SEI is formed.

Finally, the electrochemical performance of Na0.45Ni0.22-Co0.11Mn0.66O2 cathode material annealed at 750 �C was comparedwith that of the same material annealed at 800 �C, which has beenused in our previous works [15,16]. Therefore, both materials havebeen investigated either in the ionic liquid- or the organiccarbonate-based electrolytes within the potential range of 4.6e1.5 V vs. Na/Naþ. The potential profiles recorded in the fifth cycleare shown in Fig. 5.

Fig. 6. X-ray diffraction pattern and SEM images of Na0.45Ni0.22Co0.11Mn0.66O2

annealed at a) 750 �C and b) 800 �C for 6 h, in air. The results of the Rietveld refine-ment are also reported.

Fig. 7. Comparison of operating voltage ranges and energy versus deliv

L.G. Chagas et al. / Journal of Power Sources 247 (2014) 377e383382

Overall, the IL-based electrolyte exhibits also here a superiorelectrochemical performance compared to the PC-based cells asexplained above and it can be seen that the material synthesized at750 �C exhibits a better electrochemical performance in terms ofcapacity and reversibility. The discharge process reveals the higherresistance for the material synthesized at 800 �C. In both electro-lytic solutions, the high potential plateau (located between 3.9 and4.0 V) as well as the low potential plateau (located between 1.7 and1.9 V) are located at much lower potentials and are, additionally,shorter compared to the sample synthesized at 750 �C.

The different electrochemical performance can be understoodobserving the X-ray diffraction patterns of the materials synthe-sized at different annealing temperatures (Fig. 6). The diffractionpatterns generally confirm the hexagonal layered structure of bothmaterials, with the one synthesized at 800 �C showing a pure P2-type phase (space group P63/mmc) and that synthesized at 750 �Cshowing a mixture of the same P2-type phase and P3-type phase(space group R3 m). Although these two phases show reflectionsoccurring at the same diffraction angles, for instance the P2 (002),(004), (104) and (110) and P3 (003), (006), (009) and (110), how-ever, the five additional peaks located at about 36.5�, 42�, 45�, 53�

and 57� (Fig. 6b) are typical for the P3-type structure (space groupR3 m) [31].

Overall, the IL-based cell shows a superior electrochemicalperformance and it is clear that the Na0.45Ni0.22Co0.11Mn0.66O2

annealed at 750 �C is able to (de-)intercalate Naþ cations morereversibly between 4.6 and 1.5 V vs. Na/Naþ than that annealed at800 �C. The comparison of the operating voltage and specific ca-pacity of Na0.45Ni0.22Co0.11Mn0.66O2 in the IL-based system withother layered sodium cathode materials is shown in Fig. 7. Theaverage delivered capacity of Na0.45Ni0.22Co0.11Mn0.66O2 annealedat 750 �C (IL-based cell) is about 200 mAh g�1 and an averagevoltage of 2.7 V vs. Na/Naþ, leading to an extraordinary hightheoretical specific energy density of about 550 Wh kg�1 for thecathode active material. To the best of our knowledge, theseextraordinary high values are better than those obtained withpresent layered sodium cathode materials and is even comparableto LiFePO4, which theoretically delivers 530 Wh kg�1 vs. Li [9,11].

4. Conclusions

The layered Na0.45Ni0.22Co0.11Mn0.66O2 cathode material waselectrochemically characterized in two different electrolytic

ered capacity of the P2 and O3 layered sodium insertion materials.

L.G. Chagas et al. / Journal of Power Sources 247 (2014) 377e383 383

solutions, either organic carbonate- or ionic liquid-based. The elec-trochemical (de-)sodiation process exhibits a higher reversibility forthe ionic liquid than for the organic carbonate-based system. Theapparent higher reversibility of the P2eO2 phase transition at highpotentials (>4.2 V) can be attributed to a higher electrochemicalstability of the ionic liquid-based electrolyte while the improvedperformance at lower potentials (<2.2 V) is due to a lower solubilityof manganese into the ionic liquid-based electrolyte, as confirmedby ICP-OES results. Post mortem electrode characterization by SEMindicated the formation of a thin anduniform SEI layer only in the IL-based electrolyte. This layer appears to be beneficial for the activematerial operation, leading to better reversibility.

The effect of the annealing temperature during synthesis wasinvestigated for materials prepared at 750 �C and 800 �C. Thematerial synthesized at the lower temperature shows the bestelectrochemical performance especially in the lower (<2.2 V) andhigher (>4.0 V) potential regions. The extraordinary high dischargecapacities, suitable average voltage and high cycling stability clearlyexpress as Na0.45Ni0.22Co0.11Mn0.66O2 is a promising cathode ma-terial for the application in Na-ion batteries.

Acknowledgments

L. G. C. acknowledges the Conselho Nacional de Desenvolvi-mento Científico e Tecnológico (CsF-CNPq, Brazil) for the financialsupport. D.B. thanks VW & Rockwood Lithium for the financialsupport. TIMCAL is acknowledged for kindly supplying SuperC 65.

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