Crystallite Size Control of Prussian WhiteAnalogues for Nonaqueous Potassium-IonBatteriesGuang He† and Linda F. Nazar*

Department of Chemistry and the Waterloo Institute of Nanotechnology, University of Waterloo, Waterloo, Ontario N2L 3G1,Canada

*S Supporting Information

ABSTRACT: Nonaqueous potassium-ion batteries have emerged as possible low-cost alternatives to Li-ion batteries for large-scale energy storage, owing to theirability to use graphitic carbon as the negative electrode. Positive electrode materialsremain a challenge. Here, we report control of the crystal dimensions of thePrussian white hexacyanoferrate (HCF), K1.7Fe[Fe(CN)6]0.9, using solutionchemistry to obtain either nano, submicron, or micron crystallites. We observe avery strong effect of crystallite size on electrochemical behavior. The optimalcathode material comprised of 20 nm crystallites delivers a close-to-theoreticalreversible capacity of 140 mAh g−1 with two well-defined plateaus at 4.0 and 3.2 V vsK/K+ upon discharge. Slightly inferior electrochemical behavior is observed forcrystallites up to ∼160−200 nm in diameter, but unlike the analogous Na HCFs, micron-sized crystals show very limitedcapacity. For the nanosized crystallites, however, the energy density of ∼500 Wh kg−1 is comparable to that of the best NaHCF cathode materials. At a relatively high current density of 100 mA g−1, half-cells cycled with ethylene carbonate/diethylcarbonate (EC/DEC) and 5% fluoroethylene carbonate (FEC) demonstrate an initial discharge capacity of 120 mAh g−1

with a capacity retention of 85% after 100 cycles and 65% after 300 cycles.

Much interest in alternatives to lithium-ion batterieshas emerged recently, focused on potentially lowercost systems based on alkali and alkaline earth-ion

batteries based on Na+, K+, Mg2+, and Zn2+. Much of the workhas concerned sodium-ion batteries (NIBs), which exhibitadvantages such as the low cost of sodium-based electrodes andelectrolyte salts and good mobility of Na+ in solid-statelattices.1−7 While potassium-ion batteries (KIBs) have beenactively investigated with aqueous electrolytes,8−10 reports ofpotassium intercalation batteries utilizing organic electrolytesare still rare. This is due to the low capacities and poor K+-ionmobility of many candidate positive electrodes. New motivationfor studying such materials arises from recent exciting advancesat the negative electrode. In the last two years, highly reversiblepotassium (de)intercalation in graphite electrodes was achievedin nonaqueous electrochemical cells.11−13 This is in sharpcontrast to sodium, which does not intercalate into graphiticcarbon. Discharge results in the product KC8, corresponding toa capacity on the order of 300 mAh g−1 below 0.25 V vs K/K+,showing that the practical aspects of LIBs could potentially beextended to KIBs. Hard carbons are also suitable candidates forlow-potential K insertion.14 KIBs have a further advantagebecause the K/K+ couple has a lower standard potentialcompared to Na/Na+ (SHE −2.94 vs −2.71 V), which cansignificantly offset the lower gravimetric capacity of the heavieralkali ion.

Inspired by the above, a few studies on cathodes fornonaqueous KIBs have rapidly emerged. Ji et al. reportedreversible capacities of 130−200 mAh g−1 with both 3,4,9,10-perylene-tetracarboxylicacid−dianhydride (PTCDA) and poly-(anthraquinonyl sulfide) (PAQS) between 3.5 and 1.5 V vs K/K+.15,16 The overall energy density is <250 Wh kg−1 for eithercathode material. Passerini et al. studied the layered oxidecathode K0.3MnO2, which exhibited a discharge capacity of <70mAh g−1 in a similar window of 1.5−3.5 V vs K/K+ andcomplex multiple discharge steps.17 Other electrode materialsinclude KTi2(PO4)3, which exhibits a reversible capacity of 80mAh g−1 with a low potential of ∼1.7 V vs K/K+.18 The abovework proves that reversible insertion/extraction of K-ions ispossible in different frameworks.Prussian blue analogues (also called hexacyanoferrates or

HCFs) have attracted tremendous interest as hosts for largealkali ions such as Na+ and K+. HCF cathodes such asNaxMFe(CN)6·yH2O (M = Fe, Mn, Co, and Ni) have beenreported for nonaqueous NIBs,19−27 and an interesting dual ionconfiguration achieved with a K−Na liquid alloy anode showedpromising results.28 Potassium HCF analogues, however, havebeen mostly utilized for aqueous batteries.29−31 The first work

Received: February 28, 2017Accepted: April 4, 2017Published: April 4, 2017

Letterhttp://pubs.acs.org/journal/aelccp

© 2017 American Chemical Society 1122 DOI: 10.1021/acsenergylett.7b00179ACS Energy Lett. 2017, 2, 1122−1127

on a nonaqueous potassium-based HCF cathode was publishedin 2004 by Eftekhari et al.,32 who studied potassiumintercalation in a KFeFe(CN)6 thin-film electrode. Surprisingly,this work was not followed up upon until extremely recently.During the course of finishing this Letter, preliminary workappeared on K-batteries based on HCF materials. Zhang et al.reported a vacancy-ridden, hydrated HCF cathode with a verylow potassium content (K0.22Fe[Fe(CN)6]0.805□0.195·4H2O)that requires prepotassiation;33 the full cell exhibited a capacityof 70 mAh g−1 with low initial Coulombic efficiency thatincreased to 90% over cycling. Even more recently, it wascommunicated that two manganese HCFsK1.9Mn[Fe-(CN)6]0.92·0.75H2O and K1.7Mn[Fe(CN)6]0.9·1.1 H2Oex-hibited excellent reversible capacities, after initial activationcycling, of about 100 mAh g−1 vs a Na−K alloy in KClO4/PCelectrolyte with an average potential of 3.6 V.34 In the seminalreport of a full cell (vs a graphitic anode), K1.75Mn[Fe-(CN)6]0.93·0.16H2O has been reported to deliver a dischargecapacity of 60 mAh g−1 over 60 cycles.35 A communication thatsurveyed the electrochemical behavior of fully hydrated HCFmaterials with approximate compositions K1.5M[Fe(CN)6]0.92·nH2O (M = Mn, Fe, Ni, Cu; n ≈ 2−3) shows that thesePrussian white HCFs exhibit very different cycling performanceas a function of M.36 The difference was tentatively attributed

to many factors such as water content, crystal defects, andcrystal integrity.The above materials were obtained by simple precipitation of

particles, either from aqueous chlorides, from KCl solutions, orby a Na+-ion-assisted method. Herein, we report for the firsttime the crystallization of K1.69Fe[Fe(CN)6]0.90·0.4H2O by anovel citrate chelation route, which provides excellent controlof crystallize size and morphology. The factors that dictategood cell performance in nonaqueous KIBs were evaluated togain insight into the correlation between particle size andelectrochemical behavior. We find that KHCFs exhibit inferiorelectrochemical performance to their sodium NaHCFs with asimilar crystallite size; nonetheless, a high energy density of 500Wh kg−1 was obtained with an electrode comprised of HCFnanocrystallites. Long-term cycling was realized with a capacityretention of 65% over 300 cycles, with 98% Coulombicefficiency. This performance is equal to the best cathodes forboth KIBs and NIBs. Our preliminary results on HCF cathodesalso show that the reactivity of potassium metal withalkylcarbonates such as PC is an obstacle to the studies of Khalf-cells (even in the presence of fluoroethylene carbonate(FEC) as an electrolyte additive).Prussian white samples with either an ultrasmall (∼20 nm)

or intermediate (∼170−200 nm) crystallite size (denoted as

Figure 1. Physical (a−c) and electrochemical (d−f) characterization of nano-sized KFeHCF-S: (a) Local environments of monoclinicK2FeFe(CN)6 show the connectivity of FeCN6 (brown) and FeNC6 (gray) octahedra. The silver, brown, and purple balls represent nitrogen,carbon, and potassium, respectively. (b) XRD pattern and accompanying Rietveld refinement of KFeHCF-S. (c) HR-TEM images ofKFeHCF-S; the average particle size is ∼20 nm. (d) Cyclic voltammogram (at 0.05 mV s−1). (e) Initial discharge/charge profile; the insetshows the dQ/dV plot derived from the profile.

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KFeHCF-S and KFeHCF-M, respectively) were synthesized bya modified citrate-assisted coprecipitation method.37−39 Thecrystallite size was tuned by varying the amount of citrate,whereas micron-sized crystals (KFeHCF-L) were synthesizedusing K4Fe(CN)4 as both the Fe and K source (see theSupporting Information for details). On the basis of inductivelycoupled plasma (ICP) analysis coupled with thermogravimetricanalysis (TGA) below 180 °C to determine water content(Figure S1, Table S1), the compositions of the three materialsare defined as K1.69Fe[Fe(CN)6]0.90·0.4H2O (KFeHCF-S),K1.78Fe[Fe(CN)6]0.92·0.4H2O (KFeHCF-M), and K0.68Fe[Fe-(CN)6]0.89·0.7H2O (KFeHCF-L). All samples showed a smallweight loss of 2−3 wt % between 150 and 300 °C, indicatinglow levels of interstitial water in the lattice after drying undervacuum.K2FeFe(CN)6 is isostructural to monoclinic K2MnMn-

(CN)6.40 Figure 1a illustrates that the iron is either bound to

six carbon atoms of the CN ligands to form FeC6 octahedra,or bound to six nitrogen atoms of the CN ligands to formFeN6 octahedra. Potassium is located in the void space betweenthe octahedra.41,42 The X-ray diffraction (XRD) pattern of theKFeHCF-S showing a pure phase is obtained (Figure 1b),indexed in the space group P21/n. The crystallite size is ∼25nm as calculated from the Scherrer equation, consistent withthe estimation of 20 nm (Figure 1c) from high-resolutiontransmission electron microscopy (HR-TEM) where the well-defined lattice fringes confirm that the nanoparticles are highlycrystalline.The fundamentals of potassium insertion/extraction in

KFeHCF-S were evaluated by cyclic voltammetry (CV) andgalvanostatic charge−discharge protocols. The CV curvesdemonstrate two pairs of peaks at 3.6/3.1 and 4.1/3.7 V vsK/K+, which correlate with the redox reactions of Fe2+/Fe3+

couples in different coordination environments (Figure 1d).27

The low-potential peaks are derived from redox on the Fe sitewith a high-spin configuration that is coordinated to nitrogen,while the high-potential peaks are from the Fe site with a low-spin configuration that coordinates to carbon.27,43 The initialcharge and discharge capacities are 132 and 140 mAh g−1 at 10mA g−1, corresponding to 1.7 and 1.8 potassium ions performula, respectively (i.e, theoretical values). The voltage−capacity profile demonstrates two distinct plateaus; the precisepotentials obtained from the dQ/dV curve (Figure 1e inset) are3.28 and 3.93 V vs K/K+ upon discharge. By comparison, anoptimal Na2FeFe(CN)6 cathode shows a high capacity in a Na-ion cell of 160 mAh g−1 and similar two-step potential profilesat 3.00 and 3.30 V.27 While the sodium electrode has a highercapacity, the potassium electrode is superior in potential. As aresult, the two electrodes actually offer a similar energy densityof 500 Wh kg−1. Long-term cycling performance is discussedbelow.Ex situ XRD patterns of KFeHCF-S after charge (Figure 2a)

and subsequent discharge (Figure 2b) show the expectedstructural transformation upon cycling based on the Naanalogue.27,38 After full extraction of K from KFeHCF-S(“K2Fe

2+[Fe2+(CN)6]”) to form Fe3+[Fe3+(CN)6], the latticetransforms to cubic. Upon reinsertion of K-ions, the cubicsymmetry is reduced to monoclinic, and this process is highlyreversible.The bulky K+-ion has long been a concern for the

development of KIBs. Its radius of 1.40 Å is 84 and 40%larger than that for Li+ (0.76 Å) and Na+ (1.00 Å),respectively.17 This does not only impact the volumetric energy

of KIBs but also hinders the kinetics. To illustrate the latterpoint, two materials, KFeHCF-M and KFeHCF-L, with largercrystallite sizes were fabricated for comparison with theKFeHCF-S electrodes. KFeHCF-M was synthesized with thesame coprecipitation strategy as KFeHCF-S but using five-foldmore potassium citrate. Citrate is well-known to chelate totransition metal ions such as iron and thus slows thecrystallization of K2FeFe(CN)6. The KFeHCF-M materialexhibits a similar XRD pattern as KFeHCF-S (Figure S2), butthe crystallites exhibit a more pronounced cubic morphologyand a much larger size of ∼170−200 nm (Figure 3a,b). Thereaction to produce the KFeHCF-L was fairly slow even at 60°C, which allowed the cubes to grow to micron dimensions(Figure 3c).The KFeHCF-M electrode demonstrates a similar two-step

voltage profile to KFeHCF-S upon discharge, but the capacity isslightly reduced to 125 mAh/g (Figure 3e), only 8% lower thanthat for the KFeHCF-S electrode (Figure 3d) at the samecurrent density. Further increasing the particle size to thesubmicron scale, however, results in a drastically decreasedcapacity of 10 mAh g−1. The lower K-ion content of theKFeHCF-L sample may affect the performance of the electrode,at least on the first cycle. Nonetheless, typical sodium HCFcathodes with micron-scale particle size and similar Na-ioncontent to our material offer much higher capacities of >120mAh g−1.43 The difference in performance shows that HCFcathodes suffer from more severe kinetic barriers in KIBs.Nonetheless, the high energy density obtained with theKFeHCF-S cathode is comparable with that of the bestelectrodes in NIBs, and it is higher than that of other metal ionbatteries.The high reactivity between potassium metal and some alkyl

carbonate electrolytes is another challenge in KIBs in half-cellconfigurations. Simple experiments showed that contact ofpotassium metal with ethylene carbonate/diethyl carbonate(EC/DEC) resulted in no change overnight (although a film

Figure 2. (a) XRD patterns and full profile refinement of theKFeHCF-S electrodes. (b) Schematic illustration demonstrates thereversible structural transition from monoclinic K2FeFe(CN)6 tocubic FeFe(CN)6 upon potassium extraction/insertion. The purple,yellow, green, silver, and brown colors represent potassium, Fe(low-spin), Fe (high-spin), carbon, and nitrogen, respectively.

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appeared after several days); however, electrolytes such aspropylene carbonate (PC) or EC/DMC reacted rapidly overseveral hours to produce a distinct opaque film on the surfaceof the potassium. Figure 4a shows the voltage profiles of aKFeHCF-S half-cell run in EC/DEC with 0.5 M KPF6 on the1st, 10th, 50th, and 80th cycles at 100 mA g−1. Even with this“best” carbonate solvent, the cell exhibits regular charge−discharge behavior during the first cycles, but the chargeprocess becomes unstable over cycling and the cell exhibits alow average Coulombic efficiency of <90%. The orange/yellowcolor of the glass fiber separator shows evidence of electrolytereactivity (Figure S3). The addition of 2 or 5% FEC (ascommonly used for sodium metal anode cells)44 remarkablyimproved the Coulombic efficiencies of the cells to 98% (Figure4b). The cell with 2% FEC still suffered from sudden capacityfluctuation after 170 cycles, while the cell with 5% FEC cycledup to 300 cycles (Figure 4c). Nevertheless, the latter shows acapacity drop at about the 30th and 210th cycles, andelectrolyte decomposition was suggested by the orange colorof the separator after cycling. Thus, FEC enhances theCoulombic efficiency and long-term cycling but it cannotcompletely prevent electrolyte side reactions as it is successively

consumed upon cycling. Full cells35 (or different electrolytes)are necessary to explore long-term cycling to screen newpotassium cathodes.A comparison between the KFeHCF-S electrode and three

“optimal” sodium HCF cathodes is presented in Table 1.Among the four materials, the potassium cathode has a lowercapacity, but the energy density is offset by the higher cellvoltage. As a result, the cells are comparable to each other interms of energy density. The sodium HCF cathodes still exhibitsuperiority in rate and cycling performance (Figures S4 andFigure 4c), which highlights the importance of structuraloptimization for potassium cathode materials.In summary, three KFeHCF materials with different

crystallite sizes were synthesized by controlled chemistry routesand examined in potassium cells. While the electrochemicalperformance of these materials is much more sensitive tocrystallite size than that of their sodium HCF counterparts (weestimate an upper limit for optimal performance on the order of200−300 nm), a high energy density of 500 Wh kg−1 wasrealized with the electrode composed of nanocrystallites. Inhalf-cell configurations, PC is reactive with potassium, but evenwith the less reactive EC/DEC electrolyte, the addition of FEC

Figure 3. Correlation between particle size and electrochemical performance at a current density of 20 mA g−1. (a−c) TEM images and (d−f)galvanostatic discharge/charge with KFeHCF-S, KFeHCF-M, and KFeHCF-L, respectively; note the difference in the y-axis scale. HR-TEMconfirms that the average KFeHCF-S and KFeHCF-M cubes are ∼20 and <200 nm, respectively, while the KFeHCF-L cubes are >1.5 μm. Thespecific capacity was calculated using the actual composition of each sample.

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was crucial to enhance the Coulombic efficiency and long-termcycling capability. Our findings reveal that nanocrystalliteparticle dimensions for active potassium cathode materials areof utmost importance, even for relatively flexible solids such asthe HCF. Thus, searches for future novel cathode materials,such as rigid oxides, are unlikely to be fruitful unless crystallitesare controlled to small dimensions.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsenergy-lett.7b00179.

Experimental methods, thermogravimetric analysis of theelectrode active materials, XRD patterns of KFeHCF-Mand KFeHCF-L materials, electrochemical cycling in theabsence of FEC, and rate performance data (PDF)

■ AUTHOR INFORMATION

Corresponding Author*E-mail: lfnazar@uwaterloo.ca.

ORCIDLinda F. Nazar: 0000-0002-3314-8197Present Address†G.H.: Institute for New Energy Materials & Low-CarbonTechnologies, Tianjin University of Technology, China.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was supported by funding from NSERC Canadathrough the Discovery Grant and Canada Research Chairprograms.

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Figure 4. Cycling performance of KFeHCF-S cathodes with and without FEC in the electrolytes: (a) Capacity−voltage profiles (1st: blackcurves; 10th: red curves; 50th: blue curves; and 80th: pink curves) and cycling stability. The charge process becomes unstable after 20 cycles,and the cell is eventually dead after about 80 cycles as the charge cannot be completed (inset in a). (b) Cells with 2 (blue) and 5% (purple)FEC show much improved Coulombic efficiency compared to the cell with no FEC (black), but unstable charge still occurs after 170 cycles forthe cell with 2% FEC. (c) Both cells with 2 (blue) and 5% (purple) FEC demonstrate similar capacity retention of 60% after 300 cycles. Allgalvanostatic studies were conducted at 100 mA g−1.

Table 1. Electrochemical Performance of KFeHCF-SCompared to Typical Sodium HCF Cathodes

capacity(mAh g−1)

workingpotential(V)

rate perfor-mance

(mAh g−1) cycling

Na0.61FeFe(CN)621 170 at 25

mA g−13.0 120 at ∼0.53 C

(100 mA g−1)100%after160th

Na1.92FeFe(CN)627 155 at 10

mA g−13.3/3.0 155 at 1 C 72% after

1000th

Na1.89MnFe(CN)626 150 at 15

mA g−13.5 145 at 1 C 75% after

500th

K1.69FeFe(CN)6 (thiswork)

140 at 10mA g−1

3.3/3.9 120 at ∼0.62 C(100 mA g−1)

60% after300th

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ACS Energy Letters Letter

DOI: 10.1021/acsenergylett.7b00179ACS Energy Lett. 2017, 2, 1122−1127

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