Irreversible Structural Changes of Copper Hexacyanoferrate used as Cathode in Zn-Ion Batteries.

Abstract The structural changes of copper hexacyanoferrate (CuHCF), a Prussian blue analogue, which occur when used as a cathode in an aqueous Zn‐ion battery, are investigated using electron microscopy techniques. The evolution of ZnxCu1−xHCF phases possessing wire and cubic morphologies from initial CuHCF nanoparticles are monitored after hundreds of cycles. Irreversible introduction of Zn ions to CuHCF is revealed locally using scanning transmission electron microscopy. A substitution mechanism is proposed to explain the increasing Zn content within the cathode material while simultaneously the Cu content is lowered during Zn‐ion battery cycling. The present study demonstrates that the irreversible introduction of Zn ions is responsible for the decreasing Zn ion capacity of the CuHCF cathode in high electrolyte concentration.

Abstract: The structural changes of copperh exacyanoferrate (CuHCF), aP russian blue analogue, which occur when used as ac athode in an aqueous Zn-ion battery,a re investigated using electron microscopy techniques. The evolution of Zn x Cu 1Àx HCF phases possessing wire andc ubic morphologies from initial CuHCF nanoparticles are monitored after hundreds of cycles. Irreversible introduction of Zn ions to CuHCFi sr evealed locally using scanning transmission electron microscopy.Asubstitution mechanism is proposed to explain the increasing Zn content within the cathode material while simultaneously the Cu content is lowered during Zn-ion battery cycling. The presents tudy demonstrates that the irreversible introduction of Zn ions is responsible for the decreasing Zn ion capacity of the CuHCF cathodeinh ighe lectrolyte concentration.
Charging/discharging metal ions is the key process for the storageo fe lectric energy in the form of chemical energy in variousm etal-ion batteries. This process is based on the redox reactiono fe lectrode materials, accompanied by insertion and desertion of the metal cations, respectively. [1] The reversibility of the reaction determines the long-term stability and efficiency of metal-ion batteries. The inserting and desertion process result in crystal distortion and phase transformation of electrode materials with varying lattice parameters and huge volumec hanges. [1,2] Dissolution of the electrodes, subsequent releaseo ft he chemical species in the electrolyte, andt heir redeposition on the opposite electrode, limit the long-term use. [3] The understanding of the overall reaction mechanism of metal-ion battery process is inevitable to develop electrode materials with improved stability.
Prussian blue analogues( PBAs)a re polynuclear transition metal cyanides, written as AM a [M b (CN) 6 ]·x H 2 O, where Ar epresents am onovalentm etal cation, M a is ah igh-spint ransition metal ioni nM a N 6 octahedra, and M b is al ow-spin transition metal ion in M b C 6 octahedra. [4] PBAsh ave been considered promising activem aterials for various metal-ion batteries because of their capability to reversibly insert and desert several metal ions. [5] The robusta nd large 3D channel framework in PBAs allows the intercalation of even divalent Zn ions in aque-ous electrolyte, which is cheap, safe, and potentially applicable for grid-scale energy storages ystems. [6] The efficiency ands tability of PBAs can be controlled by the choice of the metal species, the number and distribution of M a ,M b ,a nd M(CN) 6 vacancies as well as interstitial water molecules. [5,7] This allows to designn ew Zn-ion battery (ZIB) electrodes with superior performance which might overcome the limitations of vanadium oxide-a nd manganese dioxide-basedc athodes suffering from low electric conductivity andm orphological-and structural changes during ZIB operation. [8] Recently,a na queous ZIB based on copperh exacyanoferrate (CuHCF), aP BA, showed promising specific energy and power density comparable to the ones of organic cellsb ased on Li 4 Ti 5 O 12 and LiFePO 4 . [9] The large cavity in the 8c site ( 1 = 4 , 1 = 4 , 1 = 4 )i nC uHCF (interstitial sites) can be used as highly reversible Zn ion storage site with chargec ompensation given by Fe 3 + /2 + redox couple.
However,CuHCF is suffering from decreasing Zn ion capacity after hundreds cycles, [10] without any obviousc athode dissolution or Zn dendrite formation. [9a, 10] This indicates that the Fe 3 + /2 + redox couple for Zn ion storagei sf ar from being ideal and there are most likely severalf actors limiting the long-term stability. The degradation of CuHCF electrodes is more severe for high electrolyte concentration. [10,11] Even though X-ray diffraction (XRD)r evealed phase changes of CuHCF during cycling, the underlying mechanism and its relationt od ecreasing Zn ion capacity are still not clear. [12] Therefore, detailed structural investigation is essential. Herein we report the morphology,c omposition,o xidation state, and crystal structurec hanges occurring in CuHCF-based cathodes in aqueous ZIB. With the help of electron microscopy techniques, we were able to obtain the key information from each individual feature formed during cycling.
Crystalline CuHCF nanoparticles were synthesized by ac ontrolled co-precipitation method. [13] The particles ize of as-prepared nanoparticles is less than 100 nm, as shown in transmission electron microscopy (TEM) and scanning TEM (STEM) images ( Figure S1a and b). As illustrated in powder XRD and electron diffraction patterns (Figure S1c and d), CuHCFn anoparticles possessaface-centered cubic crystal structure. The performance of this material is verified by applying the CuHCF nanoparticles as ZIB cathode in 100 mm of aqueous ZnSO 4 electrolyte. Specific energy and capacity fading of the cathode after 250 cycles,a ccompanied by changes in the average voltage and morphologyw ere observed ( Figure 1a and FigureS2). Althoughs pecific energy and capacity steadily decrease after 250 cycles,t he electrochemical reactionp otential of cathode, which is represented as E WE -E CE ,i ncreases first and then remains constanta fter 500 cycles. This indicates that the main (de-)intercalation reaction of Zn ions in the cathode after 500 cycles is different than before.S canning electron microscopy (SEM)i mages from cathode surfaces cycled between 0t o 1000 times are presented with the highlighted images for 0, 500, and 1000 cycles in Figure 1b and c. The CuHCFc athode surfacea fter immersion in the electrolyte for 3days but before ZIB cycling (0 cycle) reveals the same size and shape of the CuHCF nanoparticles as the nanoparticles in the initial powder. After 50, 150, and 250 cylcles, the surfaceo fc athodes still shows similar morphological features as the originalo ne with as malla mount of additional plate and rod-shapedp articles. After 500 cycles,ah ighera mount of micron-sized wire and cube-shaped morphologies as well as initial CuHCFn anoparticles are observed, indicating morphological transformation of CuHCF cathode via reactionw ith Zn ions. Wires and cubes having additional facetted crystalline features on the main cube are found on the cathode surface after 1000 cycles.
As the ZIB capacity is maintained up to 250 cycles where most part of the cathode surface is still composed of the initial nanoparticles and decreasesa fterward, the increasing amount of the wire and cube shaped structure is relatedtothe ZIB performance. The different electrochemical reaction behavior of wire and cube structuresc ompared to initial nanoparticles can be the main reason for decreasing Zn ion capacity up to 500 cycles.T he main morphological changes occur within 500 cycles and the size of the wire and cube structures increases afterward, resulting in the decrease of the Zn ion capacity while E WE -E CE remains constant.
Different chemical compositions in each morphology are monitored using energy dispersive X-ray spectroscopy (EDS). SEM-EDS( Ta ble S1) reveals that the relative amount of Cu in the cathode surface area consistingm ainly of nanoparticles decreasesw ith the number of cycles,w hile the Zn content is increasing. The Fe contentr emains similar. Wire and cube morphologies also have al ower amount of Cu and ah igher amount of Zn compared to the initial CuHCF nanoparticles. The presence of Zn even after the electrochemical desertion of Zn ions indicates that this process is not perfectly reversible or Zn ions are not only located in the interstitial sites but also in the cubic lattice.T ov erify the chemical compositionl ocally, STEM-EDS is conducted (Figure 2a-c and Ta ble1).
The TEM sample of the initial CuHCFn anoparticles and the wires obtaineda fter 1000c ycles are prepared from ac olloidal suspension andd rop casting on aT EM grid. For the micrometer sized cube features, focused ion beam (FIB) is used to obtain thin TEM lamella ( Figure S3). Wires and cubes from the 1000 cycled cathode have al ower Cu to Fe ratio compared to the initial CuHCF nanoparticles while the Zn content increases. Regarding decreasing amount of Cu, Zn ions can substitute Cu and occupy the specific sites. Noticeably,o nly cubes contain a high amount of Ki ons, implying as pecific role of Ki ons in the development of ac ube rathert han wire. One possibility is the presence of cubic potassium zinc cyanides tructure locally,s tabilizing the overall cubic morphology. [14] To investigate the substitution mechanism occurring in CuHCFu pon insertion of Zn ions, electron energy loss spectra (EELS)w ere obtained in STEM mode. Figure 2d presents the Cu-L 2,3 edge from initial CuHCF nanoparticles and aw ire from the 1000c ycled cathode. Both of them show two white-lines, indicating the presence of cationic Cu. The energy loss values of the Cu-L 2.3 edge from the wire is assignedt oamixtureo f mono-a nd divalent Cu, revealing that the Cu 2 + /1 + redox couple also works during cation (de)intercalation. The Cu 2 + /1 + as well as Fe 3 + /2 + redox couple can be the main reason of the morphological change and phases eparation of CuHCF in aqueous ZIB system. [15] The dual redoxc harge compensation mechanism of CuHCFi su nderstood by two electrons confined to ac yanide-bridged Cu and Fe unit due to the stronga ssociation with ad ivalent cation. This results in electron filling of Fe t 2g as lowest unoccupied molecular orbital (LUMO) and that of Cu e g as next LUMO. [16] Even Cu ions in initial CuHCF nanoparticles seem to be also mono-and divalenta si ndicated by the shape of Cu-L 3 line. This could stem from the intercalation of K + and acceptance of electrons simultaneouslyf rom the cyanide group which is an electron donor. [17] The Cu-L 2,3 edge from the cubic morphologyc ould not be obtained due to the low amount of Cu and high sensitivity of thin TEM lamella under electron beam bombardment.
All samples possess two distinct L 3 and L 2 peaks at the Fe-L 2,3 edge as showni nF igure 2e.T he higher the intensityr atio of the two peaks (I(L 3 )/I(L 2 ) Fe ), the higher the oxidation state of Fe species. [19] ( Table 1). The increasing I(L 3 )/I(L 2 ) Fe value indicates that the contento fF e 3 + ions increases from the initial CuHCF nanoparticles via the wire to the cube, suggesting adecreasing reduction of Fe 3 + to Fe 2 + with increasing Zn ion content. First, the presence of Fe 2 + already in the initial CuHCF nanoparticles can be understoodb yt he reduction of Fe 3 + to Fe 2 + because of K + intercalation,w hich was observed by Mçssbauer spectroscopy in an earlier study. [20] The highera mount of Zn ions in the wire and cube, however,d oes not increaseb ut decrease the amount of Fe 2 + .T his reveals that the Zn ions herea re not at interstitial sites but substitute Cu ions in case of the wire and cube structures. We propose that this substitution of Cu ions by electrochemically inactive Zn ions [21] leads to ap hase transformation and is the main reasono fc apacity degradation of CuHCF cathode in aqueous ZIB. This is because CuHCFl oses the Cu 2 + /1 + redox couple which also plays ar ole for charge compensating of divalent metal cation during intercalation, [15,22] and consequently al ower Fe 3 + /2 + redox efficiencyi s present in the wire and cubic morphologies. The severe capacity loss of the cathode is found after around 500 cycles (Figure 1), where many wires and cubes are formed. Figure S4 showst he formation of new cathodic and anodic peaks in the differential charge curve arisinga fter % 500 cycles, indicating that the Zn ion (de)intercalation chemistry of CuHCF cathode changes.C uHCF seems to transform in af orm of Zn x Cu 1Àx HCF. To explain the additional insertion of Zn ions from the interstitial site to the substitutional site, Zn ionh opping to Fe(CN) 6 vacancy sites was suggestedu sing synchrotron XRD measurements. [22] However,t his hopping mechanism was observed only at the first ZIB cycle withoutCuion loss and cannotberelated to the morphologyc hange and ZIB capacity decrease here. After Zn ions are introduced in the interstitial sites, a higherc oncentration of Zn ions in the electrolyte together with an external bias gives CuHCF ab etter chance to uptake more Zn ions to the substitutionals ite. Divalent Zn ions interact with the nitrogen of the cyanideg roup rather than with carbon duet ot heir low-spin electron configuration. In the end, new -Zn-N-b onds can form removing Cu ions from the cubic CuHCF framework. Because this reactions lowly proceeds during long-term ZIB cycles,s mall and irregular shaped initial CuHCF nanoparticles (kinetic product)t ransform to biggerw ire and cubic morphologies with higher crystallinity (thermodynamic product). [23] The nanoparticles, wires, and cubes possess similaro xygen K-edges in the EELS spectra with the first maximum centered at % 537 eV,avalue higherthan that of most metal oxides (Figure 2f). The OK-edge can be assigned to coordinated-, intercalated-, or interstitial water molecules. [4b, 24] The overall crystal structure change of the CuHCFc athode duringZ IB cycle was investigated using XRD ( Figure S5). The XRD patterns indicate changes already directly after immersing CuHCFc athode in the electrolyte even before the first ZIB cycle. An ew peak at % 2q = 278 visible as as houlder and a sharp peak at % 2q = 338 cannot be assigned. It seems that Zn ions are introduced to CuHCF and change the local crystal structure, regarding the fast cation and water exchange ability of CuHCFi ne lectrolytes. [20,22] Other new peaks with higheri ntensity are found in 500-and 1000 cycled cathodes, which can be assigned to the (200), (220), (400), and (420) planeso ft he cubic ZnHCFp hase and others to the (024), (116), (300), and (119) planes of the rhombohedral ZnHCF structure. Despite the   [10] This can be explained by the presence of two different Cu-richm orphologies in the cathodes (Figure 3). The STEM image of aF IB lamella( Figure S6a) prepared from a5 00 cycled cathode shows brighter particles % 200 nm in size surrounded by dark appearing connected particles (Figure 3a). These brightp articles are assigned to aC urich phase without Fe ions based on STEM-EDS, indicating phase separation from the initial CuHCF nanoparticles. Considering the oxygen EDS mapping, these particlesc ould not be assigned to CuO x .Itisassumedt hat Cu ions are rather well stabilized by cyanideb ridges. [25] The Cu-rich particles are highly crystalline as shown in selected area electron diffraction ( Figure 3b)a nd possess ap orouss tructure ( Figure S6b-d). The overall presence of finer Cu-rich area in the lamella is visualized using dark-field TEM imaging in lower magnification (Figure 3c). The other Cu-rich phase is shown in Figure3d. These fine particles are prepared from the colloidal suspension of the 1000 cycled CuHCFc athode. Different from the Cu-richs tructure described above, Zn as well as Cu is detectedh ere, indicating co-stabilized Cu-and Zn ions by cyanide bridging.B oth Cu-rich phasesa re formed by Cu ions released from the initial nanoparticles and are assumed to be inert towards Zn ion intercalation leadingt op erformance loss. All morphologies over 500 cycles can be summarized as Equation (1) below: KCuFeðCNÞ 6 ! aKCuFeðCNÞ 6ðinitial CuHCF nanoparticelsÞ þ bKCu 1Àx Zn x FeðCNÞ 6ðwires&cubesÞ þ cCuðCNÞ 6ðCu-rich areasÞ þ dCuZnðCNÞ 6ðCu-rich areasÞ ,w here a þ b þ c þ d ¼ 1 This work has shed new light into the structuralc hange of CuHCF, and ap reviously proposed mechanismsc an be corrected in order to take into account the releaseo fC u. [10,12] Substitution of Cu ionsb yZ ni ons in substitutional as well as in interstitial sites induces formation of wires and cubes from the initial CuHCF nanoparticles. Simultaneously,C u-rich structures are formed by the released Cu ions, whicha re inactive towards Zn ion storage. In an earlier work, we observed that the best specific charge stability was obtained for ZnSO 4 among other electrolytes such as ZnF 2 ,Zn(ClO 4 ) 2 ,a nd Zn(NO 3 ) 2 . [10] Accordingly,wee xpect that the irreversible transformationf or these electrolytesw ill occur already after even lower number of cycles. Since the electrochemical properties of PBAs are strongly dependento nt heir transition metal contenta nd defects, it is evident that optimization of the material requires detailed structural studies using electronm icroscopy techniques.