Electrochemical Reactivity with Lithium of Spinel-type ZnFe2âyCryO4

Article pubs.acs.org/JPCC

Electrochemical Reactivity with Lithium of Spinel-type ZnFe2−yCryO4 (0 ≤ y ≤ 2) Pei Fen Teh,† Stevin S. Pramana,† Chunjoong Kim,‡,# Chieh-Ming Chen,§ Cheng-Hao Chuang,§ Yogesh Sharma,∥ Jordi Cabana,‡,# and Srinivasan Madhavi*,†,⊥ †

School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States § Department of Physics, Tamkang University, New Taipei City 25137, Taiwan ∥ Department of Applied Science and Engineering, Indian Institute of Technology Roorkee, Saharanpur Campus 247667, India ⊥ CREATE , 1 CREATE Way, #10-02 CREATE Tower, 138602 Singapore ‡

S Supporting Information *

ABSTRACT: Members of the spinel solid solution series ZnFe2−yCryO4 (y = 0, 0.5, 1.0, 1.5, and 2) were synthesized using high-energy ball milling followed by annealing at 1000 °C. The structural study of the samples was performed by Fourier transform infrared spectroscopy (FTIR), X-ray absorption spectroscopy (XAS), and powder X-ray diffraction (XRD). While XRD verified the formation of single spinel phases with lattice parameters reduced by increasing Cr substitution, FTIR and XAS provided insight into the subsequently increased covalence of the chemical bonding of the spinels. The mixed transition-metal spinel oxides were employed as working electrodes in Li metal batteries. In agreement with the literature, the spinel oxides experience amorphization during the first discharge, as shown by ex situ XRD and selected area electron diffraction (SAED). The electrochemical activity of the spinel oxides was found to diminish with Cr content so that ZnCr2O4 is completely inactive even when the material is nanosized and in the presence of a large amount of conductive additive. Comparison with mixtures of ZnO and Cr2O3 led to the conclusion that the conducting band of the ternary oxide, which would be injected with electrons during reduction, is raised with respect to the individual binary oxides to the point that the overpotential required to drive a conversion reaction displaces the experimental electrochemical potential to be extremely close to, or even lower than, that of Li metal.

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INTRODUCTION Ternary transition-metal oxides with the spinel structure (ideally AB2O4, space group Fd3̅m (227)) are of wide interest in solid state chemistry because of their rich variety of crystallographic arrangements as well as their physical and chemical properties, which render them applicable as magnetic materials, catalysts, semiconductors, and pigments.1−3 The most common electron distribution is A2+B23+O4 (e.g., MgAl2O4),4 but other combinations such as A4+B22+O4 (e.g., SnZn2O4)5 and A+B23.5+O4 (e.g., LiMn2O4)6 are also wellknown. If the unit cell origin is taken at a center of symmetry 4̅3m, one-eighth of the tetrahedral sites (8a, 0, 0, 0, 4̅3m) are filled by A cations, while B cations occupy one-half of all octahedral sites (16d, 5/8, 5/8, 5/8, 3̅m) in cubic close packed arrays of oxygen atoms (32e, u, u, u, 3m) to form a normal spinel.7 If half of B cations occupy the tetrahedral sites, this ordered configuration experiences a transition from normal to 8 inverse spinel (still in a Fd3m ̅ framework). The degree of inversion significantly affects the chemical and physical properties of spinel-based materials.3,7 It can usually be tailored by controlling temperature and pressure. Spinels have received considerable interest as electrodes in lithium ion batteries (LIBs) in the last two decades. Improved © 2013 American Chemical Society

battery performance (capacity and cyclability) and tunable discharge−charge voltage plateaus were widely observed after doping or cation substitution by other metals. For instance, LiMn2O4 and its iso- or alio-valently substituted variants LiMxMn2‑xO4 (M: Ni, Al, Co, Mg, Fe)9−11 have been widely demonstrated to possess interesting electrochemical performance as battery electrodes. More recently, spinel-type compounds have also been extensively investigated as anode alternatives to commercially employed graphite (theoretical capacity =372 mAh g−1)12 because they show higher theoretical capacity and flexibility to control the working voltages.13 While their applicability is still hampered by severe fundamental issues,13 there is a growing research interest in exploring the viability of several spinel oxides as high storage capacity, safe, and long cycle life anodes in high-volume electrochemical energy storage applications.14 Among ternary spinel-based anodes, ZnFe2O4 has been the object of several studies, as it is environmentally benign and exhibits a high capacity of ∼500−800 mAh g−1 with excellent Received: September 2, 2013 Revised: October 20, 2013 Published: October 21, 2013 24213

dx.doi.org/10.1021/jp408762v | J. Phys. Chem. C 2013, 117, 24213−24223

The Journal of Physical Chemistry C

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cyclability.15−17 It has been engineered into many different nanostructures to enhance its electrode performance.15−22 For instance, NuLi et al.16 published ZnFe2O4 thin films with reversible capacity of 556 mAh g−1, whereas improved lithium uptake of particles synthesized by urea combustion method (615 mAh g−1) was reported by Sharma et al.15 The performance was further enhanced by either nanostructuring strategies to form hierarchically hollow microspheres (∼900 mAh g−1),18 nanofibers (730 mAh g−1),17 nanoparticles (800 mAh g−1),21 or by the incorporation of a conductive carbon framework to construct ZnFe2O4/C hollow spheres (841 mAh g−1)19 and carbon-coated ZnFe2O4 nanoparticles (∼1000 mAh g−1).20 On the basis of ex situ characterization of cycled electrodes, the reaction mechanism of ZnFe2O4 with lithium was proposed as follows: ZnFe2O4 + 8Li+ + 8e− → Zn + 2Fe + 4Li 2O

(first discharge)

Fe2O3 anodes to form FeO.26,27 Such inefficiency is likely one reason for capacity fading in ZnFe2O4 anodes after long-term cycling. It is now well-established that changes in capacity and working voltage can be induced on spinels as LIB electrodes by replacing the counterions in AB2O4. For instance, ZnCo2O4,28 ZnFe2O4,19 and ZnMn2O429 contain Zn in the tetrahedral sites and either Co, Fe, or Mn in the octahedral sites.1,30 However, they exhibit different specific capacity and charge (oxidation) voltage of ∼900 mAh g−1 and 1.65/2.1 V; ∼841 mAh g−1 and 1.5/1.75 V; ∼700 mAh g−1 and 1.2/1.6 V, respectively. The process in the range of ∼1.5−1.65 V can be assigned to the reoxidation of Zn to ZnO, although it was not always found to be a well-resolved peak,16,17,21 whereas the other process corresponds to the reoxidation of transition metal, B. The voltage trend largely follows the theoretical values of their corresponding binary oxides: Co3O4 (∼1.91 V), Fe2O3 (∼1.63 V), Mn2O3 (∼1.39 V).31 Because Cr2O3 theoretically has an even lower working voltage of ∼1.06 V with similar cycling behavior, comparably good capacity retention but at lower working voltage would be expected after the substitution of Fe by Cr in ZnB2O4 framework, thereby raising the theoretical energy density of a final device. Furthermore, ZnCr2O4 is also proposed to have higher electrical conductivity than ZnFe2O4, a desirable property in LIB application.1 The present work describes the battery performance of compounds in the solid solution series, ZnFe2−yCryO4 (0 ≤ y ≤ 2), with the aim of investigating their structure and verifying our hypothesis of specific capacity and operating voltage tunability.

(1)

15,17,18,21

Zn + x Li+ + x e− ↔ LixZn (x ≤ 1)

(from first discharge onward)

Zn + Li 2O ↔ ZnO + 2Li+ + 2e−

(from first charge onward)

(3)

15,17,18

Fe + Li 2O ↔ Fe(II)O + 2Li+ + 2e−

(from first charge onward)

(2)

16−18

(4)

15−17

2Fe + 3Li 2O ↔ Fe(III)2O3 + 6Li+ + 6e−

■

(5)

(from first charge onward) During the first discharge, theoretically ∼8 mols of Li react with ZnFe2O4, in accordance to eq 1.15,17,18,21 The reduction reaction is accompanied by irreversible electrode amorphization, resulting in Zn and Fe metallic nanograins of 2−5 nm uniformly embedded in an amorphous Li2O matrix.15,17,18 Small amounts of lithium were found to intercalate into ZnFe2O4 prior to this conversion reaction (eq 1).15,17,18 Chen et al.23 found that the intercalated lithium occupies vacant 16c sites in the spinel structure, but this intercalation reaction pathway highly relies on the particle size and shape of the host materials. Hollow microspheres and nanoparticles were reported to accommodate ∼2 mols of Li, whereas uptake of 1.7 mols was observed in nanofibers, and no measurable intercalation was found in ZnFe2O4 nanospheres/graphene hybrid structures.24 Zn can further reversibly alloy−dealloy with ∼1 mol of Li (eq 2). This lithiation process was experimentally verified by Guo et al.18 and NuLi et al.16 by ex situ highresolution transition microscopy (HRTEM)/SAED and X-ray photoelectron spectroscopy (XPS), respectively. From the first charge onward, reversible lithiation and delithiation follow eqs 2−5. Theoretically, a maximum number of 9 Li ions can be reversibly released and absorbed by Zn/Fe/Li2O nanocomposites. However, the (de)alloying reaction between Li and Zn is not highly reversible15,17 and incomplete reoxidation of Fe resulting in Fe(II)O or a mixture with Fe2(III)O3 were also observed in the literature.15−17 It is unclear whether these observations are the result of competition between reactions 4 and 5 or a multistep reoxidation process that may be incomplete because of poor charge transfer. These observations are not uncommon in conversion electrodes with highly oxidized metals; examples are the incomplete reoxidation of Co to Co3O4 in ZnCo2O4 anodes25 and the partial reversibility in 18,22

EXPERIMENTAL SECTION ZnFe2−yCryO4 (0 ≤ y ≤ 2) samples were prepared by mixing analytical purity ZnO, Fe 2 O 3 , and Cr 2 O 3 at desired stoichiometric ratio as starting materials. High-energy ball milling (Spex Sample Preparation Mixer) was conducted in stainless steel vials with stainless steel balls (∼5 mm in diameter) for 3 h; the ball-to-powder weight ratio was 4:1. Subsequently, the as-milled powder was annealed at 1000 °C for 3 h in air to form ZnFe2−yCryO4 (0 ≤ y ≤ 2). The morphology of the samples was examined by field emission scanning electron microscopy (FESEM, JEOL 7600F). The elemental compositions of the samples were analyzed by energy-dispersive X-ray spectroscopy (EDS, Oxford Instruments) with a detector attached to the FESEM. Structural information was gathered by collecting powder X-ray diffraction (XRD, Bruker D8 Advance, Cu Kα radiation, λ = 1.54 Å) with a step size of 0.05° over 10°−130°. Rietveld refinement was performed using the fundamental parameters approach32 implemented in TOPAS.33 A five-coefficient Chebychev polynomial background, zero shift, sample displacement, unit cell parameters, and scale factor were sequentially refined. Fourier transform infrared spectroscopy (FTIR) was carried out on samples pelletized with KBr using a PerkinElmer Spectrum GX spectrometer at a resolution of 2 cm−1 and scan number of 16. X-ray absorption spectroscopy (XAS) measurements were performed at beamline 20A at the National Synchrotron Radiation Research Center (Taiwan). A spherical grating monochromator provided a spectral range from 60 to 1250 eV with an average resolving power of 5000. The oxide powder was pressed onto In foil to improve electrical contact and then transferred into a chamber maintained under high vacuum (∼5 × 10−9 Torr). All XAS spectra were collected with 24214



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a total electron yield (TEY) detector and normalized against the background intensity at the postedge range to be unity. The electrochemical performance of ZnFe2−yCryO4 (0 ≤ y ≤ 2) was evaluated in 2016 coin cells. The electrodes were prepared by mixing 60 wt % of active materials, 20 wt % of binder (Kynar), and 20 wt % of Super P (Timcal) in N-methylpyrrolidinone (NMP, Sigma Aldrich) to form a homogeneous slurry. The slurry was coated onto etched copper foil with a doctor blade set to result in a thickness of 40 μm and dried at 80 °C for 12 h in a vacuum oven. Subsequently, the dry film was pressed between twin rollers to ensure intimate contact with the copper current collector. The electrodes were cut into circular disks of 16 mm diameter; each disk contained ∼3−4 mg of active material. The cells were assembled in an Ar-filled glovebox (MBraun, Germany) with oxygen and water content less than 1 ppm. Li metal served as a counter electrode, Celgard 2400 as the separator, and 1 M solution of LiPF6 dissolved in ethylene carbonate/diethyl carbonate (EC:DEC = 1:1 by volume, DAN VEC) as electrolyte. The lithium storage properties of ZnFe2−yCryO4 (0 ≤ y ≤ 2) were examined by galvanostatic cycling (Multichannel Battery Tester, Neware Technology Limited) and cyclic voltammetry (CV, CHI). In galvanostatic mode, the cells were charged and discharged at 60 mA g−1 in the voltage window of 0.005−3.0 V at room temperature. CV was performed in the same voltage window at a constant scanning rate of 0.1 mV s−1. For each y, a minimum of seven cells were examined to confirm the results. To understand the structural changes after galvanostatic cycling, ex situ material characterization was conducted. The electrode was removed from the coin cell and rinsed with DEC in the glovebox before being investigated by XRD (Bruker D8 Discover). Data with higher sensitivity was collected at beamline 11-3 at the Stanford Synchrotron Radiation Laboratory (SSRL). The incident beam was at 12.735 keV and had a spot size of 200 × 200 μm. Patterns were collected using a MAR345 imaging plate of 345 mm diameter with an exposure time of 30 s. Selected area electron diffraction (SAED) was taken in a transmission electron microscope (TEM, JEOL 2100F) operated at 200 kV and fitted with a lowbackground Gatan double tilt holder. Simulated diffraction rings were calculated with the JEMS program, and the structural model obtained from the Rietveld refinement was used as the input file.34

Figure 1. (a) Experimental (dots) and calculated (full line) powder XRD patterns of annealed ZnFe2−yCryO4 (0 ≤ y ≤ 2), the inset graph magnifies the 2θ region from 25° to 65°. A shift to higher 2θ region was found after substituting larger Fe3+ cations by smaller Cr3+ cations. (b) Lattice parameter (Å) of spinel as a function of Cr substitution obeys Vegard’s law. (c) EDS analysis of ZnFe2−yCryO4 (0 ≤ y ≤ 2) demonstrates the correct stoichiometric ratio between Zn, Fe, and Cr (data tabulated in Table 1).

(Figure 1c and Table 1). The microstructure of the annealed ZnFe2−yCryO4 (y = 0, 0.5, 1.0, 1.5, and 2.0) samples is shown in Figure 2; they were all composed of fine and uniform particles of 200−400 nm size. FTIR spectra were recorded from 1000 to 400 cm−1 (Figure 3) as signals in this range are generally attributed to the vibration bands of metallic ions in the crystal lattice. ZnO, Fe2O3, and Cr2O3 were also analyzed as references. The spectrum of ZnO shows characteristic absorptions by Zn2+− O2− bonds at ∼537 and 438 cm−1.38 Fe3+−O2− vibration peaks were found at ∼570 and ∼480 cm−1 for Fe2O3, whereas Cr2O3 also exhibited Cr3+−O2− bands at ∼574 and 643 cm−1, consistent with literature reports.39,40 Two intense peaks were observed between ∼650 and 400 cm−1 for each ZnFe2−yCryO4 sample, the specific shift depending on Cr substitution (y). The peaks within this range are due to different modes of concerted vibration of the bonds around both sites in the spinel structure.41 ZnCr2O4 (y = 2.0) exhibited absorption peaks at ∼640 and ∼530 cm−1, in qualitative agreement with earlier analyses by Yoon et al.1 A red shift of the whole spectra was noticed upon introduction of Fe; vibration modes were observed at ∼550 and ∼430 cm−1 when y = 0 (ZnFe2O4). This behavior suggests bonding in the structure is affected by the ratio between Fe and Cr,1 presumably reflecting changes in covalence in the compound. XAS was conducted on ZnFe2−yCryO4 to determine the local unoccupied electronic structure after transition-metal cation substitution. The spectra can be found in Figure 4a−d. O Kedge spectra of FeO, Fe3O4, Fe2O3, Cr2O3, and ZnO were also collected (Figure 4a). The signals between 526 and 532 eV (marked in Figure 4a) are assigned to the unoccupied O 2p orbitals hybridized to either Fe or Cr 3d states with t2g and eg symmetry, whereas the broader feature at higher energy can be ascribed as bonding to Fe or Cr 4s and 4p states.42 The former

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RESULTS AND DISCUSSION Material Characterization. The XRD patterns (Figure 1a) show that ZnFe2−yCryO4 are highly crystalline and phase-pure, crystallizing in a cubic spinel structure (Fd3m ̅ space group). The lattice parameters, bond lengths, and coordinate of oxygen (u) were extracted from Rietveld refinements (Table 1). The unit cell dimensions decreased with lower Fe/Cr ratio, as expressed by the shift of XRD peaks to higher 2θ. These results are in good agreement with the smaller ionic radius of Cr3+ (0.62 Å) than Fe3+ (0.645 Å),35 and with previously reported data.1,3,8,36 Figure 1b plots the change of lattice parameters as a function of Cr substitution; the trend follows Vegard’s law.37 The cation substitution of Fe by Cr caused a minor but consistent reduction in the B−O distance, while the A−O distance remained rather invariable at ∼1.97−1.98 Å (Table 1), indicating that Cr replaced Fe on the B-site and the oxides remained as normal spinels regardless of Cr (y) content. EDS analysis revealed stoichoimetric ratios of Zn, Cr, and Fe that agreed with the initial starting ratio during sample preparation 24215



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Table 1. Results of Rietveld Refinement of Annealed ZnFe2−yCryO4 Solid Solution Series: Lattice Parameter (Å), Bond Distance of A−O and B−O, u Coordinate of Oxygen, RB, and Rwpa

a

y

lattice parameter (Å)

calculated A−O distance (Å)

calculated B−O distance (Å)

u (O)

R-Bragg (RB)

R-weighted pattern (Rwp)

ratio of Zn:Fe:Cr from EDS (at %)

0 0.5 1.0 1.5 2.0

8.4436(1) 8.4186(4) 8.3915(2) 8.3498(2) 8.3266(4)

1.974(6) 1.980(4) 1.980(6) 1.971(3) 1.972(4)

2.030(3) 2.018(2) 2.008(3) 2.000(2) 1.989(2)

0.3850(4) 0.3858(3) 0.3863(4) 0.3864(2) 0.3867(3)

0.86 0.62 0.67 0.55 0.83

3.62 3.25 3.10 3.47 4.44

1:1.96:0.02 1:1.43:0.54 1:0.95:1.04 1:0.47:1.54 1:0.01:1.93

Rwp = [∑|IO − IC|2w]1/2, w = weighting factor, RB = [(∑|F20 − F2c |)/(∑F20)]1/2/100.33

Figure 2. Secondary electron micrographs of annealed ZnFe2−yCryO4 (0 ≤ y ≤ 2). The particle sizes are uniform and in the range of 200−400 nm.

ZnCr2O4 than for Cr2O3, indicating stronger hybridization and thus higher covalence in the ternary oxide. The O 2p states bonded with Zn 4s in ZnO resulted in a broad band starting at ∼532.5 eV,43 whereas the sharp peak at ∼540 eV was assigned to nondispersive O 2pz and 2px+y states.44 Thus, the O 2p unoccupied states hybridized with Zn2+ appear at a higher energy range than Fe3+ and Cr3+ because the 3d orbitals are fully occupied in Zn2+ (d10) ions. Finally, two additional peaks at ∼533.1 and 535.9 eV were consistently noticed in ZnFe2−yCryO4. They are proposed to result from O 2p−Fe/ Cr 3d and O 2p−Zn 4s intrahybridization based on a previous study of (Zn, Cr)O films.44 Figure 4b shows the Zn L-edge spectra of ZnFe2−yCryO4 and ZnO. The peak at 1029.9 eV is ascribed to the Zn 2p → 4s transition in ZnO, and it was consistently observed in the whole ZnFe2−yCryO4 series. A new peak at 1026.5 eV emerged in spinel samples; this feature was also reported in ZnxFe3−xO4.45 As presented in the inset of Figure 4b, the peak intensity increased with Cr3+ (d3) content. This observation indicates that the alteration of the FeO6 octahedra by Cr3+ replacement also affects the chemical environment of Zn2+, in agreement with the FTIR analysis (Figure 3). The higher binding strength (shorter B−O distance, refer to Table 1) of Cr3+−O2‑ compared to Fe3+−O2‑ bonds leads to more significant O 2p electron hybridization and thus covalence in ZnCr2O4. The Fe L-edge XAS spectra of ZnFe2−yCryO4, with those of FeO, Fe3O4, and Fe2O3, are shown in Figure 4c. The spectra contain L3 and L2 edges, centered at ∼703.0 and ∼715.5 eV, respectively, which are a result of spin−orbit coupling. These states are split by the crystal field, leading to multiplets.46 Fe3+ in the octahedral site of Fe2O3 shows a feature at ∼701.3 eV adjacent to the main L3 peak, which is absent in the octahedral

Figure 3. FTIR spectra of annealed ZnFe2−yCryO4. ZnO, Fe2O3, and Cr2O3 were taken as the references.

states provide an indication about the position of the Fermi level with respect to the continuum. In ZnFe2O4 (y = 0) and ZnCr2O4 (y = 2), these spectral features are reminiscent of those of Fe2O3 and Cr2O3, respectively, suggesting that Fe and Cr within the spinel structure exhibit similar structural symmetry and valence to these binary oxides. The O 2p-Fe t2g states in ZnFe2−yCryO4 appeared at ∼527.5 eV when y < 1.5, yet the second peak of the doublet is forward-shifted from 528.9 to 529.7 eV, a phenomenon that is taken as indication of 3d electron transfer from Fe-eg (d5) to Cr-t2g states (d3), which lie at higher energy (see Fe2O3 versus Cr2O3). The splitting between t2g and eg states (Δd) was measured to be ∼1.4 eV in y = 0, which is identical to the value found in Fe2O3. It became larger with increasing Cr3+ ratio: ∼1.5 eV in y = 0.5, ∼1.7 eV in y = 1.0, ∼2.1 eV in y = 1.5, and ∼3.2 eV in y = 2.0. It is worth noting that these t2g/eg states appeared at higher energy for 24216



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Figure 4. (a) O K-edge, (b) Zn L-edge, (c) Fe L-edge, and (d) Cr L-edge spectra of ZnFe2−yCryO4 collected by XAS. Standard samples (FeO, Fe3O4, Fe2O3, ZnO, and Cr2O3) were used for quantitative comparison. 24217



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Table 2), with the exception of a small peak at ∼0.2 V in y = 0, which was not resolvable in the galvanostatic curves. This feature, which was assigned to the alloying reaction between Li and Zn,48 was buried by the main reduction peak as the Cr content increased. In general, the theoretical voltage of an electrode process is related to the Gibbs free energy involved in the redox reaction of the transition-metal oxides.31,49 However, significant deviations were reported in conversion reaction due to factors that remain to be fully uncovered.13 Nonetheless, as expected, there is a direct impact on battery working voltage when the B cations in ZnB2O4 are substituted in our system of study. The gradual decrease in discharge−charge voltages is due to the increasing amount of Cr3+ over Fe3+, because Cr2O3 gets reduced at a lower voltage (∼0.2 V) than Fe2O3 (∼0.8 V).13 All in all, the analysis of the electrochemical profiles reveals the possibility of tuning the working voltage of a conversion electrode by selecting a combination of transition metals in the spinel structure. The profile on charge showed a certain degree of symmetry with respect to discharge, with a sloping region followed by a long pseudoplateau well above 1 V. This plateau also shifted to lower voltages with Cr substitution, albeit in a manner much less pronounced than on discharge (Table 2). Close inspection of the CVs again revealed an extra oxidation peak at ∼0.15 V, especially for y = 0, which was assigned to the dealloying reaction in LiZn.48 The humongous voltage hysteresis between discharge and charge is ubiquitous in reversible electrochemical conversion reactions driven by lithium.13 It appeared to decrease with increasing Fe ratio, but it is also possible that these changes were due to pure ohmic contributions rather than to a true reduction of the hysteresis. The specific capacities on charge were noticeably lower than on discharge. This loss is most likely due to a combination of irreversible electrolyte decomposition and inefficient reconversion of the transitionmetal oxides, and it points out another common source of inefficiency that cripples these materials as viable electrodes.13 Both the discharge and charge capacity of the electrodes were largely unaffected up to y ≤ 1.0, yet decreased significantly at high Cr content. Indeed, the ZnCr2O4 electrode reacted with merely ∼1.24 mols of Li ion equiv per unit formula during the first reduction, instead of the 9 mols predicted by eqs 1 and 2 mentioned above. The corresponding CV scan (Figure 5d) showed only an extremely shallow peak at ∼0.8 V with no other reduction peak could be unambiguously identified. The ratio of carbon in the electrode was doubled from 20 to 40 wt % to ensure electronic conductivity was not driving the poor performance. The result was an increase in capacity during the plateau at ∼0.8 V (Figure 6a), but this capacity was extremely irreversible. Highly irreversible processes at these voltages are commonly associated with the decomposition of the organic electrolyte on the surface of the electrode, especially the carbon additive.13,50 Thus, the increase in capacity was simply ascribed to an enhanced electrolyte reaction caused by the increment of carbon content in the electrode. It would appear that ZnCr2O4 is largely inactive in these conditions. To better understand the activity of ZnFe2O4 and ZnCr2O4, it was compared to electrodes made of physically mixed ZnO and M2O3 (M = Fe or Cr) at 1:1 molar ratio (i.e., the same Zn/ M ratio as in the spinel phase). As shown in Figure 6b, ZnO and Fe2O3 delivered comparable capacities to ZnFe2O4 at first cycle. The discharge and charge profiles were also very similar to all the processes taking place at almost the same voltages. It is interesting to note that only one broad process was found

symmetry of FeO (Fe2+) and both tetrahedral and octahedral symmetries of Fe3O4 (Fe2+ and Fe3+). The spectra of ZnFe2−yCryO4 (y = 0−1.5) exhibited similar shoulders, consistent with Fe3+ in an octahedral symmetry in the spinel structure. The rest of the spectral features in the ternary phases also ressemble those of Fe2O3. Finally, the Cr L-edge XAS spectra of ZnFe2−yCryO4 and Cr2O3 can be found in Figure 4d. The significant similarities between all spectra imply a similar chemical environment as well as valence state.46 Upon close inspection, it was found that the additional features around the main L3 peak have relatively lower intensity at 575.0 eV and higher intensity at 578.0 eV upon increasing the Cr content (see inset in Figure 4d). At higher y, the Cr spectra of ZnFe2−yCryO4 were found to be remarkably similar to that of Cr2O3. In conclusion, Cr3+ ions substitute Fe3+ and preserve the octahedral symmetry, whereas Zn2+ ions remain in a tetrahedral environment. However, Cr substitution affects their chemical environment by increasing O 2p hybridization. Electrochemical Performance in Li Metal Cells. The voltage versus specific capacity curves in the 1st, 10th, and 100th cycle of ZnFe2−yCryO4 in cells against Li metal are plotted in Figure 5a, 5b, and 5c, respectively. The first

Figure 5. Galvanostatic cycling data of ZnFe2−yCryO4 at a current density of 60 mA g−1 after (a) 1, (b) 10, and (c) 100 cycles. The specific capacities of the spinels decline significantly with increasing Cr contents. CV of ZnFe2−yCryO4 at a scan rate of 0.1 mV s−1 after (d) 1 and (e) 10 cycles.

galvanostatic discharge curve of ZnFe2O4 was characterized by a long plateau, corresponding to the process of spinel amorphization through reduction into their component transition metals (Zn and Fe) embedded in a Li2O matrix.15,16 The plateau is subsequently followed by a sloping region until the low cutoff voltage (5 mV). This step was assigned to the formation of a polymeric layer surrounding the reduced products.13,14,17,47 The voltage of the first discharge plateau decreased steadily with Cr content. Largely the same features and trends were observed by CV (Figure 5d and summarized in 24218



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Table 2. Summary of Battery Performance of ZnFe2−yCryO4 Particles at First Cycle of Galvanostatic Discharging−Charging and First CV Scan × Li

y 0 0.5 1.0 1.5 2.0 a

working voltage verified by CV

1st discharge

1st charge

10th discharge

10th charge

1st discharge

1st charge

10th discharge

10th charge

10.00 10.54 8.82 5.34 1.24

6.77 7.70 6.55 3.53 0.62

5.73 6.34 5.50 3.92 0.77

5.63 6.16 5.32 3.81 0.74

0.55 0.30 0.15 n.a.a n.a.a

1.67 1.55 1.52 1.45 n.a.a

0.88 0.69 0.67 0.67 n.a.a

1.69 1.58 1.53 1.45 n.aa

n.a: the plateau and redox peaks cannot be identified unambiguously.

process of LiZn was indicated by an anodic peak at ∼0.2 V in the derivative curve of both mixtures. Additional oxidation peaks at ∼0.54 and ∼0.64 V were proposed to be part of a multistep dealloying process.48 A very broad peak centered at ∼1.29 V can be attributed again to the smearing of Zn and Cr reoxidation voltages. None of these peaks were found in ZnCr2O4, suggesting the binary oxide composite reacts with lithium differently than the ternary oxide reacts with lithium. The trends in voltage versus capacity profiles of ZnFe2−yCryO4 at the 10th and 100th cycles are in agreement with the 1st cycle (Figure 5b and 5c), yet even lower capacities were observed. Furthermore, the charge voltages in the 1st and 10th cycle were similar, but the reduction potentials in the 10th were found to be higher than during the 1st discharge. This displacement is also common in conversion electrodes,13 but the hysteresis is still too large for application purposes. The cycling performance for all samples is plotted in Figure 7. All

Figure 7. Cycling performance of ZnFe2−yCryO4 nanoparticles at 60 mA g−1.

Figure 6. Discharge/charge behaviors of (a) ZnCr2O4 with high amount of carbon black (40 wt % of the overall electrode mass), (b) ball-milled ZnO and Fe2O3 composites before annealing, and (c) ballmilled ZnO and Cr2O3 before annealing. All measurements were conducted at current density of 60 mA g−1. Inset depicts the inverse derivative of the curves versus voltage.

ZnFe2−yCryO4 (y = 0, 0.5, 1.0, and 1.5) exhibited comparable specific capacity retention after 50 cycles with the values stabilizing at ∼612, ∼637, ∼553, and ∼504 mAh g−1, respectively. When y = 2.0, a very low capacity of ∼91 mAh g−1 was observed throughout the 50 cycles. Ex Situ Characterization of ZnFe2−yCryO4 after the Electrochemical Reactions. To understand the reaction mechanism of ZnFe2−yCryO4 anodes with lithium, XRD (Figure 8a) and SAED (Figure 9) patterns were collected ex situ for cycled electrodes. SAED was also carried out for the pristine materials to establish comparisons. The set of concentric rings in the pristine samples indicated the presence of polycrystalline spinels with d-spacings in agreement with the XRD results (Figure 1a). The XRD patterns for ZnFe2O4 (y = 0) and ZnFe1.5Cr0.5O4 (y = 0.5) after a deep discharge to 0.005 V

centered at ∼1.5 V in the first charge, which suggests that both Zn and Fe oxidize at adjacent potentials. On the other hand, ZnO and Cr2O3 electrodes showed a very high activity, in stark contrast to ZnCr2O4 (Figure 6c). The initial discharge capacity was ∼1115 mAh g−1 (∼9.7 mols of Li), which is close to the theoretical value of 1034 mAh g−1 (9 mols of Li) for full conversion with subsequent Li−Zn alloying; a lower capacity of ∼724 mAh g−1 (∼6.3 mols of Li) was obtained on charge. Two discharge plateaus at ∼0.48 and ∼0.11 V were observed during the first reduction, which can be assigned to the sequential reaction of ZnO and Cr2O3.51,52 Upon oxidation, the dealloying 24219



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uniformly dispersed in an amorphous Li2O matrix.15−17 When y ≥ 1.0, residual spinel peaks started to emerge in the XRD patterns of discharged electrodes (Figure 8a), their intensity being higher with Cr substitution. These peaks were due to unreacted material. Discharged ZnFe1.0Cr1.0O4 and ZnCr2O4 electrodes showed clear SAED patterns with distinct polycrystalline nature (Figure 9e and 9f, respectively), implying that crystalline spinel phases were preserved after the battery cycling, especially for ZnCr2O4. ZnFe1.0Cr1.0O4 showed a reversible capacity close to ZnFe2O4 (Table 2), and its corresponding ex situ SAED presented a pattern that was more diffuse than that of ZnCr2O4, indicating a portion of the spinel experienced conversion to the nanocrystalline metallic phase. It is possible that ohmic losses prevented full utilization of the oxide in the presence of a working voltage that is very close to the cutoff, resulting in domains that were converted and others that preserved the pristine structure (Figure 9e). The diffuse nature of the diffraction rings in the reduced sample compared to the spotty patterns in the pristine material probably reflects the pulverization of the electrode driven by the formation of multiple converted domains during the amorphization process.53 Extended ex situ XRD studies were carried out for ZnCr2O4 at different discharge−charge cutoff potentials (Figure 8b). No evidence of reaction was found at any voltage or after 50th cycles, confirming that the capacity values observed in ZnCr2O4 electrodes are mainly ascribed to the carbon additive and/or other undesirable side reactions. Hence, ZnCr2O4 is inactive toward electrochemical reaction with lithium and thus not a viable electrode material. In the case of ZnO and Cr2O3 composite electrodes, Figure 8c clearly shows that both hexagonal ZnO and rhombohedral Cr2O3 disappeared after first discharge to 0.005 V, and there is no evidence of any crystalline product throughout the cycling. Hence, ZnO and Cr2O3 undergo conversion reactions with amorphization when physically mixed. It is consistent with the aforementioned discussion regarding their reaction mechanism with lithium. All these results confirm the inherent electrochemical inactivity of Zn and Cr when together in a spinel structure. This inactivity must be related to electronic structure changes

Figure 8. Ex situ XRD pattern of (a) ZnFe2−yCryO4 nanoparticles after first discharge to 0.005 V. (b) ZnCr2O4 and (c) ZnO and Cr2O3 composites were discharged or charged to specific cutoff potential at 60 mA g−1, and the electrodes were recovered for ex situ characterization by XRD.

revealed the absence of peaks related to the initial spinel structure or any other new phase. A set of highly diffuse SAED rings were obtained for fully reduced ZnFe2O4 (Figure 9d); the possible phases present were very difficult to identify unambiguously because of their nanocrystalline nature, but they were proposed to be LiZn and Fe metallic nanograins

Figure 9. SAED of (top) pristine ZnFe2−yCryO4 powders and (bottom) samples after 1st discharging. (a) and (d) y = 0; (b) and (e) y = 1.0; (c) and (f) y = 2.0. Inset represents corresponding (hkl) for each diffraction ring. 24220



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result, all metals in ZnFe2−yCryO4 always get reduced simultaneously.

in the mixed ternary metal oxides versus the individual binary phases. It is worth mentioning that there is no related publication on any inactive spinel anodes yet. In general, all chromites, including ZnCr2O4, are essentially normal spinels (inversion degree less than 0.05),54 with the tetrahedral site preference of Zn2+ and a very strong octahedral site preference of Cr3+ to form [Zn2+]T[Cr3+Cr3+]OO4.1,2,36,55,56 Müller et al.56 found that ZnCr2O4 has the most exothermic enthalpy of formation from their component oxides when compared with other chromites such as Cr2O3, CoCr2O4, FeCr2O4, MgCr2O4, and NiCr2O4, indicating it is an extremely stable compound once formed. Levy et al.36 also investigated the stability of ZnCr2O4 as a function of pressure and temperature. It was proposed that a very high pressure of 18 GPa is required to decompose ZnCr2O4 spinel into ZnO and Cr2O3 at room temperature, in agreement with the ab initio computational simulations reported by Catti et al.55 Because Cr3+ has a prominent octahedral crystal field stabilization energy (CFSE) of ∼224.5 kJ mol−1,57 it results in strong orbital hybridization and a wide band gap between the top of the filled O 2p band and bottom of the transition-metal 4s and 4p conduction bands,58 as discussed in the context of the XAS data above (Figure 4b,d). The increase in covalence could explain both the inactivity of ZnCr2O4 and, more broadly, the trends in reduction potentials in ZnFe2−yCryO4. The electrochemical potential (E0) of reduction of a given solid phase is related to the position of the bottom of the conduction band with respect to a reference level (Li metal in current study), as electron injection occurs at these states.50 Because their position shifts upward, closer to the Li metal level with greater covalence, the macroscopic observation is a decrease in E0. This phenomenon could lead to values of E0 that are very close to the Li metal electrode. In the presence of severe energy penalties imposed by the conversion from crystalline oxide to metallic nanograins embedded in a Li2O matrix,59 it is feasible that the actual value of reduction potential may fall below 0 V versus Li+/Li0, so that no activity is observed in a typical electrochemical experiment such as those performed here. We believe this hypothesis applies to the voltage trends when substituting Fe by Cr in ZnFe2−yCryO4, which are consistent with the lower reduction potential of Cr2O3 compared to that of Fe2O3.13 The inactivity of ZnCr2O4 compared to ZnO/Cr2O3 is consistent with the picture painted by the O K-edge XAS data (Figure 4a), i.e., the O 2p−Cr t2g peak appeared at higher energies for ZnCr2O4 than for Cr2O3. As discussed above, this shift is a reflection of the upward move of the Fermi level closer to the continuum and, by extension, to the Fermi level of Li metal.50 It is worth noting that the existence of single reduction plateaus of ZnFe2−yCryO4, especially in the first discharge, could be considered unexpected based on the significant difference in reduction potential of the respective binary oxides (Figure 6b,c). According to the XAS results, electronic level mixing between the metals occurs and leads to overall shifts of the corresponding transitions. In the presence of such mixing, electron injection destabilizes both Cr and Fe states and causes their concurrent reduction to metal. This process would occur at a single potential, consistent with the presence of a single plateau in the first discharge of ZnFe2−yCryO4. The difference in potential between the binary oxides ZnO60 and Fe2O327 does not lead to a mechanism in which one of the two metals is preferentially extruded from the spinel structure first, followed by complete amorphization to metal particles and Li2O. As a

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CONCLUSION The solid solution series ZnFe2−yCryO4 demonstrate the flexibility in tuning the working voltages by changing the counterions (B) in the ZnB 2O4 framework. With the substitution of Fe3+ by Cr3+, a gradual decrease in this magnitude was observed. The electrode cycling properties at low and intermediate Cr contents were comparable, whereas ZnCr2O4 was found to be greatly inactive. In contrast, mixtures of the corresponding binary oxides (ZnO and Cr2O3) reversibly absorb and release Li ions according to conventional conversion reactions. A systematic study conducted by ex situ XRD and SAED confirmed that ZnCr2O4 stays pristine even after 50 cycles, and the capacity obtained in Li cells is due to irreversible decomposition of the electrolyte. The combination of electrochemical data with the thorough evaluation of the electronic structure by soft XAS provides support to the general explanation of the trends in this system as well as the hypothesis that the inactivity of ZnCr2O4 is driven by the upward shift of the Fermi level due to an increase in covalence, which appears to be higher than in Cr2O3. Because of the penalties imposed by a typical conversion process, the electrochemical potential of ZnCr2O4 must be very close to (or even below) that of Li metal, so that no activity is observed. This work also further reinforces the notion that electrochemistry can be a valuable methodology for evaluating systematic changes in bonding with a given chemical system.

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ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +65 6790 9081. Tel: +65 6790 4606. Present Address

# C.K. and J.C.: Department of Chemistry, University of Illinois at Chicago, Chicago, IL 60607.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by funding from the National Research Foundation, Clean Energy Research Project Grant NRF2009EWT-CERP001-036. The authors acknowledge Timcal and Arkema for providing Super P Li Carbon black and Kynar PVDF Binder, respectively. The materials characterization was performed in Facility for Analysis, Characterization Testing and Simulation Laboratory (FACTS), Nanyang Technological University. P.F.T. thanks the financial support from Ian Ferguson Foundation. J.C. and C.K. were supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract DE-AC02-05CH11231, under the Batteries for Advanced Transportation Technologies (BATT) Program. C.-H.C. is grateful to the National Science Council of Taiwan for financial support under Grant NSC 102-2112-M24221



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032-001. Y.S. acknowledges the financial support received from DAE-BRNS, India (Grant DAE-661-DPT).

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Electrochemical Reactivity with Lithium of Spinel-type ZnFe2âyCryO4

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