ZnAlxCo2–xO4 Spinels as Cathode Materials for Non-Aqueous Zn

Oct 20, 2017 - Full cells constructed using this new spinel as a cathode paired with a metal anode showed capacities over 100 cycles of 114 mAh/g and ...
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Cite This: Chem. Mater. 2017, 29, 9351-9359

ZnAlxCo2−xO4 Spinels as Cathode Materials for Non-Aqueous Zn Batteries with an Open Circuit Voltage of ≤2 V Chengsi Pan, Ralph G. Nuzzo,* and Andrew A. Gewirth* Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States S Supporting Information *

ABSTRACT: Rechargeable Zn batteries are promising energy storage alternatives for Li-ion batteries in part because of the high specific and volumetric capacities of Zn anodes, as well as their low cost, improved prospects for safety, and the fact that they are environmentally friendly. Development efforts, however, have focused mostly on aqueous electrolyte systems, which are intrinsically limited by the narrow electrochemical potential window of water. As a consequence, the use of alternative nonaqueous electrolytes has attracted a growing level of interest with the hope that they may provide higher operational voltages, which potentially could provide viable pathways to high-energy and high-power density Zn batteries. With regard to the latter, the considerable progress made in developing useful non-aqueous electrolyte chemistries for Zn anodes has not been matched by correlated progress regarding the development of useful cathode materials. In this work, a new series of spinels, ZnAlxCo2−xO4, are reported and their utility as cathode materials for non-aqueous Zn-ion batteries is demonstrated. Full cells constructed using this new spinel as a cathode paired with a metal anode showed capacities over 100 cycles of 114 mAh/g and an onset potential of 1.95 V, which is the highest OCV yet reported for a non-aqueous Zn-ion battery system. The data show that the Zn2+ ions reversibly intercalate into the spinel structure during the charge−discharge processes, a compositional transformation directly correlated with a reversible conversion between Co4+ and Co3+ oxidation states within the lattice. The data illustrate that the Al3+doped spinel structure is a robust candidate material for use in non-aqueous Zn batteries, suggesting guidelines for the design of more efficient multivalent cathode materials.

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INTRODUCTION

relevant experience exists, as Zn metal has been used as a battery anode for decades.5 As noted above, this metal anode carries with it other non-inconsequential advantages, such as low cost, safety, low environmental impact in its manufacture, and high natural abundance. A high-capacity, high-power Znion battery would be quite promising in terms of its prospects for providing a next-generation capability for electrochemical energy storage. A series of new materials chemistries for Zn-ion batteries have been reported recently, ones involving a Zn metal anode joined to various cathodes consisting of MnO2,6,7 Prussian blue analogues,8 Zn0.25V2O5·nH2O,9 and ZnMn2−xO4.10 All of these cathodes follow mechanisms of Zn-ion (de)intercalation as the foundation for battery operation. Most of these systems, however, utilize aqueous electrolytes. While aqueous systems are in concept cheap and safe11 relative to non-aqueous

Li-ion batteries (LIBs) are presently the most popular energy storage solutions for mobile devices, vehicles, and renewable energy resources.1 The power and energy densities of today’s LIBs, however, face significant challenges with respect to meeting the growing system-level requirements for sustainable energy consumption.2 Moreover, concerns about the safety, cost, environmental impact, and natural abundance of Li may limit further LIB use. Multivalent-ion batteries employing cations such as Mg2+, Ca2+, Zn2+, and others are considered to be some of the better prospective alternatives to LIBs because of the inherent high volumetric capacity of the zero-valent metal anode. These values (3833 mAh/mL for Mg, 2073 mAh/ mL for Ca, and 5851 mAh/mL for Zn) are substantially larger than that corresponding to a C-based LIB anode (756 mAh/ mL).3 Among the multivalent cations, Zn2+ exhibits the highest volumetric capacity and also a gravimetric capacity (820 mAh/ g) higher than that of graphite (372 mAh/g, the present anode for LIBs),4 which suggests that a Zn-ion battery could be lighter and smaller than today’s LIBs. A considerable background of © 2017 American Chemical Society

Received: August 7, 2017 Revised: October 18, 2017 Published: October 20, 2017 9351

DOI: 10.1021/acs.chemmater.7b03340 Chem. Mater. 2017, 29, 9351−9359

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Chemistry of Materials analogues, they suffer from other problems, such as the possibility of O2/H2 evolution,12 memory effects,13 anode selfcorrosion motivated by the pH,14 and the narrow electrochemical stability window of water itself.15 The latter issue strongly limits the available battery voltage and thus constrains the battery energy density to less useful limiting values.2 Non-aqueous electrolytes could in concept make available a more desirable higher-voltage Zn-ion battery. In a recent publication, Han et al.16 showed that the electrochemical window of Zn(TFSI)2 in acetonitrile can reach ∼3.8 V versus Zn2+/Zn. They also developed a Zn/V2O5 cathode system based on this electrolyte that reached an operating voltage of ∼0.9 V.17 There remains an opportunity for progress in developing alternative cathode materials that could take advantage of the large electrochemical window afforded by the Zn(TFSI)2/acetonitrile electrolyte. By analogy with its Li counterpart, ZnCo2O4 could be one of the cathode material candidates for a higher-voltage Zn-ion battery. It is known that ZnCo2O4 adopts a spinel structure with three-dimensional tunnels suitable for ionic diffusion, similar to LiMn2O4.18 More importantly, the ionic radius of Zn2+ (71 pm) is close to that of Li+ (67 pm),19 suggesting that similar driven transport dynamics might be accessible. In addition, MgCo2O4, a compound with a similar structure, has been reported as a prospective cathode material for a Mg-ionbased battery.20,21 Schematic depictions of a traditional LIB (C/LixCoO2 battery) and an analogous Zn-ion battery (including a putative charge−discharge process for a Co2O4based cathode) are shown in Figure 1. In the latter, Zn metal takes the place of the graphite anode, while the cathode invokes a similar cation intercalation mechanism. During the depicted charge process, Zn2+ is extracted from the structure and Co4+ is formed, with the spinel structure being retained in analogy with a similar process reported for LiMn2O4.18 In this structure, Zn2+ resides at the 8a site while Co3+ is at the 16d site. During the charge step of a cycle, the Zn2+ would be extracted from the structure via diffusion along the 8a−16d−8a path.22 During the reverse discharge process, Zn2+ intercalates into the spinel structure, with 2 equiv of Co4+ being reduced to a Co3+ state. There is a problem that challenges the viability of this hypothetical mechanism, however, the known instability of the CoO2 framework toward decomposition via oxygen evolution.20 The latter would lead to a series of undesirable impacts, those being most notably low Coulombic efficiency and capacity fade. The partial substitution of Al for Co provides one effective solution to the problem of CoO2 instability, as demonstrated in LiCoO2 batteries.23 It also is likely that Al substitution will facilitate Zn exchange because the Al−O bond (0.191 nm) is shorter than the Co−O bond (0.193 nm), leading to a weaker interaction between the AlO6 octahedral site and Zn2+.24 In this work, Al-doped ZnCo2O4 was prepared by a sol−gel method and developed as a new cathode for a non-aqueous, rechargeable Zn-ion battery. The structural changes induced by varying degrees of Al doping, along with the consequent electrochemical performance it engenders in a non-aqueous electrolyte, are described. The structural evolution and valence state changes seen during the Zn extraction/intercalation processes in the battery are established using combined methods of physical and spectroscopic methods of structural characterization, data that affirm in quantitative form the mechanisms shown schematically in Figure 1. When taken together, the results describe features of structural and

Figure 1. Schematic of the traditional graphite−LiCoO2 battery (a) and the Zn-ion battery (b). The enlarged part is the structure change during the electrode reaction.

dynamical guidance requiring consideration in efforts to develop higher-performance Zn cathode materials.

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EXPERIMENTAL SECTION

Chemicals were obtained from Alfa Aesar, Fisher Scientific, or SigmaAldrich and used as received unless otherwise specified. Electrochemical measurements were taken using a CHI (Austin, TX) 660D potentiostat. Zn foil (Sigma-Aldrich, >99.995% pure) was used as the counter/reference electrode. ZnAlxCo2−xO4 Preparation. ZnAl0.67Co1.33O4 was prepared using a citric sol−gel method.25 Typically, 2 mmol of Co(NO3)2·6H2O (Sigma-Aldrich, >98% pure) was dissolved in 40 mL of H2O, and then 1 mmol of Al(NO3)2·9H2O (Sigma-Aldrich, 99.9% pure), 5 mmol of citric acid monohydrate (Fisher Scientific, >99% pure), and 1.5 mmol of ZnO (Sigma-Aldrich, >99% pure) were added. Subsequently, the solution was stirred until it became transparent. The solution was heated at 200 °C to evaporate the liquid, and the resulting solid was then calcined for 30 min at 450 °C to remove the organic compounds. The powder was subsequently calcined at 600 °C for 2 h in air, yielding a dark green/black material. The other stoichiometries of ZnAlxCo2−xO4 (x = 0, 0.2, 0.5, or 1.0) were synthesized in a similar manner. ICP analysis showed that the Zn, Al, and Co content for ZnAlxCo2−xO4 (x = 0.67) was ZnAl0.656Co1.344O4. Cathode. To prepare the working electrode, the ZnAlxCo2−xO4 powder was mixed with carbon black and the polyvinylidene fluoride (PVDF) binder at a weight ratio of 70:20:10 in an N-methyl-2pyrrolidone (NMP) solvent to form a slurry. The slurry was sonicated for ∼2 h, cast onto a current collector (carbon paper, Ion Power Inc., 9352

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Chemistry of Materials GDL 35 BC), doctor-bladed to form a 15 μm thick film, and dried overnight at room temperature. The current collector was further dried at 120 °C for ∼2 h in a vacuum oven before use. Coin Cell Assembly. The current collector was punched into a 9.5 mm diameter disk to form the working electrode. The active material load in each disk was found to be 0.7−0.9 mg, corresponding to 1−1.3 mg cm−2. A Celgard 2325 membrane was used as the separator. Zn foil (9.5 mm diameter) was employed as the counter electrode. A 0.3 M solution of Zn(OTf)2 in MeCN was used as the electrolyte. The water content of the electrolyte was measured to be ∼120 ppm with a Mettler Toledo DL39 Karl Fischer coulometer. To prepare a lowwater content electrolyte (∼52 ppm), Zn(OTf)2 was dried at 100 °C in a vacuum oven for 24 h, while MeCN was dried over molecular sieves (3 Å) for 72 h before use. Characterization. Scanning electron microscopy (SEM) imaging was performed using a high-resolution scanning electron microscope (Hitachi S4700), at accelerating voltages of 10 and 20 kV. An Oxford Instruments ISIS EDS instrument attached to the scanning electron microscope was used to collect EDS data. Quantitative analysis of the weight percents of Zn, Co, and Al was performed with the software (IXRF analysis). High-resolution transmission electron microscopy (TEM) characterizations were performed by using a transmission electron microscope (JEOL 2010LaB6) at an accelerating voltage of 200 kV. A Siemens/Bruker D-5000 instrument equipped with a Cu Kα radiation source was used to obtain X-ray diffraction (XRD) data. To protect the samples from exposure to moisture and air during transfer for analysis, the samples were covered and sealed against air with a Kapton film (7 μm, Chemplex Industries Inc.). XRD refinement was performed using Jade version 9.0. The X-ray photoelectron spectroscopy (XPS) measurements were taken using a Kratos Axis Ultra spectrometer using focused monochromatized Al Kα radiation (1486.8 eV). The cathodes used for XPS were stored and transferred under Ar to avoid exposure to moisture and air. Brunauer−Emmett−Teller (BET) measurements were taken using a Quantachrome 2200e instrument.

the peak at ∼36.5° changes as a function of Al content, with an increasing level of Al being associated with a lattice constant that is smaller than that for the reference cubic ZnCo2O4 material. The ionic radius of Al3+ (53.5 pm) is smaller than that of Co3+ (54.5 pm), consistent with this trend. To establish the structure of the Al-doped ZnCo2O4, we focus on the x = 0.67 material as an exemplary case for refinement. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) results show that this compound has the formula ZnAl0.656Co1.344O4, while energy dispersive X-ray s p e c t r o s c o p y ( E D X ) g i v e s , in go o d a g r e e m e n t , ZnAl0.661Co1.303O4 (Figure S2). Both of these analyses give elemental compositions close to the theoretical ratio. The elemental results, combined with the XRD data, indicate that a significant quantity of Al is doped into the lattice. The structure of ZnAl0.67Co1.33O4 was determined by the Rietveld method.26 Typical XRD patterns analyzed using the Rietveld method (Figure S3) and the relative parameters are listed in Table S1. The Rp factor and χ2 (goodness of fit factor) are found to be 6.27% and 1.29, respectively. Similar magnitudes of the Rp and χ2 factors have been observed for similar compounds, such as Al-doped CoFe2O4 spinel27 and Fedoped ZnAl2O4 spinel.28 In ZnAl0.67Co1.33O4, the Zn ions occupy tetrahedral sites (8a) while the Co and Al ions occupy octahedral sites (16d). From the refinement, the deduced occupancy of Co is 0.677, which is close to the experimentally determined value, indicating successful substitution of Al for Co. In a control, we attempted to place the Al dopant at both Zn and Co sites. Only substitution at Co sites yielded a refinement consistent with the XRD data. Figure 2c shows the structure of ZnAl0.67Co1.33O4; the structure is similar to that of ZnCo2O4, in which Al occupies some of the Co sites of the parent ZnCo2O4 spinel homologue. We note that ZnAl2O4 and ZnCo2O4 exhibit very similar structures with a lattice mismatch of only 0.39%29 and thus may form a solid solution. Figure 3 shows SEM and TEM images of ZnCo2O4 (panels a and b, respectively, of Figure 3) and ZnAl0.67Co1.33O4 (panels c and d, respectively, of Figure 3). These images reveal a series of particles, which are agglomerated from smaller primary particles. These secondary particles are larger in the Al-doped sample than in the undoped material. There was no significant dependence of the secondary particle size seen as a consequence of the amount of Al doped into the lattice, with the exception of ZnAl0.2Co1.8O4 (Figure S4), which imbeds particles that are markedly smaller than the others. In spite of the large secondary particle size of the Al-doped samples, the primary particle size of ZnAl0.67Co1.33O4 (10−25 nm) is smaller than that of ZnCo2O4 (50−100 nm). To evaluate the suitability of the large secondary particles for use as a Zn-ion cathode, the BET surface area of ZnAl0.67Co1.33O4 was measured. The results (Figure S5) show the BET surface area of ZnAl0.67Co1.33O4 is 35.0 m2/g, which is considered to be large enough to provide good contact with the electrolyte.30 The BET isotherm exhibits a hysteresis loop, the presence of which indicates that ZnAl0.67Co1.33O4 exhibits a mesoporous structure. A Barrett−Joyner−Halenda (BJH) analysis of the isotherm (Figure S5) gives an average pore diameter of ∼6.7 nm. This suggests that the relatively large surface area deduced by the BET measurement is a consequence of the pore structure (particularly in light of the large secondary particle size of ZnAl0.67Co1.33O4).

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RESULTS AND DISCUSSION Structural Characterization. Figure 2 shows XRD data obtained as a function of Al content in a series of synthesized samples. The figure shows a series of peaks, the identities of which are understood by reference to JCPDS standards. The material is indexed to the spinel structure (Fd3m) of ZnCo2O4 (Figure 2c). The lack of additional peaks means that there are few to no ZnO or Al2O3 impurities (Figure S1). Interestingly,

Figure 2. (a and b) XRD for ZnAlxCo2−xO4 (x = 0, 0.2, 0.5, 0.67, or 1) and (c) schematic structures of ZnAl 2 O 4 , ZnCo 2 O 4 , and ZnAl0.67Co1.33O4. 9353

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Figure 3. SEM and HRTEM for ZnCo2O4 (a and b, respectively) and ZnAl0.67Co1.33O4 (c and d, respectively).

Figure 4. (a) Initial cycle CV of ZnAlxCo2−xO4 (x = 0, 0.2, 0.5, 0.67, or 1) and (b) CV of ZnAl0.67Co1.33O4 for 10 cycles. The initial cycle is colored red. The scan rate was 0.5 mV s−1.

Electrochemical Performance. We next evaluate the electrochemical properties of the Al-doped ZnAlxCo2−xO4 material. Figure 4a shows the cyclic voltammograms (CVs) measured for the compositionally varying ZnAlxCo2−xO4 series in 0.3 M Zn(OTf)2 in MeCN at a scan rate of 0.5 mV s−1. These data reveal the presence of distinct oxidation and reduction peaks for ZnAlxCo2−xO4 (x = 0, 0.2, 0.5, or 0.67), the magnitude of which increases as more Al is doped into the lattice. In a positive set of controls, no redox activities are observed for ZnAlCoO4 or the base carbon electrode when ZnAlxCo2−xO4 is absent (Figure S6a). The lack of an electrochemical response in ZnAlCoO4 likely originates from the small contribution of Al3+ to the band gap, as is found in other examples of Al-doped transition metal oxides.31 As a result, the presence of Al3+ in ZnAlCoO4 increases the degree of localization of carriers and thus lowers the conductivity of the material. As measured against an increase in Al content, the redox waves for ZnAlxCo2−xO4 (x = 0, 0.2, or 0.5) appear to be centered around 1.78/1.89, 1.76/1.98, and 1.74/2.06 V versus Zn2+/Zn across the series. The small redox peak shifts seen with an increase in Al content are likely due to differences in cation diffusion paths, as influenced by the secondary particle size.32

Figure 4b shows the CV data measured for ZnAl0.67Co1.33O4 over nine cycles following the initial cycle that is colored red. The predominate reduction and oxidation peaks are found at 1.72 and 2.07 V versus Zn2+/Zn, respectively, similar to the potential exhibited for the conversion of Co3+ to Co4+ in an aqueous Zn/Co3O4 system33 and the predicted voltage for ZnCo2O4−Zn batteries (2.1 V).34 A reduction peak at 1.55 V is present as a shoulder in the CV. We attribute this shoulder to heterogeneity in cation diffusion paths, in a manner similar to that seen in LiCoO2.35 More accurate redox peak positons for ZnAl0.67Co1.33O4 were determined using data from differential pulse voltammetry (DPV) measurements (Figure S7). The DPV method reduces the influence of the capacitive charging current.36 Figure S7 shows that the reduction and oxidation peaks obtained by using DPV occur at potentials similar to those shown in the CV (1.75 and 2.02 V versus Zn2+/Zn, respectively), indicating the capacitive charging current does not influence the redox peak position. Via integration of the CV curves in Figure 4b, the charge and discharge capacities are estimated to be 116 and 108 mAh/g, respectively, indicating a Coulombic efficiency of ∼90%. The capacity fade and potential shifts seen in these data after 10 cycles are negligible, suggesting (with modest improvements) ZnAl0.67Co1.33O4 might be a suitable candidate for a Zn-ion cathode (see below). 9354

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Figure 5. (a) Battery performance of ZnAl0.67Co1.33O4 obtained at a 0.2 C rate. (b) Typical charge and discharge curves at a 0.2 C rate. (c) C-Ratedependent capacity for a Zn−ZnAl0.67Co1.33O4 coin cell. (d) Typical charge and discharge curves obtained at 0.1, 0.2, 0.5, 1, 2, 5, and 10 C rates.

compositional series, has the highest capacity among all the Al-doped samples (Figure S8). Figure 5b shows exemplary charge and discharge curves measured using a coin cell sample as a function of cycle number. These profiles exhibit extended voltage plateaus at 1.9−2.15 and 1.65−1.90 V, respectively, consistent with the peaks observed in the CV data. In a control experiment, coin cells constructed using only carbon paper treated with CB and PVDF exhibit negligible capacity (Figure S6). Panels c and d of Figure 5 show the capacities obtained at 0.1, 0.2, 0.5, 1, 2, 5, and 10 C rates and their associated charge and discharge curves. For a 0.1 C rate, the capacity stabilizes at 118 mAh/g, which is approximately the same value as that obtained at a 0.2 C rate. Increasing the C rate to 0.5 and 1 C leads to further decreases in capacity (to 75 and 70% of the limiting 0.1 C value, respectively). Further increases in the C rate to 2, 5, and 10 C further diminish the limiting capacity (to 47, 35, and 12% of the 0.1 C value, respectively). This rate− property correlation is in fact one that reflects an underlying kinetic hysteresis, one putatively illustrating how the diffusion of Zn2+ within the consolidated solids of the cathode (see below) affects battery performance. We return to this issue and supporting data that affirm Zn2+-associated transport dynamics in the sections that follow. We simply note here that the hysteresis suggested above is most dramatically evidenced in the capacity recovery seen when the charging rate was decreased from 10 to 0.2 C. As seen in the data presented in Figure 5c, nearly 85% of the limiting capacity obtained at the 0.2 C rate is promptly recovered, indicating a good (but kinetically responsive) on-cycle stability. As part of a detailed series of control experiments, we carefully tested the sensitivity of the cycle, capacity, and rate sensitivities, as well as the CV data more generally, to specific conditions of the electrolyte. These included surveys of

The CV data shown in Figure 4b also exhibit a small (nonspinel-associated) reduction current between 1.4 and 1.5 V. This electrochemical activity is also seen in the blank control samples (Figure S6a) and has been widely described in the literature as a feature seen in the cycling of graphite or CNT composites used in capacitors and batteries. This feature is believed to be associated with the reduction of oxygencontaining functional groups on the carbon surface (such as CO and C−OH)37 and release of absorbed anions38 (Figure S6 and its associated discussion). Panels a and b of Figure 5 show several representative charge and discharge curves measured using a coin cell integrating a ZnAl0.67Co1.33O4 cathode with a Zn metal anode in 0.3 M Zn(OTf)2 in MeCN. The open circuit voltage measured on the charged coin cell is 1.95 V, while an average working voltage was found to be ∼1.7 V from the discharge profiles. To perform a test of the full cell performance, the coin cell was galvanostatically charged and discharged at a 0.2 C rate (32 mA g−1) for 100 cycles. The cell was cycled in this way between limits of 2.15 and 1.4 V. We found that exceeding these values during the charging sequence led to a dramatic reduction in the measured Coulombic efficiency. As seen in the data (Figure 5a), the first cycle yields a capacity of 134 mAh/g, which is ∼84% of the theoretical value (159 mAh/g), with an associated Coulombic efficiency (CE) of 72%. After three cycles, the discharge capacity decreased to 114 mAh/g, with the CE stabilizing at 90%. At this point, the cell cycles stably with little change in capacity being noted. We believe that the capacity loss seen in the first two cycles is likely due to irreversible forming reactions (e.g., as might result in an SEI layer) in a manner found for other cathode materials.29 It is interesting to note that the capacity of ZnAl0.67Co1.33O4 is ∼1.3 times higher than that found for ZnCo2O4 (84 mAh/g). This material, based on similar tests made across the 9355

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Figure 6. SEM for ZnAl0.67Co1.33O4 after (a) discharge and (c) charge following 10 cycles in a coin cell. (b and d) Corresponding EDX data.

ZnAl0.67Co1.33O4 cathode following cycling (Figure 6a−d). Panels a and c of Figure 6 show SEM images obtained after 10 cycles in a coin cell following discharge and charge, respectively. Relative to the pristine cathode (Figure S11), the images in Figure 6 do not exhibit major morphological changes. In particular, the pristine and cycled cathodes exhibit particles with sizes of approximately 1−5 μm. Figure 6c shows that the surface of the particles becomes somewhat rougher on charge, possibly because of a local stress change originating from lattice distortion upon removal of Zn. The lattice distortion is most likely due to Zn extraction and the resultant Zn2+ concentration gradients in ZnAl0.67Co1.33O4. A similar change in the roughness of the electrode surface on charge and discharge has been reported f or the L IB cathod es LiMn 2 O 4 4 0 and Li0.2(Mn0.54Ni0.13Co0.13)O2.41 Panels b and d of Figure 6 report the EDX data corresponding to the SEM images in the figure. The data report on the relative amounts of Zn, Co, and Al within the extent of the larger particles. The data in Figure 6b give Zn:Al:Co ratios of 1:0.67:1.32 following coin cell discharge, while the corresponding ratio from the data in Figure 6d following charging is 0.53:0.67:1.34. These data affirm that the Zn:(Co+Al) ratio changes markedly after charging the coin cell (i.e., following removal of Zn from the cathode). These exemplary data suggest changes in the Zn:(Co+Al) ratio (from 1.01:2 to 0.53:2) that correspond to a capacity of ∼104 mAh/g, which is close to the experimentally measured value for the coin cell. To more carefully document the change in Zn composition between the charge and discharge states, we performed ICPAES analyses on the cathode material. These results reveal that the charge step leads to a measured Zn:Al:Co ratio of 0.491:0.667:1.372, while after discharge, the cathode incorporates Zn to give ratios close to that measured by EDX (0.942:0.667:1.352). Taken together, these data strongly suggest the idea that the electrochemical dynamics of the cathode materials are in fact ones associated with reversible Zn intercalation processes. Diffraction data provide additional support for this conclusion.

responses seen to variations made in the concentration of the Zn(OTf)2 as well as due to the presence of water impurities in the MeCN. The sensitivities seen due to the former were unexceptional and are illustrated in summary form in materials presented in the Supporting Information (Figure S9). The sensitivities due to the concentration of water in the electrolyte are more significant (Figure S10 and the associated discussion). For the data shown above, the experiments were performed using a solvent that on testing was shown to have a water content of ∼120 ppm. We found that the water content that was either higher or lower than this value led to a very poor electrochemical performance of the cathode. To summarize the main results of the control experiments described in the Supporting Information, we found that under rigorously anhydrous conditions MeCN is subject to significant side reactions that lead to its oligomerization and deposition on the electrodes and separators as a highly colored (likely fouling) deposit. The latter reactivity, mediated via acid-catalyzed pathways, is one well-known in the literature.39 This suggests that an apparent small degree of oxidative decomposition of the MeCN yields the acidic moieties required to initiate side reactions in the solvent. The likelihood that protons serve as that source is strongly indicated. We tested and affirmed this hypothesis by adding a small concentration of the so-called proton sponge, 1,4-diazabicyclo[2.2.2]octane, to an otherwise anhydrous electrolyte. This addition restored more efficient electrodic behavior and suppressed the deposition of oligomers (Figure S10 and the associated discussion). This improvement is not one that proved to be superior to that realized with a lowparts per million water content, which proved to be sufficient to suppress the polymerization of MeCN and sustain good cycling performance. We note that identifying an improved electrolyte that affords better oxidative stability at the high operating potentials of these spinel cathodes remains an issue of interest. We turn now to the consideration of data that firmly establish the role of Zn2+ intercalation dynamics in the electrochemistry of the Al-doped ZnAlxCo2−xO4 cathodes. Structural Change and Zn2+ Intercalation during the Electrode Reaction. We next evaluated the morphological and compositional changes seen in the best-performing 9356

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Figure 7. (a) XRD for pristine (black), charged (red), discharged (blue) ZnAl0.67Co1.33O4 cathodes (tetrahedral ZrO2 as an external standard) and the blank electrode (only PVDF and carbon black cast on carbon paper). (b) Enlarged XRD patterns for (200), (311), (422), (511), and (440) peaks (from left to right, respectively) of ZnAl0.67Co1.33O4.

Figure 8. XPS (a) Co 2p and (b) O 1s spectra for the pristine, charged, and discharged ZnAl0.67Co1.33O4 cathodes. The dashed lines in panel a indicate the main and satellite peaks of Co3+.

Figure 7 shows ex situ powder X-ray diffraction (XRD) measured for pristine, charged, and discharged ZnAl0.67Co1.33O4 cathodes (taking the latter two for materials reaching the 10th cycle in a coin cell). These XRD data illustrate modifications of atomic structure that occur on cycling, ones that on the basis of the compositional data discussed above arise as a consequence of reversible Zn interaction processes. The dashed lines in Figure 7 denote diffraction coming from 10 wt % ZrO2 added as an inert standard during the electrode preparation. The peaks at 18.3°, 26.5°, 44.1°, and 54.9° are associated with the carbon support. The figure shows that the spinel structure of ZnAl0.67Co1.33O4 is retained during the sequence of charge and discharge steps as no new peaks arise in the XRD scans. Most importantly, no peaks are seen in the XRD scans that can be associated with either CoO or ZnO. A 2θ = 23.5° shoulder peak is found with the cell in the charged state. This peak is due to intercalation of the anion into the graphite that supports the ZnAl0.67Co1.33O4 cathode. A similar XRD change has also been reported for intercalation of TFSI− into similar graphitic electrodes.42,43 We note, however, that this intercalation contributes little to the cell capacity as shown by the control experiments performed using a blank electrode (Figure S6b). Figure 7b shows that, for the charged electrode, all the peaks belonging to the (200), (311), (422), (511), and (440) planes of the spinel structure shift to larger angles relative to those of the pristine electrode (a quantitative analysis is given below). These peak shifts revert back to their initial state after discharge. These correlated shifts in the 2θ scans indicate that the structural relaxations evidenced are in fact reversible

with Zn (de)intercalation. To better understand the structural evolution of the electrode, the lattice constant was refined by using the (200), (311), (422), (511), and (440) peaks for the pristine, charged, and discharged cathodes (Table S2). For the charged cathode, the lattice constant decreases by ∼0.87% due to extraction of Zn from the structure relative to the discharged cathode. The extent of lattice contraction seen here is in fact very similar to that reported for the structural transformation between LiMn2O4 and Li0.6Mn2O4 (1.0%).44 To investigate the details of the compositional changes for the electrode reaction and confirm the formation of Co4+ sites during the charging process, XPS spectra were measured. These data allow assignments relevant to the oxidation states of the Co that is present. The data shown in Figure 8b are highresolution Co 2p photoemission spectra obtained for the charged and discharged samples. To better understand the oxidation and/or bonding state changes evidenced in these data, a set of peak analyses were performed using the CasaXPS software package (Table S3). From this fitting, the spectrum of the discharged material shows the presence of two main peaks at 779.2 eV (2p3/2) and 794.3 eV (2p1/2) along with weaker satellite peaks at ∼789.1 and 803.5 eV. The latter peaks are consistent with the presence of Co3+ (being similar to that found for LiCoO2, which also features Co in the 3+ oxidation state).44 The Co 2p XPS scan obtained for the charged state of the cathode is different in several important ways. Most notably, two new broad shoulder peaks centered at 780.6 eV (2p3/2) and 795.7 eV (2p1/2) are observed (fit with a blue line in the figure). The appearance of the latter peaks and their 9357

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Chemistry of Materials binding energy shifts are consistent with an assignment to Co present in the 4+ oxidation state. The spin orbit splittings between all the Co 2p3/2 and 2p1/2 peak components were found to be ∼15.1 eV, similar to the value of 15.0 eV reported for LiCoO2.45 We infer from these data that there is a mixture of oxidation states present for the Co within the XPS sample depth for the charged sample.44,45 Control experiments show that the higher oxidation states of the Co are labile against exposure to the ambient atmosphere (and thus on transfer to the instrument for analysis). While we minimized this exposure, we cannot preclude its impact on the spectra and for this reason cannot make a definitive assignment with respect to the operando proportions of the oxidation states present for the Co, save to note that the elemental compositions (ICP for the charged cathode) described above suggest there is always Co3+ left in the lattice regardless of charge and discharge states. Finally, we note that additional evidence of the formation of the 4+ oxidation state exists in these data, specifically the presence in Figure 8b of new satellite peaks centered at ∼785.2 and 800.2 eV for the charged cathode. These features are ones consistent with the presence of Co4+ in the charged state.46 Figure 8b shows the O 1s XPS spectra obtained from the pristine, charged, and discharged states of the cathode. Two O 1s peaks, at 529.1 and 531.5 eV, are seen in these spectra. Literature assignments establish that the peak at 529.1 eV originates from lattice O, while that at 531.5 eV is due to lowcoordination surface O atoms.46 In the charged electrode, the extent of surface O is increased relative to the extent of lattice O, while the relative amount of lattice O increases during the discharge process. The change between surface and lattice O is likely due to changes in the coordination environments of these atoms as a consequence of the Zn intercalation and deintercalation process. A similar behavior has been noted for the lithiation and delithiation process of LiCoO2.47

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Andrew A. Gewirth: 0000-0003-4400-9907 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.

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REFERENCES

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CONCLUSION In summary, a series of spinel-structured ZnAlxCo2−xO4 compounds were prepared by a sol−gel method and used as a cathode material for a non-aqueous rechargeable Zn-ion battery. According to XRD, ICP-AES, and SEM−EDX analyses, the structural and compositional changes seen in a series of ZnAlxCo2−xO4 spinels on Zn charging and discharging processes during electrochemical cycling are consistent with reversible intercalation dynamics. The data further illustrate a conversion between Co3+ and Co4+ oxidation states in the cathode is responsible for the battery cycling. Among the ZnAlxCo2−xO4 series, the cathode for which x = 0.67 shows the highest capacity, a steady discharge capacity of 114 mAh/g over 100 cycles. The cell exhibits an open circuit voltage at 1.95 V, which is the highest OCV reported for a non-aqueous Zn-ion battery. The development of this composite spinel oxide system and its current efficiency as an active material for use in Zn batteries provide an encouraging milestone for progress in the development of useful multivalent metal batteries. Our future work will include improving the electrolyte stability and a separate effort to extend the voltage window through suitable doping.

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Additional SEM, XRD, and XPS experimental data and additional electrochemical measurements (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b03340. 9358

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