Structural and Electrochemical Investigation of Na+ Insertion into High

Mar 4, 2015 - The emerging interest in Na+-ion batteries should increase the relative importance of higher voltage positive electrodes as a response t...
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Structural and Electrochemical Investigation of Na+ Insertion into High-Voltage Spinel Electrodes J. R. Kim* and G. G. Amatucci Energy Storage Research Group, Department of Materials Science and Engineering, Rutgers, the State University of New Jersey, Piscataway, New Jersey 08854, United States S Supporting Information *

ABSTRACT: The emerging interest in Na+-ion batteries should increase the relative importance of higher voltage positive electrodes as a response to the lower voltage imparted by the Na/Na+ redox reaction relative to that of Li/Li+. The 4.7 V LiMn1.5Ni0.5O4 spinel is already an attractive candidate for high-voltage lithium-ion positive electrodes, and its host structure, λ-Mn 0.75 Ni 0.25 O 2 , could be of even greater importance for Na batteries. The electrochemical and structural characteristics of higher-voltage NaxMn1.5Ni0.5O4 relative to NaxMn2O4 during ion insertion are investigated for the first time. High-resolution electrochemistry, coupled with in situ and ex situ X-ray diffraction, is utilized to provide initial insights into the mechanism of Na+ insertion. Distinct electrochemical challenges brought forth by Na+ ion insertion into the spinel structure are also discussed in detail.

1. INTRODUCTION Alkali-ion intercalation materials for battery applications have been widely studied for a number of decades because of their degree of reversibility, appreciable Coulombic capacity, and utilization of sufficiently high transition metal redox potentials as positive electrodes. Lithium-ion batteries (LIBs) have seen a drastic increase in research and development over the past few decades because of the large number of crystal structures that can accommodate lithium ions in a topotactic and multiphase solid-state reaction.1−3 Despite the attention LIBs have attracted over the years, interest in sodium-ion batteries (SIBs) is increasing because of the shear abundance and economic advantages of sodium precursors compared to their lithium counterparts, making them ideal for stationary energystorage applications where volumetric and gravimetric capacities are not the sole driving factors toward commercialscale adoption.4,5 Sodium insertion has been recently and successfully investigated in both layered oxides6−16 and λ-MnO2 based spinel oxides.17,18 Figure 1 provides a schematic of some of the various polymorphic structures that MnO6 octahedra have been shown to form and offers insight into the interrelated nature of the voltage and structural features of Na+ insertion/deinsertion. Because the majority of lithium chemistries are typically derived from the ion exchange of sodium19 precursors, there are a number of studies on the electrochemical properties of layered manganese oxides16,20 in sodium cells. These manganese bronzes provide a wide variety of MnO6 skeletons conducive to the insertion/deinsertion of Na+. Initial investigations of Na+ intercalation into λ-MnO2 (Figure 1d) have demonstrated that the (Na)[Mn2]O4 phase is not thermodynamically stable © XXXX American Chemical Society

Figure 1. [0,0,1] Crystallographic projection of MnO2 polymorphs (a) α-MnO2, (b) β-MnO2, (c) γ-MnO2, and (d) [1,1,0] projection of λMnO2, illustrating relative interstitial site size.

because extended electrochemical cycling results in a change in the voltage profile indicating structural transformations within the host material.18 Cyclic voltammetry (2 mV/s) of aqueous λ-MnO2 sodium-ion cells have revealed a quasireversible insertion/extraction mechanism with observed oxidation/reduction peaks around −0.3 and 0.9 V versus SCE Received: January 14, 2015 Revised: March 2, 2015

A

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Chemistry of Materials in Na2SO4 aqueous solution.17 Equations 1 and 2 illustrate the insertion mechanism described by Zhang et al.

and Pechini-derived samples, respectively. Spinel powders were dried at 300 °C and stored in a dry room at 24 °C and 4.7 V (presumed to be vs plated Li/Li+) indicate that complete delithiation has taken place. All delithiated cells consistently exhibited capacities of 145−150 mAh/g with open circuit voltages in the 4.4−4.7 V range, suggesting extensive Li plating on the Na-metal anode. Although the concentration of Li+ in the prepared sodium cells was extremely low after delithiation (∼1 mol %), fresh sodium anodes and electrolyte were provided in rebuilt Na/Na+ cells after the initial delithiation step to ensure the following intercalation studies are free of any mixed Li+/Na+ intercalation effects. Reports on sodium-ion battery electrolytes53−58 reflect a lack of development of suitable sodium electrolytes because the perchlorate salt is most commonly used. Direct analogues from existing lithium electrolytes are rarely successful because the solvation and passivation properties of Na metal differ from those of Li. Because of these limitations, use of 1 M NaClO4PC as the baseline electrolyte was chosen because of favorable solubility of the salt and extensive use throughout other SIB studies.12−16 Although maintaining good conductivity at room temperature, stability concerns associated with residual moisture of the perchlorate salt (45.5 ppm) and thermal stability are noted, and this electrolyte is utilized for research purposes only. The 1 M NaClO4-PC electrolytes demonstrated a high measured conductivity of 6.42 mS/cm compared to 0.5 mS/cm of the low solubility 0.1 M NaBF4-EC/DMC electrolyte. However, because PC does not form a stable SEI layer with the sodium-metal anode, additives were considered

centered and laminated onto 5/8 inch carbon-coated aluminum-mesh disks that are designed to fit into the small recess of the in situ cell body, providing sufficient electrical contact and held in place by the applied pressure of the XRD window cutout. Extraction of the PC plasticizer was performed after lamination onto aluminum-mesh disks at 120 °C with 40 psi of applied pressure. The cell parts and cathodes were dried under vacuum at 120 °C prior to assembly to remove residual moisture. Cell assembly was performed as described in earlier work.47 Prior to in situ and operando XRD measurements, the 3-axis stage was aligned with the incident X-ray beam to optimize the signalto-noise ratio of the (1,1,1) spinel reflection.

3. RESULTS 3.1. X-ray Diffraction and Electrochemical Characterization. XRD patterns of standard spinel samples LiMn2O4 (LMO, ICSD no. 94339), Li1.1Mn1.5Ni0.5O3.8F0.2 (LMNOF), and Pechini-synthesized LiMn1.56Ni0.44O4 (LMNO-P, ICSD no. 186214) are shown in Figure 2. All observed peaks can be

Figure 2. X-ray diffraction patterns of (a) LMNO-P, (b) LMNOF, and (c) LMO as prepared powders with Rietveld-refined lattice parameters and crystal volumes are also presented.

Figure 3. (a) Galvanostatic discharge and (b) ex situ XRD measurements of λ-Mn0.75Ni0.25O1.9F0.1 (MNOF) and Pechini-prepared λ-Mn0.78Ni0.22O2 (MNO-P) C

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Table 1. Refined Structural Data of ex Situ XRD Measurements. Phase Compositions and Relative Weight Percentages Are Given in Bold

discharged λ-MNOF and C/100-discharged λ-MNO-P samples, the voltage of the cell was relatively invariant at approximately 3.6 V during the Na+ insertion. Figure 3b provides the ex situ XRD scans of the λ -MNOF and λ -MNO-P samples after discharging at C/100 from Figure 3a. A significant shift in all the Bragg reflections to larger d spacings are noted along with the appearance of the (2,2,0) and (4,2,2) reflections consistent with Na+ insertion on the 8a tetrahedral sites of the spinel structure. These patterns are quantified by Rietveld refinement, given in Table 1 and further discussed in detail below. 3.2. C/10-, C/25-, C/100-, and C/200-Discharged λMNOF versus Na/Na+ and C/10- and C/100-Discharged λ-MNO-P versus Na/Na+. λ-MNOF was discharged versus Na/Na+ at C/10, C/25, and C/100 rates. Afterward, the resulting phases were analyzed by ex situ XRD. Of the three

to prevent unwanted side reactions. Fluorinated ethylene carbonate (FEC) has proven to successfully passivate Na metal cells58 and a 1 M NaClO4-PC:FEC electrolyte is utilized consisting of 2 vol % FEC. Further electrolyte studies will be discussed in a future communication. Figure 3a provides the discharge voltage profiles at C/10 and C/100 rates for both λ-MNOF and λ-MNO-P spinels in rebuilt Na/Na+ cells. The solid-state-derived λ-MNOF cell exhibited a discharge specific capacity of 89.2 mAh/g at C/10 and an increased capacity of 118.8 mAh/g occurring at a C/100 discharge rate. This suggests a kinetic limitation to the insertion of Na+. Supporting this conclusion, the use of the nanocrystallite-sized Pechini-derived spinel λ-MNO-P resulted in the C/10 capacity of 120.1 mAh/g, which is a significant improvement in the rate capability of λ-MNO-P expected with a reduction in crystallite size. Between the C/100D

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Figure 4. Galvanostatic intermittent titration technique data of Pechini-derived sodium spinel cells (λ -MNO-P) with 1 M NaClO4-PC:2% FEC electrolyte. (a) First discharge and (b) subsequent charge/discharge voltage profiles.

are fitted well by a Fd3̅m spinel of moderate sodium content, as indicated by Table 1. Equation 3 illustrates the proposed insertion reaction. Movement to a Pechini synthesis route greatly improved the rate capability as evidenced by the high discharge capacities and phase purity of the ex situ XRD of the discharged product noted above. BET surface area analysis (5.8 vs 1.2 m2/g for the LMNO-P and LMNOF samples, respectively) and previous characterization of this material by FESEM and Williamson− Hall technique43 show that the Pechini synthesis route has reduced the spinel crystallite size from 200−700 nm34 to 50− 70 nm,44 while creating a mesoporous network that is more suitable for reversible ion intercalation.

discharge rates, only the C/10 discharge rate shows the existence of three phases: residual λ-MNOF spinel (a = 8.009(2) Å) because of incomplete sodiation, the sodiated spinel of expanded unit cell (a = 8.414(2) Å), and an additional phase that may also be indexed with an Fd3̅m symmetry of intermediate lattice parameter (a = 8.186(1) Å) between the vacant λ-MNOF and fully sodiated NaMn0.75Ni0.25O1.9F0.1 (NaMNOF) structures. Because of the potential of lattice instability of Na spinels, additional known layered-sodium phases were also modeled throughout the Rietveld refinements of discharged λ-MNOF versus Na/Na+ cells. Their poor fitting to the experimental data supports the retention of the spinel framework throughout Na+ insertion. A summary of the proposed structural models is given in S1 of the Supporting Information. After discharge, the C/25- and C/100-discharged λ-MNOF versus Na/Na+ cells only exhibit the existence of both the sodiated Na-MNOF spinel (a = 8.426(2) and 8.446(2) Å, respectively) and the intermediate spinel of approximate composition, the Na(0.1−0.4)Mn0.75Ni0.25O3.8F0.2 spinel (a = 8.190(3) and 8.196(7) Å, respectively). The C/200-discharged λ-MNOF versus Na/Na+ cell only resulted in the presence of the fully sodiated Na-MNOF phase, suggesting that complete insertion of Na+ onto the vacant 8a tetrahedral sites has taken place, consistent with the near theoretical specific capacities. The nanocrystalline λ-MNO-P versus Na/Na+ cells discharged at C/10 and C/100 cells reveal lattice parameters that are in close agreement with one another (a = 8.439(0) and 8.461(4) Å, respectively) with no residual or intermediate phase formation, confirming that the λ-MNO-P samples exhibit much better kinetics of Na+ insertion than the λ-MNOF samples. Refined parameters of the λ-MNOF are provided at the top of Table 1 in order to draw comparisons between the initial vacant λ-MNOF structure and that of the fully sodiated Na-MNOF. The atomic parameters for LiMn1.56Ni0.44O4 (Inorganic Crystal Structure Database, ICSD, no. 186214) are provided for comparison with those of sodiated Na-MNO-P. The fully sodiated cell results in a lattice parameter that is approximately 5.7% greater than that of the host λ-MNOF, and voltage profiles for each of the discharged λ-MNOF versus Na/ Na+ and λ-MNO-P versus Na/Na+ positive electrodes suggest that the Ni4+ → Ni2+ reaction is taking place in the 3.6 V region. The additional phases of the λ-MNOF discharged cells

( )8a [Mn1.56Ni 0.44]16d [O4 ]32e → (Na)8a [Mn1.56Ni 0.44]16d [O4 ]32e

(3)

λ-MNO-P versus Na/Na+ cells were discharged to 1.5 V (not shown) at both C/10 and C/100 rates, both of which retain the spinel Fd3m ̅ structure of the host structure. Ex situ XRD results indicate that none of the initial λ-MNO phase remains after discharging to a 1.5 V cutoff and that no structural transformations attributed to the insertion onto the 16c octahedral sites occur59 as described by eq 2. 3.3. High-Resolution Electrochemistry. GITT was utilized as a low-rate, near-equilibrium electrochemical protocol to examine whether near-theoretical specific capacities could be achieved as well as to gain insight on the voltage variation and phase progression associated with the Na+ insertion mechanism. The results are shown in Figure 4. During the initial Na+ insertion reaction (Figure 4a), the open circuit potentials are relatively invariant at 3.65 V across the majority of the discharge until ca. x = 0.8, indicating that the two-phase reaction has completed within the confines of our applied electrochemical protocol. Further discharge then appears to proceed through a single-phase reaction between 3.5 and 2 V. A final discharge capacity of 134.2 mAh/g is observed (99.53% theoretical capacity) throughout the combined two-phase and single-phase insertion reactions, demonstrating that complete Na+ insertion into the λ-MNO-P structure is possible given a sufficiently slow rate. After the initial insertion of Na+, there is significant difficulty in extracting all the sodium from the sodiated structure at a low E

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Chemistry of Materials 4.0 V cutoff (Figure 4b). Initial GITT results suggest a variation in the relaxed potential with Na+ deinsertion characteristic of a single-phase process. Figure 4b provides the following charge and discharge profiles demonstrating a reversible Na+ window between x = 0.55 and 1.0, utilizing an upper charge limitation of 4.0 V. GITT suggests that both the limited Na+ deinsertion and subsequent insertion may proceed in a single-phase reaction. A detailed investigation into the desodiation to higher voltages and optimization of λ-MNO-P structure will be published. To extract additional electrochemical evidence for the presence of single versus multiphase insertion, PITT data for a λ-MNO-P versus Na/Na+ cell is given in Figure 5. The two-

Figure 6. Working- and counter-electrode voltages vs a Na/Na+ reference electrode. λ-MNO-P cathode cycled using 1 M NaClO4PC:2%FEC electrolyte.

working potential of λ-MNO-P versus Na/Na+ counter electrode (solid curve) and λ-MNO-P versus Na/Na+ reference electrode (dashed curve) are nearly identical, indicating the negligible polarization of the negative electrode and the high level of confidence in the existing two-electrode data. The red curve in Figure 6 gives the potential of the λ-MNO-P electrode relative to that of the Na reference electrode. 3.4. Na+ Insertion: ex Situ X-ray Diffraction. Additional λ-MNO-P electrodes were galvanostatically discharged at a C/ 200 rate with Δx cutoffs of 0.4 and 0.6 in order to obtain ex situ XRD measurements suitable for lattice parameter and Rietveld analysis (not shown) at different points within the two-phase reaction regime. Refined Na+ occupancies for the sodiated phase (NaxMn1.56Ni0.44O4) of 0.894 and 0.890 along with similar lattice parameters (a = 8.391(7) and 8.412(1) Å, respectively) for the x = 0.4 and 0.6 samples support the proposed two-phase insertion reaction process described in eq 4. This conclusion is also supported by the high-resolution electrochemical techniques described above, suggesting that once the λ-MNO-P host lattice reaches complete sodium insertion to the Na0.87-Mn1.56Ni0.22O4 phase (eq 4), the insertion reaction proceeds in a single-phase manner as described by eq 5.

Figure 5. Voltage and specific current response of the sodium insertion of λ -MNO-P spinel utilizing a potentiometric intermittent titration technique protocol. Electrolyte was 1 M NaClO4-PC:2% FEC. Arrows indicate y axes for current and voltage profiles.

phase Na+ insertion reaction occurs across a few 10 mV steps in the 3.68−3.58 V range, with the majority of the capacity occurring within a very narrow voltage range: 3.65, 3.64, 3.63, 3.62, 3.61, and 3.60 V. The invariance supports a two-phase insertion behavior consistent with the GITT results presented above. Superimposed in Figure 5 is the specific current response across the applied potentials. Within the 3.65−3.60 V range of interest, each step is associated with a strong nonCottrellian behavior characteristic of multiphase reactions. Each step is associated with an increasing current that reaches a maximum, followed by a decay until the system drops below 3.58 V at which the current response then follows a Cottrellianlike t(−1/2) dependence. This transition to a single-phase reaction occurs at ca. x = 0.87, a result consistent with the GITT results. This shift in the intercalation mechanism to a single-phase reaction is observed just as the voltage drops below 3.6 V at the knee in the recorded voltage profile. Combined with existing XRD data, the overall intercalation reaction of Na+ into the λMNO-P structure is expected to occur via a two-phase intercalation reaction onto the vacant 8a tetrahedral sites up to ca. x = 0.87 followed by further Na+ insertion onto 8a sites via a single-phase reaction until full sodiation has taken place. Because there is always concern regarding the accuracy of high-resolution electrochemical techniques in a two-electrode configuration, a 3-electrode Swagelok cell46 was used to determine the contribution to the overall cell potential from the sodium-metal anode. Results are shown Figure 6. The

( )8a [Mn1.56Ni 0.44]16d [O4 ]32e → (Na 0.87)8a [Mn1.56Ni 0.44]16d [O4 ]32e at 3.65−3.60V (4)

(Na 0.87)8a [Mn1.56Ni 0.44]16d [O4 ]32e → (Na 0.87 + x)8a [Mn1.56Ni 0.44]16d [O4 ]32e

(5)

3.5. In Situ Electrochemical Cell and Operando X-ray Diffraction. To validate the reaction mechanism, operando XRD measurements were made of the λ-MNO-P versus Na/ Na+ cells at varying discharge rates of C/60 and C/40 (Figures 7 and 8, respectively). Throughout Figures 8 and 9, the (1,1,1), (3,1,1), and (4,0,0) Bragg reflections are provided. Operando XRD scans were recorded on a continuous basis with a maximum Δx of 0.031 between sequential XRD scans, which is assumed to contribute a negligible effect on the peak broadening. Displayed XRD scans are provided at intervals of x = 0.0625 in Figures 7 and 8. Vertical lines are provided at the F

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Figure 7. C/60 Operando X-ray diffraction (XRD) results of Na+ insertion into λ -MNO-P spinel. (a, b, and c) Looking at the (1,1,1), (3,1,1), and (4,0,0) Bragg reflections, respectively. (d) Corresponding discharge profile. Arrow corresponds to approximate point of transition between twophase and single-phase reaction mechanism also highlighted in the XRD scans shown in red.

Peak-shift contributions from misalignment of the in situ electrochemical cell is quickly ruled out because the aluminummesh current collector acts as an internal standard and indicates no such shift in peak position, thereby indicating that a partially sodiated spinel phase is nucleated and then proceeds by a single-phase reaction to reach a final composition of Na0.98Mn1.56Ni0.44O4 and Na0.91Mn1.56Ni0.44O4 for the C/40and C/60-discharged sodium cells, respectively. A precise refinement of the Na+ content of the nucleated sodium phase was not possible because of the data quality of the operando XRD measurements, making them unsuitable for meaningful Rietveld refinement. Reported sodium occupancies are therefore based on electrochemical data. Comparison of the C/40 and C/60 operando cells provides further insight into the possible rate dependence on the observed onset of the single-phase reaction. The C/60 and C/ 40 results show the same evidence of a mixed two/single-phase sodium intercalation reaction; however, the ratios of the capacity contribution from the respective two-phase and singlephase regions do not directly correlate, suggesting that some dependence on the discharge rate is possibly linked to the Mn4+→Mn3+ reduction. C/40 and C/60 operando cells saw a termination of the two-phase reaction at x values of 0.78, and 0.70, respectively, which differs from the PITT measurements mentioned previously (x = 0.87). This is consistent with the two-phase reaction being rate-limited in the constant current operando XRD case and the PITT offering near equilibrium conditions. Operando C/60 and C/40 λ-MNO-P versus Na/

initial 2θ positions of the nucleated sodium spinel phase, and are used as guides for the eyes in observing relative shifting in the peak positions. The arrow provided in the respective voltage profile correlates to the XRD scans highlighted in red, indicating the approximate beginning of the single-phase insertion reaction. Operando XRD measurements of Na+ insertion into λMNO-P at C/60 and C/40 discharge rates reveal a common progression of phase evolution: Initial insertion of Na+ results in the formation of a larger lattice parameter phase of high sodium content (Na0.87Mn1.56Ni0.44O4) as PITT results described above, followed by a single-phase insertion mechanism evidenced by the clear shift in diffraction peaks to higher d spacings (Figures 7a−c and 8a−c) and sloping voltage profile. The C/60-discharged λ-MNO-P versus Na/Na+ cell of Figure 7 reached a final discharge capacity of 124.04 mAh/g. In situ XRD scans taken during open circuit periods following the C/60 discharge did not exhibit any relaxation of the observed sodium spinel peaks, indicating that the reaction comes to a halt after electrochemical cycling has stopped. The vertical guidelines demonstrate the magnitude of the peak shift in the sodiated spinel Bragg reflections; as anticipated by the earlier ex situ XRD results, it appears that the onset of the Mn4+→Mn3+ reduction facilitates the dominating single-phase reaction, as can be seen by the onset of peak shift marked at the voltageprofile knee of Figures 7 and 8. G

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Figure 8. C/40 Operando X-ray diffraction (XRD) results of Na insertion into λ -MNO-P spinel. (a, b, and c) Looking at the (1,1,1), (3,1,1), and (4,0,0) Bragg reflections, respectively. (d) Corresponding discharge/charge profile. Arrow corresponds to approximate point of transition between two-phase and single-phase reaction mechanism also highlighted in the XRD scans shown in red.

Figure 9. (a and b) Measured d spacings of (1,1,1) and (3,1,1) Bragg reflections, respectively, during sodium insertion into λ -MNO-P spinels at C/ 60 discharge rate. Data taken from operando cells of Figure 8.

Na+ cells provide valuable information into the structural evolution throughout electrochemical cycling, linking the observed shift in reaction mechanism to the voltage profile. Operando data supports the proposed insertion reactions of eqs 4 and 5, indicating that the shift in reaction mechanism occurs at the knee in the voltage profile and suggesting a single-phase behavior with extended cycling.

Figure 9 compiles the measured d spacings from the C/60 in situ cell for the (1,1,1) and (3,1,1) Bragg reflections of both the vacant λ-MNO-P and sodiated Na-MNO-P spinel phases. Considerations were restricted to the (1,1,1) and (3,1,1) reflections because they provided the greatest signal-to-noise ratio, ensuring minimal convolution from additional phases or background scattering. Only the relative change between data points is of particular importance because sample height H

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Chemistry of Materials correction during operando conditions is difficult to account for given that the starting in situ XRD signals were optimized to provide the greatest signal-to-noise ratio rather than precise peak position. Because of the alignment of the in situ XRD cell, errors in the measured d spacings were calculated by calculating the sample height displacement prior to discharge. Errors in the measured d spacings are calculated to be less than 2.9%, as given by eq 6 where D is the sample displacement normal to the diffracting plane and R is the goniometer radius. The C/40 operando XRD sample was excluded from this comparison as peak intensities are insufficient to make a meaningful comparison; thus, this is provided for comparative purposes only.

Δd D cos2 θ =− d R sin θ

(6) Figure 10. Combined discharge profiles of λ-Mn0.78Ni0.22O2 vs Li/Li+ and Figure 5 demonstrating the Mn4+ → Mn3+ reduction for both Li+ and Na+ insertion.

Peak shifts of the λ-MNO-P spinel are displayed as square markers, whereas the sodiated Na-MNO-P spinel are shown by triangle markers (Figure 9). The measured d spacings of the λMNO-P spinel remain appreciably invariant throughout the considered discharge, consistent with the proposed two-phase reaction. Once the discharge voltage drops below 3.58 V, the single-phase insertion reaction begins to take over, and a clear shift in the d spacings can be seen for both the (1,1,1) and (3,1,1) reflections.

with phase pure products in contrast to the multiphase products of the λ-MNO versus Na/Na+ cells. Initial data (GITT, PITT, and operando XRD) regarding the extraction of sodium from the fully sodiated Na-MNO-P spinel shows that the extraction of Na+ occurs by a single-phase reaction (Figures 4b and 9). Further discussion on the full extraction of sodium and long-term cycling behavior are left for a future publication. 4.2. Insertion Potentials Associated with the Spinel Host Structure. As previously reported,28,29 Li+ insertion into λ-MNO occurs between 4.7 and 4.75 V. The observed voltage for insertion of Na+ into λ-MNOF and λ-MNO-P shows a significantly different behavior than the anticipated 0.3 V difference from considerations of the difference in standard reduction potentials of Li and Na. Overpotential contributions from the negative electrode were found to be negligible, indicating that the observed voltage differences between the Li and Na spinels are due solely to the λ-MNO host structure. To gain insight into the nature of the significant voltage differences between the sodium and lithium spinels, we have compared the potential of the insertion into λ-MNO-P and λMnO2 for both Li+ and Na+ (Figure 11). The overall voltage difference between the lithium and sodium insertions is approximately 1.1 V for λ-MNO-P (i.e., between Figure 11d,b, respectively) and 1.0 V for λ-MnO2 (i.e., between Figure 11c,a, respectively). This is approximately 0.7 V larger than the 0.3 V difference one would expect to be attributed to the potential difference between the Li/Li+ and Na/Na+ alone, suggesting a significant potential penalty attributed to the Na+ insertion into the spinel structure. This penalty is similar to that between both the nickel-substituted and unsubstituted spinels, and three-electrode studies have shown (Figure 6) that the overpotential of the Na/Na+ reaction is not a contributor to the overall cell potential, thereby attributing the disparity in potential to the ability of the host structure to accommodate the Na+. Because the nickel substitution within λ-MNOF and λMNO-P acts to increase consistently the observed potential by approximately 0.6 V relative to λ-MnO2 regardless of whether Li+ or Na+ is being inserted into the λ-MNO structure, we can say that this voltage penalty is characteristic of the Na+ insertion into the spinel crystal structure. This insertion occurs without structural decomposition or rearrangement, as evidenced by the comparative voltage profiles and XRD data shown herein. The

4. DISCUSSION 4.1. Reaction Mechanism. The observed two-phase Na+insertion mechanism into λ-MNO at a low rate and λ -MNO-P at all studied insertion rates should not come as a surprise because of the significant mismatch of lattice parameters and subsequent strain energy associated with the insertion of large Na+ into the 8a spinel sites. λ-MNOF revealed the initial presence of an intermediate composition spinel that was not observed in the nanostructured λ-MNO-P. This contrast is presently being investigated via operando XRD studies currently underway. The nanostructured λ-MNO-P of faster kinetics was studied in detail. High-resolution electrochemistry combined with structural studies demonstrates a two-phase reaction that occurs for the Ni4+ → Ni2+ reaction (eq 4) within an approximate voltage window of 3.68−3.58 V, followed by a single-phase reaction beginning with the onset of the Mn4+ → Mn3+ reduction (eq 5) below 3.58 V (attributed to the designed nonstoichiometry of the original spinel). The progression of the insertion reaction through a singlephase reaction occurs with minimal energy penalty because the incorporation of Na+ during the prior two-phase reaction sufficiently enlarges the spinel framework to enable better diffusion kinetics. All the preliminary evidence, although not conclusive, points to a direct correlation of the single phase reaction with the reduction of Mn4+ → Mn3+ present because of the initial intended non stoichiometry of the spinel, highlighted between the dotted guidelines of Figure 10 for both the Li+ and Na+ insertion into λ-Mn0.78Ni0.44O2 spinels. Insertion of Na+ into the λ-MNOF host lattice shows some significant kinetic barriers, evidenced by the need to employ progressively slower galvanostatic discharge rates (Figure 3a) to achieve near-theoretical capacities. The use of the nanostructured λ-MNO-P spinel derived from the Pechini method enhanced the insertion kinetics significantly because of the order of magnitude decrease in particle size. This change enabled near theoretical Na+ insertion at C/10 discharge rates I

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank N. Pereira for useful discussions.

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Figure 11. Discharge profiles of (a and b) λ-Mn0.78Ni0.22O2 and λMnO2 vs Na/Na+ (black vs. red, respectively) and (c and d) λMn0.78Ni0.22O2 and λ-MnO2 in Li/Li+ (black vs. red, respectively), both discharged at C/100 at 21 °C.

data indicate that insertion of Na+ has been occurring solely on the vacant 8a tetrahedral sites throughout this study and an overall 0.56 V difference is observed between the λ-MNO-P and λ-MnO2 sodium cells, with an overall voltage difference of 1.10 and 1.0 V demonstrated between the lithium and sodium cells for the λ-MNO and λ-MnO2 host structures, respectively.

5. CONCLUSIONS Delithiated LiMn1.56Ni0.44O4 spinels were investigated as a possible Na+ intercalation compound. Ex situ, in situ, and operando XRD, combined with GITT and PITT electrochemical analysis, reveal that Na+ intercalation occurs exclusively at 8a tetrahedral sites via a two-phase reaction at ca. 3.65 V, with the vast majority of the insertion process attributed to the Ni4+ →Ni2+ reduction leading to a nominal composition of Na0.87Mn1.56Ni0.44O4. This is followed by a small single-phase insertion on residual 8a sites, possibly associated with Mn4+ →Mn3+ until the full sodium occupancy has been reached. Rietveld analyses confirm the retention of Fd3m ̅ spinel symmetry throughout the entire discharge reaction. The twophase insertion reaction (λ-MNO → Na0.84Mn1.56Ni0.44O4) observes a 1.1 V reduction in voltage compared to the Li+ insertion reaction in the [Mn1.56Ni0.44]O4 host spinel and a 1.0 V reduction in voltage for the Na+ insertion into [Mn2]O4 compared to Li+ insertion, illustrating an increase in the reaction voltage that is nearly identical to that of nickel substitution. Significant kinetic barriers were observed for the macrosized spinel, prompting the utilization of nanostructured [Mn1.56Ni0.44]O4 spinels to consistently reach near-theoretical capacities at 3.66 V. Preliminary electrochemical and structural evidence suggest that subsequent insertion and deinsertion occurs, at least in part, through a single-phase reaction.

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ASSOCIATED CONTENT

S Supporting Information *

Summarized results of modeling the discharged λ-MNOF versus Na/Na+ positive electrodes against other known sodium compounds. This material is available free of charge via the Internet at http://pubs.acs.org. J

DOI: 10.1021/acs.chemmater.5b00283 Chem. Mater. XXXX, XXX, XXX−XXX

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