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Interplay between electrochemistry and phase evolution of the P2-type Na(Fe Mn )O cathode for use in sodium-ion batteries x

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Wei Kong Pang, Sujith Kalluri, Vanessa K. Peterson, Neeraj Sharma, Justin Kimpton, Bernt Johannessen, Hua Kun Liu, Shi Xue Dou, and Zaiping Guo Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 10 Apr 2015 Downloaded from http://pubs.acs.org on April 11, 2015

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Chemistry of Materials

Interplay between electrochemistry and phase evolution of the P2-type Nax(Fe1/2Mn1/2)O2 cathode for use in sodium-ion batteries Wei Kong Pang,†‡,⊥ Sujith Kalluri,†,⊥ Vanessa K. Peterson,*‡ Neeraj Sharma,§ Justin Kimpton,# Bernt Johannessen,# Hua Kun Liu,† Shi Xue Dou,† and Zaiping Guo*† †

School of Mechanical, Materials, and Mechatronic Engineering, Institute for Superconducting & Electronic Materials, Faculty of Engineering, University of Wollongong, NSW 2522, Australia. ‡ Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia. § School of Chemistry, University of New South Wales, Sydney NSW 2052, Australia. # Australian Synchrotron, Clayton, VIC 3168, Australia. KEYWORDS: sodium-ion battery; operando synchrotron X-ray powder diffraction; X-ray absorption; phase evolution; structure-function relation; cathode.

ABSTRACT: Sodium-ion batteries are the next-generation in battery technology; however, their commercial development is hampered by electrode performance. The P2-type Na2/3(Fe1/2Mn1/2)O2 with a hexagonal structure and P63/mmc space group is considered a candidate sodium-ion battery cathode material due to its high capacity (~ 190 mAh.g-1) and energy density (~ 520 mWh.g-1), which are comparable to the commercial LiFePO4 and LiMn2O4 lithium-ion battery cathodes, with previously-unexplained poor cycling performance being the major barrier to its commercial application. We use operando synchrotron X-ray powder diffraction to understand the origins of the capacity fade of the Na2/3(Fe1/2Mn1/2)O2 material during cycling over the relatively-wide 1.5 – 4.2 V (vs. Na) window. We found a complex phase-evolution, involving transitions from P63/mmc (P2-type at the open-circuit voltage) – P63 (OP4-type when fully-charged) – P63/mmc (P2-type at 3.4 – 2.0 V) – Cmcm (P2-type at 2.0 – 1.5 V) symmetry structures during the desodiation and sodiation of the Na2/3(Fe1/2Mn1/2)O2 cathode. The associated large cell-volume changes with the multiple two-phase reactions are likely to be responsible for the poor cycling performance, clearly suggesting to a 2.0 – 4.0 V window of operation as a strategy to improve cycling performance. We demonstrated here that the P2-type Na2/3(Fe1/2Mn1/2)O2 cathode is able to deliver ~25% better cycling performance with the strategic operation window. This significant improvement in cycling performance implies that by characterizing the phase evolution and reaction mechanisms during battery function we are able to propose these modifications to the conditions of battery use that improve performance, highlighting the importance of the interplay between structure and electrochemistry.

■ INTRODUCTION Since the commercialization of the first lithium-ion battery (LIB), featuring a LiCoO2 cathode and graphite anode, by Sony Corporation in 1991, LIBs are now extensively used for energy storage due to their relatively high energy-density, long cycle-life, and low cost.1 However, the search is underway for a replacement technology, mainly because of the limited abundance of lithium. Sodium is the fourth-highest abundant element in the Earth’s crust, making it cheaper to produce, and sodium-ion batteries (SIBs) are now attracting attention as promising alternatives for large-scale energy storage applications. Sodium is also the next lightest and smallest alkali-metal after lithium, therefore providing useful gravimetric and volumetric energy-storage density.2-5 Beyond these advantages that sodium has to offer battery technology, sodium also ex-

hibits a redox potential well-suited to battery applications (0.3 V vs. Li) with similar electrochemical function to lithium in LIBs. Taken together, these attributes make sodium a serious contender for future alternative battery chemistries.2-5 Recently, alternative cathode materials for SIBs based on sulphides, fluorides, phosphates, sulphates, and oxides (including layered transition-metal oxides) have been investigated.3,4,6 Ellis et al. indicated that the layered sodium-metal oxides, including O3-type and P2-types, using Delmas’s notation7, exhibit promising electrode properties.4 The O3-type layered oxides NaMO2 (including M = Co, Cr, V, Ni0.5Mn0.5, and Fe0.5Mn0.5, and Ni1/3Co1/3Mn1/3)8-14 are suitable cathode materials for SIBs, but suffer from poor cycle and rate performance, with an initial discharge capacity limited to ~ 160 mAh.g-1 or lower. Monoclinic NaMnO2 was also found to have an initial

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discharge capacity of ~ 160 mAh.g-1, where the two electrochemical-reactions involved lead to structural instability and poor cycling performance.15 Whilst P2-type layered oxides such as Na0.6MnO216 and Na0.7CoO217 have a better initial discharge capacity than the O3-type layered oxides, again, poor cycling performance remains a major issue. Environmental concerns are also driving research aimed at replacing toxic and costly metals such as Cr and Co with safe and abundant metals such as Mn, Fe, and Ni, possibly with these intermixed.18-20 Safer and affordable layered sodium metal-oxides containing mixed-valence Fe and Mn, such as the P2-type Nax(FeyMn1y)O2 material (x = 2/3, y = 1/2), have been obtained by solidstate, co-precipitation, auto-combustion, and sol-gel processes.20-24 Importantly, these environmentally-friendly materials deliver an excellent initial discharge capacity of ~ 190 mAh.g-1 at low current density in the 1.5-4.3 V range with an energy density of ~ 520 mWh.g-1. These electrodes are therefore comparable in performance with the LiFePO4 (~ 530 mWh.g-1) and LiMn2O4 (~ 460 mWh.g-1) cathodes used commercially in LIBs and open up a new avenue for the development of future cathode materials. Unfortunately, the poor cycling performance of the Nax(FeyMn1-y)O2 (x = 2/3, y = 1/2) materials presents a major hurdle to large-scale commercial application. The electrochemical behavior of materials is governed by their structural and chemical evolution. For example, the delithiation of the P2-LixVO2 (x = 0.80) cathode proceeds through multiple two-phase and solid-solution reactions to form P2LixVO2 (x = 0.50), leading to structural instability and poor cycling performance.10 P2-Na2/3(Fe1/2Mn1/2)O2 crystallizes in the hexagonal structure with a P63/mmc space group and is considered a candidate cathode material for SIBs, with the sodium insertion and extraction mechanisms studied using ex situ synchrotron X-ray powder diffraction (SXRPD) and X-ray absorption spectroscopy (XAS).20 It is reported that the Nax(Fe1/2Mn1/2)O2 cathode first undergoes a solid-solution reaction up to 3.8 V (from x = 0.66 to ~ 0.4) before a twophase reaction between P63/mmc (P2) and P6തm2 (OP4) is observed, alongside Fe3+/Fe4+ and Mn3+/Mn4+ transitions.20 Whilst the established mechanism is seemingly well-accepted, the ex situ study involved the removal and post-processing of composite electrodes from a coin-cell, and therefore did not directly reveal the entire mechanism of cathode function. Recent studies of the Nax(Fe1/2Mn1/2)O2 cathode using operando SXRPD reported that Nax(Fe1/2Mn1/2)O2 (x ~ 1) crystallized in the Cmcm space group (also P2-type) and first transformed to the P63/mmc symmetry (P2) through a two-phase reaction upon desodiation. Further desodiation caused the P2 structure to undergo both solid-solution (0.35 < x < 0.82) and two-phase reactions, transforming into a “Z phase” with a high degree of disorder. However, neither report has explained the structurefunction relationship that underpins the electrochemical properties of this promising SIB cathode. Since the energy density of a battery mainly depends predominately on the cathode material, the cathode is the component of focus in the consideration of new SIB systems. The properties and features of the Nax(Fe1/2Mn1/2)O2 cathode are sufficiently promising for SIB applications to warrant an indepth understanding of this material both structurally and electrochemically in the pursuit of overcoming its poor cycling performance, and thus is the subject of the present study. In particular, an understanding of the reaction pathways that control performance will enable the rational improvement of electrode materials. In the past, operando SXRPD using custom-

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ized coin-cells has successfully examined the structural evolution of electrode materials.25-28 In this study, we prepare the nano-sized P2-type Nax(Fe1/2Mn1/2)O2 (x = 2/3) material via a facile single-step sol-gel method. The nano-particulate cathode delivers excellent discharge capacity and energy density in a coin cell, but relatively poor cycling performance and energy efficiency. We study the structure-function relation of the Nax(Fe1/2Mn1/2)O2 cathode using operando SXRPD during sodium insertion and de-insertion and suggest a rational improvement in the use of the P2-type cathode. Our study reveals that the P2 hexagonal phase undergoes a solid-solution reaction during the Mn3+/Mn4+ transition and a solid-solution-like two-phase reaction at 4.1 V and above to form an OP4 phase with a P63 space group during the Fe3+/Fe4+ transition. The OP4 phase returns to the P2-type hexagonal phase upon discharge, and then transforms to a P2-type orthorhombic phase with a Cmcm space group upon discharge below 2.0 V. Importantly, we identify the significant volumetric and structural changes associated with the order-disorder (P2-OP4) and hexagonal-orthorhombic two-phase transitions, and suggest an alternative operating scheme for the battery that avoids this, enhancing cycle life.

■ EXPERIMENTAL The precursor solution was prepared by dissolving stoichiometric amounts of sodium acetate, iron nitrate, and manganese acetate (all from Sigma-Aldrich) in ethanol and N,NDimethylformamide (DMF). After stirring for an hour, 10 wt. % of polyvinylpyrrolidone (PVP, molecular weight ~ 1,300,000 g.mol-1), which acts as a gelling agent, was added to the resultant solution. The precursor solution was oven dried at 100 °C overnight before calcination at 900 °C for 2 h in air. The obtained P2-type Na2/3(Fe1/2Mn1/2)O2 powder was quenched to room temperature and stored in an Ar-filled glove box. The as-prepared P2-type Na2/3(Fe1/2Mn1/2)O2 (NFMO) was characterized using X-ray powder diffraction (XRPD) (GBC, MMA) equipped with Cu-Kα radiation and high-resolution neutron powder diffraction (NPD) using ECHIDNA29 at the Australian Nuclear Science and Technology Organisation. The wavelength of the neutron beam was 1.62380(3) Å, determined using the La11B6 NIST standard reference material (SRM) 660b. The NPD data were obtained in the 2θ angular range 6.5 to 165.2° with a step size of 0.125° over 6 h. Fullprof/WinPlotR30 were employed to perform Rietveld analysis of the high-resolution NPD and XRPD data. The parameters including the background coefficients, zero-shift, peak shape parameters, lattice parameters, oxygen positional parameter, sodium occupancy, and isotropic atomic displacement parameters (B) were refined. For the NPD measurement powders were packed into an air-tight 9mm vanadium can. The morphology and particle size of the as-prepared NFMO powder were examined using scanning electron microscopy (SEM, JEOL JSM-7500, Japan). A customized CR2032 coin cell for use in synchrotron Xray powder diffraction (SXRPD) experiments was designed and made.25-27 Electrodes were prepared by mixing the asprepared NFMO powder with carbon black (Super P, TIMCAL) and polyvinylidene fluoride (in a 80:10:10 weight ratio) in anhydrous N-methyl-2-pyrrolidinone (Sigma-Aldrich, 99.5%) to form a homogeneous slurry. The slurry was uniformly pasted onto aluminum foil before being dried in a vac-


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uum oven at 100 °C for 24 h and pressed prior to the assembly of the coin cell. The coin-cell was assembled using sodium disks (Sigma-Aldrich) as the counter electrode, porous glass fiber (Millipore) as a separator, and 1M NaClO4 in propylene carbonate as an electrolyte with 2 wt.% fluorinated ethyl carbonate electrolyte additive. Holes for permitting synchrotron X-ray beam transmission were punched in the top and bottom casing and then sealed with polyimide film (Kapton, DuPont) and wax. The cell was galvanostatically charged and dischargeed over 1.5 – 4.2 V vs. Na at constant current of 0.07 mA (equivalent to ~ 0.05 C) during data collection. SXRPD experiments were conducted on the powder diffraction beamline at the Australian Synchrotron where data were collected every 3.40 minutes at 0.68922(1) Å (determined using NIST SRM 660b) during battery cycle using a MYTHEN microstrip detector. The lattice response of P2-Na2/3(Fe1/2Mn1/2)O2 during cycle was extracted and examined using a single peak-fitting analysis with the Large-Array Manipulation Program (LAMP)31 to track the changes in peak position, intensity, and width of the 002 and 100 cathode reflections during charging and discharging. Fe and Mn K-edge X-ray absorption near-edge structure (XANES) spectra were collected using the same cells as per the SXRPD experiment on the X-ray absorption beamline at the Australian Synchrotron. The energy step size through the edge was 0.3 eV. The exposure time per step was fixed to 1s. For the Fe K-edge, FeO and Fe2O3 powders were used as Fe2+ and Fe3+ references, respectively. Fe K-edge spectra of [Fe(O)(N4Py)](ClO4)2 was sourced from Rohde et al.32 For the Mn K-edge, LiMnO2 and MnO2 were used as Mn3+ and Mn4+ references, respectively. The electrochemical properties of the P2-type Na2/3(Fe1/2Mn1/2)O2 cathode were evaluated in CR2032 coincells with a half-cell configuration as assembled in an Arfilled glove box (MBraun, Germany). Galvanostatic chargedischarge behavior was assessed using an automatic battery analyser (Land, China).

more reliable, and the refined NMFO structure is illustrated in Figure 2. In the P63/mmc hexagonal P2-type structure, Na ions occupy two prismatic sites (2b and 2d, with 24(1) and 46(2)% occupancies, respectively) that are sandwiched between layers of hybrid FeO6 and MnO6. An SEM image (Figure 3) shows the particle morphology and size and reveals the material is composed of nanoparticles without a well-defined morphology that aggregate, with particle size 200 – 700 nm.

■ RESULTS AND DISCUSSION

Figure 1. Rietveld refinement profile for the as-prepared NFMO powder using (a) XRPD and (b) NPD data. Vertical bars are reflection markers.

To understand the structure-function relation of the nanoparticulate P2-Nax(Fe1/2Mn1/2)O2 (x = 2/3) (NFMO) cathode, XRPD, high-resolution NPD, and XANES were employed to study the crystallographic and electronic structure, and its evolution. The XRPD and NPD patterns are shown in Figure 1a and 1b, respectively. The refined structures obtained using NPD and XRPD data are summarized in Table 1 and 2, respectively. Both XRPD and NPD data show that the NFMO is hexagonal and adopts the P63/mmc space group. Interestingly, the two give different lattice parameters (∆a = 0.0025(2) Å and ∆c = -0.012(1) Å, with NPD yielding a larger a and smaller c parameter) and dissimilar sodium occupancies (∆x = 0.14(2)), with the larger Na content determined from the NPD data approaching the expected value. We note that the elemental contrast in the NPD data is more advantageous for understanding the structure than the XRPD data, most notably the contrast available between the site-sharing Fe and Mn. Perhaps more importantly, the powders were isolated from air during the NPD experiment but not the XRPD measurement, where moisture is known to sometimes cause decomposition of NFMO and the partial removal of Na. The larger c and smaller a lattice parameter is consistent with a lower sodium content. For these reasons, the NPD result was considered

Figure 2. Refined crystal structure of the as-prepared NFMO powder with the unit cell shown in gray.



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Figure 3. SEM micrograph of the as-prepared NFMO powder showing the self-aggregation of nanoparticles.

Table 1. Crystallographic details of the as-prepared NFMO cathode obtained from Rietveld analysis using NPD data. Na0.70(2)(Fe1/2Mn1/2)O2 space group = P63/mmc a = b = 2.91883(1) Å and c = 11.2830(2) Å Atom

Site

x

y

z

B (Å2)

Occupancy

Na

2b

0

0

1/4

1.5(3)

0.24(1)

Na

2d

1/3

2/3

3/4

4.9(4)

0.46(2)

Mn

2a

0

0

0

0.8(1)*

1/2

Fe

2a

0

0

0

0.8(1)*

1/2

O

4f

1/3

2/3

0.0903(2)

0.54(2)

1

2

Rwp = 18.6, χ = 2.45, Bragg-R factor = 4.18 * B of Fe and Mn are constrained to be the same.

Table 2. Crystallographic details of NFMO obtained from Rietveld analysis using XRPD data.

carbonate-based electrolytes at high potential, as suggested in Yabuuchi et al.33 Figure 4a shows the charge-discharge behavior of a typical NMFO-containing coin cell for the initial, 2nd, and 80th cycles at 0.1 C. The 1st charge exhibits a flat plateau at around 4.07 V, whereas on the first discharge two plateaus are observed at 3.40 and 2.06 V (shown in the incremental capacity plot in Figure S1). In the 2nd cycle, the charge and discharge curves contain two plateau-like features at ~ 2.15 and ~ 4.05 V, and ~ 3.43 and ~ 2.07 V, respectively. The plateaus at 2.15/2.07 V correspond to the Mn3+/Mn4+ redox couple and those at 4.05/3.43 V to the Fe3+/Fe4+ couple, as reported in Yabucchi et al.20 It is notable that the ~ 2.2 V plateau presented in the 2nd cycle is absent in the 1st charge curve. The significant potential differences, especially at the 4.05/3.43 V plateaus, indicate poor energy and cycling performance. The NMFO cathode has capacity of 121.3 and 184.4 mAh.g-1 during the 1st charge and discharge, respectively. This equates to 0.467 and 0.709 Na per formula unit being inserted into and extracted from the structure, forming Na~0.203(Fe1/2Mn1/2)O2 and Na~0.912(Fe1/2Mn1/2)O2 at the charged and discharged states, respectively. It is this difference that is responsible for the abnormal energy efficiency (> 100%) of the cathode. The initial discharge-capacity is comparable to that previously reported,20 and the energy density of ~ 495 mWh.g-1 closely approaches that of the commercial LiFePO4 (~ 530 mWh.g-1) cathode and exceeds that of the LiMn2O4 (~ 450 mWh.g-1) cathode. In the second cycle the cathode delivers the reasonably-high discharge capacity of 178.3 mAh.g-1, although this occurs alongside coulombic and energy efficiencies of only ~ 89% and ~ 78%, respectively. Moreover, as shown in Figure 4b, the cathode only maintains ~ 55% of the initial capacity at the 80th cycle when cycled at 0.1 C. A similar decay phenomenon was previously observed.20 The self-aggregation of nanoparticles is commonly associated with reduced contact with the electrolyte. During the solgel synthesis nanoparticles without a well-defined morphology have a tendency to self-aggregate. Further, application of a conductive coating is expected to improve the electrochemical performance of the NFMO cathode.34,35 Nevertheless, the origin of the large capacity decay (~ 83.8 mAh.g-1 in our work) and relatively low energy efficiency (~ 78%) of the NFMO cathode remain poorly understood.

Na0.70(2)(Fe1/2Mn1/2)O2 space group = P63/mmc a = b = 2.9163(2) Å and c = 11.295(1) Å Atom

Site

x

y

z

B (Å2)

Occupancy

Na

2b

0

0

1/4

1.0(5)

0.26(1)

Na

2d

1/3

2/3

3/4

1.0(5)

0.30(1)

Mn

2a

0

0

0

1.0(5)*

1/2

Fe

2a

0

0

0

1.0(5)*

1/2

O

4f

1/3

2/3

0.06378(8)

0.1(2)

1

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2

Rwp = 36.4, χ = 6.73,Braggg- R factor = 17.5 * The B of Fe and Mn are constrained to be the same.

The electrochemical properties of the NFMO cathode were measured in coin-cells within the 1.5-4.2 V range to avoid unnecessary reactions associated with the degradation of the


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Chemistry of Materials Figure 5. Normalised XANES (a) Fe and (b) Mn K-edge spectrum of the P2-Nax(Fe1/2Mn1/2)O2 cathode within an uncycled coin-cell. Note that the Fe4+ data is taken from Rohde et al.32

We investigate the structure-function relationship that underpins the electrochemical properties of the cathode using operando SXRPD. Figure 6 shows selected regions of operando SXRPD data during charge and discharge within 1.5 and 4.2 V vs. Na, respectively, at 0.07 mA (equivalent to 0.05 C). The operando SXRPD data for the other regions are shown in Figure S3. We note that the synchrotron beam interacts with the sample in a region 2 × 1 mm in transmission geometry, and that the operando data are therefore representative of the phase evolution during charge (desodiation) and discharge (sodiation) for that part of the battery. Figure 4. (a) Charge-discharge curves of a typical NFMOcontaining coin cell at the initial and 80th cycle at 0.1 C and (b) the corresponding cycle stability. The potential range was 1.54.2 V. XANES (see Figure 5) estimates the oxidation states of Fe and Mn in the uncycled cell to be 3.4+ and 4.0+, respectively, using analysis method suggested in Berry et al.36 (Figure S2). Given that Mn4+ is electrochemically inert and the possibility of Fe3+ oxidation in SIBs20, the XANES results explain why the first charge curve of the NMFO cathode does not exhibit the 2.2 V plateau and delivers a lower charge-capacity, since only the Fe3+/Fe4+ is active and part of the Fe is 4+. Further, these results confirm that the 4.1/3.4V plateaus correspond to the Fe4+/Fe3+ redox couple and that it is therefore the Mn4+/Mn3+ redox centers responsible for the 2.2/2.0 V plateaus, in agreement with Park et al.24 We note that the oxidation states determined using XANES are for the material in the coin-cell before cycling where the material may have undergone desodiation, in contrast to the Rietveld refinementderived results which are for the as-prepared powder.

Figure 6. Contour plots of operando SXRPD data in the (a) 6.58.5 and (b) 13.5-16.8º 2θ regions.

The capacity of the custom coin-cell during the operando SXRPD experiment was lower than anticipated (Figure S4), indicating that within this sampled volume there are parts of the cathode that are inactive. The synchrotron beam passes through a hole in the outer battery casing, and it is wellestablished that this can result in poor contact between the electrode layers and cause areas of inactivity.37 Despite the lower amount of extracted and inserted Na calculated from the SXRPD data, the data show definitively the mechanism of transformations for the NFMO cathode during the chargedischarge process. Figures 7, 8, and 9 show the results from the single peakfitting routines applied to the 002 and 100 NFMO reflections. The evolution of the 002 and 100 reflections indicate changes to the c and a (= b) lattice parameters of the P2-hexagonal structure. The 002 reflection (Figure 6a and 7a) indicates an increase of the lattice parameter c during initial desodiation up



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to ~ 4.06 V, while the 100 reflection (Figure 6b and 7b) indicates a decrease of the a lattice parameter, with the latter likely a results of shrinkage of the MO6 (M = Fe, Mn) octahedra as a result of the increase in the average oxidation-state of the redox centers. The continuous c-axis expansion and a-axis contraction during desodiation up to ~ 4.06 V (the Fe3+/Fe4+ redox plateau) suggests that the Mn3+/Mn4+ redox couple proceeds through a solid-solution reaction. On further desodiation above 4.06 V the intensity of both the 002 and 100 reflections (Figure 6 and 8) change significantly alongside their shift in position. Similar changes are also observed for other reflections (Figure S3). Some reflections become nearly invisible before reappearing during the cycle. The continuous peak shift alongside changing intensity signifies concurrent solid-solution reaction and atomic rearrangement. We consider and briefly summarise previous work investigating the NFMO cathode phase evolution at high charge (low-sodium-content). Lu and Dahn studied a P2-Nax(Ni1/3Mn2/3)O2 cathode and suggested that an O2-type phase with stacking faults is formed at x ~ 1/3 during desodiation.38 Yabucchi et al. report the formation of an OP4-type phase with a P6തm2 space group after charging P2Nax(Fe1/2Mn1/2)O2 to 4.2 V as a result of gliding of the MO6 (M = Fe, Mn) slabs.20 Mortemard de Boisse et al. studied the P2-Nax(Fe1/2Mn1/2)O2 cathode with x < 0.35, suggesting a new phase with hybrid FeO6 and MnO6 octahedra-containing layers that exhibits a high degree of disorder.39 The order-disorder transition is thought to originate from instability of the prismatic sodium sites. We note that part of the cathode is unresponsive, and this inactive which is irreversible, arising from the order-disorder transition and suggesting that the upper cutoff voltage may be too high for the P2-Nax(Fe1/2Mn1/2)O2 cathode. Given that the amount of the non-responsive cathode increases with cycle, as evidenced by the increasing intensity with time in Figure 6 (and Figure S5), the unresponsive cathode may be responsible for the capacity decay of the cathode.

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Figure 7. Results of peak fitting of the NFMO (a) 002 and (b) 100 reflections illustrating changes in the 2θ-values (peak shifts). The voltage profile is also presented and the shaded regions represent two-phase transitions (purple and orange), the OP4 phase (green) and the P2-orthorhombic phase (blue).

Figure 8. Results from peak fits of the NFMO (a) 002 and (b) 100 reflections illustrating changes in the reflection intensity. The voltage profile is also presented and the shaded regions represent two-phase transitions (purple and orange), the OP4 phase (green) and the P2-orthorhombic phase (blue).


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Figure 9. Results of peak fits of the NFMO (a) 002 and (b) 100 reflections illustrating changes in the peak width. The voltage profile is also presented and the shaded regions represent twophase transitions (purple and orange), the OP4 phase (green) and the P2-orthorhombic phase (blue).

Our results reveal a dramatic change in the full-width at half-maximum (FWHM) of the NFMO 002 and 100 reflections (Figure 9) during charging at 4.1 V or above, suggesting increased structural disorder. Despite the disorder, we were able to index the reflections at the high-charge state (i.e. 4.2 V and lowest sodium content) to a hexagonal OP4 phase with a P63 space group, which is disagreement with the model suggested by Yabucchi et al. that was informed by ex situ SXRPD and XAS data.20 We find that the hybrid MO6 octahedracontaining layers are maintained, which is agreement with Mortemard de Boisse et al.39, during the P2-OP4 transition. Taken together, these results suggest that the P2-OP4 transition proceeds through a solid-solution-like two-phase reaction. The OP4 phase exhibits a distribution of Na ions at the 6c site, giving rise to disorder that is reduced during further desodiation. The OP4 phase is formed via gliding of the MO6 (M = Fe, Mn) slabs, enabled through a reduction in the repulsion between them as prismatic (6c) sodium sites are depopulated during the Fe3+/Fe4+ transition. This formation mechanism is similar to that found for the P2-Na~0.7CoO2―O3-LiCoO2 system, where the OP4 (LixNay)CoO2 crystallizes in P63/mmc,40 and where our OP4 phase has both sodium and vacancies (□) at the 6c sites and can be considered a mixed P2-NaxMO2―O3-□MO2 system. The 004 and 100 (of OP4) reflections shift dramatically to higher angle, indicating a large lattice shrinkage, with further sodium removal. This c-axis contraction likely arises due to the increasing average charge of the O-ions at the high-charge state, meaning the layers are less negatively charged and that the repulsion between them is reduced, along with the interlayer distance.41-43 Concurrently, the increasing oxidation state of the redox centers enhances the M-O bonding and causes further contraction of the MO6 octahedra, and consequently, the a-axis. Nevertheless, the time evolution of the 100 reflection position is nearly linear and its rate of change during the Mn3+/Mn4+ transition (within the P2-type structure) is smaller than that during the Fe3+/Fe4+ transition (within the OP4-type structure), further evidencing the different redox-active centers at the two voltage ranges. As one can expect from the Shannon radii of Mn (rMn3+ = 0.58 Å and rMn4+ = 0.53 Å) and Fe (rFe3+ = 0.645 Å and rFe4+ = 0.565 Å), the 2.2/2.0 and 4.1/3.4 V plateaus likely arise from the Mn3+/Mn4+ and Fe3+/Fe4+ couples, respectively.44 Upon discharge (sodiation), the reverse reac-

tions (OP4-P2) are observed. When the coin cell is discharged below 2.0 V, the P2-hexagonal phase undergoes a two-phase reaction, transforming into a P2-type orthorhombic phase with a Cmcm space group, the same structure reported by Mortemard de Boisse et al.39 Based on the operando SXRPD results, the phase evolution of the P2-Nax(Fe1/2Mn1/2)O2 cathode during charge and discharge obtained from operando SXRPD is shown in Scheme 1. The phase evolution of the cathode is accompanied by volumetric changes. Given the three different structures at 4.2, 2.7, and 1.5 V, we compare the volume per cathode formula unit of the material in Table 3, noting the ~ 12% volume change when the P2-Nax(Fe1/2Mn1/2)O2 cathode (P63/mmc) is charged (desodiated) to form the P63 structure. The relativelylarge volume change between the OP4/P2 structures is a likely contributor to structural stability of the cathode. Table 3. Comparison of volume per formula for NFMO cathode at various states. Phase

P63

P63/mmc

Cmcm

Structure type

Hexagonal

Hexagonal

Orthorhombic

~ 36.7

~ 41.6

~ 42.9

Volume per formula (Å3)

Despite the only ~ 3% volume difference between the P63/mmc and Cmcm phases, the transition between these two is structurally significant, with the symmetry changing from hexagonal to orthorhombic during desodiation. Notably, Jahnଷ ଵ Teller distortion of the Mn3+ and Fe4+ (both 3d4 (‫ݐ‬ଶ௚ ݁௚ )) destabilize the cathode at various states-of-charge, with the P63/mmc to Cmcm transition associated with considerable structural anisotropy. Notably, it is the non-cooperative JahnTeller distortion of Mn3+ during deep discharge that initiates this transition. It is likely that the poor cycling performance of the cathode originates from this complex phase/structure evolution. These results suggest that by narrowing the operating voltage of the battery from 1.5 – 4.2 to 2.0 – 4.0 V the cycle stability of the cathode may be significantly improved, and this is corroborated by reports of significantly improved cycle stability in the 1.5 – 4.0 V21 and 2.0 – 4.2 V windows. Xu et al. hypothesized that the better cycling performance obtained in the 1.5 – 4.0 V window may avoid structural distortion present in the 1.5 – 4.3 V window, and Park et al. considered that stacking-fault effects could be reduced by cycling within the 2.0 – 4.2 V window. We measured the cycling performance of the P2Nax(Fe1/2Mn1/2)O2 cathode in the 1.5 – 4.0, 2.0 – 4.0, and 2.0 – 4.2 V windows at 0.1 C, with the results shown in Figure 10. Unquestionably the P2-Nax(Fe1/2Mn1/2)O2 cathode cycling performance was best within the 2.0 – 4.0 V window, although the initial discharge capacity was only ~ 90 mAh.g-1. Within the 1.5 – 4.0 and 2.0 – 4.2 V windows, large capacity decays, i.e., 100.6 and 110.4 mAh.g-1, are observed after 80 cycles and these are attributed to the formation of the Cmcm and disordered P63 phases, respectively. Our results show that the cycling performance of the cathode can be enhanced by controlling the phase evolution, as informed by our SXRPD study. It is notably that, with 2.0 – 4.0 V window, the capacity retention rate is ~ 69%, showing ~ 25% better cycling performance when compared with the one with 1.5 – 4.0 V window. Whilst there is a significant reduction in capacity within this new



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voltage range (37%), this loss is offset by a relatively-greater gain in the cycle retention (59%). We note that this material is considered for use in not yet commercially-available sodiumion batteries, an emerging technology, where materials devel

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opment is in its infancy. Our results demonstrate that electrochemical properties of electrode materials can be optimized by a detailed knowledge of the phase and structural evolution gained using operando structural methods such as SXRPD.

Scheme 1. Phase evolution of the NFMO cathode during desodiation and sodiation.

Figure 10. (a) Discharge curves of typical NFMOcontaining coin cells at the initial and 80th cycle at 0.1 C when cycled in the 1.5 – 4.0, 2.0 – 4.0, and 2.0 – 4.2 V voltage windows, and (b) the corresponding cycle stability.

■ CONCLUSIONS The P2-Nax(Ni1/2Mn1/2)O2 nano-particulate cathode has been successfully prepared via a sol-gel method and its electrochemical properties are investigated. Although the cathode delivers a high initial capacity of ~ 180 mAh.g-1 and excellent energy-density of ~ 495 mWh.g-1, the energy efficiency and cycle stability are reasonably poor within the 1.5 – 4.2 V vs. Na operating range. To understand the origin of the capacity fade during cycling, operando synchrotron X-ray powder diffraction was used to determine the phase evolution of the cathode during charge and discharge within this voltage window. The work showed that the P2 hexagonal phase undergoes a solid-solution reaction during the Mn3+/Mn4+ redox transition and a solid-solution-like two-phase reaction at 4.1 V and above, accompanying desodiation during the Fe3+/Fe4+ transition. Importantly, by avoiding the large voltage difference between the anodic and cathodic plateaus, we show that with the alternative operating window, i.e. 2.0 – 4.0 V (vs. Na), as informed by our comprehensive understanding of the phase evolution, the cycling performance can be improved by ~25%. This work demonstrates the importance of understanding the structure-function relationship of electrode materials and the feasibility of operando synchrotron X-ray powder diffraction for studying this.


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■ AUTHOR INFORMATION Corresponding Author * Email: [email protected] (V.K.P.) Tel: +61 9717 9401 * Email: [email protected] (Z.P.G.) Tel: +61 4221 5225

Author Contributions All authors have given approval to the final version of the manuscript. ⊥ W.K.P.

and S.K. contributed equally.

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENT The authors are grateful to the financial support provided by the Commonwealth of Australia and Automotive CRC 2020. Dr Sharma would like to acknowledge AINSE support through the Research Fellowship Scheme. The use of infrastructure and facilities at ISEM and the UOW Electron Microscopy Centre is gratefully acknowledged. Part of this research was undertaken on the Powder Diffraction beamline and on the X-ray Absorption Spectroscopy beamline at the Australian Synchrotron, Victoria, Australia.

■ ASSOCIATED CONTENT Supporting information Incremental capacity plot (Figure S1); estimation of Fe and Mn oxidation states (Figure S2); contour plot of operando synchrotron diffraction data (Figure S3); charge-discharge curves of during the operando SXRPD experiment, including the capacity calculation (Figure S4); the results of peak fitting of the non-responsive NFMO 002 illustrating changes in the intensity with time. This material is available free of charge via the Internet at http://pubs.acs.org.

■ ABBREVIATIONS LIB, lithium-ion battery; SIB, sodium-ion battery; XRPD, X-ray powder diffraction; SXRPD, synchrotron X-ray powder diffraction; NPD, neutron powder diffraction; XAS, X-ray absorption spectroscopy; SEM, scanning electron microscopy; XANES, Xray absorption near-edge structure; standard reference material, SRM; NFMO, Nax(Ni1/2Mn1/2)O2.

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