Exploration of Phase Compositions, Crystal Structures, and

Subscriber access provided by University of Sunderland

Article

Exploration of phase compositions, crystal structures and electrochemical properties of NaxFeyMn1-yO2 sodium ion battery materials Steinar Birgisson, Troels Lindahl Christiansen, and Bo B. Iversen Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01566 • Publication Date (Web): 07 Sep 2018 Downloaded from http://pubs.acs.org on September 8, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Exploration of phase compositions, crystal structures and electrochemical properties of NaxFeyMn1-yO2 sodium ion battery materials

Steinar Birgisson,a Troels Lindahl Christiansena,b and Bo B. Iversena*

a

Center for Materials Crystallography, Department of Chemistry and iNANO, Aarhus University, Denmark

b

Department of Chemistry, University of Copenhagen, Denmark

*Corresponding author: [email protected]

Abstract Layered manganese oxide materials are widely used in sodium and lithium ion batteries, but significant discrepancies are encountered in the literature with respect to their electrochemical properties. This could be due to difficulties in establishing the exact phase compositions and crystal structures (typically P2, P3, O2 and O3, their distorted analogues, e.g. P’2, hydrated - PH2, or carbonated - PC2, phases) of a given synthesis product, especially when only crude crystallographic indexing is used without structural analysis. Here we report a benchmark high-resolution synchrotron powder diffraction investigation of a broad composition range of the layered NaxFeyMn1-yO2 cathode materials (x = 0.5, 0.7 and 1.0 and y = 0.3, 0.5 and 0.7) with respect to phase composition, crystal structure and electrochemical properties. Based on multiphase Rietveld refinements it is shown that crystal structure can be controlled to a certain degree for different x and y. Most synthesis products contain a complex phase mixture, but in a few cases almost phase pure P2 and O3 type materials can be produced. The P2 phase is observed to be air sensitive, whereas the O3 and P3 structures are not. Clear trends linking electrochemical performance to x and y are observed, where higher x and y result in worse performance. On the other hand, no clear trend is observed

ACS Paragon Plus Environment

Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

linking the type of layered crystal structure to electrochemical performance. Overall, the electrochemical performance of the NaxFeyMn1-yO2 samples seems to be mostly dependent on the initial oxidation state and the transition metal ratio.

Introduction Sodium ion batteries (SIB) represent a cheap and more sustainable alternative to lithium ion batteries (LIB) on account of the much greater abundance of sodium compared with lithium in the Earth’s crust.1-8 This makes SIBs an excellent candidate for large-scale applications such as grid energy storage. SIBs are less ideal for space or weight restricted applications, such as mobile electronics and electric vehicles, due to a lower gravimetric and volumetric energy density compared with LIB.9 One of the largest bottlenecks in realizing SIBs for practical application is development of suitable electrode materials.3, 10-12 An electrode material for SIBs should preferably be inexpensive and environmentally benign, but additionally exhibit suitable battery performance such as long cycle life and good rate capabilities. Layered sodium transition metal oxides, NaxMO2 (M = Mn, Fe, Co, Ni, etc.) constitute a promising class of cathode materials for SIBs, which are relatively easy to synthesize with good overall electrochemical performance.4-7 The composition can vary from a single transition metal to a mixture of two or more transition metals.1,

4-8

The layered NaxMO2 materials crystallize in a handful of different

crystalline phases. All of them feature sheets of edge sharing MO6 octahedra that stack in parallel, forming a layered structure where the sodium ions reside in between the sheets.13 The different layered phases differ in the stacking order of the close packed oxide ions and the coordination environment of the sodium ions. Here the nomenclature adopted by Delmas et al. for naming the layered crystal phases is adopted.14 The phases are named with a combination of a letter and an integer where the letter denotes the sodium ion coordination (P for prismatic, O for octahedral and T for tetrahedral) and the integer denotes the number of unique transition metal oxide layers.15 The two most common structures P2 and O3 are shown in Figure 1a. Distorted structures characterized by a symmetry lowering of the main phase are denoted


Page 2 of 29

Page 3 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with an apostrophe, e.g. P’2 or O’3. Due to the structural similarities of the different layered NaxMO2 phases, their powder X-ray diffraction (PXRD) patterns are similar with many Bragg peaks in the same or similar positions, Figure 1b. In addition, preferred orientation effects typical in layered materials can make relative peak intensities unrealiable. The most pronounced similarity between the PXRD patterns of different phases is the appearance of a strong Bragg peak at low 2θ originating from the interlayer correlation. Because of the rather similar PXRD patterns, samples with a mixture of different layered NaxMO2 phases display a high degree of peak overlap and this complicates analysis of the data.

Figure 1. a) Illustration of P2 and O3 structures constructed by MO6 octahedral sheets and intercalated prismatic or octahedral sodium.The figure is inspired by Kubota et al.4 b) Calculated PXRD patterns for P2, O3 and P3.

The different crystal phases vary in the amount of sodium ions that typically get intercalated during synthesis. As a rule of thumb the sodium content is x ≈ 0.5, 0.7, 1.0 and 0.7-1.0 for P3, P2, O’3 and O3, respectively.1,

6

The choice of transition metal, and their ratio in binary or higher order mixtures, also

greatly influences what crystal phases are obtained. It is widely believed that the crystal phase has a large influence on NaxMO2 materials in terms of performance as cathode in a battery cell.1, 16-18 However, recent studies indicate that the choice of transition metals and their original oxidation state may be more important than the crystal phase for determining the electrochemical performance.19-21 Many studies have been published on compounds containing combinations of Fe, Mn, Ni and Co,18, 22-28 but only few of these



systematically investigate the composition space.18,

27

To draw full advantage of SIBs as a cheap and

environmentally friendly alternative to LIBs, sodium must be combined with other earth abundant and environmentally benign elements.29 Thus, iron and manganese containing samples are of high interest. Here we report a systematic study of structure and electrochemical performance of NaxFeyMn1-yO2 using nine samples with target composition of x = 0.5, 0.7, 1.0 and y = 0.3, 0.5, 0.7. The effect of different sodium loading (x) and transition metal ratio (y) on the as synthesized crystal structure is studied using highresolution synchrotron PXRD and Rietveld refinement. The electrochemical performances of the produced materials are investigated using sodium ion half-cells.

Experimental Synthesis of NaxFeyMn1-yO2 Because the aim here is to investigate the effect of the elemental composition and not the synthesis procedure, all the samples were synthesized using identical conditions. The samples were synthesized via a solid state reaction of mixed transition metal oxalate precursors and Na2CO3 as introduced by Shen et al.30 This particular method provides an efficient mixing of the precursor metals resulting in homogenous materials. Mixed Fe-Mn oxalate precursors were produced in a co-precipitation synthesis step. A solution with a total transition metal concentration of 0.25 M of FeSO4·7H2O (Sigma-Aldrich) and MnSO4·H2O (SigmaAldrich) was mixed with a 0.25 M NaC2O4 (Sigma-Aldrich) solution. The ratio of transition metals in the sulfate solution matched the desired ratio in the final oxide. The mixing was done by adding both solutions dropwise into a 250 ml conical flask initially containing 50 ml demineralized water held at 70 °C using an oil bath. To ensure full precipitation of the transition metals the volumetric ratio was fixed to MSO4:Na2C2O4 = 1 : 1.1 using Aladdin programmable syringe pumps. After the mixing, the solution was further stirred for 1 h at 70 °C and 3 h at room temperature, after which the powder was washed in demineralized water, decanted and dried in a vacuum oven. The final transition metal ratio (Fe/Mn) in the oxalate material was


Page 4 of 29

Page 5 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


measured using inductively coupled plasma optical emission spectroscopy (ICP-OES) and found to be within 3% of the intended ratio, see Table S1 in the Supporting Information (SI). The different oxalate precursors were then hand-mixed with Na2CO3 (Sigma-Aldrich) in the desired molar ratio with a 10% excess of Na2CO3 and pressed into pellets. Excess of Na2CO3 was introduced to compensate for sodium evaporation at high temperature. The pellets were sintered at 900°C for 12 h in air and placed in an argon-filled glovebox after the furnace had cooled to ~250°C. Measurement of total sodium loading in the samples by ICP-OES was attempted, but instrument malfunction made the data unreliable. Experience has shown that sodium losses in the high temperature synthesis typically are less than ~5%, and the total sodium loading is therefore assumed identical to the formal loading used in the synthesis. For simplicity the samples will be abbreviated with the first letter of the constituent elements and integers to represent the ratio, e.g. Na0.7Fe0.3Mn0.7O2 will be abbreviated as N7F3M7.

Electrochemical performance Galvanostatic cycling was performed on CR2032 type half-cells with sodium metal foil as counter and reference electrode. A glass microfiber filter (Whatman GF/D, #1823-257) with a thickness of 0.67 mm was used as separator. The electrolyte was composed of 1 M NaClO4 in propylene carbonate (PC) with 2 wt% fluoroethylene carbonate (FEC) as additive. For the working electrode, the active material was mixed with acetylene black and polyvinylidene diflouride (PVDF) in the weight ratio of 80:10:10, and coated onto aluminium-foil giving a thickness of the dry layer of ~30 μm. The cells were assembled in a glove box with water and oxygen levels at ~1 ppm. To evaluate the electrochemical performance all samples were galvanostatically cycled between 1.5 V and 4.3 V with 1C ≡ 0.6 mA. The 1C current corresponds to a current density between ~170-250 mA/g depending on the active material loading of the cells. This means that electrodes with low active material loadings experienced a larger current density. This makes little difference at slow current, but it may deteriorate the high current performance of low loading electrodes. The specific active material loadings are listed in Table S2 in the SI. No correlation was observed between



electrochemical performance and mass loading. A typical cycle test was set up in two parts with 40 cycles varying the C-rate from C/10 to 10C, followed by 50 cycles at 1C (see Figure S1-S9 in the SI for further details). This procedure evaluated the rate capability, cycle life and reversible specific capacity of the samples. For one of the samples, N7F3M7, the cycle test included a constant voltage step after each charge and discharge step as discussed further in the Supporting Information.

Synchrotron PXRD High-resolution synchrotron PXRD data were collected at SPring8 in Japan at beamline BL44B2 with λ = 0.500279(5) Å and using an image plate detector in Debye Scherrer geometry.31 The powders were packed in 0.3 mm glass capillaries, which were spun during measurements to improve powder statistics. The crystalline phases present in the nine samples were analyzed using Rietveld refinement as implemented in the program FullProf.32 In all the Rietveld refinements the peak profiles were modelled using Thompson-Cox-Hasting pseudo Voigt parameters.36 Since the instrumental resolution was not determined no microstructural information was extracted. The background was modelled by interpolation between a set of 13-30 points, and the zero point displacement was refined for all PXRD patterns.

Results and discussion Crystal structure The samples contained at least four different types of layered NaxMO2 phases, namely P2, P3, O3 and O’3. The approximate structural details for each of these layered phases are given in Table 1, and these data were used as a starting point in the Rietveld refinements. Figure 2 shows the observed and calculated PXRD pattern for N7F5M5. Similar plots from Rietveld refinement of data on all other samples as well as details of refined parameters are included in the Supporting Information.


Page 6 of 29

Page 7 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Observed and calculated PXRD pattern for N7F5M5. The bars show Bragg peak position for O3, P3 and P2, respectively. The inset highlights how high-resolution synchrotron PXRD data aids in identification of phases with overlapping Bragg peaks.

Structural figures showing the void space sodium ion migration pathways for each of the structures is presented in Figure 3. The void space ion migration pathways are calculated using procrystal analysis in CrystalExplorer using the method developed by Filsø et al.32 Other methods of calculating and visualizing ion migration pathways exist, i.e. ab initio, Voronoi-Dirichlet partition and bond valance analysis.33-35 The advantages of calculating ion migration pathways using procrystal analysis are that they are fast and give visual and intuitive information about likely ion migration pathways. All the layered phases show 2D sodium ion migration between the transition metal oxide layers. Despite correlations it was possible to extract detailed structural parameters including atomic position, sodium site occupancies and anisotropic atomic displacement parameters (ADP) from Rietveld refinements, even for multi-phase samples as summarized in Table 1. This is due to the very high number of Bragg peaks and high signal to noise ratio of the PXRD data even up to high 2θ; see datasets in the SI. This minimizes correlations because these structural parameters affect the Bragg intensities in different way as a function of 2θ.

Table 1. Crystallografic information used as starting point for all Rietveld refinements.

Phase

Space group

Unit cell (Å)

Na


M

O


P2

P63/mmc (No. 194)

2.89, 2.89, 11.21

Naf: 2b, (0;0;¼), Baniso Nae: 6h, (~0.3;~0.6;¼), Biso

P3

R-3m (No. 166)

2.87, 2.87, 22.17

6c, (0;0;~0.168), Baniso

O3

R-3m (No. 166)

2.93, 2.93, 16.68

18f, (~0.12;0;0), Biso

O’3

C2/m (No. 12)

5.15, 2.81, 5.74 8j, (0.05;0.6;½), Biso 106.5o

Page 8 of 29

2a, (0;0;0), Baniso 3a, (0;0;0), Baniso 3b, (0;0;1/2), Bansio 2a, (0;0;0), Baniso

4f, (1/3;2/3;~0.091), Biso

6c, (0;0; ~0.395), Biso

6c, (0;0; ~0.234), Biso

4i, (0.28;0;0.81), Biso

32

Figure 3. Sodium ion migration pathways for P2, P3, O3 and O’3 as calculated in CrystalExplorer using the method of Filsø et al. Grey spheres: transition metal atom, red spheres: oxygen atom, grey void space: sodium ion migration pathway.

Some of the samples showed clear signs of air exposure and subsequent degradation of the P2 phase to carbonated P2 phases (PC2 and PC’2) and a hydrated P2 phase (PH2).36, 37 Since these subtle phases are difficult to quantify we include a brief discussion about the formation of these phases and their identification using PXRD.


Page 9 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Rietveld refinements of N5F5M5 to exemplify the characteristic peaks from PH2 and PC’2.

The air sensitivity of P2-Na0.67Fe0.5Mn0.5O2 and degradation to PC2 P’2 and PH2 has been studied by Duffort et.al.36 PH2 is produced when water molecules are adsorbed on the surface of the P2 crystallites and subsequently diffuse into the sodium layers. The introduction of water molecules in the sodium layers increases the interlayer distance and PH2 is therefore identified by a peak appearing at slightly lower 2θ values than the strong interlayer Bragg peak, see Figure 4. The interlayer distance typically increases from ~11 Å to ~14 Å, in good agreement with previous studies.38-39 PC2 is produced when oxygen is reduced by a transition metal ion in the presence of adsorbed carbon dioxide and water. The reaction produces a carbonate ion that intercalates into the structure forming cation vacancies.36 The formation of PC2 is fast upon air contact and due to the low scattering power of carbon the PXRD pattern of PC2 is very similar to P2. Slight shift of some Bragg peaks of the original P2 is expected due to slightly enlarged c-axis of the PC2 phase, but the peak shift is rarely noticed because the original positions are not known. A structural model of the PC2 phase is available but it is difficult to incorporate in a multi-phase Rietveld refinement of PXRD data because of only minute changes compared with P2. Therefore, the best way to detect the presence of PC2 using PXRD is when Rietveld refinements of normal P2 structure show ADPs perpendicular to the layers



for Naf and lower than expected site occupancy for the transition metal site. PC’2 forms upon prolonged air exposure when the inserted carbonate ions de-intercalate again and extract sodium ions with them forming disordered Na2CO3 on the surface of the P2 crystallites. PC’2 can be identified in a PXRD pattern by the appearance of a broad and diffuse peak at slightly lower 2θ than the interlayer correlation peak, see Figure 4. In the present study it was not possible to refine detailed structural parameters for the minute PC’2 and PH2 phases, and in most cases only scale factor and the c-axis value were refined with the a and b axis constrained to be the same as for the P2 phase. Many of the samples showed presence of additional crystalline impurity phases with the most prominent being γ-Fe2O3 in samples with high amount of iron. Very small amounts of disordered Na2CO3 was also observed in a few samples. The disorder is detected by difficulties in fitting Bragg peak intensities to the observed Na2CO3 peaks and this has been reported before.36 The low concentration and disorder of the Na2CO3 phase made it impossible to include it in the Rietveld refinements. The very high intensity of the synchrotron radiation allowed detection of other small impurity phases, based on integrated peak area of major peaks their weight fraction is 0.5 can be explained by the excess sodium added to the high temperature synthesis). This means all or nearly all the sodium is incorporated into the lattice of the layered phases. The Rietveld refinements for the P2 and P3 phase in N5F3M7 show sodium loading of ~0.47 and ~0.46, respectively. The P3 phase therefore has a lattice loading close to its expected value of ~0.5, while P2 has it too low compared with its expected value of ~0.7.1 The lower than expected Rietveld refined sodium loading indicates that the P2 structure is not formed under thermodynamic equilibrium in the given reaction conditions, and thus must be a kinetic product. Note that a proper thermodynamic and kinetic study requires variable temperature and time resolved synthesis data, but this is beyond the scope of the present study. Thus, conclusions are drawn based on local conditions not on global properties. This is also seen for the N5F5M5 sample containing


Page 14 of 29

Page 15 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


almost phase pure P2 with a sodium loading of x ≈ 0.54 according to the Rietveld refinements. The PXRD pattern of N5F5M5 shows the presence of PC’2 meaning that apart from reaction kinetics, the low amount of sodium in the lattice may also originate from extraction of sodium upon air exposure.36 The Rietveld refinements show a sodium loading of x ≈ 0.64 for the P2 phase in N5F7M3. Thus, this P2 phase has expected sodium loading and is possibly formed closer to thermodynamic equilibrium in this synthesis, whereas the rest of the iron crystallizes to Fe2O3. Most of the samples with higher formal sodium loading (i.e. the N7 and N10 samples) show Rietveld refined sodium loading significantly lower than expected from the synthesis. Assuming that loss of sodium during the high temperature synthesis is negligible, then the lacking sodium must be contained in amorphous phase(s) not detected by PXRD. The amount of amorphous phases in the samples could possible be determined by quantitative phase analysis (QPA) by introducing a known amount of fully crystalline standard in the sample. However, the results of QPA would be uncertain due to multi-phase Rietveld refinements that in some cases do not include crystalline impurity phases. Interestingly, the crystal structure changes from pure P2 to a mixture of O’3, P3 and O3 going from N7F3M7 to N10F3M7 even though the sodium loading determined by Rietveld refinements does not increase significantly. This shows that the sodium available during the synthesis, in any form, influences what layered phase is formed, independent of how much sodium is incorporated into the layered phases. This is also supported by the fact that similar and higher Rietveld refined sodium loading is observed in the mostly P2 containing samples N5F5M5 and N7F5M5 compared with the mostly O’3 containing N10F3M7 sample. The Rietveld refined sodium loading of each layered phase in N10F3M7, O3, P3 and O’3, are x ≈ 0.73, 0.47 and 0.53, respectively. This is close to the expected values for O3 and P3 (O3 has stable x ≈ 0.7-1.0), but too low for O’3 (expected x ≈ 1.0). The O’3 phase is a monoclinic distortion of the O3 structure due to Jahn-Teller distortion of MnO6 octahedra containing Mn3+.52-53 It has been reported that 33% concentration of Mn3+ ions in NaMn1/3Fe2/3O2 is enough to induce Jahn-Teller distortion and cause the O’3 phase to form.53 Therefore, it is likely that the O’3 phase in N10F3M7 contains considerable concentration of Mn3+



ions despite of the low sodium loading. This is also supported by the fact that a short low voltage Mn3+/Mn4+ plateau is visible upon the first charge of N10F3M7, see Figure S10 in the SI. The N7F5M5 sample has an average Rietveld refined sodium loading of x ≈ 0.66, which is rather close to the formal loading. In addition, all the individual phases have Rietveld refined sodium loadings close to their expected values (P2: x ≈ 0.76, P3: x ≈ 0.43 and O3: x ≈ 0.70) indicating that the synthesis of N7F5M5 was less limited by reaction kinetics than other N7 and N10 samples. The average Rietveld refined sodium loading of N10F5M5 is lower than expected, x ≈ 0.58, indicating that the reaction time was too short to fully incorporate the available sodium. The Rietveld refined sodium loading of O3, P3 and P2 are x ≈ 0.64, 0.43 and 0.49, respectively. O3 and P3 therefore have x slightly lower than their expected values, while the P2 phase probably has its crystalline sodium loading limited by reaction kinetics in this synthesis. N7F7M3 shows an exceptionally low Rietveld refined sodium loading, averaging to x ≈ 0.39 considering the Fe2O3 impurity but x ≈ 0.52 just considering the P2 phase. It is likely that Fe2O3 is an intermediate phase in the formation of layered NaxMO2 since the oxidation state of iron in Fe2O3 (+3) is lower than in NaxMO2 (+4-x). Therefore, the presence of Fe2O3 and the low crystalline sodium loading may indicate a slow formation rate of the P2 type material having high Fe/Mn ratio under the given reaction conditions. N10F7M3 has the highest Rietveld refined sodium loading of all the samples, x ≈ 0.74, but still considerably lower than expected from the synthesis. It is therefore likely that even longer reaction time could have resulted in an O3 type material with x ≈ 1.0.

Electrochemical performance The amount of sodium that is available for electrochemical cycling in a sodium ion half-cell is estimated from the extracted specific capacity during first charge. This number is probably slightly overestimated because of irreversible processes that often occur during the first cycles.54 The present results show that the sodium available for electrochemical cycling in the voltage window 1.5-4.3 V, is at best only x ≈ 0.24-


Page 16 of 29

Page 17 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.40. This is despite that the synthesis aimed for x ≈ 0.5-1.0, and the Rietveld refined sodium loadings have x ≈ 0.39-0.74. It may be possible to extract more sodium by extending the voltage range to higher voltage. However, this is problematic for the long term stability of the cells since the electrolyte is known to decompose at potentials higher than ~4.1 V.10 Charging to higher voltage may also compromise the structural stability of the layered materials since the P2 phase is known to undergo a reversible phase change to a poorly crystalline “Z” phase when charged to ~4.5 V.16, 19, 27, 30, 46, 55, 56 Even though the phase transformation is reversible it has been shown that avoiding the “Z” phase formation significantly increases cycle life.30 Iron containing O3 phases are also known to exhibit irreversible migration of the iron atoms from the layers into an adjacent tetrahedral site when the sodium loading is critically low, thereby decreasing the reversible capacity.57 The extracted sodium loading is always lower than the Rietveld refined sodium loading meaning that all the extracted capacity potentially comes from de-intercalation of sodium ions in the lattice of the layered phase(s). However, the sodium available for electrochemical cycling increases when going to higher formal sodium loading within each group while there is less correlation to crystalline sodium loading within the same sample. This indicates that at least some of the capacity extracted in the first cycle comes from extracting sodium from sodium rich amorphous phase(s) and/or other irreversible electrochemical processes.

The reversible discharge capacity, rate performance and cycle life of each sample is evaluated by the cycling program described in the experimental section. Detailed results of the cycle tests for each sample are given in Figure S1-S9 the SI by plotting the measured specific capacity as a function of cycle number. The reversible capacity-voltage curves of each sample, measured at C/10 after the rate test (cycle 36), are plotted in Figure 6. The total specific capacity generally decreases with increasing Fe/Mn ratio and increasing formal sodium loading. The voltage change during the rest period after charging and discharging indicates that the polarization generally increases in the same manner. This is in line with the overall tendency of worse electrochemical performance at higher Fe/Mn ratio and higher formal sodium loading as



further discussed below. Two different plateau-like voltage regions, separated by a relatively rapid increase/decrease in voltage, are distinguishable during both charging and discharging at ~2.2-3.0 V and ~3.5-4.0 V. The lower voltage region is relatively large at low Fe/Mn ratio, while the higher voltage region gets relatively larger at higher Fe/Mn ratio. Therefore, as has been proposed before,18, 20 the lower and higher voltage regions may be correlated to the Mn3+/Mn4+ and Fe3+/Fe4+ redox couples, respectively, and it is clear that both redox couples contribute to the reversible capacity. This result is only inferred by the observed voltage profile and definite proof of the active redox couples can only be directly measured with methods sensitive to the oxidation state of the transition metals, such as X-ray absorption spectroscopy (XAS). Notably, the polarization of the Fe3+/Fe4+ redox couple is considerably higher than the Mn3+/Mn4+ redox couple.

Figure 6. Capacity-voltage curves measured at C/10 after rate test (reversible capacity). Formal sodium loading and Fe/Mn ratio increase by going down and right, respectively.

Figure 7 summarizes the specific charge and discharge capacity obtained in the first cycle along with the reversible discharge capacity. Most of the samples show a considerably higher discharge than charge capacity in the first cycle. More detailed investigation of the capacity-voltage curves for the first charge and


Page 18 of 29

Page 19 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


discharge (see Figure S10 in the SI) reveals that the lower voltage Mn3+/Mn4+ plateau is missing or relatively small during the first charge. This means that the Mn3+/Mn4+ redox couple is inactive, or only slightly active, during the first charge. This indicates that the average oxidation state of manganese in the pristine material is equal to, or only slightly smaller than 4+. Upon subsequent discharge, and further cycling, a proportionally long low voltage Mn3+/Mn4+ plateau appears. The plateau indicates that the Mn4+ in the pristine material is reduced, using sodium from the sodium metal anode and therefore it becomes electrochemically active during cycling. This observation shows that most of the samples are able to accommodate more sodium in their structures upon electrochemical cycling than what is available in the as synthesized material. This is especially obvious for N10F3M7 where the initial discharge capacity is considerably lower that the reversible capacity. In a conventional full cell, the anode material is sodium free, and therefore the real capacity in a full cell is limited by the available sodium in the as synthesized material. If these materials were to be used in full cells, their full capacity range would only be obtained if an additional source of sodium ions would be added to the cell.58-59

Figure 7. Specific capacity for first cycle charge and discharge at C/10 for all samples. Also showing reversible discharge capacity, measured at C/10 after rate testing. Solid lines guide the eye to groups with constant Fe/Mn ratio.



All samples show acceptable reversible specific discharge capacity ranging from ~80-140 mAh/g. Higher reversible discharge capacity is in general obtained for samples having lower Fe/Mn ratio. Since γFe2O3 is electrochemically inactive in this voltage range the low specific capacity of N5F7M3 and N7F7M3 could be explained by the hematite impurity appearing in these samples. Assuming that the measured capacity solely comes from the layered phases in N5F7M3 and N7F7M3 their specific capacities are 108 and ~128 mAh/g, respectively. In general, the decrease of specific capacity with higher Fe/Mn ratio seems to be independent of crystal structure. This suggests that the specific capacity is more dependent on the active redox couple than the crystal structure. This is rather surprising since it is widely reported that the P2 structure shows superior electrochemical performance, including capacity, to O3.16-18 The reason for lower specific capacity at higher Fe/Mn ratio could be a low reaction rate of the Fe3+/Fe4+ redox couple during electrochemical cycling. Even lower cycling rate than used here (C/10) might therefore result in higher specific capacity. This is confirmed by cycling N5F7M3 at C/30, where the specific capacity increases significantly compared with the C/10 values becoming much closer to N5F3M7 cycled at C/10, see Figure S12 in Supporting Information. Other studies have shown O3 type NaFe0.7Mn0.3O2 and P2 type Na0.7Fe0.7Mn0.3O2 to have a much higher specific capacity than reported here in the same voltage windows, although at much lower C-rates.38, 49 Within the NxF3M7 and NxF7M3 sample groups the highest capacity is obtained in the P2 containing samples while the O3 samples show lower capacity. This indicates that if the Fe/Mn ratio is kept constant, the P2 structure delivers higher reversible capacity in accordance with other studies showing P2 to deliver higher reversible capacity than O3.16-18 Recent studies have shown that P2 and O3 type Na2/3FeyMn1-yO2 (y = 1/2 and 2/3) with the same elemental composition have similar electrochemical performance.19,

20

The difference between the predominately P2 and predominately

O3/O´3 containing samples observed here might therefore be related to differences in sodium loading and average oxidation state of the transition metals instead of the crystal structure. For the NxF5M5 sample group all the samples show similar specific capacity, ~105-115 mAh/g. This is despite the fact that the


Page 20 of 29

Page 21 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


samples vary significantly in composition of layered phases, and it further suggests that the exact type of layered phase is not the determining factor in the reversible specific capacity of these materials.

Figure 8. Cycle life, capacity retention [%] and initial and final specific capacity measured at 1C for all samples. Solid lines guide the eye to groups with constant Fe/Mn ratio.

The results of the cycle life testing are summarized in Figure 8 showing the initial and final discharge capacity at 1C, during the life testing conducted after the rate test, along with the ratio between them (defined as the life retention). Overall, the life retention and specific capacity decreases with increasing Fe/Mn ratio. Within the low Fe/Mn ratio group, N5F3M7 and N7F3M7 (both primarily P2) show the best performance with ~105 mAh/g initial specific capacity and life retention of ~87% and ~83%, respectively. N10F3M7 (primarily O’3) shows a slightly better life retention of 89% but much lower specific capacity at ~65 mAh/g. It is interesting to note that within each sample group the O3 (or O’3) type material shows similar or better life retention than the P2 type samples. This shows that mixing manganese with iron in the layered NaxMO2 phase efficiently reduces irreversible structural changes that have been shown to occur if O3 type NaFeO2 is charged above 3.4 V.57 Another reason for the higher than expected life retention of the O3 phases might be low reaction kinetics, resulting in low observed specific capacities. Low specific capacity



means only a small amount of sodium is moving in and out of the structure giving less structural changes and therefore less degradation of the sample. No clear trend relating crystal structure to life retention is observed. This suggests that the life retention of these materials is more dependent on the elemental composition than the crystal structure.

Figure 9. 2C Rate performance. Rate retention [%] along with reversible and 2C capacity for all samples. Solid lines guide the eye to groups with constant Fe/Mn ratio.

Figure 9 shows the specific capacity obtained at 2C and the reversible capacity along with the relative rate retention, defined as the ratio between 2C capacity and reversible capacity. 2C rate is chosen since only a part of the samples show any specific capacity at rates exceeding 2C, see Figure S1-S9 in the SI. Both rate retention and specific 2C capacity decrease with increasing Fe/Mn ratio. The rate retention seems to be rather unaffected by crystal structure since no clear trend linking rate retention and crystal structure is observed. This is further supported by the fact that the samples in the NxF5M5 series show similar rate retention even though the primary crystal structure changes from P2 to O3. Lower rate retention at higher Fe/Mn ratio indicates that the Fe3+/Fe4+ redox couple is the rate limiting factor in the redox reaction and


Page 22 of 29

Page 23 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


supports the hypothesis that the specific capacity is limited by the redox rate rather than ion diffusion in these samples.

Conclusions A systematic study of phase composition, crystal structure and electrochemical performance of NaxFeyMn1yO2

has revealed trends in formation and reaction kinetics of different layered NaxMO2 phases based on

formal sodium loading and Fe/Mn ratio. Furthermore, trends in the electrochemical performance such as initial and reversible capacity, cycle life and rate capability can be put into context with crystalline phases and elemental composition. High formal sodium content during high temperature solid-state synthesis favors formation of O3 and O’3, where a higher content of iron also helps forming a phase pure O3 product. Low and intermediate formal sodium content leads to preferred formation of the P2 phase. For these samples high iron content gives Fe2O3 as a major impurity and this indicates a lower reaction rate compared with higher manganese content samples. Incorporation of sodium into the layered phases (especially P2) during the synthesis seems to be slow since a large portion of the as prepared layered phases are sodium deficient even with excess sodium present during the synthesis. The Rietveld refined sodium loading of the P2, O3 and O’3 phases in different samples are observed to be similar. Therefore, the relative amount of sodium during the synthesis seems to be a larger factor in determining the produced layered phase than the sodium loading in the crystalline lattice. The P2 phase is observed to be air sensitive and readily degrade to PC2, PC’2 and PH2 upon short contact with air. The presence of other types of layered phases with P2 seems to inhibit the degradation of the P2 phase upon air exposure, and this might suggest a way to produce stable P2 materials better suited for production of SIB. Electrochemical performance was shown to largely depend on formal sodium loading and Fe/Mn ratio and less on the crystal structure and Rietveld refined sodium loading. Higher reversible specific capacity, better cycle life and better rate capabilities are realized for lower Fe/Mn ratio and lower sodium loading. No trend linking the electrochemical performance to the specific crystalline phase is observed.



Within the sample groups with high and low Fe/Mn ratio (NxF3M7 and NxF7M3) the P2 containing samples generally perform better than O3 (or O’3) containing samples having the same Fe/Mn ratio. However, this trend is likely related to differences in formal sodium loading and average oxidation states of the transition metals, since the intermediate Fe/Mn ratio sample group (NxF5M5) shows similar performance across different crystalline phases. All the samples studied here show a reversible specific capacity in the range ~80-140 mAh/g corresponding to inserting/removing x ≈ 0.26-0.46 sodium ions per formula unit. In general, the life and rate retention of the samples are poor. However, acceptable life retention of ~90% (after 50 cycles at 1C) and rate retention of ~40-55% (at 2C compared to reversible capacity) is obtained for the NxF3M7 sample group. Arguable the best electrochemical performance is obtained for the N5F3M7 sample (containing P2 and P3) with reversible capacity of ~144 mAh/g, life retention of ~87% and rate retention of ~55%. This is closely followed by N7F3M7 (containing air sensitive P2) with reversible capacity of ~145 mAh/g, life retention of ~86% and rate retention of ~49%.

Acknowledgements The synchrotron radiation experiment at the SPring-8 synchrotron was conducted with the approval of the Japan Synchrotron Radiation Research Institute. The RIKEN SPring8 Center is thanked for access to the BL44B2 beamline, and Anders Blichfeld, Hazel Reardon, Hidetaka Kasai, Jacob Becker and Sanna Sommer are gratefully thanked for assistance with data collection. This research was supported by the Danish National Research Foundation (DNRF93). Affiliation with the Aarhus University Center for Integrated Materials Research (iMAT) is gratefully acknowledged.

Supporting Information Table showing y in NaxFeyMn1-yO2 measured by inductively coupled plasma optical emission spectroscopy (ICP-OES). Table showing active material mass loading for all cells. Detailed Rietveld refinement results for


Page 24 of 29

Page 25 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


all materials including a figure with fitted data and difference line. Graphs showing specific capacity as a function of cycle number for all cells. Figure showing first charge and discharge CV curves for all cells. Figure showing differences between cycling programs including and excluding a constant voltage step at the end of charge and discharge. Figure showing increased capacity for N5F7M3 obtained at lower charge/discharge rate (C/30 vs C/10), the low rate capacity of N5F7M3 is also compared to N5F3M7. References 1. 2. 3. 4. 5. 6. 7. 8.

9. 10. 11. 12.

13.

14. 15. 16.

Han, M. H.; Gonzalo, E.; Singh, G. ; Rojo, T. A comprehensive review of sodium layered oxides: powerful cathodes for Na-ion batteries. Energy Environ. Sci. 2015, 8, 81-102. Wang, L. P.; Yu, L.; Wang, X.; Srinivasan, M. ; Xu, Z. J. Recent developments in electrode materials for sodium-ion batteries. J. Mater. Chem. A 2015, 3, 9353-9378. Yabuuchi, N.; Kubota, K.; Dahbi, M. ; Komaba, S. Research Development on Sodium-Ion Batteries. Chem. Rev. 2014, 114, 11636-11682. Kubota, K.; Yabuuchi, N.; Yoshida, H.; Dahbi, M. ; Komaba, S. Layered oxides as positive electrode materials for Na-ion batteries. MRS Bull., 2014 39, 416-422. Sawicki, M.; Shaw, L. L. Advances and challenges of sodium ion batteries as post lithium ion batteries. RSC Adv. 2015, 5, 53129-53154. Hwang, J.-Y.; Myung, S.-T.; Sun, Y.-K. Sodium-ion batteries: present and future. Chem. Soc. Rev. 2017, 46, 3529-3614. Kulova, T. L.; Skundin, A. M. From lithium-ion to sodium-ion battery. Russ. Chem. Bull. 2017, 66, 1329-1335. Ortiz-Vitoriano, N.; Drewett, N. E.; Gonzalo, E.; Rojo, T. High performance manganese-based layered oxide cathodes: overcoming the challenges of sodium ion batteries. Energy Environ. Sci. 2017, 10, 1051-1074. Pan, H.; Hu, Y.-S.; Chen, L. Room-temperature stationary sodium-ion batteries for large-scale electric energy storage. Energy Environ. Sci. 2013, 6, 2338-2360. Kubota, K.; Komaba, S. Review—Practical Issues and Future Perspective for Na-Ion Batteries. J. Electrochem. Soc. 2015, 162, A2538-A2550. Palomares, V.; Casas-Cabanas, M.; Castillo-Martinez, E.; Han, M. H.; Rojo, T. Update on Na-based battery materials. A growing research path. Energy Environ. Sci. 2013, 6, 2312-2337. Palomares, V.; Serras, P.; Villaluenga, I.; Hueso, K. B.; Carretero-Gonzalez, J. ; Rojo, T. Na-ion batteries, recent advances and present challenges to become low cost energy storage systems. Energy Environ. Sci., 2012, 5, 5884-5901. Paulsen, J. M.; Donaberger, R. A.; Dahn, J. R. Layered T2-, O6-, O2-, and P2-Type A2/3[M‘2+1/3M4+2/3]O2 Bronzes, A = Li, Na; M‘ = Ni, Mg; M = Mn, Ti. Chem. Mater., 2000, 12, 22572267. Delmas, C.; Fouassier, C.; Hagenmuller, P. Structural classification and properties of the layered oxides. Physica B+C, 1980, 99, 81-85. Delmas, C.; Braconnier, J.-J.; Fouassier, C.; Hagenmuller, P. Electrochemical intercalation of sodium in NaxCoO2 bronzes. Solid State Ionics, 1981, 3–4, 165-169. Yabuuchi, N.; Kajiyama, M.; Iwatate, J.; Nishikawa, H.; Hitomi, S.; Okuyama, R.; Usui, R.; Yamada, Y.; Komaba, S. P2-type Nax[Fe1/2Mn1/2]O2 made from earth-abundant elements for rechargeable Na batteries. Nat. Mater., 2012, 11, 512-517.



17. 18. 19.

20.

21.

22. 23.

24. 25.

26. 27.

28.

29. 30. 31.

32.

33. 34. 35. 36.

Shacklette, L. W.; Jow, T. R.; Townsend, L. Rechargeable Electrodes from Sodium Cobalt Bronzes. J. Electrochem. Soc., 1988, 135, 2669-2674. Thorne, J. S.; Dunlap, R. A.; Obrovac, M. N. Structure and Electrochemistry of NaxFexMn1-xO2 (1.0 ≤ x ≤ 0.5) for Na-Ion Battery Positive Electrodes. J. Electrochem. Soc., 2013, 160, A361-A367. Gonzalo, E.; Han, M. H.; Lopez del Amo, J. M.; Acebedo, B.; Casas-Cabanas, M.; Rojo, T. Synthesis and characterization of pure P2- and O3-Na2/3Fe2/3Mn1/3O2 as cathode materials for Na ion batteries. J. Mater. Chem. A, 2014, 2, 18523-18530. Han, M. H.; Acebedo, B.; Gonzalo, E.; Fontecoba, P. S.; Clarke, S.; Saurel, D.; Rojo, T. Synthesis and Electrochemistry Study of P2- and O3-phase Na2/3Fe1/2Mn1/2O2. Electrochim. Acta, 2015, 182, 10291036. Sharma, N.; Bahri, O. K. A.; Han, M. H.; Gonzalo, E.; Pramudita, J. C.; Rojo, T. Comparison of the structural evolution of the O3 and P2 phases of Na2/3Fe2/3Mn1/3O2 during electrochemical cycling. Electrochim. Acta, 2016, 203, 189-197. Doubaji, S.; Valvo, M.; Saadoune, I.; Dahbi, M.; Edström, K. Synthesis and characterization of a new layered cathode material for sodium ion batteries. J. Power Sources, 2014, 266, 275-281. D'Arienzo, M.; Ruffo, R.; Scotti, R.; Morazzoni, F.; Mari, C. M.; Polizzi, S. Layered Na0.71CoO2: a powerful candidate for viable and high performance Na-batteries. Phys. Chem. Chem. Phys., 2012, 14, 5945-5952. Buchholz, D.; Chagas, L. G.; Vaalma, C.; Wu, L.; Passerini, S. Water sensitivity of layered P2/P3NaxNi0.22Co0.11Mn0.66O2 cathode material. J. Mater. Chem. A, 2014, 2, 13415-13421. Sathiya, M.; Hemalatha, K.; Ramesha, K.; Tarascon, J. M.; Prakash, A. S. Synthesis, Structure, and Electrochemical Properties of the Layered Sodium Insertion Cathode Material: NaNi1/3Mn1/3Co1/3O2. Chem. Mater., 2012, 24, 1846-1853. Kim, D.; Lee, E.; Slater, M.; Lu, W.; Rood, S.; Johnson, C. S. Layered Na[Ni1/3Fe1/3Mn1/3]O2 cathodes for Na-ion battery application. Electrochem. Commun., 2012, 18, 66-69. Talaie, E.; Duffort, V.; Smith, H. L.; Fultz, B.; Nazar, L. F. Structure of the high voltage phase of layered P2-Na2/3-z[Mn1/2Fe1/2]O2 and the positive effect of Ni substitution on its stability. Energy Environ. Sci., 2015, 8, 2512-2523. Doubaji, S.; Ma, L.; Asfaw, H. D.; Izanzar, I.; Xu, R.; Alami, J.; Lu, J.; Wu, T.; Amine, K.; Edström, K.; Saadoune, I. On the P2-NaxCo1–y(Mn2/3Ni1/3)yO2 Cathode Materials for Sodium-Ion Batteries: Synthesis, Electrochemical Performance, and Redox Processes Occurring during the Electrochemical Cycling. ACS Appl. Mater. Interfaces, 2018, 10, 488-501. Naoaki, Y.; Shinichi, K. Recent research progress on iron- and manganese-based positive electrode materials for rechargeable sodium batteries. Sci. Technol. Adv. Mater., 2014, 15, 043501. Shen, Y.; Birgisson, S.; Iversen, B. B. A P2-NaxCo0.7Mn0.3O2 (x ≈ 1.0) cathode material for Na-ion batteries with superior rate and cycle capability. J. Mater. Chem. A, 2016, 4, 12281-12288. Nishibori, E.; Takata, M.; Kato, K.; Sakata, M.; Kubota, Y.; Aoyagi, S.; Kuroiwa, Y.; Yamakata, M.; Ikeda, N. The large Debye–Scherrer camera installed at SPring-8 BL02B2 for charge density studies. J. Phys. Chem. Solids, 2001, 62, 2095-2098. Filsø, M. Ø.; Turner, M. J.; Gibbs, G. V.; Adams, S.; Spackman, M. A.; Iversen, B. B. Visualizing Lithium-Ion Migration Pathways in Battery Materials. Chemistry – A European Journal, 2013, 19, 15535-15544. Yang, J.; Tse, J.S. Li Ion Diffusion Mechanisms in LiFePO4: An ab Initio Molecular Dynamics Study. J. Phys. Chem. A, 2011, 115(45), 13045-13049. Anurova, N. A.; Blatov, V. A. Analysis of ion-migration paths in inorganic frameworks by means of tilings and Voronoi-Dirichlet partition: a comparison. Acta Cryst., 2009, B65, 426-434. Adams, S. Bond valence analysis of structure–property relationships in solid electrolytes. J. Power Sources, 2006, 159(1), 200-204. Duffort, V.; Talaie, E.; Black, R.; Nazar, L. F. Uptake of CO2 in Layered P2-Na0.67Mn0.5Fe0.5O2: Insertion of Carbonate Anions. Chem. Mater., 2015, 27, 2515-2524.


Page 26 of 29

Page 27 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


37.

38. 39. 40. 41.

42. 43.

44. 45.

46.

47.

48. 49.

50. 51. 52. 53.

54. 55.

Eriksson, T. A.; Lee, Y. J.; Hollingsworth, J.; Reimer, J. A.; Cairns, E. J.; Zhang, X.-f.; Doeff, M. M. Influence of Substitution on the Structure and Electrochemistry of Layered Manganese Oxides. Chem. Mater., 2003, 15, 4456-4463. Lu, Z.; Dahn, J.R. In Situ X-Ray Diffraction Study of P2 - Na2/3[Ni1/3Mn2/3]O2. J. Electrochem. Soc., 2001, 11, A1225-A1229. Buchholz, D.; Chagas, L.G.; Vaalma, C.; Wu, L.; Passerini, S. Water sensitivity of layered P2/P3NaxNi0.22Co0.11Mn0.66O2 cathode material. J. Mat. Chem. A, 2014, 2, 13415-13421. Thompson, P.; Cox, D. E.; Hastings, J. B. Rietveld refinement of Debye-Scherrer synchrotron X-ray data from Al2O3. J. Appl. Crystallogr., 1987, 20, 79-83. Christiansen, T. L.; Bøjesen, E. D.; Sondergaard, M.; Birgisson, S.; Becker, J.; Iversen, B. B. Crystal structure, microstructure and electrochemical properties of hydrothermally synthesised LiMn2O4. CrystEngComm, 2016, 18, 1996-2004. Yuan, Z.; Xuehang, W.; Wenwei, W.; Kaituo, W. Synthesis and electrochemical performance of Na0.7Fe0.7Mn0.3O2 as a cathode material for Na-ion battery. Ceram. Int., 2014, 40, 13679-13682. Ding, J. J.; Zhou, Y. N.; Sun, Q.; Yu, X. Q.; Yang, X. Q. ; Fu, Z. W. Electrochemical properties of P2phase Na0.74CoO2 compounds as cathode material for rechargeable sodium-ion batteries. Electrochim. Acta, 2013, 87, 388-393. Wang, X.; Tamaru, M.; Okubo, M.; Yamada, A. Electrode Properties of P2–Na2/3MnyCo1–yO2 as Cathode Materials for Sodium-Ion Batteries. J. Phys. Chem. C, 2013, 117, 15545-15551. Wang, H.; Yang, B.; Liao, X.-Z.; Xu, J.; Yang, D.; He, Y.-S.; Ma, Z.-F. Electrochemical properties of P2Na2/3[Ni1/3Mn2/3]O2 cathode material for sodium ion batteries when cycled in different voltage ranges. Electrochim. Acta, 2013, 113, 200-204. Liu, L.; Li, X.; Bo, S.-H.; Wang, Y.; Chen, H.; Twu, N.; Wu, D.; Ceder, G. High-Performance P2-Type Na2/3(Mn1/2Fe1/4Co1/4)O2 Cathode Material with Superior Rate Capability for Na-Ion Batteries. Adv. Energy Mater., 2015, 5, 1500944. Carlier, D.; Cheng, J. H.; Berthelot, R.; Guignard, M.; Yoncheva, M.; Stoyanova, R.; Hwang, B. J.; Delmas, C. The P2-Na2/3Co2/3Mn1/3O2 phase: structure, physical properties and electrochemical behavior as positive electrode in sodium battery. Dalton Trans., 2011, 40, 9306-9312. Xu, J.; Chou, S.-L.; Wang, J.-L.; Liu, H.-K.; Dou, S.-X. Layered P2-Na0.66Fe0.5Mn0.5O2 Cathode Material for Rechargeable Sodium-Ion Batteries. ChemElectroChem, 2014, 1, 371-374. Yabuuchi, N.; Hara, R.; Kajiyama, M.; Kubota, K.; Ishigaki, T.; Hoshikawa, A.; Komaba, S. New O2/P2type Li-Excess Layered Manganese Oxides as Promising Multi-Functional Electrode Materials for Rechargeable Li/Na Batteries. Adv. Energy Mater., 2014, 4, 1301453. Ono, Y.; Kato, N.; Ishii, Y.; Miyazaki, Y.; Kajitani, T. Crystal Structure and Transport Properties of γNaxCoO2(x=0.67-0.75). J. Jpn. Soc. Powder Powder Metall., 2003, 50, 469-474. Takahashi, Y.; Akimoto, J.; Kijima, N.; Gotoh, Y. Structure and electron density analysis of Na0.74CoO2 by single-crystal X-ray diffraction. Solid State Ionics, 2004, 172, 505-508. Dick, S.; Müller, M.; Preissinger, F.; Zeiske, T. The structure of monoclinic NaNiO2 as determined by powder X-ray and neutron scattering. Powder Diffr., 1997, 12, 239-241. Mortemard de Boisse, B.; Cheng, J. H.; Carlier, D.; Guignard, M.; Pan, C. J.; Bordere, S.; Filimonov, D.; Drathen, C.; Suard, E.; Hwang, B. J.; Wattiaux, A.; Delmas, C. O3-NaxMn1/3Fe2/3O2 as a positive electrode material for Na-ion batteries: structural evolutions and redox mechanisms upon Na+ (de)intercalation. J. Mater. Chem. A, 2015, 3, 10976-10989. Ponrouch, A.; Monti, D.; Boschin, A.; Steen, B.; Johansson, P.; Palacin, M. R. Non-aqueous electrolytes for sodium-ion batteries. J. Mater. Chem. A, 2015, 3, 22-42. Mortemard de Boisse, B.; Carlier, D.; Guignard, M.; Bourgeois, L.; Delmas, C. P2-NaxMn1/2Fe1/2O2 Phase Used as Positive Electrode in Na Batteries: Structural Changes Induced by the Electrochemical (De)intercalation Process. Inorg. Chem., 2014, 53, 11197-11205.



56.

57. 58.

59.

Birgisson, S.; Shen, Y.; Iversen, B. B. In operando observation of sodium ion diffusion in a layered sodium transition metal oxide cathode material, P2 NaxCo0.7Mn0.3O2. Chem. Commun., 2017, 53, 1160-1163. Yabuuchi, N.; Yoshida, H.; Komaba, S. Crystal Structures and Electrode Performance of AlphaNaFeO2 for Rechargeable Sodium Batteries. Electrochemistry, 2012, 80, 716-719. Singh, G.; Acebedo, B.; Cabanas, M. C.; Shanmukaraj, D.; Armand, M.; Rojo, T. An approach to overcome first cycle irreversible capacity in P2-Na2/3[Fe1/2Mn1/2]O2. Electrochem. Commun., 2013, 37, 61-63. Singh, G.; Aguesse, F.; Otaegui, L.; Goikolea, E.; Gonzalo, E.; Segalini, J.; Rojo, T. Electrochemical performance of NaFex(Ni0.5Ti0.5)1−xO2 (x = 0.2 and x = 0.4) cathode for sodium-ion battery. J. Power Sources, 2015, 273, 333-339.


Page 28 of 29

Page 29 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic


Exploration of Phase Compositions, Crystal Structures, and

Recommend Documents