Unraveling the Role of Doping in Selective Stabilization of NaMnO2

Jan 31, 2018 - ... shown that boron substitution of Mn cations stabilizes monoclinic structure (α-NaMnO2) ..... structure, accounting for its reduced...
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Cite This: Chem. Mater. 2018, 30, 1257−1264

Unraveling the Role of Doping in Selective Stabilization of NaMnO2 Polymorphs: Combined Theoretical and Experimental Study Maxim Shishkin,*,† Shinichi Kumakura,¶ Syuhei Sato,¶ Kei Kubota,¶,† Shinichi Komaba,¶,† and Hirofumi Sato†,§ †

Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Nishikyo-ku, Kyoto 615-8520, Japan Department of Applied Chemistry, Tokyo University of Science Shinjuku, Tokyo 162-8601, Japan § Department of Molecular Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan ¶

S Supporting Information *

ABSTRACT: Dopants are known to modify structural, electronic, chemical, and other properties of materials; therefore, analysis of doping effects is of great interest in the fields of fundamental and applied science. However, in many functional materials, particularly transition metal (TM) compounds, such analysis could be quite complex owing to subtle interplay between possible oxidation states of various types of TM, which is hard to elucidate experimentally and difficult to model theoretically. In this work, we performed a study of the role of 3d TM and some non-TM dopants in stabilization of structural polymorphs of NaMnO2, a highly promising material for electrocatalysis and Na-ion battery applications. Our X-ray diffraction experiments and DFT+U modeling revealed the exclusive formation of α- or β-NaMnO2 polymorphs via substitutional doping of NaMnO2 by Ti or Cu cations, respectively, whereas doping with other elements results in formation of several structural polymorphs. In the most important case of stabilization of β-NaMnO2 by Cu cations, we find that geometry of this structure allows 2+ oxidation state of Cu, unlike α-NaMnO2, where Cu adopts a more artificial 3+ oxidation state, which explains lower stability of α-type polymorph.

1. INTRODUCTION The Na-based NaTMO2 layered oxides (TM are transition metal cations) are the subjects of intensive research due to their intriguing structural,1 catalytic,2 electrochemical,3 and magnetic4 properties. These properties allow application of layered oxide materials in the fields of heterogeneous catalysis (e.g., as electrocatalysts for oxygen evolution reactions2) or as cathode materials in secondary batteries.5 Additionally, some Na-based layered materials such as NaCoO2 are also attractive candidates in thermoelectric applications and superconductor design.6 In many of these applications, structural stability of the layered structures is a prerequisite for their favorable and reliable performance. For instance, effects like migration of TM cations, leading to their trapping at interstitial sites, results in the loss of catalytic activity2 or electrochemical redox properties7 of layered oxides when these are used as catalysts or as cathode materials of the Li- and Na-ion batteries. In view of an increasing employment of layered oxides as secondary battery electrodes, a great interest exists in synthesis of layered structures which contain only low cost and Earth abundant elements while still retaining desirable electrochemical properties. NaMnO2 is an example of such a material, featured by a reasonably high operating voltage and large capacity8,9 while consisting of Earth abundant and low cost elements such as Na and Mn. However, a variety of structural © 2018 American Chemical Society

polymorphs of NaxMnO2, resulting in the presence of various structural configurations at different concentrations of Na9 as well as multiple phase transitions upon charge/discharge cycling,10 can be responsible for the stepwise charge/discharge plateaux and limit the Na+ ion transport kinetics. Furthermore, β-NaMnO2 is known to gradually transform into α-NaMnO2 by electrochemical Na extraction/insertion11 whereas monoclinic layered LiMnO2 changes into a spinel-like phase12 in lithium batteries. Therefore, structural stabilization is highly desirable for more stable performance of NaMnO2-based electrodes. One of the most common ways to selectively stabilize a desired polymorph is substitutional doping. This approach has been successfully used for stabilizing layered structure of LiMnO2 over orthorhombic polymorph via substitution of Mn by Cr or Al cations (refs 13 and 14, respectively). Subsequently, DFT calculations also revealed that layered LiMnO2 can be stabilized by a wider range of substitutional dopants.15 On the basis of these DFT calculations, it has been proposed that neither the ionic size of dopants nor the Jahn−Teller distortions present in these oxides plays a decisive role in selective stabilization of studied structures.15 This conclusion Received: October 18, 2017 Revised: January 9, 2018 Published: January 31, 2018 1257

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Figure 1. XRD patterns of obtained materials, NaMn0.9Me0.1O2 with different Me dopants. The results for Ti- and Cu-doped samples where exclusive formation of α- and β-polymorphs is observed are shown in blue and red, respectively.

linear response technique.24,25 Thus, this study is also an adequacy test for DFT+U calculations where two U parameters (for Mn and the dopant cation) are determined via linear response method. Indeed, the first-principles calculations of U for all involved TM is quite essential as no prior knowledge is available about the oxidation states of dopant and Mn cations. Our DFT+U calculations have been used to elucidate the special effects of Cu and Ti on stabilization of β- and αNaMnO2 polymorphs. Additionally, computational analysis permitted to determine oxidation states of dopants and Mn cations. These findings have been used for classification of the types of dopants in terms of their influence on selective stabilization of studied polymorphs. We also analyzed the role of Jahn−Teller effects in stability of the doped structures and the impact of disturbance of antiferromagnetic ordering caused by the substitutional dopants.

has been drawn on the basis of stabilization of layered structures by dopants of small and large size alike and the presence of Jahn−Teller distortions in both competing polymorphs. It was proposed that stabilization of layered LiMnO2 is caused by disturbance of antiferromagnetic ordering of high spin Mn3+ cations by Mn4+ cations, introduced by the lower-valence dopants. These interpretations are in agreement with the ideas emerged from other studies where divalent elements (e.g., Co) were found to destabilize antiferromagnetic ordering and favor ferromagnetic orientation.16,17 It should also be noted that the effect of doping has been also studied for NaMnO2, where it has been shown that boron substitution of Mn cations stabilizes monoclinic structure (α-NaMnO2) for fully intercalated and partially (75%) desodiated cases.18 In this work, we analyze the effect of doping on stability of NaMnO2 polymorphs via replacing 10% of Mn cations by other 3d TM as well as several non-TM (Zn, Mg, and Al). As we show below, we found that only Cu doping results in exclusive formation of β-NaMnO2 polymorph. The latter conclusion is inferred from X-ray diffraction (XRD) pattern, where only βtype phase is present, whereas diffraction peaks corresponding to α-type phase are fully missed (see Figure 1). To the best of our knowledge, this is the first successful attempt at synthesizing pure β-NaMnO2 polymorph with the help of substitutional doping. Although it is certainly of great interest to study the electrochemical properties of the doped NaMnO2, for all dopants except Ti (we explain the reasons in the subsequent sections), we limit the discussion to analysis of stability of only fully intercalated structures, leaving investigation of deintercalation process to future studies. To get an insight about the role of doping in stability of NaMnO2 polymorphs, in this work we also performed ab initio calculations of electronic and magnetic properties of all doped structures. The DFT+U method, employed herein, allows study of the electronic properties of strongly correlated materials such as NaMnO2 with higher degree of accuracy as compared to that of the local DFT functionals (e.g., LDA or GGA).19 However, application of DFT+U methodology requires numerical values of Hubbard U parameters for each type of transition metal ion. To date, these are usually borrowed from the previous benchmark studies, where fitting of the results of DFT+U calculations to the experimental values has been performed for various kinds of TM ions.20−23 In view of a questionable transferability of thus obtained U parameters, we prefer to calculate the values of U from the first-principles, using ab initio

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS 2.1. Experimental Details. NaMnO2 and doped samples were synthesized by solid state reaction from starting materials of Na2CO3, Mn2O3, MgO, Al(OH)3, Sc2O3, TiO2, V2O5, Cr2O3, Fe2O3, Co3O4, Ni(OH)2, Cu2O, or ZnO. The mixtures were pressed into pellets and heated in air at 1 °C min−1 from room temperature to 1050 °C and then quenched by taking the reaction product out from the furnace at 1050 °C and immediately transferring it into an argon-filled glovebox. The samples were cooled to room temperature in the glovebox and were kept inside to avoid contact with moisture and oxygen in air. Details of synthesis conditions are described in our previous work.26 The crystal structure of NaMn0.9Me0.1O2 samples was examined by lab-scale XRD. Lab-scale XRD measurement was carried out using a diffractometer equipped with X-ray tube of Cu target with Ni filter (Rigaku, Multiflex). 2.2. Computational Details. We used the VASP computational package for all calculations performed within this work.27 For treatment of exchange-correlation effects, we employed spin-polarized PBE functional.28 The projector augmented wave (PAW) approach has been used for description of electron−nuclear interactions.29,30 In this work, the PAW potentials with minimum number of valence electrons (i.e., no semicore states) have been used for TM cations. The characteristic cut off energy of 800 eV has been used for the plane wave expansion of electronic wave functions, whereas k-point sampling was performed using 2 × 2 × 2 Monkhorst−Pack grid.31 Relaxation of lattice constants, lattice angles, and atomic coordinates was performed for all studied structures, minimizing the atomic forces to at least 0.01 eV/Å. For nondoped structures, we employed computational cells that consisted of 32 Na, 32 Mn, and 64 O. In modeling of dopant structures, we introduced four substitutional dopants in our supercells. 1258

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elsewhere. In terms of phase stability of α- and β-phase, we reached a very distinct conclusion that Cu doping strongly stabilizes β-phase; Ti doping stabilizes α-phase, and anything else shows mixed phases as well as nondoped (Me = Mn) sample. 3.2. DFT+U Analysis of Undoped NaMnO2. Comparison of energetics of all three types of studied polymorphs (α-, β-, and P′2) showed that P′2 configurations are much less thermodynamically stable than α- and β- structures (this applies to doped cases as well). The presence of P′2 polymorphs observed in XRD measurements can be possibly explained by kinetically driven mechanism of formation of such structures under chosen experimental conditions. However, as the theoretical study performed herein is focused on thermodynamics analysis exclusively, we limited our subsequent discussion to comparison of stability of α- and β- polymorphs. The atomic structures of α- and β-polymorphs were determined several decades ago via XRD measurements.35,36 In Figure 2, we show their optimized models using the same

This results in 12.5% doping of the NaMnO2 structures, which is reasonably close to the 10% doping as in experimental study. In view of the well-known deficiency of the local functionals in treatment of d-electron states of TM cations, the Hubbard corrected DFT+U framework in Dudarev’s approximation was employed.32 The localized functions used in the projections of Hubbard operator are spherical harmonics of d-states with projections evaluated within spheres of PAW potential radii.33 The effective U parameters were calculated using the linear response approach (the detailed description of this methodology can be found in ref 34). It should be emphasized that U parameters were calculated for all types of TM present in the studied materials (i.e., Mn and dopant cations). Moreover, due to the presence of dopants, we evaluated U parameters for each TM of a cell (32 ions) and averaged the calculated values for each type of TM (i.e., Mn and dopant). Our calculations were performed self-consistently, reaching agreement between the used and calculated U parameters, requiring 10−15 iterations on average. A stringent convergence (within 0.01 eV) of evaluated values of U was attained in all studied cases.

3. RESULTS AND DISCUSSION 3.1. Experimental Results. NaMn0.9MeO2 with a variety of dopants (Me) was synthesized by the conditions described in Experimental Details. These conditions were specifically optimized to obtain high temperature metastable phase.26 We fixed the synthesis conditions and did not optimize them for each dopant; however, it could be enough to obtain qualitative comparison of phase stability of β-phase because β-phase was already known as higher temperature phase than α-phase in 1971.35 Figure 1 shows the XRD patterns of doped materials synthesized at 1050 °C. In total, 3 main phases were found, α-, and β-, and P′2 phase especially seen between 12 and 18 degrees in 2θ. These diffraction peaks of 001, 002, and 001 correspond to interslab distance of β-phase (s.g.Pmnm), P′2phase (P63/mmc), and α-phase (C2/m), respectively.35 Cudoped and Ti-doped materials show single β-phase and αphase, respectively, while other samples have mixed-phases. Diffraction peaks of Cu-doped material are all assigned to βphase without any impurity peaks (see Supporting Information, Figure S1). The Rietveld analysis is not yet successful, and one possibility is preferred orientation, which is also indicated by needle-like morphology in scanning electron microscopy (SEM) images (see Figure S3). However, the distinct 011 diffraction peak at 23° indicates that Cu-doped material is the closest to the pure β-phase never reported because β-NaMnO2 containing planar defect has no 011 peak at all.9,10 We also confirm α-phase is more stable at lower temperature for all doped materials, and the XRD patterns at 800 °C are shown in Supporting Information, Figure S2. High temperature is required to form β-phase, probably because of kinetics barriers. P′2 phase is usually stable in 0.44 < x < 0.7 of NaxMO2, and thus, a certain amount of Na ions is assumed to be lost as Na2CO3 or Na2O, in particular for Cr, Co, and Ni. Na loss is probably related to thermodynamic stability of coexistence of these dopants with Na manganates. In fact, P′2 phase disappeared when the materials are synthesized with 10 mol % excess Na (see Supporting Information, Figure S2), but the trend of phase formation between α- and β-phase is not changed. Here, we focus more on formation energy of NaxMn0.9Me0.1O2 at nearly x = 1.0. Thermodynamics of α- and β-phase are discussed in the following section, especially for Cu- and Ti-doped materials, and the whole understanding of thermodynamics, including P′2 phase and other phenomena such as decomposition and phase segregation, will be reported

Figure 2. Structures of α- and β-NaMnO2 polymorphs. The boundaries of employed computational cells are indicated by the black solid line rectangular shapes.

presentation style as that employed in the work of Abakumov et al.10 (we should note that in subsequent analysis of the effects of specific dopants, we show the bonds based on distance criteria, thus omitting indication of bonding between oxygen atoms, etc.). Figure 2 clearly demonstrates the difference between α- and β-polymorphs which is manifested by straight line versus zigzag arrangements of Na atoms and MnO2 units. The computational cells used in our calculations are covered by transparent rectangular shapes with black boundaries (32 Na, 32 Mn, and 64 O ions per cell). In our calculations, we used the value of U = 3.68 eV, obtained via linear response method for both structures. To determine the most favorable magnetic ordering, we imposed ferromagnetic (FM) and antiferromagnetic (AFM) arrangements on both polymorphs. The AFM ordering was identical to the one determined by Greedan et al. for β-LiMnO2.37 Our calculations reveal that AFM ordering is more favorable than FM ordering (by about 1.5 eV), whereas β-NaMnO2 is by 0.17 eV more stable than α-NaMnO2. Given the large size of the employed cells (128 atoms), the latter difference can be considered miniscule, indicating that the two structures are degenerate in energy. We also find that the calculated values of magnetic moments of Mn ions (3.77 μB) are quite close to the respective value for the case of orthorhombic LiMnO2, measured experimentally (3.694 μB).37 3.3. Doped α- and β-NaMnO2. The values of U parameters for Mn and dopants in each case (these are 1259

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Figure 3. Relative energy ΔE for all studied dopants. The ΔE is the difference between the total energies of doped configurations of studied α- and β-polymorphs (ΔE = Eα − Eβ), which acts as a measure of a prevalent stability of one polymorph over the other. In the case of Ti-doped structures, ΔE values, corresponding to the configurations with all filled Na sites (the bottom of the white box) and the configurations with partially removed Na sodiums (bottom of the green box), are provided (see the text for further details).

deintercalation is endothermic (by 0.57 and 0.65 eV for removal of the fifth and sixth Na cation, respectively). Therefore, we assume that Ti-doped NaMnO2 can lose only the same number of Na cations as the number of Ti dopants. Upon Na removal, all Mn cations become trivalent, whereas all Ti still retain the tetravalent state. ΔE, which corresponds to such desodiated polymorphs (four Na removed), is shown by the filled bar, clearly indicating that for this case, Ti doping does not stabilize α-polymorph more substantially than several other dopants. We assume, however, that when initially formed (prior to Na removal), α-polymorph is certainly more stable (bar with unfilled cap in Figure 1) and can be possibly preserved even upon a subsequent loss of Na. We also wish to mention that our calculations have revealed that for other studied dopants, release of Na cations via reaction (1) is endothermic even with respect to removal of the first Na from the stoichiometric materials. This is not surprising, as compared to the other analyzed dopants, Ti is unique in favoring the high 4+ oxidation state under air atmosphere. In addition to the special cases of Cu and Ti doping, Figure 3 also clearly demonstrates that doping with other TM results in lower absolute value of ΔE as compared to Mg and Zn doping. Moreover, similar to TM dopants, Al-doped structures also have very small energy difference. Therefore, our analysis of Figure 3 shows that studied dopants can be classified into four groups: (1) Cu doping, favoring β-NaMnO2 formation; (2) Ti doping, favoring formation of α-NaMnO2; (3) other TM and Al dopants, which stabilize α-NaMnO2 only marginally, thus allowing formation of β-NaMnO2; and (4) non-TM, i.e. Mg and Zn, which stabilize α-NaMnO2 stronger than the third group of dopants, although with absolute value of ΔE still below the respective values for Cu and Ti dopants. The discussion of each of these dopant groups is provided in the next section. 3.4. Analysis of the Groups of NaMnO2 Dopants. 3.4.1. Cu Doping. To elucidate the reasons of greater stabilization of β-NaMnO2 by Cu doping, we analyzed the

determined using self-consistent linear response approach) are summarized in Table 1S of Supporting Information. In these calculations, we imposed antiferromagnetic ordering, equivalent to the one described for LiMnO2.37 The relative energies, defined as the difference between the total energies of α- and βNaMnO2, are shown in Figure 3. In this work, we made an attempt to search for configurations with the lowest energies for each dopant type, performing calculations of 5−7 structures with different atomic arrangements for each type of dopant in both polymorphs. The values of ΔE = Eα − Eβ, provided in Figure 3, were obtained using the structures with the lowest total energies. It should be noted that performed calculations are quite time-consuming, given the cell size, high cut off energy, and the k-point mesh in addition to the need to parametrize the DFT+U calculations for each TM cation (32 per cell). Therefore, we had to find an acceptable compromise between the large computational cost and the degree of sampling of various structural configurations in this study. Our calculations clearly show that Cu doping is unique in terms of favoring β-NaMnO2 over α-NaMnO2. Indeed, the ΔE value is not just positive in this case (in contrast to negative values for all other doped structures) but also has a high value of 1.74 eV. This provides an explanation of exclusive presence of β-type phase in the XRD pattern. For Ti-doped structures, we present two energies ΔE: for the case of fully sodified models (bar with unfilled cap) and for partially desodiated configurations (filled bar). Indeed, due to the preferred high oxidation state of Ti (4+), removal of Na cations from four NaTiO2 units is possible via the reaction under air atmosphere: 1 gO2 → 2 2TiO2 + gNa 2CO3

2NaTiO2 + gCO2 +

(1)

Our calculations showed that such reaction is exothermic by 0.67 eV with respect to release of 4 Na cations. Further 1260

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Chemistry of Materials oxidation states and magnetic moments of TM cations based on our DFT+U calculations. For Cu-doped α-NaMnO2, we find that all Mn have very similar absolute values of magnetic moments (3.79 μB), whereas magnetic moments of Cu are close to zero. It should be noted that these absolute values of magnetic moments of Mn are very close to those in undoped αNaMnO2 (which is 3.78 μB). These findings provide a clear indication that all Mn cations in Cu-doped α-NaMnO2 are trivalent, whereas Cu also adopts 3+ oxidation state. On the other hand, 4 Mn atoms of the Cu-doped β-NaMnO2 structure have much lower magnetic moments (3.15 μB) as compared to other Mn (3.78 μB), whereas magnetic moments of Cu cations are 0.62 μB. Therefore, each Cu cation adopts more common 2+ oxidation state in β-NaMnO2 with concomitant formation of four Mn4+ cations in studied cells. Our conclusions on oxidation state of Cu can also be corroborated by the analysis of PDOS of Cu cations in two polymorphs (Figure 4). Particularly, Figure 4 shows that for the

Figure 5. Oxidation states of representative cations of Cu-doped polymorphs. The bond lengths are given in Å.

oxygens of the adjacent layers (as compared to Mn−O of undoped structure, which is 2.43 Å) result in shorter distances between these oxygens and Mn cations of the next adjacent layers. These Mn cations are concomitantly oxidized to Mn4+. In contrast, the geometry of Cu-doped α-NaMnO2 does not allow such shortening of Mn−O bonds, caused by elongation of Cu−O distances (the discussed oxygen anions are present in the layers adjacent to those where referred Mn and Cu are located). Indeed, there is a much smaller difference between respective Cu−O and Mn−O distances in optimized αNaMnO2 structure as compared to optimized β-NaMnO2 (Figure 5). This explains the reason as to why all TM in αNaMnO2 (i.e., Cu and Mn) are forced to adopt trivalent oxidation state, which in turn leads to higher total energy of this structure as compared to that of the β-polymorph. 3.4.2. Ti Doping. In this section, we discuss only fully sodiated Ti-doped NaMnO2 (desodiated structures have very similar properties as the doped NaMnO2 of the third group, as discussed below). Ti doping of otherwise stoichiometric α- and β-NaMnO2 results in formation of four Mn with absolute magnetic moments substantially higher than those of the undoped structures (4.50 vs 3.78 μB as in the undoped case). The magnetic moments of Ti cations are close to zero in both polymorphs. Therefore, in contrast to Cu-doping, substitution of Mn with Ti leads to formation of Mn with lower oxidation state (Mn2+). It is clear that Ti adopts usual 4+ oxidation state in a heating process under air or oxygen atmosphere. A greater degree of stabilization of α-NaMnO2 is less obvious for Ti doping as compared to the case of β-NaMnO2 stabilization by Cu doping. As expected, Ti of β-NaMnO2 attracts a pair of oxygens in the adjacent Na-MnO2 layers (Figure 6). However, the reduced Mn2+ cations are not necessarily those that are bonded to these oxygens, although respective Mn−O distances increased due to Ti doping (Figure 6). In case of α-NaMnO2, the effect of Ti-doping on changing the Mn−O bond lengths is small, similar to Cu-doping of this polymorph as discussed above. The quite long Mn−O bonds for trivalent Mn of β-NaMnO2 (Figure 6) could be a possible reason for weaker stability of this structure as compared to α-NaMnO2. We also find that such long Mn−O bonds are not formed for Mn3+ in Ti-doped αNaMnO2. 3.4.3. Doping with Other TM and Al. We find that doping of α- and β-NaMnO2 with other 3d TM (except Cu and Ti prior to release of four Na cations) does not lead to

Figure 4. Comparison of PDOS of d-states of Cu of doped α- and βNaMnO2. Two unoccupied peaks in case of α-NaMnO2 split into still unoccupied majority peak and occupied minority peak in β-NaMnO2.

case of α-NaMnO2, majority and minority spin channels each have an unoccupied peak in PDOS of Cu d-states (these two are almost identical and fully overlap in Figure 4). On the other hand, the PDOS of Cu d-states of β-NaMnO2 shows that only the peak of majority channel is unoccupied, whereas minority channel peak is positioned below the Fermi level. This is another indication that additional electron resides on Cu cation of β-NaMnO2 structure, accounting for its reduced oxidation state (Cu2+) in contrast to Cu3+ of α-NaMnO2. Although Cu3+ cations can be present in some materials (e.g., NaCuO2), Cu is located at a square-planar site coordinated with four oxygen atoms unlike Mn at octahedral sites in α- and β-NaMnO2 and NaCuO2 where Na and Cu are located at octahedral sites has never been reported so far.38 Actually, we find that Cu-doped α-NaMnO2 is less stable due to the presence of Cu3+ cations. However, this raises another question as to why different oxidation states of Cu are realized in two studied polymorphs in spite of the same material stoichiometry. To shed light on this issue, we analyzed atomic arrangements in the vicinity of dopants in Cu-doped α- and β-NaMnO2 (Figure 5). In β-NaMnO2, much longer distances between Cu2+ and 1261

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Thus, the key difference between the effect of these two dopants and Cu is the trivalent oxidation state of Cu in αNaMnO2, which results in unfavorably high energy of this polymorph.

4. DISCUSSION Table1 provides magnetic moments and oxidation states of Mn and dopant cations for the four types of dopants, determined by Table 1. Oxidation States of Dopants and Mn Cations in αand β-NaMnO2 with Magnetic Moments for All Involved TMsa properties

Figure 6. Ti-doped NaMnO2 polymorphs. The Mn−O and Ti−O bond lengths are shown in Å.

introduction of different oxidation states of Mn, keeping all Mn trivalent as in the undoped structures. This conclusion is based on very marginal changes of Mn magnetic moments upon this type of doping (with average value of 3.78 μB vs 3.77/3.78 μB as in undoped structures). With regard to dopants, we find that their magnetic moments have very similar values in both polymorphs, which clearly shows that all dopants have the same oxidation state in these two structures. As a result, the total energies of two polymorphs differ only slightly, similar to undoped structures where all the cations have 3+ oxidation state. The same can be said about Al doping as due to trivalent Al cations all Mn atoms are in 3+ oxidation state in both doped α- and β-NaMnO2, resulting in miniscule total energy difference between these polymorphs. 3.4.4. Doping with non-TM Cations (Zn and Mg). These two kinds of dopants are divalent in both polymorphs, which is indicated by lower magnetic moments of four Mn that adopt 4+ oxidation state (Table 1S). These Mn4+ are, however, formed at different locations with respect to dopants. For instance, upon Zn doping of β-NaMnO2, Mn4+ forms acrosslayers Mn4+-O2−-Zn2+ chains similar to Cu doping, as is shown in Figure 5. In contrast, in Mg-doped β-NaMnO2, Mn4+ is formed in the layer adjacent to Mg2+ (Figure 7).

dopant type

μ (Mn)

μ (dopant)

oxidation state

Cu(α) Cu(β) Ti(α) Ti(β) Fe(α) Fe(β) Mg(α) Mg(β)

3.79 3.79/3.15 3.79/4.50 3.81/4.49 3.78 3.78 3.79/3.13 3.78/3.13

0 0.62 0 0.07/0.02 4.15 4.15

Mn3+ + Cu3+ Mn3+ + Mn4+ + Cu2+ Mn3+ + Mn2+ + Ti4+ Mn3+ + Fe3+ Mn3+ + Mn4+ + Mg2+

The values of magnetic moments are given in μB. The two values for some types of cations indicate the averaged minimum and maximum magnetic moments, caused by different oxidation states.

a

our DFT+U calculations. The Ti-doped structures described in Table 1 are not desodiated (for partially deintercalated case, all Mn are trivalent and have magnetic moments of 3.79 μB, similar to the undoped NaMnO2). For brevity, we provided only representative cases of dopants for the last two types. Table 1 shows that Cu and Ti dopants make two special cases as these cations tend to adopt oxidation states that are either lower than that of trivalent Mn in the undoped NaMnO2 (i.e., Cu2+) or higher (i.e., Ti4+). Our work also shows that technically Cu can adopt high trivalent oxidation state, enforced by α-NaMnO2 geometry, but this configuration would be much less favorable than Cu-doped β-NaMnO2. This factor explains the absence of α-NaMnO2 characteristic peak in the XRD patterns of the Cudoped sample. In contrast to Cu and Ti doping, other 3d TM and Al incorporated in NaMnO2 structures adopt trivalent oxidation states, which results in a very small difference in total energies of α- and β-polymorphs. For this reason, both α- and βNaMnO2 can be present for this type of doping, in full agreement with XRD measurements. Although Mg and Zn can only be divalent, unlike Cu they do not favor formation of β-NaMnO2. In fact, these dopants stabilize α-NaMnO2 stronger, although the difference in total energy with β-NaMnO2 is much lower than in the case of Cu doping, allowing formation of both polymorphs, in agreement with XRD data. We also wish to comment on the role of Jahn−Teller effect and impact of disturbance of magnetic ordering on selective formation of α- or β-NaMnO2 upon doping. Only in the case of divalent dopants (Cu in β-NaMnO2, Zn, and Mg), formation of Mn4+ results in shortening of the bonds with out-of-plane neighboring oxygens, thus leading to suppression of Jahn− Teller distortions. However, Cu strongly stabilizes β-NaMnO2, whereas Zn and Mg favor formation of α-NaMnO2. Therefore, in the general case, suppression of Jahn−Teller distortions does not seem to be the factor which contributes to selective

Figure 7. Mg-doped NaMnO2. 1262

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Chemistry of Materials stabilization of a specific polymorph. Moreover, because formation of Mn4+ is expected to disturb magnetic ordering of NaMnO2 structures (thereby influencing their total energies), we can also state that such disturbance of magnetic ordering is also not a key contributor to selective formation of NaMnO2 polymorphs. Overall, our findings are in a reasonable agreement with previous works on doped LiMnO2,13−15 where divalent dopants were found to stabilize the α-polymorph more strongly than the β-type structure. The exception here is Cu dopant which, according to our calculations, is trivalent in α-NaMnO2, thus favoring β-structure. Moreover, in contrast to previous work,14 we do not find Co dopants to be divalent (it is predicted to be trivalent for both types of structure, Table 1S). Although this difference in prediction of the Co oxidation state might be attributed to different alkali metal used in our study (Na vs Li in refs 15−17), another possible reason is the more rigorous computational framework employed herein, in comparison with less reliable LDA/GGA approaches utilized in refs 15−17.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Maxim Shishkin: 0000-0001-6010-7916 Shinichi Komaba: 0000-0002-9757-5905 Hirofumi Sato: 0000-0001-6266-9058 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed under a management of Elements Strategy Initiative for Catalysts and Batteries (ESICB); financial support is acknowledged.

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5. CONCLUSIONS In this work, we performed a combined XRD/DFT+U study of the effects of dopants on selective stabilization on α- and βpolymorphs of NaMnO2. On the basis of a degree of stabilization, measured as a total energy difference between the α- and β-structures, the studied dopants were divided into four groups: (1) Cu dopants, which strongly stabilize βNaMnO2; (2) Ti dopants, which strongly stabilize α-NaMnO2 prior to desodiation; (3) other TM and Al dopants, which incorporation results in a very small difference between total energies of α- and β-NaMnO2, thus allowing mixing of two polymorphs; and (4) non-TM (Mg and Zn) dopants, which also allow formation of both polymorphs with marginal enhancement of α-NaMnO2 as compared to the third group of dopants (TM and Al). We provided a detailed description of each group of dopants, analyzing the underling reasons of selective stabilization of α- or β-polymorphs as well as magnetic properties and oxidation states of all TM present in the studied structures. Our work shows that employed DFT+U methodology where U parameters are determined self-consistently using linear response approach for all TM cations, can provide adequate results, in good agreement with experimental observations. The merit of this approach is the first principles calculations of U parameters, avoiding cumbersome fitting procedure for complex materials with two types of TM. Our study also shows that pure β-NaMnO2 can be obtained via Cu-doping of NaMnO2. To the best of our knowledge, this is the first report where such pure stable β-NaMnO2 has been synthesized. The analysis of electrochemical properties of this Cu-doped β-NaMnO2 polymorph (e.g., redox potentials and stability upon cycling) as well as resistance to adverse reactions with ambient atmosphere (e.g., water molecules) is the next important step, which we plan to undertake in future studies.

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A complete list of all evaluated U parameters, magnetic moments, and the differences of energies of all doped structures for α- and β-polymorphs (ΔE) (PDF)

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