Unraveling the role of doping in selective stabilization of NaMnO2

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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 Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04394 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 2, 2018

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Chemistry of Materials

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 ‡Umicore Korea Limited, 401, Chaam-dong, Cheonan, Korea ¶Department of Applied Chemistry, Tokyo University of Science Shinjuku, Tokyo 162-8601 §Department of Molecular Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan E-mail: [email protected]

Abstract Dopants are known to modify structural, electronic, chemical and other properties of materials and 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

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battery applications. Our X-ray diffraction (XRD) 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 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 magnetic 4 properties. These properties allow application of layered oxide materials in the fields of heterogeneous catalysis (e.g. as electrocatalysts for oxygen evolution reactions 2 ) 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 activity 2 or electrochemical redox properties 7 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 capacity 8,9 while consisting of Earth abundant and low cost elements such as Na and Mn. However, a variety of structural polymorphs of Nax MnO2 , resulting in the presence of various structural configurations at different concentrations of Na 9 as well as multiple phase transi2


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tions 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/insertion 11 as monoclinic layered LiMnO2 changes into a spinel like phase 12 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 (ref. 13 and 14 respectively). Subsequently, DFT calculations also revealed that layered LiMnO2 can be stabilized by a wider range of substitutional dopants. 15 Based on these DFT calculations it has been proposed that neither the ionic size of dopants, nor the Jahn-Teller distortions, present in these oxides, play a decisive role in selective stabilization of studied structures. 15 This conclusion 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 aim at the analysis of 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 XRD spectra where only

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β-type peak is present whereas α-type peak is fully missed (see Fig. 1). To the best of over knowledge, this is the first successful attempt of 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 the future studies. In order 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 to study the electronic properties of strongly correlated materials such as NaMnO2 with higher degree of accuracy as compared to 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 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 α-NaMnO2 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

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stability of the doped structures and the impact of disturbance of antiferromagnetic ordering, caused by the substitutional dopants.

2. Experimental and computational details 2.1 Experimental details NaMnO2 and doped samples were synthesized by solid state reaction from starting materials of Na2 CO3 , Mn2 O3 , MgO, Al(OH)3 , Sc2 O3 , TiO2 , V2 O5 , Cr2 O3 , Fe2 O3 , Co3 O4 , Ni(OH)2 , Cu2 O, 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 transferred into an argon-filled glove box. The samples were cooled to room temperature in the glove box and were kept inside to avoid the contact with moisture and oxygen in air. Details of synthesis condition were described in our previous work. 26 The crystal structure of NaMn0.9 Me0.1 O2 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-polarised PBE functional. 28 The projector augmented wave (PAW) approach has been used for description of electronnuclear 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 has been performed for all studied structures, minimizing the atomic forces to at least 0.01eV/Å. For non-doped structures 5



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. 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 has been 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 have been 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 have been 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 have been 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 has been attained in all studied cases.

3. Results and discussion 3.1 Experimental results NaMn0.9 MeO2 with a variety of dopants (Me) have been synthesized by the conditions described in Experimental details. These conditions are specifically optimized to obtain high temperature meta-stable 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 since β-phase was already known as higher temperature phase than α-phase in 1971. 35 6


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Fig. 1 shows the XRD patterns of all obtained doped materials synthesised at 1050◦ C. Totally 3 main phases are found, α and β- and P’2 phase, especially seen between 12 and 18 degree in 2θ. These diffraction patterns correspond to inter-slab distance, 001, 002, and 001 directions of β-phase (s.g.Pmnm), P’2-phase (P63 /mmc) and α-phase (C2/m), respectively. 35 Cu doped and Ti doped materials show single β-phase and α-phase, respectively, while other samples have mixed-phases. Diffraction peaks of Cu doped material are all assigned to β-phase without any impurity peaks (see Supporting Information, Fig. S1) The Rietveld analysis is not yet successful and one possibility is preferred orientation which is also indicated by needle-like morphology in SEM (See Fig. S3) However, the distinct 011 diffraction peak at 23◦ indicates that Cu doped material is the closest to the pure β-phase ever 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, Fig. 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 Nax MO2 and thus, certain amount of Na ions are assumed to be lost as Na2 CO3 or Na2 O, in particular for Cr, Co, and Ni. Na loss is probably related to thermodynamic stability of co-existence of these dopants with Na manganates. In fact, P’2 phase disappeared when the materials are synthesised with 10 mol% excess Na (see Supporting Information, Fig. S2) but the trend of phase formation between α- and β-phase is not changed. Here, we will focus more on formation energy of Nax Mn0.9 Me0.1 O2 at nearly x = 1.0. Thermodynamics of α- and β-phase will be 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, phase segregation will be reported elsewhere. In terms of phase stability of α- and β-phase, we have reached very distinct conclusion that Cu doping is strongly stabilises β-phase, Ti doping stabilises α-phase, and anything else show mixed phases as well as non-doped (Me=Mn) sample.

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Figure 1: XRD patterns of obtained materials, NaMn0.9 Me0.1 O2 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.

3.2 DFT+U analysis of undoped NaMnO2 Comparison of energetics of all three types of studied polymorphs (α-, β- and P’2) have shown 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 have been determined several decades ago via XRD measurements. 35,36 In Fig. 2 we show their optimized models using the same 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). Fig.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 =3.68eV, 8


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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

Figure 2: The structures of α- and β-NaMnO2 polymorphs. The boundaries of employed computational cells are indicated by the black solid line rectangular shapes. 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 a 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 determined using selfconsistent linear response approach), are summarised in Table 1S of Supporting Information. In this 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

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α- and β-NaMnO2 are shown in Fig. 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 dopants in both polymorphs. The values of ∆E (= Eα - Eβ ), provided in Fig. 3, have been 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 parameterize 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 ∆E 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 peak in XRD spectra.

Figure 3: The relative energy ∆E for all studied dopants. The ∆E is the difference between the total energies of doped configurations of studied α and β polymorphs (∆E = Eα - Eβ ) , which acts as a measure of a prevalent stability of one polymorph over the other. In case of Ti-doped structures, the ∆E, 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).

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For Ti-doped structures we present two energies ∆E: for the case of fully sodified models (bar with unfilled cap) and for partially deintercalated 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 2N aT iO2 + gCO2 + gO2 → 2

(1)

2T iO2 + gN a2 CO3 Our calculations have shown that such reaction is exothermic by 0.67 eV with respect to release of 4 Na cations. Further 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 only lose the same number of Na cations as the number of Ti dopants. Upon Na removal all Mn cations become trivalent, whereas are Ti still retain tetravalent state. The ∆E which corresponds to such desodiated polymorphs (4 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 Fig. 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, Fig. 3 also clearly demonstrates that doping with other TM results in lower absolute value of ∆E 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 Fig. 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

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allowing formation of β-NaMnO2 ; 4) non-TM, i.e. Mg and Zn which stabilize α-NaMnO2 stronger than the third group of dopants, although with absolute value of ∆E 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 In order to elucidate the reasons of greater stabilization of β-NaMnO2 by Cu doping, we analyzed the 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 α-NaMnO2 (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, four 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 (Fig.4). Particularly, Fig.4 shows that for the 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 Fig. 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 . 12


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

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

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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 analysed atomic arrangements in the vicinity of dopants in Cu-doped α- and β-NaMnO2 (Fig. 5). In β-NaMnO2 , much longer distances between Cu2+ and oxygens of the adjacent layers (as compared to Mn-O of undoped structure, which is 2.43Å) 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 MnO 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 α-NaMnO2 structure as compared to optimized β-NaMnO2 (Fig. 5). This explains the reason as to why all TM in α-NaMnO2 (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 β-polymorph.

3.4.2 Ti doping In this section we shall 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 (4.50µB vs. 3.78µB 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

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usual 4+ oxidation state in a heating process under air or oxygen atmosphere.

Figure 6: Ti-doped NaMnO2 polymorphs. The Mn-O and Ti-O bond lengths are shown in Å. 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 (Fig. 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 (Fig. 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 (Fig. 6) could be a possible reason of 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 α-NaMnO2 . 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 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 3.78µB vs. 3.77/3.78µB as in undoped structures). With regard to dopants, we find that their magnetic moments have very similar values in both polymorphs, which clearly shows 15



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 across-layers Mn4+ -O2− -Zn2+ chains similar to Cu doping as is shown in Fig. 5. In contrast, in Mg-doped β-NaMnO2 , Mn4+ are formed in the layer adjacent to Mg2+ (Fig.7). Thus the key difference between the effect of these three dopants and Cu, is the trivalent oxidation state of Cu in α-NaMnO2 , which results in unfavorably high energy of this polymorph.

Figure 7: Mg-doped NaMnO2 .

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4. Discussion Table1 provides magnetic moments and oxidation states of Mn and dopant cations for the four types of dopants, determined by 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 3.79µB , 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 state that 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 XRD spectra of Cu doped sample. Table 1: Oxidation states of dopants and Mn cations in α- and β-NaMnO2 with magnetic moments for all involed TMs (the values of 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. Properties dopant type µ(Mn) µ(dopant) Oxidation state Cu(α) 3.79 0 Mn3+ +Cu3+ Cu(β) 3.79/3.15 0.62 Mn3+ +Mn4+ +Cu2+ Ti(α) 3.79/4.50 0 Mn3+ +Mn2+ +Ti4+ Ti(β) 3.81/4.49 0.07/0.02 Fe(α) 3.78 4.15 Mn3+ +Fe3+ Fe(β) 3.78 4.15 Mg(α) 3.79/3.13 Mn3+ +Mn4+ +Mg2+ Mg(β) 3.78/3.13

In contrast to Cu and Ti doping, other 3d TM and Al incorporated in NaMnO2 structures adopt trivalent oxidation state, which results in a very small difference in total energies of α- and β-polymorphs. For this reason, both α- and β-NaMnO2 can be present for this type of doping in full agreement with XRD measurements. 17



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 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 case of divalent dopants (Cu in β-NaMnO2 , and Zn, 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 general case, suppression of Jahn-Teller distortions does not seem to be the factor which contributes to selective 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], were divalent dopants were found to stabilize α-polymorph stronger than 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 ref. 15–17), another possible reason is a more rigorous computational framework, employed herein, in comparison with less reliable LDA/GGA approaches, utilized in ref. 15–17.

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3. Conclusions In this work we performed a combined XRD/DFT+U study of the effects of dopants on selective stabilization on α- and β- polymorphs of NaMnO2 . Based on a degree of stabilization, measured as a total energy difference between the α- and β–structures, the studied dopants have been divided into four groups: 1) Cu dopants, which strongly stabilize β-NaMnO2 ; 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 αand β-NaMnO2 , thus allowing mixing of two polymorphs; 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 the 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 the future studies.

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

Supporting Information Available A complete list of all evaluated U parameters, magnetic moments and the differences of enegies of all doped structures for α and β polymorphs (∆E) are given as a supporting information.

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Unraveling the role of doping in selective stabilization of NaMnO2

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