Quantitatively Predict the Potential of MnO2 Polymorphs as

Feb 2, 2016 - Despite tremendous efforts denoted to magnesium battery research, the realization of magnesium battery is still challenged by the lack o...
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Quantitatively Predict the Potential of MnO2 Polymorphs as Magnesium Battery Cathodes Chen Ling,* Ruigang Zhang, and Fuminori Mizuno Toyota Research Institute of North America, 1555 Woodridge Avenue, Ann Arbor, Michigan 48105, United States S Supporting Information *

ABSTRACT: Despite tremendous efforts denoted to magnesium battery research, the realization of magnesium battery is still challenged by the lack of cathode candidate with high energy density, rate capability and good recyclability. This situation can be largely attributed to the failure to achieve sustainable magnesium intercalation chemistry. In current work we explored the magnesiation of distinct MnO2 polymorphs using firstprinciples calculations, focusing on providing quantitative analysis about the feasibility of magnesium intercalation. Consistent with experimental observations, we predicted that ramsdellite-MnO2 and α-MnO2 are conversion-type cathodes while nanosized spinelMnO2 and MnO2 isostructual to CaFe2O4 are better candidates for Mg intercalation. Key properties that restrict Mg intercalation include not only sluggish Mg migration but also stronger distortion that damages structure integrity and undesirable conversion reaction. We demonstrate that by evaluating the reaction free energy, structural deformation associated with the insertion of magnesium, and the diffusion barriers, a quantitative evaluation about the feasibility of magnesium intercalation can be well established. Although our current work focuses on the study of MnO2 polymorphs, the same evaluation can be applied to other cathode candidates, thus paving the road to identify better cathode candidates in future. KEYWORDS: Mg battery, cathode, MnO2, intercalation, conversion

1. INTRODUCTION Rechargeable magnesium battery (rMB) has recently gained a lot of interest as an alternative to current Li-ion technology because of its potential to reach higher energy density in addition to the advantages such as lower cost and better safety with Mg-metal anodes.1−4 However, the realization of practical rMB highly relies on the availability of cathode that fulfils the requirements such as high energy density, rate capability and good recyclability, which unfortunately is yet discovered.4 Although Chevral phase Mo6X8 (X = S, Se) showed sustainable cyclability, its application in rMB is limited by the low voltage and capacity.1 A key gradient that contributes to the success of Li-ion battery is the intercalation chemistry, which minimizes the damages to the cathode structure through a structural invariant insertion/extraction of guest Li+ ions.5,6 Great efforts were attempted to realize the same concept of magnesium intercalation with very little success being achieved at this moment. A major challenge to realize Mg intercalation is the reduced Mg2+ mobility because of its stronger polarization to the host environment.4,7−12 Because the intercalation necessarily requires sufficient ionic mobility in the host framework, the sluggish Mg2+ mobility restricts the practical intercalation of Mg2+ to unreasonably slow rate or even kinetically prevents the intercalation.13,14 Consequently, many cathode candidates show poor capability when applied to rMB. It was recently revealed that even for classical intercalationtype cathodes used in Li-ion battery the intercalation of © XXXX American Chemical Society

magnesium may not occur in real operations. For example, the lithiation of α-MnO2 followed the intercalation path forming structural invariant α-LixMnO2.15−17 The same cathode, however, converted into amorphous mixture of magnesium and manganese oxides when used as rMB cathode.18−20 This change of reaction path was attributed to the high stability of magnesium oxide as the conversion reaction product, which thermodynamically drives the reaction away from the intercalation.9 Similar phenomenon was observed in the chemical magnesiation of α-MnO2, V2O5, and hollanditeTiO2, where magnesium oxide instead of intercalated product was identified after the magnesiation.21 It is well-known that the crystalline structure of active material plays a crucial role in determining the cathode performance. Hence by comparing the electrochemical performance of materials with different polymorphs fundamental properties associated with the electrochemistry of the cathode can be elucidated. The plentiful polymorphs of MnO2 provides a good platform for this purpose and the performances of many MnO2 polymorphs as Li-ion battery cathodes have been well documented for decades. 15,22 However, the exploration of MnO2 polymorphs as rMB cathodes was only carried out very recently.18,20,23−26 Besides α-MnO2, polymorphs including β-, γ-, and δ-phases exhibited quite similar Received: November 25, 2015 Accepted: February 2, 2016

A

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Figure 1. Crystal structure of (a) ramsdellite-, (b) CaFe2O4-analogous, (c) hollandite-, and (d) spinel- MnO2. Oxygen ions are shown as red spheres, whereas the blue octahedra represent MnO6. The one-dimensional open channels are marked with green dashes. rutile chains”, which are interconnected through vertex sharing oxygen. Between interconnected chains is the open space extended along one direction forming one-dimensional channels, which is capable to accommodate the insertion of various guest species. Based on the number of MnO6 octahedra that surround the channel, R-MnO2 and α-MnO2 have a 2 × 1 rectangular and 2 × 2 square channel, respectively, while CF-MnO2 has a triangular shaped channel, as marked with green color in Figure 1. The spinel-phase is composed of face centered close-packed oxygen with Mn and Mg filling the octahedral and tetrahedral voids, respectively. The framework of spinel-phase provides a three-dimensional connected diffusion network, through which cations migrate between neighbored octahedral and tetrahedral sites. To study the magnesiation of MnO2, we performed DFT calculations with the Vienna ab initio Simulation Package (VASP) using projector augmented waves (PAW) pseudopotentials and the exchange-correlation functionals parametrized by Perdew, Burke, and Ernzerhof for the generalized gradient approximation (GGA).28−30 Numerical convergence to less than 3 meV per MnO2 unit was ensured by using cutoff energy 500.0 eV and appropriate Gamma centered k-point mesh with the density of at least 0.04 Å−1. We use GGA+U method to treat the static correlations by introducing a Hubbard type potential to describe the d-part of the Hamiltonian. The value of U is set to be 3.9. This value works well for a number of Mn oxides in previous reports.16,31,32 Climbing nudged elastic band method was applied to calculate the energy barrier for the migration of Mg, where the calculations were initiated with five images linearly connecting the initial and final states.

discharge−charge profile with characteristically rapid capacity fading in the cycling, which strongly indicated the conversion instead of intercalation reaction during the magnesiation of these polymorphs.20,23 On the other hand, using spinel-MnO2 prepared from acid-treated LiMn2O4, Kim et al. observed the intercalation of Mg in aqueous solution.24 Spinel-MgMn2O4, however, cannot extract Mg when treated with oxidizing agent such as NO2BF4.27 The mechanism behind these different observations has yet been understood, which reflected the difficulty to analyze the complicated experimental behavior in the cell. These results highlight the complexity of magnesiation chemistry as well as the lack of fundamental knowledge that remains as a great hurdle for future research activities. Compared to the experimental approach, density functional theory (DFT) based calculations have certain advantages to break down the complex electrochemical magnesiation into physical mechanisms and provide quantitative assessment of the thermodynamics and kinetics. In current work, we performed a detailed analysis about the magnesiation chemistry of MnO2 polymorphs. Rather than looking for properties such as voltages and capacities of these materials as rMB cathodes, we aim to provide a quantitative assessment about the feasibility of Mg intercalation into the hosts. In good agreement with previous reports,20,23 our study showed that the magnesiation of α-MnO2 and ramsdellite-MnO2 follows the conversion reaction and hence they are prone to poor cyclability characterized by rapid capacity fading. While spinel-MnO2 seems to be promising to intercalate Mg,24 it is important to use nanosized material with reduced diffusion length to improve intercalation kinetics. On the basis of these results, we demonstrate that besides the sluggish Mg mobility other performance-limiting factors including product stability and structural integrity are also important to achieve sustainable intercalation. Our study provides the first example of a comprehensive analysis about the magnesiation behavior for a group of distinct polymorphs of one compound. The same evaluation can be applied to other compounds and thus pave the road for future research in this field.

3. RESULTS 3.1. Mobility of Mg2+ in MnO2 Polymorphs. Because it is widely assumed that the major hurdle to achieve Mg insertion lies with the sluggish migration of strong polarizing Mg2+, we start from evaluating the mobility of Mg2+ ions in MnO2 hosts. The activation energy barriers were calculated for the selfhopping of Mg2+ in fully charged MnO2, as well as the selfhopping of vacancy in fully discharged Mg0.5MnO2. These two limits are typically considered as the bound of the diffusion barriers in the cathode host.10 The energy profile along the diffusion path is shown in Figure 2. Surprisingly, we find that the mobility of Mg is not always extremely low, as several calculated barriers fall in the range of 0.3−0.6 eV, which is sufficient for the practical intercalation at reasonable rate.10 Even for the diffusion in close-packed oxygen lattice in spinelMnO2, the barriers, 0.50 eV for vacancy hopping and 0.78 eV for Mg hopping, may still be achievable providing necessary method to decrease the diffusion length. Our results are in fully consistent with Liu et al’s report, which suggested the potential of spinel-MnO2 as rMB cathode with sufficient Mg mobility.10 It is also interesting to note that the diffusion barrier in spinelMnO2 is higher than that in spinel-Mg0.5MnO2 by 0.28 eV. A similar trend was reported by Liu et al.10 and in the study of Li diffusion in spinel compounds.33

2. METHODS MnO2 forms a variety of polymorphs with plentiful chemistry for different applications. In our current work, the magnesiation behavior was comparatively studied for four distinct MnO2 polymorphs: ramsdellite (R-, a polymorph close to γ-phase), (Mg)Mn2 O4 isostructural to (Ca)Fe2O4 (CF−), hollandite (α-MnO2) and spinel phase. These polymorphs were either experimentally or theoretically studied as rMB cathode in earlier reports,8,20,23,27 therefore providing a good platform for current study. Figure 1 shows the crystal structures of MnO2 polymorphs considered in current study. In R-, CF- and αphases, edge-shared distorted MnO6 octahedra form so-called “doubleB

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Figure 3. (a) Activation energy barriers for Mg2+ diffusion in different configurations. Solid diamonds represent the diffusion into a preoccupied cavity while the open diamonds represent the diffusion into an open cavity. (b) Schematics of the diffusion configurations shown in (a). The solid and open circles are occupied and unoccupied site in cavities, respectively, while the red arrow denotes the hopping direction. The lower case ∈ is the small perturbation from the ideal stoichiometric composition.

Figure 2. Energy profile for the migration of Mg at dilute concentration in MnO2 (open symbols) and vacancy in Mg0.5MnO2 (solid symbols).

Although the diffusion in spinel-MnO2 via a classical tetrahedral-octahedral-tetrahedral path has been extensively studied in the literatures,7,33 the less analyzed diffusion in structures with one-dimensional channel, which in our case are R-, CF-, and α-phases, deserves additional discussion here. The anionic frameworks of these structures are typically deficient from close-packing, forming open space to allow the insertion of cationic species.15 Consequently, if the open space (cavity) is sufficiently large, it is possible to insert multiple guest cations in one cavity. In our study, α-MnO2 is capable of accepting the insertion of two Mg2+ occupying two of the four 8h sites in one cavity, whereas the insertion of more than two Mg2+ greatly destabilizes the structure.9 Only one Mg2+ can be inserted into the cavity of CF-MnO2, although the Mg−O distance seems to be appreciably larger (2.20−2.27 Å in a triangular prism) than that in MgO (2.12 Å). This weakened Mg−O bonding and the unique cooperative one-dimensional migration in the channel may explain the low diffusion barrier in CF-phase.8 In RMg0.5MnO2, the most stable configuration has inserted Mg2+ occupies one of the two 8d sites alternatively along the channel direction (see the Supporting Information for more details). The configuration of both 8d sites occupied in one cavity of the R-phase is energetically less stable. In the study of Li-ion battery cathode it is known that the diffusion behavior of Li+ highly depends on the concentration of Li+ in the lattice.33 Considering the electrostatic interaction between Mg2+ ions is not shielded by anions in the open channel structure, it is reasonable to hypothesize that the diffusion of Mg2+ is also strongly affected by Mg concentration.13,26 Hence we categorize the diffusion into two groups: the migration of Mg toward a cavity occupied by another Mg or toward an empty cavity. Figure 3 summarizes the calculated barriers with this categorization. Apparently, the existence of another Mg in the cavity strongly raises the diffusion barrier as if the diffusion is self-blocked (case D and I in Figure 3).9 Sucha “self-blocking” phenomenon significantly limits the mobility of Mg2+ along one-dimensional channel at high Mg concentration if the open cavities cannot form a percolating channel through the cathode particle.34 Additional evidence to support the self-blocking mechanism is obtained by studying the migration in R-Mg0.5MnO2. In this configuration, Mg occupies 8d site alternatively along the channel direction. As shown in Figure 4, the direct hopping

Figure 4. Two diffusion paths in R-Mg0.5MnO2. The energy profile for the direct migration of Mg as illustrated in top inset and for the sequent migration as illustrated in bottom insert is shown in black and red color, respectively.

moves one Mg2+ into a preoccupied cavity and thus forms largely unstable intermediate state. As a result the calculated energy barrier is 1.40 eV, indicating this migration path is indeed blocked. An alternative migration path includes the migration of Mg into an empty cavity followed by a sequent migration to the neighbored 8d site in the same cavity. The barriers for these two steps are 0.49 and 0.47 eV, respectively, much lower than that for the direct migration. These largely different barriers in the same lattice conclusively eliminate the possibility that the change of diffusion behavior between two groups in Figure 3 can be attributed to the structural deformation after magnesiation as being discussed later. Although more detailed study about diffusion behavior at different Mg concentrations are interesting for future work, with current data, it is sufficient to conclude that Mg2+ is not immobile in these MnO2 polymorphs if a percolating diffusion path can be established without any blocking from the inseted Mg itself. C

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ACS Applied Materials & Interfaces 3.2. Thermodynamic Reaction Pathway. Thermodynamically the magnesiation should follow the route that minimizes the free energy of system. An effective way to establish the minimum free energy path is to construct ternary Mg−Mn−O phase diagram and move the composition from MnO2 to MgxMnO2 (Figure 5a), which forms products of

The thermodynamic driven force behind the intercalation and conversion reaction is the free energy change for the reaction x MgO + x Mn2O3 + (1 − 2x)MnO2 → MgxMnO2

Here a positive ΔG of reaction 3 suggests the conversion path is more thermodynamically preferable, whereas negative ΔG suggests the preference on the intercalation path. Figure 5b shows the calculated ΔG at x = 0.5 using 1:1 MgO and Mn2O3 as the reference state. For comparison, we also included the energy to form an amorphous mixture of magnesium and manganese oxides.9 The most stable phase of Mg0.5MnO2 is spinel compound, which in fact is the global minimum at this composition as revealed by the phase diagram (Figure 5a). The polymorphs of R-, CF-, and α-Mg0.5MnO2 have positive values of ΔG, which further suggests the thermodynamic instability of the intercalated product against decomposition. The calculated ΔG of α-MnO2 is higher than that to form amorphous mixture of oxides, which agreed excellently with the experimental observation that an amorphous layer formed after discharge of α-MnO2.18,19 Several factors may affect the thermodynamic reaction path. First, we neglected the configurational entropy, which affects the free energy especially for the amorphous products. Second, the current calculations did not consider the free energy variation under applied stress. For intercalation involving large change of lattice parameters or unit-cell volume, the stress can contribute considerable free energy.35 Finally, the intercalation reaction necessarily requires the structural integrity of the crystalline lattice. However, the insertion of guest Mg2+ may cause strong deformation and destruct the host framework. Hence the analysis of the structural deformation caused by the insertion of magnesium provides additional information about the reaction path. 3.3. Quantitatively Assess Structural Deformation. To quantitatively evaluate the effect of Mg insertion on the crystalline structure of MnO 2 , we define a structural deformation score (SD) as

Figure 5. (a) Phase diagram of Mg−Mn−O2 ternary system. The red line shows the pathway for the magnesiation of MnO2. (b) Free energy to decompose Mg0.5MnO2 into crystalline mixture of 0.5MgO +0.5Mn2O3.

⎛ xij′ − xij ⎞2 ⎟⎟ SD = ∑ ∑ wij⎜⎜ x ⎝ ⎠ ij i j=1 ni

Mg2Mn3O8 and Mn2O3 (x ≤ 2/7), MgMnO3 (2/7 ≤ x ≤ 1/3) and spinel MgMn2O4 (1/3 ≤ x ≤ 1/2). Except the spinel phase, the intercalated products of R-, CF- and α-phases are not thermodynamically stable. However, it is well-known that the phase transformation is usually associated with large kinetic barrier to adjust the framework and thus requires additional energy input. The topotactic intercalation is kinetically more favorable due to its small destruction to the host framework, even if it is not the most thermodynamically reaction. A loose estimation of the reaction path for the magnesiation is to compare the intercalation with the conversion reaction.9 The classical intercalation forms structural invariant MgxMnO2 through x Mg 2 + + 2x e− + MnO2 → MgxMnO2

(4)

Here xij (xij′ ) is a measurable quantity before (after) the intercalation of Mg. ni is the total number of xij in category i. wij is the weight of each xij, which we take as 100/ni without any prior knowledge. The quantity xij considered in current work includes the Mn−O bond lengths and O−Mn−O angels, the size and joint angle of the open space, and the lattice parameters of each polymorph (see the Supporting Information for more details). Therefore, these three categories of quantities quantitatively measure the distortion of the building block of the host framework (MnO6 octahedra), the deformation of the interstitial sites and diffusion channel, and the change imposed on the entire lattice, respectively. Compared to traditional approach that only looks for the change of lattice parameters, the deformation score defined in current work clearly contains more instructive information about the influence of cation insertion to the host lattice. Indeed, as we will show below, the insertion of Mg strongly affects the local geometry around the interstitial site, which must be taken into account when evaluating the structural integrity of these materials.

(1)

The conversion reaction, on the other hand, forms mixed oxides of MgO and Mn2O3 x Mg 2 + + 2x e− + MnO2 → x MgO + x Mn2O3 + (1 − 2x)MnO2

(3)

(2) D

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As shown in Figure 6, the local deformation of the channel contributes significantly to the total deformation score, which directly results from the strong polarization of bivalent Mg2+ to the local environments. Especially the open channels in R-, CF-, and α-MnO2 experience appreciable expansion after the insertion of magnesium as listed in Table 2. The largest

Figure 6 shows the calculated deformation scores for four MnO2 polymorphs. The largest SD is recorded for R-MnO2,

Table 2. Sizes of Open Channels in MnO2 and Mg0.5MnO2 channel size phase

channel shape

MnO2

Mg0.5MnO2

RCFα-

2 × 1 rectangular triangle 2 × 2 square

4.73 × 2.38 4.36 × 4.72 × 4.72 4.62

5.08 × 2.65 4.53 × 5.07 × 5.10 4.72 × 4.86

expansion is recorded for the short side of the channel in Rphase, which is expanded by 11%. Visible deformation on the shape of the open channel is also observed for R- and α-phases (Figure 7). In R-phase, the joint angle between two neighbored

Figure 6. Deformation score of intercalated Mg0.5MnO2 for different polymorphs.

followed by α-MnO2, suggesting large structural instability of these two phases for Mg insertion. Indeed, in R-MnO2 Mn−O bond is expanded as much as to 2.6 Å after the insertion of Mg, which signals the collapse of structural integrity necessary to maintain the reversible intercalation. CF-MnO2 and spinelMnO2 have relatively small structural deformation, indicating better structural stability of these two polymorphs to accommodate Mg insertion. The scores for the distortion of MnO6 octahedra are quite similar for all four polymorphs, indicating that the distortion of MnO6 is not sensitive to the crystalline structure of MnO2. Another way to quantitatively assess the deformation of an octahedron is to calculate the parameter κ8 κ=

1 6

6

∑ i=1

(R i − R̅ )2 R̅ 2

(5)

where R and R̅ are the distance and average distance of Mn−O bonds, respectively. Table 1 reports the average Mn−O bond Table 1. Average Mn−O Bond Length and Octahedron Distortion Parameter (κ) in MnO2 and Mg0.5MnO2 phase

composition

Mn−O bond length (Å)

κ ( × 103)

R-

MnO2 Mg0.5MnO2 MnO2 Mg0.5MnO2 MnO2 Mg0.5MnO2 MnO2 Mg0.5MnO2

1.916 2.061 1.932 2.046 1.906 2.055 1.946 2.064

0.051 5.9 0.043 2.0 0.077 4.9 0 5.7

CFαspinel-

Figure 7. Deformation of the open channel from MnO2 (left) to Mg0.5MnO2 (right) for (a, b) R-, (c, d) CF-, and (e, f) α-phase.

MnO6 blocks (θR in Figure 7b) changes from 172 to 156°, whereas in α-phase the joint angle (θα in Figure 7f) changes from 90 to 109°. Consequently, the scores of the channel deformation are significantly larger for R- and α-phases than those for CF- and spinel-phases. Such large degree of framework deformation is likely to generate instability of the cathode host and damage the cycling stability. In severe cases, the strong framework deformation can induce self-relaxation of the structure and cause the failure of the cell.36 Figure 8 shows the change of lattice parameters at the compositions of Mg0.25MnO2 and Mg0.5MnO2, corresponding to the reduction of half Mn and all Mn ions, respectively. All these polymorphs show appreciable volumetric expansion after Mg insertion similar to the trend found for other cathodes.37 The smallest variation of lattice parameters is recorded for CF-

length and calculated κ value for MnO2 and Mg0.5MnO2. The insertion of Mg expands MnO6 octahedron with longer Mn−O bond length due to the larger size of reduced Mn ions. The average distortion parameter in pristine MnO2 is in the order of 10−5, while after Mg intercalation it increases by 2 orders of magnitude because of the Jahn−Teller distortion of Mn3+O6 octahedron. Such distortion is tightly associated with the valence state of Mn ions. Consequently, the variation in κ for different polymorphs is not significant, which is consistent with the deformation score presented in Figure 6. E

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DISCUSSIONS In the current work, we quantitatively evaluate the reaction path, the structural deformation caused by the insertion of Mg, and the diffusion barrier of Mg, all of which are critical to achieve reversible Mg insertion/extraction in practical operations. In order to visualize these properties we introduce a radar graph, which plots the calculated quantities of free energy change of reaction 3 (ΔG), deformation score (SD) and the diffusion barrier (Eb) in a triangle as shown in Figure 9.

Figure 8. Variation of lattice parameters (a, b, c) and unit-cell volume (V) of MnO2 polymorphs after magnesiation to Mg0.25MnO2 (open symbols) and Mg0.5MnO2 (solid symbols).

Figure 9. Triangular radar graph to quantitatively assess the free energy change (ΔG), deformation score (SD), and the diffusion barrier (Eb) for the magnesiation of MnO2 polymorphs.

phase, which has the lowest deformation score in this category, while both R- and α-phases have significantly larger deformation. The change of lattice constants can be highly asymmetric. For instance, in R-phase, the short side of the channel experiences the largest extension after the insertion of Mg. Consequently, c-axis of R-MnO2 expands by 10% at RMg0.25MnO2, whereas the a-axis (longer side of the channel) remains nearly unchanged in this stage. From R-Mg0.25MnO2 to R-Mg0.5MnO2, the a-axis extends appreciably, whereas the c-axis only slightly increases. As a result of this severe expansion, the unit-cell volume of R-MnO2 increases by 12.0% at RMg0.25MnO2, and 21.4% at R-Mg0.5MnO2, which very likely leads to poor reversibility of Mg intercalation in R-MnO2. The asymmetric variation of lattice parameters results in the change of the crystalline symmetry of α-MnO2 and spinelMnO2. After the magnesiation exceeding the concentration of Mg0.125MnO2, the tetragonal lattice α-MnO2 is distorted to orthorhombic with the expansion of a-axis and shrink of b-axis. The details of this tetragonal-to-orthorhombic deformation can be found in our earlier report.9 For the spinel-phase, spinelMnO2 has a cubic structure while spinel-Mg0.5MnO2 is tetragonal with c/a ratio of 1.12. Deep analysis shows that at the composition of Mg0.25MnO2 is energetically 29 meV more stable than tetragonal phase. At the composition of Mg0.5MnO2 the tetragonal distorted structure is 276 me V more stable. Therefore, from spinel-MnO2 to Mg0.25MnO2 the cubic structure preserves while the cubic-to-tetragonal distortion occurs after the insertion of Mg exceeds the composition of spinel-Mg0.25MnO2, where the concentration of Jahn−Teller active Mn3+ reaches the critical value (50%) to induce cooperative change of the entire lattice. This phenomenon is similar to the cubic-to-tetragonal distortion from spinelLiMn2O4 to LiMnO2.22

Apparently, an inner triangle suggests better probability to realize the practical intercalation and vice versa. Such triangular assessment easily highlights crucial challenge associated with a particular cathode candidate for rMB, thus providing instructive information for future research activities. Below, we use the study of MnO2 polymorphs as an example to make instructive discussion based on Figure 9. From Figure 9, the polymorphs that are better to achieve Mg intercalation are the spinel-phase and CF-phase. Spinel-phase has a small structural deformation and the discharged spinelMg0.5MnO2 is thermodynamic stable at this composition. Note that the structural deformation score is evaluated at the composition of spinel-Mg0.5MnO2, which suffers from large Jahn−Teller deformation of the lattice. If the capacity is sacrificed by limiting the intercalation to Mg0.25MnO2, whose structure still preserves the cubic symmetry, the value of SD is further reduced. Therefore, spinel-MnO2 is an interesting cathode that deserves more comprehensive studies. This conclusion agrees well with the experimental verification of the insertion of Mg2+ into spinel-MnO2.24 It should be noted that the diffusion barrier in spinel-MnO2 is slightly higher than the desired range,10 which suggests that necessary nanoengineering to reduce the diffusion length is crucial to achieve good electrochemical performance of the spinel cathode. Indeed, in Kim et al’s experiment the spinel-MnO2 was prepared by acid treatment of LiMn2O4, which created nanoflakes with shorter diffusion lengths to enhance the subsequent Mg insertion.24 On the other hand, the practical insertion/extraction may not be observable if the cathode particle is too large for Mg to diffuse, which explained Knight et al.’s observation that Mg was not chemically extracted from bulk MgMn2O4 particles.27 Another interesting cathode for rMB is the CF-MnO2. It has the smallest deformation score and the fastest Mg mobility. Although the discharged CF-Mg0.5MnO2 is not thermodynamiF

DOI: 10.1021/acsami.5b11460 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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in this work, R-MnO2 and α-MnO2 are predicted to be conversion-type cathodes with unstable intercalation products and large structural deformation after Mg insertion, whereas spinel-MnO2 and CF-MnO2 are shown to be promising candidates for Mg intercalation. It is necessary, however, to use nanosized spinel-MnO2 with decreased diffusion path to compensate the slightly higher diffusion barrier. These conclusions are in good agreement with current experimental observations. Furthermore, our study suggests that the sluggish Mg mobility is not the only challenge that prevents the realization of intercalation chemistry. The minimization of structural deformation caused by Mg insertion as well as improving the stability of the intercalated products is also crucial to achieve practical Mg intercalation. Although our current work focuses on the study of MnO2 polymorphs, the same evaluation can be applied to other cathode candidates thus paves the road to identify better cathode candidates with good reversibility and high sustainability in future.

cally stable against decomposition, the small deformation is beneficial for a topotactic intercalation by keeping the structural integrity. The biggest challenge to apply this cathode in rMB is the synthesis of the desired material. As being discussed in our earlier work, the CF-phase is typically obtained at pressures in the several GPa range.8 Hence attempts to obtain it with large quantity and good quality remain a hurdle to experimentally test this material in Mg cell. Although CF-MnO2 showed interesting properties as Li-ion and Na-ion battery cathode,38,39 its performance in rMB remains to be explored. R-MnO2 and α-MnO2 are both classical intercalation-type cathodes in Li-ion battery.15 However, based on current results we predict that the magnesiation of these two materials should prefer the conversion path instead of the intercalation in practical experiments. Because the final products of the conversion reaction are the same for these two polymorphs, their voltage profiles should be the same except a small shift due to the difference energy of starting MnO2 material. This prediction is consistent with the experiments, which showed very similar voltage profiles and rapid capacity fading for these two phases during cycling tests.20 For α-MnO2, the major reason is attributed to the instability of the intercalated product, which thermodynamically drives the reaction away from the intercalation route.9 Although the stability of R-Mg0.5MnO2 is improved compared to α-Mg 0.5 MnO 2 , it is still not thermodynamically stable against decomposition. Additionally, the small channel of R-MnO2 is not large enough to accommodate the insertion of Mg, whose short side experiences unaffordable expansion after magnesiation. Even at lower Mg concentration such as Mg0.25MnO2, the structural deformation has already become appreciable to damage the framework integrity, resulting in the failure of the intercalation. The mobility of Mg2+ in R-MnO2 and α-MnO2 in Figure 9 seems to be sufficient to support the intercalation at reasonable rate at least at low Mg concentrations. However, no evidence of Mg insertion was found in the experiments after ex-situ analysis of the discharge product,19 although we cannot completely rule out that the intercalation actually happens at some level in the early stage of magnesiation.9 The current data combined with experimental results unambiguously demonstrates that the magnesiation chemistry can be significantly different with the lithiation for the same cathode host. Besides the kinetic constraint brought by slow Mg migration, other factors including stronger distortion to the host structure and undesirable conversion reaction also strongly restrict the success of intercalation chemistry in rMB cathode. Thus, the bold assumption that classical intercalation-type Li-ion battery cathode should work for Mg intercalation once the mobility of Mg is sufficient must be reconsidered. With the application of the radar graph such as Figure 9 the feasibility of Mg intercalation in given cathodes can be more explicitly analyzed. We anticipate better cathode candidates with good reversibility and high sustainability can be clarified in future research using this computational approach.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b11460. Structure of magnesiated R-MnO2 and details about calculating the deformation scores (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1-7349950279. Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSIONS In current work, the magnesiation of four distinct MnO2 polymorphs are systematically studied by calculating the reaction free energy, structural deformation associated with the insertion of magnesium, and the diffusion barriers. We show that by evaluating these properties a quantitative assessment about the feasibility of intercalation reaction can be wellestablished. Among four distinct MnO2 polymorphs considered G

DOI: 10.1021/acsami.5b11460 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.5b11460 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX