Article Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX pubs.acs.org/JACS Revealing and Rationalizing the Rich Polytypism of Todorokite MnO2 † ∇ ‡ ∇ † § † Xiaobing Hu, , Daniil A. Kitchaev, , Lijun Wu, Bingjie Zhang, Qingping Meng, ∥ ⊗ § ∥ ⊥ § ∥ ⊥ § ⊥ Altug S. Poyraz, , Amy C. Marschilok, , , Esther S. Takeuchi, , , Kenneth J. Takeuchi, , ‡ # † ⊥ Gerbrand Ceder, , ,^ and Yimei Zhu*, , † Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, United States ‡ Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States § Department of Chemistry, Stony Brook University, Stony Brook, New York 11794, United States ∥ Energy Sciences Directorate, Brookhaven National Laboratory, Upton, New York 11973, United States ⊥ Department of Materials Science and Chemical Engineering, Stony Brook University, Stony Brook, New York 11794, United States # Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ^Department of Materials Science and Engineering, University of California at Berkeley, Berkeley, California 94720, United States *S Supporting Information ABSTRACT: Polytypism, or stacking disorder, in crystals is an important structural aspect that can impact materials properties and hinder our understanding of the materials. One example of a polytypic system is − todorokite MnO2, which has a unique structure among the transition-metal oxides, with large ionic conductive channels formed by the metal oxide framework that can be utilized for potential functionalization, from molecular/ion sieving to charge storage. In contrast to the perceived 3 × 3 tunneled structure, we reveal a coexistence of a diverse array of tunnel sizes in well-crystallized, chemically homogeneous one-dimensional todorokite− MnO2. We explain the formation and persistence of this distribution of tunnel sizes thermochemically, demonstrating the stabilization of a range of coherent large-tunnel environments by the intercalation of partially solvated Mg2+ cations. Based on structural behavior of the system, compared to the common well-ordered alkali-stabilized polymorphs of MnO2, we suggest generalizable principles determining the selectivity of structure selection by dopant incorporation. − 1. INTRODUCTION accommodate the ionic diffusion of diverse cations17,20 22 such + 2+ 2+ The characterization of precise nanostructural features in as Na ,Zn , and Mg , which are alternatives to Li for the transition-metal oxides is an essential component to the design of next-generation advanced battery electrodes. understanding of emergent behavior of the materials. However, natural todorokite minerals cannot be directly Polytypism, in particular, complicates this analysis as it utilized because of numerous impurities typically found in the introduces stacking disorder that is difficult to account for structure and porous crystallites, which has motivated the ff development of well-controlled syntheses of materials with the using traditional di raction methods and presents a challenge 12,23−26 − todorokite structure. Previous work on this topic has to the derivation of structure property relationships. One fl system, which we demonstrate here to be intrinsically polytypic, established the hydrothermal or re uxing syntheses for − todorokite, yielding both pure and mixed platelet and rod is todorokite MnO2. Todorokite minerals have unique 19,20,25−27 manganese oxide structures with large tunnels that were first morphologies. recognized in continental manganese ore deposits, deep-sea Although laboratory-based synthesis of todorokite has been 1,2 reported, the precise structure of this phase has not been truly nodules, and crusts. This tunnel feature allows todorokite to 27−30 ffi host various kinds of metallic ions (e.g., Ni, Co, Zn, Cu, and resolved until now. The main di culty in the character- Mg) within the nodules and is thus regarded as a polymetallic ization of todorokite lies in its generally poor crystallinity and source,3 drawing broad attention among mineralogists ever lower spatial resolution of previously used characterization methods. The todorokite mineral is conjectured to exist as a 3 since its discovery. More recently, todorokite-based structures × ∼ × have also found many functional applications in the field of 3 tunnel structure with a tunnel size of 6.9 6.9 Å, − − − catalysts,4 7 ion exchangers,8 10 molecular sieves,11 14 and − electrodes in rechargeable batteries.15 19 In particular, the Received: March 16, 2018 todorokite structure is thought to be an ideal host material to Published: May 7, 2018 © XXXX American Chemical Society A DOI: 10.1021/jacs.8b02971 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX Journal of the American Chemical Society Article 27 constituted by triple chains of edge-sharing MnO6 octahedra formation of todorokite nodules, the associated concentration (see Figure 1a schematic). The chemical formula can be process of alkaline and transition metal elements,41,42 and the unique materials properties offered by this porous struc- ture.7,8,11,15,24 Herein, we report a detailed structural analysis of synthetic magnesium todorokite samples with a rod-like morphology using aberration-corrected transmission electron microscopy (TEM). We show localized inhomogeneous nanostructural features within a single rod at the atomic scale, revealing that Mg todorokite is not a pure 3 × 3 tunnel structure but a family of polytypic structures of many different tunnel sizes. We rationalize the formation of such a polytypic structure by means of density functional theory (DFT) calculations, contrasting the driving force for the formation of the observed nanostructural features across tunnel sizes and fi 2+− con gurations of partially solvated Mg H2O complexes. Finally, we propose a general principle predicting the emergence of polytypic phases as a result of unconstrained structural degrees of freedom in the structure selection mechanism, such as a lack of constraint on tunnel size in 2+− Figure 1. Tunnel features of the τ (p × 3) todorokite family. (a, b) todorokite MnO2 stabilized by the intercalation of Mg H2O Schematics of the 3 × 3 tunnel structure viewed along the [010] complexes. Δ Δ direction. 1 and 2 indicate the projected distance of neighboring Mn columns along the [100] direction. (c, d) Schematics of the 5 × 3 2. EXPERIMENTS AND CALCULATIONS and 1 × 3 tunnel structure, respectively. (e) Structure schematic for the intergrowth of the 2 × 3 and 4 × 3 tunnels. The atomic fractional Synthesis of Todorokite. The synthesis of the todorokite material was adapted from a previous report.43 Briefly, sodium birnessite was coordinates of Mn along the [010] tunnel direction are indicated. The · − lattice vectors a, c and their intersection angle β of different tunnel prepared from MnSO4 H2O, NaOH, and an H2O2 solution. Mg buserite was obtained by treatment of the sodium birnessite with the structures are marked. · MgSO4 H2O solution and isolation of the resultant solid. Todorokite- type MgxMnO2 was prepared by hydrothermal treatment of Mg- · · buserite in 1 M MgSO4 H2O solution. approximated as AxMnO2 yH2O, where A represents various metallic cations such as Mg2+ or Cu2+ and x and y are variables. Microstructural Characterizations. Todorokite nanorods for transmission electron microscopy (TEM) observations were prepared However, structural disorder in this phase and analogues with fi by suspending them in ethanol and then transferring to holey carbon- even larger tunnels have been identi ed in both natural coated copper grids. Electron diffraction and transmission electron todorokites and synthesized Mg todorokite by means of lattice microscopy imaging, including atomic resolution high angle annular fringe imaging based on conventional transmission electron fi − dark eld (HAADF) imaging and electron energy loss spectroscopy microscopy (TEM).23,31 35 The potential existence of these (EELS) experiments were carried out using JEOL ARM 200CF with a polytypic features complicates the common analysis of cold-field emission gun and operated at 200 kV. The microscope was todorokite as a pure 3 × 3 tunnel structure as well as the equipped with double-spherical aberration correctors (CEOS GmbH) properties inferred on the basis of this simplified structural and GIF Quantum (Gatan, Inc.) with a dual EELS system. EELS data model. were recorded in scanning TEM (STEM) mode with a convergence angle of 40 mrad and a collection angle of 90 mrad. The energy Structural disorder in natural todorokite minerals may be resolution of EELS measurement was around 0.45 eV, as determined rationalized as arising from a variety of inhomogeneities, such from the full-width at half-maximum of the zero-loss peak. All as planar defects and intergrowths with other minerals, as well background signals in the EELS spectra were subtracted using a power fi as the diversity of intercalated metallic ions occupying the law tting method. The energy positions of Mn_-L2,3 were determined tunnel.36 However, these rationalizations are not applicable to by fitting the EELS profile with a combined Gaussian and Lorentz well-crystallized todorokites grown with a single cation species function. The white line ratio of L3/L2 was calculated using the 44 such as Mg2+ in the tunnel, raising the question of whether the Pearson method with double step functions. The atomic resolution polytypism persists in such laboratory-grown samples. Surpris- HAADF image simulations