Unveiling the Structural Evolution of Ag1.2Mn8O16 under

Dec 17, 2017 - XAS data of the cycled materials indicate that increased Mn–Mn atomic distances are observed for samples that demonstrated capacity d...
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Cite This: Chem. Mater. 2018, 30, 366−375

Unveiling the Structural Evolution of Ag1.2Mn8O16 under Coulombically Controlled (De)Lithiation Jianping Huang,† Xiaobing Hu,‡ Alexander B. Brady,§ Lijun Wu,‡ Yimei Zhu,‡ Esther S. Takeuchi,*,†,‡,§ Amy C. Marschilok,*,†,§ and Kenneth J. Takeuchi*,†,§ †

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 ‡

S Supporting Information *

ABSTRACT: MnO2 materials are considered promising cathode materials for rechargeable lithium, sodium, and magnesium batteries due to their earth abundance and environmental friendliness. One polymorph of MnO2, αMnO2, has 2 × 2 tunnels (4.6 Å × 4.6 Å) in its structural framework, which provide facile diffusion pathways for guest ions. In this work, a silver-ion-containing α-MnO 2 (Ag1.2Mn8O16) is examined as a candidate cathode material for Li based batteries. Electrochemical stability of Ag1.2Mn8O16 is investigated through Coulombically controlled reduction under 2 or 4 molar electron equivalents (e.e.). Terminal discharge voltage remains almost constant under 2 e.e. of cycling, whereas it continuously decreases under repetitive reduction by 4 e.e. Thus, detailed structural analyses were utilized to investigate the structural evolution upon lithiation. Significant increases in lattice a (17.7%) and atomic distances (∼4.8%) are observed when x in LixAg1.2Mn8O16 is >4. Ag metal forms at this level of lithiation concomitant with a large structural distortion to the Mn−O framework. In contrast, lattice a only expands by 2.2% and Mn−O/Mn-Mn distances show minor changes (∼1.4%) at x < 2. The structural deformation (tunnel breakage) at x > 4 inhibits the recovery of the original structure, leading to poor cycle stability at high lithiation levels. This report establishes the correlation among local structure changes, amorphization processes, formation of Ag0, and long-term cycle stability for this silver-containing α-MnO2 type material at both low and high lithiation levels.

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cycles.12 Incorporating Li2O in the tunnel improved capacity retention over 20 cycles.23 Naturally occurring α-MnO2 type materials usually contain stabilizing ions (i.e., K+, Ba2+, Sr2+) within the 2 × 2 tunnels,24 and several publications have reported electrochemistry of the NH4+, K+, and Rb+ stabilized α-MnO2 in Li batteries.25−30 The electrochemical lithiations of NH4+, K+, and Rb+ stabilized α-MnO2 have shown 80−90% utilization of Mn4+ (188−218 mAh/g) in the reduction process.25 Thus, a suitable tunnel cation has the potential to stabilize the tunnel structure without impairing lithium ion insertion. Recently, a silver-containing α-MnO2 was investigated as a possible cathode material for a Li battery.20,31,32 It was reported that the Ag+ stabilized α-MnO2 (silver hollandite) delivered an initial capacity of 180 mAh/g, and the capacity was influenced by surface oxygen vacancies, which create additional diffusion pathways for Li ions.20,32 It is notable that the dimensions of the 2 × 2 tunnel contract with

INTRODUCTION LiCoO2 has been widely used as a cathode material in Li-ion batteries due to its stable electrochemical reversibility and relatively high operating voltage (∼4 V).1,2 However, the scarcity and high cost of Co hinder large-scale application of LiCoO2 based batteries, such as in electric vehicles (EV) and hybrid electric vehicles (HEV).3,4 Thus, Mn based materials have received attention in recent years because of their high earth abundance and environmental friendliness.5−11 MnO2 materials have several polymorphs (for example, α, β, δ, λ,), and many of these materials have been studied in the context of energy storage devices, such as Li/Na/Mg batteries,12−17 lithium−air batteries,18,19 and supercapacitors.20 Specifically, α-MnO2 is a tunnel structured material, which is formed by interconnection of edge- and corner-sharing MnO6 octrahedra. The 2 × 2 tunnels (4.6 Å × 4.6 Å) in the structure are able to provide facile diffusion pathways for ions.21,22 Thus, this type of material is considered a promising cathode material for rechargeable batteries. Previous reports indicate α-MnO2 delivered an initial capacity of 220 mAh/g when discharged to 2.0 V; however, only ∼140 mAh/g was maintained after 10 © 2017 American Chemical Society

Received: August 28, 2017 Revised: December 15, 2017 Published: December 17, 2017 366

DOI: 10.1021/acs.chemmater.7b03599 Chem. Mater. 2018, 30, 366−375

Article

Chemistry of Materials

lithiation levels are successfully revealed by these techniques. Electrochemical reduction is controlled at 2 and 4 electron equivalents (per Ag1.2Mn8O16) to examine cycle stability under low and high lithiation levels. XAS data of the cycled materials indicate that increased Mn−Mn atomic distances are observed for samples that demonstrated capacity decrease with cycling.

central silver cation due to its more covalent nature in contrast to other cations (such as K) where the tunnel dimensions expand due to more ionic character.32 Reversible lithium ion insertion/deinsertion is of great importance to battery cycle life, yet there are few systematic investigations on structural stability of α-MnO2 type materials upon (de)lithiation.28,33−36 An in situ synchrotron lab X-ray diffraction (XRD) study was used to analyze the first discharge/charge process of a Li/α-MnO2 cell.34 During discharge, the (110), (200), and (211) diffraction peaks of α-MnO2 slightly shifted to lower angles, and showed significant broadening and intensity decreases. However, the diffraction peaks did not recover to their original positions (nondischarged state) and stayed at the lower angles after charging at 4.4 V.34 The results indicated an irreversible lattice expansion after lithiation, but specific lattice parameter changes were not determined. Density functional theory (DFT) calculations have been used to simulate the structural changes of α-MnO2 after lithiation.33,35 The theoretical papers have shown that the material has the largest structural deformation after 0.5 Li ion (per MnO2) insertion due to the Jahn−Teller distortion33,35 where the ratio of lattice a/b in LixMnO2 showed only 0.5% increase from MnO2 to Li0.25MnO2 with 19% increase at Li0.5MnO2.33 An in situ XRD analysis on K-α-MnO2 refined the lattice parameters upon lithiation/delithiation. The authors indicated that lattice a of K-α-MnO2 increased from 9.97 to10.04 Å after discharge to 1.5 V, and then decreased to 10.01 Å after charge to 4.2 V.28 Although the irreversible lattice expansion was verified in this study, the relatively small unit cell change (0.7%) fails to explain the capacity losses of this material during electrochemical cycling in Li batteries.27,28 A recent in situ transmission electron microscopy (TEM) study on K-αMnO2 observed that the lattice a increased from 9.84 to10.68 Å after 0.5 Li-ion intercalation (per K0.25MnO2).36 However, only highly lithiated materials (>0.5 Li per K0.25MnO2) could be measured by this in situ lithiation technique, due to lack of quantitative control of the lithiation process.36 Thus, the structural change upon lithiation of α-MnO2 type materials still suffers from lack of experimental results with Coulombically controlled levels of reduction. Notably, the correlation between long-term cycle stability and structural change has not been elucidated for this class of materials. It is generally accepted that the Jahn−Teller distortion of Mn3+ of an electrochemically active material can impact its structure as well as its cycle life. However, the impacts of Jahn−Teller distortion vary significantly from material to material. For example, in the case of the layered NaFeO2 material, it is hypothesized on the basis of DFT that the Jahn− Teller distortion of Fe4+O6 may assist sodium diffusion in NaFeO2 enabling higher performance of Na ion batteries.37 In the case of the spinel LiMn2O4, the material can be reversibly delithiated to Mn2O4.38 Thus, it is important to correlate structural change with electrochemical properties for each material class. In this paper, we use multiple techniques, including synchrotron X-ray diffraction (XPD), X-ray absorption spectroscopy (XAS), and transmission electron microscopy (TEM), to investigate Ag1.2Mn8O16 materials under Coulombically controlled reduction conditions. XAS is a characterization technique which provides both oxidation state information as well as detailed structural information. The changes of lattice parameters and atomic distances of Ag1.2Mn8O16 at different

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EXPERIMENTAL SECTION

Synthesis and Characterization. Silver hollandite was prepared using an ambient pressure reflux based reaction previously described.31 Silver permanganate (AgMnO4) and manganese sulfate monohydrate (MnSO4·H2O) were used as the starting reagents for the synthesis. After synthesis, the samples were annealed at 300 °C for 6 h. X-ray diffraction (XRD) patterns were collected using a Rigaku SmartLab X-ray diffractometer (Cu Kα radiation, λ = 1.5406 Å) with Bragg−Brentano configuration. Thermogravimetric analysis (TGA) was conducted with a TA Instruments SDT Q600 under nitrogen gas. Elemental composition was determined using inductively coupled plasma-optical emission spectroscopy (ICP-OES) with a Thermo Scientific iCAP 6300 series spectrometer. Electrochemical Characterization. Coin type cells were assembled using the cathode, Li foil, polypropylene separators, and 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) at a volume ratio of 3:7 electrolyte in an argon filled glovebox. For ex situ XAS measurements, cathodes contained silver hollandite, graphite, carbon black, and polyvinylidene fluoride (PVDF) on aluminum foil. Electrochemical reduction was performed at a current density of 10 mA/g. Galvanostatic cycling tests were controlled to 2 (64 mAh/g) and 4 (127 mAh/g) electron equivalents (e.e.) of discharge. The charge voltage was limited to 3.5 V. For ex situ diffraction measurements, silver hollandite powder was mixed with carbon black and electrochemically reduced for 1.2, 2.5, and 4.7 molar e.e. at a current density of 1.2 mA/g prior to analysis. Ex Situ XPD. X-ray powder diffraction (XPD) was performed at the NSLS-II XPD 28-ID-2 beamline at Brookhaven National Laboratory. Lanthanum hexaboride powder was used as a standard. Powder samples were sealed in polyimide capillary tubes and rotated for measurements to reduce preferred orientation effects. The beam was calibrated to a wavelength of 0.1899 Å, and detection was performed using a 16-in. amorphous silicon digital flat panel fitted with a CsI scintillator. The Fit2D software was used to convert the two-dimensional data to a one-dimensional pattern.39 Rietveld refinement of the XPD data was carried out using GSAS II.40 Ex Situ XAS. Mn K-edge and Ag K-edge X-ray absorption spectroscopies (XAS) were collected at 12-BM-B and 10-BM-B beamlines at Argonne National Laboratory, respectively. All the samples were sealed between polyimide tapes for measurement. The XAS data analyses were performed using the Athena software packages, with E0 (6.539 keV) reference values from Mn foil reference, and E0 (25.514 keV) reference values from Ag foil. Edge energy was defined as the maximum of the first derivative of χμ(E). Extended X-ray absorption fine structure (EXAFS) of the Mn edge was analyzed with the Artemis software packages, and the k range was selected from 2 to 14 Å−1 with Hanning window type.41,42 k, k2, and k3 k-weightings were used simultaneously for all fits. The Mn−O, Mn−Mnedge, and Mn−Mncorner shells were fitted over the R range 1− 3.3 Å. k2-weighted EXAFS spectra were displayed for analyses for both Mn edge and Ag edge data. TEM Characterization. The discharged samples were ground to fine powders, suspended into DMC, and transferred to a holey carbon-coated copper grid. The high resolution transmission electron microscopy (HRTEM) images, high angle annular dark field (HAADF) image in scanning transmission electron microscopy (STEM) mode, and the electron energy loss spectroscopy (EELS) analyses were performed using the double aberration-corrected JEOLARM 200CF microscope operated at 200 kV. The microscope is equipped with a cold-field emission gun and Gatan Quantum ER Energy Filter (GIF) system. EELS data were acquired in the TEM electron diffraction mode with a dispersion of 0.1 eV per channel. The 367

DOI: 10.1021/acs.chemmater.7b03599 Chem. Mater. 2018, 30, 366−375

Article

Chemistry of Materials energy resolution was 0.8 eV, as determined from the full-width at half-maximum of the zero-loss peak. The backgrounds were subtracted using a power law fitting method. The edge signals were deconvolved using the log-ratio method to remove the plural scattering effects. The energy positions of Mn L2,3 were determined by fitting the EELS profile with a combined Gaussian and Lorentz function. The white line ratio of L3/L2 was calculated on the basis of the Pearson method with double step functions.43

a significant decrease from 2.38 to 1.84 V (ΔV = 0.54 V) over the initial 10 cycles, and it continues decreasing over extended cycling, with a terminal voltage of 1.55 V at cycle 40. Such voltage fade is in sharp contrast to the minor voltage change below 2 e.e. of cycling. The loaded voltage continues to decrease during cycling thus leading to decreasing delivered energy densities (Figure 1b). Furthermore, the low Coulombic efficiency (70%) in the first cycle is also observed, and it increases to 99% at cycle 5. Therefore, an appropriate lithiation level is important to long-term cycle stability of hollandite (αMnO2) materials. In order to investigate the electrochemistry without the Coulombically controlled reduction, a cell containing Ag1.2Mn8O16 was cycled in a voltage window of 2.0−3.5 V (inset in Figure 1a). In the first cycle, a capacity of 147 mAh/g is delivered, corresponding to 4.6 e.e. of lithiation (Li4.6Ag1.2Mn8O16). However, the discharge capacity decreases to 113 mAh/g (Li3.6Ag1.2Mn8O16) at cycle 2. This capacity decrease is consistent with the voltage drop observed in the Coulombically controlled 4 e.e. cycling test (Figure 1a). The capacity retention is 68% in the initial 10 cycles (in the x (LixAg1.2Mn8O16) value of 3.2−4.6) while it is 88% from cycle 11 to cycle 50 (with x = ∼3.0). It is notable that the more significant capacity fading occurs in the x (LixAg1.2Mn8O16) value of 3.2−4.6, while the stable cycling appears at x = ∼ 3.0. This result motivated study of the degree of structural deformation at different lithiation levels. Structural Change as a Function of State of Lithiation. Lithium based electrochemical cells containing Ag1.2Mn8O16 materials were electrochemically reduced to 2 e.e., and oxidized to 3.5 V. Electrodes were recovered in the charged state after 2, 10, and 40 cycles and analyzed by XAS (Figure 2). From cycle 1 to cycle 10, the terminal voltage after

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RESULTS Material Characterization. The crystal structure of silver hollandite shows 2 × 2 tunnels formed by edge- and cornersharing MnO6 octahedra (Figure S1a). Silver ions are located in the center of these tunnels to stabilize the tunnels and reduce the a, b dimensions of the unit cell relative to a material with no metal cation located in the tunnel.32 The XRD pattern matches the standard pattern of tetragonal Ag1.8Mn8O16 (ICSD 201792) with no evidence of impurities (Figure S1b). TGA result shows that the annealed sample still contains a small amount of water (∼2.0%) (Figure S1c).12,17 On the basis of the TGA and ICP-OES results, we assign a formula of Ag1.2Mn8O15.4·0.9H2O to the silver hollandite sample. For simplicity, a notation of Ag1.2Mn8O16 is used throughout the paper. Electrochemistry. Two reduction conditions (2 and 4 e.e.) were used in Coulombically controlled galvanostatic cycling. The terminal voltages at the conclusion of each discharge cycle are compared (Figure 1a). For 2 e.e. reduction, the terminal voltage shows only a small decrease from 2.57 to 2.40 V (ΔV = 0.17 V) in the initial 10 cycles. Notably, a constant terminal voltage (2.40 V) is maintained from cycle 10 to cycle 40. When the reduction level is kept at 4 e.e., the terminal voltage shows

Figure 2. (a) Discharge/charge curves of Ag1.2Mn8O16 at 2 electron equivalents of cycling below 3.5 V. (b) XANES and (c) EXAFS spectra of oxidized Ag1.2Mn8O16 electrodes at different cycles (EXAFS fits are in dashed lines). (d) Atomic distances of Mn−Mn from EXAFS fitting for oxidized Ag1.2Mn8O16.

discharge decreases slightly from 2.57 to 2.40 V (Figure 2a). In the first cycle, 1.43 lithium ions per formula unit (1.43 e.e. for charge) can be extracted after 2 lithium ions of insertion (2 e.e. for discharge). This indicates only partial removal of the inserted lithium ions, which was also previously observed in αMnO2.13 The Coulombic efficiency increases to 89% at cycle 2,

Figure 1. (a) Terminal voltages and (b) energy densities of Li/ Ag1.2Mn8O16 electrochemical cells at 2 (black) and 4 (red) electron equivalents under Coulombically controlled cycling. Inset: Electron equivalents delivered under potential controlled cycling of Li/ Ag1.2Mn8O16 electrochemical cells in a voltage window of 2.0−3.5 V. 368

DOI: 10.1021/acs.chemmater.7b03599 Chem. Mater. 2018, 30, 366−375

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Chemistry of Materials

which are attributed to the elongation of the edge-sharing Mn−Mn (Mn−Mnedge_ab) distance. The presence of the peaks indicates that the structures remain distorted in the charged samples. Electrochemically Lithiated Materials. Samples of the Ag1.2Mn8O16 materials were electrochemically lithiated (Figure 4a). Lithiated samples were recovered after 1.2, 2.5, and 4.7

and further increases to 97% at cycle 5. From cycle 5 to cycle 10, an efficiency of >99% is maintained. Mn K-edge XAS shows the edge energies of the charged Ag1.2Mn8O16 materials after 2, 10, and 40 cycles to all be located between those of the pristine Ag1.2Mn8O16 and cycle 1 discharge Li2Ag1.2Mn8O16, indicating that Mn is not fully oxidized to the pristine Mn valence upon charge. EXAFS spectra of the oxidized samples have very similar characteristics without showing any additional peaks. Moreover, only minor decreases in peak intensities are observed suggesting only small structural changes during lithiation/delithiation. The atomic distances during cycling are compared in Figure 2d (see Supporting Information Supplementary Note 1 for EXAFS data fitting). The Mn−O distance maintains a value of 1.90 Å during cycling, and the Mn−Mnedge_ab and Mn−Mncorner only show 0.02 Å increase from cycle 0 to cycle 40. Therefore, the small structural change is conducive to the reversible lithiation/delithiation. In order to understand structural reversibility at higher lithiation levels, Ag1.2Mn8O16 materials cycled at 4 e.e. were further investigated by XAS (Figure 3). The XANES data of

Figure 4. (a) Discharge profile of Ag1.2Mn8O16 powder in Li/ Ag1.2Mn8O16 battery. (b) Ex situ X-ray powder diffraction patterns of Ag1.2Mn8O16 at 0, 1.2, 2.5, and 4.7 electron equivalents of reduction.

e.e. of reduction. The delivered capacity of the Ag1.2Mn8O16 sample to 2.0 V is 150 mAh/g (4.7 e.e.), consistent with previously reported values.32 Synchrotron based X-ray powder diffraction (XPD) patterns were recorded for each of the samples at different lithiation states (Figure 4b). All the peaks of the pristine Ag1.2Mn8O16 can be assigned to the tetragonal phase of silver hollandite (ICSD 201792). Two intense peaks at 3.54° and 4.53° are indexed as diffractions of (310) and (211) planes, respectively. Previous analysis demonstrated that the long dimension of the Ag1.2Mn8O16 nanorod is in the lattice c direction;32 therefore, the diffraction of the (00l) planes is susceptible to preferred orientation of the rod-like morphology. On the basis of these peak assignments, the peaks at 3.54° and 7.55° are direct indicators of ab planes and lattice c, respectively. After 1.2 e.e. of reduction, all the peaks maintain positions and intensities similar to those of the parent Ag1.2Mn8O16 material pattern, indicating negligible structural changes at this lithiation level. When the Ag1.2Mn8O16 sample is reduced by 2.5 e.e., the resulting XPD pattern shows decreased diffraction intensities and slightly shifted peak positions. However, no additional peaks are observed, consistent with a solid solution mechanism for lithium ion insertion. The XPD pattern of Li4.7Ag1.2Mn8O16 displays more significant diffraction intensity decrease and peak broadening, with several peaks no longer apparent. The data indicate distortion of the structure of Ag1.2Mn8O16 lithiated by 4.7 mol equiv. Two peaks, assigned as (211) and (002) diffractions, respectively, can be identified from the pattern. The (002) peak shifts to a higher angle showing a decrease of lattice c, while the (211) peak shifts to a lower angle indicating an expansion of the ab plane (tunnel dimension) due to lithium ion insertion. In order to further understand the impacts of lithiation on the Ag 1.2 Mn 8 O 16 structure, Rietveld refinements were performed on the ex situ XPD patterns (Figure 5). The refinement of Ag1.2Mn8O16 presents a good fit with an Rwp value of 4.92% (ICSD 201792), consistent with previously reported values.32 The monoclinic Ag1.2Mn8O16 structure with

Figure 3. (a) Discharge/charge curves of Ag1.2Mn8O16 at 4 electron equivalents of cycling below 3.5 V. (b) XANES and (c) EXAFS spectra of oxidized Ag1.2Mn8O16 electrodes at different cycles (EXAFS fits are in dashed lines). (d) Atomic distances of Mn−Mn from EXAFS fitting for oxidized Ag1.2Mn8O16.

oxidized samples overlay with each other at cycle 2 and cycle 10, and are slightly shifted to a lower energy at cycle 40, indicating that the electrochemically charged samples have a lower Mn oxidation state compared to the pristine material (Figure 3b). In the EXAFS spectra, the peak amplitudes dramatically decrease after repetitive reduction under 4 e.e., indicative of structural amorphization (Figure 3c). Near neighbors of Mn−O and Mn−Mn show 25−35% decreases after cycling at 4 e.e. (Figure S3). A large increase in the Mn− Mn atomic distance is observed at cycle 2 according to the fitted results (Figure 3d). In the initial two cycles, Mn− Mnedge_ab and Mn−Mncorner increase from 2.94 to 2.96 Å, and from 3.44 to 3.47 Å, respectively. From cycle 2 to cycle 40, the two Mn−Mn distances continue to gradually increase (0.02 Å increase for Mn−Mnedge_ab and 0.01 Å increase for Mn− Mncorner). Increases of 0.04 Å in the Mn−Mn distances and 0.02 Å in the Mn−O distance are observed after 40 cycles. Notably, all of the charged samples recovered after several different cycle numbers show additional peaks at ∼2.7 Å, 369

DOI: 10.1021/acs.chemmater.7b03599 Chem. Mater. 2018, 30, 366−375

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Chemistry of Materials

Figure 5. Rietveld refinement results for (a) Ag1.2Mn8O16, (b) Li1.2Ag1.2Mn8O16, and (c) Li2.5Ag1.2Mn8O16, and the XPD patterns were fitted with the tetragonal Ag1.8Mn8O16 phase. (d) Lattice constants and unit cell volumes of Ag1.2Mn8O16 at different states of reduction.

Table 1. Refined Unit Cell Parameters of Ag1.2Mn8O16 at Different States of Reduction sample

a (Å)

b (Å)

c (Å)

γ (deg)

V (Å3)

Rwp (%)

Ag1.2Mn8O16 Li1.2Ag1.2Mn8O16 Li2.5Ag1.2Mn8O16 Li4.7Ag1.2Mn8O16

9.829(4) 10.049(4) 10.247(5) 11.57(3)

9.829(4) 9.652(4) 9.715(4) 10.53(3)

2.8651(3) 2.8684(3) 2.8652(3) 2.8175(6)

90 89.49(7) 88.77(5) 85.5(2)

276.8(1) 278.2(1) 285.2(1) 342.3(9)

4.92 4.80 6.05 6.37

I2/m symmetry was used to refine lithiated Ag1.2Mn8O16 patterns. The lithiated materials show reasonable fits with Rwp values ranging from 5% to 6%. For Li1.2Ag1.2Mn8O16 and Li2.5Ag1.2Mn8O16, almost every peak can be correctly refined in both position and intensity (Figure 5b,c). However, only the two most intense peaks at 4.44° ((211) plane) and 7.73° ((002) plane) are correctly refined for peak positions and intensities for Li4.7Ag1.2Mn8O16 (Figure S4), likely due to relatively low diffraction intensities. Refined unit cell parameters and atomic positions are listed in Table 1 and Table S3, respectively. Estimated unit cell information based on the refinements is summarized in Figure 5d. Lattice a increases from 9.83 to 10.25 Å (4.2%) after 2.5 e.e. of reduction. A more significant increase (12.9%) is observed on increase in lithiation from Li2.5Ag1.2Mn8O16 to Li4.7Ag1.2Mn8O16. Notably, asynchronous unit cell expansion for lithiated α-MnO2 type material can be observed by XPD. On initial lithiation from Ag1.2Mn8O16 to Li1.2Ag1.2Mn8O16, lattice b shows a 1.8% decrease in contrast to a 2.2% increase of lattice a. Moreover, lattice c remains almost constant (2.87 Å) when the lithiation level is 3 Å) peaks. It is notable that the first shell Ag−O peaks show minor intensity decreases in Li2Ag1.2Mn8O16 and Li4Ag1.2Mn8O16. The presence of the intense first shell Ag−O peaks indicates that full transformation to Ag metal does not occur at lithiation levels less than 4 e.e. of lithiation. For the Li6Ag1.2Mn8O16 sample, the Ag−Ag peak intensity increases and a new peak occurs at ∼2.1 Å, both of which suggest the formation of Ag metal. Meanwhile, a prominent intensity decrease of the first shell Ag−O peak is observed, further demonstrating a decrease of the central Ag ion in the tunnel structure. For further characterization of the structure changes at different levels of electrochemical lithiation, ex situ TEM experiments were performed. Figure 8a,b shows a bright field TEM image and the corresponding HRTEM image for the pristine sample. The Ag hollandite has a nanorod shape and a tetragonal structure which is consistent with the XPD results. Figure 8c−e shows HRTEM images after 1.2, 2.5, and 4.7 e.e. of reduction, respectively. In comparison to the pristine sample (Figure 8b), no significant structure change was observed for 372

DOI: 10.1021/acs.chemmater.7b03599 Chem. Mater. 2018, 30, 366−375

Article

Chemistry of Materials

Prior theoretical studies on α-MnO2 have indicated that the largest increase in the lattice parameter a occurs in Li0.5MnO2 where the Mn oxidation state is 3.5+, and the large Jahn−Teller distortion breaks the 2 × 2 tunnel walls.33,35 An additional DFT study has shown that K+ stabilized α-MnO2 (K0.25MnO2) shows the largest lattice a dimension after lithiation when the Mn oxidation state reaches ∼3.1+ (5 e.e. of reduction).36 Therefore, our experimental observations on the Ag+ stabilized α-MnO2 (Ag1.2Mn8O16) are consistent with the theory predictions for lithation of K-α-MnO2. Increased amounts of Mn3+ result in a large structural change; however, the central cations appear to stabilize the structure to tolerate lower Mn valence (∼3+) in the α-MnO2 type structure. According to the previous DFT calculation results for α-MnO2 or K-α-MnO2, the 8h site (near tunnel edge) in the structure is the favored site for the Li ion when the lithium amount is below 4 per Mn8O16.33,35,36 When 8h sites near two parallel tunnel edges are occupied, Jahn−Teller distorted Mn3+ would result in an elongation of Mn−O distances in a same direction. Thus, the increase in one tunnel dimension (lattice a) for Li4Ag1.2Mn8O16 is consistent with the asynchronous lattice expansion observed in our XPD results. A schematic plot based on the XPD results is utilized to illustrate the structural evolution of Ag1.2Mn8O16 upon lithiation, Figure 10. At low

Figure 9. EELS profiles of (a) Li K-, (b) O K-, and (c) Mn-L2,3-edges for Ag1.2Mn8O16 at different lithiation states. The profiles are normalized with the same height of the Mn M2,3, main peak of O K- and Mn L2-edges, respectively, for comparison. (d) Intensity ratio of Mn L3/L2 calculated from part c and estimated valence state of Mn by assuming a linear relationship between the white line ratio and valence state.

contribution from Mn 3d electrons, indicative of a decrease in the valence state of Mn. This is clearly revealed in EELS profile for the Mn L2,3-edge (Figure 9c). The Mn L3- and L2-edges shift to a lower energy, and the intensity ratio of L3 to L2 increases slightly as the lithiation level increases. Both indicate the valence decrease of Mn, which agrees well with the O Kedge analyses. We further calculated the intensity ratio of L3/ L2, as shown in Figure 9d. The Mn valence state may be estimated by assuming a linear relationship between Mn valence and white line ratio, as shown in Figure 9d. For Ag1.2Mn8O16, the valence state of Mn by this method is estimated to be ∼3.5+. When reduced to Li4.7Ag1.2Mn8O16, the valence state of Mn decreases by 0.5 to ∼3+. Notably, Ag0 metal formation was observed in Li4.7Ag1.2Mn8O16. Reduction of the Mn4+ sites to an average oxidation state of Mn3+ occurs concomitant with partial reduction of the Ag+ sites to Ag0.

Figure 10. Schematic plot showing the structural evolution of Ag1.2Mn8O16 upon lithiation.

lithiation levels (∼2 e.e.), the structure maintains the 2 × 2 tunnels with minor structural deformation, and shows reasonable electrochemical reversibility with only ∼0.03% energy loss per cycle (Figure 1). At high lithiation levels (>4 e.e.), the larger increases in Mn−O and Mn−Mn distances lead to significant unit cell expansion and structural amorphization. The structure remains distorted upon delithiation as indicated in Figure 3, leading to decreased cycle stability with ∼10.4% energy loss after 40 cycles. Furthermore, we also provide insight into the Ag0 formation i n l i t h i a t e d A g 1 . 2 Mn 8 O 1 6 . L i 1 . 2 A g 1 . 2 Mn 8 O 1 6 a n d Li2.5Ag1.2Mn8O16 nanorods do not show detectable Ag0 nanoparticles according to the TEM results, indicating that Ag+ reduction does not occur at these lithiation levels. When the lithiation level increases to 4.7 e.e., the distorted Mn3+ leads to the expanded unit cell with open 2 × 2 tunnels (Figure S6) where significant amounts of Ag0 metal nanoparticles can be directly observed in the nanorods at Li4.7Ag1.2Mn8O16 by TEM. Li6Ag1.2Mn8O16 shows a significant contribution from Ag0 metal detectable in the EXAFS spectrum, indicating that the Ag0 metal formation at >4 e.e. of reduction is coincident with the breakage of the tunnel walls.

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DISCUSSION Both XANES and EELS results indicate that Mn4+ reduces to Mn3+ upon lithiation. At high lithiation levels (4−6 e.e.), the Mn average oxidation state in LixAg1.2Mn8O16 materials approaches 3+. Distortion associated with formation of Mn3+ in the LixAg1.2Mn8O16 structure leads to structural deformation. As indicated by diffraction data, the lattice a of the parent Ag1.2Mn8O16 unit cell expands by 17.7% with lithiation to a stoichiometry of Li4.7Ag1.2Mn8O16 and the Mn−Mn distance increases by 4.8% in Li4Ag1.2Mn8O16 as determined by X-ray absorption spectroscopy. The large structural changes at high lithiation levels result in breaking the 2 × 2 tunnels, leading to loss of crystallinity or amorphization (Figure S6). Crystal structures from Rietveld refinement for samples of Li4.7Ag1.2Mn8O16 indicate that the 2 × 2 tunnels are opened at this level of lithiation where the XPD pattern for Li4.7Ag1.2Mn8O16 is broadened with loss of intensity indicating the loss of crystallinity. The samples of Ag1.2Mn8O16 at lithiation levels of 4 mol equiv of lithiation. Through systematic investigation using complementary characterization techniques, this work unveils structural evolution of silver hollandite (Ag1.2Mn8O16) materials during lithiation/delithiation processes as a function of lithiation level. Little structural change nor evidence of Ag0 metal is noted at lithiation levels of 2 mol equiv per Ag1.2Mn8O16. In contrast, structural changes at lithiation levels of 4 mol equiv per Ag1.2Mn8O16 are coincident with formation of Ag0 and decrease in electrochemical reversibility.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported as part of the Center for Mesoscale Transport Properties, an Energy Frontier Research Center supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award DE-SC0012673. The XPD data were collected at the National Synchrotron Light Source II, Brookhaven National Laboratory, which is supported by the Department of Energy, under Contract DESC0012704. The X-ray absorption spectroscopy measurements were performed at Beamlines 12-BM-B and 10-BM-B of the Advanced Photon Source at Argonne National Laboratory, which is supported by the Department of Energy, under Contract DE-AC02-06CH11357. The authors thank Christopher J. Pelliccione for assistance with XAS data collection.

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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/acs.chemmater.7b03599. Ag1.2Mn8O16 crystal structures; X-ray diffraction pattern; thermogravimetric analysis (TGA) profile; XANES and EXAFS spectra of Ag 1.2 Mn 8 O 16 , Li 6Ag 1.2 Mn 8 O 16 , Mn3O4, and Mn2O3 Rietveld refinement parameters; and EXAFS fitting results (PDF)

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REFERENCES

(1) Mizushima, K.; Jones, P. C.; Wiseman, P. J.; Goodenough, J. B. LixCoO2 (0 < x