Effect of sub-nanoparticle architecture on cycling performance of

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Effect of sub-nanoparticle architecture on cycling performance of MnO battery cathodes through thermal tuning of polymorph composition Elahe Moazzen, Elena V. Timofeeva, James A. Kaduk, and Carlo U. Segre Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01230 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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Crystal Growth & Design

Effect of sub-nanoparticle architecture on cycling performance of MnO2 battery cathodes through thermal tuning of polymorph composition Elahe Moazzen1 *, Elena V. Timofeeva1, James A. Kaduk1, Carlo U. Segre2 1Department

of Chemistry, Illinois Institute of Technology, 3101 S Dearborn St, Chicago, IL

60616 2Department

of Physics & CSRRI, Illinois Institute of Technology, 3101 S Dearborn St,

Chicago, IL 60616 KEYWORDS MnO2 cathode, rechargeable alkaline battery, Rietveld refinement, sub-nanoparticle crystal structure, polymorph conversion, activation mechanism

ABSTRACT

Reversible cycling of MnO2 cathodes remains a challenge in alkaline rechargeable battery development. In prior work we have developed new platelet shaped MnO2 nanoparticles with stable performance in LiOH electrolyte for over 50 cycles. In this study, the effects of subnanoparticle organization of MnO2 polymorphs on cycling performance, phase activation and

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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charge/discharge mechanisms in LiOH electrolyte are investigated by ex situ x-ray powder diffraction (XRD). Different phase compositions with the same particle morphology are achieved by annealing as-synthesized nanoplatelets composed of 50% ramsdellite (R-MnO2) and 50% akhtenskite (ε-MnO2) polymorphs. Thermal treatments result in a gradual change of polymorph composition, due to reorganization of the 1 × 2 channels in ramsdellite and akhtenskite phases to 1 × 1 channels with initial appearance of the gamma phase (γ-MnO2) followed by complete conversion of ramsdellite to pyrolusite (β-MnO2) at 400℃. Electrochemical activity of thermally treated nanoparticles is correlated to the phase compositions before and after cycling. Rietveld refinement of the XRD patterns suggests material activation in the initial cycles through intercalation of Li+ and H+ ions into 1 × 2 channels resulting in lattice expansion in both akhtenskite and ramsdellite, while intercalation of ions into structural faults present in akhtenkite and gamma phases results in amorphization of the active material. The detailed mechanism of the polymorph conversion during annealing as well as phase activation and reversible redox activity are discussed. For the first time it is clearly demonstrated that the electrochemical activity of MnO2 material strongly depends not only on the lattice structure of individual polymorphs, but also on the sub-nanoparticle architecture, including surrounding polymorphs and interphases.

INTRODUCTION Despite the great interest in developing non-aqueous rechargeable batteries for high power and energy density of mobile applications, when it comes to large scale energy storage systems operational safety, low cost and lifetime of the batteries become of prime importance.1,2 Thus, aqueous rechargeable batteries have attracted a great deal of attention for the stationary energy


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storage applications.3,4 Amongst aqueous battery chemistries, manganese dioxide (MnO2) is a promising cathode material due to its high theoretical capacity (308 mAh/g in a single electron reaction), low cost, low toxicity and natural abundance. MnO2 has been used commercially in primary alkaline batteries (Zn/MnO2, KOH electrolyte) and aqueous hybrid ion batteries.5-8 Application of MnO2 as a rechargeable cathode has been demonstrated for non-aqueous Li-ion batteries, while cycling in aqueous electrolytes in most cases results in rapid capacity decrease. Amongst the variety of manganese dioxide polymorphs (α-MnO2, β-MnO2 (pyrolusite), γMnO2, ε-MnO2 (akhtenskite), δ-MnO2 (birnessite) and R-MnO2 (ramsdellite), representing different arrangements of the MnO6 octahedral units, vacancies and defects), gamma, akhtenskite and electrolytic manganese dioxide (EMD) have been used in previous studies as rechargeable battery cathodes for aqueous and non-aqueous systems. The better electrochemical performance of these highly disordered polymorphs compared to the ordered phases, α-MnO2, β-MnO2 and RMnO2, is attributed to the combination of the Mn4+ vacancies and defects facilitating ion intercalation.9-11 The structures of γ-MnO2 and ε-MnO2 are commonly described as random intergrowths of 1 × 1 (pyrolusite) and 1 × 2 (ramsdellite) channels, however ε-MnO2 possesses a much higher concentration of microtwinning defects (twinning planes in both pyrolusite and ramsdellite domains) and much smaller ordered crystalline domains. γ-MnO2 mostly has De Wolff faults (single channels in the ramsdellite matrix) that create a rather ordered structure of alternating 1 × 2 and 1 × 1 channels.9,10,12-18 EMD, typically prepared by electrodeposition, usually shows poor crystallinity with broad diffraction peaks, and mainly contains short range γMnO2 segments with random occurrence of ε-MnO2 and β-MnO2 polymorphs.10 The generally accepted mechanism for discharge of MnO2 in alkaline electrolyte (KOH) involves intercalation of H+ ions with formation of reduced oxyhydroxide (MnOOH). Deeper


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discharge results in irreversible formation of electrochemically inactive phases (Mn(OH)2, Mn2O3, Mn3O4 and ZnMn2O4) accompanied by a significant increase in the lattice parameters, and some dissolution of Mn2+ species.13,19 It is accepted that K+ ions are too large for reversible intercalation and their insertion into the structure retards the protonation mechanism and rechargeability.20 In non-aqueous Li-ion batteries, Li+ ions intercalate into MnO2 instead of protons, forming LixMnO2 on discharge.13,21,22 Therefore, the use of LiOH electrolyte instead of KOH is suggested to be beneficial for performance of MnO2 cathodes in alkaline electrolytes. Studies of commercial EMD material with x-ray powder diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) showed higher first discharge capacity in KOH electrolyte (330 mAh/g) compared to in LiOH (148 mAh/g)20, however, performance in KOH degraded by 80% in the second cycle (60 mAh/g) while much better capacity retention was achieved in the LiOH electrolyte (50% after 40 cycles, 68 mAh/g). Similar behavior (high first cycle capacity but rapid capacity fading) in KOH vs. LiOH was also reported for γ-MnO2.13 In situ and ex situ XRD and electrochemical studies suggested that H+ ions are the primary species intercalating into the lattice in the KOH electrolyte, while both H+ and Li+ species intercalate in the LiOH electrolyte. A majority of prior investigations of MnO2 cathodes in alkaline electrolytes have been conducted vs. Zn anode which upon oxidation can produce soluble Zn(OH)42- species degrading performance of the cathode through irreversible formation of ZnMn2O4.23 Formation of Mn3O4 and ZnMn2O4 spinels was reported in KOH electrolytes, while lithiated manganese oxide similar to Li[LixMn2-x]O4 defect spinels are formed in LiOH.13 Besides the effect of electrolyte on the electrochemical activity of MnO2,13,24 it was also demonstrated that polymorph composition of MnO2 nanoparticles results in significant variation of discharge capacity in LiOH electrolyte, with a higher reversible capacity for akhtenskite-rich


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materials.25 A majority of prior studies paid little attention to the precise polymorph composition, while most synthesis techniques produce mixtures of MnO2 polymorphs and wide variation in particle sizes and morphology. In many cases authors refer to a material by the dominant phase, but as can be seen from the published XRD patterns the composition of the most commonly studied EMD-MnO2 varies from gamma to akhtenskite phases.10,20,26,27 We have developed a synthesis method for nanoplatelet MnO2 nanoparticles, containing more than one polymorph, which showed stable and highly reversible performance in LiOH electrolyte without capacity fading in the first 50 cycles and which is a significant improvement compared to the previous reports on aqueous MnO2 cathodes.25 These nanoparticles provide an ideal platform to systematically study the effect of sub-nanoparticle polymorph composition on the electrochemical activity of MnO2 cathodes in LiOH electrolyte. By using mild thermal processing on nanoparticles of the same batch to gradually modify the phase composition,16 it is possible to eliminate the variables of particle morphology and synthesis method. Nanoparticles composed of 50:50 R-MnO2:ε-MnO2 were annealed to form differing fractions of ramsdellite, akhtenskite, gamma, and pyrolusite polymorphs without change in particle morphology. Electrochemical performance is correlated to the precise phase compositions from Rietveld refinement of the XRD patterns. Cycled electrodes were also studied with ex situ XRD to deduce the mechanisms of electrode activation and redox activity in different MnO2 polymorphs. EXPERIMENTAL MnO2 nanoparticle synthesis: To synthesize MnO2 nanoparticles, 3.00 g of MnSO4⋅H2O (Acros Organics) and 1.02 g of the surfactant sodium dodecyl sulfate (SDS) (Sigma-Aldrich) were dissolved in 360 mL deionized (DI) water followed by addition of 33.84 g of Na2S2O8


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(Sigma-Aldrich). The clear solution was heated to 100℃ under magnetic stirring and was held at this temperature for 1 hour. The dark brown manganese dioxide nanoparticle precipitate was filtered, washed with DI water until the pH of the filtrant was 6-7 and dried at 90℃ in an oven. The average yield of the reaction was 90±2 %. Nanoparticle characterization: Thermogravimetric analysis (TGA, SDT Q-600, TA Instruments) of as-synthesized MnO2 nanoparticles was conducted to determine temperatures for heat treatment. A ~12 mg sample was placed in a 30 μL ceramic pan and heated from 25℃ to 700℃ at 5℃/min rate under air flow (40 ml/min). Scanning electron microscopy (SEM, Hitachi S-4700) was used to investigate the size and morphology of the nanoparticles. The samples for SEM imaging were prepared by drop-casting dilute suspensions of the nanoparticles in ethanol onto a silicon wafer. The phase composition of MnO2 nanoparticles was characterized by XRD (Bruker D2 Phaser with a LynxEye detector and Cu-Kα source). The diffraction patterns were collected over the 2θ range of 10-100° with 0.03° 2θ step size and 7 s/step dwell time. Rietveld analysis (GSAS and EXPGUI software28-30) was used to quantify the phase compositions and extract structural details including lattice parameters and crystallite sizes. Thermal treatment of the MnO2 nanoparticles: Nanoparticles from the same synthesis batch were used for this study. For each thermal treatment 0.20 g of MnO2 nanoparticles were placed in a ceramic boat and treated in a tube furnace for 2 h at 170℃, 220℃, 270℃, 320℃ and 400℃ in air. The temperature ramp up from room to treatment temperature was 20 minutes and the cooling ramp down was 2 hours. The pristine MnO2 nanoparticles are labeled as SP and the


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annealed MnO2 nanoparticles as SA170, SA220, SA270, SA320 and SA400 with numbers corresponding to the treatment temperatures. Electrochemical characterization: As-synthesized (pristine) and heat treated MnO2 nanoparticles were casted onto nickel foam (1.6 mm) current collectors (MTI Corp.) and calendered to 0.1mm thickness using a roll mill. The slurries for casting were prepared from 50:30:20 weight ratio of active material, acetylene carbon black conductive filler (STREM Chemicals) and polyvinylidene fluoride (PVDF) binder (Sigma Aldrich), mixed in N-methyl pyrrolidone (NMP) solvent (Sigma Aldrich) for 12 hours using a vortex mixer. The nanoparticle performance was studied in 4.5M LiOH aqueous electrolyte using a three-electrode pouch cell with nickel foam as counter and Hg/HgO reference electrodes (CH Instruments). All electrochemical tests were performed using an EzStat Pro potentiostat/galvanostat (Nuvant Systems, Inc). To investigate the phase changes due to galvanostatic cycling, cycled electrode material was separated from the Ni foam current collector by sonication in ethanol for 1 h. The resulting suspension was drop casted onto a zero background holder for ex situ XRD characterization. RESULTS AND DESCUSION The SEM images of the as-synthesized MnO2 nanoparticles show platelet shape with the average particle sizes of 115±27 nm diameter and 28±2 nm thick estimated by ImageJ software31 (Figure 1 and S1, supporting information). Rietveld refinement of the diffraction pattern shows 50 wt.% of ramsdellite and 50 wt.% of akhtenskite phases (Table 1 and Figure S2, supporting information).


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Figure 1. SEM images of a) SP and b) SA170, c) SA220, d) SA270, e) SA320 f) SA400 nanoparticles. Table 1. Phase compositions and crystallite sizes of the polymorphs from Rietveld refinements of XRD patterns for as-synthesized and heat treated MnO2 samples.

sample

Ramsdellite

Akhtenskite

Gamma

Pyrolusite

wt.%

Crystallite Size(nm)

wt.%


wt.%


wt.%


SP

50±1

4x4

50±1

23x7

-

-

-

-

SA170

36±1

7x18

46±1

6x11

18±1

26x3

-

-

SA220

24±1

6x22

40±1

26x8

36±1

14x1

-

-

SA270

22±1

6x39

41±1

30x6

37±1

16x2

-

-

SA320

10±1

6x21

34±1

38x5

56±1

13x1

-

-

SA400

-

-

36±2

27x17

-

-

64±1

2x13


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MnO2 nanopowders synthesized through a wet chemistry route typically contain physiosorbed water on the surface and structural water in the form of OH- groups within the crystalline structure. While water is crucial for ionic conductivity and facilitates intercalation of ions, the excess of structural water reduces the density and specific capacity of the MnO2 material as presence of OH- groups instead of O2- results in Mn vacancies due to charge compensation.17 Thus thermal treatment was suggested for removal of excess water and improvement of electrochemical performance.32-35 Annealing of MnO2 results in the collapse of the 1 × 2 channels to smaller 1 × 1 channels, reducing cell parameters and increasing the density of the material.16,35-37 Thermogravimetric analysis of as-synthesized (SP) nanopowder (Figure 2) shows four distinct transformation regions. Initial mass loss between 25℃-170℃ is 0.72 wt.% and likely corresponds to the removal of the physiosorbed water. The region between 170℃-320℃ has 2 distinct drops in mass totaling 1.13 wt.%, which has been previously ascribed to water removal by condensation of OH- groups inside the crystal channels followed by evaporation (structural water removal).16 In the temperature range 320-500℃ an additional 1.15 wt.% loss is observed. At temperatures above 500℃, a large 7.30 wt.% change is observed due to conversion of the MnO2 structure to Mn2O3 and Mn5O8 with a removal of large amount of oxygen.16 Based on TGA results temperatures of 170℃, 220℃, 270℃, 320℃ and 400℃ were selected for the thermal treatment of pristine (SP) nanoparticles (Figure 2).


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Figure 2. TGA profile of as-synthesized MnO2 nanoplatelets. Thermally treated samples show a gradual change in XRD pattern (Figure 3). MnO2 heat treated at 170℃ (SA170) shows appearance of a low intensity broad peak at 23.8° which corresponds to the (120) reflection of γ-MnO2. Rietveld refinement of the pattern revealed decrease of ramsdellite and akhtenskite fractions to 36 wt.% and 46 wt.%, respectively and formation of 18 wt.% of gamma phase (Table 1). The fraction of γ-MnO2 further increases in the samples annealed at 220℃ (36 wt.%), 270℃ (37 wt.%) and 320℃ (56 wt.%) mostly through conversion of the ramsdellite phase. The removal of structural water during annealing and collapse of 1 × 2 channels in both ramsdellite and akhtenskite may result in formation of either a more ordered γ-like MnO2 phase, or a less ordered ε-MnO2 phase. Although the results show correlation between the decrease in the ramsdellite phase and the emergence of the gamma phase, it is not excluded that the akhtenskite phase could be an intermediate in the transition between ramsdellite and gamma. The sample treated at 400℃ (SA400) shows a complete conversion of ramsdellite and gamma phases to pyrolusite (broad peak at 2θ=28.7°) with 36 wt.% of akhtenskite remaining.


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Figure 3. XRD patterns of the pristine and heat treated MnO2 nanopowders along with the standard MnO2 phases from ICSD database.38 Rietveld refinement shows that crystallite sizes of as-synthesized nanoparticles (Table 1) are much smaller than the particle sizes observed with SEM, suggesting the platelets are polycrystalline, having multiple intergrown phases within each nanoparticle.25 Despite significant changes in phase composition during annealing, SEM images show no significant morphological change in the heat treated samples (Figure 1b-1f), which is in agreement with local conversion of ramsdellite and akhtenskite to gamma and pyrolusite phases. Therefore, the effect of morphology on the electrochemical performance of the nanoparticles in this study can be excluded. Analysis of the crystallite sizes from XRD (Table 1) provides additional insights to the material structure and transformations during the thermal treatment. Specifically, it can be seen that sizes of both ramsdellite and akhtenskite crystallites increase during annealing. Ramsdellite crystallites grow in the direction along the c axis while akhtenskite crystallites increase in the


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direction perpendicular to the c axis. The gamma crystallites appear as narrow regions, likely at the intergrowth lines of the original crystallites, but their sizes decrease with increase in fraction and annealing temperature. The size of the pyrolusite-phase grains is similar to that of gamma at the previous annealing temperature, suggesting that gamma domains are predominantly converted to pyrolusite. These results suggest dynamic re-arrangement of polymorphs within nanoparticle, with all phases changing and reorganizing during thermal treatment. This also suggests the possibility of segregation of a specific polymorph on the surface or in the core of the nanoparticles, which could directly affect their electrochemical activity. Initial electrochemical characterization of as-synthesized and annealed MnO2 nanoparticles was conducted by cyclic voltammetry (CV). The CVs reached a steady state after 30 cycles (Figure S3 supporting information). Pristine MnO2 nanoparticles show one broad oxidation peak at 0.48 V and two reduction peaks at 0.31 and 0.41 V, corresponding to akhtenskite and ramsdellite phases respectively (Figure 4a). In annealed samples the reduction peak at 0.41V decreases correlating with decrease in the ramsdellite fraction. Increasing intensity of the peak at 0.31V correlates with increase in gamma-phase fraction, suggesting that the redox mechanism in newly formed gamma phase is significantly different from ramsdellite and rather similar to akhtenskite, likely related to highly defective crystalline structure. Galvanostatic discharge capacities (Figure 4b) show improvement with the cycle number for all MnO2 compositions reaching a steady-state value by approximately the 30th cycle. The SA170 sample showed the highest discharge capacity (~102 mAh/g) and coulombic efficiency (76%) at the 30th cycle, slightly better than SP (~98 mAh/g and 62%), likely due to improved electrical conductivity and higher density after removal of physiosorbed and structural water (Figures 4b and S4 supporting information). The SA220, SA270, SA320 and SA400 samples all


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showed a decrease in discharge capacity (96, 76, 67 and 46 mAh/g at the 30th cycle, respectively). The discharge curve of the as-synthesized MnO2 nanoparticles shows two plateaus at 0.41 V and 0.33 V (Figure S5 and S6 and Table S1, supporting information), while in annealed samples the first plateau at 0.41 V disappears with increasing the annealing temperature, just as the feature observed at 0.41 V in CVs (Figure 4a). Another interesting effect of annealing is a shift in the voltage of the second discharge plateau (0.33 V), to lower potentials as the fraction of gamma phase increases (Table S1 and Figure S6, supporting information). Figure 5 visually presents correlation between the phase representing nanoparticles and maximum capacity achieved. While, structural water in SP and SA170 samples is considered to enable ionic conductivity within the MnO2 structure, 1 × 2 channels are more likely to accommodate intercalation of Li+ ions into the structure than 1 × 1 channels. Decrease in material capacity with conversion between ramsdellite, gamma and pyrolusite phases during annealing results in a higher concentration of 1 × 1 channels, and limits the intercalation of Li+ ions into material, affecting electrochemical performance in LiOH electrolyte.16,35


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Figure 4. a) Steady state (30th cycle) cyclic voltammograms at 1 mV/s scan rate; b) galvanostatic discharge capacity vs. cycle number, measured at C/10 rate (~31 mA/g) between 0.00 to 0.50 V in 4.5 M LiOH electrolyte (left).

Figure 5. Experimental discharge capacity at the 30th cycle vs. fraction of different MnO2 polymorphs calculated from Rietveld refinement results. To further investigate the mechanisms of reversible capacity of different MnO2 polymorphs, electrodes were extracted from the cells after the 30th cycle in fully charged and fully discharged states. The active electrode materials were separated from the current collectors and were characterized by XRD (Figure 6). It should be noted that diffraction patterns of cycled electrodes have some peaks, not related to MnO2: the peak at 2θ ∼26° is from the carbon black/graphite used for the electrode preparation, sharp peaks at 2θ of 44.9° and 52.4° are correspond to the Ni foam residue detached from the current collector during sample preparation, and the peaks at 2θ of 21.5°, 29.6° and 31.8° are from Li2CO3 formed due to the exposure to atmospheric CO2 in the


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open cell configuration. In all cases the cycled electrodes show significantly different XRD patterns compared to the starting materials. Cycling results in a shift of the R-MnO2 peak (2θ = 22.2°) towards smaller angles (2θ = 20.7°) suggesting larger interatomic distances and lattice expansion, consistent with intercalation of ions into 1 × 2 channels as the main redox mechanism in ramsdellite (Figure 6).13 Rietveld refinement of XRD patterns for pristine (SP) electrodes (Table 2, Figure S7 supporting information) show an increase in ramsdellite cell parameters along all axes with 9.9% volume expansion in the charged state and 9.3% expansion in the discharged state This lattice expansion is mostly a result of the phase activation in initial cycles (Figure 4b and Figure S5, supporting information), with only minor changes in lattice parameters between the charged and discharged state. The peak at 2θ ∼17-21° matches the characteristic reflection of LiMn2O4 (111) and can be interpreted as a lithiated amorphous phase.13 The characteristic peak of the akhtenskite phase at 2θ 56.5° is converted to a broader feature with maximum at ∼54° after cycling. This new akhtenskite-like phase has cell parameters that are slightly larger in a and b axes, but significantly decreased along the c axis, resulting in overall cell volume decrease of 4.4% in the charged state and 5.4% in the discharged state (Table 2). The two types of akhtenskite phase transformation suggest that intercalation of ions into 1 × 2 channels results in lattice change, similar to that in the ramsdellite phase, while intercalation along the defects results in phase amorphization or crystallite size reduction (Figure 5 and 6). It is interesting to note that, despite drastic crystallographic changes in the MnO2 nanoparticles during cycling, SEM examination of the cycled electrodes shows no significant change in morphology of the nanoparticles (Figure S8 supporting information).


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Similar changes in XRD are observed for all the electrodes containing ramsdellite and akhtenskite phases (SA170-SA320). Thus the gradual increase in capacity of all MnO2 cathodes during initial cycling in LiOH electrolyte (Figure 4b) can be attributed to the activation of phases through changes in the crystal lattice, which once complete provide stable redox performance without apparent capacity fading. XRD patterns also show that gamma MnO2 undergoes amorphization similar to akhtenskite during cycling which is most evident in sample with the highest concentration of the phase (SA320). Cycled SA400 electrodes show only minor changes when compared to the SA400 material. Pyrolusite phase does not change during cycling, while only a small fraction of the akhtenskite phase is converted to the akhtenskite-like phase (Figure 6). Small changes in crystalline structure are consistent with the low discharge capacity of this sample, likely due to electrochemically inactive pyrolusite and only partial activation of akhtenskite phase.


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Figure 6. XRD patterns of electrode material after 30 cycles in fully charged (black line) and discharged (red line) states, compared to the XRD of corresponding uncycled powders (blue line). Markers indicate peaks for specific polymorphs, as well as sample contamination with fragments of carbon, Ni foam and lithium carbonate. Table 2. Rietveld refinement of XRD patterns for SP-MnO2 electrode before and after cycling. Ramsdellite

Akhtenskite/Akhtenskite-like

a

b

c

volume

a

b

c

volume

9.4945

2.8306

4.4570

119.8

2.8057

2.8057

4.4096

30.06

±0.0037

±0.0014

±0.0031

±0.1

±0.0009

±0.0009

±0.0007

±0.02

SP

9.7410

2.8318

4.7753

131.7

2.8386

2.8386

4.1192

28.75

In charged state

±0.0170

±0.0050

±0.0038

±0.2

±0.0009

±0.0009

±0.00092

±0.06

SP

9.5213

2.8439

4.8383

131.0

2.8150

2.8150

4.1440

28.44

In discharged state

±0.0190

±0.0043

±0.0053

±0.3

±0.0009

±0.0009

±0.0074

±0.05

SP

The gradual change in the composition of MnO2 nanoparticles without change in the morphology, achieved in this study provides a unique opportunity analyze the electrochemical activity of individual polymorphs. If the experimental capacity of each electrode material is assumed to be a sum of contributions from each polymorph proportional to their weight fraction (Table 1), the experimental results can be presented as a system of equations: 0.50x +0.50y = 98

(Eq. 1)

0.36x + 046y + 0.18z = 102

(Eq. 2)

0.24x + 0.40y + 0.36z = 96

(Eq. 3)

0.22x + 0.41y + 0.37z = 76

(Eq. 4)


17


0.10x + 0.34y + 0.56z = 67 0.36y +0.64w = 46

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(Eq. 5)

(Eq. 6)

where x, y, z, and w are the active capacities of individual polymorphs (ramsdellite, akhtenskite, gamma and pyrolusite, respectively). Since this system of equations is based on experimental data from electrochemical tests and Rietveld analysis of the XRD patterns, it is overdetermined and doesn’t provide a unique numerical solution. However, an analytical solution with additional assumptions provides some interesting insights. Assuming the pyrolusite phase electrochemically inactive in this electrolyte based on low discharge capacity and lack of structural changes in the SA400 electrodes, the active capacity of the akhtenskite polymorph can be found from Eq. 6 as ~128 mAh/g. Further, the active capacity of the ramsdellite phase can be found from Eq. 1 as ~68 mAh/g. Using values obtained for akhtenskite and ramsdellite, the active capacity of the gamma polymorph can be calculated from Eqs. 2-5, however each of these equations provides a different solution: 104 mAh/g, 79 mAh/g, 24 mAh/g and 30 mAh/g for SA170, SA220, SA270 and SA320 samples, respectively. Such a significant change in active capacity of one phase is unlikely, and suggests that the electrochemical activity of all MnO2 polymorphs changes from sample to sample and likely depends on the local sub-nanoparticle structure: polymorph crystallite sizes, intergrowth and surrounding phases, and phases at the solid/liquid interface. This conclusion from the analysis of Eqs.1-6 finds confirmation in the varying degrees of crystalline phase transformation during cycling: complete conversion of the akhtenskite phase in samples SA170-SA320 and partial conversion in the SA400 sample (Figure 6). Variable electrochemical activity of the same MnO2 polymorphs within different nanoscale environment has not been demonstrated before and is an important result for future development of MnO2 cathodes and other intercalation based


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battery materials. Lack of attention to the nanostructure and polymorph composition in previous studies39 is likely the reason behind inconsistent reports on MnO2 performance. CONCLUSION In this paper, MnO2 nanoparticles composed of 50:50 ramsdellite (R-MnO2) and akhtenskite (ε-MnO2) polymorphs were annealed in the temperature from 170℃ to 400℃, resulting in a series of nanomaterials with the same nanoplatelet morphology, but gradually changing subnanoparticle phase composition due to conversion of R-MnO2 and ε-MnO2 phases to gamma (γMnO2) and eventually to pyrolusite (β-MnO2). This series of nanomaterials was used to study the effect of polymorph composition on the electrochemical performance of MnO2 cathodes in LiOH electrolyte. Results showed a decrease in capacity with decrease in fraction of R-MnO2 and εMnO2 and increase in gamma and pyrolusite phases. Analysis of XRD patterns before and after galvanostatic cycling showed significant changes in crystalline structure of ramsdellite, akhtenskite and gamma phases, related to material activation. In the initial cycles, intercalation of the ions into the 1 × 2 channels results in a lattice change - expansion in ramsdellite and decrease in akhtenskite phases, while the intercalation along the defects and vacancies of the akhtenskite and gamma leads to amorphization or crystallite size reduction. These crystallographic changes happen within individual nanoparticles without apparent change in particle morphology. Once activated, the electrode materials provide stable rechargeable performance without capacity fading. Our results also show that the electrochemical activity of the MnO2 material strongly depends not simply on the polymorph composition but also on the local nano-structure inside the nanoparticles, which depends on crystallite dimensions, adjacent phases, and interfaces. SUPPORTING INFORMATION


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Supporting information contains SEM images of the as-synthesized and cycled electrodes of MnO2 nanoparticles, details and examples of Rietveld refinement process of the diffraction patterns, 30 cycles of CV, coulombic efficiency, charge/discharge curves and discharge potential for pristine and heat treated MnO2 nanoparticles. CORRESPONDING AUTHOR CONTACT INFORMATION *[email protected] AUTHOR CONTRIBUTIONS The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Elahe Moazzen, Elena V. Timofeeva, James A. Kaduk, Carlo U. Segre contributed equally. ACKNOWLEDGEMENT This research was funded in part by the US Department of Energy, Advanced Research Funding Agency-Energy (ARPA-E) (award #AR000387) and used resources of the Center for Nanoscale Materials and U.S. Department of Energy (DOE) Office of Science User Facilities operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. REFERENCES 1. Posada, J. O. G.; Rennie, A. J. R.; Villar, S. P.; Martins, V. L.; Marinaccio, J.; Barnes, A.; Glover, C. F.; Worsley, D. A.; Hall, P. J. Aqueous batteries as grid scale energy storage solutions. Renewable Sustainable Energy Rev. 2017, 68, 1174-1182.


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Page 21 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2. Liu, J.; Xu, C.; Chen, Z.; Ni, S.; Shen, Z. X. Progress in aqueous rechargeable batteries. Green Energy & Environment, 2018, 3, 20-41. 3. Whitacre, J. F.; Wiley, T.; Shanbhag, S.; Wenzhuo, Y.; Mohamed, A.; Chun, S. E.; Weber, E.; Blackwood, D.; Lynch-Bell, E.; Gulakowski, J.; Smith, C. An aqueous electrolyte, sodium ion functional, large format energy storage device for stationary applications. J. Power Sources. 2012, 213, 255-264. 4. Moazzen, E.; Timofeeva, E. V.; Segre, C. U. Role of crystal lattice templating and galvanic coupling in enhanced reversible capacity of Ni(OH)2/Co(OH)2 core/shell battery cathode. Electrochim. Acta. 2017, 258, 684-693. 5. Pistoia, G. Some restatements on the nature and behavior of MnO2 for Li batteries. J. Electrochem. Soc. 1982, 129, 1861-1865. 6. Ikeda, H.; Narukawa, S. Behaviour of various cathode materials for non-aqueous lithium cells. J. Power Sources. 1983, 9, 329-334. 7. Wruck, W. J.; Reichman, B.; Bullock, K. R.; Kao, W. H. Rechargeable Zn‐MnO2 Alkaline Batteries. J. Electrochem. Soc. 1991, 138, 3560-3567. 8. Whitacre, J. F.; Shanbhag, S.; Mohamed, A.; Polonsky, A.; Carlisle, K.; Gulakowski, J.; Wu, W.; Smith, C.; Cooney, L.; Blackwood, D.; Dandrea, J. C. A polyionic, large‐format energy storage device using an aqueous electrolyte and thick‐format composite NaTi2(PO4)3/activated carbon negative electrodes. Energy Technol. 2015, 3, 20-31.


21


Page 22 of 26

9. Ingale, N. D.; Gallaway, J. W.; Nyce, M.; Couzis, A.; Banerjee, S. Rechargeability and economic aspects of alkaline zinc-manganese dioxide cells for electrical storage and load leveling. J. Power Sources. 2015, 276, 7-18. 10. Simon, D. E.; Morton, R. W.; Gislason, J. J. A close look at electrolytic manganese dioxide (EMD) and the γ-MnO2 & ε-MnO2 phases using Rietveld modeling. Adv X-ray Anal, 2004, 47, 267-280. 11. Jouanneau, S.; Sarciaux, S.; La Salle, A. L. G.; Guyomard, D. Influence of structural defects on the insertion behavior of γ-MnO2: comparison of H+ and Li+. Solid State Ionics, 2001, 140, 223-232. 12. Ding, Y. S.; Shen, X. F.; Gomez, S.; Luo, H.; Aindow, M.; Suib, S. L. Hydrothermal Growth of Manganese Dioxide into Three‐Dimensional Hierarchical Nanoarchitectures. Adv. Funct. Mater. 2006, 16, 549-555. 13. Rus, E. D.; Moon, G. D.; Bai, J.; Steingart, D. A.; Erdonmez, C.K. Electrochemical behavior of electrolytic manganese dioxide in aqueous KOH and LiOH solutions: a comparative study. J. Electrochem. Soc. 2016, 163, A356-A363. 14. Ruetschi, P. Cation‐Vacancy Model for MnO2. J. Electrochem. Soc. 1984, 131, 27372744. 15. De Wolff, P. M.; Interpretation of some γ‐MnO2 diffraction patterns. Acta Crystallogr. 1959, 12, 341-345. 16. Lv, D.; Huang, X.; Yue, H.; Yang, Y. Sodium-Ion-Assisted Hydrothermal Synthesis of γMnO2 and Its Electrochemical Performance. J. Electrochem. Soc. 2009, 156, A911-A916.


22

Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


17. Wolfenstine J.; Foster D.; Behl W.; Gilman S. Gas Evolution and Self-Discharge in Li/ MnO2 Primary Batteries. Army Research Laboratory, 1998. 18. Sarciaux, S.; La Salle, A. L. G.; Verbaere, A.; Piffard, Y.; Guyomard, D. γ-MnO2 for Li batteries: Part I. γ-MnO2: Relationships between synthesis conditions, material characteristics and performances in lithium batteries. J. Power Sources. 1999, 81, 656660. 19. McBreen, J.; The electrochemistry of β-MnO2 and γ-MnO2 in alkaline electrolyte. Electrochim. Acta. 1975, 20, 221-225. 20. Minakshi, M.; Mitchell, D. R. The influence of bismuth oxide doping on the rechargeability of aqueous cells using MnO2 cathode and LiOH electrolyte. Electrochim. Acta. 2008, 53, 6323-6327. 21. Levi, E.; Zinigrad, E.; Teller, H.; Levi, M.D.; Aurbach, D.; Mengeritsky, E.; Elster, E.; Dan, P.; Granot, E.; Yamin, H. Structural and Electrochemical Studies of 3V LixMnO2 Cathodes for Rechargeable Li Batteries. J. Electrochem. Soc. 1997, 144, 4133-4141. 22. Jung, W. I.; Sakamoto, K.; Pitteloud, C.; Sonoyama, N.; Yamada, A.; Kanno, R. Chemically oxidized manganese dioxides for lithium secondary batteries. J. Power Sources. 2007, 174, 1137-1141. 23. Paik, Y.; Bowden, W.; Richards, T.; Sirotina, R.; Grey, C. P. 2H MAS NMR and SPECS Studies of γ-MnO2 Reduction in Zinc Alkaline Primary Batteries. J. Electrochem. Soc. 2004. 151, A998-A1011.


23


Page 24 of 26

24. Cheng, F.; Zhao, J.; Song, W.; Li, C.; Ma, H.; Chen, J.; Shen, P. Facile controlled synthesis of MnO2 nanostructures of novel shapes and their application in batteries. Inorg. Chem. 2006, 45, 2038-2044. 25. Moazzen, E.; Timofeeva, E. V.; Segre, C. U. Controlled synthesis of MnO2 nanoparticles for aqueous battery cathodes: polymorphism–capacity correlation. J. Mater. Sci. 2017, 52, 8107-8118. 26. Mondoloni, C.; Laborde, M.; Rioux, J.; Andoni, E.; Lévy‐Clément, C. Rechargeable Alkaline Manganese Dioxide Batteries I. In Situ X‐Ray Diffraction Investigation of the (EMD‐Type) Insertion System. J. Electrochem. Soc. 1992, 139, 954-959. 27. Caltagirone, S.; Massingill, J. Developing γ-MnO2 Models for XRD Analysis. ECS Trans. 2008, 11, 29-35. 28. Rietveld, H. M. A Profile Refinement Method for Nuclear and Magnetic Structures. J. Appl. Crystallogr. 1969, 2, 65-71. 29. Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS); Report LAUR 86-748; Los Alamos National Laboratory: Los Alamos, NM, 2000. 30. Toby, B. H. EXPGUI, a Graphical User Interface for GSAS. J. Appl. Crystallogr. 2001, 34, 210-213. 31. Rasband, W.S.; ImageJ, U. S. National Institutes of Health, Bethesda, Maryland, USA, https://imagej.nih.gov/ij/, 1997.


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Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


32. Manev, V.; Ilchev, N.; Nassalevska, A. The Lithium-manganese dioxide cell I. Oxygen and water release during the thermal treatment of MnO2. J. Power Sources. 1989, 25, 167-175. 33. Fernandes, J. B.; Desai, B. D.; Dalal, V. K.; Manganese dioxide—a review of a battery chemical part I. Chemical syntheses and X-ray diffraction studies of manganese dioxides. J. Power Sources. 1985, 15, 209-237. 34. Dose, W. M.; Donne, S. W.; Heat treated electrolytic manganese dioxide for Li/MnO2 batteries: effect of precursor properties. J. Electrochem. Soc. 2011, 158, A1036-A1041. 35. Dose, W. M.; Donne, S. W. Heat treated electrolytic manganese dioxide for primary Li/MnO2 batteries: Effect of manganese dioxide properties on electrochemical performance. Electrochim. Acta. 2013, 105, 305-313. 36. Cheng, F.; Zhang, T.; Zhang, Y.; Du, J.; Han, X.; Chen, J. Enhancing electrocatalytic oxygen reduction on MnO2 with vacancies. Angew. Chem. 2013, 125, 2534-2537. 37. Arnott, J. B.; Williams, R. P.; Pandolfo, A. G.; Donne, S. W. Microporosity of heattreated manganese dioxide. J. Power Sources. 2007, 165, 581-590. 38. Karlsruhe, F. ICSD https://icsd.fiz-karlsruhe.de/search/index.xhtml (accessed Oct 21, 2016). 39. Shao‐Horn, Y.; Hackney, S. A.; Cornilsen, B. C. Structural Characterization of Heat‐treated Electrolytic Manganese Dioxide and Topotactic Transformation of Discharge Products in the Li‐MnO2 Cells. J. Electrochem. Soc. 1997, 144, 3147-3153.


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For Table of Contents Use Only

Effect of sub-nanoparticle architecture on cycling performance of MnO2 battery cathodes through thermal tuning of polymorph composition Elahe Moazzen1*, Elena V. Timofeeva1, James A. Kaduk1, Carlo U. Segre2

Polymorph composition of the as-synthesized and annealed MnO2 nanoparticles and the corresponding electrochemical performance (discharge capacity at 30th cycle) along with the SEM image of the pristine MnO2 nanoparticles.


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Effect of sub-nanoparticle architecture on cycling performance of

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