C ELL P ERFORMANCE

III. SOLID STATE SYNTHESIS AND WET CHEMICAL PROCESSING OF

4.4 C ELL P ERFORMANCE

4.4.1 Baseline electrochemical performance

The primary source of resistance from the electrochemical cell was the amorphous carbon substrate (5 Ω). Electrochemical performance of the cell was benchmarked against a standard three electrode experiment of identically prepared samples and electrolyte. The anodic and cathodic curves of a cyclic-voltammetry experiment were in close agreement.

A typical X-ray scattering experiment at the APS takes several hours and the electrochemical performance was time invariant with the electrolyte contained in the cell.

4.4.2 X-ray performance

A large fraction of the signal obtained though synchrotron X-ray scattering experiments results from the cell and sample environment, not the active material of interest; however, meaningful XRD patterns and PDFs can be extracted through careful background subtraction. XRD patterns and PDFs of individual components of the cell are

presented in Figure 51. Diffuse scattering from the air, cell, and electrolyte contribute a considerable amount of incoherent background the overall diffraction pattern. The glassy carbon substrate adds some coherent scattering to the background.

Figure 51. Diffraction patterns and PDFs from background components of the

25 50 75 100 125 150

Intensity (a.u)

Q (nm^-1)

Air & Cell Electrolyte Kapton Window Carbon

a)

5 10 15 20

Kapton

carbon

G(r)

r (Å)

electrolyte

b)

lattice were attributed to Jahn-Teller distortion of the Mn octahedra that occurred when Mn⁴⁺ reduced to Mn³⁺.

Figure 52. a) In-operando measurement on δ-MnO2 revealed an increase in lattice parameter on charging. b) PDFs of δ-MnO2 nanosheet electrodes showed elongation of bonds on charging.

1.5 2.0 2.5 3.0 3.5

G(r) (Å-2)

r (Å)

1st chg

1st dischg

2nd chg

2nd dischg

3rd chg

Mn – O Mn – Mn

b) a)

XANES data was collected for the Mn ion to investigate oxidation state changes during electrochemical cycling. The average Mn oxidation state reduced from 3.66 to 3.53 on charging. This confirmed Faradic redox reactions were occurring as part of the charge storage process (Figure 53).

Figure 53. In-situ XANES data of Mn ion collected with electrochemical cell

6540 6550 6560 6570

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

pH 2, In-situ re-charged

re-discharged charged

discharged

Normalized absorption

Photon energy (eV)

V. ELECTROCHEMICAL STRAIN AND CHARGE STORAGE MECHANISMS IN DEFECT ENGINEERED -MnO

2

NANOSHEETS

5.1 Introduction and Background

Energy storage in transition metal oxides with low redox potentials has been at the forefront of materials research in recent years. These materials display great promise for next generation supercapacitors with high theoretic capacitances and low charge transfer resistance. Supercapacitors are energy storage devices that fall in two categories: electric double layer capacitors (EDLCs) or pseudocapacitors. EDLCs rely on electrostatic interactions whereas pseudocapacitors use surface Faradic redox reactions to store charge.

High surface area materials are optimal for pseudocapacitors since Faradic redox reactions occur at or near the surface. MnO2 (birnessite) is one example of a high surface area redox active material that exhibits pseudocapacitive properties.

Manganese oxide is an ideal material for supercapacitor applications due to its high relative abundance, low cost, low toxicity, and high theoretic capacitance.²⁴ However, manganese oxide’s intrinsically low conductivity makes the need for a high surface area material even more vital.²⁶ Deposition of δ-MnO2 (birnessite) onto a conductive substrate is extremely important, and the colloidal nature of δ-MnO2 makes it an excellent candidate for ink-jet printing of thin films.²⁹ In fact, MnO2 thin film electrodes have achieved theoretic specific capacitance expected for a charge storage process involving Faradic redox reactions of one electron per manganese atom.²⁸ Typically if a thin film cannot be achieved, the active material is mixed with carbon black and other conductive materials to provide electronic pathways to the substrate and mitigate effects of low conductivity of MnO2 polymorphs. δ-MnO2 is a useful layered polymorph composed of edge shared MnO6

octahedra that can be exfoliated into single layer nanosheets, which is essentially a material that is 100% surface making it an ideal manganese oxide polymorph for pseudocapacitors.

Charged cation defects and vacancies in MnO2 (ramsdellite – 2 x 1 tunnel) were first linked to electrochemical performance in the 1980s where Mn reduction led to α- MnOOH formation of a similar 2 x 1 tunnel structure with hydrogen atoms in the tunnels.^52-54 Charge deficiencies, such as O^2- vacancies or Mn reduction were compensated

by “Ruetschi” protons. In the case of Mn³⁺ reduction, Ruetschi protons combined with one oxygen to form OH^- and replaced O^2- in the lattice.⁵² In addition to Ruetschi proton formation, transformation to α-MnOOH also changed the shape of the tunnels due to Jahn- Teller distorted Mn octahedra.⁵⁵ First principles calculations of mineral γ-MnO2 (2 x 1 tunnels and 1 x 1 tunnels) found defects stabilized the MnO2 structure and were more prevalent in ramsdellite (2 x 1 tunnel) rather than in pyrolusite (1 x 1 tunnel), while Ruetschi protons for charge compensation were characterized as unstable in birnessite.¹³² More recently, a defect model for δ-MnO2 revealed that a reduction from Mn⁴⁺ to Mn³⁺ caused Jahn-Teller distorted Mn³⁺O6 octahedra to displace to the surface, leaving behind a vacancy in the formerly pristine sheet.⁵⁶ This work was confirmed by Marafatto and co-workers who studied photoreduction of δ-MnO2 while revealing Jahn-Teller Mn³⁺

formation (as a result of Mn⁴⁺ reduction) took 600 fs, and migrated to the interlayer gallery within 600 ps.⁵⁷ Charged Mn³⁺ defects take the form of Zn²⁺ species present in the interlayer of chalcophanite structures. It is possible for Mn³⁺ to remain in the sheet, which is the case for the naturally occurring lithiophorite structure. The Lithiophorite structure contains MnO6 edge shared octahedra alternating with layers of Al(OH)6 edge shared octahedra. One of every three aluminum sites are substituted by a lithium octahedron.

Lithium substitutions are charge balanced by reduction of manganese from Mn⁴⁺ to Mn³⁺

(Figure 54).

Controlled introduction of these ‘surface Frenkel defects’ can be achieved through equilibration in pH regulated environments. More recent work has shown how systematically varying the defect content impacts electrochemical properties with increasing defect content causes an increase in capacitance and decrease in charge transfer resistance.^{18, 56}

In-situ studies of δ-MnO2 nanosheets often use Li⁺, Na⁺, and K⁺ containing electrolyte, although Rb⁺ and H⁺ ions are also commonly used.83, 133-135 Ionic radii range from 0.020 nm to 0.152 nm; however, hydrated radii of commonly used ions are often double these values.^{86, 136} In-situ synchrotron XRD on bulk MnO2 (birnessite) previously revealed Li⁺ electrolyte ions intercalated between MnO2 layers at the charged condition which reduced the interlayer spacing presumably due to electrostatic interactions. Previous studies concluded the shrinkage was due to differences in ionic radii of the intercalated species (Li⁺ at charged condition, and H2O at the discharged condition); however, the difference between interlayer spacing was too large to be a purely steric effect.⁶³ Similar in-situ XRD studies of bulk δ-MnO2 also revealed a reversible shrinkage of the interlayer upon ion insertion.¹⁰⁹ More recently, in-operando AFM was used to explore deformation and charge storage mechanisms of 3D WO3, a perovskite structure with A-site vacancies, containing WO6 corner shared octahedra in three dimensions, and 2D WO3•2H2O, with corner shared octahedra in two dimensions during electrochemical cycling. It was concluded that the charge storage mechanism and the rate and degree of deformation was largely dependent on morphology. Two dimensional distortion of hydrated WO3 was accompanied by a pseudocapacitive surface limited mechanism, whereas three dimensional WO3 distorted in three dimensions and relied on a battery type intercalation mechanism to store charge.¹³⁷

Morphology effects of δ-MnO2 and α-MnO2 were examined in a recent study on water desalination. Poorly crystalline materials exhibited enhanced electrochemical properties (i.e. capacitance and resistance) when compared to well crystallized materials.

This led to better desalination performance of the poorly crystalline MnO2. Charge storage mechanisms also varied as a result of morphology where crystalline MnO2 exhibited intercalation dominated charge storage, while poorly crystalline MnO2 was dominated by surface adsorption.¹³⁸ Surface redox reactions are characteristic of pseudocapacitive

materials and are known to exist in MnO2.¹⁷ In-situ XAS studies performed on nanocrystalline δ-MnO2 revealed reduction in the Mn oxidation state upon charging, and was almost completely reversible, while complete reversibility was obtained more recently on an ex-situ study of δ-MnO2 nanosheets.^{18, 139} These results suggest Faradic redox reactions are essential to charge storage in δ-MnO2, regardless of size scale or morphology.

This chapter aims to elucidate the charge storage mechanisms and causes of electrochemical strain in δ-MnO2 during electrochemical cycling.

5.2 Materials and Methods

5.2.1 Fabrication of δ-MnO2 nanostructures

K0.45MnO2 parent powder was synthesized by mixing stoichiometric amounts of K2CO3 (Alfa Aesar, 99.0% min) and MnCO3 (Alfa Aesar, 99.9% metals basis excluding Na) in isopropyl alcohol in a micronizing mill for 30 min. The resulting powders were dried on a hot plate and transferred to an alumina crucible. The powder was sintered at 800 °C in a Thermolyne (ThermoFisher Scientific, USA) furnace for 24 h with a 5 K/min ramp and cool rate.

Proton exchanged powders were made by adding 0.5 g and 45 mL 1.0 N HCl (Fisher Scientific) to 50 mL centrifuge and ultrasonicating for 4 h in a 40kHz ultrasonic bath with intermittent shaking every 20 min to redisperse powders. The powders were then washed three times with DI water (12,000 rpm 10 min). The proton exchange process was repeated three times. After the last proton exchange was complete, the powders were washed with DI water and dried on a hot plate at 55 °C. Proton exchanged powder transformed to cryptomelane if powders were left on the hot plate for too long, or if the temperature was above 55 °C. XRD was performed to check for an increase in the

DI water and washed several (5-6) more times until the suspension lacked a significant amount of nanosheets.

The nanosheet suspension was reassembled by adding 6.0 N HCl at a rate of 1mL/min until a pH of 1.7 was achieved. 1.0 N NaOH (Fisher Scientific) was then added until the suspension reached the desired pH. The resulting floccules were stirred for 24 h at 700 rpm. After 24 h the stir bar was removed, and the supernatant was decanted after most of the floccules settled to the bottom of the beaker. Typically 15-25 mL of the remaining floccules and liquid were separated into 50 mL centrifuge containers and diluted with DI water to 45 mL. Floccules were centrifuged at 12,000 rpm for 10 min and washed several times with DI water until there were no TBAOH bubbles after shaking the centrifuge tubes. Floccules were dried at 60 °C overnight (12-15 h). Freeze-dried samples usually took 6-7 days to fully dry.

5.2.2 Characterization of the samples

In-situ and in-operando high energy X-ray diffraction measurements performed on δ-MnO2 electrodes during electrochemical cycling were collected at the Advanced Photon Source on beamline 11-ID-B (APS, Argonne National Lab). Data was collected using flat plate transmission geometry, Si <311> monochromated primary beam at 58.6 keV and a silicon flat plate area detector. A custom electrochemical cell housed the electrodes during both the diffraction and PDF experiments. Scattering data for PDF extraction were collected over a Q-range 1.6–18 Å^-1, and diffraction data for all samples was collected over a Q-range of 0.02–62 Å. The 2D X-ray diffraction data were integrated to 1D using FIT2D, after appropriately calibrating detector deviations from orthogonality and masking invalid pixels.⁸⁴ CeO2 was used as the calibration standard for detector geometry. The PDF data were reduced using PDFgetX3 which includes the appropriate corrections for inelastic scattering and energy-dependent detector response, and absorption corrections, amongst others.⁸⁵ Background subtraction from environmental factors such as electrolyte, carbon substrate, air scattering from the cell, etc. were performed prior to PDF extraction in Python and fityk.¹⁴⁰

In-situ XANES measurements were gathered to study the Mn oxidation state of the fully charged and discharged electrodes inside the custom electrochemical cell.

Measurements were carried out at beamline C1 at the Cornell High Energy Synchrotron Source (CHESS, Cornell University). XANES spectra of MnO, Mn3O4, LiMn2O4, and MnO2 standard materials were taken to determine the relationship between Mn K-edge energy and Mn oxidation state.

In-situ EXAFS was performed on the Rb electrolyte ion to gather information about the ordering of electrolyte ions on the surface of δ-MnO2 at the charged and discharged conditions. In-situ measurements were executed in the custom electrochemical cell at the Advanced Photon Source on beamline 10-BM (APS, Argonne National Lab).

In-situ Raman spectra were collected using a WITec Alpha 300RA (WITec GmbH, Germany) with a power of 40 μW and a Zeiss EC Epiplan 20X objective lens. Spectra were averaged using Project FIVE+ software (Version 5.0, Build 5.0.3.43, WITec GmbH, Germany). More detail for regarding in-situ Raman measurements is provided in Appendix section 9.3

5.2.3 Electrochemical measurements

The working electrode was prepared by electrophoretic deposition of the pH treated floccules onto a glassy carbon substrate. 2 mg of floccules, 20 mL of absolute ethanol, and 5 drops of 1 M HNO3 were ultrasonicated for 20 min in a 40kHz ultrasonic bath resulting in a temporarily dispersed suspension of floccules. Glassy carbon substrates (HTW Hochtemperatur-Werkstoffe GmbH, Germany) taped to Ni foil were submerged in 10 mL of the floccule suspension 1 cm apart from the Pt anode. A GPR-60300 Laboratory DC Power Supply (GW Instek, Taiwan) was used to deposit the nanosheets onto the conductive substrate. 50 V was applied for 4-5 min as the nanosheets were deposited onto the glassy carbon, and the 10 mL suspension was used for 2 electrodes. The loading of the active

2

scans were carried out from 0 to 0.8 V at a scan rate of 1 mV/s. High cycle number in-situ Raman cyclic voltammetry measurements were performed with a scan rate of 2 mV/s.

5.3 Results and Discussion

5.3.1 -MnO2 ‘surface Frenkel’ defect content pre-electrochemical cycling The starting ‘surface Frenkel’ defect content, or the number of charged Mn defects present in the interlayer was determined through PDF refinements of -MnO2 floccules.

The defect content was also tracked in-situ during electrochemical cycling. The number of structural defects was resolvable in PDFs because the MnL – MnL distance is 2.89 Å, where as the MnL – MnIL distance is 3.45 Å (Figure 55).

Figure 55. a) Surface Frenkel defect model for MnO2. b) PDF of MnO2 floccules.¹⁸ Starting defect contents for less restacked floccules equilibrated at a pH of 2 and 4 were 27% (3%) and 20% (3%), respectively. The starting defect content for more restacked floccules was similar at 24% (4%) for pH = 2 and 21% (3%) for pH = 4. A refinement for less restacked pH = 2 floccules with a RWP of 18.4% along with partial PDFs can be seen in Figure 56. Partial PDFs gave excellent insight to contributions from individual pair correlations. The first main peak at 1.90 Å was the MnL – O (●) and MnIL – O

correlation distance (▲). Transition metal – oxygen bond lengths are typically around 2.0 Å. Small, non-physical, contributions from MnL – MnIL correlations also contributed to the intensity and area of the peak at 1.90 Å (#). The model used in PDF refinements of data for -MnO2 floccules is an approximate representation of the atomic structure and uses partial occupancies of both Mn and O atoms. This correlation represents two Mn atoms directly on top of each other along the stacking direction. The physical material of course does not have this Mn – Mn correlation, because a MnIL defect creates a vacancy in the former in-plane, MnL, site.

The small peak at 1.3 Å is a non-physical distance present in the PDF due to finite limits on the Fourier transform. The second small peak at 2.45 Å is the O – O distance (□).

The MnL – MnL distance at 2.89 Å (♦) and the MnL – MnIL distance at 3.45 Å (*) comprise the first Mn – Mn correlations in -MnO2. Almost half of the intensity of the peak at 3.45 Å also has contributions from MnL – O and MnIL – O correlations. The 3.45 Å peak is critical to quanification of the defect content.

Figure 56. PDF refinement with a RWP of 18.4% and partial PDFs of less restacked

-MnO2 floccules equilibrated at pH = 2.

5.3.2 In-situ and in-operando high energy X-ray diffraction

Upon insertion of K⁺ ions into the δ-MnO2 nanosheet electrode (charging), in-situ HEXRD measurements exhibited an expansion of the lattice in the a and b directions. The lack of a phase transition in XRD data is characteristic of pseudocapacitive materials; phase transitions did not occur during electrochemical cycling of δ-MnO2 nanosheet electrodes.

Nanosheet electrodes with less restacking and a surface Frenkel defect content of 27% experienced a lattice expansion in the lateral direction of 0.7% (A), while electrodes with a defect content of 18% underwent an in-plane lattice expansion of 0.5% (B). The greater lateral expansion of the electrode with a higher defect content is attributed to a

1 2 3 4 5 6 7

G(r) (Å-2 )

r (Å)

Mn_L-Mn_L

Mn_L-Mn_IL Mn_L-O

Mn_IL-O MnO₂ pH=2 June '18 [Mn_L] = 26.9%

RWP = 18.4%

□

●

¨

▲

*

#

larger number of Jahn-Teller distorted Mn³⁺ octahedra within the sheet. Expansion of the lattice was morphology dependent, where nanosheet floccules with a higher degree of stacking experienced a greater expansion on charging. More restacked MnO2 electrodes (C & D) with 27% and 18% surface Frenkel defects experienced 1.1% and 1.2%

expansion, respectively. Unlike floccules with little restacking, MnO2 electrodes made from floccules with a higher degree of restacking (C & D) also experienced changes to the basal direction during electrochemical cycling. Electrodes with 27% surface Frenkel defects (C) experienced 1.7% contraction along the basal direction upon charging, while electrodes with 18% surface Frenkel defects (D) experienced 0.7% shrinkage upon ion intercalation. This led to an overall volumetric expansion upon charging of 1.5% (C) and 0.8% (D) for electrodes with 27% and 18% defects, respectively. The volumetric expansion upon K⁺ intercalation for highly restacked floccules was similar to the area expansion observed in electrodes made with floccules that had less stacking order. Table IV provides a summary of the above discussion.

Table IV. -MnO2 In-situ Summary – Upon K⁺ insertion A

pH = 2 less restacked

B pH = 4 less

restacked

C pH = 2 more

restacked

D pH = 4 more

restacked Surface Frenkel

defect content 27% 20% 24% 21%

Oxidation State XANES (percent of Mn ions reduced)

17% 13% 17% 11%

Calculated Oxidation

State CV (reduction) 22% 14% 22% 14%

Basal direction

Measured contraction 0% 0% 1.7% 0.7%

Lateral direction

Measured expansion 0.7% 0.5% 1.1% 1.2%

Measured Area

expansion 1.45% 0.95% * *

Measured Volume

expansion * * 1.5% 0.8%

Calculated Area

expansion 1.47% 1.13% * *

Calculated Volume

expansion * * 2.2% 1.7%

* Area expansion upon charging was calculated for less restacked floccules (A & B), because there was no change in the z-direction at the charged condition. Volume expansion upon charging was calculated for more restacked floccules (C & D), because these materials experienced a change both laterally and in the basal direction during electrochemical cycling.

The basal direction contained broad peaks, characteristic of highly disordered, nanoscale materials. The broad (00l) peaks indicated little restacking of the nanosheets (A

& B). The disorder present in this direction led to no change in the (001) peak position

throughout electrochemical cycling. This phenomenon was only seen in δ-MnO2

nanosheets with little stacking order. The same measurement was performed on δ-MnO2

nanosheets with sharper (00l) peaks (more restacked). Highly restacked δ-MnO2 nanosheet floccules (C & D) experienced shrinkage in the basal direction upon charging the electrode.

This result coincided with literature on electrochemical cycling of bulk birnessite where positively charged intercalated potassium ions caused the distance between negatively charged sheets to decrease due to electrostatic effects. -MnO2 has very low conductivity, so upon insertion of K⁺ ion and concomitant reduction of Mn⁴⁺ to Mn³⁺, negative electrostatic charges accumulate on the surface of the sheets. Positively charged intercalating ions attract the negatively charged sheets and pull them closer together. This process is reversible upon deintercalation of K⁺ ions and reoxidation to Mn⁴⁺.⁶³ This suggests conductivity of less restacked nanosheets may be higher than bulk MnO2 or nanosheets with more stacking order (Figure 57 and 58).

Hydrated ions are roughly double the size of their unhydrated analogs, so the electrostatic effects of intercalating ions in highly disordered δ-MnO2 are counteracted by the steric response of the hydrated ion.⁸⁶ From this, it is apparent that morphology and degree of coherent restacking of the δ-MnO2 nanosheets directly impacts the electrochemical response of the lattice, similar to what was seen in WO3.⁴² In-plane expansion of MnO2 upon charging also agrees with results from Shan et. el. who performed in-situ neutron diffraction on Ni doped δ-MnO2 nanosheets. Ni doped -MnO2 electrodes experienced a ~1% and ~1.4% expansion in the a and b directions in the C2/m monoclinic structure, similar to the expansion values of highly restacked electrodes with Mn³⁺ charged defects. Ni doped -MnO2 electrodes prepared by Shan and co-workers also experienced a contraction in the (00l) direction of ~0.95%, but this may be caused by the presence of