V. ELECTROCHEMICAL STRAIN AND CHARGE STORAGE
5.3 R ESULTS AND D ISCUSSION
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)
MnL-MnL
MnL-MnIL MnL-O
MnIL-O MnO2 pH=2 June '18 [MnL] = 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
Figure 57. In-situ HEXRD pattern of MnO2 electrodes with little stacking order equilibrated at a) pH = 2 and b) pH = 4. In both cases, the <001> remain at constant d-spacings on charge and discharge. Upon charging, an expansion in the lateral direction was present in both samples.
10 20 30 40 50 60
Intensity (a.u.)
Q (nm-1)
(001)
(100)
(110) (003)
(111) (101)
1st charge
2nd charge
3rd charge 1st discharge
2nd discharge
a)
10 20 30 40 50 60
Intensity (a.u.)
Q (nm-1)
(001)
(003)
(100)
(101)
(110) (111)
1st discharge 1st charge 2nd discharge
b)
10 20 30 40 50 60
Intensity (a.u)
Q (nm-1)
1st charge 1st discharge
2nd charge (001)
(100) (101) (110) (111)
a)
10 20 30 40 50 60
Intensity (a.u.)
1st charge 1st discharge
2nd discharge
2nd charge
b)
(001)
(100)
(101) (110) (111)
correlation distances at the charged condition were attributed to an increase in the number of Jahn-Teller distorted Mn3+ octahedra. Although the concentration of Mn3+ octahedra reversibly changed throughout cycling, the number of surface Frenkel defects remained constant at 27% for the samples equilibrated at pH = 2 (A & C), and 18% for the samples equilibrated at pH = 4 (B & D).
PDFs were refined with a model developed by Manceau and co-workers.56 The model was slightly altered from a hexagonal crystal structure to allow γ to refine. The γ lattice parameter was allowed to refine to account for disorder in the structure and only deviated slightly from the hexagonal, P3m1 space group. Oxygen interlayer occupancy was refined without dependence on defect content to account for interlayer water.
Potassium was also added to the interlayer for models of the in-situ data at the charged condition. Potassium z position, occupancy, and thermal parameters were allowed to refine, while x and y positions were fixed at zero. Refinement parameters can be seen in the Appendix section 9.3. Appendix section 9.3 contains refined parameters for the in-situ measurements on electrodes made with floccules equilibrated at a pH of 2 and 4. ESDs were generated by PDFGUI. However, in some cases some unphysical ESDs are reported which are associated with lattice parameters and atom positions. Atom positions for real space data with broad peaks modeled over 7 Å is likely not accurate to six significant digits (as reported by PDFGUI). Reciprocal space methods are needed to reach such high accuracy atom positions.
The invariant surface Frenkel defect content at the charged and discharged conditions indicated reduced Mn that occurred during K+ intercalation remained within the sheet. Upon K+ intercalation, XANES measurements indicated a reduction of Mn4+ to Mn3+. Such reduction causes a Jahn-Teller distortion of the MnO6 octahedra where two Mn – O bonds elongate along one axis. The Jahn-Teller distortion causes an overall lattice expansion (seen in in-situ XRD and Table IV A-D) as well as expansion of the Mn – O and Mn – Mn correlation distances (Figure 59). Previous work showed MnO2 floccules with a higher Mn surface Frenkel defect content improved electrochemical properties by increasing capacitance, decreasing charge transfer resistance, and improving cycle stability.18 PDF refinements at the charged condition located the intercalated K+ ions directly in the middle of the δ-MnO sheets.
Figure 59. δ-MnO2 local coordination reversibly changes throughout electrochemical cycling with an expansion of the average Mn – O and Mn – Mn bond length on charging and a contraction on discharging.
5.3.4 In-situ manganese XANES
In-situ XANES was performed on the manganese cation to track the oxidation state throughout electrochemical cycling. Pseudocapacitive materials undergo reversible Faradic redox reactions throughout electrochemical cycling which allows for fast charge and discharge. At the discharged condition, the Mn oxidation state of the less restacked MnO2 floccules equilibrated at pH=2 was 3.50 (A/E). Assuming all Mn in the sample is either 3+ or 4+, 50% of the total Mn was Mn3+ and 50% was Mn4+ (A/E). PDF refinements
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
defects remained invariant throughout electrochemical cycling at 27% for the less restacked pH = 2 MnO2 floccules. The remaining 40% of the total Mn in the sample was presumably Mn3+ within the sheet (Table V).
Table V. Mn in less restacked -MnO2 at charged and discharged conditions E
pH = 2
F pH = 4
G pH = 9 Discharged condition
Total Mn4+ 50% 47% 50%
Total Mn3+ 50% 53% 50%
Mn3+ surface
Frenkel defect 27% 20% 17%
Mn3+ within the sheet 23% 33% 33%
Charged condition
Total Mn4+ 33% 34% 40%
Total Mn3+ 67% 66% 60%
Mn3+ surface
Frenkel defect 27% 20% 17%
Mn3+ within the sheet 40% 46% 43%
Reduction of the Mn cation at the charged condition provided strong evidence for a Faradic charge storage process and K+ intercalation. The change in the oxidation state throughout electrochemical cycling was reversible and a larger change in oxidation state was noticed in the floccules equilibrated at a lower pH. The less restacked δ-MnO2 sample equilibrated at pH = 4 (B/F) decreased from 3.47 to 3.34 at the charged condition, a 13%
decrease in the total number of Mn4+. Although structural deformation of pH = 9 MnO2
floccules was not measured through in-situ scattering measurements, the average oxidation state was obtained through in-situ XANES measurements to compare with pH = 2 and pH = 4 samples. The surface Frenkel defect of the less restacked pH = 9 sample (G) was determined from PDF refinements on the -MnO2 floccules because an in-situ scattering measurement during electrochemical cycling was not performed. It was assumed that
surface Frenkel defects remained invariant during electrochemical cycling. The average oxidation state of the sample equilibrated at pH = 9 decreased from 3.50 to 3.40 upon charging the electrode, a 10% decrease in the total number of Mn4+ at the charged condition. The smaller change in oxidation state corresponds to less K+ intercalation, thus less Mn reduction and electron transfer and led to lower gravimetric capacitance values.
Changes in the average oxidation state during electrochemical cycling were mostly reversible (Figure 60). More restacked floccules equilibrated at a pH of 2 and 4 (C & D) experienced a 17% and 11% reduction in oxidation state, respectively. Reduction in oxidation state at the charged condition was similar with degree of restacking. This result implied Mn reduction was the same regardless of stacking order.
Figure 60. In-situ XANES on the Mn ion revealed oxidation state decreases on the charged condition and confirmed Faradic redox reactions.
Small discrepancies between calculated Mn reduction from gravimetric capacitance
defects (A & C vs. B & D). Less restacked -MnO2 floccules equilibrated at pH = 2 (A), pH = 4 (B), and pH = 9 had a calculated reduction of 22%, 15%, and 11%, respectively.
Calculated reduction values were always greater than the measured values and assumed intercalation of one K+ ion reduced one Mn4+ to Mn3+. Calculations were based on gravimetric capacitance values calculated from CV loops, while measured values were gathered from XANES measurements. A calculation for amount of Mn reduced during cycling can be seen in Equation 24. The calculation was done for less restacked floccules equilibrated at a pH of 2 and a capacitance of 306 F/g.18 This discrepancy is likely due to another charge storage mechanism, perhaps electrostatic. Kinetics measurements will be discussed in more detail later, and there is evidence to suggest Faradic redox reactions are not the only mechanism for storing charge in layered -MnO2.
(306𝐹
𝑔) (86.94 𝑔
1 𝑚𝑜𝑙 ) (6.25 ∗ 1018 𝑒−
𝐶 ) ( 1 𝑚𝑜𝑙
6.022 ∗ 1023 𝑀𝑛 𝑎𝑡𝑜𝑚𝑠) (0.8 𝑉) (24)
= 0.22 𝑒−⁄𝑀𝑛 𝑎𝑡𝑜𝑚
5.3.5 Chemomechanical response to electrochemical stimuli
Pair distribution function measurements suggested there were 27% Mn3+ charged surface Frenkel defects in the sample equilibrated at pH = 2. There was no evidence to suggest the number of ‘surface Frenkel defects’ changed during electrochemical cycling;
however, XANES results suggested Mn reversibly reduced upon K+ insertion with 50%
Mn3+ at the discharged condition and 67% Mn3+ at the charged condition. The idea of surface Frenkel defects is well documented in MnO2; however, there is no evidence from previous studies to suggest that the defect content reversibly changes during
8.7% expansion caused by reduction to Mn3+ only occurred in 17% of the MnO6 octahedra in the sample equilibrated at a pH of 2 and 13% of the MnO6 octahedra in the sample equilibrated at a pH of 4. This leads to calculated area expansions of 1.47% and 1.13% for samples equilibrated at pH of 2 and 4, respectively for the less restacked samples (A & B).
Refer to Appendix section 9.2 for a detailed calculation of the projected area expansion.
Less restacked nanosheet floccules (A & B) solely experienced an expansion and contraction in the x-y plane during operation. From HEXRD measurements, the pH = 2 sample (A) experienced a 1.45% lateral area expansion upon charging, while δ-MnO2
samples equilibrated at a pH of 4 (B) underwent a 0.95% area expansion in the x-y plane upon charging, with no change in the z-direction. Small discrepancies in the lateral area expansion were attributed to different samples measured for HEXRD and XANES measurements.
More restacked nanosheet floccules (C & D) experienced similar reduction during operation of 17% and 13% for samples equilibrated at pH of 2 and 4, respectively. These materials also experienced lateral and basal changes during electrochemical cycling. Upon charging the sample equilibrated at a pH of 2 (C) experienced a 1.1% lateral expansion and a 1.7% basal contraction, while the sample equilibrated at a pH of 4 (D) experienced a 1.2% lateral expansion and a 0.7% basal contraction. This leads to a 1.5% volume expansion for pH = 2 and a 0.8% volume expansion for pH = 4 at the charged condition.
Calculated volume expansions for more restacked pH = 2 and pH = 4 are 2.2% and 1.7%, respectively.
5.3.6 In-situ Raman
Raman spectroscopy of layered -MnO2 has been interpreted many ways over the last few decades.37, 71, 72, 75, 133, 141, 142 Until recently, most bands were assigned to Mn – O vibrations, with some bands below 500 cm-1 assigned to interactions between interlayer species and in-sheet MnO2. DFT simulations recently attributed many of these bands to interlayer water effects.71
Most Raman band assignments of -MnO2 are referenced from two articles by C.M.
Julien and co-workers.141, 142 Most literature assigns the band around 650 cm-1 to A1g
symmetric stretching vibrations of Mn – O bonds in the C2h3 (C2/m) monoclinic space
group. More recently, the 650 cm-1 band was attributed to Mn3+ content in MnO2 where a shift to lower wavenumber signified more Mn3+. This was due to a greater average Mn3+ – O bond length caused by Jahn-Teller distortion of the Mn3+O6 octahedra which led to a lower bond energy, where bond energy is directly proportional to wavenumber. This shift in the 650 cm-1 band to lower wavenumber with a lower concentration of Mn4+ was consistent among layered and tunneled MnO2 polymorphs.37, 143 This analysis is consistent with the present data on charged versus discharged samples where reduction of Mn4+ is expected upon ion intercalation at the charged condition. However, the literature is inconsistent so that interpreting Raman spectra of floccules equilibrated at a pH of 2 and 4 become challenging. The 650 cm-1 band locations and Raman spectra of the floccules were essentially identical. Literature and analysis were also inconsistent when comparing in- situ measurements at the charged condition of samples equilibrated at a pH of 2 and 4. In fact, MnO2 floccules equilibrated at pH = 4 experienced a greater shift to lower wavenumber. Raman literature suggests this shift to lower wavenumber indicates more Mn3+ is formed in the electrode made with floccules equilibrated at pH = 4 than pH = 2.
From XANES measurements where Mn oxidation state was directly measured, the pH = 4 sample experienced a lesser reduction in oxidation state upon ion intercalation (Figure 61 and 62). The 650 cm-1 band is likely a result of other structural changes during electrochemical cycling such as interlayer water or other interlayer effects described in recent literature.
0.8 1.0
ensity (a.u)
pH = 2 pH = 4
Figure 62. Raman band at ~660 cm-1 of -MnO2 floccules equilibrated at a pH of 4 is shifted farther to the left when compared to -MnO2 floccus equilibrated at a pH of 2 (inconsistent with XANES results).
The literature assigns the band located at roughly 575 cm-1 to stretching vibrations in the basal direction.141, 142 This assignment has led many research groups to attribute shifts in the band position at 575 cm-1 to expansion or contraction to the basal spacing during electrochemical cycling.133
Bands that occur near 500 cm-1 were largely unassigned due to the small relative intensity. During the first few charge and discharge cycles (Figure 63), the 500 cm-1 band reversibly changes from a singlet to doublet upon electrolyte ion insertion (charging). One group cited this singlet to doublet transformation as a phase change from hexagonal to monoclinic birnessite due to the decreased degeneracy of the monoclinic phase.144
Raman bands with energies lower than the singlet and doublet at 500 cm-1 were largely attributed to vibrations of interlayer tetrahedrally and octahedrally coordinated structures.141, 142
In-situ Raman spectroscopy was performed during electrochemical cycling on - MnO2 nanosheet electrodes to observe structural changes during the first few cycles as well as at high cycle number. During the first few cycles a singlet to doublet transition centered around 500 cm-1 occurred at discharged and charged conditions. Previous studies attributed
200 300 400 500 600 700 800 900
0.0 0.2 0.4 0.6 0.8 1.0
Normalized Intensity (a.u.)
Raman Shift (cm-1)
pH = 2 pH = 4
this transition to changes from a hexagonal structure to a monoclinic structure during cycling; however, this singlet to doublet transition was not observed in the spectra at cycle numbers greater than or equal to 100. No hexagonal to monoclinic phase transition was observed using XRD in electrodes made with highly or less restacked floccules (A-D).
Changes to the Raman band at 500 cm-1 during the beginning phase of cycling were temporary and most likely caused by residual interlayer ions from synthesis and processing (Figure 63 and 64).
100 200 300 400 500 600 700 800 900
1st Charge
Raman Shift (cm-1) 1st Discharge
2nd Discharge 2nd Charge 3rd Discharge
3rd Charge
Figure 64. Direct comparison of in-situ Raman measurements of MnO2 nanosheets at the charged and discharged state.
5.3.7 Potential conversion to cryptomelane at high cycle number
Desirable pseudocapacitors have excellent capacitance retention at a high number of cycles. While capacitance retention was excellent after 400 cycles, the local structure of the MnO2 electrode was slightly altered as number of cycles increased (Figures 65) Broadening of the 650 cm-1 band was attributed to conversion to cryptomelane.59 There was also an increase in the intensity of the band at 720 cm-1 which is characteristic of tunnel structures (Figure 66).143 The Raman band at 720 cm-1 is more prominent in structures with large tunnels such as romanechite (2 x 3 tunnel) and todorokite (3 x 3 tunnel), but is also present in hollandite (2 x 2 tunnel) structures. Layered -MnO2 transforms to tunnel α-MnO2 at modest or even room temperature if given enough time, so it is not surprising that this conversion is also driven electrochemically.59
200 300 400 500 600 700 800 900 1000
0.2 0.4 0.6 0.8 1.0
Normalized Intensity (a.u.)
Raman Shift (cm-1)
Discharge Charge
chg 4 dischg 98
chg 98 chg 220 dischg 220
chg 315 dischg 315
chg 404 dischg 404
chg 500 dischg 500 dischg 620
chg 620
Figure 66. Raman spectra of a -MnO2 nanosheet electrode at the charged condition after 4 and 620 cycles. Significant changes to the spectra occurred throughout cycling, specifically broadening of the 650 cm-1 band and increase in the intensity of the 720 cm-1 band. These changes in the spectra are characteristic of layer to tunnel conversion.
Cryptomelane conversion was confirmed with b-value measurements. B-values are used to determine charge storage mechanisms. To be considered a true pseudocapacitor, the material must possess evidence of Faradic redox reactions, while retaining capacitive CV signatures (i.e. quasi-rectangular CV loops, and current that is directly proportional to scan rate).145 XANES measurements confirmed Faradic redox reactions during electrochemical cycling, and Figure 67 shows a CV loop typical of δ-MnO2 that is quasi- rectangular in nature. Materials that store charge via battery-type intercalation and are limited by semi-infinite diffusion display a square root dependence of current vs scan rate;
however, capacitive/pseudocapacitive materials with surface-limited kinetics exhibit a direct relationship between current and scan rate in the following equation:
𝑖 = 𝑎𝑣𝑏 (25)
where i is current, v is the CV scan rate, and a and b are constants. The b exponent varies between 0.5 and 1.0 and explains charge storage mechanisms described above. During the in-situ Raman study, the cathodic b-values calculated from scan rate analysis remained constant at around 0.88 during the first 400 cycles, but at 600 cycles the b-value dropped to 0.55. Anodic b-values were typically lower than cathodic b-values and experienced a
300 400 500 600 700 800 900
0.2 0.4 0.6 0.8 1.0
Normalized Intensity (a.u.)
Raman Shift (cm-1)
chg 4 chg 620