CHAPTER I. INTRODUCTION AND LITERATURE REVIEW
1.8 Commonly used techniques for detecting the structural defects and alkali cation
As discussed in section 1.6.2, the first studies correlate the cation vacancies with charge storage properties were published by Ruetschi for -MnO2. The Ruetschi cation vacancy fraction can be calculated by using a series of equations based on the structural water and total manganese content as described in many published papers.137,138 Although this traditional method based on the potentiometric titration is simple and easily realized in the laboratory, it can only provide a rough estimate of the cation vacancy content with low accuracy. Also, the defects are typically considered as local structures and cannot be simply characterized by traditional tools such as XRD and SEM, etc. Therefore, more advanced and sophisticated techniques are needed to investigate the defect structures, as well as for elucidating the charge storage mechanisms.
1.8.1 X-ray Absorption Spectroscopy (XAS)
In recent years, X-ray Absorption Spectroscopy, including X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS), have been widely used to understand the structural and electronic changes of many transition metal oxides. By using XANES, we are able to investigate the oxidation state change accurately. In EXAFS, we can determine the number and species of the near neighbor atoms, and their distance from the designated atom, from the oscillatory part of the absorption coefficient above the absorption edge. Thus, this technique is very sensitive, powerful and effective in detecting the local defect structures, oxidation state change, as well as surface coordination variations.
Bonil et al124 have prepared hollow γ-Fe2O3 nanoparticles, and investigated the Li+ intercalation process during electrochemical cycling by synchrotron X-ray absorption techniques. The EXAFS data (Figure 29(a)) shows that the hollow γ-Fe2O3 has significantly lower signals at the Fe-Fe distance (2-3.5 Å) compared to gamma Fe2O3, and that the solid γ-Fe2O3 has similar intensity comparing to the bulk γ-Fe2O3, indicating the presence of a large amount of Fe vacancies in the hollow γ-Fe2O3. The XANES data (Figure 29(c-d)) shows no oxidation state change of Fe during the initial stage of Li+ intercalation, indicating that the protonated oxygen sites around cation vacancies can accumulate the charges upon Li+ intercalation without affecting the oxidation state of Fe until the filling of the vacancies is complete. They further studied the intercalation of Na+ ions into structural defects in hollow γ-Fe2O3.125 Both the XANES and EXAFS data confirm the intercalation of ~1.4 Na+ ions into structural defects. Slower reduction of the Fe ions during Na+ intercalation was also found by using XANES, indicating that the protonated oxygen sites around cation vacancies may serve as the charge storage sites during Na+ intercalation.
Figure 29. (a) EXAFS data on the hollow and solid γ-Fe2O3 NPs (bulk γ-Fe2O3 was used as a reference), (b) Voltage curve at the first discharge state (up to ∼3.5 Li+ insertion)
versus metallic Li counter electrode, (c) XANES spectra measured at eight points depicted in the voltage profile shown in (b), and (d) The Fe oxidation state obtained from
XANES plots shown in (c). From Bonil, et al.124
1.8.2 X-ray Scattering and Pair Distribution Function (PDF) analysis
Another powerful technique for detecting the local defect structure in nanomaterials, especially the materials without long-range order, is X-ray scattering and pair distribution function analysis. While diffraction techniques only consider the Bragg peaks, the PDF method utilizes information of both the Bragg peaks and the diffuse scattering buried in-between the Bragg peaks. So it requires accurate determination of the diffuse and background intensities.139 Compared with EXAFS, the X-ray PDF analysis can provides not only the local structure variation but also covers the full range of atom- atom distances, and is free of Debye-Waller effects. Therefore, it can provide more reliable distances beyond the first coordination shell and with higher resolution. The r value of each peak in PDF function corresponds to different interatomic distances, and
of the peak intensities. Through fitting the experimental data with a model, more detailed information about the defect structure can be obtained.
Manceau’s group134 has done pioneering work on the pH-dependent structure of - MnO2 by using high energy X-ray scattering and PDF analysis, and their results are shown in Figure 30. The Mn ions migrate from layer to interlayer sites at acidic pH, leading to the formation of Mn vacancy with higher content, as reflected from the decreased MnL-MnL correlation with decreasing pH. Besides, the MnL-MnIL correlation increases when the pH decreases, indicating the coordination of MnO6 octahedra above or below cation vacancies. Our group25 has done some similar experiments, and our results are consistent with the model of Manceau et al.134 We have also observed an increase in Mn vacancy content with decreasing pH, as well as the appearance of a PDF peak at a distance not found in the -MnO2 structure, which corresponds to MnL-MnIL. The increase in concentration of Mn vacancies by 30 % between different pH equilibrated samples supports the hypothesis that increased proton sorption at the MnO2
surface in more acidic electrolytes expels more in-plane Mn, and therefore leads to the formation of more Mn vacancies. Li et al126 investigated F- and OH- substituted TiO2 by PDF analysis. The refinement of Ti site occupancy yields ca. 74 %, which implies the presence of large amounts of cation vacancies. The electrochemical measurements revealed improved charge storage capacity, indicating that the cation vacancies act as additional lithium hosting sites within the anatase framework. Jiang et al140 has investigated the local structures of Mn- and La-substituted BiFeO3 by synchrotron X-ray scattering and PDF analysis. They found that both La and Mn doping induce strong local structural disorder, which can be observed by the decreased peak intensity and smearing of several peaks from the extracted PDF data.
Figure 30. (a) Experimental PDFs for -MnO2 synthesized as a function of pH, showing the reverse variation of the Mn-MnL and Mn-MnIL pairs as a result of the layer-to interlayer migration of MnL at acidic pH. (b) Best-fit PDF profiles up to 8.5 Å calculated
for a model containing 17% vacancies. (c)-(e) Best-fit PDF profiles up to 7 Å calculated for a chalcophanite model. This model reproduces the short-range relaxation of the Mn-
Mn and Mn-O distances around vacancies. From Manceau, et al.134
1.8.3 Raman Spectroscopy
Raman spectroscopy can also provide valuable information for detecting the crystal structure change. Based on the inelastic scattering between photon and phonons, it is an
process can be monitored, especially by in-situ/operando Raman measurements, thereby offering direct correlations between the state of charge/discharge and the crystal phases of the electrode materials.
Figure 31. (a) The schematic diagram of the operando Raman spectroscopic test for pseudocapacitive -MnO2 thin-film electrode. (b-d) Raman spectroscopic evolution of pseudocapacitive MnO2 thin film cycled between 0 and 0.7 V in (a) 2 M LiNO3, (b) 2 M
NaNO3, and (c) 2 M KNO3 aqueous electrolyte. From Chen, et al.141
Cheng et al142 used operando Raman spectroscopy to monitor the phase changes of
-Mn0.98O2 during the charge/discharge process. They found that -Mn0.98O2 changes to the mixed phase of Na-MnO2 and Mn3O4 immediately when immersed in Na2SO4 solution. Besides, the charge storage mechanism is more complex than previously considered. H+, Na+, and even Mn2+ ions may be involved in the energy storage process.
Chen et al141 probed the structural changes of a thin-film MnO2 electrode during cycling using operando Raman spectroscopy. Their primary results are shown in Figure 31.
Through studying the changes of spectral features (e.g., band, position, width, and intensity) under different conditions (charge and discharge) in different electrolytes, they got a better insight into the cation incorporation and charge storage mechanism. This can
further help relates the cation-size effects to their electrochemical charge storage properties.
1.8.4 Other detection techniques
Other than above mentioned approaches, several different techniques are also used to detect local structural defects. Koketsu et al143 have prepared anatase TiO2 with high concentrations of titanium vacancies, which is achieved through the partial substitution of oxide ions by monovalent anions, such as fluoride and hydroxide groups. The atomic- resolution TEM images, shown in Figure 32, allow direct visualization of the titanium vacancies. The variation in atomic column intensity observed on the high-resolution image (Figure 32(b)) corresponds to a variation in the Ti atomic occupations, and thereby directly indicates the presence of vacancies. The intensity variation and dark contrast between atomic columns are further emphasized in the colored image and in the line profile in Figure 32(c).
Figure 32. (a) High-resolution TEM image of a Ti0.780.22O1.12F0.40(OH)0.48 nanoparticle.
(b) atomic-resolution image of anatase crystal oriented along the [001] axis. (c) colored high-resolution TEM image with a profile plot of a line of atoms (white rectangle), which
exhibit a clear intensity variation both of atomic columns and dark patches in between.
From Koketsu, et al.143
Li et al126 have also prepared anatase TiO2 with high concentrations of titanium
environments, depending on Ti and vacancies, with a preferential location close to vacancies.
Ariza et al144 have investigated the lithium extraction/reinsertion mechanism and the role of protons in a lithium-rich manganese oxide (Li1.6Mn1.6O4) by using inelastic neutron scattering (INS) spectroscopy, a technique that is very sensitive to protons. Their results show the presence of hydroxyls and structural water in the delithiated sample, and the water may be located in the lamellar-like regions of the defect structure. When the lithium is reinserted, most of the hydroxyls are removed, but some protons still remain in the structure mainly as structural water. The INS results confirmed that the extraction of protons and reinsertion of Li+ is more effective in Li1.6Mn1.6O4, which is in agreement with its improved lithium uptake properties.