CHAPTER III. INVESTIGATION OF THE CHARGE STORAGE
3.1 Introduction
Defective transition metal oxide nanostructures, with large amounts of cation vacancies introduced into the lattice structure either by pH equilibration25,134 or by aliovalent cation/anion doping method,126,129 has been reported effective in improving their charge storage properties compared with the defect-free oxides. As discussed in Chapter II, the Mn surface Frenkel defect content reaches 26.5 % for the δ-MnO2 nanosheet assemblies equilibrated at pH = 2 and 19.9 % for the pH = 4 sample, meanwhile the specific capacitance increased from about 200 F/g (pH = 4) to over 300 F/g (pH = 2) by intentional introduction of ~30 % more surface Frenkel defects, and the charge transfer resistance decreased from ~15 Ω to ~3 Ω. Thus the above experimental results clearly demonstrate a correlation between the quantity of defects in our 2-D δ- MnO2 nanosheet electrode system and the quantity of Na+ electrochemically inserted.
These correlations unambiguously imply that there is a structural feature of the defective δ-MnO2 nanosheet that can promote alkali cation intercalation and Faradaic charge storage process. Therefore, understanding this link will help to enable rational design of pseudocapacitive materials with larger specific energy density.
Generally speaking, several techniques, including synchrotron-based X-ray diffraction, X-ray scattering and PDF analysis, X-ray absorption spectroscopy, as well as Raman spectroscopy, are always needed and commonly used as effective tools to probe the local structure variation and oxidation states change during electrochemical cycling.
Ex-situ measurements of the charge/discharged electrodes or the in-situ/operando characterization of the electrodes during electrochemical cycling all shed light on the alkali cation intercalation and charge storage process. Several reported works in recent years have discussed the effects of defects on cation intercalation in certain transition
metal oxide nanostructures used as battery/supercapacitor electrodes, which paves the way for using these advanced techniques to elucidate the charge storage mechanisms.
Bonil et al124 have prepared hollow iron oxide nanoparticles with very high concentration of cation vacancies. In-situ XRD results indicate that the lithium ions are intercalated into octahedral Fe vacancies, leading to slightly enlarged lattice spacing without affecting the overall structure. The in-situ XANES measurements demonstrate significantly lower rate of reduction of iron cations as compared to the theoretical one.
Thus, the in-situ studies reveal that the presence of a large amount of Fe vacancies leads to the intercalation of lithium ions without structural change and reduction of iron cations. Koketsu et al143 synthesized cation-deficient anatase TiO2 through aliovalent doping with F-. The introduction of titanium vacancies provides a thermodynamically favorable driving force to insert multivalent cations, and act as microstructural voids which can accommodate Mg2+/Al3+. Ex-situ PDF analysis of magnesiated/de-magnesiated electrodes show only slight changes upon electrochemical intercalation, emphasizing a robust host framework undergoes only slight structural changes. The lattice constants obtained at each charge state from the PDF refinements show a negligible variation, indicating “zero-strain” behaviour during repeated cycling and thereby high mechanical stability. Kuo et al185 have designed a three-electrode based electrochemical cell for in- situ synchrotron XRD analysis of MnO2, and they found that the (100) peak shifts toward a lower angle during the cathodic scan, suggesting lattice expansion, and reversibly shifts back to the original position during the anodic scan. Thus they concluded that the observed lattice expansion indicates involvement of intercalation or insertion of cations into the bulk of the oxide structure rather than merely on the surface. Chen et al141 investigated the charge storage mechanism of MnO2 by probing the structural changes using operando Raman spectroscopy, and the band features are quantitatively correlated with the charge states. No new bands were observed, indicating no new phases formed during the electrochemical cycling process. The change of Raman band positions implies the replacement of water molecules with alkali cations, and the decreased band intensity
Last but not trivial, it is necessary to mention that the nature of the electrodes is critical for the X-ray scattering measurements at synchrotron facility. In most published papers, the working electrode is prepared by mixing the active material with carbon black and binder. The useful data from active material can be obtained by subtracting the carbon and binder scattering signals from the total signal. However, this is only workable when the amount of active material is large enough and thus the obtained signal is strong enough for accurate subtraction. In our case, the mass of active material within the incident beam is typically around several micrograms, meaning that the scattering signal from the active material is usually overwhelmed by that from carbon and binder, making it almost impossible to correctly subtract the scattering data from the active materials.
Therefore, the preparation of carbon and binder-free electrodes is necessary for the in-situ X-ray scattering measurements.
In this chapter, the work includes first developing an electrophoretic deposition (EPD) method to prepare binder-free -MnO2 nanosheet electrodes on various substrates.
The deposition conditions have been investigated, in order to get optimized electrochemical response. Next, ex-situ Raman spectra, collected on several identical MnO2 nanosheet electrodes interrupted at different charge states, show increased local structural distortion upon cation intercalation during the cathodic process. The in-situ XRD and PDF data, measured using our custom electrochemical cell, demonstrate reversible expansion/contraction of the nanosheet layers upon charge/discharge, as well as unchanged interlayer spacing during cycling (which is also revealed from the Raman spectroscopy). Finally, the in-situ XANES spectra clearly shows reversible reduction/oxidation of Mn upon charge/discharge, confirms that the Faradaic redox reaction is a major charge storage mechanism in the defective MnO2 nanosheet system.
However, a slower reduction rate of Mn cations as compared to the theoretical one upon charge has been observed for the MnO2 nanosheets with higher defect content, implying that a large amount of Mn vacancies in the MnO2 nanosheets may serve as new intercalation sites for potassium ions intercalation without change of Mn oxidation state.