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Chan, Candace K.

Publications and source records attributed to Chan, Candace K..

7 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

ZnxMn1-xO Solid Solutions in the Rocksalt Structure: Optical, Charge Transport, and Photoelectrochemical Properties

Theoretical predictions of ZnO:MnO solid solutions (abbreviated here as ZMO) with the rocksalt-type structure suggest improved visible light absorption and suitable band edge positions for the overall water splitting reaction, but experimental efforts to produce such phases are limited by the low solubility of Zn within this structure type. Here, we produce solid solutions of ZnxMn1-xO, with x = 0.5 and 0.3 in the metastable rocksalt phase, using high-pressure and high-temperature (HPHT) techniques. X-ray diffraction and electron microscopy methods were employed to determine the crystal structure, chemical composition, and homogeneity on the submicron scale. The solid solutions exhibit increased optical absorbance in the visible spectral range as compared to those of the parent oxides ZnO and MnO. Our theoretical calculations for ZnxMn1-xO with x = 0.5, 0.25 predict band gaps of 2.53 and 2.98 eV, respectively, with an unusually large band gap bowing. Our calculations also show small effective electron mass for these materials indicating their potential for solar energy applications. Initial photoelectrochemical tests reveal that ZMO solid solutions are suitable for water oxidation and warrant further experimental optimization.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Anodes for Lithium-Ion Batteries Based on Type I Silicon Clathrate Ba 8 Al 16 Si 30 - Role of Processing on Surface Properties and Electrochemical Behavior

Type I silicon clathrates based on Ba 8 Al y Si 46- y (8 < y < 12) have been studied as potential anodes for lithium-ion batteries and display electrochemical properties that are distinct from those found in conventional silicon anodes. Processing steps such as ball-milling (typically used to reduce the particle size) and acid/base treatment (used to remove nonclathrate impurities) may modify the clathrate surface structure or introduce defects, which could affect the observed electrochemical properties. Here, we perform a systematic investigation of Ba 8 Al y Si 46- y clathrates with y ≈ 16, i.e, having a composition near Ba 8 Al 16 Si 30 , which perfectly satisfies the Zintl condition. The roles of ball-milling and acid/base treatment were investigated using electrochemical, X-ray diffraction, electron microscopy, X-ray photoelectron and Raman spectroscopy analysis. The results showed that acid/base treatment removed impurities from the synthesis, but also led to formation of a surface oxide layer that inhibited lithiation. Ball-milling could remove the surface oxide and result in the formation of an amorphous surface layer, with the observed charge storage capacity correlated with the thickness of this amorphous layer. According to the XRD and electrochemical analysis, all lithiation/delithiation processes are proposed to occur in single phase reactions at the surface with no discernible changes to the crystal structure in the bulk. Electrochemical impedance spectroscopy results suggest that the mechanism of lithiation is through surface-dominated, Faradaic processes. Lastly, this suggests that for off-stoichiometric clathrates, as we studied in our previous work, Li + insertion at defects or vacancies on the framework may be the origin of reversible Li cycling. However, for clathrates Ba 8 Al y Si 46- y with y ≈ 16, Li insertion in the structure is unfavorable and low capacities are observed unless amorphous surface layers are introduced by ball-milling.

25 ENERGY STORAGE↗
DOE OSTI2015

Type I clathrates as novel silicon anodes: An electrochemical and structural investigation

In this study, silicon clathrates contain cage-like structures that can encapsulate various guest atoms or molecules. Here we present an electrochemical evaluation of type I silicon clathrates based on Ba 8 Al y Si 46-y for the anode material in lithium-ion batteries. Post-cycling characterization with NMR and XRD show no discernible structural or volume changes even after electrochemical insertion of 44 Li into the clathrate structure. The observed properties are in stark contrast with lithiation of other silicon anodes, which become amorphous and suffer from larger volume changes. The lithiation/delithiation processes are proposed to occur in single phase reactions at approximately 0.2 and 0.4 V vs. Li/Li + , respectively, distinct from other diamond cubic or amorphous silicon anodes. Reversible capacities as high as 499 mAh g -1 at a 5 mA g -1 rate were observed for silicon clathrate with composition Ba 8 Al 8.54 S i37.46 , corresponding to Li:Si of 1.18:1. The results show that silicon clathrates could be promising durable anodes for lithium-ion batteries.

36 MATERIALS SCIENCE↗