Christina A. Cama

Nanocrystalline iron oxide based electroactive materials in lithium ion batteries: the critical role of crystallite size, morphology, and electrode heterostructure on battery relevant electrochemistry

The importance of crystallite size control and direct synthesis of materials with desirable properties is broadly applicable for the rational design and development of new active materials for energy storage. Recently, the use of nanoparticles and crystallite size control has redefined electrode design strategies, due in part to the large surface area/volume ratios providing more pathways for ion movement within the bulk electrode. This review is structured primarily as a case study, where reports involving a specific densely structured iron oxide, magnetite, Fe3O4, and its use as an electrode in LIBs are used as examples. Due to the high theoretical capacity (924 mA h g−1), and opportunity for implementation of a low cost electrode material, magnetite was selected as the model material for this review. Notably, crystallite size, morphology, and electrode heterostructure can all play a critical role in battery relevant electrochemistry, particularly for crystallographically dense materials such as Fe3O4. Several examples of Fe3O4 based composites are described, incorporating different types of conductive materials such as carbons as part of the structure. Additionally, this review also provides a brief introduction to a newer iron oxide based material with a 2D layered structure, silver ferrite, where crystallite size control was synthetically achieved. By focusing on two specific iron oxide based nanoscale inorganic materials, this review highlights and distinguishes the contributions of electroactive material crystallite size, morphology and electrode heterostructure to electrochemical behavior, facilitating the future development of next generation of battery electrodes.

Multi-Stage Structural Transformations in Zero-Strain Lithium Titanate Unveiled by in Situ X-ray Absorption Fingerprints

Zero-strain electrodes, such as spinel lithium titanate (Li4/3Ti5/3O4), are appealing for application in batteries due to their negligible volume change and extraordinary stability upon repeated charge/discharge cycles. On the other hand, this same property makes it challenging to probe their structural changes during the electrochemical reaction. Herein, we report in situ studies of lithiation-driven structural transformations in Li4/3Ti5/3O4 via a combination of X-ray absorption spectroscopy and ab initio calculations. Based on excellent agreement between computational and experimental spectra of Ti K-edge, we identified key spectral features as fingerprints for quantitative assessment of structural evolution at different length scales. Results from this study indicate that, despite the small variation in the crystal lattice during lithiation, pronounced structural transformations occur in Li4/3Ti5/3O4, both locally and globally, giving rise to a multi-stage kinetic process involving mixed quasi-solid solution/macroscopic two-phase transformations over a wide range of Li concentrations. This work highlights the unique capability of combining in situ core-level spectroscopy and first-principles calculations for probing Li-ion intercalation in zero-strain electrodes, which is crucial to designing high-performance electrode materials for long-life batteries.

Visualization of Phase Evolution of Ternary Spinel Transition Metal Oxides (CuFe2O4) during Lithiation

Li-ion batteries (LIB) are under intense investigation because of their high energy density and power capacity. Among all candidates, nano-sized transition metal oxides (TMOs) provide incomparable electrochemical performance in terms of theoretical capacity and cycling [1,2]. Copper ferrite (CuFe2O4) has been considered as anode material for LIB due to its low toxicity, large abundance and high theoretical capacity (896mA/g). However, the symmetries of the crystal structure can vary, depending on the different cation occupancy in the tetragonal site (A-site) and octahedral site (B-site). A tetragonal CuFe2O4 (t-CuFe2O4) with disordered inverse spinel structure have all of Cu atoms sharing octahedral sites with approximately half of the Fe, while the rest half of Feoccupy the tetragonal sites. The phase evolution of TMOs with spinel structure, such as Fe3O4, during lithiation have been well reported [3]. However, the dynamic process of tetragonal CuFe2O4 during first discharge has not been fully understood yet. Since some phases that occur during reaction are meta-stable, it is critical to study the reaction as close as possible to that which would inside a real battery. Here we have used S/TEM, both in-situ and ex-situ, to directly visualize and characterize the reaction both morphologically and structurally.