Investigation of iron hydroxide and oxide-based nanomaterials as cathodes for sodium-ion batteries
<p>Na-ion batteries have received significant attention for their low-cost, resource abundance, and similarities to commercial Li-ion batteries. However, the development and commercialization of Na-ion batteries have been impeded by a lack of suitable cathodes that can accommodate the large radius of Na+ and provide high Na-ion diffusivity. Iron hydroxides/oxides have been considered as potential cathodes for Na-ion storage. However, iron hydroxide/oxide cathodes typically show low specific capacities, poor rate capabilities, and substantial capacity degradation upon cycling. Altering the degree of crystallinity or structural order provides an approach to improve the electrochemical properties of iron oxides and iron hydroxides, because an amorphous structure or a low crystallinity can facilitate Na-ion diffusion and accommodate the Na-ion insertion/de-insertion within cathodes, leading to higher specific capacities, better rate capabilities, and improved cycling stabilities. Herein, in the first part of this work, iron (III) hydroxides, Fe(OH)<sub>3</sub>, with different degrees of crystallinity were prepared by altering the precursor concentrations and synthesis temperature during the hydrothermal synthesis process. The electrochemistry results show that Fe(OH)<sub>3</sub> cathodes with low crystallinity exhibit higher reversible capacities, much better rate capabilities, and improved capacity retention upon cycling compared with crystalline counterparts. The enhanced electrochemical performance of low crystallinity Fe(OH)<sub>3</sub> cathodes is attributed to the combination of facilitated Na-ion mobility, improved ability to accommodate reversible volume changes during ion insertion/de-insertion within the structure, and a higher degree of ion storage sites within the material. However, it was also found that a semi-crystalline Fe(OH)<sub>3</sub> cathode provided improved rate capability and cycling life compared with the counterpart with a completely amorphous structure suffered from an extremely low electronic conductivity, resulting in low specific capacities at high current rates. The better electrochemical performance of semi-crystalline Fe(OH)<sub>3</sub> cathode is attributed to the multiple factors, such as morphology, electronic and ionic conductivity. However, the cycling stability of low crystalline Fe(OH)<sub>3</sub> cathodes is still needed to be further improved for practical applications.</p> <p>In the second part of this dissertation, to improve the cycling stability, low crystalline α-Fe<sub>2</sub>O<sub>3</sub>/reduced graphite oxide (rGO) nanocomposites were synthesized via a rapid microwave synthesis method to enhance the electronic conductivity and structural stability by integration of rGO compared to pristine α-Fe<sub>2</sub>O<sub>3</sub> nanoparticles. Products with low crystallinity were obtained from the short reaction times. The results show that α-Fe<sub>2</sub>O<sub>3</sub>/rGO nanocomposite cathodes exhibit superior electrochemical properties compared to highly crystalline commercial α-Fe<sub>2</sub>O<sub>3</sub> nanoparticles and low crystalline α-Fe<sub>2</sub>O<sub>3</sub> nanoparticles with rGO high-performance and low-cost cathodes for Na-ion batteries.</p>
Iron oxide, Na-ion batteries, Cathodes
Niu, S. (2019). <i>Investigation of iron hydroxide and oxide-based nanomaterials as cathodes for sodium-ion batteries</i> (Unpublished dissertation). Texas State University, San Marcos, Texas.