First Principles Study on the Polyanion based Transition Metal Oxide Materials as Rechargeable Battery Electrode and Water Oxidation Catalyst

Abstract

this material exhibits a remarkably low overpotential (η) among cobalt-based catalysts as well as long-term stability. A structural investigation based on X-ray absorption spectroscopy and Fourier-transform infrared spectra reveals that the silicate layer maintains its structure after electrolysis. In addition, density functional theory calculations indicate that the layered crystalline motif in the ACP, which is similar to that in CoOOH, is responsible for the OER mechanism

however, the local environment of the active site is significantly modulated by the silicate group, leading to a substantial reduction in the OER η. This work proposes that a new material group of phyllosilicates could be efficient OER catalysts and that tuning of the catalytic activity by controlling the redox-inert groups can pave an explored avenue toward the design of novel high-performance catalysts.

Endless increase in energy demand and the depletion of fossil fuels has accelerated progress in the development of alternative energy sources. To cope the ever-growing energy requirements and to provide sustainable energy, large energy storage systems (ESS) have become an important research area in recent years. Among the various candidates, chemical energy storage systems are regarded as the optimum choice for these applications because of the pollution free operation, the high throughput efficiency, and the flexible power and energy characteristics to meet different functions, long cycle life, and low maintenance. In this context, the search for new materials for ESSs based on cost-effective redox couples has been intensively implemented in recent years. For better materials for ESSs, my main interest is in designing new polyanionic compounds with desirable properties. In Chapter 2, introduce a new method to remove anti-site defects in olivine crystals for lithium ion battery (LIB), using electrochemical charge carrier injection process at a room temperature. The Fe anti-site defects in LiFePO4 are effectively reduced by the electrochemical recombination of Li/Fe anti-sites. The healed crystal structure of lithium iron phosphate recovers its specific capacity and high-power capabilities. In this chapter, various configuration of anti-site defects and its recombination mechanisms are discussed. In Chapter 3, a new iron-based mixed polyanion compound, Na4Fe3(PO4)2(P2O7), for NIBs is introduced. Structural characterization of a newly synthesized mixed-polyanion compound with three-dimensional Na pathways was performed using combined X-ray and neutron diffraction studies. The electrode exhibited average potential ~ 3.2 V (vs. Na+/Na) and energy density of 320 Wh kg-1. Also, the reversible electrode operation was found from ion-exchanged sample of Li3NaFe3(PO4)2(P2O7) in Li-ion cells. The electrode delivers about 92 % of theoretical capacity (~140 mAh g-1) with an average voltage of 3.4 V (vs. Li+/Li). This research firstly suggested that a significant opportunity exists to explore new open-framework electrodes for NIBs with high electrochemical performances by combination of (PO4)3- and (P2O7)4- polyanions. In Chapter 4, investigated the electrochemical mechanism of NaxFe3(PO4)2(P2O7) (1 ≤ x ≤ 4) in Na-ion cells using first principles calculations and experiments. I discovered that the de/sodiation of the NaxFe3(PO4)2(P2O7) electrode occurs via one-phase reaction with a reversible Fe2+/Fe3+ redox reaction. The electrode accompanies an exceptionally small volumetric change of less than 4% during electrochemical cycling, which is attributed to the open framework of polyanion compounds with flexible P2O7 dimer in the structure. Although the structural distortion in NaFe3(PO4)2(P2O7) reduces Na de/intercalation kinetics at the last step of charge resulting in incomplete utilization of Na (~ 82 % of theoretical capacity), high rate capability was confirmed with the negligible capacity reduction from C/20 to C/5. Also, stable cycle retention up to 20 cycles were confirmed. In situ X-ray diffraction (XRD) and differential scanning calorimetry (DSC) revealed that the partially charged electrodes, NaxFe3(PO4)2(P2O7) (1 ≤ x ≤ 4), are thermally stable up to 530 °C. The understanding of electrochemical mechanism of NaxFe3(PO4)2(P2O7) (1 ≤ x ≤ 4) shown here will give a direction to the optimization of the new Na4Fe3(PO4)2(P2O7) electrode for Na rechargeable batteries. In Chapter 5, introduce an amorphous cobalt phyllosilicate (ACP) with layered crystalline motif prepared by a simple room-temperature precipitation as a new efficient OER catalyst for hydrogen fuel