Synthesis and Electrochemical Characterization of Layered Oxides for Aqueous Energy Storage
Authors:   , Blickley Adam , Pomerantseva Ekaterina 
Published: July 2021

Energy storage devices are quickly becoming a major requirement for human society, especially with the advancement of renewable energy and the rise of electric cars. However, current energy storage technologies can be dangerous, environmentally unfriendly, and expensive. Li-ion batteries, the most common rechargeable energy storage devices used commercially, utilize flammable electrolytes and in some cases toxic electrode materials. In order to overcome these drawbacks, new rechargeable energy storage devices are being investigated. One such technology that can address many of these issues is an aqueous-based energy storage device. These energy storage systems use water as the electrolyte solvent rather than expensive, environmentally hazardous, and flammable organic compounds. Aqueous energy storage devices tend to exhibit pseudocapacitance, and because of this, are often called "pseudocapacitors." Pseudocapacitance is a form of energy storage behavior that may exhibit both surface or near-surface reactions as well as some form of intercalation mechanism. Unlike typical battery intercalation reactions, pseudocapacitive storage is not limited by the diffusion of intercalating species. The focus of this thesis research is on the effect of structure and composition of layered transition metal oxide electrodes on their intercalation-based pseudocapacitive properties in aqueous systems Chemically preintercalated vanadium oxide ([delta]-M_xV₂O₅, M = Li, Na, K, Mg, and Ca), which has been previously studied in non-aqueous Li-, Na-, and K-ion batteries, was investigated for its aqueous pseudocapacitive capabilities. First, the effect of post synthesis treatments on the initial capacitance and capacitance retention of [delta]-Na_xV₂O₅ samples was investigated in order to identify the treatment combination leading to the highest performance. It was found that [delta]-Na_xV₂O₅ samples that were aged and hydrothermally treated demonstrated the highest initial capacitance values of 230 F/g while samples that were aged and vacuum annealed exhibited the best capacitance retentions (68% after 50 cycles). The aged and hydrothermally treated and the aged and annealed post-synthesis treatment combinations were used on all five preintercalated [delta]-M_xV₂O₅, materials (M = Li, Na, K, Mg, and Ca) and the effect of preintercalated ion on pseudocapacitive performance was studied. For all five phases, and a pH study was conducted to investigate the relationship between electrolyte pH and vanadium oxide stability in aqueous electrolyte. It was found that by lowering the pH from 6.67 to 2.35, an increase in capacitance retention of up to 35% and an increase in initial capacitance of 39 F/g could be achieved. The best initial capacity of 214 F/g observed was for aged and annealed [delta]-Ca_xV₂O₅ at a pH of 2.35. The highest capacity retention observed was 96.1 % for aged and hydrothermally treated of [delta]-Li_xV₂O₅ at a pH of 2.35. The second part of this master's research was focused on the adaptation of the chemical preintercalation method developed in the Materials Science and Engineering group at Drexel for the fabrication of new layered transition metal oxides beyond vanadium oxide. For the first time, a novel family of layered tungsten oxides (M_xWO₃·nH₂O, M= Na, K, Mg, and Ca) was synthesized. Na_[0.2]WO₃·0.8H₂O phase demonstrated an initial capacitance of 60 F/g in an aqueous-based 1M H₂SO₄ electrolyte. Also, a pressure induced color change phenomenon was observed.