The demand for lithium has risen steadily in the last few decades because of the increased use of rechargeable lithium ion batteries. Lithium ion batteries are used in applications ranging from portable electronics and electric/hybrid vehicles to grid storage. Currently, about half of the world’s lithium production is from lithium brines. Lithium mining from brines is slow, land intensive, and geographically constrained to salt lake brines with the correct concentration of lithium and other dissolved materials. A batch of lithium takes 18-24 months to produce, mainly because the process applied in the concentration of the brines is solar evaporation. With the current pace of development of electric vehicles this slow ramp up time, and the production dependency on weather conditions, has caused insecurity in lithium supply and price fluctuations for lithium salts. To both unlock new lithium resources that are less geographically constrained, such as geothermal brines, and to increase the rate of production, a number of researchers have developed new methods for lithium extraction from aqueous brine solutions. In this thesis, the development of a method that is particularly suitable to extraction from geothermal brines is presented. The method is a supercritical fluid extraction process using strategically designed crown ether extractants. First, the design and synthesis of the crown ether extractants is presented. Second, their suitability as lithium extractants in a supercritical fluid extraction process is evaluated. This is done by investigating their effectiveness as lithium extractants in a liquid-liquid extraction process, in terms of overall lithium extraction and the selectivity towards lithium over other metal cations, and by researching the solubility of the crown ether extractants in the supercritical media utilized in this research, supercritical carbon dioxide. Third, the effectiveness of the crown ether extractants in a supercritical fluid extraction process, applied to a synthetic geothermal brine is evaluated, both experimentally and with molecular dynamics modeling. Overall, the results of the research performed for this thesis are favorable for the application of a supercritical fluid extraction process in lithium extraction from geothermal brines. The synthesized crown ethers function as lithium extractants, as they successfully react with lithium to form a lithium complex during the extraction process, and the complex breaks apart for the lithium to be recovered and the crown ethers to be recovered and reused, in the stripping stage of the process. Additionally, the crown ethers are an order of magnitude more soluble in supercritical carbon dioxide than previously solubilized crown ethers, with a maximum determined solubility of about 0.3 mol/L. The most selective chemical extraction system investigated in this research has a lithium extraction efficiency of 30%, a sodium extraction efficiency of 27%, and a magnesium extraction efficiency of 52%. The extraction was performed at 60°C and 250 bar, on a synthetic geothermal brine with a lithium concentration of 10 mg/L, a sodium concentration of 500 mg/L, and a magnesium concentration of 40 mg/L.