The wide applications of energy storage systems in consumer electronics, electric vehicles, and grid storage in the recent decade has created an enormous market globally. The electrochemical flow system has many applications in Li-ion battery recycling and energy storage system design. First, research work on a scalable electrochemical flow system is presented to effectively restore the lithium concentration in end-of-life Li-ion cathode materials. An effective recycling process for end-of-life lithium-ion batteries could relieve the environmental burden and retrieve valuable cathode battery materials. The design is validated in a static configuration with both cathode loose powder and cathode electrode sheet. Materials with comparable electrochemical performance to virgin cathode materials are produced after post heat treatment. Second, research contributions in sulfur-based flow battery systems for long-duration energy storage are presented. Sulfur-based redox flow batteries are promising due to their high theoretical capacity, low cost, and high abundance. The speciation of aqueous sulfur solutions with different nominal concentrations, sulfur concentrations, and pH are studied by Raman spectroscopy. Next, a promising aqueous manganese catholyte to couple with the sulfur anolyte for a full flow battery is investigated. Test protocols and quantification metrics for the catholyte are developed. The stability of the catholyte, including self-discharge rate and precipitation rate, is measured via ex-situ characterizations. The electrochemical performance of the catholyte is investigated and optimized via in-situ experiments. The reaction pathway for the precipitation of catholyte is discussed and several mitigation strategies are proposed. Finally, a semi-solid sodium-sulfur flow battery is developed. The electrochemical performance of the sodium-sulfur battery is studied first in a static configuration at an intermediate temperature (150°C). Then a Na-S semi-solid flow cell is assembled and cycled under the two-aliquots and three-aliquots intermittent flow. / Doctor of Philosophy / The market of energy storage systems has been expanding dramatically in recent years due to their wide applications in portable electronics, electric vehicles, and large-scale grid storage. First, the research on the development of an electrochemical flow system in the Li-ion batteries (LIB) recycling process is presented. The improper disposal of end-of-life LIBs will generate flammable hazardous waste. Recycling spent LIBs could ease the environmental burden and replenish valuable resources such as lithium, cobalt, and nickel, and reduce the cost of battery manufacturing. In this study, an electrochemical flow system is designed to restore the end-of-life cathode materials in LIBs. The design has the potential to scale up and is validated with a static configuration. The recycled materials show comparable electrochemical performance to virgin battery cathode materials. Life cycle analysis shows that the recycling process consumes less energy and is more environmentally friendly. Second, the research contribution in sulfur-based flow battery systems for long-duration energy storage is presented. The aqueous sulfur solutions with different nominal concentrations, sulfur concentrations, and pH are studied by Raman spectroscopy. Next, a promising aqueous manganese catholyte to couple with the sulfur anolyte for a full redox flow battery is investigated. The chemical stability of the catholyte, including self-discharge rate and precipitation rate, is measured via ex-situ characterizations. The electrochemical performance of the catholyte is studied and optimized via in-situ experiments. The reaction mechanisms for the precipitation of aqueous manganese solutions are discussed. Finally, a semi-solid sodium-sulfur (Na-S) flow battery is developed. The electrochemical performance of the sodium-sulfur battery is studied first in a static cell at intermediate temperature. Then a Na-S semi-solid flow cell is demonstrated and cycled under the two-aliquots and three-aliquots intermittent flow.