Synthesis of Multielement Nanoparticles and Applications in Polymer Electrolytes for Lithium Batteries

The needs of a growing electrified world require energy storage solutions that provide better performance in terms of higher energy density, longer cycle life, and improved safety. Solid-state materials have gained research interest to provide the necessary performance, but they suffer from poor electrochemical properties such as low ionic conductivity, high interfacial resistance, low transfer numbers, and short cycling life. These issues need to be addressed for future commercialization. This study focuses on the synthesis and applications of high entropy materials (HEM), leveraging their unique physical properties to enhance the electrochemical performance of the lithium-ion batteries. The order of this dissertation is based on the progression of research on how to best apply high entropy materials to energy storage devices. Chapter 1 focuses on the key attributes, general properties, and characteristics that define HEMs. Additionally, various prominent synthesis techniques for creating HEMs are introduced, along with their advantages and disadvantages. The use of HEMs in various battery components is also discussed. The inclusion of several key electrochemical tests for various battery components is introduced to provide a basic understanding of electrochemical testing. Chapter 2 summarizes the fundamental electrochemical measurements, experimental details, and material characterization utilized throughout the thesis. Chapter 3 details our efforts to develop a new, scalable, and adaptable method for synthesizing high entropy material that is suitable for industrial production. We report on the synthesis of high entropy hydroxides and oxides using the electrochemical synthesis (ECS) process. This method can generate high entropy hydroxides (HEH) by combining metal salt precursors, water, and electrical energy supplied by a steady-state current. Depending on the current density, the process can be adjusted to create thin films on a metal substrate, or to form a powder that is easily separable from the solution. One of the key advantages of this process is that it can be carried out at room temperature, in contrast to previous synthesis methods that require high temperatures of 800 – 1000 °C. Our research demonstrated that alternative energy sources can be used to synthesize high entropy materials, opening up new pathways for future high capacity production. The resulting HEH powder can be further processed with a heat-treated at low temperature for a short period of 3 h to produce high entropy oxide (HEO) nanoparticles. The ECS process produces high-quality nanoparticles in two variations (hydroxide or oxide) which can be easily scaled up for industrial production. This work represents a significant advancement in the field of high entropy materials synthesis, offering a promising new approach for future applications. Chapter 4 presents our efforts to use multielement materials as additives for solid polymer electrolytes and improve upon their performance. In this chapter, we report on the inclusion of a multi element oxide (MEO) as a filler for Polyethylene Oxide (PEO) polymer. The polymer electrolyte underwent 500 cycles in an LFP half-cell and successfully reduced lithium dendrite growth in the cell by means of mechanical blocking due to the addition of the MEO particles. The multiple elements found in these fillers creates improvements to the polymer through the increased association of the MEO with the Li salt anions resulting in increased conductivity of the Li salt throughout the polymer. In addition, the MEO particles helped to improve the overall morphology of the polymer and improved the lithium plating in the system. These improvements resulted in a high-capacity retention of 78.96% after 500 cycles, compared to 64.47% for when no filler is used. Additionally, the voltage stability of the polymer electrolytes increased from 4.66 V to 6.18 V vs Li+/Li, and the cycling overpotential dropped by 19.22%. However, due in part to the chosen elements, the overall ionic conductivity of the polymer electrolytes was reduced. This work offers a new strategy for mechanical blocking of Li dendrite growth by using higher entropy materials capable of suppressing growth through the use of large lattice distortions found in these materials. It paves the way for using additives with higher entropy to further reduce dendrite growth and to include additional elements that can provide higher ionic conductivities. Chapter 5 covers our work on PEDOT:PSS coatings for lithium ion batteries. In this chapter, we report on the use of PEDOT:PSS and CNF to create a composite film for use with PP separators. The CNF-PEDOT:PSS-PEO film provided increased mechanical strength to the PP separator along with significant increases to thermal stability, both from temperature and flames. The composite film was able to maintain the electrochemical performance of the electrolyte by providing similar ionic conductivity to that of the PP separator alone. Through SEM observations of the PP separator with and without the CNF-PEDOT:PSS-PEO film, the composite film was able to better maintain the pore structure of the PP separator during extended cycling. Additionally, the PEDOT:PSS layer prevented the formation of LiF crystals that were observed when only the PP separator was present. The combination of ionic conductivity and electrochemical stability resulted in a 7.91% increase in the capacity retention of the LFP coin cell when the CNF-PEDOT:PSSPEO film was used. Additionally, when the composite coating was utilized, the charge transfer resistance (Rct) in the LFP half-cell was lowered from 259.7 Ω to 162.7 Ω, a decrease of 37.4%. This work offers a new potential for applications of PEDOT:PSS to be used as part of the separatorin LIBs. Chapter 6 outlines potential future directions and ideas for further improving the electrochemical properties of the lithium-ion battery. These include two topics: (1) a polymer electrolyte formed via electrospinning and high entropy materials, and (2) a new high entropy oxide battery anode that utilize low-cost materials which avoid elements such as nickel and cobalt and includes a replacement of the PVDF binder and carbon black with PEDOT:PSS. Appendix A documents the initial application of high entropy materials for use in capacitive deionization (CDI) water treatment systems. The capacitance of the electrodes improved with the addition of a high entropy alloy (HEA), compared to the non-additive activated carbon electrodes used as a control. Unfortunately, the overall capacitance was below average for currently used systems, necessitating the need for further study. In addition to the capacitance increase, the HEA reduced the total dissolved solids (TDS) in the sample water to a greater extent than the electrodes without HEA particles. The synthesis method for the HEA nanoparticles was thermal shock synthesis, which required large processing times to produce the materials, making scalability difficult. However, current synthesis techniques have overcome the scalability issue, allowing for more detailed future testing. Appendix B documents a preliminary study of applications for high entropy materials in supercapacitors. It covers the basic concepts behind electric double layer capacitance (EDLC) and pseudocapacitance and how high entropy oxides may lead to improved performance. Initial results highlight the use of carbon nano fibers (CNF) combined with a high entropy oxide material to increase the capacitance compared to CNF alone. The CNFs were formed using an electrospinning process in conjunction with an electrochemical synthesis method for the preparation of the high entropy oxide.