Hybrid-Solid-State Electrolytes: Interface Strategy for Solid-State Lithium Metal Batteries

Abstract

Growing adoption of electrified transportation (e.g., electric vehicles and electric aircraft) is promoting research efforts into renewable and clean energy storage technologies. Hence, in recent years there has been great interest in developing next-generation battery technologies such as solid-state lithium metal batteries (SS-LMBs) that when compared to existing liquid-based lithium-ion batteries (LIBs), provide a compelling potential to boost energy density (cell energy per mass or volume) and safety characteristics. Despite the promise, the practical application of SS-LMBs is dominantly impeded by the solid-solid interface challenges between solid-state electrolytes (SSEs) and battery electrodes. Hybrid solid-state electrolytes that combine bulk SSEs with a tiny amount of liquids (e.g., gel or liquid electrolyte) are proposed as an interim approach to improve the SSEs|battery electrode interfacial contacts. In a hybrid SS-LMBs, there are different interfaces (i.e., the two-dimensional boundary between bulk SSEs and bulk electrodes) and interphases (i.e., the three-dimensional layer that is a result of irreversible reactions between electrolyte and electrodes) that need to be investigated to unravel their roles in electrochemical performance of the cells. Herein, multiscale post-mortem techniques are used to study the interfaces and interphases between ceramic-based oxide SSEs (e.g., perovskites and garnets) and the two electrodes (e.g., Li-metal anodes and Ni-rich cathodes). The experiments reveal structural changes associated with evolutions in the chemical composition of solid-electrolyte interphase (SEI), cathode-electrolyte interphase (CEI), and solid-liquid electrolyte interphase (SLEI) during cycling in different cell configurations, and the results offer guidelines for designing physiochemically stable interphases and thus prolonging cycle life. Chapter 1 introduces the background and motivation of this thesis, the thesis objective, and the thesis structure while Chapter 2 reviews the fundamentals of both liquid-state LIBs and SS-LMBs as well as explains various interface and interphase phenomena and challenges in different types of batteries. In Chapter 3, experimental methods and representative characterizations used in Chapters 4-7 are presented. In Chapter 4, the anode interface in hybrid SS-LMBs between Li metal (Li0) and perovskite-type SSEs (Li0.29La0.57TiO3, LLTO) is studied. The electrochemical stability of LLTO SSEs coated with three poly (ethylene oxide) (PEO)-based interfacial layers is also investigated. It is found that the gel PEO-lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)-succinonitrile (SN) effectively prevents LLTO reduction against Li0 while performing poorly in symmetric Li cells with a large Li nucleation overpotential. Next, in Chapters 5 and 6, the cathode interface in hybrid SS-LMBs between Ni-rich LiNi0.6Mn0.2Co0.2O2 (NMC 622) cathode and garnet-type (Li6.5La2.9Ba0.1Zr1.4Ta0.6O12, LLBZTO) SSEs is explored. The results reveal the formation and chemical composition of two interphases (e.g., SLEI and CEI) after adding a tiny quantity (21 μL cm-2, i.e., the absolute amount of liquid addition divided by the cathode surface area) of liquid electrolyte at the cathode interface. Additionally, the capacity fade mechanism in the SS-LMBs is reported, wherein the microstructural and chemical phase changes, oxygen vacancy formation associated with transition metal dissolution, as well as contributions from SLEI and CEI play key roles in the deterioration of battery performance. Finally, Chapter 7 reports how the chemical composition of the liquid electrolyte affects the physicochemical properties of the SEIs in LIBs. High concentrations of LiTFSI and lithium nitrite (LiNO3) are employed to create a concentrated precipitation electrolyte (CPE) and form an inorganic-rich (e.g., LiF, Li3N, and Li2O) SEI on the surface of Li0 anodes. The precipitations in the CPE act as a reservoir for salting-out salts, leading to stabilization of the LiTFSI and LiNO3 reduction on the Li0 surface for stable cycling performance.