Multi-pronged analysis of secondary lithium metal batteries with various cathode chemistries

Abstract

Due to permanently growing demand for safe and affordable energy, the search for sustainable alternatives to fossil fuels has become one of the most critical challenges of the 21st century. Owing to their high energy density, state-of-the-art lithium-ion batteries are widely applied in different areas of human life, from military drones carrying up to 200 kg of payload to cardioverter-defibrillators. Since current Li-ion technologies have approached the edge of their applicability in electric vehicles, there is an emerging call for developing lithium batteries with better characteristics. Post-LIB technologies, including Li-O2, Li-S, and solid-state batteries with metallic Li anode, are among the most promising alternatives to replace LIBs. However, post-LIB technologies encounter different fundamental challenges that substantially reduce the capacity retention, safety, and rate capability. Understanding and resolving some of the challenges of post-LIBs is the focus of this dissertation, with particular attention paid to correlating readily measurable battery characteristics with interfacial processes. This thesis covers multiple topics, including Li-O2 batteries, Li metal batteries with liquid and solid-state electrolytes, Li-S batteries with hybrid/liquid electrolytes, and all-solid-state batteries with high-energy-density cathodes. This work establishes multi-pronged approaches to study the efficacy of various electrolytes. A novel approach is demonstrated for stabilizing a Li+-conducting garnet solid electrolyte in Li-S batteries with electron pair donor electrolytes. Chapter 3 of the thesis presents the study of LiI as a potential charge redox mediator for nonaqueous Li-O2 batteries. The effect of LiI on oxygen electrochemistry during battery charge and discharge is investigated by combining various characterization techniques. This chapter demonstrates that the battery performance becomes less reversible when the electrolyte contains LiI. The combination of DEMS and iodometric titrations indicates that LiI lowers the yield of the target discharge product, Li2O2, and triggers redox shuttling on charge. The simultaneous presence of LiI and H2O in the electrolyte results in the irreversible 4e reduction of oxygen to LiOH. Chapters 4 – 5 are devoted to developing lithium metal batteries with different battery configurations and cathode chemistries. In Chapter 4, a comprehensive study of the solvate ionic liquid electrolytes containing LiFSI, Gn, and TTE establishes the relationship between the length of glyme solvents and the stability of Li anodes. The combination of Raman spectroscopy, AIMD simulations, and EIS illustrates that the kinetics of lithium metal anodes in solvate ionic liquid electrolytes containing long-chain Gn (G3 and G4) is impeded by the formation of the [Li(Gn)]+ chelates. In contrast, the G1(G2)-based SILs’ solvation structure is primarily comprised of strongly associated Li+ and FSI-, which reduces the interfacial resistance and activation energy barriers. However, the lithium transference number of the solvate electrolytes is limited to ~0.2, which dictates the diffusion-limited rate capability of the Li anode. The chapter demonstrates that the rate capability limit observed in the asymmetric Cu-Li and full Li||LiNCA cells agrees with the values predicted by combining the EIS results and existing Li dendrite formation models. The stability of Li anodes in solid-state cells with sulfide-based solid electrolytes is investigated in Chapter 6. This case study demonstrates that the presence of Si4+ in the structure of sulfide solid-state electrolytes triggers continuous interphase growth, which may consume a 50 micron Li anode. Meanwhile, the solid electrolytes, devoid of reducible cations, facilitate Li deposition with a CE exceeding 99 % when the current density is limited to 0.1 mA.cm-2. However, Li deposition at higher current densities is interrupted by the formation of Li dendrites. Nevertheless, the performance of Li anodes may be temporarily improved by modifying the solid electrolyte with a sacrificial dendrite scavenger or wetting the Li/solid electrolyte interface with a solvate, as developed in Chapter 4. The novel concept of the Li-S battery with a Li6.5La3Ta0.5Zr1.5O12 (LLZO) garnet solid electrolyte and an electron pair donor electrolyte, DMA, is demonstrated in Chapter 6. Numerous characterization techniques were employed to investigate the interfacial reactivity between LLZO and sulfur in DMA. Traces of LiOH and Li2CO3 at the surface of LLZO trigger the oxidation of sulfur and formation of trisulfur radical which further reacts with LLZO, yielding an insulating layer containing thiosulfate, polythionate and La-O/La-O-S species. The interfacial instability of LLZO results in a rise in the interfacial resistance (> 5000 Ohm.cm2) and rapid capacity fading of the Li-S batteries. Nonetheless, phosphorylating LLZO yields a thin (~ 10 nm) and conductive (~ 40 Ohm.cm2) layer containing Li, La, and Zr phosphates, which inhibits the side reactions. Therefore, Li-S batteries with phosphorylated LLZO and DMA deliver a stable capacity of 1400 mAh.g-1. Chapter 7 is devoted to all-solid-state batteries with LiNixCoyMn1-x-yO2 cathodes and Li-M-Cl (Li2.5Y0.5Zr0.5Cl6, Li3InCl6, L2Sc1/3In1/3Cl4) catholytes. The effect of M in Li-M-Cl on the battery performance is analyzed through DFT calculations, electrochemical methods, and ToF-SIMS. Cycling solid-state batteries with LiNi1/3Co1/3Mn1/3O2 and LiNi0.85Co0.1Mn0.05O2 demonstrates that capacity fading depends on oxygen evolution in LiNixMnyCo1-x-yO2 and the nature of M in Li-M-Cl. When LiNixMnyCo1-x-yO2 does not undergo an OER, the cell performance is dictated by the intrinsic electrochemical stability of Li-M-Cl. However, the interfacial reactivity between LiNixMnyCo1-x-yO2 and Li-M-Cl is more important when OER occurs. The chapter also establishes a multi-pronged approach to study the performance of new solid electrolytes in solid-state batteries with high-energy-density cathode active materials.