The rapidly increasing demand for energy storage necessitates the development of the next-generation rechargeable Li-ion batteries (LIBs) with high energy density, enhanced safety, long-cycling life, and low cost. Over the past few decades, tremendous effort has been devoted to understanding how LIBs work and developing high-performance battery materials. One focus is to design and discover solid electrolytes, which will replace the flammable liquid electrolytes in the current generation of rechargeable LIBs. This doctoral dissertation focuses on tracking the charge carrier, Li+, transport in composite solid electrolytes and understanding lithiation mechanisms of organic electrodes with high-resolution solid-state nuclear magnetic resonance (NMR). Composite electrolytes, combing the advantages of inorganic and polymer electrolytes, have attracted fast-growing attention due to their compatibility with electrodes, enhanced stability, and desirable mechanical properties. However, Li-ion transport pathways in these complex systems are not well understood: Li ions can transport via organic matrix, inorganic fillers, organic-inorganic interface or the combination of the three. To examine these possibilities, systematic studies have been carried out in this thesis work. Firstly, Li ions in different local structures (inorganic filler, polymer matrix, and inorganic/polymer interface) are distinguished with high-resolution 6Li NMR. Furthermore, tracer-exchange NMR is developed and has been proven to be an effective strategy to probe Li-ion transport pathways in solid electrolytes, which combines 6Li7Li isotope exchange with high-resolution 6Li NMR (Chapter 2). Chapters 3-5 discuss the Li-ion transport in representative oxide-polymer, i.e., Li7La3Zr2O12 (LLZO)-poly(ethylene oxide) (PEO), and sulfide-polymer, i.e., Li10GeP2S12 (LGPS)-PEO, composite electrolytes. Li-ion chemical environments and ion transport pathways are correlated with ion mobility, ionic conductivity, and chemical and electrochemical stability. Tracer-exchange NMR can be widely used to determine ion transport pathways in solid electrolytes for Li- or Na-ion batteries. In addition to electrolytes, electrodes are also critical to battery performance, which determine the energy and power densities of batteries. Conventional cathodes are transition-metal based inorganic compounds, consuming non-renewable resources and generate heavy metal wastes. Considering long-term sustainability, organic materials composed of light and abundant elements are considered promising as alternatives. Besides, organic cathodes are chemically diverse and can deliver much higher theoretical capacity compared with inorganic cathodes. However, the practical capacities of organic cathodes are often much lower than the theoretical values and capacity deteriorates fast during cycling. Chapter 6 and Chapter 7 aim at investigating Li-ion transformation in quinone-based organic cathodes, chloranilic acid and 7,7,8,8-tetracyanoquinodimethane, upon cycling with ex situ and in situ NMR and EPR, correlating the structural evolution with electrochemical performance, which can contribute to reveal the origin of capacity decay of organic cathodes. The inferior cycling capacity of organic electrodes originates from the incomplete lithiation/de-lithiation of organic molecules because of the relatively low electric and ionic conductivities, high solubility of organic compounds and intermediate products in liquid electrolytes, and irreversible side reaction products. In summary, these studies shed light on fundamental mechanisms of how battery materials work and provide insights for materials design, synthesis, and modification to develop the next-generation LIBs with high-performance and sustainability.
