<p dir="ltr">Lithium-ion (Li-ion) batteries are promising electrochemical energy storage and conversion systems to drive the rechargeable world toward a sustainable future. Following the breakthrough of material innovations, advanced Li-ion batteries have significantly mitigated the range and lifetime anxieties of electric vehicles (EVs) and consumer electronics. Nevertheless, state-of-the-art Li-ion chemistries still suffer from several defects, such as rapid degradations under abusive or fast-charge scenarios and unfavorable high thermal instabilities. Essentially, aging mechanisms and safety hazards of Li-ion cells are strongly coupled events. The cell safety factors are most likely to be deteriorated as degradation progresses, making the cell less safe after a long-term deployment. In this thesis, we comprehensively investigate thermo-electrochemical interactions on the safety of Li-ion batteries. Fundamental principles of Li-ion batteries, basic knowledge about material-level thermal instabilities at electrode-electrolyte interphases, thermal characterization approaches, and thermal runaway mechanisms under abusive scenarios are fully overviewed. Thermally unstable characteristics of key cell components, including inter-electrode crosstalk as a result of oxygen liberation from cathode lattice structures, significant electric energy release from massive internal short circuit due to separator collapse, anode-centric lithium-plating-induced early exotherm, and silicon-dopant-driven thermal risks of composite anodes, are specifically discussed to understand their critical role in accelerating cell-level thermal runaway catastrophes. Aging pathways of Li-ion cells under off-normal conditions, particularly overdischarge and fast charging, are thoroughly elucidated using a promising reference electrode architecture, which effectively deconvolutes the electrode behaviors from the complex full-cell performance for precise identification of the root causes in cell failure. Given the profound revelation of degradation-safety sophistication in various Li-ion chemistries, corresponding mitigation and prevention strategies are proposed to maximize cell lifetime and reliability. This thesis provides new insights into aging and safety diagnostics of cutting-edge Li-ion batteries, taking one step further in the online monitoring of battery state of health to develop adaptive battery management systems.</p>

<p> Radical species are essential in modern chemistry. In addition to fundamental chemistry, their unique chemical bonding and distinct physicochemical features serve critical functions in materials science in the form of organic electronics. Due to their high reactivity, radicals of the main group element are often transient. In recent years, remarkably stable radicals are often stabilized by π-delocalization, sterically demanding side groups, carbenes, and weakly coordinating anions. The impacts of modifications such as electron-donating, electron-withdrawing, and end-capping on the spin density distribution and thermodynamic and kinetic stability of archetypal radical-driven processes such as dimerization are not well understood. This dissertation aims to track the perturbation of spin density from EDG and EWG modifications, provide mechanistic insight into the radical-initiated reactions of conjugated radical cations, and establish correlations between molecular design and thermochemical properties and their resulting kinetic stability by computationally evaluating these characteristics against experimental data. The disclosed connections give useful new recommendations for the rational design of thermodynamically and kinetically stable novel materials.</p>