Enabling High-Voltage Cathodes in All-Solid-State Batteries

<p dir="ltr">Challenges such as high costs and safety concerns, including thermal instability and flammability, have become increasingly critical for current electrochemical batteries. In response, advanced battery materials and systems are emerging to address these drawbacks. This dissertation focuses on key innovations in materials design, interfacial engineering, and processing strategies to enable safe, affordable, and high-energy-density all-solid-state battery systems. </p><p dir="ltr">Chapter 4 presents a strategy for improving the processability and performance of garnet-type Li 6.4La 3Zr 1.4Ta 0.6O 7 (LLZTO) solid electrolytes. By incorporating LiAlO 2 (LAO) as a sintering aid, we achieve a larger grain size (~25 μm), a high relative density (~96%), a lower porosity (~3.7%), and continuous secondary phases with low ion conductivity in grain boundary regions. This improved structure results in (i) an improved Li-ion conductivity and enhanced interfacial resistance between Li metal and LLZTO by a denser structure with fewer pores and (ii) suppression of Li dendrite penetration in the electrolyte by secondary phases in grain boundary regions. </p><p dir="ltr">Chapter 5 introduces a polyoxanorbornene-based ionically conductive polymer (ICP) as an interfacial modifier between LLZTO electrolytes and high-voltage Ni-rich layered cathodes. The integration of ICP significantly improves interfacial contact and Li-ion transport, enabling high specific capacity (174.4 mAh g -1 at 0.1C) and stable long-term cycling (73.3% capacity retention at 0.3C over 300 cycles), demonstrating the practical viability of polymer-enhanced oxide-based ASSBs. </p><p dir="ltr">Chapter 6 focuses on a cobalt-free, lithium- and manganese-rich (CFLMR) layered oxide cathode paired with LLZTO electrolytes. Despite its high capacity and voltage, CFLMR suffers from poor compatibility with ionic polymers due to high oxidative reactivity. We address this by designing a surface-stable protective LiNbO 3 (LNO) coating layer that preserves the first activation of Li 2MnO 3 and enhances cycling stability. This work highlights the crucial role of surface engineering in overcoming interfacial degradation in high-voltage oxide-based ASSBs. </p><p dir="ltr">Chapter 7 shifts attention to sulfide-based ASSBs by exploring and tuning the processing parameters of LPSCl solid electrolytes, with a focus on understanding their impact on microstructure and interfacial compatibility. Through a moderate hot-pressing approach, we achieve improved densification (93% relative density), higher ionic conductivity (1.61 mS cm-1), and reduced interfacial resistance with Li metal (44.8 Ω·cm 2 ). These improvements lead to high specific capacity (150.4 mAh g -1 at C/10) and excellent cycling stability (79% retention over 100 cycles at C/3) in full cells with Ni-rich layered cathodes. </p><p dir="ltr">Chapter 8 concludes with the integration of CFLMR cathodes in sulfide-based ASSBs. Here, we reveal that direct contact with LPSCl hinders oxygen redox activation and causes CEI formation, resulting in rapid degradation. A LNO surface coating effectively mitigates these issues, restoring activation behavior and structural integrity. The resultant cathodes display higher capacities, improved cycling stability, and stabilized Mn redox states, establishing surface coating as a necessary strategy for enabling high-voltage operation in sulfide-based ASSB.</p>