DESIGN AND INVESTIGATION OF SINGLE-CRYSTAL AND COATING STRATEGIES FOR CATHODE MATERIALS IN LITHIUM-ION BATTERIES

Abstract

This thesis focuses on improving the lifetime of spinel LMO cathode materials through single-crystal synthesis and morphology control. It also investigates the role of surface coating elements in enhancing the electrochemical performance of medium-nickel NMC cathodes. The initial work focuses on developing single-crystal LMO spinel cathode materials to address the long-standing challenge of Mn dissolution by minimizing surface area. This study identifies Li₂MnO₃ as an effective sintering aid for single-crystal formation. Building on this understanding, multifaceted LMO single crystals were synthesized and identified using Laue diffraction. Their performance was benchmarked against commercial materials, but reducing surface area and controlling morphology did not prevent Mn dissolution, which remains a key barrier to the widespread adoption of spinel LMO. The next part of this work focuses on understanding the role of coating elements in improving the electrochemical performance of medium-nickel NMC cathodes at higher upper cutoff voltages. Using X-ray photoelectron spectroscopy (XPS), several common coating elements were investigated in commercial materials obtained from multiple reputable vendors. Tungsten was studied extensively, and two W-based surface compounds, Li₂WO₄ and Li₄NiWO₆, were identified using XPS. The beneficial role of the Li₄NiWO₆ surface compound was further investigated. However, this work also highlights the instability of this surface compound in NMP solvent. In addition, W-based surface compounds exhibited voltage-dependent dissolution and subsequent deposition of W on graphite during electrochemical cycling, and the impact of this deposition was demonstrated in a separate study. Another study focuses on understanding the role of cobalt rich surface layers. Cobalt rich NMC cathodes were synthesized and evaluated, showing improved performance comparable to vendor materials through minimized surface rock-salt phase formation. The final part of this work addresses air instability issues of NMC cathodes. Using XPS, surface fingerprints associated with air stability were identified, and the resulting strategy was applied to in-house NMC cathodes. Overall, this work demonstrates how single crystal engineering and coating elements can enhance the electrochemical performance and air stability of Li-ion battery cathodes.