Synthesis Design and Full Cell Development of Ni-rich Cathodes for Lithium-ion Batteries

Williams, Ethan ORCID: 0009-0001-4328-9874 (2025). Synthesis Design and Full Cell Development of Ni-rich Cathodes for Lithium-ion Batteries. University of Birmingham. Ph.D.

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Abstract

The growing demand for electric vehicles (EVs) and grid storage solutions has driven the need for lithium-ion batteries (LIBs) with higher energy densities, that can endure longer lifetimes. Nickel-rich layered oxide cathodes, such as LiNi0.9Mn0.05Co0.05O2 (NMC90-5-5), are promising candidates for high-energy LIBs, due to their high specific capacity and limited cobalt content, but their practical application is hindered by capacity fade, structural instability, and poor cycle life. This thesis explores practical large batch synthesis and doping strategies to enhance the electrochemical performance and stability of Ni-rich NMC cathodes, with a focus on firing conditions, bulk-surface engineering, and full-cell optimisation.
First, the role of boron-based fluxes (boric acid and borax) in the synthesis of NMC90-5-5, introduced to optimise firing conditions, was investigated. Boric acid reduced bulk cation mixing and surface lithium residues, improving initial capacity and charge transfer kinetics but leading to capacity fade due to an irreversible phase transition at 4.1 V. Conversely, borax-doped cathodes exhibited superior long-term stability, with 75% capacity retention after 200 cycles, highlighting the importance of the boron-based fluxes in tailoring electrochemical performance.
To further improve the structural resilience and rate capability of Ni-rich cathodes, boron (B) and tin-boron (Sn-B) co-doping strategies were implemented. Sn-B co-doped cathodes demonstrated reduced cation mixing, lattice stabilisation, and higher reversibility of the H2-H3 phase transition. This material also delivered superior rate performance across a wide temperature range (-5°C to 45°C), due to enhanced lithium-ion diffusivity and reduced interfacial resistances, resulting in faster kinetics. Furthermore, post-mortem analyses confirmed that doping stabilised the oxidation states of transition metals, reduced residual lithium compounds and mitigated surface degradation caused by electrolyte decomposition. These findings highlight the synergistic role of Sn-B co-doping in improving the structural and electrochemical resilience of Ni-rich NMC cathodes at low and high temperatures, demonstrating the potential of tailored doping strategies for improving the performance of lithium-ion batteries under extreme conditions.
Finally, full-cell studies incorporating pristine and doped NMC 90-5-5 cathodes with graphite anodes were conducted to assess practical battery performance. Full-cells containing the Sn-B co-doped cathodes exhibited superior rate capability in both coin and pouch cells, with improved lithium transport and reduced polarization. A higher N/P ratio was also identified for optimised high-rate performance, by reducing overpotentials at the graphite to prevent plating and lowering the polarisation at the cathode to limit time spent at the high voltage phase transition. High-mass-loading pouch cells (~4.5 mA h cm−2) demonstrated the potential of Sn-B doping for commercial-scale LIB applications by maintaining long-term stability and mitigating degradation under fast rate cycling.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):