Modification and degradation of Ni-rich cathode-based Li-ion batteries

The bottleneck to increasing the energy density of Li-ion batteries (LIBs) is the battery cathode material. For instance, layered Ni-rich (nickel-rich) transition metal oxides have a relatively high energy density but suffer poor cycling stability. The instability of Ni-rich cathode-based LIBs is associated with various passive reactions occurring inside batteries.

Nanostructure morphology

Material optimization strategies can mitigate these detriments. For her PhD research, Ming Jiang considered electrode material design and degradation mechanisms of Ni-rich cathode-based batteries. Ni-rich NCM cathodes and Li-metal anodes with superior battery performance are achieved through these novel optimization strategies. The corresponding degradation phenomena are investigated, and detailed mechanisms are identified.

Jiang designed an optimized Ni-rich cathode material with nanostructure morphology. With this design, the battery stability and Li-ion transportation kinetics are enhanced through a unique fabrication method. Further characterizations affirm a high exposure ratio of specific crystal facets, which is beneficial for Li-ion diffusion and structural intactness during cycling. Moreover, the improved fast-charging capability has also been realized thanks to the well-designed morphology, suggesting a practical potential of the proposed material.

Deterioration processes

In addition to new material synthesis, Jiang investigated deterioration processes in battery systems as this is essential for boosting the development of LIBs. Multiple passive reactions occur simultaneously in a battery, such as cathode dissolution, solid-electrolyte-interphase formation, and micro-cracking. These side reactions consume active Li-ions and electrode materials, which eventually causes the battery capacity to decay.

Every part in a battery system may participate in passive reactions, including electrode materials, binders, conductive carbon, and other additives. However, it is difficult to separate each part during a post-mortem study for a conventional battery configuration. Therefore, Jiang proposed a thin-film cathode with a simplified structure for degradation investigation in the Ni-rich cathode battery system. With the depth-profiling technique, the composition of the passive layer and the mechanism of performance decline have been studied in detail. The function of the protective layer coating on the Ni-rich cathode has also been investigated.

Besides studying the cathode material, Jiang also explored the surface modification of Li-metal anode in Ni-rich cathode-based LIBs. A protective coating layer is introduced to stabilize the lithium plating/stripping process during cycling. This enables prolonged battery lifetime and mitigates the passive layer formation on the anode surface. Post-mortem characterizations reveal the different deterioration processes for various Li-metal anodes, and the possible degradation mechanism is studied in the research.

Title of PhD thesis: Modification and degradation of Ni-rich cathode-based Li-ion batteries. Supervisors: P.H.L. Notten, Rüdiger A. Eichel, and Dmitri L. Danilov.