Heyin Chen: Towards sustainable Ni-rich layered oxide cathodes: A synchrotron-based study

Abstract

Rechargeable Li-ion batteries (LIBs) are essential for portable electronic devices, electric vehicles and the development of large-scale energy storage for renewable sources. Among various cathode materials in LIBs, layered Ni-rich transition metal oxides are widely used due to their high energy density. Conventionally, the toxic N-methyl-2-pyrrolidone (NMP) solvent and fluorine-containing polyvinylidene fluoride (PVdF) binder are utilized during electrode manufacturing. However, it is desirable to replace NMP with an environmentally friendly solvent and also to aim for a fluorine-free binder. Thus, this thesis aims to develop the aqueous-processing methodology for LiNi0.8Mn0.1Co0.1O2 (NMC811) electrode production.

This thesis identifies the formation of carboxylate species as a product of the irreversible reaction between the NMC811 surface and H2O vapor. Furthermore, results show that aqueous processing generates a reactive electrode surface, with subsequent electrolyte decomposition. In addition, a NiO-like rock-salt phase forms in the near-surface regions, most likely due to Li-ion leaching and Li/Ni disorder. Also, increased charge transfer resistance is observed, which likely correlate to the rock-salt phase. Building on insights into H2O’s effects on the NMC811 surface, two aqueous-processing methods for producing NMC811 electrodes are studied. To mitigate these challenges, firstly H3PO4 is added to the aqueous slurry, primarily to lower the pH and limit Al current collector corrosion. This modification to some extent stabilizes the reactive electrode surface and alleviates Li/Ni disorder, leading to improved capacity retention and enhanced reversibility of the phase transition. Secondly, with the aim to stabilize the NMC811 surface during aqueous processing, Ti is incorporated within the structure. This effectively hinders rock-salt phase formation and reduce the Li-ion transfer resistance. With inspiration from a reaction heterogeneity detected in the aqueous-processed NMC811 electrode, the study further investigates particle-scale Li-ion heterogeneity in the commercially aged LixNi0.9Co0.05Al0.05 secondary particles, suggesting a significant Li-ion heterogeneity within the particles cycled to a high state of charge.

In conclusion, this thesis elucidates the degradation mechanisms of aqueous-processed NMC811 material and demonstrates the roles of material modifications in enhancing cycling performance, offering valuable insights into the manufacturing of sustainable batteries. Furthermore, it highlights the importance of employing X-ray-based techniques for in-depth studies of battery materials.