Rechargeable lithium-ion batteries (LIBs) are crucial for transitioning towards a carbon neutral society. Nickel-rich layered oxides are the state-of-the-art material of choice, offering high practical capacities. However, accessing this high capacity induces structural degradation and parasitic electrolyte reactions at the surface. One strategy to mitigate this problem is surface modification with coating materials like Al₂O₃ – cycling life is extended but the fundamental reasons behind the protection are not well understood. Therefore, the aim of the thesis is to identify the characteristics of coatings that contribute to this protection with polycrystalline LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) coated with Al₂O₃ by atomic layer deposition (ALD) being studied. Varying the number of ALD cycles allows investigation of the influence of coating thickness (sub-nm differences) on surface degradation. Coupled with electrochemical cycling, it is revealed that an optimal coating thickness exists, balancing initial capacity with retentions. X-ray spectroscopy and diffraction confirm surface degradation is reduced with coating on the NMC811. Meanwhile, ²⁷Al solid state (ss)NMR indicates that the chemical evolution of the coating is not correlated with coating thicknesses. Detailed structural and chemical evolution of the Al₂O₃ coating at different stages of the electrochemical cycling process is tracked using a multinuclear NMR approach. Probing dipolar interactions between ²⁷Al and ¹H/¹⁹F/⁷Li shows the coating is susceptible to reactions involving acidic and protic species from the electrolyte, but no lithiated alumina phase is formed. The changes are concomitant with stabilisation of the surface oxygen electronic structure by the coating, supported by less electrolyte degradation. Finally, the heat treatment and performance of Al₂O₃ coated NMC811 is presented to provide insights into the effect of a range of coating structures. ²⁷Al ssNMR shows that heating induces Al doping into the bulk NMC structure in parallel to crystallisation of the initial amorphous phase. However, the electrochemical cycling suggests a conformal coating performs better than these modifications. From these studies, it is clear that Al₂O₃ surface modification have a chemical role in passivating the NMC811 surface, with strong implications for material engineering of long-lasting and high energy density LIBs.