[en] Highlights: • Long-term ambient storage leads to poor cycling stability of NMC-811. • Electrochemically inactive species were formed during ambient storage. • Mechanisms driving the effect of moisture on NMC-811 materials were identified. • Humid storage promotes the structural deterioration of NMC-811. -- Abstract: High-Ni cathode materials are prone to reactivity and instability upon exposure to ambient levels of humidity. This has implications for the storage and processing of cathodes for Li-ion batteries (LIBs), in order to avoid any premature degradation of the material prior to operation. NMC-811 materials were subjected to differing degrees of exposure to ambient humidity, in order to establish the impact this would have on electrochemical performance. This study used a combination of physical, chemical and electrochemical methods to investigate the operational effects that moisture can have on the battery accordingly. Longer-term cycling, d-SIMS and microscopy were used to characterise the degradation phenomena and relating to capacity fade. Post-mortem analysis revealed that the structural breakdown of the secondary particles is a dominant factor that influences charge transfer resistance increases. The study highlights the criticality of how high Ni materials are handled during storage and processing, in order to mitigate premature degradation events during operation.