Annual Review 2024-2025

S P E C T R O S C O P Y G R O U P A N N U A L R E V I E W 2 0 2 4 / 2 5 30 Distinguishing bulk and surface degradation of lithium nickel oxide cathodes in lithium-ion batteries using multidisciplinary X-ray spectroscopy techniques Layered transition metal (TM) oxides such as lithium nickel oxides (LiNiO 2 ) are the cathode materials of choice for commercial high energy density lithium-ion batteries as these materials can endure thousands of lithium intercalation cycles to increase their capacity. Furthermore, there is a growing preference for nickel-rich compositions to maximise capacity while minimising cobalt content. However, the role of the different redox centres in LiNiO 2 remains a topic of debate, and the connections between these processes and structural instabilities, along with the associated degradation, are not yet fully understood. In this work the authors from the UK, France and Canada have conducted extensive investigations into the oxygen redox processes occurring in these cathode materials using various multidisciplinary spectroscopy techniques. Their research emphasises the importance of combining bulk and surface-sensitive spectroscopy techniques, and HERFD-XAS on I20-Scanning was utilised to investigate the detailed electronic states of bulk nickel in pristine and charged LiNiO 2 . This work has demonstrated that the molecular oxygen redox processes in cathode materials are primarily bulk phenomena that contribute to reversible charge compensation, and that the surface instability of LiNiO 2 is linked to rehybridisation at high states of charge (Figure 1). This research underscores the importance of employing strategies such as cathode coatings, composition gradients, and tailored electrolyte formulations to stabilise nickel-rich cathode surfaces in contact with the electrolyte, rather than relying solely on bulk stabilisation methods to enhance the capacity of lithium-ion batteries. DOI: 10.1039/d4ee02398f Schematic representation of LiNiO 2 bulk charge compensation mechanism and surface degradation processes probed by different X-ray spectroscopy techniques.