High-Voltage Degradation Mechanisms in Mn-Rich Layered Oxides for Sodium-Ion Battery Cathodes

Sodium-ion batteries (SIBs) have emerged as a compelling alternative to lithium-ion systems, particularly due to their enhanced low-temperature performance and power density. However, the relatively lower energy density—primarily limited by cathode performance—remains a key bottleneck 1 . In this work, we explore strategies to push the upper voltage limit of layered transition metal oxides, specifically focusing on P2- and O3-type NaₓTMO₂ cathodes. Although increasing the upper cut-off voltage beyond 4.15 V (vs Na⁺/Na) theoretically enhances energy density (E=QV), it concurrently triggers complex degradation phenomena, including electrolyte oxidation, structural phase transitions, gas evolution, and transition metal dissolution. These processes have often been understood individually in reality, however we have confirmed that they occur in conjunction or consecutively within few mVs . These intertwined processes remain poorly understood so far. Different analytical tools need to be used simultaneously to deconvolute such processes. Oftentimes, one or the other degradation phenomena is overlooked due to the complexity of the active material. To address this, we employed a series of analytical method combining: operando X-ray diffraction (XRD) to monitor real-time structural evolution, differential electrochemical mass spectrometry (DEMS) to track gas evolution 2 , and ex-situ extended X-ray absorption fine structure (EXAFS) 3 and nuclear magnetic resolution (NMR) analysis to probe local sodium and transition metal environments. These advanced tools are integrated with conventional electrochemical methods—such as cyclic voltammetry 4 , GITT and differential capacity analysis—to build a coherent understanding of high-voltage instabilities. Note that it is important to combine the electrochemistry of the material to operando and ex-situ techniques in order to precisely decode the mechanisms occuring at different voltages. Furthermore, to mitigate the degradation process structural engineering of active material was done wherein, we investigate a biphasic P2/O3 5 intergrown structure to assess how phase modelling can suppress detrimental reactions and improve electrochemical stability at high voltages. The results provide critical insights into decoupling concurrent degradation pathways by combining various techniques and lay the groundwork for development of rational design of more robust cathode materials. Thus resolving key high-voltage degradation mechanisms, this study advances the fundamental understanding required to bridge the energy density gap in sodium-ion batteries. References Nayak, P. K.; Yang, L.; Brehm, W.; Adelhelm, P., From Lithium-Ion to Sodium-Ion Batteries: Advantages, Challenges, and Surprises. Angewandte Chemie - International Edition 2018, 57 (1), 102-120. Li, Y.; Mazzio, K. A.; Yaqoob, N.; Sun, Y.; Freytag, A. I.; Wong, D.; Schulz, C.; Baran, V.; Mendez, A. S. J.; Schuck, G.; Zając, M.; Kaghazchi, P.; Adelhelm, P., Competing Mechanisms Determine Oxygen Redox in Doped Ni–Mn Based Layered Oxides for Na-Ion Batteries. Advanced Materials 2024, 36 (18), 2309842. Zhang, L.; Dahn, J. R.; Xiao, P.; Metzger, M., A Novel Quantification Method for High Voltage Structural Evolution in Sodium and Lithium Layered Oxides. Advanced Energy Materials n/a (n/a), 2501405. Kaliyaraj Selva Kumar, A.; Zhang, Y.; Li, D.; Compton, R. G., A mini-review: How reliable is the drop casting technique? Electrochemistry Communications 2020, 121 , 106867. Yang, L.; del Amo, J. M. L.; Shadike, Z.; Bak, S. M.; Bonilla, F.; Galceran, M.; Nayak, P. K.; Buchheim, J. R.; Yang, X. Q.; Rojo, T.; Adelhelm, P., A Co- and Ni-Free P2/O3 Biphasic Lithium Stabilized Layered Oxide for Sodium-Ion Batteries and its Cycling Behavior. Advanced Functional Materials 2020, 30 (42). Figure 1