From Local to Long-Range—Unveiling the Multi-Elements Redox Dynamics of Sodium Manganese Oxide in Sodium-Ion Batteries via Multi In Situ Characterizations

Sodium-ion batteries (SIBs) are receiving increasing attention due to the abundance of sodium resources and their wide distribution compared to their lithium counterparts. Benefiting from the environment-friendly and cost-effective manganese resources, as well as a high theoretical specific capacity of 265 mAh g^-1, P2-type layered Na_xMn_yO₂ cathode exhibits great potential in SIBs community. However, given that the complexity of the battery system, the practical cathodic reaction mechanism is inherently complicated by several factors such as mass loading, particle size, interfacial deposition, charge transfer, electrolyte and diaphragm, which hinders the application of the P2 type layered Na_xMn_yO₂ in sodium-ion batteries. Furthermore, these factors affect the cathodic reaction mechanism across multiple scales, from short-range to long-range scales, thereby completing the analysis. However, elucidation of the actual cathode reaction and degradation mechanisms is essential for optimizing cathode performance and developing sodium-ion batteries with high energy density and long cycle life.
In this study, we employ a comprehensive micro-to macro-length characterizations approach that combines in-situ X-ray diffraction (XRD), in-situ pair distribution function (PDF) analysis, in-situ Raman spectroscopy, in-situ differential electrochemical mass spectrometry (DEMS) and HR-TEM (Transmission Electron Microscopy). This integrated characterization strategy provides a holistic understanding of the reaction mechanisms at the cathode of sodium-ion batteries. This work elucidates the degradation mechanism of the P2-type layered Na_xMn_yO2 cathode combining global structure and local-structure analysis that manganese atom migration induces severe phase transitions that lead to intense particle cracking after long-term cycling. To further analysis the origin of the manganese atom migration, in-situ Raman and in-situ DEMS techniques were employed to probe the anionic-redox reaction evolution from the local-scale to overall scale during battery cycling process. It was found that part oxygen released from Na_xMn_yO₂ at the sodium-deficient state, and these oxygen releases originate from an irreversible oxygen reaction, leading to a deterioration of its cyclic stability. Based on this finding, this work proposes a sulfuration approach to stabilize the Na_xMn_yO₂ structure, as sulfur anions are partially integrated into the oxygen sites within the lattice structure. Impressively, the oxygen redox reaction is significantly inhibited due to the higher gasification temperature of sulfur compared to oxygen. The sulfur anions within the internal lattice could reversibly participate in the muti-elements redox process and improve the integral coordination stability by mitigating undesired manganese migration. Moreover, after several cycles, there are no significant cracks in the material particles with sulfuration compared to the unmodified material. Consequently, the modified P2-Na_xMnO_2-yS_y exhibits remarkably long-term cycling stability of >1000 cycles with 98% capacity retention at 0.5 A g^-1 compared to the unmodified P2 sample which retains only 18% of its capacity after 1000 cycles. Our work not only reveals the degradation mechanism of the P2-type layered Na_xMn_yO₂ cathode but also presents a pathway for designing long-cycle-life SIB cathode materials.