High interfacial resistance in Li₇La₃Zr₂O₁₂(LLZO)/Li cells is correlated with the presence of Li₂CO₃on LLZO surfaces.

Layered Na-based oxides with the general composition NaxTMO2 (TM=transition metal) have attracted significant attention for their high compositional diversity that provides tuneable electrochemical performance for positive electrodes in in sodi-um-ion batteries. The various compositions bring forward complex structural chemistry that is decisive for the layered stack-ing structure, Na-ion conductivity and the redox-activity, potentially promising new avenues in functional material proper-ties. In this work, we have explored the maximum Na content in these P2-type layered oxides, and discovered that the high-content Na in the host enhances the structural stability, moreover, it promotes the oxidation of low-valent cations to their high oxidation states (in this case Ni2+). This can be rationalized by the increased hybridization of the O(2p)-TM(3d-eg) states, affecting both the local TM environment as well as the interactions between the NaO2 and TMO2 layers. These properties are highly beneficial for the Na storage capabilities as required for positive electrodes in sodium-ion batteries. It leads to excel-lent Na-ion mobility, a large storage capacity (>100 mAh g−1), yet preventing the detrimental sliding of the TMO2 layers (P2-O2 structural transition), as reflected by the ultra-long cycle life (3000 (dis)charge cycles demonstrated). These findings expand the horizons of high Na-content P2-type materials, providing new insights of the electronic and structural chemistry for advanced cathode materials.