Transition Metal Triggered Electrochemical Interphases in Sodium-Ion Batteries

Sodium-ion batteries are regarded as one of the most promising electrochemical energy storage devices because of their predicted advantages such as improved sustainability of sodium resources and significantly reduced cost. A large number of studies have been conducted to synthesize sodium layered oxide cathode materials that contain various transition metal elements (e.g., Ni, Mn, Co, Fe, Cu, Cr, V, Ti). Although these materials are structurally similar to the layered oxide materials in lithium ion batteries, they exhibit many distinct charging/discharging behaviors during Na + deintercalation/intercalation. For instance, the multi-phase transformation accompanied with multi-voltage plateau, which is related to Na + and vacancy ordered configuration, prevails in sodium layered oxide cathode materials and have been previously studied by synchrotron X-ray and neutron diffraction. Fe 4+ /Fe 3+ redox couple, which is electrochemically inactive in the lithium counterparts, are extensively explored and implanted into sodium layered oxide cathode materials on account of further reducing the cost of sodium ion batteries. To deliver the practical promise that sodium ion batteries hold, however, methods need to be established to tackle fundamental challenges in sodium ion batteries, including structural and chemical transformations of the electrode materials, nature of the electrode-electrolyte interfacial reactions, and rate-determining steps in the fast charging. These challenges are rooted in the interfacial charge transfer processes and determine battery characteristics such as energy density, power density, and safety. Herein, we report the interfacial descriptors such as atomic orbitals and local coordination that drive the interfacial reactions and phase transformation. We then develop a conceptually new strategy, that is controlling the characteristics of transition metal-oxygen bonds, to stabilize the interphases and accelerate the charge transfer processes.