Sodium layered oxides are considered to be cathode candidates with the most potential for large-scale energy storage because of their high reversible capacity and wide availability of sodium resources. A significant hurdle to wide application of these layered oxides lies in simultaneously satisfying high-energy density and long cycle life because of the intrinsic problems associated with their structural irreversibility. Herein, a O3/O′3-P2 core–shell composite that integrates a high specific capacity from O-type Ni-based core and good structural stability from P2-type Mn-rich shell is presented. Multiscale electron microscopy and affiliated spectroscopy analyses reveal that, in addition to the microscale O3/O′3-P2 core–shell structure, a nanoscale coherent P2/O3 intergrown structure can also be identified in the composite. Such well-tailored structures not only constrain the structural damages (microscale cracks) induced by repeated volumetric changes upon desodiation and resodiation but also facilitate fast Na ions diffusion through the exterior P2-type layered structure. This work may provide new clues into the design of high-performance cathode materials for sodium-ion batteries.

Transition metal (TM)-based layered oxides NaTMO 2 (TM = Fe, Ni, Co, Mn, etc.) have been intensively pursued as high-capacity cathode materials for Na-ion batteries. Nevertheless, they still suffer from fast capacity loss and voltage decay, as a result of the layered structure instability upon extended electrochemical cycling. The mechanism underlying such instability remains poorly understood. Here we unravel the TM migrations and structural evolution of a quaternary NaNi 0.3 Co 0.12 Mn 0.18 Fe 0.4 O 2 compound during electrochemical cycling using atomic-resolution electron microscopy and associated spectroscopies. We discover successive migrations of TM ions to Na layers that account for structure and performance degradations. The Fe ions migrate into the interstices of both tetrahedra and octahedra of the layers; on the contrary, the Ni ions migrate predominantly in the octahedral ones, and the Mn and Co ions mostly remain in the TM layers. Direct atomic-level observations of the TM migration process upon cycling offer deep insight into designing high-capacity and long-life span cathode materials for sodium-ion batteries.

No abstract

No abstract

No abstract