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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.