Fe-rich layered oxide cathode for sodium-ion batteries enabled by synergistic modulation of ion transport and structural stability

The sustainability and availability of raw materials are of critical importance for sodium-ion batteries (SIBs) to have competitiveness. Iron (Fe) as an inexpensive and electrochemically active element in SIBs layered cathodes offers unique advantages. Nonetheless, Fe-rich materials typically perform poor and most reports focus on materials with Fe content around 1/3, as higher Fe content leads to Jahn-Teller distortion, irreversible structure damage, transition metal (TM) migration, and poor air stability. Herein, for the first time we report an Fe-rich material (Fe = 0.5) that has high energy density (143.28 mA h g-1 in 2-4 V) and shows comparable cyclability with typical low-Fe materials through the synergistic modulation of ion transport and structural stability. The pillar effect of Ca in the Na layer limits the gliding of the TMO2 slab and the migration of TM ions, while the addition of Al enhances the TM(3deg*)-O(2p) hybridization, reduces the lattice distortion, and suppresses the undesired phase transition. In a sodium-ion full cell system, an excellent cyclability of 82 % capacity retention after 150 cycles can be achieved, while the unmodified Fe-rich cathode only shows a capacity retention of 38 %. This work firstly demonstrates the feasibility of using Fe-rich materials as cathode materials for SIBs.