Core-shell structured P2-type layered cathode materials for long-life sodium-ion batteries

P2-type layered Ni-Mn-based oxides are vital cathode materials for sodium-ion batteries (SIBs) due to their high discharge capacity and working voltage. However, they suffer from the detrimental P2 -> O2 phase transition induced by the O2--O2- electrostatic repulsion upon high-voltage charge, which leads to rapid capacity fade. Herein, we construct a P2-type Ni-Mn-based layered oxide cathode with a core-shell structure (labeled as NM-Mg-CS). The P2-Na-0.67[Ni0.25Mn0.75]O-2 (NM) core is enclosed by the robust P2-Na-0.67[Ni0.21Mn0.71Mg0.08]O-2 (NM-Mg) shell. The NM-Mg-CS exhibits the phase-transition-free character with mitigated volume change because the confinement effect of shell is conductive to inhibit the irreversible phase transition of the core material. As a result, it drives a high capacity retention of 81% after 1000 cycles at 5 C with an initial capacity of 78 mA h/g. And the full cell with the NM-Mg-CS cathode and hard carbon anode delivers stable capacities over 250 cycles. The successful construction of the core-shell structure in P2-type layered oxides sheds light on the development of high-capacity and long-life cathode materials for SIBs.