Aluminum manganese oxides with mixed crystal structure: High-energy-density cathodes for rechargeable sodium batteries

We report a new discovery for enhancing the energy density of manganese oxide (Na_xMnO₂) cathode materials for sodium rechargeable batteries by incorporation of aluminum. The Al incorporation results in NaAl_0.1Mn_0.9O₂ with a mixture of tunnel and layered crystal structures. NaAl_0.1Mn_0.9O₂ shows a much higher initial discharge capacity and superior cycling performance compared to pristine Na_0.65MnO₂. We ascribe this enhancement in performance to the formation of a new orthorhombic layered NaMnO₂ phase merged with a small amount of tunnel Na_0.44MnO₂ phase in NaAl_0.1Mn_0.9O₂, and to improvements in the surface stability of the NaAl_0.1Mn_0.9O₂ particles caused by the formation of Al-O bonds on their surfaces. Our findings regarding the phase transformation and structure stabilization induced by incorporation of aluminum, closely related to the structural analogy between orthorhombic Na_0.44MnO₂ and NaAl_0.1Mn_0.9O₂, suggest a strategy for achieving sodium rechargeable batteries with high energy density and stability. Going Al in: The incorporation of aluminum into Na_xMnO₂ results in aluminum manganese oxides (NaAl_0.1Mn_0.9O₂) showing higher initial discharge capacity and superior cycling performance compared to pristine Na_xMnO₂. The enhanced performance is due to the formation of a new orthorhombic layered NaMnO₂ phase merged with a small amount of tunnel Na_0.44MnO₂ phase in NaAl_0.1Mn_0.9O₂, and to increase in the surface stability of the NaAl_0.1Mn_0.9O₂ particles caused by the formation of Al-O bonds on their surfaces.