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 (NaxMnO2) cathode materials for sodium rechargeable batteries by incorporation of aluminum. The Al incorporation results in NaAl0.1Mn0.9O2 with a mixture of tunnel and layered crystal structures. NaAl0.1Mn0.9O2 shows a much higher initial discharge capacity and superior cycling performance compared to pristine Na0.65MnO2. We ascribe this enhancement in performance to the formation of a new ortho-rhombic layered NaMnO2 phase merged with a small amount of tunnel Na0.44MnO2 phase in NaAl0.1Mn0.9O2, and to improvements in the surface stability of the NaAl0.1Mn0.9O2 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 Na0.44MnO2 and NaAl0.1Mn0.9O2, suggest a strategy for achieving sodium rechargeable batteries with high energy density and stability.