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 orthorhombic 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 AlO 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.