Na0.6[Li0.2Mn0.8]O2 has been synthesized at different calcination temperatures by a solid state method. Its phase structure and electrochemical properties are investigated and discussed. Na0.6[Li0.2Mn0.8]O2 prepared at 600 °C shows a complicated structure and wonderful electrochemical performance. In the voltage range of 2.0 and 4.8 V, Na0.6[Li0.2Mn0.8]O2/Li cell delivers an initial discharge capacity of 247 mAh g−1 with an average voltage of 3.52 V, which demonstrates a very high energy density of 869 Wh kg−1. Furthermore, the material displays a stable capacity greater than 240 mAh g−1 over 30 cycles.