CHEMICAL AND ELECTROCHEMICAL INSERTION OF NA INTO THE SPINEL LAMBDA-MNO2 PHASE

We report the electrochemical reaction of sodium with the spinel, lambda-MnO2. Cycling data show that the room temperature reaction of Na induces an irreversible phase transformation to a new structure of NaxMnO2 that can reversibly cycle 0.6 Na atoms per Mn atom. This compound can also be obtained at room temperature by reacting the spinel phase with a solution of sodium iodine under reflux in acetonitrile. Upon heating, this low temperature phase changes to a second polytype above 100-degrees-C, at a temperature that depends on the heating rate, and then to a third polytype above 600-degrees-C. The phase formed above 600-degrees-C is identified as the three-dimensional phase Na0.44MnO2. The low temperature and medium-temperature phases are probably layered phases, with structure related to that of Na0.7MnO2.