Lithiation of ramsdellite-pyrolusite MnO2; NMR, XRD, TEM and electrochemical investigation of the discharge mechanism

Electrolytic manganese dioxide (EMD) is made in aqueous sulfuric acid and neutralized or ion exchanged with aqueous lithium hydroxide before use in Li batteries. Solid state Li NMR studies show that Li is present on surface and vacancy sites and migrates into Mn (III) related sites after heat treatment to remove water. During heat treatment the MnO2 rearranges from ramsdellite/pyrolusite intergrowth EMD to a defect pyrolusite heat-treated manganese dioxide (HEMD). EMD exhaustively treated with lithium hydroxide solution has 40-50% of the protons in EMD exchanged for Li ions to produce a structurally unchanged gamma-MnO2. Li magic angle spinning (MAS) NMR reveals that this lithiated material contains lithium in cation vacancy and Mn (III) related sites in the MnO2 lattice in addition to ionic Li on the surface. During heat treatment, the vacancy lithium content prevents the ramsdellite to pyrolusite rearrangement in HEMD formation. Instead, an ordered ramsdellite/pyrolusite intergrowth of lithiated manganese dioxide (LiMD) is formed with an approximate composition of 50% ramsdellite and 50% pyrolusite. Li MAS NMR of LiMD shows Li resonances near 280 and 560ppm, consistent with Li transition from surface and cation vacancy sites into the ramsdellite lattice. LiMD discharged against lithium shows two processes, one near 3.1V, the other about 2.8V. Li MAS NMR studies show the initial reduction results a lithium resonance near 560ppm associated with Li near a mixed valence Mn (III/IV) environment followed by appearance of a resonance near 100ppm consistent with a Li environment near Mn (III). TEM studies of the reduced material show initial expansion of the ramsdellite lattice accompanied by a loss in crystallinity in the 3.1V discharge process followed by disappearance of the pyrolusite content via conversion to ramsdellite in the second discharge process.