⁶Li MAS NMR Studies of the Local Structure and Electrochemical Properties of Cr-doped Lithium Manganese and Lithium Cobalt Oxide Cathode Materials for Lithium-Ion Batteries

6 Li magic-angle spinning NMR spectroscopy has been used to probe the lithium local environments in a series of layered compounds, Cr-doped LiCoO 2, Co-doped LiCrO 2, Cr-doped LiMnO 2, and the Li-rich material Li[Li 0.2 Cr 0.4 Mn 0.4 ]O 2 . A series of resonances are seen for LiCr x Co 1 - x O 2 due to Cr 3+ occupancy of the first and second cation coordination sphere surrounding Li. The shifts, which are caused by a transferred hyperfine interaction, differ in size and sign, depending on the Li−O−Cr bond angle. The shift of LiCrO 2 cannot be directly extrapolated from those measured in the magnetically dilute Cr 3+ -doped LiCoO 2 compounds, and remains constant with temperature (from room to 283 °C). This behavior is ascribed to the strong antiferromagnetic couplings that exist in LiCrO 2 . The behavior of Cr 3+ -doped layered LiMn(III)O 2 is qualitatively similar to that of Co 3+ -doped LiCrO 2, both compounds showing additional resonances caused by the dopant cations. Dramatically different NMR spectra are seen for Li[Li 0.2 Cr 0.4 Mn 0.4 ]O 2 . A series of sharper and broader resonances are observed, which are assigned to Li 2 MnO 3 -like and Mn 4+ -doped LiCrO 2 regions or domains in the solid, respectively. Lithium is found in the transition metal layers in the Li 2 MnO 3 regions of the solid, the concentration of Li in the layers decreasing substantially in the Cr-rich regions. Electrochemical deintercalation shows that the chromium-rich areas are oxidized initially, lithium local environments containing progressively more Mn 4+ being deintercalated as the potential increases. By 4.4 V, only the Li 2 MnO 3 regions in the solid and lithium local environments such as Li(OMn 4+ ) 3 (OCr)(OLi) 2 remain.