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

Li-6 magic-angle spinning NMR spectroscopy has been used to probe the lithium local environments in a series of layered compounds, Cr-doped LiCoO2, Co-doped LiCrO2, Cr-doped LiMnO2, and the Li-rich material Li[Li0.2Cr0.4Mn0.4]O-2. A series of resonances are seen for LiCrxCo1-xO2 due to Cr3+ 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 LiCrO2 cannot be directly extrapolated from those measured in the magnetically dilute Cr3+-doped LiCoO2 compounds, and remains constant with temperature (from room to 283 degreesC). This behavior is ascribed to the strong antiferromagnetic couplings that exist in LiCrO2. The behavior of Cr3+-doped layered LiMn(III)O-2 is qualitatively similar to that of Co3+-doped LiCrO2, both compounds showing additional resonances caused by the dopant cations. Dramatically different NMR spectra are seen for Li[Li0.2Cr0.4Mn0.4]O-2. A series of sharper and broader resonances are observed, which are assigned to Li2MnO3-like and Mn4+-doped LiCrO2 regions or domains in the solid, respectively. Lithium is found in the transition metal layers in the Li2MnO3 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 Mn4+ being deintercalated as the potential increases. By 4.4 V, only the Li2MnO3 regions in the solid and lithium local environments such as Li(OMn4+)(3)(OCr)(OLi)(2) remain.