Doped lithium nickel cobalt mixed oxides for the positive electrode in lithium ion batteries

Phase pure aluminium and magnesium doped lithium nickel cobalt mixed oxides Li(Ni,Co0.1–0.2M≤0.05)O2 (M=Al, Mg) were synthesised in laboratory by a synthesis procedure adopted from H.C. Stark. Structural parameters were determined by Rietveld analysis of x-ray diffraction spectra. Electrochemical characterisation took place in three-electrode teflon cells and coin-type cells versus lithium metal. Thermal stability of cathodes without electrolyte was measured by DSC. For aluminium and magnesium doped lithium nickel cobalt mixed oxides Li(Ni,Co0.1–0.2M≤0.05)O2 (M=Al, Mg) the layered structure is stabilised by both aluminium and magnesium. The lithium nickel disorder is decreased by cobalt and is nearly unaffected by aluminium. According to the Rietveld refinements, magnesium seems to reduce the lithium nickel disorder strongly, even though refinements are not totally reliable in this case. Initial capacity and reversibility in the first cycle are nearly unaffected by aluminium, but strongly inferred by magnesium. Both, aluminium and magnesium doping increase the cycling stability of lithium nickel cobalt mixed oxides. Increased thermal stability of charged electrodes without electrolyte by aluminium and magnesium doping seems to be due to limited delithiation.