A first principles investigation of transitional metal doping in lithium battery cathode materials

Abstract

The goal of this work is to understand the properties of mixed-metal intercalation oxides. Using first-principles methods, the effect of doping on the mixing, energetic, and voltage properties as well as the phase diagrams of lithium transition-metal oxides for lithium battery cathode materials was investigated. The effect of doping on the phase separation tendencies of layered transition-metal oxides was examined and it was found that for normal processing temperatures, Al is miscible in layered transition metal oxides (LiMO2) for five of the eight first-row transition metals studied. Temperature-composition phase diagrams for both Li(Al,Co)O2 and Li(Al,Cr)O2 were calculated. In these two systems, Al-doping is limited above 600°C by the formation of [gamma]-LiA1O2 and at very low temperatures owing to the existence of a miscibility gap. Reduced solubility is expected in the layered phase above 600°C for all oxides which have substantial solubility with LiA1O2 due to the formation of yLiAlO2. The effect of transition-metal doping on the average voltage properties in Mn-based spinets was calculated and the large increase in average voltage found experimentally was reproduced. A detailed analysis on the layered structure Li(Al,Co)O2 was performed, studying the energetics of different lithium sites and the effect of short-range clustering on the shape of the voltage curve. Though the average voltage is raised by Al substitution, the unexpected stability of sites with a few Al nearest neighbors leads to an initial decrease in voltage. For the Al-doped LiCoO2 system, a step in the voltage curve is found only for micro-segregated materials. When the Al and Co ions are randomly distributed in a solid solution, the voltage curve shows a continuous, gradual slope. The effect of oxygen defects in the Li(Al,Co)O2 system was investigated. A model for the effect of oxygen vacancies on the free energy of doped layered oxides was created by combining an ideal gas approximation and first-principles energy defect calculations. The results qualitatively confirm experimental studies on oxygen release in lithium battery materials.