Crystal chemistry, chemical stability, and electrochemical properties of layered oxide cathodes of lithium ion batteries
Lithium ion batteries are now widely used as power sources in mobile electronics due to their high energy density. Layered LiCoO2 is currently employed as the cathode material in commercial lithium ion batteries, but its reversible capacity is limited to only 50 % of its theoretical capacity. Co is also relatively expensive and toxic. In this regard, layered LiNi1-y-zMnyCozO2 cathodes have become appealing recently as they offer higher capacity, lower cost, and enhanced safety compared to the LiCoO2 cathode. This dissertation explores the chemical and structural factors and instabilities that control and limit the electrochemical performance parameters such as the capacity, cyclability, and rate capability of various layered LiNi1-y-zMnyCozO2 cathodes. A quantitative determination of proton contents in various chemically delithiated oxide cathodes using Prompt Gamma Ray Activation Analysis (PGAA) indicates that while the delithiated layered Li1-xCoO2, Li1-xNi1/3Mn1/3Co1/3O2, and Li1- xNi1/2Mn1/2O2 have a significant amount of proton in the lattice at deep lithium extraction, orthorhombic Li1-xMnO2, spinel Li1-xMn2O4, and olivine Li1-xFePO4 do not encounter such proton insertion. The results are complemented by mass spectrometric and thermogravimetric analysis data. The differences are attributed to the differences in the chemical instability of the various cathodes. From a systematic investigation of three series of layered LiNi1-y-zMnyCozO2 compositions (LiNi0.5-yMn0.5-yCo2yO2, LiCo0.5-yMn0.5-yNi2yO2, LiNi0.5-yCo0.5-yMn2yO2), those around LiNi1/3Mn1/3Co1/3O2 are found to have optimized electrochemical performances with high reversible capacity, good cyclability, and good rate capability. The results are explained on the basis of chemical instability in the Co-rich compositions, lithium deficiency and concurrent cation disorder in the Ni-rich compositions, and existence of the impurity phase Li2MnO3 in the Mn-rich compositions. The electrochemical rate capability is found to bear a clear relationship to the chemical lithium extraction rate, which decreases with decreasing Co content due to an increasing cation disorder. Additionally, the lithium extraction rate is found to influence the structure of the chemically delithiated end members HxNi0.5-yMn0.5- yCo2yO2; the structure changes from P3 to O1 to O3 with decreasing Co content 2y. A comparison of the chemical stability of the Na0.75-xCoO2 system shows that it maintains the theoretical value of the oxidation state of cobalt during chemical sodium extraction to low sodium contents of (0.75-x) ≈ 0.3, while Li1-xCoO2 incorporates protons for (1-x) < 0.5. The differences between two systems are discussed based on the crystal structure and the position of Co3+/4+:3d band relative to the top of the O2-:2p band.