Crystal chemistry and valence determinations for Mn, Ni and Co oxides as cathode materials in Li batteries

• Exact valence–distance correlations for Ni, Mn and Co oxides were established.
• The valence determination is unambiguous from + 2 to + 4 for Ni and Mn.
• The correlations can be used to estimate the lattice strains.
• The strains result in material instability and cation ordering.

Cation charge distribution is one of the basic problems for cathode materials in Li and Na batteries. This work analyzed the valence–distance relationship for a large number of Ni, Mn, and Co oxides. It uses high quality structural data obtained recently, and takes into account the influence of lattice strains on the bond lengths. The linear character of the valence–distance correlation for the Ni and Mn cations allows for an accurate valence determination from the average Mn–O and Ni–O distances in the charge range from + 2 to + 4. The correlation for Co is more complicated, and the unambiguous valence assignment is possible only in the range from + 2 to + 3. Although we used mainly the diffraction data to establish the valence–distance relationships, in many cases, especially for the mixed oxides, the assignment of Ni, Mn, and Co oxidation states should be done based on the local structure descriptions obtained by X-ray absorption spectroscopy. Important examples are presented to illustrate the use of the valence–distance correlations for detailed structural analysis, especially for the lattice strain determinations.