Crystal-Chemical Guide for Understanding Redox Energy Variations of M2+/3+ Couples in Polyanion Cathodes for Lithium-Ion Batteries

A crystal-chemical guide is provided for understanding how factors such as the crystal structure and covalency of the polyanion affect the M 2+/3+ redox energies in polyanion cathodes. Although there are more rigorous techniques available, our approach is precise in spite of being simple. We show that an accurate prediction can be made with regard to the voltages delivered based on a basic understanding of how the coordination of the transition-metal ion affects the covalency of the M-O bond. Additionally, a new method for assessing the covalency of the polyanion (beyond the electronegativity of the countercation) is presented and used to explain why the voltage delivered by Li 2 MP 2 O 7 cathodes is higher than that of LiMPO 4 . Furthermore, a comparison of the silicate and phosphate structures reveals that edge sharing between transition metal polyhedra and other cation polyhedra has an opposite effect on the voltage delivered by these materials. For instance, edge sharing with LiO 4 polyhedra in the silicates raises the M 2+/3+ redox energy, whereas edge sharing with PO 4 polyhedra in the phosphates lowers the M 2+/3+ redox energy. This is due to a difference in the strength of the repulsive force exerted on the transition metal by the P 5+ cation when compared to Li + . This observation is significant since edge sharing has generally been viewed as a structural feature that lowers the redox energy. Lastly, crystal field splitting consideration alone is not sufficient to understand the voltage trends for polyanion cathodes and one must consider the contributions of the structure and/or the inductive effect. Our analysis provides new insights that may prove useful in tuning the voltage of existing polyanion systems and in the design of new cathode materials.