Crystal-Chemical Guide for Understanding Redox Energy Variations of M^2+/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 M2+/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 Li2MP2O7 cathodes is higher than that of LiMPO4. 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 LiO4 polyhedra in the silicates raises the M2+/3+ redox energy, whereas edge sharing with PO4 polyhedra in the phosphates lowers the M2+/3+ redox energy. This is due to a difference in the strength of the repulsive force exerted on the transition metal by the P5+ 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.