Influence of the structure on the electrochemical performance of lithium transition metal phosphates as cathodic materials in rechargeable lithium batteries: A new high-pressure form of LiMPO₄ (M = Fe and Ni)

Materials built from MO₆ octahedra linked to XO₄ tetrahedra are good candidates for studying the different factors that determine the electrode potential. Among them, olivine-like LiMPO₄ (M = transition metal) phosphates are especially interesting. When pressure is applied to LiMPO₄ (M = Ni and Fe), a phase transition is induced. However, instead of the well-known olivine ⇔ spinel transformation, a transition to a new phase is observed (β′). The arrangements of the metal ions (including phosphorus) in the two structures are very similar; thus, the main difference between them is due to the oxygen arrangement in a similar matrix. Raman spectroscopy has confirmed the structural model proposed for the high-pressure phase, in particular the modification in the lithium coordination from 6- to 4-fold upon synthesis under pressure. Among the olivines LiMPO₄ (M = Mn, Ni, and Fe), the iron-containing one is only active up to 5.1 V. On the other hand, none of the high-pressure materials is electrochemically active; this can be explained by the change in the electrostatic field at the transition metal position.