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 candid ates for studying the different factors that determine the electrode potent ial. 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 double left right arrow spinel transformation, a transition to a new phase is observed (beta '). The arrangements of the metal ions (including phospho rus) in the two structures are very similar; thus, the main difference betw een them is due to the oxygen arrangement in a similar matrix. Raman spectr oscopy has confirmed the structural model proposed for the high-pressure ph ase, 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; thi s can be explained by the change in the electrostatic field at the transiti on metal position.