Substituted transition metal phospho olivines LiMM′PO4 (M = Mn, M′ = Fe, Co, Mg): Optimisation routes for LiMnPO4

Since transition metal phospho olivines gain increasing interest as cathode materials for lithium ion batteries in the last decades lots of publications appeared. Various synthesis methods were in the focus of interest as well as structural investigations of the pure LiMPO4 and mixed Li (MM′)PO4 phases (M, M′ = Fe, Mn, Co, Ni, Mg, Zn, Al) and their structural changes during electrochemical conversion. Lithium insertion and exsertion mechanisms have been studied with the help of e.g. structural, optical and electronic, and electrochemical characterisation methods. Likewise many efforts have been done for material optimisation concerning synthesis procedure or substitution. We tend to give an overview about Li (MM′Mn)PO4 (M, M′ = Mg, Fe, Co) on the basis of our results. For the greater topic of enhancement of performance and energy density of LiMnPO4 we discuss different solution approaches concerning the raise of specific capacity, redox potential and optimisation of material characteristics. Thus we consider effects due to the intrinsic conductivity, structural stability of the charged phase as well as its chemical stability against the electrolyte and the dynamic stability of the interface between charged phase and discharged phase during electrochemical conversion. For this purpose in our experimental part we focus on three different approaches: substitution with an electrochemically active transition metal, substitution with an electrochemically inactive metal in case for the manipulation of unit cell volume alternation and the substitution with electrochemically inactive metals for the purpose of providing a "lithium reservoir". This generated "lithium reservoir" is expected to be accessible for the utilisation of a new redox step. Phase transition in mixed transition metal phospho olivines Li(MnCo)PO4 has been investigated as example for the effect of substitution with an electrochemically active transition metal. Substitution with an electrochemically inactive metal leading to binary Li(MgMn)PO4 has been structurally and electrochemically investigated. Promising new electrochemical characteristics of binary Li(MgMn)PO4 phospho olivines are introduced for the first time. When charged to high potentials (>4.9 V) the activation of the Mn3+/Mn4+ step is reported.