Synthese und Charakterisierung von übergangsmetallbasierten Polyanionischen Kathodenmaterialien auf Phosphatbasis

Abstract

LiFePO4 with a low redoxpotential of 3.5V vs. Li/Li+ is already commercialised. Changing the transition metal from Fe to Mn results in an increase in redoxpotential of 0.6V while ionic and electronic conductivity seem to be decreased. During cycling volume change between lithiated and delithiated phase leads to a dynamic strain at the phase boundary. The Jahn-Teller ion Mn3+ is located in the delithiated phase causing a lattice distortion leading to chemical and structural instability of the delithiated phase. Several approaches to improve LiMnPO4 electrochemical performance have been investigated. Substitutions with electrochemically active and/or inactive metals (Mg, Ni, Zn, Fe, Co) have been investigated for structural effects and their influence on the electrochemical performance. Lithium ion diffusivity and the redox potential were shown to be positively affected. To gain further insight into the electrochemical lithium intercalation process chemical delithiation was performed. The LixMnPO4 materials have been characterised by a new combination of various techniques to determine chemical composition, degree of oxidation of the electrochemically active metal, valence state, structure and phase composition as well as phase stability at elevated temperatures. As the electrochemical redoxprocess Mn2+/Mn4+ is limited in the olivine structure by the Li:M ratio of one, NASICON materials with a molecular formula of Li3M2(PO4)3 provide a greater Li:M-ratio thus theoretically enabling the electrochemical utilisation of the Mn2+/Mn4+ two-electron redoxstep. As the manganese based NASICON has not been mentioned in literature yet, aim of this concept was to prove the general existence of the Li3Mn2(PO4)3 material via various synthesis routes. A possible addressation of the manganese two-electron step was key point of the electrochemical characterisation. Further phase stabilities for various Mn2+ and Mn3+ based ternary (or quarternary) (Li-)M-P-O materials are discussed.