Präparation und Charakterisierung von Hochvolt-Kathodenmaterialien für Lithium-Ionen-Akkumulatoren auf Basis von Lithium-Übergangsmetall-Phosphoolivinen

Transition metal phosphates LiMPO4 (M = Fe, Mn, Co, Ni) as cathode materials have turned out to be an alternative for the commonly used oxides. The redox process is expressed by formula (a): LiM(II)PO4 --> Li+ + e- + M(III)PO4 (a). The voltage of LiMPO4 varies with the transition metal M. The insulating properties of LiMPO4 entail low rate capabilities and discharge capacities. Partial substitution of M for divalent cations has been described as a useful concept. The cations replacing M can either be electrochemical active transition metals or inactive metals that don’t deliver capacity, but can influence the electrochemical behaviour by structure or charge density. In this work, the effect of partial substitution in LiMPO4 is tested in binary and ternary mixed systems of Mn, Fe, Co, Ni and Mg. The substitution is combined with an aqueous co-precipitation route with subsequent calcination. The materials were characterised by ICP-OES, XRD and SEM. Galvanostatic cycling was performed. The redox process in LiMPO4 is limited to the M2+/3+ step. To acquire a second redox step M3+/4+ residual Li+ has to be available in the structure. This can be achieved in LiMPO4 by partial substitution of M for Mg. Mg doesn’t deliver e- thus fixing an equivalent of Li+ in the host structure after completion of the M(II)/M(III)-step (formula (b)): Li[MgIIxMII1-x]PO4 --> Lix[MgIIxMIII1-x]PO4 + (1-x) e- + (1-x) Li+ (b). Hypothetically further oxidation is possible according to: Lix[MgIIxMIII1-x]PO4 --> [MgIIxMIVxMIII1-2x]PO4 + x e- + x Li+ (c). By cycling between 3.0 V and 5.3 V, we find an additional redox plateau at 4.8 V for Mg and Mn co-containing samples. Furthermore, the reversible capacity exceeds 100 % of the theoretical value for the M2+/3+ step of the electrochemical active elements. We don’t see these phenomena if Mn or Mg is missing. Transferring this model on LiNixM1-xPO4, with M = Fe, Mn, and with the electrochemical inactive Ni, we also find this additional redox step for M = Mn