Influence of Cobalt Introduction on the Phase Equilibrium and Sintering Behavior of Lithium-Ion Conductor Li1.3Al0.3Ti1.7(PO4)3

After the reported high sinterability of a NASICON-type lithium-ion conducting electrolyte Li1.3Al0.3Ti1.7(PO4)3 (LATP) in contact with LiCoPO4, which was found in the studies on a co-firing process for an all-solid-state battery, the present authors have investigated the influence of cobalt introduction on the sintering behavior of LATP as well as the phase equilibrium. It was indicated that 1–2 mol % cobalt is allowed to be a substitute for the constituent cation in the NASICON structure. The slightly Co-doped LATP phase shows a clearly high sinterability compared with a normal LATP, with the relative density approaching 80% after firing at 800 °C. A further increase in the cobalt content results in the separation of the excess cobalt as LiCoPO4 together with the other second phases. In the case of the nominal composition such that titanium in LATP is partly replaced by cobalt (Li1.3+2x Al0.3Co x Ti1.7‑x (PO4)3), a remarkable densification behavior was observed; e.g., a relative density approximating 100% was attained after firing at 800 °C for x = 0.1. The significantly high sinterability is ascribed to a low melting point compound LiPO3, which forms as the second phase together with LiCoPO4 and LiTiO­(PO4).