Increase in grain boundary ionic conductivity of Li1.5Al0.5Ge1.5(PO4)3 by adding excess lithium

•LAGP was prepared by solid-state reaction with excess Li.•Excess Li increased Li ionic conductivity of LAGP despite a low relative density.•The grain boundary conductivity with excess Li was comparable to that of the bulk.

Li1.5Al0.5Ge1.5(PO4)3 (LAGP) is a promising solid electrolyte for all-solid-state Li ion batteries. In this work, it was synthesized using solid-state reaction with an excess amount of Li to improve grain boundary ionic conductivity. The additional Li improved grain boundary conductivity even though the relative density decreased. This improvement may originate from a beneficial characteristic of grain boundary induced by the segregation of some of Li to the grain boundary. This segregation was indirectly observed by the change of morphology of particles in samples with excess Li. The segregation of Li may result in a facile Li transport in grain boundaries, as indicated by low activation energy and a high pre-exponential factor of the grain boundary conductivity. Through improving grain boundary conductivity with excess Li, a high total ionic conductivity of 1.9 × 10− 4 S·cm− 1 is achieved at room temperature even with a low relative density of 78%. This porous and high ionic conducting solid electrolyte can be useful in configuring the electrode composite of all-solid-state cells.