Lattice parameter and sintering temperature dependence of bulk and grain-boundary conduction of garnet-like solid li-electrolytes

The dependence of the bulk and grain-boundary lithium-ion conduction on the lattice parameter by substitution of trivalent La by divalent Mg, Ca, Sr, Sr0.5 Ba0.5, or Ba and monovalent Li in garnet-like Li5 La3 Ta2 O12, and the effect of sintering temperature were investigated. The ionic bulk conductivity increases with increasing ionic radius of the divalent alkaline earth ion and corresponding increased lattice parameter. An exception is Mg, which is too small for replacing La and forms a second phase. The lattice parameter is also found to increase with increased sintering temperature, except for the mixed Sr0.5 Ba0.5 substituted sample. The Ca, Sr, Sr0.5 Ba0.5, and Ba compounds show mainly bulk resistances with minor boundary contribution at room temperature, which decreases with increasing size of the alkaline earth ion. In contrast, the multiphase Mg-substituted sample exhibits an appreciable grain-boundary contribution to the total resistance. Microstructural investigations indicate the dependence of the grain-boundary resistance on the grain size, sinterability, and formation of transient or steady-state phase boundary compositions, which are caused by different chemical diffusion coefficients of the components. This is related to the higher conductivities of Li6 Mg La2 Ta2 O12 and Li6 Ba La2 Ta2 O12 annealed at 900°C compared to samples annealed at 950°C.