Cubic perovskite–type oxide Li3/8Sr7/16Hf1/4Ta3/4O3 (LSHT) is a promising solid electrolyte material for next–generation Li–ion batteries such as all–solid–state Li batteries and Li–redox flow batteries due to its high room–temperature Li–ion conductivity and good chemical compatibility with commercial organic liquid electrolyte. High relative density is another important requirement for these applications. Here we investigate the influence of sintering additives (Li3BO3, Li4SiO4, LiF) on the density, the total Li–ion conductivity, the interfacial conductivity against commercial liquid electrolyte, and the cycling performance of the hybrid Li-ion cells. The results show that comparing with other additives, LSHT sintered with 2wt% LiF shows the highest Li–ion conductivity (3.3×10−4 Scm−1) and the smallest interfacial resistance (125Ω cm2) against organic electrolyte at room temperature; Li/LiFePO4 batteries with LSHT pellets fired with 2wt% LiF have the best cyclability and the highest coulombic efficiency (99.6±0.04%) in 50 cycles.