High-temperature battery system operated at or below 300 °C is highly desired for geothermal borehole applications. However, it remains a challenge to find suitable electrode materials meeting the need of wide borehole temperature. In this study, the discharge performance of Li2MoO4/LiNO3-KNO3/Li-Mg-B alloy battery system was studied over the temperature range of 150~300 °C with different current densities ranging from 10 to 30 mA cm−2. The electrochemical test result showed that the single cell delivered maximum discharge capacity at 250 °C, with value of 1178.6 mAh g−1 at a current density of 10 mA cm−2. Besides, it is found that the discharge capacities of the single cell increased as the current density rose at 300 °C attributed to the enhanced ionic conductivity at high temperature. As a result, the Li2MoO4/LiNO3-KNO3/Li-Mg-B alloy system is capable of being used in instrumentation for geothermal and oil/gas borehole explore between 200 and 250 °C.