Electrochemical performance of NiCl 2 with Br-free molten salt electrolyte in high power thermal batteries

NiCl2 with high theoretical voltage and thermal decomposition temperature attracts much attention as cathode material for thermal batteries with the requirement of high power density, high energy density and long work time. Unfortunately, the practical utilization of thermal batteries with NiCl2 cathode is limited by their poor electrochemical performance under large current, even with the conventional LiF-LiCl-LiBr all-lithium molten salt electrolyte which proposes ultrahigh lithium ion conductivity. In this work, an unexpected ionic exchange reaction between NiCl2 and LiBr in LiF-LiCl-LiBr was found, which would be the main reason for the poor electrochemical behavior of thermal batteries with NiCl2 cathode and LiF-LiCl-LiBr molten salt. On this basis, LiF-LiCl-Li2SO4, another all-lithium molten salt free of LiBr, was investigated as the new electrolyte for NiCl2 cathode. For the single cell of Li(Si)/LiF-LiCl-Li2SO4/NiCl2, a discharge capacity of 377 mA h g−1 (with a cut-off voltage of 1.2 V) was achieved with large current density (500 mA cm−2) applied at 520°C, which is almost twice of that of Li(Si)/LiF-LiCl-LiBr/NiCl2(190 mA h g−1) at the same conditions.