Sustainable energy storage is the bottleneck for the integration of high-ratio renewable energy to the grid. The all-liquid-structure and membrane-free liquid metal batteries (LMBs), with the merits of low-cost, long-lifespan and easy-scale-up, are promising for large-scale energy storage applications. Previously reported lithium LMBs exhibit excellent electrochemical performance, however, the practical application of lithium LMBs was hindered by the sealing issue of highly corrosive lithium vapor, along with the very limited reserve of lithium.

Instead of lithium, due to the weaker corrosion and much higher abundance, sodium LMBs show great potential for long-term and large-capacity energy storage applications. However, sodium has a high dissolution in single-cationic molten sodium halide mixtures salt electrolytes, which causes severe self-discharge and low coulombic efficiency of batteries. Here, for the first time, high-performance molten salt electrolyte and alloy electrodes are designed for the realization of practical sodium LMBs. A multi-cationic ternary chloride (LiCl-NaCl-KCl) with a low melting point (~390 ℃) and low sodium halide content (5 mol% NaCl) is rationally designed as the electrolyte, which significantly inhibits the dissolution of sodium in the molten salt electrolyte. Based on the designed cationic electrolyte, cooperating with a dual-active Bi₉Sb alloy positive electrode, a sodium LMB is constructed and the battery stably cycles over 700 cycles with a coulombic efficiency of 97% at 100 mA cm^-2 and 450 ℃, indicating the design of coupled cation electrolyte and alloy electrode is feasible to inhibit the self-discharge of batteries. Meanwhile, the adoption of electrolytes with low active substance content in this work also provides new directions for the design of electrolytes for high-performance electrochemical cells based on molten salts for high-efficiency energy conversion and storage. The calculation based on MW energy storage system indicates that the estimated Levelized Cost of Storage (LCOS) of the sodium LMB is lower than 0.029 $/kWh. To further reduce the cost, LiCl-NaCl-CaCl₂ (28:32:40 mol%) and NaCl-CaCl₂ (50:50 mol%) have been investigated for their potential as electrolytes for sodium LMBs. The significant reduction of Li⁺ content resulted in batteries with lower cost (reduced by 32%), but the interaction of the anode (Na) with the electrolyte led to difficulties in releasing the capacity of the battery and poor stability. To address this issue, the structure and composition of the anode were optimized, which achieved the thermodynamic compatibility between the anode and the electrolyte and the stable operation of the battery. The results demonstrate that sodium LMB is a promising technology for grid-scale energy storage applications.