Addressing the Low Solubility of a Solid Electrolyte Interphase Stabilizer in an Electrolyte by Composite Battery Anode Design

Metallic sodium (Na) has been regarded as one of the most attractive anodes for Na-based rechargeable batteries due to its high specific capacity, low working potential, and high natural abundance. However, several important issues hinder the practical application of the metallic Na anode, including its high reactivity with electrolytes, uncontrolled dendrite growth, and poor processability. Metal nitrates are common electrolyte additives used to stabilize the solid electrolyte interphase (SEI) on Na anodes, though they typically suffer from poor solubility in electrolyte solvents. To address these issues, a Na/NaNO₃ composite foil electrode was fabricated through a mechanical kneading approach, which featured uniform embedment of NaNO₃ in a metallic Na matrix. During the battery cycling, NaNO₃ was reduced by metallic Na sustainably, which addressed the issue of low solubility of an SEI stabilizer. Due to the supplemental effect of NaNO₃, a stable SEI with NaN_xO_y and Na₃N species was produced, which allowed fast ion transport. As a result, stable electrochemical performance for 600 h was achieved for Na/NaNO₃||Na/NaNO₃ symmetric cells at a current density of 0.5 mA cm^-2 and an areal capacity of 0.5 mAh cm^-2. A Na/NaNO₃||Na₃V₂(PO₄)₂O₂F cell with active metallic Na of ∼5 mAh cm^-2 at the anode showed stable cycling for 180 cycles. In contrast, a Na||Na₃V₂(PO₄)₂O₂F cell only displayed less than 80 cycles under the same conditions. Moreover, the processability of the Na/NaNO₃ composite foil was also significantly improved due to the introduction of NaNO₃, in contrast to the soft and sticky pure metallic Na. Mechanical kneading of soft alkali metals and their corresponding nitrates provides a new strategy for the utilization of anode stabilizers (besides direct addition into electrolytes) to improve their electrochemical performance.