The passivation of negative electrodes is key to achieving prolonged charge–discharge cycling with Na-ion batteries. Here, we report the unusual passivation ability of superconcentrated Na-salt electrolytes. For example, a 50 mol % sodium bis(fluorosulfonyl)amide (NaFSA)/succinonitrile (SN) electrolyte enables highly reversible Na⁺ insertion into a hard carbon negative electrode without any electrolyte additive, functional binder, or electrode pretreatment. Importantly, an anion-derived passivation film is formed via preferential reduction of the anion upon charging, which can effectively suppress further electrolyte reduction. As a structural characteristic of the electrolyte, most anions are coordinated to multiple Na⁺ cations at high concentration, which shifts the lowest unoccupied molecular orbitals of the anions downward, resulting in preferential anion reduction. The present work provides a new understanding of the passivation mechanism with respect to the coordination state of the anion.

中文翻译：

---

钠离子电池中超浓缩电解质对硬碳负电极的异常钝化能力

负极的钝化是延长钠离子电池充放电循环的关键。在这里，我们报告了超浓缩钠盐电解质的非凡钝化能力。例如，50摩尔％的双（氟磺酰基）酰胺钠（NaFSA）/琥珀腈（SN）电解质无需任何电解质添加剂，功能性粘合剂或电极预处理，就可以高度可逆地将Na ⁺插入硬碳负极中。重要的是，通过在充电时优先还原阴离子来形成源自阴离子的钝化膜，这可以有效地抑制电解质的进一步还原。作为电解质的结构特征，大多数阴离子与多个Na ^+配位高浓度的阳离子，将阴离子的最低未占据分子轨道向下移动，从而导致优先的阴离子还原。目前的工作提供了一种有关阴离子配位状态的钝化机理的新认识。