Sodium (Na) metal is a promising anode for Na-ion batteries. However, the high reactivity of Na metal with electrolytes and the low Na metal cycling efficiency have limited its practical application in rechargeable Na metal batteries. High-concentration electrolytes (HCE, ≥4 M) consisting of sodium bis(fluorosulfonyl)imide (NaFSI) and ether solvent could ensure the stable cycling of Na metal with high Coulombic efficiency but at the cost of high viscosity, poor wettability, and high salt cost. Here, we report that the salt concentration could be significantly reduced (≤1.5 M) by a hydrofluoroether as an “inert” diluent, which maintains the solvation structures of HCE, thereby forming a localized high-concentration electrolyte (LHCE). A LHCE [2.1 M NaFSI/1,2-dimethoxyethane (DME)–bis(2,2,2-trifluoroethyl) ether (BTFE) (solvent molar ratio 1:2)] enables dendrite-free Na deposition with a high Coulombic efficiency of >99%, fast charging (20C), and stable cycling (90.8% retention after 40 000 cycles) of Na∥Na₃V₂(PO₄)₃ batteries.

中文翻译：

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局部高浓度电解质可实现极其稳定的钠金属电池

金属钠（Na）是钠离子电池的有希望的阳极。然而，Na金属与电解质的高反应性和低Na金属循环效率限制了其在可再充电Na金属电池中的实际应用。由双（氟磺酰基）亚胺钠（NaFSI）和醚溶剂组成的高浓度电解质（HCE，≥4M）可以确保Na金属稳定循环并具有高库仑效率，但代价是粘度高，润湿性差和粘度高盐成本。在这里，我们报告说，通过使用氢氟醚作为“惰性”稀释剂，可以将盐浓度显着降低（≤1.5M），从而保持HCE的溶剂化结构，从而形成局部的高浓度电解质（LHCE）。LHCE [2.1 M NaFSI / 1,2-二甲氧基乙烷（DME）-双（2,2,2-三氟乙基）醚（BTFE）（溶剂摩尔比1：₃ V ₂（PO ₄）₃电池。