Gradiently Sodiated Alucone as an Interfacial Stabilizing Strategy for Solid‐State Na Metal Batteries

All‐solid‐state metal batteries (ASSMBs) are attracting much attention due to their cost effectiveness, enhanced safety, room‐temperature performance and high theoretical specific capacity. However, the alkali metal anodes (such as Li and Na) are active enough to react with most solid‐state electrolytes (SSEs), leading to detrimental reactions at the metal–SSE interface. In this work, a molecular layer deposition (MLD) alucone film is employed to stabilize the active Na anode/electrolyte interface in the ASSMBs, limiting the decomposition of the sulfide‐based electrolytes (Na₃SbS₄ and Na₃PS₄) and Na dendrite growth. Such a strategy effectively improves the room‐temperature full battery performance as well as cycling stability for over 475 h in Na–Na symmetric cells. The modified interface is further characterized by X‐ray photoelectron spectroscopy (XPS) depth profiling, which provides spatially resolved evidence of the synergistic effect between the dendrite‐suppressed sodiated alucone and the insulating unsodiated alucone. The coupled layers reinforce the protection of the Na metal/electrolyte interface. Therefore, alucone is identified as an effective and bifunctional coating material for the enhancement of the metal/electrolyte interfacial stability, paving the way for rapid development and wide application of high‐energy ASSMBs.     

A Quasi-Solid-State Polyether Electrolyte for Low-Temperature Sodium Metal Batteries

Taken the unlimited Na reservoir worldwide, battery technology based on Na-ion chemistry poses as an ideal candidate for large-scale energy storage systems. Especially, with metallic Na replacing traditional carbon anodes, it's able to maximize the energy density inexpensively. Nevertheless, sodium metal batteries (SMBs) face intrinsically poor stability due to their highly-reactive nature, where low Coulombic efficiency and short lifetime are often witnessed. The situation can be further aggravated at low temperatures due to insurmountable kinetic barriers. Herein, a 1,3-dioxolane-based quasi-solid-state electrolyte (PDGE) is proposed with a high ionic conductivity of 3.68 mS cm⁻¹ even at −20 ^◦C for SMBs. Moreover, a weak solvation environment is tailored by PDGE, which possesses a high Na⁺ transference number of 0.7. Concurrently, the solid electrolyte interphase induced from PDGE presents inorganic Na₂O, NaF as the major components, which offers accelerated Na⁺ diffusion and superior stability upon long-term cycling. With such a quasi-solid-state electrolyte, the Na/Na₃V₂(PO₄)₃ full cell exhibits great stability over 1000 cycles at −20 ^◦C. This study has significant implications to the development for SMBs under low-temperature conditions.     

Regulation of Interphase Layer by Flexible Quasi-Solid Block Polymer Electrolyte to Achieve Highly Stable Lithium Metal Batteries (Adv. Funct. Mater. 27/2023)

Low safety, unstable interfaces, and high reactivity of liquid electrolytes greatly hinder the development of lithium metal batteries (LMBs). Quasi-solid-state electrolytes (QGPEs) with superior mechanical properties and high compatibility can meet the demands of LMBs. Herein, a biodegradable polyacrylonitrile/polylactic acid-block-ethylene glycol polymer (PALE) as membrane skeleton for GPEs is designed and systematically investigated by regulating the length and structure of the cross-linked chain. Benefiting from the enriched affinitive sites of polar functional groups ( CO,  C O C,  CN, and  OH) in highly cross-linked polymer structure, the designed PALE membrane skeleton exhibits flame-retardant property and ultrahigh liquid electrolyte uptake property, and the derived quasi-solid-state PALE GPEs deliver enhanced stretchability and a higher electrochemical stable window of 5.11 V. Besides, the PALE GPEs effectively protect cathodes from corrosion while allowing uniform and fast transfer of Li⁺ ions. Therefore, the Li||Li symmetrical battery and LFP or NCM811||Li full-cell using PALE GPEs exhibit excellent cycling stability coupled with compact and flat inorganic/organic interface layers. And the excellent cycling stability of pouch cells under harsh operating conditions indicates the application possibilities of PALE GPEs in flexible devices with high-energy-density.     

SnF2-Induced Highly Current-Tolerant Solid Electrolytes for Solid-State Sodium Batteries

Solid-state sodium batteries have garnered considerable interest. However, their electrochemical performance is hampered by severe interfacial resistance between sodium metal and inorganic solid electrolytes, as well as Na dendrite growth within the electrolytes. To address these issues, a uniform and compact SnF₂ film is first introduced onto the surface of the inorganic solid electrolyte Na_3.2Zr_1.9Ca_0.1Si₂PO₁₂ (NCZSP) to improve contact through an effective and straightforward process. Through experiments and computations, the in situ conversion reaction between SnF₂ and molten Na is adequately confirmed, resulting in a composite conductive layer containing Na_xSn alloys and NaF at the interface. As a result, the interfacial resistance of Na/NCZSP is significantly decreased from 813 to 5 Ω cm², and the critical current density is dramatically increased to 1.8 mA cm⁻², as opposed to 0.2 mA cm⁻² with bare NCZSP. The symmetric cell is able to cycle stably at 0.2 mA cm⁻² for 1300 h at 30 °C and exhibits excellent current tolerance of 0.3 and 0.5 mA cm⁻². Moreover, the Na₃V₂(PO₄)₃/SnF₂-NCZSP/Na full cell displays excellent rate performance and cycling stability. The SnF₂-induced interlayer proves significant in improving interfacial contact and restraining sodium dendrite propagation, thus promoting the development of solid-state sodium batteries.     

Dynamic Supramolecular Polymer Electrolyte to Boost Ion Transport Kinetics and Interfacial Stability for Solid-State Batteries

The key hurdle to the practical application of polymeric electrolytes in high-energy-density solid lithium-metal batteries is the sluggish Li⁺ mobility and inferior electrode/electrolyte interfacial stability. Herein, a dynamic supramolecular polymer electrolyte (SH-SPE) with loosely coordinating structure is synthesized based on poly(hexafluoroisopropyl methacrylate-co-N-methylmethacrylamide) (PHFNMA) and single-ion lithiated polyvinyl formal. The weak anti-cooperative H-bonds between the two polymers endow SH-SPE with a self-healing ability and improved toughness. Meanwhile, the good flexibility and widened energy gap of PHFNMA enable SH-SPE with efficient ion transport and superior interfacial stability in high-voltage battery systems. As a result, the as-prepared SH-SPE exhibits an ionic conductivity of 2.30 × 10⁻⁴ S cm⁻¹, lithium-ion transference number of 0.74, electrochemical stability window beyond 4.8 V, and tensile strength up to 11.9 MPa as well as excellent adaptability with volume change of the electrodes. In addition, no major electrolyte decomposition inside batteries made from SH-SPE and LiNi_0.8Mn_0.1Co_0.1O₂ cathode can be observed in the in situ differential electrochemical mass spectrometry test. This study provides a new methodology for the macromolecular design of polymer electrolytes to address the interfacial issues in high-voltage solid batteries.     

Ultra-Thin Single-Particle-Layer Sodium Beta-Alumina-Based Composite Polymer Electrolyte Membrane for Sodium-Metal Batteries

Inorganic/organic composite polymer electrolytes (CPEs) with good flexibility and electrode contact have been pursued for solid−state sodium-metal batteries. However, the application of CPEs for high energy density solid−state sodium-metal batteries is still limited by the low Na⁺ conductivity, large thickness, and low ion transference number. Herein, an ultra-thin single-particle-layer (UTSPL) composite polymer electrolyte membrane with a thickness of ≈20 µm straddled by a sodium beta−alumina ceramic electrolyte (SBACE) is presented. A ceramic Na⁺-ion electrolyte that bridges or percolates across an ultra-thin and flexible polymer membrane provides: 1) the strength and flexibility from the polymer membrane, 2) excellent electrolyte/electrode interfacial contact, and 3) a percolation path for Na⁺-ion transfer. Owing to this novel design, the obtained UTSPL-35SBACE membrane exhibits a high Na⁺-ion conductivity of 0.19 mS cm⁻¹ and a transference number of 0.91 at room temperature, contributing to long−term cycling stability of symmetric sodium cells with a small overpotential. The assembled quasi-solid-state cell with the as−prepared UTSPL-35SBACE membrane displays superior cycling performance with a discharge capacity of 105 mAh g⁻¹ at 0.5 °C rate after 100 cycles and excellent rate performance (82 mAh g⁻¹ at 5 °C rate) at room temperature with the potassium manganese hexacyanoferrate (KMHCF)@CNTs/CNFs cathode, where KMHCF refers to potassium manganese hexacyanoferrate.     

Largely Promoted Mechano-Electrochemical Coupling Properties of Solid Polymer Electrolytes by Introducing Hydrogen Bonds-Rich Network

Solid polymer electrolytes (SPEs) and their composites are the most promising spices to access the commercial application in all-solid-state lithium batteries, where definite requirements for SPEs should be satisfied including moderate mechanical strength, high Li-ion conductivity, and stable electrode/electrolyte interface. Herein, polyurethane-based polymer (PNPU) is designed to further construct the hybrid solid polymer electrolyte (named as PNPU-PVDF-HFP) with poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) for high energy density solid-state lithium metal batteries. The theoretical calculation and characterization demonstrate that PNPU-PVDF-HFP SPEs still maintain the multiple hydrogen bonding modes of PNPU, which contributes a significantly improved mechanical properties of the polymer membrane with compact structure. Moreover, it is corroborated that PNPU is involved to form the double Li⁺ transport paths in the hybrid electrolyte, accelerating the migration of lithium ions. Therefore, PNPU-PVDF-HFP SPEs are achieved with suitable tensile strength of 5.16 MPa and high elongation of 140.8%, high ambient ionic conductivity of 4.13 × 10⁻⁴ S cm⁻¹, excellent ductile, and stability on the interface of lithium metal anode. The Li/ LiFePO₄ and Li/Li[Ni_0.8Co_0.1Mn_0.1]O₂ solid-state batteries using PNPU-PVDF-HFP SPEs present a stable cycling performance at 30 °C. This study provides a feasible strategy to achieve mechano-electrochemical coupling stable SPEs for solid-state batteries.     

Succinonitrile as a Versatile Additive for Polymer Electrolytes

Solid polymer electrolytes with high ionic conductivities are prepared by using poly(ethylene oxide) (PEO) and poly(vinylidene fluoride‐co‐hexafluoropropylene) (P(VDF‐HFP)) as polymer matrixes, succinonitrile (SN) as an additive, and lithium bis‐trifluoromethanesulfonimide (LiTFSI) and lithium bisperfluoroethylsulfonylimide (LiBETI) as salts. In these systems, the introduction of succinonitrile into the polymer electrolytes increases the material's ionic conductivity and conveys excellent mechanical properties. The described composites, with their beneficial combination of mechanical and electric properties, are expected to have significant potential for lithium batteries.     

A Hetero-Layered,Mechanically Reinforced,Ultra-Lightweight Composite Polymer Electrolyte for Wide-Temperature-Range,Solid-State Sodium Batteries

The practical use of polyethylene oxide polymer electrolyte in the solid-state sodium metallic batteries (SSMBs) suffers from the retard Na⁺ diffusion at the room temperature, mechanical fragility as well as the oxidation tendency at high voltages. Herein, a hetero-layered composite polymeric electrolyte (CPE) is proposed to enable the simultaneous interfacial stability with the high voltage cathodes (till 4.2 V) and Na metallic anode. Being incorporated within the polymer matrix, the sand-milled Na₃Zr₂Si₂PO₁₂ nanofillers and nanocellulose scaffold collectively endow the thin-layer (25 µm), ultralightweight (1.65 mg cm⁻²) CPE formation with an order of magnitude enhancement of the mechanical strength (13.84 MPa) and ionic conductivity (1.62 × 10⁻⁴ S cm⁻¹) as compared to the pristine polymer electrolyte, more importantly, the improved dimension stability up to 180 °C. Upon the integration of the hetero-layered CPE with the iron hexacyanoferrate FeHCF cathode (1 mAh cm⁻²) and the Na foil, the cell model can achieve the room-temperature cycling stability (93.73% capacity retention for 200 cycles) as well as the high temperature tolerance till 80 °C, which inspires a quantum leap toward the surface-wetting-agent-free, energy-dense, wide-temperature-range SSMB prototyping.     

Regulating Na-ion Solvation in Quasi-Solid Electrolyte to Stabilize Na Metal Anode

Sodium metal batteries are promising for cost-effective energy storage, however, the sluggish ion transport in electrolytes and detrimental sodium-dendrite growth stall their practical applications. Herein, a cross-linking quasi-solid electrolyte with a high ionic conductivity of 1.4 mS cm⁻¹ at 25 °C is developed by in-situ polymerizing poly (ethylene glycol) diacrylate-based monomer. Benefiting from the refined solvation structure of Na⁺ with a much lower desolvation barrier, random Na⁺ diffusion on the Na surface is restrained, so that the Na dendrite formation is suppressed. Consequently, symmetrical Na||Na cells employing the electrolyte can be cycled >2000 h at 0.1 mA cm⁻². Na₃V₂(PO₄)₃||Na batteries reveal a high discharge specific capacity of 66.1 mAh g⁻¹ at 15 C and demonstrate stable cycling over 1000 cycles with a capacity retention of 83% at a fast rate of 5 C.     

Highly‐Cyclable Room‐Temperature Phosphorene Polymer Electrolyte Composites for Li Metal Batteries

Despite significant interest toward solid‐state electrolytes owing to their superior safety in comparison to liquid‐based electrolytes, sluggish ion diffusion and high interfacial resistance limit their application in durable and high‐power density batteries. Here, a novel quasi‐solid Li⁺ ion conductive nanocomposite polymer electrolyte containing black phosphorous (BP) nanosheets is reported. The developed electrolyte is successfully cycled against Li metal (over 550 h cycling) at 1 mA cm^?2 at room temperature. The cycling overpotential is dropped by 75% in comparison to BP‐free polymer composite electrolyte indicating lower interfacial resistance at the electrode/electrolyte interfaces. Molecular dynamics simulations reveal that the coordination number of Li⁺ ions around (trifluoromethanesulfonyl)imide (TFSI^?) pairs and ethylene‐oxide chains decreases at the Li metal/electrolyte interface, which facilitates the Li⁺ transport through the polymer host. Density functional theory calculations confirm that the adsorption of the LiTFSI molecules at the BP surface leads to the weakening of N and Li atomic bonding and enhances the dissociation of Li⁺ ions. This work offers a new potential mechanism to tune the bulk and interfacial ionic conductivity of solid‐state electrolytes that may lead to a new generation of lithium polymer batteries with high ionic conduction kinetics and stable long‐life cycling.     

Stable Na Metal Anode Enabled by a Reinforced Multistructural SEI Layer

Metallic sodium (Na) is one of the most promising anode candidates for next‐generation secondary batteries. The development of Na metal batteries with a high energy density and low cost is desirable to meet the requirements of both portable and stationary electrical energy storage. Unfortunately, several problems caused by the unstable Na metal anode severely hinder the practical applications of these batteries. Here reported is a facile but effective methodology to form a multistructural interphase layer containing a sodium fluoride‐rich solid electrolyte interphase (SEI) and crisscrossed Na₃Sb bars on the Na electrode surface. The reinforced Na‐alloy network and chemically/electrochemically complementary SEI formation greatly improve the interphase strength and Na⁺ conductivity. The well‐protected Na metal electrode in symmetric Na|Na cells is stable and dendrite‐free in the plating and stripping cycling processes with a negligible voltage divergence, even at a large current density of 5 mA cm^?2 or with a high deposition capacity of 10 mAh cm^?2. Moreover, this anode is especially compatible with different cathodes and demonstrates outstanding cycle performance in the full cells. It is believed that this approach provides a practical solution toward stable Na metal anodes and related battery systems.     

Multifunctionalized Safe Separator Toward Practical Sodium-Metal Batteries with High-Performance under High Mass Loading

Introducing sodium as anode to develop sodium metal batteries (SMBs) is a promising approach for improving the energy density of sodium-ion batteries. However, fatal problems, such as uncontrollable sodium dendrite growth, unstable solid electrolyte interphase (SEI) in low-cost carbonate-based electrolytes, and serious safety issues, greatly impede the practical applications. Here, a multifunctionalized separator is rationally designed, by coating PP separator (<25 µm) with a solid-state NASICON-type fast ionic conductor layer (NZSP@PP) to replace the widely used thick glass fiber separator (>200 µm) and successfully solves all of the above problems, and for the first time creats high performance SMBs by using Na₃V₂(PO₄)₃ (NVP) cathodes in pouch cell. The Na||NVP full cells can stably cycle over 1200 times with capacity retention of 80% at a high rate of 10 C and deliver a specific capacity of 80 mAh g⁻¹ even at high rate of 30 C, indicating extraordinary fast-charging characters. The full SMBs can also stably cycle 200 times with a retention of 96.4% under high NVP loading of 10.7 mg cm⁻². Most importantly, the SMB pouch cell can also deliver a long-life cycles as well as high-temperature battery performance, which guarantees the safety of SMBs in practical application.     

Engineering Solid Electrolyte Interface at Nano-Scale for High-Performance Hard Carbon in Sodium-Ion Batteries

Engineering the structure and chemistry of solid electrolyte interface (SEI) on electrode materials is crucial for rechargeable batteries. Using hard carbon (HC) as a platform material, a correlation between Na⁺ storage performance, and the properties of SEI is comprehensively explored. It is found that a “good” SEI layer on HC may not be directly associated with certain kinds of SEI components, such as NaF and Na₂O. Whereas, arranging nano SEI components with refined structures constructs the foundation of “good” SEI that enables fast Na⁺ storage and interface stability of HC in Na-ion batteries. A layer-by-layer SEI on HC with inorganic-rich inner layer and tolerant organic-rich outer flexible layer can facilitate excellent rate and cycling life. Besides, SEI layer as the gate for Na⁺ from electrolyte to HC electrode can modulate interfacial crystallographic structures of HC with pillar-solvent that function as “pseudo-SEI” for fast and stable Na⁺ storage in optimal 1 m NaPF₆-TEGDME electrolytes. Such a layer-by-layer SEI combined with a “pseudo-SEI” layer for HC enables an outstanding rate of 192 mAh g⁻¹ at 2 C and stable cycling over 1100 cycles at 0.5 C. This study provides valuable guidance to improve the electrochemical performance of electrode materials through regulation of SEI in optimal electrolytes.     

Gel Polymer Electrolyte toward Large-Scale Application of Aqueous Zinc Batteries

Aqueous zinc batteries are promising candidates for energy storage and conversion devices in the “post-lithium” era due to their high energy density, high safety, and low cost. The electrolyte plays an important role in zinc batteries by conducting and separating the positive and negative electrodes. However, the issues of zinc dendrites growth, corrosion, by-product formation, hydrogen evolution and leakage, and evaporation of the aqueous electrolytes affect the commercialization of the batteries. Moreover, the widely used aqueous electrolytes result in large battery sizes, which are not conducive to the emerging smart devices. The intrinsic properties of gel polymer electrolytes (GPEs) can solve the above problems. In order to promote the wider application of GPEs-based zinc batteries, in this review, the working principle and the current problems of zinc batteries are first introduced, andthe merits of GPEs compared to aqueous electrolytes are then summarized. Subsequently, a series of challenges and corresponding strategies faced by GPE is discussed, and an outlook for its future development is finally proposed.     

Multiscale Structural Gel Polymer Electrolytes with Fast Li+ Transport for Long-Life Li Metal Batteries

Li metal batteries (LMBs) are considered as promising candidates for future rechargeable batteries with high energy density. However, Li metal anode (LMA) is extensively sensitive to general liquid electrolytes, leading to unstable interphase and dendrites growth. Herein, a novel gel polymer electrolyte consisting of a micro-nanostructured poly(vinylidene fluoride-co-hexafluoropropylene) matrix and inorganic fillers of Zeolite Socony Mobil-5 (ZSM-5) and SiO₂ nanoparticles, is fabricated to expedite Li⁺ ions transport and suppress Li dendrite growth. Due to the Lewis acid interaction, SiO₂ can absorb amounts of PF₆⁻ and promote the dissociation of LiPF₆. The specific sub-nanometer pore structure of ZSM-5 greatly enhances the Li⁺ ion transference number. These structures can restrain the decomposition of electrolytes and build stable interphase on LMA. Therefore, the Li||Ni_0.8Co_0.1Mn_0.1O₂ full cell maintains 92% capacity retention after 300 cycles at 1 C (1 C ≈190 mAh g⁻¹) in carbonate electrolyte. This multiscale design provides an effective strategy for electrolyte exploration in high-performance LMBs.     

Stabilizing Polymer–Lithium Interface in a Rechargeable Solid Battery

Solid polymer electrolytes (SPEs) are promising candidates for developing high‐energy‐density Li metal batteries due to their flexible processability. However, the low mechanical strength as well as the inferior interfacial regulation of ions between SPEs and Li metal anode limit the suppress ion of Li dendrites and destabilize the Li anode. To meet these challenges, interfacial engineering aiming to homogenize the distribution of Li⁺/electron accompanied with enhanced mechanical strength by Mg₃N₂ layer decorating polyethylene oxide is demonstrated. The intermediary Mg₃N₂ in situ transforms to a mixed ion/electron conducting interlayer consisting of a fast ionic conductor Li₃N and a benign electronic conductor Mg metal, which can buffer the Li⁺ concentration gradient and level the nonuniform electric current distribution during cycling, as demonstrated by a COMSOL Multiphysics simulation. These characteristics endow the solid full cell with a dendrite‐free Li anode and enhanced cycling stability and kinetics. The innovative interface design will accelerate the commercial application of high‐energy‐density solid batteries.     

Ambient-Temperature All-Solid-State Sodium Batteries with a Laminated Composite Electrolyte

This study presents a sodium-ion conductive laminated polymer/ceramic-polymer solid-state electrolyte for the development of room-temperature all-solid-state sodium batteries. At the negative electrode side, a negative-electrode-benign poly(ethylene oxide) (PEO) is used as a polymer matrix into which succinonitrile (SN) is integrated to improve the room-temperature Na⁺-ion conductivity. At the positive electrode side, a cathode-friendly poly(acrylonitrile) (PAN) serves as a polymer matrix into which a NASICON-type ceramic solid-electrolyte (Na₃Zr₂Si₂PO₁₂) powder is incorporated toward both the enhancement of Na⁺-ion conductivity and the prevention of Na dendrite from penetrating through the electrolyte membrane. Through a strategical management of composition, the PAN-Na₃Zr₂Si₂PO₁₂-NaClO₄ composite and the PEO-SN-NaClO₄ polymer deliver a balanced Na⁺-ion conductivity. Combining the two electrolyte layers, the laminated PEO-SN-NaClO₄/PAN-Na₃Zr₂Si₂PO₁₂-NaClO₄ solid electrolyte provides a Na⁺-ion conductivity of 1.36 × 10⁻⁴ S cm⁻¹ at room temperature. With respect to the anodic friendly feature of the PEO-SN-NaClO₄ layer and the cathodic friendly feature of the PAN-Na₃Zr₂Si₂PO₁₂-NaClO₄ layer, the laminated solid electrolyte presents a stable electrochemical window of 0–4.8 V. Room-temperature all-solid-state sodium batteries fabricated with the laminated solid electrolyte, a Na-metal negative electrode, and a Na₂MnFe(CN)₆ positive electrode exhibit remarkably stable cyclability.     

Composition and Structure Design of Poly(vinylidene fluoride)-Based Solid Polymer Electrolytes for Lithium Batteries

Solid-state lithium batteries have become the focus of the next-generation high-safety lithium batteries due to their dimensional, thermal, and electrochemical stability. Thus, the progress of solid electrolytes with satisfactory comprehensive performances has become the key to promoting the development of solid batteries. Herein, poly(vinylidene fluoride) (PVDF) solid polymer electrolytes (SPEs) possess excellent flexibility, mechanical property, and high electrochemical and thermal stability, which show huge application potentiality in solid-state lithium batteries and obtain extensive research. But the PVDF SPEs have been suffering from low ionic conductivity, high crystallinity, and low reactive sites. The development of PVDF-based composite solid polymer electrolytes (CSPEs) has been confirmed to be a forceful strategy to optimize the performance of electrolytes. In this review, based on different design strategies, the recent progress of PVDF-based SPEs is introduced in detail, especially in the mechanism of ionic conductivity enhancement and interface regulation by modified fillers. Besides, the applications of PVDF-based SPEs in Li-S and Li-O₂ battery systems are also introduced. Finally, this review presents some insights for promoting the development of high-performance PVDF-based SPEs.     

Excellent Cycling Stability of Sodium Anode Enabled by a Stable Solid Electrolyte Interphase Formed in Ether‐Based Electrolytes

Sodium‐ion batteries have been considered one of the most promising power sources beyond Li‐ion batteries. Although the Na metal anode exhibits a high theoretical capacity of 1165 mAh g^?1, its application in Na batteries is largely hindered by dendrite growth and low coulombic efficiency. Herein, it is demonstrated that an electrolyte consisting of 1 m sodium tetrafluoroborate in tetraglyme can enable excellent cycling efficiency (99.9%) of a Na metal anode for more than 1000 cycles. This high reversibility of a Na anode can be attributed to a stable solid electrolyte interphase formed on the Na surface, as revealed by cryogenic transmission electron microscopy and X‐ray photoelectron spectroscopy (XPS). These electrolytes also enable excellent cycling stability of Na||hard‐carbon cells and Na||Na_2/3Co_1/3Mn_2/3O₂ cells at high rates with very high coulombic efficiencies.