Regulating Sodium Deposition through Gradiently-Graphitized Framework for Dendrite-Free Na Metal Anode

Binary Solvent Induced Stable Interphase Layer for Ultra-Long Life Sodium Metal Batteries

Sodium foil, promising for high-energy-density batteries, faces reversibility challenges due to its inherent reactivity and unstable solid electrolyte interphase (SEI) layer. In this study, a stable sodium metal battery (SMB) is achieved by tuning the electrolyte solvation structure through the addition of co-solvent 2-methyl tetrahydrofuran (MTHF) to diglyme (Dig). The introduction of cyclic ether-based MTHF results in increased anion incorporation in the solvation structure, even at lower salt concentrations. Specifically, the anion stabilization capabilities of the environmentally sustainable MTHF co-solvent lead to a contact-ion pair-based solvation structure. Time-of-flight mass spectroscopy analysis reveals that a shift toward an anion-dominated solvation structure promotes the formation of a thin and uniform SEI layer. Consequently, employing a NaPF6-based electrolyte with a Dig:MTHF ratio of 50% (v/v) binary solvent yields an average Coulombic efficiency of 99.72% for 300 cycles in Cu||Na cell cycling. Remarkably, at a C/2 cycling rate, Na||Na symmetric cell cycling demonstrates ultra-long-term stability exceeding 7000 h, and full cells with Na0.44MnO2 as a cathode retain 80% of their capacity after 500 cycles. This study systematically examines solvation structure, SEI layer composition, and electrochemical cycling, emphasizing the significance of MTHF-based binary solvent mixtures for high-performance SMBs.

A Critical Review on Room-Temperature Sodium-Sulfur Batteries: From Research Advances to Practical Perspectives

Room-temperature sodium-sulfur (RT-Na/S) batteries are promising alternatives for next-generation energy storage systems with high energy density and high power density. However, some notorious issues are hampering the practical application of RT-Na/S batteries. Besides, the working mechanism of RT-Na/S batteries under practical conditions such as high sulfur loading, lean electrolyte, and low capacity ratio between the negative and positive electrode (N/P ratio), is of essential importance for practical applications, yet the significance of these parameters has long been disregarded. Herein, it is comprehensively reviewed recent advances on Na metal anode, S cathode, electrolyte, and separator engineering for RT-Na/S batteries. The discrepancies between laboratory research and practical conditions are elaborately discussed, endeavors toward practical applications are highlighted, and suggestions for the practical values of the crucial parameters are rationally proposed. Furthermore, an empirical equation to estimate the actual energy density of RT-Na/S pouch cells under practical conditions is rationally proposed for the first time, making it possible to evaluate the gravimetric energy density of the cells under practical conditions. This review aims to reemphasize the vital importance of the crucial parameters for RT-Na/S batteries to bridge the gaps between laboratory research and practical applications.

Enabling Further Organic Electrolyte Infiltration of Cellulose-Based Separators via Defect-Rich Polypyrrole Modification for High Sodium Ion Transport in Sodium Metal Batteries

Sodium metal batteries (SMBs) have high-density and cost-effective characteristics as one of the energy storage systems, but uncontrollable dendrite growth and poor rate performance still hinder their practical applications. Herein, a nitrogen-rich modified cellulose separator with released abundant ion transport tunnels in organic electrolyte was synthesized by in situ polymerization of polypyrrole, which is based on the high permeability of cellulose in aqueous solution and the interfacial interaction between cellulose and polypyrrole. Meanwhile, the introduction of abundant structural defects such as branch chains, oxygen-containing functional groups, and imine-like structure to disrupt polypyrrole conjugation enables the utilization of conductive polymers in composite separator applications. With the electrolyte affinity surface on, the modified separator exhibits reinforced electrolyte uptake (254%) and extended electrolyte wettability, thereby leading to accelerated ionic conductivity (2.77 mS cm-1) and homogeneous sodium deposition by facilitating the establishment of additional pathways for ion transport. Benefiting from nitrogen-rich groups, the polypyrrole-modified separator demonstrates selective Na+ transport by the data of improved Na+ transference number (0.62). Owing to the above advantages, the battery assembled with the modified separators exhibits outstanding rate performance and prominent capacity retention two times that of the pristine cellulose separator at a high current density under the condition of fluorine-free electrolyte.

Fluorinated porous frameworks enable robust anode-less sodium metal batteries

Sodium metal batteries hold great promise for energy-dense and low-cost energy storage technology but are severely impeded by catastrophic dendrite issue. State-of-the-art strategies including sodiophilic seeding/hosting interphase design manifest great success on dendrite suppression, while neglecting unavoidable interphase-depleted Na+ before plating, which poses excessive Na use, sacrificed output voltage and ultimately reduced energy density. We here demonstrate that elaborate-designed fluorinated porous framework could simultaneously realize superior sodiophilicity yet negligible interphase-consumed Na+ for dendrite-free and durable Na batteries. As elucidated by physicochemical and theoretical characterizations, well-defined fluorinated edges on porous channels are responsible for both high affinities ensuring uniform deposition and low reactivity rendering superior Na+ utilization for plating. Accordingly, synergistic performance enhancement is achieved with stable 400 cycles and superior plateau to sloping capacity ratio in anode-free batteries. Proof-of-concept pouch cells deliver an energy density of 325 Watt-hours per kilogram and robust 300 cycles under anode-less condition, opening an avenue with great extendibility for the practical deployment of metal batteries.

Ultrahigh Nitrogen Content Carbon Nanosheets for High Stable Sodium Metal Anodes

Sodium metal, with a high theoretical specific capacity of 1165 mAh g^-1 , is the ultimate anode for sodium batteries, yet how to deal with the inhomogeneous and dendritic sodium deposition and the infinite relative dimension change of sodium metal anodes during sodium depositing/stripping is still challenging. Here, a facile fabricated sodiuphilic 2D N-doped carbon nanosheets (N-CSs) are proposed as sodium host material for sodium metal batteries (SMBs) to prevent dendrite formation and eliminate volume change during cycling. Revealing from combined in situ characterization analyses and theoretical simulations, the high nitrogen content and porous nanoscale interlayer gaps of the 2D N-CSs can not only concede dendrite-free sodium stripping/depositing but also accommodate the infinite relative dimension change. Furthermore, N-CSs can be easily process into N-CSs/Cu electrode via traditional commercial battery electrode coating equipment that pave the way for large-scale industrial applications. On account of the abundant nucleation sites and sufficient deposition space, N-CSs/Cu electrodes demonstrate a superior cycle stability of more than 1500 h at a current density of 2 mA cm^-2 with a high coulomb efficiency of more than 99.9% and ultralow nucleation overpotential, which enable reversible and dendrites-free SMBs and shed light on further development of SMBs with even higher performance.