Formation of NaF-rich Solid Electrolyte Interphase on Na Anode through Additive Induced Anion-enriched Structure of Na+ Solvation

High-capacity sodium (Na) anode suffer from the trouble of dendrite growth due to high reactivity, which can be overcome through inducing stable NaF-rich solid electrolyte interphase (SEI). Herein, we propose an additive strategy for realizing the anion-enriched structure of Na+ solvation to obtain NaF-rich SEI. The electron withdrawing acetyl group in 4-acetylpyridine (4-APD) increases the coordination number of PF6- in Na+ solvation sheath to facilitate PF6- decompose to NaF. Thus, the NaF-rich SEI with high mechanical stability and interfacial energy is formed to repress the growth of Na dendrites. With the 4-APD-contained electrolyte, the symmetric Na||Na cells show excellent cycling performance over 360 h at 1.0 mA cm-2. Meanwhile, enhanced stability is also achieved of Na||Na3V2(PO4)2O2F full cells with high Coulombic efficiency (97%) and capacity retention (91%) after 200 cycles.

Probing fluorination promoted sodiophilic sites with model systems of F16CuPc and CuPc

Sodium metal batteries (SMBs) are receiving broad attention due to the high specific capacity of sodium metal anodes and the material abundance on earth. However, the growth of dendrites results in poor battery performance and severe safety problems, inhibiting the commercial application of SMBs. To stabilize sodium metal anodes, various methods have been developed to optimize the solid electrolyte interphase (SEI) layer and adjust the electroplating/stripping behavior of sodium. Among the methods, developing anode host materials and adding electrolyte additives to build a protective layer are promising and convenient. However, the understanding of the interaction process between sodium metal and those organic materials is still limited, but is essential for the rational design of advanced anode hosts and electrolyte additives. In this study, we use copper(II) hexadecafluorophthalocyanine (F₁₆CuPc), and copper(II) phthalocyanine (CuPc), as model systems to unravel the sodium interaction with polar functional groups by in-situ photoelectron spectroscopy and density functional theory (DFT) calculations. It is found that sodium atoms prefer to interact with the inner pyrrolic nitrogen sites of CuPc, while they prefer to interact with the outer aza bridge nitrogen atoms, owing to Na-F interaction at the Na/F₁₆CuPc interface. Besides, for the both organic molecules, the central Cu(II) ions are reduced to Cu(I) ions by charge transfer from deposited sodium. The fluorine-containing groups are proven to promote the interaction process of sodium in organic materials, which sheds light on the design of functional interfaces in host materials and anode protective layers for sodium metal anodes.

A robust, highly reversible, mixed conducting sodium metal anode

Sodium metal anode holds great promise in pursuing high-energy and sustainable rechargeable batteries, but severely suffers from fatal dendrite growth accompanied with huge volume change. Herein, a robust mixed conducting sodium metal anode is designed through incorporating NaSICON-type solid Na-ion conductor into bulk Na. A fast and continuous pathway for simultaneous transportation of electrons and Na⁺ is established throughout the composite anode. The intimate contact between Na-ion conducting phase and Na metallic phase constructs abundant two-phase boundaries for fast redox reactions. Further, the compact configuration of the composite anode substantially protects Na metal from being corroded by liquid organic electrolyte for the minimization of side reactions. Benefiting from the unique configuration, the composite anode shows highly reversible and durable Na plating/stripping behavior. The symmetric cells exhibit ultralong lifespan for over 700 h at 1 mA cm^-2 with a high capacity of 5 mAh cm^-2 and outstanding rate capability up to 8 mA cm^-2 in the carbonate electrolyte. Full cells with Na₃V₂(PO₄)₃/C cathode demonstrate impressive cycling stability (capacity decay of 0.012% per cycle) and low charge/discharge polarization as well. This work provides new insights into rational design and development of robust sodium metal anode through an architecture engineering strategy for advanced rechargeable sodium batteries.

Critical Role of Ultrathin Graphene Films with Tunable Thickness in Enabling Highly Stable Sodium Metal Anodes

Sodium (Na) metal has shown great promise as an anode material for the next-generation energy storage systems because of its high theoretical capacity, low cost, and high earth abundance. However, the extremely high reactivity of Na metal with organic electrolyte leads to the formation of unstable solid electrolyte interphase (SEI) and growth of Na dendrites upon repeated electrochemical stripping/plating, causing poor cycling performance, and serious safety issues. Herein, we present highly stable and dendrite-free Na metal anodes over a wide current range and long-term cycling via directly applying free-standing graphene films with tunable thickness on Na metal surface. We systematically investigate the dependence of Na anode stability on the thickness of the graphene film at different current densities and capacities. Our findings reveal that only a few nanometer (∼2-3 nm) differences in the graphene thickness can have decisive influence on the stability and rate capability of Na anodes. To achieve the optimal performance, the thickness of the graphene film covered on Na surface needs to be meticulously selected based on the applied current density. We demonstrate that with a multilayer graphene film (∼5 nm in thickness) as a protective layer, stable Na cycling behavior was first achieved in carbonate electrolyte without any additives over 100 cycles at a current density as high as 2 mA/cm² with a high capacity of 3 mAh/cm². We believe our work could be a viable route toward high-energy Na battery systems, and can provide valuable insights into the lithium batteries as well.