SEI Formation on Sodium Metal Electrodes in Superconcentrated Ionic Liquid Electrolytes and the Effect of Additive Water

Polycarbonate-Based Solid-Polymer Electrolytes for Solid-State Sodium Batteries

Solid-polymer electrolytes comprised of polypropylene carbonate (PPC) and varied sodium bis(fluorosulfonyl)imide (NaFSI) salt concentrations are investigated for implementation as a conductive solid polymer electrolyte into solid-state cathode composites utilizing a sodium-layered oxide active material. The ionic conductivity generally increases with NaFSI salt content, reaching ≈1 mS cm-1 at 80 °C at the highest salt concentration (PPC:NaFSI = 0.5:1). Through an all-in-one slurry casting method, Na2/3Ni1/3Mn2/3O2 cathode composites are fabricated in which the dispersed PPC electrolyte acts as the primary binder. Enabled by a bilayer polymer electrolyte system, cycling performance with the PPC cathode electrolyte is optimized with respect to salt concentration and anode material. The best cyclability is achieved with a moderate salt concentration electrolyte (PPC:NaFSI = 5:1), showcasing an initial capacity of 83 mA h g-1 with a remarkable 80% capacity retention after 150 cycles at C/5 rate and 60 °C. The superior performance of the lower salt concentration electrolyte is attributed to better electrochemical stability, as confirmed by linear sweep voltammetry and online electrochemical mass spectrometry measurements. These results underscore the potential of carbonate-based polymer electrolytes and the importance of balancing electrolyte conductivity and stability in cell design.

Building Stable Anodes for High-Rate Na-Metal Batteries

Due to low cost and high energy density, sodium metal batteries (SMBs) have attracted growing interest, with great potential to power future electric vehicles (EVs) and mobile electronics, which require rapid charge/discharge capability. However, the development of high-rate SMBs has been impeded by the sluggish Na+ ion kinetics, particularly at the sodium metal anode (SMA). The high-rate operation severely threatens the SMA stability, due to the unstable solid-electrolyte interface (SEI), the Na dendrite growth, and large volume changes during Na plating-stripping cycles, leading to rapid electrochemical performance degradations. This review surveys key challenges faced by high-rate SMAs, and highlights representative stabilization strategies, including the general modification of SMB components (including the host, Na metal surface, electrolyte, separator, and cathode), and emerging solutions with the development of solid-state SMBs and liquid metal anodes; the working principle, performance, and application of these strategies are elaborated, to reduce the Na nucleation energy barriers and promote Na+ ion transfer kinetics for stable high-rate Na metal anodes. This review will inspire further efforts to stabilize SMAs and other metal (e.g., Li, K, Mg, Zn) anodes, promoting high-rate applications of high-energy metal batteries towards a more sustainable society.

How NaFTA salt affects the structural landscape and transport properties of Pyrr1,3FTA ionic liquid

Recently, it has been demonstrated that ionic liquids (ILs) with an asymmetric anion render a wider operational temperature range and can be used as a solvent in sodium ion batteries. In the present study, we examine the microscopic structure and dynamics of pure 1-methyl-1-propylpyrrolidinium fluorosulfonyl(trifluoromethylsulfonyl)amide (Pyrr_1,3FTA) IL using atomistic molecular dynamics simulations. How the addition of the sodium salt (NaFTA) having the same anion changes the structural landscape and transport properties of the pure IL has also been explored. The simulated x-ray scattering structure functions reveal that the gradual addition of NaFTA salt (up to 1.2 molal) suppresses the charge alternating feature of the pure IL because of the replacement of the Pyrr⁺ cations with the Na⁺ ions. The Na⁺ ions are majorly found near the oxygen atoms of the anions, but the probability of finding the Na⁺ ions near these atoms slightly decreases with increasing salt concentration. As expected, the Na⁺ ions stay away from the Pyrr⁺ cations. However, the probability of finding the anions around anions increases with increasing salt concentration. The simulated self-diffusion coefficients of the ions in the pure IL reveal slightly faster diffusion of the Pyrr⁺ cations as compared to the FTA^- anions. Interestingly, in the salt solution, despite having smaller size, the diffusion of the Na⁺ ions is found to be lesser than the Pyrr⁺ cations and the FTA^- anions. The analysis of the ionic conductivity and transport numbers reveals that the fractional contribution of the FTA^- anion to the overall conductivity remains nearly constant with increasing salt concentration, but the contribution of Pyrr⁺ cation decreases and Na⁺ ion increases.

Recent Progress in Rechargeable Sodium Metal Batteries: A Review

Sodium metal batteries (SMBs) have been widely studied owing to their relatively high energy density and abundant resources. However, they still need systematic improvement to fulfill the harsh operating conditions for their commercialization. In this review, we summarize the recent progress in SMBs in terms of sodium anode modification, electrolyte exploration, and cathode design. Firstly, we give an overview of the current challenges facing Na metal anodes and the corresponding solutions. Then, the traditional liquid electrolytes and the prospective solid electrolytes for SMBs are summarized. In addition, insertion- and conversion-type cathode materials are introduced. Finally, an outlook for the future of practical SMBs is provided.

Metallic Sodium Anodes for Advanced Sodium Metal Batteries: Progress, Challenges and Perspective

Sodium (Na)-based batteries, as the ideal choice of large-scale and low-cost energy storage, have attracted much attention. Na metal anodes with high theoretical specific capacity and low potential are considered to be one of the most promising anodes for next-generation Na-based batteries. However, the high reactivity of Na metal anodes makes the electrode/electrolyte phase unstable, resulting in formation of Na dendrites, short cycle life and safety problems. Herein, the contribution outlines the latest development of Na metal anodes for Na metal batteries. The design strategies for high efficiency utilization of Na metal anodes are elucidated, including sophisticated electrode construction, liquid electrolyte optimization, electrode/electrolyte interface stabilization, and solid electrolyte adaptation. Finally, the future research direction and existing problems are proposed.

High-Performance Cycling of Na Metal Anodes in Phosphonium and Pyrrolidinium Fluoro(sulfonyl)imide Based Ionic Liquid Electrolytes

We have investigated the sodium electrochemistry and the evolution and chemistry of the solid-electrolyte interphase (SEI) upon cycling Na metal electrodes in two ionic liquid (IL) electrolytes. The effect of the IL cation chemistry was determined by examining the behavior of a phosphonium IL (P_111i4FSI) in comparison to its pyrrolidinium-based counterpart (C₃mpyrFSI) at near-saturated NaFSI salt concentrations (superconcentrated ILs) in their dry state and with water additive. The differences in their physical properties are reported, with the P_111i4FSI system having a lower viscosity, higher conductivity, and higher ionicity in comparison to the C₃mpyrFSI-based electrolyte, although the addition of 1000 ppm (0.1 wt %) of water had a more dramatic effect on these properties in the latter case. Despite these differences, there was little effect in the ability to sustain stable cycling at moderate current densities and capacities (being nearly identical at 1 mA cm^-2 and 1 mAh cm^-2). However, the IL based on the phosphonium cation is shown to support more demanding cycling with high stability (up to 4 mAh cm^-2 at 1, 2, and 4 mA cm^-2 current density), whereas C₃mpyrFSI rapidly failed (at 1 mA cm^-2 /4 mAh cm^-2). The SEI was characterized ex situ using solid-state ²³Na NMR, XPS, and SEM and showed that the presence of a Na complex, identified in our previous work on C₃mpyrFSI to correlate with stable, dendrite-free Na metal cycling, was also more prominent and coexisted with a NaF-rich surface. The results here represent a significant breakthrough in the development of high-capacity Na metal anodes, clearly demonstrating the superior performance and stability of the P_111i4FSI electrolyte, even after the addition of water (up to 1000 ppm (0.1 wt %)), and show great promise to enable future higher-temperature (50 °C) Na-metal-based batteries.

Asymmetric Sodiophilic Host Based on a Ag-Modified Carbon Fiber Framework for Dendrite-Free Sodium Metal Anodes

Sodium (Na) metal is considered a promising anode material for high-energy Na batteries due to its high theoretical capacity and abundant resources. However, uncontrollable dendrite growth during the repeated Na plating/stripping process leads to the issues of low Coulombic efficiency and short circuits, impeding the practical applications of Na metal anodes. Herein, we propose a silver-modified carbon nanofiber (CNF@Ag) host with asymmetric sodiophilic features to effectively improve the deposition behavior of Na metal. Both density functional theory (DFT) calculations and experiment results demonstrate that Na metal can preferentially nucleate on the sodiophilic surface with Ag nanoparticles and uniformly deposit on the whole CNF@Ag host with a "bottom-up growth" mode, thus preventing unsafe dendrite growth at the anode/separator interface. The optimized CNF@Ag framework exhibits an excellent average Coulombic efficiency of 99.9% for 500 cycles during Na plating/stripping at 1 mA cm^-2 for 1 mAh cm^-2. Moreover, the CNF@Ag-Na symmetric cell displays stable cycling for 500 h with a low voltage hysteresis at 2 mA cm^-2. The CNF@Ag-Na//Na₃V₂(PO₄)₃ full cell also presents a high reversible specific capacity of 102.7 mAh g^-1 for over 200 cycles at 1 C. Therefore, asymmetric sodiophilic engineering presents a facile and efficient approach for developing high-performance Na batteries with high safety and stable cycling performance.

Tuning the Formation and Structure of the Silicon Electrode/Ionic Liquid Electrolyte Interphase in Superconcentrated Ionic Liquids

The latest advances in the stabilization of Li/Na metal battery and Li-ion battery cycling have highlighted the importance of electrode/electrolyte interface [solid electrolyte interphase (SEI)] and its direct link to cycling behavior. To understand the structure and properties of the SEI, we used combined experimental and computational studies to unveil how the ionic liquid (IL) cation nature and salt concentration impact the silicon/IL electrolyte interfacial structure and the formed SEI. The nature of the IL cation is found to be important to control the electrolyte reductive decomposition that influences the SEI composition and properties and the reversibility of the Li-Si alloying process. Also, increasing the Li salt concentration changes the interface structure for a favorable and less resistive SEI. The most promising interface for the Si-based battery was found to be in P₁₂₂₂FSI with 3.2 m LiFSI, which leads to an optimal SEI after 100 cycles in which LiF and trapped LiFSI are the only distinguishable lithiated and fluorinated products detected. This study shows a clear link between the nanostructure of the IL electrolyte near the electrode surface, the resulting SEI, and the Si negative electrode cycling performance. More importantly, this work will aid the rational design of Si-based Li-ion batteries using IL electrolytes in an area that has so far been neglected, reinforcing the benefits of superconcentrated electrolyte systems.