Understanding the Electrochemical Compatibility and Reaction Mechanism on Na Metal and Hard Carbon Anodes of PC-Based Electrolytes for Sodium-Ion Batteries

Synergetic Dual-Additive Electrolyte Enables Highly Stable Performance in Sodium Metal Batteries

Sodium (Na)-metal batteries (SMBs) are considered one of the most promising candidates for the large-scale energy storage market owing to their high theoretical capacity (1,166 mAh g-1) and the abundance of Na raw material. However, the limited stability of electrolytes still hindered the application of SMBs. Herein, sulfolane (Sul) and vinylene carbonate (VC) are identified as effective dual additives that can largely stabilize propylene carbonate (PC)-based electrolytes, prevent dendrite growth, and extend the cycle life of SMBs. The cycling stability of the Na/NaNi0.68Mn0.22Co0.1O2 (NaNMC) cell with this dual-additive electrolyte is remarkably enhanced, with a capacity retention of 94% and a Coulombic efficiency (CE) of 99.9% over 600 cycles at a 5 C (750 mA g-1) rate. The superior cycling performance of the cells can be attributed to the homogenous, dense, and thin hybrid solid electrolyte interphase consisting of F- and S-containing species on the surface of both the Na metal anode and the NaNMC cathode by adding dual additives. Such unique interphases can effectively facilitate Na-ion transport kinetics and avoid electrolyte depletion during repeated cycling at a very high rate of 5 C. This electrolyte design is believed to result in further improvements in the performance of SMBs.

Unraveling the underlying mechanism of good electrochemical performance of hard carbon in PC/EC-Based electrolyte

Although hard carbon in propylene carbonate / ethylene carbonate (PC/EC)-based electrolytes possesses favorable electrochemical characteristics in rechargeable sodium-ion batteries, the underlying mechanism is still vague. Numerous hypotheses have been proposed to solve the puzzle, but none of them have satisfactorily unraveled the reason at the molecular-level. In this study, we firstly attempted to address this mystery through a profound insight into the disparity of the ion solvation/desolvation behavior in electrolyte. Combining the results of density functional theory (DFT) calculations and experiments, the work explains that compared to the sole PC-based electrolyte, Na+-EC4 molecules in the PC/EC-based electrolyte preferentially undergo reduction and contribute to the emergence of a more stable protective film on the surface of hard carbon, leading to the preferable durability and rate capability of the cell. Nevertheless, applying the ion solvation/desolvation model, it also reveals that Na+-(solvent)n molecules in the PC/EC-based electrolyte can achieve faster Na+ desolvation processes than in the PC-based electrolyte alone, contributing to the enhancement of charge transfer kinetics. This research holds great importance in uncovering the possible mechanism of the remarkable electrochemical- properties of hard carbon in PC/EC-based electrolytes, and advancing its practical utilization in future sodium-ion batteries.

Research progress of organic liquid electrolyte for sodium ion battery

Electrochemical energy storage technology has attracted widespread attention due to its low cost and high energy efficiency in recent years. Among the electrochemical energy storage technologies, sodium ion batteries have been widely focused due to the advantages of abundant sodium resources, low price and similar properties to lithium. In the basic structure of sodium ion battery, the electrolyte determines the electrochemical window and electrochemical performance of the battery, controls the properties of the electrode/electrolyte interface, and affects the safety of sodium ion batteries. Organic liquid electrolytes are widely used because of their low viscosity, high dielectric constant, and compatibility with common cathodes and anodes. However, there are problems such as low oxidation potential, high flammability and safety hazards. Therefore, the development of novel, low-cost, high-performance organic liquid electrolytes is essential for the commercial application of sodium ion batteries. In this paper, the basic requirements and main classifications of organic liquid electrolytes for sodium ion batteries have been introduced. The current research status of organic liquid electrolytes for sodium ion batteries has been highlighted, including compatibility with various types of electrodes and electrochemical properties such as multiplicative performance and cycling performance of electrode materials in electrolytes. The composition, formation mechanism and regulation strategies of interfacial films have been explained. Finally, the development trends of sodium ion battery electrolytes in terms of compatibility with materials, safety and stable interfacial film formation are pointed out in the future.

Visualization of Sodium Metal Anodes via Operando X-Ray and Optical Microscopy: Controlling the Morphological Evolution of Sodium Metal Plating

Because of the abundance and cost effectiveness of sodium, rechargeable sodium metal batteries have been widely studied to replace current lithium-ion batteries. However, there are some critical unresolved issues including the high reactivity of sodium, an unstable solid-electrolyte interphase (SEI), and sodium dendrite formation. While several studies have been conducted to understand sodium plating/stripping processes, only a very limited number of studies have been carried out under operando conditions. We have employed operando X-ray and optical imaging techniques to understand the mechanistic behavior of Na metal plating. The morphology of sodium metal plated on a copper electrode depends strongly on the salts and solvents used in the electrolyte. The addition of a fluorine-containing additive to a carbonate-based electrolyte, NaClO₄ in propylene carbonate (PC):fluoroethylene carbonate (FEC), results in uniform sodium plating processes and much more stable cycling performance, compared to NaClO₄ in PC, because of the formation of a stable SEI containing NaF. A NaF layer, on top of the sodium metal, leads to a much more uniform deposition of sodium and greatly enhanced cyclability.

Hard Carbon Microsphere with Expanded Graphitic Interlayers Derived from a Highly Branched Polymer Network as Ultrahigh Performance Anode for Practical Sodium-Ion Batteries

Growing attention has been attached to hard carbon in sodium-ion batteries (SIBs). However, hard carbon from individual precursors tends to exhibit an inferior rate capability due to its limited interlayer distance. Here, a coupled strategy is designed to prepare hard carbon microspheres (HCMSs) via the pyrolysis of a highly branched polymer network formed instantaneously between two interactive precursors during the atomization of the spray drying process. The combined precursors with a tunable cross-linked structure prefer to generate a large interlayer spacing (0.399 nm) and abundant closed pore structure by suppressing the graphitization of precursors during the carbonization, relative to the individual precursor, which contributes greatly to the ion diffusion kinetics. Benefiting from the unique structure, HCMS exhibits an impressively high reversible specific capacity of 373.4 mA h g^-1 in SIBs and high initial Coulombic efficiency of 88%, retaining 90.2% of the initial capacity even after 150 cycles, which presented comparable capacities with commercial graphite in lithium-ion batteries. Besides, excellent rate capability was also demonstrated with HCMSs (250 and 117 mA h g^-1 at 300 and 600 mA g^-1). Notably, the interlayer distance and closed pore structure are tunable just by adjusting the ratio of the two precursors. The tunable and extendable fabrication process, together with its amazing high carbon yield of 48 wt % (1400 °C) and high tap density close to 0.8 g cm^-3, makes this strategy promising in the practical application for SIBs.

P2-type layered Na0.67Cr0.33Mg0.17Ti0.5O2 for Na storage applications

Na0.67Cr0.33Mg0.17Ti0.5O2 with a P2-type layered structure has been synthesized and examined as a negative electrode material for rechargeable sodium batteries. The layered oxide delivers a reversible capacity of >90 mA h g-1, which corresponds to >95% of the theoretical capacity with excellent cyclability for >450 cycles.

A Review on Applications of Layered Phosphorus in Energy Storage

Phosphorus in energy storage has received widespread attention in recent years. Both the high specific capacity and ion mobility of phosphorus may lead to a breakthrough in energy storage materials. Black phosphorus, an allotrope of phosphorus, has a sheet-like structure similar to graphite. In this review, we describe the structure and properties of black phosphorus and characteristics of the conductive electrode material, including theoretical calculation and analysis. The research progress in various ion batteries, including lithium-sulfur batteries, lithium–air batteries, and supercapacitors, is summarized according to the introduction of black phosphorus materials in different electrochemical applications. Among them, with the introduction of black phosphorus in lithium-ion batteries and sodium-ion batteries, the research on the properties of black phosphorus and carbon composite is introduced. Based on the summary, the future development trend and potential of black phosphorus materials in the field of electrochemistry are analyzed.