Electrolytes toward High-Voltage Na3V2(PO4)2F3 Positive Electrode Durable against Temperature Variation

High power and energy density, long cyclability, and tolerance for wide temperature (seasonal and daily operational temperature differences) must be considered to construct large‐scale sodium secondary batteries. In this regard, Na₃V₂(PO₄)₂F₃ (NVPF) has become a subject of interest as a high‐performance positive electrode material owing to its high energy density. However, the high operating voltage of NVPF causes continuous decomposition of electrolytes during cycles, resulting in significant capacity fading and low Coulombic efficiency. In this study, the electrochemical performance of the NVPF electrode in organic solvent electrolytes with and without additives and an ionic liquid is investigated at high voltage regimes over a wide temperature range (−20 °C to 90 °C). The results reveal that the performance of organic electrolytes is still insufficient even with additives, and the ionic liquid electrolyte demonstrates high electrochemical stability and cyclability with NVPF electrodes over a temperature range from −20 °C to 90 °C, achieving stable cycling over 500 cycles. The detailed electrochemical analysis combined with X‐ray photoelectron and energy dispersive X‐ray spectroscopy indicates that a sturdy cathode electrolyte interphase layer around the electrode protects it from capacity fading at high voltage and elevated temperature, resulting in high Coulombic efficiency

In Situ Orthorhombic to Amorphous Phase Transition of Nb₂O₅ and Its Temperature Effect on Pseudocapacitive Behavior

Niobium pentoxide (Nb₂O₅) represents an exquisite class of negative electrode materials with unique pseudocapacitive kinetics that engender superior power and energy densities for advanced electrical energy storage devices. Practical energy devices are expected to maintain stable performance under real-world conditions such as temperature fluctuations. However, the intercalation pseudocapacitive behavior of Nb₂O₅ at elevated temperatures remains unexplored because of the scarcity of suitable electrolytes. Thus, in this study, we investigate the effect of temperature on the pseudocapacitive behavior of submicron-sized Nb₂O₅ in a wide potential window of 0.01–2.3 V. Furthermore, ex situ X-ray diffraction and X-ray photoelectron spectroscopy reveal the amorphization of Nb₂O₅ accompanied by the formation of NbO via a conversion reaction during the initial cycle. Subsequent cycles yield enhanced performance attributed to a series of reversible NbV, IV/NbIII redox reactions in the amorphous LixNb₂O₅ phase. Through cyclic voltammetry and symmetric cell electrochemical impedance spectroscopy, temperature elevation is noted to increase the pseudocapacitive contribution of the Nb₂O₅ electrode, resulting in a high rate capability of 131 mAh g⁻¹ at 20, 000 mA g⁻¹ at 90 °C. The electrode further exhibits long-term cycling over 2000 cycles and high Coulombic efficiency ascribed to the formation of a robust, [FSA]−-originated solid-electrolyte interphase during cycling

Benefits of the Mixtures of Ionic Liquid and Organic Electrolytes for Sodium-ion Batteries

The successful commercialization of sodium-ion batteries is heavily contingent on the development of suitable electrolytes marked with economic feasibility and stable electrochemical performance. To this end, we present a group of hybrid electrolytes made from the [C₃C₁pyrr][FSA] (C₃C₁pyrr = N-methyl-N-propylpyrrolidinium) ionic liquid (IL) and propylene carbonate organic liquid (OL) electrolytes with Na[FSA] (FSA = bis(fluorosulfonyl)amide) and Na[ClO₄] salts are mixed with exploring the possibilities of cost reduction, high performance and inhibited flammability. The thermal stability tests reveal that the addition of IL can effectively suppress flammability. Herein, the physicochemical and electrochemical properties of the various mixing ratios of the aforementioned hybrid electrolytes (ILOL) are investigated for sodium-ion batteries. Furthermore, full cell tests using hard carbon (HC) negative and NaCrO2 (NCO) positive electrodes using the ILOL systems improve electrochemical performance and enable battery operation at 363 K

Sodium difluorophosphate: facile synthesis, structure, and electrochemical behavior as an additive for sodium-ion batteries

Despite the success of difluorophosphate (PO₂F₂⁻, DFP) electrolyte additives in lithium and potassium-ion batteries, their utilization in sodium-ion batteries remains unexplored due to difficulties in the synthesis of sodium difluorophosphates (NaDFP). Thus, in this study, NaDFP salt prepared via ion exchange of KDFP and NaPF₆ is characterized using single-crystal X-ray diffraction, Raman and infrared (IR) spectroscopy, energy dispersive X-ray analysis (EDX), and thermogravimetry-differential thermal analysis (TG-DTA). Electrochemical tests demonstrate enhanced cycle performance of a hard carbon electrode (capacity retention; 76.3% after 500 cycles with NaDFP vs. 59.2% after 200 cycles in the neat electrolyte), achieving a high coulombic efficiency (average of 99.9% over 500 cycles) when NaDFP is used as an electrolyte additive. Further, electrochemical impedance spectroscopy (EIS) using a HC/HC symmetric cell demonstrates significant reduction of the interfacial resistance upon addition of NaDFP. X-ray photoelectron spectroscopy (XPS) indicates presence of stable, Na⁺-conducting solid-electrolyte interphase (SEI) components formed in the presence of NaDFP. This work not only presents a feasible NaDFP synthesis method, but also demonstrates the use of NaDFP as a strategy for optimizing sodium-ion battery performance

Pseudo-solid-state electrolytes utilizing the ionic liquid family for rechargeable batteries

The advent of solid-state electrolytes has unearthed a new paradigm of next-generation batteries endowed with improved electrochemical properties and exceptional safety. Amongst them, Li-stuffed garnet type oxides, sulfides, and NASICON type solid-state electrolytes have emerged with fascinating ionic conductivity, electrochemical stability, and high safety standards, besides creating an avenue for using metal anodes to maximize energy density. However, the actual performance of solid-state electrolytes is heavily encumbered by unexpected metal dendrite formation and typically manifests high resistances between the metal electrodes/solid-state electrolytes or grain boundaries, thereby restricting their practical applications. Recent studies have reported several novel approaches, such as modifying solid-state electrolytes using ionic liquids to form the so-called “pseudo-solid-state electrolytes”. This class of electrolytes encompassing materials such as ionogel using ionic liquids and ionic plastic crystals has been gaining rekindled interest for their unique properties that promise great strides in battery performance and diversified utility. This minireview paper summarizes recent progress in pseudo-solid-state electrolytes utilizing ionic liquids, highlighting their fundamental properties while elaborating expedient design strategies. The realistic prospects and future challenges associated with developing pseudo-solid-state electrolyte materials present an insight into their properties to inspire far-reaching exploration into their material characteristics and functionalities

Dual-ion charge-discharge behaviors of Na-NiNc and NiNc-NiNc batteries

Dual-ion sodium-organic secondary batteries were produced with anti-aromatic porphyrinoid, NiNc, as an active electrode material, which exhibited inherent charge-discharge behavior with high discharge capacity, high stability, and high Coulombic efficiency at high current density (132.6 mA h g⁻¹ discharge capacity and 99.4% efficiency at the 100th cycle with 1 A g⁻¹ of current density and 95.3 mA h g⁻¹ discharge capacity and 99.3% efficiency at the 100th cycle with 2 A g⁻¹ of current density)

Electrode Potentials Part 1: Fundamentals and Aqueous Systems

Electrochemistry deals with the interrelationship between electrical and chemical energy. Various potentials appear in electrochemistry and pertain to one another in practical cells. Understanding the electrode potential is an important step in acquiring basic knowledge of electrochemistry and extending it to specific applications. This comprehensive paper outlines the fundamentals and related subjects of electrode potentials, including electrochemical cells and liquid junction potentials. Aqueous solution systems are ideal for connecting the theoretical background of electrode potentials to practical electrochemical measurements. Accordingly, the basic electrode chemistry in aqueous systems is described in this paper, as well as several advanced concepts introduced in recent studies

Effects of Ion Fraction in an Inorganic Ionic Liquid Electrolyte on Performance of Intermediate-Temperature Operating Sodium-Sulfur Batteries

Sodium-sulfur (Na-S) batteries are promising energy storage systems for renewable energy sources which redeem an intermittent energy source. This study reports the effects of the Na[SO₃CF₃] fraction in an inorganic Na[SO₃CF₃]-Cs[N(SO₂CF₃)₂] ionic liquid electrolyte (x(Na[SO₃CF₃]) = 0.2, 0.3, and 0.4) on the performance of Na-S batteries. Measurements of physicochemical and electrochemical properties demonstrated that decrease in the Na[SO₃CF₃] fraction decreases viscosity and increases ionic conductivity and the solubility of polysulfides into the ionic liquid, which contributes to the enhanced capacity in the low potential region during discharging

Inhibition of Aluminum Corrosion with the Addition of the Tris(pentafluoroethyl)trifluorophosphate Anion to a Sulfonylamide-Based Ionic Liquid for Sodium-Ion Batteries

Ionic liquids (ILs) based on sulfonylamide-type anions have gained widespread utility as electrolytes for secondary batteries. Although sulfonylamide-based IL electrolytes are known to form a stable passivation layer that prevents Al corrosion, the Al electrode in the Na[FSA]-[C₂C₁im][FSA] ([FSA] = bis(fluorosulfonyl)amide and [C₂C₁im] = 1-ethyl-3-methylimidazolium) IL, is found to be afflicted by pitting corrosion at potentials above 4 V vs Na⁺/Na during electrochemical measurement at 90 °C. Therefore, this study investigates the suppressive effect of [FAP]⁻ (FAP = tris(pentafluoroethyl)trifluorophosphate) on the Al corrosion behavior of the IL electrolyte. Here, the inhibited corrosion of the Al electrode is confirmed through a series of cyclic voltammetry measurements, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. Charge-discharge tests performed using a Na₃V₂(PO₄)₂F₃ positive electrode demonstrates that the addition of [FAP]⁻ into the IL enhances cycling performance at the intermediate temperature of 90 °C

Potassium Difluorophosphate as an Electrolyte Additive for Potassium Ion Batteries

The limited cyclability and inferior Coulombic efficiency of graphite negative electrodes have been major impediments to their practical utilization in potassium-ion batteries (PIBs). Herein, for the first time, potassium difluorophosphate (KDFP) electrolyte additive is demonstrated as a viable solution to these bottlenecks by facilitating the formation of a stable and K⁺-conducting solid–electrolyte interphase (SEI) on graphite. The addition of 0.2 wt % KDFP to the electrolyte results in significant improvements in the (de)potassiation kinetics, capacity retention (76.8% after 400 cycles with KDFP vs 27.4% after 100 cycles without KDFP), and average Coulombic efficiency (∼99.9% during 400 cycles) of the graphite electrode. Moreover, the KDFP-containing electrolyte also enables durable cycling of the K/K symmetric cell at higher efficiencies and lower interfacial resistance as opposed to the electrolyte without KDFP. X-ray diffraction and Raman spectroscopy analyses have confirmed the reversible formation of a phase-pure stage-1 potassium–graphite intercalation compound (KC₈) with the aid of KDFP. The enhanced electrochemical performance by the KDFP addition is discussed based on the analysis of the SEI layer on graphite and K metal electrodes by X-ray photoelectron spectroscopy