Unprotected Organic Cations─The Dilemma of Highly Li-Concentrated Ionic Liquid Electrolytes.

Highly Li-concentrated electrolytes have been widely studied to harness their uniquely varying bulk and interface properties that arise from their distinctive physicochemical properties and coordination structures. Similar strategies have been applied in the realm of ionic liquid electrolytes to exploit their improved functionalities. Despite these prospects, the impact of organic cation behavior on interfacial processes remains largely underexplored compared to the widely studied anion behavior. The present study demonstrates that the weakened interactions between cations and anions engender "unprotected" organic cations in highly Li-concentrated ionic liquid electrolytes, leading to the decomposition of electrolytes during the initial charge. This decomposition behavior is manifested by the substantial irreversible capacities and inferior initial Coulombic efficiencies observed during the initial charging of graphite negative electrodes, resulting in considerable electrolyte consumption and diminished energy densities in full-cell configurations. The innate cation behavior is ascertained by examining the coordination environment of ionic liquid electrolytes with varied Li concentrations, where intricate ionic interactions between organic cations and anions are unveiled. In addition, anionic species with high Lewis basicity were introduced to reinforce the ionic interactions involving organic cations and improve the initial Coulombic efficiency. This study verifies the role of unprotected organic cations while highlighting the significance of the coordination environment in the performance of ionic liquid electrolytes.

Entropy-Driven 60 mol% Li Electrolyte for Li Metal-Free Batteries.

Highly Li-concentrated electrolytes are acknowledged for their compatibility with Li metal negative electrodes and high voltage positive electrodes to achieve high-energy Li metal batteries, showcasing stable and facile interfaces for Li deposition/dissolution and high anodic stability. This study aims to explore a highly concentrated electrolyte by adopting entropy-driven chemistry for Li metal-free (so-called anode-free) batteries. The combination of lithium bis(fluorosulfonyl)amide (LiFSA) and lithium trifluoromethanesulfonate (LiOTf) salts in a pyrrolidinium-based ionic liquid is found to significantly modify the coordination structure, resulting in an unprecedented 60 mol% Li concentration and a low solvent-to-salt ratio of 0.67:1 in the electrolyte system. This novel 60 mol% Li electrolyte demonstrates unique coordination stricture, featuring a high ratio of monodentate-anion structures and aggregates, which facilitates an enhanced Li+ transference number and improved anodic stability. Moreover, the developed electrolyte provides a facile de-coordination process and leads to the formation of an anion-based solid electrolyte interface, which enables stable Li deposition/dissolution properties and demonstrates excellent cycling stability in the Li metal-free full cell with a Li[Ni0.8Co0.1Mn0.1]O2 (NCM811) positive electrode.

Mechanism of Reductive Fluorination by PTFE-Decomposition Fluorocarbon Gases for WO3.

Reductive fluorination, which entails the substitution of O2- from oxide compounds with F- from fluoropolymers, is considered a practical approach for preparing transition-metal oxyfluorides. However, the current understanding of the fundamental reaction paths remains limited due to the analytical complexities posed by high-temperature reactions in glassware. Therefore, to expand this knowledgebase, this study investigates the reaction mechanisms behind the reductive fluorination of WO3 using polytetrafluoroethylene (PTFE) in an Ni reactor. Here, we explore varied reaction conditions (temperature, duration, and F/W ratio) to suppress the formation of carbon byproducts, minimize the dissipation of fluorine-containing tungsten (VI) compounds, and achieve a high fluorine content. The gas-solid reaction paths are analyzed using infrared spectroscopy, which revealed tetrafluoroethylene (C2F4), hexafluoropropene (C3F6), and iso-octafluoroisobutene (i-C4F8) to be the reactive components in the PTFE-decomposition gas during the reactions with WO3 at 500 °C. CO2 and CO are further identified as gaseous byproducts of the reaction evincing that the reaction is prompted by difluorocarbene (:CF2) formed after the cleavage of C═C bonds in i-C4F8, C3F6, and C2F4 upon contact with the WO3 surface. The solid-solid reaction path is established through a reaction between WO3 and WO3-xFx where solid-state diffusion of O2- and F- is discerned at 500 °C.

Sodium Ion Batteries using Ionic Liquids as Electrolytes.

Sodium ion batteries have been developed using ionic liquids as electrolytes. Sodium is superior to lithium as a raw material for mass production of large-scale batteries for energy storage due to its abundance and even distribution across the earth. Ionic liquids are non-volatile and non-flammable, which improved the safety of the batteries remarkably. In addition, operation temperatures were extended to higher values, improving the performance of the batteries by facilitating the reaction at the electrode and mass transfer. Binary systems of sodium and quaternary ammonium salts, such as 1-ethyl-3-methylimidazolium and N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)amide, were employed as electrolytes for sodium ion batteries. A series of positive and negative electrode materials were examined to be combined with these ionic liquid electrolytes. A 27 Ah full cell was fabricated employing sodium chromite (NaCrO2 ) and hard carbon as positive and negative electrode materials, respectively. The gravimetric energy density obtained for the battery was 75 Wh kg-1 and its volumetric energy density was 125 Wh L-1 . The capacity retention after 500 cycles was 87 %. Further improvement of the cell performance and energy density is expected on development of suitable electrode materials and optimization of the cell design.

Enhanced Cycle Stability of LiNiO2 in a Highly Concentrated Ionic Liquid Electrolyte.

LiNiO2 (LNO) is a promising positive material for next-generation vehicle batteries because of its high theoretical capacity and lower cost compared to the Co analogues. However, its unstable performance such as Ni dissolution results in capacity fade and poor cycle life, impeding its practical application. Since hydrogen fluoride (HF), the hydrolysis product of LiPF6, is highly reactive with LNO positive electrodes, exploring LiPF6-free electrolytes is attractive to improve cycle stability and eliminate parasitic reactions. Herein, a series of ionic liquids (ILs) with Li[FSA] ([FSA]- = bis(fluorosulfonyl)amide) salts are investigated as electrolytes compatible with the LNO positive electrode. The use of IL electrolytes enhances cycle performance, achieving a high capacity retention of 73.1% in Li/LNO cells after 500 cycles with a high Li salt concentration. Further characterizations confirm that the cathode electrolyte interphase formed on the LNO positive electrode in the highly Li-salt concentrated ILs suppresses Ni dissolution, structural degradation, and side reactions. Meanwhile, the above electrolyte is capable of effectively alleviating Al corrosion at high potentials. This work highlights the role of electrolytes and contributes to addressing the stability concerns of positive electrode components at high voltages.

Systematic Study of Aluminum Corrosion in Ionic Liquid Electrolytes for Sodium-Ion Batteries: Impact of Temperature and Concentration.

The development of sodium-ion batteries utilizing sulfonylamide-based electrolytes is significantly encumbered by the corrosion of the Al current collector, resulting in capacity loss and poor cycling stability. While ionic liquid electrolytes have been reported to suppress Al corrosion, a recent study found that pitting corrosion occurs even when ionic liquids are employed. This study investigates the effects of temperature and Na salt concentration on the Al corrosion behavior in different sulfonylamide-based ionic liquid electrolytes for sodium-ion batteries. In the present work, cyclic voltammetry measurements and scanning electron microscopy showed that severe Al corrosion occurred in ionic liquids at high temperatures and low salt concentrations. X-ray photoelectron spectroscopy was employed to identify the different elemental components and verify the thickness of the passivation layer formed under varied salt concentrations and temperatures. The differences in the corrosion behaviors observed under the various conditions are ascribed to the ratio of free [FSA]- to Na+-coordinating [FSA]- in the electrolyte and the stability of the newly formed passivation layer. This work aims at augmenting the understanding of Al corrosion behavior in ionic liquid electrolytes to develop advanced batteries.

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.

Strategies for Harnessing High Rate and Cycle Performance from Graphite Electrodes in Potassium-Ion Batteries.

Potassium-ion batteries (PIBs) have been lauded as the next-generation energy storage systems on account of their high voltage capabilities and low costs and the high abundance of potassium resources. However, the practical utility of PIBs has been heavily encumbered by severe K metal dendrite formation, safety issues, and insufficient electrochemical performance during operations─indeed critical issues that underpin the need for functional electrolytes with high thermal stability, robust solid-electrolyte interphase (SEI)-forming capabilities, and high electrochemical performance. In a bid to establish a knowledge framework for harnessing high rate capabilities and long cycle life from graphite negative electrodes, this study presents the physical properties and electrochemical behavior of a high K+ concentration inorganic ionic liquid (IL) electrolyte, K[FSA]-Cs[FSA] (FSA- = bis(fluorosulfonyl)amide) (54:46 in mol), at an intermediate temperature of 70 °C. This IL electrolyte demonstrates an ionic conductivity of 2.54 mS cm-1 and a wide electrochemical window of 5.82 V. Charge-discharge tests performed on a graphite negative electrode manifest a high discharge capacity of 278 mAh g-1 (0.5 C) at 70 °C, a high rate capability (106 mAh g-1 at 100 C), and a long cyclability (98.7% after 450 cycles). Stable interfacial properties observed by electrochemical impedance spectroscopy during cycling are attributed to the formation of sulfide-rich all-inorganic SEI, which was examined through X-ray photoelectron spectroscopy. The performance of the IL is collated with that of an N-methyl-N-propylpyrrolidinium-based organic IL to provide insight into the synergism between the highly concentrated K+ electrolyte at intermediate temperatures and the all-inorganic SEI during electrochemical operations of the graphite negative electrode.

Octaphyrin(1.0.1.0.1.0.1.0) as an Organic Electrode for Li and Na Rechargeable Batteries.

Organic electrode materials for rechargeable batteries have come into the spotlight due to their structural tunability and diversity. In this study, it is found that bisnickel(II) meso-mesityloctaphyrin(1.0.1.0.1.0.1.0) (Oct) is exhibiting multiple oxidation states with extended π-conjugation pathways to afford an active electrode material in Li and Na-organic batteries and secure interactions with Li+ (or Na+ ) and anions enabling efficient dual ionic charge/discharge behaviors. Cyclic voltammograms of the Oct electrode elucidate constantly reversible redox processes in both Li and Na organic batteries and pseudocapacitive behaviors at high currents. Subsequent absorption transformations in CV-UV/VIS/NIR spectroscopic analysis and TD-DFT calculations upon the different redox states of Oct conclusively indicate that six electrons are involved in redox-interconversions per unit cycle with corresponding absorption transformations, which also assessed with charge-and-discharge cell capacities. Significant contributions of the pseudocapacitive processes over the diffusion-controlled processes proceeding in Li- and Na-Oct cells induced fast charge/discharge performance and long-term cyclability.

Improvement of Electrochemical Stability Using the Eutectic Composition of a Ternary Molten Salt System for Highly Concentrated Electrolytes for Na-Ion Batteries.

The increase in the concentration of electrolytes for secondary batteries has significant advantages in terms of physicochemical and electrochemical performance. This study aims to explore a highly concentrated electrolyte for Na-ion batteries using a ternary salt system. The eutectic composition of the Na[N(SO2F)2]-Na[N(SO2F)(SO2CF3)]-Na[SO3CF3] ternary molten salt system increases solubility into an organic solvent, enabling the use of highly concentrated electrolytes for Na-ion batteries. The ternary salt system achieved concentrations of 5.0 m (m = mol kg-1) with propylene carbonate (PC), 2.9 m with dimethoxyethane, 2.0 m with ethylene carbonate/dimethyl carbonate, and 3.9 m with ethylene carbonate/diethyl carbonate. The highly concentrated electrolyte of 5.0 m in PC suppressed Al corrosion and exhibited better oxidative stability. Stable electrochemical performance using hard carbon/NaCrO2 in the full-cell configuration introduces a new strategy to explore highly concentrated electrolytes for secondary batteries.

Mixed alkali-ion transport and storage in atomic-disordered honeycomb layered NaKNi2TeO6.

Honeycomb layered oxides constitute an emerging class of materials that show interesting physicochemical and electrochemical properties. However, the development of these materials is still limited. Here, we report the combined use of alkali atoms (Na and K) to produce a mixed-alkali honeycomb layered oxide material, namely, NaKNi2TeO6. Via transmission electron microscopy measurements, we reveal the local atomic structural disorders characterised by aperiodic stacking and incoherency in the alternating arrangement of Na and K atoms. We also investigate the possibility of mixed electrochemical transport and storage of Na+ and K+ ions in NaKNi2TeO6. In particular, we report an average discharge cell voltage of about 4 V and a specific capacity of around 80 mAh g-1 at low specific currents (i.e., < 10 mA g-1) when a NaKNi2TeO6-based positive electrode is combined with a room-temperature NaK liquid alloy negative electrode using an ionic liquid-based electrolyte solution. These results represent a step towards the use of tailored cathode active materials for "dendrite-free" electrochemical energy storage systems exploiting room-temperature liquid alkali metal alloy materials.

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.