Solvate Ionic Liquids for Li,Na, K,and Mg Batteries

From the viewpoint of element strategy, non‐Li batteries with promising negative and positive electrodes have been widely studied to support a sustainable society. To develop non‐Li batteries having high energy density, research on electrolyte materials is pivotal. Solvate ionic liquids (SILs) are an emerging class of electrolytes possessing somewhat superior properties for battery applications compared to conventional ionic liquid electrolytes. In this account, we describe our recent efforts regarding SIL‐based electrolytes for Li, Na, K, and Mg batteries with respect to structural, physicochemical, and electrochemical characteristics. Systematic studies based on crystallography and Raman spectroscopy combined with thermal/electrochemical stability analysis showed that the balance of competitive cation?anion and cation?solvent interactions predominates the stability of the solvate cations. We also demonstrated battery applications of SILs as electrolytes for non‐Li batteries, particularly for Na batteries.     

Maximization of Spatial Charge Density: An Approach to Ultrahigh Energy Density of Capacitive Charge Storage

Capacitive energy storage has advantages of high power density, long lifespan, and good safety, but is restricted by low energy density. Inspired by the charge storage mechanism of batteries, a spatial charge density (SCD) maximization strategy is developed to compensate this shortage by densely and neatly packing ionic charges in capacitive materials. A record high SCD (ca. 550 C cm^?3) was achieved by balancing the valance and size of charge‐carrier ions and matching the ion sizes with the pore structure of electrode materials, nearly five times higher than those of conventional ones (ca. 120 C cm^?3). The maximization of SCD was confirmed by Monte Carlo calculations, molecular dynamics simulations, and in situ electrochemical Raman spectroscopy. A full‐cell supercapacitor was further constructed; it delivers an ultrahigh energy density of 165 Wh L^?1 at a power density of 150 WL^?1 and retains 120 Wh L^?1 even at 36 kW L^?1, opening a pathway towards high‐energy‐density capacitive energy storage.     

五种1-烷基-2,3-二甲基咪唑型离子液体的合成及作为Li/LiFeO_4电池电解液的研究

合成了五种新的1-烷基-2,3-二甲基咪唑二(三氟甲基磺酰)亚胺离子液体(alkyl-DMimTFSI).以离子液体作为Li/LiFeO4电池电解液,分别考察不同烷基(正丁基、正戊基、正辛基、异辛基和正癸基)对电解液理化性质、界面性质和电池行为的影响.结果表明离子液体的电化学窗口都可以达到5.6V(-0.4～5.2Vvs.Li+/Li),显示它们具有较好的电化学稳定性.加入碳酸亚乙烯酯作为添加剂后,离子液体电解液在Li负极形成稳定的固体电解质相界面膜(SEI),从而提高了Li负极的稳定性,保护了Li片不受腐蚀.电化学阻抗和循环伏安分析进一步揭示LiFeO4正极与离子液体电解液也有良好的兼容性.此外,研究还表明离子液体中烷基种类严重影响它们的电池行为.采用butyl-DMimTFSI和amyl-DMimTFSI电解液体系的电池充放电容量和可逆性明显优于另外三种离子液体,它们的首次放电容量分别达到145和152.6mAh/g,并表现出良好的充放电循环性能.因粘度最大,采用isooctyl-DMimTFSI电解液的电池首次放电容量仅为8.3mAh/g,但添加碳酸丙烯酯(质量比1∶1)稀释后首次放电容量上升至132.4mAh/g.     

An Aqueous Redox‐Flow Battery with High Capacity and Power: The TEMPTMA/MV System

Redox‐flow batteries (RFB) can easily store large amounts of electric energy and thereby mitigate the fluctuating output of renewable power plants. They are widely discussed as energy‐storage solutions for wind and solar farms to improve the stability of the electrical grid. Most common RFB concepts are based on strongly acidic metal‐salt solutions or poorly performing organics. Herein we present a battery which employs the highly soluble N,N,N‐2,2,6,6‐heptamethylpiperidinyl oxy‐4‐ammonium chloride (TEMPTMA) and the viologen derivative N,N′‐dimethyl‐4,4‐bipyridinium dichloride (MV) in a simple and safe aqueous solution as redox‐active materials. The resulting battery using these electrolyte solutions has capacities of 54 Ah L^?1, giving a total energy density of 38 Wh L^?1 at a cell voltage of 1.4 V. With peak current densities of up to 200 mA cm^?2 the TEMPTMA/MV system is a suitable candidate for compact high‐capacity and high‐power applications.     

Solvent‐Mediated Control of the Electrochemical Discharge Products of Non‐Aqueous Sodium–Oxygen Electrochemistry

The reduction of dioxygen in the presence of sodium cations can be tuned to give either sodium superoxide or sodium peroxide discharge products at the electrode surface. Control of the mechanistic direction of these processes may enhance the ability to tailor the energy density of sodium–oxygen batteries (NaO₂: 1071 Wh kg^?1 and Na₂O₂: 1505 Wh kg^?1). Through spectroelectrochemical analysis of a range of non‐aqueous solvents, we describe the dependence of these processes on the electrolyte solvent and subsequent interactions formed between Na⁺ and O₂^?. The solvents ability to form and remove [Na⁺‐O₂^?]_ads based on Gutmann donor number influences the final discharge product and mechanism of the cell. Utilizing surface‐enhanced Raman spectroscopy and electrochemical techniques, we demonstrate an analysis of the response of Na‐O₂ cell chemistry with sulfoxide, amide, ether, and nitrile electrolyte solvents.     

An All‐Solid‐State Fiber‐Shaped Aluminum–Air Battery with Flexibility,Stretchability, and High Electrochemical Performance

Owing to the high theoretical energy density of metal–air batteries, the aluminum–air battery has been proposed as a promising long‐term power supply for electronics. However, the available energy density from the aluminum–air battery is far from that anticipated and is limited by current electrode materials. Herein we described the creation of a new family of all‐solid‐state fiber‐shaped aluminum–air batteries with a specific capacity of 935 mAh g⁻¹ and an energy density of 1168 Wh kg⁻¹. The synthesis of an electrode composed of cross‐stacked aligned carbon‐nanotube/silver‐nanoparticle sheets contributes to the remarkable electrochemical performance. The fiber shape also provides the aluminum–air batteries with unique advantages; for example, they are flexible and stretchable and can be woven into a variety of textiles for large‐scale applications.     

An All‐Solid‐State Fiber‐Shaped Aluminum–Air Battery with Flexibility,Stretchability, and High Electrochemical Performance

Owing to the high theoretical energy density of metal–air batteries, the aluminum–air battery has been proposed as a promising long‐term power supply for electronics. However, the available energy density from the aluminum–air battery is far from that anticipated and is limited by current electrode materials. Herein we described the creation of a new family of all‐solid‐state fiber‐shaped aluminum–air batteries with a specific capacity of 935 mAh g^?1 and an energy density of 1168 Wh kg^?1. The synthesis of an electrode composed of cross‐stacked aligned carbon‐nanotube/silver‐nanoparticle sheets contributes to the remarkable electrochemical performance. The fiber shape also provides the aluminum–air batteries with unique advantages; for example, they are flexible and stretchable and can be woven into a variety of textiles for large‐scale applications.     

Polymerization of the new double‐charged monomer bis‐1,3(N,N,N‐trimethylammonium dicyanamide)‐2‐propylmethacrylate and ionic conductivity of the novel polyelectrolytes

The achievement of high ionic conductivity in single‐ion conducting polymer electrolytes is one of the important aims for various electrochemical devices including modern lithium batteries. One way to enhance the ionic conductivity in polyelectrolyte systems is to increase the quantity of charge carriers in each monomer unit. Highly charged poly(bis‐1,3(N,N,N‐trimethylammonium)‐2‐propylmethacrylate) with one of the most conducting anions, namely dicyanamide, was prepared via free radical bulk polymerization or using ionic liquids as reaction medium. The cationic polymers of the double‐charged monomer have molar masses up to = 1,830,000 g/mol and the ionic conductivity equal to 5.51 × 10^?5 S / cm at 25°C. The film forming ability, crystallinity, thermal stability, and glass transition temperatures of the new polymeric ionic liquids obtained from detailed studies are presented. Copyright © 2009 John Wiley & Sons, Ltd.     

Regulating Fast Anionic Redox for High‐Voltage Aqueous Hydrogen‐Ion‐based Energy Storage

The ionic conductivity and small size of the hydrogen ion make it an ideal charge carrier for hydrogen‐ion energy storage (HES); however, high‐voltage two‐electrode configurations are difficult to construct as the result of the lack of efficient cathodic energy storage. Herein, the high potential fast anionic redox at the cathode of reduced graphene oxide (rGO) was applied by introducing redox additive electrolytes. By coupling the storing hydrogen ion in the Ti₃C₂T_x at the anode, a HES with a voltage of 1.8 V and a plateau voltage at 1.2 V was constructed. Compared with 2.2 Wh kg^?1 for the low‐voltage Ti₃C₂T_x//Ti₃C₂T_x, the specific energy of asymmetric rGO//Ti₃C₂T_x reaches 34.4 Wh kg^?1. Furthermore, it possesses an energy density of 23.7 Wh kg^?1 at high power density of 22.5 kW kg^?1. Thus, this study provides a novel guideline for constructing high‐voltage fast HES full cells.     

Understanding Structural and Transport Properties of Dissolved Li2S8 in Ionic Liquid Electrolytes through Molecular Dynamics Simulations

Lithium-sulfur batteries with high energy density are considered as one of the most promising future energy storage devices. However, the parasitic lithium polysulfides shuttle phenomenon severely hinders the commercialization of such batteries. Ionic liquids have been found to suppress the lithium polysulfides solubility, diminishing the shuttle effect effectively. Herein, we performed classical molecular dynamics simulations to explore the microscopic mechanism and transport behaviors of typical Li₂S₈ species in ionic liquids and ionic liquid-based electrolyte systems. We found that the trifluoromethanesulfonate anions ([OTf]⁻) exhibit higher coordination strength with lithium ions compared with bis(trifluoromethanesulfonyl)imide anions ([TFSI]⁻) in static microstructures. However, the dynamical characteristics indicate that the presence of the [OTf]⁻ anions in ionic liquid electrolytes bring faster Li⁺ exchange rate and easier dissociation of Li⁺ solvation structures. Our simulation models offer a significant guidance to future studies on designing ionic liquid electrolytes for lithium-sulfur batteries.     

An Ultrastable Anode for Long‐Life Room‐Temperature Sodium‐Ion Batteries

Sodium‐ion batteries are important alternative energy storage devices that have recently come again into focus for the development of large‐scale energy storage devices because sodium is an abundant and low‐cost material. However, the development of electrode materials with long‐term stability has remained a great challenge. A novel negative‐electrode material, a P2‐type layered oxide with the chemical composition Na_2/3Co_1/3Ti_2/3O₂, exhibits outstanding cycle stability (ca. 84.84 % capacity retention for 3000 cycles, very small decrease in the volume (0.046 %) after 500 cycles), good rate capability (ca. 41 % capacity retention at a discharge/charge rate of 10 C), and a usable reversible capacity of about 90 mAh g^?1 with a safe average storage voltage of approximately 0.7 V in the sodium half‐cell. This P2‐type layered oxide is a promising anode material for sodium‐ion batteries with a long cycle life and should greatly promote the development of room‐temperature sodium‐ion batteries.     

Tailored Organic Electrode Material Compatible with Sulfide Electrolyte for Stable All‐Solid‐State Sodium Batteries

All‐solid‐state sodium batteries (ASSSBs) with nonflammable electrolytes and ubiquitous sodium resource are a promising solution to the safety and cost concerns for lithium‐ion batteries. However, the intrinsic mismatch between low anodic decomposition potential of superionic sulfide electrolytes and high operating potentials of sodium‐ion cathodes leads to a volatile cathode–electrolyte interface and undesirable cell performance. Here we report a high‐capacity organic cathode, Na₄C₆O₆, that is chemically and electrochemically compatible with sulfide electrolytes. A bulk‐type ASSSB shows high specific capacity (184 mAh g^?1) and one of the highest specific energies (395 Wh kg^?1) among intercalation compound‐based ASSSBs. The capacity retentions of 76 % after 100 cycles at 0.1 C and 70 % after 400 cycles at 0.2 C represent the record stability for ASSSBs. Additionally, Na₄C₆O₆ functions as a capable anode material, enabling a symmetric all‐organic ASSSB with Na₄C₆O₆ as both cathode and anode materials.     

Redox-active electrolyte supercapacitors using electroactive ionic liquids

Here we present redox ionic liquid supercapacitors (RILSCs) which use electrolytes made from ionic liquids modified with an electroactive function to increase the energy density of activated carbon electrodes via faradaic reactions. More specifically, two different ionic liquids were made by modifying either the imidazolium cation or the bis(trifluoromethanesulfonyl)imide anion with ferrocene in order to determine the importance of the electroactive ion's polarity. The functionalization of an ionic liquid with ferrocene led to high concentrations of redox moieties in the electrolyte (2.4 M) and a large maximum operating voltage (2.5 V). An energy density of up to 13.2 Wh per kg (both electrodes) was obtained which represents an 83% increase vs. the unmodified ionic liquid. When the ionic liquid's anion is modified with ferrocene, the self-discharge at the positive electrode is fully suppressed due to the deposition of a film on the electrode. The results presented herein demonstrate that electroactive ionic liquids constitute a promising alternative to conventional solute in solvent electrolytes found in energy storage devices, and are particularly well-suited for redox-active electrolyte supercapacitors.     

A Bipolar and Self‐Polymerized Phthalocyanine Complex for Fast and Tunable Energy Storage in Dual‐Ion Batteries

Bipolar redox organics have attracted interest as electrode materials for energy storage owing to their flexibility, sustainability and environmental friendliness. However, an understanding of their application in all‐organic batteries, let alone dual‐ion batteries (DIBs), is in its infancy. Herein, we propose a strategy to screen a variety of phthalocyanine‐based bipolar organics. The self‐polymerizable bipolar Cu tetraaminephthalocyanine (CuTAPc) shows multifunctional applications in various energy storage systems, including lithium‐based DIBs using CuTAPc as the cathode material, graphite‐based DIBs using CuTAPc as the anode material and symmetric DIBs using CuTAPc as both the cathode and anode materials. Notably, in lithium‐based DIBs, the use of CuTAPc as the cathode material results in a high discharge capacity of 236 mAh g^?1 at 50 mA g^?1 and a high reversible capacity of 74.3 mAh g^?1 after 4000 cycles at 4 A g^?1. Most importantly, a high energy density of 239 Wh kg^?1 and power density of 11.5 kW kg^?1 can be obtained in all‐organic symmetric DIBs.     

Two‐Plateau Li‐Se Chemistry for High Volumetric Capacity Se Cathodes

For Li‐Se batteries, ether‐ and carbonate‐based electrolytes are commonly used. However, because of the “shuttle effect” of the highly dissoluble long‐chain lithium polyselenides (LPSes, Li₂Se_n, 4≤n≤8) in the ether electrolytes and the sluggish one‐step solid‐solid conversion between Se and Li₂Se in the carbonate electrolytes, a large amount of porous carbon (>40 wt % in the electrode) is always needed for the Se cathodes, which seriously counteracts the advantage of Se electrodes in terms of volumetric capacity. Herein an acetonitrile‐based electrolyte is introduced for the Li‐Se system, and a two‐plateau conversion mechanism is proposed. This new Li‐Se chemistry not only avoids the shuttle effect but also facilitates the conversion between Se and Li₂Se, enabling an efficient Se cathode with high Se utilization (97 %) and enhanced Coulombic efficiency. Moreover, with such a designed electrolyte, a highly compact Se electrode (2.35 g_Se cm^?3) with a record‐breaking Se content (80 wt %) and high Se loading (8 mg cm^?2) is demonstrated to have a superhigh volumetric energy density of up to 2502 Wh L^?1, surpassing that of LiCoO₂.     

Na3SbS4: A Solution Processable Sodium Superionic Conductor for All‐Solid‐State Sodium‐Ion Batteries

All‐solid‐state sodium‐ion batteries that operate at room temperature are attractive candidates for use in large‐scale energy storage systems. However, materials innovation in solid electrolytes is imperative to fulfill multiple requirements, including high conductivity, functional synthesis protocols for achieving intimate ionic contact with active materials, and air stability. A new, highly conductive (1.1 mS cm^?1 at 25 °C, E_a=0.20 eV) and dry air stable sodium superionic conductor, tetragonal Na₃SbS₄, is described. Importantly, Na₃SbS₄ can be prepared by scalable solution processes using methanol or water, and it exhibits high conductivities of 0.1–0.3 mS cm^?1. The solution‐processed, highly conductive solidified Na₃SbS₄ electrolyte coated on an active material (NaCrO₂) demonstrates dramatically improved electrochemical performance in all‐solid‐state batteries.     

Enhancing Capacity Performance by Utilizing the Redox Chemistry of the Electrolyte in a Dual‐Electrolyte Sodium‐Ion Battery

A strategy is described to increase charge storage in a dual electrolyte Na‐ion battery (DESIB) by combining the redox chemistry of the electrolyte with a Na⁺ ion de‐insertion/insertion cathode. Conventional electrolytes do not contribute to charge storage in battery systems, but redox‐active electrolytes augment this property via charge transfer reactions at the electrode–electrolyte interface. The capacity of the cathode combined with that provided by the electrolyte redox reaction thus increases overall charge storage. An aqueous sodium hexacyanoferrate (Na₄Fe(CN)₆) solution is employed as the redox‐active electrolyte (Na‐FC) and sodium nickel Prussian blue (Na_x‐NiBP) as the Na⁺ ion insertion/de‐insertion cathode. The capacity of DESIB with Na‐FC electrolyte is twice that of a battery using a conventional (Na₂SO₄) electrolyte. The use of redox‐active electrolytes in batteries of any kind is an efficient and scalable approach to develop advanced high‐energy‐density storage systems.     

A Sustainable Solid Electrolyte Interphase for High‐Energy‐Density Lithium Metal Batteries Under Practical Conditions

High‐energy‐density Li metal batteries suffer from a short lifespan under practical conditions, such as limited lithium, high loading cathode, and lean electrolytes, owing to the absence of appropriate solid electrolyte interphase (SEI). Herein, a sustainable SEI was designed rationally by combining fluorinated co‐solvents with sustained‐release additives for practical challenges. The intrinsic uniformity of SEI and the constant supplements of building blocks of SEI jointly afford to sustainable SEI. Specific spatial distributions and abundant heterogeneous grain boundaries of LiF, LiN_xO_y, and Li₂O effectively regulate uniformity of Li deposition. In a Li metal battery with an ultrathin Li anode (33 μm), a high‐loading LiNi_0.5Co_0.2Mn_0.3O₂ cathode (4.4 mAh cm^?2), and lean electrolytes (6.1 g Ah^?1), 83 % of initial capacity retains after 150 cycles. A pouch cell (3.5 Ah) demonstrated a specific energy of 340 Wh kg^?1 for 60 cycles with lean electrolytes (2.3 g Ah^?1).     

NaFe0.5Mn0.5PO4–Ge electrochemical system for sodium-ion batteries

A new electrochemical system for sodium-ion batteries, sodium iron-manganese phosphate–germanium nanowires, has been developed and tested. The laboratory batteries possessed an average discharge voltage of ∼2.0 V and an energy density based on the weight of active materials to 210 Wh kg^–1.     

Highly Concentrated Electrolyte towards Enhanced Energy Density and Cycling Life of Dual‐Ion Battery

Dual‐ion batteries (DIBs) have attracted much attention owing to their low cost, high voltage, and environmental friendliness. As the source of active ions during the charging/discharging process, the electrolyte plays a critical role in the performance of DIBs, including capacity, energy density, and cycling life. However, most used electrolyte systems based on the LiPF₆ salt demonstrate unsatisfactory performance in DIBs. We have successfully developed a 7.5 mol kg^?1 lithium bis(fluorosulfonyl)imide (LiFSI) in a carbonate electrolyte system. Compared with diluted electrolytes, this highly concentrated electrolyte exhibits several advantages: 1) enhanced intercalation capacity and cycling stability of the graphite cathode, 2) optimized structural stability of the Al anode, and 3) significantly increased battery energy density. A proof‐of‐concept DIB based on this concentrated electrolyte exhibits a discharge capacity of 94.0 mAh g^?1 at 200 mA g^?1 and 96.8 % capacity retention after 500 cycles. By counting both the electrode materials and electrolyte, the energy density of this DIB reaches up to ≈180 Wh kg^?1, which is among the best performances of DIBs reported to date.