Lithium Salt Dissociation Promoted by 18-Crown-6 Ether Additive toward Dilute Electrolytes for High Performance Lithium Oxygen Batteries

Lithium-oxygen batteries (LOBs) are well known for their high energy density. However, their reversibility and rate performance are challenged due to the sluggish oxygen reduction/evolution reactions (ORR/OER) kinetics, serious side reactions and uncontrollable Li dendrite growth. The electrolyte plays a key role in transport of Li⁺ and reactive oxygen species in LOBs. Here, we tailored a dilute electrolyte by screening suitable crown ether additives to promote lithium salt dissociation and Li⁺ solvation through electrostatic interaction. The electrolyte containing 100 mM 18-crown-6 ether (100-18C6) exhibits enhanced electrochemical stability and triggers a solution-mediated Li₂O₂ growth pathway in LOBs, showing high discharge capacity of 10 828.8 mAh g_carbon⁻¹. Moreover, optimized electrode/electrolyte interfaces promote ORR/OER kinetics on cathode and achieve dendrite-free Li anode, which enhances the cycle life. This work casts new lights on the design of low-cost dilute electrolytes for high performance LOBs.     

Direct Detection of the Superoxide Anion as a Stable Intermediate in the Electroreduction of Oxygen in a Non‐Aqueous Electrolyte Containing Phenol as a Proton Source

The non‐aqueous Li–air (O₂) battery has attracted intensive interest because it can potentially store far more energy than today′s batteries. Presently Li–O₂ batteries suffer from parasitic reactions owing to impurities, found in almost all non‐aqueous electrolytes. Impurities include residual protons and protic compounds that can react with oxygen species, such as the superoxide (O₂^?), a reactive, one‐electron reduction product of oxygen. To avoid the parasitic reactions, it is crucial to have a fundamental understanding of the conditions under which reactive oxygen species are generated in non‐aqueous electrolytes. Herein we report an in situ spectroscopic study of oxygen reduction on gold in a dimethyl sulfoxide electrolyte containing phenol as a proton source. It is shown directly that O₂^?, not HO₂, is the first stable intermediate during the oxygen reduction process to hydrogen peroxide. The unusual stability of O₂^? is explained using density functional theory (DFT) calculations.     

2,3:4,6‐Di‐O‐isopropylidene‐α‐l‐sorbofuranose and 2,3‐O‐isopropylidene‐α‐l‐sorbofuranose

In the title compounds, C₁₂H₂₀O₆, (I), and C₉H₁₆O₆, (II), the five‐membered furanose ring adopts a ⁴T₃ conformation and the five‐membered 1,3‐dioxolane ring adopts an E₃ conformation. The six‐membered 1,3‐dioxane ring in (I) adopts an almost ideal ^OC₃ conformation. The hydrogen‐bonding patterns for these compounds differ substantially: (I) features just one intramolecular O—H...O hydrogen bond [O...O = 2.933 (3) Å], whereas (II) exhibits, apart from the corresponding intramolecular O—H...O hydrogen bond [O...O = 2.7638 (13) Å], two intermolecular bonds of this type [O...O = 2.7708 (13) and 2.7730 (12) Å]. This study illustrates both the similarity between the conformations of furanose, 1,3‐dioxolane and 1,3‐dioxane rings in analogous isopropylidene‐substituted carbohydrate structures and the only negligible influence of the presence of a 1,3‐dioxane ring on the conformations of furanose and 1,3‐dioxolane rings. In addition, in comparison with reported analogs, replacement of the –CH₂OH group at the C1‐furanose position by another group can considerably affect the conformation of the 1,3‐dioxolane ring.     

Studies on the Radical Cyclization of 3‐Oxopropanenitriles and Alkenes with Cerium(IV) Ammonium Nitrate in Ether Solvents

The radical cyclization of 3‐oxopropanenitriles 1a – 1e and alkenes 2a – 2g with cerium(IV) ammonium nitrate (CAN) in ether solvents was investigated (Tables 1 and 2). In the optimization study, 1,3‐dioxolane, 1,4‐dioxane, 1,2‐dimethoxyethane, Et₂O, and THF were used as ether‐based solvents, and the latter was found to be the most effective solvent in radical cyclizations mediated by cerium(IV). This system (cerium(IV)/THF) was applied to cyclizations of various 3‐oxopropanenitriles and 1,3‐dicarbonyl compounds with alkenes resulting in the formation of 4,5‐dihydrofurans in high yields (Table 2 and Scheme 2). The results of the cerium(IV)/THF radical cyclization were compared with those obtained with manganese(III) acetate/AcOH; the cerium(IV)/THF system turned out to be much more efficient.     

Unveiling the Critical Role of Ion Coordination Configuration of Ether Electrolytes for High Voltage Lithium Metal Batteries

Albeit ethers are favorable electrolyte solvents for lithium (Li) metal anode, their inferior oxidation stability (<4.0 V vs. Li/Li⁺) is problematic for high-voltage cathodes. Studies of ether electrolytes have been focusing on the archetype glyme structure with ethylene oxide moieties. Herein, we unveil the crucial effect of ion coordination configuration on oxidation stability by varying the ether backbone structure. The designed 1,3-dimethoxypropane (DMP, C3) forms a unique six-membered chelating complex with Li⁺, whose stronger solvating ability suppresses oxidation side reactions. In addition, the favored hydrogen transfer reaction between C3 and anion induces a dramatic enrichment of LiF (a total atomic ratio of 76.7 %) on the cathode surface. As a result, the C3-based electrolyte enables greatly improved cycling of nickel-rich cathodes under 4.7 V. This study offers fundamental insights into rational electrolyte design for developing high-energy-density batteries.     

Singlet Oxygen Formation during the Charging Process of an Aprotic Lithium–Oxygen Battery

Aprotic lithium–oxygen (Li–O₂) batteries have attracted considerable attention in recent years owing to their outstanding theoretical energy density. A major challenge is their poor reversibility caused by degradation reactions, which mainly occur during battery charge and are still poorly understood. Herein, we show that singlet oxygen (¹Δ_g) is formed upon Li₂O₂ oxidation at potentials above 3.5 V. Singlet oxygen was detected through a reaction with a spin trap to form a stable radical that was observed by time‐ and voltage‐resolved in operando EPR spectroscopy in a purpose‐built spectroelectrochemical cell. According to our estimate, a lower limit of approximately 0.5 % of the evolved oxygen is singlet oxygen. The occurrence of highly reactive singlet oxygen might be the long‐overlooked missing link in the understanding of the electrolyte degradation and carbon corrosion reactions that occur during the charging of Li–O₂ cells.     

Electron Delocalization Enables Sulfone-based Single-solvent Electrolyte for Lithium Metal Batteries

Li-metal batteries (LMB), although providing high energy density, face the grand challenge of identifying good electrolyte solvents for cycling. Common solvents are either only stable against lithium metal anode or only stable against LiNi_xMn_yCo_1-x-yO₂ (NMC) cathode. There is significant effort trying to increase the cathode stability for ether electrolytes, which are in general stable against lithium metal anode. In comparison, there is much less effort trying to increase the anode stability of electrolytes that are stable against NMC cathode. One example is the sulfone-based electrolyte. It has good cathode stability but is hindered from practical application because of (1) high viscosity and poor wetting capability and (2) poor anode stability. Here, we solve these issues by modifying the sulfone molecules using resonance and electron withdrawing effect. The viscosity is significantly reduced by delocalizing the electrons through introducing additional oxygen on the molecular backbone and applying appropriate fluorination. The resulting molecule 2,2,2-trifluoroethyl mesylate (TFEM) has decreased Lewis basicity and less reactivity toward Li⁺. The electrolyte based on TFEM as single solvent enables cycling of LMB under harsh conditions of low N/P ratio (21 mg/cm² NMC811 and 50 μm Li) with 90 % capacity retention after 160 cycles at C/3 discharge rate.     

Free‐radical polymerization of dioxolane and dioxane derivatives: Effect of fluorine substituents on the ring opening polymerization

Partially fluorinated and perfluorinated dioxolane and dioxane derivatives have been prepared to investigate the effect of fluorine substituents on their free‐radical polymerization products. The partially fluorinated monomer 2‐difluoromethylene‐1,3‐dioxolane (I) was readily polymerized with free‐radical initiators azobisisobutyronitrile or tri(n‐butyl)borane–air and yielded a vinyl addition product. However, the hydrocarbon analogue, 2‐methylene‐1,3‐dioxolane (II), produced as much as 50% ring opening product at 60 °C by free‐radical polymerization. 2‐Difluoromethylene‐4‐methyl‐1,3‐dioxolane (III) was synthesized and its free‐radical polymerization yielded ring opening products: 28% at 60 °C, decreasing to 7 and 4% at 0 °C and −78 °C, respectively. All the fluorine‐substituted, perfluoro‐2‐methylene‐4‐methyl‐1,3‐dioxolane (IV) produced only a vinyl addition product with perfluorobenzoylperoxide as an initiator. The six‐membered ring monomer, 2‐methylene‐1,3‐dioxane (V), caused more than 50% ring opening during free‐radical polymerization. However, the partially fluorinated analogue, 2‐difluoromethylene‐1,3‐dioxane (VI), produced only 22% ring opening product with free‐radical polymerization and the perfluorinated compound, perfluoro‐2‐methylene‐1,3‐dioxane (VII), yielded only the vinyl addition polymer. The ring opening reaction and the vinyl addition steps during the free‐radical polymerization of these monomers are competitive reactions. We discuss the reaction mechanism of the ring opening and vinyl addition polymerizations of these partially fluorinated and perfluorinated dioxolane and dioxane derivatives. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5180–5188, 2004     

Deep‐Eutectic‐Solvent‐Based Self‐Healing Polymer Electrolyte for Safe and Long‐Life Lithium‐Metal Batteries

The deployment of high‐energy‐density lithium‐metal batteries has been greatly impeded by Li dendrite growth and safety concerns originating from flammable liquid electrolytes. Herein, we report a stable quasi‐solid‐state Li metal battery with a deep eutectic solvent (DES)‐based self‐healing polymer (DSP) electrolyte. This electrolyte was fabricated in a facile manner by in situ copolymerization of 2‐(3‐(6‐methyl‐4‐oxo‐1,4‐dihydropyrimidin‐2‐yl)ureido)ethyl methacrylate (UPyMA) and pentaerythritol tetraacrylate (PETEA) monomers in a DES‐based electrolyte containing fluoroethylene carbonate (FEC) as an additive. The well‐designed DSP electrolyte simultaneously possesses non‐flammability, high ionic conductivity and electrochemical stability, and dendrite‐free Li plating. When applied in Li metal batteries with a LiMn₂O₄ cathode, the DSP electrolyte effectively suppressed manganese dissolution from the cathode and enabled high‐capacity and a long lifespan at room and elevated temperatures.     

Electronic spectra and intersystem spin‐orbit coupling in 1,2‐ and 1,3‐squaraines

The main photophysical properties of a series of recently synthetized 1,2‐ and 1,3‐squaraines, including absorption electronic spectra, singlet‐triplet energy gaps, and spin‐orbit matrix elements, have been investigated by means of density functional theory (DFT) and time‐dependent DFT approaches. A benchmark of three exchange‐correlation functionals has been performed in six different solvent environments. The investigated 1,2 squaraines have been found to possess two excited triplet states (T₁ and T₂) that lie below the energy of the excited singlet one (S₁). The radiationless intersystem spin crossing efficiency is thus enhanced in both the studied systems and both the transitions could contribute to the excited singlet oxygen production. Moreover, they have a singlet‐triplet energy gap higher than that required to generate the cytotoxic singlet oxygen species. According to our data, these compounds could be used in photodynamic therapy applications that do not require high tissue penetration. © 2014 Wiley Periodicals, Inc.     

Cationic polymerization of cyclic ketene acetals via zwitterion formation with cyanoallene

This article deals with the polymerization of the cyclic ketene acetals (CKAs) 2‐methylene‐4‐phenyl‐1,3‐dioxolane (2), 2‐methylene‐4‐phenyl‐1,3‐dioxane (3), 4,7‐dimethyl‐2‐methylene‐1,3‐dioxepane (4), 2‐ethylidene‐4‐phenyl‐1,3‐dioxolane (5), 2‐phenylmethylene‐1,3‐dioxolane (6), and 2‐isopropylidene‐4‐phenyl‐1,3‐dioxolane (7) in the presence of cyanoallene (1). For 2 and 3, the homopolymerization of the CKAs proceeded without ring opening, and the number‐average molecular weights of the obtained polymers depended on the feed ratio of 1. However, the reactions of 1 with 4–7 afforded no polymers but did afford spirocyclic 1 : 1 adducts possessing cyclobutane rings. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2075–2081, 2000     

Unravelling the Reaction Mechanism for the Fast Photocyclisation of 2‐Benzoylpyridine in Aqueous Solvent by Time‐Resolved Spectroscopy and Density Functional Theory Calculations

A combined femtosecond transient absorption (fs‐TA) and nanosecond time‐resolved resonance Raman (ns‐TR³) spectroscopic investigation of the photoreaction of 2‐benzoylpyridine (2‐BPy) in acetonitrile and neutral, basic and acidic aqueous solvents is reported. fs‐TA results showed that the nπ* triplet 2‐BPy is the precursor of the photocyclisation reaction in neutral and basic aqueous solvents. The cis triplet biradical and the cis singlet zwitterionic species produced during the photocyclisation reaction were initially characterised by ns‐TR³ spectroscopy. In addition, a new species was uniquely observed in basic aqueous solvent after the decay of the cis singlet zwitterionic species and this new species was tentatively assigned to the photocyclised radical anion. The ground‐state conformation of 2‐BPy in acidic aqueous solvent is the pyridine nitrogen‐protonated 2‐BPy cation (2‐BPy‐NH⁺) rather than the neutral form of 2‐BPy. After laser photolysis, the singlet excited state (S₁) of 2‐BPy‐NH⁺ is generated and evolves through excited‐state proton transfer (ESPT) and efficient intersystem crossing (ISC) processes to the triplet exited state (T₁) of the carbonyl oxygen‐protonated 2‐BPy cation (2‐BPy‐OH⁺) and then photocyclises with the lone pair of the nitrogen atom in the heterocyclic ring. Cyclisation reactions take place both in neutral/basic and acidic aqueous solvents, but the photocyclisation mechanisms in these different aqueous solvents are very different. This is likely due to the different conformation of the precursor and the influence of hydrogen‐bonding of the solvent on the reactions.     

Full Dissolution of the Whole Lithium Sulfide Family (Li2S8 to Li2S) in a Safe Eutectic Solvent for Rechargeable Lithium–Sulfur Batteries

The lithium–sulfur battery is an attractive option for next‐generation energy storage owing to its much higher theoretical energy density than state‐of‐the‐art lithium‐ion batteries. However, the massive volume changes of the sulfur cathode and the uncontrollable deposition of Li₂S₂/Li₂S significantly deteriorate cycling life and increase voltage polarization. To address these challenges, we develop an ?‐caprolactam/acetamide based eutectic‐solvent electrolyte, which can dissolve all lithium polysulfides and lithium sulfide (Li₂S₈–Li₂S). With this new electrolyte, high specific capacity (1360 mAh g^?1) and reasonable cycling stability are achieved. Moreover, in contrast to conventional ether electrolyte with a low flash point (ca. 2 °C), such low‐cost eutectic‐solvent‐based electrolyte is difficult to ignite, and thus can dramatically enhance battery safety. This research provides a new approach to improving lithium–sulfur batteries in aspects of both safety and performance.     

Superoxide Stabilization and a Universal KO2 Growth Mechanism in Potassium–Oxygen Batteries

Rechargeable potassium–oxygen (K‐O₂) batteries promise to provide higher round‐trip efficiency and cycle life than other alkali–oxygen batteries with satisfactory gravimetric energy density (935 Wh kg^?1). Exploiting a strong electron‐donating solvent, for example, dimethyl sulfoxide (DMSO) strongly stabilizes the discharge product (KO₂), resulting in significant improvement in electrode kinetics and chemical/electrochemical reversibility. The first DMSO‐based K‐O₂ battery demonstrates a much higher energy efficiency and stability than the glyme‐based electrolyte. A universal KO₂ growth model is developed and it is demonstrated that the ideal solvent for K‐O₂ batteries should strongly stabilize superoxide (strong donor ability) to obtain high electrode kinetics and reversibility while providing fast oxygen diffusion to achieve high discharge capacity. This work elucidates key electrolyte properties that control the efficiency and reversibility of K‐O₂ batteries.     

An Efficient Halogen‐Free Electrolyte for Use in Rechargeable Magnesium Batteries

Unlocking the full potential of rechargeable magnesium batteries has been partially hindered by the reliance on chloride‐based complex systems. Despite the high anodic stability of these electrolytes, they are corrosive toward metallic battery components, which reduce their practical electrochemical window. Following on our new design concept involving boron cluster anions, monocarborane CB₁₁H₁₂^? produced the first halogen‐free, simple‐type Mg salt that is compatible with Mg metal and displays an oxidative stability surpassing that of ether solvents. Owing to its inertness and non‐corrosive nature, the Mg(CB₁₁H₁₂)₂/tetraglyme (MMC/G4) electrolyte system permits standardized methods of high‐voltage cathode testing that uses a typical coin cell. This achievement is a turning point in the research and development of Mg electrolytes that has deep implications on realizing practical rechargeable Mg batteries.     

Determination of the quantum yield of singlet oxygen sensitized by halogenated boron difluoride dipyrromethenes

The spectral–luminescent, photophysical, and photochemical properties of dichloro-, dibromo-, and diiodo-derivatives of boron dipyrromethenate (BODIPY) have been studied, as well as the feasibility of generating singlet oxygen (¹O₂) via its photosensitization by the dihalogenated derivatives of BF₂ dipyrromethene in solutions. Quantum yields of singlet oxygen have been determined using 1,3-diphenylisobenzofuran as the ¹O₂ trap. The lowest fluorescence quantum yields have been shown to correspond to the maximum yields of singlet oxygen. It has been found that the best ¹O₂ photosensitizer among the three test dihalotetraphenylaza- BODIPY is dibromotetraphenylaza-BODIPY, which in addition possesses the highest photostability. Diiodotetramethyl-BODIPY results in the singlet oxygen yield close to unity, but it has significantly lower photostability. The yield of singlet oxygen is affected by the solvent. Dibromtetraphenylaza-BODIPY and diiodotetramethyl-BODIPY may find use as a medium in photodynamic therapy and photocatalysis of oxidation reactions.     

应用超微电极和粉末微电极技术研究有机溶剂和正极材料对氧气还原反应和氧气生成反应的影响

采用碳纤维超微电极分别研究了O_2在二甲基亚砜、乙腈和四甘醇二甲醚3种有机溶剂中的电化学反应,结果表明,当阳离子只含四丁胺离子时,反应呈可逆的一电子转移;而阳离子只含锂离子时,O_2的还原和氧化均经历了多电子转移过程.利用超导炭黑和乙炔黑制作粉末微电极进行电化学测试,结果表明,在这2种正极材料上,氧气还原反应(ORR)过程相似,氧气生成反应(OER)过程区别明显.此外,Tafel分析结果表明,对于不同有机溶剂和正极材料,O_2还原均经历了初始的一电子转移步骤.     

A Liquid/Liquid Electrolyte Interface that Inhibits Corrosion and Dendrite Growth of Lithium in Lithium‐Metal Batteries

A proof‐of‐concept study on a liquid/liquid (L/L) two‐phase electrolyte interface is reported by using the polarity difference of solvent for the protection of Li‐metal anode with long‐term operation over 2000 h. The L/L electrolyte interface constructed by non‐polar fluorosilicane (PFTOS) and conventionally polar dimethyl sulfoxide solvents can block direct contact between conventional electrolyte and Li anode, and consequently their side reactions can be significantly eliminated. Moreover, the homogeneous Li‐ion flow and Li‐mass deposition can be realized by the formation of a thin and uniform solid‐electrolyte interphase (SEI) composed of LiF, Li_xC, Li_xSiO_y between PFTOS and Li anode, as well as the super‐wettability state of PFTOS to Li anode, resulting in the suppression of Li dendrite formation. The cycling stability in a lithium–oxygen battery as a model is improved 4 times with the L/L electrolyte interface.     

Reversible Solid–Solid Conversion of Sulfurized Polyacrylonitrile Cathodes in Lithium–Sulfur Batteries by Weakly Solvating Ether Electrolytes

Despite carbonate electrolytes exhibiting good stability to sulfurized polyacrylonitrile (SPAN), their chemical incompatibility with lithium (Li) metal anode leads to poor electrochemical performance of Li||SPAN full cells. While the SPAN employs conventional ether electrolytes that suffer from the shuttle effect, leading to rapid capacity fading. Here, we tailor a dilute electrolyte based on a low solvating power ether solvent that is both compatible with SPAN and Li metal. Unlike conventional ether electrolytes, the weakly solvating ether electrolyte enables SPAN to undergo reversibly “solid–solid” conversion. It features an anion–rich solvation structure that allows for the formation of a robust cathode electrolyte interphase on the SPAN, effectively blocking the dissolution of polysulfides into the bulk electrolyte and avoiding the shuttle effect. What's more, the unique electrolyte chemistry endowed Li ions with fast electroplating kinetics and induced high reversibility Li deposition/stripping process from 25 °C to −40 °C. Based on tailored electrolyte, Li||SPAN full cells matched with high loading SPAN cathodes (≈3.6 mAh cm⁻²) and 50 μm Li foil can operate stably over a wide range of temperatures. Additionally, Li||SPAN pouch cell under lean electrolyte and 5 % excess Li conditions can continuously operate stably for over a month.     

Unraveling the Solvent Effect on Solid-Electrolyte Interphase Formation for Sodium Metal Batteries

Ether-based electrolytes are considered as an ideal electrolyte system for sodium metal batteries (SMBs) due to their superior compatibility with the sodium metal anode (SMA). However, the selection principle of ether solvents and the impact on solid electrolyte interphase formation are still unclear. Herein, we systematically compare the chain ether-based electrolyte and understand the relationship between the solvation structure and the interphasial properties. The linear ether solvent molecules with different terminal group lengths demonstrate remarkably distinct solvation effects, thus leading to different electrochemical performance as well as deposition morphologies for SMBs. Computational calculations and comprehensive characterizations indicate that the terminal group length significantly regulates the electrolyte solvation structure and consequently influences the interfacial reaction mechanism of electrolytes on SMA. Cryogenic electron microscopy clearly reveals the difference in solid electrolyte interphase in various ether-based electrolytes. As a result, the 1,2-diethoxyethane-based electrolyte enables a high Coulombic efficiency of 99.9 %, which also realizes the stable cycling of Na||Na₃V₂(PO₄)₃ full cell with a mass loading of ≈9 mg cm⁻² over 500 cycles.