Anion Chemistry Research Articles

Cation-Modified Anionic Redox Mechanism for High-Performance Layered Oxide As Sodium-Ion Batteries Cathode Material

Sodium-ion batteries (NIBs) have been selected as a promising candidate for large-scale energy storage systems due to their abundance. Among several NIB cathode materials, P2-type transition metal layered oxides (NaxTMO2, TM = Ti, V, Cr, Mn, Fe, Co, Ni) featuring high theoretical capacity and better rate performance have attracted much attention. However, the practical applications have to endure the low energy density of NIB cathode materials compared to lithium-ion batteries. In tradition, the capacity is constrained by transition metal ions and is closed to their limits. Hence,in order to obtain extraordinarily high capacity in cathode materials, both anionic and cationic redox chemistry are utilized. Nevertheless, their performance is impeded by irreversible structure evolution and lattice oxygen emission. Therefore, it is highly urgent to develop stable anionic redox chemistry for high energy density and long-cycle-life layered oxide cathode materials.In this study, cation-doped NaxMgyCuzMn(1−y−z)O2 cathode material featuring synergistic effects of cationic and anionic redox was reported. By cations doping, the inhibited structure evolution and lattice oxygen stabilization were achieved. Moreover, the effects of cation-doped NaxMgyCuzMn(1−y−z)O2 were also studied by electrochemical measurements. Also, the mechanism of cation-doped NaxMgyCuzMn(1−y−z)O2 was confirmed by operando synchrotron X-ray absorption spectrum, operando X-ray diffraction, and density functional theory computations. Cation-doped NaxMgyCuzMn(1−y−z)O2 was synthesized through a facile sol-gel method followed by heat treatment. The cation-doped NaxMgyCuzMn(1−y−z)O2 showed high specific capacity (203 mAh g− 1 cycled at 0.1C) as well as better cycling stability, providing sodium layered oxides a new developing stage toward high-performance cathode materials in NIBs for large scale energy storage systems. Keywords: Na-ion batteries, layered oxides, anionic redox, cathode

Unraveling anion effect on lithium ion dynamics and interactions in concentrated ionic liquid electrolyte

The anion chemistry of lithium salts plays a pivotal role in determining the physicochemical and electrochemical performances of ionic liquid (IL) electrolytes. This work explores the effects of anion size and symmetry on coordination and dynamics of lithium ions (Li+) through molecular dynamics simulations. Four types of ILs composed of the same cation N-N-diethyl-N-methyl-N-(2-methoxyethyl)-ammonium (DEME+) and different anions of symmetrical bis(fluorosulfonyl)imide (FSI−) and bis(trifluoromethanesulfonyl)imide (TFSI−), as well as asymmetric (fluorosulfonyl)(trifluoromethylsulfonyl)imide (FTFSI−) and (difluoromethylsulfonyl)-(trifluoromethylsulfonyl)imide (DFTFSI−) are studied for comparison purpose. The calculation results show that the formation of Li+ aggregates in concentrated IL electrolytes could enhance Li+ transport due to the Li+ transport mechanism gradually shifts from vehicle transport to Li+ hopping through the anion ligand layers. Furthermore, it can be seen that the asymmetric anion based electrolytes possess superior performance than that of symmetric anions. In addition, the dual-anion based electrolytes all exhibit faster Li+ diffusion coefficient than mono-anion based electrolytes, which arise from the synergistic effect between mixed anions. As such, this work highlights the role of the anion structures and synergism in the rational design of concentrated IL-based electrolytes.

Iron(III) Dopant Counterions Affect the Charge-Transport Properties of Poly(Thiophene) and Poly(Dialkoxythiophene) Derivatives.

This study investigates the charge-transport properties of poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly(ProDOT-alt-biEDOT) (PE2) films doped with a set of iron(III)-based dopants and as a function of dopant concentration. X-ray photoelectron spectroscopy measurements show that doping P3HT with 12 mM iron(III) solutions leads to similar extents of oxidation, independent of the dopant anion; however, the electrical conductivities and Seebeck coefficients vary significantly (5 S cm-1 and + 82 μV K-1 with tosylate and 56 S cm-1 and +31 μV K-1 with perchlorate). In contrast, PE2 thermoelectric transport properties vary less with respect to the iron(III) anion chemistry, which is attributed to PE2 having a lower onset of oxidation than P3HT. Consequentially, PE2 doped with 12 mM iron(III) perchlorate obtained an electrical conductivity of 315 S cm-1 and a Seebeck coefficient of + 7 μV K-1. Modeling these thermoelectric properties with the semilocalized transport (SLoT) model suggests that tosylate-doped P3HT remains mostly in the localized transport regime, attributed to more disorder in the microstructure. In contrast perchlorate-doped P3HT and PE2 films exhibited thermally deactivated electrical conductivities and metal-like transport at high doping levels over limited temperature ranges. Finally, the SLoT model suggests that PE2 has the potential to be more electrically conductive than P3HT due to PE2's ability to achieve higher extents of oxidation and larger shifts in the reduced Fermi energy levels.

Ionic covalent organic frameworks with tailored anionic redox chemistry and selective ion transport for high-performance Na-ion cathodes

Employing cathode materials with multiple redox couples and electrolytes with efficient cation transport kinetics are two effective approaches to improving the electrochemical performance of batteries. In this work, for the first time, we present a design strategy of simultaneously realizing reversible cationic and anionic redox chemistries as well as selective anion/cation transport in the viologen-based COFs (BAV-COF:X, coordinated anions of X = Cl−, Br−, I−, and ClO4−) for high-performance Na-ion cathodes. Besides the cationic redox of viologen segments, the different redox activities of anions effectively tune the total capacities of the COFs. Meanwhile, electrochemical analysis and ab-initial molecular dynamics (AIMD) calculation illustrate that the anion/cation transport kinetics of electrolytes caged in the COFs’ channels can be selectively tuned by the coordinated anions. As a result, combining high-potential Br−/Br2 redox couple, cationic redox of viologen segments, and enhanced Na+ transport kinetics, the BAV-COF:Br− demonstrates stable performance with energy densities of 358.7 and 145.2 Wh kg−1 at power densities of 116.5 and 2124.1 W kg−1, respectively. This study offers new insight into the fabrication of organic cathodes with anionic redox and the advantages of COFs electrode materials in anion/cation transport selectivity for energy storage applications.

Anion chemistry enabled positive valence conversion to achieve a record high-voltage organic cathode for zinc batteries

<h2>Summary</h2> Chalcogens undergoing positive valence conversions show great potential to achieve a high discharge voltage in batteries; however, such reactions with high reversibility are difficult to achieve because element O/S/Se are inherently electron acceptors. Herein, by incorporating the chalcogens with the unique triphenylphosphine-based structure (strong electron-withdrawing groups), a high-potential triphenylphosphine selenide organic cathode (TP-Se) is developed. Facilitated by a Zn²⁺/trifluoromethanesulfonate (OTF⁻) hosting mechanism, the (TP-Se)⁻ to (TP-Se)⁰ to (TP-Se)⁺ conversion is realized. The dual-ion Zn‖TP-Se batteries exhibit a flat discharge plateau at 1.96 V and a superior discharge capacity. Benefiting from the stable triphenylphosphine molecular structures and optimized hybrid electrolytes, excellent cycling performance is also attained (up to 85.3% capacity retention after 4,300 cycles). Moreover, the Zn‖TP-Se battery also delivers a remarkable rate performance. The system is attractive due to its high discharge voltage, which is higher than ever reported for organic cathodes of zinc batteries.

Stabilizing the Anionic Redox in 4.6 V LiCoO2 Cathode through Adjusting Oxygen Magnetic Moment

AbstractThe irreversible oxygen redox and the resulting structure degradation of LiCoO2 at a high voltage cause very poor cycling performance. Herein, the anionic redox chemistry in 4.6 V LiCoO2 cathode material through manipulating the oxygen magnetic moment (OMM) with oxygen vacancy and V doping is proposed to stabilize, and the relationship between OMM and the oxidation degree of oxygen is revealed. Oxygen vacancy induces the generation of OMM, and the synergy of oxygen vacancy and V doping reduces the change of OMM during charge/discharge processes. This mitigates the oxidation degree of oxygen and improves the reversibility of oxygen redox, which greatly inhibits the irreversible oxygen escape. The oxygen vacancies can further reduce the overlap of the electron clouds and lower the O 2p band center thus decreasing the oxygen redox activity. Moreover, the introduced V also increases the energy barrier of the phase transition and suppresses the irreversible phase transition and Co migration. The irreversible O2 release is significantly inhibited and the cycling stability at 4.6 V is largely enhanced. This study presents the relationship between OMM and the oxidation degree of oxygen and provides some insights into improving the anion redox reversibility through adjusting the oxygen magnetic moment.

Take full advantage of hazardous electrochemical chlorine erosion to ultrafast produce superior NiFe oxygen evolution reaction electrode

Inexpensive yet efficient oxygen evolution reaction electrodes, working in high current densities, remain a pressing challenge for commercial hydrogen energy economy. NiFe (oxy)hydroxides are presented as strategic materials. Herein, a facile and industrially compatible electrochemical chlorine corrosion strategy for streamlined preparation of high-performance NiFe (oxy)hydroxide nanosheet arrays on commercial Ni/Fe foam not beyond 20 s with NaCl-based metal-free solutions is fathered firstly. An appealing anion competition-adsorption effect in anode-electrolyte interface boundaries is revealed that assumes strong dependence upon anion category and greatly influences chlorine corrosion behavior. We pioneeringly “counterplot” this mechanism to ingeniously convert into treasure a catastrophic chlorine-induced electrode erosion in electrochemical marine/saline water technology. Due to generating a self-consistent chlorine-phobic or -philic interface layer around anode surfaces assembled by anions, this anion-mediated mechanism enables rooting the morphologically/compositionally modulated NiFe hydroxide electrocatalysts onto substrates for the superior industrial current density-level oxygen evolution (over 800 h at 1 A cm−2 and 216 mV at 10 mA cm−2) or formulating the chlorine corrosion-immune electrolytes for the stable highly concentrated marine/saline water electrolysis (over 100 h at 0.3 A cm−2 in the 1 M NaCl + 1 KOH electrolyte). Given the rich anion chemistry, our trailblazing studies set a fresh stage towards the advanced oxygen evolution electrode or stable saline electrolysis for hydrogen production.

Delocalized Li@Mn6 superstructure units enable layer stability of high-performance Mn-rich cathode materials

<h2>Summary</h2> The search for cathode materials with high energy density, long-term cycling stability, and low cost is one of the most important challenges for current lithium-ion batteries. To address the structural instability in Mn-rich layered cathodes, we demonstrate herein through thorough experimental and theoretical studies that delocalizing Li@Mn₆ superstructure units within transition-metal layers is an effective strategy to enhance the layer stability of a Li-excess Mn-rich layered oxide (LMRO) cathode. The delocalized Li@Mn₆ superstructure units can not only increase the Mn valence to inhibit the adverse Jahn-Teller effect but also harness the anionic redox activity with suppressed O–Mn₀ species. Benefited from its stable layered structure, the LMRO cathode can retain a high capacity and energy density of 251 mA h g⁻¹ and 791 W h kg⁻¹, respectively, after 100 cycles with nearly 100% retention. This work provides a feasible route to develop the high-performance layered cathodes with stable anionic redox chemistry.

Separation of Sulfate Anion From Aqueous Solution Governed by Recognition Chemistry: A Minireview.

The sulfate anion (SO4 2–) is known as an end metabolite of cysteine and methionine, and its proper concentration is associated with the expression of key functions in the physiological system. Thus, maintaining sulfate concentration at a precise level is of great significance for biology, environments, and industrial productions. Fundamental research for sulfate anion chemistry can help understand sulfate-associated physiological processes and related applications, for example, remediation. In this minireview, we summarized recent research progresses in sulfate recognition and separation using crystallization and liquid–liquid extraction. We focused on the studies wherein molecular recognition is the key element and is considered the driving force for selective sulfate separations from aqueous solution.

First‐principles insights on anion redox activity in NaxFe1/8Ni1/8Mn3/4O2: Toward efficient high‐energy cathodes for Na‐ion batteries

AbstractIn the search for high‐energy cathode materials for Na‐ion batteries (NIBs), Fe‐doped layered transition metal oxides have been recently proposed as promising systems that can ensure improved reversible capacity at high working voltage. Exploiting the anionic redox chemistry in this class of materials represents a great advance for the energy storage community, but uncontrolled oxidation process can lead to the formation of unbound molecular oxygen, with detrimental effects on overall capacity and stability upon cycling. The higher TM–O covalency provided by Fe doping seems to prevent oxygen loss and ensure full capacity recovery. Understanding anionic processes and the underlying mechanism with atomistic details can reinforce the experimental efforts and help to outline rational design strategies for novel high‐performing NIB cathodes. To this end, we present a state‐of‐the‐art first‐principles study on the P2‐type NaxTMO2 (TM = Fe, Ni, and Mn—NFNMO) oxide. We compare structural and electronic features of stoichiometric (NaxFe0.125Ni0.125Mn0.75O2) and Mn‐deficient (NaxFe0.125Ni0.125Mn0.68O2) NFNMO to identify and discuss the contribution of each element sublattice on charge compensation processes. Although Mn deficiency is predicted to increase the cathode working voltage, we find the charge compensation being mostly exerted by the Ni and Fe sublattices. Oxygen redox is unfold via the formation of superoxide species at low Na loads with a preferential breaking of more labile Ni–O bonds and binding to Fe atoms. Our calculations predict no release of molecular O2 upon desodiation, thus highlighting the key role of Fe dopant that provides a good TM–O bond strength, preventing oxygen loss while still enabling anionic redox reactions at high voltages with extra reversible capacity.

Asymmetric Defluoroallylation of 4-Trifluoromethylpyridines Enabled by Umpolung C-F Bond Activation.

Carbon-fluorine bond activation of the trifluoromethyl group represents an important approach to fluorine-containing molecules. While selective defluorinative functionalization reactions of CF3 -containing substrates have been achieved by invoking difluorocarbocation, difluorocarboradical, or difluoroorganometallic species as the key intermediates, the transformations via fluorocarbanion mechanism only achieved limited success. Furthermore, the enantioselective defluorinative transformation of the CF3 group remained a formidable challenge. Here we report a defluorinative functionalization reaction of 4-trifluoromethylpyridines involving difluoro(pyrid-4-yl)methyl anion as the key intermediate, which was developed based upon our previous studies on the N-boryl pyridyl anion chemistry. In particular, asymmetric defluoroallylation of 4-trifluoromethylpyridines and -pyrimidines could be achieved by using Ir-catalysis to forge a difluoroalkyl-substituted chiral center.

Asymmetric Defluoroallylation of 4‐Trifluoromethylpyridines Enabled by Umpolung C−F Bond Activation**

AbstractCarbon–fluorine bond activation of the trifluoromethyl group represents an important approach to fluorine‐containing molecules. While selective defluorinative functionalization reactions of CF3‐containing substrates have been achieved by invoking difluorocarbocation, difluorocarboradical, or difluoroorganometallic species as the key intermediates, the transformations via fluorocarbanion mechanism only achieved limited success. Furthermore, the enantioselective defluorinative transformation of the CF3group remained a formidable challenge. Here we report a defluorinative functionalization reaction of 4‐trifluoromethylpyridines involving difluoro(pyrid‐4‐yl)methyl anion as the key intermediate, which was developed based upon our previous studies on theN‐boryl pyridyl anion chemistry. In particular, asymmetric defluoroallylation of 4‐trifluoromethylpyridines and ‐pyrimidines could be achieved by using Ir‐catalysis to forge a difluoroalkyl‐substituted chiral center.

Front Cover: Metallocene Anions: From Electrochemical Curiosities to Isolable Complexes (Eur. J. Inorg. Chem. 11/2022)

The Front Cover shows a depiction of a classical metallocene anion, with a red metal atom in a formal +I oxidation state sandwiched by two cyclopentadienyl rings. The central image is flanked by a range of d-transition metal(I) ions, together with a selection of derivatized five-membered carbocyclic rings, representing the d-block metallocene anions that have been structurally authenticated to date. The review covers the chemistry of metallocene anions and their derivatives, focusing on d-block examples, and provides a historical overview from their detection as intermediates in low-temperature electrochemical studies to the most recent examples of isolated complexes. The cover was designed by Corentin Magnoux. More information can be found in the Review by C. Magnoux and D. P. Mills.

Tuning Bulk O2 and Nonbonding Oxygen State for Reversible Anionic Redox Chemistry in P2-Layered Cathodes.

Improving the reversibility of oxygen redox is quite significant for layered oxides cathodes in sodium-ion batteries. Herein, we for the first time simultaneously tune bulk O2 and nonbonding oxygen state for reversible oxygen redox chemistry in P2-Na0.67 Mn0.5 Fe0.5 O2 through a synergy of Li2 TiO3 coating and Li/Ti co-doping. O2- is oxidized to molecular O2 and peroxide (O2 )n- (n<2) during charging. Molecular O2 derived from transition metal (TM) migration is related to the superstructure ordering induced by Li doping. The synergy mechanism of Li2 TiO3 coating and Li/Ti co-doping on the two O-redox modes is revealed. Firstly, Li2 TiO3 coating restrains the surface O2 and inhibits O2 loss. Secondly, nonbonding Li-O-Na enhances the reversibility of O2- →(O2 )n- . Thirdly, Ti doping strengthens the TM-O bond which fixes lattice oxygen. The cationic redox reversibility is also enhanced by Li/Ti co-doping. The proposed insights into the oxygen redox reversibility are insightful for other oxide cathodes.

Tuning Bulk O2 and Nonbonding Oxygen State for Reversible Anionic Redox Chemistry in P2‐Layered Cathodes

AbstractImproving the reversibility of oxygen redox is quite significant for layered oxides cathodes in sodium‐ion batteries. Herein, we for the first time simultaneously tune bulk O2 and nonbonding oxygen state for reversible oxygen redox chemistry in P2‐Na0.67Mn0.5Fe0.5O2 through a synergy of Li2TiO3 coating and Li/Ti co‐doping. O2− is oxidized to molecular O2 and peroxide (O2)n− (n&lt;2) during charging. Molecular O2 derived from transition metal (TM) migration is related to the superstructure ordering induced by Li doping. The synergy mechanism of Li2TiO3 coating and Li/Ti co‐doping on the two O‐redox modes is revealed. Firstly, Li2TiO3 coating restrains the surface O2 and inhibits O2 loss. Secondly, nonbonding Li−O−Na enhances the reversibility of O2−→(O2)n−. Thirdly, Ti doping strengthens the TM−O bond which fixes lattice oxygen. The cationic redox reversibility is also enhanced by Li/Ti co‐doping. The proposed insights into the oxygen redox reversibility are insightful for other oxide cathodes.

Descriptor and Scaling Relations for Ion Mobility in Crystalline Solids.

Ion mobility is a critical performance parameter not only in electrochemical energy storage and conversion but also in other electrochemical devices. On the basis of first-principles electronic structure calculations, we have derived a descriptor for the ion mobility in battery electrodes and solid electrolytes. This descriptor is entirely composed of observables that are easily accessible: ionic radii, oxidation states, and the Pauling electronegativities of the involved species. Within a particular class of materials, the migration barriers are connected to this descriptor through linear scaling relations upon the variation of either the cation chemistry of the charge carriers or the anion chemistry of the host lattice. The validity of these scaling relations indicates that a purely ionic view falls short of capturing all factors influencing ion mobility in solids. The identification of these scaling relations has the potential to significantly accelerate the discovery of materials with desired mobility properties.

Thermal decomposition of phosphonium salicylate and phosphonium benzoate ionic liquids

The thermal stability of ionic liquids is important for their use in a variety of applications. Here, reactive molecular dynamics simulations and thermogravimetric analyses were used to explore the thermal decomposition mechanisms of phosphonium salicylate and phosphonium benzoate. Experiments performed at different heating rates indicated that the decomposition temperatures of phosphonium salicylate and phosphonium benzoate were comparable, but phosphonium benzoate was less stable than phosphonium salicylate under isothermal high temperature conditions. The lower thermal stability of the benzoate compared to the salicylate was reproduced in reactive molecular dynamics simulations. The simulations also showed that cation chain length had little effect on thermal stability. The simulations revealed that thermal decomposition for both phosphonium salicylate and phosphonium benzoate occurred through many different pathways that could be broadly categorized as proton-transfer, association, and dissociation reactions. The phosphonium benzoate underwent more of these reactions and exhibited a wide range of reaction pathways in each category than the phosphonium salicylate. Multiple possible mechanisms were explored to explain this difference and it was found that the dominant factor was the presence of the hydroxyl group in salicylate that affects the ability of oxygen atoms to take part in proton-transfer reactions that are the first step of all subsequent reactions. These findings demonstrate that even subtle differences in anion chemistry may significantly affect the thermal stability of ionic liquids, suggesting avenues for tuning these properties through molecular design.

An advanced Ni–Graphite molten salt battery with 95 °C operating temperature for energy storage application

Based on the high safety and low costs, molten salt batteries (Na-S battery, liquid metal battery, ZEBRA battery, etc.) have received more and more attention as large-scale electrical storage technology. However, the high operating temperature of molten salt batteries is still one of the roadblocks that impeded their practical application. Here we present the novel Ni-graphite molten salt battery based on outstanding electrochemical reversibility of the Ni/NiCl2 redox and intercalation/de-intercalation chemistry of chloroaluminate anions in graphite. By using the low-cost, quaternary inorganic molten salt AlCl3/KCl/NaCl/LiCl as electrolyte, the as-designed Ni-graphite battery possesses low operating temperature of 95 °C, and average voltage about 1.7 V. The Ni-graphite battery delivers stable specific capacity of 174 mAh/g at 500 mA/g after 120 cycles, with the capacity retention rate of 98%. In addition, the Ni-graphite battery also shows low material costs about 113.6 $/kWh and high electrode energy density of 289 Wh/kg. This work develops an advanced molten salt battery with low operating temperature and high energy density, as well as provides an alternative means of developing future large-scale power storage equipment based on molten salt batteries.

Long-enduring oxygen redox enabling robust layered cathodes for sodium-ion batteries

Anionic redox chemistry has emerged as a promising pathway attracted extensively attention to meet the demands of high energy sodium-ion batteries. Nonetheless, suchlike cathode materials suffer from poor reversible oxygen redox reactions leading to severe capacity degradation along with sluggish kinetics and voltage decay. In this work, layered sodium transition metal oxides Na0.67Mg0.28Mn0.72-xRuxO2 (x = 0–0.6) are presented, where the optimized sample (x = 0.12) offers an attractively sustainable capacity of 222 mAh g−1 and a distinguished rate capability of 101.7 mAh g−1 at 50C, based on the anionic and cationic activity. The results reveal that the oxygen redox retention is endured at 94.1% in the optimized sample (x = 0.12) after 55 cycles, in contrast to 48.9% of the pristine sample (x = 0), which is the maximum value reported up to now. Meanwhile the lattice parameters show almost no variation, according to the in situ/ex situ X-ray diffraction experiments. Ru-substitution significantly improves the anionic redox reversibility, the structural stability and sodium diffusion, resulting in the noteworthy electrochemical performance advancement. The research performs an effective method to moderate the problems existing in anionic redox reactions and makes a reference for subsequent materials design.

Lithium hydroxyphenyl propanesulfonate imparts composite solid polymer electrolytes with ultrahigh ionic conductivity for dendrite free lithium batteries

Anion chemistry of sulfonates plays a crucial role in dictating the electrochemical performance of composite solid polymer electrolytes (CSPEs), hence affecting the cyclability of lithium-ion batteries (LIBs). The CSPEs received great attention for LIBs, however, simultaneously achieving high ionic conductivity and long cycle life in realistic batteries is still a great challenge. Herein, we report new sulfonate anion with hydrophilic aromatic structure and ether alkyl spacer of hydroxyl wing namely lithium hydroxyphenyl propanesulfonate (LHPS), and a series of CSPEs obtained by coupling of the synthesized anions via tetraethyl orthosilicate to be used in LIBs. The CSPEs comprising LHPS achieve higher ionic conductivity and excellent cyclability with impressive stability due to their decent compatibility of hydroxyl wing, and aliphatic ether spacer with the polymeric matrix. The intramolecular and intermolecular interactions of LHPS with other precursors within anion-polymer matrices via hydrogen bonding reduce anionic species migration, thus augmenting Li+ ion conduction and impeding lithium dendritic growth during the electrochemical performance. This opens a new avenue for designing conductive and long cycle life CSPEs for LIBs in the future.