Solvation Chemistry Research Articles

Tuning Zn-Ion Solvation Chemistry with Chelating Ligands toward Stable Aqueous Zn Anodes.

Changing the solvation sheath of hydrated Zn ions is an effective strategy to stabilize Zn anodes to obtain a practical aqueous Zn-ion battery. However, key points related to the rational design remain unclear including how the properties of the solvent molecules intrinsically regulate the solvated structure of the Zn ions. This study proposes the use ofa stability constant (K), namely, the equilibrium constant of the complexation reaction, as a universal standard to make an accurate selection of ligands in the electrolyte to improve the anode stability. It is found that K greatly impacts the corrosion current density and nucleation overpotential. Following this, ethylene diamine tetraacetic acid with a superhigh K effectively suppresses Zn corrosion and induces uniform Zn-ion deposition. As a result, the anode has an excellent stability of over 3000h. This work presents a general principle to stabilize anodes by regulating the solvation chemistry, guiding the development of novel electrolytes for sustainable aqueous batteries.

Solvation structure of the Hg(NO3)2 and Hg(TfO)2 salts in dilute aqueous and methanol solutions: An insight into the Hg2+ coordination chemistry

A synergic approach combining X-ray absorption spectroscopy (XAS) and Molecular Dynamics (MD) has been used to investigate the solvation properties of dilute aqueous and methanol solutions of the Hg(NO3)2 and Hg(TfO)2 (where TfO−=trifluoromethanesulfonate or triflate) salts. A new Lennard-Jones potential has been developed to be used in classical MD simulations of Hg2+-containing systems by refining the force-field parameters to reproduce the Hg2+ ion coordination in water in agreement with the XAS data. A different behavior of the Hg2+ ion in water and methanol has been highlighted by the analysis of the MD simulations carried out with the newly developed interaction potential: in aqueous solutions of the Hg(NO3)2 and Hg(TfO)2 salts, the Hg2+ first coordination shell is composed only of water molecules, while in methanol solutions of the same salts the Hg2+ cation coordinates both counterions and methanol molecules in its first solvation sphere. The MD results have been confirmed by comparison with the Hg L3-edge XAS experimental data. Independently of the formation of ion pairs with the counterions, the Hg2+ ion in solution tends to form heptacoordinated first shell complexes, with an arrangement of the ligands depending on the nature of the solvent: in water a 7-fold cluster of water molecules in a C2 symmetry is found, while in methanol the Hg2+ solvation shell is composed of both counterions and methanol molecules, with oxygen atoms coordinating Hg2+ arranged in a distorted pentagonal bipyramid geometry. These findings represent a significant step forward in the understanding of the solvation chemistry of the Hg2+ ion, which is fundamental to improve the efficiency of mercury removal procedures that are crucial to the safety of water resources.

Measurements and Utilization of Consistent Gibbs Energies of Transfer of Single Ions: Towards a Unified Redox Potential Scale for All Solvents.

Utilizing the “ideal” ionic liquid salt bridge to measure Gibbs energies of transfer of silver ions between the solvents water, acetonitrile, propylene carbonate and dimethylformamide results in a consistent data set with a precision of 0.6 kJ mol−1 over 87 measurements in 10 half‐cells. This forms the basis for a coherent experimental thermodynamic framework of ion solvation chemistry. In addition, we define the solvent independent peabsH2O ‐ and the EabsH2O values that account for the electronating potential of any redox system similar to the pHabsH2O value of a medium that accounts for its protonating potential. This EabsH2O scale is thermodynamically well‐defined enabling a straightforward comparison of the redox potentials (reducities) of all media with respect to the aqueous redox potential scale, hence unifying all conventional solvents′ redox potential scales. Thus, using the Gibbs energy of transfer of the silver ion published herein, one can convert and unify all hitherto published redox potentials measured, for example, against ferrocene, to the EabsH2O scale.

Stable cycling and fast charging of high-voltage lithium metal batteries enabled by functional solvation chemistry

Dimethyl ether (DME)-based localized high-concentration electrolytes (LHCEs) with Li+: DME ratio ∼ 1: 1.5 could ensure stable cycling of LMBs with high Coulombic efficiency but suffer from oxidation decomposition at high voltages and poor charging ability due to its low conductivity. Herein, we design an electrolyte consisting of lithium bis(fluorosulfonyl) imide (LiFSI, 1.0 M) along with lithium hexafluorophosphate (LiPF6, 0.1 M) in DME-hydrofluoroether (TTE) (with Li+: DME = 1: 4), which could both enhance the Li+ ion transportation kinetics of electrolyte and the interfacial stability of both Li metal and LiNi0.9Mn0.05Co0.05O2 (NMC90) cathode. Additive amounts of PF6− anions are found to reduce the probability of FSI− anions entering into the first solvation sheath and simultaneously strengthen the interaction between Li+ and DME, hence stabilizing the DME solvent against reduction/oxidation. Meanwhile, the polymerization of free DME is initiated by PF5, forming a flexible SEI layer on the Li metal anode. On the other hand, PF6− anions facilitate stabilizing NMC90 cathode by forming a LiF-rich interfacial layer. These enable the Li||NMC90 (40 µm Li, 20 mg cm−2 NMC90, 12 μL electrolytes) battery to achieve outstanding capacity retention of 93.7% after 250 cycles at a high charging current density of 4.0 mA cm−2 with a charge cut-off voltage of 4.3 V. These fundamental understandings emphasize the importance in the rational design of electrolyte solvation structures and chemistry to promote the interfacial stability and kinetics, and thus promoting the practical application of high-energy LMBs.

Solvation chemistry of rare earth nitrates in carbonate electrolyte for advanced lithium metal batteries

Lithium metal is recognized as the next generation anode material due to its ultra-high capacity. However, the safety hazards caused by the dendritic lithium growth hinder its practical application. In this work, rare earth nitrates, such as Y(NO3)3, are introduced as electrolyte additives because of the unfixed coordination number and complex coordination configuration for rare earth cations. Molecular dynamics simulation reveals that the rare earth ion changes the solvation structure of the lithium ion. When it is combined with fluoroethylene carbonate (FEC), the interphase generated from electrochemical reactions is favorable for the dense Li deposition. As a result, a stable and high Coulombic efficiency is achieved for Li/Cu cells in commercial carbonate electrolyte with such additive. The Li/Li symmetrical cells can operate>1000 h at current density of 1 mA cm−2. Moreover, the NCM811/Li full cells deliver superior cycling stability and better rate performance. This strategy sheds light on utilizing rare earth elements to optimize the electrolyte for lithium metal anode and it can be extended to other electrolytes aiming at sodium and potassium battery chemistry.

Constructing a Stable Interface Layer by Tailoring Solvation Chemistry in Carbonate Electrolytes for High-Performance Lithium-Metal Batteries.

Lithium-metal batteries (LMBs) are considered as promising next-generation batteries due to their high energy density. However, commercial carbonate electrolytes cannot be used in LMBs due to their poor compatibility with the lithium-metal anode and detrimental hydrogen fluoride (HF) generation by lithium hexafluorophosphate decomposition. By introducing lithium nitrate additive and a small amount of tetramethylurea as a multifunctional cosolvent to a commercial carbonate electrolyte, NO3 - , which is usually insoluble, can be introduced into the solvation structure of Li+ to form a conductive and stable solid electrolyte interface. At the same time, HF generation is suppressed by manipulating the solvation structure and a scavenging effect. As a result, the Coulombic efficiency (CE) of Li||Cu half cells using the designed carbonate electrolyte can reach 98.19% at room temperature and 96.14% at low temperature (-15°C), and Li||LiFePO4 cells deliver a high capacity retention of 94.9% with a high CE of 99.6% after 550cycles. This work provides a simple and effective way to extend the use of commercial carbonate electrolytes for next-generation battery systems.

Safe and Stable Lithium Metal Batteries Enabled by an Amide-Based Electrolyte

HighlightsA novel amide-based nonflammable electrolyte is proposed. The formation mechanism and solvation chemistry are investigated by molecular dynamics simulations and density functional theory.An inorganic/organic-rich solid electrolyte interphase with an abundance of LiF, Li3N and Li–N–C is in situ formed, leading to spherical lithium deposition.The amide-based electrolyte can enable stable cycling performance at room temperature and 60 ℃.The formation of lithium dendrites and the safety hazards arising from flammable liquid electrolytes have seriously hindered the development of high-energy-density lithium metal batteries. Herein, an emerging amide-based electrolyte is proposed, containing LiTFSI and butyrolactam in different molar ratios. 1,1,2,2-Tetrafluoroethyl-2,2,3,3-tetrafluoropropylether and fluoroethylene carbonate are introduced into the amide-based electrolyte as counter solvent and additives. The well-designed amide-based electrolyte possesses nonflammability, high ionic conductivity, high thermal stability and electrochemical stability (> 4.7 V). Besides, an inorganic/organic-rich solid electrolyte interphase with an abundance of LiF, Li3N and Li–N–C is in situ formed, leading to spherical lithium deposition. The formation mechanism and solvation chemistry of amide-based electrolyte are further investigated by molecular dynamics simulations and density functional theory. When applied in Li metal batteries with LiFePO4 and LiMn2O4 cathode, the amide-based electrolyte can enable stable cycling performance at room temperature and 60 ℃. This study provides a new insight into the development of amide-based electrolytes for lithium metal batteries.

Electrolyte solvation structure as a stabilization mechanism for electrodes

Rechargeable batteries with high capacity, power and safety are urgently required for current and future technological demands. The solid electrolyte interphase (SEI) layer has a dominant impact on battery cyclability and the solvate is the key factor that determines the SEI layer. In this perspective, we first review the recent advances in understanding the influences of electrolyte composition on the solvation chemistry and SEI layer. The solvation structure of electrolytes seems to be the root cause of the stability of electrodes during cell cycling. We then discuss the strategy to manipulate the solvation chemistry by adjusting the compositions of the electrolytes, including the solvent, salt, concentration and additive. Finally, we concisely discuss the challenges in characterizing the structure of the solvates at the electrode|electrolyte interface. This review refreshes our current understanding of the key factors for stable electrode|electrolyte interfaces in the pursuit of high-performance battery systems.

Tailoring Solvation Chemistry in Carbonate Electrolytes for All-Climate, High-Voltage Lithium-Rich Batteries

Tailoring Solvation Chemistry in Carbonate Electrolytes for All-Climate, High-Voltage Lithium-Rich Batteries

Stable Long Cycling of Small Molecular Organic Acid Electrode Materials Enabled by Nonflammable Eutectic Electrolyte.

Small molecule organic acids as electrode materials possess the advantages of high theoretical capacity, low cost, and good processability. However, these electrode materials suffer from poor cycling stability due to the inevitable dissolution of organic molecules in the electrolytes. Here, a eutectic mixture of lithium bis(trifluoromethanesulfonyl)imide and N-methylamine is employed as a eutectic electrolyte in Li-ion batteries with small molecule organic acids as electrodes. To enhance the cycling stability of the electrolyte, fluoroethylene carbonate is used as an additive. The electrolyte exhibits nonflammability, high ionic conductivity, and good electrochemical stability. Molecular dynamics simulations and density functional theory are performed to further investigate the solvation chemistry of the eutectic electrolyte. The well-designed eutectic electrolyte inhibits the dissolution of terephthalic acid effectively and displays superior performance with a capacity retention of ≈84% after 2000 cycles at a high current density of 1 A g-1 . It also enables stable cycling of more than 900 cycles at a high current density of 2 A g-1 at 60°C. This study provides a strategy to enhance the cycling stability and safety of Li-ion batteries with organic electrode materials.

Realizing wide-temperature Zn metal anodes through concurrent interface stability regulation and solvation structure modulation

Stable cycling of Zn metal anodes under thermal extremes remains a grand challenge with the corresponding failure mechanisms largely unexplored. Here, we unravel the origin of thermal instability during Zn plating/stripping. The low temperature renders deteriorative dendrites growth, while a high temperature causes aggravating parasitic reactions. Zn metal/electrolyte interface and electrolyte solvation chemistry are then regulated via the introduction of oligomer poly(ethylene glycol) dimethyl ether as a competitive-solvent into the aqueous electrolyte to circumvent these issues. Complementary experimental and theoretical studies demonstrate that the competitive-solvent shifts water-occupied interface into oligomer one through preferential Zn surface adsorption, enabling dendrite-free Zn morphologies. Furthermore, such solvent alters the electrolyte interaction by re-constructing oligomer/water hydrogen bonds and participating in the solvation sheath of Zn ions, which highly alleviates parasitic reactions. Consequently, Zn metal anodes deliver more than 1600 h Zn cyclic lifetime at all the tested temperatures of 0, 25 and 50 °C, over 10-fold enhancement than in pristine electrolytes. Application-wise, competitive-solvent suppresses the fast cathode dissolution because of highly reduced water activities and realizes the stable Zn/V2O5 full cells over a wide temperature range from -15 to 65 °C.

Advanced Redox Flow Battery Chemistries

Redox flow batteries (RFBs) have increasingly being recognized as a prominent candidate for large-scale energy storage due to their unique advantages of high safety, decoupling of power and energy, long lifespan, quick response, and potentially low cost. Electrolyte is de facto the most critical component in a RFB system. This presentation describes development of new electrolyte chemistries at Pacific Northwest National Laboratory. Solvation chemistry of the different electrolyte systems will be discussed, which provide a greater understanding of dynamic interactions between solvent-solvent, ion-solvent, and ion-ion at the molecular level. Such understanding is pivotal in developing new redox flow system with higher energy density and temperature stability. New RFB chemistries based on a variety of redox active materials will also be presented.

Regulating Solvent Molecule Coordination with KPF6 for Superstable Graphite Potassium Anodes.

Graphite is one of the most attractive anode materials due to its low cost, environmental friendliness, and high energy density for potassium ion batteries (PIBs). However, the severe capacity fade of graphite anodes in traditional KPF6-based electrolyte hinders its practical applications. Here, we demonstrate that the cycling stability of graphite anodes can be significantly improved by regulating the coordination of solvent molecules with KPF6 via a high-temperature precycling step. In addition to the solvents being electrochemically stable against reduction, a stable and uniform organic-rich passivation layer also forms on the graphite anodes after high-temperature precycling. Consequently, the PIBs with graphite anodes could operate for more than 500 cycles at 50 mA g-1 with a reversible capacity of about 220 mAh g-1 and an average Coulombic efficiency greater than 99%. Furthermore, full batteries based on Prussian blue cathodes and high-temperature precycled graphite anodes also exhibit excellent performance. Molecular dynamics simulations were performed to explore the solvation chemistry of the electrolytes used in this study.

Accelerated design of vanadium redox flow battery electrolytes through tunable solvation chemistry

Operational stability of electrolytes is a persistent impediment in building redox flow battery technology. Stabilizing multiple vanadium oxidation states in aqueous solution is a primary challenge in designing reliable large-scale vanadium redox flow batteries (VRBs). Here we demonstrate that rationally selected ionic additives can stabilize the aqua vanadium solvate structures through preferential bonding and molecular interactions despite their relatively low concentrations (≤0.1 M). The competing cations (NH4+ and Mg2+) and bonding anions (SO42−, PO43−, and Cl−) introduced by bi-additives are used to tune the vanadium solvation chemistry and design an optimal electrolyte for VRB technology. Such molecular engineering of VRB electrolytes results in enhancement of the operational temperature window by 180% and energy density by more than 30% relative to traditional electrolytes. This work demonstrates that tunable solvation chemistry is a promising pathway to engineer an optimal electrolyte for targeted electrochemical systems.

The Neutral Complex (NH3)Ag(NCO)

Dissolution of solid AgNCO (silver isocyanate) in aqueous ammonia (25 %) and subsequent crystal growth at T = –9 °C furnished the new ammoniate (NH3)Ag(NCO) as colorless crystals [P21/c (no. 14); a = 4.1817(3) Å, b = 14.445(1) Å, c = 6.1988(5) Å, β = 102.0(4)°, V = 365,6(2) Å3; Z = 4]. In the molecular monammine complex, which is only stable at temperatures below T = 0 °C, silver is in a twofold, however, asymmetrical coordination by the isocyanate anion and ammonia. At the reaction conditions applied, AgNCO does not form an ionic diammine species (e.g. [Ag(NH3)2]+) as known from related silver salts. In this sense, the solvation chemistry of AgNCO exhibits a rarely observed feature.

Electrolyte solvation chemistry for lithium–sulfur batteries with electrolyte-lean conditions

Sparingly solvating electrolytes exhaust free solvents to render a quasi-solid sulfur conversion, while highly solvating electrolytes favor the polysulfide formation and stabilize unique radicals to accelerate redox kinetics. • Sparingly and highly solvating electrolytes in Li–S batteries are reviewed. • Sparingly solvating electrolytes are featured by exhausting free solvents. • Highly solvating electrolytes favor the polysulfide formation. • This Review discusses technical and scientific issues of unique electrolytes. • Electrolyte solvation chemistry perhaps promises energy-dense Li–S batteries. Lithium–sulfur (Li–S) batteries possess overwhelming energy density of 2654 Wh kg −1 , and are considered as the next-generation battery technology for energy demanding applications. Flooded electrolytes are ubiquitously employed in cells to ensure sufficient redox kinetics and preclude the interference of the electrolyte depletion due to side reactions with the lithium metal anode. This strategy is capable of enabling long-lasting, high-capacity and excellent-rate battery performances, but it mask the requirements of practical Li–S batteries, where high-sulfur-loading/content and lean electrolyte are prerequisite to realize the energy-dense Li–S batteries. Sparingly and highly solvating electrolytes have emerged as effective yet simple approaches to decrease the electrolyte/sulfur ratio through altering sulfur species and exerting new reaction pathways. Sparingly solvating electrolytes are characterized by few free solvents to solvate lithium polysulfides, rendering a quasi-solid sulfur conversion and decoupling the reaction mechanisms from electrolyte quantity used in cells; while highly solvating electrolytes adopt high-donicity or high-permittivity solvents and take their advantages of strong solvation ability toward polysulfide intermediates, thereby favoring the polysulfide formation and stabilizing unique radicals, which subsequently accelerate redox kinetics. Both solvation chemistry approaches have their respective features to allow the operation of cells under electrolyte-starved conditions. This Review discusses their unique features and basic physicochemical properties in the working Li–S batteries, presents remaining technical and scientific issues and provides future directions for the electrolyte chemistry to attain high-energy Li–S batteries.

Toward the Sustainable Lithium Metal Batteries with a New Electrolyte Solvation Chemistry

AbstractHerein, a new solvation strategy enabled by Mg(NO3)2 is introduced, which can be dissolved directly as Mg2+ and NO3− ions in the electrolyte to change the Li+ ion solvation structure and greatly increase interfacial stability in Li‐metal batteries (LMBs). This is the first report of introducing Mg(NO3)2 additives in an ester‐based electrolyte composed of ternary salts and binary ester solvents to stabilize LMBs. In particular, it is found that NO3− efficiently forms a stable solid electrolyte interphase through an electrochemical reduction reaction, along with the other multiple anion components in the electrolyte. The interaction between Li+ and NO3− and coordination between Mg2+ and the solvent molecules greatly decreases the number of solvent molecules surrounding the Li+, which leads to facile Li+ desolvation during plating. In addition, Mg2+ ions are reduced to Mg via a spontaneous chemical reaction on the Li metal surface and subsequently form a lithiophilic Li–Mg alloy, suppressing lithium dendritic growth. The unique solvation chemistry of Mg(NO3)2 enables long cycling stability and high efficiency of the Li‐metal anode and ensures an unprecedented lifespan for a practical pouch‐type LMB with high‐voltage Ni‐rich NCMA73 cathode even under constrained conditions.

Azeotropic isotopologues

Isotopologues are molecular species differing only in their stable isotope composition. For example, the isotopologues of water include HOH, DOD, and HOD. The possibility of reactive azeotropy among isotopologues of the same molecule is explored within the ideal gas/ideal solution assumption. The chemical equilibrium enabling azeotropy may involve isotope exchange reactions or may involve more complicated dissociation/solvation chemistry. Azeotropy among isotopologues appears possible in the presence of isotope exchange reactions and with simple dissociation or combination reactions. However, for such chemical reactions, the isotope fractionation factors must be unity, that is, isotope ratios are identical in the vapor and liquid phases. This is not consistent with experimental observations that isotope fractionation factors can differ from unity at an azeotrope. More complex solvation chemistry, wherein the isotopic composition in the solvation shell differs slightly from that of the bulk solvent, can lead to an azeotrope for which the isotope fractionation factor is not unity. Possible azeotropy among the isotopologues of water is then considered. Experiments to directly quantify water isotopologue compositions at the elevated temperatures and pressures anticipated for azeotropy are described. The paper concludes with a discussion of the potential impact of azotropic isotopologues on the azeotropic extraction and isotopic analysis of soil water and on the interpretation of isotopic signatures in geological and cosmological samples.

Lithium Nitrate Solvation Chemistry in Carbonate Electrolyte Sustains High-Voltage Lithium Metal Batteries.

The lithium metal anode is regarded as a promising candidate in next-generation energy storage devices. Lithium nitrate (LiNO3 ) is widely applied as an effective additive in ether electrolyte to increase the interfacial stability in batteries containing lithium metal anodes. However, because of its poor solubility LiNO3 is rarely utilized in the high-voltage window provided by carbonate electrolyte. Dissolution of LiNO3 in carbonate electrolyte is realized through an effective solvation regulation strategy. LiNO3 can be directly dissolved in an ethylene carbonate/diethyl carbonate electrolyte mixture by adding trace amounts of copper fluoride as a dissolution promoter. LiNO3 protects the Li metal anode in a working high-voltage Li metal battery. When a LiNi0.80 Co0.15 Al0.05 O2 cathode is paired with a Li metal anode, an extraordinary capacity retention of 53 % is achieved after 300 cycles (13 % after 200 cycles for LiNO3 -free electrolyte) and a very high average Coulombic efficiency above 99.5 % is achieved at 0.5 C. The solvation chemistry of LiNO3 -containing carbonate electrolyte may sustain high-voltage Li metal anodes operating in corrosive carbonate electrolytes.

Lithium Nitrate Solvation Chemistry in Carbonate Electrolyte Sustains High‐Voltage Lithium Metal Batteries

AbstractThe lithium metal anode is regarded as a promising candidate in next‐generation energy storage devices. Lithium nitrate (LiNO3) is widely applied as an effective additive in ether electrolyte to increase the interfacial stability in batteries containing lithium metal anodes. However, because of its poor solubility LiNO3 is rarely utilized in the high‐voltage window provided by carbonate electrolyte. Dissolution of LiNO3 in carbonate electrolyte is realized through an effective solvation regulation strategy. LiNO3 can be directly dissolved in an ethylene carbonate/diethyl carbonate electrolyte mixture by adding trace amounts of copper fluoride as a dissolution promoter. LiNO3 protects the Li metal anode in a working high‐voltage Li metal battery. When a LiNi0.80Co0.15Al0.05O2 cathode is paired with a Li metal anode, an extraordinary capacity retention of 53 % is achieved after 300 cycles (13 % after 200 cycles for LiNO3‐free electrolyte) and a very high average Coulombic efficiency above 99.5 % is achieved at 0.5 C. The solvation chemistry of LiNO3‐containing carbonate electrolyte may sustain high‐voltage Li metal anodes operating in corrosive carbonate electrolytes.