Molecular Dynamics Simulations of Polymer–Ionic Liquid (1-Ethyl-3-methylimidazolium Tetracyanoborate) Ternary Electrolyte for Sodium and Potassium Ion Batteries

Flame-Retardant Nonaqueous Electrolytes for High-Safety Potassium-Ion Batteries

Potassium-ion batteries (PIBs) with conventional organic-based flammable electrolytes suffer from serious safety issues with a high risk of ignition and burning especially under harsh conditions, which significantly limits their widespread applications. Flame-retardant electrolytes (FREs) are considered as one of the most effective strategies to address these safety issues. Therefore, it's much necessary to summarize the challenges, recent progress, and design principles of flame-retardant nonaqueous electrolytes for PIBs to guide their development and future applications. In this review, an in-depth introduction and explanation of the origins of electrolyte flammability are first presented. Particularly, the state-of-the-art design principles of FREs for PIBs are extensively summarized and emphasized, including the electrolyte flame-retardant solvents/additives, highly concentrated electrolytes (HCEs), localized high-concentration electrolytes (LHCEs), ionic liquids-based electrolytes and solid-state electrolytes. Moreover, the advantages and drawbacks of each approach are systematically presented and discussed, following by proposed perspectives to guide the rational development of next-generation high-safety PIBs for practical applications.

Polymerized and Colloidal Ionic Liquids─Syntheses and Applications

The breadth and importance of polymerized ionic liquids (PILs) are steadily expanding, and this review updates advances and trends in syntheses, properties, and applications over the past five to six years. We begin with an historical overview of the genesis and growth of the PIL field as a subset of materials science. The genesis of ionic liquids (ILs) over nano to meso length-scales exhibiting 0D, 1D, 2D, and 3D topologies defines colloidal ionic liquids, CILs, which compose a subclass of PILs and provide a synthetic bridge between IL monomers (ILMs) and micro to macro-scale PIL materials. The second focus of this review addresses design and syntheses of ILMs and their polymerization reactions to yield PILs and PIL-based materials. A burgeoning diversity of ILMs reflects increasing use of nonimidazolium nuclei and an expanding use of step-growth chemistries in synthesizing PIL materials. Radical chain polymerization remains a primary method of making PILs and reflects an increasing use of controlled polymerization methods. Step-growth chemistries used in creating some CILs utilize extensive cross-linking. This cross-linking is enabled by incorporating reactive functionalities in CILs and PILs, and some of these CILs and PILs may be viewed as exotic cross-linking agents. The third part of this update focuses upon some advances in key properties, including molecular weight, thermal properties, rheology, ion transport, self-healing, and stimuli-responsiveness. Glass transitions, critical solution temperatures, and liquidity are key thermal properties that tie to PIL rheology and viscoelasticity. These properties in turn modulate mechanical properties and ion transport, which are foundational in increasing applications of PILs. Cross-linking in gelation and ionogels and reversible step-growth chemistries are essential for self-healing PILs. Stimuli-responsiveness distinguishes PILs from many other classes of polymers, and it emphasizes the importance of segmentally controlling and tuning solvation in CILs and PILs. The fourth part of this review addresses development of applications, and the diverse scope of such applications supports the increasing importance of PILs in materials science. Adhesion applications are supported by ionogel properties, especially cross-linking and solvation tunable interactions with adjacent phases. Antimicrobial and antifouling applications are consequences of the cationic nature of PILs. Similarly, emulsion and dispersion applications rely on tunable solvation of functional groups and on how such groups interact with continuous phases and substrates. Catalysis is another significant application, and this is an historical tie between ILs and PILs. This component also provides a connection to diverse and porous carbon phases templated by PILs that are catalysts or serve as supports for catalysts. Devices, including sensors and actuators, also rely on solvation tuning and stimuli-responsiveness that include photo and electrochemical stimuli. We conclude our view of applications with 3D printing. The largest components of these applications are energy related and include developments for supercapacitors, batteries, fuel cells, and solar cells. We conclude with our vision of how PIL development will evolve over the next decade.

Effect of Zwitterionic Additives on Solvation and Transport of Sodium and Potassium Cations in (Ethylene Oxide)10: A Molecular Dynamics Simulation Study

Sodium- (Na+) and potassium- (K+) ion batteries are cost-effective alternatives to lithium-ion (Li+) batteries due to the abundant sodium and potassium resources. Solid polymer electrolytes (SPEs) are essential for safer and more efficient Na+ and K+ batteries because they often exhibit low ionic conductivity at room temperature. While zwitterionic (ZW) materials enhance Li+ battery conductivity, their potential for Na+ and K+ transport in batteries remains unexplored. In this study, we investigated the effect of three ZW molecules (ChoPO4, i.e., 2-methacryloyloxyethyl phosphorylcholine, ImSO3, i.e., sulfobetaine ethylimidazole, and ImCO2, i.e., carboxybetaine ethylimidazole) on the dissociation of Na+ and K+ coordination with ethylene oxide (EO) chains in EO-based electrolytes through molecular dynamics simulations. Our results showed that ChoPO4 possessed the highest cation-EO10 dissociation ability, while ImSO3 exhibited the lowest. Such dissociation ability correlated with the cation-ZW molecule coordination strength: ChoPO4 and ImSO3 showed the strongest and the weakest coordination with cations. However, the cation-ZW molecule coordination could slow the cationic diffusion. The competition of these effects resulted in accelerating or decelerating cationic diffusion. Our simulated results showed that ImCO2 enhanced Na+ diffusion by 20%, while ChoPO4 and ImSO3 led to a 10% reduction. For K+, ChoPO4 reduced its diffusion by 40%, while ImCO2 and ImSO3 caused a similar decrease of 15%. These findings suggest that the ZW structure and the cationic size play an important role in the ionic dissociation effect of ZW materials.

Molecular Dynamics Simulation Study of the Far-Infrared Spectrum of a Deep Eutectic Solvent

Deep eutectic solvents (DESs) are similar to ionic liquids (IL) in terms of physicochemical properties and technical uses. In ILs, far-infrared (FIR) spectroscopy has been utilized to reveal ionic interactions and even to produce a signature of the strengthening of the cation-anion hydrogen bond. However, for the situation of the DES, where the mixing of a salt and a molecular species makes the interplay between multiple intermolecular interactions even more complex, a full investigation of FIR spectra is still absent. In this work, the FIR spectrum of the DES, often referred to as ethaline, which is a 1:2 mixture of choline chloride and ethylene glycol, is calculated using classical molecular dynamics (MD) simulations and compared to experimental data. To explore the induced dipole effect on the computed FIR spectrum, MD simulations were run with both nonpolarizable and polarizable models. The calculation satisfactorily reproduces the position of the peak at ∼110 cm^-1 and the bandwidth seen in the experimental FIR spectrum of ethaline. The MD simulations show that the charge current is the most important contributor to the FIR spectrum, but the cross-correlation between the charge current and dipole reorientation also plays a role in the polarizable model. The dynamics of the chloride-ethylene glycol correlation span a wide frequency range, with a maximum at ∼150 cm^-1, but it participates as a direct mechanism only in the charge current-dipole reorientation cross-term. Anion correlations, whose dynamics are regulated via correlation with both ethylene glycol and choline, make the most significant contribution to the charge current mechanism. The MD simulations were also utilized to investigate the effect on the FIR spectrum of adding water to the DES and switching to a 1:1 composition.

Molecular Dynamics Insight into the Role of Water Molecules in Ionic Liquid Mixtures of 1-Butyl-3-methylimidazolium Iodide and Ethylammonium Nitrate

Imidazolium-based ionic liquids are well known for their versatility as solvents for various applications such as dye-sensitized solar cells, fuel cells, and lithium-ion batteries; however, their complex interactions continue to be investigated to further improve upon their design. Ionic liquids (ILs) are commonly mixed with co-solvents such as water, organic solvents, or other ionic liquids to tailor their physiochemical properties. To better predict these properties and fundamentally understand the molecular interactions within the electrolyte mixtures, molecular dynamics (MD) simulations are often employed. In this study, MD simulations are performed on ternary solutions containing ionic liquids of 1-butyl-3-methylimidazolium iodide ([BMIM][I]) and ethylammonium nitrate ([EA][NO₃]) with increasing concentration of water. As previously reported, these ternary solutions displayed a wide temperature window of thermal stability and electrochemical conductivity. Utilizing MD simulations, the complex intermolecular interactions are identified, and the role of water as a co-solvent is disclosed to correlate with changes in their bulk properties. The MD results, including simulation box snapshots, radial distribution functions, and self-diffusion coefficients, reveal the formation of heterogeneous regimes with increasing water concentration, hydrogen bonding between iodide-water, iodide-[EA]⁺, and a change in IL ordering when in mixtures containing water. The simulations also display the formation of water aggregates and networks at high water concentrations, which can contribute to the thermal behavior of the respective mixtures. As the design of IL-based electrolytes grows in demand with increasing complexity, this work demonstrates the capability of MD simulations containing multiple constituents and their necessity in material development through identification of microscopic structure-property relationships.

Fine-Tuning the Polarizable CL&Pol Force Field for the Deep Eutectic Solvent Ethaline

Polarizable force fields are gradually becoming a common choice for ionic soft matter, in particular, for molecular dynamics (MD) simulations of ionic liquids (ILs) and deep eutectic solvents (DESs). The CL&Pol force field introduced in 2019 is the first general, transferable, and polarizable force field for MD simulations of different types of DESs. The original formulation contains, however, some problems that appear in simulations of ethaline and may also have a broader impact. First, the originally proposed atomic diameter parameters are unbalanced, resulting in too weak interactions between the chlorides and the hydroxyl groups of the ethylene glycol molecules. This, in turn, causes an artificial phase separation in long simulations. Second, there is an overpolarization of chlorides due to strong induced dipoles that give rise to the presence of peaks and antipeaks at very low q-vector values (2.4 nm^-1) in the partial components of the structure factors. In physical terms, this is equivalent to overestimated spatial nanoscale heterogeneity. To correct these problems, we adjusted the chloride-hydroxyl radial distribution functions against ab initio data and then extended the use of the Tang-Toennis damping function for the chlorides' induced dipoles. These adjustments correct the problems without losing the robustness of the CL&Pol force field. The results were also compared with the nonpolarizable version, the CL&P force field. We expect that the corrections will facilitate reliable use of the CL&Pol force field for other types of DESs.

Molecular Dynamics and Emerging Network Graphs of Interactions in Dinitrile-Based Li-Ion Battery Electrolytes

Advancements in battery research have shown interesting formulations of battery electrolytes that have helped improve the efficiency of Li-ion batteries over the decades. However, the quest for a safer and affordable battery electrolyte still proceeds with more unique formulations reported in the literature regularly. The dinitriles, especially adiponitrile and glutaronitrile, have caught the attention of the research community as part of this quest. In this work, we performed molecular dynamics simulations of dinitrile electrolytes with lithium bistrifluorosulfonimide (LiTFSI) as the electrolyte salt at varying concentrations and temperatures. On analysis of our simulations, we find that the densities of the mixtures follow the same trend as that of experimental values. The solvation properties were explored using the radial distribution functions. The connectivity of the Li⁺ with the dinitrile molecules and anions is established for all of the electrolyte concentrations using network graphs. We observe that the electrolytes form highly networked structures as the concentration increases without being affected by the rise in temperature. The networking of ionic interactions was quantified by calculating the average degree of each graph. Ionic conductivity calculations were computed using three methods: Nernst-Einstein relation, correlated method, and current autocorrelation function. We report the importance of accounting for the correlated motion of ions while estimating the ionic conductivity. The correlated conductivity and current autocorrelation function calculations provide a satisfactory estimation of the ionic conductivity compared to the experimental values.

Water-Salt Oligomers Enable Supersoluble Electrolytes for High-Performance Aqueous Batteries

Aqueous rechargeable batteries are highly safe, low-cost, and environmentally friendly, but restricted by low energy density. One of the most efficient solutions is to improve the concentration of the aqueous electrolytes. However, each salt is limited by its physical solubility, generally below 21-32 mol kg^-1 (m). Here, a ZnCl₂ /ZnBr₂ /Zn(OAc)₂ aqueous electrolyte with a record super-solubility up to 75 m is reported, which breaks through the physical solubility limit. This is attributed to the formation of acetate-capped water-salt oligomers bridged by Br^- /Cl^- -H and Br^- /Cl^- /O-Zn²⁺ interactions. Mass spectrometry indicates that acetate anions containing nonpolarized protons prohibit the overgrowth and precipitation of ionic oligomers. The polymer-like glass transition temperature of such inorganic electrolytes is found at ≈-70 to -60 °C, without the observation of peaks for salt-crystallization and water-freezing from 40 to -80 °C. This supersoluble electrolyte enables high-performance aqueous dual-ion batteries that exhibit a reversible capacity of 605.7 mAh g^-1 , corresponding to an energy density of 908.5 Wh kg^-1 , with a coulombic efficiency of 98.07%. In situ X-ray diffraction and Raman technologies reveal that such high ionic concentrations of the supersoluble electrolyte enable a stage-1 intercalation of bromine into macroscopically assembled graphene cathode.

Electrolytes and Interphases in Potassium Ion Batteries

Potassium ion batteries (PIBs) are recognized as one promising candidate for future energy storage devices due to their merits of cost-effectiveness, high-voltage, and high-power operation. Many efforts have been devoted to the development of electrode materials and the progress has been well summarized in recent review papers. However, in addition to electrode materials, electrolytes also play a key role in determining the cell performance. Here, the research progress of electrolytes in PIBs is summarized, including organic liquid electrolytes, ionic liquid electrolytes, solid-state electrolytes and aqueous electrolytes, and the engineering of the electrode/electrolyte interfaces is also thoroughly discussed. This Progress Report provides a comprehensive guidance on the design of electrolyte systems for development of high performance PIBs.