Lithium Solvation Research Articles

Advancing Electrolyte Stability in Lithium Metal Batteries: A Strategy to Suppress Ionic Liquid Decomposition through Lithium Solvation Power Optimization

Non-flammable and electrochemically stable ionic liquid electrolytes (ILEs) have emerged as promising candidates for thermally stable high-voltage lithium metal batteries (LMBs). To mitigate the high viscosity of ILEs and improve their applicability, research has increasingly focused on incorporating non-solvating co-solvents (NSCs) that dilute ILEs without altering the local Li+ solvation structure. However, NSC with ILEs often undergo extensive reductive decomposition. Herein, we investigated the reductive behavior of NSC with ILEs and proposed highly stable hybrid electrolyte systems that integrate ILEs with weakly solvating co-solvents (WSCs). Our comparative analysis of various co-solvents, based on their solvation power, elucidates that the interaction between the n-propyl-n-methylpyrrolidinium cation (Pyr13+) and the co-solvents is governed by the co-solvents’ solvation power, which triggers the reductive decomposition of the electrolytes. The incorporation of strongly solvating co-solvents (SSCs) into ILEs, shifts the preferential interaction towards Li+ rather than Pyr13+, thereby minimizing Pyr13+ interaction and reducing reductive decomposition. Furthermore, replacing SSCs with WSCs in ILs results in a reduction in free solvent species, which improves chemical stability and compatibility with Li metal anode. As a result, a novel finding of our study is the effectiveness of an WSC with ILEs system in inhibiting electrolyte decomposition. Specifically, Employing the 1,2-diethoxyethane (DEE) with ILEs system, which incorporates DEE as a representative WSC, we assembled characterized by a high-nickel layered oxide cathode (3.0 mAh cm-2) and thin lithium foil (20 μm). This configuration enables us to achieve an impressive initial battery-level energy density exceeding 380 Wh kg-1 and the capacity retention was 95% for 180 cycles. On the other hand, when TTE, non-solvating co-solvent was added to ILEs, the capacity retention was only 51% for 50 cycles. This research not only advances our understanding of co-solvents with ILE systems but also paves the way for the development of ILs for LMBs with improved performance and stability. Figure 1

Insights into the Lithium Solvation Environments of Localized High Concentration Electrolytes by NMR and Effects on NMC811/Gr Cell Performance

Advanced battery technologies require advanced electrolytes to support high voltage and long cycle life at a wide range of temperatures and charging rates, and enhanced safety. The lithium solvation environment within the electrolyte can have a large impact on cell performance, influencing ion transport in the electrolyte as well as ion transfer at the electrode-electrolyte interfaces.1 By optimizing lithium-ion solvation, cell kinetics can be improved to facilitate low temperature and fast charge performance, while also promoting stable electrode interfaces during high temperature operation. Localized high concentration electrolytes (LHCEs)- composed of lithium salt(s), solvents, and a non- or minimally-solvating diluent -are designed to enhance the local concentration of salt around the lithium ion. These anion-involved lithium solvation environments can increase ion transport in the electrolyte and lead to more stable, inorganic-rich interfacial species.2 While LHCEs have shown promise in improving performance for lithium metal, high voltage, fast charge, and low temperature applications,2,3 further understanding of solvation in LHCEs and development of new diluent molecules is needed. The development of diluents should include solvation and physical property characterization, as well as performance testing in battery cells.Previous studies have demonstrated that nuclear magnetic resonance spectroscopy (NMR) can be used to provide insight into the solvation environments of electrolytes with standard carbonate solvents as well as novel solvent species.4,5 In this work, we utilized NMR to build understanding of the cation (Li: 7Li-NMR), anion (PF6: 19F-NMR), and solvent (carbonates: 13C-NMR) solvation environments of standard electrolytes as a function of salt concentration. We then applied this understanding to NMR analysis of electrolytes with a known diluent to establish a baseline of how solvation changes for LHCEs. Finally, this body of knowledge was used to investigate a novel fluorinated ether species, KDC-403, proposed as a diluent for LHCEs. Furthermore, we demonstrate improved low temperature cycling and more stable fast charge cycling with reduced lithium plating in electrolytes containing KDC-403, supporting previous studies that conclude LHCEs can provide superior battery performance.

A self fire-extinguishing and high rate lithium-fluorinated carbon battery realized by ethoxy (pentafluoro) cyclotriphosphazene electrolyte additive design

The lithium/carbon fluoride (Li/CFx) battery has attracted significant attention due to its highest energy density among all commercially available lithium primary batteries. However, its high energy density also poses a significant risk during thermal runaway events, and its poor electrochemical performance at high discharge current densities limits its application in high-power devices. In this study, we propose a flame-retardant and interface-affinity enhancing electrolyte additive, ethoxy (pentafluoro) cyclotriphosphazene (PFPN), to regulate the electrolyte in Li/CFx batteries. The PFPN molecules participate in the lithium solvation shell, and its fluorine groups shows better affinity with the carbon fluoride cathode compared to traditional carbonate electrolytes. Furthermore, as a bulky molecule, PFPN reduces the desolvation energy of Li+ ions after participating in lithium ion coordination, making it easier for Li+ ions to be utilized at the carbon fluoride cathode, significantly enhancing the battery's rate performance. In a carbonate electrolyte containing 16 % PFPN additive (1 M LiBF4/EC+DMC, volume ratio 1:1), the CFx cathode delivers 277 % higher capacity at a current density of 1 A g−1 compared to a CFx cathode without the additive. Additionally, due to the ability of PFPN to produce fluorine and phosphorus radicals at high temperatures that interrupt the combustion chain reaction, the electrolyte can achieve up to 10 instances of zero-second self-extinguishing, reducing the battery self-extinguishing time by 90 %. This study is the first to investigate the safety and flame-retardant electrolyte design of carbon fluoride batteries, providing a method to improve the power performance and safety of Li/CFx batteries at the same time.

NMR investigation of multi-scale dynamics in ionic liquids containing Li+ and La3+: From vehicular to hopping transport mechanism

The dynamics in mixtures of ionic liquid and monoatomic cations has been studied at different time scales ranging from the nanosecond up to the second. The mixtures were composed of cholinium bis(trifluoromethanesulfonyl)imide ([Chol][TFSI]) and LiTFSI, with LiTFSI mole fraction, ▪, spanning from 0 to 0.5 (saturated solution), and [Chol][TFSI] and ▪ from 0 to 0.12. The translational self-diffusion coefficients of ▪, ▪ and ▪ have been measured, along with NMR their relaxation times at various magnetic fields, in order to decipher the intertwined dynamics between the ions, and to reveal how the local dynamics impact the long range translational diffusion. When the concentrations of lithium and lanthanum are increased in the liquid, the long range dynamics of all the ions drop. In the case of LiTFSI, the self-diffusion coefficient of lithium becomes higher than the one of TFSI at high concentration, revealing a change in lithium transport mechanisms. The NMR relaxation data confirm this change, showing a clearer transition at ▪. It is interpreted as a change from a vehicular transport mechanism of the lithium below ▪ to a hopping mechanism above. A similar crossover seems to occur in the lanthanum solutions. This phenomenon seems correlated to the departure of the hydroxyl group of the organic cation from the lithium solvation shell.

Control of Lithium Salt Partitioning, Coordination, and Solvation in Vitrimer Electrolytes

Control of Lithium Salt Partitioning, Coordination, and Solvation in Vitrimer Electrolytes

Effect of Fluorinated Carboxylic Acid Ester on Lithium Solvation as an Additive in Electrolyte and Low-Temperature Insight on Battery Performance

The fluorinated carboxylic acid esters are effective additives to improve the performance of lithium-ion batteries (LIBs). Among them, 2,2,2-trifluoroethylbutyrate (TFEB) has been reported as a good additive to enhance battery performance, especially at low temperatures. It becomes a consensus that the solvation structure around lithium ion plays a crucial role in LIBs. However, it remains a mystery how such additives impact lithium solvation and how the impact changes at low temperatures. In this work, classical molecular dynamics (MD) simulations were conducted to investigate microcosmic mechanisms of the influence of TFEB on a typical commercial electrolyte composed of lithium hexafluorophosphate (LiPF6)/ethylene carbonate (EC)/ethyl methyl carbonate (EMC) in LIBs. Although TFEB is a minority species, it can significantly modify the lithium solvation structure. The introduction of TFEB facilitates the participation of anion in the solvation shell, due to its weaker coordination ability compared to EC/EMC, which was elucidated by the detailed analyses including radial distribution functions, cluster analysis, cage residence time, etc. We further investigated a series of TFEB analogs (TFEA, ETFA, and MPFP) and found that a moderately weak coordination ability is essential in the design of electrolytes. Furthermore, it was uncovered not only that the coordination number of the anions will decrease at low temperatures but also that their orientations are significantly changed. We demonstrated that the introduction of TFEB will alleviate this tendency, which may be advantageous to low-temperature performance. In summary, the critical role of fluorinated additives is uncovered by our MD simulations in modulating the Li+ solvation structure and improving the low-temperature performance of LIB, which provides references to the design strategy of novel low-temperature fluorine-containing additive-based electrolytes.

Understanding Lithium-Ion Dynamics in Single-Ion and Salt-in-Polymer Perfluoropolyethers and Polyethyleneglycol Electrolytes Using Solid-State NMR

Lithium metal batteries with high energy densities can enable a revolution in energy storage and accelerate shifts in electric transportation and electricity generation. However, several morphological and electro-chemo-mechanical challenges impede their development. Solid-state electrolytes such as those based on polymers show great promise in replacing liquid electrolytes in lithium metal batteries. Polyether-based polymer electrolytes are the most investigated but are plagued by low room-temperature ionic conductivity and poor oxidative stability. Hence, there is great need for the development and understanding of ion transport in new classes of polymer electrolytes. Perfluoropolyether (PFPE)-based electrolytes have shown improved oxidative stability, but little is understood about their lithium solvation and transport mechanism. In this work, we use multinuclear solid-state magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy to investigate the lithium cation environment and mobility in crosslinked single-ion and salt-in-polymer PFPE electrolytes and compare it directly to that of well-known polyethylene glycol (PEG) electrolytes. We show that the interaction of the lithium cation with the polymer backbone is weaker in PFPE systems compared to PEG, likely resulting in stronger ion pairing in the PFPE systems. Line shape analyses show lower lithium mobility in PFPE electrolytes despite lower activation energies being derived from spin-lattice relaxation (T1) measurements as compared to those for the PEG systems. The rapid relaxation is instead ascribed to the local fluctuations caused by polymer backbone mobility. By studying different modes of ion binding (single-ion vs salt-in-polymer), we show that differences across polymer backbones (PFPE vs PEG) have a greater effect on mobility than differences in ion binding modes within each polymer class (especially when single-ion conducting site density is not high). Our ability to use MAS NMR to study polymer electrolytes in their native state opens new opportunities to develop and understand novel polymer or hybrid solid-state electrolytes for next-generation lithium metal batteries.

Desolvation Synergy of Multiple H/Li-Bonds on an Iron-Dextran-Based Catalyst Stimulates Lithium-Sulfur Cascade Catalysis.

Traditional lithium-sulfur battery catalysts are still facing substantial challenges in solving sulfur redox reactions, which involve multistep electron transfer and multiphase transformations. Here, inspired by the combination of iron dextran (INFeD) and ascorbic acid (VC) as a blood tonic for the treatment of anemia, a highly efficient VC@INFeD catalyst is developed in the sulfur cathode, accomplishing the desolvation and enrichment of high-concentration solvated lithium polysulfides at the cathode/electrolyte interface with the assistance of multiple H/Li-bonds and resolving subsequent sulfur transformations through gradient catalysis sites where the INFeD promotes long-chain lithium polysulfide conversions and VC accelerates short-chain lithium polysulfide conversions. Comprehensive characterizations reveal that the VC@INFeD can substantially reduce the energy barrier of each sulfur redox step, inhibit shuttle effects, and endow the lithium-sulfur battery with high sulfur utilization and superior cycling stability even under a high sulfur loading (5.2mg cm-2 ) and lean electrolyte (electrolyte/sulfur ratio, ≈7µL mg-1 ) condition.

Chemometric and Machine Learning Analysis of Lithium Concentration and Solvation Behavior in Li-Ion Battery Electrolytes

The demand for batteries is rapidly growing across a range of technologies. The increasingly diverse use cases for batteries require various capabilities, particularly requirements for high energy densities, that are currently unmet by traditional Li-ion batteries. Electrolyte stability proves to be a bottleneck for battery advancement towards energy dense chemistries beyond Li-ion, including metal anodes. In situ spectroscopy tools, such as Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and X-ray spectroscopy, have provided insight into critical molecular-level interactions in batteries during cycling. These in situ tools have yielded continuous improvement of electrolyte properties. Spectroscopy datasets, however, contain many nuances that challenge meaningful human understanding. Artificial intelligence and chemometric tools can be coupled with in situ spectroscopy to find relevant interpretations of spectral datasets and elucidate complex molecular phenomena. In this research, an analysis was performed on FTIR spectroscopy data from an electrolyte composed of LiPF6 in ethylene carbonate (EC) and ethyl methyl carbonate (EMC) to discern solvation behavior using principal component analysis (PCA) and a convolutional neural network (CNN). PCA pinpointed exact band locations of solvation shifting behavior in the IR spectra and improved understanding of the relationship between lithium concentrations and peak changes. The CNN was trained with spectral datasets of electrolytes with known lithium concentrations and then could predict lithium concentrations from spectral datasets with extraordinarily high accuracy. Additionally, the CNN interpreted FTIR spectral datasets from a graphite half-cell with EC/EMC/LiPF6 electrolyte and accurately determined the lithium concentration in the bulk electrolyte. The CNN also observed lithium depletion events (up to 10% lithium depletion) in the graphite anode during fast-charging cycles of the galvanostatic intermittent titration technique. This research breaks new ground on using advanced computational tools for in situ spectroscopic analysis of battery electrolytes and demonstrates an improved understanding of complex molecular-level phenomena in electrolytes.

Decomposition Pathways of EC and Effects of Additives: Accounting for Liquid, Gas, and Solid Phase Reactions during the First Charge

The majority of lithium-ion batteries (LIBs) utilize carbonate-based electrolytes, due to the good lithium solvation, electrochemical stability, and electrode passivation.1 Studies regarding decomposition pathways and other reaction mechanisms2-4 of carbonates in LIBs are abundant in the literature but few attempt to quantify total losses of these solvents. Quantitative measurement of solvent loss from the electrolyte would significantly inform the comprehensive picture of electrolyte reaction mechanisms in the cell. Furthermore, this approach allows for the study of any effects additives may have on carbonate solvent decomposition. For example, organosilicon (OS) additives have been shown to reduce gassing and improve high temperature cycling in carbonate-based LIBs,5 but their quantitative impact on EC decomposition is not known.In this poster, we detail our methodology for quantitatively tracking and characterizing different forms of ethylene carbonate (EC) decomposition in a LIB pouch cell as well as the effects of organosilicon additives on these reactions. This study focused on the loss of EC after the first charge when the majority of SEI formation occurs. Liquid phase composition, including both EC and its decomposition products, was quantified by NMR analysis (1H and 19F) of extracted electrolyte using internal standards (LiPF6 and 1,4-bis(trifluoromethyl)benzene). Gas phase decomposition was characterized by a combination of the Archimedes method (to quantify gas volume) and GC-MS analysis (to determine gas composition). Finally, the impact of OS additives on the decomposition pathways of EC was examined. Inclusion of OS molecules reduces EC decomposition during the first charge, predominately by reducing the amount of liquid phase decomposition. 1Xu, K. Chem. Rev. 2004, 104, 4303-4417. 2 Campion, C.; Li, W.; Lucht, B. Thermal Decomposition of LiPF6-Based Electrolytes for Lithium-Ion Batteries. Journal of the Electrochemical Society 2005, 152, A2327. 3 Seo, D.; Chalasani, D.; Parimalam, B.; Kadam, R.; Nie, M.; Lucht, B. Reduction Reactions of Carbonate Solvents for Lithium Ion Batteries. EC Electrochemistry Letters 2014, 3, A91-A93. 4 Xing, L.; Li, W.; Wang, C.; Gu, F.; Xu, M.; Tan, C.; Yi, J. Theoretical Investigation on Oxidative Stability of Solvents and Oxidative Decomposition Mechanism of Ethylene Carbonate for Lithium Ion Battery Use. J. Phys. Chem. 2009, 113, 16596-16602. 5 Guillot, S.L.; Usrey, M.L.; Peña-Hueso, A.; Kerber, B.M.; Zhou, L.; Du, P.; Johnson, T. Reduced Gassing in Lithium-Ion Batteries with Organosilicon Additives. J. Electrochem. Soc. 2021, 168, 030533-030543.

Dual breaking of ionic association in water-in-LiTFSI electrolyte for low temperature battery applications

This paper deals with the low temperature performance of Li-ion aqueous battery in water-in-LiTFSI electrolyte. Two simultaneous approaches have been adopted to reduce the ionic interaction of lithium cation with TFSI anion. First, the LiTFSI eutectic mixture has been prepared with trifluoroacetamide (TFA) that shows liquid-like properties due to the strong interaction of amide-group with TFSI−. The eutectic behavior is explained by the interaction of trifluoroacetamide with bulky TFSI− anion that reduced the melting point of mixture to −55.3 °C. Despite such eutectic behavior, the low conductivity still remains an issue which is solved by increasing water content that improves the hydration degree of lithium. Molecular dynamic simulation (MD) studies show that the addition of water at low molar ratio is sufficient for lithium ion hydration and about five times improves the conductivity of electrolyte. A dual approach to break the ionic association in LiTFSI strongly influences the activation barrier for the movement of ions and is facilitated by water, which is fully utilized for the solvation of lithium and no free water is available to participate in faradaic reactions.

Lithium Solvation and Mobility in Ionic Liquid Electrolytes with Asymmetric Sulfonyl-Cyano Anion

Lithium Solvation and Mobility in Ionic Liquid Electrolytes with Asymmetric Sulfonyl-Cyano Anion

Quasi-solid electrolytes with tailored lithium solvation for fast-charging lithium metal batteries

Using quasi-solid electrolytes (QSEs) can make lithium metal batteries (LMBs) considerably safer. However, formulating QSEs with rate capabilities rivaling those of liquid electrolytes is extremely difficult. Herein, we develop a QSE characterized by safety, high conductivity above 2 × 10 −3 S cm −1 , and high lithium-ion transference number ( t Li + ) above 0.8. Computations and experiments reveal that these characteristics are achieved by adjusting the Li + solvation through a weak interaction with the polymer skeleton and strong coordination with NO 3 − . The further inclusion of fluoroethylene carbonate leads to the formation of a Li 3 N- and LiF-rich solid electrolyte interphase on the lithium metal. Consequently, QSE-based LMBs are capable of fast charging and high-power density, as well as stable and long cycling. By adjusting Li + solvation and lithium metal stability, this work rationally develops QSEs for fast-charging LMBs. • Quasi-solid electrolyte prepared with high-donor-number solvent and LiNO 3 • In-depth investigations on lithium solvation and ion transport by atomic simulations • Fast-charging lithium metal battery based on the QSE • Safe, flexible, and non-flammable pouch-type lithium metal battery Zhou et al. design a quasi-solid electrolyte with tailored lithium solvation and diffusion for fast-charging lithium metal batteries. The impact of the high-donor number solvent and LiNO 3 on solvation and charge-transfer mechanisms is studied experimentally and computationally.

Validating Concentrated Lithium-Ion Electrolyte Transport Models with in-Situ MRI

Spatiotemporal understanding of chemical species concentrations and electrochemical potentials, on both micro- and macroscopic scales, is vital to the enhancement of lithium-ion and lithium-metal battery performance. To understand transport in a concentrated binary electrolyte system comprising lithium hexafluorophosphate in ethyl methyl carbonate (LiPF6:EMC), we compared the responses of the concentration distribution and cell voltage captured with in situ MRI to simulations based on concentrated-solution transport theory. The transport simulations were parameterized using a suite of indirect thermodynamic, voltammetric, and rheometric measurements, including measurements of liquid-junction potentials, restricted-diffusion and Hittorf experiments, conductometry, impedance spectroscopy, and densitometry [1]. Close agreement between the simulations and experiment (see fig 1) suggests that the local states within super-concentrated electrolytes under applied currents can be predicted using a multicomponent transport theory that accounts for pairwise diffusional interactions, thermodynamic activity, and solution-volume effects [2]. The exceptionally close agreement between the measured and simulated transient states during the relaxation step of pulse-relaxation experiments suggests that simulation results can be used to extract kinetic overpotentials from experimental cell voltages during the pulse step. Experimental data is used to elucidate charge-transfer kinetics at lithium-metal electrode interfaces. Moving beyond binary electrolytes, we will also discuss the validity of pseudo-binary assumptions used to model current state-of-the-art co-solvated electrolytes, wherein preferential ion solvation by one of the solvents may impact salt activity [3].[1] Wang, A et al, Shifting-Reference Concentration Cells to Refine Composition-Dependent Transport Characterization of Binary Lithium-Ion Electrolytes. Electrochim. Acta (2020)[2] Liu, J.; Monroe, C. W. Solute-Volume Effects in Electrolyte Transport. Electrochim. Acta (2014)[3] O. Borodin et al., Competitive lithium solvation of linear and cyclic carbonates from quantum chemistry. Phys Chem Chem Phys (2016) Figure 1

Cation-Assisted Lithium-Ion Transport for High-Performance PEO-based Ternary Solid Polymer Electrolytes.

N‐alkyl‐N‐alkyl pyrrolidinium‐based ionic liquids (ILs) are promising candidates as non‐flammable plasticizers for lowering the operation temperature of poly(ethylene oxide) (PEO)‐based solid polymer electrolytes (SPEs), but they present limitations in terms of lithium‐ion transport, such as a much lower lithium transference number. Thus, a pyrrolidinium cation was prepared with an oligo(ethylene oxide) substituent with seven repeating units. We show, by a combination of experimental characterizations and simulations, that the cation's solvating properties allow faster lithium‐ion transport than alkyl‐substituted analogues when incorporated in SPEs. This proceeds not only by accelerating the conduction modes of PEO, but also by enabling new conduction modes linked to the solvation of lithium by a single IL cation. This, combined with favorable interfacial properties versus lithium metal, leads to significantly improved performance on lithium‐metal polymer batteries.

Cation‐Assisted Lithium‐Ion Transport for High‐Performance PEO‐based Ternary Solid Polymer Electrolytes

AbstractN‐alkyl‐N‐alkyl pyrrolidinium‐based ionic liquids (ILs) are promising candidates as non‐flammable plasticizers for lowering the operation temperature of poly(ethylene oxide) (PEO)‐based solid polymer electrolytes (SPEs), but they present limitations in terms of lithium‐ion transport, such as a much lower lithium transference number. Thus, a pyrrolidinium cation was prepared with an oligo(ethylene oxide) substituent with seven repeating units. We show, by a combination of experimental characterizations and simulations, that the cation's solvating properties allow faster lithium‐ion transport than alkyl‐substituted analogues when incorporated in SPEs. This proceeds not only by accelerating the conduction modes of PEO, but also by enabling new conduction modes linked to the solvation of lithium by a single IL cation. This, combined with favorable interfacial properties versus lithium metal, leads to significantly improved performance on lithium‐metal polymer batteries.

Solvation and transport of lithium ions in deep eutectic solvents.

Lithium based deep eutectic solvents (DESs) are excellent candidates as eco-friendly electrolytes for lithium ion batteries. While some of these DESs have shown promising results, a clear mechanism of lithium ion transport in DESs is not yet established. This work reports the study on the solvation and transport of lithium in a DES made from lithium perchlorate and acetamide using Molecular Dynamics (MD) simulation and inelastic neutron scattering. Based on hydrogen bonding (H-bonding) of acetamide with neighboring molecules/ions, two states are largely prevalent: (1) acetamide molecules that are H-bonded to lithium ions (∼36%) and (2) acetamide molecules that are entirely free (∼58%). Analyzing their stochastic dynamics independently, it is observed that the long-range diffusion of the former is significantly slower than that of the latter. This is also validated from the neutron scattering experiment on the same DES system. Furthermore, the analysis of the lithium dynamics shows that the diffusion of acetamide molecules in the first category is strongly coupled to that of lithium ions. On an average, the lithium ions are H-bonded to ∼3.2 acetamide molecules in their first solvation. These observations are further bolstered through the analysis of the H-bond correlation function between acetamide and lithium ions, which shows that ∼90% of lithium ionic transport is achieved by vehicular motion where the ions diffuse along with their first solvation shell. It is also observed that the ionic motions are largely uncorrelated and the conductivity of lithium ions in the DES is found to be 11 mS/cm. The findings of this work are an important advancement in understanding solvation and transport of lithium in the DES.

Pyrrolidinium Ionic Liquid Electrolyte with Bis(trifluoromethylsulfonyl)imide and Bis(fluorosulfonyl)imide Anions: Lithium Solvation and Mobility, and Performance in Lithium Metal–Lithium Iron Phosphate Batteries

A mixture of n-methyl-n-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [PYR13][TFSI] and n-propyl-n-methylpyrrolidinium bis(fluorosulfonyl)imide, [PYR13][FSI] ionic liquids (ILs) is investigated for lithium–metal batteries. Specifically, the relation among conductivity, solvation structure, and Li⁺ mobility is investigated in a Li/IL/IL type ternary mixture. Li⁺ anion coordination numbers with both [TFSI] and [FSI] in the ternary mixtures are derived from Raman analysis. The Li⁺ transference number was measured by a combination of potentiostatic polarization and electrochemical impedance spectroscopy (EIS) techniques. The electrochemical stability and transport property of the developed ternary mixture were confirmed with Li–Li symmetrical and Li–LiFePO₄ cells. The ternary system exhibited improved rate capability compared to binary parent electrolytes as well as the state-of-the art carbonate-based electrolyte at −20 °C, and slightly better cycling stability at 25 °C. This study demonstrates the flexibility in tailoring physical properties and the Li⁺ solvation environment by ternary Li/IL/IL mixtures for enhanced battery performance.

Structure and dynamics in the lithium solvation shell of nonaqueous electrolytes

The solvation of a lithium ion has been of great importance to understand the structure and dynamics of electrolytes. In mixed electrolytes of cyclic and linear carbonates, the lithium solvation structure and the exchange dynamics of solvents strongly depend on the mixture ratio of solvents, providing a connection of the rigidity of the lithium solvation shell with the solvent composition in the shell. Here we study the dynamical properties of solvents in the solvation sheath of a lithium ion for various solvent mixture ratios via molecular dynamics simulations. Our results demonstrate that the exchange dynamics of solvents exhibits a non-monotonic behavior with a change in the mixture ratio, which keeps preserved on both short and long time scales. As the fraction of cyclic carbonate increases, we find that the structural properties of cyclic and linear carbonates binding to a lithium ion show different responses to a change in the fraction. Furthermore, we find that the rotational dynamics of cyclic carbonate is relatively insensitive to the mixture ratio in contrast to the rotational dynamics of linear carbonate. Our results further present that an anion shows different properties in structure and dynamics from solvents upon changing the mixture ratio of solvents.

Effect of standard light illumination on electrolyte\u2019s stability of lithium-ion batteries based on ethylene and di-methyl carbonates

Combining energy conversion and storage at a device and/or at a molecular level constitutes a new research field raising interest. This work aims at investigating how prolonged standard light exposure (A.M. 1.5G) interacts with conventional batteries electrolyte, commonly used in the photo-assisted or photo-rechargeable batteries, based on 1 mol.L−1 LiPF6 EC/DMC electrolyte. We demonstrate the intrinsic chemical robustness of this class of electrolyte in absence of any photo-electrodes. However, based on different steady-state and time-resolved spectroscopic techniques, it is for the first time highlighted that the solvation of lithium and hexafluorophosphate ions by the carbonates are modified by light exposure leading to absorbance and ionic conductivity modifications without detrimental effects onto the electrochemical properties.