Microscopic Structure and Dynamics of LiBF4 Solutions in Cyclic and Linear Carbonates

Correlating the Solvating Power of Solvents with the Strength of Ion-Dipole Interaction in Electrolytes of Lithium-ion Batteries

The solvation structure of Li+ plays a significant role in determining the physicochemical properties of electrolytes. However, to date, there is still no clear definition of the solvating power of different electrolyte solvents, and even the solvents that preferentially participate in the solvation structure remain controversial. In this study, we comprehensively discuss the solvating power and solvation process of Li+ ions using both experimental characterizations and theoretical calculations. Our findings reveal that the solvating power is dependent on the strength of the Li+ -solvent (ion-dipole) interaction. Additionally, we uncover that the anions tend to enter the solvation sheath in most electrolyte systems through Li+ -anion (ion-ion) interaction, which is weakened by the shielding effect of solvents. The competition between the Li+ -solvent and Li+ -anion interactions ultimately determines the final solvation structures. This insight into the fundamental understanding of the solvation structure of Li+ provides inspiration for the design of multifunctional mixed-solvent electrolytes for advanced batteries.

Interplay of Structure and Dynamics in Lithium/Ionic Liquid Electrolytes: Experiment and Molecular Simulation

Despite their promising use in electrochemical and electrokinetic devices, ionic-liquid-based electrolytes often exhibit complex behavior arising from a subtle interplay of their structure and dynamics. Here, we report a joint experimental and molecular simulation study of such electrolytes obtained by mixing 1-butyl 3-methylimidazolium tetrafluoroborate with lithium tetrafluoroborate. More in detail, experiments consisting of X-ray scattering, pulsed field gradient NMR, and complex impedance spectroscopy are analyzed in the light of molecular dynamics simulations to probe the structural, dynamical, and electrochemical properties of this ionic-liquid-based electrolyte. Lithium addition promotes the nanostructuration of the liquid as evidenced from the appearance of a scattering prepeak that becomes more pronounced. Microscopically, using the partial structure factors determined from molecular dynamics, this prepeak is shown to correspond to the formation of well-ordered positive/negative charge series and also large aggregates (Li_n(BF₄)_4-m)^(4-m+n)-, which develop upon lithium addition. Such nanoscale ordering entails a drastic decrease in both the molecular mobility and ionic conductivity. In particular, the marked association of Li⁺ cations with four BF₄^- anions and long ion pairing times, which are promoted upon lithium addition, are found to severely hinder the Li⁺ transport properties.

Structures and dynamic properties of the LiPF6 electrolytic solution under electric fields - a theoretical study

Fluorinated carbonates have attracted increasing attention in high-voltage lithium ion battery applications. Under free and electric fields, their structure-related solvent dynamic properties such as charge and discharge rate, however, are rarely reported. Herein, solutions including linear fluorinated carbonates have been simulated using joint MD/DFT calculations. For the first time, the dielectric constants of the four pure fluorinated carbonates have been predicted to be ca. 5.4 (4: ethyl-2,2,2-trifluoroethyl carbonate)-12.1 (7: di-1,1,2,2,2-pentafluoroethyl carbonate), which are dependent on the number of fluorine atoms. Minor difference in the solvation free energies and few contact ion associations were observed in the ETFEC and DTFEC solvents. Their comparable viscosity values were found through calculations of ion diffusivity and conductivity. In the electric fields, the Li⁺ mobilities in the two solutions have similar values, showing that the dynamic properties of the electrolytic solutions are almost independent of the number of fluorine atoms on these carbonates. There emerges an oriented and ordered arrangement of the solvent molecules and thus the largely decreased dielectric constants under the electric fields, even though the field strength is very low. These interesting phenomena should be relevant in the formation of contact and aggregate ion pairs, leading to unexpected reduction of the charge-discharge rate-related ion mobility in mixed solvents.

General Trend of a Negative Li Effective Charge in Ionic Liquid Electrolytes

We show that strong cation-anion interactions in a wide range of lithium-salt/ionic liquid mixtures result in a negative lithium transference number, using molecular dynamics simulations and rigorous concentrated solution theory. This behavior fundamentally deviates from that obtained using self-diffusion coefficient analysis and explains well recent experimental electrophoretic nuclear magnetic resonance measurements, which account for ion correlations. We extend these findings to several ionic liquid compositions. We investigate the degree of spatial ionic coordination employing single-linkage cluster analysis, unveiling asymmetrical anion-cation clusters. We formulate a way to compute the effective lithium charge and show that lithium-containing clusters carry a negative charge over a remarkably wide range of compositions and concentrations. This finding has significant implications for the overall performance of battery cells based on ionic liquid electrolytes. It also provides a rigorous prediction recipe and design protocol for optimizing transport properties in next-generation highly correlated electrolytes.

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.

Investigation of Ion-Solvent Interactions in Nonaqueous Electrolytes Using in Situ Liquid SIMS

Ion-solvent interactions in nonaqueous electrolytes are of fundamental interest and practical importance, yet debates regarding ion preferential solvation and coordination numbers persist. In this work, in situ liquid SIMS was used to examine ion-solvent interactions in three representative electrolytes, i.e., lithium hexafluorophosphate (LiPF₆) at 1.0 M in ethylene carbonate (EC)-dimethyl carbonate (DMC) and lithium bis(fluorosulfonyl)imide (LiFSI) at both low (1.0 M) and high (4.0 M) concentrations in 1,2-dimethoxyethane (DME). In the positive ion mode, solid molecular evidence strongly supports the preferential solvation of Li⁺ by EC. Besides, from the negative spectra, we also found that PF₆^- forms association with EC, which has been neglected by previous studies due to the relatively weak interaction. In both LiFSI in DME electrolytes, however, no evidence shows that FSI^- is associated with DME. Furthermore, strong salt ion cluster signals were observed in the 1.0 M LiPF₆ in EC-DMC electrolyte, suggesting that a significant amount of Li⁺ ions stay in the vicinity of anions. In sharp comparison, weak ion cluster signals were detected in dilute LiFSI in DME electrolyte, suggesting most ions are well separated, in agreement with our molecular dynamics simulation results. These findings indicate that with virtues of little bias on detecting positive and negative ions and the capability of directly analyzing concentrated electrolytes, in situ liquid SIMS is a powerful tool that can provide key evidence for improved understanding on the ion-solvent interactions in nonaqueous electrolytes. Therefore, we anticipate wide applications of in situ liquid SIMS on investigations of various ion-solvent interactions in the near future.

Computer simulation study of the solvation of lithium ions in ternary mixed carbonate electrolytes: free energetics, dynamics, and ion transport

A ternary mixed carbonate electrolyte composed of ethylmethyl carbonate, dimethyl carbonate, ethylene carbonate, and Li⁺PF₆⁻ salt.

Ion speciation of lithium hexafluorophosphate in dimethyl carbonate solutions: an infrared spectroscopy study

The speciation of lithium hexafluorophosphate (LiPF₆) in dimethyl carbonate as function of the concentration is studied via IR spectroscopy and DFT computations.

A salient effect of density on the dynamics of nonaqueous electrolytes

The mobility and solvation of lithium ions in electrolytes are crucial for the performance and safety of lithium ion batteries. It has been known that a single type of solvent cannot satisfy the requirements of both mobility and solvation simultaneously for electrolytes. Therefore, complex solvent mixtures have been used to optimize both properties. Here we present the effects of density on the dynamics and solvation of organic liquid electrolytes via extensive molecular dynamics simulations. Our study finds that a small variation in density can induce a significant effect on the mobility of electrolytes but does not influence the solvation structure of a lithium ion. It turns out that an adjustment of the density of electrolytes could provide a more effective way to enhance mobility than a control of the solvent mixture ratio of electrolytes. Our study reveals that the density change of electrolytes mainly affects the residence time of solvents in the first solvation shell of a lithium ion rather than the structural change of the solvation sheath. Finally, our results suggest an intriguing point for understanding and designing electrolytes of lithium ion batteries for better performance and safety.

A comparison of the solvation structure and dynamics of the lithium ion in linear organic carbonates with different alkyl chain lengths

The structure and dynamics of electrolytes composed of lithium hexafluorophosphate (LiPF₆) in dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate were investigated using a combination of linear and two-dimensional infrared spectroscopies.

The multiscale coarse-graining method. XI. Accurate interactions based on the centers of charge of coarse-grained sites

It is essential to be able to systematically construct coarse-grained (CG) models that can efficiently and accurately reproduce key properties of higher-resolution models such as all-atom. To fulfill this goal, a mapping operator is needed to transform the higher-resolution configuration to a CG configuration. Certain mapping operators, however, may lose information related to the underlying electrostatic properties. In this paper, a new mapping operator based on the centers of charge of CG sites is proposed to address this issue. Four example systems are chosen to demonstrate this concept. Within the multiscale coarse-graining framework, CG models that use this mapping operator are found to better reproduce the structural correlations of atomistic models. The present work also demonstrates the flexibility of the mapping operator and the robustness of the force matching method. For instance, important functional groups can be isolated and emphasized in the CG model.

Solvation of the fluorine containing anions and their lithium salts in propylene carbonate and dimethoxyethane

Electrolyte solutions based on the propylene carbonate (PC)-dimethoxyethane (DME) mixtures are of significant importance and urgency due to emergence of lithium-ion batteries. Solvation and coordination of the lithium cation in these systems have been recently attended in detail. However, analogous information concerning anions (tetrafluoroborate, hexafluorophosphate) is still missed. This work reports PM7-MD simulations (electronic-structure level of description) to include finite-temperature effects on the anion solvation regularities in the PC-DME mixture. The reported result evidences that the anions appear weakly solvated. This observation is linked to the absence of suitable coordination sites in the solvent molecules. In the concentrated electrolyte solutions, both BF4(-) and PF6(-) prefer to exist as neutral ion pairs (LiBF4, LiPF6).

Li(+)-molecule interactions of lithium tetrafluoroborate in propylene carbonate + N,N-dimethylformamide mixtures: an FTIR spectroscopic study

FTIR (Fourier transformed infrared) spectra have been collected and analyzed for solutions of lithium tetrafluoroborate in propylene carbonate (PC), N,N-dimethylformamide (DMF), and PC+DMF mixtures. The band splitting and symmetric ring deformation for PC and O=C-N deformation for DMF suggest that there is a strong interaction between lithium cations and solvent molecules. The solvent molecules have been assigned to two types, the free and complexed molecules. By a comparison of the intensity for the corresponding bands, it has been concluded that Li(+) cations are preferentially solvated by DMF molecules in the LiBF4/PC+DMF solutions. This has been explained by the difference in values of donor number (DN).

FTIR spectroscopic studies of lithium tetrafluoroborate in propylene carbonate+diethyl carbonate mixtures

FTIR (Fourier transformed infrared) spectra have been collected and analyzed for solutions of lithium tetrafluoroborate in propylene carbonate (PC), diethyl carbonate (DEC), and PC+DEC mixtures. It has been shown that the carbonyl stretch bands of PC and DEC, the ring of PC and the ether oxygen stretch bands of DEC are all very sensitive to the interaction between Li(+) and the solvent molecules. New shoulders appear and the original bands split with the addition of LiBF4, indicating that a strong interaction between Li(+) and molecules of PC and DEC exists through the oxygen group of C=O and ring of PC and both C=O oxygen and ether oxygen atoms of DEC. In addition, no preferential solvation of Li(+) in LiBF4/PC+DEC solutions was detected.