Controlling Gas Generation of Li-Ion Battery through Divinyl Sulfone Electrolyte Additive.

The focus of mainstream lithium-ion battery (LIB) research is on increasing the battery's capacity and performance; however, more effort should be invested in LIB safety for widespread use. One aspect of major concern for LIB cells is the gas generation phenomenon. Following conventional battery engineering practices with electrolyte additives, we examined the potential usage of electrolyte additives to address this specific issue and found a feasible candidate in divinyl sulfone (DVSF). We manufactured four identical battery cells and employed an electrolyte mixture with four different DVSF concentrations (0%, 0.5%, 1.0%, and 2.0%). By measuring the generated gas volume from each battery cell, we demonstrated the potential of DVSF additives as an effective approach for reducing the gas generation in LIB cells. We found that a DVSF concentration of only 1% was necessary to reduce the gas generation by approximately 50% while simultaneously experiencing a negligible impact on the cycle life. To better understand this effect on a molecular level, we examined possible electrochemical reactions through ab initio molecular dynamics (AIMD) based on the density functional theory (DFT). From the electrolyte mixture's exposure to either an electrochemically reductive or an oxidative environment, we determined the reaction pathways for the generation of CO2 gas and the mechanism by which DVSF additives effectively blocked the gas's generation. The key reaction was merging DVSF with cyclic carbonates, such as FEC. Therefore, we concluded that DVSF additives could offer a relatively simplistic and effective approach for controlling the gas generation in lithium-ion batteries.

A Computer Simulation Study of Thermal and Mechanical Properties of Poly(Ionic Liquid)s.

Thermal and mechanical properties of poly(ionic liquid)s (PILs), an epoxidized ionic liquid-amine network, are studied via molecular dynamics simulations. The poly(ionic liquid)s are designed with two different ionic liquid monomers, 3-[2-(Oxiran-2-yl)ethyl]-1-{4-[(2-oxiran-2-yl)ethoxy]phenyl}imidazolium (EIM2) and 1-{4-[2-(Oxiran-2-yl)ethyl]phenyl}-3-{4-[2-(oxiran-2-yl)ethoxy]benzyl}imidazolium (EIM1), each of which is networked with tris(2-aminoethyl)amine, paired with different anions, bis(trifluoromethanesulfonyl)imide (TFSI-) and chloride (Cl-). We investigate how ionic liquid monomers with high ionic strength affect structures of the cross-linked polymer networks and their thermomechanical properties such as glass transition temperature (Tg) and elastic moduli, varying the degree of cross-linking. Strong electrostatic interactions between the cationic polymer backbone and anions build up their strong structures of which the strength depends on their molecular structures and anion size. As the anion size decreases from TFSI- to Cl-, both Tg and elastic moduli of the PIL increase due to stronger electrostatic interactions present between their ionic moieties, making it favorable for the PIL to organize with stronger bindings. Compared to the EIM2 monomer, the EIM1 monomers and TFSI- ions generate a PIL with higher Tg and elastic moduli. This attributes to the less flexible structure of the EIM1 monomer for the chain rotation, in which steric hindrance by ring moieties in the EIM1-based PIL enhances their structural rigidity. The π-π stacking structures between the rings are found to increase in EIM1-based PIL compared to the EIM2-based one, which becomes stronger with smaller Cl- ion rather than TFSI-. The effect of the degree of the cross-linking on thermal and mechanical properties is also examined. As the degree of cross-linking decreases from 100% to 60%, Tg also decreases by a factor of 10-20%, where the difference among the given PILs becomes decreased with a lower degree of cross-linking. Both the Young's (E) and shear (G) moduli of all the PILs decrease with degree of cross-linking, which the reduction is more significant for the PIL generated with EIM2 monomers. Transport properties of anions in PILs are also studied. Anions are almost immobilized globally with very small structural fluctuations, in which Cl- presents lower diffusivity by a factor of ~2 compared to TFSI- due to their stronger binding to the cationic polymer backbone.

Prediction and Interpretation of Polymer Properties Using the Graph Convolutional Network.

We present machine learning models for the prediction of thermal and mechanical properties of polymers based on the graph convolutional network (GCN). GCN-based models provide reliable prediction performances for the glass transition temperature (T g), melting temperature (T m), density (ρ), and elastic modulus (E) with substantial dependence on the dataset, which is the best for T g (R 2 ∼ 0.9) and worst for E (R 2 ∼ 0.5). It is found that the GCN representations for polymers provide prediction performances of their properties comparable to the popular extended-connectivity circular fingerprint (ECFP) representation. Notably, the GCN combined with the neural network regression (GCN-NN) slightly outperforms the ECFP. It is investigated how the GCN captures important structural features of polymers to learn their properties. Using the dimensionality reduction, we demonstrate that the polymers are organized in the principal subspace of the GCN representation spaces with respect to the backbone rigidity. The organization in the representation space adaptively changes with the training and through the NN layers, which might facilitate a subsequent prediction of target properties based on the relationships between the structure and the property. The GCN models are found to provide an advantage to automatically extract a backbone rigidity, strongly correlated with T g, as well as a potential transferability to predict other properties associated with a backbone rigidity. Our results indicate both the capability and limitations of the GCN in learning to describe polymer systems depending on the property.

Comparison of Machine Learning Methods towards Developing Interpretable Polyamide Property Prediction.

Polyamides are often used for their superior thermal, mechanical, and chemical properties. They form a diverse set of materials that have a large variation in properties between linear to aromatic compounds, which renders the traditional quantitative structure-property relationship (QSPR) challenging. We use extended connectivity fingerprints (ECFP) and traditional QSPR fingerprints to develop machine learning models to perform high fidelity prediction of glass transition temperature (Tg), melting temperature (Tm), density (ρ), and tensile modulus (E). The non-linear model using random forest is in general found to be more accurate than linear regression; however, using feature selection or regularization, the accuracy of linear models is shown to be improved significantly to become comparable to the more complex nonlinear algorithm. We find that none of the models or fingerprints were able to accurately predict the tensile modulus E, which we hypothesize is due to heterogeneity in data and data sources, as well as inherent challenges in measuring it. Finally, QSPR models revealed that the fraction of rotatable bonds, and the rotational degree of freedom affects polyamide properties most profoundly and can be used for back of the envelope calculations for a quick estimate of the polymer attributes (glass transition temperature, melting temperature, and density). These QSPR models, although having slightly lower prediction accuracy, show the most promise for the polymer chemist seeking to develop an intuition of ways to modify the chemistry to enhance specific attributes.

Computer simulation study of fluorocarbon phosphate surfactant based aqueous reverse micelle in supercritical CO2: roles of surfactant functional groups, ionic strength, and phase changes in CO2.

Structural and dynamic properties of an aqueous micelle organized from fluorocarbon phosphate surfactant molecules in supercritical carbon dioxide (CO2) are investigated via molecular dynamics computer simulations. The roles of the functional groups and ionic strength of the surfactants on the formation of reverse micelles in supercritical CO2, and related water dynamics characterized as translational and reorientational dynamics, are systematically demonstrated by employing three different phosphate-based surfactants paired with sodium cations. The strong electrostatic interactions between the phosphate head groups and sodium cations result in formation of an aqueous core inside the surfactant aggregates, where water molecules are bonded together with loss of the tetrahedral hydrogen bonded network found in bulk water. It is found that all the three surfactants with CO2-philic fluorocarbon double tails build up well-stabilized reverse micelles in supercritical CO2, avoiding direct contacts between CO2 and water molecules. Despite this, the surfactant with a carboxylic ester linkage between the phosphate head and fluorocarbon tail group tends to coordinate water molecules toward sustaining the inter-water hydrogen bonds, indicating better efficiency at covering the aqueous core with hydrophobic groups compared to one without a carboxylic ester group. As for water molecules confined in the reverse micelle, their translational and reorientational motions, and fluctuating dynamics of the inter-water hydrogen bonds, significantly slow down compared to bulk water at ambient temperature. The water dynamics become more restricted with an increase in ionic strength of the anionic surfactant; this is attributed to divalent surfactant heads and sodium cations being more tightly bound together with bonding to water compared to monovalent ones. Lastly, the structural and dynamic changes of the reverse micelle caused by a phase change in CO2 are monitored with gradually decreasing temperature and pressure from the supercritical to gaseous state for CO2. The average reverse micelle structure equilibrated in supercritical CO2 is found to remain stable over a time period of 0.2 ms through a depressurization process to gaseous CO2. We note that the diverse pathways of surfactant self-aggregation in gaseous CO2 could be controlled by the preceding solvation procedure in the supercritical regime which governs the final aggregated structures in gaseous CO2.

First-principles study of the surface reactions of aminosilane precursors over WO3(001) during atomic layer deposition of SiO2.

Atomic layer deposition (ALD) has emerged as a critical technique to deposit highly conformal and uniform thin films for advanced semiconductor devices. The development of ALD processes relies on ALD precursor design to meet the required properties of thin films. In this study, we report the ALD mechanisms of silicon oxide over the tungsten oxide substrate using density functional theory (DFT) methods. To analyze the ligand effects of precursors, we compare the surface reactions of different aminosilane precursors with a varying number of amino ligands such as diisopropylaminosilane (DIPAS), bis(diethylamino)silane (BDEAS), and tris(dimethylamino)silane (TDMAS) over the hydroxyl-terminated WO3 (001) surface. BDEAS shows the lowest energy barrier in the rate-determining step and the overall reaction energetics of DIPAS and BDEAS decomposition are more exothermic than that of TDMAS. For this reason, BDEAS is estimated to provide the fastest growth rate. However, the binding strength of the leaving amine molecule of DIPAS on WO3(001) is weaker than those of BDEAS and TDMAS, and thus DIPAS is more likely to reduce surface impurities during the ALD process.

Reductive reactions via excess Li in mixture electrolytes of Li ion batteries: an ab initio molecular dynamics study.

The electro-reduction of battery electrolytes plays a critical role in the formation of solid-electrolyte interphase (SEI) layers on the surfaces of negative electrodes. These layers have a significant influence on the performance of rechargeable battery cells. Using ab initio molecular dynamics, we demonstrate the electro-reduction of mixture electrolytes computationally by adding a certain number of excess Li+ first to form the solvation structure and the same number of electrons later for reductive reactions. Our method enables direct observations of the ring opening of one cyclic carbonate followed by merging with another solvent molecule as well as gas generation. When we examined FEC- and EC-based electrolytes, we were able to observe the differences in terms of reaction products. In particular, the two gaseous products that are generated the most are in accordance with recent in situ gas measurements in the literature. The different reaction products of each electrolyte also match well with the SEI constituents reported experimentally. By tracing reaction pathways, we found that Li+ ions facilitate many otherwise difficult electrochemical reactions, presumably by lowering energy barriers. We also found that the excess Li+ forms cationic clusters of Li2PF6+, which enable the reductive decomposition of salt anions and which do not occur easily simply by increasing the electronic occupation. Based on the reaction products of FEC-based electrolytes, here we propose a possible mechanism of polymerization through aldehyde intermediates that are known to bond with surrounding radical anions.

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

A ternary mixture of ethylmethyl carbonate (EMC), dimethyl carbonate (DMC), and ethylene carbonate (EC) is a common solvent used in lithium ion batteries. The effect of cyclic ethylene carbonate (EC) on the solvation structure, free energetics and related dynamics of the ternary mixed carbonate electrolyte with lithium hexafluorophosphate (Li+PF6-) salt is systematically investigated by molecular dynamics simulations. The EC co-solvent tends to decrease the structural heterogeneity and solvent fluctuation in the solvation of Li+ ions. Despite the large difference in dielectric constants according to the EC concentration, the effective polarity of ternary mixed electrolytes, measured as the solvation-induced stabilization of Li+ ions, is marginally affected by EC co-solvent. As the EC concentration χEC increases, the solvation structural relaxation of Li+ ions accelerates in the subpicosecond regime, whereas the diffusional motion of ions slows down. The viscosity of the electrolyte solution and the solvation structure changed by EC co-solvent contribute to the ionic conductivity in an opposite manner, and as a result, the ionic conductivity of the electrolyte solution exhibits non-monotonic behavior according to χEC.

Atomic layer deposition of diisopropylaminosilane on WO3(001) and W(110): a density functional theory study.

The decomposition reactions of the Si precursor, diisopropylaminosilane (DIPAS), on W(110) and hydroxylated WO3(001) surfaces are investigated to elucidate the initial reaction mechanism of the atomic layer deposition (ALD) process using density functional theory (DFT) calculations combined with ab initio molecular dynamics (AIMD) simulations. The decomposition reaction of DIPAS on WO3(001) consists of two steps: Si-N dissociative chemisorption and decomposition of SiH3*. It is found that the Si-N bond cleavage of DIPAS is facile on WO3(001) due to hydrogen bonding between the surface OH group and the N atom of DIPAS. The rate-determining step of DIPAS decomposition on WO3(001) is found to be the Si-H dissociation reaction of the SiH3* reaction intermediate which has an activation barrier of 1.19 eV. On the contrary, sequential Si-H dissociation reactions first occur on W(110) and then the Si-N dissociation reaction of the C5H7NSi* reaction intermediate is found to be the rate-determining step, which has an activation barrier of 1.06 eV. As a result, the final products in the DIPAS decomposition reaction on WO3(001) are Si* and SiH*, whereas Si* atoms remain with carbon impurities on W(110), which imply that the hydroxylated WO3 surface is more efficient for the ALD process.

Excitation-energy dependence of solvation dynamics in room-temperature ionic liquids.

Influence of the excitation energy of a probe solute molecule on its solvation dynamics and emission spectrum in 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI(+)PF6 (-)) is studied via molecular dynamics simulations using a coarse-grained model description. By exciting the probe at different energies, each with an extremely narrow distribution, ensuing solvent relaxation and its dynamic variance are monitored using the isoconfigurational ensemble method. Resulting Stokes shift function, S(t), indicates that long-time solvent relaxation becomes slower with the decreasing excitation energy and approaches the equilibrium correlation function, C(t), of solvent fluctuations. This suggests that the system excited at the red-edge of the spectrum observes linear response better than that at the blue-edge. A detailed analysis of nonequilibrium trajectories shows that the effect of initial configurations on variance of relaxation dynamics is mainly confined to short times; it reaches a maximum around 0.1 ≲ t ≲ 1 ps and diminishes as time further increases. The influence of the initial velocity distribution, on the other hand, tends to grow with time and dominates the long-time variations of dynamics. The emission spectrum shows the red-edge effect in accord with previous studies.

Computer Simulation Study of Graphene Oxide Supercapacitors: Charge Screening Mechanism.

Graphene oxide supercapacitors in the parallel plate configuration are studied via molecular dynamics (MD) simulations. The full range of electrode oxidation from 0 to 100% is examined by oxidizing the graphene surface with hydroxyl groups. Two different electrolytes, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI(+)BF4(-)) as an ionic liquid and its 1.3 M solution in acetonitrile as an organic electrolyte, are considered. While the area-specific capacitance tends to decrease with increasing electrode oxidation for both electrolytes, its details show interesting differences between the organic electrolyte and ionic liquid, including the extent of decrease. For detailed insight into these differences, the screening mechanisms of electrode charges by electrolytes and their variations with electrode oxidation are analyzed with special attention paid to the aspects shared by and the contrasts between the organic electrolyte and ionic liquid.

Theoretical study of interactions of a Li(+)(CF3SO2)2N(-) ion pair with CR3(OCR2CR2)nOCR3 (R = H or F).

Interactions of a lithium bis(trifluoromethane sulfonyl)imide (Li(+)Tf2N(-)) ion pair with oligoethers are investigated via density functional theory (DFT). As a model for polymer electrolytes polyethyleneoxide (PEO) and perfluoropolyether (PFPE), CR3(OCR2CR2)n=1-5OCR3 (R = H or F) is considered. Topographical analysis of the molecular electrostatic potential (MESP) is performed to determine preferential binding sites of Li(+). Our study shows that the MESP value near the oxygen sites of the polymer backbone is more negative for PEO than for PFPE. This result indicates that substitution of hydrogen by fluorine in polyethers leads to reduction in Li(+)-polymer interactions, in concert with the experimental ionic conductivity results. S-O stretching vibrations of Tf2N(-) are calculated for the lithium salt in the presence and absence of electrolytes. The blue and red shifts predicted for S-O stretching are further explained by natural bond orbital analysis and molecular electron density topography. The S-O stretching vibrations can be used as a useful tool to understand the ion pair interactions and thus ion transport phenomena in polymer electrolytes.

Modulation of the Dirac point voltage of graphene by ion-gel dielectrics and its application to soft electronic devices.

We investigated systematic modulation of the Dirac point voltage of graphene transistors by changing the type of ionic liquid used as a main gate dielectric component. Ion gels were formed from ionic liquids and a non-triblock-copolymer-based binder involving UV irradiation. With a fixed cation (anion), the Dirac point voltage shifted to a higher voltage as the size of anion (cation) increased. Mechanisms for modulation of the Dirac point voltage of graphene transistors by designing ionic liquids were fully understood using molecular dynamics simulations, which excellently matched our experimental results. It was found that the ion sizes and molecular structures play an essential role in the modulation of the Dirac point voltage of the graphene. Through control of the position of their Dirac point voltages on the basis of our findings, complementary metal-oxide-semiconductor (CMOS)-like graphene-based inverters using two different ionic liquids worked perfectly even at a very low source voltage (V(DD) = 1 mV), which was not possible for previous works. These results can be broadly applied in the development of low-power-consumption, flexible/stretchable, CMOS-like graphene-based electronic devices in the future.

Dielectric relaxation and solvation dynamics in a room-temperature ionic liquid: temperature dependence.

Dielectric relaxation, related polarization and conductivity, and solvation dynamics of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMI(+)PF6(-)) are studied via molecular dynamics computer simulations in the temperature range 300 K ≤ T ≤ 500 K. Two main bands of its dielectric loss spectrum show differing temperature behaviors. As T increases, the absorption band in the microwave region shifts to higher frequencies rapidly, whereas the location of the bimodal far-IR band remains nearly unchanged. Their respective intensities tend to decrease and increase. The static dielectric constant of BMI(+)PF6(-) is found to decrease weakly with T. The ultrafast inertial component of solvation dynamics remains largely unchanged, while their dissipative relaxation component becomes faster. Roles played by ion reorientations and translations in governing dynamic and static dielectric properties of the ionic liquid are examined. A brief comparison with available experimental results is also made.

Graphene-based supercapacitors in the parallel-plate electrode configuration: ionic liquids versus organic electrolytes.

Supercapacitors with two single-sheet graphene electrodes in the parallel plate geometry are studied via molecular dynamics (MD) computer simulations. Pure 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI+BF4-) and a 1.1 M solution of EMI+BF4- in acetonitrile are considered as prototypes of room-temperature ionic liquids (RTILs) and organic electrolytes. Electrolyte structure, charge density and associated electric potential are investigated by varying the charges and separation of the two electrodes. Multiple charge layers formed in the electrolytes in the vicinity of the electrodes are found to screen the electrode surface charge almost completely. As a result, the supercapacitors show nearly an ideal electric double layer behavior, i.e., the electric potential exhibits essentially a plateau behavior in the entire electrolyte region except for sharp changes in screening zones very close to the electrodes. Due to its small size and large charge separation, BF4- is considerably more efficient in shielding electrode charges than EMI+. In the case of the acetonitrile solution, acetonitrile also plays an important role by aligning its dipoles near the electrodes; however, the overall screening mainly arises from ions. Because of the disparity of shielding efficiency between cations and anions, the capacitance of the positively-charged anode is significantly larger than that of the negatively-charged cathode. Therefore, the total cell capacitance in the parallel plate configuration is primarily governed by the cathode. Ion conductivity obtained via the Green-Kubo (GK) method is found to be largely independent of the electrode surface charge. Interestingly, EMI+BF4- shows higher GK ion conductivity than the 1.1 M acetonitrile solution between two parallel plate electrodes.

Carbon nanotubes in benzene: internal and external solvation.

The structure and dynamics of benzene inside and outside of single-walled carbon nanotubes (SWNTs) in the (n,n) armchair configuration are studied via molecular dynamics computer simulations. Irrespective of the nanotube diameter, benzene molecules form cylindrical solvation shell structures on the outside of the nanotubes. Their molecular planes near the SWNTs in the first external solvation shell are oriented parallel to the nanotube surface, forming a π-stacked structure between the two. By contrast, the benzene distributions in the interior of the SWNTs are found to vary markedly with the nanotube diameter. In the case of the (7,7) and (8,8) nanotubes, internal benzene forms a single-file distribution, either in a vertex-to-vertex (n = 7) or face-to-face (n = 8) orientation between two neighboring molecules. Inside a slightly wider (9,9) nanotube channel, however, a cylindrical single-shell distribution of benzene arises. A secondary solvation structure, which begins to appear inside (10,10), develops into a full structure separate from the first internal solvation shell in (12,12). The ring orientation of internal benzene is generally parallel to the nanotube wall for n = 9-12, while it becomes either slanted with respect to (n = 7), or perpendicular to (n = 8), the nanotube axis. The confinement inside the small nanotube pores exerts a strong influence on the dynamics of benzene. Both translational and rotational dynamics inside SWNTs are slower and more anisotropic than in liquid benzene. It is also found that reorientational dynamics of internal benzene deviate dramatically from the rotational diffusion regime and change substantially with the nanotube diameter.

MD study of solvation in the mixture of a room-temperature ionic liquid and CO(2).

Solvation structure and dynamics of a saturated solution of carbon dioxide in the room-temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMI+PF6-) at 313 K and 0.15 kbar are investigated via molecular dynamics computer simulations by employing a diatomic probe solute. It is found that the mixture shows preferential solvation, which is mainly controlled by the solute-BMI+PF6- electrostatic interactions and thus dictates differing roles for CO2 as the solute charge distribution varies. The local structure and density of BMI+PF6- and CO2 in the vicinity of the solute become enhanced and reduced, respectively, as its dipole moment increases. As a result, equilibrium solvation dynamics of a nonpolar solute in the mixture have a strong CO2 character, whereas those of a dipolar solute are very similar to, albeit faster than, solvation dynamics in pure BMI+PF6-. Related nonequilibrium solvent response couched in dynamic Stokes shifts and accompanying solvation structure relaxation, in particular, CO2 structure reorganization, shows interesting dependence on the solute charge distribution. Ion transport in the mixture is much faster than in pure BMI+PF6-, indicating that the addition of cosolvent CO2 reduces the viscosity of the ionic liquid, significantly. The effective polarity of the mixture, measured as solvation-induced stabilization of a dipolar solute, is found to be comparable to that of neat BMI+PF6-, consonant with solvatochromic measurements.

Nanoporous carbon supercapacitors in an ionic liquid: a computer simulation study.

Supercapacitors composed of carbon nanotube (CNT) micropores in the room-temperature ionic liquid (RTIL) 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI+BF4(-)) are studied via molecular dynamics (MD)computer simulations. It is found that the distribution of RTIL ions inside the micropore varies significantly with the pore size. Internal solvation of small (6,6) and (7,7) CNTs with an electrified interior wall is effected almost exclusively via counterions. Surprisingly, these counterions, even though they all have the same charge, lead to a charge density characterized by multiple layers with alternating signs. This intriguing feature is attributed to the extended nature of RTIL ion charge distributions, which result in charge separation through preferential orientation inside the electrified nanotubes. In the case of larger (10,10) and (15,15) CNTs, counterions and coions develop multilayer solvation structures. The specific capacitance normalized to the pore surface area is found to increase as the CNT diameter decreases from (15,15) to (7,7). As the pore size further reduces from (6,6) to(5,5), however, the specific capacitance diminishes rapidly. These findings are in excellent agreement with recent experiments with carbon-based materials. A theoretical model based on multiple charge layers is proposed to understand both the MD and experimental results.

Adiabatic electron transfer in a room-temperature ionic liquid: reaction dynamics and kinetics.

Adiabatic electron transfer (ET) in the room-temperature ionic liquid 1-butyl-3-methyldicyanamide (BMI(+)DCA(-)) and in aprotic acetonitrile is studied with molecular dynamics (MD) computer simulation techniques using a model diatomic reaction complex. The influence of barrier crossing dynamics on ET kinetics is examined directly via constrained reaction coordinate MD, while the corresponding effect arising from activation and deactivation processes in the reactant and product states is analyzed with the aid of simulation results on solvation dynamics. The departure from the transition state theory (TST) rate constant caused by barrier crossing is found to be moderate and comparable in BMI(+)DCA(-) and acetonitrile despite a huge difference in their viscosity. A theoretical analysis shows that the Grote-Hynes theory yields a reasonable agreement with the MD results on barrier crossing in both solvents, whereas the Kramers theory fails completely in BMI(+)DCA(-). The influence of activation and deactivation dynamics on ET kinetics in BMI(+)DCA(-) varies markedly with reaction free energetics because of the biphasic nature of solvation dynamics, viz., ultrafast subpicosecond relaxation followed by slow subnanosecond decay. This indicates that dynamic factors controlling adiabatic ET in BMI(+)DCA(-) transition from barrier crossing to activation/deactivation as the barrier height for the forward and/or backward reaction decreases. This regime change of ET dynamics is accompanied by the breakdown of TST as the reaction becomes activation-limited in BMI(+)DCA(-). By contrast, activation and deactivation dynamics do not play a major role in acetonitrile.