Ion Transport in the EMITFSI/PVDF System at Different Temperatures: A Molecular Dynamics Simulation.

We used all-atom molecular dynamics simulations to study the ion transport in the 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide/poly(vinylidene fluoride) (EMITFSI/PVDF) system with 40.05 wt % EMITFSI at different temperatures. The glass-transition temperature (T g = 204 K) of this system shows a good agreement with the experimental value (200 K). With the increase of temperature, the peaks of the pair correlation function show an increasing trend. Interestingly, the coordination numbers of ion pairs and the degree of independent ion motion are mainly affected by the binding energy between ion pairs as the temperature increases. In addition, the ion transport properties with increasing temperature can be studied by the ion-pair relaxation times, ion-pair lifetimes, and diffusion coefficients. The simulation results illustrate that the ion transport is intensified. Especially, the cations can always diffuse faster than the anions. The power law shows that mobilities of anions and cations are seen to exhibit a "superionic" behavior. With the increase of temperature, transference numbers of anions decrease first and then increase and transference numbers of cations show the opposite changes; ionic conductivity increases gradually; and viscosity decreases gradually, indicating that the diffusion resistance of ions decreases. In general, after adding PVDF into the EMITFSI system, the glass-transition temperature and viscosity increase, the ionic conductivity and degree of independent ion motion decrease, and diffusion coefficients of cations decrease faster than those of the anions.

Introduction

An ionic liquid (IL), defined as salts composed entirely by cations and anions, has interesting physical–chemical properties, including negligible vapor pressure regarded as nonvolatile, the characteristics of low melting point, good thermal and chemical stabilities, strong flame retardancy, high conductivity, and a wide electrochemical stability window.[1−3] Therefore, ionic liquids/polymer composites are the most popular as electroactive polymers (EAPs).[4] Among them, the dry actuator is the most popular,[5,6] called the bucky gel actuator (BGA), which is a sandwich structure consisting of an ionic liquid/polymer electrolyte layer sandwiched between two gel electrode layers,[7] and achieves large deformation actuation at low voltages (<3 V) based on the ion diffusion.[8]

There are different polymers that can be used in ionic liquids/polymer composites, such as poly(ethylene oxide) (PEO),[9] poly(dimethylsiloxane) (PDMS),[10] thermoplastic polyurethane (TPU),[11] polyether-segmented polyurethaneurea (PEUU),[12] and poly(vinylidene fluoride) (PVDF).[13−15] Especially, PVDF is outstanding among all reported actuator polymers[16−19] because it has large dielectric constant, high polarity, high voltage coefficient, good thermal stability, strong chemical resistance, biocompatibility, easy processibility, and high mechanical strength.[20−22] Different anions and cations have been used in ionic liquids/polymer composites, the former including chloride ions (Cl–), bromide ions (Br–), hexafluorophosphate (PF6–), tetrafluoroborate (BF4–), ethyl sulfate (EtSO4–), bis(fluorosulfonyl)imide (FSI–), bis(trifluoromethanesulfonyl)imide (TFSI–), and trifluoromethanesulfonate (TfO–) and the latter including imidazolium, pyrrolidinium, piperidinium, and quaternary ammonium.[23−26]

The performance is mainly dependent on ion diffusion and mobility in the BGA,[27] and the migration of ions in the electrolyte is rate-determining as the adsorption/desorption process occurs much slower than on the electrode.[25,28] Therefore, it is of great importance to study the ion transport properties in the IL/PVDF electrolyte layer for a comprehensive understanding of the actuation mechanism of the BGA. In this paper, we studied the mechanical properties, electrical conductivity, and bending efficiency of the BGA based on PVDF with 40 wt % 1-ethyl-3-methylimidazolium chloride (EMICl), 1-hexyl-3-methylimidazolium chloride (HMICl), 1-decyl-3-methylimidazolium chloride (DMICl), EMITFSI, HMITFSI, and DMITFSI and obtained the best overall performance in 40 wt % EMITFSI.[29] However, to the best of our knowledge, only a few papers researching the IL/PVDF system have been reported using theoretical calculations.[30−32] Especially, we have previously studied the effect of different anions on the ion transport in six IL/PVDF systems (EMICl/PVDF, EMIBr/PVDF, EMIBF4/PVDF, EMIPF6/PVDF, EMITfO/PVDF, and EMITFSI/PVDF systems) at different temperatures using MD simulations.[32]

The experiments studied the dielectric modulus and conductivity of different IL/PVDF systems at different temperatures.[33] In addition, the ion transport properties are greatly affected by the temperature[34−37] and directly related to the structural relaxation times in the pure IL and polymer ionic liquid (polyIL).[38−40] In this paper, we used MD simulations to study the ion transport properties in the EMITFSI/PVDF system at different temperatures. Specifically, the relevant basic concepts and theories as well as molecular dynamics modeling and simulation methods were introduced. Next, we obtained the glass-transition temperature (Tg) of the EMITFSI/PVDF system and analyzed the pair correlation function (PCF), coordination number (CN), and binding energy (Ebind) at different temperatures. Then, we calculated and discussed ion-pair relaxation times (τC) and lifetimes (τS), diffusion coefficients (D), and transference numbers of ions, ideal conductivity (σNE), true ionic conductivity (σ), degree of independent ion motion (α), and viscosity (η).

Methods

Force Field

Both ab initio molecular dynamics (AIMD) and force field molecular dynamics (FFMD), which can study ILs at the molecular level, have been used to determine the dynamics and structure.[41] FFMD is far less time-consuming and can also simulate the larger time intervals compared with AIMD. Therefore, some papers have presented many force fields for different kinds of ILs within the last two decades,[42−52] for example, 0.8*2009IL force field is a partial charge assignment for ILs based on a noninteger molecular charge scaling of ±0.8 e,[49] OPLS-VSIL force field has been undertaken, where RMIM+ cation parameters are created that utilizes a virtual site bisecting the ring nitrogen atoms,[50] CL&P force field is modified upon inclusion of the Drude dipoles,[51] and the BILFF force field can accurately describe the hydrogen bonding between biopolymers and ILs, with a special emphasis on the microstructure and dynamics.[52] In this work, we used a parameter setting originated from the OPLS-AA force field.[46−49,53,54] The force field parameters for molecules and ions are listed in the Supporting Information (SI). The potential functions used in this MD simulation, including the contributions of valence (bonds, angles, dihedral angles) and nonbond terms, are shown below.The valence terms arewhere kb, is the harmonic bond constant, r represents the bond length, and r0, represents the equilibrium bond length; kθ, represents the harmonic angular constant, θ is the angle, and θ0, is the equilibrium angle; and V represents the Fourier coefficient and ϕ is the dihedral angle. The nonbond terms arewhere q, q represent the partial atomic charges, σ is the Lennard-Jones radii, and ε is the well-depth. We utilized geometric combining rules for these parameters such that and .

Pair Correlation Function

The pair correlation function (PCF) is a useful mathematical tool to study the material structure by a measure of local spatial ordering, which is the conditional probability number density gαβ(r) of finding a β particle between the spheres of radius r and r + dr.where ρβ is the average number density of β particles and Gαβ(r) is the radial distribution function.where |Rα – Rβ| is the distance between particles α and β and Nβ is the total number of β particles.

In addition, we calculate the relevant coordination number (CN) of ion pairs by numerically integrating the PCF, which is expressed as[55,56]where ρ is the average number density and g(r) is the PCF, where the cutoff radius (rc) is usually chosen to be the first local minimum of the corresponding PCF, that is, the first solvation shell.

Time Correlation Functions

Two time correlation functions (TCFs) are used to characterize the dynamic properties between ion pairs, which are the intermittent time correlation function, C(t), and the continuous time correlation function, S(t).[40,57−59] The cutoff distances defining ion pairs (IPs) are determined by the first coordination shell of the PCF. In addition, the trajectory is saved every 1 ps to optimize the storage resources required and analysis efficiency when evaluating C(t) and S(t). A previous paper has verified that this time interval qualitatively agreed with a trajectory-saving frequency of finer, as small as 0.01 ps.[60]

C(t) is defined aswhere the population variable h(t) is unity if an ion pair that is present at initial t = 0 remains intact at time t and zero otherwise. ⟨  ⟩ represents an ensemble average over all pairs and time origins.

S(t) is defined aswhere the population variable H(t) is unity if an ion pair that is present at initial t = 0 remains intact during the time duration t. ⟨ ⟩ represents an ensemble average over all pairs and time origins.

C(t) and S(t) can be fitted by a single Kohlrausch–Williams–Watts (KWW)[40,61,62] stretched exponential function of the formwhere a0, α, and β are the fitting parameters. Then, ion-pair relaxation times, τC, are obtained fromwhere Γ denotes the γ function (a similar definition applies to ion-pair lifetimes τS).

Diffusion Coefficients

Diffusion coefficients (D), which can be calculated by the Einstein relation, are an important parameter for characterizing the diffusion ability of ions and molecules in the system.[63]where d is the system dimensionality (i.e., d = 1, 2, 3), MSD = ⟨|ri(t) – ri(0)|2⟩ is the mean square displacement, ri(t) and ri(0) represent the position of particle i at time t and 0, respectively. When calculating the diffusion coefficient of ions, the time interval used for MSD fitting is 50 ps.

Transference Numbers

Transference numbers, which may be estimated from D of both anions and cations,[64] are used to evaluate the contributions of anions and cations to the charge transport in the EMITFSI/PVDF system and are also an important factor to measure the performance of electrolytes.[65] The greater the transference numbers, the greater the relative contribution to the charge transport. The transference numbers of anions are defined aswhere N– (N+) and D– (D+) are the numbers and the diffusion coefficients of anions (cations), respectively (a similar definition applies to t+).

Ionic Conductivity

Ionic conductivity (σ) is how easy it is for charged particles to flow, which is calculated using the Green–Kubo formula as[66−68]where j(t) = e∑zivi(t) is the autocorrelation function of the charge flux, e is the electronic charge, vi(t) is the velocity of ion i, zi is the charge of ion i, T represents the temperature, kB is the Boltzmann constant, V is the volume of the system, and ⟨ ⟩ represents the ensemble average. In MD simulations, it is often useful to write the above eq in the form of an equivalent Einstein relationIn an ideal condition, the ideal conductivity (σNE) can be obtained by D of anions and cations based on the Nernst–Einstein (NE) relation.[69]where z– (z+), N– (N+), and D– (D+) are the charge, numbers, and the diffusion coefficients of anions (cations), respectively.

The degree of independent ion motion (α) measures the deviation of σ from σNE. It is defined aswhere α = 1 indicates that all ions move independently of each other, while α = 0 indicates that all ions move together.[70,71]

Viscosity

Viscosity (η) is a measure of fluid viscosity, which can be estimated using the diffusion coefficients by the Stokes–Einstein equation[72,73]where Di is the self-diffusion coefficient of ion i and r is the hydrodynamic radius of ion i (Stokes radius).

Models and Simulation Methods

The EMITFSI/PVDF system with 40 wt % EMITFSI can obtain the best overall performance, such as mechanical properties, electrical conductivity, and bending efficiency of the BGA,[29] so the EMITFSI content of 40.05 wt % was selected in MD simulations. In detail, 6 PVDF chains, 33 EMI+, and 33 TFSI– were placed in a cubic periodic box with a length of 37.69 Å, in which the total number of atoms was 2934, and the modeling details are presented in Section S2. In all simulations, the model was conducted in the constant pressure/temperature (NPT) ensemble at 250–440 K, and all temperatures and pressures of the system were maintained by the Nosé–Hoover thermostat and barostat, respectively.[74,75] The equations of motion were integrated with a step of 1 fs, and there were no constraints on bond lengths and angles. The van der Waals (vdW) interactions were truncated at 10 Å with long-range tail correction, and the long-range electrostatic interaction was evaluated by the PPPM method.[76] The converged density of the amorphous EMITFSI/PVDF cell was obtained using the high–low pressure dynamics simulation method,[77] where high-pressure dynamics with P = 1 GPa was applied for 5 ns and low-pressure dynamics with P = 0.0001 GPa (1 atm) was applied for 5 ns. After equilibration run, production runs of 10 ns were performed at different temperatures (250, 280, 300, 320, 340, 360, 380, 400, 420, and 440 K) and 1 atm pressure, during which trajectories were collected for analysis. Converged densities of the EMITFSI/PVDF system at different temperatures are shown in Table S10. In the analyses of the PCF, τC, and τS, atom N was taken as the cationic EMI+ center, and atom N was also taken as the anionic TFSI– center. All MD simulations were performed using the LAMMPS (large-scale atomic/molecular massively parallel simulator)[78] simulation package except that two time correlation functions were obtained by analyzing the trajectories through self-written scripts.

Results and Discussion

Glass-Transition Temperature

The glass-transition temperature (Tg) is very important to the ionic conductivity and diffusion coefficients (D) in the pure IL and the IL/PVDF system,[32,79] so it is necessary to study the Tg of the system. We have used NPT simulations to obtain the equilibrium density of the EMITFSI/PVDF system at different temperatures, and Figure shows the specific volume (Vsp) as a function of temperature T. The inflection point of Tg can be roughly estimated by the visual inspection of the computed data points.[80,81] Then, in Figure , linear fittings are performed on the region below (glassy) and above (rubbery), and R2 represents the coefficient of determination. The intersection of these two lines is Tg = 204 K of the EMITFSI/PVDF system, which shows a good agreement with the reported experimental value (200 K).[82] Therefore, it can be considered that the selected force field parameters are suitable for the EMITFSI/PVDF system in this study. Additionally, Tg (204 K) of the EMITFSI/PVDF system is greater than Tg (182.91 K)[83] of the EIMTFSI system, which indicates that Tg will increase when PVDF is added to the EMITFSI system.

Figure 1

Computed specific volume (Vsp) as a function of temperature T for the EMITFSI/PVDF system.

Pair Correlation Function

In this paper, the pair correlation function (PCF) is used to investigate the local spatial structure of ion pairs in the EMITFSI/PVDF system at different temperatures. PCF curves are shown in Figure , which have a lower peak near 4.35 Å and then the higher peak near 5.55 Å, and both peaks show an increasing trend as the temperature increases. The reason may be that as the temperature increases, the motion of anions and cations gradually increases, so there is enough energy to overcome the repulsive force between ion pairs. Therefore, the distance between the ion pairs becomes closer. The coordination number (CN) of ion pairs, which represents the number of ion pairs in the local space, is calculated by eq in the EMITFSI/PVDF system at different temperatures. As shown in Figure , we can observe that the CN of ion pairs has irregular fluctuation as the temperature increases. With the increase of temperature, the CN of ion pairs is the smallest at 300 K and the largest at 380 K, indicating that the number of ion pairs is the smallest at 300 K and the largest at 380 K.

Figure 2

PCF of ion pairs at different temperatures for the IL/PVDF system.

Figure 3

CN of ion pairs as a function of temperature T for the EMITFSI/PVDF system.

Binding Energy

The binding energy (Ebind) between ion pairs, which can well reflect the strength of the binding force, is very important to understand the ion transportation in the IL/PVDF system.[32]Ebind is the negative value of the interaction energy (−Einter) between particles, which is nonbonding energy, and has been long-range-corrected. We used the group/group command in LAMMPS to calculate Einter between anions and cations, where all anions were in the first group and all cations were in the second group. Figure shows Ebind between ion pairs as a function of temperature T for the EMITFSI/PVDF system. As shown in Figure , we can observe that Ebind between ion pairs has irregular fluctuation as the temperature increases. With the increase of temperature, Ebind between ion pairs is the smallest at 300 K and the largest at 380 K. That is to say, the binding force between ion pairs is the smallest at 300 K and the largest at 380 K. Therefore, we can conclude that Ebind between ion pairs is the main factor affecting the CN of ion pairs (Section ) as the temperature increases. The reason may be that when Ebind between ion pairs is large, there is enough energy to overcome the repulsive force between ion pairs so that more cations (anions) can be attracted around the anions (cations).

Figure 4

Ebind as a function of temperature T for the EMITFSI/PVDF system.

Ion Pairs Kinetic Properties

Ion pairs kinetic properties is an important method of studying the ion transport for the EMITFSI/PVDF system. The intermittent time correlation function, C(t), and the continuous time correlation function, S(t), can be calculated by eqs ,8, respectively, at different temperatures. Figure 3Sa,b shows the C(t) and S(t) as a function of time (t), respectively. It can be seen from Figure 3S that the decay rate of C(t) and S(t) curves gradually increases as the temperature increases, but the decay rate of S(t) is faster, which is consistent with the conclusion of the previous paper.[32] We used eq to fit C(t) curves in Figure S3a, and the fitted parameters were substituted into eq to obtain ion-pair relaxation times (τC). In the same way, we can also obtain ion-pair lifetimes (τS) from Figure S3b. Figure presents τC and τS as a function of temperature T for the EMITFSI/PVDF system. As can be seen from Figure , with the increase of temperature, τC and τS gradually decrease, and τC is always greater than τS, and the temperature T has a similar impact on τC and τS. Therefore, it can be inferred from the above conclusions that the motion of anions and cations is gradually intensified as the temperature increases.

Figure 5

τC and τS as a function of temperature T for the EMITFSI/PVDF system.

The mean square displacement (MSD) of anions and cations for the EMITFSI/PVDF system was obtained by MD simulations at different temperatures, and the results are shown in Figure S4. The initial and terminal nonlinear parts should not be included when we performed a linear fit to the MSD to obtain the slope, so the region from 2 to 8 ns was selected. Then, the slope was substituted into eq to further calculate the diffusion coefficients of anions (D–) and diffusion coefficients of cations (D+) at the corresponding temperature. It can be seen from Table S12 that when the temperature is the same, D– (D+) for EMITFSI is always greater than D– (D+) for the EMITFSI/PVDF system. Therefore, we can see that D– and D+ decrease after adding PVDF into the EMITFSI system, indicating that the transport of anions and cations becomes worse.

Figure shows D– and D+ as a function of temperature T for the EMITFSI/PVDF system. With the increase of temperature, D– and D+ monotonously increase, indicating that the transport of anions and cations gradually increases, which is the same as the conclusions of published papers.[32,60] In addition, it can be seen from Figure that D+ is always greater than D–, which concludes that the cations can transport stronger than the anions over the whole temperature ranges for the EMITFSI/PVDF system. This is identical with the behavior of the BMI+-based IL with different anionic structures and TFSI–-based IL with different cationic structures.[35,84]

Figure 6

D– and D+ as a function of temperature T for the EMITFSI/PVDF system.

Figure a,b shows D as a function of τC–1 and τS–1, respectively. We can see that D– and D+ gradually increase with the increase of τC–1 and τS–1, respectively. Some papers have concluded that the power law, namely, D ∝ τC–γ and D ∝ τS–δ, exists in the pure IL, IL/PVDF, and polyIL systems.[32,85,86] Specifically, the ionic mobilities are seen to exhibit an “‘ionic”’ behavior when γ is equal to 1, and ionic mobilities are seen to exhibit a “superionic” behavior when γ is less than 1.[37,87] Interestingly, it can be seen from Figure that D also shows correlation with τC–1 and τS–1 for the EMITFSI/PVDF system, respectively. We can obtain D– – τC–0.93193 (black solid line), D+ – τC–0.80391 (red dashed line), D– – τC–2.43303 (blue solid line), and D+ – τS–1.85072 (green dashed line) from the power law. Obviously, all γ are less than 1, so the mobilities of anions and cations are seen to exhibit a superionic behavior for the EMITFSI/PVDF system.

Figure 7

EMITFSI/PVDF system: (a) D ∝ τC–1 and (b) D ∝ τS–1. The power law shows D– – τC–0.93193 (black solid line), D+ – τC–0.80391 (red dashed line), D– – τC–2.43303 (blue solid line), and D+ – τS–1.85072 (green dashed line).

The transference numbers of anions and cations in the EMITFSI/PVDF system are calculated by eq at different temperatures, as shown in Figure . We can obtain that the transference numbers of anions (t–) are in the range of 0.34–0.51, and the transference numbers of cations (t+) are in the range of 0.49–0.65. In addition, when the temperature is less than 360 K, t– and t+ decrease and increase, respectively, as the temperature increases, indicating that D+ increases faster than D–. However, when the temperature is greater than 360 K, the opposite result appears. On the whole, t+ is always greater than t–. It can be inferred that D+ is always greater than D–, which is consistent with the conclusion obtained from Figure .

Figure 8

t– and t+ as a function of temperature T for the EMITFSI/PVDF system.

When the temperature increases from 263 to 353 K, t+ is between 0.60 and 0.65 for the EMITFSI system.[88] We have used MD simulations to obtain that t+ is between 0.49 and 0.65 for the EMITFSI/PVDF system in the temperature range of 280–360 K. It can be seen that t+ decreases after adding PVDF into the EMITFSI system. In conjunction with Section , we can obtain the conclusion that the D+ decreases faster than D–.

Ionic Conductivity

In this paper, the ideal conductivity (σNE) and true ionic conductivity (σ) of the EMITFSI/PVDF system are calculated by eqs and 15, respectively, at different temperatures, and the results are shown in Figure , which indicates that σNE and σ increase as the temperature increases. It is reported that σ is between 0.016 and 0.389 S/m in the temperature range of 313–416 K for the EMITFSI/PVDF system.[82] This MD simulation shows that σ is between 0.0495 and 0.701 S/m in the temperature range of 320–420 K for the EMITFSI/PVDF system. We can see that the simulated value is close to the actual value. In addition, when the temperature increases from 263 to 353 K, σ of the EMITFSI system is in the range of 0.25–4 S/m.[88] When the temperature increases from 280 to 360 K, we used MD simulations to obtain that σNE is in the range of 0.0453–0.351 S/m and σ is in the range of 0.0208–0.158 S/m. Therefore, it can be concluded that the ionic conductivity decreases after adding PVDF into the EMITFSI system.

Figure 9

σNE and σ as a function of temperature T for the EMITFSI/PVDF system.

The degree of independent ion motion (α) is calculated by eq for the EMITFSI/PVDF system at different temperatures. As shown in Figure , we can observe that α has an irregular fluctuation as the temperature increases. With the increase of temperature, α is the largest at 300 K and the smallest at 380 K, indicating that the ion-correlated motion is the weakest at 300 K and the strongest at 380 K. This result can be explained by the binding energy between ion pairs in Section . Especially, with the increase of temperature, when the binding energy between ion pairs increases, the ion-correlated motion increases (α decreases), and vice versa. Therefore, we can conclude that α is mainly affected by the binding energy between ion pairs as the temperature increases. When the temperature increases from 263 to 353 K, α of the EMITFSI system is in the range of 0.7–0.8.[88] When the temperature increases from 280 to 360 K, α of the EMITFSI/PVDF system is in the range of 0.45–0.51 in this MD simulation. Therefore, it can be concluded that α decreases after adding PVDF into the EMITFSI system, which is good for mass transport but not charge transport. This is consistent with the results reported in some literature studies.[89,90]

Figure 10

α as a function of temperature T for the EMITFSI/PVDF system.

The volume of TFSI– is 0.230 nm3,[91] so we calculated its Stokes radius to be 0.380 nm. Viscosity (η) is calculated by eq for the EMITFSI/PVDF system at different temperatures. Figure presents η as a function of temperature T. With the increase of temperature, η of the EMITFSI/PVDF system decreases monotonously; specifically, it decreases rapidly first and then gradually decreases, which indicates that η is more sensitive to temperature changes at low temperatures. Therefore, as the temperature increases, the diffusion resistance of ions gradually decreases in the EMITFSI/PVDF system. When the temperature increases from 283 to 353 K, η of the EMITFSI system is in the range of 65–7 cP.[88] When the temperature increases from 280 to 360 K, we used MD simulations to obtain that η is in the range of 175.5–26.7 cP for the EMITFSI/PVDF system. We can obtain that η increases after adding PVDF into the EMITFSI system, which leads to greater ion diffusion resistance for the EMITFSI/PVDF system.

Figure 11

η as a function of temperature T for the EMITFSI/PVDF system.

Conclusions

In this paper, the EMITSFI/PVDF system with 40.05 wt % content of EMITFSI was selected to study the ion transport properties at different temperatures using MD simulations.

Our simulated Tg value is 204 K for the EMITFSI/PVDF system, which shows a good agreement with the experimental value (200 K). Therefore, the force field parameters are suitable for the EMITFSI/PVDF system in this MD simulation, and Tg increases after adding PVDF into the EMITFSI system. With the increase of temperature, there may be enough energy to overcome the repulsive force between ion pairs, which makes the distance between the ion pairs closer, so the peaks of PCF show an increasing trend for the EMITFSI/PVDF system. Especially, the binding energy between ion pairs is the main factor affecting the CN of ion pairs as the temperature increases.

We have used MD simulations to study the ion transport properties in the EMITFSI/PVDF system. First, we have found that τC and τS decrease as the temperature increases, so it is known that ion motion is intensified. Second, we can obtain that D– and D+ decrease after adding PVDF into the EMITFSI system, indicating that the transport of anions and cations becomes worse. With the increase of temperature, D– and D+ monotonously increase, indicating that the transport of anions and cations gradually increases, and D+ is always greater than D–, which concludes that the cations can transport stronger than the anions. The D – τC–1 and D – τS–1 show the universal correlation, and the power law shows that the mobilities of anions and cations are seen to exhibit a superionic behavior. Third, based on the diffusion coefficients, we have calculated the transference numbers of ions, ionic conductivity, and viscosity. As the temperature increases, t– first decreases and then increases, which indicates that D– increases first slower than D+, and then, D– increases faster than D+. Through the analysis of D and transference numbers of ions, we can get that D+ decreases faster than D– after adding PVDF into the EMITFSI system. Finally, σNE and σ increase as the temperature increases and decrease after adding PVDF into the EMITFSI system. α is mainly affected by the binding energy between ion pairs as the temperature increases and decreases after adding PVDF into the EMITFSI system, which is good for mass transport but not charge transport. In addition, η decreases as the temperature increases, indicating that the diffusion resistance of ions gradually decreases. η increases after adding PVDF into the EMITFSI system, which leads to greater ion diffusion resistance for the EMITFSI/PVDF system.