The presence of a specific Coulombic interaction between the lithium cation and the separator membrane in lithium secondary batteries was proven in this study, through evaluating the mobilities and microviscosities of the mobile ions in electrolyte solutions outside and within the separator membrane. The magnitude of the interaction depends on the solvation structure of the lithium cation, whose net charge is affected by the type and number of solvating species due to their shielding effect. Lithium cations with larger or more strongly coordinated solvating species are less attracted to the membrane; therefore, they display higher mobility compared to weakly solvated cations that are more strongly attracted to the membrane. We confirmed that the mobility of lithium cations in the separator membrane is controlled by their solvated structure in the electrolyte, as well as by the surface charge of the separator membrane. This knowledge could lead to the systematic design of battery performance appropriate for the battery loading system.

Microviscosity components influencing the ionic mobility of lithium electrolyte solution in separator membranes were evaluated and the ionic mobility was correlated with membrane morphology. The microviscosity component responsible for the anion–cation Coulombic interactions (α) was anomalously large in low porosity membranes compared to that in the free electrolyte solution, owing to restricted ionic motion in the narrow pore spaces within the membrane. With increase in the membrane porosity, α diminished owing to the enlarged space and motional freedom for the ionic species, whereas βcation, which is responsible for the cation/membrane interactions, appeared. This behavior may be due to the specific charge on the walls of the pore spaces and the increased collision frequency for the ions. Consequently, the cationic mobility (Dcation) selectively decreased, whereas the anionic mobility (Danion) increased in high porosity membranes. Despite the positive correlation between the diffusion coefficient distribution width (σ) and averaged diffusion value for the anion and solvent species, a clear correlation between σ and Dcation was not observed, which is attributed to the fact that the cation is specifically affected by the membrane, perhaps via Coulombic interactions, as evidenced by the appearance of βcation. Controlling the microviscosities of the ions in the electrolyte by controlling the porous morphology of the separator membrane is significant for designing high-performing battery systems.

The interactions between the mobile ions and membrane substrate were investigated for lithium electrolyte solutions confined within the separator membrane in lithium batteries through NMR and electrochemical measurements as well as theoretical analyses. Fundamental dynamic properties such as ionic mobility, dissociation degree of the salt, and microviscosities of the ion, which are responsible for ionic mobility, were different for the electrolyte confined within the membranes compared with the corresponding values for the free electrolyte. We also found that the interactions between the ion and membrane substrate depend on the type of membrane. The results of this study suggest that the ionic mobility of the electrolyte in a lithium battery can be controlled by selecting separator membranes with suitable chemical or physical properties, which would be significant for improving the performance of the battery system.

The migration properties of cations and anions in lithium electrolyte solutions through separator membranes were evaluated on the basis of diffusion coefficients. The observed diffusion coefficients for the cationic (DLi), anionic (DF), and solvent species (DH) of the electrolyte solution in the membrane are lower than those of the free electrolyte solution. Two main effects are responsible for the reduction in D. One is the physical barrier effect of the membrane substrate. The magnitude of this effect depends on the size, configuration, and the total volume of the pore spaces which hold the solution. The other is a chemical interaction effect, which is associated with polar groups or sites in the membrane substrate. For example, DF values are anomalously lower than DLi values for solutions in polyvinylidene difluoride- (PVDF-) based membranes, leading to an increase in the apparent cation transport number. This would indicate that the anions selectively interact with the membrane substrate through a Coulombic effect contributed by the PVDF chemical structure. These results suggest that separator membranes could be intentionally designed to control the structural stability and mobility of ionic species in the electrolyte, which underpin the output performance of battery systems.

We prepared new lithium polymer gel electrolytes with Lewis acid ionic groups on the PVB polymer chain (ionic PVB gel) with the goal of attracting anions to restrict their mobility. The cation and anion diffusion coefficients (Dcation and Danion), dissociation degree of the salt (x), interactive forces between the ion and polar groups on the polymer (βcation and βanion), and interactive forces between the cation and anion (α) were estimated based on measurements of the diffusion coefficients (DLi, DF, and DH) and ionic conductivity (σ). The Dcation and Danion of the ionic PVB gel increased and decreased, respectively, compared to the values in the original PVB gel that did not contain an ionic group, which led to the enhanced cation transport number. The interactions between the mobile ions and polar groups depended on the solvation structure of the lithium cation, Li(EC)n+, which is associated with the polarity of the gel solvent. The solvating EC species around Li+ is the barrier for weakening Li+/OH– and Li+/TFSI– interactions and consequently activating the TFSI–/ionic group interaction.

Ionic mobility of electrolyte materials is essentially determined by the nanoscale interactions, the ion–ion interactions and ion–solvent interactions. We quantitatively evaluated the interactive situation of the lithium polymer gel electrolytes through the measurements of ionic conductivity and diffusion coefficients of the mobile species of the lithium polymer electrolytes. The interactive force between the cation and anion in the gel depended on the mixing ratio of the binary solvent, ethylene carbonate plus dimethyl carbonate (EC/DMC). The gel with the solvent (3:7 EC:DMC) showed minimal cation–anion interaction, which is the cause of the highest ionic mobility compared with those of the other gels with different solvents. This suggests that the cation–anion interaction does not simply depend on the dielectric constant of the solvent but is associated with the solvation condition of the lithium. In the case of the gel with the 3:7 EC/DMC solvent, most of the EC species strongly coordinate to a lithium ion, forming the stable solvated lithium, Li(EC)3+, and there are no residual EC species for exchange with them. As a result, the solvating EC species would be a barrier that restricts the anion attack to the lithium leading to the smallest cation–anion interaction. On the other hand, interaction between the cation and polar sites, hydroxyl and oxygen groups of ether of the polyvinyl butyral (PVB) and polyethylene oxide (PEO) polymer, respectively, in the gels was another dominant factor responsible for cation mobility. It increased with increasing polar site concentration per lithium. In case of the PVB gels, cation–anion interaction increased with an increasing polymer fraction of the gel contrary to the independent feature of PEO gels with the change of the polymer fraction. This indicates that the cation–anion interaction is associated with the polymer structure of the gel characterized by the kind and configuration of polar groups, molecular weight, and network morphology of the polymer.

Conduction properties of the proton conductive polymer gel electrolytes were investigated by the measurements of the sulfonicacid site concentration, proton concentration after the dissociation from the site, proton conductivity, and diffusion coefficient of the 1H species. The proton mobility, μH+, estimated from the conductivity and proton concentration, as a function of the water fraction of the gel, showed a change with a maximum. At the higher water content over the maximum point, μH+ converged to the proton mobility in the bulk water. This changing feature reveals that there is an optimum concentration of the gel ([H+]/[H2O]) responsible for the μH+ maximum. A specific region at the interface between the polymer chains and water molecules could be formed which permits the efficient proton hopping. This is confirmed by the measurement of the 1H relaxation time, T1. Correlated with the μH+ increase with increasing the water content of the gel, T1’s increase is restricted, showing the T1-flat region. This anomalous region would be associated with the structural features of the polymer. One is the nanolevel feature, concentration of the acid site at which the sulfonate group after the proton dissociation would play the part of the base for keeping the water molecules close to the chains. The other is the macroscopic aspect of the polymer, the free volume space of the polymer molecules. Associated with the size and form of the space, the layered region of the water molecules around the polymer chains could be formed, which is effective for proton hopping transport taking advantage of the mobile water and sulfonate groups.