How does lithium ion migrate in organic electrolytes?

Organic electrolyte

As shown in Figure 1, the electrolyte acts as a carrier inside the lithium battery which provides a transport path for ion transport between the positive and negative materials. Simply taking the charging process as an example, Li+ is removed from the positive active material, and the Li+ concentration on the surface of the solid phase particles of the positive electrode is lowered, so that a concentration difference occurs between the inside and the surface of the particle, so that the Li+ generates particles to diffuse from the inside to the outside. At the same time, Li+ generated by electrochemical reaction on the surface of the particles enters the electrolyte, and the local concentration of the interface region in the solution phase is increased, causing a difference in concentration inside the solution phase, resulting in diffusion and migration of Li+ from the inside to the outside. In the negative electrode region, since the negative electrode particles electrochemically react with Li+ in the electrolyte, Li+ in the solution phase is consumed, and the Li+ concentration in the solution phase is lowered, resulting in a difference in concentration, resulting in the generation of Li+ in the solution phase from the outside to the inside diffusion and migration.

At the same time, an electrochemical reaction occurs on the surface of the negative electrode particles, and Li+ is intercalated to cause a difference in concentration inside the particles, which causes Li+ to diffuse from the outside to the inside of the particles. At the separator, due to the difference in concentration caused by the positive and negative electrodes, Li+ in this region causes diffusion and migration from the positive electrode to the negative electrode, and the discharge process is opposite to the above process. It can be seen from the above process that the normal and efficient operation of the lithium battery is mainly determined by the migration of lithium ions inside the battery. The migration of lithium ions is restricted by the properties of the electrolyte, and the properties of the electrolyte are mainly affected by the following factors.

Lithium salt dissolution

The electrolyte consists of a solute and a solvent. The solute is generally selected from a liquid of a combination of a plurality of organic solvents. When LiPF6 is dissolved in the solvent, lithium ions and PF6 negative ions are formed. The dissolution of the lithium salt is closely related to the dielectric constant of the solvent, and the greater the dielectric constant, the stronger the solubility of the lithium salt. When lithium ions are completely surrounded by solvent molecules, the effect of negative ions on lithium ions is weakened, so-called dissolution occurs. For lithium salts, the larger the anion, the better the ionic conductivity of the electrolyte and its own dissolution, because the larger the anion, the easier it is to disperse its negative charge and prevent the pairing of cations.

2. Electrolyte viscosity

The viscosity of the electrolyte has an important effect on the movement of ions, and the lower the viscosity, the more favorable the movement of ions.

As described above, lithium ions are transported and transferred under the influence of the dissolution and viscosity of the electrode liquid. In formula 1, t+ is the number of transports, i+ and i- represent the current formed by the cation and the anion, respectively, it represents the total current, u± represents the mobility of the anion and cation, and D± represents the diffusion coefficient of the anion and cation.

In fact, the ionic resistance is not only related to anion and cation, but also related to the solvent. The number of ion migrations can be expressed by Equation 2:

Among them, TLi++ represents the number of lithium ion migration, ΔV is the polarization voltage, I(∞) is the steady state current after polarization, and Rb and Rct are the bulk resistance and charge transfer resistance.

The electrolyte of the single-phase solvent system is difficult to have both high conductivity and low viscosity. Therefore, the commonly used electrolyte solvent is formulated by a variety of solvents, such as a binary electrolyte. (Lithium salt) + (1-w) (solvent A) + w (solvent B), the lithium salt m unit is generally a molar concentration, mol / kg, and w is the mass fraction of the solvent. For unit electrolytes, there is no reliable theory to predict the viscosity and ionic conductivity of the electrolyte. Jones–Dole (JD) and Debye–Hückel–Onsager (DHO) have proposed two empirical formulas, Equation 3 and Equation 4.

Where μr is the relative viscosity, μ is the solution viscosity, μ0 is the pure solvent viscosity, C is the lithium salt concentration, A, B, and D are coefficients, Λ is the molar conductivity, and Λ0 is the molar conductivity in the infinite dilution state. S is a parameter that is affected by the physical properties of the solvent and the properties of the electrolyte, and C is the concentration of the solute, if the type of lithium salt and solvent changes, the empirical formula also needs to be modified. For mixed system electrolytes, the formula is more complicated.

Therefore, when a new multi-component electrolyte is configured, the performance of the electrolyte needs to be tested to be determined, and the pre-estimation cannot be performed. Although ionic conductivity has a great influence on battery performance, other factors such as the formation and performance of SEI are also very critical factors, and stability, toxicity, and the like of the electrolyte at high magnification should also be considered. In short, all factors related to the actual production application should be considered before considering the ionic conductivity parameters

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