How are lithium ions transported in liquid and solid electrolytes?

Li+ transport mechanism in liquid electrolyte

In the liquid electrolytes we are familiar with (such as LiPF6), the transport of lithium ions is mainly through three methods:

• Migration;
• Diffusion;
• Convection.

As a non-contact force field, the electric field exists between charges and exerts a force on the charges. In a solution environment, the action of this electric field force causes the ions in the solution to begin to move in a directional manner, forming the so-called electromigration phenomenon.

At the same time, since the migration number of lithium ions in liquid electrolytes is less than 1 (usually between 0.2 and 0.4), PF₆^– in the solution will migrate and accumulate in the opposite direction, which will lead to the formation of a lithium ion concentration gradient. The concentration gradient will cause diffusion. In addition to migration and diffusion, convection is also an important factor causing lithium ion transmission, which is usually caused by external forces (such as gravity, pressure gradient or external mechanical force). The three transmission mechanisms are shown in the figure below:

According to the dilute solution theory assumption, the net flux density of lithium ions is:

Li+ transport mechanism in solid electrolyte

Unlike liquid electrolytes, the ion transport mechanism in solid electrolytes is significantly different. In a crystal, lithium ions must move from a stable lattice position (local energy minimum point) to an adjacent position. There is a “bottleneck point” with the highest energy between these two positions, like a mountain that must be climbed between two valleys. At the same time, lithium ions also need vacancies, that is, “landing points”, to move.

The ionic conductivity of a solid depends on the number of interstitial ions, vacancies and partially occupied sites at the lattice sites or gaps. The more defects there are, the more paths lithium ions can move through, and the higher the conductivity.

Interstitial ions and vacancies can be generated by substitution of unequal cations. For example, doping Ta⁵⁺ in LLZO (garnet electrolyte) forces the lattice to produce lithium vacancies, thereby improving the conductivity of lithium ions.

Lithium ion conductivity in a crystalline solid can be described by the product of the number of mobile lithium ions per unit volume, the square of the charge of each lithium ion, and the absolute mobility of the lithium ions. Lithium ion conductivity can be expressed as:

Here, E_A is the diffusion activation energy or migration energy.

The most typical garnet-type Li₇La₃Zr₂O₁₂ (LLZO) lithium-rich electrolyte is represented (see the figure below). It has a high lithium ion content and forms a continuous three-dimensional network channel, which is conducive to the transition and transmission of lithium ions. Therefore, the room temperature ionic conductivity can exceed 10^-4 S/cm. It is relatively stable to air and moisture, has a wide electrochemical window, and has good compatibility with lithium negative electrodes. It is considered to be one of the most attractive solid electrolyte materials.

Garnet-type Li₇La₃Zr₂O₁₂ (LLZO) lithium-rich electrolyte structure and three-dimensional lithium ion transport channel Li₇La₃Zr₂O₁₂

The three-dimensional vacancy hopping of lithium ions in solid electrolytes is essentially the result of the combined effects of crystal structure design (high symmetry, interconnected interstitial sites), defect chemistry regulation (doping-induced vacancies) and low-barrier dynamics (cooperative migration).

This mechanism not only breaks through the low conductivity limitation of traditional solid electrolytes, but also provides a theoretical basis for the design of high-energy-density and high-safety solid-state batteries.

Characteristics	Liquid electrolyte	Solid electrolyte
Transport carrier	Li+ solvation, migration, diffusion and convection, reverse migration with anions	Li+ single ion conduction, anion fixed, migration number ≈ 1
Activation energy	Low (mainly diffusion)	High (need to overcome lattice barriers)
Interface issues	Good liquid wettability, but easy to form dendrites	High solid-solid contact impedance, interface engineering required
Safety	Flammable (organic solvent)	High temperature resistance, dendrite suppression