The academic and industrial communities have been striving to achieve superior electrochemical performance in lithium-ion batteries, including rate capability, high and low temperature charge/discharge characteristics, and cycle life. However, from the perspective of mature products, there is always a "seesaw" problem: improvements in certain performance characteristics of lithium-ion batteries are often accompanied by decreases in others. How do we find a balance? Let's start by discussing the basic electrochemical characteristics of lithium-ion batteries.
When a lithium-ion battery is working, both electron conduction and ion migration occur simultaneously. Figure 1 shows a schematic diagram of the lithium-ion battery charging process.
Figure 1. Schematic diagram of electron transfer and ion migration processes during lithium-ion battery charging.
During charging and discharging, the positive electrode of a lithium-ion battery undergoes an oxidation reaction, and electrons are transferred through a conductive network such as a conductive agent to the current collector and then to the negative electrode. Lithium ions are extracted from the lattice of the positive electrode material and combine with solvent molecules in the electrolyte to form solvated lithium ions. Driven by the electric field and the difference in ion concentration, they pass through the separator to the negative electrode, gain electrons, undergo a reduction reaction, and embed themselves in the negative electrode material.
The discharge process, on the other hand, involves impedance formation during electron transfer through active materials, conductive agents, and current collectors, as well as during lithium ion diffusion in the solid phase and migration in the solution, leading to a drop in battery voltage. This manifests as electrochemical polarization, concentration polarization, and internal resistance loss. The formula for the operating voltage is shown in Figure 2.
Figure 2. Schematic diagram of lithium-ion battery operating voltage breakdown.
The impedance of a lithium-ion battery consists of three main parts: ionic impedance, electronic impedance, and interfacial impedance, which can be further subdivided into the following components:
Therefore, improving the performance of lithium-ion batteries focuses on reducing various internal impedances of the battery.
From a materials perspective, taking cathode materials as an example (Table 1), the diffusion coefficient and conductivity are related to the crystal structure. Lithium cobalt oxide and other 2D layered structures have high diffusion coefficients and good conductivity. In contrast, lithium iron phosphate materials with 1D unidirectional tunnel structures have low diffusion coefficients and poor conductivity. Comparatively, the former exhibits superior rate performance and a higher discharge platform.
Table 1 Properties of Commonly Used Cathode Materials
The following measures can be taken to improve the diffusion coefficient of cathode materials:
Doping – altering crystal structure parameters facilitates lithium-ion insertion and extraction.
Coatings that are conductive or ion-conductive facilitate ion transport.
Reduce particle size - reduce ion diffusion distance
The diffusion coefficient of the negative electrode material can be improved by:
Moderate oxidation
Metal deposition
Surface polymer or carbon coating
Boron doping
It is worth mentioning that reducing particle size is not feasible in anode materials because the specific surface area of the anode increases with decreasing particle size, leading to more Li consumption and the formation of the SEI layer.
The migration ability of lithium ions in the liquid phase is closely related to the solvent and lithium salt type of the electrolyte. The most important parameters of the electrolyte are dielectric constant and viscosity. The former reflects the ability to form solvated lithium ions, while the latter reflects the resistance to ion migration. Ionic conductivity can be improved by optimizing the type of electrolyte solvent and additives.
For the positive electrode, negative electrode, and electrolyte, the solid-phase or liquid-phase diffusion of lithium ions is greatly affected by the ambient temperature, conforming to the Arrhenius equation. Materials can be optimized and grouped for lithium-ion batteries used in different temperature ranges.
