The charge/discharge rate performance of lithium-ion batteries is directly related to the migration ability of lithium ions at the positive and negative electrodes, the electrolyte, and the interfaces between them. Any factor affecting the migration speed of lithium ions (these influencing factors can also be equated to the battery's internal resistance) will affect the charge/discharge rate performance of lithium-ion batteries. Furthermore, the heat dissipation rate inside the battery is also a crucial factor affecting rate performance. If the heat dissipation rate is slow, the heat accumulated during high-rate charge/discharge cannot be dissipated, severely impacting the safety and lifespan of the lithium-ion battery. Therefore, researching and improving the charge/discharge rate performance of lithium-ion batteries primarily focuses on improving both the lithium-ion migration speed and the internal heat dissipation rate of the battery.
1. Internal resistance of lithium-ion batteries
Conductive agents are typically added inside the positive electrode active material to reduce the contact resistance between active materials and between the active material and the positive electrode substrate/current collector, thereby improving the conductivity (ionic and electronic conductivity) of the positive electrode material and enhancing rate performance. Different materials and shapes of conductive agents will affect the internal resistance of lithium-ion batteries, thus affecting their rate performance.
The current collectors (tabs) at the positive and negative electrodes are the carriers through which lithium-ion batteries transfer electrical energy to the outside world, and the resistance of the current collectors has a significant impact on the rate performance of the battery. Therefore, by changing the material, size, lead-out method, and connection process of the current collectors, the rate performance and cycle life of lithium-ion batteries can be improved.
In addition, the liquid absorption rate and porosity of the separator also have a significant impact on the permeability of lithium ions, and will also affect the rate performance of lithium-ion batteries to a certain extent (relatively small).
The degree of wetting between the electrolyte and the positive and negative electrode materials in a lithium-ion battery affects the contact resistance at the electrolyte-electrode interface, thus impacting the battery's rate performance. Factors such as the total amount of electrolyte, viscosity, impurity content, and porosity of the positive and negative electrode materials all alter the contact impedance between the electrolyte and electrodes, representing an important research direction for improving rate performance.
2. Ionic conductivity of electrolytes
The ionic conductivity of an electrolyte, much like the resistance of water, significantly affects the swimming speed of lithium ions. Currently, the organic electrolytes used in lithium-ion batteries, whether liquid or solid, do not have very high ionic conductivity. The electrolyte's resistance becomes a crucial component of the overall battery resistance, and its impact on the high-rate performance of lithium-ion batteries cannot be ignored.
In addition to improving the ionic conductivity of the electrolyte, it is also important to pay attention to its chemical and thermal stability. During high-rate charge and discharge, the electrochemical window of the battery varies over a very wide range. If the electrolyte has poor chemical stability, it is prone to oxidation and decomposition on the surface of the cathode material, affecting the ionic conductivity of the electrolyte.
