How much do you know about the low-temperature technology of lithium-ion batteries? (2)-Principles

by:CTECHi     2021-07-08

The reason for the poor low-temperature performance of lithium-ion batteries can be explained from two perspectives: the material perspective and the electrochemical perspective. Let's talk about the interpretation of the material perspective first.

For the electrolyte, the solvent is mainly composed of some cyclic esters and chain esters. The common feature of these solvents is that the fluidity will become worse at low temperatures, and some electrolytes may even Partially solidified at -30℃~-40℃. As a result, the conduction speed of lithium ions in the electrolyte at low temperatures will slow down, thereby reducing the low-temperature charge and discharge performance of the battery. The following figure lists the melting points of solvents commonly used in electrolytes. The lower the melting point, the solvent is more suitable for use at low temperatures:

For positive and negative materials, lithium The discharge process of an ion battery is the process in which lithium ions are extracted from the negative electrode and inserted into the positive electrode. When the battery is discharged under low temperature conditions, the impedance of lithium ions from the negative electrode and the resistance to the positive electrode will increase, thereby increasing the overall reaction. resistance.

At this time, a careful friend may ask: What is the reason that low-temperature charging will cause irreversible lithium evolution? Contrary to the discharge process, the charging process is a process in which lithium ions are extracted from the positive electrode and inserted into the negative electrode. At low temperatures, the impedance of lithium ions inserted into the negative electrode at low temperature will increase sharply, and is so large that lithium ions would rather be directly deposited on the surface of the negative electrode , It will not be 100% embedded inside the negative electrode. We can also make a qualitative understanding of the phenomenon of lithium evolution in the negative electrode charged at low temperature: for the negative electrode material, its 'original state' is the state in which no lithium ions are intercalated. When charging at low temperature, the negative electrode has obvious The trend of maintaining the original state makes it more difficult to insert lithium ions.

After the introduction of the material perspective, the editor will give you some long-know principles. Please be patient and read on. Taking discharge as an example, the 'Performance' has introduced to you that the voltage of low-temperature discharge will be significantly lower than that of normal temperature. So what is the electrochemical principle that produces this phenomenon? The answer is the increase in 'polarization' of the battery when discharged at low temperatures. Polarization in a battery refers to the difference between the battery's equilibrium state (when it is left) during charging and discharging. Daily charge and discharge will cause the polarization of the battery. One of the results of polarization is to produce a voltage different from the equilibrium state. For example, a battery with a balanced state voltage of 3.9V will instantly drop to about 3.8V when discharged at 0.5C at room temperature, and will drop momentarily at 0.5C at low temperature. To about 3.7V, the corresponding two pressure differences (3.9V-3.8V at normal temperature and 3.9V-3.7V at low temperature) are the result of polarization, and the name of the pressure difference is called overpotential.

When the battery is charged and discharged from the rest state, the change in voltage is called the overpotential, and the magnitude of the overpotential is a measure of the degree of polarization.

Polarization in a battery can be divided into two parts. The first part is called electrochemical polarization, which is mainly caused by the resistance of electrochemical reactions. For lithium-ion batteries, the factors that cause electrochemical polarization include the resistance encountered by the conduction of lithium ions in the electrolyte, the resistance encountered by the insertion and extraction of lithium ions in the positive and negative electrodes, current collectors and tabs. The resistance and so on. When a lithium ion battery is charged and discharged at low temperature, the conduction speed of lithium ions in the electrolyte will decrease, and the resistance of insertion and extraction in the positive and negative electrodes will also increase. Therefore, the electrochemical polarization of low-temperature charging and discharging will increase significantly compared with normal temperature. Big.

Another cause of polarization is concentration polarization. Take the low-temperature discharge of a lithium-ion battery as an example. This is a lithium ion extracted from the negative electrode into the electrolyte and inserted from the electrolyte. The process to the positive electrode. In this process, the electrolyte near the negative electrode has a higher lithium ion concentration due to the continuous entry of lithium ions extracted from the negative electrode, while the electrolyte near the surface of the positive electrode has a lower lithium ion concentration on the contrary In the process of continuous discharge, the concentration difference of lithium ions in the electrolyte near the positive and negative electrodes continues to exist. Because lithium ions are positively charged, in order to continue to maintain this difference in lithium ion concentration, it is necessary to continue to apply An extra voltage, this extra voltage is the result of concentration polarization. And as the temperature decreases, the diffusion of lithium ions in the electrolyte will be slower, so this concentration difference will also become greater, resulting in greater polarization. The schematic diagram of the concentration polarization during discharge is as follows:

The polarization in the battery is composed of electrochemical polarization and concentration polarization.

Summary: The reasons for the poor performance of lithium-ion batteries at low temperatures are mainly due to the decrease in the ion conductivity of the electrolyte at low temperatures from the material point of view, and the extraction and insertion of lithium ions in the positive and negative electrodes at low temperatures. The increase in impedance; from an electrochemical point of view, it is the increase in battery polarization at low temperatures, and polarization can be divided into two parts: electrochemical polarization and concentration polarization.

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