How much do you know about the low-temperature technology of lithium-ion batteries? (1)-Performance

For lithium-ion batteries, whether it is a national standard or an enterprise standard, there is a strict limit on the lower limit temperature of discharge: not less than -20°C. As for the charging temperature, not only the minimum temperature will be specified like discharging, but also that it can only be charged at a low rate at low temperatures and cannot be fully charged (for example, only 0.2C can be charged at 0 to 15 degrees, and the upper limit voltage is 4.0. V), for fear that the user will overstep the thunder pool. So the question is, what users need is an all-around fighter who must take into account both high and low temperature performance, but why do lithium battery manufacturers set these harsh restrictions?

Let us start with the performance of lithium-ion batteries at low temperatures. For the discharge at the same rate, the lower the external temperature, the lower the discharge voltage of the lithium-ion battery. As shown in the figure below:

It can be seen from the figure that when the temperature decreases, the battery discharge voltage is also greatly reduced, so that the battery will reach faster when discharged at a low temperature Discharge cut-off voltage, resulting in low temperature discharge capacity significantly lower than normal temperature capacity. What needs to be explained to you is that the capacity of lithium-ion batteries at low temperatures does not disappear, but it can’t be fully discharged within the normal voltage range (≥3.0V). If the discharge cut-off voltage can be continued to be extended, then the remaining capacity can be reduced. The capacity is released. But the problem is that too low voltage can no longer maintain the normal use of electrical equipment, so too low lower limit voltage (<2.5V) generally does not have much discussion significance.

Compared with low-temperature discharge, the performance of lithium-ion battery low-temperature charging is even more unsatisfactory. First, charging at low temperature will quickly reach the constant voltage stage, and will reduce the charging capacity to a certain extent, while increasing The charging time is shown in the figure below:

Not only that, when the lithium-ion battery is charged at low temperature, lithium ions may not have time to be embedded in the graphite negative electrode, thereby depositing metal lithium on the surface of the negative electrode. Dendrites, this reaction will consume the lithium ions in the battery that can be repeatedly charged and discharged, and greatly reduce the battery capacity. The precipitated metal lithium dendrites may also pierce the separator, thereby affecting safety performance. The picture of lithium evolution on the negative electrode surface of lithium-ion battery after low-temperature charging is as follows: It can be restored after charging and discharging, which is a reversible capacity loss; but low-temperature charging will cause lithium evolution, which is a permanent capacity loss.

Because low-temperature charging is more harmful to lithium, the low-temperature charging of lithium-ion batteries is more stringent than the low-temperature discharge control. At present, many battery manufacturers use the 'step charging' method to describe the charging conditions of lithium-ion batteries. The following is a similar example:

The above description of step charging is for customers. It's a bit of a 'pitSince the voltage plateau of lithium-ion batteries is lower during low-temperature discharge, it will cause a significant decrease in discharge capacity at low temperatures, but this loss will be automatically compensated as the charge and discharge at room temperature proceed, which is a reversible loss. But for low temperature charging, too low temperature or too high rate will cause irreversible formation of lithium dendrites and irreversible capacity loss of the battery, and affect the safety performance of the battery.

Source: Zhixing Lithium Battery