Low temperature precipitation of lithium ion battery was analyzed

The performance of lithium ion batteries is greatly affected by the kinetic characteristics. Because Li+ needs to be dissolvated first when embedded into graphite material, it needs to consume certain energy, which prevents Li+ from diffusing into graphite. On the contrary, when Li+ discharges graphite and enters the solution, it will first undergo a solvation process, which does not require energy consumption. Li+ can quickly dissociate graphite, leading to the fact that the charging acceptance of graphite is significantly worse than that of discharge [1].

At low temperature, the kinetic properties of the graphite anode become worse and worse, so the electrochemical polarization of the anode is obviously intensified in the process of charging, which easily leads to the precipitation of lithium metal on the surface of the anode. Christiane vonlu of the technical university of Munich in Germany. Ders, studies have shown that under - 2 ℃, the charging ratio more than 2 C/returned to significantly increase the amount of precipitation of lithium metal, such as C / 2 ratio, the number of the cathode surface plating lithium about 5.5% of the whole charge capacity, but under the 1 C rate will reach 9% [2]. The precipitated lithium metal may further develop and eventually become lithium dendrites, penetrating the membrane and causing a short circuit between the positive and negative electrodes. Therefore, we need to try to avoid lithium ion battery charging at low temperature, when the battery must be under the low temperature charging, need as far as possible choose low current lithium ion battery for charging, and in charge of lithium ion battery fully suspended, thus ensuring the cathode precipitation of metal lithium can react with graphite, to embed the graphite anode inside.

The technical university of Munich VeronikaZinth and others by using neutron diffraction, the lithium ion battery in the analysis of the lithium - 20 ℃ low temperature behavior were studied. Neutron diffraction method is a new detection method in recent years. Compared with XRD, neutron diffraction is more sensitive to light elements (Li, O, N, etc.), so it is very suitable for non-destructive testing of lithium ion batteries.

In the experiment, VeronikaZinth used NMC111/ graphite 18650 battery to study the lithium evolution behavior of lithium ion battery at low temperature. During the test, the battery was charged and discharged according to the process shown below.

The figure below shows the phase change of the negative electrode under different SoC during the second charging cycle at C/30 times. It can be seen that the phase of the negative electrode was mainly composed of LiC12, li1-xc18 and a small amount of LiC6 when the SoC was 30.9%SoC. After SoC exceeded 46%, the diffraction strength of LiC12 continued to decrease, while the LiC6 strength continued to increase. However, even when the charging was finally completed, only 1503mAh (room temperature capacity 1950mAh) was charged at low temperature, so LiC12 continued to exist in the negative electrode. If the charging current is reduced to C/100, the battery can still achieve a capacity of 1950mAh at low temperature, which indicates that the capacity reduction of lithium ion battery at low temperature is mainly caused by poor dynamic conditions.

Below for under - 20 ℃ low temperature, according to the C / 5 ratio in the process of charging, the cathode graphite phase change, you can see, compared with 30 charging rate, C/graphite phase change has obvious different, can see from the graph, in the SoC > 40%, the C / 5 battery charging ratio in LiC12 phase intensity decrease significantly slower, LiC6 phase intensity increases significantly weaker than the C / 30 charging ratio, this suggests that under the ratio of C / 5 relatively high, less LiC12 intercalated-li continuously, converted to LiC6.

The figure below shows the comparison of phase changes of graphite anode when charging at C/30 and C/5 magnification respectively. It can be seen from the figure that for two different charging magnification, li1-xc18 of lean lithium phase is very similar, and the difference is mainly reflected in LiC12 and LiC6 phases. It can be seen from the figure that the phase change trend of the negative electrode is relatively close under the two charging multiples at the initial stage of charging. For LiC12 phase, when the charging capacity reached 950mAh (49%SoC), the change trend began to differ. When the charging capacity reached 1100mAh (56.4%SoC), a significant difference began to appear in LiC12 phase under the two rates. When charging at a low rate of C/30, the LiC12 phase had a very fast falling speed, but the LiC12 phase had a much slower falling speed at a C/5 rate. Correspondingly, the LiC6 phase increased very rapidly at a small ratio of C/30, but at a much slower rate of C/5. This suggests that under the C / 5 ratio, less Li embedded in the graphite crystal structure, but it is interesting to note C / 5 charge ratio in battery charging capacity (mAh) 1520.5 rather than 30 charging ratio of C/capacity (mAh) 1503.5 but a little higher, the more not embedded in the graphite anode in Li are likely to be in the form of metallic lithium on the graphite surface precipitation, charging stand from the end of the process from the side also illustrates this point.

The figure below is the phase structure diagram of the graphite negative electrode after charging and after using it for 20h. It can be seen that at the end of charging, the phase of the graphite negative electrode is very different under the two charging rates. At high C/5 ratio, the ratio of LiC12 and LiC6 in the graphite negative electrode was higher, but the difference between them had become very small after 20h.

The figure below shows the phase change of the graphite negative electrode during the 20h shelving process. As can be seen from the figure, although there was a great difference between the two phases of the negative electrode at the beginning, with the increase of shelving time, the phase change of the graphite negative electrode under the two charging rates had become very close. In the process of using, LiC12 could also be transformed into LiC6 continuously, indicating that Li was still embedded in graphite during the process of using, and this part of Li was probably lithium metal precipitated on the surface of graphite anode at low temperature. Further analysis shows that at the end of charging at C/30 times, the degree of lithium embedded in the graphite negative electrode is 68%, but after the use of lithium embedded degree increased to 71%, increased by 3%. At the end of charging at C/5 times, the lithium embedded degree of the graphite negative electrode was 58%, but it increased to 70% after being used for 20h, a full increase of 12%.

When charging, the study showed that low temperature variation due to the dynamic condition, not only to cause a decline in the capacity of battery, also because the graphite intercalated-li speed is reduced, and the precipitation in the cathode surface metal lithium, although after a period of time aside, this part of the metal lithium can also be embedded in the graphite inside again, but in the practical use, are short on time, can not guarantee all of metallic lithium can be embedded in the interior of the graphite again, so it may cause metal lithium persist the cathode surface, not only affects the capacity of lithium ion batteries, and may produce endangering the safety of lithium ion batteries, lithium dendrite, Therefore, to avoid charging the lithium ion battery at low temperature as far as possible, it is necessary to use as little current as possible when charging at low temperature, and ensure sufficient shelving time after charging to eliminate the lithium metal with graphite negative electrode.

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