Source: lw • Author: New Energy Leader

       The performance of lithium-ion batteries is greatly affected by temperature. High temperatures will aggravate the side reactions at the interface of the positive and negative electrodes, causing accelerated decline of lithium-ion batteries. Low temperatures will cause the deterioration of the dynamic conditions of lithium-ion batteries, which will cause the decline of battery performance, especially Charging at low temperature may cause the metal Li to precipitate out of the negative electrode, resulting in low battery coulombic efficiency. In severe cases, it may even cause a short circuit of the positive and negative electrodes, causing a safety accident.

       The 18650 battery is a very widely used lithium ion battery structure. The 18650 batteries introduced by different manufacturers are usually similar in structure, but there is one thing, different manufacturers have obvious differences. This is the steel core in the middle of the battery core. Manufacturers will use steel cores, but others do not. The steel core is usually added to support the battery core and avoid the collapse of the battery core during cycling, which affects the battery performance. Some studies have shown that when the 18650 battery thermal runaway occurs, the steel core can prevent the battery core from collapsing, which is beneficial to the gas discharge, thereby avoiding the battery The accumulation of internal pressure causes the battery to explode. Recent studies by Rachel Carter (first author) and CoreyT. Love (corresponding author) of the US Navy experiment have found that the 18650 battery steel core also has a positive effect on improving the low-temperature performance of lithium-ion batteries.

The 18650 battery used in the experiment has a steel core and a steel core. The positive electrode system of the battery with steel core is NMC532 mixed with LCO, and the negative electrode is graphite. The positive electrode of the battery without steel core uses NMC532 material and the negative electrode is Graphite, the capacity of both batteries is 2.6Ah, the structure of the two batteries is shown in the figure below.
The figure below shows the cycle (0.5C rate) performance curve of two 18650 batteries. From the figure below, we can see that both batteries show very good cycle performance at room temperature. After 90 cycles of the battery without a steel core The capacity declines by 5%, and the battery with steel core declines to 3%, but if we control the temperature of the battery cycle to 0 ℃, there is a clear gap between the two batteries. After 90 cycles of the battery without the steel core The capacity decline is over 35%, while the capacity decline of the battery with steel core is only about 7%.

It can be seen from the changes in the coulombic efficiency of the two batteries in the following figures b and c during the cycle. The coulombic efficiency of the battery without a steel core is obviously divided into three stages during the cycle (Figure b below). The first stage is 0-40 times, the Coulomb efficiency is relatively stable, but in the second stage (40-80 times), the batterys Coulomb efficiency begins to decline linearly. After 80 cycles, it enters the third stage, and the batterys Coulomb efficiency exhibits nonlinearity. Decline. In contrast, the battery with a steel core maintained a stable Coulomb efficiency of 99.95% throughout the cycle, without significant degradation.

Yang et al.s research found that lithium precipitation at the negative electrode often occurs at the same time as rapid capacity decay and Coulomb efficiency reduction. Therefore, from the above test results, the 18650 battery without steel core may appear obvious in the third stage In the second stage, there may be a slight anode lithium precipitation.

The following figure a is the change of the charge and discharge curve of the battery without a steel core at room temperature and 0°C, where the dotted line is the change of the charge and discharge curve of the battery during the battery cycle at normal temperature. From the figure, you can see that these curves during the cycle No obvious changes have occurred, indicating that the battery declines little during normal temperature cycling. The solid line is the voltage curve of the battery during cycling at 0°C. From the figure, it can be seen that there is basically no significant decline in the cell during the first 40 cycles, and then the decline in battery capacity begins to accelerate significantly, and the battery The polarization has also increased significantly.

In order to further analyze the decay mechanism of batteries without steel cores at low temperature cycles, the author used charge and discharge data to draw the capacity difference curves of the battery at different cycle periods, where each peak represents a reaction, as can be seen from the figure. The characteristic peaks of the batterys reaction did not change significantly in the first 40 cycles. In the process of cycling to 40-80 times, we can see that the intensity of the characteristic peak of the reaction began to decrease significantly, and the position of the reaction peak also began to shift slightly, indicating that the loss of active material began to appear at this time. After 80 cycles, the intensity of the reaction characteristic peak of the battery continued to decrease, and the reaction peak began to shift significantly, indicating that at this time the battery began to lose active material and active Li at the same time.

The following figure c is the change of the batterys polarization voltage when the battery is discharged. From the figure, you can see the initial stage of the cycle. Because the batterys temperature is low, the batterys polarization has increased by 0.02V compared to normal temperature, but in the second stage The polarization of the battery increased significantly, indicating that at this time, due to the collapse of the battery, part of the active material was lost, and in the third stage, due to the simultaneous loss of active material and loss of active Li, we can see the polarization of the battery Further increase.

Therefore, it is not difficult to see from the above analysis that the battery without a steel core is divided into three distinct processes during the low-temperature cycle. In the first process, the polarization of the battery is significantly increased due to the effect of low temperature. The second process The cell collapses and deforms toward the middle round hole, causing the loss of the positive and negative active materials; in the third process, the negative electrode begins to deposit a large amount of lithium, which is accompanied by the loss of active material. The following picture is the authors CT scan image of an 18650 battery without a steel core. From the figure, it can be seen that the shape of the cell before the cycle is regular, but after the 0°C cycle, the cell collapses to the central circular hole. High-resolution analysis of the collapsed area can find that the positive electrode has obviously peeled off, and the negative electrode has also suffered severe damage, which is basically consistent with our previous analysis results. Generally, we believe that the precipitation of metallic lithium in the negative electrode will cause deformation of the negative electrode or flaking of the active material, and the loss of Li element in the positive electrode will also lead to a decrease in mechanical properties, which is more likely to cause particle breakage and electrode flaking.

The following figure A shows the battery voltage curve of the 18650 battery with a steel core during normal temperature and low temperature cycling. It can be seen that the capacity decline of the battery is relatively small at normal temperature and low temperature, and there is no obvious impedance increase. . It can also be seen from the voltage difference curve that there is no obvious change in the reaction characteristic peak of the battery during the entire low temperature cycle, and in terms of voltage polarization, we see that except for the low temperature, the battery voltage is polarized by 0.06V In the cycling process, the voltage polarization of the battery hardly increased, indicating that the 18650 battery with a steel core has very stable cycling performance at low temperatures.

We analyzed the structural stability of the 18650 battery with a steel core structure before and after low temperature cycling through CT scanning. We found that the 18650 battery with a steel core structure did not collapse inwardly after a 0°C low temperature cycle. The structure of the battery cell remains intact.

The research by Rachel Carter et al. shows that the steel core structure inside the 18650 battery can play a very good role in supporting the battery core and prevent the battery core from deforming and collapsing, especially when the battery is cycling at low temperature. A certain amount of strain accumulates inside the core, making the battery more prone to structural deformation, and the presence of the steel core suppresses this deformation, thereby reducing the loss of active material caused by the deformation of the battery core and achieving the purpose of improving the low-temperature performance of the 18650 battery.