About Li-ion battery voltage analysis

The first thing we want to know is: What is the relationship between the voltage of a full battery and a half battery?

This question is actually not difficult to understand: the full battery is composed of two half-cells in the positive and negative poles, and at the same time there is the condition that the positive electrode voltage>the negative electrode voltage. Therefore, the full battery voltage is composed of positive and negative electrodes. It is obtained by subtracting the voltage of the negative two half-cells.

Dynamic picture of the relationship between full-cell voltage and half-cell voltage:

In the above animation, we will take the discharge curve of the lithium cobalt oxide half-cell The charging curve of the graphite half-cell is drawn in the same figure. The discharge of the lithium cobalt oxide half-cell is a lithium ion intercalation process, and the charging of the graphite half-cell is a lithium ion deintercalation process. Together, it happens to be a full-cell discharge reaction. . The figure clearly shows the relationship between the full-cell voltage and the half-cell voltage.

Of course, the full-battery discharge curve fitted by this method is slightly different from the full-battery discharge curve we measured. The main manifestation is that the fitted curve has a higher discharge platform. The reason is related to the following points: 1) The areal density of the whole battery is often larger, and the size of the pole piece is also larger, so the voltage platform is lowered; 2) The first time the efficiency of the positive and negative electrodes of the whole battery is different, and there is a negative electrode. Excessive, so lithium ions will not be 100% inserted and deintercalated in the positive and negative electrodes, which will have a certain impact on the voltage platform of the full battery; 3) The positive and negative electrode formula and compaction of the full battery may be different from the half There are differences during battery testing.

For a full battery, we naturally hope that the higher the voltage, the better, because with the same capacity, the higher the voltage, the more energy it can provide. Now that we know that the full battery voltage is obtained by subtracting the positive and negative half-cell voltages, it is theoretically possible to increase the full battery voltage by looking for higher voltage cathode materials and lower voltage anode materials.

Is this idea feasible? The direction is indeed correct, but the actual operational space is already very small. For the positive electrode, too high voltage will cause the decomposition of the electrolyte solvent. Unless the solvent is greatly improved, it is difficult to promote. As for the negative electrode, the possibility of further reducing its voltage is very slim. The reason is that the current deintercalation lithium potential of carbon-based materials is only about 0.2V higher than the potential of lithium ions to form metallic lithium. If a negative electrode material with a lower potential than lithium ions precipitates into metallic lithium is used, it will definitely cause lithium ions during charging. After arriving on the surface of the negative electrode, it precipitates directly on the surface of the negative electrode instead of being embedded in the negative electrode.

We can imagine the full battery charging process as a 'lithium ion step down' process: during charging, the reaction voltage of lithium ion deintercalation from the positive electrode is very high, we compare it to a height of 3.7 The step of V; lithium ions are faced with two choices after being deintercalated from the positive electrode. One is to insert the negative electrode, the height corresponding to this step is 0.2V (that is, the average voltage of the carbon negative half-cell), and the second is to extract lithium, which corresponds to this step. The height is 0V. But lithium ion is a coward. When it finds that there are two options for the next step, it will preferentially choose to fall on a step with a smaller height difference. Therefore, lithium ions extracted from the positive electrode will preferably be embedded in the carbon negative electrode. . Even if we can find a negative electrode with a lower lithium insertion potential, it will be difficult to apply because it will greatly increase the risk of lithium ion precipitation in the negative electrode.

Low-temperature charging will cause lithium precipitation in the negative electrode. The principle is the same as described in this article: when charging at low temperature, the polarization of the negative electrode increases. The potential of lithium ions inserted into the negative electrode is lower than usual. When its value is lower than the lithium ion precipitation potential of 0V, lithium ions will be directly precipitated. The following is the voltage change curve of the full battery and the voltage change curve of the graphite half-cell when the full battery is charged at room temperature and low temperature:

The above two full battery curves are easy to understand: when the full battery is charged at a low temperature, due to the increase in polarization, the charging voltage is higher than the normal temperature. This is what we have talked about in the low temperature series of articles. Here are the two graphite voltage curves in the figure: when the temperature is lower, the increase in polarization will cause a decrease in the lithium intercalation potential of graphite (the closer the lithium evolution reaction potential is to 0V, the greater the risk of lithium evolution). When charging to about 50%, the potential drops suddenly and is lower than 0V. At this time, the cowardly of lithium ions feels that the direct precipitation will make the reaction voltage span smaller, so if you continue to charge the battery at this time, lithium will be significantly separated. Look, the phenomenon of lithium precipitation in the negative electrode can actually be explained by the law of half-cell voltage change.

My friends all know that the safety of lithium titanate anode is much higher than that of graphite anode. The reason is that the voltage (1.5V) at which lithium ions are inserted into lithium titanate is much higher than the lithium extraction voltage (0V) After lithium ions come to the negative electrode of lithium titanate, it is found that the voltage span generated by intercalation of lithium titanate is much smaller than that of lithium evolution. The coward will naturally choose a more 'safe' path (intercalation of lithium titanate), so there is almost no Lithium evolution will occur.

Summary: The full battery voltage is obtained by subtracting the positive and negative half-cell voltages; when the full battery is charged, when lithium ions are released from the positive electrode to the negative electrode, the path with a small voltage span is preferred for reaction Therefore, the negative lithium insertion voltage caused by various conditions is too low, which can be said to be the direct cause of lithium precipitation.

Source: Zhixing Lithium Battery