Short circuit phenomenon in symmetric batteries in lithium metal battery research!

In the current research on the modification of lithium metal negative electrodes, the most commonly used method to evaluate the lifespan of lithium metal negative electrodes and determine the modification effect is the cycle life assessment of Li/Li batteries. The analysis of test results generally considers factors such as overpotential during lithium deposition, stability of voltage response during cycling, and cycle life in the event of short circuit signals.
However, the criteria and criteria for judging short-circuit signals are still not unified, and there are still some cases of misjudgment in previous research. For example, in symmetric lithium metal batteries, due to the activation process that exists in the early stages of cycling, the response potential shows a gradually decreasing trend, which is easily confused with the short circuit process, especially the soft short circuit process caused by only local dendrite piercing. The misjudgment in the conclusion of whether dendrites are generated or not leads to inaccurate evaluation of short-circuit life, making it difficult to objectively demonstrate the effect of negative electrode modification. This article summarizes and studies the short-circuit phenomenon during the cycling process of lithium symmetric batteries, aiming to explore the judgment basis of short-circuit signals and provide reference for the evaluation of electrode life in lithium metal negative electrode research.
Experimental section
The experiment uses lithium sheets purchased from Tianjin Zhongneng Lithium Industry, with a thickness of 100 μ m. Stamped into circular plates with a diameter of 13mm for assembling button batteries. The assembly of the buckle type battery (CR2032) is carried out entirely in a glove box. The electrolyte is an EC/DEC/DMC solution of 1 mol/L LiPF6. The constant current charge and discharge test was conducted using Wuhan Blue Electric CT2001A, in situ optical cell observation using HIROX RH-2000, and the external electrochemical workstation was Biologic VMP3.
Results and Discussion
Physical processes in symmetric batteries
The button type lithium symmetric batteries currently used in research have both positive and negative electrodes made of lithium foil. When using liquid electrolytes, the positive and negative electrodes are separated by a separator and immersed in the electrolyte as a whole; Solid electrolytes serve as both separators and electrolyte separators for positive and negative electrodes. Due to the fact that the positive and negative electrodes in a symmetrical battery are of the same substance, the open circuit potential of the assembled battery should be 0V. However, in actual testing, due to the cleanliness of the positive and negative battery shells, the pressure of battery assembly, and the accuracy of battery testing equipment, the actual open circuit potential tested may be displayed within positive and negative 10mV. When evaluating the lifespan of electrodes, a constant current is used to charge and discharge the battery, causing lithium to continuously deposit and dissolve between the positive and negative electrodes. After a certain number of cycles, the deposited lithium metal exhibits dendritic structures, piercing the diaphragm and causing a short circuit. During this process, the decrease in overpotential of the battery (often a sudden drop) is often used as a basis for determining the cycle life of the lithium negative electrode.
As shown in Figure 1, the cycling process of lithium symmetric batteries can be decomposed into the deposition and dissolution process of lithium metal on the positive and negative electrode surfaces. Taking the charging process as an example, an external power source forcibly increases the positive electrode potential, reduces the negative electrode potential, injects electrons from the positive electrode into the negative electrode, causing the negative electrode potential to decrease, while the positive electrode loses electrons and the potential increases; The lithium metal on the positive electrode surface loses electrons and dissolves to form lithium ions, while the lithium ions on the negative electrode surface receive electrons and undergo deposition. The above reverse process occurs during discharge. Due to the symmetry of the battery, a completely consistent physical process occurs during charging and discharging, except for the deposition and dissolution of lithium metal on different electrode surfaces. During the cycling process, the voltage signal reflects the potential difference between the positive and negative electrodes, which comes from the difference in overpotential between lithium dissolution and lithium deposition.

The fresh lithium metal surface formed during the deposition process continuously reacts with the electrolyte, generating more SEI on the surface. The encapsulation of lithium by SEI results in some lithium not having electrochemical activity. When this electrode dissolves under reverse current, the overpotential will continue to increase. Only when the voltage reaches the overpotential of fresh lithium on the electrode surface, will lithium continue to dissolve. In the subsequent charging and discharging cycle, the positive and negative poles will continuously repeat the above process. During the charging and discharging cycle, the voltage changes in each cycle are similar, and in the initial stage of the cycle, the continuous decrease in the average voltage value per week may be due to the decrease in overpotential caused by the electrode activation process.

As shown in Figure 1, in the initial stage of the symmetrical battery cycle, there is a gradually decreasing overpotential, but the overall shape of the voltage signal remains consistent. At the end of charging or discharging, there is a clear voltage rise signal caused by polarization. If there is a sudden drop in voltage during the cycling process, it indicates that there is a short circuit caused by lithium dendrite piercing inside the battery, causing direct contact between the positive and negative electrodes, thus reducing the potential difference. It should be noted that the short circuit process can be an irreversible hard short circuit, followed by a constant low voltage signal (close to 0V), which depends on the current applied during the cycle. As the battery at this time acts as a resistor, the voltage value is linearly proportional to the current value; The short circuit process can also be a soft short circuit caused by local contact, and this form of short circuit is reversible to a certain extent. When a soft short circuit occurs, although the dendrites come into contact with the opposite electrode after piercing the diaphragm, in the subsequent process, as the opposite electrode dissolves or the dendrites detach from the electrode, the positive and negative poles no longer come into contact, both can manifest as the recovery of the short circuit. At this point, the voltage signal will experience a slight decrease, corresponding to the moment of contact between the positive and negative poles due to dendrites. Afterwards, the voltage will gradually change on a new platform, but there will be no obvious sudden drop; Even a certain degree of voltage rebound can occur, and then maintain stable changes.

This article will focus on the identification of recoverable short circuits, providing a basis for determining whether modified lithium dendrites grow and whether the modification is effective.
Visualization of 2-symmetric battery short circuit
To further elucidate the voltage signal changes and corresponding physical processes of lithium symmetric batteries during cycling, we observed the lithium deposition dissolution process through in-situ optical batteries. The positive and negative electrodes of the in-situ battery are both lithium metal, and the specific composition and structure can be found in our previous research work. As the applied current changes, the voltage response of the battery corresponds to the physical process under an optical microscope, as shown in Figure 2. During the charging process, as lithium deposition proceeds, a large amount of dendritic lithium metal forms on the negative electrode surface (Figure 2b). The formation of dendrites consumes a large amount of electrolyte, and the generated gas continuously increases the volume of bubbles inside the battery (a → d); And due to the growth of dendrites, the actual surface area of the electrode is increased, and the overpotential shows a significant downward trend (a → b); During the subsequent discharge process, some of the lithium dendrites on the negative electrode surface dissolve, and the remaining "dead lithium" no longer has electrochemical activity and does not participate in the dissolution process. However, the external circuit continues to provide electrons. Therefore, after certain activation, the lithium in the negative electrode that did not participate in the reaction crosses the potential barrier generated by the dissolution potential and enters the stage of easier dissolution (b → c); As the cycle progresses, this signal alternates between positive and negative poles (a → c, c → e).

After two cycles, obvious dendrites or dead lithium formed on the surfaces of the electrodes on both sides (Figure 2e). At this point, lithium deposition is continuously carried out on one side of the negative electrode at a higher current, and the dendrites grow rapidly and extend to the other side, making contact with the positive electrode. The voltage signal shows a significant decrease (Figure 2f), corresponding to a short circuit caused by positive and negative electrode contact. At this point, although the battery has already experienced a short circuit, due to the continuous dissolution of the positive electrode at the point of dendrite contact, the contact area has not further expanded. It is worth noting that the voltage signal returns to normal after a sudden drop, corresponding to the contact position disconnection caused by the dissolution of the positive electrode. This indicates that in the testing of symmetrical batteries, if a voltage drop occurs and the signal returns to normal, a short circuit has actually occurred inside the battery. After recovering from the short circuit, after a certain period of lithium deposition, new contact points were formed between the positive and negative electrodes again, and a step like drop in voltage occurred (Figure 2g), corresponding to the short circuit caused by the new contact position. During the g-h process, the growth of dendrites caused continuous contact between the positive and negative electrodes, so the voltage signal could not be restored to normal.
The above results indicate that the potential drop caused by local short circuits may be extremely weak, and may not even affect the continued cycling of the battery. However, the short circuit caused by internal dendrites has already occurred, posing a threat to the safety of the battery. The previous research results of Bai et al. (21) also demonstrated the voltage reduction and dendrite growth caused by soft short circuits, indicating that such short circuits have a certain destructive effect in lithium symmetric batteries, which is of great significance for predicting the growth of dendrites. Therefore, in the cycling process of a symmetrical battery, if there is a voltage signal disorder or a small sudden drop, it means that there are lithium dendrites inside and a short circuit phenomenon has occurred. Even if the subsequent signal displays normally, it should be considered that the negative electrode has not achieved the modification effect of "no dendrites". This is particularly important for determining the effectiveness of lithium metal negative electrode modification.

3 symmetrical battery short circuit signal
In the published research work, some research results have obvious analytical errors in determining the timing of short circuit occurrence during charging and discharging processes, such as: a short circuit has occurred but is interpreted as an activation process; Or a soft short circuit may have occurred, but it is still believed that the battery is stable and the electrode has not produced dendrites. This article analyzes the possible misinterpretation of symmetric battery cycle data, providing some reference for identifying whether symmetric batteries have short circuits. As shown in Figure 3, symmetrical batteries exhibit two different forms of short circuits during a single charging process.

The battery in Figure 3a experiences a sudden voltage drop (as low as 20mV) during the charging process, after which the voltage remains low and unchanged. This is a typical hard short circuit, corresponding to the situation in Figure 3c where the lithium dendrites pierce the separator and the two poles come into direct contact. The voltage of the battery in Figure 3b decreases and then rebounds during the charging process, and changes repeatedly. The overall response signal is chaotic, but the voltage value ultimately remains around 0.3 V, which is a typical soft short circuit. It corresponds to the short circuit recovery caused by lithium dendrites falling off or contact point disconnection in Figure 3d.

Compared with the battery cycle data in previous studies, a large number of modified negative electrodes exhibit voltage response signal disturbances similar to those in Figure 4 during the cycling process, and the subsequent voltage response lacks diffusion polarization process (in liquid electrolyte), with voltage values as low as tens of millivolts (depending on current). This indicates that the process described in Figure 4c has occurred in the battery, where dendrite growth leads to continuous micro short circuits inside, but also continuous short circuit recovery; When the number of dendrites accumulates to a certain extent, the battery experiences irreversible positive and negative pole contact, resulting in a complete short circuit of the battery (Figure 4d). It should be noted that for liquid electrolytes, another possible reason why the voltage response becomes straight and does not exhibit diffusion polarization is due to the drying up of the electrolyte caused by dendritic growth; However, if a short circuit does not occur due to the electrolyte drying up, the overpotential should be high (with a high interface impedance), so a low voltage value can be determined as a short circuit. Although some research results have not shown a linear voltage response, there is a clear disorder in the voltage response signal, and the possibility of a short circuit should also be considered, rather than simply being classified as an activation process.

Based on the above interpretation of short-circuit data, it can be directly observed from the voltage response curve that if there is a significant voltage drop within one cycle, it can be considered as a short-circuit to determine whether a lithium symmetric battery has experienced a short-circuit. According to the performance after the sudden drop, hard short circuit and soft short circuit can also be distinguished: the voltage drop of hard short circuit is irreversible, and the voltage is fixed after the sudden drop, which is manifested as a straight response signal. If the current is changed, the voltage response signal increases proportionally to a fixed value and remains straight (at this time, the battery as a whole is equivalent to a constant resistance); If it is a soft short circuit, the amplitude of voltage drop is small, and after the decrease, the voltage response curve can recover to normal as the charging and discharging process proceeds, and the voltage does not show a fixed value.

4. Other methods for determining short circuits
In addition to the voltage response curve, AC impedance spectroscopy can also assist in determining whether a battery has experienced a short circuit. If a hard short circuit occurs in the battery, there will be a clear inductive impedance signal in the high frequency range of the AC impedance spectrum, and the intersection point value (impedance real part mode) between the high frequency range and the x-axis will be small, and irregular random points will appear in the low frequency range. Soft short circuit is manifested as a normal AC impedance spectrum, but the electrostatic resistance of the battery is significantly reduced, and the charge transfer resistance is also reduced. Due to the widespread activation phenomenon during the testing process of lithium symmetric batteries, and the increase in active surface caused by lithium dendrite growth, the decrease in battery impedance may also be caused by this. Therefore, it is difficult to determine soft short circuits solely through AC impedance spectroscopy, and it is necessary to combine the voltage response signal of the battery to explain. In addition, commonly used microscopic analyses, such as scanning electron microscopy, can visually observe lithium dendrites, but due to the randomness of the display sites, it cannot fully indicate whether the entire electrode undergoes dendrite growth in a specific micro region. In addition to the common electrochemical and material characterization methods mentioned above, the use of in-situ batteries is also a direct way to determine the effectiveness of modification.

conclusion
This article summarizes the common short circuit forms and corresponding physical processes in the research process of lithium metal negative electrodes, and summarizes the basis for determining whether internal short circuits occur in lithium symmetric batteries. A method for distinguishing between soft short circuit and battery activation process was proposed by studying the signal manifestations of soft short circuit and hard short circuit. This work provides a reference for determining the formation of dendrites and the effectiveness of modification methods in the study of lithium metal negative electrodes.