How does lithium precipitation happen?

When the lithium-ion battery is charging, Li+ is de-intercalated from the positive electrode and embedded in the negative electrode; but in some abnormal situations: such as insufficient space for lithium intercalation in the negative electrode, too much resistance of Li+ intercalation in the negative electrode, and Li+ is de-intercalated from the positive electrode too quickly but cannot be embedded in the same amount. When an abnormality such as the negative electrode occurs, the Li+ that cannot be embedded in the negative electrode can only obtain electrons on the surface of the negative electrode, thereby forming a silver-white metallic lithium element, which is often referred to as lithium precipitation. Lithium precipitation not only degrades battery performance and greatly shortens cycle life, but also limits the battery’s fast charge capacity and may cause catastrophic consequences such as combustion and explosion. In this paper, we discuss from macroscopic lithium-ion batteries, working conditions, gradients existing inside batteries, electrochemical tests, safety tests, etc.), microscopic (electrodes, particles, microstructures, etc.) and atoms (atoms, ions, molecules, activation energy barriers, etc.)


1. When does the lithium deposition side reaction occur?

When a lithium-ion battery is charged, Li+ is deintercalated from the positive electrode, and these Li+ diffuses in the electrolyte to the surface of the negative electrode and intercalates into the negative electrode material. Taking the graphite negative electrode as an example, when the negative electrode potential drops to 200-65 mV vs. Li+/Li, the lithium intercalation process occurs; as the charging continues, the negative electrode potential drops below 0 V vs. Li+/Li, and lithium occurs. The deposition side reaction, at this time, the lithium deposition side reaction of the negative electrode and the lithium intercalation reaction are carried out simultaneously. Considering the effect of polarization, the lithium deposition side reaction occurs when the sum of the equilibrium potential and the overpotential (from ohmic resistance, charge transfer, and diffusion processes) is negative with respect to the Li+/Li pair.

2. Factors affecting the side reactions of lithium deposition?

· The positive and negative electrodes of the lithium-ion battery and the metal lithium reference electrode are formed into a three-electrode system as shown in Figure 2(b) for charging test, and the change of the negative electrode potential with the full battery voltage is shown in Figure 2(a) . The study found that when the state of charge (SOC) and charging current density are larger, and the test temperature is lower, the potential of the graphite negative electrode will be more negative, and the side reaction of lithium deposition on the negative electrode surface will be more likely to occur.

· Lithium battery level: Increasing the N/P ratio within a certain range helps limit the state of charge of the negative electrode to a lower level, thereby reducing the battery aging rate and making the battery internal resistance increase more slowly.

· Anode reaction kinetics: The lithium evolution reaction is also affected by the type, morphology, and conductivity of the anode material. They affect the degree of anode polarization from the perspective of diffusion mass transfer or charge transfer, thereby affecting the anode potential and anode reaction.

· Activation energy: The activation energy that solvated lithium ions need to overcome when diffusing in the electrolyte is negligible, while the activation energy that solvated lithium ions need to overcome during desolvation, diffusion through the SEI membrane, and charge transfer is negligible. Highest. With the progress of the charging process, the number of Li+ embedded in the negative electrode gradually increases, the activation energy that needs to be overcome when Li+ diffuses in the negative electrode active material increases, and the solid-phase diffusion becomes more difficult.

· Temperature: According to the Arrhenius formula, when the battery is cycled at low temperature, the lithium evolution reaction has a larger reaction rate than the lithium intercalation process, that is, the negative electrode is more prone to lithium evolution reaction at low temperature. This has been verified by experimental observations of more negative potentials of graphite anodes at low temperatures. In addition, the charge transfer and solid-phase diffusion at low temperature are also slower, and the reaction rate between the metal lithium deposited on the surface of the negative electrode and the electrolyte also decreases.

· Charging rate: The charging current rate determines the lithium ion flux on the negative electrode material per unit area. When the solid-phase diffusion process of Li+ in the negative electrode is slow (for example, when the temperature is too low, the state of charge is high, or the diffusion of Li+ in the material needs to overcome a large activation energy), and the charging current density is too large, the negative electrode surface Lithium evolution reaction occurs. When other conditions remain unchanged, and the current density increases to a certain threshold, the negative electrode potential becomes negative, accompanied by the onset of the lithium evolution reaction.

· Others: Whether the lithium evolution reaction occurs on the surface of the negative electrode is determined by the three factors of charging rate, temperature and state of charge. For example: (1) Charging at low temperature does not mean that the negative electrode will inevitably undergo a lithium evolution reaction. Lithium evolution occurs only when the state of charge and/or current density exceeds a certain threshold. (2) During the charging process of lithium-ion batteries, if a higher charging current density is adopted when the state of charge is low, and a lower charging current density is adopted when the state of charge is high, the lithium precipitation reaction can be obtained. effective suppression.

3. Collecting experimental evidence for lithium deposition side reactions from different perspectives

Dr. Margret Wohlfahrt-Mehrens selected five commercial lithium-ion batteries that have been widely studied in recent years, and numbered them as battery 1-battery 5, and collected experimental evidence for the side reactions of lithium deposition from the following four perspectives: (1) Aging characteristics; (2) voltage curve; (3) physicochemical properties of batteries; (4) physicochemical properties of electrodes.

By comparing lithium-ion batteries of the same type, the researchers found that the side reaction of lithium deposition made the battery have a faster aging rate, and its battery capacity, energy density, and energy efficiency were significantly attenuated.

4. Gradients of various factors in lithium-ion batteries and the morphology of metal lithium deposited on the negative electrode

In the actual charging or aging process, the temperature, state of charge, and charging current density are often unevenly distributed on the surface of the negative electrode inside the lithium-ion battery. The gradient formed by these factors inside the battery causes the lithium deposition side reactions to proceed at different rates at various points on the surface of the negative electrode, resulting in the deposition of non-uniform lithium layers. When the localized temperature of the negative electrode surface is low, the state of charge is high, and the current density is high, the lithium deposition side reaction tends to proceed rapidly in this region, resulting in more severe lithium deposition than other regions. In practical Li-ion batteries, the metal lithium deposited on the surface of the negative electrode often exhibits the macroscopic morphology shown in Figure 5.

Metal lithium is uniformly deposited on the surface of the negative electrode

Under low temperature conditions (0 ℃~-20 ℃), when the charge-discharge cycle is performed at a low current rate, the lithium-ion battery may form a metal lithium thin layer with uniform thickness and uniform distribution on the surface of the negative electrode, and its macroscopic morphology is shown in Figure 5 ( a) shown. This means that there is no significant temperature gradient in the x-y plane. However, by disassembling the cylindrical 26650 lithium-ion battery, Petzl et al. found that the battery

The metallic lithium deposited in the core is much less than the metallic lithium deposited near the battery casing. This is because the metal casing of the battery tends to have higher thermal conductivity and slower radial heat transfer. In a cylindrical battery, the heat from the core needs to be conducted away through multiple layers of material. This makes the core temperature of the cylindrical battery higher than that of the outer casing, suppressing the side reaction of lithium deposition in the core, resulting in a significant difference in the anode potential in the radial direction and the thickness of metallic lithium.

Similarly, the temperature gradient near the casing of the pouch cell is significantly higher than that of the core. where the temperature gradient near the casing is proportional to the current density. The greater the current density, the greater the temperature gradient at the battery casing. It is worth noting that a thicker Li layer is always deposited near the current collector of the pouch cell because of the larger localized current density on the current collector, which exacerbates the Li deposition side reaction.

The GD-OES test showed that metallic lithium was mostly deposited on the surface of the negative electrode, that is, between the negative electrode and the separator. Computational simulation results show that this is because the overpotential is the largest at the anode-separator interface.

Part of the metallic lithium deposited on the surface of the negative electrode is also embedded in the negative electrode material (~10 μm), which increases the thickness of the negative electrode and the total thickness of the battery. This should be taken into account when measuring the thickness of cells or electrodes.

5. Safety hazards caused by side reactions of lithium deposition

In Li-ion batteries, the process of thermal runaway caused by Li deposition side reactions is shown in Figure 8. In the process of thermal runaway, the main reasons for the continuous increase of the temperature of the lithium-ion battery are: (1) the continuous growth of lithium dendrites on the surface of the negative electrode, piercing the diaphragm, and causing an internal short circuit; (2) the metal lithium deposited on the surface of the negative electrode and The “dead lithium” inside the battery reacts with the electrolyte, releasing a lot of heat; (3) when overcharged, the large current density further aggravates the side reaction of lithium deposition and increases the temperature of the battery; (4) when the temperature continues to rise The gas generated by the decomposition of the electrolyte continuously increases the internal pressure of the battery, eventually causing the battery to outgas and the metal lithium to melt. During this process, water and oxygen in the air react violently with metallic lithium, leading to combustion and even explosion.

· Detection technology of internal short circuit of lithium ion battery

Internal short circuits in Li-ion batteries are usually caused by Li dendrites. In order to study the growth law of lithium dendrites, early researchers have done a lot of work in surface science experiments and computational simulations, and found that the growth of lithium dendrites conforms to a positive feedback mechanism. Monte Carlo simulations show that the electric field at the tip of Li dendrites accelerates the diffusion of Li ions in the electrolyte, thereby accelerating the growth process of Li dendrites.

Macroscopically, the internal short circuit of lithium-ion battery can be divided into soft short circuit and hard short circuit. Among them, soft shorts are caused by high-resistance slender Li dendrites, which disappear with the dissolution of Li dendrites during the discharge stage. When a soft short circuit occurs, the large short-circuit current density causes the negative electrode voltage to drop sharply, and there is a significant local temperature rise. At the same time, the metal lithium deposited on the negative electrode reacts with the electrolyte to release a lot of heat, which promotes the melting of the metal lithium and the separator. Battery failure caused by a soft short is not catastrophic.

When the temperature continues to rise, the soft short circuit becomes a hard short circuit. Unlike soft shorts, hard shorts are characterized by low resistance and have a higher short-circuit current density than soft resistances. A hard short causes the battery to locally heat up faster, making it more likely to cause catastrophic thermal runaway.

In practical applications, the internal short circuit of lithium-ion batteries can cause catastrophic consequences such as fire and explosion, but the probability of its occurrence is very small (about the order of ppm). Therefore, artificial methods are usually used to cause internal short circuits in experimental research, such as drilling holes on the battery casing, or plugging the micropores on the separator with low-melting alloys.

In order to prevent the occurrence of thermal runaway, the following measures can be taken: (1) For a lithium-ion battery with a capacity of about 1 Ah, when the internal resistance of the battery decreases, its voltage deviates significantly from the normal value. An internal short circuit can be detected by detecting the voltage. (2) For a lithium-ion battery with a larger capacity (such as 40 Ah), its internal resistance is smaller, and the voltage deviation during short circuit is not obvious, so it is more difficult to detect, and an element that amplifies the signal needs to be used. (3) After an internal short circuit is detected, it is necessary to isolate the failed single cell in the battery pack.

In 2014, Wu et al. invented a sandwich-structured composite diaphragm. The structure is that a conductive film is sandwiched between two ordinary diaphragms. By measuring the voltage between the negative electrode and the conductive film, the existence of lithium dendrites can be detected. Measuring the resistance between the two can also detect the presence of pinholes in the diaphragm. However, there are difficulties in mass production of such diaphragms, so industrial production has not yet been realized.

In addition, the lithium deposited on the surface of the separator may also cause an internal short circuit, and the deposited lithium may not be in a dendrite form at this time. This is more likely to happen when lithium is deposited near the current collector or at the edge of the anode.

From a thermal conduction point of view, the size of the battery has a large impact on thermal runaway. When heat accumulates, the small size of the battery is more conducive to heat conduction, the overall temperature rise is more obvious, the local temperature rise is relatively slow, and the entire battery only needs to install a temperature sensor. On the contrary, large-sized batteries have more obvious local heating, and the overall heating rate is relatively small. In practical applications, multiple temperature sensors need to be installed.

· Detection of exothermic reactions caused by lithium deposition

Lithium dendrites have a large specific surface area and release a lot of heat when they react with the electrolyte, so they can be detected by ARC, DSC, TGA and other methods.

Three parameters related to safety can be measured by ARC, namely, the self-heating onset temperature TSH, the thermal runaway onset temperature TTR, and the self-heating rate SHR. Under the quasi-adiabatic conditions of the ARC test, the heat released by the reaction within the battery cannot be transferred to the environment, reflecting the most severe thermal runaway condition.

The information related to exothermic reaction, endothermic reaction and component evaporation can be measured by DSC, and the mass loss of the corresponding process can be measured by combining DSC with TGA.

The negative electrode of the lithium ion battery was dismantled to carry out the above test, and it was found that the self-heating onset temperature TSH and self-heating rate SHR of the negative electrode material depended on the charge-discharge cycle history of the negative electrode. When the negative electrode undergoes charge-discharge cycles at low temperature, its self-heating onset temperature decreases significantly and its self-heating rate increases significantly. For example, the negative electrode that performs charge-discharge cycles at -10 °C enters the self-heating stage from 30 °C in the ARC test.

The TGA test showed that the negative electrode subjected to charge-discharge cycles at low temperature experienced more severe lithium deposition and aging process, its thermal stability deteriorated, and its weight loss increased at 33-200 °C. Above 200 °C, the molten lithium is more reactive, the gaseous products are further increased, and the weight loss measured by TGA increases.

In conclusion

Lithium precipitation reaction is the fatal culprit that causes the increase of internal resistance, capacity attenuation and Coulomb efficiency drop of lithium-ion batteries. In severe cases, it may even lead to safety hazards such as fire and explosion. Based on the previous research on 5 types of commercial lithium-ion batteries, this paper introduces the causes, phenomena and product morphology of lithium deposition side reactions, and discusses the use of different characterization methods to collect direct experimental evidence and indirect experiments for lithium deposition side reactions. method of evidence. Subsequently, the review presents battery aging mechanisms involving lithium deposition side reactions and summarizes methods for detection and prevention of related safety hazards.

In fact, in order to reflect the real life of Li-ion batteries as accurately as possible under practical application conditions, electrochemical tests need to be performed under a combination of various aging mechanisms, such as cycle-life aging tests after the calendar-life aging phase. Such cases are more complex and require further investigation. In addition, optimizing the charging and discharging conditions, designing the battery configuration and designing new anode materials are also important ways to alleviate the lithium evolution reaction.

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