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Recently, the topic of reducing the capacity of lithium-ion batteries due to lithium-ion loss has sparked a heated discussion in the communication group. One of the main reasons for the decrease in capacity of lithium-ion batteries is the irreversible loss of lithium elements (compounds and ions), which forms irreversible lithium compounds or lithium metals. Irreversible lithium compounds are one of the main components that form SEI films, while irreversible lithium metal mainly forms dendritic lithium and dead lithium. For us beginners, how can we understand lithium dendrites more easily?

This article mainly discusses the following issues based on literature and practical work experience. If there are any inaccuracies, please correct them. At the same time, we also hope to spark discussions and attract experts to better explain the issue of lithium dendrites.

How are lithium dendrites formed?

What are the characteristics of lithium dendrites?

3. Factors affecting lithium dendrites?

How to avoid the formation of lithium dendrites?

How are lithium dendrites formed

As early as the 1970s, researchers conducted detailed observations on the deposition of metallic lithium. However, the growth mechanism of lithium dendrites involves fields such as electrochemistry, crystallography, kinetics, and thermodynamics, which are very complex. Therefore, there is currently no universal theory for dendrite growth.

The problem of lithium dendrites in batteries is similar to the electroplating production in the electrochemical industry, such as electroplating Cu, Ni, and Zn, which also faces the problem of metal dendrite growth. Therefore, the experience accumulated during the electroplating process can serve as a reference for understanding the growth of lithium crystals. Previous experience has shown that during the electroplating process, there is a concentration gradient of cations in the electrolyte, which is limited by the diffusion rate of lithium ions. When the current density reaches a specific value, the current can only be maintained for a period of time, known as beach time. After that, the cations are exhausted in the electrolyte near the deposition electrode, which breaks the surface electrical neutral balance of the deposition electrode and forms a local space charge, leading to the formation of dendrites during electroplating.

With the help of electroplating experience and previous research, M. Rosso et al. proposed a Monroe Newman model for lithium dendrites, considering the effects of deposition rate, ion concentration, current density, overpotential, and surface tension on the embedding and ion deposition processes

Discussing the Formation Mechanism and Prevention of Lithium Dendrites

In the formula: e is the basic unit of charge; Co is the initial concentration; D is the diffusion constant; J is the current density; μ c is the cation concentration; μ a is the concentration of anions. The experiment shows that as J2 increases, τ cc decreases.

Additionally, some theories suggest that due to the uneven surface and numerous protrusions on the metal lithium negative electrode, the distribution of electron charges at the protrusions increases, leading to more Li+being attracted and depositing to form lithium dendrites.

In summary, the growth of lithium dendrites is a complex electrochemical problem involving many factors, making it difficult to describe with a single model or theory. However, research and exploration of theoretical models for lithium dendrite nucleation and growth are still ongoing.

What are the characteristics of lithium dendrites

The morphology of lithium dendrites varies in different battery environments and at different times, such as mossy lithium, filamentous lithium, needle like lithium, whisker like lithium, shrub like lithium, dendritic lithium, etc. Due to different descriptions by researchers, this phenomenon of diversified names has emerged. It can be briefly divided into three categories: ① no branching, single growth, such as filamentous lithium, needle like lithium, whisker like lithium; ② Cluster shaped, growing in a manner similar to the fermentation process of dough, such as moss like lithium and shrub like lithium; ③ Obvious branching structure is visible, with sparse branches, making it the most dangerous dendritic structure that is prone to puncturing membranes, such as dendritic lithium.

Discussing the Formation Mechanism and Prevention of Lithium Dendrites

The formation of dendrites can be divided into three stages

The initial nucleation and growth process of lithium dendrites can be divided into three stages.

In the first stage, after battery assembly, due to the high reactivity of lithium metal, an instantaneous reaction can occur when it comes into contact with organic solvents and other components in the electrolyte, forming an SEI film, which occurs earlier than the formation of dendrites. The dense SEI film can prevent further reactions between the electrolyte and metallic lithium, making it a good ion conductor but an electronic insulator. Li+can pass through this SEI film and deposit on the electrode surface, but its deposition distribution is uneven due to the inherent characteristics of lithium, electrolyte, SEI film, and the influence of charge and discharge conditions.

The second stage, the nucleation stage, refers to the continuous accumulation of uneven precipitation, which causes certain areas to bulge until the original SEI film is broken.

Finally entering the growth stage, piercing the original SEI film and continuing to grow in the length direction, becoming visible dendrites. At the same time, the SEI film continuously reacts and proliferates with the growth of lithium metal dendrites, but always covers the surface of lithium metal.

Generally speaking, the number of dendrites is mainly determined by the nucleation stage, while the morphology of dendrites is mainly determined by the growth stage.

Discussing the Formation Mechanism and Prevention of Lithium Dendrites

In situ electron microscopy observation of the growth process of lithium dendrites

How are lithium dendrites formed

According to the Monroe Newman model and practical work experience, the reasons why lithium deposition is prone to occur are summarized as follows:

When overcharged, the negative lithium is saturated, and excess lithium has precipitated as metal

At the negative electrode where the copper foil leaks, due to low polarization, it is prone to lithium alloying and lithium deposition

Charging with 3 high currents, lithium on the negative electrode surface does not have time to diffuse internally, resulting in precipitation on the electrode surface

4 electrode edges, especially when aligned, are affected by edge effects, resulting in high current density and easy lithium deposition in the negative electrode

In addition, the redundancy design of the positive and negative electrodes is insufficient, and factors such as low-temperature charging of the battery, poor gas contact between the positive and negative electrode plates, poor infiltration of the negative electrode electrolyte, SEI film, electrolyte type, solute concentration, effective distance between the positive and negative electrodes may all lead to lithium deposition in the negative electrode.

1. Uneven surface of positive and negative electrodes

There are many reasons for uneven surface of the positive and negative electrodes, such as uneven coating, heavy coating of the positive electrode or light coating of the negative electrode, impurities mixed in the active material, and excessive thickness of the positive or negative electrode head.

The roughness of the positive and negative electrode surfaces affects the formation of dendritic lithium. The rougher the surface, the more favorable it is for the formation of dendritic lithium. The formation of dendritic lithium involves four major aspects: electrochemistry, crystallography, thermodynamics, and kinetics, which are described in detail in David R. Ely's article.

Discussing the Formation Mechanism and Prevention of Lithium Dendrites

Discussing the Formation Mechanism and Prevention of Lithium Dendrites

2. Lithium ion concentration gradient and distribution

After lithium ions are released from the positive electrode material, they pass through the electrolyte and separator, and receive electrons at the negative electrode. During the charging process, the lithium ion concentration of the positive electrode gradually increases, while the lithium ion concentration of the negative electrode decreases due to continuous electron acceptance. In a dilute solution with high current density, the ion concentration will become 0. The Monroe Newman model shows that when the ion concentration decreases to 0, the negative electrode will form local space charges and dendritic structures, with the growth rate of dendritic structures being the same as the ion migration rate in the electrolyte.

Discussing the Formation Mechanism and Prevention of Lithium Dendrites

Self repair of electrostatic field theory, metal cations will adsorb at protrusions to form a positive electric field, thereby repelling lithium ions with the same charge and reducing protrusions

3. Current density

Discussing the Formation Mechanism and Prevention of Lithium Dendrites

Discussing the Formation Mechanism and Prevention of Lithium Dendrites

How to avoid the formation of lithium dendrites

According to the formation and influencing factors of dendritic lithium, the formation of dendritic lithium can be avoided from the following aspects:

1. Control the flatness of the positive and negative electrode current collectors after coating.

Because it has already been discussed before, it will not be elaborated here.

2. The size of the negative electrode particles should be smaller than the critical thermodynamic radius.

During the growth process of lithium dendrites, lithium nuclei are formed with a thermodynamic critical radius raq and a kinetic critical radius rk:

Discussing the Formation Mechanism and Prevention of Lithium Dendrites

In the formula, is the surface energy of the lithium electrolyte interface, Ω is the molar volume of lithium, z is the number of charges, F is the Faraday constant, and Gf is the molar volume conversion free energy. The growth of lithium dendrites must first overcome the thermodynamic critical radius in order to have sufficient energy for nucleation. Secondly, a single crystal nucleus can only grow if it is greater than the kinetic critical radius, otherwise the nucleus will gradually disappear.

3. Add electrolyte additives that stabilize the negative electrode electrolyte interface

Additives decompose, polymerize, or adsorb on the surface of lithium negative electrodes, participating as reactants in the formation of SEI films to change the composition and structure of SEI films, modify the physical and chemical properties of SEI. In addition, they can also act as surfactants to change the reactivity of lithium negative electrode surfaces, regulate the current distribution during lithium deposition, and achieve uniform lithium deposition. Additives can even improve lithium deposition morphology and cycling efficiency at ppm levels in the electrolyte. Therefore, using electrolyte additives to modify lithium negative electrodes is the most economical and convenient method.

4. Replace the liquid electrolyte with high strength gel/solid electrolyte

Solid state electrolytes have a high modulus, which can prevent the growth and spread of lithium dendrites. Lithium dendrites are difficult to penetrate the electrolyte and conduct positive and negative electrodes, greatly improving safety. Therefore, they are considered the optimal choice for lithium metal batteries.

Discussing the Formation Mechanism and Prevention of Lithium Dendrites

5. Establish a high-strength lithium negative electrode surface protective layer

The modulus of inorganic ceramic solid electrolytes is generally high, which can prevent the growth and spread of lithium dendrites. However, it is worth noting that inorganic ceramics with high modulus have poor contact, which can lead to poor contact with electrodes and excessive interface resistance. Therefore, when selecting inorganic ceramic materials, a balance must be struck between high modulus and surface contact.

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