Stanford University Qin Jian "AFM": Finally understood! Molecular simulations reveal how polymer coatings inhibit the deposition of lithium chic crystals on the negative surface of metal lithium.

Lithium metal is an ideal negative material for lithium-ion batteries: electrochemically low (extremely -3.04 V relative to standard hydrogen) and theoretically higher than capacity (3860 mAh.g
-1
). However, this negative material in the use of the process will produce a deadly "lithium branch crystal", not only will lead to short circuit of the battery to bring safety problems, these crystals will also be irreversible side reaction with electrolytes, reducing battery capacity and Coulomb efficiency, so that this ideal negative material has not yet achieved commercial applications.
how to eliminate or inhibit the deposition of lithium branch crystals on the surface of lithium metal has become the focus of

. Some researchers start with modified SEI membranes to solve problems by preparing more uniform, flexible, and mechanically stronger SEI membranes, while others use solid electrolytes to inhibit the growth of branch crystals, such as ceramic-based electrolytes, segment copolymers, or layered polymer coatings.
lithium-ion battery, polymer coating bonding performance is good, flexible performance is excellent, the cost is not high, and easy to coat, more importantly, can be very good to inhibit the growth of the branch crystals. The research shows that the thickness, dielectic constant and surface of polymer coating can determine the growth rate and shape of lithium chic crystals on the negative surface.
to understand in principle how these factors affect the growth of lithium chit crystals is essential to promote the commercial application of lithium metal negative poles, but few studies have been conducted in this area. Experiments to solve this problem appear to be insulable, simulation calculation has become the preferred method for researchers.
in the simulation calculation of lithium chip deposition, linear stability analysis is undoubtedly the most widely used method, but this method uses many simplified assumptions and can only be used to calculate 2D system. Molecular dynamics (MD) model is simple, can get a lot of information about the sedimentary dynamics of branch crystals and branch crystal form, has been widely concerned by researchers, but the existing research is mostly focused on 2D systems, the study of polymer coating is rarely reported.
Results Introduction
Based on the above analysis,
Stanford University
Professor Qin Jian, Kong Xian
Task Force
proposed a 3D coarse particle molecular simulation model for polymer systems to study the deposition process
of lithium chic crystals on the metal negative pole coated with polymers. It was found that when the polymer coating structure is not good, the simulation is carried out to 50 milliseconds and 125 milliseconds, the random formation of the tip of the branch crystal will pierce the polymer;
coating thickness should be appropriate, not less than two layers, but should not exceed nine layers.
results provide theoretical guidance for the rational design of polymer coating in lithium-ion batteries and the effective suppression of the growth of branch crystals.
3D coarse-grain molecular model hypothesis and setting of
Figure 1. Model schematic and initial sedimentary state, simulating space 20×20×25 nm, lithium ion randomly distributed at the top (z s 25 nm).
the researchers applied an electric field in the simulated space of the 3D coarse particle molecular model, and the lithium ions were deposited on the lithium metal negative poles through liquid electrolytes and solid polymer coatings. In order to make the simulation calculation better, the researchers made some assumptions and condition settings for the model. Assuming that the set fluid is a flat surface, the surface of which is in close contact with the plane z-0, periodic boundary conditions are used in the x and y directions to minimize the effect of limited analog space size on the results. The solvent and anion are implicitly modeled, and the dielem constant is fixed. The polymer coating is a network of removable beads linked to each other, while lithium ions are removable spheres with a basic charge e, lithium ions are deposited into fixed spheres with the same potential as electrodes, and the movement of lithium-ion and polymer beads is described by Brown motion. The top boundary of the analog space is positive, the potential is fixed to the scathode, the negative polar fluid and the deposited lithium potential is fixed to the sanode, the potential difference is 0.5 V when the polymer coating is used, and 0.1 V when the polymer coating is not used.
2. In the form of lithium branch crystals, the dielectic constant of the polymer strongly affects the potential field, and the arrow indicates the strength of the electric field.
potential field near the structure of the
(a) branch crystal, and (b) the potential field in the presence of a polymer coating.
The researchers found that the distribution of potential field in simulated space was influenced by the depositional lithium branch crystal form (Figure 2a) and the dieleccharide constant of polymer coating (Figure 2b): near the tip of the branch crystal, the potential contour distribution was denser, indicating that the potential field here changed faster, the electric field was stronger, and lithium ions were more likely to gather at the tip under electrostectronic gravity;
of lithium ions on different polymer coatings
3. Deposition curves in the presence of polymer coatings.
the simulation, the researchers quantitatively studied the sedimentary dynamics of lithium branch crystals by calculating the number of lithium-ion depositions. The researchers studied the sedimentary dynamics of lithium ions on electrodes without polymer coatings and coated with three different properties. It was found that as the simulation progressed, the number of lithium deposits on all electrodes increased over time, and in the absence of a coating, lithium ions moved first towards the nucleus tip, and the deposited lithium metal rapidly grew into branches. Polymer-coated electrodes deposit more lithium metal, and the dynamics vary significantly depending on coating performance: for orange and green curves, the initial lithium deposition is uniform, but at 50 milliseconds and 125 milliseconds, the randomly formed tip of the branch is punctured Polymer coating, after rupture, polymer coating will hinder the deposition of lithium, and exposed lithium tip will take precedence for deposition, for the red curve, no coating rupture, the simulation process has been maintained even deposition, indicating that the ideal coating can inhibit the formation of the branch crystal.
The effect of polymer coating on the crystal shape of lithium branch
The appearance of lithium branch crystal deposited is influenced by the rigidity of polymer coating, relaxation time, dielectic constant and coating thickness, and the researchers studied the influence of these factors on lithium branch crystal deposition, and through the optimization of the polymer coating, the purpose of inhibiting the growth of branch crystals can be achieved.
effects of polymer rigidity on lithium deposition
Figure 4. Lithium deposition is maximum at medium bonding strength, and the deposition rate is constant relative to bonding strength.
(a) the sedimentation of lithium over time curve,
(b) lithium deposition and bonding strength of the relationship curve,
(c) polymer mechanical properties on the growth and morphology of the branch crystal map.
in the process of lithium deposition, the randomly occurring lithium tip curvature is very high, which can apply a large strain to the polymer coating and can easily lead to rupture if the polymer is not flexible. The researchers studied the effects of polymer rigidity on lithium deposition by changing the bonding strength between polymer beads. Prior to coating rupture, the deposition curve of lithium on the negative surface coated with different rigid polymers was almost exactly the same, indicating that the effect of polymer stiffness on the deposition rate was negligible. When the bonding strength of the coating is too low or too high, the amount of lithium deposited is not high.
When the coating bonding strength is low, the growing deposition of lithium tip can easily puncture the polymer, leading to the growth of the branch crystals, the simulation begins soon after the coating ruptures, and eventually multiple branch-like branch crystals pierce the polymer coating to continue to grow, forming a "forest-like" structure. The behavior of the liquid electrolyte can be represented when the bonding strength is 0.
If the bonding strength of the coating is too high, the polymer coating can not adapt to the morphological changes of lithium deposition, in the deposition process, the polymer coating is similar to a flat rigid body with the slow movement of lithium deposition, rigid polymer coating plays the role of porous membrane, lithium deposited in the membrane hole. These holes can improve ion conductivity, but they are also potential growth pathways for branch crystals. Once the deposited lithium penetrates the polymer coating, it leads to explosive growth of the lychee, eventually forming a "mushroom-like" form.
When the coating bonding strength is moderate, lithium is deposited evenly, and the polymer coating can be adapted to the evolving lithium tip, which, by shrinking the local structure, will guide the lithium ion away from the randomly occurring tip, and the coating strength is sufficient to prevent the branch from puncturing the polymer coating.
effect of polymer relaxation time on lithium deposition
Figure 5. Polymer relaxation time changes the deposition stability of the sprigs.
(a) the relaxation time of the polymer varies with the bonding strength and polymer motion,
(b) the relaxation time of the no-scale varies with the bonding strength curve, and
(c) optimizes the deposition of lithium over a narrow relaxation time range.
researchers studied the effects of polymer coating relaxation time on lithium deposition. They combined the initial attenuation fit in the coating average height self-correlation function as an exponential, as a relaxation time for the coating height fluctuations. By changing bond strength and bead migration rates over four orders of magnitude, the relaxation time could vary between 0.01 and 100 ms. As expected, relaxation time decreases monotony as the bead migration rate increases or the bonding strength increases. When the coating bonding strength is higher than the minimum threshold (k> 1 eV.nm
-2
), the amount of lithium deposited increases rapidly over a short relaxation period and the amount of lithium deposited during the coating relaxation time is 0.05 to 0.5 ms, revealing a clear relationship between polymer viscosity and lithium deposition pattern and stability.
effect of polymer dieleccharide constant on lithium deposition
Figure 6. The deposition of branch crystals increases with the increase of polymer dieleccharide constant, but the deposition rate decreases gradually.
(a) the ratio of polymer to electrolyte dieleculate constant has a significant effect on lithium deposition, and
(b) branch deposition has a significant effect on the ratio of polymer to electrolyte dielectic constant.
of the lithium tip is mainly driven by a strong electric field near the tip, the strength of which depends on the dielectic response of the polymer coating. Therefore, increasing the coating dielectural constant reduces the directional movement of lithium ions near the tip of the lithium branch crystal, which is conducive to reducing the probability of the coating being punctured. With the increase of the dielectic constant of polymer coating, the amount of lithium deposited evenly increases gradually, effectively inhibiting the directional movement of lithium ions toward the tip of the branch crystal. Compared with non-polar polymers, polymer coatings containing polar function groups promote the uniform deposition of lithium ions, and PVDF coatings with high polarity β phase perform better than low polar α phase coatings.
effect of polymer coating thickness on lithium deposition
Figure 7. The number of branch crystal deposition increases with the increase of coating thickness, but the deposition rate decreases gradually.
(a) the number of branch depositions and the simulation time relationship curve, coating thickness
(polymer layer number) has a significant impact on lithium deposition;
researchers studied the effects of polymer coating thickness on lithium deposition. Thicker polymer coatings are found to reduce the motion and deposition rates of lithium ions, and when the coating is thin (such as one or two layers of structure in a model), almost any random tip of the branch can penetrate it, even though the collecting fluid has been completely covered. When the thickness of the coating exceeds nine layers, there is little change in the sedimentary form of the branch crystals. Therefore, the researchers believe that when preparing lithium-ion batteries, the polymer coating should use the best thickness, without significantly reducing the deposition current, can effectively inhibit the growth of the branch crystals.
Sute knot
In order to clarify the deposition of lithium chic on the metal lithium negative pole coated with polymer in lithium-ion battery, Professor Qin Jian of Stanford University proposed a 3D coarse particle molecular simulation model, and found that the rigidity of the coating, relaxation time, dielectic constant and coating thickness had a significant effect on the sedimentary dynamics and appearance of lithium branch crystals. When the polymer coating structure is poor, at 50 milliseconds and 125 milliseconds, the randomly formed tip of the branch will pierce the polymer coating; With minimal deposition, the polymer coating reduces the directional movement of lithium ions near the tip of the lithium branch crystal by increasing the coating dielectural constant (e.g., the introduction of polar erroneum groups), which helps to reduce the probability of the coating being punctured, and when the coating thickness is one or two layers, almost any randomly occurring tip of the branch can penetrate it, but it cannot be too high to exceed nine layers.
.