Stanford University reveals the origin and regulation of lithium dendrites in solid-state batteries

On January 31, 2023, Beijing time, the team of Professor William Chueh of Stanford University in the United States published a new study entitled “Mechanical regulation of lithium intrusion probability in garnet solid electrolytes” in the journal Nature Energy.

The team reported the latest work on the origin and regulation of lithium dendrites in the field of solid-state batteries. The study proved that nanocracks in ceramic solid electrolytes were the main cause of lithium dendrites, and it was found that the weak strain of 0.070% was sufficient to control and change the propagation direction of lithium dendrites. This basic research on the mechanism of lithium dendrites provides important theoretical guidance for the safe design and production of solid-state batteries.

The corresponding authors of the paper are Geoff McConohy, Xu Xin, and William C. Chueh. The first authors are Geoff McConohy, Xu Xin, and Cui Teng.

The lithium dendrites produced in the solid electrolyte are one of the important factors affecting the safety and efficiency of solid-state batteries (“dendrites” in the solid electrolyte are not the only form, but in order to simplify the discussion, this paper uses “lithium dendrites” as a discussion). Especially in the process of fast charging, the formation of lithium dendrites will cause premature failure or even short circuit of the battery, resulting in serious safety hazards. However, the scientific question of why such “soft” lithium metal grows into the “hard” ceramic electrolyte is inconclusive. The two currently popular hypotheses suggest that the main causes of lithium dendrites lie in 1) pre-existing mechanical defects in the electrolyte; 2) Excessively high electronic conductivity in the electrolyte. However, the relevant research lacks quantitative experimental data support, and the mechanism of lithium dendrite production needs to be investigated.

The researchers used a focused ion beam/scanning electron microscopy (FIB/SEM)-based microprobe platform with simultaneous pressure control and electrochemical measurements. Researchers have conducted more than 60 in situ lithium metal growth experiments on garnet-type solid electrolyte (LLZO). Through the statistical analysis of experimental data, it is found that the probability of lithium dendrite and the diameter of lithium deposition are consistent with the Weakest Link Model, which indicates that the main factor inducing lithium dendrites lies in the material defects in the deposition area. In addition, increasing the contact pressure between the probe and the electrolyte, the probability of lithium dendrite appearing is significantly increased. This shows that the new defect is due to a higher tip pressure (5 mN vs. 0.1 mN) and that the nature of this defect is mechanical. Finally, the researchers designed a cantilever bending experimental platform, which proved that only 0.070% of the weak strain was enough to change the propagation direction of lithium dendrites, and it was found that the application of an external force field to the electrolyte could achieve effective manipulation of lithium dendrites.

The specific work is as follows:

1. Microprobe platform and cantilever bending platform

In Figure 1, the researchers designed two in situ electrochemical-force coupling experimental platforms to explore the dynamic process of lithium dendrite generation and propagation under local and global stresses. The microprobe stage is equipped with a spring table to measure the pressure exerted by the tip on the electrolyte surface. In a cantilever bending platform, one end of the electrolyte is fixed and an upward pressure is artificially applied at the other end, resulting in a global surface pressure strain with gradient changes on the electrolyte surface (the strain at the fixed end is maximum, and the strain at the free end is zero). In the experiment, a negative voltage relative to the counter electrode is applied to the probe, and the lithium ions will be reduced to metallic lithium on the surface of the electrolyte and deposited on the surface of the electrolyte. When the voltage increases to a certain value, lithium dendrites begin to produce and cause the solid electrolyte to rupture.

Figure 1: Microprobe manipulation platform and cantilever bending experimental platform, and scanning electron microscopy images of lithium deposition dynamics (0.1 mN contact pressure).

2. Probability and mechanism of lithium dendrites

In Figure 2, the researchers performed statistical analysis of the observed lithium deposition diameter under two different pressure conditions (5 mN vs. 0.1 mN), and found that the occurrence probability of lithium dendrites and lithium deposition diameter met the Weibull distribution and conformed to the weakest link model, which showed that the main factors inducing lithium dendrites were defects in the sedimentary area, and their behavior was consistent with the fracture mechanics theory of brittle materials. In addition, lithium dendrites under high pressure conditions (5 mN) can be generated in a smaller deposition area (i.e., lithium dendrites appear earlier), indicating that the pressure from the tip induces new defects at the contact surface.

Figure 2: The occurrence probability and sedimentation diameter of lithium dendrites meet the Weberian distribution, and local defects are the main triggers.

3. Defect type analysis

In Figure 3, the researchers conducted an in-depth analysis of the defect type, and through nanoindentation and finite element analysis, it was found that the stress generated by the tungsten probe itself could not cause the plastic deformation of the solid electrolyte LLZO. And regardless of whether the plastic deformation of LLZO is considered, the pressure-displacement curve of finite element simulation is higher than the experimental results of nanoindentation, which shows that at large contact pressures (5mN), the type of defect produced by LLZO is not plastic deformation, but more likely nanoscale cracks.

Figure 3: Scanning electron microscopy images of lithium dendrites (5 mN contact pressure), ectopic nanoindentation, and finite element analysis.

4. Control the propagation direction of lithium dendrites

In the cantilever bending experiment designed by the researchers in Figure 4, three different regions were used for dynamic deposition of lithium metal, namely free end (0 surface pressure strain), middle end (0.033% surface pressure strain), and fixed end (0.070% surface pressure strain). It is observed that with the increase of surface pressure strain, lithium dendrites will propagate in the direction of strain, because surface pressure strain will make it more difficult to open cracks perpendicular to the strain direction, thereby inhibiting the propagation of lithium dendrites perpendicular to strain. This experiment shows the mechanical nature of lithium dendrite propagation, and the effective regulation of lithium dendrite propagation can be achieved by applying an external force field.

Figure 4: The mechanical pressure strain of 0.070% can regulate the propagation direction of lithium dendrites, which proves the mechanical nature of lithium dendrite propagation.

This work revealed the origin and mechanical controllability of lithium dendrites in solid electrolytes, and proved that local nanoscale cracks were the main causes of lithium dendrites through in situ probe experiments and statistical analysis. In this study, it is found that local compressive stress can produce new defects in the solid electrolyte, thereby inducing the production of lithium dendrites. This has important guiding significance for the production process of solid-state batteries, for example: the impurity particles mixed between the layers of battery materials can produce local extremely high compressive stress during the battery pressing process, and then produce microcracks to induce lithium dendrites. In addition, global compressive stress has been found to be used to inhibit the generation of new cracks, thereby preventing the propagation of lithium dendrites. Therefore, how to reduce the occurrence of local compressive stress in solid electrolyte and introduce the corresponding global compressive stress is an important measure to prevent the propagation of lithium dendrites and reduce the probability of battery failure. (Source: Science Network)

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