Not only leads to degraded battery performance and significantly shortened cycle life, but also limits fast charging capabilities, and even poses safety hazards such as combustion,battery swelling,and even explosions. This article will introduce the classification,causes,and solutions to prevent lithium plating.
1.What is Lithium Deposition on the Negative Electrode?
The lithium intercalation potential of graphite is between 65–200 mV (vs. Li+/Li0). When the potential of the negative electrode approaches or drops below the deposition potential of metallic lithium, lithium ions will be deposited as metallic lithium on the surface of the negative electrode. Experiments have shown that during the charging process, some lithium ions are deposited as metallic lithium on the surface of the negative electrode, while the remaining lithium ions intercalate into the graphite or other negative electrode materials. During discharge, both ion de-intercalation and the stripping of deposited lithium metal occur simultaneously. In the process of lithium metal stripping, "dead lithium" is formed.
In short, the phenomenon of lithium deposition on the negative electrode refers to the simultaneous intercalation and deposition of lithium ions during the charge and discharge processes, causing lithium to deposit as metallic lithium on the surface of the negative electrode, and resulting in the formation of "dead lithium" that cannot be reused.
Dead lithium" reacting with the electrolyte is the main cause of capacity loss and shortened cycle life in lithium-ion batteries. The deposition of lithium on the negative electrode is due to charge transfer limitations (CTL) and solid-state diffusion limitations (SDL).
As charging progresses, the available spaces for lithium ions to intercalate between the graphite layers gradually decrease, limiting the diffusion of lithium ions within the solid graphite phase, which in turn reduces the corresponding lithium ion intercalation current.
At the same time, since the diffusion rate of lithium ions from the electrolyte is much higher than their intercalation rate into the graphite, more and more lithium ions accumulate on the surface of the graphite, causing the negative electrode potential to approach the lithium deposition potential, leading to lithium deposition on the negative electrode.
Edge Lithium Deposition: In the design of lithium-ion batteries, to ensure safety and prevent the dissolution of lithium from the negative electrode, the area of the negative electrode is made larger than that of the positive electrode. Specifically, the edges of the negative electrode extend 1-3 millimeters beyond the positive electrode, and the region where the negative electrode extends beyond the positive electrode is referred to as the "excess portion" or "overhang.
2.The main reasons for edge lithium eposition are twofold:
1.The excess portion is designed too large, resulting in an excess of lithium ions at the edge of the positive electrode. During charging, the excess portion of the negative electrode cannot intercalate the surplus lithium ions from the positive electrode, leading to lithium deposition.
2.During the coating process of the negative and positive electrodes, edge effects may cause a mismatch in surface density in the edge regions. For example, the surface density at the edge of the positive electrode may be too high, or the surface density at the edge of the negative electrode may be too low, ultimately leading to lithium deposition.
To reduce the negative impact of edge lithium deposition, the design of the excess portion should be properly optimized during the design phase, and the coating process for both the positive and negative electrodes should ensure uniform and consistent surface density.
Partial lithium deposition refers to the distribution of lithium ions on the surface of the negative electrode in a relatively random manner, without fixed areas, and appears as discontinuous spots. The main causes of partial lithium deposition include external forces acting on part of the battery cell (such as compression, deformation, etc.) and local defects in the electrodes and separator.
In addition, insufficient electrolyte wetting, as well as the presence of residual gases in the separator and negative electrode, can also lead to lithium deposition during the charging process of the negative electrode.
4.Uniform Lithium DepositionUniform lithium deposition means that lithium metal is evenly distributed over the entire surface of the negative electrode.
Uniform lithium deposition is related to the uniformity of current distribution during charging, which in turn depends on the quality of the electrode, such as pore distribution, refractive index, surface morphology, conductive network, etc. Additionally, the uniformity of current distribution is also affected by the position and number of current collectors (tabs).
5.Causes of Anode Lithium DepositionN/P Ratio Changes:
The N/P ratio is the ratio of the capacity of the positive electrode to the capacity of the negative electrode in a lithium-ion battery, also known as the battery balance (CB) value. The N/P ratio is an important factor affecting battery safety. A lower N/P ratio can cause the negative electrode's lithium potential to reach the deposition potential of lithium, leading to lithium deposition on the negative electrode during charging.
On the other hand, a high N/P ratio, while suppressing the occurrence of lithium deposition under a given cutoff voltage, can cause excessive de-intercalation of lithium in the positive electrode, which not only destabilizes the crystal structure of the positive electrode but also induces oxidation reactions of the electrolyte in the positive electrode.
During battery use, the N/P ratio constantly changes, and its variation is related to factors such as the charging rate, cutoff voltage, environmental temperature, and number of charge-discharge cycles.
Additionally, the N/P ratio change is associated with the chemical system of the battery. For example, with high-nickel positive electrode materials, the N/P ratio tends to increase with the number of cycles due to structural collapse and metal ion dissolution. For silicon-based negative electrode materials, the N/P ratio tends to decrease due to large volume expansion, delamination, particle cracking, and the formation of a new solid electrolyte interphase (SEI) layer.
In summary, many factors influence the variation of the N/P ratio, such as the types of positive and negative active materials, charging rate, and charge-discharge cutoff voltage. Therefore, during the battery design process, it is important to consider the characteristics of N/P ratio changes to avoid lithium deposition on the negative electrode caused by a decrease in the N/P ratio.