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 DepositionThe 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.
From a thermodynamic perspective, as the ambient temperature decreases, the charge transfer resistance increases, and the anode potential lowers. When the potential drops to the lithium deposition potential, lithium ions are deposited as metallic lithium on the anode surface. According to a kinetic analysis, as the temperature decreases, the chemical reaction rate also slows down.
When charging at low temperatures, the diffusion rate of lithium ions in the electrolyte, SEI film, and solid-phase graphite all decrease. With the energy barrier remaining unchanged, the probability of lithium intercalation decreases. As a result, a large number of lithium ions on the anode acquire electrons and undergo lithium deposition.
Therefore, when using lithium-ion batteries at low temperatures, it is necessary to reduce electrode polarization resistance and increase the diffusion rate of lithium ions in the electrolyte, SEI film, and solid-phase graphite to avoid lithium deposition on the anode.
7.Fast ChargingDuring ultra-fast charging, the current density per unit area on the electrode surface is high, meaning the concentration of lithium ions is also high. The driving force for lithium ions to intercalate into the solid phase of the graphite anode is the concentration gradient.
When the lithium ion transfer rate is slow (due to low temperature, high state of charge (SOC), or materials with higher energy barriers) and the current density is relatively high during charging, lithium deposition will occur.
In addition, fast charging can drive the anode to reach the lithium deposition potential, leading to lithium deposition. Therefore, at low state of charge (SOC), if the battery is charged at a high rate, as the SOC increases, it is advisable to switch to a lower current charging method to avoid lithium deposition.
After a certain period of time, the deposited metallic lithium will re-intercalate into the graphite crystal structure to reduce the loss of active lithium.
8.OverchargingOvercharging refers to the behavior where the charging voltage exceeds the upper cutoff voltage after the battery is fully charged. The degree of overcharging in a lithium-ion battery is typically represented by the battery's state of charge (SOC). When the SOC exceeds 185%, the surface of the anode becomes completely covered with metallic lithium.
For power batteries, including products from the top ten lithium iron phosphate (LFP) battery manufacturers, it is necessary to use individual cells in series and parallel. If the voltage, internal resistance, and capacity consistency of each individual cell are poor, it is easy for a single cell to become overcharged, leading to lithium deposition on the anode surface and potentially causing safety incidents.
The overcharge issue of lithium-ion batteries can be controlled from two aspects:
1.Control through the Battery Management System (BMS).
2.Iternally, by increasing the oxidation potential of the electrolyte and raising the initial temperature for thermal runaway of the battery.
The lithium ion flow phenomena in the anode active region and the overhang (excess) region are closely related to changes in battery capacity and the deposition of lithium on the anode.
For example, during the charging process, due to the presence of the overhang, when charging is complete, the overhang of the anode is not fully lithified, resulting in a gradient distribution of lithium at the edge of the anode. During subsequent resting periods, the lithium embedded in the anode plate will diffuse from the center towards the edge.
The overhang region still contains uninserted lithium, which means that during discharge, the edge of the cathode not only receives lithium ions directly from the anode region opposite it but also from the lithium ions removed from the overhang of the anode.
As the number of charge-discharge cycles increases, the lithium concentration at the edge of the cathode will become higher, leading to an increased tendency for lithium deposition at the edge of the anode during charging. Therefore, within the manufacturing quality and precision limits of the electrode sheets and manufacturing equipment, the overhang should be designed to be as small as possible to avoid lithium deposition.
10.Methods to Address Anode Lithium Deposition1.Battery Structure Optimization: The structure of the battery is closely related to the lithium deposition window on the anode. For example, reducing the overhang can prevent a large amount of lithium ions from migrating from the cathode edge to the anode edge during charging, thus reducing edge lithium deposition.
Using a multi-tab design can ensure a more uniform current density distribution during the charging process, preventing localized lithium deposition caused by excessive local current density. In addition, an appropriate N/P ratio (the ratio of the number of positive to negative electrodes) is also an effective measure to suppress anode lithium deposition.
2.Electrode Quality Control: The manufacturing steps of the electrode sheets include slurry preparation, electrode coating, and electrode calendaring. These steps affect the porosity, refractive index, and surface density of the electrodes, which in turn influence the current distribution during the battery's charging process.
The impact of the electrode sheets (both cathode and anode) on anode lithium deposition is mainly reflected in: lithium deposition caused by insufficient slurry mixing or coating defects, and large-area lithium deposition due to inadequate anode lithium insertion kinetics caused by excessive calendaring of the electrode sheets.
Avoiding lithium deposition on the anode can be achieved by reducing the overpotential of the graphite anode and increasing the overpotential for lithium deposition on the anode surface. Lithium deposition is an electrochemical crystallization process, where nucleation occurs first, followed by growth. The driving force for growth is the difference in interfacial energy between the anode surface and metallic lithium.
By depositing a nanometer-thick metallic layer on the anode surface using magnetron sputtering, the overpotential for lithium metal deposition can be increased, thereby weakening the driving force for lithium metal growth, which helps improve lithium deposition control.
Additionally, using laser ablation to create a pit array on the anode surface can effectively reduce the diffusion resistance and charge transfer resistance of lithium ions at low temperatures, thereby reducing the risk of lithium deposition.
12.Anode Material OptimizationThe lithium insertion kinetics of the graphite anode can be described by the energy barrier. The energy barrier for lithium ions inserting into the graphite crystal from the edge and basal plane are 0.3–0.7 eV and 10 eV, respectively. Even if defects are present on the basal plane of the graphite, the energy barrier for lithium insertion into the basal plane is still an order of magnitude higher (2.36–6.35 eV) compared to the edge.
Therefore, lithium ions are more likely to insert between the graphite layers from the edge. The edges of the graphite are divided into two types: chair-edge and zigzag-edge.
Research on doping boron (B) and nitrogen (N) on the chair and zigzag edges has shown that doping boron (B) on the chair-edge type of the edge reduces the Fermi level and increases the adsorption energy, which is beneficial for improving the lithium insertion kinetics of graphite
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