Negative Electrode Lithium Plating Issues and Solutions

 

The lithium intercalation potential of graphite is 65–200 mV (vs. Li⁺/Li⁰). When the negative electrode's potential approaches or falls below the deposition potential of metallic lithium, lithium ions deposit as metallic lithium on the negative electrode surface.

 

Experiments have found that the lithium plating reaction on the negative electrode surface and the lithium intercalation reaction into graphite occur simultaneously. During the charging process, a portion of lithium ions deposit as metallic lithium on the negative electrode surface, while the remaining portion intercalates into the graphite. During the discharging process, lithium deintercalation and the stripping of deposited metallic lithium occur. During the stripping of metallic lithium, "dead lithium" can form. The reaction of "dead lithium" with the electrolyte is a major cause of capacity loss and shortened cycle life in lithium-ion batteries. Negative electrode lithium plating is the result of Charge Transfer Limitation (CTL) and Solid-state Diffusion Limitation (SDL). As charging proceeds, the available sites for lithium intercalation within the graphite layers gradually decrease, limiting the diffusion of lithium ions within the graphite solid phase, and the corresponding intercalation current also gradually decreases. Simultaneously, because the rate at which lithium ions diffuse from the electrolyte to the negative electrode is much faster than the rate at which they intercalate into the graphite, more and more lithium ions accumulate on the graphite surface. This drives the negative electrode potential towards the lithium plating potential, leading to lithium plating on the negative electrode.

 

 

Typically, to observe whether lithium plating has occurred on the negative electrode or to assess its severity, batteries are fully charged, disassembled, and the interface condition is examined. The most common distribution patterns include edge plating, localized plating, strip-like plating, and uniform plating.

 

 

In the design process of lithium-ion batteries, to ensure the positive electrode is fully utilized, the negative electrode is usually designed to be slightly in excess of the positive electrode, i.e., the negative electrode extends beyond the positive electrode dimensions by 1–3 mm. The area where the negative electrode extends beyond the positive electrode is called the Overhang. Edge plating is primarily caused by two factors: First, an Overhang design that is too large leads to an excess of lithium ions at the positive electrode edge, causing the negative electrode Overhang area to be unable to intercalate the excessive lithium ions from the positive electrode during charging, resulting in plating. Second, mismatched area densities at the edges of the positive and negative electrodes due to the "thick edge effect" during the coating process, such as a higher area density at the positive electrode edge or a lower area density at the negative electrode edge, can also cause plating.

 

 

Localized plating is distributed relatively randomly, with unfixed locations, often appearing as discontinuous, spot-like patterns. Causes for this type of plating mainly include: localized deformation of the cell, local defects in the electrode sheets, local defects in the separator, air bubbles between the separator and electrode sheets, and insufficient electrolyte wetting.

 

 

Uniform plating refers to the uniform appearance of lithium plating across the surface of the negative electrode sheet. This is mainly caused by several reasons: an excessively large charging current during testing; overcharge or over-discharge; excessive compaction density of the negative electrode; or positive electrode coating near the upper specification limit or negative electrode coating near the lower specification limit.

 

 

 

The N/P ratio is the ratio of the negative electrode capacity to the positive electrode capacity in a lithium-ion battery: N/P = (q_negative specific capacity * m_negative) / (q_positive specific capacity * m_positive)

 

The N/P ratio is an important factor affecting battery safety. A lower N/P ratio prevents the internal structure of the negative electrode from fully accommodating all lithium ions coming from the positive electrode, causing them to plate on the negative electrode surface. Conversely, a higher N/P ratio can, to some extent, prevent lithium plating, but it may lead to excessive delithiation of the positive electrode. Furthermore, the N/P ratio constantly changes during battery cycling. For example, with high-nickel positive electrode materials prone to structural collapse and dissolution, the N/P ratio tends to increase as cycles progress. For silicon-based negative electrode materials that experience volume expansion and particle cracking, the N/P ratio may decrease.

 

 

During high-rate charging, the electrode surface experiences a high current density per unit area. The driving force for lithium ions moving from the positive electrode to embed into the graphite solid phase at the negative electrode surface is the concentration gradient. At this point, the graphite structure cannot quickly accommodate so many lithium ions. Especially in graphite structures near the current collector, lithium ions may not have intercalated yet, so they plate on the negative electrode surface.

 

 

The lower the temperature, the higher the charge transfer resistance and the slower the chemical reaction rate. The diffusion rates of lithium ions in both the electrolyte and within the graphite solid phase decrease. Therefore, with the energy barrier remaining constant, the probability of lithium intercalation also decreases.

 

 

At first glance, a larger Overhang should make lithium plating less likely because a larger negative electrode makes it easier for lithium from the positive electrode to intercalate. So, what causes plating in this case? The main reason is that lithium diffusion within the negative electrode and lithium ion deintercalation are not entirely perpendicular processes.

 

For example, during the charging process of a battery with an Overhang region, when charging is complete, the negative electrode Overhang area is not fully lithiated. A pronounced lithium concentration gradient forms at the edge of the negative electrode sheet. During subsequent rest periods, the lithium intercalated in the negative electrode sheet diffuses from the center towards the edges. After discharging, lithium that has not deintercalated remains in the Overhang area. This indicates that during discharge, the edge of the positive electrode sheet not only receives lithium ions from the negative electrode area directly opposite it but also receives lithium ions deintercalating from the negative electrode Overhang area. As cycling increases, the lithium concentration at the edge of the positive electrode sheet becomes higher and higher, making it prone to cause lithium plating at the negative electrode edge during charging. Therefore, a larger Overhang area is not necessarily better. It should be minimized as much as possible while meeting design requirements to avoid lithium plating.

 

 

Overcharge refers to the act of continuing to charge a battery after it is fully charged, causing the charging voltage to exceed the upper cutoff voltage. Full charge corresponds to 100% SOC; exceeding this constitutes overcharge and can cause serious damage to the battery. Normally, during single-cell testing, overcharge typically does not occur. However, for batteries connected in series or parallel to form modules, if there is poor consistency within the batch and significant capacity differences exist, it is easy for some batteries to still be charging while others have already been overcharged.

 

 

 

The design quality of the cell structure significantly impacts negative electrode lithium plating. We can reduce lithium plating by minimizing the Overhang area and setting a reasonable N/P ratio. Additionally, employing a multi-tab design helps to uniformly distribute the current density within the cell during charging, avoiding localized plating caused by excessive local current.

 

 

Electrode sheet manufacturing includes: slurry mixing, coating, and calendering. All three processes significantly impact battery performance. The quality of slurry mixing affects material dispersion. Uneven dispersion can lead to local defects in the electrode sheet, causing localized plating. Simultaneously, unstable slurry viscosity can cause issues during the coating process, affecting the electrode sheet's area density and potentially leading to large-area plating. Excessive compaction of the electrode sheet can lead to insufficient lithium intercalation kinetics in the negative electrode, causing widespread plating.

 

 

Negative electrode lithium plating is influenced by negative electrode polarization and lithium intercalation kinetics. These influencing factors are related to the mechanical properties, chemical stability, and ionic conductivity of the SEI (Solid Electrolyte Interphase) film. Functional additives (film-forming agents) in the electrolyte help improve the quality of the SEI film. Therefore, developing suitable film-forming agents is also an effective approach to solving negative electrode lithium plating.

 

 

As mentioned earlier, high-rate rapid charging/discharging, overcharging, and low-temperature charging can easily cause negative electrode lithium plating. Based on these patterns, we can solve the lithium plating problem by optimizing the charging procedure.

 

 

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