Lithium Plating Model Analysis

I. Overview of Lithium Plating Mechanism

Lithium plating is a phenomenon where lithium ions from the electrolyte are deposited as metallic lithium on the anode, rather than being intercalated into the anode particles. This occurs when the anode potential drops below that of Li/Li+. For graphite, the intercalation potential is between 65 to 200 mV (vs. Li+/Li0). When the anode’s potential approaches or falls below the metallic lithium’s deposition potential, lithium ions precipitate on the anode surface as metallic lithium.

Key Points of Lithium Plating

1. Formation of Lithium Droplets: Initially, lithium metal forms droplets to minimize surface energy. It then reacts quickly with the electrolyte to create a Solid Electrolyte Interphase (SEI) membrane. As more lithium is deposited beneath the SEI until it ruptures, a new SEI forms. The lithium salt concentration decreases, and lithium metal begins to grow perpendicularly to the anode surface, forming lithium dendrites. Dendrite growth is considered one of the most detrimental side reactions, as it can lead to internal short circuits and rapid heating if it pierces the separator and reaches the cathode.
2. Simultaneous Reactions: Experiments have shown that the precipitation of lithium ions on the anode surface and their intercalation into graphite occur simultaneously. During charging, some lithium ions are deposited as metallic lithium on the anode surface, while the rest are intercalated into graphite. During discharging, deintercalation of ions and stripping of the deposited metallic lithium take place, leading to the formation of “dead lithium.” This “dead lithium” reacts with the electrolyte, causing capacity loss and a reduction in the battery’s cycle life.

Research Models

Researchers have proposed models for observing lithium plating, including the P2D model-based lithium plating model by Fuller, Doyle, and Newman, and the reversible lithium re-embedding process by Arora, Doyle, and White. Perkins proposed a reduced-order model for control, while Hein and Latz introduced a three-dimensional microstructure analysis model. Ren considered both reversible lithium re-embedding and the reaction of irreversible lithium (dead lithium) during battery charging.

II. Chemical Reactions of Lithium Plating

During the lithium ion charging process, if metallic lithium is precipitated on the surface of the graphite anode, the following four chemical reactions occur:

1. Intercalation Reaction:
2. Lithium Metal Precipitation Reaction:
3. Reaction between Precipitated Lithium and Unsaturated Graphite, Forming Reversible Lithium:
4. Reduction Reaction of Precipitated Metallic Lithium with Electrolyte Solvent, Forming a Solid Electrolyte Interphase (SEI) Membrane, Forming Irreversible Lithium:

III. Detection of Lithium Plating

Non-destructive observation of lithium plating is crucial for practical battery applications. Common characterization methods include SEM, TEM, NMR, and XRD, but they require battery destruction or special battery configurations. Non-destructive lithium plating observation often utilizes external battery characteristics, such as aging rate, lithium re-embedding voltage platform, and model predictions. Detection methods based on aging characteristics include:

• Arrhenius Equation: A method used to predict the rate of chemical reactions based on temperature.
• Capacity and Impedance Change Analysis: Analyzing the changes in capacity and impedance during the decay process.
• Nonlinear Frequency Domain Response Analysis: Examining the battery’s response in the frequency domain.
• Coulombic Efficiency Analysis: Assessing the efficiency of lithium ion transfer during charging and discharging cycles.

Some precipitated lithium can re-embed into the anode or dissolve during discharging. The process of lithium precipitation and re-embedding may also cause abnormal heat peak values, signaling lithium plating. An increase in battery thickness may lead to lithium plating, but further research is needed to understand the mechanism.

IV. Factors Leading to Lithium Ion Precipitation During Charging

Several factors can lead to lithium ion precipitation during the charging process of lithium-ion batteries:

• Insufficient Anode Capacity: When the anode capacity is insufficient to accommodate all the lithium ions that are deintercalated from the cathode, lithium ions will precipitate on the anode surface.
• Improper Charging Mechanisms: Such as low-temperature or high-rate charging, which can prevent timely lithium ion intercalation into the anode.
• Abnormal Intercalation Paths: Issues with the separator and anode material can cause lithium ion precipitation on the anode surface.
• Charging at Low Temperatures: Low temperatures are prone to lithium plating.
• Abnormal Battery Materials: High anode material compaction density, mismatch with the electrolyte, damage to the anode material, or insufficient anode material surface density margin.
• Improper Charger Selection and Charging Methods: Using the wrong charger model can lead to excessive current charging and increased risk of lithium plating.
• Improper Use and Maintenance of the Battery: Deformation from external forces, excessive gas production at high temperatures, or changes in the internal material contact interface due to battery aging.

To prevent lithium plating, it is recommended to use the appropriate charger, avoid charging at extreme temperatures, and regularly check the battery’s health. Battery manufacturers can also reduce the risk of lithium plating by improving battery design and manufacturing processes and using materials and technologies suitable for fast charging.