What is Lithium Plating?

Lithium plating is a mechanism of degradation in lithium-ion batteries (LIBs). It describes the accumulation of metallic lithium on the surface of the anode (usually graphite powder). Lithium ions gather at the anode surface and form metallic deposits. This happens when the electrochemical potential of the anode is equal to or lower than the potential of metallic lithium.

Typically, lithium plating occurs when the battery is undergoing rapid charging or at extreme temperatures. In extreme cases, lithium plating leads to the formation of long dendritic strings of lithium which can penetrate the polymer separator or even break off within the LIB. As a result, the battery is likely to internally short or worse, suffer dangerous failures such as fires.

Understanding how and why lithium plating forms is crucial for the future development of LIBs. Identifying the degradation mechanism in real-time is difficult as it requires the detection of lithium plating during battery use.

What Causes Lithium Plating?

---

When the anode potential is close to or less than the Li/Li+ potential then lithium plating occurs. This is because lithium ions undergo reduction, to form solid lithium rather than intercalating into the anode material. Graphite is the most widely used anode material in LIBs due to its excellent structural stability, high energy density, and ability to reversibly intercalate lithium ions. During charging, lithium ions move from the cathode to the anode and intercalate into the graphite layers. However, the electrochemical potential of fully lithiated graphite is very close to that of metallic lithium, making it susceptible to lithium plating under certain conditions.

Key Factors Leading to Lithium Plating

• Low Anode Potential
  • Lithium plating occurs when the anode potential drops close to or below the Li/Li^⁺ equilibrium potential (0V vs. Li/Li^⁺).
  • This happens when the lithium-ion intercalation rate is too slow, causing excess lithium ions to deposit as metallic lithium instead of intercalating into the graphite structure.
• High Charging Rates (Fast Charging)
  • Rapid charging increases the lithium-ion flux towards the anode, reducing the time available for proper intercalation.
  • If the intercalation process is not fast enough, excess lithium ions accumulate on the anode surface and undergo reduction to form metallic lithium.
• Low Temperature
  • At lower temperatures, lithium-ion diffusion into graphite is slower due to reduced ionic mobility and sluggish electrode kinetics.
  • This increases the likelihood of lithium plating since lithium ions are not efficiently intercalated into the anode material.
• Overcharging or Overvoltage Conditions
  • Applying an excessive voltage during charging can drive an excessive amount of lithium towards the anode, overwhelming its capacity to intercalate lithium.
  • This can lead to lithium deposition on the surface instead of proper intercalation.
• High Lithium Concentration Near the Anode Surface
  • Poor electrolyte transport properties or local depletion of lithium ions can create an imbalance where more lithium ions arrive at the anode surface than can be accommodated, leading to plating.
• Aging and Degradation of Battery Materials
  • As LIBs age, structural and chemical changes in the graphite anode can make lithium intercalation less efficient.
  • Degradation of the solid electrolyte interphase (SEI) layer can expose fresh electrode surfaces, increasing the risk of lithium plating.

Why is Lithium Plating an Issue?

---

Lithium plating is a big issue in the development of fast-charging lithium-ion batteries, particularly those found in electric vehicle or other high-demand portable devices. Rapid charging requires high current densities, which can lead to an influx of lithium ions within the graphite anode. This can cause a significant drop in the anode potential, eventually leading to lithium plating on the surface of the anode instead of proper intercalation into the graphite structure.

Temperature extremes further exacerbate this issue by negatively affecting the anode's electrochemical performance:

• At high temperatures, there is impedance on the anode as solid electrolyte interphase formation (SEI) is kinetically more favorable, often causing unstable or uneven growth. 
• At low temperatures, there is impedance at the anode as the intercalation of lithium ions is kinetically less favorable.

At both extremes, the conditions favor lithium plating and battery decomposition. This not only contributes to a loss in battery capacity but also raises significant safety concerns.

Large dendritic formation of lithium during plating can pierce the separator and even reach the cathode. At this point, an internal short circuit is formed which can result in rapid cell heating, swelling and even rupture. One safety concern here is thermal runaway where the short circuit or exothermic decomposition reactions can trigger a self-heating chain reaction. In the most extreme cases this can lead to fires and even explosions.

How to Monitor Lithium Plating?

---

It is difficult to monitor lithium plating as battery cells are closed environments when they are in use. Degradation is complex and can involve a combination of variables depending on cell design, materials used, operating conditions, battery age and the number of charge cycles it has endured.

However, there are some key battery readings that can be monitored whilst it is in use, including:

• Voltage
• Time
• Temperature

These measurements can be translated into lithium plating parameters for any type of lithium-ion battery:

• Plating Energy
• Plating Period
• Plating Power

“Plating Energy” is the amount of energy consumed by the anode during plating and is used to quantify lithium plating within a battery cell. The plating energy can be determined by monitoring the anode potential profile during charge. Plating energy assumes all lithium plating is irreversible to determine the maximum amount of plating experience by the electrode using the following equation:

C

Constant of reversibility (C=1 - completely irreversible, C=0 - completely reversible)

Va Ia

Anode Potential (V) Anode Current (Ah)

Anode potential can be measured using a reference electrode in a three-electrode setup. In this configuration, the working electrode is the anode, the counter electrode is the cathode, and the reference electrode is Li/Li^⁺. The reference electrode is typically positioned near the anode to ensure accurate potential measurement.

The profiles below compare two scenarios: one where lithium plating occurs and one where it does not:

Lithium plating occurs when the anode potential drops below the Li/Li^⁺ equilibrium potential (0V vs. Li/Li^⁺). Plating may continue as long as the anode remains at or below this potential, and it stops when conditions (e.g., charge progression or increased anode potential) favor lithium intercalation instead.

Anode Active Materials

Browse now

Learn More

---