The SEI layer or solid electrolyte interphase layer is a component of lithium-ion batteries, formed from the decomposition materials associated with the electrolyte of the battery.

The interfacial reactions relating to the SEI layer are a vital factor in battery function, particularly the reactivity of the electrode material and the oxidation/reduction reactions that occur on the surface particles of the respective anode and cathode. Therefore, the SEI layer has important implications for the performance of lithium-ion batteries in terms of cycle life limitations, the capacity for reversibility and safety.

Atomic layer deposition (ALD) coating has been demonstrated to improve lithium-ion batteries by stabilizing or preventing the SEI layer. Forge Nano has developed ALD coating techniques that reduce the SEI layer in lithium-ion batteries leading to a greater cycle life.

Importance of the SEI Layer to Lithium Ion Battery Design and Functioning

The formation of the passivating SEI layer is a fundamental factor in the design and functioning of high-performance batteries. The role of the SEI layer involves the prevention of further electrolyte decomposition to maintain cycling ability. This requires that the SEI layer is well adhered to the electrode material, has good electronic insulation properties and the ability to conduct lithium ions.

To enhance lithium-ion battery operation, the quality of the SEI layer on both the positive and negative electrode surfaces must be tuned. This is because variation in porosity and thickness affect the conductivity of lithium ions through the SEI layer.

SEI Layers and the Interfacial Reactions of Anode and Cathode Particles

Interfacial reactions on the surface particles of both the anode and cathode affect the performance of lithium ions through the SEI layer. The irreversible reactions occurring on the electrode surface can be split into cathodic reduction reactions and anodic oxidation reactions.

When traces of water and oxygen are present in the electrolyte, reduction of water and oxygen occurs, followed by the reduction of the electrolyte components of solvent and salt into an SEI layer. The formation of the SEI layer can also occur on the positive anode through electrolyte oxidation.

The SEI layer formation on the cathode particle surface from electrolyte decomposition is formed in the first five charge-discharge cycles of carbonaceous electrodes. The composition and stability of the SEI layer is dependent on the type of electrolyte and the electrode material.

The formation of the SEI layer on positive anode particles has been less well studied due to the thin layer that is produced on positive electrodes. The SEI layer formed on the anode has more limited passivation ability as it is primarily composed of organic compounds.

The oxidation of the electrolyte forms an SEI layer that is a few nanometers in thickness. The layer is therefore not dense enough to provide a barrier between the electrolyte and oxidizing environment, contrasting with the SEI layer formed on the cathode.

The Effect of ALD Coatings on the Rate of SEI Layer Growth

Atomic layer deposition (ALD) coatings are proven to stabilize and prevent the formation of SEI layers. The ability to add sub-nanometer thickness coatings of conducting material to form a passivated layer increases the power capability of lithium-ion batteries. Previous studies have highlighted the range of cathode particles that can be deposited by ALD, though the deposition of anode materials by ALD is more limited.

The high conductivity coatings developed by Forge Nano have been found to inhibit the rate of SEI layer growth on anode surfaces. The reduction in the SEI layer allows for lower impedance profiles to be maintained with the retention of power density and a higher cycle life. The fact that these results are independent of operating temperature means that the tailored ALD coatings developed by Forge Nano can improve the performance of graphite anode materials at low temperatures.

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