Li-ion Battery Degradation: Understanding SEI Growth and Aging Processes

What is SEI Growth?

SEI stands for Solid Electrolyte Interface, a thin layer that forms on the surface of lithium-ion batteries during their first charge-discharge cycle. This layer primarily consists of lithium salts and electrolyte components. Its formation is crucial as it protects the battery from short circuits but also contributes to its degradation over time.

The SEI layer grows with each charge-discharge cycle, leading to increased internal resistance and reduced battery capacity.

Factors Affecting Battery Degradation

Battery degradation is influenced by several factors including temperature, depth of discharge (DoD), and charging rate. Higher temperatures accelerate SEI growth and chemical reactions within the cell, leading to faster degradation. Cycling at higher DoDs or faster rates also accelerates these processes.

Calendar aging refers to the gradual loss of battery capacity over time even when not in use, primarily due to chemical changes within the battery components.

Arrhenius Temperature Dependence

The Arrhenius equation describes how reaction rates increase with temperature. In Li-ion batteries, this means that higher temperatures can significantly accelerate SEI growth and other degradation processes. The equation is typically expressed as k = A * exp(-Ea/RT), where k is the rate constant, A is a pre-exponential factor, Ea is the activation energy, R is the gas constant, and T is the temperature in Kelvin.

By understanding this relationship, engineers can design batteries that perform optimally within specific temperature ranges to minimize degradation.

Cycle Fade and Calendar Ageing

Cycle fade refers to the loss of battery capacity over cycles. This occurs due to various factors such as SEI growth, electrolyte decomposition, and mechanical stress from repeated expansion and contraction during charging and discharging.

Calendar ageing is the degradation that happens even when a battery is not in use. It involves chemical changes within the battery components, leading to a gradual loss of capacity over time.

Frequently asked questions

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