Mechanisms of Lithium - battery Cell Capacity Fading

The capacity fading of lithium - battery cells is a complex phenomenon that significantly impacts the performance and lifespan of lithium - ion batteries, and understanding its underlying mechanisms is crucial for developing strategies to mitigate this issue. Several interrelated physical and chemical processes contribute to the gradual decline in cell capacity over time.

One of the primary mechanisms is the degradation of the solid - electrolyte interface (SEI) film. The SEI film forms on the anode surface during the initial charging process of the lithium - battery cell. It serves as a protective layer, preventing the electrolyte from further reacting with the anode material. However, during repeated charge - discharge cycles, the SEI film continuously grows and deteriorates. As lithium ions shuttle between the cathode and anode, small amounts of lithium are consumed in the formation and growth of the SEI film. Additionally, volume changes in the anode material during lithium - ion insertion and extraction can cause the SEI film to crack and break, exposing fresh anode surface and leading to further SEI formation. This continuous consumption of lithium ions and SEI degradation reduces the available lithium for electrochemical reactions, ultimately resulting in capacity fading.

Another significant factor is the structural changes in the electrode materials. In the cathode, materials such as lithium - cobalt - oxide (LCO), lithium - nickel - manganese - cobalt - oxide (NMC), and lithium - iron - phosphate (LFP) undergo phase transitions during charging and discharging. These phase transitions can cause lattice distortion and structural collapse over time, especially when the battery is subjected to high - voltage charging, high - temperature operation, or deep - discharge conditions. The structural degradation reduces the ability of the cathode material to intercalate and deintercalate lithium ions effectively, leading to a decrease in capacity. Similarly, in the anode, graphite - based anodes experience volume expansion and contraction during lithium - ion insertion and extraction. Repeated volume changes can cause the anode particles to crack and break, disrupting the electrical contact and reducing the active surface area available for lithium - ion storage, which also contributes to capacity fading.

Side reactions within the electrolyte also play a role in capacity fading. At elevated temperatures or under extreme charging and discharging conditions, the electrolyte can decompose, generating gas and other by - products. These side reactions not only consume electrolyte components but also can deposit on the electrode surfaces, blocking the diffusion of lithium ions and further degrading the battery's performance. Moreover, the presence of impurities in the electrolyte or electrode materials can catalyze these side reactions, accelerating the capacity - fading process. In addition, the growth of lithium dendrites on the anode surface during overcharging or fast - charging can cause internal short - circuits, leading to sudden capacity loss and potential safety hazards. By comprehensively understanding these capacity - fading mechanisms, researchers can develop targeted solutions, such as improved electrode materials, optimized electrolyte formulations, and advanced battery management strategies, to slow down the capacity - fading process and extend the lifespan of lithium - battery cells.

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