What is the capacity degradation mechanism of thick electrodes in lithium-ion batteries?

　　The thick electrode design of lithium-ion batteries is aimed at increasing the energy density of the battery to meet the needs of high range new energy vehicles. However, during long-term cyclic use, the electrochemical performance of thick electrodes will significantly deteriorate, mainly due to the following mechanisms:

　　1. Lithium ion transport limitation: In thick electrodes, lithium ions need to migrate over long distances inside the electrode, which leads to limitations in lithium ion transport. Especially on the side of the electrode close to the current collector, due to the slower migration rate of lithium ions, the electrochemical reaction rate is lower, while on the side close to the electrode liquid, due to the higher concentration of lithium ions and electrons, the reaction can proceed better.

　　2. Electronic transmission limitation: In addition to lithium ion transmission, the transmission of electrons in the electrode may also become a limiting factor. On the side of the electrode near the current collector, electron migration is hindered, resulting in a decrease in the electrochemical reaction rate in that area.

　　3. SOC non-uniformity: Due to the non-uniformity of SOC (State of Charge) caused by material transport, thick electrodes undergo a relatively high proportion of electrochemical reactions near the top of the diaphragm, while reactions are scarce near the bottom of the current collector. This non-uniformity intensifies with cycling, leading to a decline in battery performance.

　　4. Formation of current hotspots: In thick electrodes, due to the non-uniformity of SOC, current hotspots may form in certain areas of the electrode. The high current density in these areas can damage the active material, causing cracks or irreversible phase transitions.

　　5. Particle rupture and interface side reactions: High current density may also form a large gradient of solid-phase lithium ion concentration distribution in secondary particles, generating a stress field that leads to particle breakage, exposure of new interfaces, side reactions, and formation of thick organic layers at new interfaces, causing impedance increase and ultimately resulting in battery failure.

　　6. Electrolyte decomposition: Electrolyte decomposition is also an important factor leading to battery capacity degradation. The decomposition of electrolyte may lead to the loss of active materials, thereby affecting the capacity and cycling stability of the battery.

　　7. The impact of overcharging cycles: Overcharging cycles can exacerbate capacity degradation and safety issues in lithium-ion batteries. Overcharging can lead to electrolyte decomposition, loss of active materials, and growth of lithium dendrites, all of which have a negative impact on the performance and lifespan of the battery.

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