One of the main concerns regarding Li-ion batteries is their aging process, which affects their capacity and overall life span. Several factors contribute to the aging process at the material particle, cell, and battery pack levels, as listed below:
1. Solid Electrolyte Interphase (SEI): SEI is a passivation layer formed on the electrode surfaces. While it's essential for stable battery operation, its continuous growth consumes lithium ions and reduces the battery's capacity. Additionally, an unstable SEI can increase internal resistance.
2. Lithium Plating: During charging, especially at low temperatures or high rates, lithium can deposit onto the graphite anode instead of intercalating within. This metallic lithium plating reduces cyclable lithium, leading to capacity loss and raising safety concerns due to the risk of internal shorts.
3. Electrode Cracking: Repeated lithiation and delithiation can cause volume changes in the electrodes. Over time, this can lead to mechanical stress and cause cracking in the electrode materials, further leading to capacity fade and impedance rise.
4. Electrode Dissolution: Transition metal ions from cathodes, especially from high-capacity cathodes like NMC or NCA, can dissolve into the electrolyte. This can lead to deposition on the anode, contaminating it and harming the battery’s performance.
5. Electrolyte Decomposition: High voltages and elevated temperatures can cause the electrolyte to decompose. Decomposition products can deposit on electrodes, increasing the internal resistance and reducing ion mobility.
6. Non-uniform Current: Uneven current distribution can lead to localized overcharging or over-discharging, causing accelerated aging in specific areas of the battery.
7. Non-uniform Thermal Management: Inefficient thermal management can lead to hotspots. Elevated temperatures can accelerate side reactions, resulting in faster degradation.
8. Particle Cracking: Active material particles, under the stress of ion insertion and extraction, can crack, reducing the active surface area and impeding ion transport.
9. Particle Dissolution: Some active material particles may dissolve into the electrolyte, especially at high voltages. This can result in capacity loss and harm the SEI layer.
10. Particle Phase Transition & Gas Evolution: Changes in the crystalline structure of electrode materials can occur during cycling. Some phase transitions might be irreversible, leading to capacity loss. Additionally, certain reactions within the battery can generate gases, causing swelling and even rupture.
Addressing these complex issues demands advancements in material science, battery design, and management systems to prolong the life of lithium-ion batteries.