Capacity degradation in lithium-ion batteries is categorized into reversible capacity loss and irreversible capacity loss. Reversible capacity loss is relatively "mild" and can be partially recovered by adjusting charge-discharge protocols (e.g., optimizing charging current, voltage limits) and improving usage conditions (e.g., temperature/humidity control). In contrast, irreversible capacity loss arises from irreversible changes within the battery, leading to permanent capacity reduction. According to GB/T 31484-2015 standards for cycle life testing: "During standard cycle life testing, the discharge capacity shall not fall below 90% of the initial capacity after 500 cycles, or 80% after 1,000 cycles." If the battery exhibits rapid capacity decline within these standard cycle ranges, it is classified as capacity fade failure, typically involving irreversible degradation mechanisms.
I. Material-Related Factors
1. Cathode Material Structural Degradation
Cathode materials undergo complex physical and chemical changes during charge-discharge cycles. Taking spinel-structured LiMn₂O₄ as an example, its structure distorts due to the Jahn-Teller effect during cycling. This distortion accumulates with repeated cycles and may eventually cause cathode particle fracture. Fractured particles degrade electrical contact between particles, hindering electron transport and reducing capacity. Additionally, irreversible phase transitions and structural disordering occur in some cathode materials. For instance, under high voltage, certain cathode materials transition from stable crystal structures to phases unfavorable for lithium-ion intercalation/de-intercalation, impeding lithium-ion mobility and accelerating capacity loss.
2. Excessive SEI Growth on Anode Surfaces
For graphite anodes, interactions between the surface and electrolyte are critical. During the initial charging process, components in the electrolyte undergo reduction reactions on the graphite surface, forming a solid electrolyte interphase (SEI) layer. Normally, the SEI layer is ionically conductive but electronically insulating, protecting the anode from continuous electrolyte corrosion. However, excessive SEI growth poses significant issues. First, SEI formation consumes lithium ions, reducing the available Li⁺ for normal charge-discharge processes and causing capacity loss. Second, transition metal impurities (e.g., from cathode dissolution) deposited on the anode surface can catalyze further SEI growth, accelerating lithium depletion.
3. Electrolyte Decomposition and Degradation
The electrolyte plays a vital role in ion transport. Common lithium salts like LiPF₆ exhibit poor chemical stability and decompose under high temperatures or voltages, reducing available Li⁺ and generating harmful byproducts (e.g., PF₅, which reacts with solvents). Trace moisture in the electrolyte reacts with LiPF₆ to produce hydrofluoric acid (HF), a corrosive agent that attacks cathode/anode materials and current collectors. Poor battery sealing allows external moisture/oxygen ingress, accelerating electrolyte oxidation. Degraded electrolytes exhibit increased viscosity, discoloration, and drastically reduced ionic conductivity, severely impairing battery performance.
4. Current Collector Corrosion
Current collectors (e.g., aluminum foil for cathodes, copper foil for anodes) collect and conduct current. Failures include corrosion and weakened adhesion. Corrosion mechanisms include:
5. Trace Impurities in the Battery System
Transition metal impurities (Fe, Ni, Co) introduced via raw materials may participate in redox reactions, catalyze electrolyte decomposition, or compete with Li⁺ intercalation. These impurities also destabilize SEI layers, exacerbating anode side reactions.
II. Operational Environmental Factors
1. Temperature Effects
• High temperatures accelerate electrolyte decomposition and SEI restructuring. LiPF₆ degradation generates PF₅, which reacts with solvents, while SEI layers thicken into inorganic-dominated films with higher ionic resistance. For example, EVs operating in hot climates exhibit accelerated capacity fade.
2. Charge-Discharge Rates (C-Rates)
High C-rates during charging cause uneven lithium deposition, forming dendrites that consume Li⁺ and risk internal shorts. High-rate discharging exacerbates polarization, reducing usable energy and accelerating capacity loss. Power tools requiring frequent high-current discharge demonstrate shortened battery lifespans.
3. Overcharge/Over-Discharge
• Overcharge forces excessive delithiation of cathodes, causing structural collapse and violent electrolyte oxidation (gas generation, swelling, or thermal runaway).• Over-discharge over-lithiates anodes, destabilizing their structure and inducing electrolyte reduction. Early smartphones without protection circuits showed rapid capacity loss under such abuse.
Consequences of Battery Failure
Severe capacity degradation manifests as insufficient runtime (e.g., short device operation after charging) or abnormal charging behavior (e.g., slow charging). In critical applications:
• Electric vehicles: Battery failure reduces driving range and may strand vehicles.• Grid-scale energy storage: Failed batteries destabilize power supply reliability, threatening grid security.
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