Why Does Lithium Battery Capacity Suddenly ＂Plummet＂? An In-Depth Analysis of Six Failure Modes

The primary reasons for sudden lithium ion battery capacity degradation ("nosedive") include:

1.  Anode Interface Failure  

SEI Film Dynamic Breakdown/Reformation: During initial cycles, the continuous destruction and reformation of the Solid Electrolyte Interphase (SEI) consume active lithium, causing rapid reversible capacity loss.  

Lithium Dendrite Formation: Under conditions like low temperature, overcharging, or insufficient N/P ratio (low anode design capacity), lithium ions deposit on the anode surface forming dendrites. Subsequent cycles can trigger internal short circuits, leading to catastrophic capacity failure.

2.  Cathode Active Material & Interface Degradation  

Cation Mixing & Phase Transition: High-nickel cathode materials (e.g., NCM811) are prone to Li⁺/Ni²⁺ cation mixing at high voltages. This generates rock-salt phases (e.g., NiO), reducing available lithium intercalation sites.  

Transition Metal Dissolution: Manganese (e.g., in LiMn₂O₄) undergoes Mn³⁺ disproportionation at high temperatures. Dissolved Mn²⁺ migrates to the anode, damaging the SEI film and accelerating lithium consumption.

3.  Electrolyte System Failure  

Insufficient Electrolyte Retention: Electrolyte depletion (e.g., due to separator absorption degradation or inadequate initial filling) causes electrode/electrolyte interface contact failure, hindering Li⁺ transport. This often triggers sudden capacity drops in later cycles.  

Electrochemical Decomposition & Additive Incompatibility: Carbonate-based electrolytes oxidatively decompose at high voltages, generating gaseous by-products (e.g., CO₂). This leads to electrode pore blockage and interface thickening.

4.  Mechanical Stress-Induced Structural Damage  

Electrode Particle Cracking & Delamination: Volume expansion in silicon-based anodes (>300%) and layer spacing changes in graphite cause material pulverization, detaching active particles from the current collector.  

Separator Mechanical Fatigue: Long-term cycling reduces separator porosity (PP/PE collapse) or causes rupture due to lithium dendrite penetration. This induces a sudden increase in charge transfer impedance.

5.  Manufacturing Process Defects  

Imbalanced Positive/Negative Electrode Redundancy: An N/P ratio below 1.1 fails to buffer lithium deposition risks, causing capacity nosedives even in initial cycles.  

Conductive Additive Agglomeration: Poor slurry dispersion causes conductive additives to clump, drastically reducing active material utilization and leading to "stepwise" capacity decay early in cycling.

6.  Environmental & Usage Conditions  

High-Rate Discharge Aging: 3C discharge rates double SEI film thickening rates on the anode, causing Li⁺ diffusion impedance to rise exponentially. Capacity retention often falls below 50% in later cycles.  

High-Temperature Storage: After 90 days at 55°C, the SEI film thickness on LFP battery anodes increases to 164% of its initial value, contributing over 70% of irreversible lithium loss.

Summary of Capacity Plunge Triggers:

Chemical Failure (Early-Stage Dominant):  

SEI Reformation / Electrolyte Decomposition

Structural Failure (Mid-Life Dominant):  

Electrode Pulverization / Separator Rupture

Electrochemical Failure (Late-Stage Sudden Drop):  

Internal Short Circuits / Lithium Dendrites

These conclusions are consistently supported by experimental data (e.g., ICP elemental analysis, XPS interface characterization, EIS/DV curve interpretation) and case studies, demonstrating that sudden capacity failure results from the coupling of multiple degradation mechanisms.