1. Material system and electrochemical properties
The positive and negative electrode materials determine the voltage window: different positive and negative electrode materials have different electrochemical stability windows. For example:
Lithium iron phosphate (LFP) system: The charging cut-off voltage is usually 3.65V, and the discharging cut-off voltage is 2.5V (room temperature) or 2.0V (low temperature).
Ternary material (NCM/NCA) system: charging cutoff voltage is 4.2V, and discharging cutoff voltage is 2.75V–3.0V.
Lithium titanate (LTO) system: charging cut-off voltage is 2.9V, and discharging cut-off voltage is 1.5V.
Overcharge/over-discharge risk:
Excessive charging voltage can lead to damage to the positive electrode structure, oxygen evolution, and electrolyte decomposition.
Too low a discharge voltage can cause the negative electrode SEI film to rupture and the current collector to corrode.
2. Security Protection Mechanism
Hierarchical protection design:
Charging termination voltage: such as 3.65V (LFP), when the charging is reached, the BMS terminates charging.
Level 1 overcharge protection: If ≥3.8V (LFP), charging will be forcibly terminated.
Level 2 overcharge protection: If ≥4.0V (LFP), lock the BMS to prevent thermal runaway.
Discharge termination voltage: such as 2.5V (LFP), discharge will stop when this voltage is reached.
Level 1 over-discharge protection: If ≤2.0V (LFP), the discharge will be forcibly terminated.
Level 2 over-discharge protection: If ≤1.8V (LFP), lock the BMS and require manual repair.
3. Temperature adaptability
Low temperature adjustment: The discharge cutoff voltage decreases as the temperature decreases, for example:
T>0°C: 2.5V (LFP).
T ≤ 0°C: 2.0V (LFP) to avoid premature cutoff due to polarization at low temperatures.
4. Lifespan and performance optimization
Cycle life impact:
Excessively high charging cutoff voltage (such as increasing LFP from 3.65V to 4.0V) will accelerate capacity decay.
A discharge cutoff voltage that is too low (such as LFP below 2.85V) will lead to the loss of active lithium on the negative electrode.
Formation process: The pre-charge cutoff voltage needs to be controlled within the SEI film formation range (e.g., 2.8–3.0V) to avoid impurity reactions.
Summarize
The determination of the cutoff voltage is a comprehensive result of material properties, safety, lifespan, temperature adaptability, and standard specifications. Manufacturers verify the boundary values through electrochemical testing, and the BMS performs graded protection accordingly to ensure that the battery operates within a safe window.
The above content is based on my daily work, communication, and literature review. Due to my limited abilities, there may be omissions in the viewpoints presented. I welcome colleagues in the industry to actively exchange ideas and make progress together!
References:
1. Research on State Estimation and Lifetime Prediction Methods for Lithium Iron Phosphate Batteries under Abnormal Operating Conditions, Wei Meng
2. ATL Project Engineering Department Engineering Manual
3. The impact of formation process on lithium-ion battery performance, Wang Lingling
4. GB/T 36276—2023 Lithium-ion batteries for power storage
