Definition and core function of K-value in lithium batteries

The K -value is the voltage drop per unit time of a lithium battery under static conditions (measured in mV/h or mV/d). It quantifies the battery's self-discharge rate and reflects its internal chemical stability and reliability . Its importance lies in:

• Self-discharge performance assessment : The smaller the K value, the stronger the battery capacity retention capability (e.g., K value < 0.2 mV/h for high-quality cells) ;
• Battery consistency screening: The difference in K value within the group must be controlled within 0.05mV/h to avoid battery pack failure due to uneven voltage decay;
• Safety hazard warning: An abnormally high K value may indicate potential problems such as micro-short circuits, electrolyte decomposition, or impurity contamination.

Key factors affecting the K value

I. Material and Structural Properties

• Ternary material batteries generally have a higher K-value than lithium cobalt oxide and lithium iron phosphate batteries, resulting in different self-discharge rates due to differences in activity .

• Positive electrode metal impurities (such as copper and iron particles) can easily cause micro-short circuits, significantly increasing the K value .

• When the electrolyte decomposes in water or contains impurities, it will corrode the electrodes and accelerate side reactions (such as gas production and lithium dendrite growth) .
• Insufficient membrane thickness or uneven porosity makes it easy for metal dendrites to puncture it, forming a physical micro-short circuit.

II. Production Process and Storage Conditions

Manufacturing defects :

• Poor sealing allows moisture to enter, accelerating electrolyte decomposition ;

• Incomplete filtration of the electrode slurry can lead to the introduction of metallic foreign matter (such as Cu, which poses the greatest risk) and cause micro-short circuits .

• High temperatures (>40℃) accelerate SEI film rupture and electrolyte decomposition, while low humidity causes electrolyte drying, both of which significantly increase the K value .

• Long-term storage causes an imbalance in the capacity of the positive and negative electrodes, leading to the accumulation of irreversible side reactions .

III. Test Conditions and Equipment Interference

• Polarization effect : If polarization is not sufficiently eliminated by resting, or if the SOC is in a steep region of the voltage curve (such as the high/low charge range), the K value may be artificially inflated ;
• Test process error: Inconsistency between the two voltage measurement devices or fluctuations in temperature and humidity may cause the K value to be distorted (or even negative).
• Initial voltage selection: The initial voltage in the non-plateau region (such as >4.2V or <3.0V) will cause measurement deviation due to the change in the slope of the voltage-capacity curve.

Methods for handling abnormal K values

I. Material and Process Optimization

• Low-activity cathode materials (such as lithium iron phosphate) and high-purity electrolytes are used to reduce side reactions ;

• Apply coated diaphragms or ceramic composite diaphragms to prevent dendrite penetration .

• Fully automated production lines reduce the introduction of foreign metal objects and enhance the electrode slurry filtration process .

• Optimize the packaging process (such as laser welding seal inspection) to prevent moisture intrusion .

II. Storage and Test Management

• Storage temperature should be controlled at 20-25℃, and humidity <60% to slow down chemical side reactions ;
• Perform capacity balancing maintenance regularly to avoid capacity imbalance caused by prolonged inactivity.

• Extend the settling time to more than 48 hours to eliminate polarization interference ;

• Select a voltage plateau range (e.g., 3.6-3.8V) as the initial test point to reduce slope error .
  
  

III. Screening and Repair of Abnormal Battery Cells

• Capacity testing and static screening : Through multiple capacity tests combined with static monitoring (such as 3.0V process), cells with excessive K value are eliminated ;
• Rework and activation process: Repeatedly charge and discharge cells with slight K-value abnormalities to restore electrode interface stability;
• Intelligent monitoring system: Implantable sensors monitor internal temperature/strain signals in real time, providing early warning of micro-short circuit risks.