The interface resistance of lithium batteries is a complex problem with multiple scales and factors, which directly affects the rate performance, low-temperature performance, cycle life, and safety of the battery.

1. Definition and composition of interface resistance

Interface resistance usually refers to the resistance of two key parts inside the battery:

(1) Contact resistance between the current collector and the coating: affects the efficiency of electron transfer from the current collector to the coating.

(2) The electronic transmission resistance inside the coating is determined by the electronic channels formed between active material particles through a network of conductive agents and binders.

2. Influencing factors and improvement methods

(1) Formula design:

Reasonable formula design can directly reduce the internal resistance between interfaces, including material selection and dosage. 

Conductive agents and binders: 

determine the quantity and quality of electronic transmission channels, directly affecting rate performance, low-temperature internal resistance, and heating behavior. Simply pursuing low resistivity is not always advantageous. Increasing the proportion of active substances or replacing conductive agents may improve the surface, but in reality, it may lead to uneven interface resistance, especially in silicon doped negative electrode systems where silicon has poor electron transfer and frequent expansion and contraction, requiring an appropriate conductive network to be matched.

(2) Process control:

Roll pressing: Proper compaction can improve particle contact and reduce resistance; However, excessive compaction may lead to particle breakage, which in turn reduces electron and ion channels. 

Baking: The floating of adhesive (such as SBR) can cause the area near the current collector to loosen, increasing the contact resistance. 

Other factors such as coating stability, carbon coating quality on the surface of the current collector, and welding tightness also significantly affect the interface resistance.

3. Testing method

(1) Four probe method: Quickly screen for batch uniformity, but cannot measure the contact resistance between the current collector and the coating.

(2) EIS (Electrochemical Impedance Spectroscopy): By separating contact impedance information in the high-frequency region, a reliable equivalent circuit model is required. 

(3) Symmetric battery or three electrode half battery: used for mechanism research to separate interface resistance from overall impedance, but not suitable for production lines. 

(4) Combination strategy: First, use four probes for rapid screening, then analyze the frequency response using EIS, and finally observe the cross-section using SEM or focused ion beam to comprehensively evaluate the interface state.

4. Conclusion

The control of interface resistance requires comprehensive consideration of formula, process, and testing methods to avoid blindly pursuing low resistivity, and should focus on the actual effectiveness of electronic and ion channels. Through multidimensional testing and process optimization, interface resistance can be effectively reduced and battery performance can be improved.

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