Risk and safety technology of lithium ion battery (2)

3. Security technology

Although lithium ion batteries have many hidden dangers, under specific conditions of use and with certain measures, they can effectively control the occurrence of side reactions and violent reactions in the battery cells to ensure their safe use. The following is a brief introduction to several commonly used safety technologies for lithium ion batteries.

(1) Select raw materials with higher safety factor

Positive and negative polar active materials, diaphragm materials and electrolytes with higher safety factor shall be selected.

a) Selection of positive material

The safety of cathode materials is mainly based on the following three aspects:

1. Thermodynamic stability of materials;

2. Chemical stability of materials;

3. Physical properties of materials.

b) Selection of diaphragm materials

The main function of the diaphragm is to separate the positive and negative electrodes of the battery, to prevent short circuit caused by contact between the positive and negative electrodes, and to enable electrolyte ions to pass through, that is, it has electronic insulation and ion conductivity. The following points should be noted when selecting the diaphragm for lithium ion batteries:

1. It has electronic insulation to ensure the mechanical isolation of positive and negative electrodes;

2. It has a certain aperture and porosity to ensure low resistance and high ionic conductivity;

3. The diaphragm material shall have sufficient chemical stability and must be resistant to electrolyte corrosion;

4. The diaphragm shall have the function of automatic shutdown protection;

5. The thermal shrinkage and deformation of diaphragm shall be as small as possible;

6. The diaphragm shall have a certain thickness;

7. The diaphragm shall have strong physical strength and enough puncture resistance.

c) Selection of electrolyte

Electrolyte is an important part of lithium ion battery, which plays the role of transmitting and conducting current between the positive and negative electrodes of the battery. The electrolyte used in lithium ion batteries is an electrolyte solution formed by dissolving appropriate lithium salts in organic aprotic mixed solvents. It shall generally meet the following requirements:

1. Good chemical stability, no chemical reaction with electrode active substance, collector fluid and diaphragm;

2. Good electrochemical stability, with a wide electrochemical window;

3. High lithium ion conductivity and low electronic conductivity;

4. Wide range of liquid temperature;

5. It is safe, non-toxic and environmentally friendly.

(2) Strengthen the overall safety design of the cell

The battery cell is the link that combines various materials of the battery, and the integration of positive pole, negative pole, diaphragm, lug and packaging film. The design of the cell structure not only affects the performance of various materials, but also has an important impact on the overall electrochemical performance and safety performance of the battery. The selection of materials and the design of the core structure are just a kind of relationship between the local and the whole. In the design of the core, the reasonable structure mode should be formulated according to the material characteristics.

In addition, some additional protective devices can be considered for the lithium battery structure. Common protective mechanisms are as follows:

a) The switch element is adopted. When the temperature inside the battery rises, its resistance value will rise accordingly. When the temperature is too high, the power supply will be automatically stopped;

b) Set a safety valve (that is, the air vent at the top of the battery). When the internal pressure of the battery rises to a certain value, the safety valve will open automatically to ensure the safety of the battery.

Here are some examples of the safety design of the electric core structure:

1. Positive and negative pole capacity ratio and design size slice

Select appropriate capacity ratio of positive and negative electrodes according to the characteristics of positive and negative electrode materials. The ratio of positive and negative electrode capacity of the cell is an important link related to the safety of lithium ion batteries. If the positive electrode capacity is too large, metal lithium will deposit on the surface of the negative electrode, while if the negative electrode capacity is too large, the capacity of the battery will be greatly lost. Generally, N/P=1.05-1.15, and appropriate selection shall be made according to the actual battery capacity and safety requirements. Large and small pieces shall be designed so that the position of the negative paste (active substance) encloses (exceeds) the position of the positive paste. Generally, the width shall be 1~5 mm larger and the length shall be 5~10 mm larger.

2. Allowance for diaphragm width

The general principle of diaphragm width design is to prevent internal short circuit caused by direct contact between positive and negative electrodes. As the thermal shrinkage of the diaphragm causes deformation of the diaphragm in the length and width direction during battery charging and discharging and under thermal shock and other environments, the polarization of the folded area of the diaphragm increases due to the increase of the distance between positive and negative electrodes; The possibility of micro short circuit in the stretching area of the diaphragm is increased due to the thinning of the diaphragm; Shrinkage at the edge of the diaphragm may lead to direct contact between the positive and negative electrodes and internal short circuit, which may cause danger due to thermal runaway of the battery. Therefore, when designing the battery, its shrinkage characteristics must be taken into account in the use of the area and width of the diaphragm. The isolation film should be larger than the anode and cathode. In addition to the process error, the isolation film must be at least 0.1mm longer than the outer side of the electrode piece.

3.Insulation treatment

Internal short circuit is an important factor in the potential safety hazard of lithium-ion battery. There are many potential dangerous parts that cause internal short circuit in the structural design of the cell. Therefore, necessary measures or insulation should be set at these key positions to prevent internal short circuit in the battery under abnormal conditions, such as maintaining necessary spacing between the positive and negative electrode ears; Insulating tape shall be pasted at the non paste position in the middle of the single end, and all exposed parts shall be covered; Insulating tape shall be pasted between positive aluminum foil and negative active substance; The welding part of the lug shall be completely covered with insulating tape; Insulating tape is used on the top of the electric core.

4.Setting safety valve (pressure relief device)

Lithium ion batteries are dangerous, usually because the internal temperature is too high or the pressure is too high to cause explosion and fire; The reasonable pressure relief device can rapidly release the pressure and heat inside the battery in case of danger, and reduce the explosion risk. The reasonable pressure relief device shall not only meet the internal pressure of the battery during normal operation, but also automatically open to release the pressure when the internal pressure reaches the danger limit. The setting position of the pressure relief device shall be designed considering the deformation characteristics of the battery shell due to the increase of internal pressure; The design of the safety valve can be realized by flakes, edges, seams and nicks.

(3) Improve process level

Efforts should be made to standardize and standardize the production process of the cell. In the steps of mixing, coating, baking, compaction, slitting and winding, formulate standardization (such as diaphragm width, electrolyte injection volume, etc.), improve process means (such as low pressure injection method, centrifugal packing method, etc.), do a good job in process control, ensure process quality, and narrow the differences between products; Set special work steps in key steps that affect safety (such as deburring of electrode piece, powder sweeping, different welding methods for different materials, etc.), implement standardized quality monitoring, eliminate defective parts, and eliminate defective products (such as deformation of electrode piece, diaphragm puncture, active material falling off, electrolyte leakage, etc.); Keep the production site clean and tidy, implement 5S management and 6-sigma quality control, prevent impurities and moisture from mixing in production, and minimize the impact of accidents in production on safety.


Post time: Nov-16-2022