I. the danger of lithium-ion batteries.

Lithium-ion battery is a potentially dangerous chemical power source because of its own chemical characteristics and system composition.

(1) high chemical activity.
Lithium is the main group element of the second cycle of the periodic table, which has extremely active chemical properties.

(2) High energy density.
The specific energy of lithium-ion battery is extremely high (≥ 140Wh/kg), which is several times higher than that of Ni-CD, Ni-MH and other secondary batteries. If the thermal runaway reaction occurs, it will give off high heat and easily lead to unsafe behavior.

(3) using organic electrolyte system.
The organic solvent of organic electrolyte system is hydrocarbon, the decomposition voltage is low, it is easy to oxidize, and the solvent is flammable; if leakage occurs, it will cause the battery to catch fire, even burn and explode.

(4) the probability of side reaction is high.
During the normal use of lithium-ion battery, there is a positive chemical reaction between electric energy and chemical energy. However, under some conditions, such as over-charging, over-discharge or over-current work, it will easily lead to a chemical side reaction inside the battery; after the side reaction is intensified, it will seriously affect the performance and service life of the battery, and may produce a large amount of gas, which will lead to safety problems caused by explosion and fire after the rapid increase of the pressure inside the battery.

(5) the structure of electrode material is unstable.

The overcharging reaction of lithium-ion battery will change the structure of the cathode material and make the material have a strong oxidation effect, so that the solvent in the electrolyte will be strongly oxidized, and this effect is irreversible. If the heat caused by the reaction accumulates, there is a risk of thermal runaway.

II. cause analysis of safety problems of lithium-ion battery products.
After 30 years of industrial development, the safety technology of lithium-ion battery has made great progress, which effectively controls the occurrence of side reactions in the battery and ensures the safety of the battery. However, with the more and more extensive use of lithium-ion batteries and higher energy density, there are still repeated incidents such as explosion injury or product recall due to safety risks in recent years. We summarize the main reasons for the safety problems of lithium-ion battery products are as follows:

(1) the problem of cell material.
The materials used in the battery include: positive active material, negative active material, diaphragm, electrolyte and shell, etc. the safety performance of the cell is determined by the selection of materials and the matching of the system. In the selection of positive and negative active materials and diaphragm materials, the manufacturers did not assess the characteristics and matching of raw materials, resulting in congenital deficiencies in the safety of the core.

(2) production process problems.
Lax inspection of cell raw materials and poor production environment lead to the mixing of impurities in production, which is not only disadvantageous to the capacity of the battery, but also has a great impact on the safety of the battery; in addition, if too much water is mixed into the electrolyte, a side reaction may occur to increase the internal pressure of the battery, which will affect the safety. Due to the limitation of the production process, the products can not achieve good consistency in the production process of the electrode. for example, poor smoothness of the electrode matrix, shedding of the electrode active material, mixing of other impurities in the active material, unstable welding temperature, burr on the edge of the electrode and no use of insulating tape in key parts may adversely affect the safety of the core.

(3) the design defect of the battery, the safety performance is reduced.
In structural design, many key points affecting safety have not been paid attention to by manufacturers, such as no insulation tape in key parts, no margin or insufficient margin in diaphragm design, unreasonable design of positive and negative electrode capacity ratio, unreasonable design of positive and negative active material area ratio, unreasonable design of pole ear length, etc., all these may bury hidden dangers to the safety of the battery. In addition, in the process of cell production, some battery manufacturers try to save and compress raw materials in order to save costs and improve performance, such as reducing diaphragm area, thinning copper foil, aluminum foil and not using pressure relief valve, insulation tape and so on. all these will reduce the safety of the battery.

(4)the energy density is too high.
At present, the market is in pursuit of higher capacity battery products, manufacturers in order to increase the competitiveness of products, continue to improve the volume specific energy of lithium-ion batteries, which greatly increases the risk of batteries.

III. Security technology.
Although lithium-ion battery has many hidden dangers, under specific use conditions, and the adoption of certain measures can effectively control the occurrence of side reactions and severe reactions in the cell and ensure its safe use. The following is a brief introduction to several commonly used safety technologies for lithium-ion batteries.

1> choose raw materials with higher safety factor.
Choose positive and negative active materials, diaphragm materials and electrolytes with higher safety factor.
A) selection of cathode materials.
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 the material.

B) selection of diaphragm materials.
The main function of the diaphragm is to separate the positive and negative electrodes of the battery, prevent the positive and negative electrodes from contacting and short-circuit, and have the ability to make electrolyte ions pass through, that is, electronic insulation and ion conductivity. The following points should be paid attention to when choosing diaphragms for lithium-ion batteries:
(1) has electronic insulation to ensure the mechanical isolation of positive and negative electrodes.
(2) has a certain pore diameter and porosity to ensure low resistance and high ionic conductivity.
(3) The diaphragm material has sufficient chemical stability and must be resistant to electrolyte corrosion.
(4) the diaphragm should have the function of automatic shut-off protection.
(5) the thermal shrinkage and deformation of the diaphragm should be as small as possible.
(6) the diaphragm should have a certain thickness.
(7) the diaphragm should have strong physical strength and strong enough anti-puncture ability.

C) selection of electrolytes.
Electrolyte is an important part of lithium-ion battery, which transmits and conducts current between the positive and negative electrodes of lithium-ion battery. The electrolyte used in lithium-ion battery is an electrolyte solution formed by dissolving appropriate lithium salt in organic non-proton mixed solvent. It should usually meet the following requirements:
(1) good chemical stability, no chemical reaction with electrode active material, current collector and diaphragm.
(2) good electrochemical stability and wide electrochemical window.
(3) High lithium ion conductivity and low electronic conductivity.
(4) wide range of liquid temperature.
(5) safe, non-toxic and friendly to the environment.

Strengthen the overall safety design of the cell.
The battery is the link that combines all kinds of materials of the battery, and it is the integration of positive electrode, negative electrode, diaphragm, electrode ear and packaging film and so on. The structure design of the battery 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 cell structure is a kind of relationship between the part and the whole. In the design of battery, a reasonable structure mode should be established according to the material characteristics.

In addition, some additional protection devices can be considered in the structure of lithium battery. the common protection mechanism designs are as follows: 1 the switch element is used, and its resistance increases when the temperature in the battery rises, and when the temperature is too high, will automatically stop the power supply. (2) set up the safety valve (that is, the vent at the top of the battery). When the internal pressure of the battery rises to a certain value, the safety valve opens automatically to ensure the safety of the battery.

The following are some examples of the safety design of the core structure:
A) capacity ratio of positive and negative electrodes and design size.
The appropriate ratio of positive and negative electrode capacity is selected according to the characteristics of positive and negative electrode materials. the ratio of positive and negative electrode capacity is an important link related to the safety of lithium ion battery. if the positive capacity is too large, the metal lithium will be deposited on the surface of the negative electrode. the capacity of the battery with too large negative electrode will have a great loss. In general, N/P=1.05~1.15, and make appropriate choices according to the actual battery capacity and safety requirements. The size of the film is designed so that the position of the negative paste (active material) is larger than that of the positive paste, the width should be 1 mm larger and the length should be 5 mm larger.
B) there is a margin in the width of the diaphragm.
The general principle of the design of the width of the diaphragm is to prevent the internal short circuit caused by the direct contact of the positive and negative electrodes. The thermal shrinkage of the diaphragm leads to the deformation of the diaphragm in the direction of length and width during battery charge and discharge and in the environment of thermal shock. The area of the diaphragm fold increases the polarization due to the increase of the distance between the positive and negative electrodes, and the area of the diaphragm tension increases the possibility of micro-short circuit due to the thinning of the diaphragm. The contraction of the edge area of the diaphragm may lead to internal short circuit caused by direct contact between positive and negative electrodes, which can make the battery dangerous due to thermal runaway. Therefore, when designing the battery, the shrinkage characteristics of the diaphragm must be considered in the use of the area and width of the diaphragm, and the separator is larger than the anode and cathode. Considering that in addition to the process error, the isolation film must be at least 0.1mm longer than the outside of the electrode.
C) Insulation treatment.
Internal short circuit is an important factor in the safety hidden danger of lithium ion battery. there are many potential dangerous places that cause internal short circuit in the structural design of lithium ion battery, so necessary measures or insulation should be set up in these key locations. in order to prevent the occurrence of short circuit in the battery in abnormal circumstances, such as keeping the necessary distance between positive and negative ears. Insulating tape should be affixed to the middle non-paste position of the ending side, and all the exposed parts should be wrapped; insulating tape should be pasted between the positive aluminum foil and the negative active material; the welding part of the electrode ear should be completely covered with insulating tape; the top of the core should be covered with insulating tape, etc.
D) set up safety valve (pressure relief device).
The danger of lithium-ion battery is often caused by explosion and fire caused by excessive internal temperature or pressure; setting up a reasonable pressure relief device can quickly release the pressure and heat inside the battery when the danger occurs and reduce the risk of explosion. A reasonable pressure relief device is required not only to meet the internal pressure of the battery when it is working normally, but also to automatically open and release the pressure when the internal pressure reaches the dangerous limit. The setting position of the pressure relief device needs to consider the deformation characteristics of the battery shell due to the increase of internal pressure. The design of the safety valve can be realized by thin sheets, edges, seams and scratches.
(3) improve the level of technology.
Strive to do a good job in the standardization and standardization of the battery production process. In the steps of mixing, coating, baking, compaction, cutting and winding, standardize (such as diaphragm width, electrolyte injection, etc.), improve process means (such as low pressure liquid injection method, centrifugal shell method, etc.), do a good job in process control, ensure process quality, and reduce the differences between products. In the key steps affecting safety, set up special steps (such as depolarizing burr, sweeping powder, using different welding methods for different materials, etc.), implement standardized quality control, eliminate defective parts, eliminate defective products (such as electrode deformation, diaphragm puncture, active material shedding and electrolyte leakage, etc.). Keep the production site clean and clean, implement 5S management and 6-sigma quality control, prevent the mixing of impurities and moisture in production, and minimize the impact of accidents on safety.