Studying the safety issues of 3.7v 3000mah 18650 battery

　　Since this summer, domestic electric vehicle fire accidents have occurred frequently. The number of electric vehicle fire accidents in August and September alone has exceeded the total number of electric vehicle fire accidents in 2017. In the context of the rapid growth of domestic electric vehicles, , what is exposed behind the occurrence of electric vehicle safety accidents is that currently battery companies and OEMs are pursuing higher energy density to obtain more subsidies, but ignore the most fundamental safety attributes of power batteries, and recent frequent accidents The emergence should sound an alarm for the domestic electric vehicle industry that is pursuing rapid development.

　　Statistics on domestic accidents from January to August

　　According to public information, in the first half of 2018 (January to June), a total of 8 fire accidents occurred in new energy battery vehicles in China, which was basically the same as in 2017. Among them, 7 accidents occurred after May, indicating that new energy vehicle fires are mainly concentrated. In summer. In terms of shape and brand, domestic and foreign pure electric vehicles such as Tesla and Jiangling are on the list. However, according to the latest data, there have been as many as 12 public fire incidents since August and September, which is much higher than the number of public fires in 2016 and 2017. Since the main cause of fires in electric vehicles is that the battery overheats and the heat cannot be dissipated in time, the battery catches fire due to high temperature. Therefore, it is reasonable that it occurs more frequently in the summer when high temperatures, heavy rains and other weather conditions are frequent.

　　Based on the accident data in the first half of the year, from the perspective of fire causes, there were a total of 5 fire accidents caused by charging, accounting for 50%, becoming the first cause of fire accidents; followed by two cases each of collision fires and spontaneous combustion while driving. , each accounting for 20%. Judging from the state of the vehicle, standing still and charging are in the same category, and the battery may catch fire both when it is standing still and when in use.

　　Analysis of the causes of lithium-ion battery fires

　　As the energy source of pure electric vehicles, the main cause of fire in lithium-ion batteries is thermal runaway caused by overheating of the battery. This kind of overheating is most likely to occur during the battery charging and discharging process. Since the lithium-ion battery itself has a certain internal resistance, when the output power is used to power pure electric power, it will generate a certain amount of heat, which will increase its own temperature. When its own temperature exceeds its normal operating temperature range, it will damage the entire battery. longevity and safety. In pure electric vehicles, the power battery system is composed of multiple power battery cells. During the working process, a large amount of heat is generated and accumulated in the small battery box. If the heat cannot be quickly dissipated in time, the high temperature will affect the power. The battery life may even suffer from thermal runaway, causing accidents such as fires and explosions. In principle, the causes of thermal runaway mainly include the following four aspects:

　　(1) Mechanical abuse

　　It mainly occurs when a car collides. Due to the action of external forces, the lithium battery cells and battery packs are deformed, and different parts of the battery are relatively displaced, causing the battery separator to be torn and internal short circuit occurs; the leakage of flammable electrolyte eventually causes a fire. Among mechanical abuse, puncture injuries are the most serious, as they may insert conductors into the battery body, causing a direct short circuit between the positive and negative poles. In contrast, collisions, extrusions, etc. only cause internal short circuits in a probabilistic manner. The heat generated during the puncture process is more intense, and the probability of thermal runaway is higher.

　　(2) Electricity abuse

　　Electrical abuse is mainly caused by improper use of batteries, including external short circuit, overcharging and over-discharging. Among them, over-discharge causes the least harm, but the growth of copper dendrites caused by over-discharge will reduce the safety of the battery and increase the chance of thermal runaway. An external short circuit is the result of two conductors with a voltage difference connected outside the battery cell. When an external short circuit occurs and the heat generated by the battery cannot be dissipated well, the battery temperature will also rise, and the high temperature triggers heat. out of control.

　　Overcharging is one of the most harmful forms of electrical abuse. Due to excessive lithium intercalation, lithium dendrites grow on the anode surface. Secondly, excessive deintercalation of lithium causes the cathode structure to collapse due to heat generation and oxygen release (oxygen release of NCA cathode). The release of oxygen accelerates the decomposition of the electrolyte, producing a large amount of gas. Due to the increase in internal pressure, the exhaust valve opens and the battery begins to exhaust. After the active material in the battery core comes into contact with the air, it reacts violently and releases a large amount of heat, which can cause the battery pack to burn and catch fire.

　　(3) Thermal abuse

　　Thermal abuse mainly refers to local overheating in the battery, which rarely exists independently. It is often developed through mechanical abuse and electrical abuse, and is a situation that ultimately directly triggers accidents such as thermal runaway. Thermal abuse is generally caused by a short circuit caused by excessive heat in the battery due to high external environment or ineffective temperature control system, thus causing thermal runaway. In terms of causes, thermal abuse is the most complex. Collision and damage to the battery pack, the internal structure and performance of the battery, or failures of other thermal management systems and air conditioning systems may all lead to thermal abuse.

　　(4) Internal short circuit

　　Internal short circuit is caused by direct contact between the positive and negative electrodes of the battery. Of course, the degree of contact is different, and the subsequent reactions caused are also very different. Large-scale internal short circuits usually caused by mechanical and thermal abuse will directly lead to thermal abuse. The causes of internal short circuits are equally complex. For example, overcharging of lithium-ion batteries may cause dendrites to accumulate to a certain extent and pierce the battery separator, resulting in internal short circuits or direct contact between the positive and negative electrodes after collision or puncture damage, leading to thermal runaway. Compared with internal short circuits caused by external factors, internal short circuits caused by spontaneous defects in the battery manufacturing process are relatively mild. The heat generated by the innate internal short circuit is very small and will not immediately trigger thermal runaway. And this inherent defect will evolve into a mild internal short circuit over a period of time.

　　In response to the thermal runaway situation of lithium-ion batteries, the current domestic mainstream solutions mainly improve from two aspects: external protection and internal improvement. External protection mainly refers to system upgrades and improvements, while internal improvements refer to improvements to the battery itself.

　　(1) Improvement of cooling methods

　　The thermal management system is mainly responsible for controlling the temperature to ensure that the battery is always at a reasonable operating temperature. Usually, the thermal management system is controlled by the vehicle controller. When the temperature of the battery pack is abnormal, the air conditioning system can dissipate heat or heat it in time to ensure battery safety and life. The heat dissipation methods of batteries are divided into four categories based on different heat conduction methods and media: air cooling (air cooling), liquid cooling (water cooling), phase change materials (solid), and combined cooling (air cooling/water cooling + solid cooling). kind.

　　(2) Improvement of internal materials and structures

　　Internal improvement refers to the transformation of the material structure inside the battery core, so that the battery has better heat resistance and heat dissipation performance. In terms of current research hotspots, developing solid electrolytes; structurally modifying the positive and negative electrodes; and introducing safer separator materials are all mainstream methods to improve battery thermal performance from within.

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