CR2032 button cell battery

Safety research on CR2032 button cell 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. Under the rapid growth of domestic electric vehicles, the safety accidents of electric vehicles have exposed the fact that current battery companies and OEMs are pursuing higher energy density to obtain more subsidies, but have ignored the most fundamental safety attributes of power batteries. The recent frequent accidents should sound the alarm for the domestic electric vehicle industry that pursues rapid development.

Statistics of domestic accidents from January to August

According to public data, in the first half of 2018 (January to June), there were 8 fire accidents 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 molding and brand, Tesla, Jiangling and other domestic and foreign pure electric vehicles are on the list. However, according to the latest data, there have been as many as 12 public fires since August and September, which is much higher than the number of public fires in 2016 and 2017. Since the main cause of electric vehicle fires is battery overheating, the heat cannot be dissipated in time, resulting in high-temperature battery fires. Therefore, it is reasonable that there are more fires in summer with frequent high temperatures and heavy rains.

Based on the accident data in the first half of the year, from the cause of fire, there were 5 fires caused by charging, accounting for 50%, becoming the first cause of fire accidents; followed by collision fires and spontaneous combustion during driving, each accounting for 20%. From the perspective of vehicle status, static and charging are one type, and batteries may catch fire when static or in use.

Analysis of the causes of lithium-ion battery fires

As the energy source of pure electric vehicles, the main cause of lithium-ion battery fires is thermal runaway caused by battery overheating, which is most likely to occur during battery charging and discharging. Since lithium-ion batteries have a certain internal resistance, they will generate a certain amount of heat while outputting electricity to provide power for pure electric vehicles, causing their own temperature to rise. When their own temperature exceeds their normal operating temperature range, the life and safety of the entire battery will be damaged. In pure electric vehicles, the power battery system is composed of multiple power battery cells. During operation, a large amount of heat is generated and accumulated in the narrow battery box. If the heat cannot be dissipated quickly and in time, the high temperature will affect the life of the power battery and even cause thermal runaway, thereby causing accidents such as fire and explosion. In principle, the causes of thermal runaway are mainly the following four aspects:

(1) Mechanical abuse

It mainly occurs when the car collides. Due to the action of external forces, lithium battery cells and battery packs are deformed, and different parts of themselves are relatively displaced, causing the battery diaphragm to be torn and internal short circuit to occur; flammable electrolyte leakage eventually causes fire. In mechanical abuse, puncture damage is the most serious. It may insert the conductor into the battery body, causing a direct short circuit between the positive and negative electrodes. In contrast, collision, extrusion, etc., only have a probability of internal short circuit. The heat generated during the puncture process is more intense, and the probability of causing thermal runaway is higher.

(2) Electrical abuse

Electrical abuse is mainly caused by improper use of batteries, and there are several types, including external short circuit, overcharging and over-discharging. Among them, the harm caused by over-discharging is the least, but the growth of copper dendrites caused by over-discharging will reduce the safety of the battery and increase the probability of thermal runaway. External short circuit is the result of two conductors with a pressure difference being connected outside the battery cell. When an external short circuit occurs, the heat generated by the battery cannot be dissipated well, and the battery temperature will also rise accordingly, and the high temperature will trigger thermal runaway.

Overcharging is the most harmful type of electrical abuse. Due to excessive lithium insertion, lithium dendrites grow on the surface of the anode. Secondly, excessive lithium deintercalation causes the cathode structure to collapse due to heat and oxygen release (oxygen release of the NCA cathode). The release of oxygen accelerates the decomposition of the electrolyte and produces 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 substances in the battery cell come into contact with the air, a violent reaction occurs, releasing a large amount of heat, which causes the battery pack to burn and catch fire.

(3) Thermal abuse

Thermal abuse mainly refers to local overheating in the battery. It rarely exists independently and is often developed through mechanical and electrical abuse. It is also a situation that directly triggers accidents such as thermal runaway. Thermal abuse is generally caused by excessive battery heat due to high external environment or ineffective temperature control system, which leads to short circuit and thermal runaway. In terms of causes, the causes of thermal abuse are the most complicated. Collision and damage of battery packs, internal structure and performance of batteries, or failure 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. Usually, large-scale internal short circuits caused by mechanical and thermal abuse will directly lead to thermal abuse. The causes of internal short circuits are also complicated. For example, when lithium-ion batteries are overcharged, dendrites accumulate to a certain extent and pierce the battery diaphragm, causing internal short circuits or collision and puncture damage directly leading to contact between the positive and negative electrodes and thermal runaway. Compared with the internal short circuit caused by external factors, the internal short circuit caused by spontaneous defects in the battery manufacturing process is relatively mild. The heat generated by the inherent internal short circuit is very small and will not immediately trigger thermal runaway. Moreover, this internal defect will evolve into a mild internal short circuit after a period of time.

For the thermal runaway of lithium-ion batteries, the current mainstream solutions in China are mainly improved from two aspects: external protection and internal improvement. External protection mainly refers to the upgrade and improvement of the system, and internal improvement refers to the improvement of the battery itself.

(1) Improvement of cooling method

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 is used to dissipate heat or heat in time to ensure the safety and life of the battery. The heat dissipation method of the battery is divided into four types according to the different heat conduction methods and media: air cooling (air cooling), liquid cooling (water cooling), phase change material (solid), and combined cooling (air cooling/water cooling + solid cooling).

(2) Improvement of internal materials and structures

Internal improvement refers to the modification of the material structure inside the battery cell, so that the battery has better heat resistance and heat dissipation performance. In terms of current research hotspots, the development of solid electrolytes, structural modification of positive and negative electrodes, and the introduction of safer diaphragm materials are all mainstream methods to improve the thermal performance of batteries from the inside.

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