What is the thermal safety study of the spinel structure positive material powered lithium battery?

Abstract: in this paper, the present research status of thermal safety of lithium-ion power battery is introduced, and the research on thermal safety of spinel structure positive material dynamic lithium-ion battery is emphasized. The surface heat generation of the battery during charging and discharging was studied quantitatively by using infrared thermal imager. The temperature and voltage trends of power cells during hot boxes, short circuits and acupuncture are tested by means of thermal safety testing devices and other devices, and it is finally proved that the subjects are reliable and stable in terms of thermal safety. It also provides a quantitative basis for the next optimization.

Keywords: spinel structure; Power cells; Thermal safety

The increase of atmospheric pollution and environmental protection pressure has made people pay close attention to the development of efficient and energy-efficient energy. Governments of various countries have also successively introduced measures and policies to promote the development of related industries. In terms of energy conservation, electric vehicles have received strong support from the people and the government because of their advantages in terms of pollution, low noise, and low energy consumption. As one of the key technologies in electric vehicles, power batteries have become a barometer for the development of electric vehicles. In the development of power cell industrialization, people not only pay attention to the improvement of electrochemical performance of batteries but also pay more attention to the reliability of safety performance. With the excellent performance of 50 Olympic buses with lithium-manganese powered lithium-ion batteries during the Olympic Games, people are full of hope and expectation for the widespread use of lithium-ion batteries with spinel structure positive materials. At the same time, CITIC Guoan Alliance Guli Corporation (MGL) has never stopped its work on safety performance improvement and has improved the thermal safety of batteries through various methods and processes. As a result, the safety performance test of the battery has passed the test of the North Automobile Quality Supervision and Inspection Laboratory (201) for five consecutive years.

1 Status of research

The key factors that affect battery thermal safety are positive and negative electrode materials, electrolytic fluid type, diaphragm, and battery structure design. Jinhuifen[ 1] The thermal stability of the commercialized LiCoO2 / graphite system was studied with the ARC(accelerated calorimeter). The results showed that the negative electrode began to heat up at 60 °C, the positive electrode began to heat up at 110 °C, and eventually with the increase of the internal pressure of the battery. Causes thermal runaway; Tangzhiyuan[ 2] This paper expounds how these factors affect the thermal safety of batteries from the aspects of positive, negative, electrolyte, etc.. Author Chil-HoonDoh[ 3] In the safety experiment(overcharge and acupuncture), the effects of thermal and electrochemical properties of LiCoO2/C system batteries are proposed. These are mainly used to explain the safety of batteries through microscopic means. At the same time literature[ 4-6] Using finite element analysis and thermal simulation method, the thermal safety of the reaction cell is intuitively studied by the whole cell and heat dissipation of the battery.

At present, there is no macro study of the thermal safety of spinel positive materials powered lithium batteries. In this paper, the thermal safety of monomer battery is studied by thermal imaging and quantitative test of safety performance.

Research Method of 2 Thermal Safety

2.1 Subjects studied

This paper takes 100 Ah produced by MGL as the research object

2.2 Methods of study

The thermal imaging and calculation methods were used to study the thermal properties of monomer batteries.

During the battery charging and discharging process, the battery is fixed on the steel frame, and the battery surface is almost completely in contact with the air and is in a state of natural convection heat dissipation. The positive and negative poles of the battery are connected to the channel of the test bed, as shown in Figure 2. Before the experiment, the room temperature was controlled at(23 ± 2) °C, and the battery was placed in equilibrium with the ambient temperature for a long time.

Among them, the thermal imaging results show different colors showing changes in temperature. The 10 areas shown in the surface temperature map of the battery during the test were marked with AR1, AR2, AR3, AR4, AR5, AR6, AR7, AR8, AR9, and AR10, respectively.

Where IL is the working current; UL is the operating voltage; E0 is the battery balanced electromotive force, which is approximated by the open circuit voltage Uoc when calculating; T is the battery temperature; DE0 / DT is the temperature effect coefficient of the battery balanced electromotive force; VB is the battery volume. The first IL(E0-UL) / VB on the right side of formula(1-1) describes heat generation due to internal resistance and other irreversible effects of the battery, and the second is heat generation due to electrochemical reactions within the battery.

When conducting battery safety experiments(short circuit, hot box, acupuncture), self-designed measurement equipment is used, including industrial control machines, data acquisition cards, temperature sensors, voltage sensors, and current sensors

3 Research results

3.1 Heat Generation Capacity of Single Battery

Thermal imaging equipment was used to test the surface temperature rise when the battery was charged under 200A current discharge and 100A current charge

During the discharge process of the battery, the temperature rises fastest at the positive polar ear, followed by the negative polar ear. This shows that the polar ear may be the heat source for the entire battery when the battery is discharged, especially in the case of high current discharge. A large amount of heat generated at the polar ear will be transmitted to the interior of the battery, inducing a series of exothermic reactions inside the battery, causing the thermal failure of the battery; In the charging process, the temperature at the positive and negative polar ears is lower than in other regions. Since the battery charging process itself is an endothermic process, there will be no safety accidents due to a large amount of heat generated when the operation is reasonable.

Throughout the process, in the short-circuit time(there is current), the acquisition points 3 and 4 show different temperature changes, and the temperature changes at the collection points 1, 2, and 5 are basically the same. The main reason for this change is that MGL has improved its battery design to effectively prevent thermal failure caused by high current passing. Throughout the short circuit, the battery has no change in appearance. The battery has not burned or exploded and meets safety standards.

3.3 Heat box test of batteries

In the process of the hot box, the voltage changes only at the end of the hot box, and the voltage does not change at other stages. In terms of temperature, the trend of changes in the collection points is the same, with a maximum temperature of 137 °C.

The battery in the heat box behind only the phenomenon of inflation, no fire, no explosion, in line with National safety standards.

3.4 Acupuncture test of batteries

During the acupuncture process, the battery voltage decreased first and then increased, and the temperature at each collection point gradually increased. The maximum temperature reached 40 °C. The battery did not catch fire and did not explode during the entire process, meeting National safety standards.

4.Summary and outlook

There are many reasons for battery safety problems, including abuse(overcharge, overdischarge, etc.), unreasonable use and many other factors. Under the condition of ensuring the proper operation of the battery, improving the thermal safety of the battery by improving the battery materials and processes is one of the important ways. In this paper, the quantitative thermal safety analysis of the improved battery is carried out to objectively characterize the heat generation and safety characteristics of the battery, and to provide a better idea for the next step to improve the thermal safety of the battery.

The improvement of battery thermal safety is an eternal topic in the process of battery development. With the development of technology and the Advancement of analog technology, the comprehensive use of experimental and analog evaluation methods is an inevitable choice. This is not only from the perspective of cost savings. It is the result of comprehensive consideration from the perspective of information feedback speed.

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