Why is the battery heat out of control? Is it the reason for short circuit?

著者：Iflowpower – Provedor de central eléctrica portátil

High-than-energy power lithium battery is the darling of the battery industry today, however, the greater the energy, the safety is also different. The failure mechanism of the battery is also very complicated. When studying the thermal out-control mechanism of the battery, people have previously been able to analyze the thermal response to a single battery module, such as cathodes, anodes, diaphragms, and electrolytes.

For example, diaphragm shrinkage or incomplete closing is bound to increase the current density, resulting in local overheating or even cause the thermal out of control of the battery. Therefore, the preparation of a high thermal stabilizing diaphragm is one of the ways to improve battery safety. However, the problem is, how is the high-fever stabilizing diaphragm to solve the safety problem of the battery? Today's highly sought-in full solid electrolyte (solid ceramic and polymer electrolyte replaces traditional diaphragm and electrolyte), can it thoroughly reduce battery safety? Recently, the Qinghua University Ouyang Ming Te Team pointed out that the safety design of the power lithium battery is not only to improve the safety of a single material, but should start from the system level.

The author takes into account the factors of the battery level and material level, and the thermal out-control mechanism of the power lithium battery is studied. The study is based on graphite as an anode, single crystal layer Li0.5Mn0.

3CO0.2O2 (NMC532) is a 25aH-type power lithium battery of the PET / ceramic nonwoven fabric as a diaphragm. The authors analyze the cathode gas, structural (phase) transformation and heat generation when there is no anode, when there is an active state, and the cathode is analyzed.

It is proposed that if the cathode release O2 can react with an anode, it will be a serious potential security problem! Through the two parts of the design safety mechanism experiment, the author first conducts Arc testing of the whole battery, followed by the various components of the battery. Among them, the EV-ARC test records the entire thermal out-of control process of the battery, and it is found that the heat loss temperature of the battery is even lower than the heat shrinkage temperature of the membrane, indicating that the battery has not occurred due to the failure of the passage. Large area short circuit.

This is the first report without the thermal out of control in the case of internal short circuit. In order to study specific heat-conducting mechanisms, the team uses DSC technology to analyze the heat flow (cathode, anode, electrolyte, and combination thereof) of battery different components; use online high temperature XRD technology to determine the cathode during heating and thermal decomposition, Further, the author uses synchronous thermal analysis technology (DSC-TG) and mass spectrometry (MS), detecting heat, weight loss and gas release process. In order to further confirm the conclusion, the author will quickly freeze the liquid nitrogen in 206 ¡ã C, a series of subsequent tests, including SEM, ICP-OES and XPS tests, including SEM, ICP-OES and XPS tests.

Figure 1 Basic properties of the power lithium battery: a. Circulating performance and Kurun efficiency; b. Ultra-rate performance The power lithium battery exhibits good cycle performance and magnification performance.

The first week discharge capacity is 25.04ah, still 24.08ah after 292 weeks, and the capacity retention rate is as high as 96%.

Even at 4C, there are still 21.5ah capacity. Figure 2 Measuring the thermal out of control of the power lithium battery using EV + ARC.

The small picture is the autopilot phase (0-105s) authors use EV + ARC to monitor the thermal out of control of the power lithium battery. T1 is the starting temperature of the self-heating, T2 is the thermal out of control temperature (TR), T3 is the highest temperature. T1 is 115.

2 ¡ã C. Exact record of the instrument under this temperature rise process (T1→T2) chemical side reaction. First, the SEI film decomposition of the anode causes the exposed anode anode to the electrolyte to form a new SEI film, and heat is also heat; SEI is constantly repeated, resulting in disappearing and inorganic components of anode surface carbonate component Rising; the side reaction occurs, causing temperature to rise until the heat loss temperature TR (T2 = 231 ¡ã C).

At this time, the battery temperature index is rising, and the exothermic reaction is extremely intense. With the battery release a large amount of smoke; in addition, the volume expansion of the battery is very obvious, which proves that the exothermic side reaction of this process is caused by a gas. After reaching T2, the battery temperature is within a few seconds, rapidly increased to 815 ¡ã C to achieve the highest value T3.

Figure 325AHSC-NMC532 / graphite thermal out-of control characterization A. The relationship between the heat loss rate, the temperature rise rate, battery voltage and internal resistance and absolute temperature; B. Before the heat loss, the internal resistance follows the absolute temperature change (part of the figure) When the thermal out-of control, the battery voltage varies with absolute temperatures into a more comprehensive study of the internal reaction of the battery.

When the authors are tested, the real-time changes of voltage and internal resistance are recorded. Figure 3A, after the turning point of the temperature rise rate occurs after 160 ¡ã C, T1 (115.2 ¡ã C), which is related to the repeated formation of the SEI and the decomposition of LiPF6, this process accelerates heat and gas production.

The voltage change shows that the thermal out of control (T2 = 231 ¡ã C) occurs, the voltage is maintained above 2.0V, which proves that there is no short circuit. Changes in battery internal resistance are divided into four phases: stage I ( <145 ¡ã C), the internal resistance is slow to 22.

1M. Less dependence; stage II (145175 ¡ã C), internal resistance 22.1m→143.

3m. The battery bag is broken at 145 ¡ã C, accelerates the electrolyte attack, causing an increase in the internal resistance; the increase in the cathode impedance also increases the internal resistance of the battery; the decomposition of the anode surface SEi causes new increasing inorganic components, reducing ion conductance, also Resulting in increased battery internal resistance; stage III (180231 ¡ã C), internal resistance 143.3m→56.

5m. Before the heat loss, the reduction in internal resistance is due to the dissolution of the transition metal and the decomposition of the lithium salt, will be confirmed later; stage IV (> 231 ¡ã C), internal resistance 56.5m→1011.

2m. After the heat loss, the battery combusts, the voltage is rapidly dropped in a few seconds, and the internal resistance is then rapidly riser to 1011.2m.

At this time, the diaphragm is disintegrated, the battery is completely failed. Figure 4PET-ceramic non-woven sealing structure and thermal stability: PET-ceramic nonwoven fabrics after thermal stability test (room temperature 450 ¡ã C), SEM scan, room temperature and 450 ¡ã C morphology and elements mapping After 500 ¡ã C at room temperature, the DSC heat flow and TGA weight loss of the diaphragm were lost, the temperature increase rate was 10 ¡ã C / min; PET-ceramic nonwoven fabric diaphragm, the illustration of an enlarged SEM photo of Al2O3; cross-sectional view, Al2O3 particles The PET nonwoven fabric fibers wrapped in conventional PP, PE diaphragm showed excellent thermal stability. As shown in Figure 4A, in 230 ¡ã C for 30 min, only very small heat shrinkage occurs (1.

2%). As shown in Figure 4B, PET is melted with heat transfer at 257 ¡ã C, and degradation accompanying weight at 432 ¡ã C. The SEM of Figure 4C indicates that non-woven PET nanofibers are embedded in ceramic particles, not a double-sided coating of ceramic particles.

Figure 4C section SEM indicates that the diaphragm is 19.5μM Figure 5 Test the heat-conducting conditions of each component of the charge state battery through the DSC: a. When the electrolytic solution is present, the charge electrode (Ce); b.

In the presence of electrolyte, the charge electrode. An anode; CA cathode; ELE electrolyte; CE charger electrode Figure 5a indicates that the cathode and the anode are far less than the heat generation of the cathode and the anode coexistence; Figure 5B shows that the presence or absence of electrolyte There is no significant impact. Therefore, regardless of whether the electrolyte is present, mix the cathode and the anode, there will be a lot of heat.

The author speculates that there is a mutual use between the cathode anode, which may be a chemical reaction. Figure 6 Structural transformation, heat generation and O2 release of the charge cathode material: a. High temperature XRDB.

At different temperatures, the on-site heat and release of the DSC and TGA-MS system is measured when high temperature, Dithodium NMC Not stable, structural transformation will accompany O2 release. The author speculates that the cause of thermal out-of control is the release of the mutual use between the O2 and the anode. Figure 6A shows that the NMC 532 begins to change the transition of the layered structure to the spinel structure at 350 ¡ã C until 350 ¡ã C.

Figure 6B shows that the MS curve of the study of heat DSC curves and the release of O2 is consistent with the structural variation, and there is a peak at 276 ¡ã C, meaning serious phase transformation. Figure 7 is between the charging state cathode and the anode, interference between the chemical reaction level: a separate charge cathode, a peak of strong oxygen release; however, when the cathode and anode of the charge state are present, there is basically no dosage However, in the same temperature interval, the production of heat is significantly larger, and the charging state is released at high temperatures based on the mutual interference schematic of the chemical reaction, and only a small amount of heat is released; when there is an anode, O2 heat is out of control. Therefore, for the safety of the mechanical lithium battery system, the thermal management system should take intervention before the thermal out-of control, otherwise it is difficult to prevent the battery from being fired even if the liquid nitrogen having the strongest heat dissipation function.

Figure 8 During the occurrence of heat loss, the characterization of liquid nitrogen is frozen by liquid nitrogen, the change curve of liquid nitrogen, battery temperature and voltage is added at 206 ¡ã C, and the illustration is the front surface (I) and the side surface of the battery after liquid nitrogen cooling (II) Photo; Z-type lamination structure and battery after cooling in liquid nitrogen at 206 ¡ã C, photo liquid nitrogen in the internal cathode, anode and diaphragm, rapidly down to -100 ¡ã C in the battery of 206 ¡ã C, although the side of the pre-battery bag is Bracked (Figure 8A). Figure 8B shows the decomposed battery module, no visible hole or damage surface surface, indicating that it maintains voltage stability and effectively prevents short circuit; the cathode surface and the corresponding diaphragm are black belt, which is Ni, CO, MN transition metal deposition No signifi.