Mingyi Chen

Investigation on the thermal hazards of 18650 lithium ion batteries by fire calorimeter

In applications of lithium ion batteries, it is a requisite to precisely appraise their fire and explosion hazards. In the current study, a fire calorimeter is utilized to test the combustion performance of two commercial 18650 lithium ion batteries (LiCoO2 and LiFePO4) at different state of charge (SOC). Characteristics on thermal hazards of lithium ion batteries including surface temperature, time to ejection, mass loss, and heat release rate (HRR) are measured and evaluated. In case of thermal runaway, all the lithium ion batteries will rupture the can and catch fire even explode automatically. The solid electrolyte interface layer decomposition and the polymer separator shrinking are direct causes of the lithium ion battery fire. The experimental results show that the HRR and total heat generally rise as the SOC increases, whereas the time to first ejection and the time gap between first and second ejection decrease. LiCoO2 18650 battery shows higher explosion risk than LiFePO4 18650, as the former has released much more oxygen. The experimental combustion heats calculated and modified in the oxygen consumption method reveal that the internally generated oxygen have significant effect on the estimate of the heat, where the largest modified rate is 29.9 for 100xa0% SOC LiCoO2 18650 battery. The results can provide scientific basis for fire protection during the storage and distribution of lithium ion batteries.

Impacts of ceiling height on the combustion behaviors of pool fires beneath a ceiling

To examine the effect of varying ceiling height on the combustion behaviors of pool fires whose flame impinged the ceiling, a series of n-heptane pool fires was performed beneath a ceiling in the range of Hef/Dxa0=xa00.43–2.5 using a scaled-down calorimeter. Typical parameters including the burning intensity, combustion yields and combustion efficiency were quantitatively analyzed. It was found that with the decreasing ceiling height, the burning intensity initially increased to a peak at Hef/Dxa0=xa01.38 and then decreased, exhibiting a parabolic variation tendency. The maximum concentration increments of CO and CO2 showed the parallel results to the burning intensity, while the total amount of CO yield monotonously increased with the declining ceiling height, implying increasing toxicity. Besides, the carbon conversion ratio produced the similar result as the combustion efficiency, and both of them showed the parabolic variation law.

Experimental study on the thermal decomposition and combustion characteristics of nitrocellulose with different alcohol humectants

Although the thermal behaviors including thermal instability of nitrocellulose (NC) and its mixtures with some humectants have been comprehensively examined previously in the literature, their combustion characteristics have not been systematically studied. To address the issue, the combustion properties of NC with alcohol humectants are investigated by the means of the ISO 5660 cone calorimeter. Two kinds of NC-humectant mixtures with 30wt.% isopropanol and 30wt.% ethanol, respectively, were employed as samples. The tests were conducted under different external radiations, ranging from 0-15kW/m2. The experimental results indicate that the external radiation positively influences the peak heat release rate (HRR) intensity and the maximum mass loss rate (MLR), while the total heat release (THR) decreases with the elevated external radiation. Comparatively, the sample with isopropanol exhibits a higher fire risk, characterized by the higher peak HRR, THR and maximum MLR. Auxiliary investigating methods, including Scanning Electron Microscopy and Differential Scanning Calorimeter-Thermal Gravimetric Analysis, were applied to examine the micro structure and thermal behavior of NC-humectant mixtures. The results helped to explain the burning characteristics observed in the cone calorimeter tests.

Combustion characteristics of primary lithium battery at two altitudes

To increase the safety margin, the fire hazard of lithium battery should be considered. In this research, the experimental results of lithium battery fires were provided, expecting to offer guidance to facilitate the safe handling of battery packs and cells under normal and high-altitude conditions. Single and bundles of primary lithium battery experiments were performed to study the fire behaviors of primary lithium batteries. The same configured calorimeter was built in a sea-level city Hefei (100.8xa0kPa/24xa0m) and a high-altitude city Lhasa (64.3xa0kPa/3650xa0m), respectively, to investigate the effect of altitude on the fire behaviors. From the results, the mass loss exhibits as a linear function of cell numbers in both Hefei and Lhasa. The mass loss also increases as altitude decreases for the same number of cells. The study also shows that the heat release rate, effective heat of combustion and heat flux decrease at higher altitude. The combustion efficiency in Lhasa is lower than that in Hefei.

An experimental study about the effect of arrangement on the fire behaviors of lithium-ion batteries

Safety problem is always a significant consideration before wider field of application such as mobile phones, computers and new energy vehicles. However, the knowledge on the battery combustion behavior is limited. To increase the safety margin, the fire hazard of lithium-ion batteries should be considered. An experimental study of different arrangements: horizontal 4xa0×xa01, horizontal 2xa0×xa02 and vertical 2xa0×xa02 lithium-ion batteries fire behaviors was conducted. The photographs of fire processes and the batteries before and after the fire tests were directly shown to describe the fire hazard. The mass loss rate, heat release rate and heat flux were used to analyze the combustion behavior more detailed. Based on the results, lithium-ion batteries are volatile and burning with potentially deadly explosions. The arrangements can affect the ignition time, heat release rate, released heat and the heat flux, while the way of mass loss and the total mass loss are similar. The results indicate that batteries with bigger heating area have more risky and fierce fires compared with others.

Experimental study on the thermal behaviors of lithium-ion batteries under discharge and overcharge conditions

To have a better understanding of the thermal behaviors of lithium-ion batteries (LIBs) under discharge and overcharge conditions, some tests were conducted by a cone calorimeter. Several parameters were measured such as the battery surface temperature, voltage, the time to thermal runaway, the time to maximum temperature, heat release rate and total heat released. The results indicate that the lithium-ion battery will have obvious warming up during discharge owing to the reversible heat and irreversible heat. It was observed that the current treatment (discharge) has a significant influence on the thermal behaviors of LIBs. It can accelerate the warming up, result in earlier thermal runaway, and reduce the heat released. In addition, overcharge will make LIBs more unstable and easier to attain the thermal runaway.

Combustion calorimetry of carbonate electrolytes used in lithium ion batteries

In this article, some of the combustion properties of three carbonate solvent mixtures commonly used as the electrolytes of lithium ion batteries are considered by means of the ISO 5660 cone calorimeter. Experimental findings reveal that the heat release rate, the most important parameter in fire science, exhibits a significant variation among the carbonate mixtures. Other key parameters governing the fire-induced hazards such as total heat release, mass loss rate, combustion efficiency, and concentration of the major exhaust gases are also determined and analyzed. Furthermore, as some researchers argue that oxygen consumption calorimetry is likely to over-predict the chemical heat release for lithium ion cells, another thermal chemistry method based on stoichiometry for heat release rate calculation is adopted. Heat release rate results of the three carbonate solvent mixtures obtained by these two separate methods are found to be in good agreement. Thus, oxygen consumption calorimetry is considered to be an appropriate technique to determine the heat release in fires in relation to electrolytes of lithium ion batteries.

Prediction of heat release rate of shredded paper tapes based on profile burning surface

A series of shredded paper fire experiments were conducted by means of a calorimeter. The mass loss rate and heat release rate were measured. The flame spread process was recorded, which shows that the flame spread process can be divided into four typical stages, and the mean spread rates along different directions were obtained from the observed combustion process. Based on the mean flame spread rate, a mathematical model for predicting the burning surface as a function of time during the four stages is established. Combining this model with the effective heat of combustion calculated from measured mass loss rate and heat release rate, an improved model to predict the heat release rate as a function of time was developed. In this model, the linear relationship between heat release rate and burning surface is found, and the predicted result agrees well with the measured heat release rate.

Experimental study of high altitude effect on heat release rates of pool fires using calorimeters

With the aid of same configured calorimeters as dimensions of 40% of that in ISO 9705 in Hefei (24xa0m, 100.8xa0kPa) and Lhasa (3650xa0m, 64xa0kPa), the influence of high altitude on heat release rate and combustion efficiency were investigated. Two groups of liquid pool fires of moderate sizes (Dxa0=xa00.15, 0.25xa0m) with fuel level maintaining system were tested at two sites, respectively. Typical fuels with different sooting levels, i.e., N-heptane and Jet-A, were selected. The ambient air pressure effects were introduced by modifying the standard calculation method of heat release rate in ISO 9705. Experimental results indicated that the dimensionless burning intensity in the quasi-steady stage for both fuels could be accorded to pressure modeling with acceptable accuracy. And it could be correlated with radiation modeling well for Jet-A, while that of n-heptane failed and this may be explained by the flame convection feedback which could not be neglected for moderate sooty fuel of moderate sizes. The combustion efficiency at high altitude is slightly higher than that at atmospheric pressure, and it will gradually increase with the decreasing pool dimension regardless of the ambient pressure.

A study on the fire behaviors of 18650 battery and batteries pack under discharge

In order to have a better understanding of the fire behaviors of lithium-ion battery (LIB) and batteries pack under discharge, a series of fire tests were conducted. The fire behaviors of LIB were characterized by parameters including the flame photographs, surface temperature, mass loss, radiative heat flux and hydrogen fluoride (HF) mass flow. The results indicate that the number of batteries will affect the fire behaviors of LIB. Pack with larger number of LIB possesses a faster temperature rise, a higher peak heat flux and total radiative heat, leading to a quicker thermal runaway. Additionally, the discharging treatment also has a significant impact on LIB. It can accelerate the warming-up process, increase the fire risks of LIB, decrease the combustion efficiency of LIB and reduce the radiative heat. This is the result of the heat generation during discharge and the more reactive materials inside the battery under discharge. Besides, the HF generated during tests is discussed in the paper to note the toxicity of the released gases in LIB fire.