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Effects of thermal hazard on 18650 lithium ion battery under different states
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of charge
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4
Wei-Chun Chen．Jian-De Li．Chi-Min Shu．Yih-Wen Wang*
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Abstract Lithium ion (Li-ion) battery is an important power storage system with efficient
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energy densitiesand long life cycles characteristics. However, its potential safe issue still need
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to be further discussed. This study used an adiabatic calorimeter, vent sizing package 2 (VSP2),
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to appraise the thermal runaway behaviours of 18650 lithium ion battery on various charging
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levels. The batteries were tested for states of charge at 30, 50, 80, and 100%. By calorimetric
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experimental trails, we can determine the thermal hazard features, such as apparent exothermic
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initial temperatures (T0), maximum temperatures (Tmax), pressure, temperatures, maximum
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pressure (Pmax), self-heating rates (dT dt–1), pressure rise rates (dP dt–1), and runaway patterns.
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During the thermal runaway, Tmax and Pmax under full chargeable Li-ion battery were 774.9 K
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and 1,519.6 KPa, respectively. These experimental results could assist in estimating uncontrolled
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behaviours and thermokinetic parameters for various charged states of the 18650 Li-ion battery.
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They could be used in proactive design, and as the ultimate objective, they could provide the
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process safety parameters to forestall commercial batteries from thermal damage packs.
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Keywords Lithium ion batteries．Process safety parameters．States of charge．Thermal
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hazard features．Vent sizing package 2 (VSP2)
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W.-C. Chen
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1
Graduate School of Engineering Science and Technology, National Yunlin University of
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Science and Technology (YunTech), 123, University Rd., Sec. 3, Douliou, Yunlin 64002,
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Taiwan, ROC
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J.-D. Li．C.-M. Shu
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Department of Safety, Health, and Environmental Engineering, YunTech, Douliou, Yunlin
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64002, Taiwan, ROC
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Y.-W. Wang (
)
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Department of Occupational Safety and Health, College of Public Health, China Medical
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University, 91, Hsueh-Shih Rd., Taichung 40402, Taiwan, ROC
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e-mail: even0937@yahoo.com.tw
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List of symbols
A
Frequency factor (min–1)
Cp
Capacity of test cell (J g–1 K–1)
Ea
Apparent activation energy (kJ mol–1)
k
Heat transfer coefficient (W m–1 K–1)
Kb
Boltzmann constant (J K–1)
m
Sample mass (g)
t
Time (sec)
T
Absolute temperature (°C)
T0
Apparent exothermic onset temperatures (°C)
∆H
Heat of reaction (J g–1)
∆Tad
Adiabatic temperature rise (°C)
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3
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1
Introduction
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Lithium ion (Li-ion) batteries have a long cycle life and a high power density without adverse
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memory effects. In practice, a Li-ion battery can be changed by shape, such as cylindrical,
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including 18650 or 26650, button type, square, and plastic flexible packaging. The volumetric
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energy densities and efficacy are superior to Ni-Cd batteries. Many Li-ion batteries can be used
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as series and parallel connections in the battery pack and are largely applied in energy storage
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systems for 3C electronic products and electric vehicles [1–5].
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Li-ion batteries are composed of a cathode, anode, separator film, an electrolyte, and the
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protective devices, such as battery management system (BMS), vent device, positive
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temperature coefficient (PTC), thermal fuse, etc. There are sources of the electrochemical
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reaction via the cathode and anode. However, the separator film may crack or melt, resulting in
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internal short circuit or outer crash, and the cathode and anode will break down, causing catalytic
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burning. Li-ion batteries are similar to a chemical reactor because their temperature, materials,
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and release heat should be well governed. However, many improper applications or defects of
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materials have happened, which have caused disastrous damage, such as fire or explosion. State
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of charge (SOC) means that the remaining electric capacity in the batteries by factoring in the
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variables as the discharge rate and time, temperature, charge rate and duration. Many methods
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can estimate SOC, including discharge test, ampere hour counting, etc. [6–9]. The aim of this
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study was to compare the thermal stability influence of Li-ion batteries under four different
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SOCs: 30, 50, 80, and 100%.
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By experimental calorimetry tests, we can determine the thermal hazard features, such as
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apparent exothermic initial temperatures (T0), maximum temperatures (Tmax), pressure,
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temperature, maximum pressure (Pmax), self-heating rates (dT dt–1), pressure rise rates (dP dt–1),
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and runaway patterns with kinetic analysis equations. The results could be used in proactive loss
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prevention design, and as the ultimate goal, they could provide electrochemical safety
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parameters of Li-ion cells to prevent thermal damage from commercial battery packs [10–14].
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In addition to the safety conditions of the applications, the damage sources should be eliminated
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and removed or the high risk conditions could be made less risky and more acceptable.
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Experimental methods
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Samples
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The Li-ion batteries had a diameter of 18.0 mm and a length of 65.0 mm, as broadly used in
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various areas worldwide, such as series connections or parallel connections in electric vehicles.
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We used commercial 18650 Li-ion battery (LG DB218650) as the test sample, which cathode
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material was LiCoO2, and the nominal capacity of the Li-ion battery was 2600 mAh. In general,
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the cathode and anode were isolated by separator film. Separator film played an interposed
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important role in Li-ion batteries. It was not only prevented to contact with cathode and anode,
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but also ionized ions activated in the electrolyte. The electrochemical reaction initially proceeds
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slowly, but it accelerates exponentially as the temperature increases at the start of the runaway
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reaction. The proposed reaction mechanisms of LiCoO2 cathode and ethylene carbonate (EC)
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electrolyte as an example [15, 16]:
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1
LiCoO 2  Li  Co 3O 4  O 2
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3
(1)
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1
Co 3O 4  3CoO  O 2
2
(2)
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1
1
Co 3O 4  C  3CoO  CO 2
2
2
(3)
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1
CoO  Co  O 2
2
(4)
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3
2LiCoO 2  CO 2  Li 2 CO 3  2Co  O 2
2
(5)
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Δ
2Li  C3 H 4 O3 (EC) 
Li 2 CO 3  C 2 H 4
(6)
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Programmable DC power DSP-030-025HD and Prodigit instrument professional 3302F have
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been carried out various SOCs of a battery. In our previous studies, we were using full charged
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level (4.2 V) and un-charged level (3.7 V) to compare the thermal hazards on Li-ion batteries [2,
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5, 9]. SOC is a useful estimation for battery applications, which can prevent the battery failure
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from being over-charged/over-dischargrd operation. For the application of 3C electronic
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products and electric vehicles, the control of charge level is advantageous to obtain close to
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100% charge efficiency and charge acceptance rather than maximum battery capacity. The SOC
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estimation could evaluate the relationship between the remaining battery capacity and thermal
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runaway ranking for Li-ion battery. After charge-discharge cycling at a constant rate of 0.5 C-
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rate, the Li-ion batteries had SOCs of 30, 50, 80, and 100%, as listed in Table 1. In practice
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the SOC of the charging process can be derived from Equation (7) [17]:
𝑄releasable
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SOC =
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in which SOC is the capacity of the battery, Qreleasable is the releasable capacity, and Qrate is the
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battery rated capacity.
𝑄rated
× 100%
(7)
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Vent sizing package 2 (VSP2)
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Vent sizing package 2, an adiabatic calorimeter with a pressure and temperature system, is
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manufactured by Fauske & Associates, LLC (Burr Ridge, Illinois, USA). It has PC-control
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to govern and balance the internal and external pressure as well as the temperature in the
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tank. We designed the test cell to handle high pressure and temperature. VSP2 is a typical
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adiabatic calorimeter with three heat-wait-search (HWS) steps in the experimental test. After
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obtaining the temperature, we waited ten minutes and searched for the Li-ion batteries’
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exothermic rate to determine whether it exceeded 0.2 °Cmin–1. Fire exposure, abnormal
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heating, loss of cooling, and the other factors can be evaluated. The temperature and pressure
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were all measured, as accurate temperature and pressure data were received directly from
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the reaction system in the adiabatic environment [15].
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The exothermic reaction enthalpy is important for safety issue because it depicts the
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thermal runaway reaction for the Li-ion battery. 18650 cylindrical batteries have been tend
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to cause internal heat accumulation. Practically speaking, there is lower heat loss to the
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surroundings in an exothermic reaction condition, and the reaction energy is increasing the
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self-heating of Li-ion batteries to result in potential thermal runaway. The effortless thermal
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analytical equations characterise the exothermic reaction heat of Li-ion batteries as
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determined by VSP2 experiments. Li-ion batteries are described such that the heat capacity
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of test cells (Cp), the heat of reaction ( ∆ H), the sample mass (m), and the adiabatic
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temperature rise (∆Tad) of the Li-ion batteries can be calculated as Eq. (8) [18, 19]:
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∆𝐻 = 𝐶p × 𝑚 × ∆𝑇ad
(8)
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Results and discussion
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This study focused on the thermal runaway reaction of 18650 Li-ion battery through external
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heating and induced the electrochemical exothermic behaviours. The electrochemical potential
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difference of the various SOCs could convert the chemical energy to electric energy or reverse
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situation in the batteries. There were the dramatic electrochemical reactions in higher SOC Li-
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ion battery associated with elevated temperature and pressure conditions.
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The adiabatic results showed exothermic profiles, along with thermokinetic parameters,
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such as apparent exothermic onset temperatures (T0), maximum temperatures (Tmax), maximum
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pressure (Pmax), self-heating rates (dT dt–1), and pressure rise rates (dP dt–1). We compared the
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related thermal instability of various SOCs on Li-ion batteries under adiabatic conditions for the
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purpose of the thermal runaway reaction. The SOCs of 30, 50, 80, and 100% charging voltage
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were 2.84, 3.62, 3.90, and 4.18 V, respectively. The 18650 lithium-ion batteries lost 9 g of
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mass after the adiabatic experiment, and the charging voltage lost efficacy because of the
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exploded cell.
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The thermal decomposition of 18650 Li-ion batteries in the SOCs of 30, 50, 80, and 100%
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is shown in Figs. 1–3. The T0 of 18650 Li-ion batteries in the SOCs of 30, 50, 80, and 100%
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were determined to be 457.8, 447.8, 444.9, and 424.0 K, respectively. According to the literature,
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the separator would crack and then cause an internal short circuit at 80–120 °C. When an
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internal short circuit takes place, the lithium ion batteries are going to
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induce self-heating reactions in Eqs 1–6.
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The separator crack or melt may trigger the short-circuiting of positive and negative
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electrodes to result from battery failure. When the temperature exceeded 457.8 K at SOC
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30%, and the decomposed cathode material reacted with the electrolyte, which caused the
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temperature to reach 473 K. When the cathode reacted with the electrolyte, it resulted in an
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oxidation reaction, which courted a runaway reaction. The Tmax quickly rose to 715.5 K, and the
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pressure and temperature increased simultaneously, followed by a release of an enormous
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amount of gas and heat. With increasing SOC, the corresponding T0 was also initiated at the
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earlier temperature, and the Tmax, as well as the experimental reactions, were significant.
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The Pmax values varied from 396.4 to 1519.6 KPa, with changes up to 5–fold from the
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SOCs of 30 to 100%. The self-exothermic runaway happened after 180 min in the SOCs of 80
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and 100%; times of more than 270 min occurred for the SOCs of 30 and 50%. Although Tmax
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values were similar, the pressure results were different, with rises of approximately 3–fold in the
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SOCs of 80 and 100%. The T0 occurred earlier when the 18650 Li-ion battery under the SOC
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of 100%. The circuit might provide an indication of SOCs and safety features in the case of
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short-circuiting and over-heating situations on battery failure. The electrolyte was composed
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of organic solvent, and cathode materials included lithium ion and metal ions. If organic
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solvent mix with metal ions, it will occur incompatible reaction to induce thermal runaway.
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The results show that the uncontrolled battery quickly generated enormous amounts of heat and
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gases in a shorter time to trigger explosion, fire, or releasing toxic gas. This heat generation is
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due to the violent reactions accompanied by the pressure and temperature in Figs. 1 and 2.
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Defective tightness of the test cell could lead to leaking of gas. Nevertheless, Li-ion battery
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reactions fall into two main stages. The first stage occurs when the battery temperature rise
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reaches the apparent exothermic onset temperature, indicating that the separator has broken
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down to induce an exothermic phenomenon. The second stage occurs when the temperature rises
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to the maximum temperature during the runaway excursion.
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1
As the temperature reached the apparent exothermic onset temperature, the adiabatic
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temperature rise was more than 473 K. During the thermal runaway reaction, the temperature
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and pressure of self-reactive batteries suddenly reach the maximum values. As shown in Figs.
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4 and 5, although the SOC of 30% has a low charging voltage, the self-heating rate (dT dt–1)
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and pressure rising rate (dP dt–1) increase rapidly, up to 3102.9 K min–1 and 1412.7 KPa min–
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1
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reaction induces a significant increase in temperature and pressure. It is demonstrated that the
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SOC of 100% of the Li-ion battery has a higher thermal hazard than 30, 50, and 80%.
, respectively. Therefore, the broken Li-ion batteries are unsafe and risky because the runaway
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VSP2 can be applied with experimental data to calculate the slope, apparent activation
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energy (Ea), and frequency factor (A). Among them, the Arrhenius method can be based on the
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self-heating rate versus the reciprocal of temperature, as shown in Eq. (9):
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𝑘 = 𝐴exp(𝑘 𝑇𝑎)
−𝐸
(9)
b
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The experimental data from the VSP2 can be used to obtain Ea and A using the Arrhenius
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method; these results are shown in Table 3 and Fig. 6. The apparent activation energies of 18650
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Li-ion batteries were 3.5, 2.1, 1.7, and 2.4 eV, corresponding to SOC of 30, 50, 80, and 100%,
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respectively.
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According to linear analysis results, we can obtain ln((dT/dt)/(Tf–T)) versus the reciprocal
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of absolute temperature [20]. Then, these data can be used to obtain the slope to calculate the
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apparent activation energy, as shown in Eq. (7). The thermokinetic parameters of 18650 Li-ion
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batteries were 6.0, 1.8, 1.7, and 1.2 eV, corresponding to SOCs of 30, 50, 80, and 100%,
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respectively, as shown in Fig. 8 and Table 3. The values are the same when using two apparent
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activation energy methods for the SOCs of 50 and 80%.
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ln𝑘 = ln𝑘0 − 𝐾 a𝑇 = ln
𝐸
b
d𝑇/d𝑡
(10)
Tf −𝑇 n
Cn−1
0 (𝑇 −T ) (Tf −T0 )
f 0
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1
When 18650 Li-ion batteries are exposed to high ambient temperature, the separator film
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may crack and cause the short-circuiting of the cathode and anode. The electrolyte was
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composed of organic solvent, and cathode materials included lithium ion and metal ions. If
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organic solvent mix with metal ions, it will occur incompatible reaction to induce thermal
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runaway. Furthermore, the electrochemical reactions of a Li-ion battery contribute heat
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generating and gas expanding. The runaway reaction mechanisms are complicated and
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disorder [16]. The runaway behaviors depended on SOCs of Li-ion battery and should control
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the battery thermal management.
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Conclusions
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The thermal runaway parameters, such as T 0, Pmax, dT/dt and dP/dt, were sequentially
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increasing with SOCs of 30%, 50%, 80%, and 100% on 18650 Li-ion battery. Moreover, the
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exothermic reaction form the adiabatic calorimetry experiments could estimate the kinetic
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data to an individual battery. The calorimetry methodology is proposed to evaluate the safety
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issue on thermal management of battery pack and module. The thermal runaway of 18650 Li-
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ion batteries at higher SOCs should be considered with caution during applications, particularly
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under abnormal conditions.
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Acknowledgements
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The authors are grateful to Mr. Can-Yuan Jhu and Mr. Hong-Hsi Lin for the experimental
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suggestions and to China Medicine University (CMU), Taiwan, ROC (grant no. CMU102-N-09)
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for the financial support of this study.
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Table captions
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Table 1 Information on the various SOCs in SE US18650GR of 18650 Li-ion batteries
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Table 2 VSP2 adiabatic experimental data for the various SOCs in 18650 Li-ion batteries
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Table 3 Apparent activation energy analysis graphs for 18650 Li-ion batteries
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6
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Figure captions
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Fig. 1 Temperature–time curves for the adiabatic runaway system of the 18650 Li-ion batteries
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in SOC of 30, 50, 80, and 100%
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Fig. 2 Pressure–time curves for the adiabatic runaway system of the 18650 Li-ion batteries in
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SOC of 30, 50, 80, and 100%
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Fig. 3 Temperature–pressure curves for the adiabatic runaway system of the 18650 Li-ion
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batteries in SOC of 30, 50, 80, and 100%
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Fig. 4 Self-heating rate for the thermal decomposition of the 18650 Li-ion batteries in SOC of
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30, 50, 80, and 100%
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Fig. 5 Pressure rise rate for the thermal decomposition of the 18650 Li-ion batteries in SOC of
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30, 50, 80, and 100%
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Fig. 6 18650 Li-ion batteries in SOC of 30, 50, 80, and 100% of apparent activation energy
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analysis graph by Arrhenius equation
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15
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Fig. 7 18650 Li-ion batteries in SOC of 30, 50, 80, and 100% of apparent activation energy
analysis graph
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