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Quantitative Comparison of Thermal Influence among Laser Irradiation and Other Methods for Thermal Runaway of Traction Lithium Ion Battery

Initiation methods for thermal propagation tests are being discussed in the EVS-GTR. In this paper, laser irradiation was compared with nail penetration and heater heating using a prismatic Li-ion battery module from the viewpoint of heat energy transfer to adjacent cells. Such transfer with laser irradiation was 0.025% of that of heater heating and the adjacent cell’s temperature rise was less than 1°C by thermal runaway of the target cell. This result demonstrates that laser irradiation is a rational method in terms of replicating single cell thermal runaway only due to the cell’s own energy. ,


Introduction
The trend toward stricter environmental regulations is accelerating in various countries. Some European countries have declared that sales of vehicles with internal combustion engines will be prohibited in the future, and the popularization of electric vehicles (EV) is expected to accelerate. Many current EVs use lithium ion batteries (LIBs) as the traction batteries. LIBs have a higher energy density than lead-acid batteries and Ni-MH batteries [1] . However, smoking and ignition accidents have occurred in the market for both traction and consumer-use LIBs [2][3][4][5] , and further enhancement of safety is necessary for the popularization of EVs. Meanwhile, longer EV cruising ranges are also demanded, and there are expectations for even higher energy densities for traction LIBs in order to increase cruising ranges. High-energy density designs for LIBs generally tend to result in lower safety, which calls for advanced battery design to increase the energy density while still securing safety.
For this reason, the "thermal propagation test method" is being discussed in the UNECE EVS-GTR (Electric Vehicle Safety -Global Technical Regulation) as a method of verifying the safety of EV batteries. This test method forcibly induces thermal runaway of one cell on the hypothesis that an internal short circuit occurred in the cell due to some issue, and assesses whether vehicle occupants are exposed to a hazardous environment resulting from a thermal propagation [6] . Forced thermal runaway initiation methods of a target cell are currently being examined in the EVS-GTR. The forced internal short circuit method by nail penetration, the heater heating method, and the overcharge method are noted as initiation methods in the EVS-GTR Phase 1 document published in 2018.
In the forced internal short circuit method by nail penetration, external heat energy is not applied to the target cell, which makes this a rational method in that it replicates thermal runaway that occurs only due to the cell's own energy starting from an internal short circuit.
However, it is necessary to physically penetrate a nail into the target cell, so target cell selection is restricted by the battery pack layout.
The heater heating method offers easy test preparation due to the high degree of freedom of heater and cable installation, and has the merit of a greater degree of freedom of target cell selection. However, the cell is heated by external heat energy input, so there is the issue of heat transfer to adjacent cells before thermal runaway of the target cell.
As a result, the heater heating method may result in a more severe assessment compared with the thermal runaway mode starting with the internal short circuit of a single cell.
The overcharge method has a high degree of freedom of target cell selection if electrical connection is available. Like heater heating, however, it may result in a more severe assessment due to external energy input. Furthermore, the overchage method is not applicable to cells with a current interrupt device.
This study examined the initiation method using laser irradiation.
The laser irradiation method has already been adopted as a thermal propagation test initiation method in the industrial storage battery standard IEC 62619:2017 [7] . Moreover, as it is a heating method with local and high power density, it is expected that excessive heat input to adjacent cells, which is an issue with the heater heating method, will be suppressed.
Various research is underway on the thermal runaway initiation method with laser irradiation [8][9][10][11] , but there are no known reports of quantitative evaluation of the amount of heat transfer to adjacent cells.
This study used a traction prismatic LIB and quantitatively evaluated the heat energy transferred to an adjacent cell until thermal runaway initiation of the target cell with the laser irradiation, nail penetration, and heater heating methods as the initiation method.
In addition, the results of morphological analysis of a discharged cell after laser irradiation and analysis of the heat generation characteristics of the cell constituent materials were used to examine the process leading from the laser-irradiated part to thermal runaway of the entire cell.

Evaluation of Heat Transfer Amount to an Adjacent Cell
Using an Assembled Battery An assembled battery consisting of five unit cells connected serially as shown in Fig. 1 was fabricated to evaluate the thermal influence on adjacent cells in the assembled battery state. Table 1 shows the specifications of the cells used in this study.

Nail penetration
The nail penetration test used a steel nail with a diameter of 3 mm and a tip angle of 60°. This nail penetrated to a depth of 25 mm from the cell case surface at a speed of 1 mm/s.

Heater heating
Heater heating used a 16 mm  35 mm planar heater. This heater was fixed to the narrow face of the cell case and heating control was performed to maintain a heater temperature of 600°C.
Judgment of thermal runaway of the target cell conformed to the EVS-GTR. The judgment condition was set as satisfying both a cell voltage of less than 2 V and a temperature rise of more than 1°C/s at the temperature measurement point.

Analysis of Mechanism of Thermal Runaway Occurrence by
Laser Irradiation The mechanism of LIB thermal runaway occurrence by laser irradiation has been reported as originating from separator contraction near the irradiated part resulting in an internal short circuit [8] . In that report, laser irradiation was performed in the direction perpendicular to the electrode stacking direction in a cylindrical battery. By contrast, this study performed laser irradiation in the direction horizontal to the electrode stacking direction in a prismatic battery. As the cell shape and the laser irradiation direction to the electrode differ, this study sought to confirm that thermal runaway occurs by the same mechanism, and examined the process by which thermal runaway occurs with low energy input.
A laser irradiation test using a discharged cell was performed to reproduce the cell internal state when thermal runaway occurs due to laser irradiation. The internal structure of the laser-irradiated cell was analyzed by X-ray computed tomography (CT) and the cell was disassembled to directly observe the electrode and separator.
To extrapolate the process from local heating by laser irradiation to cell thermal runaway, differential scanning calorimetry (DSC) was Measurement was performed at a temperature rise rate of 10°C/s over the temperature range from 0°C to 400°C.

Evaluation of Heat Transfer Amount to an Adjacent Cell Using
an Assembled Battery   a high X-ray absorption rate were present around the hole. This is thought to be due to melting and re-coagulation of the copper current collector foil and other material melted by the laser irradiation. Figure 6 shows a photograph of an electrode extracted from the disassembled discharged cell. The white part is the separator between the cathode and anode, the black part is the anode active material layer, and the copper foil on the bottom is the anode current collector foil.
In addition, the arrow in the figure indicates the laser irradiation direction. Thermal damage to the separator in a concentric pattern from the irradiated part was observed, which confirmed that heat is transferred to the surroundings centering on the irradiated part. This  separator has a multilayer structure consisting of polyethylene (melting point: approximately 130°C [12] ) and polypropylene (melting point: approximately 160°C [12] ) as shown in Table 1  A report by Matthew et al. [13] using an internal short-circuit instigator confirmed that even if a state with a separator partially missing in the cell is reproduced, a sudden voltage drop does not occur as a result of a short circuit between the cathode active material layer and the anode active material layer, and only a gradual voltage drop occurs．This is mainly due to the high electron resistance of the cathode active material layer, and indicates that a hard short circuit does not occur simply due to partial omission of the separator.
In this study as well, a hard short circuit leading to a voltage drop of the entire cell was not observed in the discharged cell with only partial melting of the separator and metal melting around the hole.
From the above, it is thought that joule heat due to a soft short circuit is generated when a laser irradiates the electrode in a charged cell, and that heat generation occurs first on the anode heated by the laser beam near the irradiated part, and then on the cathode.
Furthermore, it is presumed that the heat generated from the electrode around the irradiated part heats the surrounding electrodes, and heat generation propagates inside the cell, leading to thermal runaway of the entire cell.
The laser irradiation method can input heat energy intensively to the irradiated part and induce heat generation from the electrode element locality. This enables inducing of cell thermal runaway with energy input of just 0.40% of the electrical energy of the cell. This method replicates thermal runaway by only the cell's own energy generated by an internal short circuit, so it can be considered rational.
This method is expected to contribute to appropriate battery evaluation without excessive testing demands on products. The red arrow shows the direction of laser beam.

Conclusion
A method that uses laser irradiation to initiate thermal runaway of a traction LIB was compared with the nail penetration and heater heating methods from the viewpoint of evaluating the amount of heat transfer to adjacent cells until the start of thermal runaway of the target cell. In the heater heating method, the temperature of the adjacent cell rose by 67°C from the initial level by the start of thermal runaway of the target cell, and eventually thermal propagation occurred. In the laser irradiation and nail penetration methods, the temperature rise of adjacent cells was less than 1°C at the time of thermal runaway initiation of the target cell, and thermal propagation did not occur.
In addition, a method was demonstrated quantitatively measuring the amount of heat flowing to adjacent cells before thermal runaway of the trigger cell occurs, and the amount of heat transfer to an adjacent cell was quantitatively evaluated for each initiation method.
The results confirmed that laser irradiation enables reduction of the amount of heat transfer to an adjacent cell until thermal runaway of the target cell to 0.025% that of heater heating.
As the laser irradiation method can induce local heat generation with low energy input, it can thus be considered a rational method from the view point of replicating thermal runaway by only the cell's own energy generated by an internal short circuit.
This paper is written based on a proceeding presented at 2021 JSAE Congress (Autumn).