2.5.1. Methods to Prevent Thermal Runaway

There are several ways of acting against the thermal runaway and these can be divided into three categories, taking into account the effects on the process. The first category regards preventive measures, such as the addition of flame retardants. The second category involves fail-safe measures that stop or decrease the damage caused by thermal runaway, such as separators or cell venting techniques. The third category concerns actions for extinguishing fires once the thermal runaway has occurred [49]. There is another classification about the way to prevent thermal runaway or reduce its damages. Three main ways of acting corresponding to three levels of protection have been identified: cell-to-cell, module-to-module, and battery pack level.

• Cell-to-cell. This is the highest level of protection using engineered materials between every single cell. Obviously, if we consider space constraints, this represents a challenge but the advantages are relevant: in case of thermal runaway, the material surrounding the cells absorbs heat and minimize the propagation of thermal

effects to the adjacent cells [47]. Thanks to their heat-absorbing nature, phase-change materials (PCMs) are often used in cell-to-cell protection. Moreover, if they change phase from liquid to gas, they also bring with them the cell gases out from the battery modules. The shape of the cells has to be taken into account because it influences the PCM that it is possible to use. For cylindrical cells, the PCM can be solid; it is not the same for pouch batteries that need liquid phase material as they expand and contract continuously.


#### 2.5.2. Emergency Spray Cooling

There are some emergency situations during which rapid battery pack cooling is required. With a common BTMS, it is usually hard to deal with these events with the right timing. An efficient solution for these issues is adding a supplemental refrigerant spray cooling system [48], not only useful to let the temperature drop fast but also to suppress oxygen, one of the possible causes of the exothermic reaction. When the thermal sensors detect an incoming thermal runaway, the refrigerant is sprayed inside the battery box, and it gasifies due to the high temperature strengthening the heat convection. In this way, a rapid temperature decrease occurs and the generated refrigerant gasses push the oxygen out of the box. Vents positioned in the right places can also contribute to the oxygen flow out. Several tests have been carried on about different spray modes: continuous, fixedinterval intermittent, and non-fixed-interval intermittent mode. With the continuous mode, the result is the most efficient cooling method and the maximum value of temperature difference uniformity. On the other hand, oxygen suppression is not better than the one obtained with the intermittent modes. With intermittent modes, the higher is the frequency of the spray, the better is the efficiency of the cooling. Concerning the temperature difference uniformity, the non-fixed-interval intermittent mode is less influenced by the spray frequency than the fixed-interval intermittent one. Another advantage of the intermittent modes is that using them it is easier to maintain the low-oxygen status.

#### **3. CFD Analysis of a Hybrid Solution**

The battery arrangement studied in this section is a hybrid solution with staggered cylindrical cells embedded in solid support and cooled by a fluid that flows in wavy channels. This study is part of a deep investigation of the performances of battery packs conducted by our research group in the automotive field, with dedicated experimental campaigns in which the theoretical results shown in this work are deployed and validated in the laboratory. The novelty of this solution is the strict coupling of solid plastic matrix and channels for liquid cooling [57]. The geometry of a portion of the hybrid solution is shown in Figure 3.

In the figure, the gray part is the solid plastic support with the function to fix the cells, and the blue zones are the wavy channels where the coolant flows. This study aims to optimize the distance *p* between two cells and the width *c* of the channels. Twelve combinations of *p* and *c* have been chosen for the optimization, shown in Table 2.

**Figure 3.** Section of a portion of the battery arrangement (**left**) and a zoomed-in image of the sketch (**right**).


**Table 2.** Parameters chosen for the optimization of the parameters *p* and *c*.

A representative part of the battery pack has been chosen for the simulations, as shown in Figure 4, with two lines of cells cut in their symmetry plane, with the channel between the two lines.

**Figure 4.** Fundamental unit analyzed in the simulations.

The real geometry of the battery is obtained by repeating periodically this part in the direction perpendicular to the flow. The number of cells in the direction parallel to the coolant flow was varied from 8 to 16 to show the linear behavior of the cell temperature as a function of the distance from the coolant inlet. In the present paper, the cells have a format 21,700, but the approach can be easily up-scaled to cells with a different format. The Li-ion cells are considered as made by homogeneous material with density 2680 kg/m3, thermal conductivity 3.4 W/(m K), and heat capacity 1280 J/(kg K). Two materials are compared for the cell support, a common plastic with thermal conductivity *k*<sup>1</sup> = 0.28 W/(m K) and density 1380 kg/m3 (here called "plastic 1"), and a conductive plastic with additives with thermal conductivity *k*<sup>2</sup> = 1 W/(m K) and density 1450 kg/m3 (here called "plastic 2"). The two materials have specific heat 1100 J/(kg K). These values refer to plastic material that are used in our laboratory to realize the support for the cell in the battery pack. The CFD code STAR-CCM+ has been employed to solve the steady-state Reynolds-averaged

Navier–Stokes (RANS) equations with realizable k-*ε* turbulence model. Bidimensional meshes with polygonal elements have been created for each configuration, as shown in Figure 5.

**Figure 5.** Mesh of a portion of the battery arrangement (**top**) and zoom of the mesh refinement and prism layer (**bottom**).

A base size of 0.8 mm and a surface grow rate of 1.3 have been set for each mesh. Once the target surface size is defined, a mesh refinement is necessary to concentrate spacial resolution in the fluid flow zone. The mesh in the fluid zone mesh has a finer resolution with a base size of 0.5 mm in order to get at least 10 elements along the channel height. In addition, a boundary layer has been modeled in the fluid zone with five layers with a total thickness of 33% of the base size, as shown in Figure 5. The final mesh has 14,939 cells. The studied cases are steady-state problems for incompressible flows, with conjugate heat transfer. The equations that describe this problem are the incompressible Navier–Stokes equations, together with the energy conduction for the solid region and convection equations for the fluid region. The continuity equation, together with the momentum balance equation, were solved for the liquid phase:

$$\frac{\partial u\_i}{\partial t} + u\_j \frac{\partial u\_i}{\partial x\_j} = -\frac{1}{\rho} \frac{\partial p}{\partial x\_i} + \nu \left[ \bigtriangledown^2 u\_i + \frac{1}{3} \breve{\bigtriangleleft} (\breve{\bigtriangleleft} \cdot \vec{u}) \right] \tag{1}$$

The energy equation in temperature formulation has been solved for the flow region:

$$\frac{\partial T}{\partial t} + u\_i \frac{\partial T}{\partial x\_j} = \mathfrak{a} \bigtriangledown \mathfrak{T} + \frac{q\_\mathcal{K}}{\rho \mathbf{c}} + \frac{\nu}{\rho \mathbf{c}} \Phi \tag{2}$$

where *α* = *<sup>k</sup> <sup>ρ</sup><sup>c</sup>* is the fluid thermal diffusivity and Ψ = *νφ* is called the *Dissipation Function* and is defined as *νφ* = *Dijτij*. The governing equation for the solid is

$$k\frac{\partial^2 T}{\partial x\_i^2} = 0\tag{3}$$

where *k* is the fluid thermal conductivity and *c* is the specific heat. The temperature at the interface between the solid and liquid region is equal, while the heat flux entering one region at one side of the interface is equal to the flux leaving the other region on the other side of the interface. A heat source is defined at the center of each cell so that the temperature of the system rises according to the governing equations and the materials' properties. Contact interface conditions are applied at interfaces between solid and fluid regions. Additionally, a thermal contact resistance of 0.001921 W/m2K is considered between the plastic support and the cells for mechanical backlash. In all the cases, the inlet fluid has a temperature of 20 ◦C. The cases of air-cooling and water-cooling have been considered in order to evidence the main strengths and challenges related to the two approaches. Different cooling-fluid velocity ranges have been considered in the two cases in order to have similar pressure drop ranges along the channels. A steady thermal power *Q* = 4 W has been assigned to each cell, because these values were measured in our laboratory for some extreme automotive regimes [57].
