**1. Introduction**

Driven by the need to lower the pollution at street level, the interest in electric vehicles is growing increasingly. Lithium-ion cells are the most commonly used batteries in electric vehicles (EVs) due to their high power density (255 Wh/kg for commercial products, and, in the short term, cells with 270 Wh/kg are expected on the market), low self-discharge rate, high recyclability, light weight and compact size, which is very helpful when space is lacking. They also have a longer life cycle with respect to the other rechargeable batteries [1–3]. On the other hand, Li-ion cells show highly non-isotropic thermal properties due to the different layers of the electrodes [4]. In addition, the automotive field necessity of having compact battery packs leads to modules with small space among the cells and between the strings. This yields high temperatures and non-uniform thermal distribution situations, which make the battery efficiency worse and can cause them to degenerate in dangerous and uncontrolled environments. For instance, thermal runaway is caused by successive processes that can lead to fire and combustion [5]. Today, 18,650 Li-ion cells (with diameter 18 mm and length 65 mm) are among the most used batteries in the automotive field because of their small size: as the cells contain a limited amount of energy, if a failure event occurs, the effect is much less than that expected from a larger cell [6]. These cells were introduced by Tesla more than ten years ago on the pioneer Roadster model, and now they

**Citation:** Falcone, M.; Palka Bayard De Volo, E.; Hellany, A.; Rossi, C.; Pulvirenti, B. Lithium-Ion Battery Thermal Management Systems: A Survey and New CFD Results. *Batteries* **2021**, *7*, 86. https://doi.org/ 10.3390/batteries7040086

Academic Editors: Seung-Wan Song, Kai Peter Birke and Duygu Karabelli

Received: 4 August 2021 Accepted: 8 December 2021 Published: 14 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

are massively used in the 21,700 format on Tesla Model X and Model 3. Recent diffusion of cylindrical cells in realizing large packs are mainly driven by the exploitation of simple integration technology, allowing the realization of modules containing between 50 and 100 cells in parallel. Many efforts have been made to increase the battery efficiency. Research in this area started with investigations about why Li-ion batteries should have represented the best choice for EVs [7], then proceeded with a considerable number of simulations (for example, Huang et al. used 3D simulations to study the convection in the spacing between the cells and what their best arrangement is [8]). The final step was the validation of the simulations (for instance, experiments about the effects of different arrangements and shapes of the cells [9–11]). In the context of battery pack cooling management, it is necessary to take into consideration several methods and systems to control the temperature of a battery pack, maintaining it in a precise range. In this work, battery thermal management systems (BTMSs) are considered. Many approaches are involved in various aspects of BTMSs: air cooling, hydrogen cooling, helium cooling, ammonia cooling, liquid cooling, phase change materials [12,13], and hybrid cooling, all used to reduce energy waste and to achieve the battery best performances. Each method has its own pros and cons and is more suitable for a precise application rather than another. The best operating temperature range for Li-ion batteries is between 15 ◦C and 40 ◦C, as suggested by many recent studies [14]. If the temperature rises above 50 ◦C, the charging efficiency and the longevity of batteries decreases [15]. The purpose of a BTMS is to keep the cell temperature within the optimal range while maintaining the temperature's uniformity within the modules [16]; heating is also essential in those rare situations when the battery temperature is lower than the minimum acceptable temperature. At low temperatures, discharging of a charged Li-ion battery is easier than charging of a discharged one [17], and the capacity of Li-ion batteries can decrease up to 95% when the Energy Storage System (ESS) is operating at −10 ◦C [18]. Due to the exothermic nature of the chemical reactions inside the batteries, few studies have been done about heating [19]. However, there are some promising studies about internal self-heating strategies to overcome the poor performances at low temperatures [20] and the reduction of battery life [21]. While studying the heat dissipation performances of a battery pack, two main indexes are usually taken into account: the maximum temperature increase (*Tris*,*max*) and the maximum temperature difference (*Tdi f* ,*max*). The maximum temperature rising is the maximum difference value between the battery pack temperature and the environmental temperature. The maximum temperature difference is the maximum difference value recorded inside the battery pack. If *Tris*,*max* is too high, it means that the environmental temperature is relatively low for the battery pack and that it is not possible to remove the heat generated by the battery pack through the cooling system. If *Tdi f* ,*max* is too high, it means that there is no uniform temperature distribution inside the battery pack. An appropriate cooling system design is necessary to reduce both *Tris*,*max* and *Tdi f* ,*max*. It is desirable to have the maximum temperature increase be less than 10 ◦C and the maximum temperature difference be less than 5 ◦C [22]. Concerning air-cooling methods, ref. [23], Zhou et al. [16] show the advantages of an air distribution pipe thermal manage system, and Xu et al. [22] studied the optimization of the forced air cooling, testing the effects of different airflow duct modes; indeed, Xie et al. [24] do so by modifying factors such as the air inlet-angle and the outlet-angle. There are also studies that propose a multi-parameter control strategy for air-based BTMSs [25] to monitor the state of health (SOH) of the battery and energy consumption in function of the temperature fluctuations and the air mass-flow rates. Many experiments have been carried out on not only to optimize the whole BTMS, but also to analyze the thermal conditions inside the single battery modules improving the temperature uniformity. Several methods have been explored, with analytical and experimental tests, such as the installation of a baffle plate [26], different cell arrangement structures [10], the spacing optimization between the battery cells [27], and the installation of a secondary vent [28] in an air-cooled BTMS. Other examples are the addition of an inlet plenum [29], special axial-flow air-cooling systems [30], nanofluid-based cooling techniques with forced air-flow to remove the heat from the battery arrangement [31]. Among

all the liquid cooling BTMSs, the cooling plates are those on which it has been written less considering to cool the cylindrical 18650 lithium-ion batteries often used in the automotive field. Several studies have been carried on about prismatic cells cooled by cooling plates, about how this system can be designed [32], improved, and optimized [33], focusing on which parameters more influence the optimum working point. Recently, some studies [34] have shown that it makes sense to compare cylindrical cells to much larger prismatic ones. This is first because the governing chemistry reactions inside both kinds of the batteries are the same. Secondly, it has to be assumed that the experiments are performed with strict temperature control, which means considering an extensive thermal mass test system with exceptional temperature control sensors. Cooling plates were shown to be a good solution in particular for those applications where high thermal conductivity and compact design are required. This is why they are a common choice in the automotive field. An advanced liquid cooling system can involve the use of heat pipes [35,36], leading to energy saving in electric vehicles, with better BTMS performances and increasing heat flux loads [37]. In particular, recent studies have explored heat pipes used in innovative systems such as inclined U-shaped flat micro heat pipes [38] and hybrid oscillating heat pipes (OHPs) with an ethanol aqueous solutions of carbon nanotubes as working fluid [39]. Other innovative studies have focused their attention on the thermal performance achievable using a two-phase refrigerant BTMS, comparing it with an ordinary liquid cooled one [40]. The thermal performances of liquid-based BTMSs coupled with heat-pump air-conditioning systems (HPACSs) were investigated by Tang et al. [41] to predict the cooling capability of the liquid system on a basis of a machine learning method. Hydrogen-based cooling systems were realized by Alzareer et al. [42] to maximize the BTMS cooling efficiency in hybrid fuel cell electric vehicles (HEVs) with prismatic battery packs. These systems are also able to increase the driving range of these vehicles in which the hydrogen is used both as coolant in aluminum cold plates and as fuel. Among the HEV BTMSs, Alipour et al. [43] studied the cooling capability of a helium-based cooling system (suitable for EVs as well) on pouch cells, comparing it with an ordinary air-based BTMS and optimizing its efficiency in function of several factors such as the inlet flow rate (the most influential one), the flow direction, and the inlet and outlet diameters. Ammonia is also considered as a refrigerant to cool the future HEV batteries: Al-Zareer et al. [44] create an electrochemical thermal model to study an innovative semi-immersive ammonia-based system for cylindrical cells. The system exploits the ammonia boiling process that involves the parts of the batteries immersed into the coolant, and then the natural convection process when the ammonia vapor cools the part of the cells out from the ammonia pool. Recently, hybrid BTMSs that combine natural air convection with liquid and thermoelectric cooling systems (TEC) have been introduced [45], together with innovative cooling methods such as the adoption of liquid immersive solutions by TESLA: these systems perform well both in hot and in cold environments [46]. Finally, a panoramic view of several emergency strategies needed to prevent the thermal runaway (TR) is given. The inception of TR is generally a consequence of an internal short circuit (ISC) inside a single cell that is caused by either mechanical or electrical abuse. It is important in that case to lower the damages or, in the worst cases, to extinguish fire [47–49]. This paper aims to give an overview of the different approaches for thermal management of lithium battery packs. In this context, a BTMS for cylindrical cells is presented, where the cells are arranged in staggered lines and embedded in a solid structure between wavy channels. A Computational Fluid Dynamics (CFD) approach is presented to simulate the thermal behavior of battery cells and to optimize the distance between the cells and the width of the channels with different materials used for the cells support and with different cooling fluids. The arrangement shown in this work is novel, both for the characteristics of the solid material in which the cells are embedded and for the coupling between thermal conduction and convective cooling methods adopted. Moreover, the CFD-based optimization approach to find the best performances of the BTMS is original and shows interesting results for a cells arrangement typical for automotive and that can be easily generalized. The temperature distribution on the cells is obtained with very high

heat loads. The optimal coolant conditions that give the best thermal performances for the arrangement of the cells are obtained. It is shown that the use of materials with additives to increase the thermal conductivity enhances the heat overall heat transfer for this type of BTMS and gives lower temperatures of the cells. The best performances are obtained by using water instead of air, with small channel widths and with an optimal distance between cells in the direction parallel to the flow.
