Thermal analysis of a 6s4p Lithium-ion battery pack cooled by cold plates based on a multi-domain modeling framework

Highlights

• 3D thermal modeling of 6s4p Li-ion battery pack with 14 cells is performed.

• A multi-channel parallel cold plate is designed for the cooling of the battery pack.

• The cooling performance of the battery pack under different conditions is modeled.

• Thermal responses of Li-ion battery under external shorting are simulated.

• Multi-scale multi-domain modeling is demonstrated for large Li-ion battery pack.

Abstract

Lithium-ion (Li-ion) batteries are the most promising power source for pure electric vehicles (EVs) and hybrid electric vehicles (HEVs) due to the batteries’ high specific energy, low self-discharge rate, low weight, long lifecycle, and no memory effect. The enormous heat generation, however, limits the performance and even causes safety problems. Thermal control of the battery cells remains a challenging issue although much research has been conducted on this topic. In this study, a three-dimensional analysis of Li-ion battery cells and a 6s4p (6 serial and 4 parallel batteries in a stage) battery pack consisting of 24 prismatic batteries was performed using a multi-domain modeling framework. The well-known Newman, Tiedemann, Gu, and Kim (NTGK) model was used for subscale electrochemical modeling and the problem of heat generation due to electrical resistance, electrochemical reactions, and temperature was solved in the cell domain. The temperature evolutions at a high discharge rate and during external shorting were obtained. Strategies for modifying the cooling water states or designing cold plates with special channels to release the generated heat were proposed. It was found that although the temperature of the running battery increased quickly to 80 °C, which could trigger a thermal runaway, the cell temperature and temperature gradients were maintained at a tolerable level at a suitable coolant inlet velocity and temperature, even at a 5C discharge rate and under external shorting conditions. For a large-scale battery pack, the heat generated by the Li-ion cells accumulates inside the module, which poses a high risk of thermal runaway. The cold water flowed into the center of the battery pack through channels and the predicted maximum cell temperature and maximum temperature difference in the pack were maintained below 40 °C and 5 °C respectively at a 5C discharge rate.

Introduction

Due to energy shortages and environmental pollution, the transportation industry faces many challenges [1]. Pure electric vehicles (EVs) and hybrid electric vehicles (HEVs) are more energy-efficient and cleaner than conventional vehicles [2], [3]. EVs have already been widely accepted in the automotive industry because they represent the most promising replacements of conventional vehicles due to lower CO₂ emissions in light of global environmental issues [4]. Among available battery technologies, Lithium-ion (Li-ion) batteries are the core power source of current EVs and HEVs due to their many advantages, such as high specific energy, low self-discharge rate, low weight, long lifecycle, and no memory effect [5], [6]. However, Li-ion batteries generate a lot of heat in the discharge process. If the heat cannot be dissipated in a timely manner, the performance of the battery is greatly affected due to a sharp increase in the temperature. Moreover, localized high temperatures caused by large temperature differences also affect the performance of large-format Li-ion batteries and battery packs. Thus, proper cooling of Li-ion battery packs is needed to prevent high temperatures and temperature non-uniformity in battery packs [7], [8]. The acceptable operating temperature range for Li-ion batteries is −20 to 60 °C [9] and to maintain optimal performance, a narrow temperature range between 15 °C and 35 °C has been recommended by Pesaran et al. [10]. Aside from the concern of deleterious aging mechanisms in the normal discharge process of Li-ion batteries, preventing excessively high temperatures is also important. A thermal runaway can occur in a Li-ion battery cell when the temperature exceeds 80 °C [11]. A series of exothermic reactions occur in sequence and cause a battery fire or even an explosion in Li-ion batteries during a thermal runaway. Thus, the proper design of Li-ion batteries and battery pack cooling is essential for maintaining the Li-ion battery life and battery safety. Different cooling systems have been proposed including the most prevalent air cooling [12], [13], [14], [15], [16], a cold plate with liquid coolant [17], [18], phase change [19], [20], heat pipes [21], [22], and cooling systems combined with the aforementioned methods [23], [24], [25], [26].

Thermal modeling of Li-ion batteries and battery packs is needed for better insights into the thermal behavior. Thermal modeling of Li-ion batteries and battery packs under different EV-related conditions can reduce the number of experiments and prototyping. Numerical simulations improve our understanding of those conditions and provide an efficient and low-cost method, especially for modeling thermal conditions. The difficulty of modeling a Li-ion battery is related to its multi-domain multiphysics nature. Vastly different battery length scales associated with different physical conditions complicate the problem. The goal of thermal analysis is to determine the temperature distribution at the battery length scale. The Li-ion transport occurs in the anode-separator-cathode sandwich layers (at the scale of the electrode pair length). The Li-ion transport in an active material occurs at the atomic scale. The multi-scale multi-domain (MSMD) approach deals with multiphysics in different solution domains [27]. For the MSMD model, a wide range of electrochemical sub-models have been reported in the literature, from simple empirically-based models to fundamental physics-based models. Several main approaches, including the Newman, Tiedemann, Gu, and Kim (NTGK) model, the equivalent circuit model (ECM), and the Newman Pseudo-two-dimensional (P2D) model. These models have been integrated into different numerical modeling platforms for Two-dimensional (2D) or Three-dimensional (3D) Li-ion battery thermal modeling and realistic modeling applications for various Li-ion batteries and battery packs.

The NTGK model is a simple semi-empirical electrochemical model, which is useful for highly efficient modeling of batteries. It was proposed by Kwon et al. [28] and has been used by others [29], [30]. Current-voltage (I-V) curves under different C-rates (C-rate is the measurement of the charge and discharge speed, which equals the current required for the battery to charge or discharge its rated capacity at a specified time) were predicted with the NTGK model and the results matched well with the experimental results. In the model formulation, the volumetric current transfer rate is a function of the potential difference between the positive and negative electrodes. The two parameters are assumed to follow a polynomial form and the discharge depth is a variable in this empirical function. In concrete terms, the polarization curves can be obtained experimentally for a given battery cell and the two parameters can be determined by curve fitting the experimental data. Thus, the NTGK model has been used extensively for 3D thermal modeling of Li-ion batteries, especially for thermal modeling of prismatic Li-ion battery cells [31], [32]. In the ECM, the electric behavior of the battery is simulated by an electrical circuit. Chen and Rincon-Mora [33] established an ECM model capable of predicting the battery runtime and I-V performance; the circuit consisted of three resistors and two capacitors. The corresponding resistances, capacitances, and open-circuit voltage were assumed to be a function of the state of charge (SOC). Newman’s group developed a physics-based model using a porous electrode and concentrated solution theories [34]. The model was able to accurately capture the Li-ion migration in the battery by considering physical processes such as the Li-ion release from solid active particles to the electrolyte, the Li-ion insertion into solid active particles, the electrochemical reaction on the surface of the solid active particles, as well as the Li-ion transport in the electrolyte solution based on the concentrated solution transport theory. Thus, the P2D model has been widely used for many fundamental thermal modeling studies of Li-ion batteries [35], [36], [37]. Recently, these models of Li-ion batteries have been successfully integrated into different modeling platforms, including COMSOL Multiphysics [38], [39], ANSYS Fluent [40], [41], and other numerical platforms [42], [43].

3D thermal modeling has been applied to investigate the Li-ion battery cooling system. Lan et al. [44] developed a novel design for a battery thermal management system (BTMS) based on aluminum mini-channel tubes and applied it to a single prismatic Li-ion cell under different discharge rates. Parametric studies were conducted to investigate the performance of the BTMS using different flow rates and configurations. Panchal et al. [45] investigated the thermal modeling and validation of temperature rise in a prismatic lithium-ion battery with LiFePO₄ (also known as LFP) cathode material. The proposed model is validated with the experimental data collected in terms of temperature and voltage profiles. The surface temperature distributions on the principal surface of the battery are studied under various discharge/charge profiles with varying boundary conditions (BCs) and average surface temperature distributions. To provide significant quantitative data on the thermal behavior of lithium-ion batteries, Pachal et al. [46] designed a battery thermal management system with water cooling. The results indicated that increased discharge rates and decreased operating temperature resulted in increased heat fluxes at the three locations as experimentally measured. Basu et al. [47] established a coupled 3D electrochemical thermal model for a proposed Li-ion battery pack and introduced a novel liquid coolant-based thermal management system for the 18,650 battery pack. This system was designed to be compact and economical without compromising safety. Jilte and Kumar [48] provided insights into the 3D transient thermal response, flow field, and thermal regimes in the battery module. Different air temperature profiles were confirmed in the flow direction and across the width and depth of the battery. Patil et al. [49] investigated the cooling performance characteristics of 20 Ah lithium-ion pouch cell with cold plates along both surfaces by varying the inlet coolant mass flow rates and the inlet coolant temperatures. The study showed that enhanced cooling energy efficiency was accompanied with low inlet coolant temperature, low inlet coolant mass flow rate, and a high number of the cooling channels. Rao et al. [50] designed a novel liquid cooling-based thermal management system for a cylindrical Li-ion battery module with a variable contact surface. The cooling system relies on an aluminum block, which effectively transfers heat from the battery to the cooling water. The results indicated that the system with a variable contact surface significantly improved the temperature uniformity. Lazrak et al. [51] proposed a new solution to integrate and improved the heat transfer of a phase change material (PCM) inside a BTMS and investigated the effect of the PCM melting temperature on the system performance. In general, 3D thermal simulations provide a better understanding of the thermal behavior in Li-ion battery packs. However, this type of modeling study is limited to relatively small-scale Li-ion battery packs.

Although 3D thermal modeling has been applied to model Li-ion batteries and battery packs cooled by a cold plate, several knowledge gaps remain in current research. First, there are relatively few studies that have focused on cold-plate cooling for batteries under fast heat release conditions or abusive thermal conditions. The current study investigates Li-ion battery thermal responses under fast discharge and external shorting conditions. Second, although cold-plate cooling of Li-ion batteries has been widely used in EVs, the temperature distribution of Li-ion batteries resulting from the interaction of fast heat release and cold-plate cooling has not been analyzed in detail. In this study, an analysis is conducted on the temperature distributions and the underlying mechanism is evaluated by predicting temperature contours using 3D thermal modeling. Third, the literature indicates a lack of large-scale numerical simulations for Li-ion battery packs, especially for Li-ion battery packs cooled by a cold plate. In EV and HEVs, the Li-ion battery module usually consists of tens of individual batteries. Thus, in this study, a 21.6 V battery pack consisting of 24Li-ion battery cells and 24 channels is simulated to develop a realistic cooling solution for an automotive Li-ion battery pack. This provides a demonstration of 3D thermal modeling of an automotive battery pack and is applicable to the thermal design of battery packs.