Efficient thermal management of high‐power lithium-ion batteries (LiBs) is critical for ensuring safety, longevity, and performance in electric vehicles (EVs). Battery thermal management systems (BTMS) play a crucial role in regulating temperature, as LiBs are highly sensitive to thermal fluctuations. Excessive heat generation during charging and discharging can degrade battery performance, reduce lifespan, and pose safety risks.
Traditional cooling methods, such as air and liquid cooling, often require additional power and complex components, making them less effective for high-energy-density batteries. As a result, recent advancements focus on immersion, indirect, and hybrid cooling solutions. Among these, phase change material (PCM)‐based BTMS has emerged as a promising passive cooling approach. PCMs efficiently absorb and store heat, maintaining optimal battery temperature without external power. Their thermal performance is further enhanced by integrating expanded graphite (EG) fillers, metal foams, or fins, improving heat dissipation.
This review examines recent progress (2019-2024) in BTMS technologies, with a particular focus on PCM applications in fast-charging conditions. It also discusses BTMS performance under extreme environments, such as high temperatures, sub-zero conditions, and abuse scenarios.
The Importance of Battery Thermal Management
The automotive sector has recently gained prominence as a vital global industry, primarily due to its significant environmental consequences. The sector's significant reliance on fossil fuels results in widespread air pollution and substantially contributes to the emission of GHGs, with CO2 being the most prevalent, underscoring the pressing need to switch to greener and more sustainable options. Among the different energy storage systems, LiBs are the most favorable technology for NEVs, including HEVs and pure BEVs.
LiBs are widely utilized because of their superior energy density and affordability. Regardless of these benefits, the limited driving range of EVs, typically between 200 and 350 km on a full charge, remains a significant drawback. In summary, the energy storage scheme is vital for EVs, requiring durability, affordability, lightweight design, safety, and efficiency.
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Batteries produce substantial heat during fast charging and discharging cycles due to electrochemical reactions and high current flow. Excessive heat can lead to serious risks, including overheating or potential explosions. Additionally, extreme temperatures can negatively impact battery performance, reducing the vehicle's driving range. To mitigate these challenges, a VTM is implemented to enhance safety and efficiency. Multiple factors affect an EV battery pack, including efficiency, with temperature playing a crucial role. A typical lithium‐ion (Li‐ion) EV battery pack functions most efficiently between 15 and 35 °C temperature.
BTMS is critical in ensuring electric vehicles' safe and efficient operation (EVs). The design of BTMS plays a vital role in influencing costs, optimizing heat transfer efficiency, managing energy effectively, preserving battery health, and increasing energy density. This study recommends creating an effective BTMS for installing high‐energy-density LiBs in NEVs. This article also examines the integration of BTMS with VTM systems in greater facets. Last, issues and potential paths for BTMS are discussed, emphasizing avoiding thermal runaway.
Basics of Lithium-Ion Batteries
This section deals with the basics of LiB, starting with the LiB's fundamental and chemical reactions causing heat generation in the LiBs with the consequences of rising temperatures. The side effects of this rising temperature are further discussed. Finally, different problems associated with the rising cell temperature are discussed.
During discharging, the anode releases electrons to the external circuit while the cathode obtains electrons from the circuit. The separator prevents the direct flow of electrons between the electrodes while minimally interfering with other processes. Furthermore, a thin passivation layer called the solid electrolyte interphase (SEI) forms on the carbon anode during the initial charge. Temperature affects several LiB deterioration processes. At higher temperatures, the anode's SEI layer expands rapidly, becoming increasingly porous and unstable.
Conversely, lower temperatures hinder ion diffusion and intercalation, elevating the risk of lithium plating and dendritic lithium growth. On the cathode side, high temperatures during cycling lead to phase transitions, metal dissolution, binder decomposition, and the creation of the cathode electrolyte interphase. The electrolyte, commonly composed of ethylene carbonate (EC) and dimethyl carbonate (DMC) in equal proportions with 1 m LiPF6, is prone to decomposition at high temperatures, producing gaseous byproducts such as CO2.
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In general, elevated temperatures accelerate most degradation processes. While lowering the temperature can mitigate degradation by slowing the diffusion of active species and modifying reaction dynamics, it may also intensify deprivation if metallic lithium accumulates on the anode. The development of the SEI at the interface between the anode and electrolyte is a key factor contributing to lithium‐ion battery degradation across various cycling conditions. This protective layer increases the cell's impedance, leading to a reduction in power output.
The lithium consumed during SEI formation is irreversible, contributing to gradual capacity loss. Initially, the SEI layer serves as a protective barrier, but with repeated cycling, it becomes unstable, thickens, and may obstruct the electrode's pores or even penetrate the separator. This increases impedance due to reduced active surface area and potential safety risks. The thermal breakdown of the carbonate‐based electrolyte yields several byproducts, including CO2, CO, C2H4, hydrogen, alkylfluorides, and H3PO3F, in addition to the SEI that forms on the graphite anode.
For a battery subjected to high temperatures for prolonged periods of time, electrode delamination and particle cracking become major issue when parasitic reaction byproducts build up. Carbon additives in the cathode are electrochemically reactive with PF6‐ions, resulting in structural changes to the cathode. Thermal runaway can happen when the temperature reaches a critical threshold. An uncontrolled positive feedback loop is created when the pace of exothermic reactions and the ensuing temperature increase contribute to thermal runaway.
While most polymer separators dissolve at temperatures above 130 °C, some are specifically engineered to preserve their structural integrity and avert short circuits. For instance, trilayer polypropylene-polyethylene-polypropylene (PP‐PE‐PP) separators can be designed to deactivate once the temperature surpasses a definite threshold. An alternative method for inducing thermal shutdown involves placing an additional material layer amongst the cathode and the current collector. At high temperatures (such as 70 °C in their study, though this can vary depending on the polymer matrix and the nickel‐to‐polymer ratio), the electrical k of the material decreases drastically by seven to eight orders of magnitude within just 1 s, successfully turning off the cell and averting thermal runaway.
If thermal runaway is not clogged before the separator melts, the cell experiences an internal short circuit that damages the cathode. Electrolyte decomposition becomes severe between 250 and 350 °C during thermal runaway, rapidly producing gaseous byproducts.
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Impact of Temperature and Voltage on LiB Performance
The performance of LiBs is heavily influenced by operating temperature and voltage. LiBs operate optimally within a specific temperature range. When the temperature rises, these reactions speed up, leading to greater capacity and power output. Initially, SEI decomposes, followed by reactions between the electrode and electrolyte, which produce combustible gases. At 130 °C, the separator melts, causing a short circuit between the electrodes.
Battery performance declines significantly as the operating temperature drops. Extremely low temperatures can freeze the battery, reducing its capacity, while very high temperatures can harm the active chemical apparatuses of the battery. Furthermore, they emphasize that the need for battery cooling is determined mainly by the heat produced within the cells and packs, especially during high‐current use. Battery's performance can vary significantly with temperature. As observed, the battery's capacity drops sharply as the operating temperature decreases.
Heat Generation in LiBs
The battery's temperature rises significantly as a result of this heat generation. The C‐rate governs the rates at which current is added/removed from the battery throughout these cycles. A higher C‐rate corresponds to a larger current, resulting in a faster heat buildup. Since fast charging involves higher charging currents, it leads to increased heat generation due to the quadratic relationship between the rate of irreversible heat generation and the current.
Their research revealed that total heat generation across all tested cells remained within a similar range. At low C‐rates, reversible heat generation was predominant, whereas, at high C‐rates, irreversible heat became the main contributor. The research demonstrated that lowering the average temperature from 35 to 20 °C during storage or operation can almost double the battery's lifespan.
In some applications, battery systems operate under harsh conditions, such as rapid charging/discharging and extreme temperatures, which can increase the risk of system failure. A BTMS is essential to regulate the battery's temperature in an optimum range. The choice of heat transfer medium plays a vital role in deciding the performance and cost‐efficiency of the BTMS. According to foundational research, multiple cooling methods are available for the battery system.
Prismatic cells seem the best option for cooling automobiles because of their comparatively large surface area, making transferring heat from the cell's interior to the outside easier. Prismatic cells are considered the most suitable choice for cooling vehicles due to their larger surface area, which enhances heat dissipation from the cell's interior to its exterior. Cylindrical cells are commonly used primarily due to their cost-effectiveness, durability, safety, availability, and advanced production techniques. Companies like Tesla and BMW Mini notably rely on these cells. In vehicle applications, cells are organized in various configurations and integrated with safety and control mechanisms to construct a battery module.
Cooling Strategies for Lithium-Ion Batteries
Further, the different cooling strategies of LiB, such as air, water, and hybrid cooling, have been discussed.
Air Cooling
Air‐cooling methods are commonly used in BTMS for NEVs because of their simple structure, lightweight characteristics, and comfort of upkeeping. Due to its enhanced performance in BTMS, forced convection is widely used in the automotive industry. On the other hand, the active configuration utilizes pre‐conditioned air sourced from either a heater or the HVAC system's evaporator. More efficient thermal regulation is provided by this pre‐treated air, particularly in harsh environments like below‐freezing or above‐45 °C temperatures. Moreover, the air duct design plays a vital role in the effectiveness of the BTMS.
Their findings suggest that a BTMS utilizing parallel ventilation achieves lower peak temperatures and a more balanced temperature dispersal than series ventilation systems. The conventional air‐cooling arrangement of BTMs. Currently, advancements in computer numerical simulation technology and experimental testing aid in optimizing air‐cooling BTMS. Several studies have examined the cooling performance of these systems using numerical techniques like CFD simulations.
For BTMS III, the variation in airflow velocity within the cooling channels is most noticeable, leading to the highest temperature variations. Through experimentation and optimization of factors like air intake angle, air exit angle, and battery spacing. They found that increasing the radial gap between cylindrical batteries raised the average temperature but improved temperature uniformity. A higher airflow rate also minimized temperature disparities within the battery pack. Fins are occasionally added to battery surfaces to enhance air‐cooling systems' thermal efficiency. Fins add weight, negating the advantages of classical air‐cooling, which is frequently thought of as the simplest...
Battery Thermal Management System for Electric Vehicles | BTMS
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