1. Challenges in Managing the Thermal Aspect of Batteries
1.1. Novel Battery Materials for Higher Energy and Power Density Demand
From 2008 up to now,
lithium-ion batteries (LIB
s) technology has been improved by an increase in the volumetric energy density to more than five times, with nearly a 90% cost reduction in the battery pack level. This substantial progress was achieved due to research and development in active materials, electrode processing, and cell manufacturing
[41][1]. Early LIB commercialization was conducted by Sony in 1990 for electronic devices, in which a LiCoO
2 (LCO) cathode and a petroleum coke anode developed by the Goodenough and Asahi Kasei Corporation, respectively, were combined to create a fully rechargeable battery with an energy density of 80 Wh/kg and a volumetric energy density of 200 Wh/L
[42,43][2][3]. The creation of LIBs with a graphite anode, proposed by Sanyo Electric, together with ethylene carbonate (EC) as the co-solvent in the liquid electrolyte based on Dahn’s work, could increase the voltage and volumetric energy to 4.2 V and 400 Wh/L, respectively. For its increased oxidation stability, Guyomard and Tarascon published a novel electrolyte formulation, namely LiPF
6 in EC/
dimethyl carbonate (DMC
), in 1993
[44][4]. This electrolyte is still widely used today and allows LCO-based LIBs to have three times the energy density (250 Wh/kg and 600 Wh/L) of Sony’s first-generation batteries
[44][4]. Such an immense improvement is expected to be progressively performed for considerable opportunities in improving LIB technology
[45][5]. Despite this, at present, the technology still cannot beat internal combustion engine (ICE) vehicles in terms of energy density.
Other cathode materials besides LCO have also been introduced for commercial EVs to ensure robustness and reliability during operation. They are LiFePO
4 (LFP), LiNi
xMn
yCo
1–x–yO
2 (NMC), LiNi
xCo
yAl
1–x–yO
2 (NCA)
[46][6], LiMn
2O
4 (LMO) and LiN
0.5Mn
1.5O
4 (LNMO)
[47][7]. Each cathode has superiorities over others. For example, LFP is considered the safest cathode but has the lowest energy density. LFP is usually used for low-cost EVs, light EVs, and electric motorcycles. LFP does not require a complex cooling system because it might not explode if a thermal event occurs
[48][8]. In contrast, NCA has the highest energy density, but its explosive content requires a special battery packing design. Due to its characteristics, NCA batteries are used for luxurious cars requiring high energy and power densities. The safety issues are solved by implementing a more advanced BTMS, which might be expensive and complicated but reliable in their design. Furthermore, depending on its nickel content, NMC has moderate energy density and safety
[49][9]. NMC-based batteries are suitable for heavy vehicles, such as electric buses and trucks, which require high density and moderate risk due to their size and the related implications when an accident happens. Spinel-type cathode materials, such as LMO and LNMO, were introduced as cobalt-free cathodes to counter the scarcity and toxicity of cobalt
[50,51][10][11]. LMO cathodes are cheap and environmentally friendly but have a low energy density, making them less preferable for practical applications
[52][12]. Nickel-substituted spinel cathodes, LMNO, known for their high voltage stability, have a high power-density and fast Li
+ diffusion
[47,53][7][13]. On the other hand, the impurity of Li
xNi
1−xO in LNMO drops the electrical performance, which makes the material difficult to prepare
[54][14].
Significant work has been conducted by Murashko et al., in which they investigated the heat properties of LIB materials based on LFP, NMC, and NCA cathodes with graphite anodes. The specific heat capacity and thermal conductivity as a function of the SoC of the batteries were found to be different
[55][15]. NMC and LFP have a higher specific heat capacity and a lower thermal conductivity, so they are more challenging to heat up and cool down, but this gives them more thermal stability. NCA, in comparison, has a lower specific heat capacity and a higher thermal conductivity. This means it is less thermally stable and is easy to heat up, but it is also easier to cool down. Such behavior needs to be accommodated by the BTMS to keep the LIBs in the optimal temperature range and with a minimum temperature difference.
In general, the higher energy density of LIBs results in a longer usage time, leading to more heat generation. The cooling load requirements for BTMSs are increasing along with the development of batteries with a high energy density. Different LIB constituent materials result in different heat generation rates and thermal properties. The main reason that NMC and NCA are more active than LFP is the adoption of nickel as the reactive material
[49][9]. It might ensure high energy and power densities, but at the same time, it can easily generate exothermic reactions
[56][16]. Golubkov et al. showed that NCA produced more flammable CO, CO
2, and H
2 gases than LFP during a temperature ramp test, indicating that NCA was more reactive than LFP
[57][17]. The potential for flammable gas production is bigger when the SoC of the battery is full or overcharged.
1.2. Limited Vehicle Compartment Space and Safety
BTMSs of EVs need to have robust cooling and heating performances. This can be realized by increasing the system size to maximize the heat transfer area and the cooling medium. However, when designing the battery packing of EVs,
where
haveit has only a limited space for the vehicles to work with ergonomically; hence, the design needs to be simultaneously as compact as possible and still sufficiently reliable. The limited space in EVs also makes the battery packs susceptible to heat accumulation, especially during fast charging and discharging
[67][18]. Weng et al. showed that in a limited space with a limited cooling medium rate, increasing the heat transfer area does not always give a better cooling efficiency
[68][19]. Indeed, a compact battery pack design is also required to ensure the high energy density of EVs and to compete with other vehicle types, such as ICE and hybrid vehicles.
Three common types of battery cells are available in the market that can accommodate the limited space for battery packing, namely cylindrical cells, prismatic cells, and pouch cells
[71][20]. A cylindrical cell uses steel to wrap the battery with various diameters and lengths, which ensures full protection from mechanical loading
[72][21]. The size is considerably small to ensure the battery cell has good thermal stability. The small size also helps the cell to be arranged within the available space. Furthermore, the small size also makes the cell adaptive to various battery modules or pack designs. However, these advantages must be paid with their low effective volumetric and gravimetric energy densities. In contrast, pouch batteries use aluminum polymer foils to wrap the cell
[73][22]. This makes the cell have an effective high gravimetric energy density, but at the same time the strength and rigidity of the battery become questionable
[74][23]. Such battery cells also require additional packs when they are used for EVs, reducing their densities. To improve the density, a prismatic cell is introduced with a relatively bigger size than other cells. This might be promising for EVs requiring a high energy capacity. However, their bigger size causes the cell to be not favorable for thermal management. The quality of each cell might also be different since it is more difficult to control the quality during the manufacturing process of these battery cells.
Besides the battery cell design, battery modules and packing are optimally designed to meet the available space and to provide good thermal management. The battery cells must unavoidably be arranged in series and parallel connections. A series connection is created to fulfill the required voltage, whereas a parallel connection is created to fulfill the required energy capacity
[75][24]. In a series connection, the nonuniform charging/discharging process of each cell is unavoidable. Consequently, each cell has a different SoC, which implies different temperatures and different levels of Joule heating
[76][25]. This phenomenon makes designing BTMS more complicated, which must be handled by optimized design strategies. It is worth noting that in designing battery packing, thermal management is only one of the main issues that must be handled. Other issues, such as vibration and crash safety, must also be considered during the design process
[77][26].
1.3. Climate and Seasonal/Environmental Temperature
For obtaining the best performance and lifetime, LIBs should generally be operated at a temperature between 25 and 40 °C with a maximum difference between the batteries of 5 °C
[1,2][27][28]. The optimum operating temperature might depend on the battery’s active materials
[78][29]. The battery’s cooling and heating requirements should be considered depending on environmental conditions such as the climate, season, and driving circumstances. To improve the performance and maintain the battery’s ideal state of health, the temperature across the battery pack should be kept inside the optimal range and as uniform as possible. As a result, a sophisticated BTMS with accurate temperature management capabilities in locations with varying temperatures throughout the year is greatly desired for EVs
[79][30]. Since the heat transfer rate and specific heat of air significantly decrease during a hot summer, an air-cooled BTMS might not be suitable for year-round usage. An air-cooling system cannot provide the necessary cooling load to the battery pack
[80][31]. A hot environmental temperature can trigger the battery to accelerate the redox chemical reactions, which directly causes an abundance of heat generation (thermal event). In the long run, the phenomenon can cause thermal runaway in the battery.
For subtropical climates, EVs require a heating system to ensure the battery does not freeze. A low temperature makes the battery lose its performance due to the increased liquid electrolyte viscosity, causing high internal resistance
[81,82][32][33]. This makes the chemical reactions in the battery slow. As a result, the current output can be low, causing the electric motor of the EV drive train to not have the power to rotate. At an environmental temperature of 0 °C, the discharging capacity can drop by more than 20%, and the dropped capacity proportionally increases as the temperature decreases
[83][34]. Even though a low temperature is usually less hazardous for a battery than a high temperature, the malfunctioning of the battery makes the EV unable to operate normally, which is less favorable than ICE vehicles.
2. Recent Advancements in Research and Development of LIBs
As a summary of the discussion in the previous section, three fundamental problems must be solved in designing battery packing with superior thermal management in EVs. These problems have become design constraints that might be difficult to solve simultaneously. For example, considering the current technology of the constituent material of batteries, high-energy-capacity cathodes (NMC or NCA) release a lot of heat during the charging and discharging processes, increasing the thermal runaway risk
[86,87][35][36]. Extreme climate or season conditions also require robust and complicated battery packing designs that might increase the vehicle weight and consume a lot of energy
[79,88][30][37]. Optimizing a battery design that accommodates all constraints is usually suggested to solve such problems. In this section,
we explain the significant findings
will be explained from the recent decade that can solve these problems. Some ideas are still under ongoing research, which might become promising game changers in battery packing design and technology, even though they still need to be further proven before their implementation.
The thermal problems of LIBs, which are high heat generation and varying environmental temperatures, are caused by the narrow working temperature of LIBs. Recent advancements in battery technology give potential solutions to these problems, for instance by implementing a functional co-solvent for the electrolytes—or utilizing solid-state batteries. Functional co-solvents and electrolyte additives have been vastly studied and developed, with significant progress being achieved in recent decades such as in obtaining a longer cycle life, allowing a fast charging process, hindering electrolyte decomposition at the electrodes, enhancing the energy density, enabling a wider temperature range of operation, or producing lower-cost LIBs
[89,90,91,92,93,94,95][38][39][40][41][42][43][44]. Lu et al.
[96][45] proposed 2,2,2-trifluoroethyl N-caproate (TFENH) as a co-solvent for LIB electrolytes. The TFENH co-solvent improved the low- and high-temperature performance of a LiCoO
2/graphite battery by keeping the volume ratio of TFENH within 17 to 25 vol%. An X-ray photoelectron spectroscopy (XPS) test showed that a film of CH
3(CH
2)
4COOLi was formed, which decomposed from the TFENH. The film reduced the other reduction reaction products from the carbonate solvent and lithium salt, which in turn improved the low-temperature performance and cycling stability of the LIB. Another co-solvent application was proposed by Ouyang et al.
[97][46], which used a combination of fluoroethylene carbonate (FEC) and dimethyl carbonate (DMC) at a ratio of 3:7, named FD37. The co-solvent was used for an NMC LIB and showed a significant improvement in cycling features due to the formation of a cathode–electrode interface. The interface inhibited the electrolyte decomposition and further improved the stability of the electrode/electrolyte interface. The co-solvent also improved the thermal stability of the LIB, which was shown in high-temperature cycling and storage tests. The LIBs with an FD37 co-solvent showed a better capacity retention and Coulombic efficiency than the traditional LIB. However, employing this strategy in the LIBs with liquid-based electrolytes does not mean fully omitting the natural characteristics of liquid electrolytes that tend to have a consequence in easier fire ignition when it reacts with particular chemicals, even if they are non-flammable
[98][47]. For small-scale applications, such as power sources for portable devices, attempting this strategy could be favorable and promising, while the drawback case might be conveniently handled or prevented. On the contrary, aside from the enhancement of the electrochemical performances required for large-scale demands, especially in EV applications, this technology needs more concern regarding safety, owing to the severe impacts, such as catastrophic fires or explosions, which will result in the case where the battery gets into trouble.
Therefore, in recent LIB development, it is believed that solid-state battery (SSB) technology could replace the LIB technology based on liquid electrolytes currently commercialized in the market, considering the exponential progress of SSB research pursued in the last decades, which is much improved compared to the initially revealed idea many years past
[99,100][48][49]. Furthermore, a functional additive that previously accounted for significant advantages in LIBs could also be employed in solid electrolytes, allowing SSBs to become more promising candidates for next-generation power sources, especially for EV applications
[101][50]. The use of materials such as oxide inorganic solid-electrolyte NASICON-type LATP (Li
1+xAl
xTi
2–x(PO
4)3), NASICON-type LGPS (Li
10GeP
2S
12), NASICON-type LAGP (Li
1+xAl
xGe
2–x(PO
4)3) or garnet-type LLZO (La
3Li
7O
12Zr
2) instead of liquid electrolytes with a separator enables the significant enhancement of a battery’s capacity by up to four times due to generally of having a wide electrochemical stability window
[102][51]. The main capacity of LIBs is in the range of 150–200 mAh
[103][52]. Liu et al. recently constructed a solid-state Li–air battery with lithium foil anode, LAGP and a solid electrolyte, and an LAGP–nanoparticle composite/single-walled carbon nanotubes (SWCNTs) as the air electrode
[104][53]. A remarkably high capacity of 2800 mAh/g was shown in the first cycle despite the good cycling performances being limited to 1000 mAh/g, and the electrochemical process may vastly differ between an air and a pure oxygen atmosphere. Tao et al. reported that the use of LLZO-nanoparticle-filled poly(ethylene oxide) electrolytes could generate favorable capacities of >900, 1210, and 1556 mAh/g at successive temperatures of 37, 50, and 70 °C, respectively, even though at the first charge/discharge cycle they ran into an irreversible electrochemical reaction, which lead to capacity decay
[105][54]. Accordingly, the use of solid electrolytes also allows the battery to have a wider operational temperature owing to the high thermal resistance, and this substantially increases the battery safety significantly, even in harsh environments
[105,106,107,108,109][54][55][56][57][58]. The current method proposed to extend the temperature range is thus available by coupling the types of solid electrolyte material and implementing such filler/substitute material. These superiority aspects lead SSBs to be the primary candidates for solving the low-density and safety concerns that mostly arise in conventional batteries. Furthermore, the manufacturing process of SSBs can be considerably easier when producing a solid electrolyte using additive manufacturing, spark plasma sintering, or conventional sintering
[106][55]. The installation of solid electrolyte in SSBs could also prevent leakage that commonly arises from the liquid electrolyte, which can cause a short circuit and thermal runaway
[107][56].
The promising nature of SSBs does not mean they can be implemented immediately. In fact, one remaining fundamental problem must be solved, i.e., mechanical damages that always appear after several charging–discharging cycles
[108][57]. These damages occur due to the solid electrolytes that always bear the pressure loading from the expansion–shrinkage of electrodes, which is different to liquid electrolytes that can redistribute the pressure around the cell pack. The most common solutions to avoid these damages involve using zero-strain cathodes such as LTO or limiting the SoC in the battery application to ensure the solid electrolyte is not subjected to overpressure. The high internal resistance of SSBs also occurs due to the low quality of the manufacturing process. Toyota has spent research funding to solve the SSB problem so that they can be implemented in their cars
[110][59]. Bolloré Group launched the BlueIndy electric-car-sharing program to demonstrate an EV prototype installed SSB of 30 kWh
[111][60]. Other established companies such as Volkswagen, BMW, Daimler, and Hyundai also keep paying attention to the research and development of SSB technology
[112][61]. The immediate solution to overcome the high heat generation of high-energy-density batteries and their environmental problems is through advanced BTMSs, which have better thermal management performances. The advancement of BTMS research has resulted in hybrid BTMSs, which combine one type of BTMS with another to utilize the advantages of both systems while overcoming the weaknesses of each system. Yang et al.
[30][62] combined mini-channel liquid and air cooling to improve the BTMS cooling performance for cylindrical batteries. The maximum temperature and temperature distribution were reduced to 2.22 and 2.04 K, respectively. Another hybrid BTMS combining liquid cooling with a heat pipe was proposed by Jang et al. for a prismatic battery
[31][63]. The liquid cooling utilized a mini-channel heat sink placed on top of the battery, while a heat pipe was placed on the front side of each battery. The heat pipe transferred the heat generated by the batteries up to the heat sink area to be further transferred by the liquid coolant out of the battery pack. The proposed BTMS successfully reduced the maximum temperature of the battery up to 9.4 °C. A BTMS with a combination of a liquid PCM and a heat pipe for a pouch battery was proposed by Zhou et al.
[32][64]. The PCM took the heat from the battery through convection, and then the heat pipe transferred the heat out of the battery pack by air. The system lowered the temperature difference between the batteries by up to 67% compared to a forced air-cooled BTMS.
The mentioned research
[30,31,32][62][63][64] proved that hybrid BTMSs could improve the cooling performance of ordinary BTMSs. However, the drawback of hybrid BTMSs is that they require a higher energy consumption and a bigger compartment space. Therefore, due to their compactness (handling the limited compartment space) and minimum power consumption, without sacrificing safety, direct-cooling BTMSs have been introduced in the last decades
[80][31]. Their cooling design is superior to air- and liquid-cooling designs. In addition, they can be integrated with the air conditioning system of an EV to achieve a compact design without sacrificing the cooling performance
[113,118,119][65][66][67]. The basic idea of direct cooling BTMSs is that the battery requires an operational temperature similar to the comfort temperature of the passenger. Thus, the cooling system of the battery and cabin can be integrated. A direct cooling system solves the basic problem of air-cooling system, which is usually unreliable without implementing a complicated structure, such as liquid cooling. Together with an advanced control system, direct cooling BTMSs could be promising for solving the thermal management of EV batteries. At this time, the Nissan Leaf and Toyota Prius use a direct cooling system and show favorable performance.
A battery preheating mechanism was a method introduced and proposed by researchers for EV applications in cold environmental temperatures, which can be further categorized into external and internal preheating. External heating uses a heat source outside the battery to heat the battery by transferring the heat through a medium, such as air, liquid, or PCM. Air preheating uses power from the battery to heat a resistance heater and then transfers the heat to the battery via air convection produced by a fan
[33][68]. In contrast, Wang et al.
[34][69] proposed an immersive preheating system by immersing prismatic batteries in a flowing heat transfer liquid. The system used an external heat exchanger to heat the liquid for heating the battery. Ruan et al.
[36][70] proposed an internal preheating method by direct current discharge due to its simplicity and high heat generation. The research successfully optimized the heating time to 103 s and reduced the capacity degradation to 1.4%. Zhang et al.
[37][71] proposed a preheating mechanism using a sinusoidal alternating current, causing the battery to be only heated by the irreversible heat of Joule heating, since the reversible heat of the electrochemical reaction was canceled out after one period. The study found that the heating process can take less than 15 min with no capacity loss after repeated preheating, which was slower than DC preheating but had less capacity degradation. Moreover, a pulse internal preheating system heats the battery by pulse excitation, which Wu et al.
[38][72] successfully optimized by varying the amplitude and frequency. The results showed that the heating time was 308 s with a capacity degradation of 0.035% after 30 cycles.
Advanced control systems always become the main solution to control complicated BTMSs, considering that heat generation in the battery caused by internal resistance (Joule heating) and redox chemical reactions cannot be avoided. The same thing also goes for the non-uniform temperature of each cell, which is also unable to be averted since the battery must be arranged in both series and parallel connections. Hence, a control system was implemented to ensure a constant and uniform battery temperature, which can be approached by two methods. The first approach is to control the SoC and voltage level of each cell using a battery management system (BMS) so that the battery can generate heat uniformly.
In addition, a BMS can also restrict the excessive current flow during the charging–discharging process, which commonly raises the heat generation drastically. The second approach is installing a temperature sensor, heat flow sensor, fuse, and gas sensor for identifying a heating spike. The data obtained from the sensor can then be analyzed to determine the necessary action of a cooling system set. The development of machine learning in this situation can plentifully help researchers to find robust control systems that are urgently needed, especially for extreme climates and weather.