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Abstract

Thermal management is a crucial design aspect for the safety and performance of the Lithium-ion battery pack used in electric vehicles. Phase change material (PCM) based battery thermal management is one of the appropriate passive or hybrid thermal management. The present paper highlights the selection of PCM depends on various factors related to application operating conditions like ambient temperature, the heat released by the battery, and the type of battery pack design. In selecting PCM, a thorough understanding of various PCMs requires an overall review of multiple types and properties of organic and inorganic PCM, using phase change equilibrium diagrams to comprehend phase change transformations. Further, the present work summarizes the various studies on PCM durability, different thermal performance enhancement techniques, and methodology for selecting PCM; as a result, assisting the researchers and engineers in choosing the appropriate PCM for the various EV battery thermal management applications in terms of chemical, thermal, and economic aspects

Graphical Abstract

Keywords

Phase change material battery thermal management composite phase change material lithium-ion battery electric vehicle

Highlights

• Detailed review on physical, chemical and thermal properties of various PCM materials

• The aspects of PCM material durability and compatibility are elaborated

• To characterize the PCM materials, Phase change equilibrium diagram is a very useful tool

• A Novel PCM selection methodology proposed with respect to Electric vehicle application

Introduction

In recent years, the world has been taking measures to reduce pollution by controlling emissions of Greenhouse gases (GHG) by depending mainly on renewable energy sources. To mandate the former initiative, all countries signed a Paris climate agreement that aims to decrease global warming by maintaining temperature rise below 2°C. One of the initiatives countries take to reduce pollution is to deploy electric vehicles (EV) through subsidy schemes. EVs and renewable energy will pave the way for a strong foothold in decreasing pollution.

At present, Lithium-ion batteries (LIB) are found to be a suitable energy source for electric vehicles due to their potential of having a high energy density (Wh/kg) and its extended life span, and also energy on the vehicle accounts for 30%–40% of overall vehicle cost. Therefore, Maximizing the life of the battery and maintaining safety of the battery are the critical issues to be addressed while building an electric vehicle to meet performance. The safety of the battery is ensured by the proper electrical design, structural design, and thermal design around the cells in a pack and mainly it can be achieved by suitable thermal management systems. Since the battery is sensitive to temperature and demands a need of strict thermal control to be in a temperature range between 15°C to 35°C¹ approximately. Operating outside the mentioned range will lead to the following adverse effects. (Figure 1)

Figure 1.

Temperature effect on lithium-ion battery.²

Most safety-related consequences can be controlled by using the proper battery thermal management systems (BTMS). Various thermal management technologies are used globally, as seen in Figure 2. Primarily, classification is based on external power input, namely active BTMS and passive BTMS. In active BTMS, with the help of external power, the cooling operation will be carried out. Whereas, in passive BTMS, without external power, cooling will be done by phase change and heat pipe equipment.⁵ PCM-based battery thermal management is passive thermal management systems that are economical and environment-friendly BTMS. Phase change materials (PCM) that absorb or release heat during the phase transition processes is a promising alternative to the active cooling system for battery thermal management. The goals of proper BTMS are like temperature uniformity from cell to cell can be achieved by the PCM/enhanced PCM with Graphite and shows greater efficiency than air cooling, and helps to maintain the optimum temperature range for an individual cell to achieve more significant life. PCM offers the dual quality of buffering and conducting with the simplest design. Thus, in the advancements of Li-ion BTMS, the PCM-based BTMS became a crucial component. Even in environments with high ambient temperatures of 45°C to 50°C, PCM-based thermal management keeps the battery pack's internal temperature well below the prescribed limit. PCM-based BTMS protects the system with high latent heat and thermal conductivity in stressed conditions. For example, at the ambient temperature of 45°C and discharge current rate of 6.67C, the maximum temperature difference between the cells is not increased beyond 0.5°C. In casual operating conditions, the maximum temperature differences are negligible.⁶

Figure 2.

Thermal management cooling and heating classification.^3,4

A PCM based passive thermal management system makes it possible to achieve uniform temperatures under abnormal/stressed and normal conditions. Passive thermal management also provides better safety than the complexity involved in active cooling systems. The enhanced PCM, like the PCM-Graphite matrix can control the thermal runaway by conducting and absorbing the heat within the cells. In Plugin hybrid electric vehicle's PCM based technology for cooling has become an excellent alternative to forced air cooling with compact packs.⁷

The PCM shows excellent efficiency even in cold conditions and space applications. The PCM will absorb the excess heat, When the battery module temperature is more than the melting point temperature of the PCM and release it to the atmosphere when the ambient temperature is lesser than the PCM melting point temperature. The structure of the PCM for these applications is very simple, space-saving, and lighter than the active BTMS systems. The PCM-based BTMS are available at cheaper cost and hence they become more attractive in the recent years.

The various researchers working on the PCM-based BTMS with different phase change materials are discussed in Table 1.

Table 1.

BTMS related review articles.

S.No.	Reference	Highlights
1	Jaguemont et al.⁸ (2018)	• Detailed review on the PCM application in automotive field
		• Elaborated advance modeling of the battery with combined PCM
		• Various cooling techniques discussed thoroughly
2	Jiang et al.⁹ (2020)	• Reviewed on liquid-solid phase transition based BTMS
		• Reviewed on liquid-gas phase transition based BTMS
		• Reviewed on enhanced organics and its application on BTMS
3	Liu et al.¹⁰ (2020)	• Reviewed about all properties of PCM
		• Reviewed about passive BTMS with enhancing techniques
		• Detailed explanation about the importance of PCM in BTMS
4	Zu-Guo et al.¹¹ (2021)	• Reviewed about various BTMS and stated various merits and demerits
4	Zu-Guo et al.¹¹ (2021)	• Elaborated about the performance enhancement of PCM based BTMS.
5	Wu et al.¹² (2022)	• Experimentation about the flexible composite phase change material application in the BTMS.
		• Variety of batteries with different voltages has been considered were observed with different flexible composite temperature range
		• Tested experimentally with different type of cell (prismatic, pouch and cylindrical)

A few other notable experimental works in the application of PCM thermal management, in particular to the PCM stability are Xiao et al.¹³ synthesized a crosslinking polymeric structure having a crosslinking skeleton gives well-defined properties like high latent heat of 98.8 j/g in the absence of leakage and volume expansion. The skeleton structure showed high thermal stability under extremely high-temperature conditions. This polymetric structure will be helpful in the actual application without any comprise on lower thermal conductivity, Li et al.¹⁴ designed a methodology of solvent-evaporation to obtain a new kind of flexible CPCM under room temperature by considering ethylene-vinyl acetate (EVA), cyclohexane as the supporting material and solvent useful for BTMS. This new methodology simplifies the preparation even under harsh environmental conditions. Flexible CPCM-EVA allows a more compact installation, decreasing the contact resistance between the cell surface. CPCM and EVA-PCM show excellent properties at room temperature, like anti-leakage properties and flexibility. In consequence, compared with CPCM, great improvement can be seen in temperature-control performance. Ye et al.¹⁵ proposed a novel CPCM with dual phase change temperature regions. Dual temperature regions will be helpful to provide a suitable optimum working temperature range for the battery, indirectly enhancing the performance by controlling the maximum temperature under harsh working environments. As well as improving the stability of the material. At the same time, Lv et al¹⁶ have made serpentine PCM layers which will leave the provision for secondary cooling by a fan, thus increasing the overall energy density of the module from 107.8 to 121.6 Wh/kg, Lv et al.¹⁷ also proven that with PCM, the life of the module increased by 65.3%. The PCM application to the battery can extend to other energy storage like supercapacitors. Hasan et al.¹⁸ experimentally designed the electrode materials by doping. The co-doped polyaniline chains and nitrogen-doped reduced graphene oxides improve the capacitive performance of the supercapacitors by having unique surface properties, high processability, cycle rate capability, and life cycle. After going through all the works of literature about various properties, PCM materials, and respective advantages and disadvantages, a novel methodology proposed on how to select the suitable PCM material for a given thermal application, which will be helpful for the engineers and researchers in the experimentation field.

Based on different literature reviews, it is clear that all have considered multiple ways of conveying PCM usage in BTMS and defined extensively various properties of PCM, explaining the PCM using phase change equilibrium diagrams, thermal performance enhancement techniques and whereas, in this paper, the methodology of selecting the PCM is defined as a step-by-step approach via flow chart and contribution of this paper is given in Table 2 by comparing with recent reviews about PCM based works for EV applications. Tables 3–8, shows consolidated properties of PCM materials taken from various literature.

Table 2.

Contribution of present article in comparison to the recent reviews on PCM for EV applications.

S.No	Reference	Properties of PCM	Phase change equilibrium diagrams	Durability of PCM	Thermal performance enhancement techniques	Methodology to select PCM	Discussion on the BTMS application
1	Jaguemont et al⁸	YES	NO	NO	NO	NO	YES
2	Jiang et al⁹	NO	YES	NO	YES	NO	YES
3	Liu et al¹⁰	YES	NO	NO	YES	NO	YES
4	Zu-Guo et al¹¹	NO	NO	NO	YES	NO	YES
5	AdilWazeer et al¹⁹	YES	NO	NO	YES	NO	YES
6	Khan Z et al²⁰	YES	NO	YES	YES	NO	NO
7	Zhou et al²¹	YES	NO	NO	YES	NO	NO
8	Liu et al²²	NO	NO	NO	YES	NO	YES
9	Zalba B et al²³	YES	NO	YES	YES	NO	NO
10	Shamseldin et al²⁴	YES	YES	YES	YES	NO	NO
11	Okogeri et al²⁵	YES	NO	NO	NO	NO	NO
12	Present article	YES	YES	YES	YES	YES	YES

Table 3.

Properties, limitations and enhancing techniques of different PCMs.^26–35

PCM		PROPERTIES	LIMITATIONS	ENHANCEMENT TECHNIQUES
ORGANICS	PARAFFINS	1. Structure stability after 1500 cycles	1. Flammability, slow to oxidization	1. Adaptation of extended surfaces or fins
		2. Less volumetric expansion	2. Leakproof enclosure needed	2. Multiple PCMs
		3. No super cooling (surely solidifies below freezing point)	3. Seepage of solid-liquid PCM	3. Encapsulation of PCM.
		4. Good compatibility with shell polymers	4. Low thermal conductivity	4. Addition of carbon additives
		5. Non-toxic in nature	5. Self-nucleating	5. Shape stabilization
		6. Non-corrosive in nature
		7. Non-explosive compounds
	NON PARAFFINS (FATTY ACIDS)	1.Higher heat capacity	1. Individual melting temperatures are higher	1. Nano encapsulation
		2. Proper melting temperature range	2. Low thermal conductivity	2. Esterification of fatty acids
		3. Low volumetric change	3. Undesirable odor after specific melt/freeze cycles
		4. No super cooling	4. High sublimation rate
		5. Non-toxic, low-cost
		5. Non-flammability
		6. Non-corrosive to metal containers
		7. Good thermal and chemical stability. Thermally stable up to 3000 cycles
INORGANICS	SALT HYDRATES	1. High thermal conductivity	1. Melt incongruently	1. Adding excess water
		2. High volumetric storage density	2. Under go sub-cooling	2. Mechanical stirring of TES
		3. Non-explosive	3. Aggressive corrosion with the container	3. Mixing with nucleating agents
		4. Non-flammable		4. Adding thickening additives
				5. Microencapsulation, adding porous material
EUTECTICS	MIXTURES	1. Narrow melting point temperature	1. Thermo-physical properties not well defined	1. Modified into nanostructures
		2. High density	2. Low total latent heat capacity	2. Addition of gelling agents
		3. No phase segregation	3. Super-cooling effect	3. Addition of nano-grapheme platelets
		4. Congruent phase change	4. Strong odor
			5. High cost

Table 4.

Commonly used PCM properties range.

S.No	Material	Melting point (°C)	Specific heat capacity (Kj/Kg.k)	Latent heat of fusion (kj/Kg)	Density (kg/m³)	Thermal conductivity (W/mK)	Application	Reference
ORGANICS
1	Paraffin waxes	60	1.85(solid)	214	856(solid)	0.4	Energy storage	Reddy et al³⁷
			2.384(liquid)		775(liquid)
2	Polyglycol E600	22		127.2	1126 (liquid, 25°C)	0.1897(liquid,38.6°C)	Wide range	Cabeja et al³⁸
INORGANICS
1	Salts:Glaubers salts+ additives	32	1760(solid)3320(liquid)	254	1485	0.6	Energy storage in buildings	Wasmi et al³⁹
EUTECTICS
1	Stearic acid	54–56	2.83(solid)	186.50	1.08(solid)	0.18	Energy storage	Sari et al⁴⁰
			2.38(liquid)		1.15(liquid)
2	Eutectics:MgCl₂.6H₂O/Mg(NO₃)₂.6H₂O	59	NA	132.2	1550 (liquid, 50°C)	0.510 (liquid, 65.0°C)	Wide range	Zalba et al²³
					1630 (solid, 24°C)	0.678 (solid, 38.0°C)
HYDRATED SALTSClick or tap here to enter text
1	MgCl₂.6H₂O	117	NA	167	1442(liquid,78°C)	0.598(liquid,140°C) 0.704 (solid 110°C)	Energy storage	Cabeja et al³⁸
					1569 (solid,20°C)
2	Mg(NO₃)₂.6H₂O	89	1.84(solid)	162.8	1550 (liquid, 94°C) 1636 (solid, 25°C)	0.490 (liquid, 95°C)	Energy storage	Cabeja et al³⁸
			2.51(liquid)³⁴			0.611 (solid, 37°C)
3	Na₂SO₄·10H₂O	32.4	1.76(solid)	287	1460(solid)	0.7(solid)	Latent heat storage	Qu et al⁴¹
			3.3(liquid)		1330(liquid)	0.544(liquid)

Table 5.

Detail description of the various works on PCMs as storage material.

Type of PCM	Sub-category	Material	Melting point (°C)	Specific heat capacity (kj/kg k)	Density (kg/m³)	Thermal Conductivity (w/m k)	Latent heat Capacity (kj/kg)	Refe--ence	Type of work	Inference
Organics	Paraffin	n-C₁₉H₄₀ (nona decane)	32	2.27_liquid	785	0.213–0.152	222	Agrawal et al⁴²	.Exp	1. Leakage is mitigated by epoxy resin
										2. Carbon nano fibers, carbon nanotubes, boron nitride, microparticles, or boron nitride nanotubes enhance thermal conductivity
										3. With a 10% weight boron nitride composition it gives good thermal stability
	Fatty acid	n-C₁₀H₂₀O₂ (Deconic acid)	32.0 ±1.5	2.83	850	0.15	145 ±15	Noel et al⁴³	Exp	1. PCM selection tools
										2. Compared with ester isomer
										3. Increasing molar mass $\propto$ raise in melting temperatures $\propto$ high enthalpy change
										4. Long alkyl chains $\to$ entropy change $\to$ enthalpies of fusion
	Polymer	Poly ethylene glycol l(PEG)-6000	62.1 ±0.6	NA	NA	0.187 ± 0.002	180.1 ± 4.5	Paberit et al⁴⁴	Exp	1. Examined PEGS molecules with different molecular weights (400–6000g/mol)
										2. The evolution of phase transition thermal behavior under various circumstances
										3. Through DSC life cycles and prior properties were drawn
Inorganics	Salt hydrate	CaCl₂6H₂O	24	1.4_solid	1470	1.09_solid	140	Tyagi et al⁴⁵	Exp	The latent heat of fusion and melting temperatures were calibrated for 1000 stable cycles of CaCl₂6H₂O
	Molten salt	NaNO₃	309.3	1.798–1.812	NA	0.514	174.13	Zhao et al⁴⁶	Exp	The binary molten salt of the mixture NaNO₃-KNO₃ thermal conductivity is calibrated
	Eutectic salt	53% NaNO₃, 40% NaNO₂, 7% NaNO₃ (wt%) =Hitech salt	142	1.40_solid	1790	0.7	65–67	Xiao et al⁴⁷	Exp	1. Nanoparticles and copper foam were applied to Hitech salt Al₂O₃ to retrieve the limitation (eutectic mixtures with single enhancement properties)
				1.56_liquid						2. Effective thermal conductivity increased by 10 times
										3. Melting and freezing points differed from the pure salt
	Metal	Galium	186	4.389	515	41.3	433.78	Ge at al⁴⁸	Theory	1. Low melting point liquid metal-based PCM explained
										2. Summarize the basic properties of conventional PCMs and the recent advancements of metal PCMs explained
	Alloy	Bi₅₈Sn₄₂	138	0.201	8560	19_liquid	44.8	Wang et al⁴⁹	Exp	1. Alloy Sn–Bi–Zn–Ga was examined
										2. Alloy with different compositions by the addition of Galium was observed
										3. Galium reduces the essential peak temperature and raises the alloys' heat capacity
										4. When the temperatures increase, Galium increases the thermal conductivity of alloys and decreases the density

(Exp- Experimental).

Table 6.

Commonly used organics thermal stability and properties.

S.NO	Type of PCM	Melting point temperature (°C)	Latent heat of fusion (kJ/kg)	Experimentation	Thermal cycles	Reference
1	Paraffin low grade	53	146	DSC	300	Sharma et al⁶¹
2	Paraffin B grade (60-62)	59	109	DSC	600	Shukla et al⁶²
3	Paraffin (C_22.2 H_44.1)	47.1	166	Computer controlled measuring system	900	Hadjieva et al⁶³
4	Expanded perlite/n-eicosane (C₂₀)	36.18	275.9	DSC	10000	Karaipekli et al⁶⁴
5	Paraffin (70 wt%) + Polypropylene (30 wt%)	44.77	136.16	DSC and TGA	3000	Alkan et al⁶⁵

Table 7.

Commonly used salt hydrates thermal stability and properties.

S.NO	Type of PCM	Melting point temperature (°C)	Latent heat of fusion (kJ/kg)	Experimentation	Thermal cycles	Reference
1	CaCl₂·6H₂O	—	—	Resistance thermo meter	5650	Porisini et al⁶⁶
2	MgSO₄7H₂O	48.5	202	DSC		Mehling et al⁶⁷
3	Zn₃(OH)₄(NO3)₂	25–42	143	Water bath, T-history	1000	N. Kumar et al⁶⁸
4	Ba(OH)₂.8H₂O	30–90	238	Durability testing chamber	400	Xiao et al⁶⁹
5	Sodium acetate trihydrate	70	250	Water bath, Stainless steel tube	400	Wada et al⁷⁰

Table 8.

Commonly used Eutectics thermal stability and properties.

S.NO	Type of PCM	Melting point temperature(°C)	Latent heat of fusion (kJ/kg)	Experimentation	Thermal cycles	Reference
1	Capric acid (98% pure) + myristic acid (97% pure)	20.86	156.99	TGA	200	Jebasingh et al⁷¹
2	Lauricacid (80.8%) + stearic acid (19.2%)	38.2–33.7	180.5	DSC	360	Sari et al⁷²
3	Binary mixture of polyethylene glycol 2000 and 10000	48	160.02	DSC	1500	Ansu et al⁷³
4	Tetradodecanol (71.84 wt%) + myristic acid (28.16 wt%)	37.75	125.70	DSC	1000	Jingyu H et al⁷⁴
5	Caprylic acid (70 wt%) + 1-dodecanol (30 wt%)	6.52	171.06	DSC	120	Zuo et al⁷⁵

Details about phase change materails

The latent heat storage system stores or releases energy with the help of PCM, which can store lots of energy. Their high energy storage potential got activated while phase changing at the isothermal condition. Unlike other materials, PCM undergoes a phase to keep the energy.

Classification of PCM’s

Thermal storage exists in physical and chemical modes. The physical storage is further categorized as sensible heat and latent heat. The PCM stores the energy in latent heat form, maintaining a constant temperature. PCM undergoes different phases as per existing material, i.e., solid to liquid, liquid to liquid, and solid to solid, as shown in Figure 3. Out of these, the latent heat carrying capacity is more than in the solid-to-liquid transition phase; hence the BTMS utilizes the PCM in the solid-to-liquid phase. The solid-to-liquid PCM was classified as per the existing number of carbon atoms as Organic, Inorganic, and Eutectics. The classification details are given in Figure 4.

Figure 3.

Various phases of PCM and the related mechanisms.

Figure 4.

Classification of PCM.

Selection of the material

The selection of PCM related to storage applications, mainly in electric vehicles, is critical. In all aspects of chemical, thermophysical, kinetic, and economic sense, PCM will be selected as per the application demand. To start with, melting temperature is the significant parameter in the selection of PCM, and it is recommended to be within the operating range of Li-ion battery (20°C-60°C), i.e., and often the selected PCM should have a melting temperature below 40°C for Indian Conditions¹⁹ further, PCM should be non-polluted throughout service life, low environmental impact, and must possess recycling capability.

Each type of individual PCM has its own advantage and disadvantage. To evaluate the thermo physical performance of PCM, many calibration techniques are available, namely, the Differential scanning calorimetric method (DSC), Differential thermal analysis (DTA), and T-history method. Thermal conductivity can be calibrated by hot disk apparatus, and thermal stability by Thermo gravimetric analysis (TGA). Fourier can analyze chemical properties transformed by infrared spectroscopy (FT-IR).²¹ Calibration of nano-structured PCM can be performed by electrochemical impedance spectroscope (EIS) and X-ray diffractometer (XRD).

Description of PCM

Organics

Organics are alkaline containing carbon-attached hydrogen or C-H bonds. Paraffin and non-paraffin are additional classifications for organic PCM. A straight, saturated hydrocarbon chain with the formula C_nH_2n+2 makes up the paraffin structure. The properties like melting point (°C) and latent heat of fusion (kJ/Kg) differ with inorganics based on the configuration of carbon with hydrogen ions. Paraffin has many other worthy properties like different melting temperatures, phase transformation temperatures, low vapor pressure, no super cooling, chemically stable, inert to metal containers, and available at reasonable prices. Besides the advantages, paraffin has a few limitations, like low thermal conductivity and the nature of flammability, and shows incompatibility with plastic containers. Non-paraffin organic PCMs have weak flame resistance, poor thermal conductivity, low combustion temperatures, and momentary toxicity.

In organics

Inorganic paraffins are described with the chemical formula CH3 (CH2)2nCOOH (fatty acids) and the basic property of latent heat is comparable with organic paraffins. In comparison to organics, inorganic materials have some advantages, like high heat storage capacity, better thermal conductivity, greater phase change enthalpy, and lower cost compared to organics.²¹ The applications of the inorganics are medium to high melting temperature ranges. Demerits of inorganics are highly corrosive in nature, have no specific phase transition due to lack of stability, and behave under cooling.²² Broadly inorganics are classified as molten salts, salt hydrates, and metals.

Eutectics

Eutectics are the mixture of two or more components having minimum melting point composition. Eutectics tend to freeze to form a nice blend of crystals. Due to this, eutectic is not segregated during melting and freezing.²³ Eutectics exist with the combination of organics and inorganics.

Temperature range of various phase change materials

Even though eutectics have more latent heat values due to their negative melting temperature ranges, they are not suitable PCM for the EV battery application.

Organics have the lowest density out of all the types of PCM; hydrated salts are a good choice in terms of density.

Most of the PCM specific heat will lie between 2–2.5 kJ/kgK except eutectic PCM. Still, eutectics are not suitable for EV battery applications because of the negative melting temperatures.

Organics are the second-highest maximum operating temperature materials in the case of PCM. HT PCM has the highest operating temperature, ideal for thermal runaway cases in a battery. Still, the Phase change temperatures of HT would not be a perfect case to use in EV battery applications. (Figures 5–10)

Figure 5.

Ideal properties of PCM.²⁰

Figure 6.

Operating temperature range of various PCM.³⁶

Figure 7.

The latent heat versus different types of PCM. (Graph reproduced using the literature data).³⁶

Figure 8.

The density versus different types of PCM. (Graph reproduced using the literature data).³⁶

Figure 9.

The specific heat versus different types of PCM. (Graph reproduced using the literature data).³⁶

Figure 10.

Maximum operating temperature versus different types of PCM. (Graph reproduced using the literature data).³⁶

Brief description of phase equilibrium diagrams to characterize different PCM

Material mixtures can exist with many components. The basic mixture starts with one individual pure component, showing its phase change at a particular temperature. A binary mixture is the simplest multicomponent mixture have desirable phase equilibrium characteristics. Based on the observed desirable changes in the PCM, there were nine different types of phase change equilibrium systems were illustrated and described below in Figures 11–13. Each system is miscible in the liquid phase and the condensed systems are assumed at constant pressure condition by neglecting the vapor phase.

Figure 11.

(a) Congruent isomorphous lowest melting temperature, (b) congruent Isomorphous highest melting temperature, (c) Partially congruent isomorphous lowest melting temperature.⁵¹

Figure 12.

(a) Partially existed isomorphous eutectic (b) partially existed isomorphous peritectic (c) partially existed isomorphous congruent melting compound (additionally with eutectic and peritectic contents).⁵¹

Figure 13.

(a) Non-isomorphous eutectic (b) non-isomorphous peritectic, (c) non-isomorphous congruent melting compound formation (additionally with eutectic and peritectic contents).⁴⁵

Solid solutions, eutectics, and compounds are the three different types of phase variations that exist in binary solid mixtures. Solid solutions are described as single-phase solutions made of more than single component, generally called solute and solvent, these components made homogeneous composition. Eutectics are homogeneous mixtures of different compositions which melts or solidifies at lower temperature than one of the components melting points. Compound solutions are mixtures formed with fixed stoichiometric proportioned atoms of more than one element.⁵⁰

Figure 11 describes the phase equilibrium of the miscible solution formed with components A and B in solid and liquid phases. These all are characterized by the tangential contacts between liquid and solid curves at congruent melting points. Liquidus curves represent the temperatures at which different alloy components begin to freeze while cooling and finish melting. In contrast, the solidus curves represent the temperature points at which the alloy components of various proportions begin to freeze when exposed to a cooling medium, and initiate melting when subjected to a heating medium.⁵² The definition of congruent is solid phase and liquid phase existing in equilibrium with the same composition and, ideally, having a one precise melting point with no phase separation. The term isomorphous indicates that all components present in A and B are completely mutually miscible in all proportions. Figure 11(a) and (b) show congruent isomorphous melting for solid solutions at their lowest and highest melting temperatures. Adding one element lowers the melting point temperature of another element in the isomorphous congruent minimum melting instance.

In the isomorphous congruent maximum melting case, the addition of one element will increase another element's melting point temperature. Figure 11(c) describes the isomorphous congruent melting partially where the miscibility gap exists below the melting point temperature within the miscible solid solution of A and B which is very similar to the (a) in ideal conditions.

Figure 12 displays the Compound mixtures of eutectic and peritectic compounds are formed due to their reversible nature, which prevents phase separation. As a result, when these mixtures solidify, two distinct solid phases with different compositions emerge from the liquid phase. Eutectics does not accurately match congruent melting systems even though it exhibits similarity with them through the presence of sharp melting points. PCM exhibits perfect behavior in eutectic conditions. This solid composition will show phase separation in case one solid exposes to supercooling. Eutectics can occur in partially isomorphous or non-isomorphous systems, but not in fully isomorphous systems. Figure 12(a) shows two solid solutions combined composition is equal to the liquid phase composition at eutectic isotherm, indicating that sharp point with E. Figure 12(b) shows the solid solution of formed peritectic is the subset of eutectics. Peritectic exhibit incongruent melting by changing a solid and a liquid into a solid. Peritectic reactions can experience coring, which hinders them from reaching the complete stage of phase transformation. Coring reduces the effective freezing temperature and hinders the attainment of equilibrium, leading to significant phase separation rather than a sharp, centered point. Figure 12(c) represents a compound mixture of components A and B. Components are partially isomorphically mixed and melted. As a result, congruently, eutectics and peritectic are used to create compound formulations with solid solutions (S_A,S_B), compounds (S_AB, S_AB + Liquid), eutectics(E), peritectic(P).

Figure 13 is representing the non- isomorphous eutectics, peritectic and compounds mixtures. In this system even if peritectic exists, there will always be at least one eutectic. Figure 13(a) represents a simple eutectic structure formed by the solid components A and B. Very sharp unique melting point(eutectic) created with two pure components in the manner of non-isomorphous system due to those two solid components are complete immiscible with each other. The existence of a eutectic point in a non-isomorphous system is always conceivable due to the reason that a component's adoption lowers another component's melting temperature. Similarly Figure 13(b) depicts the peritectic formation from the two solid components (A, B). Along with eutectic, a different substance (P) form. Figure 13(c) represents the creation of a straightforward congruent melting compound from two solid components (A, B).

In the field of PCM-based BTMS, various researchers used different kinds of PCMs to achieve lesser temperatures in the battery pack. The base PCM for many of the experiments is paraffin due to its advantages shown in Table 2. In order to study the thermal behavior of lithium-titanate (LTO) battery cells during a high C-rate discharging operation, Behi et al.⁸⁰ employed paraffin as PCM and a heat pipe cooling system. The basic phase shift transition diagram of the binary combination of paraffins C14H30 and C16H34 is shown in Figure 15.

Figure 14.

Pseudo-phase diagrams of binary wax blends for Bees wax and paraffin.⁵⁵

Putra et al.⁵⁵ used beeswax to determine the cooling system’s effectiveness at the temperature range of 25°C–55°C in the application of EVs. Figure 14 shows the basic eutectic blend of beeswax and paraffin wax. In order to attain the ideal temperature range with the aid of bionic liquid channels, Yang et al.⁵⁶ used polyethylene glycol-1000 (PEG) as PCM and 16% ethylene glycol solution as a coolant in a PCM-based BTMS. Figure 16 displays the diagram of the volume% of the pure ethylene glycol phase transition. In order to attain the battery module's ideal temperature range, Sihang Hu et al.⁸¹ reported the passive thermal management work on a battery module using a composite PCM combination (Lauric acid + expand graphite + graphene). Costa et al.⁵⁷ gave a clear explanation of the binary mixtures at the solid-liquid equilibrium of the fatty acids and triacylglycerols. Figure 17 shows the reaction between lauric and myristic acids, which led to the formation of a non-isomorphous congruent melting compound with six distinct locations. The phase diagram of the mixture is thus divided into six regions: region A, where the liquid phase is located above the locus of the liquidus point; region B, where the solid myristic acid coexists with the liquid phase; region C, where the solid lauric acid coexists with the liquid phase; region D, where a pure solid combination of the compound (myristic acid+ lauric acid) coexists with the liquid mixture; and regions E and F, where the pure solid lauric acid and myristic acid respectively.

Figure 15.

C₁₄H₃₀ and C₁₆H₃₄ binary mixture liquid –solid phase diagram.⁸⁰

Figure 16.

Phase diagram for ethylene glycol water solutions at 100 kPa.⁵¹

Figure 17.

Phase diagram of lauric + myristic acids.⁵⁴

The corrosive nature of the salt hydrate PCM might cause other materials to lose their structural integrity and shorten the battery pack's lifespan.¹⁰ As a result, salt hydrates are rarely used in the BTMS application.

Examples: phase equilibrium diagrams of inorganic salt hydrates systems.

Miscellaneous properties of the PCM

Selection of the material provides a brief description of the factors to be taken into account when choosing a PCM, and it highlights the key factors that are necessary to comprehend PCM fully. This discussion concerns thermal stability, including robustness and compatibility with the containers.

Thermal stability of PCM

The material's durability is determined by how well it holds together after repetitive use and thermal cycling in storage application intended for melting and freezing. After long-run thermal cycles, the PCM should be chemically, thermally, and physically stable. The prior required properties for BTMS application are melting point temperature and latent heat of fusion. As a result, the initial stability test of PCMs should be carried out in accordance with their real usage. PCM’s thermal stability calibrating thermostatic chamber/bath, constant temperature oven, electric hot plate, and DSC, TGA.

Calibration of thermal stability of PCM

DSC is the widely used equipment for the application of PCM-based BTMS, on the basis of thermo analytical principal DSC works. A PCM sample and reference are considered, throughout the experiment and are necessary to keep a constant temperature. Using DSC analysis, heat is applied to the PCM sample and reference substance to keep their respective temperatures constant, after a certain time the PCM sample undergoes phase transitions and it requires sufficient heat to maintain a constant temperature with the reference. The requirement of heat for the PCM sample based on process is exothermic or endothermic. By measuring the difference in heat flow between the PCM sample and the reference, the DSC will determine if there are heat transactions of absorbing or releasing. A curve known as the DSC curve is a plot between heat flow and temperature. The area under the peak of that curve represents the latent heat of fusion. Drawing onset linen with a curve fitting to the raising part of the peak results in the phase transition temperature and phase transition range is the region between the curve's onset and peak temperature⁶⁰

To calibrate the change of latent heat fusion and temperature of phase transition after every specified cycle, i.e., 500 or 1000 cycles, a relative percentage difference (RPD) is to be noted.⁶¹ The RPD of any property i of the PCM at any number of cycles n and the 0th cycle may be defined as

R P D = \frac{X_{n, i} - X_{0, i}}{X_{0, i}} \times 100 (%)

Where X_n,i = onset and peak temperatures and the latent heat of the PCM after n cycles

X_0,i = onset and peak temperatures of the PCM at 0^th cycle.

Various works of literature explained the thermal stability of PCM in detail. Many of the sources are from experimental results. Some of the regularly used PCMs are shown in the below tables.

PCM compatibility with container

For the long-term stability of PCM, compatibility with adjacent support structures is important. Hence, corrosion is not present in the container while it is in contact with PCM, and the progress of the chemical reaction also slows down. Castellon et al.[77] experimentally shown the behavior of plastics with PCMs. Plastics considered in this work are HDPE, EGYPTENE, HD6070UA, LDPE RIBLENE FM34, and Exxon Mobil PP 7011L1, PCM as commercial-grade organic kinds of paraffin are RT20, RT25, and RT27, inorganic salt hydrate. Plastics were immersed in PCM-contained beakers and performed thermal cycles by keeping them in the oven. After 400 days, Different tests measured material compatibility and stability and concluded that HDPE showed better results and that paraffin is more compatible than salt hydrate. Gerard Ferrer et al.[78]explained the corrosion effect on metal containers using PCM in energy storage applications. In this work, five different metals, namely aluminum, copper, carbon steel, stainless steel 304, and stainless steel 316 were selected with four different PCM arrangements of two eutectic mixtures Capric acid (73.5%) + myristic acid (26.5%), Capric acid (75.2%) + palmitic acid (24.8%), Ester Salt Pure Temp 23, Inorganic Salt SP21E were examined and concluded that Stainless steel 316 and stainless steel 304 showed great corrosion resistance (0-1 mg/cm2yr) and it is compatible any of the studied PCM. Khan et al. [20] gave a detailed review of the performance enhancement of PCMs based on latent heat energy storage in contact with PCMs stability and compatibility. This review says that the long-term productivity of latent heat storage systems is proportionate to Thermo physical stability and compatibility with withholding materials. Khan explained various works done on paraffin and salt hydrates stability and the impact of the compatibility with plastics and other metals. Rawson et al.[79] concerned with finding alloys suitable for energy storage applications. In this view, the CALPHAD (Calculation of phase Diagrams) approach is implemented in the TCAL5 and TCOX8 databases offered by Thermo-Calc Software in order to choose the proper containers for such alloys utilizing automated macro creation. It concludes that databases identified nine container materials compatible with many pure and eutectic mixtures at equilibrium. Also, Thermal properties for hundred eutectic compositions were listed, and this work contributes to finding the compatible phases for identified eutectic compositions and pure mixtures. Nagano et al.[76] considered six different metals Cu, C-steel, Brass, Al, and two different proportions of stainless steel, SUS304, and SUS316, used in the corrosion test. This corrosion test helped determine the compatibility of the salt hydrate PCM mixture of magnesium nitrate hexahydrate (Mg (NO3)2. 6H2O) and additive magnesium chloride hexahydrate (MgCl2.6H2O). (Figures 18–20) The test results showed that SUS316 and Al might be utilized as the storage container material and heat exchangers for this PCM mixture after examining each metal for 90 days in a glass tube at a temperature of 95°C. Figure 21 shows the results of the corrosion test.

Figure 18.

Gabuler’s salt phase diagram.⁵⁸

Figure 19.

Phase diagram of sodium hydroxide–water system.⁵⁹

Figure 20.

DSC measurement of the latent heat of fusion and the melting temperature of paraffin.⁶⁰

Figure 21.

Magnesium nitrate hexa hydrate with additive magnesium chloride hexahydrate compatibility test with metals surface condition after 90 days observation at the temperature of 95°C.⁷⁶

Figure 22.

Methodology for selecting suitable PCM for battery thermal management.

Methodology to select the right PCM and implementation in battery thermal management

The following methods may be used as a guide for applying PCM-based BTMS to choose the PCM for BTMS. Prior concerns such as the needed melting temperature, required density, and latent heat of fusion were considered while choosing the appropriate material. Additionally, performance-improving approaches are taken into account. Many charging and discharging cycles must be carried out after choosing the PCM in order to evaluate the material's durability or life cycle. One of the most important things to think about is if the PCMs are compatible with the containers because many PCMs display corrosion and flammability at higher temperatures. The following technique was developed based on knowledge of numerous literary works (Figure 22).

Conclusion and future scope

Thermal management is crucial and plays a significant role in maintaining the performance and safety of lithium-ion batteries. This work focuses extensively on passive thermal management using phase change materials (PCM). The properties of various PCMs are listed, compared using phase change equilibrium diagrams, and the stability of the PCM is discussed in relation to its lifespan alongside the battery.

Perspectives for selecting the right PCM

• Organic PCM and hydrated salts are the most suitable PCM options as their melting temperatures are closer to the appropriate working temperatures of lithium-ion batteries.

• Organic PCM is less corrosive compared to other PCM types.

• All PCM materials inherently have low thermal conductivity, which can be mitigated by using thinner layers or by enhancing conductivity through thermal conductivity enhancers.

• Composite organic PCMs exhibit higher thermal stability compared to other PCMs.

• The durability cycles of the selected PCM should be equivalent to the battery pack's lifespan.

• PCM compatibility is crucial because the properties can drastically change when interacting with different materials, even if the individual materials perform well independently.

Future scope in using PCM in (BTMS)

• The durability of PCMs under mechanical vibration and shocks, which are important factors in real-world battery applications, needs to be extensively studied.

• Quantitative studies should be conducted to determine the advantages of PCM usage. As PCM adds mass to the battery and the vehicle, its impact on mileage and energy savings should be evaluated for selecting the appropriate PCM.

• Numerical simulations are valuable tools for optimizing the design of BTMS and approximating the liquid fraction of PCM over time. This information is crucial for determining the suitability of PCM in a given application, which can be challenging to determine through experimentation alone.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Moda Geetha Rani

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