Electric Vehicles as a Promising Trend: A Review on Adaptation, Lubrication Challenges, and Future Work

Abstract

The increased energy efficiency of electrified vehicles and their potential to reduce CO₂ emissions through the use of environmentally friendly materials are highlighted as reasons for the shift to electrified vehicles. Brief trends on the development of electric vehicles (EVs) have been discussed, presenting outstanding improvement towards the actualization of the green economy. The state of the art in lubrication has been thoroughly investigated as one of the factors influencing energy efficiency and the lifespan of machine components. As a result, many reports on the effectiveness of specific lubricants in electric vehicle applications have been developed. Good thermal and corrosion-resistant lubricants are necessary because of the emergence of several new tribological difficulties, especially in areas that interact with greater temperatures and currents. To avoid fluidity and frictional problems that may be experienced over its lifetime, a good viscosity level of lubricant was also mentioned as a crucial component in the formulation of EV lubricant. New lubricants are also necessary for the gearbox systems of electric vehicles. Furthermore, battery electric vehicles (BEVs) require a suitable cooling system for the batteries; thus, a compatible nano-fluid is recommended. Sustainable battery cooling options support global energy efficiency and carbon emission reduction while extending the life of EV batteries. The path for future advancements or the creation of the most useful and efficient EV lubricants is provided by this review study.

1. Introduction

In response to issues with automobile internal combustion emissions, a significant number of countries have accepted the legally binding global climate change agreement made at COP21 in Paris in 2015 [1,2]. The treaty, which joined the United Nations Framework Convention on Climate Change (UNFCCC) in November 2016, seeks to keep global warming well below 2 °C and preferably at or below 1.5 °C, in comparison to pre-industrial levels [2]. The five-year strategies for combating climate change, known as Nationally Defined Contributions (NDCs), are created by each party to the agreement. According to predictions in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), transportation emissions may increase at a faster rate than those from all other energy end-use sectors [3].

The European Commission has recognized the need to provide a framework for the shift to low-emission mobility, since about 94% of the energy used for transportation in the EU comes from oil products [3]. Electric vehicles (EVs) are a key technology within the electrification of the transportation fleet, which is a leading option under discussion [4]. The majority of vehicles are still powered by more efficient conventional gasoline and diesel engines, with the exception of a small number of electric vehicles, such as full battery electric vehicles (BEVs), plug-in hybrid electric vehicles (PHEVs), or mild hybrid electric vehicles (MHEVs). A relatively small battery that is powered by the internal combustion engine is a feature of mild hybrid cars, such as the first Toyota Prius. However, Europe and China will not be able to fulfill their 2025 fuel consumption targets unless their fleets of passenger cars are significantly electrified [3].

Present-day discussion on electric vehicle usage primarily concentrates on the environmental advantages and technological developments, while the possible drawbacks are largely ignored. According to recent studies, EVs require more extraction operations because their material footprint is substantially larger than that of internal combustion engine vehicles (ICEVs) [4]. Furthermore, according to recent predictions, EV users might drive more, which could have detrimental impacts on the environment and society. However, recent advancements in EV batteries allow them to be used as energy storage devices, which makes it easier to integrate renewable energy sources like solar and wind into the electrical grid. By lowering the emission of pollutants like nitrogen oxides, EVs can help improve urban air quality and lessen air pollution inequities [4].

Multidisciplinary research is necessary to develop ideas about how to attain fully sustainable lubrication of electric vehicles given the many ramifications of contemporary transportation systems. By synthesizing existing studies on the less obvious negative consequences of the enhancement of EV lubrication, this study adds to the body of literature. It is challenging to develop lubricants for electric vehicles (EVs), since original equipment manufacturers have different electric motor designs, necessitating a particular lubricant to meet their requirements for optimal performance. The technical requirements for EV lubricants are higher than those for lubricants for ICEs [5]. Important requirements like anti-wear performance, friction reduction, efficiency, electrical compatibility and insulation, and cooling of electric motors and battery packs must all be addressed by the lubricants. Lubricants are necessary for essential electrical parts of EVs, including brake fluids, gear oils for the differentials, the chassis, the gear reducer, and the wheels, as well as for coolants for the car batteries and grease for other EV parts [4].

2. Brief Trends on the Development of EVs

Early in the nineteenth century, the idea of an electric vehicle (EV) was developed, and by the late nineteenth century, commercial EVs had evolved [5]. The Toyota Prius’ introduction in 1997 marked a turning point in the advancement of hybrid cars (HEVs) [6]. Since then, the number of EVs and HEVs has increased. Reports have indicated that the sales of electric vehicles will continue to rise globally [7]. Figure 1a depicts the history of significant occasions that influenced EV/HEV research and development.

In order to produce sustainable EVs, manufacturers provide a wide range of models, such as plug-in hybrid electric cars (PHEVs), hybrid electric vehicles (HEVs), battery-powered electric vehicles (BEVs), fuel cell electric vehicles (FCEVs), and photovoltaic EVs, as illustrated in Figure 1b. Recently, a pure BEV without fuel has emerged as a promising technology that is paving the way for the environment to become carbon-free [8]. BEVs are anticipated to have a significant market penetration soon due to a sluggish social acceptability. This is due to the fact that the performance of these vehicles is crucial to energy storage systems (ESSs), control plans, and energy management (EM) techniques, and they have the potential to become the future mode of transportation [4,5]. However, to actualize the dream of EV services and to optimize their performance, adequate lubrication with suitable lubricant needs serious research, which this proposal intends to explore. The adoption of EVs, among other vehicles, is a promising step in line with the sustainability goals due to its maximum ecofriendly impact and reliability.

Although a few papers have reviewed research on electric vehicles as a current innovation for a sustainable society [1,9,10], they lack detail on how to lubricate an EV with suitable lubricants in order to improve system performance. A thorough analysis of acceptable lubricants that ought to be compatible with the system batteries and other reactive components is also lacking, which the manufacturers need to understand. As a result, this article aims to compile research on various EV lubrication issues, as well as future aspirations for fully active, series-reconfigurable analysis situs for battery-powered EVs. On the basis of performance evaluations including efficiency, longevity, range furtherance, and cost, thorough implementation of EV lubricants and lubrication techniques, such as rule-based and optimization-based approaches, is provided.

Nevertheless, the primary barriers to the broad adoption of EVs in the automobile industry are their low driving range, poor battery efficiency and energy output, and high pricing. It is anticipated that EVs will cost more than ICE vehicles because of the relatively new and sophisticated technology under investigation [11], but the average operating cost for EVs is significantly cheaper. The battery technology that is used is what accounts for the great operating range and inexpensive cost of EVs. The majority of lithium-ion batteries, which are the most promising and readily available, contain cobalt, a relatively uncommon metal that drives up costs [12]. On the other hand, lubricating components of electric vehicles need to be thoroughly studied to meet the application without causing more loss of energy and damage, since machines cannot perform optimally without proper lubrication. To actualize this, energy losses in EMs, power electronics, battery charging and discharging, cabin heating and ventilation, air dragging, and friction ought to be reduced. To prevent thermal runway (TR), adequate cooling of the battery system is essential, as well as the prevention of contaminants that could raise the battery temperature. Tribological solutions could serve to minimize the ultimate cause of loss. Also, since EVs include additional tribologically sensitive components that could increase friction losses and adversely affect efficiency and durability, other friction losses should be taken into account. All of these issues require suitable lubricants that possess all the requirements, without altering the operation of EVs’ working systems.

3. EV Lubrication State of the Art

Automobiles frequently utilize lubricants for a variety of purposes. It is essential that all the elements involved in the power transmission process be adjusted to increase the vehicle’s performance and efficiency. Researchers have reported recent advancements in lubrication in areas like bio-lubricants [13,14,15], mineral oil-based lubricants [16], nanoparticle additives [17,18], and carbon nanotube-based lubricants [19,20,21], among others, with huge positive results. The goal of lubricant research has been to increase copper corrosion resistance and compatibility with the polymers used in EV/HEV electronic components [22,23]. According to Cann PM. [24], lithium grease offers the benefits of good adhesion, non-corrosiveness, and moisture resistance, making it suitable for a variety of applications. Andrew [25] ascertains that aluminum and urea greases also function effectively, but their manufacturing is accompanied by risky handling and limitations on process balance. This entails the creation of brand-new, industry-recognized test procedures to gauge EV characteristics [23,26]. The high-tech powertrains of electrified vehicles are the only ones for which the electric vehicle fluid was specifically created to meet a wide variety of performance requirements successfully and efficiently. Although there are many other kinds of lubricants (solid, synthetic, and mineral), bio-lubricants and grease lubricants will be the main emphasis of this study.

Del Río et al. [27] investigated the effectiveness of lubricant tribological enhancement using magnetic nanoparticles: Nd alloy compared to trimethylolpropane trioleate base. In comparison to TMPTO, the Nd alloy compound’s nano-dispersion had the highest tribological performance, with decreases in friction and WSD of 29% and 67%, respectively. However, rolling conditions tests were used to examine the wear depth of nano-lubricants, and it was found that the wear depth of Fe₃O₄ was 59%, while the Nd alloy nano-lubricant, with a wear depth of 67%, reduced friction in comparison to TMPTO. Because of this, understanding lubricants is crucial and needs thorough research. Engine oil, transmission fluids, and grease are all utilized as lubricants both in hybrid EVs and single EVs for effective operations, along with a variety of other performance-improving and protective activities against wear in metal-to-metal contact, cooling components, and more. Whether it is an automatic stepping gearbox (AT), continuously variable transmission (CVT), or dual-clutch transmission (DCT), the transmission fluid serves the same general purposes: to generate hydraulic pressure, disperse heat, and guard against wear on the metal gears and other components requiring optimum lubrication, as shown in Figure 2.

Additional important areas of focus include achieving low-viscosity and enhancing electric and thermal characteristics [28]. The use of low-viscosity oils, vapor phase lubrication, ionic liquids, and anti-wear and friction lubricants based on nanotechnology have proven to be the most effective strategies [29]. Another area of research interest includes electric discharge, as well as bearing currents, lubricant instability, common mode voltages, and common mode currents [30,31]. It is crucial to conduct research on the viscosity of EV lubricant. According to Gupta et al. [32], a low-viscosity oil increased engine efficiency in EV mode by 17% when compared to factory gearbox oil. Compared to conventional internal combustion engines, electric vehicles’ engines require entirely different engine fluids depending on the working conditions. Traditional lubricants are not an option for EVs since they can cause a variety of issues, from viscosity-related issues to the corrosion of other design parts. All these issues will substantially jeopardize the performance and longevity of the vehicle.

Sun et al. [33] conducted an experiment on speed plan under lubricant grease application, using three up-and-down speed ramps labeled Sweep 1, Sweep 2, and Sweep 3. According to the investigation, during the running-in period, the clamp force marginally dropped as the housing warmed up, reaching the load representative for the e-motor bearings, which in turn had an impact on system performance. Again, Ref. [34], in related research on long sweeps, found that the friction torques of Lithium Complex (LiX) and Polypropylene (PP) are more equivalent to one another when the speed is decreasing than when it is increasing. The friction for both greases reduced with the speed increments up to 300 knDm during the first half of sweeps 1 and 2 (which involved rising speed). The friction torque in the case of PP grease remained consistent at a plateau of 8–9 N mm during the first sweep and 7–8 N mm during the second sweep, with changes in speed between 200 and 400 knDm having no impact.

With the use of low-viscosity greases, the occurrence of this plateau, which has less friction and spans a wide speed range, was observed [35]. As opposed to this, the LiX grease’s plateau of friction torque at increasing speeds (12 N mm at Sweep 1 and 11 N mm at Sweep 2) demonstrated significant hysteresis, with torque falling even lower to 9 N mm once the downward speed sequence was started. The fact that the PP grease had no decrease in friction suggests that the LiX grease is more sensitive to variations in speed and is therefore dependent on the speed history. Challenges in a grease lubrication experiment might be increased by unreliable variables such dirt, incorrect installation, or improper lubrication [36]. The influence of thickeners was mostly related to thickeners entering the contact at low speeds (lower than 100 knDm) in prior experiments [37,38,39]. However, the findings of this investigation indicate that friction torque values, even at high speeds of up to 600 knDm, are controlled by the type of thickener.

Fluids for electric vehicles are necessary to maintain the proper temperature of the battery and power electronics, as well as to lubricate and cool the powertrain and transmission systems [23,40]. This is in contrast to dielectric grease, which is intended to reduce contact levels in electric vehicles [25], while heat transfer fluid is used to maintain the battery temperature of EVs. Sankarkumar and Natarajan [9] asserted that electric vehicle fluids contribute to improved EV transmission efficiency and a decrease in energy losses in the drivetrain system. Fluids for electric vehicles provide insulation to prevent any arcing caused by close contact with the vehicle’s electrical components. In addition, they provide insulation against electric current and cooling e-motors and gears, thereby increasing machine efficiency. Analysis on materials/lubricants in reactions towards corrosions, thermal conductivity, cooling efficiency, and general performance are presented in

Table 1. Some analysis of materials/lubricants’ reactions towards corrosion, thermal conductivity, cooling efficiency, and their performance.

No	Lubricants/ Materials	Analysis Technique	Thermal Conductivity	Corrosion Resistance	Tribological Performance	Remark	Ref.
1	TiN/TiCN/TiCN/TiC/Al2O3, and TiN/TiCN/TiCN/TiC/TiN coated samples	Samples prepared using CVD system, while the tribology test was performed under dry testing using pin-on-disk	N/A	TiN/TiCN/TiCN/TiC/Al2O3 coating exhibited more corrosion resistance than all other samples. This is because Al2O3 is more corrosion-resistant than TiN.	TiN/TiCN/TiCN/TiC/TiN-coated sample yielded more wear profile compared to TiN/TiCN/TiCN/TiC/Al2O3-coated sample.	Using at oxidative area, TiN/TiCN/TiCN/TiC/Al2O3-coated sample is recommended to avoid unwanted corrosion, which will lead to high COF and wear.	[41]
2	Additive Sn-free steel and additive Sn-containing steel	Electrochemical tests on weathering steel (WS)	N/A	Additive of Sn enhanced the corrosion resistance of WS in a simulated tropical marine atmosphere	N/A	When the WS is corroded, Sn participates in the formation of rust layer in the form of SnO2 and is mainly distributed in the inner rust layer.	[42]
3	polytetrafluoroethylene with polyimide and boron nitride	Dry sliding friction of a block-on-ring tribometer	PTFE has a low thermal conductivity of about 0.24 W/(m·K).	N/A	The wear rate and COF of the 10:10:80 BN/PI/PTFE composite reduced to almost 1/300 and 80% of those of pure PTFE, respectively.	In blending BN/PI/PTFE, the concentration of PTFE should be higher as to defend the operation from attack of corrosion.	[43]
4	polyol ester blended with sesame oil	Four ball tribometer	Maximum thermal conductivity was obtained at 85 °C for the blend B35 and is 25.73% more than that of the base POE oil	Resistance of corrosion under sesame oil blended than base form	Sesame oil blends have lower values of CoF and wear scar radius when compared to the base POE oil.	The use of the POE/sesame oil blended lubricants in contact with copper gives no risk of corrosion, thus could be applied in EVs and refrigeration.	[44]
5	Copper oxide (Cuo) nanoparticles and Titanium oxide (TiO2) nanoparticles	Pin-on-disk tribometer	Increase in concentration is liable for improvement of thermal conductivity of nano-bio-lubricants but better with copper oxide	N/A	With 0.5% Cuo, nanoparticles have lower coefficient of friction and specific wear rate compared to the rest of the nano-bio-lubricants.	Since the outcome yielded enhanced thermal conductivity, this implies that application on EVs close to the battery is not recommended to avoid current flow.	[45]

.

Due to the complex configuration of modern EVs, materials are designed to promote their best performance and prevent explosions. Chen et al. [46] stated that in lubrication, nanotechnology, synthetic base oils (Bos), and thickeners have shown to improve lubricity, increase service life, and reduce friction torque when used with grease. Furthermore, Ref. [47] asserted that numerous lubricants, including nanoparticle additives, synthetics, mineral oil-based lubricants, grease, and bio-lubricants, have been researched in terms of sustainability, thus demonstrated huge success. Chen et al. [11] emphasized that high viscous lubricant and thick film during lubrication lead to a high friction coefficient due to the dragging effect between the sliding contact, and affects the operation, since lubricants influence the overall performance of every machine. As a matter of fact, lower-viscosity lubricant applied on EVs is expected to enhance the performance, and thus needs to be investigated. Gupta et al. [32] affirmed that a low-viscosity oil increased engine efficiency in ICEV mode by 17% when compared to original equipment manufacturer (OEM) transmission oil; however, some areas in EVs demonstrated better performance under grease lubricant application.

As presented in Figure 3, considering the components of batteries and the currents transmissions during operations, applied lubricants should have characteristics to prevent the short-circuiting of motor components and the transmission fluid in HEVs must have insulating qualities (low electrical conductivity). Since EV lubricants will come into direct contact with the e-motor and/or other electrical parts of the automobile, they must have greater electrical insulation to prevent arcing. High temperatures, increased oxidation, and particle abrasion are all possible in EV operation environments. The lubricants should have consistent dielectric stability throughout to function under these circumstances. The electric engine and other components of power electronics have a range of operating temperatures where they are most reliable and effective. For temperatures as high as 180 °C, the lubricant’s function is to offer excellent heat dissipation. A component’s fracture, swelling, cracking, corrosion, vibration, etc., may result from non-compatibility of lubricants with contact materials. Since copper has a high electrical conductivity, most of these components are constructed from copper. Therefore, it is crucial that the lubricant has high compatibility with copper, seals, and every other component to avoid catastrophe.

Apart from reducing friction and wear, the goal of lubricant research has been to develop lubricants that are more resistant to copper corrosion and compatible both with the polymer used and electronic components of EVs. The use of low-viscosity oils, vapor phase lubrication, ionic liquids, and anti-wear and friction lubricants based on nanotechnology have been the most effective strategies [29,48].

The performance of cooling is thought to be influenced by BOs and their viscosities, but the electrical conductivity of the EV is greatly influenced by additives. But it is also noted that additives might have a slight impact on cooling effectiveness [11]. Bio-based lubricants have shown promise as substitutes for conventional oils due to their low levels of volatile organic compounds (VOCs), low compressibility, high dielectric strength, and good emulsification. Also, they outperform traditional lubricants with chemical modifications (for high thermal stability and oxidative stability) and the application of the right additives for load-bearing and friction qualities [13].

Therefore, investigation of lubricant electric discharge, bearing currents, lubricant instability, and common mode voltages needs serious attention, which this study intends to provide. Hakim El Bahi [40] affirmed that an ideal dispersant/detergent ratio used in the formulation of a mineral oil-based hybrid car transmission fluid allowed the fluid to achieve good anti-rust properties and low electrical conductivity, which has not been fully investigated for bio-base lubricant. According to [49], high adhesion, non-corrosiveness, and moisture resistance have all been demonstrated to be advantages of some bio-lubricants and lithium grease, making them suitable for a variety of applications. Grease made of aluminum and urea also performs well, with the primary drawback of being incompatible for some applications, and thus restrictions have been placed [25]. Based on the reviewed works, expectations from the use of poor lubricants and incompatibility with the machine elements are illustrated in Figure 4, as well as their resultant effects.

4. Major Bio-Lubricant Challenges

The features of bio-lubricants make them an excellent choice for industrial application, especially due to their ecofriendly nature. There are more than 300 oil-bearing plants that can take the place of traditional mineral lubricants. These feedstocks have varied degrees of vulnerability for industrial applications depending on the region of production, but few will be covered for the purpose of this research. Erhan et al. [50] reported that lubricants have inherent advantages in terms of their technical qualities, such as high lubricity, high anti-wear properties, high viscosity index, high flash point, etc. Bio-lubricants have numerous advantages [51], but they also have certain drawbacks, such as poor cold-flow characteristics and oxidation stability, leading to polymerization and deterioration. Human food makes up the majority of the biomass resources used to create bio-lubricants, which could cause a food scarcity catastrophe. However, there are several inedible biomass feedstocks that can be chosen to reduce this danger [50,51].

Due to a number of variables, bio-based lubricants currently only make up a small portion of the market. The obvious causes would be related to crude vegetable oils’ poor low-temperature characteristics and oxidative stability. Regarding pour point, various studies have found that at temperatures below 10 °C, vegetable oils first become cloudy, then precipitate, and finally solidify, making it difficult for them to flow under low temperature conditions. This resulted from the consistent stacking of the triglyceride backbone “bend” during the development of macro-crystalline structures. Due to the loss of kinetic energy experienced by each molecule during self-stacking, the presence of these macro-crystals would eventually limit the capacity of vegetable oil-based lubricants to flow easily [50,52].

According to Scrimgeour [53], Fatty acid (FA) alkyl chains have low oxidative stability because they are easily oxidized at their double bonds and the nearby alkylic carbons. Equations (1)–(9) describe a simplified reaction of the FA autoxidation process and a description of the oxidation products [53]. The production of oxidation products has a significant impact on the viscosity of vegetable oil. The byproducts of secondary oxidation include molecules with high molecular weight, including ketones (RCOR). According to Lubis et al. [54], these chemicals can engage in additional chemical processes to produce carboxylic acids that can thicken oil. The finding on thickening tendency of bio-lubricant carboxylic acids was supported by Shahbazi et al. [55].

Jagadeash et al. [56] affirmed that oxidation-induced free FA production (a byproduct of hydrolysis and hydrogenation) increased wear; thus, the corrosive effect on metal surfaces is accelerated by free FAs. They also ascertained that FFAs are released from triglycerides by hydrolysis and the removal of hydrogen processes; products of polymerization were formed at high pressure and temperature at the latter stages of the oxidation process. Due to their acidic character, the formulations of these products enhance boundary lubrication capabilities while increasing wear on surfaces.

• Initiation (formation of free radicals)

$$RH\to \dot{R+{HOO}^{.}}$$

(1)

2.

Propagation or branching (chain reactions of free radicals)

$$R^{.}+O_2\to \dot{{ROO}^{.}+{RO}^{.}+H_{2}O}$$

(2)

$${ROO}^{.}+RH\to ROOH+R^{.}$$

(3)

$$ROOH\to \dot{{RO}^{.}+{OH}^{.}}$$

(4)

$${RO}^{.}+RH\to \dot{ROH+R^{.}}$$

(5)

$$2ROOH\to \dot{{ROO}^{.}+{RO}^{.}+H_{2}O}$$

(6)

3.

Termination (formation of nonradical products)

$${ROO}^{.}+{ROO}^{.}\to ROOH+O_2$$

(7)

$${ROO}^{.}+R^{.}\to ROOH$$

(8)

$${RO}^{.}+{RO}^{.}\to 2R$$

(9)

4.

Peroxide decomposition: ROO → Various lower molecular weight compounds

$$Polymerization:ROOH \to Various higher molecular weight compounds$$

The problem of agglomeration as a result of inadequate solubility during operation is another significant challenge with lubricants [21,43,57]. According to Lijesh et al. [58], poor solubility and agglomeration of MWCNTs in aqueous fluids (caused by their high surface area and surface activity) has been a significant barrier to their use in tribological applications (such as bearings, gears, and transmission). Agglomerated nanoparticles produce micron-sized particles of irregular shape, which wear down the system more quickly than base oil and cause it to operate less efficiently. In solving this, Nasreen et al. [59] and Gulzar et al. [60] affirmed that surfactant has been found to be effective in improving the stability of nanoparticles in fluids.

5. Major EV Components and Lubricant-Associated Challenges

5.1. Batteries and Motors

Despite the benefits electric vehicles (EVs) have over conventional combustion engine vehicles, such as zero emissions, lower operating costs, better comfort, and the lack of cooling circuits, gear shifts, and other components, there are significant problems with the management of the batteries and motors. Corrosion and heat oxidation are two important issues that hamper EV systems and are shown in Figure 5. Corrosion, mostly caused by lubricants, has become a key focus for the EV industry [61]. As mentioned above, vehicles’ batteries and electrical components are much more prone to corrosion as a result of being exposed to severe working conditions with increased levels of dust, dirt, moisture, and salts. Significant temperature variations that occur during production, transportation, and operational procedures also raise the possibility of corrosion. Since all of these elements raise the risk of EV corrosion, a thorough corrosion control system needs to be put in place. Electric vehicles (EVs) use an electric motor as a generator to produce electricity when the vehicle is in motion [23]. Voltage regulators, motor inverters, and controllers for regulating energy flow and regenerative braking are additional parts that support motors. These components enable the electric vehicle to run smoothly and recover energy as efficiently as possible. The parts must be devoid of corrosive substances, such as lubricants that could develop during connection, because of their sensitivity. When it comes to battery protection, slow flame propagation and robust sealing against humidity are the most crucial requirements. Water seeping into a battery cell increases the likelihood of a malfunction. The battery can easily reach temperatures above 1000 °C in the event of a malfunction, which could result in a fire.

In mitigating the corrosion effect, vapor corrosion inhibitors (VCIs) should be applied to the coating of top and bottom of EV batteries, as illustrated in Figure 5. According to Kim et al. [62], vapor corrosion inhibitors reduce contact with and impact on the battery by creating a vapor that fills the air inside a sealed container or enclosure, preventing corrosion. The vapor forms a strong bond with the metal surfaces, promoting and maintaining the natural and healthy passive oxide layer on the metal surface [61,63,64]. As described by ZERUST [61], this vapor forms a thin, invisible film that protects the metal from corrosion; thus, it is useful for protecting metal surfaces that are in contact with water or other corrosive substances. For electric vehicles, VCIs protect various metal components, such as wiring, connectors, and other electrical components, from corrosion.

Through the interaction of inadequate lubricant with machine elements, metallic materials undergo electrochemical corrosion processes, which can be further subdivided into oxidation (anodic reaction) and reduction (cathodic reaction) reactions. However, the creation of numerous tiny compounds, such as peroxide, alcohols, aldehydes, acids, esters, and hydroxy acids during the thermal oxidation of lubricating oil has significant effects on lubrication performance [65,66,67]. Additionally, these tiny molecules are polymerized into larger molecules, which form sludge or carbon deposits [67]. According to Sun et al.’s [68] research, adding heterogeneous Cu and Fe oxidizing catalysts sped up lubricating oil degradation. Due to an increase in frictional force at the sliding parts and rapid occurrence of corrosion, higher lubricating oil viscosity and acid value may eventually have a negative effect on the performance of EVs [69,70,71]. Thus, it is essential to examine the oxidation processes of EV lubricating oil and the appropriate strategy for reducing the incidents. Figure 6a,b show the corrosion reaction of iron and copper materials when they are lubricated with oil that contains a lot of water.

Again, as described by Opia et al. [15] and Tung et al. [72] in their research on the functions of lubricants, which include cooling, lubricating contact bodies to reduce the heating effect, cleaning, etc., EV lubricant, however, provides a contribution to addressing the TR problem, despite the fact that the advancement of EVs has resulted in significant improvements in their fluid cooling system. The temperature of the EV battery may rise as a result of excessive friction from the lubricating parts of EVs, owing to increased frictional energy [73]. Several circumstances that result in uncontrolled heating can generate battery TR, as in a lithium-ion cell, described as an interpretation of the heat generation mechanism from T1 to T2. Accordingly, SYSANELE was the primary source of heat generation between T1 and T2. Therefore, between T1 and T2, the exothermic reactions occurring within SYSANELE serve as the primary source of heat for both the cells with the NCM cathode and the cells with the LFP cathode [74].

Apart from crushing or piercing incidents and cell abuse, such as overcharging or overdischarging, which results in copper plating at the cathode or lithium plating at the anode, contact with leaked conductive lubricant can all lead to internal short circuits. Additionally, an external short circuit has been documented to occur through the release of corrosion, water, or the interaction with conductive lubricant into the battery [75,76]. In the initial state of SYSANELE, there is a graphite surface, which is normally covered by a continuous SEI coating. The SEI film breaks at roughly T1 in temperature. The electrolyte and intercalated lithium react with one another to produce apparent heat, which the pulverized SEI sheet cannot avert [75]. A cell enters the heat–temperature–reaction (HTR) loop when its temperature rises above the point at which it can function safely. This self-amplifying process occurs when heat causes chemical reactions that result in heat-related exotherms, which in turn cause more heating, which prompts yet more reactions. The temperature increases exponentially as a result of the HTR loop’s output, TR. According to Galushkin et al. [77], the exothermic breakdown processes are controlled by Arrhenius reaction kinetics according to Equation (10).

Q(T) = Q_(0^exp)·(−Ea/RT)

(10)

where Qo is the pre-exponential constant, Ea is the activation energy, R is the universal gas constant, and Q(T) is the heat transferred at a particular temperature (T).

5.2. Elastomers Seals

Different EV parts, such as the gearbox, steering gear box, wheel bearings, actuators, etc., require elastomers seals, either for reciprocating or rotating motion, as presented in Figure 7. They have the ability to push fluid (oil, grease, coolant, water, etc.) towards the opposite side of the seal, forming an elasto-hydrodynamic lubricating film between the seal and shaft surfaces with relative motion [78]. In these systems, the torque sensor, assist electric motor, controller, pinion-and-rack or worm gear reduction mechanism, steering column unit (consisting of the steering wheel, torsion bar, and universal joints), servo unit (consisting of these components), and tie rods are usually the key parts [47,79]. Commercially accessible seals for automotive applications come in a variety of types and materials. The most popular materials used to make seals today include rubber, ethylene-propylene-diene monomer (EPDM) [80], nitrile-butadiene rubber (NBR), neoprene (poly-chloroprene or chloroprene rubber, CR), fluorelastomer (FKM) [81], silicone rubber (vinyl-methyl silicone, VMQ), and polytetrafluoroethylene (PTFE) [43,48].

The compatibility between the seal and the lubricant is what determines performance [23,82,83]. With time, poor compatibility causes swelling and seal breakdown. Therefore, compatibility should be the main factor taken into account when choosing or developing a seal or lubricant. The greatest notable improvements in dynamic seal performance recently came from using surface texturing techniques on shafts or seals [36,79,84,85]. The properties of these materials might be taken into account for the creation of new seal choices for EV component parts. Modifying the roughness of surfaces, creating new composite seal materials, and using sophisticated lubricants can all help reduce friction and boost seal longevity. However, while designing novel seal materials or lubricants, research into their compatibility under actual working conditions should be given priority.

Figure 7. Some important EV components requiring good elastomer for better protection from damage [83].

Figure 7. Some important EV components requiring good elastomer for better protection from damage [83].

5.3. Constant-Velocity Joints

Constant-velocity joints are made and utilized to transmit power across two shafts that are axially misaligned [40,86]. All modern vehicles, including the majority of EVs, use them to carry power from the primary driveshaft to the wheels. The connections are made up of a variety of rolling parts, kept together by a retaining cage situated between two raceways. The entire assembly is grease-filled and enclosed in a rubber boot. The cage operates through contact between the raceway grooves; the rolling components can be balls, as demonstrated in Figure 8.

The rolling components move back and forth with minimal oscillation, placing a heavy pressure on the ball race contact and moving at a slow speed, which might be ruled by the boundary lubrication (BL) regime. These operating settings lead to relatively high friction, failures brought on by contact fatigue, and raceway and rolling-element wear [38,87]. However, as a result of the performance and compatibility of the components with lubricant/grease, damage to the boot/elastomer seal may occur, leading to failure in these mechanical components [37]. Through optimizing the rolling components, especially the lubricants, the lubricity and service life of grease will be achieved, thus enhancing the boot’s durability. From this, it is possible to reduce friction and extend the service life of constant-velocity joints.

Due to substantial advancements in materials and technology, such as composites [88,89], low-friction bearings [24,90], magnetic bearings [91], power electronics, and control approaches, flywheel systems have lately come back into focus as a promising application for the optimal performance of EVs. In order to attain the best possible performance of the specified characteristics, appropriate low-viscosity lubricants are used. The reduction in friction losses in bearings and the improvement of power transmission efficiency, which still need a lot of research, are the primary tribological problems. The introduction of suitable materials could minimize these losses, which are electrical (hysteresis, eddy current, copper), mechanical (drag, bearing, friction), and power converter-related (switching and conduction) [9,92].

6. Formulation of Suitable Lubricants for EV Application

For an EV/HEV lubricant formulation that is thermally and energetically efficient, there are many factors to take into account. The thickness of the film will be lowered by the use of low-viscosity fluids. Lynne Peskoe-Yang [93] comfirmed that a thinner coating will operate at a higher temperature, which will shorten the bearings’ fatigue life. Narita and Takekawa [16] stated that due to intermolecular collisions, using organic molecules with a longer chain and less branching will improve heat transfer in lubricant formulation. Korcek et al. [94] discovered that low levels of phosphorus or sulfur can have a significant negative impact on components. As a result, dialkyldithiophosphates and other anti-wear and antioxidant additives may not be employed in future formulations of EV lubricants. As comfirmed by Syahir et al. [13], bio-based lubricants have shown promise as substitutes for conventional oils due to their low levels of volatile organic compounds (VOCs), low compressibility, high dielectric strength, and good emulsifiability. The adopted technique, together with the required characteristics needed for functional lubricants for EV application, is demonstrated in Figure 9. Vegetable lubricants can perform better than standard lubricants with chemical modifications (for high thermal stability and oxidative stability) and the addition of suitable additives for load-bearing and friction qualities [13].

Due to the prevalence of grease use, it will be crucial to comprehend the basic grease lubrication mechanism and the theoretical techniques used to anticipate future EV/HEV performance. For greases, a better blend of thickeners, BOs, and additives is desired to provide lower torque characteristics. There will be a need for new grease compositions that can tolerate extreme temperature changes and strong shear. The use of environmentally friendly and biodegradable greases will increase. Reformulating the greases, coolants, and gear oils is necessary due to the novel and variable EV/HEV design. The longevity, water resistance, load-bearing capacity, corrosion resistance, and low-temperature performance of the grease will all be critical considerations in electric vehicles (EVs), which will make grease usage very important [93]. Furthermore, the mechanical (hardness, crack resistance, and tensile strength) and electrical properties of EV/HEV component parts should not be altered by grease formulations. Additionally, due to the variety of EV components and designs, it is highly desirable to have application-specific specialized greases rather than manufacturing generic ones [39].

7. Future Work

Concern over the environmental effects of fossil fuels is growing, and a new era of clean energy and transportation is dawning on the world. The use of renewable energy sources, such as solar and wind, is steadily increasing, causing a shift toward this new era of green energy. In recent years, the commercialization of HEVs, PHEVs, and BEVs has increased exponentially, reaching 2.3 million sales in 2020 [95]. This is a significant indication of this growing “green conscience.” Future advancements in battery and lubricant technology are necessary as green energy and EV commercialization continue to expand. Although further research is needed to completely eliminate cobalt in order to manufacture cobalt-free batteries, high-nickel cathodes seem like promising materials for future LIBs. The lack of cobalt guarantees better LIB performance and energy output, as well as a reduction in mass and cost. It is anticipated that LMNO and LMR, two new cathodes, will not be used in EVs until the late 2020s. This is because of several research obstacles related to metal dissolution and unstable surface chemistry, as well as the absence of significant advancements in electrolyte and separator technology. However, recent studies have shown the advantages of Fe doping for LNMO59 and PVP ((C₆H₉NO)_n) molecule bridging/coating for stability and cycle improvements; thus, the potential use of these cathodes is still encouraging.

Future advancements in battery and lubricant technology are necessary as green energy and EV commercialization continue to expand with required compositions as presented in Figure 10. There has been a discernible shift in recent decades toward identifying the best additives, particularly nano-compounds. The percentage of nano-additives, which improve a variety of lubricant characteristics such as density, viscosity, specific heat capacity, anti-wear, COF, and thermal conductivity, is shown in Figure 11. The goal of contemporary additive research is to address the issues of instability and inadequate dispersion. The electrochemical and thermal characteristics of greases, which have not been thoroughly investigated during use in ICEVs, will be the primary determinants of EV lubrication.

Figure 10. Major areas needing improvement for EV performance enhancement.

Figure 10. Major areas needing improvement for EV performance enhancement.

Figure 11. Expected proportion of additives for adequate EV lubricant formulation [96,97].

Figure 11. Expected proportion of additives for adequate EV lubricant formulation [96,97].

The focus of tribology in the EV industry is on characterization centered on desirable properties, such as viscosity index, electrical and thermal conductivity, and specific heat capacity, even though the basic structure of greases remains intact, with the lubricant being made up of BO and additives. Important additives for EV lubricants will undoubtedly be developed in the similarly expanding field of nanotechnology; TiO₂ and MoS₂ are already showing promise. As the EV business ages, its future focus will be on a combination of the best surface-coating materials, battery compositions, and electrolyte salts. As sufficient batteries start to emerge from development, other technologies, such as battery cooling systems and dynamic wireless power transmission, will also start to receive more attention.

8. Conclusions

Several reasons have been highlighted to explain the growing electrification of the fleet of passenger cars, the most prominent among them being significantly increased energy efficiency; potential reductions in CO₂ emissions; local emissions of NOx and particulate matter being potentially zero; lower total cost of ownership; and the swift advancement of battery technologies to keep up with the current global sustainability initiatives. Along with this technological drive, tribological problems arise with BEVs and PHEVs, especially for hybrid electric vehicles and the gearbox of an electric car. Despite not requiring engine oil, a BEV typically requires thermal fluid to maintain the battery’s optimal temperature range while both driving and fast charging. New fluids are therefore required for these applications, in order to address these issues. For tribologists and fluid developers, the ever-evolving automotive ecosystem will provide new problems, and future research on the effects of electric and magnetic fields on lubricants and fluids is expected to grow.