Additive and Base Oil Trends in Electric Vehicle Applications

According to the US Environmental Protection Agency (EPA), the world emitted approximately 50 billion tons of greenhouse gases in 2020, and the transportation sector is a significant contributor of greenhouse gas, accounting for about 7 billion tons [1]. Around 60% of the 7 billion tons is related to road transportation by passenger cars, and a study shows that electrifying the road transportation sector can potentially reduce carbon emissions by 11.9% [1]. As a response, electric vehicles (EVs) and hybrid cars are gaining popularity worldwide. As an example, “The Inflation Reduction Act of 2022” added a “clean vehicles credit,” which promotes the purchase of EVs and fuel-efficient vehicles [2].

This global growth in EVs and hybrid cars has encouraged many global automotive manufacturers to increase their investment in electric vehicle research and production capability. Research predicts that by 2025, electric vehicles will constitute 10% of global passenger vehicle sales, escalating to 28% by 2030 and surging to 58% by 2040 [3,4]. However, one of the main concerns associated with EVs is their low driving range (versus the traditional gasoline engine re-fueling rate) and higher relative cost versus internal combustion engines (ICEs). Therefore, research should continue to strive to make EVs more affordable while improving the energy efficiency of these designs.

The International Energy Agency found that around 25% of global energy is consumed due to friction loss. Moreover, approximately 2–3% of the European Union's GDP is expended on recovering the loss caused by these frictions [5–7]. Friction loss in EVs consumes about 57% of their total energy demand. Therefore, the necessity of designing and optimizing mechanically stable, efficient lubricants is very important [8]. Over the years, lubricants and grease technologies have been developed to protect ICE systems, which are used to reduce friction between surfaces so that the vehicle can run more efficiently and lower the risk of engine wear/damage. When conventional engines are properly lubricated, they require less energy to activate the pistons, and as the pistons glide with less effort, the vehicle can operate by using less fuel and running at a lower operating temperature [9]. Unlike traditional lubricants and greases used in ICE vehicles, lubricants play a new role in EV engines: EVs require oil for the gears, which translates mechanical force through the EV's transmission, as well as a fluid designed specifically for the electric motor to improve cooling [10]. In EVs, lubricants must also withstand high temperatures (∼120 °C) caused by the operation of power electronics and contact pressure from 1.5 to 2.5 GPa [11,12] while still providing excellent wear protection for the gears. Therefore, designing lubricants optimized for EVs is a unique technical challenge stemming from the sophisticated integration of EV powertrains. Future demands may necessitate a single lubricant that not only safeguards hardware against wear under high loads but also ensures compatibility with copper and electronics components. It must effectively dissipate motor-generated heat and possess specific conductivity properties to prevent leakage and arcing.

Addressing these multifaceted requirements calls for an innovative approach to lubricant development, considering a broad spectrum of factors crucial for the optimal functioning of EVs. This comprehensive exploration begins by defining the specialized requirements of EV lubricants, emphasizing the need for high-performance solutions compatible with the intricate architecture of EV powertrains. A thorough analysis of existing lubricant solutions follows, spotlighting advanced materials, each offering unique advantages and challenges. The review then shifts to optimizing base oil selection, particularly focusing on polyalphaolefin (PAO) base stocks and their synergistic combinations with various additive technologies, highlighting recent innovations and contemplating future trends. Venturing beyond conventional paradigms, the discussion culminates in a visionary perspective on lubricants, advocating a transformative approach toward high-efficiency lubrication solutions in the realm of electric vehicles.

Many proposed lubricants for EVs cannot satisfy all the various design requirements for this application. Compared to traditional lubricants, lubricants used in EVs must work in multiple places, across various temperatures and torque loads, and in complex electrical environments. Researchers must consider the influence of both temperature and electric current when designing these lubricants. Lubricant designers must consider several challenges when creating an effective EV lubricant. The requirements, properties, and additive solutions are summarized in Table 1.

The requirement of electrical conductivity for e-mobility lubricants is stringent. If the lubricants' electrical conductivity is too high, current leakage can occur. Tang et al. concluded that lubricants attached to electric motor components, like the stator's electrical windings, must exhibit low electrical conductivity to avert short circuiting through the lubricant/assembly interface [29]. In contrast, if the conductivity of the lubricant in the engine is too low, electrical arcing can occur, accelerating the oxidation of the lubricant and potentially causing pitting wear. This in turn will reduce its effectiveness, leading to oil degradation, reducing the protectiveness of the fluid [30]. Lubricant formulators must use caution in balancing lubricant design properties to ensure an optimal level of electrical conductivity. Materials like graphene, silver powder, and carbon black can be incorporated into lubricants to tune appropriate electric conductivity. Graphene and silver powder provide excellent electrical properties, while carbon black is known for its ability to maintain resistivity.

Because of the unusual application conditions, lubricants for EVs must remain stable and effective not only under wide temperature ranges but also possibly in a fluctuating electrically charged environment. As Fig. 1(a) displays, if the lubricants are not stable under high temperatures, the whole system (lubricant and mechanical components) will degrade to failure [31]. At the same time, under a high electric charge, the lubricant film could collapse and cause lubricating starvation. When exposed to high-temperature conditions, these collapses will lead to the generation of microbubbles between two lubricant layers (Fig. 1(b)). The formation and bursting of microbubbles disrupt stable lubrication, resulting in increased noise and vibration within the bearing [32–34]. Lithium or calcium complexes, chlorinated paraffins, and red phosphorus can be used to resolve the issue. These additives improve the lubricant's ability to withstand higher temperatures and electrical loads while reducing the risk of fire.

Maintaining grease/lubricant stability for a long-duty cycle is another primary target for lubricant formulators. Some materials perform well initially, but after a long-running interval, some undesirable chemical reactions occur, which might lead to the denaturation of lubricants that influence performance. In addition, for EVs, preventing copper corrosion is a critical performance parameter. EVs require a substantial amount of copper used in batteries, windings, electric motors, wiring, busbars, and charging infrastructure [35]. Most copper-based components are contained on the grease-lubricated surfaces. If the lubricant induces copper corrosion in these parts, it will lead to contamination and, ultimately, failure of both the lubricant and the corresponding mechanical system.

Lubricating greases need to be designed with excellent chemical and mechanical stability, while protecting the equipment against corrosion. In addition, lubricating greases need strong oxidation stability and need to be capable of maintaining effective lubrication in the presence of water.

Oxidation resistance is the lubricant's ability to resist degradation when in contact with oxygen at higher temperatures. Adequate water handling is crucial to maintain lubricity and to protect the metal surfaces against rust and corrosion, while chemical and mechanical stability ensures that the lubricant does not degrade over time leading to the formation of undesired deposit and/or varnish.

Effective strategies to address these potential issues in a grease formulation include the selection of an optimized thickener system. For example, aluminum complex greases are the best at handling water, but because of an inherently lower dropping point, such greases are not ideal for higher temperature applications. However, calcium sulfonate thickener systems are preferred for applications demanding extreme pressure (EP) or high load resistance, corrosion protection, and optimal water handling. However, calcium sulfonates do have some limitations when it comes to lower temperature applications. Lithium-based greases (simple or complex) are the most widely used thickener systems due to balanced performance across most load conditions and applications. Finally, the role of the polyurea thickener should be considered. Polyurea greases are excellent in handling higher temperature applications, have good shear/mechanical stability, and are often selected in fill-for-life applications. Polyurea greases do not contain metals in the thickener matrix, and as a result, they are more durable and therefore suitable for some fill-for-life applications. Without a significant amount of metals embedded in the lubricant, polyurea greases do not catalyze decomposition reactions later in the life of the grease. Some of the downsides associated with polyurea greases are the generally higher cost and the manufacturing process, which involves toxic reagents.

Once the grease thickener is selected, the role of the additives will refine the performance of the finished product, tailoring it for a specific application. Therefore, when thinking about the needs of a grease designed for EV applications, it will require a certain amount of corrosion inhibitors such as imidazoline, which are used to improve copper protection. Aminic anti-oxidants are known to provide strong oxidation resistance, and finally, ZDDP (zinc dialkyldithiophosphate) and MoS₂ can contribute synergistically to overall chemical and mechanical stability [36,37]. These generic concepts can help define the starting point of a new development program aiming to find the right balance between the chemical structure of the thickener matrix and the selection of the various additives to meet the requirements of a specific application.

Furthermore, enhancing the efficiency and extending the lifespan of EV powertrain components necessitates reducing friction and wear. This goal requires lubricants that maintain a stable friction coefficient and promote effective tribofilm formation. A consistent friction coefficient is crucial for performance and energy efficiency, while tribofilm formation helps create a protective layer on surfaces, thereby minimizing direct contact and wear. 2D materials, such as MoS₂, graphene, and MXene, are particularly effective in achieving these requirements [18,24,26,38]. Known for their excellent lubricating properties, these materials provide both friction reduction and wear protection by forming robust protective layers that significantly reduce wear.

In the lubricant market, thickeners are essential, distinguishing lubricant properties and applications with two primary classifications: soap based and nonsoap based. Soap-based thickeners, including the widely used lithium based, are renowned for good water resistance, high-temperature performance, and shear stability with variants like lithium complex and lithium hydroxy stearate. Contrastingly, nonsoap-based thickeners like clay, calcium sulfonate, and silica offer excellent higher temperature resistance with clay also exhibiting remarkable water absorption, making these nonsoap-based thickeners ideal for extreme-pressure applications and high-heat environments like oven chains and bearings. Calcium sulfonate greases are experiencing a new wave of attention in the industry as an alternative to lithium-based greases, since lithium availability has contracted due to demand into other applications. However, calcium sulfonate greases are usually recommended for extreme applications requiring high loads but operating at low/medium speeds. They are also often preferred to lithium-based greases in applications related to the food industry since they do not require performance additives containing zinc or phosphorous. This is because the calcite thickener can confer those benefits inherently. Calcium sulfonate greases are not likely to be considered in EV applications, which are often characterized by high speed and lower loads compared to what is generally needed in the heavy-duty industry. In contrast, polyurea stands out in this category, delivering high shear stability and resistance to oxidation and chemical breakdown, thus being preferred for demanding settings like high-speed bearings and gears.

When focusing on EV lubrication, the choice between soap-based lithium thickeners and nonsoap-based polyurea thickeners is crucial. Lithium thickeners are the most used in lubricating rolling bearings, and their thermal stability makes them an excellent contender for lubricating EVs. Currently, the lubricant industry is the third largest lithium consumer and lithium is the primary thickener system for EV greases [39–41]. Lithium greases are produced by the saponification of lithium hydroxide and hydroxy stearic acid, which forms a simple lithium thickener matrix. This matrix can be further reacted with a complex diacid molecule such as diazaleic acid to form a complex lithium grease. Depending on the application, the lithium complex will have stronger mechanical stability compared to a simple lithium grease. Both lithium and lithium complex enable a high dropping point, increased resistance to moisture, and adequate thermal stability [40]. These systems offer a balanced blend of higher temperature endurance, shear stability, and water resistance, suitable for a broad range of EV applications. The popularity, the history of experience, and the formulation flexibility of lithium-based thickener systems enable the specific lubrication needs to be met with precision. In contrast, while polyurea-based thickeners excel in high-temperature performance, shear stability, and oxidation resistance, their higher cost and limited availability make them less favorable. Despite the advanced capabilities of polyurea, the practicality, affordability, and established dependability of lithium-based thickeners give them a distinct advantage, making them the current preferred solution in EV lubrication.

Graphene is an ideal additive for EV lubricants due to its lubricity, high thermal and electrical conductivity, and exceptional mechanical strength, which collectively enhance efficiency, wear resistance, and thermal management [26,42,43]. In addition, chemical stability and compatibility with advanced materials also ensure long-term durability and effective integration with modern EV components.

In a recent study, Lin et al. proposed using graphene with lithium-based grease to reduce friction and enhance antiwear (AW) properties. At a lower rotational speed, lithium-based grease can bleed enough oil into the system to avoid wear; however, the addition of graphene makes it mechanically stable in high temperatures and speeds by retaining more oil and forming a protective layer between two interacting surfaces [26]. The study used functional graphene nanomaterials (FGNs), which slow the sedimentation process and disperse well in the lubricants. Preparing graphene at high temperatures homogenized the mixture, and the hydrophilic nature of this mixture enabled the FGNs to dissolve into the grease easily. The four-ball test confirmed the nano grease to be uniform and therefore ensured the antifriction properties [26]. Figures 2(c) and 2(d) show that a homogeneous and condensed network was created due to the presence of graphene. Also, the thickener fibers from Figs. 2(a) and 2(b) have collapsed, forming a denser structure that heavily impacted the grease characteristics.

Furthermore, the result in Fig. 3(a) illustrates that for slow-moving, heavily loaded (SMHL) objects, the coefficient of friction (COF) and wear scar diameter (WSD) decreased significantly as graphene (0.07 wt%) was added to the grease, compared to the control lubricants. However, as shown in Figs. 3(b) and 3(c), for median speed, heavily loaded (MHL) objects, and high-speed loaded (HSL) objects, 0.15 wt% and 0.3 wt% of graphene produced the best results [26].

Since vehicles similar to EVs are reported to require friction reduction at speeds up to 25,000 rpm [44], it can be inferred that the addition of 0.15 wt% of graphene could improve reliability results for EVs as well. Apart from that, EV powertrains involve significant electrical components, and graphene's electrical conductivity can help prevent static buildup and ensure smooth operation of the electric motor and other components [45,46]. However, in a rotating bearing, as the rotational speed increases, the contact frequency between two surfaces increases, and the centrifugal force on the grease increases as well, potentially leading to grease starvation. In such a situation, an optimal amount of graphene retains extra grease and minimizes wear scars [47]. Figure 3(d) supports this claim because wear volume is lowest when the rotational speed is the highest after adding 0.3 wt% of the graphene, where HSL got a 0.088 wear volume.

Moreover, graphene with a metal oxide composite can perform even better under heavy loads. A study led by Jin et al. proposed using Mn₃O₄/graphene, and the result improved significantly. They synthesized the nanoadditive by electromagnetic stirring, ultrasound vibration, and three-roll milling [48], following the protocol summarized in Fig. 4(a). Mn₃O₄/graphene can be synthesized through the in situ green method, which introduces Mn to the graphene oxide (GO) synthesis process and improves the stability under low load, but the effectiveness of such synthesis for heavy loads has not been examined [49]. Here, the hydrothermal process was utilized to prepare the additives to ensure mechanical and thermal stability under high pressure and a heavy load by ensuring high crystallinity, uniform nanoparticle distribution, and strong interfacial bonding.

Tribological evaluations were performed employing the SRV4 tester (Optimol Instruments, Germany) under a ball–disc contact configuration. Friction coefficients across varying loads were quantitatively assessed, and subsequent wear scar topographies were examined utilizing a 3D white light interferometer. The study found significant improvement when nanoadditive was introduced to the system, and low concentrations (0.02 and 0.03 wt%) of additives effectively reduced friction and wear scars under loads of 700 N. However, they lost their stability under heavier loads, as shown in Fig. 4(b). Commercial graphene (0.1 wt%) was able to reduce COF by 9.3%, whereas 0.1 wt% of Mn₃O₄/graphene reduced the friction by 43.5%, proving its superior antifriction properties. Load conditions markedly affect the lubrication efficiency of Mn₃O₄#G additives. Figure 4(c) illustrates the tribological behaviors of grease under loads from 600 to 900 N, with 50 N increments. Grease with 0.1 wt% Mn₃O₄#G maintained a stable mean COF (0.11–0.12), whereas the average COF of plain grease (PG) varied significantly (0.14–0.18). Below 750 N, Mn₃O₄#G decreased the mean COF by about 37.1%, but this reduction tapered off at higher loads.

The wear spots on the spherical sample serve as direct indicators of the greases' lubrication quality. Figure 5 presents a clear depiction of the wear scars' original morphology and wear spots, focusing on three Mn₃O₄#G additive concentrations (0.0 wt%, 0.05 wt%, 0.1 wt%). The wear spot diameter diminishes significantly by 35.2% (from 1043.2 to 676.3 µm), with the incorporation of 0.1 wt% Mn₃O₄#G, compared to PG as shown in Figs. 5(b) and 5(f). With 0.05 wt% additive, oxidation lessens, yet “burned” oxidation surfaces surround the wear, as presented in Fig. 5(d). Conversely, at 0.1 wt% additives, only minor oxidation and small scratching occur on the wear spot's inner and outer surfaces. Incorporating 0.1 wt% Mn₃O₄/graphene notably reduced wear scar width by 39% and depth by 86.1% under heavy loads, although higher concentrations adversely affected tribological properties. The 0.1 wt% addition significantly curbed oxidation, suggesting the potential for corrosion reduction in EV applications.

Additionally, graphene's electrical conductivity could foster well-balanced formulations, optimizing material compatibility and electrical properties for EV powertrains [50]. The observed benefits primarily stem from a synergistic tribofilm effect, where graphene's island effect acts as a barrier and the additives fill in, minimizing surface interactions. This positions graphene as a promising EV lubricant candidate. However, the study's limitation lies in overlooking additives' thermal stability, focusing mainly on moderate temperatures. Consequently, further research should explore these properties and tribological behaviors across varying temperatures to draw conclusive insights.

Every vehicle relies on the seamless movement of ball joints and tie knots, necessitating the use of robust lubricants to facilitate this motion and ensure component longevity. In this context, the strategic integration of additives into grease plays a crucial role. These additives, particularly in the form of nanoparticles, are pivotal in enhancing the lubricant's tribological properties – improving wear resistance, reducing friction, and ultimately extending the operational life of vehicular components.

Metal oxide nanoparticles are highly suitable as additives in EV lubricants due to their unique properties that enhance the performance and longevity of the powertrain. Metal oxide nanoparticles, such as titanium dioxide (TiO₂) and zirconium dioxide (ZrO₂), are known for their high hardness and mechanical strength [51,52]. When incorporated into lubricants, these nanoparticles create a protective layer on metal surfaces, which minimizes direct contact between moving parts and significantly reduces wear. In addition, metal oxide nanoparticles can endure high temperatures without breaking down, making them ideal for the high-temperature conditions commonly found in EV powertrains. This thermal stability ensures that the lubricant retains its protective qualities even under extreme conditions. Furthermore, metal oxides like zinc oxide (ZnO) offer strong oxidation resistance even at high temperatures [53]. This is crucial because oxidation can lead to corrosion, potentially damaging components and shortening the lifespan of the powertrain. The robust protection against oxidation provided by these nanoparticles helps to prevent such damage.

In a comparative study, tribological characteristics of lithium grease with nanoparticle additives were quantitatively assessed, with findings depicted in Fig. 6. Results indicated a reduction in coefficients of friction for greases with nanoparticle additives compared to pure lithium grease, with optimal concentrations identified at 1.5 wt% for CuO and 1.0 wt% for TiO₂, as shown in Fig. 6(a). The CuO-enhanced grease demonstrated superior antiwear performance compared to TiO₂, as detailed in Fig. 6(b), a phenomenon attributed to the lower hardness of CuO nanoparticles facilitating their penetration into the contact surface. A study conducted by Wozniak et al. suggests that TiO₂ and ZrO₂ mixed with lithium lubricants can enhance the performance of the tie rod ends [54]. The result in Fig. 6(c) showed that the addition of 1 wt% TiO₂ (32 mm³) leads to less wear compared to ZrO₂ (48 mm³), which is almost two times less than pure lithium grease. This also reduced COF by at least 50% when 1 wt% of ZrO₂ was added, and the addition of 1 wt% of TiO₂ decreased COF by 20%. Since the study integrated the component of the tie rod-end with a polymer insert and allowed the cylindrical rotational motion, the design of the experiment emulated “real-life” mechanical conditions. In addition, other academic/industry studies have used similar mechanisms and claimed similar results [54–56]. These additive/grease combinations are available for adoption in the EV lubricant market today.

Harnessing the unique advantages of core-shell nanoparticle structures presents a revolutionary approach in lubrication technology, promising enhanced performance and longevity of mechanical components. The study introduced soft-core with hard-shelled composite nanoparticles (ZnO@SiO₂) synthesized via chemical deposition, marking a novel addition to grease additives. These composite nanoparticles exhibited improved dispersion and tribological properties over their physically mixed ZnO/SiO₂ counterparts. The friction test results, presented in Fig. 6(d), indicated that both nanoparticle types reduced the mean COF of lithium grease, with the most significant reduction occurring at a concentration of 0.6 wt%. Specifically, lithium grease containing 0.6 wt% ZnO@SiO₂ composite nanoparticles showed a 15.6% decrease in COF compared to the original formulation. Moreover, at all examined concentrations, ZnO@SiO₂ nanoparticles demonstrated superior performance in lowering COF compared to the physically mixed nanoparticles [57].

The growing interest in using 2D materials for tribological applications stems from their unique features, such as large surface area, strong and flexible structure, low friction, and high resistance to heat and chemicals, significantly improving the durability and efficiency of moving parts. The application of 2D materials such as graphene, MoS₂, MXene, and boron nitride (BN) in electric vehicle lubrication provide significant advantages due to their unique properties. These materials have a layered structure, where the layers are attached by weak van der Waals forces [15]. This structure provides lubricity, which allows the layers to easily slide over each other, therefore reducing friction losses [57]. In addition, the strong covalent bonds within each layer ensure the formation of robust protective films, minimizing wear and thus extending the lifespan of powertrain components [26]. Furthermore, 2D materials exhibit high thermal conductivity due to the strong covalent bonding within the plane and the efficient phonon transport, which aids in effective heat dissipation [58]. This thermal conductivity is crucial for maintaining optimal operating temperatures in EV powertrains, preventing overheating and ensuring consistent performance. Collectively, these properties demonstrate support for consideration and selection of 2D materials over traditional additives in EV lubrication applications.

One study led by Wu et al. proposed using NbSe₂ and BN to improve the electrical conductivity and enhance the tribological performance of lithium-based grease [15]. Both NbSe₂ and BN have hexagonal layered crystal structures, which enable them to have a high degree of crystallinity, helping them to achieve good electrical and thermal conductivity. The electrical conductivity of the lubricant is reliant on the concentrations and conductivity of the additives; therefore, increased concentrations (2 wt%–4 wt%) of NbSe₂ reduced the volume resistivity of the lubricant by tenfold compared to pure grease as shown in Fig. 7(a). BN increased the volume resistivity by 1.6 times. It shows that NbSe₂ has high conductivity since fewer electron traps were created [15], which is described well by the permeation theory.

In addition, the study looked at the effect of BN and NbSe₂ on the COF and wear properties, as shown in Figs. 7(b) and 7(d). It was found that 2 wt% of NbSe₂ was the optimal concentration, which reduced COF by 28.2% and wear scar by 18.7%. The result showed that these additives were very stable under heavy loads, and they did not increase the wear scar significantly as displayed in Figs. 7(c) and 7(d). One of the additional challenges associated with lubricants is their thermal instability, but NbSe₂ presented exceptional thermal stability in the finished grease at 2 wt% concentration [15].

Lithium-based greases are one of the most conventional and commercial choices for the e-mobile industry. However, as the lithium price increases and the elements' availability continues to decrease, the EV industry should proceed with caution to embrace it fully [40,59]. There should be more research to find the most cost-effective and efficient grease. Also, the environmental effect of such greases needs to be considered, and holistic research should be conducted to incorporate previous findings with the new results.

Ionic liquids (ILs) possess a set of remarkable properties, making them ideal candidates for high-performance lubricants. These include exceptional thermo-oxidative stability, absence of ash content, intrinsic polarity, nonflammability, and low volatility [60,61]. Ionic liquids exhibit outstanding resistance to thermal and oxidative degradation. This stability is crucial in EV powertrains, which can experience high temperatures during operation. Unlike traditional lubricants that may break down and form sludge under high thermal and oxidative stress, ILs maintain their integrity, ensuring consistent lubrication performance over a wide temperature range. This property not only extends the service life of the lubricant but also enhances the overall reliability of the powertrain. The intrinsic polarity of ILs significantly enhances their lubricating performance. Polarity allows IL molecules to strongly adhere to metal surfaces, forming stable tribolayers. These tribolayers act as protective films that reduce direct metal-to-metal contact, thereby lowering friction and wear. ILs exhibit stable viscosity over a broad temperature range. This ensures that the lubricant remains effective in reducing friction under both low- and high-temperature conditions, which is particularly important in the varying operational environments of EV powertrains.

The study synthesized and evaluated three protic ionic liquids, specifically dodecylamine salts with dialkyldithiocarbamate (DTC) by varying alkyl chain lengths: ethyl (DDED), butyl (DDBD), and octyl (DDOD). These compounds were assessed for their efficacy as additives in lithium complex grease intended for steel/steel contact. Their tribological characteristics were assessed using an Optimol SRV-I friction/wear tester and an MRS-10A four-ball tester. The investigation, detailed in Fig. 8, focused on the coefficients of friction and wear volumes of grease with varying concentrations (1–5 wt%) of these additives. Results indicated a notable reduction in COF, likely due to the formation of adsorbed films by the IL groups and lower shear strength tribochemical films by the DTC groups. The friction-reducing effects were similar across the additives, suggesting a minimal impact of DTC chain lengths on this property. Moreover, friction coefficients stabilized at concentrations above 1 wt% as shown in Fig. 8(a). Wear volume analysis in Fig. 8(b) revealed significant reductions upon additive incorporation, with similar trends across different DTC chain lengths, indicating that chain length does not significantly affect the antiwear properties during sliding friction.

Due to ILs' inherent polarity, rendering them immiscible with nonpolar hydrocarbon oils (groups I, II, and III), researchers have predominantly examined ILs' behavior in these oils as oil–IL emulsions [58,59]. Deviating from traditional approaches, one study synthesized three phosphonium-based oil-soluble ionic liquids with varied molecular structures, employing them as additives in PAO lithium-based grease [59]. Unlike typical additives, these synthesized ionic liquids engaged in the saponification process, integrating seamlessly into the entanglement network of the grease's thickener. The tribological behavior of these novel greases was assessed through a four-ball friction tester, concentrating on the synthesized ionic liquids: [P₈₈₈₁₄][DEHP], [P_888p][DEHP], and [P₈₈₈₁₄][AOT]. Tribological outcomes, illustrated in Fig. 8(c), demonstrated that the base PAO grease initially presented high and fluctuating friction coefficients, with a WSD near 0.76 mm. However, the introduction of [P₈₈₈₁₄][DEHP] and [P_888p][DEHP] significantly lowered and stabilized the friction coefficient to approximately 0.05, and notably reduced the WSD and volume. When compared to greases containing the conventional EP antiwear additive ZDDP, the greases with [P₈₈₈₁₄][DEHP] and [P_888p][DEHP] showed superior performance in terms of lower friction and wear as shown in Fig. 8(d). Meanwhile, incorporating [P₈₈₈₁₄][AOT] into the grease notably enhanced its properties in reducing friction and resisting wear, yet its performance in terms of WSD and volume was somewhat inferior when compared to ZDDP, indicating a relative limitation in wear protection efficiency.

Optimizing lubricants and greases for EV engines and transmission systems requires new approaches and innovative base stocks and additives. One common approach to developing lubricants for EVs is leveraging specific additives to enhance the properties of the base oil mixture. However, particularly in the case of a grease lubricant, it is important to be aware that the primary component that impacts overall performance is the base stock. The additives provide some directional improvements, but the base stocks will determine the overall performance and expected life of the grease and the equipment. Shah et al. emphasized that the base stocks for EV lubricants should possess strong molecular structures and maintain stable viscosity across diverse temperature ranges; other shortcomings in the base oil mixture properties will be overcome by incorporating a range of additives [33]. This conclusion encourages researchers to focus on two main fields to improve EV lubricant performance – finding suitable additives and selecting the proper base stock combinations to enable the desired performance properties for this application.

The standard of suitable dispersant/detergent additives is stringent and complicated. In 2013, early efforts were underway to provide the basic requirement for EVs using lubricant additives. To satisfy these standards, Tang et al. analyzed the influence of calcium detergents and dispersant additives synthesized from the reaction between hydro-carbyl-dicarboxylic acid or anhydride and polyamine, examining both electrical conductivity and antirust performance. The base stock utilized in this study was mineral oil, which was selected to control the cost of the finished fluids. Finally, a hybrid electric vehicle transmission fluid, HEVTF was designed by his team. The formulation based on mineral oil featured low viscosity, outstanding antiwear, and pressure capabilities, minimal electrical conductivity (1700 pS/m), and adequate copper/rust protection. More importantly, compared with traditional lubricants, the properties of HEVTF demonstrated far greater resiliency upon varying temperatures and aging [29].

Despite these early observations, as the growth and innovation of electric vehicles have improved, the requirements for additives have also become more complicated. One significant purpose of using lubricants in EVs is cooling. Therefore, lubricants must have outstanding thermal conductivity and thermal capacity. According to Christensen et al. [13], these properties are primarily examined by the molecular structure of the base stocks, and that is the reason why researchers are now recognizing the benefit of improving thermal properties by leveraging synthetic base stocks.

The early studies geared toward the development of fluids suitable for EV application were often conducted using mineral-based stocks to control the cost of the finished product [29]. However, as hybrid and electric equipment became more sophisticated, the demand for improved performance in terms of electrical conductivity, and thermal capacity combined with a need for increased hardware protection and excellent energy efficiency drove the researchers to consider synthetic base stocks like PAO.

A finished lubricant formulated with PAO base stocks will leverage the benefits of the PAO molecules offering lower inherent MTM (mini traction machine) traction/coefficient of friction, ensuring superior oxidative stability, and exceptional thermal performance. In addition, PAO-based grease can also offer benefits in energy efficiency, which translates into lower energy consumption and supports longer vehicle driving range intervals.

Lotfi [60] has tested a novel low viscosity, low volatility (LVLV) PAO across a range of comparative tests and determined that MTM traction/coefficient of friction benefits can be observed at both 40 °C and 80 °C versus Grp II+/III + equivalent viscosity blend. These data represent ∼50% reduction in traction coefficient at 35% slide to roll ratio (SRR), 40 °C versus Grp II+/III + blend, and 64% reduction at 80 °C. The MTM results at 80 °C are shown in Fig. 9(a).

Furthermore, transmission fluids formulated with either LVLV PAO or conventional PAO 4 are compared evaluating oxidation stability using the CEC L48 test method. The results are summarized in Fig. 9(b) where the novel PAO material clearly outperformed the conventional PAO 4 blend measuring the ΔKV40 °C and ΔKV100 °C, after the test was run for 384 h at 170 °C.

Moreover, Calderon Salmeron and Leckner at KTH in Sweden developed a novel bearing test rig to illustrate preliminary energy efficiency benefits measured under different operating conditions, sweeping from low to high speed, aiming to simulate the conditions experienced by the bearings in an electric motor [61]. In this example, they analyzed the effect of the grease thickener, comparing two candidates manufactured in PAO with either polypropylene or simple lithium. The methodology is very promising, as they were able to differentiate two very similar greases and extract the energy efficiency as an indirect function of the measured power loss. In this instance, the polypropylene grease showed a 21.5% lower energy consumption as compared to the lithium-based grease (Figs. 9(c) and 9(d)). A more recent publication by the same group expanded their comparison of a different thickener system to include polyurea in combination with polypropylene and lithium. They analyzed the efficiency of each simple grease under dynamic conditions rather than conducting the experiment under static conditions. In this second phase of their work, Calderon Salmeron et al. observed that the polyurea grease was the one with the lowest bleeding, which resulted in the highest energy consumption. Polypropylene-based grease exhibited the best performance, with lithium-based greases following very closely [62].

Future work from this group is looking at the effects of the base stock selection while keeping the thickener selection constant in this matrix. Some of the candidates being evaluated include LVLV PAO molecule versus PAO 6, and Grp III (ISO VG 30, Polyurea thickener system).

In addition, synthetic base stocks like PAO offer excellent electrical properties. Liu et al. [63] studied the effect of low-viscosity PAO, which also offers low permittivity and good thermal properties, and they observed a significant improvement in terms of friction and breakdown field strength. They started their analysis by comparing PAO 4 versus air cool/lubricated ball on a disc, and then they explored the effect of viscosity going from PAO 4 to PAO 8 and PAO 10. Not surprisingly, the efficiency dropped when using higher viscosity fluids, but they also observed a reduction of the triboelectrical signal. There is a fine delicate balance to be identified based on the need to protect the equipment while pursuing the best possible efficiency output (Fig. 10).

The combination of traction benefits, conductivity stability, and thermal performance properties illustrates the multidimensional benefits of PAO for EV lubricants/greases; these base stocks offer cutting-edge performance and could be complemented further by novel additive technologies as discussed earlier [63].

The benefits of using, in most cases neat PAO, can be expected to manifest in the finished grease formulations with comparable magnitude. For example, a work conducted by Kuzhyil and Tian at ExxonMobil showed how the friction benefits offered by a PAO-based oil mixture are significant as compared to the friction observed when using a mineral-based oil mixture, as illustrated in Fig. 11(a). Similar benefits are observed in Fig. 11(b) when the same mixtures are used to manufacture a Lithium Complex grease (LiX Grease) [64].

The role of the more traditional additives is going to be critical in the development of next-generation lubricants. Yang et al. evaluated the feasibility of using triazine derivatives as a PAO additive. The study revealed that the triazine derivatives exhibited remarkable antiwear and extreme-pressure characteristics, with the additives maintaining stability and not decomposing until temperatures exceeded 200 °C [65]. At the same time, the development of nanotechnology provides a new view to help researchers make improvements for lubricant additives. Nanotechnology, which operates on structures less than 100 nm, enables the meticulous design and functionalization of molecules at a microscale. This field notably enhances the potential of 2D materials like MXene and graphene, each heralded for their distinctive properties and wide-ranging applications in various domains [18,45,66]. Shah et al. pointed out that more recent fluid engineering efforts are exploring the utilization of nanomaterials that can help to manipulate the characteristics of the standard base oil [33]. This indicates that the combination of nanoparticles and lubricants will probably become a novel direction for the future work. Some researchers have already shown that nanotechnology can make a tremendous contribution to improving lubricants' performance by targeting EV applications. For instance, some early experiments have found that adding nanoparticles to lubricants can significantly increase the thermal conductivities of driveline lubricants [18,56]. This can be considered a very valuable tool that can help a manufacturer to tune in the desired level of conductivity based on the specific demands of the target application.

The development of carbon nanotubes (CNTs) and graphene also attracted fluid engineering researchers' attention. CNTs and graphene have various excellent chemical properties, such as high thermal conductivities and outstanding stability, which are suitable to be used as lubricant additives. Choi et al. specifically investigated the impact of introducing suspended carbon nanotubes, noting a substantial 160% enhancement in the thermal conductivity of PAOs [67]. The significance of this particular result can become more obvious when looking at Fig. 12(a). The dotted line at the bottom, which is also zoomed in on the insert, represents the thermal conductivity increase expected when using a certain treat rate of CNT, ranging from 0% to 1%. The expected improvement was expected to be about 10% when using 1% of the experimental CNT. However, the measurement shows a very significant increase up to 160%. This behavior clearly shows that CNT can be used to deliver a specific conductivity profile of the finished lubricant using a low amount of this additive.

2D nanomaterials are recognized for their exceptional thermal conductivity, primarily due to the robust covalent bonds within their atomic layers. These strong interatomic connections facilitate efficient vibrational energy transfer across the planes, significantly enhancing thermal conductivity [68]. In the context of novel 2D materials such as Ti₃C₂T_x, a member of the MXene family, notable increases in thermal conductivity have been observed, as presented in Fig. 12(b) [18]. Specifically, PAO-Ti₃C₂T_z nanofluids exhibited a 7.1% enhancement in thermal conductivity with a 0.01 wt% Ti₃C₂T_z infusion, reaching a peak increase of 23% at a 0.04 wt% loading. In studies focusing on hexagonal boron nitride (hBN), the thermal diffusivity of nanofluids compared to pure PAO was measured using a laser flash apparatus, varying with temperature [20]. This allowed for the calculation of the nanofluids' thermal conductivity, which showed significant enhancements with the addition of hBN, as shown in Fig. 12(c). For instance, a 1 vol% hBN infusion elevated the thermal conductivity of PAO by over 20%. In contrast, metal oxides are generally less conductive, a trait attributable to their electronic structure where electrons are tightly bonded to atoms, creating stable ionic or covalent bonds. This bonding pattern generates a band gap between the valence and conduction bands, typical of insulators or semiconductors [69,70]. However, certain conditions can induce conductivity improvements in metal oxides. For example, a study revealed a noticeable thermal conductivity enhancement in a 10 wt% ZrO₂ nanofluid sample with nanoparticles averaging 10.9 nm in diameter [19]. This enhancement was uniquely observed in the 10 wt% ZrO₂ concentrate sample, as depicted in Fig. 12(d), indicating a distinctive behavior among metal oxide-based nanofluids.

Another new approach to enhance the performance of EV lubrication additives is functionalization, a process that finely tunes their properties to meet specific requirements. It is possible to design a new set of nanoadditives that can satisfy the various needs of EVs using lubricants. For example, the functionalization and modification specialization of CNTs and graphene have been proven effective and applied to some fields [71,72]. Ali et al. analyzed how copper/graphene additives influenced grease's friction performance and got a positive result as shown in Fig. 13(a): Cu/graphene nanolubricants can make up to a 32.6% reduction of COF [73]. Particularly, it is very interesting to see the real-life effects of the Cu/graphene additives and how they influenced the COF over time, simulating a piston stroke. Similarly, in Fig. 13(b), it is possible to observe how a multiwall carbon nanotube or other type of nanoparticles can improve the thermal conductivity of the bulk lubricant at very low treat rates.

However, using such an additive for greases designed for EV/electrified conditions is still not available since the carbon nanotubes and graphene will also bolster the lubricant's electric conductivity to a potentially undesired level [13]. As is often the case, the formulation of a lubricant requires striking a delicate balance of positive performance while minimizing the side effects of an additive. Therefore, we cannot ignore the benefits associated with the utilization of graphene and carbon nanotubes, which do merit further evaluation in combination with certain base oils and additives to maximize their potential impact.

In recent years, research has focused on developing biolubricants as replacements for conventional petroleum-based lubricants such as mineral and synthetic oil [75]. Compared with pursuing pure performance improvements, the advantages of biolubricants are related to their biodegradability attributes, while still being suitable for EV applications. Biolubricants are often derived from either bio-sourced or renewable raw material and are therefore, inherently more biodegradable, compared to conventional petroleum-based lubricants.

However, biolubricants in this application still face some critical obstacles. According to Shah et al., because of the structural nature of triglycerides, or vegetable oils, biolubricants have the shortcomings of low thermal stability and poor lubrication at low temperatures [33]. Biolubricants must be compatible with the materials used in EV powertrains, including metals and polymers. This requires careful formulation to prevent degradation or adverse reactions. Research to overcome biolubricant disadvantages is ongoing, and as of today, most bio-based lubricant technologies may need to rely on the utilization of nanoadditives [33]. Further research is being conducted focusing on improving their thermal stability, low-temperature performance, and overall efficiency through advanced additive technologies. For instance, the performance of vegetable oil can be improved by using additives such as graphite nanoplatelets (GNPs) and hBN.

GNPs have emerged as a pivotal nanoadditive for enhancing the lubrication of renewable lubricants like vegetable oils [76]. Pin-on-disk tests revealed that integrating GNPs into canola oil reduces the coefficient of friction by minimizing direct surface contact, as illustrated in Fig. 14(a). Notably, as GNP concentration increases, COF decreases, highlighting GNPs' role in filling inter-asperity valleys and forming a tribolayer, aligning parallel to motion and easing shearing stress. As depicted in Fig. 14(b), wear-rates in samples with GNP-infused nanolubricants are significantly lower than those with neat oil, across various loads. This reduction escalates with higher GNP concentrations because GNPs fill microgaps and nanogaps on surfaces, smoothing and shielding them, hence reducing wear. For instance, at 15 N load, 0.1 and 0.3 vol% GNP concentrations reduce wear volume by 78% and 99%, respectively, versus neat oil. The wear-rate versus GNP volume percentage graph is U shaped, indicating an optimum GNP concentration for minimal wear in canola oil at 0.3 vol%. Beyond this, wear-rates rise due to excessive GNPs leading to agglomeration, impeding the protective tribolayer, and increasing wear [76].

Samples of nanofluid, comprising functionalized hBN nanoparticles dispersed in castor oil, were prepared, utilizing Silane coupling agent A-151 to bolster compatibility and stabilize the suspension within the base oil [77]. The coefficient of friction was highest for pure castor oil, and it notably dropped for the 1 wt% hBN-infused nanofluid. As depicted in Fig. 14(c), the coefficient of friction for unmodified castor oil averaged at 0.0692; however, the 1 wt% nanofluid significantly lowered this value to 0.0483, marking a 30.2% decrease. The COFs for the 2 wt% and 5 wt% hBN nanofluids were 0.0587 and 0.059, showing reductions of 15.2% and 14.7%, respectively. Wear analysis demonstrated that castor oil alone resulted in the most substantial wear, as depicted in Fig. 14(d). The cross-sectional wear profiles showed maximum depths of 70.64 µm for unmodified castor oil and notably less for the nanofluids: 38.56 µm for 1 wt% and approximately 53.26 µm for both 2 wt% and 5 wt%. The 1 wt% nanofluid exhibited the least wear, 51.74% lesser than that of pure castor oil, with the 2 wt% and 5 wt% nanofluids reduce wear by nearly 28.5% compared to pure oil. At lower hBN nanoparticle concentrations, a rolling effect predominates, diminishing both friction and wear. However, as hBN levels rise, a polishing effect emerges, increasing both friction and wear. The interplay between these rolling and polishing mechanisms dictates the overall friction behavior, varying with hBN nanoparticle concentration [77].

Grease, composed of base oil, thickener, and additives, is pivotal in lubrication systems. While there is a trend toward using vegetable oil as a base stock, attention is also turning to metal-free thickeners to complement this shift. The lubricant industry is increasingly concentrating on developing base stocks from renewable resources, aiming for entirely bio-based grease formulations [78,79]. This necessitates not just renewable base stocks but also thickeners derived from sustainable materials. In line with this approach, cellulose pulp from the Eucalyptus Globulus plant has been explored as a raw material for creating a biodegradable thickener system, suitable for grease manufacturing [80]. Further research has also delved into optimizing this cellulose pulp, proposing it as a viable candidate for producing bio-based thickeners. Moreover, natural wax, extracted from agricultural waste, presents itself as a promising option for biolubricants, aligning with the industry's objectives [78].

In recent years, water-based lubricants have been gaining popularity due to their high fluidity, inherently lower toxicity nature, and exceptional cooling properties [71]. In vehicles, the radiator plays an important role in maintaining the optimal temperature as mechanical loads increase. Water-based lubricants have excellent thermal conductivity, which aids in efficient heat dissipation and helps maintain optimal operating temperatures in EV powertrains. Moreover, electrically conductive grease significantly boosts performance, extends longevity, and fosters cost-efficiency for electric vehicles [14,81]. This grease is crucial for parts requiring not just reduced friction and wear but also electrical and thermal conductivity, such as power transmission components in transformation equipment and integrated circuits [13]. For instance, conductive grease in electronics serves as a grounding medium. When applied to ball bearings, it prevents static charge accumulation, directing any buildup to dissipate through the bearing, ensuring operational safety and equipment protection [13]. In such cases, water lubricants might play an essential role in maintaining heat transfer and electric conductivity for appropriate operating systems [14,82,83].

One solution for conducting grease is a glycerol aqueous solution, renowned for its excellent low-temperature performance and antimicrobial properties, which outshines mineral oils in applications [13,84]. This mixture achieves remarkably low resistivity by optimizing hydrogen bond interactions between glycerol and water. For instance, a 1:1 water–glycerol solution with a 4.5 wt% hydroxyl-grafted multiwalled carbon nanotube (MWNT-OH) concentration registers a resistivity of 20 Ω cm [13]. Notably, a glycerol–water mixture (3:1) with the same MWNT-OH concentration achieves the lowest resistivity at 10.0 Ω cm, significantly lower than traditional PAO oil with 7.5 wt% MWNT-OH, which shows a resistivity of 4500 Ω cm [13]. Conductive grease ensures efficient electrical conductivity between components, which is crucial for maintaining the integrity of electrical connections in EV powertrains. By minimizing electrical resistance, conductive grease helps improve the efficiency of the electrical system, reducing energy loss and enhancing overall performance. Conductive grease aids in the dissipation of heat generated by electrical components, preventing overheating and maintaining optimal operating temperatures. In electric vehicles, conductive grease is especially important for high-voltage and high-current applications. It ensures that these critical components function reliably and efficiently, contributing to the overall performance and safety of the vehicle.

Incorporating dextran-modified MoS₂ (MoS₂-dex) and GO into a 1:1 water–glycerol solution significantly influenced friction coefficient and wear reduction [84]. The stable aqueous dispersion of GO and MoS₂-dex prompted an investigation into their combined effects in glycerol-based lubricants. Figure 15(a) depicts that 0.5 wt% GO and MoS₂ individually performed similarly, initially reducing friction for 1200 s before reverting to baseline levels, suggesting the formation of protective but fragile films on metal surfaces. However, 0.5 wt% MoS₂-dex, especially when mixed with GO, markedly diminishes friction, with the optimal blend of 0.5 wt% MoS₂-dex and GO halving the glycerol solution's average friction coefficient to 0.075. This blend also significantly enhances the AW property, as demonstrated in Fig. 15(b), improving it by over 76% with MoS₂-dex and 92% with the MoS₂-dex/GO mixture, surpassing the wear reduction of standalone GO (21%) and MoS₂ (35%). The superior tribological performance of MoS₂-dex and its mix with GO is likely due to MoS₂-dex's small size and high dispersion stability, enabling it to fill surface gaps and form protective films. Besides, it involves GO's extensive surface area supporting MoS₂-dex on wear surfaces and inhibiting MoS₂ oxidation, while MoS₂-dex prevents GO fragmentation [84].

Another promising approach leverages a mixture of water and ethylene glycol (EG), known for its high boiling point and effective heat transfer properties, which becomes particularly beneficial in cooling systems, such as battery packs in electric vehicles [13,83]. Its inherent electrical conductivity adds to its utility. A mixture comprising 25% water, 50% glycerol, and 25% ethylene glycol, when combined with 4.5% MWNT-OH, presents a resistivity of 16.8 Ω cm [13]. Pure ethylene glycol with a 12.53% MWNT concentration demonstrates a resistivity of 46.0 Ω cm, illustrating the potential of these fluid mixtures in applications requiring electrical conductivity [13]. In the radiator system study, the performance of EG–water-based GO nanofluids was evaluated, with Fig. 15(c) detailing the impact of GO nanofluid mixtures at various EG–water ratios (60:40, 30:70, and 20:80) on the rate of heat transfer across various flowrates. Among these, the 60:40 ratio demonstrated the most favorable performance, showing a notable increase in the heat transfer rate, especially at higher volumetric flowrates. This enhancement is primarily due to the nanoparticles facilitating thermal energy exchange, the Brownian motion of particles, and an increase in thermal conductivity [83]. Another study focused on the lubrication performance of EG aqueous solutions at different concentrations, with results shown in Fig. 15(d) [85]. For EG concentrations between 10 and 50 wt%, the coefficient of friction starts high, similar to water, but decreases over time due to an effective running-in process, eventually dropping below 0.01, indicating high lubricity. However, for EG concentrations exceeding 50 wt%, the COF swiftly stabilizes, resembling pure EG's experimental outcomes.

Various kinds of additives can be added to water-based lubricants to enhance their performance [86–88]. One study led by Fan et al. proposed in situ preparation of multifunctional water-based additives using benzotriazole (BTAH) and cationic materials tetrabutyl ammonium hydroxide (N4444OH) or tetrabutyl phosphonium hydroxide (P4444OH), which can be used to enhance the tribological performance of water lubricants without significant added costs [22]. To understand its tribological impacts, they performed friction and wear tests under moderate to heavy loads, and a corrosion test was performed to investigate its resistivity under different contact angles. The proposed additive is hydrophilic and, hence, easily soluble in water, making it convenient to be used with water-based lubricants. From Figs. 16(a) and 16(b), it can be concluded that both P4444BTA/water and N4444BTA/water, at any concentration, showed their ability to reduce COF compared to pure water. The optimal concentration for both additives was 0.03 mol/L because it provided the lowest COF and wear level. While testing antiwear properties, it was found that any amount of P4444BTA/water and N4444BTA/water outperformed conventional water-only lubricants. The contact angle between surfaces was lowest when 0.03 mol/L P4444OH/H₂O and N4444OH/H₂O were added, mostly due to the absorption capability of OH- on the metal surfaces. This leads to the formation of protective layers of the friction pair surface, which reduces the COF. In addition, benzotriazole is an effective corrosion inhibitor, and its presence in the additives lowered the level of corrosion in interacting surfaces, as shown in Fig. 16(c). Due to the sensitive nature of EV lubricants in the presence of unique metals and materials, anticorrosion properties are one of the most sought-after characteristics of lubricants used in EVs.

Water-based lubricants for EVs face several significant challenges. One major difficulty is corrosion. These lubricants can promote the corrosion of metal components if not properly formulated with adequate corrosion inhibitors. In addition, maintaining consistent viscosity in water-based lubricants is challenging, especially under varying temperature conditions, which can impact its performance. Furthermore, water-based lubricants generally exhibit lower lubricity compared to oil-based counterparts, potentially leading to increased wear and reduced efficiency. Microbial growth is another concern. The presence of water can foster microbial contamination, degrading the lubricant over time. By focusing on these difficulties, the development of water-based lubricants for EVs can lead to more efficient and reliable lubrication solutions.

The importance of developing EVs, and the lubricants that support them, is recognized worldwide. The market share of electric vehicles has also expanded under the policy encouragement of the government and the investment of both R&D and production capacity of leading automobile manufacturers. This trend emphasizes the criticality of developing the lubricants and greases that harmonize and amplify the capabilities of the electric vehicle systems that they lubricate. Qualified, optimized lubricants can improve energy savings, stability, safety, and driving experience of EVs. Suitable greases must have various outstanding properties, such as high thermal conductivity, high-heat capacity, and proper conductivity, to satisfy the basic working requirement. Furthermore, lubricant designers must also ensure that the lubricants remain stable under high-temperature operating conditions and the presence of electric currents. The development of nanotechnologies, such as carbon nanotubes, graphene, and other nanoparticles, has also injected vitality into the research of improving the performance of lubricating oil additives. In addition, the utilization of polyalphaolefin base stock technology, including novel molecules such as LVLV PAO, provides new opportunities to optimize lubricants for these unique conditions. In general, developing unique and fit-for-application EV lubricants and greases is necessary for enhanced efficiency, reliability, and vehicle driving range.

In conclusion, this review navigates through the evolving landscape of EV lubrication, highlighting cutting-edge materials and strategies, and underscores the pivotal role of innovative, lubricants in advancing the efficiency of electric vehicles.

There are no conflicts of interest.

The authors attest that all data for this study are included in the paper.