There is no doubt that a rapid transition from traditional internal combustion engine vehicles to electric vehicle (EV)-based transportation can help to curtail greenhouse gas emissions and their harmful effects1. However, even though EVs have fewer moving parts than internal combustion engine vehicles, their electric motors and driveline components, power electronics and battery packs bring critical mechanical, tribological, thermal and electrical challenges2. The most desirable materials for EV components must address these complex challenges.

EV system requirements

In place of the engine, fuel tank and some ancillary systems of internal combustion engine vehicles, EVs use single, dual, or in-wheel motors, a battery pack and/or a set of hydrogen fuel cells, and a set of power electronics2. The EV powertrain consists of an electric motor coupled to a single- or multi-speed transmission. The electric motors and power electronics inherently generate stray currents that flow through the shaft, which interfere with tribological elements such as bearings and gears and cause serious damage at moving interfaces in the forms of electric discharge pitting, scuffing, scoring and fluting, as well as heat-induced weakening of the lubricant film, hence increasing friction, wear and fatigue. Collectively, these forms of damage can severely limit the proper functioning of the moving interfaces of tribological components and compromise EVs’ long-range durability and energy efficiency3.

At the same time, the use of high-current-density batteries and all-power electronics —including a DC–DC converter or DC–AC inverter, and a controller and charger, that control the electric motor and the energy management — generates a large amount of heat. Keeping the temperature of the battery pack, power electronics and powertrain optimal and stable is critical to battery life and performance. Thus, EVs require cooling fluids and material systems that quickly and efficiently dissipate the generated heat. This is achieved mainly by a cooling circuit, in which a coolant (or thermal management fluid) circulates through the battery pack, power electronics and powertrain by a network of vessels or channels. Consequently, the thermal properties (particularly, specific heat capacity and thermal conductivity), material compatibility (with wiring resins, metals and other polymers) and dielectric properties of the cooling fluids used in EVs are all key to desirable performance.

In other moving components like tyres and brakes, the existing materials challenges call for novel composites or compositions because the weight distribution of EVs is impacted by the location of the heavy battery pack and the added mass, which leads to higher torques and, consequently, slippage experienced during acceleration and deceleration.

Relevant characteristics of 2D materials

To this end, 2D materials — such as graphene, transition metal dichalcogenides (TMDs, including MoS2, WS2 and so on), hexagonal boron nitride and MXenes, among others — offer a wide range of useful characteristics (see Supplementary Table 1). Unlike typical materials used in vehicle components, which perform or specialize in one function (for example, oils that reduce friction and wear of rings and liner in internal combustion engines by forming a protective boundary film, or coolants that protect engines from overheating), 2D materials can be integrated into affected components and surfaces to protect or provide multiple functions in many different ways. They can be used as additives in the forms of suspended few-layer flakes or multi-layer particles in EV oils and greases, as protective solid lubricant coatings on surfaces and at interfaces, or as fillers in composite parts of the EV assembly (Fig. 1).

Fig. 1: 2D materials for tribological issues in electric vehicles.
figure 1

Given their versatility, 2D materials offer protection against friction, wear and corrosion in many forms depending on the specific requirements of the key components in an electric car. 2D materials can be used as additives in lubricants and coolants, or they can be applied as coatings and composite fillers directly at the contacts experiencing tribological issues.

Full size image

2D materials have remarkable mechanical and tribological properties, including high mechanical strength (for example, graphene’s intrinsic strength is 130 GPa (ref. 4)), wear resistance (for example, graphene enables three to four orders of magnitude reduction in wear of steel surfaces4) and easy-to-shear ability (for example, coefficient of friction for MXenes and MoS2 reaches 0.01 under dry conditions (Supplementary Table 1)). Given their versatility, they can protect against friction, wear and corrosion and substantially enhance the functionality of moving interfaces of EV drivetrains4,5.

In addition, 2D materials offer favourable thermal and electrical properties, even under extreme conditions6,7. For instance, graphene possesses high thermal (4,000 W m–1 K–1) and electrical (graphene is a zero band-gap material) conductivities when deposited as a coating (Supplementary Table 1), allowing for efficient heat dissipation and rapid electricity discharge to prevent arcing8,9. At the same time, most 2D materials can withstand elevated temperatures, contact pressures and electrical discharges, thus providing extended lubricity and maintaining their functionality even if the base lubricants or greases break down owing to these harsh conditions (for example, temperatures >180 °C can occur in electric motors owing to variable and harsh driving cycles).

2D materials are also known for their excellent chemical stabilities and high oxidation resistance. Tribo-corrosion (material oxidation and degradation processes promoted by loading and shearing), which may induce accelerated wear, especially under electrified conditions, is a common problem in EVs, given the severity of the mechanical, electrical and thermal stresses the components endure. 2D materials can considerably enhance protection against corrosion and tribocorrosion in these harsh operating environments by protecting the surfaces from oxygen access when integrated in the form of coatings and enhancing heat dissipation from the contacting interfaces as additives in coolants4 (Supplementary Table 2). Films made of 2D materials can also act as impermeable barriers for corrosive species (oxygen, water and electrolytes) when applied on rolling bearings and gears and electrical connectors and contacts, thus protecting the underlying substrate materials and preventing their degradation10. Moreover, the electronic properties and surface chemistries of many 2D materials are easily tuned by functionalization, allowing their charge transfer properties to be tailored to hinder the onset of corrosion and electrical discharges. These chemical stability and tunable charge transfer characteristics are of particular interest for EVs because variable driving conditions (that is, intensive torque-speed peaks for the e-motor) and regenerative braking tend to produce high levels of heat, which increases the oxidation of the lubricants being used. Compared with other common additives, the high chemical and thermal stability of 2D materials, when added to coolants, oils and greases, ensures the longevity of the lubricants and maintains their performance over an extended period.

2D material prospects in EVs

2D materials should be considered for specific functions and components based on their individual characteristics (Supplementary Table 2). For instance, highly electrically insulating 2D materials like MoS2, hexagonal boron nitride and MXenes are ideal for protecting bearings and gears from electrical discharges at the contact interfaces. The battery and power electronics require high-heat-capacity and thermally conducting coolants with high dielectric strength, ultra-low viscosity, and high boiling and low freezing points; MXene or graphene additives may be able to confer these characteristics to a base coolant. To resolve or prevent corrosion and wear at electrical connectors and contacts, coatings are needed with high electrical conductivity, good tribological characteristics and corrosion protection, which are achievable with graphene, graphene-based materials, MXenes or black phosphorous. These contacts can be additionally protected from adverse environmental impacts with electrically insulating and thermally conductive liquid or semi-solid lubricants, which can be further improved by hexagonal boron nitride, certain TMDs and MXenes. Some of these 2D materials may also be considered as higher-performing fillers to reinforce the materials for tyres and brakes.

Outlook and future challenges

EVs’ thermal, electrical and tribological challenges are wide and intertwined in many ways, rendering a single or straightforward solution impossible. In such a complex and harsh operating environment, we anticipate that the design and development of multifunctional materials offering thermal, electrical and tribological advantages will improve the efficiency, reliability and safety of future EVs. Fundamental research on 2D materials has already demonstrated their outstanding performance in these properties, laying a strong case for their use in EVs. However, to actually exploit them in these vehicles, more application-oriented research that examines their performance under more realistic EV working conditions is necessary. In this context, more specialized laboratory equipment and component- and system-level test rigs need to be developed to adequately mimic the coupled electrical and tribological stresses of EVs.