Two-dimensional Electrides Demonstrate 30% Friction Reduction Via Interlayer Electron Redistribution

Friction represents a significant global energy loss, motivating the search for materials exhibiting superlubricity, and recent research focuses on two-dimensional electrides as promising candidates. Jingcheng Qi, Giuliana Materzanini from Université Catholique de Louvain, Gian-Marco Rignanese from Université Catholique de Louvain and WEL Research Institute, Maria Clelia Righi from University of Bologna, and Junjie Wang from Northwestern Polytechnical University, investigated the frictional behaviour of these layered materials, revealing a surprising link between interlayer electrons and reduced friction. The team demonstrates that the frictional force correlates with the charge of the material’s layers and how that charge shifts during movement, and they discovered that barium nitride (Ba2N) exhibits unexpectedly low friction despite strong adhesion between its layers. This anomalous behaviour stems from the redistribution of electrons during sliding, which dissipates energy and enables remarkably smooth movement, even achieving near-frictionless sliding under specific conditions, and establishing 2D electrides as a potentially transformative platform for designing energy-efficient materials.

Barium Nitride’s Electronic Structure Enables Superlubricity

This research investigates the potential for superlubricity in barium nitride (Ba2N), a two-dimensional (2D) electride material. The study reveals that Ba2N’s unique electronic structure, specifically excess electrons forming a 2D electron gas, can lead to exceptionally low friction. Researchers demonstrate that these excess electrons significantly influence interactions between layers and the energy landscape, reducing the potential energy barrier for sliding and lowering friction. Applying pressure further enhances this superlubricity by modifying the electronic structure and creating a more favorable energy landscape for sliding, potentially eliminating friction altogether. The electronic reconstruction occurring between Ba2N layers plays a crucial role, as the excess electrons redistribute to minimize repulsive forces. The team highlights the possibility of tuning friction properties through doping, offering a pathway to control and optimize its superlubricating behavior, potentially surpassing the performance of materials like graphene and MoS2.

Electride Layering Dictates Friction and Energy Loss

This study pioneers a novel approach to achieving superlubricity using two-dimensional (2D) electrides, materials with layered structures and confined electrons acting as anions. Researchers established a strong correlation between interlayer friction and both the charge of the cationic layers and how charge redistributes during sliding, revealing that electron redistribution dominates energy dissipation. By quantifying charge redistribution, the team observed a high correlation between charge evolution and energy barriers during sliding. To investigate stacking configurations, scientists developed a machine learning force field validated against accurate calculations, enabling the simulation of twisted bilayer Ba2N containing a large number of atoms.

Researchers constructed numerous Ba2N bilayer models with varying twist angles, demonstrating that incommensurate twisted interfaces, with angles between 2° and 58°, achieve structural superlubricity by suppressing deformation and energy fluctuations. Notably, a critical normal load enables barrier-free sliding in aligned Ba2N, resulting in an ultralow shear-to-load ratio, suggesting the potential for scalable superlubricity through twist engineering, load adaptation, or electrostatic gating. This work establishes 2D electrides as a transformative platform for energy-efficient tribology and advances understanding of electron-mediated friction.

Barium Nitride Rivals Graphite’s Low Friction

This research establishes two-dimensional electrides as a promising new platform for achieving remarkably low friction and, potentially, superlubricity. Scientists systematically investigated a series of compounds, including barium, strontium, and calcium electrides with nitrogen, phosphorus, and arsenic, revealing unique frictional properties governed by structural parameters and electron redistribution. Calculations demonstrate that barium nitride (Ba2N) exhibits surprisingly lower interlayer friction than graphite, defying conventional tribological understanding. The team discovered that the key to Ba2N’s performance lies in the redistribution of electrons during sliding, which becomes the dominant pathway for energy dissipation rather than mechanical resistance.

Deep potential molecular dynamics simulations revealed that twisting the Ba2N bilayer to angles between 2° and 58° effectively suppresses deformation, enabling structural superlubricity. Manipulating the electron density within these 2D electrides through doping effectively reduces interlayer friction by modulating the energy differences between different stacking configurations, opening possibilities for scalable superlubricity through twist engineering, load adaptation, or electrostatic gating. This work advances fundamental understanding of electron-mediated friction and positions Ba2N as a model system for designing cost-effective, high-performance materials for energy-efficient tribology.

Electride Layers Enable Near Frictionless Motion

This research establishes two-dimensional electrides as a promising new platform for achieving superlubricity, a state of nearly frictionless motion. Scientists have demonstrated that interlayer friction in these materials correlates directly with the charge of the cationic layers and how that charge redistributes during sliding. Notably, barium nitride (Ba2N) exhibits unexpectedly low friction despite strong interlayer adhesion, defying conventional understanding of material friction. Investigations reveal that this behavior stems from electron redistribution becoming the dominant pathway for energy dissipation during sliding.

Through a combination of advanced computational methods, the team discovered that twisting the layers of Ba2N between 2 and 58 degrees achieves structural superlubricity by preventing deformation and energy fluctuations. Furthermore, applying a specific normal load enables virtually frictionless sliding in aligned Ba2N layers, resulting in an exceptionally low shear-to-load ratio. The researchers also found that introducing additional electrons into the material effectively reduces friction by modifying the stacking energies. While the study focuses on Ba2N, the findings suggest that this approach can be broadly applied to other 2D electrides.