# Ultrafast lithium diffusion in bilayer graphene

## TL;DR

This paper demonstrates that bilayer graphene acts as a single-phase mixed conductor with ultrafast lithium diffusion, surpassing bulk graphite and even liquid water diffusion, using innovative device architecture and magnetotransport measurements.

## Contribution

It introduces bilayer graphene as a true single-phase mixed conductor with ultrafast lithium diffusion, and develops a novel electrochemical cell architecture for direct diffusion measurement.

## Key findings

- Lithium diffusion in bilayer graphene exceeds that in bulk graphite by an order of magnitude.
- Diffusion in bilayer graphene surpasses sodium chloride in water.
- The device architecture enables direct in-plane diffusion kinetics measurement.

## Abstract

Solid mixed conductors with significant ionic as well as electronic conduction play a pivotal role for mass transfer and storage as required in battery electrodes. Single-phase materials with simultaneously high electronic and ionic conductivity at room temperature are hard to come by and therefore multi-phase systems with separate ion and electron channels have been put forward instead. Here, we explore bilayer graphene as a true single phase mixed conductor and demonstrate ultrafast lithium diffusion exceeding diffusion in bulk graphite by an order of magnitude and even surpassing diffusion of sodium chloride in liquid water. To this end, an innovative electrochemical cell architecture has been developed where the redox-reaction forcing lithium intercalation is localized at a protrusion of the device only. Its remainder consists of pristine bilayer graphene unperturbed by an electrolyte. The geometry lends itself to the use of magnetotransport machinery known from mesoscopic low-dimensional physics. Time dependent Hall measurements across spatially displaced Hall probes deliver a direct view on the in-plane diffusion kinetics. The device layout with a perimeterial electrochemical cell is transferable to other 2D materials as well as thin films and may promote a paradigm shift on the use of electrolytes in on-chip experiments.