An interesting example of this multi-functionality is shown with high volume expansion materials such as Si and Ge. The development of 0D core-shell nanoparticles and 3D ‘pomegranate' structures from them, where carbon serves as a protecting and electron-conducting shell, (Fig. 4a,d) were proven to be particularly effective in improving cycle life. In this case, active material nanoparticles are able to ‘breathe' inside the carbon shell during charge–discharge without deterioration of the electrochemical properties5152 1D composite structures that use a similar approach have been designed (Fig. 4b)51. Highly conductive 2D materials, such as graphene or MXene, can serve as a backbone decorated by nanoparticles22 or alternatively some ‘spacers' can be integrated between the 2D layers to avoid restacking and improve electrochemical performance5153. Metal oxide nanoparticles encapsulated in conductive mesoporous carbon templates serve as an example of a 3D composite structure54.