Carbon materials play a key role in the development of electrochemical energy storage solutions. Regarding high power solutions, porous carbons are the preferred electrode materials as they can combine a high electronic conductivity with a high specific surface area for ion adsorption. In the search for high energy solutions beyond lithium, disordered carbon materials are gaining interest as graphite substitutes on account of their ability to store monovalent and multivalent cations through multiple mechanisms. Metal-ion capacitors, which are constructed using a battery-type electrode and a capacitor-type electrode (generally in the negative and positive sides, respectively), is an emerging high-power technology that aims to merge in a single device the merits of both energy storage systems, i.e., a good response at fast charge and discharge, a stable cyclability, and an energy density closer to that of batteries. In particular, dual carbon metal-ion capacitors, employing carbon materials in both electrodes, constitutes an appealing approach due to the easy tunability of the morphology, microstructure or chemical composition of carbon materials that allows to boost each energy storage mechanism, combined with the possibility of using sustainable resources and methods for their production. Remarkably, the last feature aligns well with growing awareness of environmental protection and health risk expressed in the international policies that claim for a more sustainable industry.

Herein we propose an environmentally sound and simple method for the production of energy grade carbons, that allows simultaneously for the production of high-performance, capacitor-type or battery-type carbons. This procedure entails the use of renewable carbon sources and benign chemicals, such as MgSO₄ and an alkaline chloride (NaCl or KCl). We show that the sulphate is able to act as a highly effective sulfur dopant for the production of S-doped hard carbons whilst, with the increase of the temperature, it also acts as a porogenic agent, giving rise to highly porous carbons (S_BET>2000 m² g^-1). The assistance of an inert salt helps modulate the morphology of the resultant carbon materials to reduce diffusion resistance (both through the porous system or in the solid state). The melting of KCl at high temperatures has also been shown to boost the porogenic effect of the sulphate. As a proof of concept, both types of materials were jointly tested as Na-ion capacitor electrodes, exhibiting a good energy/power performance (38 Wh kg^-1 at 22 kW kg^-1), as well as a very low capacity fade of 0.00078 % per cycle.