  Citation: Nieto, N.; Noya, O.; Iturrondobeitia, A.; Sanchez- Fontecoba, P.; Pérez-López, U.; Palomares, V.; Lopez- Urionabarrenechea, A.; Rojo, T. On the Road to Sustainable Energy Storage Technologies: Synthesis of Anodes for Na-Ion Batteries from Biowaste. Batteries 2022, 8, 28. https://doi.org/10.3390/ batteries8040028 Academic Editor: Torsten Brezesinski Received: 14 January 2022 Accepted: 16 March 2022 Published: 22 March 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). batteries Article On the Road to Sustainable Energy Storage Technologies: Synthesis of Anodes for Na-Ion Batteries from Biowaste Nekane Nieto 1 , Olatz Noya 2 , Amaia Iturrondobeitia 1 , Paula Sanchez-Fontecoba 1 , Usue Pérez-López 3 , Verónica Palomares 1,4 , Alexander Lopez-Urionabarrenechea 2, * and Teófilo Rojo 1 1 Organic and Inorganic Chemistry Department, Science and Technology Faculty, University of the Basque Country UPV/EHU, 48080 Bilbao, Spain; nekane.nieto@ehu.eus (N.N.); amaia.iturrondobeitia@ehu.eus (A.I.); paula.sanchez@ehu.eus (P.S.-F.); veronica.palomares@ehu.eus (V.P.); teo.rojo@ehu.eus (T.R.) 2 Chemical and Environmental Engineering Department, University of the Basque Country UPV/EHU, 48013 Bilbao, Spain; olatznoya@gmail.com 3 Plant Biology and Ecology Department, Science and Technology Faculty, University of the Basque Country UPV/EHU, 48080 Bilbao, Spain; usue.perez@ehu.eus 4 BCMaterials—Basque Center for Materials, Applications and Nanostructures, UPV/EHU Science Park, 48940 Leioa, Spain * Correspondence: alex.lopez@ehu.eus Abstract: Hard carbon is one of the most promising anode materials for sodium-ion batteries. In this work, new types of biomass-derived hard carbons were obtained through pyrolysis of different kinds of agro-industrial biowaste (corncob, apple pomace, olive mill solid waste, defatted grape seed and dried grape skin). Furthermore, the influence of pretreating the biowaste samples by hydrothermal carbonization and acid hydrolysis was also studied. Except for the olive mill solid waste, discharge capacities typical of biowaste-derived hard carbons were obtained in every case (≈300 mAh·g −1 at C/15). Furthermore, it seems that hydrothermal carbonization could improve the discharge capacity of biowaste samples derived from different nature at high cycling rates, which are the closest conditions to real applications. Keywords: sustainable batteries; Na-ion battery; biowaste; hard carbon; anode 1. Introduction The European Green Deal is the strategy of the European Union (UE) to make Europe climate neutral to 2050 and, at the same time, a world leader in climate issues. This plan is based on four main pillars: industrial innovation, bioeconomy, energy innovation and circular economy [1]. In short, there is an urgent need to use innovation to change the current status quo into a scenario where: (1) biological raw materials are used as a source of products and energy, (2) energy comes from renewable energy sources that are climate- neutral, and (3) waste is reintroduced into the economy in the form of secondary raw materials. On the road to innovation in renewable energy, it is necessary to advance in electrochemical energy storage technologies, with the aim of overcoming the limitations of intermittent production of solar, wind or wave energy. Innovation in this field currently requires the design of sustainable batteries that, among other aspects, are not based on critical materials such as lithium or that incorporate materials fabricated from biological feedstock [2]. For the past few years, sodium-ion batteries (SIBs) have received increased attention as an alternative to lithium-ion batteries (LIBs) for stationary energy storage, due to the fact that sodium and lithium have similar chemical properties, such as the intercalation chemistry and, contrary to the case of lithium, there are abundant reserves of sodium on earth [3–5]. Even if fundamental principles of SIBs and LIBs are almost the same, Na- ion batteries usually exhibit lower specific capacities, shorter cycle life and poorer rate Batteries 2022, 8, 28. https://doi.org/10.3390/batteries8040028 https://www.mdpi.com/journal/batteries