Materials and Society: The Circular Economy (SAM13) Edited by Jean-Pierre Birat, Gael Fick, Mauro Chiappini, Valentina Colla, Andrea Declich, Barbara Fornai, Dominique Millet and Leiv Kolbeinsen REGULAR ARTICLE A review of hard carbon anode materials for sodium-ion batteries and their environmental assessment Jens F. Peters 1,* , Mohammad Abdelbaky 1 , Manuel Baumann 2 , and Marcel Weil 1,2 1 Helmholtz Institute Ulm (HIU), Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany 2 ITAS, Institute for Technology Assessment and Systems Analysis, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany Received: 20 August 2019 / Accepted: 5 November 2019 Abstract. Sodium-ion batteries are increasingly being promoted as a promising alternative to current lithium- ion batteries. The substitution of lithium by sodium offers potential advantages under environmental aspects due to its higher abundance and availability. However, sodium-ion (Na-ion) batteries cannot rely on graphite for the anodes, requiring amorphous carbon materials (hard carbons). Since no established market exists for hard carbon anode materials, these are synthesised individually for each Na-ion battery from selected precursors. The hard carbon anode has been identified as a relevant driver for environmental impacts of sodium-ion batteries in a recent work, where a significant improvement potential was found by minimising the impacts of the hard carbon synthesis process. In consequence, this work provides a detailed process model of hard carbon synthesis processes as basis for their environmental assessment. Starting from a review of recent studies about hard carbon synthesis processes from different precursors, three promising materials are evaluated in detail. For those, the given laboratory synthesis processes are scaled up to a hypothetical industrial level, obtaining detailed energy and material balances. The subsequent environmental assessment then quantifies the potential environmental impacts of the different hard carbon materials and their potential for further improving the environmental performance of future Na-ion batteries by properly selecting the hard carbon material. Especially organic waste materials (apple pomace) show a high potential as precursor for hard carbon materials, potentially reducing environmental impacts of Na-ion cells between 10 and 40% compared to carbohydrate (sugar) based hard carbons (the hard carbon material used by the current reference work). Waste tyres are also found to be a promising hard carbon precursor, but require a more complex pre-treatment prior to carbonisation, why they do not reach the same performance as the pomace based one. Finally, hard carbons obtained from synthetic resins, another promising precursor, score significantly worse. They obtain results in the same order of magnitude as the sugar based hard carbon, mainly due to the high emissions and energy intensity of the resin production processes. Keywords: sodium ion battery / process modelling / pyrolysis / life cycle assessment / anode material / hard carbon / environmental impact 1 Introduction Sodium-ion batteries (SIB) are a recent development in the field of post-lithium batteries. These aim at overcoming limitations of existing lithium-ion batteries (LIB) in terms of resource availability, costs or performance. Following the same working principle as LIB (Fig. 1), they are considered a drop-in technology, based on similar electro- chemical processes, materials and manufacturing process- es. For SIB, the motivation for their development is the possibility of substituting lithium by sodium for the cathode and electrolyte, and of substituting copper by aluminium for the anode current collector (unlike lithium, sodium does not alloy with aluminium at the anode) [1]. The use of very abundant sodium promises advantages in terms of cost (sodium salts are a very cheap raw material), environmental impacts (lithium salt mining is more complex and requires higher inputs than sodium salt mining), resource supply (concentration of lithium in earth’s crust is 20 ppm compared to 2.4% for sodium) and safety (SIB are, unlike most LIB, not prone to thermal runaway and explosion) [2]. However, due to the higher specific weight of sodium in comparison with lithium, the achievable energy densities are lower. This was identified as a major drawback in terms of economic competitiveness with LIB, requiring further research for obtaining better performance and further reducing costs [3]. In terms of * e-mail: j.peters@kit.edu Matériaux & Techniques 107, 503 (2019) © SCF, 2020 https://doi.org/10.1051/mattech/2019029 Matériaux & Techniques Available online at: www.mattech-journal.org