Pristine and engineered biochar as Na-ion batteries anode material: A comprehensive overview

The sodium-ion battery (Na-ion battery, NIB) is considered the most promising post-lithium energy storage technology, taking advantage of using the same manufacturing technology as Li-ion batteries (LIBs), while enabling the use of more abundant and economic, thus more sustainable, raw materials. Due to the inability of Na+ ions to be intercalated within the graphene-layered structure of graphite-based electrodes (the state of art anode material in LIBs), highly disordered and microporous carbons, known as hard carbons, are considered the anode material of choice for NIB technology. Biomass-derived biochar (BC) is one of the most relevant classes of hard carbons, exhibiting a good combination of sustainable fabrication, structural-morphological features and electrochemical performances. In this review, the main achievements on BC are rigorously reported from the production to the application into NIBs, with particular emphasis on the strategies to improve the electrochemical behaviour of BC by activating it and tailoring its chemical and structural properties. These strategies include selecting specific feedstocks, modulation of the pyrolysis temperature, pre- and post-production treatments, and materials engineering. The possible role of BC in sustainable NIBs development is also briefly discussed, together with some insights of its use in other post-Li energy storage systems and some concluding remarks and future direction of the research.