Rechargeable lithium-ion batteries (LIBs) have been recognized as the most successful energy storage devices with an energy density increase of about 7-8 Wh kg^-1 per year, now reaching between 260 and 270 Wh kg^-1. However, the massive growing market of portable electronic devices, electric vehicles, stationary grid storage, etc. and the safety problems resulting from this technology involve demand for alternatives. Among the intensely studied technologies (i.e., Na-ion batteries, K-ion batteries, Li-sulphur batteries, solid state batteries, Li/Na-air batteries, etc.), Na-ion batteries (NIBs) represent a promising alternative to Li-ion batteries because sodium is widely available and exhibits similar chemistry to that of LIBs.

However, a major challenge of this technology lies in the development of anode materials because graphite, the reference anode used in Li-ion batteries, intercalates only small amounts of Na ions. Hard carbon (HC) has been found to be more suitable anode candidates, being able to reach specific capacities of around 300 mAh g^-1. This is possible due to its disordered complex structure combining graphitic domains and micropores, being recognized as a non-graphitizable carbon. Moreover, they can be synthesized from a large number of bio-sourced precursors (i.e., cellulose, carbohydrates, bio-wastes, plants, etc.), offering thus the possibility to develop eco-friendly anodes.

In this context, our team proposed to develop hard carbons from a series of bio-sourced precursors: natural polyphenols¹ and biopolymers. More precisely, five natural tannin based-polyphenols and five biopolymers were selected to synthesize hard carbons, following a single pyrolysis process at 1500°C, under Ar. Their carbon content, morphology, structure and texture were studied by several techniques. These complementary analyzes confirmed the formation of typical hard carbon structure but also the presence of some residual inorganic compounds, inherently induced by the parent precursor.

HCs electrochemical performance were evaluated versus Na metal, in coin cells. Reversible capacities of around 300 mAh g^-1 were obtained for most of the materials with an initial Coulombic efficiency between 81% and 88%.

Finally, the electrochemical performance was plotted versus different physico-chemical properties of the hard carbon materials and several strong correlations could be found.

This simple approach provides new possibilities for the use of bio-sourced precursors as electrode materials for NIBs, as they provide a reliable, natural, renewable, nontoxic, and low-cost resource for hard carbon production.