2.1.2. Sodium-Ion Batteries

Recently, there have been several reports published in the literature concerning the use of hard carbon (HC), obtained from fruit wastes and fruit peels, as an abundant and lowcost material for the production of sodium-ion batteries (SIBs) [10,46–51]. SIBs are cheap and environmentally friendly energy storage tools that are alternatives to the frequently used lithium-ion batteries (LIBs). Moreover, the vast abundance of sodium resources (the sixth most abundant element in the world) compared to the limited abundance of lithium and other elements commonly used in batteries, e.g., copper or nickel, also contribute to an increasing amount of research work being published [52,53]. In LIBs, graphite is used as an anode (negative electrode) material, while in SIBs, graphite is thermodynamically unstable with sodium ions [54]. A work published by Stevens and Dahn in the year 2000 [55] started the interest in hard carbon materials as potential anode materials for SIBs. In their work, the scientists demonstrated that this type of anode delivered a reversible capacity of 300 mAh·g<sup>−</sup>1, close to that obtained for graphite in LIBs (372 mAh·g−1) [55]. Hard carbon is usually prepared by the pyrolysis of organic precursors (most often from vegetable biomass, coal or petroleum) at temperatures between 1000 ◦C and 1500 ◦C, depending on the type of feedstock [56]. There are also some recent literature reports on the use of fruit waste as a source of hard carbon in SIBs. For example, in the study of Wu et al. [10], the electrochemical properties of apple waste-derived hard carbon electrodes were reported. Material for the electrodes (hard carbon) was obtained by a two-step dehydration process of wild apples followed by heat treatment (at 1100 ◦C) under an argon atmosphere. Then, the hard carbon electrodes (with a final composition of 80 wt.% HC) were prepared. The obtained electrodes demonstrated a very stable capacity of around 245 mAh·g−<sup>1</sup> (at current rates of 0.1C) with full retention after 80 cycles, and good long-term cycling stability (1000 cycles at 5C). Moreover, the HC electrodes showed a promising rate capability with 112 mAh·g−<sup>1</sup> at 5C [10]. Another study, conducted by Dou et al. [46], showed that pectin-free apple pomace waste-derived HC have a good overall performance during the galvanostatic long-term cycling at 0.1C (the capacity was around 285 mAh·g−<sup>1</sup> after 230 cycles). Moreover, the specific capacities at 1.0–0.12 V (slope) and 0.12–0.02 V (plateau) (at 0.1C) during the fifth discharge were recorded. The obtained HC in the slope-like region showed a capacity of 110 mAh·g−1, while in the plateau region, a capacity of 175 mAh·g−<sup>1</sup> was observed [46]. Interestingly, these results were quite different from those obtained for HC derived from apple pomace containing pectin in other works of the same author. HC from apple waste containing pectin delivered much lower capacity within the plateau as compared to HC from pectin-free apple waste (108 and 175 mAh·g<sup>−</sup>1, respectively). In the slope-like region, very similar capacities were recorded (112 and 110 mAh·g−1). This indicates the differences in the sodium storage mechanism of HCs [46,57].
