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.

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Over the last years, hard carbon (HC) has been the most promising anode material for sodium‐ion batteries due to its low voltage plateau, low cost and sustainability. In this study, three biomass wastes (spent coffee grounds, sunflower seed shells and rose stems) were investigated as potential materials for hard carbon preparation combining a two‐step method consisting on Hydrothermal Carbonization (HTC), to remove the inorganic impurities and increase the carbon content, and a subsequent pyrolysis process. The use of HTC as pretreatment prior to pyrolysis improves the specific capacity in all the materials compared to the ones directly pyrolyzed by more than 100% at high C‐rates. The obtained capacity ranging between 210 and 280 mAh g‐1 at C/15 is similar to the values reported in literature for biomass‐based hard carbons. Overall, HC obtained from sunflower seed shell performs better than that obtained from the other precursors with an Initial Coulombic Efficiency (ICE) of 76% and capacities of 120 mAh g‐1 during 1000 cycles at C with a high capacity retention of 86‐93%.

High energy density anodes for Li-ion technology are the next step to achieve battery-based electric transportation with longer driving range. The substitution of graphite by Si-based materials could lead to a 20% improvement on the volumetric energy density of the battery, from 700 Wh/L (graphite anode) to 900 Wh/L (Si-based anode). However, Si anodes present some problems that must be solved in order to achieve generalized commercial market acceptance. The huge volume change that Si undergoes when alloying with lithium (about 300%) causes electrode cracking and pulverization and limits the cycle life of the electrode. This work compares the efficiency of two different Si-based composites to prevent electrode degradation and enhance the cycle life. Both composite materials were prepared with similar proportions of the same Si nanopowder (~100 nm) supplied by Tekna[1]. The first sample consists in a mixture of Si nanopowder with graphite, whereas the second material comprises Si/amorphous carbon/reduced graphene oxide (rGO). The synthesis process of the latter sample includes the mixture of Si nanopowder with an organic carbon source in a water-based suspension of graphene oxide that is subsequently annealed in inert atmosphere at 900º C. Structural and morphological characterization of the prepared materials includes X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy. The degree of order present in the carbon of the samples was obtained from XRD and Raman spectroscopy. The comparison of these data showed the very different nature of the carbon in both composites; graphite was significantly ordered while the carbon generated from an organic source was much closer to a hard carbon. Analysis of the SEM images showed the distribution of Si nanoparticles in the carbonaceous matrix (graphite or amorphous C/rGO). Electrochemical galvanostatic tests were also performed in 2032 coin cells in half cell configuration vs. metallic lithium at C/3 rate for 150 cycles. Initial coulombic efficiency at C/20 formation cycle and specific capacity, cycle life and capacity retention were examined for both composite materials (Fig. 1). The differences found in the electrochemical performance between the two composites were related to the observed structural and morphological features of the carbonaceous fraction. The high initial capacity loss observed (53 %) for the Si/C/rGO composite and the substantially low electroactivity of this carbonaceous fraction (vs. specific capacity of 350 mAh/g for graphite) hindered the overall energy density of this composite anode electrode. On the other hand (Fig.1c), the Si/C/rGO electrode retained above 99% of the initial specific capacity after 150 cycles at C/3 (2190 mAh/g-Si) in comparison with 65% capacity retention for the Si/Gr anode (421 mAh/g-Si/Gr) with equivalent low Si content (10 wt%). These preliminary findings on increased cycle life by the preparation of Si/C/rGO materials will be applied to higher Si content composite anodes. [1] http://www.tekna.com/ Figure 1 . Formation cycle voltage profiles at C/20 current rate for the (a) Si/Gr and (b) Si/C/rGO anode electrodes with low Si content (~10 wt%). Capacity retention (c) at C/3 cycling. Figure 1