Use of Hydrothermal Carbonization to Improve the Performance of Biowaste-Derived Hard Carbons in Sodium Ion-Batteries.

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, biomass waste (spent coffee grounds, sunflower seed shells and rose stems) was investigated as potential material for hard carbon preparation combining a two-step method consisting of 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 %.

Lithium Iron Phosphate/Carbon (LFP/C) Composite Using Nanocellulose as a Reducing Agent and Carbon Source.

Lithium iron phosphate (LiFePO4, LFP) is the most promising cathode material for use in safe electric vehicles (EVs), due to its long cycle stability, low cost, and low toxicity, but it suffers from low conductivity and ion diffusion. In this work, we present a simple method to obtain LFP/carbon (LFP/C) composites with different types of NC: cellulose nanocrystal (CNC) and cellulose nanofiber (CNF). Microwave-assisted hydrothermal synthesis was used to obtain LFP with nanocellulose inside the vessel, and the final LFP/C composite was achieved by heating the mixture under a N2 atmosphere. The resulting LFP/C indicated that the NC in the reaction medium not only acts as the reducing agent that aqueous iron solutions need (avoiding the use of other chemicals), but also as a stabiliser of the nanoparticles produced in the hydrothermal synthesis, obtaining fewer agglomerated particles compared to synthesis without NC. The sample with the best coating-and, therefore, the best electrochemical response-was the sample with 12.6% carbon derived from CNF in the composite instead of CNC, due to its homogeneous coating. The utilisation of CNF in the reaction medium could be a promising method to obtain LFP/C in a simple, rapid, and low-cost way, avoiding the waste of unnecessary chemicals.

Iron-Doped Sodium-Vanadium Fluorophosphates: Na3V2-yO2-yFey(PO4)2F1+y (y < 0.3).

The sodium-vanadium fluorophosphate family has been actively investigated recently, but few examples tackle chemical doping or the substitution of vanadium. This work presents a series of iron-doped compounds Na3V2-yO2-yFey(PO4)2F1+y (y ≤ 0.3) prepared by hydrothermal synthesis with low iron content. The amount of iron in the structure is confirmed by X-ray and neutron powder diffraction, electronic paramagnetic resonance, magnetic susceptibility measurements, and solid-state nuclear magnetic resonance (ssNMR). The degree of vanadium substitution, together with the solubility limit for iron in sodium-vanadium fluorophosphates, has been calculated by ssNMR and magnetic susceptibility measurements to be y = 0.3 based on the synthetic route used here. The introduction of small amounts of Fe3+ to the structure leads to the reduction of a fraction of V4+ to V3+, and the voltage profiles do not change with the introduction of iron to the structure. In situ synchrotron X-ray diffraction demonstrates that the electrochemical-structural changes during charge and discharge are very similar to those observed in the V3+/V4+ mixed-valent Na3V2O1.6(PO4)2F1.4, which could be related to the existence of both iron dopant and V3+ in the phase.

Magnetic, Magnetoelastic and Corrosion Resistant Properties of (Fe-Ni)-Based Metallic Glasses for Structural Health Monitoring Applications.

We have performed a study of the magnetic, magnetoelastic, and corrosion resistance properties of seven different composition magnetoelastic-resonant platforms. For some applications, such as structural health monitoring, these materials must have not only good magnetomechanical properties, but also a high corrosion resistance. In the fabricated metallic glasses of composition Fe73-xNixCr5Si10B12, the Fe/Ni ratio was varied (Fe+Ni=73% at.) thus changing the magnetic and magnetoelastic properties. A small amount of chromium (Cr5) was added in order to achieve the desired good corrosion resistance. As expected, all the studied properties change with the composition of the samples. Alloys containing a higher amount of Ni than Fe do not show magnetic behavior at room temperature, while iron-rich alloys have demonstrated not only good magnetic properties, but also good magnetoelastic ones, with magnetoelastic coupling coefficient as high as 0.41 for x=0 in the Fe73Ni0Cr5Si10B12 (the sample containing only Fe but not Ni). Concerning corrosion resistance, we have found a continuous degradation of these properties as the Ni content increases in the composition. Thus, the corrosion potential decreases monotonously from 46.74 mV for the x=0, composition Fe73Ni0Cr5Si10B12 to -239.47 mV for the x=73, composition Fe0Ni73Cr5Si10B12.