The performance as anode in sodium-ion batteries of a carbon xerogel (CX-500) using NaPF6 salt-based electrolytes with different carbonate solvent mixtures (EC:DEC, EC:DMC, EC:EMC) is investigated by prolonged galvanostatic cycling with the support of the Electrochemical Impedance Spectroscopy (EIS). To this end, the CX500 semi-cell internal ohmic (flow of ions by the electrolyte, Rs) and faradic (solid-electrolyte interface layer, RSEI; charge transfer at the electrode/electrolyte interface, RCT) resistances as well as the parameters associated with the diffusion processes of the Na+ ions at the electrode/electrolyte interface and inside the electrode (Warburg coefficient, sigma; diffusion coefficient, D) along cycling were calculated by this non-destructive technique. The long-term electrochemical stability of the carbon xerogel anode in sodium-ion batteries was found to be highly dependent on electrolyte composition. By comparing the evolution of the capacity and the EIS parameters of the CX-500 semi-cell during cycling, it can be inferred that the use of EC:EMC carbonate mixture in the electrolyte is the most suitable since the other mixtures, EC:DEC and EC:DMC, induce abrupt increases of the internal resistances, particularly as regards RSEI and RCT, whose increases are related to the loss of active area accessible to the electrolyte as a consequence of the continuous growth of the SEI layer, thus leading to a significant capacity fading as a consequence of electrode degradation. These findings establish quantitative relationships between solvent chemistry and electrochemical degradation mechanisms in hard carbon anodes, advancing mechanistic understanding of electrolyte design for sodium-ion battery optimization.

Carbon xerogels with a pore size of approximately 500 nm, synthesized through an energy-efficient microwave-assisted process and subsequently thermally treated at temperatures ranging from 1000 degrees C to 2800 degrees C, are investigated as anodes in sodium-ion batteries using a glyme-based electrolyte. The present study focuses on determining the optimal balance between the textural and structural properties of the resulting materials to enhance their electrochemical performance. Particular attention is paid to the material interlayer spacing and porosity, aiming to achieve a suitable compromise between ordered and disordered carbon structures. This balance is crucial to maximize the electrochemical storage of the Na+ ions in the materials via surface adsorption (defects, pores), insertion/intercalation/co-intercalation into the aromatic layers of graphitic domains, and/or nanopore filling mechanisms. Among the carbon xerogels studied, CX500-1800 which features very low surface area, an almost negligible micropore volume, and an interlayer spacing of approximately 0.37 nm emerged as the most suitable candidate for this application. This material delivers a capacity of 233 mAh g(-)& sup1; after 200 cycles at 37.2 mA g(-)& sup1; with a coulombic efficiency of 94%. The enhanced performance of this carbon material suggests a synergistic effect of its optimized textural and structural properties. Specifically, the absence of microporosity minimizes irreversible sodium trapping, while the moderate increase in the structural order-achieved through thermal treatment at 1800 degrees C-facilitates efficient Na+ ion transport and storage mainly through intercalation into the graphitic domains.

Sodium dual-ion batteries combine economic and environmental benefits by using carbon materials in both electrodes and sodium compounds in the electrolyte. Among other factors, their successful implementation for energy storage relies on optimization of the properties of the carbon electrode materials. To this end, carbon materials with a wide range of textural and structural properties were prepared by simply heat treating a single porous carbon in the absence or presence of a low-cost highly effective iron-based catalyst. These materials were investigated as anode or cathode in the sodium dual-ion batteries by prolonged galvanostatic cycling. The optimal textural and structural properties for carbon materials to achieve the best performance as electrodes in sodium dual-ion batteries were identified as having a high degree of graphitic structural order combined with minimal microporosity in the cathode and a non-graphitic structure with a layer spacing of around 0.37 nm and moderate microporosity in the anode.

The implementation of sodium-ion batteries for renewable energy storage requires the development of sustainable electrode materials. Usually, these materials are produced through complex energy-intensive processes that are challenging to scale and involve expensive and/or toxic reagents. In this study, sustainable hard carbon materials, some doped with iron, synthesized from sucrose using a simple, fast, and cost-effective two-step eco-friendly process, are investigated as anodes for sodium-ion batteries. The influence of physicochemical and structural material properties on electrode reversible capacity, cycling stability, and efficiency is analyzed. The SC900 material, which exhibits a certain development of graphite-like structure, though not strictly graphitic, showed the best electrochemical performance, providing discharge capacities exceeding 100 mAh g−1 after 400 cycles with excellent cycling stability and high coulombic efficiency. The capacity of the materials increases as d002 decreases, (i.e., as the degree of structural order increases), to the optimum value of ~0.3700 nm. However, a further decrease in d002 to values characteristic of quasi-graphitic materials, as a consequence of the catalytic effect of iron, hinders Na+-ion storage, which, in addition to the low electrochemical activity of the iron oxides present, leads to much lower capacities.

This study presents groundbreaking results in the field of rechargeable aluminium-ion batteries, achieving stable capacities exceeding 300 mAh g-1 for more than 300 cycles. The key to this achievement lies in the utilization of tailor-made carbon materials and a urea-AlCl3-based electrolyte. The article investigates the optimal physicochemical properties of the active material necessary for effective electrodes for these aluminium-ion batteries. This investigation employs a wide range of materials characterization techniques (XRD, SEM-EDX, N2 adsorption-desorption isotherms, Hg porosimetry, XPS, FTIR, Raman and TEM-EDX) and electrochemical performance analyses to delve into the subject. These findings represent a significant improvement in the capacity of aluminium-ion batteries, bringing us closer to their implementation and commercialization. This achievement is attributed to the utilization of readily available, cost-effective, and non-corrosive materials. The ability to customize carbon xerogels and the use of the urea-AlCl3 electrolyte offer promising avenues for the practical implementation of these advanced battery technologies, leading to further enhancements in their performance and widespread adoption in various applications. This study reveals ground-breaking advancements in rechargeable aluminium-ion batteries, achieving remarkable stability with capacities surpassing 300 mAh g-1 over 300 cycles. Using tailor-made carbon materials and a urea-AlCl3 electrolyte, the research investigates optimal physicochemical properties, employing various characterization techniques. These findings promise significant progress in commercializing aluminium-ion batteries, driven by cost-effective and customizable materials. image

Natural graphite is a critical raw material and its substitution in some applications will contribute to reduce its supply risk. Coal ash, which is still produced in high amounts, contains a carbonaceous solid residue (char) that may be a substitute of natural graphite. For this purpose, char may be graphitized and used in electrocatalysis reactions, however char properties depend on coal rank, composition and combustion conditions that affect the graphitization process. Research on industrial coal char graphitization is limited, resulting in restricted comprehension of the contributing factors. In order to provide novel insights, industrial residual chars (derived from Poland, Portugal, Romania and South Africa) subjected to high-temperature treatments (carbonization at 1000 C-degrees followed by treatment at 2600 C-degrees under environmental pressure) were examined regarding their microstructural and microtextural transformations. The samples were studied by focussing on its heterogeneity, namely its specific properties and interactions, including elemental composition, optical character, and structure. Furthermore, the evolution of structural order of selected char morphotypes in samples derived from coal of similar rank was assessed using Raman microspectroscopy. The results highlighted a set of aspects that could have influenced the transformations experienced by the different chars during high temperature treatments (graphitization). It was found that Hydrogen might play a role in the graphitization ability of isotropic chars derived from low rank coal. The prior preferential orientation of the Basic Structural Units (BSU) contributes to achieve a higher graphitization degree, but it can be enhanced or hindered by other factors, e.g., hydrogen content. Raman microspectroscopy showed that char morphotypes from the same group underwent different transformations during graphitization, indicating that morphotypes or sections with unfused optical character may be more prone to graphitization. However, the existence of disordered domains and specific microtextures, such as polyhedral pores identified under TEM, likely hindered further graphitization. Further research on this topic is needed.

The prismatic and basal plane surfaces of carbon materials dictate most of their anisotropic physicochemical properties. For many applications where interfacial interactions are key, high-temperature treatments are performed to achieve their graphitization. Such treatment changes edge and basal plane configuration, impacting the energetical behavior of the carbon surfaces, particularly for carbon nanomaterials, with consequences for their properties. Therefore, efforts should be devoted to probing the prismatic and basal plane surfaces of such materials to understand their surface properties for the development of high-performance carbon materials. Herein, we investigate the effect of high-temperature graphitization (3073 K) on the structural, textural, chemical, and magnetic properties of graphitic carbon nanomaterials presenting different prismatic/basal surfaces. The evolution of the prismatic/basal surfaces has been probed by nitrogen adsorption, temperature-programmed desorption, and X-ray photoelectron spectroscopy (XPS). Although these three techniques are in agreement for the starting materials, they diverge in the case of materials that have undergone thermal annealing. This is linked in particular to the formation of loops following the heat treatment, which are identified as belonging to the prismatic surface by XPS and modified the N2 adsorptive potentials. Formed small vacancies on closed loops and nonperfect closure of certain loops can contribute to the accumulation of very reactive defects at the loop level. The thermal annealing also has a pronounced influence on the magnetic properties of these materials. Interestingly, we show that a positive correlation exists between the spin density of the annealed graphitic carbons and their prismatic and basal surfaces.

A detailed study of the intercalation/de-intercalation mechanisms of PF6- anions in a graphite cathode of sodium dual-ion batteries is carried out by cyclic voltammetry. The influence of the continuous anion intercalation/de-intercalation on the graphite structure is investigated by ex-situ XRD after cycling. It was concluded that the intercalation/de-intercalation occurs through a combination of diffusion-controlled and pseudocapacitive mechanisms. Initially, the contribution of the diffusion-controlled is significant, specifically at the highest voltage and at the lowest scan rates which agrees with the slow kinetic of this mechanism and its usual prevalence in carbon materials with a graphitic structure. However, the continuous anion intercalation/de-intercalation causes some deterioration of the graphite structural order, even from the initial cycles, as it was demonstrated by the evolution of the crystalline parameters, interlayer spacing, d002, and crystallite size, Lc. This deterioration hinders the diffusion-controlled anion intercalation/de-intercalation and, as a consequence, the pseudocapacitive becomes the main mechanism along cycling. Therefore, since graphite is capable of intercalating PF6- anions at high voltages through a capacitive mechanism, with its consequent rapid kinetics, this carbon material is an excellent candidate for using as cathode in high power sodium dual-ion batteries, in which, kinetically fast mechanisms are required.

Carbon xerogel with different mean pore sizes (10, 50, 100, 500 nm) but analogous structure, chemical composition, surface area, and microporosity, which are produced by an energy-effective microwave-based process, and activated carbon xerogels (100 nm-pore size) with higher microporosity were investigated as anodes for sodium dual-ion batteries. The objective of this study was to optimize the material textural properties for this specific application to be further matched with a carbon xerogel-based cathode in a full battery configuration. To this end, the role of these properties was evaluated, specifically the pore size and the microporosity since the storage of the Na+ ions in these carbon xerogels was demostrated to occur mainly by a pseudocapacitive mechanism based on adsorption on surface, defects (i.e. microporosity) and pores. The balance between the pore size and the associated external surface area of the carbon xerogels was a determining factor for their anodic performance. In this context, a material pore size of similar to 100 nm and an associated external surface area of similar to 100 m(2) g(- 1) was the optimum balance. For a given material pore size, the increase of the microporosity by physical activation improved the anode capacity as well as the cycling stability. However, the development of the microporosity in relation to the external surface area must be optimized to avoid an undesirable increase in the first cycle irreversible capacity. Overall, the physically activated carbon xerogel with a pore size of 100 nm, a micropore volume of 0.47 cm(3) g(- 1) and an external surface of 123 m(2) g(- 1) was the most suitable active anode material for sodium dual-ion batteries. This material provided a specific discharge capacity of similar to 154 mAh g(- 1) after 300 discharge/charge cycles with excellent cycling stability and coulombic efficiency.

In recent years, the research on lithium-ion batteries (LIBs) to improve their lifetime, efficiency and energy density has led to the use of silicon-based materials as a promising anode alternative to graphite. Specifically, crystalline silicon (cSi) and silicon carbide (SiC) obtained from deposition or reduction processes (e.g., magnesiothermal reduction) stand out for their electrochemical properties. However, the synthesis routes proposed until now have limitations that make them difficult to afford or operate on a large scale. For this reason, in this work, carbon-silicon (C-Si) hybrid materials synthesized through an efficient route are evaluated as the potential precursor for the obtention of both cSi and SiC species in a single material. The feasibility and influence of the magnesiothermal reduction process were evaluated, and materials with 10 wt.% of reduced Si and 10–26 wt.% of SiC were obtained. Both species play a role in the improvement of the performance of silicon-based materials as anodes in lithium-ion batteries. In comparison with materials obtained by the reduction of silica gels and composites, the reduced C-Si hybrid gels stand out thanks to the homogeneous distribution and stability of the species developed.

Natural graphite is a critical raw material and its substitution in some applications will contribute to reduce its supply risk. Coal ash, which is still produced in high amounts, contains a carbonaceous solid residue (char) that may be a substitute of natural graphite.For this purpose, char may be graphitized and used in electrocatalysis reactions, however char properties depend on coal composition and combustion conditions that affect the graphitization process. To give new insights into coal char graphitization, the microstructural and microtextural transformations were studied by focussing on its heterogeneity, namely its specific properties and interactions, including elemental composition, optical character and structure using a combination of characterization techniques: proximate and elemental analysis, carbon forms, optical microscopy, scanning electron microscopy, transmission electron microscopy (TEM), and Raman microspectroscopy.The results obtained highlighted a set of aspects that could have influenced the transformations experienced by the different chars during high temperature treatments (graphitization). It was found that hydrogen might play an important role in the graphitization ability of isotropic chars derived from low rank coal. The prior preferential orientation of the Basic Structural Units (BSU) seems to contribute to achieve a higher graphitization degree, but it can be enhanced or hindered by other factors, e.g., hydrogen content. Raman microspectroscopy results showed that char morphotypes from the same group underwent different transformations during graphitization, suggesting that morphotypes or sections with unfused optical character may be more prone to graphitization. On the other hand, the existence of disordered domains and specific microtextures, such as polyhedral pores identified under TEM, may have hindered further graphitization. Further research on this topic is needed.

Abstract The objective of this study is to optimize the performance of a graphitized carbon xerogel doped with graphene oxide (GXGO) as cathode for sodium dual‐ion batteries. Carbon xerogels are synthetic materials produced in large amounts, with well‐controlled chemistry, porosity and structure, by a simple and fast microwave process that has been scaled‐up. The intercalation of PF6− anions from various NaPF6 salt‐based electrolytes in a GXGO‐based cathode is investigated by considering the influence of the upper cut‐off voltage (UCOV) and of the electrolyte on the electrochemical parameters. The best results were attained in an electrolyte with a 2.0 M NaPF6 concentration in EC : EMC carbonate solvent mixture with added FEC, at a UCOV of 5.0 V. GXGO cathode provides a discharge capacity of 47 mAh g−1 after 2500 cycles with a coulombic efficiency of 97 %. Moreover, it shows great cycling stability and capacity retention of essentially 100 % after the initial cycles.

Silicon-based anodes are widely studied as an alternative to graphite anodes for lithium-ion batteries. Nevertheless, their practical application is mainly limited by the huge volume change that silicon particles undergo due to alloying and de-alloying with lithium ions during discharge/charge processes, which result in cracks and electrode degradation. In the present study, porous silicon-carbon composites are investigated as anode materials for next-generation lithium-ion batteries. These composites are prepared by a cost-effective, easily-scalable method based on a microwave assisted approach for the carbon matrix, followed by dispersion of the silicon in 2-propanol. The electrochemical behavior of the Si/C composites with different proportions of silicon is evaluated in terms of alloying and de-alloying mechanisms of lithium ions, battery reversible capacity, irreversible capacity in the first cycle, retention of capacity along cycling, and cycle efficiency. The composite with 30 wt.% of silicon presents specific discharge capacity as high as 917 mAh g(-1) after 200 cycles and excellent stability in the long-term at high current density, which makes it a promising candidate for the lithium-ion battery market.

The electrochemical intercalation of PF6− anions into graphitic nanomaterials of renewable origin with different degrees of structural order to be subsequently used as cathodes for sodium dual-ion batteries is herein investigated for the first time. Overall, the electrochemical performance of the biogas-derived carbon nanofibers depends on their graphitic structure, specifically on crystallite height which was found to be the determining parameter for the scope of electrochemical intercalation of PF6− anions into these nanomaterials.

The intercalation of PF6− anions in graphite from various sodium salt-based electrolytes with organic carbonate mixtures as solvents is investigated. The purpose was to optimize the electrochemical performance of the graphite as cathode in terms of specific capacity, capacity stability, and coulombic efficiency to be coupled in the future with a hard carbon-based anode in a full sodium dual-ion battery. To this end, a detailed study was made of the influence of applied current density, upper cut-off voltage (UCOV), and electrolyte—in terms of both salt concentration and solvent mixture—on the intercalation/de-intercalation of PF6− anions in the graphite cathode. Low (37.2 mA g−1) and high (372.0 mA g−1) currents, UCOVs from 4.8 to 5.2 V, electrolytes with NaPF6 salt concentrations in the range of 0.2–1.2M and EC:DEC, EC:DMC and EC:EMC solvent mixtures were studied. The best graphite cathode performance was attained in 1.2MNaPF6/EC:EMC electrolyte at the highest current density of 372.0 mA g−1 and for the potential range between 2.9 and 5.0 V vs. Na/Na+. In these conditions, a discharge capacity of 79 mAh g−1 after 1000 cycles with a coulombic efficiency of 99 % and a remarkable capacity retention throughout cycling were determined.

Six solid bitumen samples from Paja Formation (Cretaceous of Colombia) were optically, chemically and structurally characterized using bulk chemistry, reflectance indicating surface (RIS), micro-Raman and X-ray diffraction (XRD) parameters. The volatile matter and carbon contents and high random reflectance (Ro%) placed these solid bitumens in the category of cata-impsonites according to Jacob's classification. RIS axes (RMAX, RINT and RMIN) form almost a sphere, and consequently, the anisotropy (Ram or bireflectance values) of these materials is very weak, comparatively to other carbonaceous materials (coals) of similar degree of evolution. The weak anisotropy character of the solid bitumen is related to the absence of pressure in the system, which did not promote the orientation of the basic structural units (BSUs) in a three-dimensional arrangement. The degree of ordering evaluated by Raman parameters, such as full width at half maximum (FWHM) of the D1 and G bands, and XRD parameters, including d002 and crystallite sizes Lc and La, showed that the solid bitumens of the Paja Formation have a low structural order equivalent to high-rank coals, specifically anthracites B (3.0% ≤ Ro < 4.0%), although their reflectance values placed them in a higher rank (Ro > 4.0%). The discrepancy between high reflectance and low degree of structural order can explain why some of the equations used to estimate equivalent vitrinite reflectance (Ro eq. Vite) from solid bitumens do not work universally. Taking in consideration several equations available in the literature, Jacob's equation (Jacob, 1989) was the one that fitted the Ro eq. Vite values in the range of 3.0% to 4.0%, which parallels with the degree of structural order obtained for the solid bitumens. Consequently, Jacob's equation can now be extended to solid bitumen in the range of cata-impsonites.

Coal ash char concentrates from four countries (Portugal, Poland, Romania, and South Africa) were prepared, characterised, and graphitized under the scope of the Charphite project (Third ERA-MIN Joint Call (2015) on the Sustainable Supply of Raw Materials in Europe). Coal ash chars may be a secondary raw material to produce synthetic graphite and could be an alternative to natural graphite, which is a commodity with a high supply risk. The char concentrates and the graphitized material derived from the char concentrates were characterised using proximate analysis, X-ray fluorescence, X-ray diffraction (structural), Raman microspectroscopy, solid-state nuclear magnetic resonance, scanning electron microscopy, and petrographic analyses to determine if the graphitization of the char was successful, and which char properties enhanced or hindered graphitization. Char concentrates with a lower proportion of anisotropic particles and a higher proportion of mixed porous particles showed greater degrees of graphitization. It is curious to see that embedded Al2O3 minerals, such as glass and clay, influenced graphitization, as they most likely acted as catalysts for crystal growth in the basal direction. However, the graphitized samples, as a whole, do not compare well against a reference natural graphite sample despite some particles in select char concentrates appearing to be graphitized following graphitization.

The electrochemical performance of novel nano-silicon/biogas-derived carbon nanofibers composites (nSi/BCNFs) as anodes in lithium-ion batteries was investigated, focusing on composition and galvanostatic cycling conditions. The optimization of these variables contributes to reduce the stress associated with silicon lithiation/delithiation by accommodating/controlling the volume changes, thus preventing anode degradation and therefore improving its performance regarding capacity and stability. Specific capacities up to 520 mAh g−1 with coulombic efficiency > 95% and 94% of capacity retention are achieved for nSi/BCNFs anodes at electric current density of 100/200 mA g−1 and low cutoff voltage of 80 mV. Among the BCNFs, those no-graphitized with fishbone microstructure, which have a great number of active sites to interact with nSi particles, are the best carbon matrices. Specifically, a nSi:BCNFs 1:1 weight ratio in the composite is the optimal, since it allows a compromise between a suitable specific capacity, which is higher than that of graphitic materials currently commercialized for LIBs, and an acceptable capacity retention along cycling. Low cutoff voltage in the 80–100 mV range is the most suitable for the cycling of nSi/BCNFs anodes because it avoids formation of the highest lithiated phase (Li15Si4) and therefore the complete silicon lithiation, which leads to electrode damage.

Carbon xerogels (CXs) with the same chemical composition and BET surface area but different pore sizes (10-200 nm), which had been easily produced in large amounts via a cost-effective microwave-based process, are investigated as anodes for sodium-ion batteries (SIBs). The role of textural properties of CXs in the process of sodium ions storage was evaluated. The most suitable anode for SIBs was CX-100 with a pore size of 100 nm, the largest micropore volume and the lowest external surface area (S-ext), which gives an idea of the most accessible surface of the material, along with relatively high open porosity. Larger pore sizes facilitate electrolyte penetration, thus improving Na+ ions diffusion inside the electrode, while microporosity is crucial in increasing electrode capacity since Na+ ions storage on CXs is mainly due to absorption on the surface and in structural defects (i.e., micmporosity). Moreover, lowering S-ext leads to a decrease in the Na+ ions used in the formation of the SEI layer and irreversibly absorbed during initial cycles, therefore improving electrode performance. In summary, an optimal combination of textural properties, including pore structure and S-ext, should be considered in order to effectively design CXs for SIBs.

Novel graphitic materials of renewable origin, biogas-derived carbon nanofibers (BCNFs), are investigated as anodes for sodium-ion batteries in a glyme-based electrolyte. These materials show a unique combination of electrochemical properties, including suitable capacity (~100 mAh g), high rate capability, excellent cycle stability and coulombic efficiency as well as long cycle life (~6000 cycles up to 7.5 A g) which make them adequate candidates for this application. The sodiation of BCNFs occurs through different combinations of diffusioncontrolled intercalation and capacitive intercalation processes. Overall, the quantitative contribution of the capacitive current to the total stored sodium in BCNF electrodes is noteworthy (28-71 %), which account for their ultrahigh rate performance, comparable to that of supercapacitors, because of the improvement of the transportability of the Na ions through the graphene layers. * Corresponding author E-mail address: icamean@incar.csic.es