Microcrystalline Hybridization Enhanced Coal-Based Carbon Anode for Advanced Sodium-Ion Batteries

Coal-derived boron and phosphorus co-doped activated carbon with expanded interlayer space for high performance sodium ion capacitor anode

Aiming at the key problem of Na+ insertion difficulty and low charge transfer efficiency of activated carbon materials. It is an effective strategy to increase the lattice spacing and defect concentration by doping to reduce the ion diffusion resistance and improve the kinetics. Hence, anthracitic coal is used to prepare activated carbon (AC) and B,P-doped activated carbon (B,P-AC) as the cathode and anode materials for high-performance all-carbon SICs, respectively. AC cathode material has high specific surface area and reasonable micropore structure, which shows excellent capacitance performance. B,P-AC anode material has the advantages of extremely high specific surface area (1856.1 m2/g), expanded interlayer spacing (0.40 nm) and uniform distribution of B and P heteroatoms. Hence, B,P-AC anode achieves a highly reversible Na+ storage capacity of 243 mAh/g at a current density of 0.05 A/g. Density functional theory (DFT) calculations further verify that B,P-AC has stronger Na+ storage performance. The final assembled B,P-AC//AC SIC offers a high energy density of 109.78 Wh kg-1 and a high-power density of 10.03 kW kg-1. The high-performance coal-derived activated carbon of this work provides a variety of options for industrial production of electrode materials for sodium ion capacitors.

Enhancing the sodium storage performance of hard carbon by constructing thin carbon coatings via esterification reactions

Hard carbons derived from pitch are considered a competitive low-cost anode for sodium-ion batteries. However, the preparation of pitch-based hard carbon (PHC) requires the aid of a pre-oxidation strategy, which introduces unnecessary defects and oxygen elements, which leads to low initial Coulombic efficiency (ICE) and poor cycling stability. Herein, we demonstrate a new surface engineering strategy by grafting chemically active glucose molecules on the PHC surface via esterification reactions, which can achieve low-cost nano-scaled carbon coating. Thin glucose coating can be carbonized at a lower temperature, which results in a more closed pore structure and fewer functional groups. The as prepared PHC exhibits a high reversible capacity of 328.5 mAh/g with a high ICE of 92.08 % at 0.02 A/g. It is noteworthy that the PHC can be adapted to a variety of cathode materials for full-cell assembling without pre-sodiation, which maintains the characteristics of high capacity and excellent cycling stability. The performance of resin-based hard carbon coated with a similar method was also improved, demonstrating the universality of the technique.

Demineralization activating highly-disordered lignite-derived hard carbon for fast sodium storage

Lignite, as one of the coal materials, has been considered a promising precursor for hard carbon anodes in sodium-ion batteries (SIBs) owing to its low cost and high carbon yield. Nevertheless, hard carbon directly derived from lignite pyrolysis typically exhibits highly ordered microstructure with narrow interlayer spacing and relatively unreactive interfacial properties, owing to the abundance of polycyclic aromatic hydrocarbons and inert aromatic rings within its molecular composition. Herein, an innovative demineralization activating strategy is established to simultaneously modulate the interfacial properties and the microstructure of lignite-derived carbon for the development of high-performance SIBs. Demineralization process not only creates numerous void spaces in the matrix of lignite precursor to assist aromatic hydrocarbon rearrangement, thereby reducing the ordering and expanding interlayer spacing, but also exposes more interfacial oxygen-containing functional groups to effectively increasing the sodium storage active sites. As a result, the optimal demineralized lignite-derived hard carbon (DLHC 1300) delivers a high reversible capacity of 335.6 mAh g-1 at 30 mA g-1, superior rate performance of 246.3 mAh g-1 at 6 A g-1 and nearly 100 % capacity retention after 1100 cycles at 1A g-1. Furthermore, the optimized DLHC 1300 material functions as an outstanding anode in sodium ion full cells. This work significantly advances the development of low-cost, high-performance commercial hard carbon anodes for SIBs.

Revealing an Extended Adsorption/Insertion-Filling Sodium Storage Mechanism in Petroleum Coke-Derived Amorphous Carbon

Amorphous carbon holds great promise as anode material for sodium-ion batteries due to its cost-effectiveness and good performance. However, its sodium storage mechanism, particularly the insertion process and origin of plateau capacity, remains controversial. Here, an extended adsorption/insertion-filling sodium storage mechanism is proposed using petroleum coke-derived amorphous carbon as a multi-microcrystalline model. Combining in situ X-ray diffraction, in situ Raman, theoretical calculations, and neutron scattering, the effective storage form and location of sodium ions in amorphous carbon are revealed. The sodium adsorption at defect sites leads to a high-potential sloping capacity. The sodium insertion process occurs in both the pseudo-graphite phase (d002 > 0.370 nm) and graphite-like phase (0.345 ≤ d002 < 0.370 nm) rather than the graphite phase, contributing to low-potential sloping capacity. The sodium filling into accessible closed pores forms quasi-metallic sodium clusters, contributing to plateau capacity. The threshold of the effective interlayer spacing for sodium insertion is extended to 0.345 nm, breaking the consensus of insertion interlayer threshold and enhancing understanding of closed pore filling. The extended adsorption/insertion-filling mechanism explains the sodium storage behavior of amorphous carbon with different microstructures, providing theoretical guidance for the rational design of high-performance amorphous carbon anodes.

Multieffect Preoxidation Strategy to Convert Bituminous Coal into Hard Carbon for Enhancing Sodium Storage Performance

Preoxidation is an effective strategy to inhibit the graphitization of coals during carbonization. However, the single effect of the traditional preoxidation strategy could barely increase surface-active sites, hindering further enhancement of sodium storage. Herein, a multieffect preoxidation strategy was proposed to suppress structural rearrangement and create abundant surface-active sites. Mg(NO3)2·6H2O helps to introduce oxygen-containing functional groups into bituminous coal at 450 °C, which acted as a cross-linking agent to inhibit the rearrangement of carbon layers and promote structural cross-linking during the subsequent thermal carbonization process. Besides, the residue solid decomposition product MgO would react with carbon to create surface-active sites. The obtained coal-based hard carbon contained more pseudographitic domains and sodium storage active sites. The optimized sample could deliver an excellent capacity of 287.1 mAh g-1 at 20 mA g-1, as well as remarkable cycling stability of capacity retention of 96.1% after 200 cycles at 50 mA g-1, and notable capacity retention of 88.9% after 1000 cycles at 300 mA g-1. This work provides an effective and practical strategy to convert low-cost bituminous coal into advanced hard carbon anodes for sodium-ion batteries (SIBs).

One-Step Construction of Closed Pores Enabling High Plateau Capacity Hard Carbon Anodes for Sodium-Ion Batteries: Closed-Pore Formation and Energy Storage Mechanisms

Closed pores play a crucial role in improving the low-voltage (<0.1 V) plateau capacity of hard carbon anodes for sodium-ion batteries (SIBs). However, the lack of simple and effective closed-pore construction strategies, as well as the unclear closed-pore formation mechanism, has severely hindered the development of high plateau capacity hard carbon anodes. Herein, we present an effective closed-pore construction strategy by one-step pyrolysis of zinc gluconate (ZG) and elucidate the corresponding mechanism of closed-pore formation. The closed-pore formation mechanism during the pyrolysis of ZG mainly involves (i) the precipitation of ZnO nanoparticles and the ZnO etching on carbon under 1100 °C to generate open pores of 0.45-4 nm and (ii) the development of graphitic domains and the shrinkage of the partial open pores at 1100-1500 °C to convert the open pores to closed pores. Benefiting from the considerable closed-pore content and suitable microstructure, the optimized hard carbon achieves an ultrahigh reversible specific capacity of 481.5 mA h g-1 and an extraordinary plateau capacity of 389 mA h g-1 for use as the anode of SIBs. Additionally, some in situ and ex situ characterizations demonstrate that the high-voltage slope capacity and the low-voltage plateau capacity stem from the adsorption of Na+ at the defect sites and Na-cluster formation in closed pores, respectively.

Hard Carbon Derived From Different Precursors for Sodium Storage

Due to the almost unlimited resource and acceptable performance, Sodium-ion batteries (SIBs) have been regarded as a promising alternative for lithium-ion batteries (LIBs) for grid-scale energy storage. As the key material of SIBs, hard carbon (HC) plays a decisive role in determining the batteries' performance. Nevertheless, the micro-structure of HCs is quite complex and the random organization of turbostratically stacked graphene layers, closed pores, and defects make the structure-performance relationship insufficiently revealed. On the other hand, the impending large-scale deployment of SIBs leads to producing HCs with low-cost and abundant precursors actively pursued. In this work, the recent progress of preparing HCs from different precursors including biomass, polymers, and fossil fuels is summarized with close attention to the influences of precursors on the structural evolution of HCs. After a brief introduction of the structural features of HCs, the recent understanding of the structure-performance relationship of HCs for sodium storage is summarized. Then, the main focus is concentrated on the progress of producing HCs from distinct precursors. After that, the pros and cons of HCs derived from different precursors are comprehensively compared to conclude the selection rules of precursors. Finally, the further directions of HCs are deeply discussed to end this review.

Microstructure engineering in sulfuretted coal tar pitch by varying the Cross-Link state for enhanced sodium storage

Sulfur (S) is an efficient dopant to enhance the sodium storage of carbon, yet the conventional in-situ/post treatments cause unstable S configuration or lower S content, and hence unsatisfied electrochemical performance. Herein, we investigate sulfurization at various cross-link state of coal tar pitch (CTP) (pristine, coke, and carbonized states), and the microstructure of the products (SCTP). Experimental and calculational results reveal that introducing S in the coke state of CTP is essential for achieving abundant and stable C-Sx-C bonds between carbon layers. Moreover, this innovative strategy not only achieves a high S content, but also avoids the liquid carbonization, resulting in a hierarchically porous structure with a small particle size. As a result, the SCTP delivers a sodium storage capacity of 318 mA h g-1 at 0.1 A g-1 after 200th cycle, and the capacity maintains 207 mA h g-1 with capacity retention of 99 % after 1000th cycle at 2.0 A g-1, in half-cells. Moreover, the sample shows a considerable discharge capacity of 328 mA h g-1anode at 0.05 A g-1 in full-cells. Consequently, this approach offers a novel pathway for large-scale production of thermoplastic-derived carbons in battery industry.

Weakly Solvating Few-Layer-Carbon Interface toward High Initial Coulombic Efficiency and Cyclability Hard Carbon Anodes

The carbonaceous anodes in sodium ion batteries suffer from low initial Coulombic efficiency (ICE) and poor cyclability due to rampant solid electrolyte interface (SEI) growth. The concept of the weakly solvating electrolyte (WSE) has been popularized for SEI regulation on the anode by adjusting the cation solvation structure. Nevertheless, the effects on the solvation sheath from the electrode/electrolyte interface are ignored in most WSE applications. In this work, we extend the WSE from the bulk electrolyte to the electrolyte/carbon interface. By recycling asphalt wastes into sp2 C enriched few-layer carbon on hard carbon, a weakly solvating interface is fabricated with lower adsorption energy to electrolyte solvent molecules than a pristine anode (-0.89 vs -1.08 eV for Na/diglyme). Accordingly, more anionic groups are attracted into the solvent-weakened solvation sheath during sodiation (2.30 vs 1.96 coordination number for PF6-). The anion-mediated contact ion pairs facilitate a thin, inorganic-rich SEI layer with a homogeneous distribution, which confers a high ICE of 97.9% and a high capacity of 335.6 mA h g-1 at 1 C (89.5% retention, 1000 cycles). The full battery also manifests an energy density of 209 W h kg-1. This interfacial design is applicable in both ether- and ester-based electrolytes, which is promising in cost-effective modification for carbonaceous electrodes.

Engineering Ultrathin Carbon Layer on Porous Hard Carbon Boosts Sodium Storage with High Initial Coulombic Efficiency

Hard carbon (HC) has been widely adopted as the anode material for sodium ion batteries (NIBs). However, it is troubled by a low initial Coulombic efficiency (ICE) due to its porous structure. Herein, a graphitized and ultrathin carbon layer coating on HC is proposed to solve this challenge. The as-prepared porous carbon material coated with an ultrathin carbon layer composite (PCS@V@C) exhibits a cavity structure, which is prepared by using bis(cyclopentadienyl) nickel (CP-Ni) as the carbon source for outer coating, glucose carbon spheres as porous carbon, and introducing a silica layer to facilitate the coating process. When utilized as the anode for NIBs, the material shows an ICE increase from 47.1% to 85.3%, and specific capacity enhancement at 0.1 A g-1 from 155.3 to 216.7 mA h g-1. Moreover, its rate capability and cycling performance are outstanding, demonstrating a capacity of 140.3 mA h g-1 at 10 A g-1, and a retaining capacity of 225.6 mA h g-1 after 300 cycles at 0.1 A g-1 with the Coulombic efficiency of 100% at the second cycle. The excellent electrochemical performance of the PCS@V@C is attributed to the ultrathin carbon layer, which is beneficial for the formation of a stable solid electrolyte interphase (SEI) film. Therefore, this study provides a feasible surface modification method for the preparation of anode materials for NIBs with high specific capacity and ICE.

Vanadium Nitride Nanoparticles Grown on Carbon Fiber Cloth as an Advanced Binder-Free Anode for the Storage of Sodium and Potassium Ions

The escalating demand for sustainable and high-performance energy storage systems has led to the exploration of alternative battery technologies for lithium-ion batteries. Sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) have emerged as promising candidates because of their abundant Na/K resources, inexpensive costs, and similar chemistries to lithium-ion batteries. However, inherent challenges, such as large ionic radii, sluggish kinetics, and serious volume expansion, necessitate the development of robust and efficient anode materials for SIBs and PIBs. Vanadium nitride has attracted increasing attention as a viable anode due to its high electronic conductivity and potential capacity. In this study, we report on a flexible electrode for SIBs and PIBs that creates binder-free anodes by synthesizing vanadium nitride nanoparticles grown directly on carbon fiber cloths (VN/CFC). The unique architecture and binder-free nature of this anode ensure a robust electrode-electrolyte interface and enhance its electron/ion transport kinetics. The results demonstrate that the material exhibits an outstanding specific discharge capacity of 227 mAh g-1 after undergoing 1000 cycles at a current density of 2 A g-1 for SIBs. An electrochemical analysis indicated that the excellent performance of the material is attributed to the bind-free structure of carbon fiber cloth and the fast kinetics of surface pseudo-capacitive contribution. Furthermore, the material continues to demonstrate an impressive performance, even for PIBs, with a specific discharge capacity of 125 mAh g-1 after 1000 cycles at a current density of 1 A g-1. This study provides a new perspective for designing and developing advanced binder-free anodes for the storage of sodium and potassium ions, paving the way for high-performance energy storage applications.

Toward High Performance Anodes for Sodium-Ion Batteries: From Hard Carbons to Anode-Free Systems

Sodium-ion batteries (SIBs) have been deemed to be a promising energy storage technology in terms of cost-effectiveness and sustainability. However, the electrodes often operate at potentials beyond their thermodynamic equilibrium, thus requiring the formation of interphases for kinetic stabilization. The interfaces of the anode such as typical hard carbons and sodium metals are particularly unstable because of its much lower chemical potential than the electrolyte. This creates more severe challenges for both anode and cathode interfaces when building anode-free cells to achieve higher energy densities. Manipulating the desolvation process through the nanoconfining strategy has been emphasized as an effective strategy to stabilize the interface and has attracted widespread attention. This Outlook provides a comprehensive understanding about the nanopore-based solvation structure regulation strategy and its role in building practical SIBs and anode-free batteries. Finally, guidelines for the design of better electrolytes and suggestions for constructing stable interphases are proposed from the perspective of desolvation or predesolvation.

Phosphate-Induced Reaction to Prepare Coal-Based P-Doped Hard Carbon with a Hierarchical Porous Structure for Improved Sodium-Ion Storage

The use of coal as a precursor for producing hard carbon is favored due to its abundance, low cost, and high carbon yield. To further optimize the sodium storage performance of hard carbon, the introduction of heteroatoms has been shown to be an effective approach. However, the inert structure in coal limits the development of heteroatom-doped coal-based hard carbon. Herein, coal-based P-doped hard carbon was synthesized using Ca₃(PO₄)₂ to achieve homogeneous phosphorus doping and inhibit carbon microcrystal development during high-temperature carbonization. This involved a carbon dissolution reaction where Ca₃(PO₄)₂ reacted with SiO₂ and carbon in coal to form phosphorus and CO. The resulting hierarchical porous structure allowed for rapid diffusion of Na⁺ and resulted in a high reversible capacity of 200 mAh g^-1 when used as an anode material for Na⁺ storage. Compared to unpretreated coal-based hard carbon, the P-doped hard carbon displayed a larger initial coulombic efficiency (64%) and proportion of plateau capacity (47%), whereas the unpretreated carbon only exhibited an initial coulombic efficiency of 43.1% and a proportion of plateau capacity of 29.8%. This work provides a green, scalable approach for effective microcrystalline regulation of hard carbon from low-cost and highly aromatic precursors.

An ultrastable sodium-ion battery anode enabled by carbon-coated porous NaTi2(PO4)3 olive-like nanospheres

NaTi₂(PO₄)₃ (NTP) is a promising anode material for sodium-ion batteries (SIBs). It has drawn wide attention because of its stable three-dimensional NASICON-type structure, proper redox potential, and large accommodation space for Na⁺. However, the inherent low electronic conductivity of the phosphate framework reduces its charge transfer kinetics, thus limiting its exploitation. Therefore, this paper proposes a material with carbon-coated porous NTP olive-like nanospheres (p-NTP@C) to tackle the issues above. Based on experimental data and theoretical calculations, the porous structure of the material is found to be able to provide more active sites and shorten the Na⁺ diffusion distance. In addition, the carbon coating can effectively improve the electron and Na⁺ diffusion kinetics. As the anode material for SIBs, the p-NTP@C olive-like nanospheres exhibit a high reversible capacity (127.3 mAh g^-1 at 0.1 C) and ultrastable cycling performance (84.8% capacity retention after 10,000 cycles at 5 C). Furthermore, the sodium-ion full cells, composed of p-NTP@C anode and Na₃V₂(PO₄)₂F₃@carbon cathode, also deliver excellent performance (75.7% capacity retention after 1000 cycles at 1 C). In brief, this nanostructure design provides a viable approach for the future development of long-life and highly stable NASICON-type anode materials.