Exploring Carbonization Temperature to Create Closed Pores for Hard Carbon as High-Performance Sodium-Ion Battery Anodes.

Biomass-derived porous carbon materials are meaningful to employ as a hard carbon precursor for anode materials of sodium-ion batteries (SIBs) from a sustainability perspective. Here, a straightforward approach is proposed to develop rich closed pores in pinenut-derived carbon, with the aim of improving Na+ plateau storage by adjusting the pyrolysis temperature. The optimized sample, namely the pinenut-derived carbon at 1300 °C, demonstrates remarkable reversible specific capacity of 278 mAh g-1, along with a high initial Coulomb efficiency of 85% and robust cycling stability (with a capacity retention of 89% after 800 cycles at 0.2 A g-1). In situ and ex situ analyses unveil that the developed closed pores play a significant role in enhancing the plateau capacity, providing compelling evidence for the "adsorption-filling" mechanism. Moreover, the corresponding full-cell achieves a high energy density of 245.7 Wh kg-1 (based on the total weight of both electrode active materials) and exhibits outstanding rate capability (191.4 mAh g-1 at 3 A g-1).

Steric Hindrance Engineering to Modulate the Closed Pores Formation of Polymer-Derived Hard Carbon for High-Performance Sodium-Ion Batteries.

The closed pores play a critical role in improving the sodium storage capacity of hard carbon (HC) anode, however, their formation mechanism as well as the efficient modulation strategy at molecular level in the polymer-derived HCs is still lacking. In this work, the steric hindrance effect has been proposed to create closed pores in the polymer-derived HCs for the first time through grafting the aromatic rings within and between the main chains in the precursor. The experimental data and theoretical calculation demonstrate that steric-hindrance effect from the aromatic ring side group can increase backbone rigidity and the internal free volumes in the polymer precursor, which can prevent the over graphitization and facilitate the formation of closed pores during the carbonization process. As a result, the as-prepared HC anode exhibits a remarkably enhanced discharge capacity of 340.3 mAh/g at 0.1 C, improved rate performance (210.7 mAh/g at 5 C) as well as boosted cycling stability (86.4 % over 1000 cycles at 2 C). This work provides a new insight into the formation mechanisms of closed pores via steric hindrance engineering, which can shed light on the development of high-performance polymer-derived HC anode for sodium-ion batteries.

Recent Advances of Pore Structure in Disordered Carbons for Sodium Storage: A Mini Review.

Disordered carbons as the most promising anode materials for sodium ion batteries (SIBs) have attracted much attention, due to the widely-distributed sources and potentially high output voltage when applied in full cells owing to the almost lowest voltage plateau. The complex microstructure makes the sodium storage mechanism of disordered carbons controversial. Recently, many studies show that the plateau region of disordered carbons are closely related to the embedment of sodium ion/semimetal in nanopores. In this regard, the classification, characterization and construction of nanopores are exhaustively discussed in this review. In addition, perspectives about the controllable construction of nanopores are presented in the last section, aiming to catch out more valuable studies include not only the construction of closed pores to enhance capacity but also the design of carbon materials to understand Na storage mechanism.

Understanding the Sodium Storage Behavior of Closed Pores/Carbonyl Groups in Hard Carbon.

Hard carbon (HC) is a promising anode material for sodium-ion batteries. However, the intrinsic relationship between the closed pores/surface groups and sodium storage performance has been unclear, leading to difficulties in targeted regulation. In this study, renewable tannin extracts were used as raw materials to prepare HC anodes with abundant tunable closed pores and carbonyl groups through a pyrolytic modulation strategy. Combining ex situ characterizations reveals that closed pores and carbonyl groups are regulated by the pyrolytic process. Further, it is demonstrated that the plateau region is mainly contributed by the closed pores; highly stable fluorine-rich solid electrolyte interphase compositions are produced through carbonyl-induced interfacial catalysis. The optimized HC anode displays good cycling stability, exhibiting a high reversible capacity (360.96 mAh g-1) at 30 mA g-1 and capacity retention of up to 94% after 500 cycles at 1 A g-1. Moreover, the full battery assembled with Na3V2(PO4)3/C demonstrates a stable cycling performance. These findings provide a fresh knowledge of the structural design of high-performance HC anode materials and the mechanism of sodium storage in HC.

Resolution of the reciprocity between radical species from precursor and closed pore formation in hard carbon for sodium storage.

Hard carbon (HC) has emerged as a highly promising anode material for sodium ion batteries, drawing tremendous interest in producing this material with low-cost and easily accessible precursors. The determination of the crucial parameters of precursors influencing the formation of key structures, such as closed pores, in the HC is of paramount importance. Considering the potential role of free radicals in the structural evolution of the precursors, we, for the first time, delve into the impact of radical species on the development of closed pores by electron paramagnetic resonance spectroscopy, with petroleum asphalt as the model system. Our findings reveal that carbon centred radicals, with the g value close to that of the free electron (2.0023), exhibit a propensity to form long-range, well-ordered graphitic structures with lower sodium storage capacity. Conversely, the deliberately incorporated oxygen radicals with the g value over 2.005 require a higher energy for ordering the graphitic structures, leading to the creation of closed pores. As a result, the optimal sample showcases a four-fold increase in plateau capacity for sodium ion storage due to the pore filling process. Our research underscores the pivotal role of employing electron paramagnetic resonance spectroscopy studying the critical structural evolution of functional carbon materials.

Structural regulation of asphalt-based hard carbon microcrystals based on liquid-phase crosslinking to enhance sodium storage.

Air-oxidation is an effective strategy to obtain promising carbon materials from asphalt for sodium-ion batteries. However, this method would generate a vast amount of gaseous pollutant, which pose challenges for recycling. Herein, a simple, cost-effective and environmentally friendly liquid-phase oxidation method is proposed. The oxygen-containing functional groups (-NO2) are introduced into asphalt, which effectively prevents the melting of asphalt and rearrangement of carbon layers during subsequent carbonization process. As a result, a carbon material with notable disorder degree, large interlayer spacing and abundant closed pores, is prepared. The as-prepared product demonstrates an impressive initial Coulombic efficiency of 88.3 % and an enhanced specific capacity of 317.0 mA h g-1, which is 2.6 times that of the pristine product. Moreover, when assembled with a Na3.32Fe2.34(P2O7)2 cathode, the full-cell delivers a high reversible capacity of 271.7 mA h g-1 at 30 mA g-1 with superb cycle life. This study offers a novel oxidation strategy and provides a solution for producing highly disordered carbon anodes from soft carbon precursors.

O-Targeted Carbon Hybrid Orbital Conversion to Produce sp2-Rich Closed Pores for Sodium-Storage Hard Carbon.

Biomass-based hard carbon has the advantages of a balanced cost and electrochemical performance, making it the most promising anode material for sodium-ion batteries. However, due to the structural limitations of biomass (such as macropores and impurities), it still faces the problems of low specific capacity and initial Coulombic efficiency (ICE). Herein, an integrated strategy of biomass liquefaction and oxidation treatment is proposed to fabricate hard carbon with low ash content and sp2-rich closed pores. Specifically, liquefaction treatment can break through the inherent constraints of biomass, while oxidation treatment with O-targeted effect can directionally convert C─C/C─O bonds into C═O/O═C─O bonds, which would promote the formation of closed pores and the rearrangement into sp2-carbon within the graphene layer. Moreover, it is well demonstrated that the hard carbon interface rich in sp2 hybridization can induce the generation of an inorganic-rich solid electrolyte interface, contributing to fast ion migration and excellent interfacial stability. As a result, the optimized hard carbon with maximum closed pore volume and sp2/sp3 ratio can exhibit a high capacity of 347.3 mAh g-1 at 20 mA g-1 with the ICE of 90.5%, and a capacity of 110.4 mAh g-1 at 5.0 A g-1 after 10 000 cycles.

Regulating Pseudo-Graphitic Domain and Closed Pores to Facilitate Plateau Sodium Storage Capacity and Kinetics for Hard Carbon.

Hard carbon anode demonstrates exceptional potential in sodium-ion batteries due to their cost-effectivenss and superior plateau capacity. However, the proximity of the plateau capacity to the cut-off voltage of battery operation and the premature cut-off voltage response caused by polarization at high rates greatly limit the exploitation of plateau capacities, raising big concerns about inferior rate performance of high-plateau-capacity hard carbon. In this work, a facile pre-oxidation strategy is proposed for fabricating lignin-derived hard carbon. Both high-plateau capacity and sodiation kinetics are significantly enhanced due to the introduction of expanded pseudo-graphitic domains and high-speed closed pores. Impressively, the optimized hard carbon exhibits an increased reversible capacity from 252.1 to 302.0 mAh g-1, alongside superior rate performance (174.7 mAh g-1 at 5 C) and stable cyclability over 500 cycles. This study paves a low-cost and effective pathway to modulate the microstructure of biomass-derived hard carbon materials for facilitating plateau sodium storage kinetics.

Enhancing Sodium-Ion Energy Storage of Commercial Activated Carbon by Constructing Closed Pores via Ball Milling.

Mechanical ball milling is a prevalent technology for material preparation and also serves as a post-treatment method to modify electrode materials, thus enhancing electrochemical performances. This study explores the microstructure modification of commercial activated carbon through mechanical ball milling, proving its efficacy in increasing sodium-ion energy storage. The evolution of activated carbon's physical and chemical properties during ball milling was systematically examined. It was observed that the quantity of closed pores and the graphitization degree in activated carbon increased with extended ball milling duration. The sodium storage mechanism in activated carbon transitions to an insertion-pore filling process, significantly elevating platform capacity. Additionally, ball-milled activated carbon demonstrates remarkable long-term cycling stability (92% capacity retention over 200 cycles at 200 mA g-1) and rate performance. This research offers a novel approach to developing advanced anode materials for sodium-ion batteries.

Self-Encapsulation of Biomacromolecule Drugs in Porous Microscaffolds with Aqueous Two-Phase Systems.

At present, the most commonly used methods of microencapsulation of protein drugs such as spray drying, multiple emulsification, and phase separation, can easily cause the problem of protein instability, which leads to low bioavailability and uncontrolled release of protein drugs. Herein, a novel method to encapsulate protein drugs into porous microscaffolds effectively and stably was described. Ammonium hydrogen carbonate (NH4HCO3) was employed to prepare porous microscaffolds. α-Amylase was encapsulated into the porous microscaffolds without denaturing conditions by an aqueous two-phase system (PEG/Sulfate). The pores were closed by heating above the glass transition temperature to achieve a sustained release of microscaffolds. The pore-closed microscaffolds were characterized and released in vitro. The integrity and activity of protein drugs were investigated to verify that this method was friendly to protein drugs. Results showed that the pores were successfully closed and a high loading amount of 9.67 ± 6.28% (w/w) was achieved. The pore-closed microscaffolds released more than two weeks without initial burst, and a high relative activity (92% compared with native one) of protein demonstrated the feasibility of this method for protein drug encapsulation and delivery.

Fabrication and Compressive Properties of Low to Medium Porosity Closed-Cell Porous Aluminum Using PMMA Space Holder Technique.

In recent years, closed-cell porous Aluminum (Al) has drawn increasing attention, particularly in the applications requiring reduced weight and energy absorption capability such as in the automotive and aerospace industries. In the present work, porous Al with closed-cell structure was successfully fabricated by powder metallurgy technique using PMMA as a space holder. The effects of the amount of PMMA powder on the porosity, density, microstructure and compressive behaviors of the porous specimens were systematically evaluated. The results showed that closed-cell porous Al having different porosities (12%-32%) and densities (1.6478 g/cm³, 1.5125 g/cm³ and 1.305 g/cm³) could be produced by varying the amount of PMMA (20-30 wt %). Meanwhile, the compressive behavior results demonstrated that the plateau stress decreased and the energy absorption capacity increased with increasing amount of PMMA. However, the maximum energy absorption capacity was achieved in the closed-cell porous Al with the addition of 25 wt % PMMA. Therefore, fabrication of closed-cell porous Al using 25 wt % PMMA is considered as the optimal condition in the present study since the resultant closed-cell porous Al possessed good combinations of porosity, density and plateau stress, as well as energy absorption capacity.