Boosting Sodium Storage Performance of Hard Carbon Anodes by Pore Architecture Engineering

Regulating the structure of hard carbon derived from industrial waste to boost sodium storage performance

Biomass-derived hard carbon materials stand out in the research of anode materials for sodium-ion batteries (SIBs) due to their low cost, high performance, and broad range of raw materials. However, the low carbonization yields of biomass-derived hard carbons and the waste produced during industrial production processes contribute to the high production cost of these hard carbons. Furthermore, it is essential to discuss the use of various electrolytes aimed at enhancing the ion storage capacity within SIBs. Herein, the high-temperature carbonization was proposed to recycle waste precursors for fabricating the hard carbon materials and regulating its ordering degree to boost the sodium storage ability. Meanwhile, it was found that ether-based electrolytes have positive effects on sodium storage by in-situ Raman characterization compared to ester-based electrolytes. Moreover, the assembled pouch cells and 18,650 cylindrical cells utilizing obtained hard carbon materials exhibit excellent reversible capacities of 406 and 674 mAh, respectively, indicating their promising potential for applications. This work presents simple recycling methods and compares the differences between the materials in ester-based and ether-based electrolytes, providing guidance for reducing material costs and developing high-performance anodes for SIBs.

Melamine-formaldehyde resin-derived hard carbon spheres: Morphology-preserved synthesis and closed pore formation for sodium storage

Melamine-formaldehyde resin spheres (MFS) often suffer from foaming and structure collapse during pyrolysis. The nitrogen's role in closed pore development of hard carbons for sodium storage is also barely investigated. Herein, through pre-stabilizing of MFS at 150 °C using ammonium chloride (NH4Cl) as a curing agent, we obtained nitrogen-rich (38.3 ± 1.7 at.%) spherical precursor at 400 °C and hard carbon spheres via further 800-1500 °C treatment. During the curing phase, the continuous decomposition of NH4Cl generates hydrogen chloride, which catalyzes sufficient condensation of the molecular structure, enabling the uniform shrinkage of spherical framework during subsequent pyrolysis and carbonization. In addition, the nitrogen release creates numerous defect sites, which hinders the ordered microcrystal development. Above 1000 °C, nitrogen is largely eliminated and intermittent microcrystals crosslink to form closed pores. Ultimately, the closed pore surface area greatly increases from 6 to 736 m2 g-1 as the carbonization temperature rises from 800 to 1500 °C, contributing to a remarkable enhancement in low-potential plateau sodium storage capacity from 15.9 to 206.1 mAh g-1. Therefore, this work provides a reference for preparing MFS derived hard carbon spheres, demonstrating that nitrogen in the precursors favors the formation and development of closed pores during high-temperature carbonization.

Boosting Sodium Storage in Pitch-Derived Hard Carbon via MgO Catalytic Preoxidation

Pitch-based hard carbon (HC) materials at low cost and high carbon yield represent a promising anode for sodium-ion batteries. However, their limited capacity poses a significant challenge to their practical use. Here, we report a universal strategy to boost sodium storage of pitch-based HC materials via catalytic oxidation with magnesium oxide to produce a high-oxygen pitch. The oxygenic groups suppress pitch softening and molecular rearrangement during carbonization, resulting in a highly disordered structure and substantially closed pores in the obtained HC materials. As a result, the optimized HC materials achieve a specific capacity of 321.7 mAh g-1 with a significant 80.5% increase over typical pitch-based carbon materials, along with a high initial Coulombic efficiency up to 88.5 and 88.3% capacity retention after 600 cycles. This study provides new insights into the rational design of high-capacity pitch-based HC and holds potential for application in other carbon-based materials.

Benchmarking the Match of Porous Carbon Substrate Pore Volume on Silicon Anode Materials for Lithium-Ion Batteries

Silicon (Si) is one of the most promising anode materials for high-energy-density lithium-ion batteries. However, the huge volume expansion hinders its commercial application. Embedding amorphous Si nanoparticles in a porous carbon framework is an effective way to alleviate Si volume expansion, with the pore volume of the carbon substrates playing a pivotal role. This work demonstrates the impact of pore volume on the electrochemical performance of the silicon/carbon porous composites from two perspectives: 1) pore volume affects the loadings of Si particles; 2) pore volume affects the structural stability and mechanical properties. The smaller pore volume of the carbon substrate cannot support the high Si loadings, which results in forming a thick Si shell on the surface, thereby being detrimental to cycling stability and the diffusion of electrons and ions. On top of that, the carbon substrate with a larger pore volume has poor structural stability due to its fragility, which is also not conducive to realizing long cycle life and high rate performance. Achieving excellent electrochemical performances should match the proper pore volume with Si content. This study will provide important insights into the rational design of the silicon/carbon porous composites based on the pore volume of the carbon substrates.

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).

Interfacial-Catalysis-Enabled Layered and Inorganic-Rich SEI on Hard Carbon Anodes in Ester Electrolytes for Sodium-Ion Batteries

Constructing a homogenous and inorganic-rich solid electrolyte interface (SEI) can efficiently improve the overall sodium-storage performance of hard carbon (HC) anodes. However, the thick and heterogenous SEI derived from conventional ester electrolytes fails to meet the above requirements. Herein, an innovative interfacial catalysis mechanism is proposed to design a favorable SEI in ester electrolytes by reconstructing the surface functionality of HC, of which abundant CO (carbonyl) bonds are accurately and homogenously implanted. The CO (carbonyl) bonds act as active centers that controllably catalyze the preferential reduction of salts and directionally guide SEI growth to form a homogenous, layered, and inorganic-rich SEI. Therefore, excessive solvent decomposition is suppressed, and the interfacial Na⁺ transfer and structural stability of SEI on HC anodes are greatly promoted, contributing to a comprehensive enhancement in sodium-storage performance. The optimal anodes exhibit an outstanding reversible capacity (379.6 mAh g^-1 ), an ultrahigh initial Coulombic efficiency (93.2%), a largely improved rate capability, and an extremely stable cycling performance with a capacity decay rate of 0.0018% for 10 000 cycles at 5 A g^-1 . This work provides novel insights into smart regulation of interface chemistry to realize high-performance HC anodes for sodium storage.

Superfast Mass Transport of Na/K Via Mesochannels for Dendrite-Free Metal Batteries

Fast ion diffusion in anode hosts enabling uniform distribution of Li/Na/K is essential for achieving dendrite-free alkali-metal batteries. Common strategies, e.g. expanding the interlayer spacing of anode materials, can enhance bulk diffusion of Li but are less efficient for Na and K due to their larger ionic radius. Herein, a universal strategy to drastically improve the mass-transport efficiency of Na/K by introducing open mesochannels in carbon hosts is proposed. Such pore engineering can increase the accessible surface area by one order of magnitude, thus remarkably accelerating surface diffusion, as visualized by in situ transmission electron microscopy. In particular, once the mesochannels are filled by the Na/K metals, they become the superfast channels for mass transport via the mechanism of interfacial diffusion. Thus-modified carbon hosts enable Na/K filling in their inner cavities and uniform deposition across the whole electrodes with fast kinetics. The resulting Na-metal anodes can exhibit stable dendrite-free cycling with outstanding rate performance at a high current density of up to 30 mA cm^-2 . This work presents an inspiring attempt to address the sluggish transport issue of Na/K, as well as valuable insights into the mass-transport mechanism in porous anodes for high-performance alkali-metal storage.

Biomass-Derived Hard Carbon with Interlayer Spacing Optimization toward Ultrastable Na-Ion Storage

Hard carbons as a kind of nongraphitized amorphous carbon have been recognized as potential anode materials for sodium-ion batteries (SIBs) due to its large interlayer spacing. However, the issues in terms of onerous synthetic procedure and elusive working mechanism remains critical bottlenecks for practical implement. Herein, we report a facile production of tubular hard carbon through direct carbonization of platanus flosses (FHC) for the first time. Through optimizing the pyrolysis temperatures, the FHC obtained at 1300 °C possesses a key balance between the interlayer spacing and surface area, which can maintain the substantial active sites as well as reduce the irreversible sodium storage. Accordingly, it can deliver a reversible capacity of 324.6 mAh g^-1 with a high initial Coulombic efficiency of 80%, superb rate property of 107.2 mAh g^-1 at 2 A g^-1, and long operating stability over 1000 cycles. Furthermore, the in situ Raman spectroscopic studies certify that sodium ions are stored in FHC following the "adsorption-insertion" mechanism. Our study could provide a promising route for large-scale development of the biomass-derived carbonaceous anodes for high-performance SIBs.

Metal Selenides Anode Materials for Sodium Ion Batteries: Synthesis, Modification, and Application

The powerful and rapid development of lithium-ion batteries (LIBs) in secondary batteries field makes lithium resources in short supply, leading to rising battery costs. Under the circumstances, sodium-ion batteries (SIBs) with low cost, inexhaustible sodium reserves, and analogous work principle to LIBs, have evolved as one of the most anticipated candidates for large-scale energy storage devices. Thereinto, the applicable electrode is a core element for the smooth development of SIBs. Among various anode materials, metal selenides (MSe_x ) with relatively high theoretical capacity and unique structures have aroused extensive interest. Regrettably, MSe_x suffers from large volume expansion and unwished side reactions, which result in poor electrochemistry performance. Thus, strategies such as carbon modification, structural design, voltage control as well as electrolyte and binder optimization are adopted to alleviate these issues. In this review, the synthesis methods and main reaction mechanisms of MSe_x are systematically summarized. Meanwhile, the major challenges of MSe_x and the corresponding available strategies are proposed. Furthermore, the recent research progress on layered and nonlayered MSe_x for application in SIBs is presented and discussed in detail. Finally, the future development focuses of MSe_x in the field of rechargeable ion batteries are highlighted.

Sulfur Encapsulation and Sulfur Doping Synergistically Enhance Sodium Ion Storage in Microporous Carbon Anodes

MOF-based materials are a class of efficient precursors for the preparation of heteroatom-doped porous carbon materials that have been widely applied as anode materials for Na-ion batteries. Thereinto, sulfur is often introduced to increase defects and act as an active species to directly react with sodium ions. Although the sulfur introduction and high surface area can synergistically improve capacity and rate capability, the initial Coulombic efficiency (ICE) and electrical conductivity of carbon material are inevitably reduced. Therefore, balancing sodium storage capacity and ICE is still the bottleneck faced by adsorbent carbon materials. Here, sulfur-encapsulated microporous carbon material with nitrogen, sulfur dual-doping (NSPC) is synthesized by postprocessing, achieving the reduced specific surface area by encapsulating sulfur in micropores, and the increased active sites by edge sulfur doping. The synergy between encapsulation and sulfur doping effectively balances specific capacity, rate capability, and ICE. The NSPC material exhibits capacities of 591.5 and 244.2 mAh g^-1 at 0.5 and at 10 A g^-1, respectively, and the ICE is as high as 72.3%. Moreover, the effect of nitrogen and sulfur on the improvement of electron/ion diffusion kinetics is resonantly demonstrated by density functional theory calculations. This synergistic preparation method may reveal a feasible thought for fabricating excellent-performance adsorption-type carbon materials for Na-ion batteries.

Tailoring the surface chemistry of hard carbon towards high-efficiency sodium ion storage

Hard carbon (HC) is most likely to be a commercialized anode material for sodium-ion batteries (SIBs). However, its low initial coulombic efficiency (ICE) impedes its further large-scale industrialization. Since the ICE is greatly related to the side reactions of the electrolyte on the HC surface, herein, we focus on tailoring the surface chemistry of HC via a facile low-temperature oxygen plasma (LTOP) treatment technique. The modified HC after a suitable treatment time possesses a highly ordered and low defect surface without a negligible change in layer spacing, thus facilitating Na⁺ deinsertion/insertion and reducing the HC/electrolyte side reactions. Moreover, LTOP treatment also brings oxygen functional groups (CO) to the HC surface to enrich Na⁺ storage active sites. Consequently, the modified HC reveals a higher ICE of 80.9% compared to 60.6% in the bare HC. Also, the modified HC delivers an ultrahigh specific capacity of 331.0 mA h g^-1 at 0.1 A g^-1 and exhibits superior rate performance with a high specific capacity of 211.0 mA h g^-1 at 5 A g^-1. This work provides a feasible strategy to tailor the surface chemistry of HC for high-efficiency Na-storage and provides a novel avenue to construct high-efficiency SIBs.

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.

Ether-based electrolytes for sodium ion batteries

Sodium-ion batteries (SIBs) are considered to be strong candidates for large-scale energy storage with the benefits of cost-effectiveness and sodium abundance. Reliable electrolytes, as ionic conductors that regulate the electrochemical reaction behavior and the nature of the interface and electrode, are indispensable in the development of advanced SIBs with high Coulombic efficiency, stable cycling performance and high rate capability. Conventional carbonate-based electrolytes encounter numerous obstacles for their wide application in SIBs due to the formation of a dissolvable, continuous-thickening solid electrolyte interface (SEI) layer and inferior stability with electrodes. Comparatively, ether-based electrolytes (EBEs) are emerging in the secondary battery field with fascinating properties to improve the performance of batteries, especially SIBs. Their stable solvation structure enables highly reversible solvent-co-intercalation reactions and the formation of a thin and stable SEI. However, although EBEs can provide more stable cycling and rapid sodiation kinetics in electrodes, benefitting from their favorable electrolyte/electrode interactions such as chemical compatibility and good wettability, their special chemistry is still being investigated and puzzling. In this review, we provide a thorough and comprehensive overview on the developmental history, fundamental characteristics, superiorities and mechanisms of EBEs, together with their advances in other battery systems. Notably, the relation among electrolyte science, interfacial chemistry and electrochemical performance is highlighted, which is of great significance for the in-depth understanding of battery chemistry. Finally, future perspectives and potential directions are proposed to navigate the design and optimization of electrolytes and electrolyte/electrode interfaces for advanced batteries.