Influence of Pore Architecture and Chemical Structure on the Sodium Storage in Nitrogen-Doped Hard Carbons

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.

Dual Activation for Tuning N, S Co-Doping in Porous Carbon Sheets Toward Superior Sodium Ion Storage

Porous carbon has been widely focused to solve the problems of low coulombic efficiency (ICE) and low multiplication capacity of Sodium-ion batteries (SIBs) anodes. The superior energy storage properties of two-dimensional(2D) carbon nanosheets can be realized by modulating the structure, but be limited by the carbon sources, making it challenging to obtain 2D structures with large surface area. In this work, a new method for forming carbon materials with high N/S doping content based on combustion activation using the dual activation effect of K2SO4/KNO3 is proposed. The synthesized carbon material as an anode for SIBs has a high reversible capacity of 344.44 mAh g-1 at 0.05 A g-1. Even at the current density of 5 Ag-1, the capacity remained at 143.08 mAh g-1. And the ICE of sodium-ion in ether electrolytes is ≈2.5 times higher than that in ester electrolytes. The sodium storage mechanism of ether/ester-based electrolytes is further explored through ex-situ characterizations. The disparity in electrochemical performance can be ascribed to the discrepancy in kinetics, wherein ether-based electrolytes exhibit a higher rate of Na+ storage and shedding compared to ester-based electrolytes. This work suggests an effective way to develop doubly doped carbon anode materials for SIBs.

Agar-Derived Slope-Dominated Carbon Anode with Puparium Like Nano-Morphology for Cost-Effective SIBs

The microstructure of hard carbons (HCs) including interlayer distance and lateral ab direction and pore size distribution plays a key role in regulating the sodium ions storage performance. Herein, by employing the gelatinous agar as a model precursor, series P-doping HCs (P-HC-x, x = 1, 2, 3, 4) are facilely prepared in batches via controllably regulating its crosslinking state by phytic acid (PA) at a low carbonization temperature of 750 °C, in which PA plays three roles (acid, flame retardant, and P-doping precursor) in promoting the final structure of P-HC-x. Among those, the puparium like P-HC-2 with expanded carbon interlayer distance of 3.91 Å and shortened lateral ab direction of 9.4 nm delivers a high reversible capacity of 394 mAh g-1 at 0.1 A g-1 with high increased slope capacity of 363 mAh g-1 as well as an ultrafast charge-discharge feature and a superlong cycle life. Pairing with the Na3V2(PO4)3 cathode, the fabricated sodium-ion full cells exhibit the 132 mAh g-1 reversible capacity at 0.1 A g-1, and 86% capacity retention after 100 cycles. This work successfully develops slope-dominated high-performance carbon anode, which will provide new insights for the microstructure regulation and design of other precursor-derivedHCs.

Electron paramagnetic resonance as a tool to determine the sodium charge storage mechanism of hard carbon

Hard carbon is a promising negative electrode material for rechargeable sodium-ion batteries due to the ready availability of their precursors and high reversible charge storage. The reaction mechanisms that drive the sodiation properties in hard carbons and subsequent electrochemical performance are strictly linked to the characteristic slope and plateau regions observed in the voltage profile of these materials. This work shows that electron paramagnetic resonance (EPR) spectroscopy is a powerful and fast diagnostic tool to predict the extent of the charge stored in the slope and plateau regions during galvanostatic tests in hard carbon materials. EPR lineshape simulation and temperature-dependent measurements help to separate the nature of the spins in mechanochemically modified hard carbon materials synthesised at different temperatures. This proves relationships between structure modification and electrochemical signatures in the galvanostatic curves to obtain information on their sodium storage mechanism. Furthermore, through ex situ EPR studies we study the evolution of these EPR signals at different states of charge to further elucidate the storage mechanisms in these carbons. Finally, we discuss the interrelationship between EPR spectroscopy data of the hard carbon samples studied and their corresponding charging storage mechanism.

Enhanced Sodium Ion Batteries' Performance: Optimal Strategies on Electrolytes for Different Carbon-based Anodes

Carbon-based materials have emerged as promising anodes for sodium-ion batteries (SIBs) due to the merits of cost-effectiveness and renewability. However, the unsatisfactory performance has hindered the commercialization of SIBs. During the past decades, tremendous attention has been put into enhancing the electrochemical performance of carbon-based anodes from the perspective of improving the compatibility of electrolytes and electrodes. Hence, a systematic summary of strategies for optimizing electrolytes between hard carbon, graphite, and other structural carbon anodes of SIBs is provided. The formulations and properties of electrolytes with solvents, salts, and additives added are comprehensively presented, which are closely related to the formation of solid electrolyte interface (SEI) and crucial to the sodium ion storage performance. Cost analysis of commonly used electrolytes has been provided as well. This review is anticipated to provide guidance in future rational tailoring of electrolytes with carbon-based anodes for sodium-ion batteries.

Probing Sodium Storage Mechanism in Hollow Carbon Nanospheres Using Liquid Phase Transmission Electron Microscopy

Carbonaceous materials are promising sodium-ion battery anodes. Improving their performance requires a detailed understanding of the ion transport in these materials, some important aspects of which are still under debate. In this work, nitrogen-doped porous hollow carbon spheres (N-PHCSs) are employed as a model system for operando analysis of sodium storage behavior in a commercial liquid electrolyte at the nanoscale. By combining the ex situ characterization at different states of charge with operando transmission electron microscopy experiments, it is found that a solvated ionic layer forms on the surface of N-PHCSs at the beginning of sodiation, followed by the irreversible shell expansion due to the solid-electrolyte interphase (SEI) formation and subsequent storage of Na(0) within the porous carbon shell. This shows that binding between Na(0) and C creates a Schottky junction making Na deposition inside the spheres more energetically favorable at low current densities. During sodiation, the SEI fills the gap between N-PHCSs, binding spheres together and facilitating the sodium ions' transport toward the current collector and subsequent plating underneath the electrode. The N-PHCSs layer acts as a protective layer between the electrolyte and the current collector, suppressing the possible growth of dendrites at the anode.

High-Capacity Sb/Fe2S3 Sodium-Ion Battery Anodes Fabricated by a One-Step Redox Reaction, Followed by Ball Milling with Graphite

Antimony (Sb) has been considered a promising anode for sodium-ion batteries (SIBs) owing to its high theoretical capacity (660 mA h g^-1) and low redox voltage (0.2-0.9 V vs Na⁺/Na). However, the capacity degradation caused by the volumetric variation during battery discharge/charge hinders the practical application. Herein, guided by the DFT calculation, Sb/Fe₂S₃ was fabricated by annealing Fe and Sb₂S₃ mixed powder. Next, Sb/Fe₂S₃ was blended with 15 wt % graphite by ball milling, yielding nano-Sb/Fe₂S₃ anchored on an exfoliated graphite composite (denoted as Sb/Fe₂S₃-15%). When applied as an anode of SIBs, Sb/Fe₂S₃-15% delivered reversible capacities of 565, 542, 467, 366, 285, and 236 mA h g^-1 at current rates of 1, 2, 4, 6, 8, and 10 A g^-1, respectively, surpassing most of the Sb-based anodes. The co-existence of highly conductive Fe₂S₃ and Sb minimizes the polarization of the anode. Our experiments proved that the Sb and Fe₂S₃ phases were reversible during discharge/charge cycling, and the exfoliated graphite can accelerate the Na⁺ diffusion and e^- conduction. The proposed synthesis method of this work can also be applicable to synthesize various antimony/transition metal sulfide heterostructures (Sb/M_1-xS), which may be applied in a series of fields.

Surface Stretching Enables Highly Disordered Graphitic Domains for Ultrahigh Rate Sodium Storage

Hard carbons (HCs) with high sloping capacity are considered as the leading candidate anode for sodium-ion batteries (SIBs); nevertheless, achieving basically complete slope-dominated behavior with high rate capability is still a big challenge. Herein, the synthesis of mesoporous carbon nanospheres with highly disordered graphitic domains and MoC nanodots modification via a surface stretching strategy is reported. The MoO_x surface coordination layer inhibits the graphitization process at high temperature, thus creating short and wide graphite domains. Meanwhile, the in situ formed MoC nanodots can greatly promote the conductivity of highly disordered carbon. Consequently, MoC@MCNs exhibit an outstanding rate capacity (125 mAh g^-1 at 50 A g^-1 ). The "adsorption-filling" mechanism combined with excellent kinetics is also studied based on the short-range graphitic domains to reveal the enhanced slope-dominated capacity. The insight in this work encourages the design of HC anodes with dominated slope capacity toward high-performance SIBs.

Noncrystalline Carbon Anodes for Advanced Sodium-Ion Storage

Developing an anode with excellent rate performance, long-cycle stability, high coulombic efficiency, and high specific capacity is one of the key research directions of sodium-ion batteries. Among all the anode materials, noncrystalline carbon (NCC) has great possibilities according to its supreme performance and low cost, but with the complexity and variability of the structure. With the in-depth study of the sodium storage behaviors of NCC in recent years, three modes of interlayer intercalation, clustering into micropores, and adsorption are reported and summarized. Although the storage mechanism has gradually become more evident, the complex behavior of the ions at different voltage regions, especially in the low-voltage (plateau) region, still remains controversial. It is essential to understand further the relationship between ions and NCC structure during energy storage processes. Based on the summary of previous works, this article has reviewed the storage mechanism of sodium ions in NCC and evaluated the structure-behavior relationship between sodium-ion storage and the carbon structure.

Application of Thermal Response Measurements to Investigate Enhanced Water Adsorption Kinetics in Ball-Milled C2 N-Type Materials

Sorption-based water capture is an attractive solution to provide potable water in arid regions. Heteroatom-decorated microporous carbons with hydrophilic character are promising candidates for water adsorption at low humidity, but the strong affinity between the polar carbon pore walls and water molecules can hinder the water transport within the narrow pore system. To reduce the limitations of mass transfer, C2 N-type carbon materials obtained from the thermal condensation of a molecular hexaazatriphenylene-hexacarbonitrile (HAT-CN) precursor were treated mechanochemically via ball milling. Scanning electron microscopy as well as static light scattering reveal that large pristine C2 N-type particles were split up to a smaller size after ball milling, thus increasing the pore accessibility which consequently leads to faster occupation of the water vapor adsorption sites. The major aim of this work is to demonstrate the applicability of thermal response measurements to track these enhanced kinetics of water adsorption. The adsorption rate constant of a C2 N material condensed at 700 °C remarkably increased from 0.026 s-1 to 0.036 s-1 upon ball milling, while maintaining remarkably high water vapor capacity. This work confirms the advantages of small particle sizes in ultramicroporous materials on their vapor adsorption kinetics. It is demonstrated that thermal response measurements are a valuable and time-saving method to investigate water adsorption kinetics, capacities, and cycling stability.

Insight into a Nitrogen-Doping Mechanism in a Hard-Carbon-Microsphere Anode Material for the Long-Term Cycling of Potassium-Ion Batteries

To investigate the alternatives to lithium-ion batteries, potassium-ion batteries have attracted considerable interest due to the cost-efficiency of potassium resources and the relatively lower standard redox potential of K⁺/K. Among various alternative anode materials, hard carbon has the advantages of extensive resources, low cost, and environmental protection. In the present study, we synthesize a nitrogen-doping hard-carbon-microsphere (N-SHC) material as an anode for potassium-ion batteries. N-SHC delivers a high reversible capacity of 248 mAh g⁻¹ and a promoted rate performance (93 mAh g⁻¹ at 2 A g⁻¹). Additionally, the nitrogen-doping N-SHC material also exhibits superior cycling long-term stability, where the N-SHC electrode maintains a high reversible capacity at 200 mAh g⁻¹ with a capacity retention of 81% after 600 cycles. DFT calculations assess the change in K ions’ absorption energy and diffusion barriers at different N-doping effects. Compared with an original hard-carbon material, pyridinic-N and pyrrolic-N defects introduced by N-doping display a positive effect on both K ions’ absorption and diffusion.

Enabling Fast Na+ Transfer Kinetics in the Whole-Voltage-Region of Hard-Carbon Anodes for Ultrahigh-Rate Sodium Storage

Efficient electrode materials, that combine high power and high energy, are the crucial requisites of sodium-ion batteries (SIBs), which have unwrapped new possibilities in the areas of grid-scale energy storage. Hard carbons (HCs) are considered as the leading candidate anode materials for SIBs, however, the primary challenge of slow charge-transfer kinetics at the low potential region (<0.1 V) remains unresolved till date, and the underlying structure-performance correlation is under debate. Herein, ultrafast sodium storage in the whole-voltage-region (0.01-2 V), with the Na⁺ diffusion coefficient enhanced by 2 orders of magnitude (≈10^-7 cm² s^-1 ) through rationally deploying the physical parameters of HCs using a ZnO-assisted bulk etching strategy is reported. It is unveiled that the Na⁺ adsorption energy (E_a ) and diffusion barrier (E_b ) are in a positive and negative linear relationship with the carbon p-band center, respectively, and balance of E_a and E_b is critical in enhancing the charge-storage kinetics. The charge-storage mechanism in HCs is evidenced through comprehensive in(ex) situ techniques. The as prepared HCs microspheres deliver a record high rate performance of 107 mAh g^-1 @ 50 A g^-1 and unprecedented electrochemical performance at extremely low temperature (426 mAh g^-1 @ -40 °C).

Schiff-bases for sustainable battery and supercapacitor electrodes

The quest for more efficient ways to store electrical energy prompted the development of different storage devices over the last decades. This includes but is not limited to different battery concepts and supercapacitors. However, modern batteries rely on electrochemical principles that often involve transition metals which can for instance suffer from toxicity or limited availability. More sustainable alternatives are needed. This sparked the search for organic electrode materials. Nevertheless, compared to their inorganic counterparts, organic electrode materials remain less intensely investigated. Besides the often more complicated electrochemical principles, one likely reason for that are the complex synthetic skills required to develop novel organic materials. Here we review materials synthesized by an old and comparably simple reaction from the field of organic chemistry, namely Schiff-base formation. This reaction can often yield materials under relatively mild conditions, making them especially interesting for the formation of sustainable electrodes. The main weakness of Schiff-base materials, susceptibility to hydrolysis, is of limited concern in most of the battery systems as they mostly anyways require water-free conditions. This review gives an overview of some selected nanomaterials obtained from Schiff-base formation as well as their carbonized derivatives which are of interest for energy storage. Firstly, the general chemistry of Schiff-bases is introduced, followed by an in-depth survey of the most important breakthroughs in the formation of organic battery electrodes that involve materials based on Schiff-base reaction. Lastly, an outlook considering the main hurdles as well as future perspectives of this research area is given.

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

Hard carbon (HC) displays great potential for high-performance sodium-ion batteries (SIBs) due to its cost-effective, simple fabrication and most likely to be commercialized. However, the complicated microstructures of HC lead to difficulties in deeply understanding the structure-performance correlation. Particularly, evaluation of influence of pore structure on Na storage performances is still causing disputes and rational strategies of designing pore architecture of HC are still necessary. In this work, the skillful and controllable phase-inversion method is applied to construct porous HC with abundantly interconnected and permeable tunnel-like pores, which can promote ionic diffusion and improve electrode-electrolyte interfacial affinity. Structure-performance investigation reveals that porous HC with cross-coupled macropore architecture can boost Na storage performances comprehensively. Compared to pristine HC with negligible pores, well-regulated porous HC anodes show an obvious enhancement on initial Coulombic efficiency (ICE) of 68.3% (only 51.5% for pristine HC), reversible capacity of 332.7 mAh g^-1 at 0.05 A g^-1, rate performance with 67.4% capacity retention at 2 A g^-1 (46.5% for pristine HC), and cycling stability with 95% capacity maintained for 90 cycles (86.4% for pristine HC). Additionally, the ICE can be optimized up to 76% by using sodium carboxymethyl cellulose as a binder. This work provides an important view of optimizing Na storage performances of HC anodes by pore engineering, which can be broadened into other electrode materials.

Revisit Electrolyte Chemistry of Hard Carbon in Ether for Na Storage

Hard carbons (HCs) as an anode material in sodium ion batteries present enhanced electrochemical performances in ether-based electrolytes, giving them potential for use in practical applications. However, the underlying mechanism behind the excellent performances is still in question. Here, ex situ nuclear magnetic resonance, gas chromatography-mass spectrometry, and high-resolution transmission electron microscopy were used to clarify the insightful chemistry of ether- and ester-based electrolytes in terms of the solid-electrolyte interphase (SEI) on hard carbons. The results confirm the marked electrolyte decomposition and the formation of a SEI film in EC/DEC but no SEI film in the case of diglyme. In situ electrochemical quartz crystal microbalance and molecular dynamics support that ether molecules have likely been co-intercalated into hard carbons. To our knowledge, these results are reported for the first time. It might be very useful for the rational design of advanced electrode materials based on HCs in the future.

Insights into the sodiation mechanism of hard carbon-like materials from electrochemical impedance spectroscopy

To render the sodium ion battery (SIB) competitive among other technologies, the processes behind sodium storage in hard carbon anodes must be understood. For this purpose, electrochemical impedance spectroscopy (EIS) is usually undervalued, since fitting the spectra with equivalent circuit models requires an a priori knowledge about the system at hand. The analysis of the distribution of relaxation times (DRT) is an alternative, which refrains from fitting arbitrarily nested equivalent circuits. In this paper, the sodiation and desodiation of a hard carbon anode is studied by EIS at different states of charge (SOC). By reconstructing the DRT function, highly resolved information on the number and relative contribution of individual electrochemical processes is derived. During the sloping part of the sodiation curve, mass transport is found to be the most dominant source of resistance but rapidly diminishes when the plateau phase is reached. An equivalent circuit model qualitatively reproducing the experimental data of the sloping region was built upon the DRT results, which is particularly useful for future EIS studies on hard carbon SIB anodes. More importantly, this work contributes to establish EIS as a practical tool to directly study electrode processes without the bias of a previously assumed model.