Density-dependent sodium-storage mechanisms in hard carbon materials.

Sodium-ion batteries (SIBs) have significant potential for applications in portable electric vehicles and intermittent renewable energy storage due to their relatively low cost. Currently, hard carbon (HC) materials are considered commercially viable anode materials for SIBs due to their advantages, including larger capacity, low cost, low operating voltage, and inimitable microstructure. Among these materials, renewable biomass-derived hard carbon anodes are commonly used in SIBs. However, the reports about biomass hard carbon from basic research to industrial applications are very rare. In this paper, we focus on the research progress of biomass-derived hard carbon materials from the following perspectives: (1) sodium storage mechanisms in hard carbon; (2) optimization strategies for hard carbon materials encompassing design, synthesis, heteroatom doping, material compounding, electrolyte modulation, and presodiation; (3) classification of different biomass-derived hard carbon materials based on precursor source, a comparison of their properties, and a discussion on the effects of different biomass sources on hard carbon material properties; (4) challenges and strategies for practical of biomass-derived hard carbon anode in SIBs; and (5) an overview of the current industrialization of biomass-derived hard carbon anodes. Finally, we present the challenges, strategies, and prospects for the future development of biomass-derived hard carbon materials.

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

Against the backdrop of global energy transition toward low‐carbon systems and the implementation of China's “Dual Carbon” strategic objectives, sodium‐ion batteries (SIBs) have emerged as a critical complementary technology to lithium‐ion batteries due to their resource abundance, cost‐effectiveness, and superior low‐temperature performance. Among key components, hard carbon anode materials are regarded as the most promising candidates for SIBs owing to their low sodiation potential (<0.1 V vs Na/Na + ), high reversible capacity, long cycle life, and economic feasibility. This review focuses on biomass‐derived hard carbon materials, systematically summarizing research progress in sodium storage mechanisms, raw material sources (e.g., cellulose, starch, shells), preparation processes (pretreatment, carbonization strategies, post‐treatment), and advanced characterization techniques (e.g., interlayer spacing, closed pores, sodium clusters) employed across studies. The necessity of combining advanced characterization with experiments is further emphasized in the text to quantitatively analyze the correlation between microstructure characteristics and electrochemical performance. Only by establishing a plateau that collaboratively links the sources of biomass precursors, synthesis process parameters, and energy storage mechanisms, covering the entire process from “precursor → process → structure → performance”, can the hard carbon derived from biomass be elevated from an accidental product to a mature anode material for SIBs with adjustable structure and designable performance, and promote the practical application process of SIBs.

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.

Hard carbon (HC) is an attractive anode material for sodium-ion batteries (NIBs) because it can be made from biomass or plastic, has a high capacity of ~300 mAh/g, and an extremely low average voltage near 0V. When fabricated with cathodes free of Li, Ni, and Co, NIBs using HC anodes are a promising approach for grid storage because they are energy-dense, sustainable, and have a low cost of energy. In order for these batteries to become a practical long-life solution, the issues of low first coulombic efficiency and poor rate performance of HC in conventional carbonate electrolytes need to be addressed. Two mechanisms that potentially cause these issues to occur are changes to the HC structure affecting sodium storage and the formation of the solid electrolyte interface (SEI). The proper selection of electrolyte is critical for controlling these mechanisms. To observe and characterize these mechanisms, we compared HC using a conventional carbonate electrolyte, 1M NaPF6 in propylene carbonate (PC), with a high performing glyme based electrolyte, 1M NaBF4 in tetraethylene glycol dimethyl ether (TEGDME). Compared to the poor rate capability of HC in PC electrolyte, HC in TEGDME electrolyte shows an excellent rate capability of ~265 mAh/g at C/20 and ~260 mAh/g at C/3. Titration gas chromatography and Raman spectroscopy were used to show that the HC with either electrolyte had the same sodium storage mechanism with no “stuck” sodium in the HC structure after the first cycle. These observations suggest that SEI formation is responsible for the irreversible capacity differences. The SEI formation was explored with scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), and x-ray photoelectron spectroscopy (XPS). The PC electrolyte’s SEI is found to be thick, fluffy, irregular and composed of polyesters, alkyl carbonates, NaxPFyOz, and NaF. The TEGDME electrolyte’s SEI is found to be thin, conformal, uniform, and composed of polyethers, sodium alkoxides, NaxBFyOz, and NaF. Additionally, the PC electrolyte’s SEI is rate dependent with more unstable products formed at high rates whereas the TEGDME electrolyte’s SEI is not rate dependent. These findings show that HC anode’s rate capability is controlled by the quality of the SEI formed by the electrolyte rather than the HC material structure. This model of rate and electrolyte dependent SEI indicates that SEI engineering is a pathway to improve HC as an anode for long-life grid-level NIBs. Figure 1

Hard carbon materials have emerged as promising anode materials for sodium-ion batteries (SIBs) due to their abundant resources, low cost, and tunable pore structures. However, the relationship between preparation conditions, material structure, and electrochemical performance remains unclear. In this study, coal-based hard carbon was synthesized via a one-step carbonization method. By characterizing the structure of hard carbons derived from different carbonization temperatures and analyzing their electrochemical performance, we have established the correlation between carbonization temperature, graphite microcrystal structure, and sodium storage behavior. The as-prepared sample at a carbonization temperature of 900 °C showed abundant graphite microcrystals with a layer spacing of 0.383 nm, and CO surface functional groups. It showed a high reversible capacity of 277.7 mA h g-1, a long plateau discharge capacity of 111.76 mA h g-1, and an excellent rate capacity retention of 76% at 1 A g-1. This work offers guidance for the production of coal-based hard carbon materials, and provides the possibility of high energy density SIBs.

The current strong interest in electromotive mobility and the need to transition to an energy grid with sustainable storage devices has led to a renewed interest in sodium ion batteries (SIBs). Hard carbons (HCs) are promising candidates for high-capacity negative electrode materials in SIBs. Their high capacities, however, are often accompanied with high irreversible capacity losses during the initial cycles.[1]Generally, large losses can be connected to irreversible reactions, e.g., the formation of the solid electrolyte interphase (SEI), during the initial sodiation of the HC-material. However, high irreversible capacity values are often in contradiction to the experimentally determined low surface area of the sample material.[1] A better understanding of the structure-property relationship should enable quantification and understanding of the potential of hard carbon materials for SIBs. Hence, the aim is to use analytical techniques to establish a correlation between the structure and the electrochemical characteristics of HC-materials. This has previously been difficult, in part because the sodium storage mechanism is not stoichiometric and due to the disordered structure of HCs.In order to tackle the aforementioned challenges, our approach is to explore whether a core-shell structure can separate sodium storage and solid electrolyte interphase formation. Thus, storage capacity and irreversible losses could be investigated and finally optimized separately. The synthesis of a selection of porous carbon structures serving as the core material, will be attempted. Simultaneously, sodium-conducting shell structures will be developed to allow for separation of sodium ions and electrolyte molecules. Subsequently, the combination of core and shell materials will be undertaken. These anodes should enable high capacities accompanied with low irreversible losses due to optimized SEI-formation.Literature:[1] Matsukawa, Y.; Linsenmann, F.; Plass, M. A.; Hasegawa, G.; Hayashi, K.; Fellinger, T.-P. Beilstein J. Nanotechnol. 2020, 11, 1217–1229.

Macro-micro structure engineering of bio-spore-derived hard carbon as an efficient anode in sodium ion batteries

Developing supermechanically resilient hard carbon materials that can quickly accommodate sodium ions is highly demanded in fabricating durable anodes for wearable sodium-ion batteries. Here, an interconnected spiral nanofibrous hard carbon fabric with both remarkable resiliency (e.g., recovery rate as high as 1200 mm s-1 ) and high Young's modulus is reported. The hard carbon nanofabrics are prepared by spinning and then carbonizing the reaction product of polyacrylonitrile and polar molecules (melamine). The resulting unique hard carbon possesses a highly disordered carbonaceous structure with enlarged interlayer spacing contributed from the strong electrostatic repulsion of dense pyrrolic nitrogen atoms. Its excellent resiliency remains after intercalation/deintercalation of sodium ions. The outstanding sodium-storage performance of the derived anode includes excellent gravimetric capacity, high-power capability, and long-term cyclic stability. More significantly, with a high loading mass, the hard carbon anode displays a high-power capacity (1.05 mAh cm-2 at 2 A g-1 ) and excellent cyclic stability. This study provides a unique strategy for the design and fabrication of new hard carbon materials for advanced wearable energy storage systems.

Hard carbon has the potential to serve as a high‐capacity anode material for sodium‐ion batteries (SIBs), however, its Na+ storage mechanism, particularly on the low potential plateau, remains controversial. To overcome this issue, two types of hard carbons with different microstructures are employed and the relationship between the microstructures and Na+ storage behaviors is evaluated. By the combination of operando X‐ray diffraction, ex situ Raman spectroscopy, NMR, and theoretical calculation, it is found that the sodium storage capacities of the hard carbons in the low potential plateau region contain the concurrent contributions from both interlayer intercalation and micropores filling, and the ratio of the two contributors greatly depends on the microstructure of hard carbon materials. Moreover, an electrochemical pointer (potential inflection point at the end of the discharge curve) is found to distinguish the dominance of interlayer intercalation and the micropores filling processes of sodium ions in the low potential plateau region. Based on this new finding, a microstructure‐dependent mechanism (“adsorption‐intercalation/filling” hybrid mechanism) is proposed to achieve an overall understanding of the sodium storage behaviors in different hard carbon materials, which may provide deep insight into the rational design of hard carbon structures as high‐performance anode materials for advanced SIBs.

Sodium‐ion batteries (SIBs) hold great potential in the application of large‐scale energy storage. With the coming commercialization of SIBs, developing advanced anode of particularly hard carbon is becoming increasingly urgent yet challenging. Hard carbon still suffers from unclear sodium storage mechanism, unsatisfactory performance, and low initial Coulombic efficiency (ICE). Herein, the current state‐of‐the‐art advances in designing hard carbon anodes for high‐performance SIBs is summarized. First, the formation process of hard carbon and typical sodium storage models of “insertion–adsorption,” “adsorption–insertion,” “adsorption–pore filling,” and “adsorption–insertion–pore filling” are introduced systematically. Then, the key strategies including morphological engineering, heteroatom doping, and graphitic structure regulation are presented to enhance the capacity of hard carbon based on the in‐depth understanding of sodium storage behaviors. Subsequently, to promote the practical application of hard carbon, more attention is paid to the methods of ICE improvement, including electrolyte optimization, defect and surface engineering, and presodiation. Whereafter, hard‐carbon‐based SIBs and their intriguing applications are briefly sketched. Finally, future directions and challenging perspectives of hard‐carbon anodes for SIBs are proposed from the viewpoints of storage mechanisms, electrode structures, and presodiation techniques.

Insight to defects regulation on sugarcane waste-derived hard carbon anode for sodium-ion batteries

Hard carbon (HC) materials are suitable anodes for sodium-ion batteries (SIBs) but still suffer from insufficient initial Coulombic efficiency (ICE). Promoting sodium storage via the pore filling mechanism is an effective way to improve the ICE, and the key here is regulating the pore structures of HC. In this work, coal-derived HC is successfully engineered with abundant accessible closed nanopores by treating the coal precursors with a facile destructive oxidation strategy. Investigations demonstrate that the destructive oxidation strategy can not only introduce abundant oxygen-containing functional groups (OFGs) but also decrease the size of graphitic microcrystals. Thus, the OFGs significantly enhance the crosslinking of small graphitic microcrystals and stimulate the formation of accessible closed nanopores during carbonization, which eventually improves the ICE by promoting the pore filling mechanism. The optimized HC exhibits so far the highest ICE (92.2%) among coal-derived SIB anode materials, together with a considerable capacity of 328.5 mAh g-1 at 90mA g-1 and a capacity retention of 95.1% after 150 cycles. The results provide guidelines for developing high-performance HC materials toward the large-scale application of SIBs, which is of great significance for future energy storage systems.

Hard carbon materials are considered as the most promising anode for sodium-ion batteries (SIBs). However, the high cost and poor rate performance hinder their application in SIBs. Moreover, the controversial mechanism of Na-ion storage restricts the improvement of hard carbon anodes. Herein, hard carbon micro-nano tubes (HCMNTs) from low-cost biomass kapok fibers are prepared as a promising anode for SIBs. Benefitting from the micro-nano structure, which offers low surface area and short Na+ diffusion path, 1400HCMNT possesses a good initial Coulombic efficiency of 80%, a high reversible capacity of 290 mA h g-1, and an excellent rate capacity. Furthermore, electron paramagnetic resonance and thermogravimetric analysis were applied to investigate the Na-ion storage mechanism in the HCMNTs. Sodium is stored in the hard carbon in an ionic state in the slope region and as quasi-liquid metallic sodium clusters in the low-voltage plateau.

Biomass-derived hard carbon (HC) materials, celebrated for their superior sodium storage capability and economic viability, have emerged as a promising candidate for anode materials in sodium-ion batteries (SIBs). However, the electronic conductivity, cycling stability, and initial Coulombic efficiency (ICE) of HC materials remain significant challenges hindering their broader application. In this study, we successfully developed a HC anode material using Indian trumpetflower seeds (ITS) as precursors, enhanced by coating with pitch on their surface. This ITS-based HC material not only delivers an impressive reversible capacity of 430.3 mAh g-1 @ 300 mA g-1 but also maintains a capacity retention of 63% at a current density of 1500 mA g-1, along with a remarkable ICE of 90.92%. After 900 cycles, the capacity retention remains high at 77%, demonstrating exceptional cycling stability. These performance enhancements are attributed to the appropriate concentration of carbonized oxygen-doped pitch (CODP) coating, which avoids direct contact between the electrolyte and ITS and introduces oxygen atoms to provide more active sites. This work not only provides a promising precursor for the fabrication of high-performance HC anodes for SIBs but also introduces an innovative synthetic strategy, injecting momentum into the commercialization process of SIBs.