Exploring the Storage Mechanism of Alkali Ions in Non-Graphitic Hard Carbon Anodes

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.

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

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.

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.

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.

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

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 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.

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

Graphite, graphitizable carbon (soft carbon) and non-graphitizable carbon (hard carbon) have been investigated as anode materials for lithium ion secondary batteries. Graphite and hard carbon are now mainly put to practical use and both have their merits and demerits. In this article, characteristics and properties of graphite and hard carbon as lithium intercalation hosts are described and the possibility is discussed of novel hard carbon anode materials prepared from agricultural wastes such as bamboo saw dusts or coffee grounds.

Recently demand for Li-ion batteries is growing as power sources for various applications such as electric vehicles and electric energy storage systems, because their high energy density and light weight properties are suitable for these applications. However, lithium resources may be insufficient to meet the increasing demand for these applications. Na-ion batteries could be a promising alternative to Li-ion batteries thanks to the natural abundance and lower cost of sodium. Non-graphitized carbon materials such as hard carbon are well-studied as high capacity anode materials for Na-ion batteries. The discharge capacity of 200 - 300 mAh/g, which is lower than conventional graphite anode materials for Li-ion batteries, has been reported [1,2]. In addition, the insertion mechanism of Na-ion into non-graphitized carbon materials is proposed that Na-ions are first inserted between stacked graphene layers in the voltage range from 2.0 V to 0.2 V (vs. Na metal), and then inserted into nano-pores in the voltage range from 0.2 V to 0.0 V (vs. Na metal) [2]. In this study, we focused on the pore structure of carbon materials and investigated the relationship between the carbon structures and electrochemical properties to clarify the optimum pore structure and propose a new carbon material as anode for high energy Na-ion batteries. Non-graphitized carbon samples were synthesized by heating various raw materials at between 1000 and 2600 °C under flowing argon. The pore structure of the synthesized carbon samples was characterized by using argon and nitrogen adsorption measurement and helium gas densitometry. Electrochemical properties were evaluated by using laminate-type cell with Na metals as the counter electrode and the reference electrode. Open and closed pore volumes of the synthesized carbons were calculated and the relationship between the reversible capacity and pore volume was investigated. As seen in Figure 1, an almost linear relationship between the reversible capacity in the voltage range from 0.2 to 0.0 V (vs. Na metal) and the closed pore volume was confirmed. Some carbon materials with large closed pore volume on the basis of this relationship were designed and successfully synthesized. Figure 2 shows charge/discharge curves of one of synthesized carbon samples. This carbon sample had the closed pore volume of 0.37 cc/g and its discharge capacity reached 413 mAh/g. This result suggests that Na-ion could be inserted reversibly in the closed pore and non-graphitized carbons with large closed pore volume may be promising for high capacity anode materials for Na-ion batteries. [1] D. A. Stevens et al, J. Electrochem. Soc., 148 (2001) A803 [2] S.Komaba et al, ACS Appl. Mater. Interfaces, 3 (2011) 4165 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.

Understanding the sodium-storage mechanism in hard carbon (HC) anodes is crucial for advancing sodium-ion battery (SIB) technology. However, the intrinsic complexity of HC microstructures and their interactions with sodium remain not fully elucidated. We present a multiscale methodology that integrates grand-canonical Monte Carlo (GCMC) simulations with a machine-learning interatomic potential based on the Gaussian approximation potential (GAP) framework to investigate sodium insertion mechanisms in hard carbons with different levels of porosity, achieved by simulating structural models with densities ranging from 0.7 to 1.9 g cm-3. Structural and thermodynamic analyses reveal the interplay between pore size and accessibility and the relative contributions of adsorption, intercalation, and pore filling to the overall storage capacity. Low-density carbons favor pore-filling, achieving extremely high capacities at near-zero voltages, whereas high-density carbons primarily store sodium through adsorption and intercalation, leading to lower but more stable capacities. Intermediate-density carbons (1.3-1.6 g cm-3) provide the most balanced performance, combining moderate capacity (480 and 310 mAh g-1), safe operating voltages, and minimal volume expansion (<10%). These findings establish a direct correlation between carbon density and electrochemical behavior, providing atomic-scale insight into how hard carbon morphology governs sodium storage. The proposed framework offers a rational design principle for optimizing HC-based SIB anodes toward high energy density and long-term cycling stability.

Unveiling the impact of ultramicropores in carbonaceous electrode materials to boost surface-driven capacitive potassium ion storage in hybrid supercapacitors

The increasing demand for sodium‐ion batteries has risen due to sodium resources that are inexpensive and abundant compared with its counterpart of lithium‐ion batteries. Development of anode materials with high performance is central issue for sodium‐ion batteries. Plant‐based hard carbon material is a promising anode material for sodium ion batteries due to its green preparation process, favorable ions transport channel, and special porous structure. Previous studies have not systematically correlated the material's inherent structure limitation with its electrochemical properties, resulting in a complex classification and a lack selection of pretreatment methods. In this review, the intrinsic limitations in the structure of plant‐based precursors are divided into four parts, including impure components, single chemical structure, uncontrollable crystalline texture, and irregular porous structure. The previous pretreatment methods are reclassified, including purification component aiming at plant‐based precursors, optimized structure, and precabornized process aiming at intermediate products. Furthermore, the pretreatment strategies are discussed according to the preferred structures of plant‐based hard carbon and the relationship between structure and performance is systematically expounded. This review provides deep understanding of the mechanism of pretreatment and a guide for selecting appropriate pretreatment strategies to optimize the structure of plant‐based hard carbon for sodium ion batteries.