Low‐Defect and Low‐Porosity Hard Carbon with High Coulombic Efficiency and High Capacity for Practical Sodium Ion Battery Anode

Hard carbon is regarded as the most promising anode material for commercialization of Na ion batteries because of its high capacity and low cost. At present, the practical utilization of hard carbon anodes is largely limited by the low initial Coulombic efficiency (ICE). Na ions have been found to adopt an adsorption–insertion storage mechanism. In this paper a systematic way to control the defect concentration and porosity of hard carbon with similar overall architectures is shown. This study elucidates that the defects in the graphite layers are directly related to the ICE as they would trap Na ions and create a repulsive electric field for other Na ions so as to shorten the low‐voltage intercalation capacity. The obtained low defect and porosity hard carbon electrode has achieved the highest ICE of 86.1% (94.5% for pure hard carbon material by subtracting that of the conductive carbon black), reversible capacity of 361 mA h g^?1, and excellent cycle stability (93.4% of capacity retention over 100 cycles). This result sheds light on feasible design principles for high performance Na storage hard carbon: suitable carbon layer distance and defect free graphitic layers.     

Low‐Temperature Growth of Hard Carbon with Graphite Crystal for Sodium‐Ion Storage with High Initial Coulombic Efficiency: A General Method

Practical application of hard carbon materials in sodium‐ion batteries (SIBs) is largely limited by their low initial coulombic efficiency (ICE), which may be improved by increasing the graphitization degree. However, biomass‐derived hard carbon is usually nongraphitizable and extremely difficult to graphitize by direct heating even at 3000 °C. Herein, a general strategy is reported for fabricating hard carbon materials with graphite crystals at 1300 °C promoted by external graphite that serves as a crystal template for the growth of graphite crystals. The graphite crystals enable the contacted pseudographitic domains with a high‐level ordered structure, large domain size, and low defects, leading to an enhanced ICE. The obtained hard carbon materials with graphite crystals, using the carbonized eggshell membranes, and sucrose‐derived microsphere as precursors, achieve very high ICE of 89% and 91% with reversible capacity of 310 and 301 mA h g^?1, respectively. Therefore, using external graphite to promote high‐level ordering pseudographitic domains at low temperature is quite useful to improve ICE for SIB applications.     

Boosting Fast Sodium Storage of a Large‐Scalable Carbon Anode with an Ultralong Cycle Life

Sodium‐ion batteries (SIBs) are considered to be a promising alternative for large‐scale electricity storage. However, it is urgent to develop new anode materials with superior ultralong cycle life performance at high current rates. Herein, a low‐cost and large‐scalable sulfur‐doped carbon anode material that exhibits the best high‐rate cycle performance and the longest cycle life ever reported for carbon anodes is developed. The material delivers a reversible capacity of 142 mA h g^?1 at a current rate up to 10 A g^?1. After 10 000 cycles the capacity is remained at 126.5 mA h g^?1; 89.1% of the initial value. Density functional theory computations demonstrate that the sulfur‐doped carbon has a strong binding affinity for sodium which promotes sodium storage. Meanwhile, the kinetics analysis identifies the capacitive charge storage as a large contributor to sodium storage, which favors ultrafast storage of sodium ions. These results demonstrate a new way to design carbon‐based SIBs anodes for next‐generation large‐scale electricity storage.     

Rational Assembly of Hollow Microporous Carbon Spheres as P Hosts for Long‐Life Sodium‐Ion Batteries

This paper reports the rational assembly of novel hollow porous carbon nanospheres (HPCNSs) as the hosts of phosphorous (P) active materials for high‐performance sodium‐ion batteries (SIBs). The vaporization‐condensation process is employed to synthesize P/C composites, which is elucidated by both theories and experiments to achieve optimized designs. The combined molecular dynamics simulations and density functional theory calculations indicate that the morphologies of polymeric P₄ and the P loading in the P/C composites depend mainly on the pore size and surface condition of carbon supports. Micropores of 1–2 nm in diameter and oxygenated functional groups attached on carbon surface are essential for achieving high P loading and excellent structural stability. In light of these findings, HPCNS/amorphous red phosphorus composites with enhanced structural/functional features are synthesized, which present an extremely low volume expansion of ≈67.3% during cycles, much smaller than the commercial red P's theoretical value of ≈300%. The composite anodes deliver an exceptional sodium storage capacity and remarkable long‐life cyclic stability with capacity retention over 76% after 1000 cycles. This work signifies the importance of rational design of electrode materials based on accurate theoretical predictions and sheds light on future development of cost‐effective P/C composite anodes for commercially viable SIBs.     

Carbon Anodes for Nonaqueous Alkali Metal‐Ion Batteries and Their Thermal Safety Aspects

Since their commercialization by Sony in 1991, graphite anodes in combination with various cathodes have enabled the widespread success of lithium‐ion batteries (LIBs), providing over 10 billion rechargeable batteries to the global population. Next‐generation nonaqueous alkali metal‐ion batteries, namely sodium‐ion batteries (SIBs) and potassium‐ion batteries (PIBs), are projected to utilize intercalation‐based carbon anodes as well, due to their favorable electrochemical properties. While traditionally graphite anodes have dominated the market share of LIBs, other carbon materials have been investigated, including graphene, carbon nanotubes, and disordered carbons. The relationship between carbon material properties, electrochemical performance, and charge storage mechanisms is clarified for these alkali metal‐ion batteries, elucidating possible strategies for obtaining enhanced cycling stability, specific capacity, rate capability, and safety aspects. As a key component in determining cell performance, the solid electrolyte interphase layer is described in detail, particularly for its dependence on the carbon anode. Finally, battery safety at extreme temperatures is discussed, where carbon anodes are susceptible to dendrite formation, accelerated aging, and eventual thermal runaway. As society pushes toward higher energy density LIBs, this review aims to provide guidance toward the development of sustainable next‐generation SIBs and PIBs.     

Extended “Adsorption–Insertion” Model: A New Insight into the Sodium Storage Mechanism of Hard Carbons

Hard carbons (HCs) are promising anodes of sodium‐ion batteries (SIBs) due to their high capacity, abundance, and low cost. However, the sodium storage mechanism of HCs remains unclear with no consensus in the literature. Here, based on the correlation between the microstructure and Na storage behavior of HCs synthesized over a wide pyrolysis temperature range of 600–2500 °C, an extended “adsorption–insertion” sodium storage mechanism is proposed. The microstructure of HCs can be divided into three types with different sodium storage mechanisms. The highly disordered carbon, with d₀₀₂ (above 0.40 nm) large enough for sodium ions to freely transfer in, has a “pseudo‐adsorption” sodium storage mechanism, contributing to sloping capacity above 0.1 V, together with other conventional “defects” (pores, edges, heteroatoms, etc.). The pseudo‐graphitic carbon (d‐spacing in 0.36–0.40 nm) contributes to the low‐potential (<0.1 V) plateau capacity through “interlayer insertion” mechanism, with a theoretical capacity of 279 mAh g^?1 for NaC₈ formation. The graphite‐like carbon with d₀₀₂ below 0.36 nm is inaccessible for sodium ion insertion. The extended “adsorption–insertion” model can accurately explain the dependence of the sodium storage behavior of HCs with different microstructures on the pyrolysis temperature and provides new insight into the design of HC anodes for SIBs.     

Hard Carbon Anodes: Fundamental Understanding and Commercial Perspectives for Na-Ion Batteries beyond Li-Ion and K-Ion Counterparts

Hard carbon (HC) is recognized as a promising anode material with outstanding electrochemical performance for alkali metal-ion batteries including lithium-ion batteries (LIBs), as well as their analogs sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs). Herein, a comprehensive review of the recent research is presented to interpret the challenges and opportunities for the applications of HC anodes. The ion storage mechanisms, materials design, and electrolyte optimizations for alkali metal-ion batteries are illustrated in-depth. HC is particularly promising as an anode material for SIBs. The solid-electrolyte interphase, initial Coulombic efficiency, safety concerns, and all-climate performances, which are vital for practical applications, are comprehensively discussed. Furthermore, commercial prototypes of SIBs based on HC anodes are extensively elaborated. The remaining challenges and research perspectives are provided, aiming to shed light on future research and early commercialization of HC-based SIBs.     

3D Nanowire Arrayed Cu Current Collector toward Homogeneous Alloying Anode Deposition for Enhanced Sodium Storage

Alloying electrodes are regarded as promising anodes for lithium/sodium storage thanks to their multielectron reaction capacity, moderate voltage plateau, and high electrical conductivity. However, huge volume change upon cycling, especially for sodium storage, usually causes the loss of electrical connection between active components and their delaminations from traditional current collectors, thus leading to rapid capacity decay. Herein, a unique 3D current collector is assembled from 1D nanowire arrays anchored on 3D porous Cu foams for constructing core‐shelled Cu@Sb nanowires as advanced sodium‐ion battery (SIB) anodes. The so‐formed hierarchical 3D anode with interconnected 3D micrometer sized pores and abundant voids between nanowires not only effectively accommodates the structural strains during repeated cycling but also ensures the structural integrity and contributes to a uniform ion/electron scattered distribution throughout the whole surface. When employed as anodes for SIBs, the obtained electrode shows a high capacity of 605.3 mAh g^?1 at 330 mA g^?1, and demonstrates a high capacity retention of 84.8% even at a high current density of 3300 mA g^?1. The 3D nanowire arrayed Cu current collector in this work can offer a promising strategy for designing and building advanced alloy anodes for lithium/sodium storage.     

Deactivating Defects in Graphenes with Al2O3 Nanoclusters to Produce Long‐Life and High‐Rate Sodium‐Ion Batteries

Carbon materials are the most promising anodes for sodium‐ion batteries (SIBs), but low initial Coulombic efficiency (ICE) and poor cyclic stability hinder their practical use. It is shown herein, that an effective but simple remedy for these problems can be achieved by deactivating defects in the carbon with Al₂O₃ nanocluster coverage. A 3D porous graphene monolith (PGM) is used as the model material and Al₂O₃ nanoclusters around 1 nm are grown on the defects of graphene. It is shown that these Al₂O₃ nanoclusters suppress the decomposition of conductive sodium salt in the electrolyte, resulting in the formation of a thin and homogenous solid electrolyte interphase (SEI). In addition, Al₂O₃ nanoclusters appear to reduce the detrimental etching of the SEI by hydrogen fluoride (HF) and improve its stability. Therefore, after the introduction of Al₂O₃ nanoclusters, the ICE, cyclic stability, and rate capability of the PGM are greatly improved. A higher ICE (70.2%) and capacity retention (82.9% after 500 cycles at 0.5 A g^?1) than those of normally reported for large surface area carbons are achieved. This work indicates a new way to deactivate defects and modify the SEI of carbon materials, and hopefully accelerate the commercialization of carbon materials as anode materials for SIBs.     

Layered Oxide Cathodes for Sodium‐Ion Batteries: Phase Transition,Air Stability,and Performance

The increasing demand for replacing conventional fossil fuels with clean energy or economical and sustainable energy storage drives better battery research today. Sodium‐ion batteries (SIBs) are considered as a promising alternative for grid‐scale storage applications due to their similar “rocking‐chair” sodium storage mechanism to lithium‐ion batteries, the natural abundance, and the low cost of Na resources. Searching for appropriate electrode materials with acceptable electrochemical performance is the key point for development of SIBs. Layered transition metal oxides represent one of the most fascinating electrode materials owing to their superior specific capacity, environmental benignity, and facile synthesis. However, three major challenges (irreversible phase transition, storage instability, and insufficient battery performance) are known for cathodes in SIBs. Herein, a comprehensive review on the latest advances and progresses in the exploration of layered oxides for SIBs is presented, and a detailed and deep understanding of the relationship of phase transition, air stability, and electrochemical performance in layered oxide cathodes is provided in terms of refining the structure–function–property relationship to design improved battery materials. Layered oxides will be a competitive and attractive choice as cathodes for SIBs in next‐generation energy storage devices.     

Hard Carbon Microtubes Made from Renewable Cotton as High‐Performance Anode Material for Sodium‐Ion Batteries

Sodium‐ion batteries (SIBs) have attracted more and more attention for scalable electrical energy storage due to the abundance and wide distribution of Na resources. However, the anode still remains a great challenge for the application of SIBs. Here the production of uniform hard carbon microtubes (HCTs) made from natural cotton through one simple carbonization process and their application as an anode are reported. The study shows that the electrochemical performance of the HCTs is seriously affected by the carbonization temperature due to the difference in their microstructure and heteroatomic content. The HCTs carbonized at 1300 °C deliver the highest reversible capacity of 315 mAh g^?1 and good rate capability due to their unique tubular structure. This contribution not only provides a new approach for the preparation of hard carbon materials with unique tubular microstructure using natural inspiration, but it also deepens the fundamental understanding of the sodium storage mechanism.     

A Safe Organic/Inorganic Composite Anode for Sodium-Ion Batteries

Sodium-ion batteries (SIBs), based on hard carbon anodes and Na⁺-intercalation compound cathodes, have gained significant attention. Nonetheless, hard carbon anodes involve the storage of Na⁺ at a low potential, typically below 0.1 V (vs Na/Na⁺), which increases the risk of dendritic Na growth on the anode surface during overcharging. Herein, a safe organic/inorganic composite anode containing tetrasodium 3,4,9,10-perylenetetracarboxylate (Na₄PTC) and Metallic bismuth (Bi) with a weight ratio of 7:2, which exhibits an average potential of 0.7 V (vs Na⁺/Na) and a capacity of 150 mAh g⁻¹ is proposed. The electrode reaction involves a reversible coordination reaction within the organic host and alloying reactions within the metallic Bi component. Importantly, the organic component efficiently buffers the volume changes in Bi during the alloying reaction, while the metallic Bi enhances the electronic conductivity of the organic material. As a result, this composite anode shows high cycle stability and rate performance, even under high mass loadings ranging from 10 to 50 mg cm⁻². Furthermore, it is demonstrated that the Na-ion full cell, consisting of the composite anode and the Na₃V₂O₂(PO₄)₂F cathode, exhibits minimal capacity degradation over 100 cycles while maintaining a high areal capacity of 1.1 mA cm⁻².     

Codoped Holey Graphene Aerogel by Selective Etching for High‐Performance Sodium‐Ion Storage

The pursuit of more efficient carbon‐based anodes for sodium‐ion batteries (SIBs) prepared from facile and economical methods is a very important endeavor. Based on the crystallinity difference within carbon materials, herein, a low‐temperature selective burning method is developed for preparing oxygen and nitrogen codoped holey graphene aerogel as additive‐free anode for SIBs. By selective burning of a mixture of graphene and low‐crystallinity carbon at 450 °C in air, an elastic porous graphene monolith with abundant holes on graphene sheets and optimized crystallinity is obtained. These structural characteristics lead to an additive‐free electrode with fast charge (ions and electrons) transfer and more abundant Na⁺ storage active sites. Moreover, the heteroatom oxygen/nitrogen doping favors large interlayer distance for rapid Na⁺ insertion/extraction and provides more active sites for high capacitive contribution. The optimized sample exhibits superior sodium‐ion storage capability, i.e., high specific capacity (446 mAh g^?1 at 0.1 A g^?1), ultrahigh rate capability (189 mAh g^?1 at 10 A g^?1), and long cycle life (81.0% capacity retention after 2000 cycles at 5 A g^?1). This facile and economic strategy might be extended to fabricating other superior carbon‐based energy storage materials.     

Partially Reduced Holey Graphene Oxide as High Performance Anode for Sodium‐Ion Batteries

The current Na⁺ storage performance of carbon‐based materials is still hindered by the sluggish Na⁺ ion transfer kinetics and low capacity. Graphene and its derivatives have been widely investigated as electrode materials in energy storage and conversion systems. However, as anode materials for sodium‐ion batteries (SIBs), the severe π–π restacking of graphene sheets usually results in compact structure with a small interlayer distance and a long ion transfer distance, thus leading to low capacity and poor rate capability. Herein, partially reduced holey graphene oxide is prepared by simple H₂O₂ treatment and subsequent low temperature reduction of graphene oxide, leading to large interlayer distance (0.434 nm), fast ion transport, and larger Na⁺ storage space. The partially remaining oxygenous groups can also contribute to the capacity by redox reaction. As anode material for SIBs, the optimized electrode delivers high reversible capacity, high rate capability (365 and 131 mAh g^?1 at 0.1 and 10 A g^?1, respectively), and good cycling performance (163 mAh g^?1 after 3000 cycles at a current density of 2 A g^?1), which is among the best reported performances for carbon‐based SIB anodes.     

Regulating Pore Structure of Hierarchical Porous Waste Cork‐Derived Hard Carbon Anode for Enhanced Na Storage Performance

Porous structure design is generally considered to be a reliable strategy to boost ion transport and provide active sites for disordered carbon anodes of Na‐ion batteries (NIBs). Herein, a type of waste cork‐derived hard carbon material (CC) is reported for efficient Na storage via tuning the pore species. Benefiting from the natural holey texture of this renewable precursor, CCs deliver a novel hierarchical porous structure. The effective skeletal density test combined with small angle X‐ray scattering analysis (SAXS) is used to obtain the closed pore information. Based on a detailed correlation analysis between pore information and the electrochemical performance of CCs, improving pyrolysis temperature to reduce open pores (related to initial capacity loss) and increase closed pores (related to plateau capacity) endows an optimal CC with a high specific capacity of ≈360 mAh g^?1 in half‐cells and a high energy density of 230 Wh kg^?1 in full‐cells with a capacity retention of 71% after 2000 cycles at 2C rate. The bioinspired high temperature pore‐closing strategy and the new insights about the pore structure–performance relationship provide a rational guide for designing porous carbon anode of NIBs with tailored pore species and high Na storage capacity.     

Heteroatom‐Doped Mesoporous Hollow Carbon Spheres for Fast Sodium Storage with an Ultralong Cycle Life

Carbon materials have attracted significant attention as anode materials for sodium ion batteries (SIBs). Developing a carbon anode with long‐term cycling stability under ultrahigh rate is essential for practical application of SIBs in energy storage systems. Herein, sulfur and nitrogen codoped mesoporous hollow carbon spheres are developed, exhibiting high rate performance of 144 mA h g^?1 at 20 A g^?1, and excellent cycling durability under ultrahigh current density. Interestingly, during 7000 cycles at a current density of 20 A g^?1, the capacity of the electrode gradually increases to 180 mA h g^?1. The mechanisms for the superior electrochemical performance and capacity improvement of the cells are studied by electrochemical tests, ex situ transmission electron microscopy, X‐ray diffraction, X‐ray photoelectron spectroscopy, and Raman analysis of fresh and cycled electrodes. The unique and robust structure of the material can enhance transport kinetics of electrons and sodium ions, and maintain fast sodium storage from the capacitive process under high rate. The self‐rearrangement of the carbon structure, induced by continuous discharge and charge, lead to the capacity improvement with cycles. These results demonstrate a new avenue to design advanced anode materials for SIBs.     

Lithium‐Pretreated Hard Carbon as High‐Performance Sodium‐Ion Battery Anodes

Hard carbon (HC) is the state‐of‐the‐art anode material for sodium‐ion batteries (SIBs). However, its performance has been plagued by the limited initial Coulombic efficiency (ICE) and mediocre rate performance. Here, experimental and theoretical studies are combined to demonstrate the application of lithium‐pretreated HC (LPHC) as high‐performance anode materials for SIBs by manipulating the solid electrolyte interphase in tetraglyme (TEGDME)‐based electrolyte. The LPHC in TEGDME can 1) deliver > 92% ICE and ≈220 mAh g^?1 specific capacity, twice of the capacity (≈100 mAh g^?1) in carbonate electrolyte; 2) achieve > 85% capacity retention over 1000 cycles at 1000 mA g^?1 current density (4 C rate, 1 C = 250 mA g^?1) with a specific capacity of ≈150 mAh g^?1, ≈15 times of the capacity (10 mAh g^?1) in carbonate. The full cell of Na₃V₂(PO₄)₃‐LPHC in TEGDME demonstrated close to theoretical specific capacity of ≈98 mAh g^?1 based on Na₃V₂(PO₄)₃ cathode, ≈2.5 times of the value (≈40 mAh g^?1) with nontreated HC. This work provides new perception on the anode development for SIBs.     

3D Amorphous Carbon with Controlled Porous and Disordered Structures as a High‐Rate Anode Material for Sodium‐Ion Batteries

Sodium‐ion batteries (SIBs) have a promising application prospect for energy storage systems due to the abundant resource. Amorphous carbon with high electronic conductivity and high surface area is likely to be the most promising anode material for SIBs. However, the rate capability of amorphous carbon in SIBs is still a big challenge because of the sluggish kinetics of Na⁺ ions. Herein, a three‐dimensional amorphous carbon (3DAC) with controlled porous and disordered structures is synthesized via a facile NaCl template‐assisted method. Combination of open porous structures of 3DAC, the increased disordered structures can not only facilitate the diffusion of Na⁺ ions but also enhance the reversible capacity of Na storage. When applied as anode materials for SIBs, 3DAC exhibits excellent rate capability (66 mA h g^?1 at 9.6 A g^?1) and high reversible capacity (280 mA h g^?1 at a low current density of 0.03 A g^?1). Moreover, the controlled porous structures by the NaCl template method provide an appropriate specific surface area, which contributes to a relatively high initial Coulombic efficiency of 75%. Additionally, the high‐rate 3DAC material is prepared via a green approach originating from low‐cost pitch and NaCl template, demonstrating an appealing development of carbon anode materials for SIBs.     

Nanosheets‐Assembled CuSe Crystal Pillar as a Stable and High‐Power Anode for Sodium‐Ion and Potassium‐Ion Batteries

Thanks to low costs and the abundance of the resources, sodium‐ion (SIBs) and potassium‐ion batteries (PIBs) have emerged as leading candidates for next‐generation energy storage devices. So far, only few materials can serve as the host for both Na⁺ and K⁺ ions. Herein, a cubic phase CuSe with crystal‐pillar‐like morphology (CPL‐CuSe) assembled by the nanosheets are synthesized and its dual functionality in SIBs and PIBs is comprehensively studied. The electrochemical measurements demonstrate that CPL‐CuSe enables fast Na⁺ and K⁺ storage as well as the sufficiently long duration. Specifically, the anode delivers a specific capacity of 295 mA h g^?1 at current density of 10 A g^?1 in SIBs, while 280 mA h g^?1 at 5 A g^?1 in PIBs, as well as the high capacity retention of nearly 100% over 1200 cycles and 340 cycles, respectively. Remarkably, CPL‐CuSe exhibits a high initial coulombic efficiency of 91.0% (SIBs) and 92.4% (PIBs), superior to most existing selenide anodes. A combination of in situ X‐ray diffraction and ex situ transmission electron microscopy tests fundamentally reveal the structural transition and phase evolution of CuSe, which shows a reversible conversion reaction for both cells, while the intermediate products are different due to the sluggish K⁺ insertion reaction.     

Advanced Organic Electrode Materials for Rechargeable Sodium‐Ion Batteries

Benefiting from the high abundance and low cost of sodium resource, rechargeable sodium‐ion batteries (SIBs) are regarded as promising candidates for large‐scale electrochemical energy storage and conversion. Due to the heavier mass and larger radius of Na⁺ than that of Li⁺, SIBs with inorganic electrode materials are currently plagued with low capacity and insufficient cycling life. In comparison, organic electrode materials display the advantages of structure designability, high capacity and low limitation of cationic radius. However, organic electrode materials also encounter issues such as high‐solubility in electrolyte and low conductivity. Here, recently reported organic electrode materials, which mainly include the reactions based on either carbon‐oxygen double bond or carbon‐nitrogen double bond, and doping reactions, are systematically reviewed. Furthermore, the design strategies of organic electrodes are comprehensively summarized. The working voltage is regulated through controlling the lowest unoccupied molecular orbital energies. The theoretical capacity can be enhanced by increasing the active groups. The dissolution is inhibited with elevating the intermolecular forces with proper molecular weight. The conductivity can be improved with extending conjugated structures. Future research into organic electrodes should focus on the development of full SIBs with aqueous/aprotic electrolytes and long cycling stability.