Self-Supporting, Flexible, Additive-Free, and Scalable Hard Carbon Paper Self-Interwoven by 1D Microbelts: Superb Room/Low-Temperature Sodium Storage and Working Mechanism

New frontiers in alkali metal insertion into carbon electrodes for energy storage

With rising interest in new electrodes for next-generation batteries, carbon materials remain as top competitors with their reliable performance, low-cost, low voltage reactions, and diverse tunability. Depending on carbon's structure, it can attain high cyclability as with Li+ at crystalline graphite or exceptional capacities with Na+ at amorphous, porous hard carbons. In this review, we discuss key results and research directions using carbon electrodes for alkali ion storage. We start the first section with hard carbon (HC), a leading material of interest for next-generation Na-ion batteries. Methods for tuning the HC structure towards a high capacity pore-filling mechanism are examined. The rate performance of hard carbon electrodes is further discussed. We finish this section with soft carbons that mostly remain as low performing materials compared to other carbons. In the second section, we discuss alkali ion insertion into graphite and graphite-like materials. Though graphite has a long history with Li-ion batteries, it also shows promising characteristics for K-ion batteries. We discuss the significant progress made on improving the electrolyte for high cyclability of graphite with K+. Thereafter, we evaluate B/C/N materials that have a similar structure to graphite but can attain higher capacities for both Li+ and Na+. Finally, we touch on the recent developments using alternative solvents for Na+ cointercalation at graphite and deeper knowledge on the intercalant structure. Despite steady progress, carbon electrodes continue to improve as a key group of materials for alkali energy storage.

Solvation Effect: The Cornerstone of High-Performance Battery Design for Commercialization-Driven Sodium Batteries

Sodium batteries (SBs) emerge as a potential candidate for large-scale energy storage and have become a hot topic in the past few decades. In the previous researches on electrolyte, designing electrolytes with the solvation theory has been the most promising direction is to improve the electrochemical performance of batteries through solvation theory. In general, the four essential factors for the commercial application of SBs, which are cost, low temperature performance, fast charge performance and safety. The solvent structure has significant impact on commercial applications. But so far, the solvation design of electrolyte and the practical application of sodium batteries have not been comprehensively summarized. This review first clarifies the process of Na+ solvation and the strategies for adjusting Na+ solvation. It is worth noting that the relationship between solvation theory and interface theory is pointed out. The cost, low temperature, fast charging, and safety issues of solvation are systematically summarized. The importance of the de-solvation step in low temperature and fast charging application is emphasized to help select better electrolytes for specific applications. Finally, new insights and potential solutions for electrolytes solvation related to SBs are proposed to stimulate revolutionary electrolyte chemistry for next generation SBs.

Polyphenol-Derived Carbonaceous Frameworks with Multiscale Porosity for High-Power Electrochemical Applications

With the escalating global demand for electric vehicles and sustainable energy solutions, increasing focus is placed on developing electrochemical systems that offer fast charging and high-power output, primarily governed by mass transport. Accordingly, porous carbons have emerged as highly promising electrochemically active or supporting materials due to expansive surface areas, tunable pore structures, and superior electrical conductivity, accelerating surface reaction. Yet, while substantial research has been devoted to crafting various porous carbons to increase specific surface areas, the optimal utilization of the surfaces remains underexplored. This review emphasizes the critical role of the fluid dynamics within multiscale porous carbonaceous electrodes, leading to substantially enhanced pore utilization in electrochemical systems. It elaborates on strategies of using sacrificial templates for incorporating meso/macropores into microporous carbon matrix, while exploiting the unique properties of polyphenol moieties such as sustainable carbons derived from biomass, inherent adhesive/cohesive interactions with template materials, and facile complexation capabilities with diverse materials, thereby enabling adaptive structural modulations. Furthermore, it explores how multiscale pore configurations influence pore-utilization efficiency, demonstrating advantages of incorporating multiscale pores. Finally, synergistic impact on the high-power electrochemical systems is examined, attributed to improved fluid-dynamic behavior within the carbonaceous frameworks, providing insights for advancing next-generation high-power electrochemical applications.

Tailored Hollow Mesoporous Carbon Nanospheres from Soft Emulsions Enhance Kinetics in Sodium Batteries

Mesoporous materials endowed with a hollow structure offer ample opportunities due to their integrated functionalities; however, current approaches mainly rely on the recruitment of solid rigid templates, and feasible strategies with better simplicity and tunability remain infertile. Here, we report a novel emulsion-driven coassembly method for constructing a highly tailored hollow architecture in mesoporous carbon, which can be completely processed on oil-water liquid interfaces instead of a solid rigid template. Such a facile and flexible methodology relies on the subtle employment of a 1,3,5-trimethylbenzene (TMB) additive, which acts as both an emulsion template and a swelling agent, leading to a compatible integration of oil droplets and composite micelles. The solution-based assembly process also shows high controllability, endowing the hollow carbon mesostructure with a uniform morphology of hundreds of nanometers and tunable cavities from 0 to 130 nm in diameter and porosities (mesopore sizes 2.5-7.7 nm; surface area 179-355 m2 g-1). Because of the unique features in permeability, diffusion, and surface access, the hollow mesoporous carbon nanospheres exhibit excellent high rate and cycling performances for sodium-ion storage. Our study reveals a cooperative assembly on the liquid interface, which could provide an alternative toolbox for constructing delicate mesostructures and complex hierarchies toward advanced technologies.

Challenges and Breakthroughs in Enhancing Temperature Tolerance of Sodium-Ion Batteries

Lithium-based batteries (LBBs) have been highly researched and recognized as a mature electrochemical energy storage (EES) system in recent years. However, their stability and effectiveness are primarily confined to room temperature conditions. At temperatures significantly below 0 °C or above 60 °C, LBBs experience substantial performance degradation. Under such challenging extreme contexts, sodium-ion batteries (SIBs) emerge as a promising complementary technology, distinguished by their fast dynamics at low-temperature regions and superior safety under elevated temperatures. Notably, developing SIBs suitable for wide-temperature usage still presents significant challenges, particularly for specific applications such as electric vehicles, renewable energy storage, and deep-space/polar explorations, which requires a thorough understanding of how SIBs perform under different temperature conditions. By reviewing the development of wide-temperature SIBs, the influence of temperature on the parameters related to battery performance, such as reaction constant, charge transfer resistance, etc., is systematically and comprehensively analyzed. The review emphasizes challenges encountered by SIBs in both low and high temperatures while exploring recent advancements in SIB materials, specifically focusing on strategies to enhance battery performance across diverse temperature ranges. Overall, insights gained from these studies will drive the development of SIBs that can handle the challenges posed by diverse and harsh climates.

Defect engineering unveiled: Enhancing potassium storage in expanded graphite anode

Expanded graphite (EG) stands out as a promising material for the negative electrode in potassium-ion batteries. However, its full potential is hindered by the limited diffusion pathway and storage sites for potassium ions, restricting the improvement of its electrochemical performance. To overcome this challenge, defect engineering emerges as a highly effective strategy to enhance the adsorption and reaction kinetics of potassium ions on electrode materials. This study delves into the specific effectiveness of defects in facilitating potassium storage, exploring the impact of defect-rich structures on dynamic processes. Employing ball milling, we introduce surface defects in EG, uncovering unique effects on its electrochemical behavior. These defects exhibit a remarkable ability to adsorb a significant quantity of potassium ions, facilitating the subsequent intercalation of potassium ions into the graphite structure. Consequently, this process leads to a higher potassium voltage. Furthermore, the generation of a diluted stage compound is more pronounced under high voltage conditions, promoting the progression of multiple stage reactions. Consequently, the EG sample post-ball milling demonstrates a notable capacity of 286.2 mAh g-1 at a current density of 25 mA g-1, showcasing an outstanding rate capability that surpasses that of pristine EG. This research not only highlights the efficacy of defect engineering in carbon materials but also provides unique insights into the specific manifestations of defects on dynamic processes, contributing to the advancement of potassium-ion battery technology.

High concentration in situ polymer gel electrolyte for high performance lithium metal batteries

A high concentration gel polymer electrolyte (GPE) was prepared by simply using LiFSI-LiNO3 dissolved in 1,3-dioxolane. The Li‖Li cell achieves stable battery cycling for over 3200 h. Furthermore, the Li‖Cu cell demonstrates a high CE of 99.2%. Even at a high current density of 8 mA cm-2, a high CE of 98.5% was still achieved. Notably, in a Li‖LiFePO4 cell, this electrolyte enables high capacity retention of 94.5% and an average CE of 99.8% over 500 cycles, showing promising prospects for high-performance lithium metal batteries.

Hard carbon for sodium-ion batteries: progress, strategies and future perspective

Because of its abundant resources, low cost and high reversible specific capacity, hard carbon (HC) is considered as the most likely commercial anode material for sodium-ion batteries (SIBs). Therefore, reasonable design and effective strategies to regulate the structure of HCs play a crucial role in promoting the development of SIBs. Herein, the progress in the preparation approaches for HC anode materials is systematically overviewed, with a special focus on the comparison between traditional fabrication methods and advanced strategies emerged in recent years in terms of their influence on performance, including preparation efficiency, initial coulombic efficiency (ICE), specific capacity and rate capability. Furthermore, the advanced strategies are categorized into two groups: those exhibiting potential for large-scale production to replace traditional methods and those presenting guidelines for achieving high-performance HC anodes from top-level design. Finally, challenges and future development prospects to achieve high-performance HC anodes are also proposed. We believe that this review will provide beneficial guidance to actualize the truly rational design of advanced HC anodes, facilitating the industrialization of SIBs and assisting in formulating design rules for developing high-end advanced electrode materials for energy storage devices.

Consummating ion desolvation in hard carbon anodes for reversible sodium storage

Hard carbons are emerging as the most viable anodes to support the commercialization of sodium-ion (Na-ion) batteries due to their competitive performance. However, the hard carbon anode suffers from low initial Coulombic efficiency (ICE), and the ambiguous Na-ion (Na+) storage mechanism and interfacial chemistry fail to give a reasonable interpretation. Here, we have identified the time-dependent ion pre-desolvation on the nanopore of hard carbons, which significantly affects the Na+ storage efficiency by altering the solvation structure of electrolytes. Consummating the pre-desolvation by extending the aging time, generates a highly aggregated electrolyte configuration inside the nanopore, resulting in negligible reductive decomposition of electrolytes. When applying the above insights, the hard carbon anodes achieve a high average ICE of 98.21% in the absence of any Na supplementation techniques. Therefore, the negative-to-positive capacity ratio can be reduced to 1.02 for full cells, which enables an improved energy density. The insight into hard carbons and related interphases may be extended to other battery systems and support the continued development of battery technology.

Emerging Chemistry for Wide-Temperature Sodium-Ion Batteries

The shortage of resources such as lithium and cobalt has promoted the development of novel battery systems with low cost, abundance, high performance, and efficient environmental adaptability. Due to the abundance and low cost of sodium, sodium-ion battery chemistry has drawn worldwide attention in energy storage systems. It is widely considered that wide-temperature tolerance sodium-ion batteries (WT-SIBs) can be rapidly developed due to their unique electrochemical and chemical properties. However, WT-SIBs, especially for their electrode materials and electrolyte systems, still face various challenges in harsh-temperature conditions. In this review, we focus on the achievements, failure mechanisms, fundamental chemistry, and scientific challenges of WT-SIBs. The insights of their design principles, current research, and safety issues are presented. Moreover, the possible future research directions on the battery materials for WT-SIBs are deeply discussed. Progress toward a comprehensive understanding of the emerging chemistry for WT-SIBs comprehensively discussed in this review will accelerate the practical applications of wide-temperature tolerance rechargeable batteries.

Microporous Sulfur-Carbon Materials with Extended Sodium Storage Window

Developing high-performance carbonaceous anode materials for sodium-ion batteries (SIBs) is still a grand quest for a more sustainable future of energy storage. Introducing sulfur within a carbon framework is one of the most promising attempts toward the development of highly efficient anode materials. Herein, a microporous sulfur-rich carbon anode obtained from a liquid sulfur-containing oligomer is introduced. The sodium storage mechanism shifts from surface-controlled to diffusion-controlled at higher synthesis temperatures. The different storage mechanisms and electrode performances are found to be independent of the bare electrode material's interplanar spacing. Therefore, these differences are attributed to an increased microporosity and a thiophene-rich chemical environment. The combination of these properties enables extending the plateau region to higher potential and achieving reversible overpotential sodium storage. Moreover, in-operando small-angle X-ray scattering (SAXS) reveals reversible electron density variations within the pore structure, in good agreement with the pore-filling sodium storage mechanism occurring in hard carbons (HCs). Eventually, the depicted framework will enable the design of high-performance anode materials for sodium-ion batteries with competitive energy density.

Regulation of anion-Na+ coordination chemistry in electrolyte solvates for low-temperature sodium-ion batteries

High-performance sodium storage at low temperature is urgent with the increasingly stringent demand for energy storage systems. However, the aggravated capacity loss is induced by the sluggish interfacial kinetics, which originates from the interfacial Na+ desolvation. Herein, all-fluorinated anions with ultrahigh electron donicity, trifluoroacetate (TFA-), are introduced into the diglyme (G2)-based electrolyte for the anion-reinforced solvates in a wide temperature range. The unique solvation structure with TFA- anions and decreased G2 molecules occupying the inner sheath accelerates desolvation of Na+ to exhibit decreased desolvation energy from 4.16 to 3.49 kJ mol-1 and 24.74 to 16.55 kJ mol-1 beyond and below -20 °C, respectively, compared with that in 1.0 M NaPF6-G2. These enable the cell of Na||Na3V2(PO4)3 to deliver 60.2% of its room-temperature capacity and high capacity retention of 99.2% after 100 cycles at -40 °C. This work highlights regulation of solvation chemistry for highly stable sodium-ion batteries at low temperature.

Revealing the closed pore formation of waste wood-derived hard carbon for advanced sodium-ion battery

Although the closed pore structure plays a key role in contributing low-voltage plateau capacity of hard carbon anode for sodium-ion batteries, the formation mechanism of closed pores is still under debate. Here, we employ waste wood-derived hard carbon as a template to systematically establish the formation mechanisms of closed pores and their effect on sodium storage performance. We find that the high crystallinity cellulose in nature wood decomposes to long-range carbon layers as the wall of closed pore, and the amorphous component can hinder the graphitization of carbon layer and induce the crispation of long-range carbon layers. The optimized sample demonstrates a high reversible capacity of 430 mAh g-1 at 20 mA g-1 (plateau capacity of 293 mAh g-1 for the second cycle), as well as good rate and stable cycling performances (85.4% after 400 cycles at 500 mA g-1). Deep insights into the closed pore formation will greatly forward the rational design of hard carbon anode with high capacity.

Recycling spent masks to fabricate flexible hard carbon anode toward advanced sodium energy storage

The massive discard of spent masks during the COVID-19 pandemic imposes great environmental anxiety to the human society, which calls for a reliable and sustainable outlet to mitigate this issue. In this work, we demonstrate a green design strategy of recycling the spent masks to fabricate hard carbon fabrics toward high-efficient sodium energy storage. After a simple carbonization treatment, flexible hard carbon fabrics composed of interwoven microtubular fibers are obtained. When serving as binder-free anodes of sodium-ion batteries, a large Na-ion storage capacity of 280 mAh g^-1 is achieved for the optimized sample. More impressively, the flexible anode exhibits an initial coulombic efficiency of as high as 86% and excellent rate/cycling performance. The real-life practice of the flexible hard carbon is realized in the full-cells. The present study affords an enlightening approach for the recycling fabrication of high value-added hard carbon materials from the spent masks for advanced sodium energy storage.

Toward High Performance Anodes for Sodium-Ion Batteries: From Hard Carbons to Anode-Free Systems

Sodium-ion batteries (SIBs) have been deemed to be a promising energy storage technology in terms of cost-effectiveness and sustainability. However, the electrodes often operate at potentials beyond their thermodynamic equilibrium, thus requiring the formation of interphases for kinetic stabilization. The interfaces of the anode such as typical hard carbons and sodium metals are particularly unstable because of its much lower chemical potential than the electrolyte. This creates more severe challenges for both anode and cathode interfaces when building anode-free cells to achieve higher energy densities. Manipulating the desolvation process through the nanoconfining strategy has been emphasized as an effective strategy to stabilize the interface and has attracted widespread attention. This Outlook provides a comprehensive understanding about the nanopore-based solvation structure regulation strategy and its role in building practical SIBs and anode-free batteries. Finally, guidelines for the design of better electrolytes and suggestions for constructing stable interphases are proposed from the perspective of desolvation or predesolvation.

Vanadium fluorophosphates: advanced cathode materials for next-generation secondary batteries

Next-generation secondary batteries including sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) are considered the most promising candidates for application to large-scale energy storage systems due to their abundant, evenly distributed and cost-effective sodium/potassium raw materials. The electrochemical performance of SIBs (PIBs) significantly depends on the inherent characteristics of the cathode material. Among the wide variety of cathode materials, sodium/potassium vanadium fluorophosphate (denoted as MVPF, M = Na and K) composites are widely investigated due to their fast ion transportation and robust structure. However, their poor electron conductivity leads to low specific capacity and poor rate capacity, limiting the further application of MVPF cathodes in large-scale energy storage. Accordingly, several modification strategies have been proposed to improve the performance of MVPF such as conductive coating, morphological regulation, and heteroatomic doping, which boost the electronic conductivity of these cathodes and enhance Na (K) ion transportation. Furthermore, the development and application of MVPF cathodes in SIBs at low temperatures are also outlined. Finally, we present a brief summary of the remaining challenges and corresponding strategies for the future development of MVPF cathodes.

Boosting the Reversible, High-Rate Na+ Storage Capability of the Hard Carbon Anode Via the Synergistic Structural Tailoring and Controlled Presodiation

Hard carbons (HCs) are extensively investigated as the potential anodes for commercialization of sodium-ion batteries (SIBs). However, the practical deployment of HC anode suffers from the retarded Na⁺ diffusion at the high-rate or low-temperature operation scenarios. Herein, a multiscale modification strategy by tuning HC microstructure on the particle level as well as replenishing extra Na⁺ reservoir for the electrode through a homogeneous presodiation therapy is presented. Consequently, the coulombic efficiency of HC anode can be precisely controlled till the close-to-unit value. Detailed kinetics analysis observes that the Na⁺ diffusivity can be drastically enhanced by two orders of magnitude at the low potential region (< 0.1 V vs. Na⁺ /Na), which accelerates the rate-limiting step. As pairing the presodiated HC anode (≈5.0 ± 0.2 mg cm^-2 ) with the NaVPO₄ F cathode (≈10.3 mg cm^-2 ) in the 200 mAh pouch cell, the optimal balance of the cyclability (83% over 1000 cycles), low-temperature behavior till -40 °C as well as the maximized power output of 1500 W kg^-1 can be simultaneously achieved. This synergistic modification strategy opens a new avenue to exploit the reversible, ultrafast Na⁺ storage kinetics of HC anodes, which thus constitutes a quantum leap forward toward high-rate SIB prototyping.

Zinc Single-Atom-Regulated Hard Carbons for High-Rate and Low-Temperature Sodium-Ion Batteries

Hard carbons, as one of the most commercializable anode materials for sodium-ion batteries (SIBs), have to deal with the trade-off between the rate capability and specific capacity or initial Columbic efficiency (ICE), and the fast performance decline at low temperature (LT) remains poorly understood. Here, a comprehensive regulation on the interfacial/bulk electrochemistry of hard carbons through atomic Zn doping is reported, which demonstrates a record-high reversible capacity (546 mAh g^-1 ), decent ICE (84%), remarkable rate capability (140 mAh g^-1 @ 50 A g^-1 ), and excellent LT capacity (443 mAh g^-1 @ -40 °C), outperforming the state-of-the-art literature. This work reveals that the Zn doping can generally induce a local electric field to enable fast bulk Na⁺ transportation, and meanwhile catalyze the decomposition of NaPF₆ to form a robust inorganic-rich solid-electrolyte interphase, which elaborates the underlying origin of the boosted electrochemical performance. Importantly, distinguished from room temperature, the intrinsic Na⁺ migration/desolvation ability of the electrolyte is disclosed to be the crucial rate-determining factors for the SIB performance at LT. This work provides a fundamental understanding on the charge-storage kinetics at varied temperatures.

Conformal carbon nitride thin film inter-active interphase heterojunction with sustainable carbon enhancing sodium storage performance

Sustainable, high-performance carbonaceous anode materials are highly required to bring sodium-ion batteries to a more competitive level. Here, we exploit our expertise to control the deposition of a nm-sized conformal coating of carbon nitride with tunable thickness to improve the electrochemical performance of anode material derived from sodium lignosulfonate. In this way, we significantly enhanced the electrochemical performances of the electrode, such as the first cycle efficiency, rate-capability, and specific capacity. In particular, with a 10 nm homogeneous carbon nitride coating, the specific capacity is extended by more than 30% with respect to the bare carbon material with an extended plateau capacity, which we attribute to a heterojunction effect at the materials' interface. Eventually, the design of (inter)active electrochemical interfaces will be a key step to improve the performance of carbonaceous anodes with a negligible increase in the material weight.

Recent Progress and Perspective: Na Ion Batteries Used at Low Temperatures

With the rapid development of electric power, lithium materials, as a rare metal material, will be used up in 50 years. Sodium, in the same main group as lithium in the periodic table, is abundant in earth's surface. However, in the study of sodium-ion batteries, there are still problems with their low-temperature performance. Its influencing factors mainly include three parts: cathode material, anode material, and electrolyte. In the cathode, there are Prussian blue and Prussian blue analogues, layered oxides, and polyanionic-type cathodes in four parts, as this paper discusses. However, in the anode, there is hard carbon, amorphous selenium, metal selenides, and the NaTi₂(PO₄)₃ anode. Then, we divide the electrolyte into four parts: organic electrolytes; ionic liquid electrolytes; aqueous electrolytes; and solid-state electrolytes. Here, we aim to find electrode materials with a high specific capacity of charge and discharge at lower temperatures. Meanwhile, high-electrical-potential cathode materials and low-potential anode materials are also found. Furthermore, their stability in air and performance degradation in full cells and half-cells are analyzed. As for the electrolyte, despite the aspects mentioned above, its electrical conductivity in low temperatures is also reported.

Step-by-step desolvation enables high-rate and ultra-stable sodium storage in hard carbon anodes

Hard carbon is regarded as the most promising anode material for sodium-ion (Na-ion) batteries, owing to its advantages of high abundance, low cost, and low operating potential. However, the rate capability and cycle life span of hard carbon anodes are far from satisfactory, severely hindering its industrial applications. Here, we demonstrate that the desolvation process defines the Na-ion diffusion kinetics and the formation of a solid electrolyte interface (SEI). The 3A zeolite molecular sieve film on the hard carbon is proposed to develop a step-by-step desolvation pathway that effectively reduces the high activation energy of the direct desolvation process. Moreover, step-by-step desolvation yields a thin and inorganic-dominated SEI with a lower activation energy for Na⁺ transport. As a result, it contributes to greatly improved power density and cycling stability for both ester and ether electrolytes. When the above insights are applied, the hard carbon anode achieves the longest life span and minimum capacity fading rate at all evaluated current densities. Moreover, with the increase in current densities, an improved plateau capacity ratio is observed. This step-by-step desolvation strategy comprehensively enhances various properties of hard carbon anodes, which provides the possibility of building practical Na-ion batteries with high power density, high energy density, and durability.

Hard Carbons as Anodes in Sodium-Ion Batteries: Sodium Storage Mechanism and Optimization Strategies

Sodium-ion batteries (SIBs) are regarded as promising alternatives to lithium-ion batteries (LIBs) in the field of energy, especially in large-scale energy storage systems. Tremendous effort has been put into the electrode research of SIBs, and hard carbon (HC) stands out among the anode materials due to its advantages in cost, resource, industrial processes, and safety. However, different from the application of graphite in LIBs, HC, as a disordered carbon material, leaves more to be completely comprehended about its sodium storage mechanism, and there is still plenty of room for improvement in its capacity, rate performance and cycling performance. This paper reviews the research reports on HC materials in recent years, especially the research process of the sodium storage mechanism and the modification and optimization of HC materials. Finally, the review summarizes the sterling achievements and the challenges on the basis of recent progress, as well as the prospects on the development of HC anode materials in SIBs.

Interfacial Stabilization of Li2O-Based Cathodes by Malonic-Acid-Functionalized Fullerenes as a Superoxo-Radical Scavenger for Suppressing Parasitic Reactions

The utilization of an anionic redox reaction as an innovative strategy for overcoming the limitations of cathode capacity in lithium-ion batteries has recently been the focus of intensive research. Li₂O-based materials using the anionic (oxygen) redox reaction have the potential to deliver a much higher capacity than commercial cathodes using cationic redox reactions based on transition-metal ions. However, parasitic reactions attributed to the superoxo species (such as LiO₂), derived from the Li₂O active material of the cathode, deteriorate the stability of the interface between the cathode and electrolyte, which has limited the commercialization of Li₂O-based cathodes. To address this issue, malonic-acid-functionalized fullerenes (MC₆₀) were applied in the electrolyte as an additive for scavenging the superoxo radicals (O₂^1- in LiO₂) that trigger parasitic reactions. MC₆₀ can efficiently capture superoxo radicals using the π-conjugated surface and the malonate functionality on the surface. As a result, MC₆₀ considerably enhanced the available capacity and cycling performance of the Li₂O-based cathodes, decreased the interfacial layer formed on the cathode surface, and hindered the generation of byproducts, such as Li₂CO₃, CO₂, and C-F₃, derived from parasitic reactions. In addition, the loss of Li₂O from the cathode surface during cycling was also suppressed, validating the ability of MC₆₀ to capture superoxo radicals. This result confirms that the introduction of MC₆₀ can effectively alleviate the parasitic reactions at the cathode/electrolyte interface and improve the electrochemical performance of Li₂O-based cathodes by scavenging the superoxo species.

Mechanistic Insight into Ultrafast Kinetics of Sodium Cointercalation in Few-Layer Graphitic Carbon

Fast-charging sodium ion batteries remain deeply challenged by the lack of suitable carbonaceous anodes that exhibit intercalation plateau with fast kinetics. Here we develop a few-layer graphitic carbon with nanoscale architecture, which enables shortened Na⁺ ion diffusion path and fast formation of fully intercalated phase at the same time. Combined in situ Raman and electrochemical test reveal that this graphitic carbon with highly crystalline few layers follows surface-controlled intercalation rather than typical diffusion-controlled kinetics observed in natural graphite. As a result, a few-layer graphitic carbon anode maintains the reversible capacity of 106 mAh g^-1 at 10 A g^-1 and achieves 87% capacity retention even after 10 000 cycles at 1 A g^-1. This work provides new insight on the Na storage mechanism in fast-charging graphitic carbon as well as the design of carbon anodes for high-rate sodium ion batteries.

Eldfellite-structured NaCr(SO4)2: a potential anode for rechargeable Na-ion and Li-ion batteries

Recent global concerns over continuously increasing air pollution and the related health risks due to automobile exhaust have shifted our attention towards green transportation. Recent decades have witnessed a revolution in portable energy-storage systems, mainly lithium-based energy-storage devices. However, the uneven distribution of global lithium reserves and its scarcity lead to huge price differences and geopolitical imbalances, and hence the research in energy-storage materials has shifted towards the development of cost-effective, abundant electrode materials. Here, NaCr(SO₄)₂, a transition metal-based polyanionic layered material with low cost and high stability during the charge/discharge process vs. Na, operating on the basis of the Cr³⁺/²⁺ redox couple, is presented. The test materials were characterized by techniques like XRD, FTIR, SEM, UV, XPS, TGA-DTA, and a detailed electrochemical analysis of the charge/discharge capacity of the materials is presented here. Here, the findings provide insights towards achieving a Cr³⁺/Cr²⁺ redox-couple-based sodium-ion battery with a specific capacity of 75 mA h g^-1 and 150 mA h g^-1 at operating voltages of 0.95 V vs. Na and 1.05 V vs. Li, respectively, with 100% coulombic efficiency. Cr²⁺ is a very special oxidation of Cr that cannot be obtained easily and CrTa₂O₆ is the only known oxide where Cr exists in the 2+ state. Here, a shift in the redox energy of the Cr^3+/2+ couple was obtained due to its bonding with (SO₄)^2- polyanions in eldfellite that made the accessibility of Cr^3+/2+ possible, resulting in the superior intercalation/deintercalation of Na and Li and the superior energy-storage capacity of the NaCr(SO₄)₂vs. Na/Li cell.

Construction of a Few-Layered COF@CNT Composite as an Ultrahigh Rate Cathode for Low-Cost K-Ion Batteries

Potassium-ion batteries (PIBs) are attracting great interest for large-scale energy storage owing to the abundant resources and low redox potential of K⁺/K. However, the large volume changes and slow kinetics caused by the larger ionic radius of K⁺ for cathode materials remain a critical challenge for PIBs. Herein, we construct few-layered covalent organic frameworks integrated with carboxylated carbon nanotubes (DAAQ-COF@CNT) as cathode materials for PIBs. The synthesized DAAQ-COF@CNT features numerous active sites, a stable conductive framework, and an appropriate surface area with nanopores, which can render high electrical conductivity, shorten the ion/electron diffusion distance, and accelerate K⁺ diffusion. In consequence, the DAAQ-COF@CNT delivers a high reversible capacity of 157.7 mAh g^-1 at 0.1 A g^-1, an excellent rate capability of 111.2 mAh g^-1 at 1 A g^-1, and a long cycling stability of 77.6% capacity retention after 500 cycles at 0.5 A g^-1. The integrated characterization of ex situ X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and theoretical simulation discloses that the storage mechanism of DAAQ-COF@CNT is based on the reversible reaction between electroactive C═O groups and K⁺ during two successive steps. This work provides a promising high-performance cathode material for PIBs and encourages the development of new types of covalent organic frameworks for PIBs.

Microporous Carbon Nanospheres with Fast Sodium Storage Capability Enabled by Dominant Capacitive Behavior

Hard carbon is considered one of the most promising anode candidates for sodium ion batteries but suffers from a moderate rate performance. Here, we design microporous carbon nanospheres using a novel hybrid monomer that simultaneously involves an organic moiety and an inorganic moiety as the starting unit. The inorganic moiety forms a continuous network, which serves as a 3D scaffold and a nanometer-scale template, then supports the off-collapse of the carbon skeleton and creates a well-developed microporous structure. In addition, the graphite microcrystal structure can be tailored by adjusting the heating treatment temperatures. The electrochemical study demonstrates that the microporous carbon nanospheres show dominant capacitive sodium storage behavior, thus presenting an outstanding rate performance. Even if a very high current density of 10 A g^-1 is applied, the hard carbon anode can deliver a large capacity of 127 mAh g^-1 with a considerable plateau capacity of 53 mAh g^-1, which has rarely been obtained in previous publications. Besides, the carbon anode has a good cycling stability, and the capacity reached 210 mAh g^-1 after 1000 cycles with a current density of 1 A g^-1, showing no dramatic capacity loss.

BiOI Nanopaper As a High-Capacity, Long-Life and Insertion-Type Anode for a Flexible Quasi-Solid-State Zn-Ion Battery

The development of intercalation anodes with high capacity is key to promote the progress of "rocking-chair" Zn-ion batteries (ZIBs). Here, layered BiOI is considered as a promising electrode in ZIBs due to its large interlayer distance (0.976 nm) and low Zn²⁺ diffusion barrier (0.57 eV) obtained by density functional theory, and a free-standing BiOI nanopaper is designed. The process and mechanism of Zn(H₂O)_n²⁺ insertion in BiOI are proved by ex situ X-ray diffraction, Raman, and X-ray photoelectron spectroscopy. The suitable potential (0.6 V vs Zn/Zn²⁺), high reversible capacity (253 mAh g^-1), good rate performance (171 mAh g^-1 at 10 A g^-1), long cyclic life (113 mAh g^-1 after 5000 cycles at 5 A g^-1), and dendrite-free operation of BiOI nanopaper prove its potential as a superior anode. When it is coupled with Mn₃O₄ cathode, the quasi-solid-state battery exhibits a high initial capacity of 149 mAh g^-1 (for anode) and a good capacity retention of 70 mAh g^-1 after 400 cycles. The self-assembled flexible battery also shows stable charge-discharge during the cyclic test. This work shows the feasibility of BiOX anode for dendrite-free ZIBs.

Electrode-Electrolyte Interfacial Chemistry Modulation for Ultra-High Rate Sodium-Ion Batteries

Sodium-ion batteries capable of operating at rate and temperature extremes are highly desirable, but elusive due to the dynamics and thermodynamics limitations. Herein, a strategy of electrode-electrolyte interfacial chemistry modulation is proposed. The commercial hard carbon demonstrates superior rate performance with 212 mAh g^-1 at an ultra-high current density of 5 A g^-1 in the electrolyte with weak ion solvation/desolvation, which is much higher than those in common electrolytes (nearly no capacity in carbonate-based electrolytes). Even at -20 °C, a high capacity of 175 mAh g^-1 (74 % of its room-temperature capacity) can be maintained at 2 A g^-1 . Such an electrode retains 90 % of its initial capacity after 1000 cycles. As proven, weak ion solvation/desolvation of tetrahydrofuran greatly facilitates fast-ion diffusion at the SEI/electrolyte interface and homogeneous SEI with well-distributed NaF and organic components ensures fast Na⁺ diffusion through the SEI layer and a stable interface.

Introducing the Solvent Co-Intercalation Mechanism for Hard Carbon with Ultrafast Sodium Storage

As the most successful anode material for sodium-ion batteries, hard carbon has attracted extensive attention from researchers. However, its storage mechanism is still controversial. In this paper, a solvent co-intercalation mechanism into hard carbon is proposed and is proved by in situ XRD and ex situ TEM XPS results successfully. Thanks to the co-intercalation of solvent, the platform capacity of hard carbon maintains well at very high current densities. It can even exhibit 245 mAh g^-1 at 5 A g^-1 , which is the best rate performance obtained for hard carbon anode as far as it is known. The full battery assembled with Na₃ V₂ (PO₄ )₃ has a high energy density of 157 Wh kg^-1 at 3800 W kg^-1 (relative to the electrode). This finding brings new insights with regard to the design of hard carbon materials and sodium storage mechanisms.

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).

Interconnected 3D carbon network with enhanced reaction kinetics and architecture stability for advanced potassium-ion hybrid capacitors

Due to their high energy/power densities and ultralong cycle lifespan, potassium-ion hybrid capacitors (PIHCs) have attracted increasing research interest for large-scale energy storage systems. However, the kinetics mismatch between the battery-type anodes and capacitor-type cathodes severely hampers the further development of PIHCs. Herein, the kinetics-enhanced N-doped amorphous porous carbon with an interconnected three-dimensional (3D) network (marked as NPC) is reported. The existence of an amorphous configuration can provide numerous storage potassium sites, while the interconnected 3D network contributes to electron transfer, thus improving the reversible capacity and reaction kinetics of NPC. The expanded carbon interlayer spacing, well-established porous structure and plentiful active sites induced by N-doping greatly boost the structural stability and further increase kinetics. Benefiting from these structure merits, the NPC electrode delivers a high capacity (257.7 mA h g^-1 at 0.5 A g^-1), an excellent rate capability (199.5 mA h g^-1 at 2 A g^-1), and an extraordinary cycling stability over 3000 cycles at 2 A g^-1. Moreover, coupling with activated carbon (AC) cathode and NPC anode, the assembled PIHCs exhibit ultra-large energy/ultra-high power density (177.3 W h kg^-1 and 19348.3 W kg^-1) with a long cycling life (81.6% capacity retention after 3000 cycles).

Hierarchical porous carbon derived from coal tar pitch by one step carbonization and activation combined with a CaO template for supercapacitors

Hierarchical porous carbon is synthesized through a one-step carbonization and activation method from coal tar pitch using CaO as the hard template.

Biomass-derived porous activated carbon from anacardium occidentale shell as electrode material for supercapacitors

Anacardium occidentale shell (AOS) biowaste was chemically activated using KOH at various temperatures to produce AC. Interestingly, this study also presents a novel strategy for achieving value-added usage of cashewnut shell in the energy storage field.

Ether-Based Electrolyte Chemistry Towards High-Voltage and Long-Life Na-Ion Full Batteries

Although ether-based electrolytes have been extensively applied in anode evaluation of batteries, anodic instability arising from solvent oxidability is always a tremendous obstacle to matching with high-voltage cathodes. Herein, by rational design for solvation configuration, the fully coordinated ether-based electrolyte with strong resistance against oxidation is reported, which remains anodically stable with high-voltage Na₃ V₂ (PO₄ )₂ O₂ F (NVPF) cathode under 4.5 V (versus Na⁺ /Na) protected by an effective interphase. The assembled graphite//NVPF full cells display superior rate performance and unprecedented cycling stability. Beyond that, the constructed full cells coupling the high-voltage NVPF cathode with hard carbon anode exhibit outstanding electrochemical performances in terms of high average output voltage up to 3.72 V, long-term cycle life (such as 95 % capacity retention after 700 cycles) and high energy density (247 Wh kg^-1 ). In short, the optimized ether-based electrolyte enriches systematic options, the ability to maintain oxidative stability and compatibility with various anodes, exhibiting attractive prospects for application.

Constructing electronic interconnected bimetallic selenide-filled porous carbon nanosheets for stable and highly efficient sodium-ion half/full batteries

Owing to their large theoretical capacity and relatively high electronic conductivity, transition metal selenides have been investigated as potential anodes for energy storage applications. On the other hand, the quick capacity decline induced by volume expansion during cycling and unconnected conducting network of the transition metal selenide-based electrode severely limit their employment in sodium-ion batteries (SIBs). Herein, a simple solvent ultrasonic technique and pyrolysis selenation process were used to make a porous N-doped carbon nanosheet-supported FeSe₂/CoSe₂ electrode. The electrochemical kinetics could be improved, and the stress generated by volume expansion could be efficiently adjusted by exquisitely constructed boundary of the FeSe₂/CoSe₂-CN electrode. As expected, the FeSe₂/CoSe₂-CN porous nanosheets exhibited a high Na⁺ storage capacity of 350 mA h g^-1 (10 A g^-1, 1000 cycles). Kinetic studies were conducted to explore the Na⁺ storage mechanism of FeSe₂/CoSe₂-CN. The as-constructed full sodium-ion batteries, when combined with Na₃V₂(PO₄)₂O₂F, have a phenomenal energy density (109 W h kg^-1), encouraging the exploration of energy-related components with the high-energy density properties.

Elucidating the Mechanism of Fast Na Storage Kinetics in Ether Electrolytes for Hard Carbon Anodes

The sodium storage performance of a hard carbon (HC) anode in ether electrolytes exhibits a higher initial Coulombic efficiency (ICE) and better rate performance compared to conventional ester electrolytes. However, the mechanism behind faster Na storage kinetics for HC in ether electrolytes remains unclear. Herein, a unique solvated Na⁺ and Na⁺ co-intercalation mechanism in ether electrolytes is reported using designed monodispersed HC nanospheres. In addition, a thin solid electrolyte interphase film with a high inorganic proportion formed in an ether electrolyte is visualized by cryo transmission electron microscopy and depth-profiling X-ray photoelectron spectroscopy, which facilitates Na⁺ transportation, and results in a high ICE. Furthermore, the fast solvated Na⁺ diffusion kinetics in ether electrolytes are also revealed via molecular dynamics simulation. Owing to the contribution of the ether electrolytes, an excellent rate performance (214 mAh g^-1 at 10 A g^-1 with an ultrahigh plateau capacity of 120 mAh g^-1 ) and a high ICE (84.93% at 1 A g^-1 ) are observed in a half cell; in a full cell, an attractive specific capacity of 110.3 mAh g^-1 is achieved after 1000 cycles at 1 A g^-1 .

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.

Iron Phosphide Confined in Carbon Nanofibers as a Free-Standing Flexible Anode for High-Performance Lithium-Ion Batteries

Iron phosphide with high specific capacity has emerged as an appealing candidate for next-generation lithium-ion battery anodes. However, iron phosphide could undergo conversion reactions and generally suffer from a rapid capacity degradation upon cycling due to its structure pulverization. Chemomechanical breakdown of iron phosphide due to its rigidity has been a challenge to fully realizing its electrochemical performance. To address this challenge, we report here on an enticing opportunity: a flexible, free-standing iron phosphide anode with Fe₂P nanoparticles confined in carbon nanofibers may overcome existing challenges. For the synthesis, we introduce a facile electrospinning strategy that enables in situ formation of Fe₂P within a carbon matrix. Such a carbon matrix can effectively minimize the structure change of Fe₂P particles and protect them from pulverization, allowing the electrodes to retain a free-standing structure after long-term cycling. The produced electrodes showed excellent electrochemical performance in lithium-ion half and full cells, as well as in flexible pouch cells. These results demonstrate the successful development of iron phosphide materials toward high capacity, light weight, and flexible energy storage.

Electrochemical Lithium Storage Performance of Molten Salt Derived V2SnC MAX Phase

MAX phases are gaining attention as precursors of two-dimensional MXenes that are intensively pursued in applications for electrochemical energy storage. Here, we report the preparation of V₂SnC MAX phase by the molten salt method. V₂SnC is investigated as a lithium storage anode, showing a high gravimetric capacity of 490 mAh g^-1 and volumetric capacity of 570 mAh cm^-3 as well as superior rate performance of 95 mAh g^-1 (110 mAh cm^-3) at 50 C, surpassing the ever-reported performance of MAX phase anodes. Supported by operando X-ray diffraction and density functional theory, a charge storage mechanism with dual redox reaction is proposed with a Sn-Li (de)alloying reaction that occurs at the edge sites of V₂SnC particles where Sn atoms are exposed to the electrolyte followed by a redox reaction that occurs at V₂C layers with Li. This study offers promise of using MAX phases with M-site and A-site elements that are redox active as high-rate lithium storage materials.

Vacant Manganese-Based Perovskite Fluorides@Reduced Graphene Oxides for Na-Ion Storage with Pseudocapacitive Conversion/Insertion Dual Mechanisms

Na-ion capacitors (NICs) and Na-based dual-ion batteries (Na-DIBs) have been considered to be promising alternatives to traditional lithium-ion batteries (LIBs) because of the abundance and low cost of the Na-ion, but their energy density, power density and life cycle are limited. Herein, dual-vacancy (including K⁺ and F^- vacancies) perovskite fluoride K_0.86 MnF_2.69 @reduced graphene oxide (rGO; recorded as Mn-G) as anode for NICs and Na-DIBs has been developed. The special conversion/intercalation dual Na-ion energy storage mechanism and pseudocapacitive dynamics are analyzed in detail. The Mn-G//AC NICs and Mn-G//KS6 Na-DIBs delivered a maximum energy density of 92.7 and 187.6 W h kg^-1 , a maximum power density of 20.2 and 21.12 kW kg^-1 , and long cycle performance of 61.3 and 68.4 % after 1000 cycles at 5 A g^-1 , respectively. Moreover, Mn-G//AC NICs and Mn-G//KS6 Na-DIBs can work well over a wide range of temperatures (-20 to 40 °C). These results make it competitive in Na-ion storage applications with high energy/power density over a wide temperature range.