Anode For Sodium-ion Batteries Research Articles

Multieffect Preoxidation Strategy to Convert Bituminous Coal into Hard Carbon for Enhancing Sodium Storage Performance.

Preoxidation is an effective strategy to inhibit the graphitization of coals during carbonization. However, the single effect of the traditional preoxidation strategy could barely increase surface-active sites, hindering further enhancement of sodium storage. Herein, a multieffect preoxidation strategy was proposed to suppress structural rearrangement and create abundant surface-active sites. Mg(NO3)2·6H2O helps to introduce oxygen-containing functional groups into bituminous coal at 450 °C, which acted as a cross-linking agent to inhibit the rearrangement of carbon layers and promote structural cross-linking during the subsequent thermal carbonization process. Besides, the residue solid decomposition product MgO would react with carbon to create surface-active sites. The obtained coal-based hard carbon contained more pseudographitic domains and sodium storage active sites. The optimized sample could deliver an excellent capacity of 287.1 mAh g-1 at 20 mA g-1, as well as remarkable cycling stability of capacity retention of 96.1% after 200 cycles at 50 mA g-1, and notable capacity retention of 88.9% after 1000 cycles at 300 mA g-1. This work provides an effective and practical strategy to convert low-cost bituminous coal into advanced hard carbon anodes for sodium-ion batteries (SIBs).

Bimetallic Sulfide Sb2S3/FeS2 Heterostructure Anchored in a Carbon Skeleton for Fast and Stable Sodium Storage.

Sodium-ion batteries (SIBs) are a promising substitute for lithium batteries due to their abundant resources and low cost. Metal sulfides are regarded as highly attractive anode materials due to their superior mechanical stability and high theoretical specific capacity. Guided by the density functional theory (DFT) calculations, 3D porous network shaped Sb2S3/FeS2 composite materials with reduced graphene oxide (rGO) through a simple solvothermal and calcination method, which is predicted to facilitate favorable Na+ ion diffusion, is synthesized. Benefiting from the well-designed structure, the resulting Sb2S3/FeS2 exhibit a remarkable reversible capacity of 536 mAh g-1 after 2000 cycles at a current density of 5 A g-1 and long high-rate cycle life of 3000 cycles at a current density of 30 A g-1 as SIBs anode. In situ and ex situ analyses are carried out to gain further insights into the storage mechanisms and processes of sodium ions in Sb2S3/FeS2@rGO composites. The significantly enhanced sodium storage capacity is attributed to the unique structure and the heterogeneous interface between Sb2S3 and FeS2. This study illustrates that combining rGO with heterogeneous engineering can provide an ideal strategy for the synthesis of new hetero-structured anode materials with outstanding battery performance for SIBs.

Rational design of mangosteen-like bismuth nanospheres coated by N-doped carbon shell as superb composite anode for high-performance sodium-ion batteries

Metallic bismuth (Bi) anode materials are promising candidates for alkali-ion batteries due to its high theoretical gravimetric/volumetric capacity. However, the pronounced volume change from Bi to full sodiation phase Na3Bi, inducing the structural collapse, and ionic conduction interruption upon cycling, hinders their implementation in sodium-ion batteries (SIBs). Herein, a mangosteen-like bismuth nanosphere coated by N-doped carbon shell (Bi@NC) was designed and synthesized to address these limitations. Due to the nanosize of Bi core and in situ, generated N-doped carbon shell, this Bi@NC anode can not only effectively accommodate the volume change during the alloying/dealloying process but also provide high-speed channels for electron/ion transport to the highly active bismuth nanospheres. The Bi@NC electrode exhibits outstanding cycling stability (1000 cycles at 5 A g−1 with an impressive capacity retention of 96.5 %) and rapid sodium storage performance (392.8 mAh g−1 at 20 A g−1). Importantly, ex-situ techniques and kinetic analysis have revealed the distinctively sharp multiphase transitions and the “battery-capacitor dual-mode” sodium-storage mechanism. This research introduces a new approach to enhance the performance of bismuth-based anodes in advanced SIBs and demonstrates potential for practical applications.

3D MoS2/graphene oxide integrated composite as anode for high-performance sodium-ion batteries

Sodium-ion batteries (SIBs) are emerging as a promising alternative to conventional lithium-ion technology, due to the abundance of sodium resources. The major drawbacks for the commercial application of SIBs lie in the slow kinetic processes and poor energy density of the devices. Molybdenum sulfide (MoS2), a graphene-like material, is becoming a promising anode material for SIBs, because of its high theoretical capacity (670 mAh g–1) and layered structure that suitable for Na+ intercalation/extraction. However, the intrinsic properties of MoS2, such as low conductivity, slow Na+ diffusion kinetics and large volume change during charging/discharging, restrict its rate capability and cycle stability. Here, molybdenum disulfide and graphene oxide (3D MoS2/GO) with excellent conductivity were fabricated through layer-by-layer method using amino-functionalized SiO2 nanospheres as templates. The 3D MoS2/GO composite demonstrates excellent cycling stability and capacity of 525 mA h g–1 at 500 mA g–1 after 100 cycles, which mainly due to the integrated MoS2/GO components and unique 3D macroporous structure, facilitating the material conductivity and Na+ diffusion rate, while tolerating the volume expansion of MoS2 during the charge/discharge processes.

Carbon-Coated MOF-Derived Porous SnPS3 Core-Shell Structure as Superior Anode for Sodium-Ion Batteries.

Metal thiophosphites have recently emerged as a hot electrode material system for sodium-ion batteries because of their large theoretical capacity. Nevertheless, the sluggish electrochemical reaction kinetics and drastic volume expansion induced by the low conductivity and inherent conversion-alloying reaction mechanism, require urgent resolution. Herein, a distinctive porous core-shell structure, denoted as SnPS3@C, is controllably synthesized by synchronously phosphor-sulfurizing resorcinol-formaldehyde-coated tin metal-organic framework cubes. Thanks to the 3Dporous structure, the ion diffusion kinetics are accelerated. In addition, SnPS3@C features a tough protective carbon layer, which improves the electrochemical activity and reduces the polarization. As expected, the as-prepared SnPS3@C electrode exhibits superior electrochemical performance compared to pure SnPS3, including excellent rate capability (1342.4 and 731.1mAhg-1 at 0.1 and 4Ag-1, respectively), and impressive long-term cycling stability (97.9% capacity retention after 1000 cycles at 1Ag-1). Moreover, the sodium storage mechanism is thoroughly studied by in-situ and ex-situ characterizations. This work offers an innovative approach to enhance the energy storage performance of metal thiophosphite materials through meticulous structural design, including the introduction of porous characteristics and core-shell structures.

Confining WS2 hierarchical structures into carbon core-shells for enhanced sodium storage

The growing demand for clean energy has heightened interest in sodium-ion batteries (SIBs) as promising candidates for large-scale energy storage. However, the sluggish reaction kinetics and significant volumetric changes in anode materials present challenges to the electrochemical performance of SIBs. This work introduces a hierarchical structure where WS2 is confined between an inner hard carbon core and an outer nitrogen-doped carbon shell, forming HC@WS2@NCs core–shell structures as anodes for SIBs. The inner hard carbon core and outer nitrogen-doped carbon shell anchor WS2, enhancing its structural integrity. The highly conductive carbon materials accelerate electron transport during charge/discharge, while the rationally constructed interfaces between carbon and WS2 regulate the interfacial energy barrier and electric field distribution, improving ion transport. This synergistic interaction results in superior electrochemical performance: the HC@WS2@NCs anode delivers a high capacity of 370 mAh g−1 at 0.2 A/g after 200 cycles and retains261 mAh g−1 at 2 A/g after 2000 cycles. In a full battery with a Na3V2(PO4)3 cathode, the Na3V2(PO4)3//HC@WS2@NC full-cell achieves an impressive initial capacity of 220 mAh g−1 at 1 A/g. This work provides a strategic approach for the systematic development of WS2-based anode materials for SIBs.

Triphasic heterostructured Zn–Sn–S hollow nanoboxes encapsulated by N, S-codoped carbon as anodes for high-performance sodium-ion batteries

Metal sulfides have been expected huge practical potential for sodium-ion batteries (SIBs), which are mainly owing to their admirable merits of natural abundance, low price, and high theoretical capacity. However, the inferior electrical conductivity and enormous volume variation originating from sodiation/desodiaziton reactions usually result in unsatisfied rate and cycling properties. In this work, a coprecipitation with a following sulfurization method has been rationally designed to prepare the triphasic heterostructured hollow Zn–Sn–S nanoboxes encapsulated by N, S-codoped carbon (ZSS@NCS) as anodes for SIBs. The coexistence of triphasic ZSS heterostructures that consist of ZnS, SnS2, and Sn2S3 effectively facilitates the fast Na + diffusion. The N, S-codoped carbon (NSC) derived from the polydopamine is coated outside of the ZSS heterostructures, which provides infinite affinity between ZSS and NSC that efficiently accelerates the electron transport and maintains the structural stability via the confinement effect. The synergistic effects of the heterostructured ZSS and NSC endow the ZSS@NSC anode with favorable sodium storage properties including decent discharge capacity (683.8 mAh g−1 at 0.1 A g−1), satisfying rate (232.6 mAh g−1 at 10.0 A g−1) and cycling properties (402.2 mAh g−1 over 150 cycles).

CoSe2-Modified multidimensional porous carbon frameworks as high-Performance anode for fast-Charging sodium-Ion batteries

Transition metal selenides (TMSs) possess relatively high theoretical capacity and unique structures emerge as promising anode materials for sodium-ion batteries (SIBs). However, TMSs anode materials suffer from challenges such as substantial volume expansion, undesired side reactions, and poor active reaction dynamics. Herein, we propose a novel synthesis method that integrates metal salt-assisted polymer-blowing technique with in situ selenization strategy to fabricate multidimensional porous 3D carbon nanosheet frameworks modified by cobalt diselenide nanoparticles (CoSe2-CNF). Such unique architecture bestows intimate structural interconnectivity with high mechanical strength and abundant ion diffusion channels on CoSe2-CNF, exhibiting remarkable reversible capacity and fast-charging capability within diverse electrolyte systems. For instance, the CoSe2-CNF anode in ether electrolyte exhibits prominent rate performance of 349 mAh/g at 15 A/g. Sodium storage mechanism and fast electrochemical kinetics in the discharge/charge processes are investigated by in situ XRD and in situ EIS techniques. Additionally, DFT calculations are utilized to analyze the electrochemical differences observed in various electrolyte systems. When coupled with Na3V2(PO4) (NVP) cathodes, the full batteries also afford enhanced reversible capacity of 105 mAh/g over 100 cycles. The proposed structural engineering for CoSe2-CNF is beneficial for designing fast-charging energy storage devices.

Enhancing the sodium storage performance of hard carbon by constructing thin carbon coatings via esterification reactions

Hard carbons derived from pitch are considered a competitive low-cost anode for sodium-ion batteries. However, the preparation of pitch-based hard carbon (PHC) requires the aid of a pre-oxidation strategy, which introduces unnecessary defects and oxygen elements, which leads to low initial Coulombic efficiency (ICE) and poor cycling stability. Herein, we demonstrate a new surface engineering strategy by grafting chemically active glucose molecules on the PHC surface via esterification reactions, which can achieve low-cost nano-scaled carbon coating. Thin glucose coating can be carbonized at a lower temperature, which results in a more closed pore structure and fewer functional groups. The as prepared PHC exhibits a high reversible capacity of 328.5 mAh/g with a high ICE of 92.08 % at 0.02 A/g. It is noteworthy that the PHC can be adapted to a variety of cathode materials for full-cell assembling without pre-sodiation, which maintains the characteristics of high capacity and excellent cycling stability. The performance of resin-based hard carbon coated with a similar method was also improved, demonstrating the universality of the technique.

Bimetallic Sulfide Hollow Nanocubes Heterostructure Promotes Dual Coupling of Conversion and Alloying/Dealloying Reactions to Achieve Durable Potassium-Ion Battery Anode.

Conversion and alloying-type transitional metal sulfides have attracted significant interests as anodes for Potassium-ion batteries (PIBs) and Sodium-ion batteries (SIBs) due to their high theoretical capacities and low cost. However, the poor conductivity, structural pulverization, and high-volume expansions greatly limit the performance. Herein, Co1-xS/ZnS hollow nanocube-like heterostructure decorated on reduced graphene oxide (Co1-xS/ZnS@rGO) composite is fabricated through convenient hydrothermal and post-heat vulcanization techniques. This unique composite can provide a more stable conductive network and shorten the diffusion length of ions, which exhibits a remarkable initial charge capacity of 638.5mA h g-1 at 0.1 A g-1 for SIBs and 606mA h g-1 at 0.1 A g-1 for PIBs, respectively; It is worth noting that the composite presents remarkable long stable cycle performance in PIBs, which initially delivered 274mA h g-1 and sustained the charge capacity up to 245mA h g-1 at high current density of 1 A g-1 after 2000 cycles. A series of in situ/ex situ detections and first principle calculations further validate the high potassium ions adsorption ability of Co1-xS/ZnS anode materials with high diffusion kinetics. This work will accelerate the fundamental construction of bimetallic sulfide hollow nanocubes heterostructure electrodes for energy storage applications.

(Invited) Novel Q Carbon Anodes for Sodium-Ion Batteries

The main goal of this research is to develop low-cost anodes for sodium-ion batteries. Lack of standard carbon intercalation/insertion anode for sodium-ion batteries has greatly hindered its application. Two types of Q carbons have been studied as a potential anode material for sodium-ion batteries. Q1 carbon exhibits a high initial columbic efficiency of 81% but has a low-capacity retention of less than 60%. Whereas Q2 carbon has a low initial columbic efficiency of 58% but has a high-capacity retention of 81%. Q2 exhibited a stable capacity of 168 mAh/g at a cycling rate of C/3 which is comparable to other high performing hard carbon anodes. This work will discuss in detail about the synthesis procedure, microstructural and electrochemical performance of the Q carbons. This study reports the successful pathway of using Q carbon as a promising anode for sodium ion batteries.ACKNOWLEDGEMENTSThis research was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.

Operando Stress Evolution in Hard Carbon Anodes – Insight into the Mechanical Degradation Induced Failure Mode in Rechargeable Sodium Ion Batteries

Microstructural instabilities in electrodes arising from cycling induced stress is a primary failure mode in rechargeable batteries. Therefore, a quantitative knowledge of the cycling induced stress evolution in rechargeable battery electrodes is important for understanding the electrode microstructural instabilities during cycling that can potentially inform the electrode design to prolong battery cycle life. The cycling induced stress in electrodes typically arises from the volume changes associated with insertion/extraction of the electroactive species in the electrode matrix. For example, operando measurement of lithiation-delithiation induced stress in graphite electrodes that typically show 10% volume change has previously resulted in useful information that correlates well with the lithiation-delithiation mechanism in graphite anodes1. The size of the shuttling-electroactive species can be taken as an indicator of the cycling induced stress magnitude that drives the electrode microstructural instabilities. In this regard, quantification of the cycling induced stress in rechargeable Na-ion battery electrodes is warranted for understanding mechanical degradation induced failure modes owing to the larger size of the electroactive species, i.e., Na ions (vs. Li-ion).Hard carbons as potential anodes in rechargeable sodium ion batteries has gained considerable attention in the recent past due to their superior Na-ion storage capability (vs. graphite that is a canonical anode in rechargeable lithium-ion batteries). However, hard carbon anodes typically show significant capacity fade that can potentially arise from cycling induced mechanical degradation of the anodes. Operando stress measurements coupled with comprehensive electrode microstructural characterization will provide valuable insights in this context. Here we report on the operando stress evolution in hard carbon anodes in rechargeable sodium ion batteries as probed by Multiple Optical-beam Sensing (MOS) technique that basically involves monitoring the spacings among a group of laser spot (Fig. 1). Preliminary measurements have shown that the stress correlates with the potential with a maximum around 12 MPa in compression during sodiation and tensile stress of ~ 2MPa during desodiation. Operando stress measurements are performed in hard carbon anodes in two different electrode configurations – typical composite anodes (with binder and conductive agents) and hard carbon thin film anodes (without binder and conductive agents). The thin film configuration is considered to evaluate intrinsic stress response in hard carbon anodes in absence of any secondary phase. A comparison of operando stress response in these two types of hard carbon anodes along with comprehensive electrode microstructural characterization will be presented. Additionally, attempts will be made to shed light on the current debate in the mechanism of sodium storage in hard carbon anodes by comparing operando stress response of hard carbons from multiple sources along with their structural characterization (Raman, XPS, surface area etc.). V. A. Sethuraman, N. Van Winkle, D. P. Abraham, A. F. Bower, and P. R. Guduru, Journal of Power Sources, 206, 334–342 (2012). Figure 1

Effect of Electrolytes on Performance of Tire Derived Carbon for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have recently become prominent in the battery ecosystem due to their low cost and use of earth abundant materials such as Sodium, Manganese, Aluminum, Iron, and Prussian blue (iron cyanide). [1] Graphite is the standard anode used for LIBs but cannot be used in SIBs due to interlayer distance being too small to accommodate Na+ ion. Hard carbons are suitable candidates for SIB due to their low cost, low working potential, high capacity, and ability to be synthesized at lower pyrolysis temperatures. In our past work we have reported a low-cost, scalable waste tire-derived carbon as an anode for SIBs [2]. In this work we test the tire derived carbon with 6 different electrolytes and understand the impact each has on the performance. The best electrolyte was found to be sodium hexafluorophosphate in ethylene carbonate (EC)–ethyl methyl carbonate (EMC) (NaPF6-EC/EMC) which showed a capacity of 108 mAh/g after 200 cycles at charge/discharge rate of C/3. The initial columbic efficiency (ICE) was found to be 52% and to mitigate this problem we tried a scalable contact pre-sodiation method. The pre-sodiation method showed high initial columbic efficiency drastically improving capacity retention of the battery.

(Battery Student Slam 8 Award Winner) Evaluating Electrodeposited Tin and Antimony-Based Anodes for Sodium-Ion Batteries

The widescale adaption of intermittent renewable energy sources, such as wind and solar, must be accompanied by a grid storage network to store energy when it is abundant and redistribute it as needed for on-demand use. Short-term storage in the form of rechargeable batteries is an attractive avenue for meeting these storage needs. The dominant technology in today’s market, the lithium-ion battery (LIB), is cost-prohibitive due to the need for rare-earth materials. Using the same working principle as LIBs, sodium-ion batteries (NIBs) utilize earth-abundant, low-cost sodium compounds instead of lithium compounds. Tin (Sn) and antimony (Sb) alone are promising anode materials for sodium-ion batteries with high theoretical capacities (847 and 660 mAh/g for Sn and Sb with Na, respectively) relative to current LIB anode chemistry (372 mAh/g) [1,2]. When combined, SnSb has shown to have increased stability relative to Sn or Sb alone [3]. Here, we synthesized Sn-Sb thin films with varied Sn and Sb atomic ratios by co-electrodeposition from a deep eutectic solvent by modifying solution stoichiometry and electrodeposition parameters. Using a combination of X-ray photoelectron spectroscopy (XPS), X-ray diffraction, and energy dispersive X-ray spectroscopy (EDS), phases and compositions of Sn-Sb synthesized by electrodeposition were identified. Once synthesized, we sought to determine the influence of morphology, surface chemistry, and composition on the capacity retention and rate capabilities of Sn and Sb-based anodes in NIBs. An optimized electrode composition was identified using a carbonate-based electrolyte, which resulted in stable capacity retention cycling at a rate of C/2 for 270 cycles.[1] W. T. Jing, C. C. Yang, and Q. Jiang, Journal of Materials Chemistry A, 8, 2913–2933 (2020).[2] S. Megahed and B. Scrosati, Journal of Power Sources, 51, 79–104 (1994).[3] W. P. Kalisvaart, H. Xie, B. C. Olsen, E. J. Luber, and J. M. Buriak, ACS Applied Energy Materials, 2, 5133–5139 (2019).

Coal-derived flaky hard carbon with fast Na+ transport kinetic as advanced anode material for sodium-ion batteries

Hard carbon (HC) has demonstrated a significant potential in anode for sodium-ion batteries (SIBs) due to its superior electrochemical properties. Coal is considered a promising hard carbon precursor due to its high carbon yield. However, the widespread adoption of coal-derived HC faces challenges such as limited storage capacity and decreased rate performance, primarily attributed to the dense structure of coal. In this study, a molten salt-assisted approach was developed to adjust the microstructure of coal-derived HC. The dense coal is etched into a flaky structure by molten salt, which is conducive to the diffusion and transport of sodium ions. The optimized HC materials possess large interlayer spacing and abundant pseudo-graphitic domains, exhibiting a remarkable reversible capacity of 303.6 mA h g−1 at 30 mA g−1 and 82.1 mA h g−1 at 500 mA g−1. Besides, a full cell with the configuration of as-prepared coal derived HC versus Na3V2(PO4)2O2F (NVPOF) is presented with a high initial coulombic efficiency of 76.15 % and capacity of 100.7 mA h g−1. The results indicate that our synthesis strategy is feasible for the application of coal derived carbon materials in SIBs.

A hollow Core-Shell Cu2S@C nanoboxes for High-Performance electrochemical sodium storage

Aiming at the poor intrinsic conductivity and huge volume expansion of transition metal sulfides, carbon-coated cuprous sulfide hollow nanoboxes (Cu2S@C) with core–shell structure were designed via co-precipitation, facial carbon coating and in-situ sulfurization methods, and exhibited significantly improved electrochemical properties applied as sodium ion battery anode. The uniformly coated carbon shell can enhance the electrolyte infiltration, promote charge transfer efficiency, accelerate electrochemical conversion redox kinetics. The conversion reaction reversibility of restricted Cu2S cores show a significant improvement benefitting from accelebrating ion diffusion and spatial limitation. During the sulfidation, robust chemically and electronically bonded connections such as C-S moieties formed between the two phases, creating interconnected channels that facilliate rapid charge transfer kinetics and optimize structural durability. Moreover, the hollow core–shell structure can provide a buffer space for tolerating volume expansion and preventing agglomeration arising from the repeating Na+ insertion/desertion process, which can availably enhance cyclic stability. Thanks to the synergistic effect between strong interfacial coupling and hollow core–shell structure, the Cu2S@C composite can deliver an improved Na+ storage performance. At the high current density of 1.0 A/g, the Cu2S@C anode could exhibit a specific capacity of 100.08 mAh/g, which is significant ameliorative than Cu2S counterpart (only 11.6 mAh/g).

Regulating the active hydroxyl group of starch: Revealing the evolution of hard carbon structure and sodium storage behavior

The active hydroxyl group of starch has a crucial effect in regulating the microstructure of hard carbon anodes for sodium-ion batteries. By optimizing the chemical structure of starch and controlling the pyrolysis process, high-performance hard carbon materials can be obtained. These active hydroxyl groups are primarily located on the terminal and ring structures of glucose units, exerting significant influence on the formation and performance of hard carbon. Our results demonstrate that low-temperature oxidation treatment effectively modifies the terminal hydroxyl groups in precursor materials, facilitating chain cross-linking and leading to a more robust hard carbon structure. This cross-linking effect helps suppress excessive foaming during pyrolysis, thereby enhancing the stability of the hard carbon structure. Furthermore, by modifying the hydroxyl group on the ring structure of precursors, we can control the pyrolysis process, promote pore closure, and further improve the low plateau capacity of hard carbon anode materials from 153.95 to 219.58 mAh g −1. This study offers novel insights into harnessing and understanding starch's active hydroxyl groups in preparing hard carbon materials—a significant contribution towards optimizing their performance as anode materials.

Two-dimensional metallic Be2Al monolayer as a potential anode material for lithium-ion/sodium-ion batteries

Developing anode materials that combine excellent robustness, high-volume capacity, and superior electrical conductivity is essential for advancing high-performance ion batteries. Through comprehensive first-principles calculations, this paper illustrates the possibility of a two-dimensional Be2Al monolayer as an anode candidate for lithium/sodium-ion batteries. The Be2Al monolayer exhibits a high theoretical energy storage capacity (2382/2382 mAh/g for Li/Na) and an ultra-low diffusion barrier (124/77 meV for Li/Na). Its average open-circuit voltage (0.35/0.52 V for Li/Na) falls within the desired ideal range. Additionally, the Be2Al monolayer demonstrates excellent electrical conductivity, facilitating efficient electron transport. These promising outcomes indicate that the Be2Al monolayer can be an effective anode for next-generation lithium-ion and sodium-ion batteries.

Dual-Gradient Concentration Distribution in Sb-Cu Alloy Nanoarrays for Robust Sodium-Ion Storage.

Alloy-type materials are attractive for anodes in sodium-ion batteries (SIBs) owing to their high theoretical capacities and overall performance. However, the accumulation of stress/strain during repeated cycling results in electrode pulverization, leading to rapid capacity decay and eventual disintegration, thus hindering their practical applications. Herein, we report a 3D coral-like Sb-Cu alloy nanoarray with gradient distribution of both elements. The array features a Sb-rich bottom and a Cu-rich top with increasing Sb and decreasing Cu concentrations from top to bottom. The former is the active component that provides the high capacity, whereas the latter serves as an inert additive that acts against volume variation. The gradual transition in composition within the electrode introduces a ladder-type volume expansion effect, facilitating a smooth distribution and effective release of stress, thereby ensuring the wanted mechanical stability and structural integrity. The as-developed nanoarray affords a high reversible capacity (460 mAh g-1 at 0.5 C), stable cycling (89 % retention over 120 cycles at 1.0 C), and superior rate capability (354 mAh g-1 at 10 C). The concentration dual-gradient strategy paves a new pathway of designing alloy-type materials for SIBs.

Engineering multiple heterogeneous Co/CoSe/MoSe2 embedded in carbon nanofibers with Co/Mo–Se–C bonding towards high performance sodium ion capacitors

MoSe2, as a representative two-dimensional transition selenide with a broad interlayer distance of 0.65 nm and a high theoretical capacity of 422 mAh g−1, is considered an optimal anode material for sodium-ion batteries (SIBs). However, its unsatisfactory electrical conduction and structural instability often lead to severe capacity decay during cycling, hindering its large-scale application in SIBs. To overcome these challenges, coupling heterostructures has been identified as an effective solution. The establishment of multiple heterointerfaces can minimize interface diffusion resistance, significantly enhancing ion and electron delivery dynamics compared to a single component. In this work, multiple heterogeneous Co/CoSe/MoSe2 uniformly distributed in carbon nanofibers (CNF) via Co/Mo–Se–C bonding are successfully synthesized using electrospinning and subsequent calcination. Importantly, the Co/CoSe and Co/MoSe2 heterointerfaces can greatly accelerate charge transfer and improve Na + adsorption capability compared to the CoSe/MoSe2 heterostructure, as corroborated by theoretical calculations. Given the high rate performance of Co/CoSe/MoSe2@CNF as an anode in SIBs (267.9 mAh g−1 at 5 A g−1), a rocking-chair sodium-ion capacitor (SIC) incorporating a Co/CoSe/MoSe2@CNF anode and a Na3V2(PO4)3 cathode is proposed, delivering high energy densities of 199.1 Wh kg−1 at 180 W kg−1 and 126.8 Wh kg−1 at 3280 W kg−1.