Electrochemical Sodium Storage Research Articles

Improving electrochemical sodium storage performance and insight into the sodium ion diffusion in the high-pressure polymorph β-V2O5

The high-pressure form of vanadium pentoxide, β-V2O5, possesses promising sodium storage properties, featuring reversible sodium intercalation of one Na+ per formula unit, yielding an appealing capacity of 147 mAh g−1. However, its short cycle life in conventional carbonate-based electrolytes remains a significant drawback. In this work, we demonstrate that using non-carbonate-based electrolytes markedly enhances key electrochemical performances. Additionally, reducing the particle size of β-V2O5 through milling significantly increases the specific capacity, particularly at high current rates. Milled V2O5 maintains a respectable capacity of 73 mAh g−1 at 1 C rate, compared to the negligible capacity observed in non-milled V2O5. The milling process also alters the energy storage mechanism. Interestingly, after milling, sodium diffusion coefficient (DNa+) increased from 1.78 × 10−13 to 1.73 × 10−11 cm2 s−1, likely due to induced near-surface defects. Sodium storage exhibits dominant faradaic behavior at slow current rates, while, at high current rates, capacitive processes predominate. The synergy of improved sodium diffusion and additional capacitive charge storage leads to significantly improved electrochemical performance at high current rates. Furthermore, ionic liquid-based electrolytes promote long-life cycling, advancing this material toward practical application as a cathode in sodium-ion cells.

Ultra-Fast-Charging, Long-Duration, and Wide-Temperature-Range Sodium Storage Enabled by Multiwalled Carbon Nanotube-Hybridized Biphasic Polyanion-Type Phosphate Cathode Materials.

Sodium-ion batteries (SIBs) represent a promising energy storage technology with great safety. Because of their high operating potential, superior structural stability, and prominent thermal stability, polyanion-type phosphates have garnered significant interest in superior prospective cathode materials for SIBs. Nevertheless, the disadvantages of poor intrinsic electronic conductivity, sluggish kinetics, and volume variation during sodiation/desodiation remain great challenges for satisfactory rate performance and cycle stability, which severely hinder their further practical applications. In this work, by adjusting the amounts of pretreated multiwalled carbon nanotubes (CNT) added intentionally at the beginning of the preparation, biphasic polyanion-type phosphate materials (marked as NFC) are synthesized through a one-pot solid state reaction methodology, which are composed of CNT-interwoven Na3V2(PO4)2F3 (NVPF) and a small amount of Na3V2(PO4)3 (NVP). Benefiting from the improved electronic conductivity and unique composition and structure, the optimized sample (labeled as NFC-2) illustrates exceptional cycle stability and remarkable rate performance. The discharge capacities of the NFC-2 electrode are 114.8 and 78.6 mAh g-1 tested at 20 and 5000 mA g-1, respectively. Notably, such an electrode still gives out 75.7% capacity retention upon 10 000 cycles at 5000 mA g-1. In situ X-ray diffraction analysis demonstrates that the NFC-2 cathode has outstanding structural reversibility during charge/discharge cycles. More importantly, such a biphasic material has achieved impressive electrochemical performance within a wide operating temperature range of -20-50 °C. When temperature is decreased to -20 °C, the NFC-2 electrode still delivers an initial discharge capacity of 102.4 mAh g-1 and exhibits a remarkable capacity retention of 97.8% even after 500 cycles at 50 mA g-1. In addition, the sodium-ion full cell assembled by integrating NFC-2 cathode and hard carbon anode shows a satisfying energy density of 431.3 Wh kg-1 at 20 mA g-1 with a better long-term cycle performance. The synergistic effect among high energy NVPF, conductive CNT, and stable NVP may lead to the great improvement in the electrochemical sodium storage performance of the NFC-2 sample. Such biphasic polyanion-type phosphate materials will inject new ideas into the material design for SIBs with excellent electrochemical performance and further promote practical applications of this advanced energy storage technology.

Preparation of N-doped carbon-modified Na3V2(PO4)2F3 based on ZIF-8 as a template and its electrochemical properties

N-doped porous carbon material (N-PC) was generated through high-temperature cracking, using ZIF-8 as a template, and preparation of Na3V2(PO4)2F3/N-PC composites were designed and prepared as cathode materials for sodium-ion batteries, and the electrochemical sodium storage performance was systematically investigated. The physical characterization results showed that the addition of N-PC played a role in increasing the electrical conductivity of Na3V2(PO4)2F3 material and improving the specific surface area. The electrochemical test results showed that the discharge specific capacity of Na3V2(PO4)2F3/N-PC composite was 109.56 mAh/g at 0.1C, and the capacity retention rate was 85.10 % after 100 cycles, and the RSEI of Na3V2(PO4)2F3/N-PC composite (81.55 Ω) is better than that of Na3V2(PO4)2F3 pure-phase material (142.5 Ω), which indicates that the ion transfer impedance of the modified material has been effectively reduced, and the ion de-embedding is more fluent.

Demystifying the Influence of Precursor Structure on S‐Doped Hard Carbon Anode: Taking Glucose, Carbon Dots, and Carbon Fibers as Examples

AbstractHeteroatom doping is a promising strategy for adjusting the microstructure of hard carbon (HC) to promote its electrochemical sodium storage performance. However, clarifying the doping sites of heteroatoms and effectively regulating their doping levels remain serious challenges. Herein, this work reveals the impact of three distinct structural precursors on S‐doped hard carbon: namely glucose (small organic molecule), carbon dots (CDs, intermediate state between organic and inorganic), and graphitized carbon fibers (inorganic carbon materials). It is demonstrated that the S‐doped HC derived from CDs possesses a more significant number of C─S bonds within its carbon framework, which is attributed to the preferential bonding between sulfur and short polymeric chains abundant in unsaturated functional groups. And these chains cluster prominently on the surface of CDs, enhancing the affinity of sulfur. Furthermore, as a prominent feature of CDs, the extremely small size inherently distinguishes them from other precursors, enabling them to serve as fundamental units for constructing various carbon microstructures, such as three‐dimensional (3D) structure. In summary, this study explores the influence of different precursor structures on heteroatom doping, with CDs identified as the most useful precursor.

Internal Vanadium Doping and External Modification Design of P2-Type Layered Mn-Based Oxides as Competitive Cathodes toward Sodium-Ion Batteries.

P2-type layered manganese-based oxides have attracted considerable interest as economical, cathode materials with high energy density for sodium-ion batteries (SIBs). Despite their potential, these materials still face challenges related to sluggish kinetics and structural instability. In this study, a composite cathode material, Na0.67Ni0.23Mn0.67V0.1O2@Na3V2O2(PO4)2F was developed by surface-coating P2-type Na0.67Ni0.23Mn0.67V0.1O2 with a thin layer of Na3V2O2(PO4)2F to enhance both the electrochemical sodium storage and material air stability. The optimized Na0.67Ni0.23Mn0.67V0.1O2@5wt %Na3V2O2(PO4)2F exhibited a high discharge capacity of 176 mA h g-1 within the 1.5-4.1 V range at a low current density of 17 mA g-1. At an increased current density of 850 mA g-1 within the same voltage window, it still delivered a substantial initial discharge capacity of 112 mAh g-1. These findings validate the significant enhancement of ion diffusion capabilities and rate performance in the P2-type Na0.67Ni0.33Mn0.67O2 material conferred by the composite cathode.

Facile flame synthesis of S-doped Fe2O3 on carbon cloth as superior anode material for sodium-ion battery

Take advantage of its superior theoretical specific capacity, Fe2O3 based nanocomposites are promising candidates for sodium ion batteries (SIBs). However, the complicate synthetic procedure hinders their further application. Herein, a facile ethanol diffusion flame synthesis approach is developed to prepare S-doped Fe2O3 composite on carbon cloth. The morphology of the Fe2O3 can be easily controlled by tuning the composition of the fuel and duration time of the flame treatment. Optimized composite with flake-like structure and heteroatom S-doped Fe2O3 (S/Fe2O3@CC-2) displayed enhanced electrochemical sodium storage performances. Specifically, sample S/Fe2O3@CC-2 maintained a reversible discharge capacity of 464.3 mAh g−1 after 150 cycles at 0.2 A g−1. Besides, at current densities of 0.4, 0.8, 1.6 and 3.2 A g−1, capacities of 393.8, 311.2, 277.5 and 218.9 mAh g−1 could be maintained, respectively. Kinetic analysis implies that S/Fe2O3@CC-2 manifested an improved capacitive-controlled sodium storage process. This work will shed light on facile and low-cost approach for the synthesis of high-performance anode materials for SIBs.

Metal-organic framework derived Co1-xS@C nanoboxes for long-life Na+ storage

Transition metal sulfides appear excellent electrochemical properties in the realm of anode for sodium-ion batteries and hold significant potential for practical application in large-scale devices in the future. However, their development is hindered by inferior inherent conductivity and tremendous volume strain during discharge and charge processes. In this study, Co1-xS@C nanoboxes were synthesized utilizing Co-Co PBA as a precursor. The electrochemical sodium storage performance of Co1-xS@C electrode was characterized, revealing an extremely long-life and stable performance. At 1.0 A g−1 and 2.0 A g−1, the electrode delivered capacities of 255.8 mAh g−1 and 197.6 mAh g−1 after 5000 and 10,000 cycles respectively, with negligible capacity attenuation. The pseudo-capacitance behavior of Co1-xS@C NBs electrode in sodium storage was quantitatively calculated through CV test at varying sweep speeds. The significant contribution of pseudo-capacitance resulted in its exceptional rate capacity and electrochemical stability. Furthermore, kinetic analysis revealed that the existence of a carbon shell enhanced the Na+ diffusion kinetics and charge transfer kinetics of Co1-xS@C NBs. This strategy provides a reference for enhancing the Na+ storage performance of other transition metal sulfides.

Electrochemical Sodium Storage in Hard Carbon Powder Electrodes Implemented in an Improved Cell Assembly: Insights from In-Situ and Ex-Situ Solid-State NMR.

In this work, we report on an improved cell assembly of cylindrical electrochemical cells for 23 Na in-situ solid-state NMR (ssNMR) investigations. The cell set-up is suitable for using powder electrode materials. Reproducibility of our cell assembly is analyzed by preparing two cells containing hard carbon (HC) powder as working electrode and sodium metal as reference electrode. Electrochemical storage properties of HC powder electrode derived from carbonization of sustainable cellulose are studied by ssNMR. 23 Na in-situ ssNMR monitors the sodiation/desodiation of a Na|NaPF6 |HC cell (cell 1) over a period of 22 days, showing high cell stability. After the galvanostatic process, the HC powder material is investigated by high resolution 23 Na ex-situ MAS NMR. The formation of ionic sodium species in different chemical environments is obtained. Subsequently, a second Na|NaPF6 |HC cell (cell 2) is sodiated for 11 days achieving a capacity of 220 mAh/g. 23 Na ex-situ MAS NMR measurements of the HC powder material extracted from this cell clearly indicate the presence of quasi-metallic sodium species next to ionic sodium species. This observation of quasi-metallic sodium species is discussed in terms of the achieved capacity of the cell as well as of side reactions of sodium in this electrode material.

Ultra‐stable cycling of organic carboxylate molecule hydrogen bonded with inorganic Ti3C2Tx MXene with improved redox kinetics for sodium‐ion batteries

AbstractThe application of small organic molecules for sodium‐ion batteries is generally plagued by their high solubility, poor conductivity, and sluggish redox dynamics in organic electrolyte, thus developing efficient strategies to restrain solubilization while obtaining fast charge transfer becomes a challenge. Herein, a rational hybridization strategy through hydrogen bond between organic molecule and inorganic substrate has been proposed, employing the terminal –C═O of trisodium 1, 2, 4‐benzenetricarboxylate (TBC) molecule and –OH groups of inorganic Ti3C2Tx MXene, respectively. In general, such a design evidently mitigates the aggregation of both TBC molecules and Ti3C2Tx MXene. Furthermore, the robust hydrogen bonding significantly mitigates the dissolution of TBC and guarantees the robust coupling between them, thus contributing to the integrity of electrode and modifying the electrochemical sodium storage in both half and full cells. Moreover, the systematic kinetic analysis and mechanism detection reveal improved charge transportation and robust two‐electron electrochemical reversibility of the hybrid TBC/Ti3C2Tx. Taken together, this work demonstrates a potential novel strategy toward stable and practical organic battery chemistries through hydrogen bonding with inorganic compounds.

4,4′-Biphenyldicarboxylic acid as an anode for sodium-ion batteries: Different electrochemical behaviors in ester and ether-based electrolytes

Organic electrode materials have been regarded as sustainable alternatives for sodium-ion batteries (SIBs) due to their high theoretical specific capacity, large reserves and wide sources from biomass. However, the variety of presently reported organic electrode materials is very limited and their essential electrochemical sodium storage behaviors also deserve further investigating. In this paper, we explore the electrochemical sodium storage behavior of a conjugated aromatic acid (4,4′-biphenyldicarboxylic acid, H2bpdc) in esters and ethers-based electrolytes. The results illustrate that in esters-based electrolytes, H2bpdc just shows limited capacity and poor electrochemical sodium storage kinetics. By contrast, in ethers-based electrolytes, H2bpdc experiences a process similar to conversion type mechanism from pristine acid to sodium salt, displaying fast kinetics and delivers a high reversible capacity of 280 mA h g−1. This research would provide basic and novel insights to the electrochemical sodium storage behaviors of promising organic electrode materials and accelerate the new materials exploration process for SIBs.

Constructing Rich Interfacial Structure by Carbon Dots to Improve the Sodium Storage Capacity of Sb/C Composite

AbstractDesigning Sb/carbon composite is an effective strategy to enhance the cycle stability and rate capability of Sb anodes for sodium ion batteries. However, the introduction of carbon often leads to a loss in specific capacity and initial Coulombic efficiency (ICE). In this work, a new perspective is adopted by focusing on improving the “sloping capacity” contribution of carbon (C) in Sb/C composites, rather than solely increasing the platform capacity of Sb as traditionally done. Theoretical predictions indicate that constructing Sb/C composites with high Sb and carbon interfacial areas can provide more Na adsorption sites, thus enhancing the “sloping capacity” of amorphous carbon. Guided by DFT, a rational Sb/C/NSCDs‐2(N) is successfully developed by self‐embedding Sb nanodots into 2D‐amorphous carbon sheets doped with N/S. Sb/C/NSCDs‐2(N) exhibits abundant Sb/carbon interfacial areas and excellent nanoscale effects, demonstrating ideal electrochemical sodium storage performances. Impressively, the Sb/C/NSCDs‐2(N) provides a “sloping capacity” close to 160 mAh g−1. Significantly, owing to the fact that these interfaces and defects are not in direct contact with the electrolyte, the side reaction of electrolyte is effectively inhibited, leading to no decrease of ICE.

Na3VMn0.5Ti0.5(PO4)3/C with active Na+ hopping sites for high-rate and durable sodium-ion batteries

This study established the carbon-coated Na3VMn0.5Ti0.5(PO4)3/C (NVMTP/C) as a high-energy cathode material of the NASICON family for next-generation sodium-ion batteries (SIBs). When subjected to electrochemical sodium storage testing, the NVMTP/C cathode provided a stable specific capacity of 160.3 mAh g-1 with three electron reactions (average working voltage ∼3.4 V) during the Na+ (de)insertion routine, achieving a superior energy output of 544 Wh kg−1 among the NASICON family of SIBs. Benefiting from the characteristic NASICON structure and established multiple redox couples, the cathode delivered excellent rate capability (82 mAh g-1 at 17 C, 1 C = 176 mAh g-1), in addition to excellent cycling permanence at low and high current surges (104 mAh g-1 after 500 cycles at 0.85 C and 84 mAh g-1 after 3000 cycles at 11.34 C, respectively). Computational studies based on first principle calculations and density of states (DoS) attributed the superior electrochemical properties of NVMTP/C cathode to its substantial Na+ hopping sites facilitating the unhindered multi-electron transfer reactions (V2+/3+, V3+/4+, Ti3+/4+, and V4+/5+) reversibly at their respective redox potentials. The detailed DoS analysis revealed that the varying electronic configuration due to the co-swapping of Ti and Mn significantly uplifts the electronic conductivity of the NVMTP/C cathode. Further calculation studies clearly indicate lower activation energy for active Na+ hopping and hence faster Na-diffusion thus suggesting that the present NVMTP/C cathode can have definite commercial prospects.

Two-Dimensional Covalent Organic Framework with Synergistic Active Centers for Efficient Electrochemical Sodium Storage

Benefiting from the adjustable molecule structures, abundant functional units, large specific surface areas, and ordered pores, covalent organic frameworks (COFs) are highly desirable for electrochemical energy storage. Herein, a two-dimensional COF (denoted as HHTP-TABQ) with a fully π-conjugated framework and C═N and C═O dual-active sites has been synthesized and applied as the anode in organic rechargeable sodium-ion batteries (SIBs). During the sodium-storage process, it delivered remarkable rate capability, satisfactory ultra-long cycle stability (84.5% capacity retention ratio after 1000 cycles at 5000 mA g–1), and facile charge exchange kinetics (2.49 × 10–7 cm2 s–1), which is superior among most reported COF-based electrodes. Furthermore, the sodiation mechanism was investigated by density functional theory calculations and ex situ characterizations, which confirms that the electrochemical reaction is dominated by the synergistic reaction of C═N and C═O groups. These results suggest that COFs with stable π-conjugated structures, insoluble characteristics, and abundant active sites would have great potential for practical applications in rechargeable SIBs.

Design of double-shelled NiS-FeS@NC hollow nanocubes for high-performance sodium-ion batteries

Mixed metal sulfides (MMSs) are highly noticeable as prospective materials for sodium storage due to their prominent electrochemical reversibility and excellent redox activity. Nevertheless, the fast capacity decay and poor rate capability hinder their application in sodium-ion batteries (SIBs). In this work, a facile template method, using Prussian blue analogue as the precursor, was employed to fabricate the double-shelled hollow structure with nitrogen (N)-doped carbon layer coated on the outer shell of NiS-FeS hollow nanocubes (NiS-FeS@NC). Specifically, the NiS-FeS@NC composite provides a glorious reversible capacity (395.8 mAh g−1 after 200 cycles at 1.0 A g−1) and superior rate capability (322.2 mAh g−1 at 5.0 A g−1) when employed as electrode material for SIBs. In addition, the designed NiS-FeS@NC= |Na3V2(PO4)2 @C full battery can maintain an excellent reversible capacity (68.7 mAh g−1 after 200 cycles at 1.0 A g−1) as well as robust cycling stability. The improvement of the sodium storage performance can be mainly attributed to the synergistic coupling of the N-doped carbon coating layer and the hollow nanostructure, which can effectively maintain the integrity of the nanocube structure and accelerate the transportation of electrons/ions, thus contributing to improving the electrochemical sodium storage performance. Importantly, the unique modification strategy reported in this paper, combining with the mechanism and kinetics revealing, is expected to provide insights for facilitating high-performance electrodes for SIBs.

Functional surface modification of P2-type layered Mn-based oxide cathode by thin layer of NASICON for sodium-ion batteries

P2-type layered manganese-based oxides, as promising high-energy-density and low-cost cathodes, have been triggered considerable attentions in sodium ion batteries (SIBs). Nevertheless, P2-type manganese-based oxides still suffer from the sluggish kinetics and unfavorable Jahn-Teller effect. Herein, motivated by the high ionic conductivity and rigid three-dimension framework of NASICON-type materials, the well-known Na3Zr2Si2PO12 thin layer was coated on P2-type Na0.612K0.056MnO2 surface to modify electrochemical sodium storage and structural stability. The optimal Na0.612K0.056MnO2@Na3Zr2Si2PO12 exhibited a high discharge capacity of 203 mA h g−1 within 1.8 − 4.3 V at a low current density of 10 mA g−1. When the current density enhances into 500 mA g−1, it delivers a high initial discharge capacity of 145.7 mA h g−1 within 1.8 − 4.3 V at 500 mA g−1 and maintains 69% capacity retention after 100 cycles. Besides, at low temperature of −20 °C, it offers 100.1 mA h g−1 after 100 cycles within 1.8 − 4.3 V at 100 mA g−1, which is much higher than that of NKMO (65.2 mA h g−1). The Na3Zr2Si2PO12 can decrease Mn3+ content and avoid the Jahn-Teller effect of Na0.612K0.056MnO2 in structural level, enabling the enhanced Na+ diffusion coefficient of Na0.612K0.056MnO2. Such strategy NASICON coating on cathode would pave a new pathway on rational design of high-performance cathode for SIBs.

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.

Heterostructure engineering of MnO/TiO2 embedded in N-doped hollow carbon nanofibers for superior sodium storage

TiO2-based materials are viewed as promising anodes for sodium-ion batteries due to their high theoretical capacity and superior structural stability, but they suffer from low electron conductivity and sluggish Na+ diffusion kinetics, which leads to inferior rate performance and cyclability. Herein, a novel MnO/TiO2 embedded into N-doped hollow carbon nanofibers (MnO/TiO2@N-C) is proposed to address this challenge via combining the heterostructure engineering and nanostructure designing. The integrated design greatly enhances the Na+ transfer and adsorption, provides more active interfacial sites and promotes the electric conductivity. The first-principle density functional theory calculations manifest that the constructed hetero-interfaces between MnO and TiO2 induce strong electric fields and accelerate Na+ transfer. Due to these merits, the MnO/TiO2@N-C exhibits a high reversible discharge capacity of 406.5 mA h g−1 at 0.3 A/g, superior rate capability (213.3 mA h g−1 at 3 A/g), and exceptional cycling performance (99.3 % capacity retention after 10,000 cycles at 3 A/g). In addition, the Na full cell based on MnO/TiO2@N-C anode exhibits a promising energy density of 332.8 Wh kg−1 at 677.1 W Kg−1. This work offers a novel and an interesting strategy to improve electrochemical sodium storage of TiO2-based materials, which is significant for the practical applications of sodium-ion batteries.

In situ growth Zn2GeO4 nanorods network on 3D conductive foam as free standing electrode for sodium-ion batteries

• Zn 2 GeO 4 nanorods were anchored on nickel foam as a binder free anode for SIBs. • Zn 2 GeO 4 @NF anode shows 3D interlinked porous network structure. • The Zn 2 GeO 4 @NF showed excellent electrochemical sodium storage performances. Large-scale energy storage and scarcity of lithium bring exploration interest of high-performance anode materials for sodium-ion batteries (SIBs). Herein, Zn 2 GeO 4 nanorods was anchored directly on nickel foam to form a free standing, binder free anode for SIBs, though a facile hydrothermal method. Benefiting from composite effect of nanorod network structure, conductive network and mechanical stability, the Zn 2 GeO 4 @NF reaches an initial discharge capacity of 882.0 mAh g -1 and maintains a capacity of 435.6 mAh g -1 after 400 cycles at 0.1A g -1 . At a higher current density (2 A g -1 ) Zn 2 GeO 4 @NF also delivers a reversible discharge capacity of 239.7 mAh g -1 , which is higher than that for the Zn 2 GeO 4 particles.

Waste cotton fiber-based graphitized carbon/Fe7S8 composites derived from sulfurization of pyrolyzate Fe-Fe3C intermediates as anodes for high-performance sodium-ion batteries

Converting waste biomass resources into electrochemical energy storage materials has been regarded as a valuable contribution to implementing sustainable energy. Herein, waste cotton fiber-based graphitized carbon/Fe7S8 (CFGC/Fe7S8) composites were successfully synthesized from sulfurization of Fe-Fe3C intermediates, produced by simple pyrolysis of waste cotton fabric and low-cost iron compound. The as-prepared CFGC/Fe7S8 composites were composed of partly graphitized carbon fiber embedded with well-dispersed Fe7S8 nanoparticles. This structure offers excellent performance for electrochemical sodium storage. Acting as an anode in sodium-ion battery, CFGC/Fe7S8 composites deliver a high discharge specific capacity of 661 mAh g−1 after 100 cycles at 0.1 A g−1 and an initial Coulombic efficiency of 85.8% and achieve a discharge specific capacity of 328 mAh g−1 after 800 cycles even at a high current density of 2 A g−1. These results demonstrate that Fe-Fe3C intermediate derived-Fe7S8 nanoparticles combined with cotton fiber-based graphitic carbon plays a significant role in the electrochemical performance of sodium ion storage. These findings present a novel strategy for transferring waste textile into high-performance sodium storage materials.

Investigation of the sodium storage mechanism of iron fluoride hydrate cathodes using X-ray absorption spectroscopy and mossbauer spectroscopy

Elucidation of a reaction mechanism is the most critical aspect for designing electrodes for high-performance secondary batteries. Herein, we investigate the sodium insertion/extraction into an iron fluoride hydrate (FeF3⋅0.5H2O) electrode for sodium-ion batteries (SIBs). The electrode material is prepared by employing an ionic liquid 1-butyl-3-methylimidazolium-tetrafluoroborate, which serves as a reaction medium and precursor for F− ions. The crystal structure of FeF3⋅0.5H2O is observed as pyrochlore type with large open 3-D tunnels and a unit cell volume of 1129 Å3. The morphology of FeF3⋅0.5H2O is spherical shape with a mesoporous structure. The microstructure analysis reveals primary particle size of around 10 nm. The FeF3⋅0.5H2O cathode exhibits stable discharge capacities of 158, 210, and 284 mA h g−1 in three different potential ranges of 1.5–4.5, 1.2–4.5, and 1.0–4.5 V, respectively at 0.05 C rate. The specific capacities remained stable in over 50 cycles in all three potential ranges, while the rate capability was best in the potential range of 1.5–4.5 V. The electrochemical sodium storage mechanism is studied using X-ray absorption spectroscopy, indicating higher conversion at a more discharged state. Ex-situ Mössbauer spectroscopy strengthens the results for reversible reduction/oxidation of Fe. These results will be favorable to establish high-performance cathode materials with selective voltage window for SIBs.