Sodium-ion Half-cells Research Articles

Synthesis of ultra-high nitrogen doped hollow mesoporous carbon nanospheres via a universal solid–liquid interface self-assembly strategy

Functional mesoporous polymers, particularly their derived hollow mesoporous carbon spheres (HMCS), have introduced new opportunities in the field of energy storage. Traditional synthesis methods for HMCS typically rely on chemical etching, which often results in uniform chemical properties and structural control, thereby limiting their application in large-scale production and diverse fields. Therefore, in this study, we report a novel strategy for obtaining HMCS through the direct pyrolysis of a core matrix, melamine–formaldehyde resin (MF), to achieve hollow structures and ultra-high nitrogen doping (approximately 14.8 wt%). This new solid–liquid interfacial self-assembly approach enables the production of HMCS with remarkable electrochemical performance and practical application prospects in sodium-ion half-cells and full cells, thanks to the high nitrogen doping and unique structure. Notably, this strategy has been validated for its general applicability across 1D, 2D, and 3D solid matrices. Additionally, by adjusting the calcination temperature, this method allows for precise control over both the core size and nitrogen content of the egg yolk-shell structure mesoporous carbon spheres. In summary, the mesoporous materials developed in this study demonstrate flexibility, simplicity, versatility, and structural diversity, offering significant insights for future applications in energy storage and other fields.

Electrodeposition of Tin and Antimony-Based Anode Materials for Sodium-Ion Batteries

Tin antimonide (SnSb) is a promising alloying anode for sodium-ion batteries due to its high theoretical capacity and relative stability. The material is popular in the battery field, but, to our knowledge, few studies have been conducted on the influence of altering Sn and Sb stoichiometry on anode capacity retention and efficiency over time. Here, Sn-Sb electrodes were synthesized with compositional control by optimizing electrodeposition parameters and stoichiometry in solution and the alloys were cycled in sodium-ion half-cells to investigate the effects of stoichiometry on both performance and electrochemical phenomena. Higher concentrations of antimony deposited into the films were found to best maintain specific capacity over 270 cycles in the tin-antimony alloys, with each cell showing a slow, gradual decrease in capacity. We identified that a 1:3 ratio of Sn:Sb retained a specific capacity of 486 mAh g−1 after 270 cycles, highlighting a need to explore this material further. These results demonstrate how control over stoichiometry in Sn-Sb electrodes is a viable method for tuning performance.

Confining Co1.11Te2 nanoparticles within mesoporous hollow carbon combination sphere for fast and ultralong sodium storage

Co1.11Te2 nanoparticles are in-situ uniformly grown within mesoporous hollow carbon combination sphere (MHCCS@Co1.11Te2) using a hard-template and spray drying process, solution impregnation and pyrolysis tellurization. Material characterizations reveal that Co1.11Te2, with a diameter of ∼ 20 nm, is attached to the internal walls of the unit spheres or embedded in the mesopore shells of the unit spheres, presenting a distinctive “ships-in-combination-bottles” nanoencapsulation structure. In sodium-ion half-cells, MHCCS@Co1.11Te2 exhibits excellent cycling stability, achieving reversible capacities of 257 mAh/g at 0.5 A/g after 250 cycles, 235 mAh/g at 1.0 A/g after 300 cycles and 161 mAh/g at 10.0 A/g after 1900 cycles. Electrochemical kinetic analyses and ex-situ characterizations reveal rapid electron/Na+ transport kinetics, prominent surface pseudocapacitive behavior, robust nanocomposite structure, and multi-step conversion reactions of sodium polytellurides. In sodium-ion full-cells, MHCCS@Co1.11Te2 still demonstrates stable cycling performance at 1.0 and 5.0 A/g and excellent rate capability. The superior electrochemical performance is associated with the nanoencapsulation structure based on mesoporous hollow carbon combination spheres, which promotes electron conduction and Na+ transport. The space-confined effect maintains the high electrochemical activity and cycling stability of Co1.11Te2.

Lamellar sodium titanium silicate assembled by nanostrips decorated with N-rich 3D carbon for sodium dual-ion batteries

Sodium dual-ion batteries have received tremendous attention owing to their appealing operating voltage (i.e. >4.0 V) and eco-friendly features. However, the exploitation of efficient sodium ion storage anode materials is one of the keys for driving the application of sodium dual-ion batteries in the grid-scale energy storage. Herein, a lamellar sodium titanium silicate (NTSO) woven by plentiful nanostrips grown in situ on the surface of N-rich 3D carbon has been prepared via a convenient synthetic strategy. Based on the synergistic effect both micro-nano composite spatial structure and energy storage mechanism based on conversion reaction of material bulk phase, the NTSO/3DC-N composite as anode delivers a high special capacity of 540.6 mAh/g at 0.05 A/g and 163.1 mAh/g at 5.0 A/g in sodium ion half-cell. More impressively, the sodium dual-ion batteries built with NTSO/3DC-N as anode and a N-rich 3D carbon as cathode exhibits a high reversible capacity of about 123.6 mAh/g at 0.05 A/g. The elaborate designing an interconnecting framework composed of layered titanium silicate sodium and heterogeneous atom doping carbon substrate provides a generally applicable strategy for enhancing the anode performance of sodium dual-ion batteries.

Kinetically well-matched porous framework dual carbon electrodes for high-performance sodium-ion hybrid capacitors

Sodium-ion hybrid capacitors (SIHCs) have attracted extensive interest due to their applications in sodium-ion batteries and capacitors, which have been considered expectable candidates for large-scale energy storage systems. The crucial issues for achieving high-performance SIHCs are the reaction kinetics imbalances between the slow Faradic battery-type anodes and fast non-Faradaic capacitive cathodes. Herein, we propose a simple self-template strategy to prepare kinetically well-matched porous framework dual-carbon electrodes for high-performance SIHCs, which stem from the single precursor, sodium ascorbate. The porous framework carbon (PFC) is obtained by direct calcination of sodium ascorbate followed by a washing process. The sodium-ion half cells with PFC anodes exhibit high reversible capacity and fast electrochemical kinetics for sodium storage. Moreover, the as-obtained PFC can be further converted to porous framework activated carbon (PFAC) with rich porosity and a high specific surface area, which displays high capacitive properties. By using kinetically well-matched battery-type PFC anodes and capacitive PFAC cathodes, dual-carbon SIHCs are successfully assembled, which can work well in 0–4 V. The optimal PFC//PFAC SIHC exhibits high energy density (101.6 Wh kg−1 at 200 W kg−1), power density (20 kW kg−1 at 51.1 Wh kg−1), and cyclic performance (71.8 % capacitance attenuation over 10,000 cycles).

Novel Bismuth Nanoflowers Encapsulated in N-Doped Carbon Frameworks as Superb Composite Anodes for High-Performance Sodium-Ion Batteries.

Bismuth (Bi) has attracted attention as a promising anode for sodium-ion batteries (SIBs) owing to its suitable potential and high theoretical capacity. However, the large volumetric changes during cycling leads to severe degradation of electrochemical performance and limits its practical application. Herein, Bi nanoflowers are encapsulated in N-doped carbon frameworks to construct a novel Bi@NC composite via a facile solvothermal method and carbonization strategy. The well-designed composite structure endows the Bi@NC with uniformly dispersed Bi nanoflowers to alleviate the attenuation while the N-doped carbon frameworks improve the conductivity and ion transport of the whole electrode. As for sodium-ion half-cell, the electrode exhibits a high specific capacity (384.8mAhg-1 at 0.1Ag-1 ) and excellent rate performance (341.5mAhg-1 at 10Ag-1 ), and the capacityretention rate still remains at 94.9% after 5000 cycles at 10Ag-1 . Furthermore, the assembled full-cell with Na3 V2 (PO4 )3 cathode and Bi@NC anode can deliver a high capacity of 251.5mAhg-1 at 0.1Ag-1 , and its capacity attenuates only 0.009% in each cycle after 2000 times at 5.0Ag-1 . This work offers a convenient, low-cost, and eco-friendliness approach for high-performance electrodes in the field of sodium ion electrochemical storage technology.

All-solid-state sodium-ion batteries operating at room temperature based on NASICON-type NaTi2(PO4)3 cathode and ceramic NASICON solid electrolyte: A complete in situ synchrotron X-ray study

All-solid-state sodium-ion batteries that work at ambient temperature are a potential approach for large-scale energy storage systems. Nowadays, ceramic solid electrolytes are gaining attention because of their good ionic conductivity and excellent mechanical and chemical stabilities. Furthermore, a good interface between electrode and solid electrolyte is also required to achieve successful cell performances. In this work, sintered ceramic layer electrolyte Na3.16Zr1.84Y0.16Si2PO12, with high ionic conductivity (0.202 mS/cm at room temperature), are prepared by using uniaxial pressing followed by a sintering process. The conductive carbon coated NASICON material (NaTi2(PO4)3/C) exhibits, as cathode material, enhanced rate capability and stability for sodium ion batteries for high carbon (18.95 %) coated sample. At C/10, the optimized cathode (with higher carbon content) achieves a remarkable initial discharge capacity of 107.3 mAh/g (reversible capacity of 101.4 mAh/g), a sufficient rate capability up to a rate of 10C, and a long cycle life (capacity retention of 58% after 950 cycles). The one-stage reversible biphasic reaction mechanism and potential-dependent structure–property of NaTi2(PO4)3 can be explained by employing in situ X-ray synchrotron method. Sequential Rietveld refinements of the in situ data show the evolution of the Na-poor NaTi2(PO4)3 and Na-rich Na3Ti2(PO4)3 phase fractions (wt%), unit cell characteristics, and unit cell volume. The design of an all-solid-state sodium ion half-cell with a NaTi2(PO4)3/C cathode and a Na3.16Zr1.84Y0.16Si2PO12 solid-state electrolyte interface results in stable capacity of 83.6 mAh/g at C/10 and excellent reversible capacity at high C-rate. The results show that sintered NASICON-based electrolytes can significantly contribute for the fabrication of all-solid-state sodium-ion battery due to the superior conductivity and stability.

Nitrogen-coordinated antimony atom anchored on carbon matrix as efficient active sites to enhance sodium/potassium ion storage

Carbon-based anode materials have become a research hotspot for alkali metal ion batteries. Crucially, the electrochemical performance of carbon materials must be improved by appropriate means such as micro-nano structure design and atomic doping. Herein, antimony doped hard carbon materials are prepared by anchoring Sb atoms on nitrogen-doped carbon (SbNC). The coordination of non-metal atoms can better disperse Sb atoms on the carbon matrix, and the synergistic effect between Sb atoms, coordinated non-metal atoms, and hard carbon matrix endows SbNC anode with good electrochemical performance. When used in sodium-ion half-cells, the SbNC anode showed high rate capacity of 109 mAh g−1 at 20 A g−1 and good cycling performance (254 mAh g−1 at 1 A g−1 after 2000 cycles). In addition, when used in potassium-ion half-cells, the SbNC anode exhibited initial charge capacity of 382 mAh g−1 at 0.1 A g−1 and rate capacity of 152 mAh g−1 at 5 A g−1. This research shows that compared with ordinary nitrogen doping, Sb-N coordination active sites on carbon matrix can provide much more adsorption capacity, improve ion filling and diffusion properties as well as enhance the kinetics of electrochemical reaction for the sodium/potassium storage.

Flexible gel polymer electrolyte comprising high flash point solvent adiponitrile with ethylene carbonate as co-solvent for sodium-ion batteries

Due to the unique properties like high flash point, low vapor pressure and high electrochemical stability, the adiponitrile (ADN) as co-solvent with ethylene carbonate (EC) is proposed as an electrolyte solvent for sodium-ion batteries (NIBs). Here, we report a flexible thick film of a gel polymer electrolyte (GPE) containing a Na-salt (sodium triflate), dissolved in a mixture of ADN:EC, entrapped in poly(vinylidene fluoride-co-hexafluoropropylene) for use in NIBs. The structural/morphological studies of the GPE film are performed by X-ray diffraction, scanning electron microscopy and Fourier-transform infrared spectroscopy. The electrolyte offers excellent performance characteristics with high ionic conductivity (σRT ∼ 4.6 × 10−3 S cm−1) and Na+-transport number (tNa+ ∼ 0.77), wide electrochemical potential range (∼5.0 V vs. Na/Na+) at room temperature, and sufficient thermal stability (up to ∼120 °C), making the GPE-film attractive for sodium-ion storage. The Na-stripping/plating measurements for a long duration show excellent reversibility of the electrolyte with Na-electrodes, revealing low and stable polarization. The sodium-ion half-cell (Na0.44MnO2/GPE/Na-metal) shows prominent charge–discharge cycling stability, with a discharge capacity of ∼85.5 mAh g−1 at a current rate of 0.1C, and good rate capability up to 1C. The GPE-film has also been tested in a full sodium ion-cell fabricated with sodiated bismuth powder and Na0.44MnO2 as anode and cathode materials, respectively. The full cell offers discharge capacity of ∼55.7 mAh g−1 at 0.1C with stable cycling performance. The results indicate that the GPE film has potential applications as high performance electrolyte for Na-storage.

Heterojunction and carbon coating dual strategy mechanism: WSe2/NiSe@C nanosheets for efficient sodium storage

Based on the design strategy of heterojunction and carbon coating, a nanosheet structure composed of heterojunction bimetallic selenide and carbon layer (denoted as WSe2/NiSe@C) was successfully designed and synthesized. The creation of heterojunctions structure can benefit adjusting the electron structure and accelerate migration rate of ions, which provides the possibility to improve reaction kinetics of sodium storage. In addition, in order to alleviate the matter of material volume pulverization during charging and discharging, a method of carbon coating is proposed to solve the problem. As expected, WSe2/NiSe@C showed superior sodium storage property. In the sodium ion half-cell, WSe2/NiSe@C delivered sustainable discharge specific capacity (535.2 mAh g−1), outstanding rate performance, prominent initial Coulombic efficiency (78.1%) and excellent cycling capacity (361.3 mAh g−1 after 100 cycles) at 0.5 A g−1. More surprisingly, when the WSe2/NiSe@C is applied to the full sodium ion cell, it can maintain 105 mAh g−1 after 200 cycles at 0.5 A g−1, showing remarkable practical application value.

Rational design of glass fiber-cellulose composite separator for sodium-ion batteries

Using glass fiber scraps as matrix and highly dispersed cellulose as enhancer, composite separators are prepared by a simple slurry sieving process. The effects of different composite proportions on the microstructure, mechanical properties, ionic conductivity and electrochemical properties are studied by SEM, tensile test, AC impedance and charge-discharge test, etc. As mass composite ratio is 1:1, the thickness of composite separator is 73.9 um, tensile strength is 29.5 Mpa and ion conductivity is 0.45 × 10−3 S cm−1, respectively. Sodium-ion half-cell (NaxNi[Fe(CN)6] as cathode) assembled by above composite separator exhibits excellent cycle stability (93.3% of capacity retention after 500 cycles at 1 C), rate capability (85.1% of capacity retention at 10 C) and maintains fine interfacial stability. Compared with glass fiber separator, some performance indicators of this new composite separator have been greatly improved, suggesting its potential application prospects in sodium-ion batteries.

Temperature-assisted phase pure sodium trivanadate thin film as a potential electrode for sodium-ion storage applications

The Sodium trivanadate thin films were successfully deposited by the cost-efficient spray pyrolysis technique on flexible stainless steel as substrates for the first time. The XRD and Raman analysis of the deposited thin film reveals the crystallinity and structural properties. SEM result unveils the morphology of the films. The chemical composition was determined by EDAX analysis. The XPS analysis of the film discloses the oxidation states of the vanadium present in the film. Supercapacitor studies were performed using a three-electrode setup in which 1M NaNO3 was used as an electrolyte throughout the study. Among the deposited films, the film annealed at 400 ˚C has an exceptional specific capacitance of around 255 F/g at a current density of 0.25 A/g. An asymmetric supercapacitor device has been fabricated for the better-performed film in three-electrode studies and it delivered a specific capacitance of 12.3 F/g at 0.25 A/g current density. Sodium-ion half cells were also constructed and tested for the same film. The cells obtained a capacity of 78 mAh/g at a high current density of 15 mA/g with a capacity retention of 80% even after 50 cycles. The contribution to the film's capacity comes from the pseudocapacitance-type behaviour of the thin film cathodes. The prepared additive and binder-free sodium trivanadate thin film electrodes could exhibit excellent Na-ion storage properties. Thus, the successful assembly of Na-ion batteries and supercapacitors by utilizing a single electrode bodes well for the growth of hybrid energy storage and delivery systems in the future.

Cellulose filter papers derived separator featuring effective ion transferring channels for sodium-ion batteries

As a basic component of batteries, separators are proposed to apart the two electrodes while allowing the transport of ionic charge carriers. Cellulose separators have attracted tremendous attention due to their excellent thermal stability and wettability. In this study, we prepare all-cellulose composite (ACC) separators coated with Zn5(OH)8Cl2·H2O (ZHC) particles. The surface of the cellulose filter paper is partially dissolved by the 65% ZnCl2 solution, forming a tortuous and porous all-cellulose composite membrane. At the same time, ZHC is generated in situ on the surface during the removal of residual ZnCl2 from the cellulose, which effectively promotes the passage of a sodium ion (Na+) and substantially improves the cycling performance of the batteries. Density functional theory (DFT) calculations demonstrate that suitable adsorption energy on ZHC is favorable for promoting Na+ transport. The corresponding ACC separator exhibits high ionic conductivity (2.75 mS/cm) and Na+ transference number (0.82), correspondingly the assembled Na/hard carbon half-cells show an excellent reversible specific capacity (315 mA h/g at 25 mA/g) and cycling stability (the retention is 97.77% after 50 cycles at 50 mA/g), outperforming the glass fiber separator in both sodium-ion half-cell and full-cell.

Recyclable molten-salt-assisted synthesis of N-doped porous carbon nanosheets from coal tar pitch for high performance sodium batteries

N-doped porous carbon nanosheets (NPCS) are fabricated by a facile recyclable molten salt strategy with low-cost coal tar pitch as precursor. The obtained NPCS is featured with a continuous conductive network with a high nitrogen content of 5.22 at.%, facilitating the migration of electrons and ions in the process of sodium ion insertion/de-insertion. Benefiting from its appropriate structure and chemical composition, NPCS exhibits a high reversible capacity of 313.6 mAh g-1 at 0.1 A g-1 in sodium-ion half cells and excellent rate capability of 131.5 mAh g-1 at large current of 10 A g-1. Meanwhile, the full cell assembled by Na3V2(PO4)3 cathode and NPCS anode can deliver a high capacity of 220.1 mAh g-1 at 0.1 A g-1, and its capacity only attenuates 0.003 % in each cycle after 1000 times at 1.0 A g-1. The cleanness and massive utilization of coal tar pitch via eco-friendliness approach will promote the development of high-performance electrodes in the field of sodium ion electrochemical storage technology.

Biomass derived erythrocyte-like hard carbon as anodes for high performing full sodium-ion batteries

Herein, a novel hard carbon is prepared via one-step pyrolysis process from waste Ganoderma spores. We explore four pyrolysis temperatures and obtain the hard carbon product (NG-700) with the best performance. Ganoderma spores are hard that the prototype structure is reserved after pyrolysis process. The NG-700 is erythrocyte-like shape with hollow structure which can hold electrolyte. Many nitrogen atoms doping in NG-700 improve the conductivity of the material and provide more absorption sites. Moreover, the porous structure of NG-700 is conducive to entering of electrolyte and relieving volume expansion during cycling. In sodium-ion half-cells, the NG-700 electrode maintains 165 mA h g−1 for 10,000 cycles at 2 A g−1. In sodium-ion full-cells, the NG-700 anode can keep the capacity of 94 mA h g−1 for 200 cycles at 0.1 A g−1 paired with Na3V2(PO4)3 cathodes. Under severe environmental conditions (−25 °C), the electrode's capacity retention rate is 50 %.

Fabricating multi-porous carbon anode with remarkable initial coulombic efficiency and enhanced rate capability for sodium-ion batteries

Due to the abundant sodium reserves and high safety, sodium ion batteries (SIBs) are foreseen a promising future. While, hard carbon materials are very suitable for the anode of SIBs owing to their structure and cost advantages. However, the unsatisfactory initial coulombic efficiency (ICE) is one of the crucial blemishes of hard carbon materials and the slow sodium storage kinetics also hinders their wide application. Herein, with spherical nano SiO2 as pore-forming agent, gelatin and polytetrafluoroethylene as carbon sources, a multi-porous carbon (MPC) material can be easily obtained via a co-pyrolysis method, by which carbonization and template removal can be achieved synchronously without the assistance of strong acids or strong bases. As a result, the MPC anode exhibited remarkable ICE of 83% and a high rate capability (208 mAh/g at 5 A/g) when used in sodium-ion half cells. Additionally, coupling with Na3V2(PO4)3 as the cathode to assemble full cells, the as-fabricated MPC//NVP full cell delivered a good rate capability (146 mAh/g at 5 A/g) as well, implying a good application prospect the MPC anode has

VS4 nanoarrays pillared Ti3C2Tx with enlarged interlayer spacing as anode for advanced lithium/sodium ion battery and hybrid capacitor

Aiming to improve the sluggish ion insertion/extraction at the battery-type anode in lithium-ion/sodium-ion hybrid capacitor (LIC/SIC), herein, a VS4@L-Ti3C2Tx composing of VS4 nanoarrays and MXene with enlarged interlayer spacing is developed through a facile hydrothermal approach. Benefiting from the merits of building blocks, the VS4@L-Ti3C2Tx based lithium-ion half-cell exhibits superior specific capacity (1328 mAh g−1 after 170 cycles at 0.1 A g−1), rate performance (808 mAh g−1 after 10 cycles at 20 A g−1) and cycling stability (721 mAh g−1 after 500 cycles at 1.0 A g−1), surpassing most of the reported VS4 based counterparts. The sodium-ion half-cell with VS4@L-Ti3C2Tx anode also shows good performances. Moreover, LIC and SIC based on VS4@L-Ti3C2Tx anodes deliver high energy densities of 122.5 Wh kg−1 and 117.0 Wh kg−1 at a low power density 100.3 W kg−1, respectively. Additionally, they exhibit excellent cycling lifespans with capacity retention of 60% after 20000 cycles for LIC at a high current rate 1.0 A g−1, outperforming most of the reported analogues. This study may shed light on the development of other layered anode materials for energy storage devices.

Graphene wrapped Cobalt/Tin bimetallic sulfides composites with abundant active facets and extended interlayer spacing for stable sodium/potassium storage

In this work, graphene wrapped Cobalt/Tin bimetallic sulfides composites (Co1Sn6.75-sulfides/rGO, CSSG) were prepared via soft template-assisted solvothermal reactions, the electrostatic assembly and high-temperature calcination strategies. Compared with pure SnS, Co1Sn6.75-sulfides show extended interlayer spacings and abundant active facets, which can accelerate the intercalation/extraction of Na+/K+ and improve the diffusion kinetics. Graphene can increase the electrical conductivity of composites and induce the formation of stable electrode/electrolyte interfaces. Therefore, CSSG exhibit superior cycle performance (391.7 mAh/g at 2.0 A/g after 1000 cycles, 240 mAh/g at 5.0 A/g over 5000 cycles) and outstanding rate capability (326.3 mAh/g at 10.0 A/g) for sodium ion half-cells. In addition, Na3V2(PO4)3||CSSG full-cells deliver a high reversible capacity of 339.3 mAh/g at 0.5 A/g after 100 cycles. Similarly, CSSG still exhibits high cycle capacities for potassium ion half-cells (306.6 mAh/g at 0.5 A/g after 1000 cycles). A variety of analysis methods and ex-situ characterizations are adopted to explore the diffusion kinetics of Na+/K+ and potassium storage mechanism. The structural evolution of CSSG during the long-term cycle process for sodium storage is investigated to analyze the capacity growth and attenuation.

Exploration of a Na3V2(PO4)3/C –Pb full cell Na-ion prototype

Recent research focusses typically on sodium-ion half cells, and the progress of high performance and cost-effective full cells remains a critical obstacle to commercialize sodium-ion batteries. Here, we rationally design a full sodium-ion battery based on carbon coated Na3V2(PO4)3 (NVP-C) cathode and Pb anode materials. Carbon coated NVP is prepared via easy and one-step solid-state method. With 8 wt% carbon content, the obtained NVP-C core-shell structure provides an excellent reversible capacity of 104 mAh/g at C/10 rate, approaching towards its theoretical capacity. The NVP-C electrode even at a high 5 C rate delivers a primary reversible capacity of 87 mAh/g with 99.9% capacity retention almost after 3500 cycles. As an anode, Pb delivers an initial reversible capacity of 477 mAh/g at a fixed current density of 13 mA/g with a good cyclability and capacity retention of 98.5% over 50 cycles. The full cell constructed from the as-prepared materials exhibits a preliminary reversible capacity of 233 mAh/g at C/10 rate. In addition, the good rate capability including discharge energy density in the range of 170–180 Wh/kg makes the full cell a potential candidate in practical sodium-ion battery application. The excellent electrochemical behavior with simple, low-cost properties of fabricated full cell mark it as attractive option for high-energy energy storage applications.

Unveiling the Electrochemical Mechanism of High-Capacity Negative Electrode Model-System BiFeO3 in Sodium-Ion Batteries: An In Operando XAS Investigation.

Careful development and optimization of negative electrode (anode) materials for Na-ion batteries (SIBs) are essential, for their widespread applications requiring a long-term cycling stability. BiFeO3 (BFO) with a LiNbO3-type structure (space group R3c) is an ideal negative electrode model system as it delivers a high specific capacity (770 mAh g-1), which is proposed through a conversion and alloying mechanism. In this work, BFO is synthesized via a sol-gel method and investigated as a conversion-type anode model-system for sodium-ion half-cells. As there is a difference in the first and second cycle profiles in the cyclic voltammogram, the operating mechanism of charge-discharge is elucidated using in operando X-ray absorption spectroscopy. In the first discharge, Bi is found to contribute toward the electrochemical activity through a conversion mechanism (Bi3+ → Bi0), followed by the formation of Na-Bi intermetallic compounds. Evidence for involvement of Fe in the charge storage mechanism through conversion of the oxide (Fe3+) form to metallic Fe and back during discharging/charging is also obtained, which is absent in previous literature reports. Reversible dealloying and subsequent oxidation of Bi and oxidation of Fe are observed in the following charge cycle. In the second discharge cycle, a reduction of Bi and Fe oxides is observed. Changes in the oxidation states of Bi and Fe, and the local coordination changes during electrochemical cycling are discussed in detail. Furthermore, the optimization of cycling stability of BFO is carried out by varying binders and electrolyte compositions. Based on that, electrodes prepared with the Na-carboxymethyl cellulose (CMC) binder are chosen for optimization of the electrolyte composition. BFO-CMC electrodes exhibit the best electrochemical performance in electrolytes containing fluoroethylene carbonate (FEC) as the additive. BFO-CMC electrodes deliver initial capacity values of 635 and 453 mAh g-1 in the Na-insertion (discharge) and deinsertion (charge) processes, respectively, in the electrolyte composition of 1 M NaPF6 in EC/DEC (1:1, v/v) with a 2% FEC additive. The capacity values stabilize around 10th cycle and capacity retention of 73% is observed after 60 cycles with respect to the 10th cycle charge capacity.