Candidate For Sodium-ion Batteries Research Articles

Synergistic engineering of heteroatomic N-doping and PPy modification in flower-like vanadium sulfide anode enables stable sodium storage

Vanadium sulfide, an appealing anode candidate for sodium ion batteries (SIBs), still encounters considerable volume fluctuation and sluggish Na+ diffusion kinetics, which inevitably gives rise to unsatisfactory rate capability and severe capacity decay. Herein, we successfully designed and constructed an outstanding sodium-storage feature and exceedingly stable anode for SIBs in the form of three-dimensional flower-like architecture composing of phosphorus doped VS2 wrapped by conductive elastic polypyrrole (PPy) layer (i.e., P-VS2@PPy). Through leveraging the multifunctional synergistic effect of composition and structure, involving enhanced electronic conductivity, reduced volumetric fluctuation, accelerated charge/ion transfer efficiency, and remarkable surface-capacitive behaviors can guarantee significantly enhanced electrochemical sodium-storage ability of P-VS2@PPy composites. As a consequence, such P-VS2@PPy is endowed with desirable rate characteristic accompanied with long-period cyclic lifespan (436.7 mAh/g at 20.0 A/g after 2600 cycles), which far outperform contrastive P-VS2 and pure VS2 samples. Meticulous kinetic analysis combined with theoretical simulations conclude that phosphorus doping and PPy coating have substantial effects on strengthening ionic and electron diffusion kinetics. Thorough examination and analysis of the insertion and conversion mechanisms of P-VS2@PPy was conducted through ex situ tests, revealing superior reversible phase transformation between original VS2, intermediate NaVS2, and final Na2S/V. Additionally, a practical application test demonstrates that sodium ion full-cell and hybrid capacitor based on P-VS2@PPy anode showcase significant capacity contribution and outstanding specific energy output, capable of effortlessly lighting up a LED screen.

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.

Boosting sodium-ion batteries performance by N-doped carbon spheres featuring porous and hollow structures.

Carbon materials are considered among the most promosing candidates for sodium ion batteries because of their competitive performance. Nevertheless, they suffer from low initial coulombic efficiencies (ICEs) and limited electrochemical performance. Herein, nitrogen-doped hollow carbon spheres (NHCSs) with a distinct porous structure are developed by a template-assisted carbonization of dopamine, followed by a template removal procedure. This advanced structural design, coupled with the surface chemistry of carbonized polydopamine, leads to an impressive ICE of 89.18% and a reversible capacity stabilized at ∼700 mA h g-1 at 50 mA g-1 after 100 cycles. Compared to commercial hard carbon anodes, NHCSs demonstrate superior rate performance, delivering a capacity of ∼200 mA g h-1 at 5 A g-1 with minimal capacity fading of ∼0.057 mA h g-1 per cycle over 1000 cycles. These findings highlight the potential of NHCSs as a high-performance anode material for sodium-ion batteries, offering both high efficiency and excellent cycling stability.

Hollow CoSe2-ZnSe microspheres inserted in reduced graphene oxide serving as advanced anodes for sodium ion batteries

Transition metal selenides are promising anode candidates for sodium ion batteries (SIBs) because of their higher theoretical capacity and conductivity than metal oxides. However, the disadvantages of severe capacity degradation and poor magnification performance greatly limit their commercial applications. Herein, we have developed a new hollow bimetallic selenides (CoSe2-ZnSe)@reduced graphene oxide (rGO) composite with abundant heterointerfaces. The rGO could not only alleviate the volume variations of hollow CoSe2-ZnSe microspheres during cycling, but also improve the conductivity of composite. The presence of the heterointerfaces could help to accelerate ionic diffusion kinetics and improve electron transfer, resulting in the improved sodium storage performance. As an advanced anode for SIBs, the CoSe2-ZnSe@rGO exhibits an enhanced initial coulombic efficiency of 75.1% (65.2% of CoSe2@rGO), extraordinary rate capability, and outstanding cycling stability (540.3 mAh/g at 0.2 A/g after 150 cycles, and 395.2 mAh/g at 1 A/g after 600 cycles). The electrochemical mechanism was also studied by kinetic analysis, showing that the charging/discharging process of CoSe2-ZnSe@rGO is mostly related to a capacitive-controlled behavior.

New Protective Layer for Long Time Stable Sodium Metal Anodes

Facing the challenges of degradation processes in sodium metal batteries, a lot of new anode materials were prepared and tested. Especially Hard Carbon is a promising candidate for sodium ion batteries.[1] However, the competitive battery performance as gravimetric and volumetric energy density cannot be achieved without metallic sodium as an anode.[2]Another way to stabilize a sodium metal battery is the formation of an artificial solid-electrolyte interface (SEI). In the recent years, a lot of efforts were devoted for development of organic and inorganic protective layers. Most of the processes to form an artificial SEI are not easy for application, or result in a weak long-term stability of a sodium metal battery.[3]Here, we can introduce a new artificial SEI, which can easily be formed on top of metallic sodium and is also stable upon a long-term cycling. The capacity retention in a full cell setup (Swagelok cell, layered sodium cobalt titanium oxide as a cathode material, 1 M NaClO4 in EC/PC as an electrolyte) is of nearly 85 % after 500 cycles with a 1C current density. A reference full cell with non-protected sodium only reaches 20 % after the same cycling conditions.A possible explanation for the observed improvement in the cell performance was done based on comprehensive electrochemical, chemical and structural studies. Acknowledgment This work was supported by the Bundesministerium für Bildung und Forschung is carried out as part of the HeNa project (03XP0390C). Additional information Patent pending.

P2/O3 biphase integration promoting the enhancement of structural stability for sodium layered oxide cathode

O3-type sodium layered oxides have garnered significant attention as highly promising candidates for sodium ion batteries due to their high theoretical capacity. However, they often encounter challenges associated with complex and irreversible phase transitions and sluggish kinetics. To overcome these issues, herein we integrate the O3-type phase with the P2-type phase and systematically investigate the influence of different ratios of P2 phase in O3-type NaxLi0.05Mn0.55Ni0.3Cu0.075Mg0.025O2 pentanary layered oxides on the electrochemical performance by tuning the sodium contents. The LMC3-P2/O3-32 with optimized P2 phase ratio of 32.3 % exhibits a high reversible capacity of 157.0 mAh g−1 at a rate of 0.2C and excellent capacity retention rate of 63.5 % after 1000 cycles at 10C. The LCM3-P2/O3-32 also demonstrates good cycle life and rate capability in full cells when coupled with a hard carbon anode. In situ X-ray diffraction analyses confirm that the detrimental phase transformations to Na-poor O1 phase and the irreversible phase transitions occurred in the pure O3 cathode can be effectively suppressed in the P2/O3 biphasic cathode. The less Jahn-Teller active ions, enhanced phase transition reversibility and improved diffusion kinetics of Na+ ions are responsible for the improved cyclability and rate capability in the P2/O3 nanoscale intergrowth cathode. Our work shed light on the leverage of P2 and O3 phases through phase integration in layered oxides cathodes, which can serve as a guideline for the further development of high-performance cathode materials for advanced sodium ion batteries.

Quantum Physics and Deep Learning to Reveal Multiple Dimensional Modified Regulation by Ternary Substitution of Iron, Manganese, and Cobalt on Na3V2(PO4)3 for Superior Sodium Storage

AbstractNa3V2(PO4)3 is regarded as a promising candidate for sodium ion batteries. Nevertheless, the poor electronic conductivity, low capacities, and unstable structure limit its further investigations. Herein, a new type of Fe/Mn/Co co‐substituted Na3V2(PO4)3 with nitrogen‐doped carbon coating (NFMC) by a facile sol‐gel route is synthesized. The introduced elements feature in both crystal bulk and carbon coating layer. Suitable heteroatom substitution activates more effective Na+ to participate in electrochemical process and reinforce the structure. An extra high voltage platform at 3.8 V resulting from the multi‐element synergy (Mn2+/Mn3+/Mn4+; Co2+/Co3+; V4+/V5+) is stably and reversibly existed in NFMC to supply added capacities, which is investigated by quantum physics calculations. The high flux paths for Na+ migration and spin quantum state distribution in NFMC are demonstrated by molar magneton calculation. Significantly, the generated polyatomic coordination environment of MNC (M = Fe/Co/Mn) in carbon layer is first proposed. The most optimized combination structures are obtained from 69 possible structures and demonstrated by X‐ray absorption spectroscopy. The superior electrochemical performance is precisely forecasted by innovative deep learning. Predicted values with high precision are obtained based on a small number of operating data, extremely short development period, and provide real‐time status references for safer use.

Layer-by-layer hetero-carbon modifying ZnS nanocubes anode with improved long-term life for sodium-ion batteries

Zinc sulfide (ZnS) anode has gripped much focus in face of the various candidates for sodium-ion batteries (SIBs) because of its high capacity, abundant resource as well as environmental friendliness. Nevertheless, the structural collapse of ZnS and the dissolution of polysulfides will irreversibly lead to inferior Na-storage properties. Herein, layer-by-layer hetero-carbon modifying ZnS nanocubes anode (ZnS@NDC) composite has been rationally designed and prepared using pre-carbonized ZIF-8 as a matrix framework, C3N4 and polypyrrole as the coated carbon sources. The initial carbon layer derived from C3N4 can change the surface properties of ZnS nanocubes improving the electronic affinities and provide sufficient buffer matrix successfully inhibiting the agglomeration of ZnS nanocubes. The second carbon layer derived from polypyrrole can improve the ionic diffusion and electronic conductivity owing to the high conductivity and strong charge storage capacity of polypyrrole, as well as further accommodate volume expansion of the ZnS nanocubes. Synergistic effects of layer-by-layer hetero-carbon modifying can also refrain from dissolution of polysulfides improving the structural stability and electrochemical kinetics. When denoted as the anode for SIBs, ZnS@NDC electrode displays admirable long-term life stability and rate capability with capacities of 518.6 mA h g−1 at 1.5 A g−1 after 1000 cycles and 192.4 mA h g−1 at 10 A g−1, respectively. The idea of synthetic ZnS@NDC material supply experimental basis for the farther development of Na-storage devices and provide an innovative way for the synthesis of other metal compounds based on layer-by-layer construction.

Mg/Ti doping co-promoted high-performance P2-Na0.67Ni0.28Mg0.05Mn0.62Ti0.05O2 for sodium-ion batteries

Transition-metal layered oxides (such as P2-Na2/3Ni1/3Mn2/3O2) are suggested as one type of the most potential cathode candidates for sodium ion batteries (SIBs) owing to their high capacity and low cost; however, they suffer from the structural damage and sluggish Na+ kinetics resulting from the undesirable phase transformation of P2−O2 and the Na+/vacancy ordering, respectively. Herein, a Mg/Ti co-doped P2-Na0.67Ni0.28Mg0.05Mn0.62Ti0.05O2 layered oxide is demonstrated as a high-efficiency cathode material for SIBs. The cathode delivers a high reversible capacity of 135.5 mAh g−1, good cycling stability (82.7 mAh g−1 upon 100 cycles at 0.1C), and an attractive energy density of 479.4 Wh Kg−1. Furthermore, the phase transition from the undesirable P2−O2 to the reversible P2−OP4 demonstrated by in situ XRD and the partially suppressed Na+/vacancy ordering as well as the improved electronic and ionic conductivities all give rise to the enhancement. These results show the important role of cationic co-doping in designing and preparing high-efficiency layered oxide cathode materials for SIBs.

An ultralong-life SnS-based anode through phosphate-induced structural regulation for high-performance sodium ion batteries

As a star representative of transition metal sulfides, SnS is viewed as a promising anode-material candidate for sodium ion batteries due to its high theoretical capacity and unique layered structure. However, the extremely poor electrical conductivity and severe volume expansion strongly hinder its practical application while achieving a high reversible capacity with long-cyclic stability still remains a grand challenge. Herein, different from the conventional enhancement method of elemental doping, we report a rational strategy to introduce PO43− into the SnS layers using phytic acid as the special phosphorus source. Intriguingly, the presence of PO43− in the form of Sn–O–P covalent bonds can act as a conductive pillar to buffer the volume expansion of SnS while expanding its interlay spacing to allow more Na+ storage, supported by both experimental and theoretical evidences. Profiting from this effect combined with microstructural metrics by loading on high pyridine N-doped reduced graphene oxide, the as-prepared material presented an unprecedented ultra-long cyclic stability even after 10,000 cycles along with high reversible capacity and excellent full-cell performances. The findings herein open up new opportunities for elevating electrochemical performances of metal sulfides and provide inspirations for the fabrication of advanced electrode materials for broad energy use.

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.

Self-source silicon embedded in 2D biomass-based carbon sheet as anode material for sodium ion battery

In this research, 2D self-sourced silicon-embedded carbon sheets were prepared through pre-treatment combined with a high-temperature carbonization strategy. Through X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), specific surface area and pore size analysis, scanning electron microscope (SEM) and transmission electron microscope (TEM) and other technical means to obtain the electrode material structure and morphology. The results show that the pretreatment of the acidic system can effectively ensure the two-dimensional sheet while ensuring the precipitation of the raw material silicon. At the same time, the energy storage capacity and electrochemical performance of obtaining anode in sodium ion batteries (SIBs) were studied. The anode shows high specific capacity, excellent rate performance and stable cycle performance. At a current density of 100 mA.g−1, a specific capacity of 180mAh.g−1 was obtained. After 1000 cycles, it still has a coulomb efficiency of 99.7% and a capacity retention rate of 94.6%. Furthermore, the rapid reaction kinetics capability of this material was analyzed by calculating the proportion of pseudo-capacitance contribution in the system and the sodium ion diffusion coefficient. Its stable energy storage capacity and cycle stability make it a promising candidate for sodium ion batteries.

Enabling high-performance Na4MnV(PO4)3 cathode via synergetic strategy of carbon encapsulation and nanoengineering

NASICON-type materials with three-dimensional ionic migration pathways are proposed as the most important cathode candidates for sodium ion batteries. To promote the real applications, Na4MnV(PO4)3 (NMVP) with low cost and high energy density is highly recommended by partial substitution of V with earth-abundant active element Mn. Nevertheless, NMVP still encounters the problems of poor electrical conductivity and fair reversibility of Mn2+/Mn3+ redox couple, resulting in unsatisfactory cycling stability. Herein, carbon-encapsulated NMVP nanoroads are synthesized by a facile solid-state reaction in molten hydrocarbon media. The synergistic effects of carbon encapsulation and nanoengineering enable high electronic conductivity and large electrolyte-accessible surface areas of NMVP/C cathode, as well as highly reversible phase transitions upon repeated Na+ insertion/extraction. Thus, the NMVP/C nanoroads deliver excellent electrochemical performances, including high reversible capacity (112.3 mA h g−1 at 0.02 A g−1), excellent rate capability (58.3 mA h g−1 at 5 A g−1), and good cycling stability (85.1% of capacity retention over 1200 cycles at 1 A g−1). Moreover, the storage mechanism of Na+ is investigated by ex situ X-ray diffraction, and the fast ionic/electronic transport kinetics are revealed by combining electrochemical methods with density functional theory (DFT) calculations.

Simultaneous defect regulation by p-n type co-substitution in a Na3V2(PO4)3/C cathode for high performance sodium ion batteries.

The Na3V2(PO4)3 (NVP) cathode is deemed to be a promising candidate for sodium ion batteries due to its strong structural stability and high theoretical capacity. Nevertheless, its poor intrinsic conductivity restricts further development. To overcome these shortcomings, a dual modification strategy of Mn2+/Ti4+ co-substitution is proposed for the first time. Significantly, Mn doping can efficiently accelerate the transmission speed of electrons by introducing beneficial holes derived from the low valence state of +2, presenting the classical p-type doping modification. Moreover, the presence of Mn2+ with a larger ionic radius can support the crystal to stabilize the Na superionic conductor (NASICON) framework of the NVP system. Ti4+ is introduced for perfect charge compensation. Accordingly, the addition of Ti4+ can generate excess electrons due to the n-type substitution, which contributes to the favorable electronic conductivity. In addition, conductive carbon nanotubes (CNTs) are utilized to construct an efficient network to improve the rate capability of the NVP composite. Meanwhile, CNTs can inhibit particle growth and thus reduce particle size, shortening the transport path of Na+ and promoting the diffusion of Na+. Comprehensively, the optimized Na3V2-xMnxTix(PO4)3/C@CNTs (x = 0.15) deliver high capacities of 70.3 and 68.2 mA h g-1 at 90C and 180C, maintaining 58 and 53.8 mA h g-1 after 1000 cycles with high capacity retention of 82.5% and 78.9%.

Highly-conductive Ti3C2 sheets in boosting sodium-ion storage performances of Sn2S3 anode

Sn2S3 is a promising anode candidate for sodium ion batteries (SIBs). Nevertheless, it is very hard for the Sn2S3 anode to reach its theoretical capacity even discharged at low current density. Besides, suffering from large volume expansion/shrinkage during discharge/charge processes, the utilization rate of Sn2S3 is very low and the capacity drop ratio is rather high. Enhancing the electronic conductivity and compounding with rigid second-phase material is an effective way to boost the reversible capacity and prolong the cycle life. In this contribution, super conductive and rigid Ti3C2 sheets was used to anchor the Sn2S3/Carbon (Sn2S3/C) particles. Theoretical calculation demonstrated that the negatively charged Ti3C2 sheets can efficiently boost Na + diffusion rate. Upon served the Sn2S3/C/Ti3C2 as anode of SIBs, the reversibility, rate capacity and cycle-life were all improved overwhelmingly compared to recent-reported Sn2S3-based anode. The electrochemical tests showed that Na + diffuse kinetics was enhanced by incorporating Ti3C2 sheets, which was also predicted by theoretical calculations. As expected, Sn2S3/C/Ti3C2 delivered about 920, 725, 560, 440, 330, 205 mA h g−1 at 0.1, 0.2, 0.5, 1, 2, 4 A g−1, respectively, and the utilization ratio of Sn2S3 at 0.1 A g−1 and 60th cycle surprisingly reached ∼90%. Considering the ease of operating strategy proposed in this work, it may trigger new enthusiasm for designing high-performance SIBs anode materials.

A novel red phosphorus/reduced graphene oxide-C3N4 composite with enhanced sodium storage capability

• N -enriched rGO-C 3 N 4 hybrid network is proposed as matrix for phosphorus composites. • Interfacial compatibility between rGO-C 3 N 4 and red phosphorus is realized by P N bond. • Interfacial bonding is revealed by theoretical calculations. • Enhanced kinetics is facilitates by the formation of P N bonds. Red phosphorus (RP) has attracted wide attention as a promising anode candidate for sodium ion batteries due to its admirable specific capacity, safe working potential and low cost. However, its large volumetric expansion during desodiation/sodiation process and poor electronic conductivity hinder the progress of red phosphorus electrodes. Here, the N -enriched reduced graphene oxide-C 3 N 4 (rGO-C 3 N 4 ) network is utilized to facilitate the electronic conductivity and construct a robust architecture through P N bonds. Admirable bonding strength between phosphorus and the C 3 N 4 hybrid framework is revealed by the calculations via the first principles based on density function theory. The red phosphorus/rGO-C 3 N 4 anode for sodium ion batteries obtained via a facile ball-milling method exhibits a discharge capacity of 652.6 mA h g −1 after 100 cycles. This method holds great promise on developing red phosphorus anode on sodium-ion batteries.

Improving Electronic Conductivity and Electrochemical Properties of Lepidocrocite Titanates via Self-Assembly

Sodium titanates with layered or tunnel structures that reversibly intercalate sodium ions at low potentials are promising anode candidate for sodium ion batteries. Although titanates are attractive anodes due to their low cost, low toxicity, and high abundancy, their low intrinsic electronic conductivity and poor cycling performance are major hurdles. Large amounts of carbon additives are often used to improve electrical conductivity in the electrode composites. Another way to overcome the low electrical conductivity of sodium titanates is to manipulate the electrode structure. To overcome electronic transport issues in sodium ion batteries, we have designed and synthesized 2D heterostructures of lepidocrocite titanate (K0.8Ti1.73Li0.27O4, LTO). In these heterostructures, oxide layers alternate with conductive layers to make heterostructures with improved properties over the native oxides.In this study, we used a self-assembly approach to prepare van der Walls heterostructures of LTO with carbonized dopamine. Positively charged dopamine molecules were used as carbon precursor and intercalated between negatively charged exfoliated titania sheets through electrostatic interaction. Dopamine-intercalated LTO was then annealed under Argon to form conductive carbon layers between titania sheets. Electrodes that were prepared by using as-carbonized heterostructures without any carbon additives exhibit higher capacities, excellent capacity retention over 100 cycles and lower impedance when compared to parent material in sodium half-cells configurations.

PVP-assisted synthesis of ultrafine transition metal oxides encapsulated in nitrogen-doped carbon nanofibers as robust and flexible anodes for sodium-ion batteries

Transition metal oxides (TMOs) with high theoretical capacities are promising anode candidates for sodium ion batteries (SIBs), which yet suffers from inferior cycling stability and rate capability due to large volume change and sluggish transport kinetics during the sodiation/desodiation processes. Herein, a general strategy is developed to fabricate ultrafine TMOs nanoparticles encapsulated in nitrogen-doped carbon nanofibers (uf-TMOs@N-CNFs, M = Fe, Mn, Zn, etc.) as advanced anodes for SIBs. The uf-TMOs@N-CNFs are facilely synthesized via an in-situ electrospinning and subsequent carbonization strategy, among which polyvinylpyrrolidone (PVP) is employed as an effective dispersant to suppress metal agglomeration and control the size of TMOs. Benefiting from the intimate interaction between ultrafine TMOs and conductive N-CNFs substrates, the uf-TMOs@N-CNFs can efficiently mitigate the aggregation and pulverization of TMOs, facilitate electron/ion transfer, and boost pseudocapacitive charge storage, leading to superior sodium storage performance, including satisfactory reversible capacity, excellent rate capability, and durable cycling stability. Interestingly, the obtained uf-TMOs@N-CNFs membranes also exhibit excellent flexibility to serve as self-supported and flexible electrodes, demonstrating great potential for flexible SIBs. The present work provides a general and feasible method to construct robust and flexible electrodes for energy storage devices.

Stabilizing fluorine to achieve high-voltage and ultra-stable Na3V2(PO4)2F3 cathode for sodium ion batteries

Na3V2(PO4)2F3 is a promising cathode candidate for sodium ion batteries due to its high working voltage. However, an “unknown” low-voltage plateau around ~3.3 V lowers its operating voltage and puzzles researchers. Herein, the reasons for its appearance are explored. It is found that the formation of the low-voltage plateau originates from the extra-generated [VO6] induced by the fluorine loss during the synthesis. To enable better performance, a feasible strategy of stabilizing fluorine is proposed to eliminate the low-voltage plateau. It is demonstrated that this strategy can effectively guarantee the existence of fluorine and modulate the local electronic structure of V to maintain [VO4F2] in Na3V2(PO4)2F3 phase, thus eliminating the undesirable plateau and achieving an operating-voltage increase of ~100 mV. Moreover, the enhanced structural stability and kinetics of the optimized Na3V2(PO4)2F3 cathode are confirmed by in-situ XRD. As expected, the Na3V2(PO4)2F3 cathode delivers higher energy density (446.4 Wh kg−1), better rate capability and longer cycling performance (89.2%@30 C after 1000 cycles). Furthermore, its excellent adaptability to wide temperature range (−25 °C–55 °C) and strong competence in Na3V2(PO4)2F3||hard carbon full-cell are verified. These excellent properties further qualify Na3V2(PO4)2F3 as a competitive cathode for sodium ion batteries.

Facile synthesis of Co9S8@NC anode with enhanced sodium storage and long cycling life

Transition metal sulfides (TMSs) are presently under extensive research and have been regarded as the prospective anode candidates for sodium ion batteries (SIBs). However, the poor intrinsic electrical conductivity and huge volume expansion of metal sulfides electrodes during the discharge-charge process eventually cause pulverization of active materials and quick capacity decay, which severely limit their further practical application. In this study, ultrafine Co9S8 nanoparticles embedded in N-doped carbon layer (Co9S8@NC) were facilely synthesized via a simple one-step solid phase calcination method with glucose as the carbon source and thiourea served as N, S-containing precursor. The Co9S8@NC hybrid with small nanoparticle size could aid in the quick penetration of the electrolyte and shorten the pathway of Na ions diffusion; Meanwhile, the N-doped disordered carbon layer could maintain structural integrity and favor the electron transport of the nanocomposite during the electrochemical reaction. The Co9S8@NC composite is firstly investigated as anode for SIBs. They exhibit a satisfied discharge/charge specific capacity of 285.3/276.3 mAh g-1 after 160 cycles at 170 mA g−1, and its discharge/charge specific capacity is capable to maintain 160.8/158.5 mAh g−1 even after 1000 cycles at 1000 mA g−1. Further kinetics analyses based on CV demonstrate that the partial pseudocapacitive behaviors partially accounts for the remarkable Na storage performances.