Advances in sodium-ion batteries at low-temperature: Challenges and strategies

With the continuing boost in the demand for energy storage,there is an increasing requirement for batteries to be capable of operation in extreme environmental conditions.Sodium-ion batteries(SIBs) have emerged as a highly promising energy storage solution due to their promising performance over a wide range of temperatures and the abundance of sodium resources in the earth's crust.Compared to lithiumion batteries(LIBs),although sodium ions possess a larger ionic radius,they are more easily desolvated than lithium ions.Fu rthermore,SIBs have a smaller Stokes radius than lithium ions,resulting in improved sodium-ion mobility in the electrolyte.Nevertheless,SIBs demonstrate a significant decrease in performance at low temperatures(LT),which constrains their operation in harsh weather conditions.Despite the increasing interest in SIBs,there is a notable scarcity of research focusing specifically on their mechanism under LT conditions.This review explores recent research that considers the thermal tolerance of SIBs from an inner chemistry process perspective,spanning a wide temperature spectrum(-70 to100℃),particularly at LT conditions.In addition,the enhancement of electrochemical performance in LT SIBs is based on improvements in reaction kinetics and cycling stability achieved through the utilization of effective electrode materials and electrolyte components.Furthermore,the safety concerns associated with SIBs are addressed and effective strategies are proposed for mitigating these issues.Finally,prospects conducted to extend the environmental frontiers of commercial SIBs are discussed mainly from three viewpoints including innovations in materials,development and research of relevant theoretical mechanisms,and intelligent safety management system establishment for larger-scale energy storage SIBs.

Difficulties, strategies, and recent research and development of layered sodium transition metal oxide cathode materials for high-energy sodium-ion batteries

Energy-storage systems and their production have attracted significant interest for practical applications.Batteries are the foundation of sustainable energy sources for electric vehicles(EVs),portable electronic devices(PEDs),etc.In recent decades,Lithium-ion batteries(LIBs) have been extensively utilized in largescale energy storage devices owing to their long cycle life and high energy density.However,the high cost and limited availability of Li are the two main obstacles for LIBs.In this regard,sodium-ion batteries(SIBs) are attractive alternatives to LIBs for large-scale energy storage systems because of the abundance and low cost of sodium materials.Cathode is one of the most important components in the battery,which limits cost and performance of a battery.Among the classified cathode structures,layered structure materials have attracted attention because of their high ionic conductivity,fast diffusion rate,and high specific capacity.Here,we present a comprehensive review of the classification of layered structures and the preparation of layered materials.Furthermore,the review article discusses extensively about the issues of the layered materials,namely(1) electrochemical degradation,(2) irreversible structural changes,and(3) structural instability,and also it provides strategies to overcome the issues such as elemental phase composition,a small amount of elemental doping,structural design,and surface alteration for emerging SIBs.In addition,the article discusses about the recent research development on layered unary,binary,ternary,quaternary,quinary,and senary-based O3-and P2-type cathode materials for high-energy SIBs.This review article provides useful information for the development of high-energy layered sodium transition metal oxide P2 and O3-cathode materials for practical SIBs.

Recent Progress in Improving Rate Performance of Cellulose-Derived Carbon Materials for Sodium-Ion Batteries

Cellulose-derived carbon is regarded as one of the most promising candidates for high-performance anode materials in sodium-ion batteries;however,its poor rate performance at higher current density remains a challenge to achieve high power density sodium-ion batteries.The present review comprehensively elucidates the structural characteristics of cellulose-based materials and cellulose-derived carbon materials,explores the limitations in enhancing rate performance arising from ion diffusion and electronic transfer at the level of cellulose-derived carbon materials,and proposes corresponding strategies to improve rate performance targeted at various precursors of cellulose-based materials.This review also presents an update on recent progress in cellulose-based materials and cellulose-derived carbon materials,with particular focuses on their molecular,crystalline,and aggregation structures.Furthermore,the relationship between storage sodium and rate performance the carbon materials is elucidated through theoretical calculations and characterization analyses.Finally,future perspectives regarding challenges and opportunities in the research field of cellulose-derived carbon anodes are briefly highlighted.

High-efficiently doping nitrogen in kapok fiber-derived hard carbon used as anode materials for boosting rate performance of sodium-ion batteries

The engineering of plant-based precursor for nitrogen doping has become one of the most promising strategies to enhance rate capability of hard carbon materials for sodium-ion batteries;however,the poor rate performance is mainly caused by lack of pyridine nitrogen,which often tends to escape because of high temperature in preparation process of hard carbon.In this paper,a high-rate kapok fiber-derived hard carbon is fabricated by cross-linking carboxyl group in 2,6-pyridinedicarboxylic acid with the exposed hydroxyl group on alkalized kapok with assistance of zinc chloride.Specially,a high nitrogen doping content of 4.24%is achieved,most of which are pyridine nitrogen;this is crucial for improving the defect sites and electronic conductivity of hard carbon.The optimized carbon with feature of high nitrogen content,abundant functional groups,degree of disorder,and large layer spacing exhibits high capacity of 401.7 mAh g^(−1)at a current density of 0.05 A g^(−1),and more importantly,good rate performance,for example,even at the current density of 2 A g^(−1),a specific capacity of 159.5 mAh g^(−1)can be obtained.These findings make plant-based hard carbon a promising candidate for commercial application of sodium-ion batteries,achieving high-rate performance with the enhanced pre-cross-linking interaction between plant precursors and dopants to optimize aromatization process by auxiliary pyrolysis.

Boron-doped high-entropy oxide toward high-rate and long-cycle layered cathodes for wide-temperature sodium-ion batteries

03-type layered metal oxides hold great promise for sodium-ion batteries cathodes owing to their energy density advantage.However,the severe irreversible phase transition and sluggish Na^(+)diffusion kinetics pose significant challenges to achieve high-performance layered cathodes.Herein,a boron-doped03-type high entropy oxide Na(Fe_(0.2)Co_(0.15)Cu_(0.05)Ni_(0.2)Mn_(0.2)Ti_(0.2))B_(0.02)O_(2)(NFCCNMT-B_(0.02))is designed and the covalent B-O bonds with high entropy configuration ensure a robust layered structure.The obtained cathode NFCCNMT-B_(0.02)exhibits impressive cycling performance(capacity retention of 95%and 82%after100 cycles and 300 cycles at 1 and 10 C,respectively)and outstanding rate capability(capacity of 83 mAh g^(-1)at 10 C).Furthermore,the NFCCNMT-B_(0.02)demonstrates a superior wide-temperature performance,maintaining the same capacity level(113,4 mAh g^(-1)@-20℃,121 mAh g^(-1)@25℃,and 119 mAh g^(-1)@60℃)and superior cycle stability(90%capacity retention after 100 cycles at 1 C at-20℃).The high-entropy configuration design with boron doping strategy contributes to the excellent sodium-ion storage performance.The high-entropy configuration design effectively suppresses irreversible phase transitions accompanied by small volume changes(ΔV=0.65 A3).B ions doping expands the Na layer distance and enlarges the P3 phase region,thereby enhancing Na^(+)diffusion kinetics.This work offers valuable insights into design of high-performance layered cathodes for sodium-ion batteries operating across a wide temperature.

Manipulating Na occupation and constructing protective film of P2-Na_(0.67)Ni_(0.33)Mn_(0.67)O_(2) as long-term cycle stability cathode for sodium-ion batteries

P2-Na_(0.67)Ni_(0.33)Mn_(0.67)O_(2)(NNMO)is promising cathode material for sodium-ion batteries(SIBs)due to its high specific capacity and fast Na+diffusion rate.Nonetheless,the irreversible P2-O_(2)phase transformation,Na+/vacancy ordering,and transition metal(TM)dissolution seriously damage its cycling stability and restrict its commercialization process.Herein,Na occupation manipulation and interface stabilization are proposed to strengthen the phase structure of NNMO by synergistic Zn/Ti co-doping and introducing lithium difluorophosp(LiPO_(2)F_(2))film-forming electrolyte additive.The Zn/Ti co-doping regulates the occupancy ratio of Nae/Nafat Na sites and disorganizes the Na+/vacancy ordering,resulting in a faster Na+diffusion kinetics and reversible P2-Z phase transition for P2-Na_(0.67)Ni_(0.28)Zn_(0.05)Mn_(0.62)Ti_(0.05)O_(2)(NNZMTO).Meanwhile,the LiPO_(2)F_(2)additive can form homogeneous and ultrathin cathode-electrolyte interphase(CEI)on NNZMTO surface,which can stabilize the NNZMTO-electrolyte interface to prevent TM dissolution,surface structure transformation,and micro-crack generation.Combination studies of in situ and ex situ characterizations and theoretical calculations were used to elucidate the storage mechanism of NNZMTO with Li PO_(2)F_(2)additive.As a result,the NNZMTO displays outstanding capacity retention of 94.44%after 500 cycles at 1C with 0.3 wt%Li PO_(2)F_(2),excellent rate performance of 92.5 mA h g^(-1)at 8C with 0.1 wt%Li PO_(2)F_(2),and remarkable full cell capability.This work highlights the important role of manipulating Na occupation and constructing protective film in the design of layered materials,which provides a promising direction for developing high-performance cathodes for SIBs.

Liquid metal in prohibiting polysulfides shuttling in metal sulfides anode for sodium-ion batteries

Metal sulfides are a class of promising anode materials for sodium-ion batteries(SIBs)owing to their high theoretical specific capacity.Nevertheless,the reactant products(polysulfides)could dissolve into electrolyte,shuttle across separator,and react with sodium anode,leading to severe capacity loss and safety concerns.Herein,for the first time,gallium(Ga)-based liquid metal(LM)alloy is incorporated with MoS_(2)nanosheets to work as an anode in SIBs.The electron-rich,ultrahigh electrical conductivity,and self-healing properties of LM endow the heterostructured MoS_(2)-LM with highly improved conductivity and electrode integrity.Moreover,LM is demonstrated to have excellent capability for the adsorption of polysulfides(e.g.,Na_(2)S,Na_(2)S_(6),and S_(8))and subsequent catalytic conversion of Na_(2)S.Consequently,the MoS_(2)-LM electrode exhibits superior ion diffusion kinetics and long cycling performance in SIBs and even in lithium/potassium-ion battery(LIB/PIB)systems,far better than those electrodes with conventional binders(polyvinylidene difluoride(PVDF)and sodium carboxymethyl cellulose(CMC)).This work provides a unique material design concept based on Ga-based liquid metal alloy for metal sulfide anodes in rechargeable battery systems and beyond.

Achieving structurally stable O3-type layered oxide cathodes through site-specific cation-anion co-substitution for sodium-ion batteries

O3-type layered oxides have garnered great attention as cathode materials for sodium-ion batteries because of their abundant reserves and high theoretical capacity.However,challenges persist in the form of uncontrollable phase transitions and intricate Na^(+)diffusion pathways during cycling,resulting in compromised structural stability and reduced capacity over cycles.This study introduces a special approach employing site-specific Ca/F co-substitution within the layered structure of O_(3)-NaNi_(0.5)Mn_(0.5)O_(2) to effectively address these issues.Herein,the strategically site-specific doping of Ca into Na sites and F into O sites not only expands the Na^(+)diffusion pathways but also orchestrates a mild phase transition by suppressing the Na^(+)/vacancy ordering and providing strong metal-oxygen bonding strength,respectively.The as-synthesized Na_(0.95)Ca_(0.05)Ni_(0.5)Mn_(0.5)O_(1.95)F_(0.05)(NNMO-CaF)exhibits a mild O3→O3+O'3→P3 phase transition with minimized interlayer distance variation,leading to enhanced structural integrity and stability over extended cycles.As a result,NNMO-CaF delivers a high specific capacity of 119.5 mA h g^(-1)at a current density of 120 mA g^(-1)with a capacity retention of 87.1%after 100 cycles.This study presents a promising strategy to mitigate the challenges posed by multiple phase transitions and augment Na^(+)diffusion kinetics,thus paving the way for high-performance layered cathode materials in sodium-ion batteries.

Review and prospects on the low-voltage Na_(2)Ti_(3)O_(7) anode materials for sodium-ion batteries

Due to its low cost and natural abundance of sodium,Na-ion batteries(NIBs)are promising candidates for large-scale energy storage systems.The development of ultralow voltage anode materials is of great significance in improving the energy density of NIBs.Low-voltage anode materials,however,are severely lacking in NIBs.Of all the reported insertion oxides anodes,the Na_(2)Ti_(3)O_(7) has the lowest operating voltage(an average potential of 0.3 V vs.Na^(+)/Na)and is less likely to deposit sodium,which has excellent potential for achieving NIBs with high energy densities and high safety.Although significant progress has been made,achieving Na_(2)Ti_(3)O_(7) electrodes with excellent performance remains a severe challenge.This paper systematically summarizes and discusses the physicochemical properties and synthesis methods of Na_(2)Ti_(3)O_(7).Then,the sodium storage mechanisms,key issues and challenges,and the optimization strategies for the electrochemical performance of Na_(2)Ti_(3)O_(7) are classified and further elaborated.Finally,remaining challenges and future research directions on the Na_(2)Ti_(3)O_(7) anode are highlighted.This review offers insights into the design of high-energy and high-safety NIBs.

Cycling performance of layered oxide cathode materials for sodium-ion batteries

Layered oxide is a promising cathode material for sodium-ion batteries because of its high-capacity,high operating voltage,and simple synthesis.Cycling performance is an important criterion for evaluating the application prospects of batteries.However,facing challenges,including phase transitions,ambient stability,side reactions,and irreversible anionic oxygen activity,the cycling performance of layered oxide cathode materials still cannot meet the application requirements.Therefore,this review proposes several strategies to address these challenges.First,bulk doping is introduced from three aspects:cationic single doping,anionic single doping,and multi-ion doping.Second,homogeneous surface coating and concentration gradient modification are reviewed.In addition,methods such as mixed structure design,particle engineering,high-entropy material construction,and integrated modification are proposed.Finally,a summary and outlook provide a new horizon for developing and modifying layered oxide cathode materials.

Two-dimensional layered In_(2)P_(3)S_(9): A novel superior anode material for sodium-ion batteries

Developing reliable and efficient anode materials is essential for the successfully practical application of sodium-ion batteries.Herein,employing a straightforward and rapid chemical vapor deposition technique,two-dimensional layered ternary indium phosphorus sulfide(In_(2)P_(3)S_(9)) nanosheets are prepared.The layered structure and ternary composition of the In_(2)P_(3)S_(9) electrode result in impressive electrochemical performance,including a high reversible capacity of 704 mA h g^(-1) at 0.1 A g^(-1),an outstanding rate capability with 425 mA h g^(-1) at 5 A g^(-1),and an exceptional cycling stability with a capacity retention of88% after 350 cycles at 1 A g^(-1).Furthermore,sodium-ion full cell also affords a high capacity of 308 and114 mA h g^(-1) at 0.1 and 5 A g^(-1).Ex-situ X-ray diffraction and ex-situ high-resolution transmission electron microscopy tests are conducted to investigate the underlying Na-storage mechanism of In_(2)P_(3)S_(9).The results reveal that during the first cycle,the P-S bond is broken to form the elemental P and In_(2)S_(3),collectively contributing to a remarkably high reversible specific capacity.The excellent electrochemical energy storage results corroborate the practical application potential of In_(2)P_(3)S_(9) for sodium-ion batteries.

A 30-year overview of sodium-ion batteries

Sodium-ion batteries(NIBs)have emerged as a promising alternative to commercial lithium-ion batteries(LIBs)due to the similar properties of the Li and Na elements as well as the abundance and accessibility of Na resources.Most of the current research has been focused on the half-cell system(using Na metal as the counter electrode)to evaluate the performance of the cathode/anode/electrolyte.The relationship between the performance achieved in half cells and that obtained in full cells,however,has been neglected in much of this research.Additionally,the trade-off in the relationship between electrochemical performance and cost needs to be given more consideration.Therefore,systematic and comprehensive insights into the research status and key issues for the full-cell system need to be gained to advance its commercialization.Consequently,this review evaluates the recent progress based on various cathodes and highlights the most significant challenges for full cells.Several strategies have also been proposed to enhance the electrochemical performance of NIBs,including designing electrode materials,optimizing electrolytes,sodium compensation,and so forth.Finally,perspectives and outlooks are provided to guide future research on sodium-ion full cells.

Access to advanced sodium-ion batteries by presodiation:Principles and applications

Sodium-ion batteries(SIBs)are expected to offer affordability and high energy density for large-scale energy storage system.However,the commercial application of SIBs is hurdled by low initial coulombic efficiency(ICE),continuous Na loss during long-term operation,and low sodium-content of cathode materials.In this scenario,presodiation strategy by introducing an external sodium reservoir has been rationally proposed,which could supplement additional sodium ions into the system and thereby markedly improve both the cycling performance and energy density of SIBs.In this review,the significance of presodiation is initially introduced,followed by comprehensive interpretation on technological properties,underlying principles,and associated approaches,as well as our perspectives on present inferiorities and future research directions.Overall,this contribution outlines a distinct pathway towards the presodiation methodology,of significance but still in its nascent phase,which may inspire the targeted guidelines to explore new chemistry in this field.

Degradation analysis and doping modification optimization for high-voltage P-type layered cathode in sodium-ion batteries

Advancing high-voltage stability of layered sodium-ion oxides represents a pivotal avenue for their progress in energy storage applications.Despite this,a comprehensive understanding of the mechanisms underpinning their structural deterioration at elevated voltages remains insufficiently explored.In this study,we unveil a layer delamination phenomenon of Na_(0.67)Ni_(0.3)Mn_(0.7)O_(2)(NNM)within the 2.0-4.3 V voltage,attributed to considerable volumetric fluctuations along the c-axis and lattice oxygen reactions induced by the simultaneous Ni^(3+)/Ni^(4+)and anion redox reactions.By introducing Mg doping to diminished Ni-O antibonding,the anion oxidation-reduction reactions are effectively mitigated,and the structural integrity of the P2 phase remains firmly intact,safeguarding active sites and precluding the formation of novel interfaces.The Na_(0.67)Mg_(0.05)Ni_(0.25)Mn_(0.7)O_(2)(NMNM-5)exhibits a specific capacity of100.7 mA h g^(-1),signifying an 83%improvement compared to the NNM material within the voltage of2.0-4.3 V.This investigation underscores the intricate interplay between high-voltage stability and structural degradation mechanisms in layered sodium-ion oxides.

A Molecular-Sieving Interphase Towards Low-Concentrated Aqueous Sodium-Ion Batteries

Aqueous sodium-ion batteries are known for poor rechargeability because of the competitive water decomposition reactions and the high electrode solubility.Improvements have been reported by saltconcentrated and organic-hybridized electrolyte designs,however,at the expense of cost and safety.Here,we report the prolonged cycling of ASIBs in routine dilute electrolytes by employing artificial electrode coatings consisting of NaX zeolite and NaOH-neutralized perfluorinated sulfonic polymer.The as-formed composite interphase exhibits a molecularsieving effect jointly played by zeolite channels and size-shrunken ionic domains in the polymer matrix,which enables high rejection of hydrated Na^(+)ions while allowing fast dehydrated Na^(+)permeance.Applying this coating to electrode surfaces expands the electrochemical window of a practically feasible 2 mol kg^(-1) sodium trifluoromethanesulfonate aqueous electrolyte to 2.70 V and affords Na_(2)MnFe(CN)_(6)//NaTi_(2)(PO_(4))_(3) full cells with an unprecedented cycling stability of 94.9%capacity retention after 200 cycles at 1 C.Combined with emerging electrolyte modifications,this molecular-sieving interphase brings amplified benefits in long-term operation of ASIBs.

Experimental and computational optimization of Prussian blue analogues as high-performance cathodes for sodium-ion batteries:A review

In this review,we discuss the electrochemical properties of Prussian blue(PB)for Na^(+)storage by combining structural engineering and electrolyte modifications.We integrated experimental data and density functional theory(DFT)in sodium-ion battery(SIB)research to refine the atomic arrangements and crystal lattices and introduce substitutions and dopants.These changes affect the lattice stability,intercalation,electronic and ionic conductivities,and electrochemical performance.We unraveled the intricate structure-electrochemical behavior relationship by combining experimental data with computational models,including first-principles calculations.This holistic approach identified techniques for optimizing PB and Prussian blue analog(PBA)structu ral properties for SIBs.We also discuss the tuning of electrolytes by systematically adjusting their composition,concentration,and additives using a combination of molecular dynamics(MD)simulations and DFT computations.Our review offers a comprehensive assessment of strategies for enhancing the electrochemical properties of PB and PBAs through structural engineering and electrolyte modifications,combining experimental insights with advanced computational simulations,and paving the way for next-generation energy storage systems.

A review on anode materials for lithium/sodium-ion batteries

Since lithium-ion batteries(LIBs) have been substantially researched in recent years, they now possess exceptional energy and power densities, making them the most suited energy storage technology for use in developed and developing industries like stationary storage and electric cars, etc. Concerns about the cost and availability of lithium have prompted research into alternatives, such as sodium-ion batteries(SIBs), which use sodium instead of lithium as the charge carrier. This is especially relevant for stationary applications, where the size and weight of battery are less important. The working efficiency and capacity of these batteries are mainly dependent on the anode, cathode, and electrolyte. The anode,which is one of these components, is by far the most important part of the rechargeable battery.Because of its characteristics and its structure, the anode has a tremendous impact on the overall performance of the battery as a whole. Keeping the above in view, in this review we critically reviewed the different types of anodes and their performances studied to date in LIBs and SIBs. The review article is divided into three main sections, namely:(i) intercalation reaction-based anode materials;(ii) alloying reaction-based anode materials;and(iii) conversion reaction-based anode materials, which are further classified into a number of subsections based on the type of material used. In each main section, we have discussed the merits and challenges faced by their particular system. Afterward, a brief summary of the review has been discussed. Finally, the road ahead for better application of Li/Na-ion batteries is discussed, which seems to mainly depend on exploring the innovative materials as anode and on the inoperando characterization of the existing materials for making them more capable in terms of application in rechargeable batteries.

Ionic liquid electrolytes for sodium-ion batteries to control thermal runaway

Sodium-ion batteries are expected to be more affordable for stationary applications than lithium-ion batteries,while still offering sufficient energy density and operational capacity to power a significant segment of the battery market.Despite this,thermal runaway explosions associated with organic electrolytes have led to concerns regarding the safety of sodium-ion batteries.Among electrolytes,ionic liquids are promising because they have negligible vapor pressure and show high thermal and electrochemical stability.This review discusses the safety contributions of these electrolyte properties for high-temperature applications.The ionic liquids provide thermal stability while at the same time promoting high-voltage window battery operations.Moreover,apart from cycle stability,there is an additional safety feature attributed to modified ultra-concentrated ionic liquid electrolytes.Concerning these contributions,the following have been discussed,heat sources and thermal runaway mechanisms,thermal stability,the electrochemical decomposition mechanism of stable cations,and the ionic transport mechanism of ultra-concentrated ionic liquid electrolytes.In addition,the contributions of hybrid electrolyte systems consisting of ionic liquids with either organic carbonate or polymers are also discussed.The thermal stability of ionic liquids is found to be the main contributor to cell safety and cycle stability.For high-temperature applications where electrolyte safety,capacity,and cycle stability are important,highly concentrated ionic liquid electrolyte systems are potential solutions for sodium-ion battery applications.

Engineering homotype heterojunctions in hard carbon to induce stable solid electrolyte interfaces for sodium-ion batteries

Developing effective strategies to improve the initial Coulombic efficiency(ICE)and cycling stability of hard carbon(HC)anodes for sodium-ion batteries is the key to promoting the commercial application of HC.In this paper,homotype heterojunctions are designed on HC to induce the generation of stable solid electrolyte interfaces,which can effectively increase the ICE of HC from 64.7%to 81.1%.The results show that using a simple surface engineering strategy to construct a homotypic amorphous Al_(2)O_(3) layer on the HC could shield the active sites,and further inhibit electrolyte decomposition and side effects occurrence.Particularly,due to the suppression of continuous decomposition of NaPF 6 in ester-based electrolytes,the accumulation of NaF could be reduced,leading to the formation of thinner and denser solid electrolyte interface films and a decrease in the interface resistance.The HC anode can not only improve the ICE but elevate its sodium storage performance based on this homotype heterojunction composed of HC and Al_(2)O_(3).The optimized HC anode exhibits an outstanding reversible capacity of 321.5mAhg^(−1) at 50mAg^(−1).The cycling stability is also improved effectively,and the capacity retention rate is 86.9%after 2000 cycles at 1Ag^(−1) while that of the untreated HC is only 52.6%.More importantly,the improved sodium storage behaviors are explained by electrochemical kinetic analysis.

Realizing high-performance Na_(3)V_(2)(PO_(4))_(2)O_(2)F cathode for sodium-ion batteries via Nb-doping

Na_(3)V_(2)(PO_(4))_(2)O_(2)F(NVPOF)has received considerable interest as a promising cathode material for sodium-ion batteries because of its high working voltage and good structural/thermal stability.However,the sluggish electrode reaction resulting from its low intrinsic electronic conductivity significantly restricts its electrochemical performance and thus its practical application.Herein,Nb-doped Na_(3)V_(2-x)Nb_(x)(PO_(4))_(2)O_(2)F/graphene(rGO)composites(x=0,0.05,0.1)were prepared using a solvothermal method followed by calcination.Compared to the un-doped NVPOF/r GO,doping V-site with high-valence Nb element(Nb^(5+))(Na_(3)V_(1.95)Nb_(0.05)(PO_(4))_(2)O_(2)F/r GO(NVN05POF/rGO))can result in the generated V4^(+)/V3^(+)mixed-valence,ensuring the lower bandgap and thus the increased intrinsic electronic conductivity.Besides,the expanded lattice space favors the Na^(+)migration.With the structure feature where NVN05POF particles are attached to the rGO sheets,the electrode reaction kinetics is further accelerated owing to the well-constructed electron conductive network.As a consequence,the as-prepared NVN05POF/r GO sample exhibits a high specific capacity of~72 m Ah·g^(-1)at 10C(capacity retention of 65.2%(vs.0.5C))and excellent long-term cycling stability with the capacity fading rate of~0.099%per cycle in 500 cycles at 5C.