高性能三元NASICON型Na3.5?xMn0.5V1.5?xZrx(PO4)3/C鈉離子電池正極材料

關(guān)鍵詞：鈉離子電池；NASICON；正極材料；循環(huán)穩(wěn)定性；倍率性能

1 Introduction

Owing to their cost-effectiveness and the widespreadavailability of sodium resources， rechargeable batteries based onNa+ as charge carriers have been extensively studied forpotential applications in grid energy storage 1–5. However， theircycle life and energy density still trail behind those of lithiumionbatteries， primarily due to limitations in cathode and anodematerials. Cathode materials for SIBs can generally becategorized into three types： transition metal oxides， polyanioniccompounds， and Prussian blue analogues 6–9. Among these，sodium superionic conductor （NASICON） materials showpromise due to their adjustable composition， structure， andpotential applications 10–14. NASICON materials arecharacterized by the general formula AMM'（XO4）3， where Atypically represents alkali or alkaline earth metal ions， M （andM'） sites are occupied by transition metals with various valencestates， and X is predominantly Si or P 15，16. These materials offerflexibility in both composition and structure 17–19.

As cathodes for SIBs， the A site in the NASICON structure istypically occupied by Na ions or vacancies， which enablereversible extraction and insertion during charge and dischargecycles， accompanied by electron transfer 20–22. The NASICONstructure is composed of alternating MO6 octahedra and XO4tetrahedra sharing vertices， thereby forming a spacious threedimensionalchannel 23–26. This arrangement facilitates the rapidextraction and migration of sodium ions within the material 27，28.By selecting different transition metal elements， the voltagerange of NASICON materials can be adjusted， typically between2 and 5 V （vs. Na/Na+） 29，30. One exemplary NASICON-typecathode material is Na3V2（PO4）3 （NVP）， known for its stablestructure and electrochemical performance， featuring a capacityof 117 mAh?g?1 at 3.4 V 31–33. Despite these attributes， NVPfaces challenges such as low conductivity and limited energydensity， hindering its practical use 34–36. Moreover， vanadium，which constitutes over 90% of the cathode material costs， is toxicand costly 37–40. To address these issues and enhanceelectrochemical performance while reducing costs， substitutingvanadium partially with other transition metals has beenexplored. Among alternatives like Mn， Mg， and Ni， manganese（Mn） stands out due to its robust bonding with oxygen， whichimproves the material's structural stability 41–43.

Manganese-based materials undergo Mn4+/Mn3+/Mn2+ redoxreactions， allowing for multiple electron transfers and promisinghigher energy density in new electrode designs 44–46. InNa3+xMnxV2?x（PO4）3 materials， increasing Mn content raises thedischarge voltage platform from 3.38 V to 3.56 V 47. In 2016，Goodenough et al. 48 introduced the Mn/V mixed-ionNa4MnV（PO4）3 cathode material， exhibiting a high capacity of～101 mAh?g?1 with an average of 3.5 V. Upon charging to 4.3V， V4+ oxidizes further to V5+， forming a platform around 4.1 Vand extracting 2.8 Na ions from the Na4MnV（PO4）3 crystalstructure， yielding approximately 150 mAh?g–1 charge capacity.However， this process induces irreversible phase transitions，reducing discharge capacity. Anishchenko et al. 49 revealed theinfluence of Mn doping on the kinetics of Na3+xMnxV2?x（PO4）3electrodes， noting that increasing Mn content decreases the Na+diffusion coefficient by an order of magnitude and introducesgreater voltage hysteresis in charge/discharge curves. Zhang etal. 50 investigated Na3.5Mn0.5V1.5（PO4）3 （NMVP） as a cathodematerial， achieving 130 and 109 mAh?g?1 for the first cycle.Despite these promising capacities， this material suffers fromslow charge transfer kinetics and low electronic conductivity.

Doping and substitution strategies have emerged as effectiveapproaches to enhance the performance of SIBs 51，52. Thesestrategies can significantly enhance the structural stability， iondiffusion pathways， and electronic conductivity of cathodematerials. In this study， we focus on the unique advantages ofZr4+ doping in NASICON Na3.5?xMn0.5V1.5?xZrx（PO4）3/C（denoted as NMVZP-x）. The introduction of Zr not onlystabilizes the crystal structure through the formation of strongZr―O bonds but also creates additional Na? vacancies thatenhance ion mobility. Moreover， the larger ionic radius of Zr4+leads to an expansion of the lattice structure， providing a broadermigration pathway for Na+ ions. The partial substitution ofvanadium with Zr in the NMVZP-0.1 cathode material resultedin a high initial discharge capacity of 119 mAh?g?1 at 0.5C.Additionally， this material demonstrated impressive long-termstability， retaining 89.9% of its capacity after 800 cycles at 10C.

2 Results and discussion

The crystal structure of the samples was examined usingXRD， as shown in Fig. S1， which presents the XRD patterns forNa3.5?xMn0.5V1.5?xZrx（PO4）3 with x values of 0 to 0.15. Allsamples exhibit similar diffraction patterns， belonging to the R3cspace group. The sharp and well-defined peaks suggest highcrystallinity and phase purity， with no evidence of impurities.The crystal structure of NMVZP-0.1 was further refined usingthe Rietveld method. NMVZP-0.1 exhibits a NASICON-typestructure， characterized by a three-dimensional network ofcorner-sharing [MO6] octahedra and [PO4] tetrahedra. Asdepicted in Fig. S1， the increase in Zr content leads to a shift indiffraction peaks to lower angles， reflecting lattice expansioncaused by the larger radius of Zr4+ （0.72 ?）. The refinementresults show that Zr4+ occupies the 12c position in the crystallattice. This substitution leads to a noticeable increase in the aand b lattice parameters， with the c parameter exhibiting a moresignificant expansion， as depicted in Fig. 1c. The increased cellvolume is a direct consequence of the larger size of Zr4+ ions.Additionally， the substitution of Zr4+ for V3+ results in a reducedoccupancy of sodium sites， which creates additional sodiumvacancies. These vacancies enhance the mobility of sodium ionswithin the structure， contributing to improved electrochemicalperformance. XPS analysis was further utilized to identify thevalence states of the elements in the synthesized samples. Thesurvey XPS spectrum （Fig. S2） verifies the presence ofelemental composition. The Mn 2p spectrum （Fig. 1d） showsdistinct peaks at Mn 2p1/2 （652.6 eV） and Mn 2p3/2 （640.8 eV），along with their satellite peaks， indicative of the Mn2+ state. Fig.1e illustrates the deconvoluted V 2p spectrum， which featurespeaks at V 2p1/2 （522.8 eV） and V 2p3/2 （516 eV）， confirming thepresence of V3+. In addition， Fig. 1e presents the Zr 3d spectrumwith peaks at Zr 3d3/2 （185.57 eV） and Zr 3d5/2 （183.17 eV），confirming the incorporation of Zr4+ into the NMVZP structure.The intensity of the Zr 3d peaks increases with higher Zrconcentrations in the Na3.5?xMn0.5V1.5?xZrx（PO4）3 series （0 ≤ x ≤0.15）， indicating successful doping of Zr into the material.

The SEM images （Figs. S3a–d and 2a） show that the samplesconsist of irregularly shaped particles， with sizes ranging from 1to 2 μm due to agglomeration. Fig. 2b provides the HR-TEMimage of NMVZP-0.1， which reveals clear lattice fringes and athin surface layer approximately 3 nm thick on the particles. TheFast Fourier Transform （FFT） pattern corresponds to the [111]observation axis， confirming the high crystallinity of NMVZP-0.1 and its rhombohedral symmetry （R3c）. Raman spectroscopy（Fig. S4） confirms the presence of residual carbon in thesamples. The ID/IG ratio， representing the peak intensity ratio ofthe D peak （1340 cm?1） to the G peak （1600 cm?1）， is 1.02 forNMVP and 0.99 for NMVZP-0.1. A lower ID/IG value indicatesbetter conductivity due to higher graphitization. The measuredcarbon content for NMVP and NMVZP-0.1 is 3.8% and 3%，respectively （Fig. S5）. Nitrogen adsorption-desorption curves（Fig. S6a，b） show that the specific surface area of NMVZP-0.1（32.4 m2?g?1） is slightly larger than that of NMVP （25.6 m2?g?1）.

Fig. 2c presents the high-angle annular dark-field （HAADF）image of NMVZP-0.1， revealing a well-ordered atomic-levelstructure observed along specific crystallographic directions.The bright dots in the HAADF images can be assigned to P andM （where M can be Mn， V， or Zr）， displaying a periodicarrangement consistent with the NASICON crystallographicframework. This confirms that the NASICON structuremaintains its integrity after Zr substitution， with Zr successfullyoccupying the designated M site within the framework. Acomparison of HAADF-STEM images （Fig. 2d，f） and intensityprofile analysis （Fig. 2e，g） reveals that the distances between theM atom columns in NMVZP-0.1 （0.522 nm） are greater thanthose in NMVP （0.499 nm）. This unit cell expansion is due tothe larger ionic radius of Zr4+ compared to V3+. The increasedinteratomic distances resulting from Zr substitution enlarge theionic channels， thereby facilitating more efficient Na+ iondiffusion. Additionally， the EDS analysis （Fig. 2h） confirms thatall elements are uniformly distributed within the particles.

Fig. 3a，b show the initial charge/discharge curves of theNMVZP-0.1 electrode at 0.5C. The electrode achieves highinitial capacities of 128 and 119 mAh?g?1， respectively， with aCoulombic efficiency （C.E.） of 93%. The voltage profilesfeature two plateaus： the lower voltage plateau （～3.5 V）corresponds to the redox reactions of Mn2+/Mn3+ and V3+/V4+during the Na+ insertion/deinsertion， while the short plateau atthe higher voltage （～3.9 V） indicates the V4+/V5+ redox couple.This indicates that suitable Mn substitution can activate thereversible V4+/V5+ reaction， providing additional capacity. Fig.3c illustrates the CV curves of electrodes. Both electrodesexhibit distinct electrochemical behavior with two pairs ofanodic and cathodic peaks， revealing the presence of multipleredox reactions in both materials. A smaller distance （ΔE）between peak values corresponds to better reversibility andlower polarization. The ΔE value for NMVZP-0.1 is 0.42 V，significantly smaller than 0.66 V for pristine NMVP， indicatingsuperior reversibility and smaller polarization. Thisimprovement is attributed to the optimized electronic structureand enhanced sodium ion mobility due to Zr substitution， leadingto more efficient charge transfer and better overallelectrochemical performance.

The long cycle performance of NMVP and NMVZP-0.1cathodes at 10C is shown in Fig. 3d. The results indicate thatboth cathode materials have an initial C.E. above 96%. TheNMVZP-0.1 cathode material demonstrates a high initialspecific capacity of 101 mAh?g–1 and maintains 91 mAh?g–1 after800 cycles， retaining 90% of its initial capacity. This slow decayin discharge specific capacity and good cycle stability are closelyrelated to the stable structure of the NMVZP-0.1 cathodematerial. In contrast， the NMVP cathode material shows asignificant decline in discharge capacity， dropping from 96mAh?g?1 to 53 mAh?g?1， leading to a capacity retention of just55.2% after cycling. XRD analysis （Fig. S7） of the electrodeafter 200 cycles reveals that the pristine NMVP shows theappearance of impurity phases at positions of 17.7° and 33.2°after prolonged cycling， indicating the instability of the electrodeafter long cycles. The superior long-term cycling stability ofNMVZP-0.1 underscores the beneficial effects of Zr substitutionin enhancing the structural integrity and electrochemicalperformance of the cathode material.

Fig. 3e–g compares the rate performance of NMVP andNMVZP-0.1 cathodes. The NMVP cathode initially provides adischarge capacity of 121 mAh?g?1 at 0.2C， but its capacitydiminishes rapidly with increasing rates， reflecting its limitedrate capability. In contrast， the NMVZP-0.1 cathode exhibitsmarkedly better rate performance， maintaining a dischargecapacity of 90 mAh?g?1 at 10C and 84 mAh?g?1 at 20C. This issignificantly higher compared to NMVP， which delivers only 78mAh?g?1 at 10C and drops to 38 mAh?g?1 at 20C. The enhancedrate performance of NMVZP-0.1 is further highlighted in Fig.3f，g， which display the voltage profiles of the electrodes atvarious C-rates. Notably， NMVZP-0.1 demonstrates a smallerpolarization voltage， especially at rates above 5C. This reductionsuggests that Zr4+ doping effectively mitigates polarization andimproves conductivity， thereby reducing the internal resistanceof the electrode at high rates. The improved performance ofNMVZP-0.1 at elevated C-rates can be attributed to theincreased Na? ion mobility and enhanced electronic conductivityresulting from Zr4+ substitution. This makes NMVZP-0.1 apromising candidate for applications requiring high-ratecapabilities and better overall performance in sodium-ionbatteries.

The Nyquist plots （Fig. S8） further confirm that NMVZP-0.1has a smaller semicircle in the high-frequency region，corresponding to the charge transfer resistance， and a steeperWarburg impedance line in the low-frequency region， reflectingmore efficient sodium-ion diffusion within the electrodematerial. The results reveal that the NMVZP-0.1 electrodeexhibits a significantly lower charge transfer impedancecompared to the pristine NMVP electrode. This reduction inimpedance indicates an improved charge transfer process at theelectrode-electrolyte interface， which can be attributed to theenhanced ionic conductivity and optimized electron transportpathways resulting from the Zr4+ substitution. The superiorperformance of NMVZP-0.1 highlights the beneficial impact ofZr substitution in optimizing the electrode’s performance，making it a promising candidate for high-performance sodiumionbatteries.

Fig. 4 shows that the initial XPS spectrum of the electrodereveals peaks at approximately 641 eV for Mn 2p and 516 eVfor V 2p， corresponding to Mn2+ and V3+ states， respectively.Upon charging to 3.65 V， the observed shift in these peaks tohigher positions signifies the oxidation of Mn2+ to Mn3+ and V3+to V4+. The higher charging voltage of 4.2 V further shifts the V2p peaks， indicating continued oxidation to V4+ and the presenceof V5+， while the Mn3+ state remains stable. These spectralchanges confirm the sequential oxidation of vanadium speciesfrom V3+ to V4+ and V5+ during the charge process. Upondischarging back to 2.5 V， the Mn and V peaks revert to theiroriginal positions， reflecting the reduction of Mn3+ to Mn2+ andV5+ to V3+. This reversible shift in oxidation states underscoresthe excellent stability and reversibility of the NMVZP-0.1cathode material. The stable oxidation states and theirreversibility throughout the charge-discharge cycle indicate thatthe material maintains its structural integrity and electrochemicalactivity.

Fig. 5 compares the electrode reaction kinetics of NMVP andNMVZP-0.1. The CV curves （Fig. 5a，d） for both electrodes wererecorded at scan rates ranging from 0.1 to 2 mV?s?1. As the scanrate increases， the NMVP electrode shows a growing potentialdifference between the anodic and cathodic peaks. The Na+diffusion coefficient （DNa+） was determined using the Randles-Sevcik equation and the linear relationship between the squareroot of the scan rate （v1/2） and the peak current （Ip）， as shown inFig. 5c. For the NMVZP-0.1 cathode material， the DNa+ duringsodium extraction and insertion were found to be 6.32 × 10?12cm2?s?1 and 9.16 × 10?12 cm2?s?1， respectively. In contrast， theinitial NMVP cathode material exhibited DNa+ values of 5.78 ×10?13 cm2?s?1 and 8.24 × 10?13 cm2?s?1， respectively. Theseresults indicate that Zr4+ doping significantly enhances the Na+diffusion coefficient， thereby improving electrochemicalkinetics. Fig. 5d，e compare the GITT curves and thecorresponding DNa+ of NMVP and NMVZP-0.1 cathodes withinthe voltage range of 2.5–4.2 V. The GITT curves display twodistinct inclined and flat voltage plateaus at ～3.4 V and ～3.9 V.The DNa+ of the NMVZP-0.1 cathode （Fig. 5f） is estimated to beapproximately 3.9 × 10?11–6.4 × 10?11 cm2?s?1， which is higherthan that of NMVP. This improvement of DNa+ can be attributedto enhanced conductivity， broadened sodium channels， and thecreation of Na vacancies in the framework.

DFT simulations were further performed to compare the Na+diffusion barriers in NMVP and NMVZP-0.1， aiming tounderstand the impact of Zr4+ doping on ionic conductivity andelectrochemical performance. In NMVP， the Na? ion migrationfollows a path along the two nearest MO6 octahedra and passesthrough a triangular bottleneck， which can restrict ionmovement. The optimized Na? ion migration pathways areillustrated in Fig. 5g. To assess how Na? ion concentrationimpacts diffusivity， we compared the Na+ hopping barriers alongthe Na1-Na2 and Na2-Na1-Na2 pathways in both NMVP andNMVZP-0.1. This comparison was carried out using the CINEBmethod， which allows us to evaluate the influence of Zrdoping on Na? ion mobility and migration efficiency. ForNMVP， the direct Na1-Na2 hopping barrier was found to be0.232 eV， and the Na2-Na1-Na2 knockoff hopping barrier was0.328 eV （Fig. 5h）. After Zr doping， these barriers weresignificantly reduced to 0.083 eV for direct Na1-Na2 hoppingand 0.310 eV for Na2-Na1-Na2 knockoff hopping （Fig. 5i）. Thisreduction in the diffusion barrier suggests that Zr4+ dopingfacilitates Na+ ion migration within the lattice. The lowerdiffusion barrier in NMVZP-0.1 can be attributed to the creationof sodium vacancies and lattice expansion， thereby enhancingionic conductivity and improving electrochemical performance.

3 Conclusion

In this study， a series of Na3.5?xMn0.5V1.5?xZrx（PO4）3 （0 ≤ x ≤0.15） cathode materials were successfully synthesized using asol-gel method combined with high-temperature solid-statesynthesis. The incorporation of Zr into the NASICON structuresignificantly enhanced the electrochemical performance. Amongthe synthesized compositions， NMVZP-0.1 demonstratedoutstanding electrochemical properties， achieving an initialcapacity of 119 mAh?g?1 at 0.5C. Notably， after 800 cycles at 10C， NMVZP-0.1 retained 89.9% of its capacity and delivered adischarge capacity of 84 mAh?g?1 at an ultra-high rate of 20C，indicating excellent performance even under high current rates.The enhanced electrochemical kinetics attributed to Zr dopingfacilitated improved ion diffusion pathways and structuralstability， positioning these materials as promising candidates foradvanced sodium-ion battery applications.

Author Contribution： Conceptualization， P.H. and Z.G.;Methodology， D.X.; Software， S.Z.; Validation， J.M.， B.L. andP.H.; Formal Analysis， G.Z.; Investigation， J.M.; Resources，P.H.; Data Curation， J.M. and B.L.; Writing-Original DraftPreparation， J.M.; Writing-Review amp; Editing， P.H.;Visualization， D.X.; Supervision， P.H. and G.Z.; ProjectAdministration， P.H.; Funding Acquisition， P.H.

Declaration of Competing Interest： The authors declare no conflict of interest.

Supporting Information： available free of charge via the internet at https：//www.whxb.pku.edu.cn.