Triggering the Reversible Reaction of V3+/V4+/V5+ in Na3V2(PO4)3 by Cr3+ Substitution

Cu2+ substitution regulating Na3V2(PO4)3 with solid SEI membrane for superior electrochemical performance

Na3V2(PO4)3 (NVP) suffers from poor ionic and electronic conductivity. Herein, a dual-optimized design with Cu2+ doping and wrapping in tubular carbon nanotubes (CNTs) is proposed for the first time. This strategy not only modifies the internal electronic structure but also regulates the morphological features of NVP material. Notably, Cu2+ occupying V3+ sites introduces favorable p-type doping effects. Consequently, newly generated holes can act as charge carriers to improve electronic conductivity. Meanwhile, to conserve the charge balance of the whole system, a series of distinctive Na3+xV2-xCux(PO4)3 cathode materials are designed. The Na-rich scheme maintains charge integrity, as well as supplying more active Na+ to take part in the reversible de-intercalation process. Due to the larger ionic radius of Cu2+, Cu2+ doping plays a great role as a pillar to support the crystal skeleton and then expand the Na+ migration channels, thus significantly elevating the ionic transport rate. Furthermore, moderate CNTs are wrapped around the active grains, functioning together with coated carbon layers to construct a highly conductive framework, enhancing electronic transfer. Meanwhile, the tubular CNTs and porous morphology effectively increase the contact areas between active particles and electrolyte, providing more active sites. Furthermore, in situ EIS measurement demonstrates that a stable SEI membrane covers the cycled Na3.07V1.93Cu0.07(PO4)3@CNTs grains to maintain electrode stability and prevent the occurrence of side-effects. Comprehensively, the Na3.07V1.93Cu0.07(PO4)3@CNTs sample releases 124.2 mA h g-1 at 0.1 C. It releases 101.9 and 98.6 mA h g-1 at 10 and 50 C, suggesting superior rate capability.

The Role of Fluorine in Polyanionic Cathode Materials for Sodium-Ion Batteries

With the growing global demand for renewable energy and the increasing scarcity of lithium resources, sodium-ion batteries have received extensive attention and research as a potential alternative. Among many cathode materials for sodium-ion batteries, polyanion materials are favored for their high operating voltage, stable cycling performance, and good safety. However, the low electronic conductivity and low energy density of polyanionic materials limit their potential for large-scale commercial applications. To overcome this challenge, various strategies have been explored to improve their electrochemical performance. Among them, fluorine doping has been proven to be an effective means. In this study, we have systematically explored the effects of trace fluorine doping and mass fluorine substitution on the structure, dynamics, and electrochemistry of polyanionic cathode materials for sodium-ion batteries and deeply analyzed their reaction mechanisms. The analysis results show that trace fluorine doping can effectively improve the electronic conductivity of the material, thus enhancing its electrochemical performance. A large amount of fluorine substitution can effectively improve the voltage plateau of the material, thus enhancing its energy density. However, the environmental and safety challenges associated with the introduction of fluorine should also be addressed. Overall, the introduction of fluorine in polyanionic cathode materials can further optimize the electronic structure and electrochemical performance, thus realizing the wide application of high-performance sodium-ion batteries and making them a competitive battery technology.

Activating Reversible V4+/V5+ Redox Couple in NASICON-Type Phosphate Cathodes by High Entropy Substitution for Sodium-Ion Batteries

Vanadium-based Na superionic conductor (NASICON) type materials (NaxVM(PO4)3, M = transition metals) have attracted extensive attention when used as sodium-ion batteries (SIBs) cathodes due to their stable structures and large Na+ diffusion channels. However, the materials have poor electrical conductivity and mediocre energy density, which hinder their practical applications. Activating the V4+/V5+ redox couple (V4+/V5+≈4.1 V vs Na+/Na) is an effective way to elevate the energy density of SIBs, whereas the irreversible phase transition of V4+/V5+ and severe structural distortion will inevitably result in fast capacity fading and unsatisfactory rate capability. Herein, a high entropy regulation strategy is proposed to optimize the detailed crystal structure and improve the reversibility of crystalline phase transformation and electrical conductivity of the material. With the activated reversible V4+/V5+ redox couple, stable structure, and fast electrochemical kinetics, the high entropy material Na3.2V1.5Fe0.1Al0.1Cr0.1Mn0.1Cu0.1(PO4)3 (NVMP-HE) exhibits an outstanding electrochemical performance with highly reversible specific capacity of 120.1 mAh g-1 at 0.1 C and excellent cycling stability (92.4% retention after 1000 cycles at 20 C). Besides, the in situ X-ray diffraction (XRD) measurement reveals that a smooth three-phase transition reaction is involved in this high-entropy cathode and the existence of mesophase facilitates a fast phase transition.

A High-Voltage Cathode Material with Ultralong Cycle Performance for Sodium-Ion Batteries

Vanadium-based polyanionic materials are promising electrode materials for sodium-ion batteries (SIBs) due to their outstanding advantages such as high voltage, acceptable specific capacity, excellent structural reversibility, good thermal stability, etc. Polyanionic compounds, moreover, can exhibit excellent multiplicity performance as well as good cycling stability after well-designed carbon covering and bulk-phase doping and thus have attracted the attention of multiple researchers in recent years. In this paper, after the modification of carbon capping and bulk-phase nitrogen doping, compared to pristine Na3V2(PO4)3, the well optimized Na3V(PO3)3N/C possesses improved electromagnetic induction strength and structural stability, therefore exhibits exceptional cycling capability of 96.11% after 500 cycles at 2 C (1 C = 80 mA g-1) with an elevated voltage platform of 4 V (vs Na+/Na). Meanwhile, the designed Na3V(PO3)3N/C possesses an exceptionally low volume change of ≈0.12% during cycling, demonstrating its quasi-zero strain property, ensuring an impressive capacity retention of 70.26% after 10,000 cycles at 2 C. This work provides a facial and cost-effective synthesis method to obtain stable vanadium-based phosphate materials and highlights the enhanced electrochemical properties through the strategy of carbon rapping and bulk-phase nitrogen doping.

Improved Electrochemical Performance of NASICON Type Na3V2-xCox(PO4)3/C (x = 0-0.15) Cathode for High Rate and Stable Sodium-Ion Batteries

In recent years, the Na-ion SuperIonic CONductor (NASICON) based polyanionics are considered pertinent cathode materials in sodium-ion batteries due to their 3D open framework, which can accommodate a wide range of Na content and can offer high ionic conductivity with great structural stability. However, owing to the inferior electronic conductivity, these materials suffer from unappealing rate capability and cyclic stability for practical applications. Therefore, in this work we investigate the effect of Co substitution at the V site on the electrochemical performance and diffusion kinetics of Na3V2-xCox(PO4)3/C (x = 0-0.15) cathodes. All the samples are characterized through Rietveld refinement of the X-ray diffraction patterns, Raman spectroscopy, transmission electron microscopy, etc. We demonstrate improved electrochemical performance for the x = 0.05 electrode with a reversible capacity of 105 mAh g-1 at 0.1 C. Interestingly, the specific capacity of 80 mAh g-1 is achieved at 10 C with retention of about 92% after 500 cycles and 79.5% after 1500 cycles and having nearly 100% Coulombic efficiency. The extracted diffusion coefficient values through the galvanostatic intermittent titration technique and cyclic voltammetry are found to be in the range of 10-9 to 10-11 cm2 s-1. The post-mortem studies show excellent structural and morphological stability after testing for 500 cycles at 10 C. Our study reveals the role of optimal dopant of Co3+ ions at the V site in improving the cyclic stability at a high current rate.

Facile Al2O3 coating suppress dissolution of Mn2+ in Mn-substituted Na3V2(PO4)3 with outstanding electrochemical performance for full sodium ion batteries

Na3V2(PO4)3 (NVP) encounters significant obstacles, including limited intrinsic electronic and ionic conductivities, which hinder its potential for commercial feasibility. Currently, the substitution of V3+ with Mn2+ is proposed to introduce favorable carriers, enhancing the electronic conductivity of the NVP system while providing structural support and stabilizing the NASICON framework. This substitution also widens the Na+ migration pathways, accelerating ion transport. Furthermore, to bolster stability, Al2O3 coating is applied to suppress the dissolution of transition metal Mn in the electrolyte. Notably, the Al2O3 coating serves a triple role in reducing HClO4 concentration in the electrolyte, inhibiting Mn dissolution, and functioning as the ion-conducting phase. Likewise, carbon nanotubes (CNTs) effectively hinder the agglomeration of active particles during high-temperature sintering, thereby optimizing the conductivity of NVP system. In addition, the excellent structural stability is investigated by in situ XRD measurement, effectively improving the volume collapse during Na+ de-embedding. Moreover, the Na3V5.92/3Mn0.04(PO4)3/C@CNTs@1wt.%Al2O3 (NVMP@CNTs@1wt.%Al2O3) possesses unique porous structure, promoting rapid Na+ transport and increasing the interface area between the electrolyte and the cathode material. Comprehensively, the NVMP@CNTs@1wt.%Al2O3 sample demonstrates a remarkable reversible specific capacity of 122.6 mAh/g at 0.1 C. Moreover, it maintains a capacity of 115.9 mAh/g at 1 C with a capacity retention of 90.2 mAh/g after 1000 cycles. Even at 30 C, it achieves a capacity of 87.9 mAh/g, with a capacity retention rate of 84.87 % after 6000 cycles. Moreover, the NVMP@CNTs@1wt.%Al2O3//CHC full cell can deliver a high reversible capacity of 205.5 mAh/g at 0.1 C, further indicating the superior application potential in commercial utilization.

Boosting sodium-ion battery performance by anion doping in NASICON Na4MnCr(PO4)3 cathode

Na superionic conductor (NASICON)-structured Na4MnCr(PO4)3 (NMCP) possessing unique three-electron transfer process renders admirable energy density for sodium ion batteries (SIBs). However, the current issues like its sluggish Na+ diffusion kinetics, deficient intrinsic conductivity, and unsatisfactory structural stability, hinder its practical application. Herein, a selective replacement of O elements in PO4 group by Cl anions in the NMCP system was developed to significantly enhance its electrochemical performance. The results affirm that the enhanced performance of Cl doped samples can be attributed to the enlargement of cell size, the creation of Na vacancies and the weakness of Na2O bond after Cl doping. The as-prepared Na3.85□0.15MnCr(PO3.95Cl0.05)3/C (NMCPC - 15/C) cathode delivers a high capacity (128.0 mAh/g at 50 mA g-1) and excellent rate performance (73.0 mAh/g at 1000 mA g-1) in contrast to NMCP/C that merely provides 105.2 mAh/g at 50 mA g-1 and reduces to 47.4 mAh/g at 1000 mA g-1. Meanwhile, NMCPC - 15/C shows a capacity retention of 60.7 % at 1000 mA g-1 after 500 cycles, while only 37.1 % for NMCP/C in the same test conditions. Moreover, the satisfactory performance and energy density of NMCPC - 15/C||hard carbon (HC) full cell confirm the potential practicality of NMCPC - 15. Therefore, chloride ions doping into NMCP has practical application prospects in the preparation of high-performance cathode materials and our work also offers new inspiration to apply anion doping strategies in promoting the performance of the other NASICON-structured cathodes for SIBs.

Machine-Learning-Driven High-Throughput Screening for High-Energy Density and Stable NASICON Cathodes

The Na super ionic conductor (NASICON), which has outstanding structural stability and a high operating voltage, is an appealing material for overcoming the limits of low specific energy and larger volume distortion of sodium-ion batteries. In this study, to discover ideal NASICON cathode materials, a screening platform based on density functional theory (DFT) calculations and machine learning (ML) is developed. A training database was generated utilizing the previous 124 545 electrode databases, and a test set of 3126 potential NASICON structures [NaxMyM'1-y(PO4)3] with 27 dopants at the metal site and 6 dopants at the polyanion central site was constructed. The developed ML surrogate model identifies 796 materials that satisfy the following criteria: formation energy of <0.0 eV/atom, energy above hull of ≤0.025 eV/atom, volume change of ≤4%, and theoretical capacity of ≥50 mAh/g. The thermodynamically stable configurations of doped NASICON structures were then selected using machine learning interatomic potential (MLIP), enabling rapid consideration of various dopant site configurations. DFT calculations are followed on 796 screened materials to obtain energy density, average voltage, and volume change. Finally, 50 candidates with an average voltage of ≥3.5 V are identified. The suggested platform accelerates the exploration for optimal NASICON materials by narrowing the focus on materials with desired properties, saving considerable resources.

Synergetic impact of high-entropy microdoping modification in Na3V2(PO4)3

High entropy polyanionic Na3V1.9(Ca,Mg,Cr,Ti,Mn)0.1(PO4)3 was synthesized, which activated a reversible redox reaction of V4+/V5+ and reached a remarkable capacity of 116 mA h g-1 at 1C and maintained 90% capacity retention after 1000 cycles at 10C. Structure evolution and sodium storage mechanisms were revealed by an in situ XRD method, which disclosed the high-entropy effect of material design. Together with the full battery tests, these findings promote the wide applications of high-entropy polyanionic cathodes in SIBs.

NASICON-Type Na3V1.5Cr0.4Fe0.1(PO4)3: High-Voltage and High-Rate Cathode Materials for Sodium-Ion Batteries

Cationic alteration related to a sodium super ion conductor (NASICON)-structured Na3V2(PO4)3 (NVP) is an effective strategy for formulating high-energy and stable cathodes for sodium-ion batteries (SIBs). In this study, we altered the structure of NVP with dual cations, namely, Cr and Fe, to develop Na3V1.5Cr0.4Fe0.1(PO4)3 cathodes for SIBs with high-rate capability (∼71 mAh g-1 at 100 C) and an extreme cycle life output (∼75 mAh g-1 with 95% capacity retention for 10,000 cycles). These excellent electrochemical properties can be ascribed to the synergistic effects of Cr and Fe in the NVP structure, as verified experimentally and theoretically. Therefore, the proposed cosubstitution method can enhance the performance of SIBs by improving their structural stability, electronic conductivity, and phase-change behavior.

Simultaneous modification of dual-substitution with CeO2 coating boosting high performance sodium ion batteries

Na3V2(PO4)3 (NVP) is highly valued based on the stable construction among the polyanionic compounds. Nevertheless, the drawback of low intrinsic conductivity has been impeded its further application. In this paper, the internal channels of the crystal structure are extended by the introduction of larger radius Ce3+, which increases the transport rate of Na+. The introduction of Mo6+ replacing the V site leads to a beneficial n-type doping effect and facilitates the transportation of electrons. Besides, CeO2 cladding is introduced to further enhance the electronic conductivity of NVP system. Initially, CeO2 serves as an n-type semiconductor and functions as a conductive additive to significantly enhance the electronic conductivity of the electrode, thereby improving the electrochemical characteristics. Moreover, CeO2 functions as an oxygen buffer, aiding in the maintenance of active metal dispersion during operation and enabling efficient electron transfer between CeO2 and [VO6] octahedra in NVP, thus fostering outstanding electrical connectivity between the oxides. CeO2 cladding can be effectively integrated with the carbon layer to stabilize the NVP system. Comprehensively, the modified Na3V1.79Ce0.07Mo0.07(PO4)3/C@8wt.%CeO2 (CeMo0.07@8wt.%CeO2) composite exhibits excellent rate and cycling properties. It delivers a capacity of 113.4 mAh/g at 1C with a capacity retention rate of 80.3 % after 150 cycles. Even at 10C and 40C, it also submits high capacities of 84.7 mAh/g and 76 mAh/g, respectively. Furthermore, the CHC//CeMo0.07@8wt.%CeO2 asymmetric full cell possesses excellent sodium storage property, indicating its prospective application potentials.

Enhanced Electrochemical Performance of Ca-Doped Na3V2(PO4)2F3/C Cathode Materials for Sodium-Ion Batteries

Na3V2(PO4)2F3 (NVPF) with a NASICON structure has garnered attention as a cathode material owing to its stable 3D structure, rapid ion diffusion channels, high operating voltage, and impressive cycling stability. Nevertheless, the low intrinsic electronic conductivity of the material leading to a poor rate capability presents a significant challenge for practical application. Herein, we develop a series of Ca-doped NVPF/C cathode materials with various Ca2+ doping levels using a simple sol-gel and carbon thermal reduction approach. X-ray diffraction analysis confirmed that the inclusion of Ca2+ does not alter the crystal structure of the parent material but instead expands the lattice spacing. Density functional theory calculations depict that substituting Ca2+ ions at the V3+ site reduces the band gap, leading to increased electronic conductivity. This substitution also enhanced the structural stability, preventing lattice distortion during the charge/discharge cycles. Furthermore, the presence of the Ca2+ ion introduces two localized states within the band gap, resulting in enhanced electrochemical performance compared to that of Mg-doped NVPF/C. The optimal NVPF-Ca-0.05/C cathode exhibits superior specific capacities of 124 and 86 mAh g-1 at 0.1 and 10 C, respectively. Additionally, the NVPF-Ca-0.05/C demonstrates satisfactory capacity retention of 70% after 1000 charge/discharge cycles at 10 C. These remarkable results can be attributed to the optimized particle size, excellent structural stability, and enhanced ionic and electronic conductivity induced by the Ca doping. Our findings provide valuable insight into the development of cathode material with desirable electrochemical properties.

Reversible Multielectron Redox Chemistry in a NASICON-Type Cathode toward High-Energy-Density and Long-Life Sodium-Ion Full Batteries

Na-superionic-conductor (NASICON)-type cathodes (e.g., Na3 V2 (PO4 )3 ) have attracted extensive attention due to their open and robust framework, fast Na+ mobility, and superior thermal stability. To commercialize sodium-ion batteries (SIBs), higher energy density and lower cost requirements are urgently needed for NASICON-type cathodes. Herein, Na3.5 V1.5 Fe0.5 (PO4 )3 (NVFP) is designed by an Fe-substitution strategy, which not only reduces the exorbitant cost of vanadium, but also realizes high-voltage multielectron reactions. The NVFP cathode can deliver extraordinary capacity (148.2 mAh g-1 ), and decent cycling durability up to 84% after 10 000 cycles at 100 C. In situ X-ray diffraction and ex situ X-ray photoelectron spectroscopy characterizations reveal reversible structural evolution and redox processes (Fe2+ /Fe3+ , V3+ /V4+ , and V4+ /V5+ ) during electrochemical reactions. The low ionic-migration energy barrier and ideal Na+ -diffusion kinetics are elucidated by density functional theory calculations. Combined with electron paramagnetic resonance spectroscopy, Fe with unpaired electrons in the 3d orbital is inseparable from the higher-valence redox activation. More competitively, coupling with a hard carbon (HC) anode, HC//NVFP full cells demonstrate high-rate capability and long-duration cycling lifespan (3000 stable cycles at 50 C), along with material-level energy density up to 304 Wh kg-1 . The present work can provide new perspectives to accelerate the commercialization of SIBs.

Heterojunction of Y3+-substituted Na3V2(PO4)3-NaYO2 accelerating kinetics with superior performance for full sodium-ion batteries

The inherent poor ionic and electronic conductivity and relatively low capacity seriously limit the development of Na3V2(PO4)3 (NVP). Currently, Y3+ substitution is proposed for the first time and corresponding Y-doped NVP samples are successfully synthesized by traditional solid phase method. By replacing V3+ (0.64 Å) of Y3+ (0.9 Å) with large ionic radius, the transport channel of Na+ is expanded, accelerating the migration rate of Na+. Meanwhile, the strong Y-O chemical bond reinforces the crystal structure of NVP and improves its stability. It is worth noting that Y3+ reacts with slightly excess Na+ during the sintering process to form a new conductive phase NaYO2 coated on the surface of NVP particles. NaYO2 possesses excellent semiconductor properties and can form a double conductive construction together with the amorphous carbon layer to improve the kinetics of NVP system. Moreover, the unique heterojunction could be generated between the boundaries of Na3V2(PO4)3-NaYO2, further elevating the ionic conductivity. According to the investigations of ex-situ XRD and in-situ EIS, NaYO2 can provide active Na+, participating in the electrochemical process to supply extra reversible capacity. Comprehensively, the optimized NVP-Y0.07/C releases a high capacity of 125.2 mAh/g at 0.1 C, far exceeding the theoretical value of NVP (117.6 mAh/g). Even at 60 C, a high specific capacity of 94.2 mAh g-1 can be achieved. When cycled at 250 C, it maintains 85.5 % after 2000 cycles. Besides, the assembled NVP-Y0.07/C//hard carbon full cell can deliver a reversible value of 118.2 mAh/g, suggesting the superior practical application potentials.

Nanodesigns for Na3V2(PO4)3-based cathode in sodium-ion batteries: a topical review

With the rapid development of sodium-ion batteries (SIBs), it is urgent to exploit the cathode materials with good rate capability, attractive high energy density and considerable long cycle performance. Na₃V₂(PO₄)₃(NVP), as a NASICON-type electrode material, is one of the cathode materials with great potential for application because of its good thermal stability and stable. However, NVP has the inherent problem of low electronic conductivity, and various strategies are proposed to improve it, moreover, nanotechnology or nanostructure are involved in these strategies, the construction of nanostructured active particles and nanocomposites with conductive carbon networks have been shown to be effective in improving the electrical conductivity of NVP. Herein, we review the research progress of NVP performance improvement strategies from the perspective of nanostructures and classifies the prepared nanomaterials according to their different nano-dimension. In addition, NVP nanocomposites are reviewed in terms of both preparation methods and promotion effects, and examples of NVP nanocomposites at different nano-dimension are given. Finally, some personal views are presented to provide reasonable guidance for the research and design of high-performance polyanionic cathode materials of SIBs.

Optimizing Vanadium Redox Reaction in Na3V2(PO4)3 Cathodes for Sodium-Ion Batteries by the Synergistic Effect of Additional Electrons from Heteroatoms

Na3V2(PO4)3 (NVP) is one of the most potential cathode materials for sodium-ion batteries (SIBs), but its actual electrochemical performance is limited by the defects of large electron and ion transfer resistance. Multicomponent design is considered an effective method to optimize the conductivity of NVP electrodes. Therefore, Cr and Si are added in NVP to form a multielement component of Na3V1.9Cr0.1(PO4)2.9(SiO4)0.1 (NVP-CS). It is confirmed that 3d electrons of Cr are beneficial for improving the conductivity and increasing the average potential by activating V4+/V5+. Theoretical calculations show that the introduction of Si changes the electronic structure of V and O, thus promoting the electrochemical reaction of V3+/V4+ to exert higher capacity. Due to the coordination of the two elements, a lower migration barrier is obtained in NVP-CS. Specifically, NVP-CS retains the advantages of single-doped electrodes very well (capacity retention of 90% after 300 cycles at 1 C and a high capacity of 94.1 mA h g-1 at 5 C, compared to NVP with only 82.6% capacity retention at 1 C and 59.4 mA h g-1 at 5 C). The excellent electrochemical performance results show that NVP can be successfully optimized by the introduction of Cr and Si. This work can provide some inspiration for multicomponent material research of cathode materials.

Vanadium dissolution at 30 °C and its suppression at a low temperature of -20 °C for a Na3V1.5Cr0.5(PO4)3 cathode

Na₃V_1.5Cr_0.5(PO₄)₃ cycled at 30 °C shows fast capacity decay, due to the migration of V ions into the electrolyte and the loss of V³⁺/V⁴⁺ redox. A low temperature of -20 °C attenuates V-ion dissolution, retains V³⁺/V⁴⁺ redox and improves the electrochemical performance, favorable for use in cold climates and high-altitude drones.

O3-NaFe(1/3-x)Ni1/3Mn1/3AlxO2 Cathodes with Improved Air Stability for Na-Ion Batteries

Na-ion batteries (NIBs) have been considered as potential candidates for large-scale energy storage, where O3-type Na-based layered oxide cathodes have attracted great attention due to their high capacity and low cost. However, O3-Na_xTMO₂ materials still suffer from insufficient air stability, which could lead to deteriorative electrochemical properties and thus hinder their practical application. In this work, a series of Al-doped O3-NaFe_(1/3-x)Ni_1/3Mn_1/3Al_xO₂ cathodes prepared by a co-precipitation method were investigated to enhance their electrochemical performance and air stability through stabilizing their structural and interface chemical properties. The Al-doped O3-NaFe_(1/3-0.01)Ni_1/3Mn_1/3Al_0.01O₂ (NFNMA_0.01) cathode delivers a comparable capacity of 138 mAh g^-1 and keeps a capacity retention of 85.88% after 50 cycles at 0.2 C, while the undoped O3-NaFe_1/3Ni_1/3Mn_1/3O₂ (NFNM) can only keep a capacity retention of 71.02%, although with an initial capacity of 141 mAh g^-1 at 0.2 C. For the air stability, the capacity decay rates are 58.87 and 5.07% for the undoped NFNM and Al-doped NFNMA_0.01 after the air exposure for 30 days, respectively. The greatly decaying electrochemical performance could be due to the formation of carbonates during air exposure, which can be efficiently suppressed by Al doping. The doped Al³⁺ has been confirmed to be inserted into the NFNM crystal lattice, inducing the reduced values of lattice parameters a and c due to the smaller ionic radius of Al³⁺ (53.5 pm) vs Fe³⁺ (55.0 pm). This study demonstrates that Al doping plays an important role in the air stability and cycling stability for layered cathode materials, which offers an efficient strategy to optimize the material design for their practical application in NIBs.

Importance of Crystallographic Sites on Sodium-Ion Extraction from NASICON-Structured Cathodes for Sodium-Ion Batteries

The V⁴⁺/V³⁺ (3.4 V) redox couple has been well-documented in cathode material Na₃V₂(PO₄)₃ for sodium-ion batteries. Recently, partial cation substitution at the vanadium site of Na₃V₂(PO₄)₃ has been actively explored to access the V⁵⁺/V⁴⁺ redox couple to achieve high energy density. However, the V⁵⁺/V⁴⁺ redox couple in partially substituted Na₃V₂(PO₄)₃ has a voltage far below its theoretical voltage in Na₃V₂(PO₄)₃, and the access of the V⁵⁺/V⁴⁺ redox reaction is very limited. In this work, we compare the extraction/insertion behavior of sodium ions from/into two isostructural compounds of Na₃VGa(PO₄)₃ and Na₃VAl(PO₄)₃, found that, by DFT calculations, the lower potential of the V⁵⁺/V⁴⁺ redox couple in Na₃VM(PO₄)₃ (M = Ga or Al) than that in Na₃V₂(PO₄)₃ is because of the extraction/insertion of sodium ions through the V⁵⁺/V⁴⁺ redox reaction at different crystallographic sites, that is, sodium ions extracting from the Na(2) site in Na₃VM(PO₄)₃ while from the Na(1) site in Na₃V₂(PO₄)₃, and further evidenced that the full access of the V⁵⁺/V⁴⁺ redox reaction is restrained by the excessive diffusion activation energy in Na₃VM(PO₄)₃.