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

High-entropy doping in NASICON cathodes: Activating the V4+/V5+ redox couple and inducing a reversible single solid-solution phase reaction for advanced sodium ion batteries

Na3V2(PO4)3 (NVP) with typical NASICON structure is highly regarded as one of the most appealing cathodes for sodium-ion batteries (SIBs) for their excellent structural stability; however, the poor electronic conductivity and irreversible phase transition at high voltage (∼4 V) for the material result in poor rate capabilities and significant capacity degradation during electrochemical reactions. Herein, the high-entropy doping strategy of introducing six types of metal ions into the V-sites in Na3V2(PO4)3 is used to design a high-performance cathode of Na3V1.73(Mn, Ca, Mg, Al, Zr)0.05Nb0.02(PO4)3 (HE-NVP) for SIBs. After the high-entropy doping, a reversible V4+/V5+ redox pair is activated at high voltage of ∼4 V, which significantly contributes to the enhancement of energy density and operating voltage. Moreover, after high entropy doping, the single solid solution phase transition in high voltage range is induced, effectively improving the material's structural stability and exhibiting very small volume change of 1.1 % during the Na+ extraction/insertion. Based on the unique advantages mentioned above, the high-entropy HE-NVP exhibits a superior discharge capacity of 122.1 mAh g-1 at a current rate of 0.5C and maintains a remarkable capacity retention of 90.1 % after 500 cycles at a rate of 5C. This work provides a useful reference for designing advanced cathodes for SIBs by regulating high entropy doping.

High-Performance Alluaudite-Type Na2.54(Fe0.97Mg0.03)1.73(SO4)3 Microspheres Cathode for Sodium-Ion Batteries with an Ultrahigh Rate Capability and 9000 Cycle Life

Na2+2xFe2-x(SO4)3 (NFSO) is a promising cathode material for sodium-ion batteries (SIBs) due to its low cost and high operating potential (≈3.8 V). However, poor intrinsic electronic conductivity and sluggish kinetics are major drawbacks to its practical application. Herein, magnesium doped Na2+2xFe2-x(SO4)3 microspheres (Na2.54(Fe0.97Mg0.03)1.73(SO4)3) are synthesized via a designed spray-drying process. The optimized cathode material (NFSO@C-Mg0.02) possesses excellent rate performance up to 50C (50.8 mAh g-1 at 50C) and long-cycle stability (capacity retention of 78.3% even after 9000 cycles at 10C). Despite an increase in the mass loading to 10 mg cm-2, the electrode continues to represent a reversible capacity of 72.1 mAh g-1 at 3C. Furthermore, NFSO@C-Mg0.02║HC full cell demonstrates superior cycling stability (80% capacity retention over 8000 cycles at 5C) and high energy density (≈310 Wh kg-1, based on the cathode). In situ X-ray diffraction (XRD) results reveal that the Mg doping strategy successfully mitigates the variation in lattice volume. The density functional theory (DFT) calculations verify that the prominent rate performance is attributed to the enhanced Na+ diffusion kinetics and low ionic-migration energy barrier. This work provides an effective strategy and a fundamental understanding to enhance the electrochemical performance of cathode materials for SIBs.

Na3V2(PO4)3 cathode materials for advanced sodium-ion batteries: Modification strategies and density functional theory calculations

With the rapid development of electric vehicles and smart grids, the demands for energy supply systems such as secondary batteries are increasing exponentially. Despite the world-renowned achievements in portable devices, lithium-ion batteries (LIBs) have struggled to meet the demands due to the constraints of total lithium resources. As the most promising alternative to LIBs, sodium-ion batteries (SIBs) are generating widespread research enthusiasm around the world. Among all components, the cathode material remains the primary obstacle to the practical application of SIBs due to its inability to match the performance of other components. Na3V2(PO4)3 (NVP) stands out as a promising cathode material for SIBs, given its suitable theoretical specific capacity, appropriate operating voltage, robust structural stability, and excellent ionic conductivity. In this article, we first review recent modification strategies for NVP, including conductive substance coating, ion doping (single-, dual- and multi-site doping) and morphology modulation (from zero-dimensional (0D) to three-dimensional (3D)). Subsequently, we summarize five ways in which density functional theory (DFT) calculations can be applied in guiding NVP modification studies. Furthermore, a series of emerging studies combining DFT calculations are introduced. Finally, the remaining challenges and the prospects for optimization of NVP in SIBs are presented.

Constructing Multi-Electron Reactions by Doping Mn2+ to Increase Capacity and Stability in K3.2V2.8Mn0.2(PO4)4/C of Phosphate Cathodes for Potassium-Ion Batteries

Vanadium-based phosphate cathode materials (e.g., K3V2(PO4)3) have attracted widespread concentration in cathode materials in potassium-ion batteries owing to their stable structure but suffer from low capacity and poor conductivity. In this work, an element doping strategy is applied to promote its electrochemical performance so that K3.2V2.8Mn0.2(PO4)4/C is prepared via a simple sol-gel method. The heterovalent Mn2+ is introduced to stimulated multiple electron reactions to improve conductivity and capacity, as well as interlayer spacing. Galvanostatic intermittent titration technique (GITT) and in situ X-ray diffraction results further confirm that Mn-doping in the original electrode can obtain superior electrode process kinetics and structural stability. The prepared K3.2V2.8Mn0.2(PO4)4/C exhibits a high-capacity retention of 80.8% after 1 500 cycles at 2 C and an impressive rate capability, with discharge capacities of 87.6 at 0.2 C and 45.4 mA h g-1 at 5 C, which is superior to the majority of reported vanadium-based phosphate cathode materials. When coupled K3.2V2.8Mn0.2(PO4)4/C cathode with commercial porous carbon (PC) anode as the full cell, a prominent energy density of 175 Wh kg-1 is achieved based on the total active mass. Overall, this study provides an effective strategy for meliorating the cycling stability and capacity of the polyanion cathodes for KIB.

Effect of Temperature on Intermediate Phases of Na3V2(PO4)3 during Cycling by Operando X-ray Diffraction

Na3V2(PO4)3 (NVP) has gained a lot of attention due to its remarkable properties, such as its robust crystal structure, cycle life, rate capabilities, and so on. Nevertheless, NVP undergoes a substantial decrease in its rate capability at low temperatures, which limits its practical applications. In this study, the performance of NVP at low, room, and high temperatures during cycling is thoroughly investigated using synchrotron operando X-ray diffraction. The (de)insertion of two sodium ions from Na3V2(PO4)3 to Na1V2(PO4)3 appeared to occur via two intermediate phases (Na2V2(PO4)3 and Na1.64V2(PO4)3). The Na1.64V2(PO4)3 phase which is observed for the first-time during operando XRD measurements of NVP, exhibited limited stability at high temperatures. The increase in the quantity of these intermediate phases from high to low temperatures, especially at high C-rates, could be anticipated to be one of the contributing factors of poor rate capabilities of NVP at low temperatures. This study encourages the exploration of suitable strategies to enhance the performance of NVP at low temperatures and high C-rates.

Optimization Strategies of Na3V2(PO4)3 Cathode Materials for Sodium-Ion Batteries

Na3V2(PO4)3 (NVP) has garnered great attentions as a prospective cathode material for sodium-ion batteries (SIBs) by virtue of its decent theoretical capacity, superior ion conductivity and high structural stability. However, the inherently poor electronic conductivity and sluggish sodium-ion diffusion kinetics of NVP material give rise to inferior rate performance and unsatisfactory energy density, which strictly confine its further application in SIBs. Thus, it is of significance to boost the sodium storage performance of NVP cathode material. Up to now, many methods have been developed to optimize the electrochemical performance of NVP cathode material. In this review, the latest advances in optimization strategies for improving the electrochemical performance of NVP cathode material are well summarized and discussed, including carbon coating or modification, foreign-ion doping or substitution and nanostructure and morphology design. The foreign-ion doping or substitution is highlighted, involving Na, V, and PO43- sites, which include single-site doping, multiple-site doping, single-ion doping, multiple-ion doping and so on. Furthermore, the challenges and prospects of high-performance NVP cathode material are also put forward. It is believed that this review can provide a useful reference for designing and developing high-performance NVP cathode material toward the large-scale application in SIBs.

Doping Engineering in Electrode Material for Boosting the Performance of Sodium Ion Batteries

In recent years, sodium ion batteries (SIBs) emerged as promising alternative candidates for lithium ion batteries (LIBs) due to the high abundance and low cost of sodium resources. However, their commercialization has been hindered by inherent limitations, such as low energy density and poor cycling stability. To address these issues, doping methodology is one of the most promising approaches to boosting the structural and electrochemical properties of SIB electrodes. This review provides a comprehensive overview of recent advancements in doping strategies, focusing on the improvement of the performance of SIBs. Various dopants including s- and p-block elements, transition metals, oxides, carbonaceous materials, and many more dopants are discussed in terms of their effects on enhancing the electrochemical properties of SIBs. Furthermore, the mechanisms responsible for the improvement in the performance of doped SIBs materials are also discussed. It also highlights the importance of doping sites in the crystal lattice, which also play a crucial role in doping in optimizing electrode structure, enhancing ion diffusion kinetics, and stabilizing electrode/electrolyte interfaces. The review ends by looking at the recent studies in simultaneous multiple heteroatom doping, offering valuable perspectives for a high performance SIB. This study provides valuable insight into the researchers and battery industries striving for advancements in energy storage technologies.

Unraveling the modified mechanism of ruthenium substitution on Na3V2(PO4)3 with superior rate capability and ultralong cyclic performance

Na3V2(PO4)3(NVP) is an ideal cathode material for sodium ion battery due to its stable three-dimensional frame structure and high operating voltage. However, the low intrinsic conductivity and serious structural collapse limit its further application. In this work, a simultaneous optimized Na3V1.96Ru0.04(PO4)3/C@CNTs cathode material is synthesized by a simple sol-gel method. Specifically, the ionic radius of Ru3+ is slightly larger than that of V3+ (0.68 Å vs 0.64 Å), which not only ensures the feasibility of Ru3+ replacing V3+ site, but also appropriately expands the migration channel of sodium ions in NVP and stabilizes the structure, effectively improving the diffusion efficiency of sodium ions. Moreover, CNTs construct a three-dimensional conductive network between the grains, reducing the impedance at the interface and effectively improving the electronic conductivity. Ex-situ XRD analysis at different SOC were performed to determine the change in the crystal structure of Ru3+doped Na3V2(PO4)3, and the refinement results simultaneously demonstrate the relatively low volume shrinkage value of less than 3 % during the de-intercalation process, further verifying the stabilized crystal construction after Ru3+ substitution. Furthermore, the ex-situ XRD/SEM/CV/EIS after cycling indicate the significantly improved kinetic characteristics and enhanced structural stability. Notably, the modified Na3V1.96Ru0.04(PO4)3/C@CNTs reveals superior rate capability and ultralong cyclic performance. It submits high capacities of 82.3/80.9 mAh g-1 at 80/120C and maintains 71.3/59.6 mAh g-1 after 14800/6250 cycles, indicating excellent retention ratios of 86.6 % and 73.6 %, respectively. This work provides a multi-modification strategy for the realization of high-performance cathode materials, which can be widely applied in the optimization of various materials.

A Comprehensive Review on Strategies for Enhancing the Performance of Polyanionic-Based Sodium-Ion Battery Cathodes

The significant consumption of fossil fuels and the increasing pollution have spurred the development of energy-storage devices like batteries. Due to their high cost and limited resources, widely used lithium-ion batteries have become unsuitable for large-scale energy production. Sodium is considered to be one of the most promising substitutes for lithium due to its wide availability and similar physiochemical properties. Designing a suitable cathode material for sodium-ion batteries is essential, as the overall electrochemical performance and the cost of battery depend on the cathode material. Among different types of cathode materials, polyanionic material has emerged as a great option due to its higher redox potential, stable crystal structure, and open three-dimensional framework. However, the poor electronic and ionic conductivity limits their applicability. This review briefly discusses the strategies to deal with the challenges of transition-metal oxides and Prussian blue analogue, recent developments in polyanionic compounds, and strategies to improve electrochemical performance of polyanionic material by nanostructuring, surface coating, morphology control, and heteroatom doping, which is expected to accelerate the future design of sodium-ion battery cathodes.

Low-Dimensional Vanadium-Based High-Voltage Cathode Materials for Promising Rechargeable Alkali-Ion Batteries

Owing to their rich structural chemistry and unique electrochemical properties, vanadium-based materials, especially the low-dimensional ones, are showing promising applications in energy storage and conversion. In this invited review, low-dimensional vanadium-based materials (including 0D, 1D, and 2D nanostructures of vanadium-containing oxides, polyanions, and mixed-polyanions) and their emerging applications in advanced alkali-metal-ion batteries (e.g., Li-ion, Na-ion, and K-ion batteries) are systematically summarized. Future development trends, challenges, solutions, and perspectives are discussed and proposed. Mechanisms and new insights are also given for the development of advanced vanadium-based materials in high-performance energy storage and conversion.

A high-energy-density NASICON-type Na3V1.25Ga0.75(PO4)3 cathode with reversible V4+/V5+ redox couple for sodium ion batteries

The stable three-dimensional framework and high operating voltage of sodium superionic conductor (NASICON)-type Na3V2(PO4)3 has the potential to work with long cycle life and high-rate performance; however, it suffers from the poor intrinsic electronic conductivity and low energy density. Herein, Ga3+ is introduced into Na3V2(PO4)3 to activate the V4+/V5+ redox couple at a high potential of 4.0 V for enhancing energy density of the materials (Na3V2-xGax(PO4)3). After the partial substitution of Ga3+ for V3+, three redox couples (V2+/V3+, V3+/V4+ and V4+/V5+) of V are reversibly converted in the voltage range of 1.4-4.2 V, suggesting multi-electrons (>2e-) involved in the reversible reaction, and simultaneously the electronic conductivity of the materials is effectively enhanced. As a result, the cathode with x = 0.75 exhibits excellent electrochemical properties: in the voltage range of 2.2-4.2 V, delivering an initial capacity of 105 mAh/g at 1C with a capacity retention rate of 92.3% after 400 cycles, and providing a stable reversible capacity of 88.3 mAh/g at 40C; and in the voltage range of 1.4-4.2 V, presenting the reversible capacity 152.3 mAh/g at 1C (497.6 Wh kg-1), and cycling stably for 1000 cycles at 20C with a capacity decay of 0.02375% per cycle. It is found that the Na3V2-xGax(PO4)3 cathodes possess the sodium storage mechanism of single-phase and bi-phase reactions. This investigation presents a useful strategy to enhance the energy density and cycling life of NASICON-structured polyanionic phosphates by activating high-potential V4+/V5+ redox couple.

Boosted Redox Kinetics Enabling Na3V2(PO4)3 with Excellent Performance at Low Temperature through Cation Substitution and Multiwalled Carbon Nanotube Cross-Linking

The open NASICON framework and high reversible capacity enable Na3V2(PO4)3 (NVP) to be a highly promising cathode candidate for sodium-ion batteries (SIBs). Nevertheless, the unsatisfied cyclic stability and degraded rate capability at low temperatures due to sluggish ionic migration and poor conductivity become the main challenges. Herein, excellent sodium storage performance for the NVP cathode can be received by partial potassium (K) substitution and multiwalled carbon nanotube (MWCNT) cross-linking to modify the ionic diffusion and electronic conductivity. Consequently, the as-fabricated Na3-xKxV2(PO4)3@C/MWCNT can maintain a capacity retention of 79.4% after 2000 cycles at 20 C. Moreover, the electrochemical tests at -20 °C manifest that the designed electrode can deliver 89.7, 73.5, and 64.8% charge of states, respectively, at 1, 2, and 3 C, accompanied with a capacity retention of 84.3% after 500 cycles at 20 C. Generally, the improved electronic conductivity and modified ionic diffusion kinetics resulting from K doping and MWCNT interconnecting endows the resultant Na3-xKxV2(PO4)3@C/MWCNT with modified electrochemical polarization and improved redox reversibility, contributing to superior performance at low temperatures. Generally, this study highlights the potential of alien substitution and carbon hybridization to improve the NASICON-type cathodes toward high-performance SIBs, especially at low temperatures.