Ultrafast Charge-Discharge Capable and Long-Life Na3.9 Mn0.95 Zr0.05 V(PO4 )3 /C Cathode Material for Advanced Sodium-Ion Batteries

Tuning TM-O Bond Covalency to Boost Cationic Activity and Reversibility of Na4Fe1.5Mn1.5(PO4)2P2O7

The pursuit of cost-effectiveness stimulates great interest in the Na4Fe1.5Mn1.5(PO4)2P2O7 (NMFPP) cathode. However, its cationic redox activity and reversibility are hardly up to expectation, accompanied by poor conductivity and rapid structural degradation. These issues can be attributed to the high ionization degree of TM-O bonds in the polyanion crystal field, which intensifies electronic localization and degrades the stability of TMO6 octahedra under the Jahn-Teller effect. Herein, a strategy is proposed to enhance the covalency of TM-O bonds. Specifically, Ti4+ with strong electrophilicity is introduced to alter the local electronic structure of TM-O bonds, including band structure and bonding strength. Ultimately, both intrinsic conductivity and lattice stability of Ti modified Na4Mn1.3Fe1.5Ti0.1(PO4)2P2O7 (NMFTPP) are well optimized, upgrading the activity and reversibility of cationic redox. This work reveals the potential mechanism between TM-O bond covalency and the intrinsic conductivity/structural stability of polyanion materials, opening up a feasible path for the high-performance development of sodium ion batteries.

High-entropy doping for high-performance Na3V2(PO4)3@C cathode materials in sodium-ion and hybrid lithium/sodium-ion batteries

The rising demand for cost-effective and safe energy storage systems has ignited strong global interest in advanced cathodes that surpass lithium-ion batteries in practical applications. Herein, we propose a composite material in which five ions, Ti4+, Mn2+, Fe2+, Zr4+, and Mo6+, are doped into the lattice of Na3V2(PO4)3 and a uniform carbon-coating layer is applied to its surface, resulting in the formation of high-entropy doped Na3V2(PO4)3@C materials (HENVP@C). When employed as the cathode material for Sodium-ion batteries (SIBs) and hybrid lithium/sodium-ion batteries (HLSIBs), HENVP@C shows remarkable Li+/Na+ storage capabilities. The results of electrochemical performance tests on half-cells reveal that HENVP@C exhibits a high capacity, along with excellent cycle life and stability. High-entropy doping effectively stabilizes the NASICON structure of HENVP@C, enhances ionic conductivity, and optimizes ion diffusion kinetics, particularly improving its dual-ion pseudocapacitive storage capability. In addition, the surface carbon coating improves the electronic conductivity of the material. The insights presented in this work provide new ideas for designing cathode materials for SIBs and HLSIBs.

Ternary NASICON-Type Na3.25VMn0.25Fe0.75(PO4)3/NC@CNTs Cathode with Reversible Multielectron Reaction and Long Life for Na-Ion Batteries

Na superionic conductor (NASICON)-structure Na4MnV(PO4)3 (NVMP) electrode materials reveal highly attractive application prospects due to ultrahigh energy density originating from two-electron reactions. Nevertheless, NVMP also encounters challenges with its poor electronic conductivity, Mn dissolution, and Jahn-Teller distortion. To address this issue, utilizing N-doped carbon layers and carbon nanotubes (CNTs) for dual encapsulation enhances the material's electronic conductivity, creating an effective electron transport network that promotes the rapid diffusion and storage of Na+. On this basis, partially substituting Mn in NVMP with Fe, a new sodium superionic conductor (NASICON) structured cathode material has been designed to alleviate Jahn-Teller distortion and prolong the cycling life. The synergistic effect of N-doped double nanocarbon encapsulation and multielectron reactions is employed to promote the optimized Na3.25VMn0.25Fe0.75(PO4)3/NC@CNTs (NVMn0.25Fe0.75P/NC@CNTs) electrode material to deliver fast Na+ diffusion kinetics, high reversible capacity (110.2 mAh g-1 at 0.1 C), and long-term cyclic stability (80.1% of the capacity at 10 C over 2000 cycles). Besides, the electrochemical properties of NVMn0.25Fe0.75P/NC@CNTs composites were investigated in detail at high loads and high window voltages to evaluate their potential for practical applications. The reduction/oxidation processes involved in Fe2+/Fe3+, Mn2+/Mn3+, and V3+/V4+ redox couples and a solid-solution and biphasic reaction mechanism upon repeated de- and re-intercalation processes are revealed via ex-situ XRD and XPS characterization. Finally, the assembled NVMn0.25Fe0.75P/NC@CNTs ∥ hard carbon full cell manifests high capacity (101.1 mAh g-1 at 0.1 C) and good cycling stability (98.2% capacity retention at 1 C after 100 cycles). The rational design with multimetal ion substitution regulation has the potential to open up new possibilities for high-performance sodium-ion batteries.

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.

Exceeding Three-Electron Reactions in Polyanionic Cathode To Achieve High-Energy Density for Sodium-Ion Batteries

Activating multielectron reactions of sodium superionic conductor (NASICON)-type cathodes toward higher energy density remains imperative to boost their application feasibility. However, multisodium storage with high stability is difficult to achieve due to the sluggish reaction kinetics, irreversible phase transitions, and negative structural degradation. Herein, a kind of NASICON-type Na2.5V1.5Ti0.5(PO4)3/C (NVTP-0.5) hierarchical microsphere consisting of abundant primary nanoparticles is designed, realizing a reversible 3.2-electron reaction with high stability. The optimized NVTP-0.5 cathode demonstrates an ultrahigh discharge capacity of 192.42 mAh g-1, energy density of up to 497.3 Wh kg-1 at 20 mA g-1, and capacity retention ratio of 94.1% after 1000 cycles at 1 A g-1. Additionally, the NVTP-0.5 cathode delivers excellent tolerance to extreme temperatures while also achieving a high-energy density of 400 Wh kg-1 (based on the cathode mass) in a full-cell configuration. Systematic in situ/ex situ analysis results confirm the multisodium storage processes of NVTP-0.5 involving successive redox reactions (V2+/V3+, Ti3+/Ti4+, and V3+/V4+ redox couples) and reversible structure evolution (solid-solution and biphasic mechanisms), which contribute to the high capacity and excellent cycling stability. This work indicates that the rational regulation of components with different functions can unlock more possibilities for the development of NASICON-type cathodes.

Synergistic Strain Suppressing and Interface Engineering in Na4MnV(PO4)3/C for Wide-Temperature and Long-Calendar-Life Sodium-Ion Storage

A Na4MnV(PO4)3 (NMVP) cathode is regarded as a promising cathode candidate for sodium-ion batteries (SIBs). However, issues such as low electronic conductivity and partial cation dissolution contribute to high polarization and structure distortion. Herein, we engineered the local electron density and reaction kinetic properties of NMVP cathodes with varying oxygen vacancies by introducing varying amounts of Zr doping and carbon coating. The optimized sample exhibited a high-rate capacity of 71.8 mAh g-1 at 30 C (83.1% capacity retention after 1000 cycles) and excellent performance over a wide temperature range (84.1 mAh g-1 at 60 °C and 61.4 mAh g-1 at -30 °C). In situ X-ray diffraction technology confirmed a redox solid solution and a two-phase reaction mechanism, revealing minor changes in cell volume and slight strain variations after Zr doping, effectively suppressing the structural distortion. Theoretical calculations illustrated that Zr doping largely shrinks the band gap of NMVP, enriches local electron density, and slightly alters the local element distribution and bond lengths. Moreover, full-cells have shown high energy density (259.9 Wh kg-1) and outstanding cycling stability (200 cycles). The work provides fresh insights into the synergistic effect of strain suppressing and interface engineering in promoting the development of wide temperature range and long-calendar-life SIBs.

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.

High-Energy-Density Cathode Achieved via the Activation of a Three-Electron Reaction in Sodium Manganese Vanadium Phosphate for Sodium-Ion Batteries

Sodium superionic conductor (NASICON)-type Na3 V2 (PO4 )3 has attracted considerable interest owing to its stable three-dimensional framework and high operating voltage; however, it suffers from a low-energy density due to the poor intrinsic electronic conductivity and limited redox couples. Herein, the partial substitution of Mn3+ for V3+ in Na3 V2 (PO4 )3 is proposed to activate V4+ /V5+ redox couple for boosting energy density of the cathodes (Na3 V2- x Mnx (PO4 )3 ). With the introduction of Mn3+ into Na3 V2 (PO4 )3 , the band gap is significantly reduced by 1.406 eV and thus the electronic conductivity is greatly enhanced. The successive conversions of four stable oxidation states (V2+ /V3+ , V3+ /V4+ , and V4+ /V5+ ) are also successfully achieved in the voltage window of 1.4-4.0 V, corresponding to three electrons involved in the reversible reaction. Consequently, the cathode with x = 0.5 exhibits a high reversible discharge capacity of 170.9 mAh g-1 at 0.5 C with an ultrahigh energy density of 577 Wh kg-1 . Ex-situ x-ray diffraction (XRD) analysis reveals that the sodium-storage mechanism for Mn-doped Na3 V2 (PO4 )3 consists of single-phase and bi-phase reactions. This work deepens the understanding of the activation of reversible three-electron reaction in NASICON-structured polyanionic phosphates and provides a feasible strategy to develop high-energy-density cathodes for sodium-ion batteries.