A Na3V2(PO4)3 cathode material for use in hybrid lithium ion batteries

Crystal Chemistry and Ionic Conductivity of the NASICON-Related Phases in the Li3–xNaxV2(PO4)3 System

In the present work, we studied crystal phases in the Li_3-xNa_xV₂(PO₄)₃ system over a wide range of x prepared by four synthesis methods: mechanochemically assisted solid-state synthesis, 'soft chemistry' sol-gel approach, chemical (CIE) and electrochemical (EIE) ion exchange starting from Li₃V₂(PO₄)₃ (anti-NASICON, P2₁/c S.G.), and Na₃V₂(PO₄)₃ (NASICON, C2/c S.G.). EIE was studied by operando and ex situ XRD in Li₃V₂(PO₄)₃ vs Na and Na₃V₂(PO₄)₃ vs Li electrochemical cells. It was shown that both mechanochemically assisted solid-state and sol-gel synthesis methods do not result in the single-phase Na_3-xLi_xV₂(PO₄)₃. In contrast, CIE and EIE lead to deep substitution degrees and proceed much easier in the NASICON framework (Na₃V₂(PO₄)₃), where more than 2/3 of Na⁺ ions per f.u. are replaced with Li⁺ resulting in Na_0.6Li_2.4V₂(PO₄)₃ (R3̅ S.G.), while in the anti-NASICON framework (Li₃V₂(PO₄)₃), only 1/3 of Li⁺ ions are replaced with Na⁺ resulting in Li₂NaV₂(PO₄)₃ (Pbcn S.G.), which was shown to be a metastable phase, and after high-temperature treatment, it decomposes into two NASICON-type compounds. The ionic conductivity was analyzed both theoretically and experimentally, and the results show that in the NASICON framework, the migration of both Na⁺ and Li⁺ ions is realized, while in the anti-NASICON framework, the Li⁺ migration is preferable. The contribution of the electronic component to total conductivity was determined.

Na2ZrFe(PO4)3─A Rhombohedral NASICON-Structured Material: Synthesis, Structure and Na-Intercalation Behavior

A NASICON-structured earth-abundant mixed transition metal (T_M) containing Na-T_M-phosphate, viz., Na₂ZrFe(PO₄)₃, has been prepared via a sol-gel route using a low-cost Fe³⁺-based precursor. The as-prepared material crystallizes in the desired rhombohedral NASICON structure (space group: R3̅c) at room temperature. Synchrotron X-ray diffraction (XRD), transmission electron microscopy, X-ray absorption spectroscopy, etc., have been performed to determine the crystal structure, associated details, composition, and electronic structures. In light of the structural features, as one of the possible functionalities of Na₂FeZr(PO₄)₃, Na-intercalation/deintercalation has been examined, which indicates the occurrence of reversible electrochemical Na-insertion/extraction via Fe²⁺/Fe³⁺ redox at an average potential of ∼2.5 V. The electrochemical data and direct evidences from operando synchrotron XRD indicate that the rhombohedral structure is preserved during Na-insertion/extraction, albeit within a certain range of Na-content (i.e., ∼2-3 p.f.u.), beyond which rhombohedral → monoclinic transformation takes place. Within this range, Na-insertion/extraction takes place via solid-solution pathway, resulting in outstanding cyclic stability, higher Na-diffusivity, and good rate-capability. To the best of the authors' knowledge, this represents the first in-depth structural, compositional, and electrochemical studies with Na₂ZrFe(PO₄)₃, along with the interplay between those, which provide insights into the design of similar low-cost materials for various applications, including sustainable electrochemical energy storage systems.

Dual-Ion Intercalation Chemistry Enabling Hybrid Metal-Ion Batteries

To outline the role of dual-ion intercalation chemistry to reach sustainable energy storage, the present Review aimed to compare two types of batteries: widely accepted dual-ion batteries based on cationic and anionic co-intercalation versus newly emerged hybrid metal-ion batteries using the co-intercalation of cations only. Among different charge carrier cations, the focus was on the materials able to co-intercalate monovalent ions (such Li⁺ and Na⁺ , Li⁺ and K⁺ , Na⁺ and K⁺ , etc.) or couples of mono- and multivalent ions (Li⁺ and Mg²⁺ , Na⁺ and Mg²⁺ , Na⁺ and Zn²⁺ , H⁺ and Zn²⁺ , etc.). Furthermore, the Review was directed on co-intercalation materials composed of environmentally benign and low-cost transition metals (e. g., Mn, Fe, etc.). The effect of the electrolyte on the co-intercalation properties was also discussed. The summarized knowledge on dual-ion energy storage could stimulate further research so that the hybrid metal-ion batteries become feasible in near future.

Research Progress on Na3V2(PO4)3 Cathode Material of Sodium Ion Battery

Sodium ion batteries (SIBs) are one of the most potential alternative rechargeable batteries because of their low cost, high energy density, high thermal stability, and good structure stability. The cathode materials play a crucial role in the cycling life and safety of SIBs. Among reported cathode candidates, Na₃V₂(PO₄)₃ (NVP), a representative electrode material for sodium super ion conductor, has good application prospects due to its good structural stability, high ion conductivity and high platform voltage (~3.4 V). However, its practical applications are still restricted by comparatively low electronic conductivity. In this review, recent progresses of Na₃V₂(PO₄)₃ are well summarized and discussed, including preparation and modification methods, electrochemical properties. Meanwhile, the future research and further development of Na₃V₂(PO₄)₃ cathode are also discussed.

Effect of Mixed Li+/Na+-ion Electrolyte on Electrochemical Performance of Na4Fe3(PO4)2P2O7 in Hybrid Batteries

The mixed sodium-iron ortho-pyrophosphate Na4Fe3(PO4)2P2O7 (NFPP) is a promising Na-containing cathode material with the highest operating voltage among sodium framework structured materials. It operates both in Na and Li electrochemical cells. When cycled in a hybrid Li/Na cell, a competitive co-intercalation of the Li+ and Na+ ions occurs at the cathode side. The present study shows that this process can be tuned by changing the concentration of the Na+ ions in the mixed Li+/Na+-ion electrolyte and current density. It is shown that if the Na concentration in the electrolyte increases, the specific capacity of NFPP also increases and its high-rate capability is significantly improved.

α-VPO4: A Novel Many Monovalent Ion Intercalation Anode Material for Metal-Ion Batteries

In this paper, we report on a novel α-VPO₄ phosphate adopting the α-CrPO₄ type structure as a promising anode material for rechargeable metal-ion batteries. Obtained by heat treatment of a structurally related hydrothermally prepared KTiOPO₄-type NH₄VOPO₄ precursor under reducing conditions, the α-VPO₄ material appears stable in a wide temperature range and possesses an interesting "sponged" needle-like particle morphology. The electrochemical performance of α-VPO₄ as the anode material was examined in Li-, Na-, and K-based cells. The carbon-coated α-VPO₄/C composite exhibits 185, 110, and 37 mA h/g specific capacities respectively at the first discharge and around 120, 80, and 30 mA h/g at consecutive cycles at a C/10 rate. The considerable capacity drop after the first cycle in Li and Na cells is presumably due to irreversible alkali ion consumption taking place upon alkali-ion de/insertion. The EDX analysis of the recovered electrodes revealed an uptake of ∼23% of Na after the first discharge with significant cell parameter alteration validated by operando XRD measurements. In contrast to the known β-VPO₄ anode materials, both Li and Na de/insertion into the new α-VPO₄ proceed via an intercalation mechanism with the parent structural framework preserved but not via a conversion mechanism. The dimensionality of alkali-ion migration pathways and diffusion energy barriers was analyzed by the BVEL approach. Na-ion diffusion coefficients measured by the potentiostatic intermittent titration technique are in the range of (0.3-1.0)·10^-10 cm²/s, anticipating α-VPO₄ as a prospective high-power anode material for Na-ion batteries.

A first-principles investigation of the influence of polyanionic boron doping on the stability and electrochemical behavior of Na3V2(PO4)3

Na₃V₂(PO₄)₃ (NVP) is one of the most promising candidates for use as cathodes in room-temperature sodium ion batteries owing to its high structural stability and rapid Na⁺ transportation kinetics. The cationic doping of foreign ions at Na or V sites in the NVP lattice has proven to be an effective approach for enhancing the electrochemical performance of NVP. In this work, we present a first-principles density functional theory investigation of the impact of polyanionic boron doping at P sites on the structural and electrochemical behavior of NVP. Our simulation results suggest that B doping considerably increases the structural stability of NVP while shrinking its lattice size to some extent. Since B donates far fewer electrons to connected O atoms, the surrounding V atoms become more positive, causing the operating voltage to increase with B content. However, the reduction in lattice size is not beneficial for the Na⁺ transportation kinetics. As demonstrated by a search for the transition state, a concerted ion-exchange mechanism is preferred for Na⁺ transportation, and increased B doping leads to a higher Na⁺ diffusion barrier. Improvements in electrochemical performance due to B doping see (Hu et al. Adv Sci 3(12):1600112, 2016) appear to originate mainly from the resulting increased electrical conductivity.

A Multi-Ion Strategy towards Rechargeable Sodium-Ion Full Batteries with High Working Voltage and Rate Capability

Sodium-ion batteries (SIBs) are a promising alternative for the large-scale energy storage owing to the natural abundance of sodium. However, the practical application of SIBs is still hindered by the low working voltage, poor rate performance, and insufficient cycling stability. A sodium-ion based full battery using a multi-ion design is now presented. The optimized full batteries delivered a high working voltage of about 4.0 V, which is the best result of reported sodium-ion full batteries. Moreover, this multi-ion battery exhibited good rate performance up to 30 C and a high capacity retention of 95 % over 500 cycles at 5 C. Although the electrochemical performance of this multi-ion battery may be further enhanced via optimizing electrolyte and electrode materials for example, the results presented clearly indicate the feasibility of this multi-ion strategy to improve the electrochemical performance of SIBs for possible energy storage applications.

Surface Modification of Na3V2(PO4)3 by Nitrogen and Sulfur Dual-Doped Carbon Layer with Advanced Sodium Storage Property

Nitrogen and sulfur dual-doped carbon layer wrapped Na₃V₂(PO₄)₃ nanoparticles (NVP@NSC) have been successfully fabricated by the facile solid-state method. In this hierarchical structure, the Na₃V₂(PO₄)₃ nanoparticles are well dispersed and closely coated by nitrogen and sulfur dual-doped carbon layer, constructing an effective and interconnected conducting network to reduce the internal resistance. Furthermore, the uniform coating layers alleviate the agglomeration of Na₃V₂(PO₄)₃ as well as mitigate the side reaction between electrode and electrolyte. Because of the excellent electron transfer mutually enhancing sodium diffusion for this extraordinary structure, the NVP@NSC composite delivers an impressive discharge capacity of 113.0 mAh g^-1 at 1 C and shows a capacity retention of 82.1% after 5000 cycles at an ultrahigh rate of 50 C, suggesting the remarkable rate capability and long cyclicity. Surprisingly, a reversible capacity of 91.1 mAh g^-1 is maintained after 1000 cycles at 5 C under the elevated temperature of 55 °C. The approach of nitrogen and sulfur dual-doped carbon-coated Na₃V₂(PO₄)₃ provides an effective and promising strategy to enhance the ultrahigh rate and ultralong life property of cathode, which can be used for large-scale commercial production in sodium ion batteries.

Layered hybrid phase Li2NaV2(PO4)3/carbon dot nanocomposite cathodes for Li+/Na+ mixed-ion batteries

Hybrid phase Li₂NaV₂(PO₄)₃ (H-LNVP) is one of the most promising cathode materials for Li⁺/Na⁺ mixed-ion batteries.

Hollow K0.27MnO2 Nanospheres as Cathode for High-Performance Aqueous Sodium Ion Batteries

Hollow K0.27MnO2 nanospheres as cathode material were designed for aqueous sodium ion batteries (SIBs) using polystyrene (PS) as a template. The samples were systematically studied by X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy. As cathode materials for aqueous SIBs, the hollow structure can effectively improve the sodium storage property. A coin cell with hollow K0.27MnO2 as cathode and NaTi2(PO4)3 as anode exhibits a specific capacity of 84.9 mA h g(-1) at 150 mA g(-1), and the capacity of 56.6 mA h g(-1) is still maintained at an extremely high current density of 600 mA g(-1). For full cell measurement at the current density of 200 mA g(-1), 83% capacity retention also can be attained after 100 cycles. The as-designed hollow K0.27MnO2 nanospheres demonstrate long cyclability and high rate capability, which grant the potential for application in advanced aqueous SIBs.

Rechargeable dual-metal-ion batteries for advanced energy storage

Possible configurations of hybrid-ion batteries based on dual-metal-ions are summarized: these could be promising rechargeable battery systems as they combine the respective advantages of each single-metal-ion.

A study into the extracted ion number for NASICON structured Na₃V₂(PO₄)₃ in sodium-ion batteries

Excellent C-rate and cycling performance with a high specific capacity of 117.6 mA h g(-1) have been achieved on NASICON-structure Na3V2(PO4)3 sodium-ion batteries. Two different Na sites, namely Na(1) and Na(2), are reported in the open three-dimensional framework, of which the ions at the Na(2) sites should be mainly responsible for the electrochemical properties. It is vitally important and interesting to find that there are two kinds of possible ion occupation of Na ions in Na3V2(PO4)3 and the investigation of ion-extraction number is firstly explored by discussing ion occupations with the help of first-principles calculations. The ion occupation of 0.75 for all Na sites is suitable for the configuration of [Na3V2(PO4)3]2, and the two-step extraction process accompanied by structure reorganization can account for the theoretical capacity of Na3V2(PO4)3.

Mechanistic investigation of ion migration in Na3V2(PO4)2F3 hybrid-ion batteries

The ion-migration mechanism of Na₃V₂(PO₄)₂F₃ is investigated in Na₃V₂(PO₄)₂F₃–Li hybrid-ion batteries through a combined computational and experimental study.

Morphology and surface properties of LiVOPO4: a first principles study

First principles calculations were used to investigate the surface energies, equilibrium morphology, surface redox potentials, and surface electrical conductivity of LiVOPO4. Relatively low-energy surfaces are found in the (100), (010), (001), (011), (111), and (201) orientations of the orthorhombic structure. Thermodynamic equilibrium shape of the LiVOPO4 crystal is built with the calculated surface energies through a Wulff construction. The (001) and (111) orientations are the dominating surfaces in the Wulff shape. Similar calculations for VOPO4 display a larger decrease in surface energies for the (100) surface rather than those in the other surfaces. It suggests that the Wulff shape of LiVOPO4 is closely related to the chemical environment around. Surfaces (100), (010) and (201) present lower Li surface redox potentials in comparison with the bulk material. Therefore, the Li migration rate on surfaces could be effectively increased by maximizing the exposure of these low redox potential surfaces. In addition, lower surface band gaps are found in all orientations compared to the bulk one, which indicates that electrical conductivity can be improved significantly by enlarging surfaces with relatively low band gaps in the particle. Therefore, synthesizing (201) and (100) nanosheets will greatly improve the electrochemical properties of the material.

Investigation of the sodium ion pathway and cathode behavior in Na₃V₂(PO₄)₂F₃ combined via a first principles calculation

The electrochemical properties of Na3V2(PO4)2F3 cathode utilized in the sodium ion battery are investigated, and the ion migration mechanisms are proposed as combined via the first principles calculations. Two different Na sites, namely, the Na(1) and Na(2) sites, could cause two sodium ions of Na3V2(PO4)2F3 to be extracted or inserted by a two-step electrochemical process accompanied by structural reorganization that could be responsible for the redox reaction of V(3+/4+). Because the calculated average voltage (V(avg)) of the second charging plateau is 4.04 V for the optimized system but 4.38 V for the unoptimized one, the reorganization of the cathode system can make a stable configuration and lower the extraction energy. Three designed pathways for sodium ions along the x, y, z directions in Na3V2(PO4)2F3, known as a 3D ions transport tunnel, have activation energies (Ea) of 0.449, 0.2, and 0.323 eV, respectively, by using DFT calculations, demonstrating the different feasibilities of the migration directions.

Sodium/Lithium storage behavior of antimony hollow nanospheres for rechargeable batteries

Sodium-ion batteries (SIBs) have come up as an alternative to lithium-ion batteries (LIBs) for large-scale applications because of abundant Na storage in the earth's crust. Antimony (Sb) hollow nanospheres (HNSs) obtained by galvanic replacement were first applied as anode materials for sodium-ion batteries and exhibited superior electrochemical performances with high reversible capacity of 622.2 mAh g(-1) at a current density of 50 mA g(-1) after 50 cycles, close to the theoretical capacity (660 mAh g(-1)); even at high current density of 1600 mA g(-1), the reversible capacities can also reach 315 mAh g(-1). The benefits of this unique structure can also be extended to LIBs, resulting in reversible capacity of 627.3 mAh g(-1) at a current density of 100 mAh g(-1) after 50 cycles, and at high current density of 1600 mA g(-1), the reversible capacity is 435.6 mAhg(-1). Thus, these benefits from the Sb HNSs are able to provide a robust architecture for SIBs and LIBs anodes.

A promising Na3V2(PO4)3 cathode for use in the construction of high energy batteries

High-energy batteries need significant cathodes which can simultaneously provide large specific capacities and high discharge plateaus. NASICON-structured Na3V2(PO4)3 (NVP) has been utilised as a promising cathode to meet this requirement and be used in the construction of high energy batteries. For a hybrid-ion battery by employing metallic lithium as an anode, NVP exhibits an initial specific capacity of 170 mA h g(-1) in the voltage range of 1.6-4.8 V with a long discharge plateau around 3.7 V. Three Na(2) sites for NVP are found capable to be utilised through the application of a wide voltage window but only two of them are able to undergo ions exchange to produce a NaLi2V2(PO4)3 phase. However, a hybrid-ion migration mechanism is suggested to exist to describe the whole ion transport in which the effects of a Na-ion "barrier" results in a lowered ion diffusion rate and observed specific capacity.