A surface-modified Na3V2(PO4)2F3 cathode with high rate capability and cycling stability for sodium ion batteries

High voltage, high rate, and cycling-stable cathodes are urgently needed for development of commercially viable sodium ion batteries (SIBs). Herein, we report a facial ball-milling to synthesize a carbon-coated Na3V2(PO4)2F3 composite (C-NVPF). Benefiting from the highly conductive carbon layer, the C-NVPF material exhibits a high reversible capacity (110.6 mA h g-1 at 0.1C), long-term cycle life (54% of capacity retention up to 2000 cycles at 5C), and excellent rate performance (35.1 mA h g-1 at 30C). The present results suggest promising applications of the C-NVPF material as a high-performance cathode for sodium ion batteries.

Na3V2(PO4)3-decorated Na3V2(PO4)2F3 as a high-rate and cycle-stable cathode material for sodium ion batteries

Since Na3V2(PO4)3 (NVP) possesses modest volume deformation and three-dimensional ion diffusion channels, it is a potential sodium-ion battery cathode material that has been extensively researched. Nonetheless, NVP still endures the consequences of poor electronic conductivity and low voltage platforms, which need to be further improved. On this basis, a high voltage platform Na3V2(PO4)2F3 was introduced to form a composite with NVP to increase the energy density. In this study, the sol-gel technique was successfully used to synthesize a Na3V2(PO4)2.75F0.75/C (NVPF·3NVP/C) composite cathode material. The citric acid-derived carbon layer was utilized to construct three-dimensional conducting networks to effectively promote ion and electron diffusion. Furthermore, the composites' synergistic effect accelerates the quick ionic migration and improves the kinetic reaction. In particular, NVP as the dominant phase enhanced the structural stability and significantly increased the capacitive contribution. Therefore, at 0.1C, the discharge capacity of the modified NVPF·3NVP/C composite is 120.7 mA h g-1, which is greater than the theoretical discharge capacity of pure NVP (118 mA h g-1). It discharged 110.9 mA h g-1 of reversible capacity even at an elevated multiplicity of 10C, and after 200 cycles, it retained 64.1% of its capacity. Thus, the effort produced an optimized NVPF·3NVP/C composite cathode material that may be used in the sodium ion cathode.

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.

Unravelling Li+ Intercalation Mechanism and Cathode Electrolyte Interphase of Na3 V2 (PO4 )3 and Na3 (VOPO4 )2 F Cathode as Robust Framework Towards High-Performance Lithium-Ion Batteries

Although lithium-ion batteries (LIBs) are promising towards high energy density and superior safety energy storage systems (ESS), severe depletion of Li reserve cannot meet the ever-growing demand for LIBs due to the uneven distribution and limited amount of Li resource. Li-free polyanionic cathodes, such as Na₃ V₂ (PO₄ )₃ (NVP) and Na₃ (VOPO₄ )₂ F (NVOPF), show intriguing electrochemical performances with prospective future for LIBs due to their appropriate crystallographic sites, robust host structure, and abundant Na resource. In this work, NVP and NVOPF were systematically investigated as cathodes for LIBs using different voltage windows of 2.5-4.3, 2.0-4.3, and 1.5-4.8 V, along with their electrochemical mechanisms, cathode electrolyte interphase properties, and electrode morphologies for comparison. Ex-situ X-ray diffraction, ex-situ X-ray photoelectron spectroscopy, and post-mortem scanning electron microscopy revealed that their mechanisms shifted from a predominant Na⁺ intercalation/deintercalation in the first charging/discharging to a mixed Li⁺ /Na⁺ intercalation/deintercalation at the subsequent cycling. Due to the residual Na⁺ acting as pillar in the structure, NVP and NVPF could serve as robust host framework, providing appropriate crystallographic sites for repeated Li⁺ /Na⁺ intercalation/deintercalation. NVP electrode delivered a higher discharge capacity of 107.6 mAh g^-1 with superior capacity retention of 84.3 % after 1000 cycles (2.5-4.3 V, 100 mA g^-1 ) than NVOPF electrode (97.3 mAh g^-1 , 68.8 %). Electrode polarization and kinetic analysis manifested one energetically similar and two energetically nonequivalent crystallographic Na sites within the R 3 ‾ c and I4/mmm polyanionic structure of NVP and NVOPF. This work comprehensively demonstrates the feasibility and prospect of sodium-based NVP and NVOPF polyanions serving as advanced Li-free cathodes for LIBs, which provides novel insights into seeking Li-free candidates as prospective cathodes for LIBs towards a more sustainable society and a cost-effective battery manufacturing system.

Enabling high-performance all-solid-state hybrid-ion batteries with a PEO-based electrolyte

All-solid-state hybrid-ion batteries exhibiting a synergistic Na⁺/Li⁺ de/intercalation mechanism were designed and assembled, by using modified PEO-based solid polymer electrolyte, Na₂V₂(PO₄)₂O₂F cathode, and Li metal anode. The batteries exhibited a high average working voltage of 3.88 V, and an energy density of 432.37 W h kg^-1, providing a new avenue for the development of high-safety and low-cost secondary batteries.

Na3V2(PO4)2F3@bagasse carbon as cathode material for lithium/sodium hybrid ion battery

The nano-scale spherical Na₃V₂(PO₄)₂F₃ with a NASICON structure phase was prepared with a spray drying technique, and the bagasse in Guangxi, China was selected as the carbon source to prepare Na₃V₂(PO₄)₂F₃/C. The optimal preparation conditions of the composite determined using thermogravimetry, X-ray diffraction, scanning electron microscopy and electrochemical testing were: a calcination temperature of 650 °C and a 20% carbon source. The Na₃V₂(PO₄)₂F₃/C has obvious redox peaks, determined by cyclic voltammetry (CV), at 3.90 V and 3.75 V, 4.32 V and 4.15 V. These two pairs of redox peaks correspond to the escape/intercalation of the two pairs of Li⁺/Na⁺. Notably, compared with pure Na₃V₂(PO₄)₂F₃, the specific discharge capacity of Na₃V₂(PO₄)₂F₃/C-20%, which were used as a cathode material for lithium-sodium hybrid ion batteries, increased from 55 mA h g^-1 to 125 mA h g^-1, which was an improvement of twofold.

Revealing the Multi-Electron Reaction Mechanism of Na3 V2 O2 (PO4 )2 F Towards Improved Lithium Storage

Na₃ V₂ O₂ (PO₄ )₂ F (NVOPF) as an attractive electrode material has received much attention based on the one-electron reaction of V⁴⁺ /V⁵⁺ . However, the electrochemical reactions involving lower vanadium valences were not investigated till now. Herein, a composite of graphene decorated nanosheet-assembled NVOPF microflowers (NVOPF/G) was synthesized and the multi-electron reaction of NVOPF/G was conducted by controlling the operation voltage windows. The reaction mechanism, structural changes, and vanadium valences during the insertion/extraction of Li ions (from 2 to 6) were elucidated clearly by in-situ X-ray diffraction and ex-situ X-ray photoelectron spectroscopy. Theoretical computations also revealed the Li-ion locations in the structure of NaV₂ O₂ (PO₄ )₂ F. Due to the additional redox couple of V³⁺ /V⁴⁺ , NVOPF/G displayed a much higher initial capacity of 183.3 mAh g^-1 in the wider voltage window of 1.0-4.8 V than that of 2.5-4.8 V (129.3 mAh g^-1 ). Moreover, excellent Li-storage performance of NVOPF/G at a lower voltage (≤2.5 V) with the active reaction of V²⁺ /V³⁺ /V⁴⁺ was obtained for the first time, demonstrating the high potential of NVOPF/G as an anode material for Li ion storage.

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 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.

Cross-Linking Hollow Carbon Sheet Encapsulated CuP2 Nanocomposites for High Energy Density Sodium-Ion Batteries

Sodium-ion batteries (SIB) are regarded as the most promising competitors to lithium-ion batteries in spite of expected electrochemical disadvantages. Here a "cross-linking" strategy is proposed to mitigate the typical SIB problems. We present a SIB full battery that exhibits a working potential of 3.3 V and an energy density of 180 Wh kg^-1 with good cycle life. The anode is composed of cross-linking hollow carbon sheet encapsulated CuP₂ nanoparticles (CHCS-CuP₂) and a cathode of carbon coated Na₃V₂(PO₄)₂F₃ (C-NVPF). For the preparation of the CHCS-CuP₂ nanocomposites, we develop an in situ phosphorization approach, which is superior to mechanical mixing. Such CHCS-CuP₂ nanocomposites deliver a high reversible capacity of 451 mAh g^-1 at 80 mA g^-1, showing an excellent capacity retention ratio of 91% in 200 cycles together with good rate capability and stable cycling performance. Post mortem analysis reveals that the cross-linking hollow carbon sheet structure as well as the initially formed SEI layers are well preserved. Moreover, the inner electrochemical resistances do not significantly change. We believe that the presented battery system provides significant progress regarding practical application of SIB.

Phosphate Framework Electrode Materials for Sodium Ion Batteries

Sodium ion batteries (SIBs) have been considered as a promising alternative for the next generation of electric storage systems due to their similar electrochemistry to Li-ion batteries and the low cost of sodium resources. Exploring appropriate electrode materials with decent electrochemical performance is the key issue for development of sodium ion batteries. Due to the high structural stability, facile reaction mechanism and rich structural diversity, phosphate framework materials have attracted increasing attention as promising electrode materials for sodium ion batteries. Herein, we review the latest advances and progresses in the exploration of phosphate framework materials especially related to single-phosphates, pyrophosphates and mixed-phosphates. We provide the detailed and comprehensive understanding of structure-composition-performance relationship of materials and try to show the advantages and disadvantages of the materials for use in SIBs. In addition, some new perspectives about phosphate framework materials for SIBs are also discussed. Phosphate framework materials will be a competitive and attractive choice for use as electrodes in the next-generation of energy storage devices.

Mechanistic Insight into the Stability of HfO2 -Coated MoS2 Nanosheet Anodes for Sodium Ion Batteries

It is demonstrated for the first time that surface passivation of 2D nanosheets of MoS2 by an ultrathin and uniform layer of HfO2 can significantly improve the cyclic performance of sodium ion batteries. After 50 charge/discharge cycles, bare MoS2 and HfO2 coated MoS2 electrodes deliver the specific capacity of 435 and 636 mAh g(-1) , respectively, at current density of 100 mA g(-1) . These results imply that batteries using HfO2 coated MoS2 anodes retain 91% of the initial capacity; in contrast, bare MoS2 anodes retain only 63%. Also, HfO2 coated MoS2 anodes show one of the highest reported capacity values for MoS2 . Cyclic voltammetry and X-ray photoelectron spectroscopy results suggest that HfO2 does not take part in electrochemical reaction. The mechanism of capacity retention with HfO2 coating is explained by ex situ transmission electron microscope imaging and electrical impedance spectroscopy. It is illustrated that HfO2 acts as a passivation layer at the anode/electrolyte interface and prevents structural degradation during charge/discharge process. Moreover, the amorphous nature of HfO2 allows facile diffusion of Na ions. These results clearly show the potential of HfO2 coated MoS2 anodes, which performance is significantly higher than previous reports where bulk MoS2 or composites of MoS2 with carbonaceous materials are used.

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.

A phase-transfer assisted solvo-thermal strategy for low-temperature synthesis of Na3(VO1−xPO4)2F1+2x cathodes for sodium-ion batteries

A series of Na₃(VO_1−xPO₄)₂F_1+2x (x = 0, 0.5 and 1) compounds with superior Na storage performance can be synthesized by a phase-transfer assisted solvo-thermal strategy at a rather low temperature (120 °C) in one simple step.

A new high-energy cathode for a Na-ion battery with ultrahigh stability

Large-scale electric energy storage is a key enabler for the use of renewable energy. Recently, the room-temperature Na-ion battery has been rehighlighted as an alternative low-cost technology for this application. However, significant challenges such as energy density and long-term stability must be addressed. Herein, we introduce a novel cathode material, Na1.5VPO4.8F0.7, for Na-ion batteries. This new material provides an energy density of ~600 Wh kg(-1), the highest value among cathodes, originating from both the multielectron redox reaction (1.2 e(-) per formula unit) and the high potential (~3.8 V vs Na(+)/Na) of the tailored vanadium redox couple (V(3.8+)/V(5+)). Furthermore, an outstanding cycle life (~95% capacity retention for 100 cycles and ~84% for extended 500 cycles) could be achieved, which we attribute to the small volume change (2.9%) upon cycling, the smallest volume change among known Na intercalation cathodes. The open crystal framework with two-dimensional Na diffusional pathways leads to low activation barriers for Na diffusion, enabling excellent rate capability. We believe that this new material can bring the low-cost room-temperature Na-ion battery a step closer to a sustainable large-scale energy storage system.

A multifunctional 3.5 V iron-based phosphate cathode for rechargeable batteries

In the search for new positive-electrode materials for lithium-ion batteries, recent research has focused on nanostructured lithium transition-metal phosphates that exhibit desirable properties such as high energy storage capacity combined with electrochemical stability. Only one member of this class--the olivine LiFePO(4) (ref. 3)--has risen to prominence so far, owing to its other characteristics, which include low cost, low environmental impact and safety. These are critical for large-capacity systems such as plug-in hybrid electric vehicles. Nonetheless, olivine has some inherent shortcomings, including one-dimensional lithium-ion transport and a two-phase redox reaction that together limit the mobility of the phase boundary. Thus, nanocrystallites are key to enable fast rate behaviour. It has also been suggested that the long-term economic viability of large-scale Li-ion energy storage systems could be ultimately limited by global lithium reserves, although this remains speculative at present. (Current proven world reserves should be sufficient for the hybrid electric vehicle market, although plug-in hybrid electric vehicle and electric vehicle expansion would put considerable strain on resources and hence cost effectiveness.) Here, we report on a sodium/lithium iron phosphate, A(2)FePO(4)F (A=Na, Li), that could serve as a cathode in either Li-ion or Na-ion cells. Furthermore, it possesses facile two-dimensional pathways for Li+ transport, and the structural changes on reduction-oxidation are minimal. This results in a volume change of only 3.7% that--unlike the olivine--contributes to the absence of distinct two-phase behaviour during redox, and a reversible capacity that is 85% of theoretical.

Mechanism of Water Photooxidation Reaction at Atomically Flat TiO2(Rutile) (110) and (100) Surfaces: Dependence on Solution pH

The mechanism of water photooxidation reaction at atomically flat n-TiO(2) (rutile) surfaces was investigated in aqueous solutions of various pH values, using photoluminescence (PL) measurements. The PL bands, which peaked at around 810 and 840 nm for the (110) and (100) surfaces, respectively, were assigned to radiative transitions between conduction-band electrons and surface-trapped holes (STH), [Ti-O=Ti(2)](s)+, formed at triply coordinated (normal) O atoms at the surface lattice. The PL intensity (I(PL)) decreased stepwise with increasing solution pH, namely, it sharply decreased at around pH 4, near the point of zero charge of TiO(2) (about 5), and then rapidly decreased to zero near pH 13. The first sharp decrease around pH 4 is attributed to the increased rate of nucleophilic attack of a water molecule to a hole at a site of surface bridging oxygen (Ti-O-Ti), the density of which increases with increasing pH. The nucleophilic attack is regarded as the main initiating step of the water oxidation reaction in low and intermediate pH. The high PL intensity at low pH is ascribed to slow nucleophilic attack owing to a very low density of Ti-O-Ti by its protonation at the low pH. The second sharp decrease near pH 13 is attributed to formation of surface anionic species like Ti-O- which can be readily oxidized by photogenerated holes. Interrelations between reaction intermediates proposed in this work and those reported by time-resolved laser spectroscopy are discussed.