Superior Na‐Storage Performance of Low‐Temperature‐Synthesized Na3(VO1−xPO4)2F1+2x (0≤x≤1) Nanoparticles for Na‐Ion Batteries

Na‐ion batteries are becoming comparable to Li‐ion batteries because of their similar chemical characteristics and abundant sources of sodium. However, the materials production should be cost‐effective in order to meet the demand for large‐scale application. Here, a series of nanosized high‐performance cathode materials, Na₃(VO_1?xPO₄)₂F_1+2x (0≤x≤1), has been synthesized by a solvothermal low‐temperature (60–120 °C) strategy without the use of organic ligands or surfactants. The as‐synthesized Na₃(VOPO₄)₂F nanoparticles show the best Na‐storage performance reported so far in terms of both high rate capability (up to 10 C rate) and long cycle stability over 1200 cycles. To the best of our knowledge, the current developed synthetic strategy for Na₃(VO_1?xPO₄)₂F_1+2x is by far one of the least expensive and energy‐consuming methods, much superior to the conventional high‐temperature solid‐state method.     

Co0.50VOPO4·2H2O

The crystal structure of cobalt vanadophosphate dihydrate {systematic name: poly[diaqua‐μ‐oxido‐μ‐phosphato‐hemicobalt(II)vanadium(II)]}, Co_0.50VOPO₄·2H₂O, shows a three‐dimensional framework assembled from VO₅ square pyramids, PO₄ tetrahedra and Co[O₂(H₂O)₄] octahedra. The Co^II ions have local 4/m symmetry, with the equatorial water molecules in the mirror plane, while the V and apical O atom of the vanadyl group are located on the fourfold rotation axis and the P atoms reside on sites. The PO₄ tetrahedra connect the VO₅ polyhedra to form a planar P–V–O layer. The [Co(H₂O)₄]²⁺ cations link adjacent P–V–O layers via vanadyl O atoms to generate an unprecedented three‐dimensional open framework. Powder diffraction measurements reveal that the framework collapses on removal of the water molecules.     

Preparation of ammonium intercalated vanadyl phosphate by redox intercalation and ion exchange

Redox intercalation of NH₄⁺ into vanadyl phosphate dihydrate (VOPO₄·2H₂O) leads to a two-phase (NH₄)_xVOPO₄·H₂O (x=0.2−0.9) compound with interlayer distances of 6.7 and 6.4 Å. Ammonium ions can be incorporated into the interlayer space of VOPO₄ also by an ion exchange, starting from alkali-metal redox-intercalated vanadyl phosphate Me_xVOPO₄·yH₂O (Me=Li, Na, K, Rb). Several phases are formed during the ion exchange, one of them with the interlayer distance corresponding to the original Me_xVOPO₄·yH₂O phase, the second one corresponding to formed (NH₄)_xVOPO₄·H₂O. In addition, a third phase is formed by the ion exchange when Li_0.98VOPO₄·1.98H₂O or Rb_0.60VOPO₄·1.3H₂O are used as starting compounds. An opposite ion exchange of NH₄⁺ for Me⁺ starting from (NH₄)_xVOPO₄·H₂O does not proceed.     

Synergistic Dual‐Additive Electrolyte Enables Practical Lithium‐Metal Batteries

A rechargeable Li metal anode coupled with a high‐voltage cathode is a promising approach to high‐energy‐density batteries exceeding 300 Wh kg^?1. Reported here is an advanced dual‐additive electrolyte containing a unique solvation structure and it comprises a tris(pentafluorophenyl)borane additive and LiNO₃ in a carbonate‐based electrolyte. This system generates a robust outer Li₂O solid electrolyte interface and F‐ and B‐containing conformal cathode electrolyte interphase. The resulting stable ion transport kinetics enables excellent cycling of Li/LiNi_0.8Mn_0.1Co_0.1O₂ for 140 cycles with 80 % capacity retention under highly challenging conditions (≈295.1 Wh kg^?1 at cell‐level). The electrolyte also exhibits high cycling stability for a 4.6 V LiCoO₂ (160 cycles with 89.8 % capacity retention) cathode and 4.95 V LiNi_0.5Mn_1.5O₄ cathode.     

Reversible Ammonium Ion Intercalation/de-intercalation with Crystal Water Promotion Effect in Layered VOPO4⋅2 H2O**

The non-metal NH₄⁺ carrier has attracted tremendous interests for aqueous energy storage owing to its light molar mass and fast diffusion in aqueous electrolytes. Previous study inferred that NH₄⁺ ion storage in layered VOPO₄⋅2 H₂O is impossible due to the removal of NH₄⁺ from NH₄VOPO₄ leads to a phase change inevitably. Herein, we update this cognition and demonstrated highly reversible intercalation/de-intercalation behavior of NH₄⁺ in layered VOPO₄⋅2 H₂O host. Satisfactory specific capacity of 154.6 mAh g⁻¹ at 0.1 A g⁻¹ and very stable discharge potential plateau at 0.4 V based on reference electrode was achieved in VOPO₄⋅2 H₂O. A rocking-chair ammonium-ion full cell with the VOPO₄⋅2 H₂O//2.0 M NH₄OTf//PTCDI configuration exhibited a specific capacity of 55 mAh g⁻¹, an average operating voltage of about 1.0 V and excellent long-term cycling stability over 500 cycles with a coulombic efficiency of ≈99 %. Theoretical DFT calculations suggest a unique crystal water substitution process by ammonium ion during the intercalation process. Our results provide new insight into the intercalation/de-intercalation of NH₄⁺ ions in layered hydrated phosphates through crystal water enhancement effect.     

Vanadyl phosphate dihydrate,a solid acid: the role of water in VOPO4·2H2O and its sodium derivatives Na x (VIV x VV 1−x O)PO4·(2−x)H2O

Sodium-containing intercalates having as general formula Na _x VOPOP₄·(2–x)H₂O (0.25x<0.50) have been obtained and characterized. Orthorhombic phases, which essentially maintain the structure of the layered oxide hydrate VOPO₄·2H₂O result. Intercalated sodium ions act as pillars. The presence of H₃O⁺ ions in the parent VOPO₄·2H₂O and also in some reduced phases, is detected. The understanding of the structural role of the water molecules is advanced and the topotactic dehydration/rehydration processes are studied. The formation of a new metastable VOPO₄·H₂O phase is established.     

Electron paramagnetic resonance study of doped synthetic crystals of struvite and its zinc analogue

The electron paramagnetic resonance (EPR) technique has been used to study the Mn²⁺ paramagnetic impurity complexes in synthetic struvite (MgNH₄PO₄β6H₂O) and the zinc isomorph (ZnNH₄PO₄β6H₂O). EPR of VO²⁺ ion complexes in vanadyl doped crystals of the zinc isomorph of struvite has also been studied. Two differently oriented, but otherwise identical complexes of both Mn²⁺ ion and VO²⁺ ion are found in these crystals. The spin Hamiltonian parameters indicate a large orthorhombic distortion of the [Mn²⁺(H₂O)₆] octahedra and an axial symmetry of the vanadyl complexes. The results indicate that in both manganese and vanadyl complexes, the metal ions have covalent bonding with the ligands.     

Rechargeable Room‐Temperature Na–CO2 Batteries

Developing rechargeable Na–CO₂ batteries is significant for energy conversion and utilization of CO₂. However, the reported batteries in pure CO₂ atmosphere are non‐rechargeable with limited discharge capacity of 200 mAh g^?1. Herein, we realized the rechargeability of a Na–CO₂ battery, with the proposed and demonstrated reversible reaction of 3 CO₂+4 Na?2 Na₂CO₃+C. The battery consists of a Na anode, an ether‐based electrolyte, and a designed cathode with electrolyte‐treated multi‐wall carbon nanotubes, and shows reversible capacity of 60000 mAh g^?1 at 1 A g^?1 (≈1000 Wh kg^?1) and runs for 200 cycles with controlled capacity of 2000 mAh g^?1 at charge voltage <3.7 V. The porous structure, high electro‐conductivity, and good wettability of electrolyte to cathode lead to reduced electrochemical polarization of the battery and further result in high performance. Our work provides an alternative approach towards clean recycling and utilization of CO₂.     

Ion-Exchange Properties of Alkali-Metal Redox-Intercalated Vanadyl Phosphate

Ion exchange of alkali metals in M_xVOPO₄·yH₂O (M=H, Na, K, Rb, Cs) is reported. The role of valence, size, and affinity of the cations in the exchange process is discussed. The interlayer distance in the H_1-xK_xVOPO₄·yH₂O system is discussed in terms of finite layer rigidity theory. Different behavior is observed for K_xNa_1-xVOPO₄·yH₂O dependening on the starting compound used. When potassium in KVOPO₄·H₂O is exchanged for Na⁺, one phase compound is formed. In contrast, K_xNa_1-xVOPO₄·yH₂O formed from NaVOPO₄·H₂O and K⁺ is a multiphase system. Ion exchange does not proceed when exchanging ions differ distinctly from each other in size, e.g., sodium and cesium.     

Non-concentrated aqueous electrolytes with organic solvent additives for stable zinc batteries

Rechargeable aqueous zinc batteries (RAZBs) are promising for large-scale energy storage because of their superiority in addressing cost and safety concerns. However, their practical realization is hampered by issues including dendrite growth, poor reversibility and low coulombic efficiency (CE) of Zn anodes due to parasitic reactions. Here, we report a non-concentrated aqueous electrolyte composed of 2 m zinc trifluoromethanesulfonate (Zn(OTf)₂) and the organic dimethyl carbonate (DMC) additive to stabilize the Zn electrochemistry. Unlike the case in conventional aqueous electrolytes featuring typical Zn[H₂O]₆²⁺ solvation, a solvation sheath of Zn²⁺ with the co-participation of the DMC solvent and OTf⁻ anion is found in the formulated H₂O + DMC electrolyte, which contributes to the formation of a robust ZnF₂ and ZnCO₃-rich interphase on Zn. The resultant Zn anode exhibits a high average CE of Zn plating/stripping (99.8% at an areal capacity of 2.5 mA h cm⁻²) and dendrite-free cycling over 1000 cycles. Furthermore, the H₂O + DMC electrolytes sustain stable operation of RAZBs pairing Zn anodes with diverse cathode materials such as vanadium pentoxide, manganese dioxide, and zinc hexacyanoferrate. Rational electrolyte design with organic solvent additives would promote building better aqueous batteries.

Involvement of dimethyl carbonate and trifluoromethanesulfonate anions in a hybrid aqueous electrolyte enables the formation of a new Zn²⁺-solvation structure and a ZnF₂–ZnCO₃-rich interphase that stabilizes the Zn battery chemistry.     

Hydrophobic Organic-Electrolyte-Protected Zinc Anodes for Aqueous Zinc Batteries

Aqueous Zn batteries are promising energy-storage devices. However, their lifespan is limited by irreversible Zn anodes owing to water decomposition and Zn dendrite growth. Here, we separate aqueous electrolyte from Zn anode by coating a thin MOF layer on anode and filling the pores of MOF with hydrophobic Zn(TFSI)₂-tris(2,2,2-trifluoroethyl)phosphate (TFEP) organic electrolyte that is immiscible with aqueous Zn(TFSI)₂–H₂O bulk electrolyte. The MOF encapsulated Zn(TFSI)₂-TFEP forms a ZnF₂-Zn₃(PO₄)₂ solid electrolyte interphase (SEI) preventing Zn dendrite and water decomposition. The Zn(TFSI)₂-TFEP@MOF electrolyte protected Zn anode enables a Zn||Ti cell to achieve a high average Coulombic efficiency of 99.1 % for 350 cycles. The highly reversible Zn anode brings a high energy density of 210 Wh kg⁻¹ (of cathode and anode mass) and a low capacity decay rate of 0.0047 % per cycle over 600 cycles in a Zn||MnO₂ full cell with a low capacity ratio of Zn:MnO₂ at 2:1.     

The Development of Vanadyl Phosphate Cathode Materials for Energy Storage Systems: A Review

Various cathode materials have been proposed for high-performance rechargeable batteries. Vanadyl phosphate is an important member of the polyanion cathode family. VOPO₄ has seven known crystal polymorphs with tunneled or layered frameworks, which allow facile cation (de)intercalations. Two-electron transfer per formula unit can be realized by using V^V/V^IV and V^IV/V^III redox couples. The electrochemical performance is closely related to the structures of VOPO₄ and the types of inserted cations. This Review outlines the crystal structures of VOPO₄ polymorphs and their lithiated phases. The research progress of vanadyl phosphate cathode materials for different energy storage systems, including lithium-ion batteries, sodium-ion batteries, potassium-ion batteries, multivalent batteries, and supercapacitors, as well as the related mechanism investigations are summarized. It is hoped that this Review will help with future directions of using vanadyl phosphate materials for energy storage.     

A Long‐Cycling Aqueous Zinc‐Ion Pouch Cell: NASICON‐Type Material and Surface Modification

Aqueous zinc‐ion batteries (ZIBs) have become the highest potential energy storage system for large‐scale applications owing to the high specific capacity, good safety and low cost. In this work, a NASICON‐type Na₃V₂(PO₄)₃ cathode modified by a uniform carbon layer (NVP/C) has been synthesized via a facile solid‐state method and exhibited significantly improved electrochemical performance when working in an aqueous ZIB. Specifically, the NVP/C cathode shows an excellent rate capacity (e. g., 48 mAh g^?1 at 1.0 A g^?1). Good cycle stability is also achieved (e. g., showing a capacity retention of 88% after 2000 cycles at 1.0 A g^?1). Furthermore, the Zn²⁺ (de)intercalation mechanism in the NVP cathode has been determined by various ex‐situ techniques. In addition, a Zn||NVP/C pouch cell has been assembled, delivering a high capacity of 89 mAhg^?1 at 0.2 A g^?1 and exhibiting a superior long cycling stability.     

How Water Accelerates Bivalent Ion Diffusion at the Electrolyte/Electrode Interface

The effect of H₂O in electrolytes and in electrode lattices on the thermodynamics and kinetics of reversible multivalent‐ion intercalation chemistry based on a model platform of layered VOPO₄ has been investigated. The presence of H₂O at the electrolyte/electrode interface plays a key role in assisting Zn²⁺ diffusion from electrolyte to the surface, while H₂O in the lattice structure alters the working potential. More importantly, a dynamic equilibrium between bulk electrode and electrolyte is eventually reached for H₂O transport during the charge/discharge cycles, with the water activity serving as the key parameter determining the direction of water movement and the cycling stability.     

Controlled synthesis of single-crystal VOx·nH2O nanoribbons via a hydrothermal reduction method

VO_x·nH₂O (2.0?x?2.5) nanoribbons have been synthesized by direct hydrothermal processing of the aqueous solution of NH₄VO₃ and polyethylene glycol 400 (PEG-400) at pH 3.5-5.5. Techniques of XRD, SEM, TEM, HRTEM, ED, and XPS have been used to characterize the structure, morphology, and composition of the nanoribbons. The VO_x·nH₂O nanoribbons are up to ∼200 μm in length, 100-150 nm in width, 20-30 nm in thickness, and grow along the [010] direction. The ratios of V⁴⁺ to V⁵⁺ in the products can be readily controlled by carefully adjusting the periods of reaction time. PEG carries the roles of both transport and reducing agent. A coordination self-assembly mechanism was proposed to elucidate the formation of the VO_x·nH₂O nanoribbons.     

High capacity and excellent cyclability of vanadium (IV) oxide in lithium battery applications

A VO₂ · 0.43H₂O powder with a flaky particle morphology was synthesized via a hydrothermal reduction method. It was characterized by scanning electron microscopy, electron energy loss spectroscopy, and thermogravimetric analysis. As an electrode material for rechargeable lithium batteries, it was used both as a cathode versus lithium anode and as an anode versus LiCoO₂, LiFePO₄ or LiNi_0.5Mn_1.5O₄ cathode. The VO₂ · 0.43H₂O electrode exhibits an extraordinary superiority with high capacity (160 mAh g^?1), high energy efficiency (95%), excellent cyclability (142.5 mAh g^?1 after 500 cycles) and rate capability (100 mAh g^?1 at 10 C-rate).     

Reversible Oxygen Redox Chemistry in Aqueous Zinc‐Ion Batteries

Rechargeable aqueous zinc‐ion batteries (ZIBs) are promising energy‐storage devices owing to their low cost and high safety. However, their energy‐storage mechanisms are complex and not well established. Recent energy‐storage mechanisms of ZIBs usually depend on cationic redox processes. Anionic redox processes have not been observed owing to the limitations of cathodes and electrolytes. Herein, we describe highly reversible aqueous ZIBs based on layered VOPO₄ cathodes and a water‐in‐salt electrolyte. Such batteries display reversible oxygen redox chemistry in a high‐voltage region. The oxygen redox process not only provides about 27 % additional capacity, but also increases the average operating voltage to around 1.56 V, thus increasing the energy density by approximately 36 %. Furthermore, the oxygen redox process promotes the reversible crystal‐structure evolution of VOPO₄ during charge/discharge processes, thus resulting in enhanced rate capability and cycling performance.     

Triple-function Hydrated Eutectic Electrolyte for Enhanced Aqueous Zinc Batteries

Aqueous rechargeable zinc-ion batteries (ARZBs) are impeded by the mutual problems of unstable cathode, electrolyte parasitic reactions, and dendritic growth of zinc (Zn) anode. Herein, a triple-functional strategy by introducing the tetramethylene sulfone (TMS) to form a hydrated eutectic electrolyte is reported to ameliorate these issues. The activity of H₂O is inhibited by reconstructing hydrogen bonds due to the strong interaction between TMS and H₂O. Meanwhile, the preferentially adsorbed TMS on the Zn surface increases the thickness of double electric layer (EDL) structure, which provides a shielding buffer layer to suppress dendrite growth. Interestingly, TMS modulates the primary solvation shell of Zn²⁺ ultimately to achieve a novel solvent co-intercalation ((Zn-TMS)²⁺) mechanism, and the intercalated TMS works as a “pillar” that provides more zincophilic sites and stabilizes the structure of cathode (NH₄V₄O₁₀, (NVO)). Consequently, the Zn||NVO battery exhibits a remarkably high specific capacity of 515.6 mAh g⁻¹ at a low current density of 0.2 A g⁻¹ for over 40 days. This multi-functional electrolytes and solvent co-intercalation mechanism will significantly propel the practical development of aqueous batteries.     

Towards Highly Stable Storage of Sodium Ions: A Porous Na3V2(PO4)3/C Cathode Material for Sodium‐Ion Batteries

A porous Na₃V₂(PO₄)₃ cathode material coated uniformly with a layer of approximately 6 nm carbon has been synthesized by the sol–gel method combined with a freeze‐drying process. The special porous morphology and structure significantly increases the specific surface area of the material, which greatly enlarges the contact area between the electrode and electrolyte, and consequently supplies more active sites for sodium ions. When employed as a cathode material of sodium‐ion batteries, this porous Na₃V₂(PO₄)₃/C exhibits excellent rate performance and cycling stability; for instance, it shows quite a flat potential plateau at 3.4 V in the potential window of 2.7–4.0 V versus Na⁺/Na and delivers an initial capacity as high as 118.9 and 98.0 mA h g^?1 at current rates of 0.05 and 0.5 C, respectively, and after 50 cycles, a good capacity retention of 92.7 and 93.6 % are maintained. Moreover, even when the discharge current density is increased to 5 C (590 mA g^?1), an initial capacity of 97.6 mA h g^?1 can still be achieved, and an exciting capacity retention of 88.6 % is obtained after 100 cycles. The good cycle performance, excellent rate capability, and moreover, the low cost of Na₃V₂(PO₄)₃/C suggest that this material is a promising cathode for large‐scale sodium‐ion rechargeable batteries.     

Natural graphite enhanced the electrochemical performance of Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> cathode material for lithium ion batteries

Natural graphite treated by mechanical activation can be directly applied to the preparation of Li₃V₂(PO₄)₃. The carbon-coated Li₃V₂(PO₄)₃ with monoclinic structure was successfully synthesized by using natural graphite as carbon source and reducing agent. The amount of activated graphite is optimized by X-ray diffraction, scanning electron microscope, transmission electron microscope, Raman spectrum, galvanostatic charge/discharge measurements, cyclic voltammetry, and electrochemical impedance spectroscopy tests. Our results show that Li₃V₂(PO₄)₃ (LVP)-10G exhibits the highest initial discharge capacity of 189 mAh g^?1 at 0.1 C and 162.9 mAh g^?1 at 1 C in the voltage range of 3.0–4.8 V. Therefore, natural graphite is a promising carbon source for LVP cathode material in lithium ion batteries.