A Cation and Anion Dual Doping Strategy for the Elevation of Titanium Redox Potential for High‐Power Sodium‐Ion Batteries

Titanium‐based polyanions have been intensively investigated for sodium‐ion batteries owing to their superior structural stability and thermal safety. However, their low working potential hindered further applications. Now, a cation and anion dual doping strategy is used to boost the redox potential of Ti‐based cathodes of Na₃Ti_0.5V_0.5(PO₃)₃N as a new cathode material for sodium ion batteries. Both the Ti³⁺/Ti⁴⁺ and V³⁺/V⁴⁺ redox couples are reversibly accessed, leading to two distinctive voltage platforms at ca. 3.3 V and ca. 3.8 V, respectively. The remarkably improved cycling stability (86.3 %, 3000 cycles) can be ascribed to the near‐zero volume strain in this unusual cubic symmetry, which has been demonstrated by in situ synchrotron‐based X‐ray diffraction. First‐principles calculations reveal its well‐interconnected 3D Na diffusion pathways with low energy barriers, and the two‐sodium‐extracted intermediate NaTi_0.5V_0.5(PO₃)₃N is also a stable phase according to formation energy calculations.     

A High-Energy NASICON-Type Na3.2MnTi0.8V0.2(PO4)3 Cathode Material with Reversible 3.2-Electron Redox Reaction for Sodium-Ion Batteries

Na superionic conductor (NASICON) structured cathode materials with robust structural stability and large Na⁺ diffusion channels have aroused great interest in sodium-ion batteries (SIBs). However, most of NASICON-type cathode materials exhibit redox reaction of no more than three electrons per formula, which strictly limits capacity and energy density. Herein, a series of NASICON-type Na_3+xMnTi_1−xV_x(PO₄)₃ cathode materials are designed, which demonstrate not only a multi-electron reaction but also high voltage platform. With five redox couples from V^5+/4+ (≈4.1 V), Mn^4+/3+ (≈4.0 V), Mn^3+/2+ (≈3.6 V), V^4+/3+ (≈3.4 V), and Ti^4+/3+ (≈2.1 V), the optimized material, Na_3.2MnTi_0.8V_0.2(PO₄)₃, realizes a reversible 3.2-electron redox reaction, enabling a high discharge capacity (172.5 mAh g⁻¹) and an ultrahigh energy density (527.2 Wh kg⁻¹). This work sheds light on the rational construction of NASICON-type cathode materials with multi-electron redox reaction for high-energy SIBs.     

Development and Investigation of a NASICON-Type High-Voltage Cathode Material for High-Power Sodium-Ion Batteries

Herein, we introduce a 4.0 V class high-voltage cathode material with a newly recognized sodium superionic conductor (NASICON)-type structure with cubic symmetry (space group P2₁3), Na₃V(PO₃)₃N. We synthesize an N-doped graphene oxide-wrapped Na₃V(PO₃)₃N composite with a uniform carbon coating layer, which shows excellent rate performance and outstanding cycling stability. Its air/water stability and all-climate performance were carefully investigated. A near-zero volume change (ca. 0.40 %) was observed for the first time based on in situ synchrotron X-ray diffraction, and the in situ X-ray absorption spectra revealed the V^3.2+/V^4.2+ redox reaction with high reversibility. Its 3D sodium diffusion pathways were demonstrated with distinctive low energy barriers. Our results indicate that this high-voltage NASICON-type Na₃V(PO₃)₃N composite is a competitive cathode material for sodium-ion batteries and will receive more attention and studies in the future.     

Development and Investigation of a NASICON‐Type High‐Voltage Cathode Material for High‐Power Sodium‐Ion Batteries

Herein, we introduce a 4.0 V class high‐voltage cathode material with a newly recognized sodium superionic conductor (NASICON)‐type structure with cubic symmetry (space group P2₁3), Na₃V(PO₃)₃N. We synthesize an N‐doped graphene oxide‐wrapped Na₃V(PO₃)₃N composite with a uniform carbon coating layer, which shows excellent rate performance and outstanding cycling stability. Its air/water stability and all‐climate performance were carefully investigated. A near‐zero volume change (ca. 0.40 %) was observed for the first time based on in situ synchrotron X‐ray diffraction, and the in situ X‐ray absorption spectra revealed the V^3.2+/V^4.2+ redox reaction with high reversibility. Its 3D sodium diffusion pathways were demonstrated with distinctive low energy barriers. Our results indicate that this high‐voltage NASICON‐type Na₃V(PO₃)₃N composite is a competitive cathode material for sodium‐ion batteries and will receive more attention and studies in the future.     

Towards Reversible High-Voltage Multi-Electron Reactions in Alkali-Ion Batteries Using Vanadium Phosphate Positive Electrode Materials

Vanadium phosphate positive electrode materials attract great interest in the field of Alkali-ion (Li, Na and K-ion) batteries due to their ability to store several electrons per transition metal. These multi-electron reactions (from V²⁺ to V⁵⁺) combined with the high voltage of corresponding redox couples (e.g., 4.0 V vs. for V³⁺/V⁴⁺ in Na₃V₂(PO₄)₂F₃) could allow the achievement the 1 kWh/kg milestone at the positive electrode level in Alkali-ion batteries. However, a massive divergence in the voltage reported for the V³⁺/V⁴⁺ and V⁴⁺/V⁵⁺ redox couples as a function of crystal structure is noticed. Moreover, vanadium phosphates that operate at high V³⁺/V⁴⁺ voltages are usually unable to reversibly exchange several electrons in a narrow enough voltage range. Here, through the review of redox mechanisms and structural evolutions upon electrochemical operation of selected widely studied materials, we identify the crystallographic origin of this trend: the distribution of PO₄ groups around vanadium octahedra, that allows or prevents the formation of the vanadyl distortion (O^…V⁴⁺=O or O^…V⁵⁺=O). While the vanadyl entity massively lowers the voltage of the V³⁺/V⁴⁺ and V⁴⁺/V⁵⁺ couples, it considerably improves the reversibility of these redox reactions. Therefore, anionic substitutions, mainly O²⁻ by F⁻, have been identified as a strategy allowing for combining the beneficial effect of the vanadyl distortion on the reversibility with the high voltage of vanadium redox couples in fluorine rich environments.     

Sodium‐ and Potassium‐Hydrate Melts Containing Asymmetric Imide Anions for High‐Voltage Aqueous Batteries

Aqueous Na‐ or K‐ion batteries could virtually eliminate the safety and cost concerns raised from Li‐ion batteries, but their widespread applications have generally suffered from narrow electrochemical potential window (ca. 1.23 V) of aqueous electrolytes that leads to low energy density. Herein, by exploring optimized eutectic systems of Na and K salts with asymmetric imide anions, we discovered, for the first time, room‐temperature hydrate melts for Na and K systems, which are the second and third alkali metal hydrate melts reported since the first discovery of Li hydrate melt by our group in 2016. The newly discovered Na‐ and K‐ hydrate melts could significantly extend the potential window up to 2.7 and 2.5 V (at Pt electrode), respectively, owing to the merit that almost all water molecules participate in the Na⁺ or K⁺ hydration shells. As a proof‐of‐concept, a prototype Na₃V₂(PO₄)₂F₃|NaTi₂(PO₄)₃ aqueous Na‐ion full‐cell with the Na‐hydrate‐melt electrolyte delivers an average discharge voltage of 1.75 V, that is among the highest value ever reported for all aqueous Na‐ion batteries.     

A Shuttle-Free Solid-State Cu−Li Battery Based on a Sandwich-Structured Electrolyte

Cu−Li batteries leveraging the two-electron redox property of Cu can offer high energy density and low cost. However, Cu−Li batteries are plagued by limited solubility and a shuttle effect of Cu ions in traditional electrolytes, which leads to low energy density and poor cycling stability. In this work, we rationally design a solid-state sandwich electrolyte for solid-state Cu−Li batteries, in which a deep-eutectic-solvent gel with high Cu-ion solubility is devised as a Cu-ion reservoir while a ceramic Li_1.4Al_0.4Ti_1.6(PO₄)₃ interlayer is used to block Cu-ion crossover. Because of the high ionic conductivity (0.55 mS cm⁻¹ at 25 °C), wide electrochemical window (>4.5 V vs. Li⁺/Li), and high Cu ion solubility of solid-state sandwich electrolyte, a solid-state Cu−Li battery demonstrates a high energy density of 1 485 Wh kg_Cu⁻¹and long-term cyclability with 97 % capacity retention over 120 cycles. The present study lays the groundwork for future research into low-cost solid-state Cu−Li batteries.     

Electrical Properties of NASICON-type Structured Li1.3Al0.3Ti1.7(PO4)3 Solid Electrolyte Prepared by 1,2-Propylene glycol-assisted Sol-gel Method

Lithium-ion conductor Li_1.3Al_0.3Ti_1.7(PO₄)₃ with an ultrapure NASICON-type phase is syn-thesized by a 1,2-propylene glycol (1,2-PG)-assisted sol-gel method and characterized by differential thermal analysis-thermo gravimetric analysis, X-ray diffraction, scanning elec-tron microscopy, electrochemical impedance spectroscopy, and chronoamperometry test.Due to the use of 1,2-PG, a homogeneous and light yellow transparent precursor solu-tion is obtained without the precipitation of Ti⁴⁺ and Al³⁺ with PO₄^3-. Well crystallizedLi_1.3Al_0.3Ti_1.7(PO₄)₃ can be prepared at much lower temperatures from 850 oC to 950 oC within a shorter synthesis time compared with that prepared at a temperature above 1000 oC by a conventional solid-state reaction method. The lithium ionic conductivity of the sintered pellets is up to 0.3 mS/cm at 50 oC with an activation energy as low as 36.6 kJ/mol for the specimen pre-sintered at 700 oC and sintered at 850 oC. The high conductivity, good chemi-cal stability and easy fabrication of the Li_1.3Al_0.3Ti_1.7(PO₄)₃ provide a promising candidate as solid electrolyte for all-solid-state Li-ion rechargeable 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.     

Synthesis and electrochemical properties of Li9V3 − x Ti x (P2O7)3(PO4)2/C compounds via wet method for lithium-ion batteries

A series of Ti⁴⁺-doped Li₉V_3???x Ti _x (P₂O₇)₃(PO₄)₂/C compounds have been prepared by using wet method. X-ray diffraction measurement shows that single phase region can be expressed as x?≤?0.10. The effects of substitution of Ti for V on the electrochemical properties of Li₉V_3???x Ti _x (P₂O₇)₃(PO₄)₂ compounds have been studied. Our investigations show that Ti doping can improve the electrochemical performance. The Li₉V_2.95Ti_0.05(P₂O₇)₃(PO₄)₂/C exhibits the best cycle performance and the highest first discharge capacity of 120.7 mAh g^?1 at 0.2 C. The electrochemical impedance spectroscopy indicates that the charge transfer resistance initially decreases with x and then for x?>?0.05 increases monotonically with Ti⁴⁺ content.     

Designed One-Pot Strategy for Dual-Carbon-Protected Na3V2(PO4)3 Hybrid Structure as High-Rate and Ultrastable Cathode for Sodium-Ion Batteries

Sodium-ion batteries have attracted tremendous attention due to their much lower cost and similar working principle compared with lithium-ion batteries, which have been invited great expectation as energy storage devices in grid-level applications. The sodium superionic conductor Na₃V₂(PO₄)₃ has been considered as a promising cathode candidate; however, its intrinsic low electronic conductivity results in poor rate performance and unsatisfactory cycling performance, which severely impedes its potential for practical applications. Herein, we developed a facile one-pot strategy to construct dual carbon-protected hybrid structure composed of carbon coated Na₃V₂(PO₄)₃ nanoparticles embedded with carbon matrix with excellent rate performance, superior cycling stability and ultralong lifespan. Specifically, it can deliver an outstanding rate performance with a 51.5 % capacity retention from 0.5 to 100 C and extraordinary cycling stability of 80.86 % capacity retention after 6000 cycles at the high rate of 20 C. The possible reasons for the enhanced performance could be understood as the synergistic effects of the strengthened robust structure, facilitated charge transfer kinetics, and the mesoporous nature of the Na₃V₂(PO₄)₃ hybrid structure. This work provides a cost-effective strategy to effectively optimize the electrochemical performance of a Na₃V₂(PO₄)₃ cathode, which could contribute to push forward the advance of its practical applications.     

Single-Ion Conducting Polymer Electrolyte for Superior Sodium-Metal Batteries

Sodium-metal batteries (SMBs) are considered a potential alternative to high-energy lithium-metal batteries (LMBs). However, the high reactivity of metallic sodium towards common liquid organic electrolytes renders such battery technology particularly challenging. Herein, we propose a multi-block single-ion conducting polymer electrolyte (SIPE) doped with ethylene carbonate as suitable electrolyte system for SMBs. This novel SIPE provides a very high ionic conductivity (2.6 mS cm⁻¹) and an electrochemical stability window of about 4.1 V at 40 °C, enabling stable sodium stripping and plating and excellent rate capability of Na||Na₃V₂(PO₄)₃ cells up to 2 C. Remarkably, such cells provide a capacity retention of about 85 % after 1,000 cycles at 0.2 C thanks to the very high Coulombic efficiency (99.9 %), resulting from an excellent interfacial stability towards sodium metal and the Na₃V₂(PO₄)₃ cathode.     

Electrospun Li3V2(PO4)3 Nanobelts: Synthesis and Electrochemical Properties as Cathode Materials of Lithium‐Ion Batteries

Novel Li₃V₂ (PO₄)₃ nanobelts, which was confirmed by the peaks of X‐ray diffraction, were prepared by a facile and environmentally friendly electrospinning method. A distinct nanobelt structure, with an average width of 2.5 µm and a thickness of 200 nm, is observed by scanning electron microscopy (SEM), while the specific surface area of 140.8 m²/g is estimated by a specific surface area analyzer. Moreover, the unique Li₃V₂(PO₄)₃ nanobelts exhibited a specific discharge capacity of 155.6 mAh/g at 0.2 C rate when they were used as cathode material in lithium‐ion batteries, on testing from 3.0 to 4.8 V. Remarkably, the batteries containing Li₃V₂(PO₄)₃ nanobelts displayed excellent cycling performance, with only a 0.02% fading rate per cycle after 50 cycles in the range 30–4.3 V. These outstanding electrochemical performances could be ascribed to the particular morphology, large surface area, homogeneous particle size distribution, and the one‐dimensional microstructure of Li₃V₂(PO₄)₃ nanobelts.     

An Aqueous Symmetric Sodium‐Ion Battery with NASICON‐Structured Na3MnTi(PO4)3

A symmetric sodium‐ion battery with an aqueous electrolyte is demonstrated; it utilizes the NASICON‐structured Na₃MnTi(PO₄)₃ as both the anode and the cathode. The NASICON‐structured Na₃MnTi(PO₄)₃ possesses two electrochemically active transition metals with the redox couples of Ti⁴⁺/Ti³⁺ and Mn³⁺/Mn²⁺ working on the anode and cathode sides, respectively. The symmetric cell based on this bipolar electrode material exhibits a well‐defined voltage plateau centered at about 1.4 V in an aqueous electrolyte with a stable cycle performance and superior rate capability. The advent of aqueous symmetric sodium‐ion battery with high safety and low cost may provide a solution for large‐scale stationary energy storage.     

High-Voltage Polyanion Positive Electrode Materials

High-voltage generation (over 4 V versus Li⁺/Li) of polyanion-positive electrode materials is usually achieved by Ni³⁺/Ni²⁺, Co³⁺/Co²⁺, or V⁴⁺/V³⁺ redox couples, all of which, however, encounter cost and toxicity issues. In this short review, our recent efforts to utilize alternative abundant and less toxic Fe³⁺/Fe²⁺ and Cr⁴⁺/Cr³⁺ redox couples are summarized. Most successful examples are alluaudite Na₂Fe₂(SO₄)₃ (3.8 V versus sodium and hence 4.1 V versus lithium) and β₁-Na₃Al₂(PO₄)₂F₃-type Na₃Cr₂(PO₄)₂F₃ (4.7 V versus sodium and hence 5.0 V versus lithium), where maximizing ΔG by edge-sharing Fe³⁺-Fe³⁺ Coulombic repulsion and the use of the 3d²/3d³ configuration of Cr⁴⁺/Cr³⁺ are essential for each case. Possible exploration of new high-voltage cathode materials is also discussed.     

Carbon coated Na3V2(PO4)3 as novel electrode material for sodium ion batteries

A Na₃V₂(PO₄)₃ sample coated uniformly with a layer of 6 nm carbon has been successfully synthesized by a one-step solid state reaction. This material shows two flat voltage plateaus at 3.4 V vs. Na⁺/Na and 1.63 V vs. Na⁺/Na in a nonaqueous sodium cell. When the Na₃V₂(PO₄)₃/C sample is tested as a cathode in a voltage range of 2.7-3.8 V vs. Na⁺/Na, its initial charge and discharge capacities are 98.6 and 93 mAh/g. The capacity retention of 99% can be achieved after 10 cycles. The electrode shows good cycle performance and moderate rate performance. When it is tested as an anode in a voltage range of 1.0-3.0 V vs. Na⁺/Na, the initial reversible capacity is 66.3 mAh/g and the capacity of 59 mAh/g can be maintained after 50 cycles. These preliminary results indicate that Na₃V₂(PO₄)₃/C is a new promising material for sodium ion batteries.     

A new high voltage alluaudite sodium battery insertion material

Large-scale stationary storage forms a key sector that can be economically served by sodium-ion batteries. In realizing practical sodium-ion batteries, discovery and development of novel cathodes is essential. In this spirit, alluaudite-type Na₂Fe₂(SO₄)₃ was reported in 2014 to have the highest Fe³⁺/Fe²⁺ redox potential (～3.8 V vs. Na). This finding led to reports on various PO₄^3? and SO₄^2? based alluaudite compounds exhibiting high energy densities. In 2017, MoO₄^2? based alluaudite, Na_2.67Mn_1.67(MoO₄)₃, was found as a 3.45 V cathode material. Exploring molybdenum chemistry further, this work reports alluaudite type Na_3.36Co_1.32(MoO₄)₃ (NCMo) as a novel versatile electroactive cathode for Li-ion and Na-ion batteries. It was synthesized by a wet solution-combustion route with a restricted annealing duration of 1 min at 600 °C. Calorimetric study revealed the formation enthalpy from component oxides (ΔH°_f,ox = ?575.49 ± 7.75 kJ/mol) to be highly exothermic. Unlike the sulfate class of alluaudites, this material is highly stable in air and moisture (ΔH_ds = 537.42 ± 0.78 kJ/mol). Having an ionic conductivity of 6.065 × 10^?8 S/cm (at 50 °C), it offers a pseudo two-dimensional Na⁺ migration pathway. Without any material optimization, NCMo was found to work as a high-voltage insertion cathode (ca. 4.0 V vs. Na/Na⁺ and 4.1 V vs. Li/Li⁺) in sync with theoretically predicted potential of 3.98 V (vs. Na/Na⁺). Ex-situ X-ray diffraction and photoelectron spectroscopy studies revealed the occurrence of solid-solution redox mechanism solely involving Co³⁺/Co²⁺ redox centre. It benchmarks Na_3.36Co_1.32(MoO₄)₃ as a novel electrochemically active Mo-based alluaudite-type polyanionic cathode insertion material.     

Vanadium–Manganese Redox Flow Battery: Study of MnIII Disproportionation in the Presence of Other Metallic Ions

The Mn^III/Mn^II redox couple with a standard potential of +1.51 V versus the standard hydrogen electrode (SHE) has attracted interest for the design of V/Mn redox flow batteries (RFBs). However, Mn^III disproportionation leads to a loss of capacity, an increase in pressure drop, and electrode passivation caused by the formation of MnO₂ particles during battery cycling. In this work, the influence of Ti^IV or/and V^V on Mn^III stability in acidic conditions is studied by formulating four different electrolytes in equimolar ratios (Mn, Mn/Ti, Mn/V, Mn/V/Ti). Voltammetry studies have revealed an EC_i process for Mn^II oxidation responsible for the electrode passivation. SEM and XPS analysis demonstrate that the nature and morphology of the passivating oxides layer depend strongly on the electrolyte composition. Spectroelectrochemistry highlights the stabilization effect of Ti^IV and V^V on Mn^III. At a comparable pH, the amount of Mn^III loss through disproportionation is decreased by a factor of 2.5 in the presence of Ti^IV or/and V^V. Therefore, V^V is an efficient substitute for Ti^IV to stabilize the Mn^III electrolyte for RFB applications.     

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.     

A Simple Prelithiation Strategy To Build a High‐Rate and Long‐Life Lithium‐Ion Battery with Improved Low‐Temperature Performance

Lithium‐ion batteries (LIBs) are being used to power the commercial electric vehicles (EVs). However, the charge/discharge rate and life of current LIBs still cannot satisfy the further development of EVs. Furthermore, the poor low‐temperature performance of LIBs limits their application in cold climates and high altitude areas. Herein, a simple prelithiation method is developed to fabricate a new LIB. In this strategy, a Li₃V₂(PO₄)₃ cathode and a pristine hard carbon anode are used to form a primary cell, and the initial Li⁺ extraction from Li₃V₂(PO₄)₃ is used to prelithiate the hard carbon. Then, the self‐formed Li₂V₂(PO₄)₃ cathode and prelithiated hard carbon anode are used to form a 4 V LIB. The LIB exhibits a maximum energy density of 208.3 Wh kg⁻¹, a maximum power density of 8291 W kg⁻¹ and a long life of 2000 cycles. When operated at −40 °C, the LIB can keep 67 % capacity of room temperature, which is much better than conventional LIBs.