Hollow-Sphere-Structured Na4Fe3(PO4)2(P2O7)/C as a Cathode Material for Sodium-Ion Batteries

Photogenerated Holes Induced Deep Sodium Storage of TiO2/CdSe/NFPP Cathode for High-Efficiency Photorechargeable Sodium Batteries

Resource-friendly photorechargeable sodium batteries (PRSBs) integrate energy storage devices with solar cells, offering a promising path for sustainable energy. Herein, a novel TiO2/CdSe/Na3Fe2(PO4)P2O7 (NFPP) cathode was prepared layer-by-layer utilizing resource-abundant commercialized NFPP and photoactive CdSe. The aligned energy levels with type II band structure ensure effective transfer of photogenerated holes from CdSe (-5.71 eV) to higher valence band of NFPP (-5.10 eV). Experimental results reveal that, during charging, the induced holes in NFPP accelerate the transition of Fe2+ to Fe3+ with a change of O-Fe hybrid orbitals. The calculations of bond valence sum and energy distribution reveal that NFPP-holes possesses broad Na+ transport path with reduced transport barrier (from 0.512 to 0.428 eV), improving Na+ extraction efficiency. Additionally, photogenerated holes could regulate surface charge distribution on NFPP and thus form a film-forming agent fluoroethylene carbonate (FEC)-dominated electric double layer. Finally, it converts to a thinner (9.75 nm illumination) NaF-rich cathode interphase layer, avoiding subsequent excessive electrolyte decomposition. As a result, the NFPP under illumination delivers high capacity of 119.1 mAh g-1 at 1 C, showing 41.11% improvement compared to dark conditions.

Spherical Mg/Cu Co-Doped Na4Fe3(PO4)2P2O7 Cathode Materials with Mitigated Diffusion-Induced Stresses and Enhanced Cyclic Stability

Na4Fe3(PO4)2P2O7 (NFPP) has been regarded as the promising cathode material for sodium-ion batteries (SIBs). However, the practical applications of NFPP are hindered by its high-volume changes, poor intrinsic electron conductivity and sluggish Na+ ions diffusion kinetics. Herein, a spray-drying and solid-state reaction method have been utilized to fabricate the spherical trace amount Mg/Cu co-doped Na4Fe3(PO4)2P2O7 (NFMCPP). The Mg/Cu co-doping can effectively mitigate the lattice volume change and promote the electronic conductivity of NFMCPP by reducing band gap between the conduction and valence bands. While, the unique spherical structured NFMCPP with a carbon film even coated on its surface ensures rapid electron transport. Moreover, small NFMCPP particles with spherical geometry demonstrate an alleviated diffusion-induced stress and enhanced structural stability, due to the high sphericity structure enables fluent Na+ extraction/insertion, leads to a low high-stress concentration and uniform stress/strain distribution during extensive (de)sodiation process. Consequently, the optimized spherical NFMCPP cathode materials exhibit an excellent rate capability and cyclic stability.

Vacancy and Low-Energy 3p-Orbital Endow Na4Fe3(PO4)2(P2O7) Cathode with Superior Sodium Storage Kinetics

Iron-based phosphate Na4Fe3(PO4)2(P2O7) (NFPP) has been regarded as the most promising cathode for sodium-ion batteries (SIBs) thanks to its cost-effectiveness and eco-friendliness. However, it is in a predicament from the intrinsic low ionic/electronic conductivity, becoming a great challenge for its practical application. Herein, the significant roles of the low-energy 3p-orbital and transition metal vacancies are emphasized in facilitating charge rearrangement and reconstructing ion-diffusion channels, from the perspectives of crystallography and electron interaction for the first time, and the modification mechanism is fully explored by various characterizations and theoretical calculations. As proof of this concept, the designed Na4Fe2.85Al0.1(PO4)2(P2O7) (NF2.85A0.1PP) delivers prominent electrochemical performance, achieving high energy density (≈350 Wh kg⁻¹), superior kinetics (62 mAh g⁻¹ at 10 A g⁻¹), excellent power density (23 kW kg⁻¹, 143 Wh kg⁻¹), and extraordinary cycling stability (with negligible attenuation after 10 000 cycles). This work provides a brand-new perspective for designing ultra-endurable high-rate polyanion cathodes.

Hollow Core-Shelled Na4Fe2.4Ni0.6(PO4)2P2O7 with Tiny-Void Space Capable Fast-Charge and Low-Temperature Sodium Storage

Iron-based mixed polyanion phosphate Na4Fe3(PO4)2P2O7 (NFPP) is recognized as a promising cathode for Sodium-ion Batteries (SIBs) due to its low cost and environmental friendliness. However, its inherent low conductivity and sluggish Na+ diffusion limit fast charge and low-temperature sodium storage. This study pioneers a scalable synthesis of hollow core-shelled Na4Fe2.4Ni0.6(PO4)2P2O7 with tiny-void space (THoCS-0.6Ni) via a one-step spray-drying combined with calcination process due to the different viscosity, coordination ability, molar ratios, and shrinkage rates between citric acid and polyvinylpyrrolidone. This unique structure with interconnected carbon networks ensures rapid electron transport and fast Na+ diffusion, as well as efficient space utilization for relieving volume expansion. Incorporating regulation of lattice structure by doping Ni heteroatom to effectively improve intrinsic electron conductivity and optimize Na+ diffusion path and energy barrier, which achieves fast charge and low-temperature sodium storage. As a result, THoCS-0.6Ni exhibits superior rate capability (86.4 mAh g-1 at 25 C). Notably, THoCS-0.6Ni demonstrates exceptional cycling stability at -20 °C with a capacity of 43.6 mAh g-1 after 2500 cycles at 5 C. This work provides a universal strategy to design the hollow core-shelled structure with tiny-void space cathode materials for reversible batteries with fast-charge and low-temperature Na-storage features.

Demystifying In Situ Pyrolysis Chemistry for High-Performance Polyanionic Cathodes in Sodium-Ion Batteries

The carbon coating strategy has emerged as an indispensable approach to improve the conductivity of polyanionic cathodes. However, owing to the complex reaction process between precursors of carbon and cathode, establishing a unified screening principle for carbonaceous precursors remains a technical challenge. Herein, we reveal that carbonaceous precursor pyrolysis chemistry undeniably influences the formation process and performance of Na3V2(PO4)3 (NVP) cathodes from in situ insights. By investigating three types of carbonaceous precursors, it is found that O/H-containing functional groups can provide more bonding sites for cathode precursors and generate a reducing atmosphere by pyrolysis, which is beneficial to the formation of polyanionic materials and a uniform carbon coating layer. Conversely, excessive pyrolysis of functional groups leads to a significant amount of gas, which is detrimental to the compactness of the carbon layer. Furthermore, the substantial presence of residual heteroatoms diminishes graphitization. In this case, it is demonstrated that carbon dots (CDs) precursors with suitable functional groups can comprehensively enhance the Na+ migration rate, reversibility, and interface stability of the cathode material. As a result, the NVP/CDs cathode displays outstanding capacity retention, maintaining 92% after 10,000 cycles at a high rate of 50 C. Altogether, these findings provide a valuable benchmark for carbon source selection for polyanionic cathodes.

Solid-State Synthesis of Na4Fe3(PO4)2P2O7/C by Ti-Doping with Promoted Structural Reversibility for Long-Cycling Sodium-Ion Batteries

The cathode material Na4Fe3(PO4)2P2O7 (NFPP) has shown great potential for sodium-ion batteries (SIBs) due to its cost-effectiveness, prolonged cycle life, and high theoretical capacity. However, the practical large-scale production of NFPP is hindered by its poor intrinsic electron conductivity and the presence of a NaFePO4 impurity. In this study, we propose a mutually reinforcing approach involving Ti doping, mechanical nano treatment, and in situ carbon coating to produce Ti-NFPP via the solid-state methods of synthesis. Ti doping strengthens the covalent Fe-O interaction, hence accelerating the electron transfer and the redox reactions Fe2+/Fe3+. In situ carbon coating improves electrical conductivity and allows for accommodating the volumetric variation. Nanosized treatment promotes the uniform progression of solid-state reactions. The synthesized Na4Fe2.98Ti0.01(PO4)2P2O7 material (Ti-NFPP) exhibits promising electrochemical properties with an initial discharge specific capacity of 112.5 mA h g-1 at 0.1 C. A volumetric change of only 2.98% was observed during the de/sodiation process, indicating an enhanced reversibility of the crystal lattice. Moreover, it demonstrates exceptional cycling stability with a capacity retention rate of 97.2 mA h g-1 at 10 C over 5000 cycles. These findings offer a promising pathway for the large-scale production of Ti-NFPP in SIBs.

Progress and perspectives on iron-based electrode materials for alkali metal-ion batteries: a critical review

Iron-based materials with significant physicochemical properties, including high theoretical capacity, low cost and mechanical and thermal stability, have attracted research attention as electrode materials for alkali metal-ion batteries (AMIBs). However, practical implementation of some iron-based materials is impeded by their poor conductivity, large volume change, and irreversible phase transition during electrochemical reactions. In this review we critically assess advances in the chemical synthesis and structural design, together with modification strategies, of iron-based compounds for AMIBs, to obviate these issues. We assess and categorize structural and compositional regulation and its effects on the working mechanisms and electrochemical performances of AMIBs. We establish insight into their applications and determine practical challenges in their development. We provide perspectives on future directions and likely outcomes. We conclude that for boosted electrochemical performance there is a need for better design of structures and compositions to increase ionic/electronic conductivity and the contact area between active materials and electrolytes and to obviate the large volume change and low conductivity. Findings will be of interest and benefit to researchers and manufacturers for sustainable development of advanced rechargeable ion batteries using iron-based electrode materials.

Three-Dimensional Holey Graphene Modified Na4 Fe3 (PO4 )2 (P2 O7 )/C as a High-Performance Cathode for Rechargeable Sodium-Ion Batteries

Polyanion-type Na₄ Fe₃ (PO₄ )₂ (P₂ O₇ ) (NFPP) is a promising cathode material for sodium-ion batteries due to its low cost and high safety. Herein, a three-dimensional (3D) holey graphene (HG) modified NFPP/C material (NFPPCHG) has been successfully prepared by a simple and scalable ball milling strategy with sodium phytate and ferrous oxalate as precursors. The introduction of HG can obviously improve the specific surface area, electronic conductivity, and ions transport performance of NFPPCHG and largely enhance its electrochemical properties. The prepared NFPPCHG delivers a high reversible capacity of 118 mAh g^-1 at 0.2 C and keeps a considerable capacity of 53 mAh g^-1 even at an ultrahigh rate of 100 C. NFPPCHG also shows excellent performance at 55 °C and -20 °C. Moreover, in situ distribution of relaxation time analysis further demonstrates NFPPCHG has superior electrochemical kinetics. In addition, the HC//NFPPCHG full cell displays good performance, suggesting great potential of the prepared material for practical applications.

Co2P-Assisted Atomic Co-N4 Active Sites with a Tailored Electronic Structure Enabling Efficient ORR/OER for Rechargeable Zn-Air Batteries

Oxygen reduction and evolution reactions (ORR and OER, respectively) are vital steps for metal-air batteries, which are plagued by their sluggish kinetics. It is still a challenge to develop highly effective and low-cost non-noble-metal-based electrocatalysts. Herein, a simple and reliable method was reported to synthesize a Co2P-assisted Co single-atom (Co-N4 centers) electrocatalyst (Co2P/Co-NC) via evaporative drying and pyrolysis processes. The Co2P nanoparticles and Co-N4 centers are uniformly distributed on the nitrogen-doped carbon matrix. Notably, Co2P/Co-NC showed excellent activities in both ORR (initial potential, 1.01 V; half-wave potential, 0.88 V) and OER (overpotential, 369 mV at 10 mA cm-2). The above results were comparable to those of commercial catalysts (such as Pt/C and RuO2). Based on the experimental and theoretical analyses, the impressive activity can be ascribed to the tailored electronic structure of Co-N4 centers by the adjacent Co2P, enabling the electron transfer from the Co atom to the neighboring C atoms, leading to a downshift of the d-band center, and improved reaction kinetics were achieved. The assembled Zn-air batteries using Co2P/Co-NC as the air cathode showed a peak power density of 187 mW cm-2 and long-life cycling stability for 140 h at 5 mA cm-2. This work may pave a promising avenue to design hybrid bifunctional electrocatalysts for highly efficient ORR/OER.

Active-Site-Specific Structural Engineering Enabled Ultrahigh Rate Performance of the NaLi3Fe3(PO4)2(P2O7) Cathode for Lithium-Ion Batteries

Iron-based mixed-polyanionic cathode Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) has advantages of environmental benignity, easy synthesis, high theoretical capacity, and remarkable stability. From NFPP, a novel Li-replaced material NaLi₃Fe₃(PO₄)₂(P₂O₇) (NLFPP) is synthesized through active Na-site structural engineering by an electrochemical ion exchange approach. The NLFPP cathode can show high reversible capacities of 103.2 and 90.3 mA h g^-1 at 0.5 and 5C, respectively. It also displays an impressive discharge capacity of 81.5 mA h g^-1 at an ultrahigh rate of 30C. Density functional theory (DFT) calculation demonstrates that the formation energy of NLFPP is the lowest among NLFPP, NFPP, and NaFe₃(PO₄)₂(P₂O₇), indicating that NLFPP is the easiest to form and the conversion from NFPP to NLFPP is thermodynamically favorable. The Li substitution for Na in the NFPP lattice causes an increase in the unit cell parameter c and decreases in a, b, and V, which are revealed by both DFT calculations and in situ X-ray powder diffraction (XRD) analysis. With hard carbon (HC) as the anode, the NLFPP//HC full cell shows a high reversible capacity of 91.1 mA h g^-1 at 2C and retains 82.4% after 200 cycles. The proposed active-site-specific structural tailoring via electrochemical ion exchange will give new insights into the design of high-performance cathodes for lithium-ion batteries.