Spherical Shell with CNTs Network Structuring Fe-Based Alluaudite Na2+2 δ Fe2- δ (SO4 )3 Cathode and Novel Phase Transition Mechanism for Sodium-Ion Battery

Fast Screening Suitable Doping Transition Metals to Na3V2(PO4)2F3 for Sodium-Ion Batteries with High Energy Density in Wide-Temperature Range

Screening the suitable doping elements for Na3V2(PO4)2F3 (NVPF) through the traditional trial-and-error method to enhance its intrinsic electronic conductivity and electrochemical performance is a time-exhausted task. Here, a new strategy of theoretical prediction-assisted chemical synthesis is proposed to fast filter the suitable doping elements to NVPF by first calculating the band gaps of various transition metals doped NVPF and then verifying by the experimental results. Single crystal NVPF-M (Na3V1.85M0.15(PO4)2F3, M = Ru, Fe, Ni, Ti, and Cd, etc.) materials are synthesized to compare their electrochemical performances. Excellent cycling performance (2000 cycles with high Coulombic efficiencies), remarkable rate capacity (20 C), and wide-temperature range (-30-60 °C) application capability are witnessed in the NVPF-Ru/Fe cathodes in both half and full cells. In situ X-ray diffraction patterns have confirmed that they followed the consisting of multi-phase reactions (Na3 ↔ Na2.4 ↔ Na2.2 ↔ Na1) and a solid-solution reaction (Na1.8 ↔ Na1.3) with small changes of lattice volume and strains. Compromising the cost and performance, the NVPF-Fe cathode is regarded as the optimized cathode for sodium-ion batteries with a high energy density and wide temperature application features.

Unlocking Phase Purity of Sodium Iron Sulfate for Low-Cost and High-Performance Sodium-Ion Batteries

The alluaudite-type sulfate Na2Fe2(SO4)3 has gained significant attention as a promising cathode material for sodium-ion batteries (SIBs). However, the inevitable formation of impurities during synthesis and the irreversible structural distortion caused by Fe-Na exchange during electrochemical reactions severely hinder its electrochemical performance. Herein, we tackle these challenges by engineering an enlarged Fe-Fe distance in the lattice through partial PO43- substitution. This strategic modification significantly alleviates the Coulombic repulsion between Fe ions and effectively prevents Fe-migration during the electrochemical reaction. Moreover, the unique ion state within the structure ensures enhanced ion/electron transport kinetics, minimal volume change, and a stable framework conducive to long cycling life. Notably, the novel Fe-fully occupied phase-pure Na2.5Fe2(SO4)2.5(PO4)0.5 [also denoted as Na5Fe4(SO4)5(PO4)] electrode delivers a record-high discharge capacity of 112 mA h g-1 at 0.2C, coupled with exceptional cycling stability with 88.8% capacity retention over 10,000 cycles at 10C. Additionally, the enhanced adsorption energy of Na2.5Fe2(SO4)2.5(PO4)0.5 cathode toward H2O contributes to its outstanding air stability in humid atmosphere. This finding offers valuable insights for the development of advanced, low-cost materials for SIBs.

Colloidal Synthesis of Na2Fe2(SO4)3 Nanocrystals as the Cathode Toward High-Rate Capability and High-Energy Density Sodium-ion Batteries

Alluaudite-type Na2+2xFe2-x(SO4)3 (NFS) with high theoretical energy density is regarded as the promising cathode of sodium-ion batteries (SIBs), while practical rate and cyclic performances are still hindered by intrinsic poor conductivity. Here, a facile method is developed, collaborating high-boiling organic solvents assisted colloidal synthesis (HOS-CS) with sintering for tailoring Na2Fe2(SO4)3 nanocrystals decorated by conductive carbon network toward high-rate-capability cathode of SIBs. Impressively, the as-prepared Na2Fe2(SO4)3@MC provides 60.6 and 46.9 mAh g-1 of reversible capacities even at ultrahigh rates of 20 and 30 C, respectively, ranking the superior state among the current NFS-based cathode. More importantly, Na2Fe2(SO4)3@MC achieves 73% of capacity retention at 20 C after 500 cycles, highlighting its potential for application as a fast chargeable cathode. As a bonus, the full-cell configuration constructed with Na2Fe2(SO4)3@MC cathode and commercial hard carbon (HC) anode delivers 45.6 mAh g-1 at 10 C and 68.3 mAh g-1 of initial capacity with ≈79.4% of retention after 100 cycles at 2 C. Also, Na2Fe2(SO4)3@MC||HC full cell supplies as high as 140 Wh kg-1 of practical energy density. This work offers a novel approach to prepare NFS cathode for SIBs with both high energy density and fast-charging ability.

Uncovering the Nonequilibrium Evolution Mechanism between Na2+2 δFe2- δ(SO4)3 Cathode and Impurities in the Na2SO4-FeSO4·7H2O Binary System for High-Voltage Sodium-Ion Batteries

Sodium-ion batteries are increasingly recognized as ideal for large-scale energy storage applications. Alluaudite Na2+2 δFe2- δ(SO4)3 has become one of the focused cathode materials in this field. However, previous studies employing aqueous-solution synthesis often overlooked the formation mechanism of the impurity phase. In this study, the nonequilibrium evolution mechanism between Na2+2 δFe2- δ(SO4)3 and impurities by adjusting ratios of the Na2SO4/FeSO4·7H2O in the binary system is investigated. Then an optimal ratio of 0.765 with reduced impurity content is confirmed. Compared to the poor electrochemical performance of the Na2.6Fe1.7(SO4)3 (0.765) cathode, the optimized Na2.6Fe1.7(SO4)3@CNTs (0.765@CNTs) cathode, with improved electronic and ionic conductivity, demonstrates an impressive discharge specific capacity of 93.8 mAh g-1 at 0.1 C and a high-rate capacity of 67.84 mAh g-1 at 20 C, maintaining capacity retention of 71.1% after 3000 cycles at 10 C. The Na2.6Fe1.7(SO4)3@CNTs//HC full cell reaches an unprecedented working potential of 3.71 V at 0.1 C, and a remarkable mass-energy density exceeding 320 Wh kg-1. This work not only provides comprehensive guidance for synthesizing high-voltage Na2+2 δFe2- δ(SO4)3 cathode materials with controllable impurity content but also lays the groundwork of sodium-ion batteries for large-scale energy storage applications.

Synthesis of a Sodium-Ion Cathode Material Na3Fe2(SO4)3F with the Help of Fluorine Element

The development of cathode materials has always been one of the most crucial areas of research in the field of sodium-ion batteries. Sulfate-based polyanionic materials, known for their high working voltage characteristics, have received widespread attention. In this work, a fluoro-sulfate sodium-ion battery cathode material, Na3Fe2(SO4)3F modified with carbon nanotubes, was developed using a low-temperature solid-state annealing method. This Na3Fe2(SO4)3F cathode exhibits an exceptionally high voltage of 3.77 V, excellent discharge capacity (102 mAh/g at 0.1C), and good rate capability. This material broadens the research directions for cathode materials and holds promise as a foundation for the further development of high-performance sodium-ion batteries.

N-Containing Na2VTi(PO4)3/C for Aqueous Sodium-Ion Batteries

Phosphates featuring a 3D framework offer a promising alternative to aqueous sodium-ion batteries, known for their safety, cost-effectiveness, scalability, high power density, and tolerance to mishandling. Nevertheless, they often suffer from poor reversible capacity stemming from limited redox couples. Herein, N-containing Na2VTi(PO4)3 is synthesized for aqueous sodium-ion storage through multi-electron redox reactions. It demonstrates a capacity of 155.2 mAh g-1 at 1 A g-1 (≈ 5.3 C) and delivers an ultrahigh specific energy of 55.9 Wh kg-1 in a symmetric aqueous sodium-ion battery. The results from in situ X-ray diffraction analysis, ex situ X-ray photoelectron spectroscopy analysis, and first-principle calculations provide insights into the local chemical environment of sodium ions, the mechanisms underlying capacity decay during cycling, and the dynamics of ion and electron transfer at various states of charge. This understanding will contribute to the advancement of electrode materials for aqueous sodium-ion batteries.