Engineering Crystal Growth and Surface Modification of Na3 V2 (PO4 )2 F3 Cathode for High-Energy-Density Sodium-Ion Batteries

Na₃ V₂ (PO₄ )₂ F₃ (NVPF) is a suitable cathode for sodium-ion batteries owing to its stable structure. However, the large radius of Na⁺ restricts diffusion kinetics during charging and discharging. Thus, in this study, a phosphomolybdic acid (PMA)-assisted hydrothermal method is proposed. In the hydrothermal process, the NVPF morphologies vary from bulk to cuboid with varying PMA contents. The optimal channel for accelerated Na⁺ transmission is obtained by cuboid NVPF. With nitrogen-doping of carbon, the conductivity of NVPF is further enhanced. Combined with crystal growth engineering and surface modification, the optimal nitrogen-doped carbon-covered NVPF cuboid (c-NVPF@NC) exhibits a high initial discharge capacity of 121 mAh g^-1 at 0.2 C. Coupled with a commercial hard carbon (CHC) anode, the c-NVPF@NC||CHC full battery delivers 118 mAh g^-1 at 0.2 C, thereby achieving a high energy density of 450 Wh kg^-1 . Therefore, this work provides a novel strategy for boosting electrochemical performance by crystal growth engineering and surface modification.

Ionic Liquid-Acrylic Acid Copolymer Derived Nitrogen-Boron Codoped Carbon-Covered Na3V2(PO4)2F3 as Cathode Material of High-Performance Sodium-Ion Batteries

In this study, a nitrogen-boron codoped carbon layer, Na₃V₂(PO₄)₂F₃ sample, obtained by using an ionic liquid-acrylic acid copolymer as the nitrogen-boron source was used as the cathode material for sodium-ion batteries. The optimized and modified nitrogen and boron codoped carbon layer, Na₃V₂(PO₄)₂F₃ (denoted as NVPF-PCNB-20), illustrated better rate capability and cycling performance. The discharge capacities of NVPF-PCNB-20 at 0.5C and 10C were 109 and 90 mAh g^-1, respectively, and the capacity retention rate was 93.2% after 100 cycles at 0.5C and 92.8% after 750 cycles at 10C. Through in situ X-ray diffraction analysis of NVPF-PCNB-20, the results show that the modified Na₃V₂(PO₄)₂F₃ has excellent cycle reversibility. The scanning electron microscopy and transmission electron microscopy images reveal that NVPF-PCNB-20 particles were finer and covered by a uniform coating. The results show that the ionic liquid-acrylic acid copolymer not only make the material dispersion more uniform but also enhance the electronic conductivity and sodium storage performance of Na₃V₂(PO₄)₃F₃ effectively. This study may provide an effective way to synthesize nitrogen and boron codoped carbon-coated Na₃V₂(PO₄)₂F₃ with excellent electrochemical performance.

Realizing outstanding electrochemical performance with Na3V2(PO4)2F3 modified with an ionic liquid for sodium-ion batteries

Na₃V₂(PO₄)₂F₃ is a typical NASICON structure with a high voltage plateau and capacity. Nevertheless, its applications are limited due to its low conductivity and poor rate performance. In this study, nitrogen-boron co-doped carbon-coated Na₃V₂(PO₄)₂F₃ (NVPF-CNB) was prepared by a simple sol-gel method using an ionic liquid (1-vinyl-3-methyl imidazole tetrafluoroborate) as a source of nitrogen and boron for the first time. The morphology and electrochemical properties of NVPF-CNB composites were investigated. The results show that a nitrogen-boron co-doped carbon layer could increase the electron and ion diffusion rate, reduce internal resistance, and help alleviate particle agglomeration. NVPF-CNB-30 exhibited better rate performance under 5C and 10C charge/discharge with initial reversible capacities of 99 and 90 mA h g^-1, respectively. Furthermore, NVPF-CNB-30 illustrates excellent cyclic performance with the capacity retention rate reaching 91.9% after 500 cycles at 5C, as well as a capacity retention rate of about 95.5% after 730 cycles at 10C. The evolution of the material's structure during charge/discharge processes studied by in situ X-ray diffraction confirms the stable structure of nitrogen-boron co-doped carbon-coated Na₃V₂(PO₄)₂F₃. Co-doping of nitrogen and boron also provides more active sites on the surface of Na₃V₂(PO₄)₂F₃, revealing a new strategy for the modification of sodium-ion batteries.

Rechargeable Na/Cl2 and Li/Cl2 batteries

Lithium-ion batteries (LIBs) are widely used in applications ranging from electric vehicles to wearable devices. Before the invention of secondary LIBs, the primary lithium-thionyl chloride (Li-SOCl₂) battery was developed in the 1970s using SOCl₂ as the catholyte, lithium metal as the anode and amorphous carbon as the cathode^1-7. This battery discharges by lithium oxidation and catholyte reduction to sulfur, sulfur dioxide and lithium chloride, is well known for its high energy density and is widely used in real-world applications; however, it has not been made rechargeable since its invention^8-13. Here we show that with a highly microporous carbon positive electrode, a starting electrolyte composed of aluminium chloride in SOCl₂ with fluoride-based additives, and either sodium or lithium as the negative electrode, we can produce a rechargeable Na/Cl₂ or Li/Cl₂ battery operating via redox between mainly Cl₂/Cl^- in the micropores of carbon and Na/Na⁺ or Li/Li⁺ redox on the sodium or lithium metal. The reversible Cl₂/NaCl or Cl₂/LiCl redox in the microporous carbon affords rechargeability at the positive electrode side and the thin alkali-fluoride-doped alkali-chloride solid electrolyte interface stabilizes the negative electrode, both are critical to secondary alkali-metal/Cl₂ batteries.