Achieving High Rate and Long Cycle Performance of Na₂FePO₄F Cathode Through Co‐Modification of Ti Doping and Carbon Coating

Abstract

Layered Na2FePO4F (NFPF) cathode material has received widespread attention due to its green nontoxicity, abundant raw materials, and low cost. However, its poor inherent electronic conductivity and sluggish sodium ion transportation seriously impede its capacity delivery and cycling stability. In this work, NFPF by Ti doping and conformal carbon layer coating via solid‐state reaction is modified. The results of experimental study and density functional theory calculations reveal that Ti doping enhances intrinsic conductivity, accelerates Na‐ion transport, and generates more Na‐ion storage sites, and pyrolytic carbon from polyvinylpyrrolidone (PVP) uniformly coated on the NFPF surface improves the surface/interface conductivity and suppresses the side reactions. Under the combined effect of Ti doping and carbon coating, the optimized NFPF (marked as 5T‐NF@C) exhibits excellent electrochemical performance, with a high capacity of 108.4 mAh g−1 at 0.2C, a considerable capacity of 80.0 mAh g−1 even at high current density of 10C, and a high capacity retention rate of 81.8% after 2000 cycles at 10C. When assembled into a full cell with a hard carbon anode, 5T‐NF@C also show good applicability. This work indicates that co‐modification of Ti doping and carbon coating makes NFPF achieve high rate and long cycle performance for sodium‐ion batteries.

Electronic/Ionic Dual Functional Layer-Coated Na3V2(PO4)2F3 Cathode with High Sodium Storage Performance

“…Therefore, the selection and modification of electrode materials for SIBs can refer to those for LIBs.. As known, cathode material is the critical component that affects battery performance. Thus, many attempts have been made to find suitable cathodes for Na + storage, including transition-metal oxides, polyanionic compounds, Prussian blue analogues, and organic compounds. − Among them, polyanionic cathode materials have received widespread attention due to their stable structure and high safety. Especially, NASICON-structured Na 3 V 2 (PO 4 ) 2 F 3 (NVPF) is a promising candidate because of its large theoretical capacity (∼128 mAh g –1 ) and high average working voltage (∼3.8 V), , which is more advantageous than other polyanionic compounds. − NVPF consists of [PO 4 ] tetrahedrons and [V 2 O 8 F 3 ] double octahedrons, and its open and stable three-dimensional channel structure is conducive to rapid extraction/intercalation of Na + . , Nevertheless, the low intrinsic electronic conductivity of NVPF results in poor capacity performance, thus limiting its application. , Besides, the inevitable side reactions between NVPF and the electrolyte will reduce the cycle performance .…”

Achieving High-Performance Na3V2(PO4)2F3 Cathode Material through a Bifunctional N-Doped Carbon Network

“…In recent years, due to the limited and uneven distribution of lithium resources, the price of lithium salts fluctuates greatly, which has brought adverse effects on the development of lithium-ion batteries (LIBs). , As a complementary technology to LIBs, sodium-ion batteries (SIBs) are increasingly valued due to their rich raw materials, good safety, excellent low-temperature performance, and fast charging characteristics. − Cathode material is a critical component that affects the working voltage and energy density of SIBs. Significant efforts have been made in the modification of cathode materials for SIBs, including transition-metal oxides, polyanionic compounds, Prussian blue analogs, and organic compounds. − Among them, NASICON-type Na 3 V 2 (PO 4 ) 2 F 3 (NVPF), as a typical polyanionic compound, is a promising cathode material for SIBs owing to its high working voltage (∼3.8 V) and large theoretical capacity (∼128 mA h g –1 ). , NVPF has an open three-dimensional (3D) frame structure composed of [PO 4 ] tetrahedron and [V 2 O 8 F 3 ] double-octahedron, which is conducive to Na ions diffusion . Nevertheless, its low electronic conductivity always leads to inferior capacity, which greatly hinders its utilization .…”