Electrospun NaVPO₄F/C Nanofibers as Self‐Standing Cathode Material for Ultralong Cycle Life Na‐Ion Batteries

Abstract

Cathode materials mainly include transition metal oxide compounds, [11][12][13][14][15] polyanionic compounds, [16][17][18][19][20][21][22][23] Prussian blue analogues, and organic materials. [24][25][26][27][28] Among the various cathode materials for SIBs, polyanion-based cathode materials possess stable 3D host framework structure due to strong covalent bonding of oxygen atom in the polyanion polyhedra, resulting in their excellent thermal stability and long cycle life. [29] More importantly, this open 3D framework could provide enough interstitial channels for Na + transit and buffer severe volume change during Na + insertion/extraction. Particularly, NaVPO 4 F has attracted a great deal of interests owing to the low-cost raw materials, safe application, and high working potential. NaVPO 4 F was first proposed by Barker et al., [30] which possesses a tetragonal symmetry structure (space group I4/mmm). The crystal structure is consistent with the sodium aluminum fluorophosphate (Na 3 Al 2 (PO 4 ) 2 F 3 ). When used as cathode for Na-ion batteries, it delivered a discharge capacity of 82 mA h g −1 . However, the capacity faded more than 50% after 30 cycles. To improve the cyclability and rate performance, many strategies, including coating with conductive materials, fabricating pores, and doping alien ions have been attempted. [31][32][33] Although these attempts improve the electrochemical property of NaVPO 4 F in a certain degree, the capacity of the reported NaVPO 4 F materials is far below its theoretical specific capacity and still could not meet the application requirements in Na-storage. The root problem is traditional technology for preparing NaVPO 4 F mainly based on the high-temperature solid-state reaction, sol-gel method, and hydrothermal method, which often produce bulk or micrometer-sized NaVPO 4 F particles with insufficient carbon coating, leading to rapid capacity fading since this structure is unfavorable to electron transfer and the permeation of electrolyte. [31,32,34] Hence, it is significant to enhance the kinetics of Na-ion transfer in NaVPO 4 F. In order to achieve this goal, strategies mainly include decreasing the crystallite size and altering morphology of the material. [7,22,35,36] As far as we know, electrospinning is a versatile technique to prepare various 1D carbon-containing composites and produce flexible membrane, [6,8,[37][38][39] which encourages us to fabricate NaVPO 4 F with novel morphology combined the method of electrospinning to improve its electrochemical performance.Herein, we first synthesized 1D NaVPO 4 F/C nanostructure via an electrospinning method. Such a structure combines a variety of advantages for battery electrodes: (I) the small nanoparticles (≈6 nm) shorten the length of Na-ion transport; (II) NaVPO 4 F has received a great deal of attention as cathode material for Na-ion batteries due to its high theoretical capacity (143 mA h g −1 ), high voltage platform, and structural stability. Novel NaVPO 4 F/C nanofibers are successfully prepared via a feasible el...

A Simple One‐Pot Strategy for Synthesizing Ultrafine SnS2 Nanoparticle/Graphene Composites as Anodes for Lithium/Sodium‐Ion Batteries

“…The spectrac urves contain as emicircle in the high-frequency region and as traight line in the low-frequencyr egion, indicating that the electrochemical process is controlled by both charget ransfer and lithium ion diffusion. [45] The inclinedl ine in the low-frequency region is associated with the Warburg impedance (Z W ), whichi sr elated to the lithium ion diffusion between the electrode and the electrolyte. [45] The inclinedl ine in the low-frequency region is associated with the Warburg impedance (Z W ), whichi sr elated to the lithium ion diffusion between the electrode and the electrolyte.…”

A New Strategy for High‐Voltage Cathodes for K‐Ion Batteries: Stoichiometric KVPO4F

“…[26][27][28][29][30]37] In fluorophosphates, F − can often be substituted with some O 2− (oxygenation), in which case charge compensation is achieved by changing the oxidation state of nearby transition metals.…”

“…Fluorophosphates, including AVPO 4 F (A = Li and Na) [25][26][27][28] and Li 2 MPO 4 F (M = Fe and Mn) [29][30] , continue to attract considerable attention as cathode materials for LIBs and NIBs [ 31] This recent discovery motivates our investigation of the structural and electrochemical properties of KVPO 4 F for use as a KIB cathode. Specifically, in this work, we investigate the structure of as-synthesized KVPO 4 F using 31 P solid-state nuclear magnetic resonance (NMR) spectroscopy and X-ray diffraction (XRD), and the structural evolution of the material during K de/intercalation using ex situ XRD and ab-initio calculations.…”

Rational Architecture Design Enables Superior Na Storage in Greener NASICON‐Na4MnV(PO4)3 Cathode

“…Therefore, it is reasonable to conclude that the outstanding cycling stability of NMVP@C@GA should be ascribed to the structure of robust interconnected GA framework-supported in situ carbon-coated NASICON-Na 4 MnV(PO 4 ) 3 , which can not only endure the current attack even at high rate of 20 C, but can also buffer the lattice strain and volume change caused by Na + diffusion behaviors to prevent the electrode from structural degradation. [43] NMVP@C@GA displays the smallest slope, reflecting the fastest sodium diffusion realized by the architecture design. Energy Mater.…”