Unraveling the Influence of Hetero Atom Doped Carbon Coating and Morphology Control on Electrochemical Behaviour of Na3V2(PO4)2F3 Cathode Material
Authors: Gupta Priyanka , Maddar Faduma , Basu Suddhasatwa , Hasa Ivana 
Published: December 2024

The rapid growth of the global battery demand has accelerated the search for efficient low-cost and sustainable batteries. Among them, sodium-ion batteries (SIBs) represent one of the most promising electrochemical energy storage systems for stationary and light vehicle applications. The growing interest in SIBs has been catalyzed by the adoption of this battery technology by several companies worldwide.

Despite the great achievements recently announced, several challenges persist. Among them, is the limited energy density of SIBs especially when compared to lithium-ion batteries. To tackle the challenges, research has been focused on the rational design of more powerful sodium-ion battery materials. While hard carbon seems to be the anode of choice, several cathode chemistries have been proposed so far. Out of all, polyanionic compounds have garnered widespread attention as promising reversible cathode materials given their rich chemistry, ability to tolerate volumetric stress upon cycling, and high operating potential. Lattice doping with three fluoride ions onto NASICON-type Na3V2(PO4)3 material yields the formation of a fluorophosphate Na3V2(PO4)2F3 (NVPF) with enhanced theoretical capacity and operating potential of 128 mAh g-1 and 3.95V vs. Na+/Na respectively. Nevertheless, performance-limiting traits such as inferior electronic conductivity and the restricted reversible capacity obstruct the overall practical application for high energy density SIB.

To address the listed challenges, we have examined the synergistic role of structural modulation and surface modification strategies through the synthesis of boron-doped carbon-coated hollow microsphere Na3V2(PO4)2F3 (NVPF) materials deploying solvothermal, chemical vapor deposition with a subsequent solution-mixing technique. The prepared materials were analyzed via physical-chemical characterizations to elucidate the phase identification, crystal structure, surface morphology, optimal coating thickness, and prevailing oxidation states. Further, the electrochemical activity of the as-prepared materials was demonstrated in half-cell configuration using rate capability and galvanostatic charge-discharge (GCD) cycling within a potential window of 2.5V- 4.3V.

In this work, we discuss the comparison between a pristine version of the NVPF (S1) material with two carbon-coated materials, namely a 10 wt% boron-doped carbon-coated NVPF (S2) and 20 wt% boron-doped carbon-coated NVPF (S3). It is found that the optimal conditions for improved rate performance and cyclability are identified for the composite NVPF (S2) material with capacity retention of 97% after 300 cycles when compared to NVPF (S1) and coated NVPF (S3) which displayed capacity holding of 64% and 94%. Further, an in-depth analysis through impedance spectroscopy and post-mortem characterization has been carried out to shed light on the effect of the boron-doped carbon coating on interfacial and structural stability upon cycling.