Titanium Doping Induced the Suppression of Irreversible Phase Transformation at High Voltage for V‐based Phosphate Cathodes of Na‐Ion Batteries

Abstract

Polyanionic material, specifically the NASICON‐type material, is considered a promising cathode material for Na‐ion batteries (SIBs) because of its stable structure and high operating voltage. Further, it improves the energy density correlated with the well utilization of all Na in the compound. For Na3V2(PO4)2F3 (NVPF), the extraction of the third Na, reported as the electrochemical inactivated, can be realized at a high voltage region while forming an irreversible tetragonal phase. In this study, we introduce Ti doping to the Na2VTi(PO4)2F3 (NVTPF) material; we reveal that the Ti‐doped NVTPF could effectively suppress the irreversible phase transformation, thus successfully harnessing Na in a wide voltage range. Experimental study discloses that the Ti‐substituted Na2VTi(PO4)2F3 could take up the Na+ from Amam phase to Cmc21 phase between 1.0 V and 4.8 V reversibly accounting for the 2 Na+ transportation that shows favorable Na+ kinetics and structural stability. Our research provides the strategy to stabilize the polyanion structure upon charging at a high voltage range and inspires the utilization of full sodium in the polyanionic materials, which could be considered as a material design for future conventional applications.

Exceeding Three-Electron Reactions in Polyanionic Cathode To Achieve High-Energy Density for Sodium-Ion Batteries

“…Specifically, the NVTP-0.5 exhibits a smaller cell volume and lattice parameter ( a ) compared to undoped NVTP-0, which is caused by less Na + entering the lattice and introduction of Ti 4+ with the smaller ionic radius (60.5 pm vs 64 pm for V 3+ ) . The average lengths of Ti–O bonds are shorter than those of V–O bonds, indicating that the structure becomes more stable by Ti substitution . Consistent with the design of NVTP-0.5, the incorporation of Ti leads to the substitution of 25% of V. The inset of Figure b also shows the visualization of the crystal structure and atomic occupation of NVTP-0.5, where the octahedral transition-metal (TM) sites are occupied by V and Ti.…”

“…Figure a shows the X-ray diffraction (XRD) patterns of NVTP-0, NVTP-0.5, and NVTP-1 samples, which can be indexed to the trigonal crystal phase with the space group R 3̅ c (PDF#00-062-0345). The great similarity of XRD patterns indicates the full solubility of V and Ti in the framework. − The enlarged image shows a progressive shift of the diffraction peak toward higher angles as the content of Ti increases. This phenomenon is attributed to the alteration of the TM type and the corresponding adjustment of the sodium content.…”

“…As mentioned before, V–O and Ti–O in NVTP-0.5 provide stronger bond strength compared with the counterpart (V–O) in NVTP-0, which establishes its superior structural stability. With the strengthening of the bond interaction of TM-O, the bonding actions can effectively divert the strain variation by diversified TM-O in NVTP-0.5 . Accordingly, the robust frame of NVTP-0.5 guarantees the phase transition reversibility by virtue of the Ti–O and V–O participation.…”

A zero-strain Na-deficient NASICON-type Na2.8Mn0.4V1.0Ti0.6(PO4)3 cathode for wide-temperature rechargeable Na-ion batteries

“…5–9 However, the high cost and intense toxicity of V-based raw materials weaken the significant cost advantage of NIBs, limiting their future application in large-scale expansion. 10–14…”

Synergetic impact of high-entropy microdoping modification in Na3V2(PO4)3

“…9 Therefore, further increasing the capacity and energy density of NVP, and overcoming its inherent drawbacks of low electronic and ionic conductivity would greatly promote its application. To surpass the challenging issues mentioned above, a lot of effort has been made, involving cation doping 10–14 and carbon coating. 15,16 Recently, the concept of a high-entropy doping tactic with the advantage of synergetic utilization of multiple dopants was proposed and proved to be effective in the SIB cathode materials.…”