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

Graphical The improvement of the energy density of Na 3 V 2 (PO 4 ) 2 F 3 (NVPF) is now related to the utilization of the third Na + , which was reported to be electrochemically inaccessible. Our research revealed that Ti-doped Na 3 V 2 (PO 4 ) 2 F 3 (NVTPF) could be overcharged at approximately 4.9 V without forming the irreversible metastable phase, which means the electrochemical inactive Na + could be effectively attracted without affecting the subsequent electrochemical behavior. 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 Na 3 V 2 (PO 4 ) 2 F 3 (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 Na 2 VTi(PO 4 ) 2 F 3 (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 Na 2 VTi(PO 4 ) 2 F 3 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.