Observation on the electrochemical reactions of Li3-xNaxV2(PO4)3 (0 ≤ x ≤ 3) as cathode materials for rechargeable batteries

In this work, we propose a facile solid state reaction to prepare Li 3-x Na x V 2 (PO 4 ) 3 /C (0 ≤ x ≤ 3), which are rarely explored as possible cathode materials in sodium-ion batteries. Their phase composition, surface morphology and electrochemical property are described and compared in detail by using various physical/chemical methods. Seen from phase analysis results, it is known that Li 2.5 Na 0.5 V 2 (PO 4 ) 3 is a three-phase mixture (monoclinic Li 3 V 2 (PO 4 ) 3 , rhombohedral Li 3 V 2 (PO 4 ) 3 and rhombohedral Na 3 V 2 (PO 4 ) 3 ). In contrast, Li 2 NaV 2 (PO 4 ) 3 , Li 1.5 Na 1.5 V 2 (PO 4 ) 3 , LiNa 2 V 2 (PO 4 ) 3 and Li 0.5 Na 2.5 V 2 (PO 4 ) 3 are two-phase mixtures consisting of rhombohedral Na 3 V 2 (PO 4 ) 3 and rhombohedral Li 3 V 2 (PO 4 ) 3 . Evaluated as cathode materials in half-cells, Li 3-x Na x V 2 (PO 4 ) 3 /C (0 ≤ x ≤ 3) can deliver the initial reversible capacities of 94.3 mAh g −1 (x = 0.0), 103.6 mAh g −1 (x = 0.5), 115.1 mAh g −1 (x = 1.0), 112.7 mAh g −1 (x = 1.5), 99.6 mAh g −1 (x = 2.0), 94.0 mAh g −1 (x = 2.5) and 91.9 (x = 3.0) mAh g −1 , respectively. After 45 cycles, the reversible capacities of 88.9 mAh g −1 (x = 0.0), 88.2 mAh g −1 (x = 0.5), 80.6 mAh g −1 (x = 1.0), 99.6 mAh g −1 (x = 1.5), 87.2 mAh g −1 (x = 2.0), 72.7 mAh g −1 (x = 2.5) and 72.3 mAh g −1 (x = 3.0) can be still kept for these samples. The electrochemical results suggest that Li/Na co-existence in M 3 V 2 (PO 4 ) 3 /C may be beneficial to enhance the electrochemical performance . As a result, Li 1.5 Na 1.5 V 2 (PO 4 ) 3 /C shows the best sodium storage capability as cathode material in sodium-ion batteries.